Groundwater Modelling Report · Groundwater Modelling Contents Abbreviations vi Glossary of Terms...

Cairns Water

Appendix DGroundwater Modelling Report

42/15610/98344 Mulgrave River Aquifer Feasibility StudyGroundwater Modelling

Contents

Abbreviations vi

Glossary of Terms viii

1. Introduction 1

1.1 Scope of Work 1

1.2 Previous Studies 2

1.3 Regional Conditions 3

1.4 Models Used 3

1.5 Nature and Limitations of this Report 4

2. Background 6

2.1 Study Area 6

2.2 Location and Topography and Drainage 6

2.3 Drainage 7

2.4 Climate 8

2.5 Vegetation and Land Use 8

2.6 Soils 12

2.7 Geology 13

2.8 Surface Water 16

3. Hydrogeology 19

3.1 Field Investigations 19

3.2 Inferred Hydrostratigraphy 21

3.3 Aquifer Tests and Hydraulic Properties 24

3.4 Groundwater Levels 24

3.5 Fresh/Saline Groundwater Interfaces 32

3.6 Groundwater Usage 33

3.7 Groundwater Recharge 36

3.8 Baseflow Analysis 37

3.9 Preliminary Catchment Water Balance 41

4. Model Construction 43

4.1 Code Selection 43

4.2 Model Datasets and Extent 43

4.3 Boundary Conditions 47


4.4 Near Surface Processes and Groundwater Recharge 47

4.5 Groundwater and Surface Water Abstraction 48

4.6 Aquifer Parameters 49

5. Model Calibration 54

5.1 Calibration Quality 55

5.2 Calibration Sensitivity Analysis 58

5.3 Calibrated Model Water Balance 59

5.4 Model Limitations 61

6. Numerically Modelled Impact Assessment 62

6.1 Background and Approach 62

6.2 Bore Field Design 67

6.3 Predicted River Baseflow Impacts 67

6.4 Predicted Surface Water Impacts in Context 70

6.5 Predicted Drawdown Impacts 70

6.6 Predictive Model Sensitivity Analysis 74

6.7 Potential Impacts of Climate Change 75

7. Conclusions 77

7.1 Results 77

7.2 Model Summary 77

7.3 Potential Impacts 78

8. Recommendations 83

9. References 84

Table IndexTable 1 Mulgrave River Groundwater Resource Potential 13Table 2 Investigation Sites 19Table 3 Aquifer Test Results 24Table 4 Surface Water Analysis Summary 40Table 5 Preliminary Water Balance – Mulgrave River

Alluvium 41Table 6 Mulgrave Alluvium Water Balance – Using

Calibrated Groundwater Model 60Table 7 Existing Groundwater Users Potentially Affected by

the Proposed Abstraction 71


Table 8 Existing Groundwater Users Potentially Affected bythe Proposed Abstraction Assuming the LowestLevel of River Aquifer Connection 74

Table 9 Predicted Impacts – Dry Season, Average ClimaticYear (1997-98) 80

Table 10 Predicted Impacts – After Several Dry ClimaticYears (June 2001 to November 2003) 81

Table 11 Predicted River Flow Impacts – Dry Season,Average Climatic Year (1997-98) 81

Table 12 Predicted River Flow Impacts – Aquifer Several DryClimatic Years (June 2001 to November 2003) 82

Table 13 Soil Parameters for Subsidence Calculations 103Table 14 Calculated Subsidence (m) 104

Figure IndexFigure 1 Location Plan 9Figure 2 Digital Terrain Model 10Figure 3 Long Term Average Annual Rainfall 11Figure 4 Soils 14Figure 5 Geology 15Figure 6 Hydrologic Monitoring 18Figure 7 Geological Surface Depth of Bedrock 22Figure 8 Geological Surface – Top of Model Layer 2 23Figure 9 Groundwater Level Hydrographs, 1973 –2004 25Figure 10 Average June to July Groundwater Levels 26Figure 11 Average Wet Season Groundwater Levels 27Figure 12 Average Dry Season Groundwater Levels 28Figure 13 Average Annual Groundwater Level Fluctuation 29Figure 14 Average June to July Depth to Water Table 30Figure 15 Existing Groundwater Users – Primary Bore

Purpose 34Figure 16 Existing Groundwater Users – License Allocation 35Figure 17 Measured Flow and Salinity Relationships 38Figure 18 Groundwater Model Grid and Boundaries 45Figure 19 Aquifer Test Calibration Grid 46Figure 20 Hydraulic Conductivity Layer 1 50Figure 21 Hydraulic Conductivity Layer 2 51Figure 22 Storage Coefficient Layer 1 52


Figure 23 Storage Coefficient Layer 2 53Figure 24 Calibrated Steady State Watertable Contours 57Figure 25 Calibration Sensitivity Analysis 59Figure 26 Modelled Drawdown Response Scenario 1 63Figure 27 Modelled Drawdown Response Scenario 2 64Figure 28 Scenario 1 Sensitivity Analysis: 1500 ML/year

Abstractions, Modelled Dry Season Impacts,Minimal River/Aquifer Connection 65

Figure 29 Modelled Drawdown Following Successive DryYears 66

Figure 30 Abstraction Volume and Baseflow Impacts –Sensitivity Analysis 73

Figure 31 Modelled Drawdown following successive dry years 75

AppendicesA Geophysical Logging InventoryB Observed and Modelled Groundwater Level HydrographsC Observed and Modelled Aquifer Test DrawdownsD Detailed Water Balance Calculations – Mulgrave River AlluviumE PERFECT Model – Assigned Soil PropertiesF Residual Statistics – Steady State CalibrationG Residual Statistics – Transient CalibrationH Land Subsidence Analysis

vi42/15610/98344 Mulgrave River Aquifer Feasibility StudyGroundwater Modelling

Abbreviations

ADWG Australian Drinking Water Guidelines

AHD Australian Height Datum

ANZECC Australian and New Zealand Environmental Conservation Council

APSIM Agricultural Production Systems sIMulator

AS/NZS Australian Standard / New Zealand Standard

ASS Acid Sulfate Soils

BFI Baseflow Index

CMB Conductivity Mass Balance

CSIRO Commonwealth Scientific and Industrial Research Organization

DERM Department of Environment and Resource Management

DF Direct Filtration

DNR Department of Natural Resources

DNRMW Department of Natural Resources, Mines and Water

DNRW Department of Natural Resources and Water

DO Dissolved Oxygen

DTM Digital Terrain Model

EC Electrical Conductivity

Eh Redox Potential

EPA Environmental Protection Agency

GHB General Head Boundaries

GPS Global Positioning System

HACCP Hazard and Critical Control Point

HDPE High Density Polyethylene

LIDAR Light Detection and Ranging

LTA Long Term Average

MCPA 2-methyl-4-chlorophenoxyacetic acid

MF Microfiltration

ML Millilitres

NEPM National Environment Protection Measures

NRM Natural Resources and Mines

vii42/15610/98344 Mulgrave River Aquifer Feasibility StudyGroundwater Modelling

nRMS normalised Root Mean Squared

PAC Powdered Activated Carbon

PACl Poly Aluminum Chloride

PAH Phenoxyacetic Acid Herbicides

PASS Potential Acid Sulfate Soils

QASSIT Queensland Acid Sulfate Soil Investigation Team

QWQG Queensland Water Quality Guidelines

RO Reverse Osmosis

STP Sewage Treatment Plant

SW Surface Water

SWL Standing Water Level

TDS Total Dissolved Solids

USGS United States Geological Survey

WQ Water Quality

WTP Water Treatment Plant

WWTP Wastewater Treatment Plant

viii42/15610/98344 Mulgrave River Aquifer Feasibility StudyGroundwater Modelling

Glossary of Terms

Abstraction The process of taking water from any source, either temporarily or permanently

Aeration The process by which air is circulated through, mixed with or dissolved

Alluvial aquifer An area of water-bearing sand

Alluvium Soil or sediments deposited by a river or a moving water body. It typically consists of silt, clay, sand

and/or gravel.

Aquifer A geological formation that is made up of water-bearing permeable rock or unconsolidated material

allowing the storage and transmission of significant volumes of water.

Basalt A common mafic extrusive volcanic rock that is grey to black and fine grained due to rapid cooling of

lava. It may contain larger crystals.

Base flow Portion of streamflow that comes from groundwater and not from a runoff

Bore A hole or a passage made by drilling that is used for water sampling

Borefield An area that contains the bores or wells through which the water is extracted

Catchment An area which water is collected / captured, with catchment area referring to an area that is drained

by a river

Chlorination The process of adding the element chlorine to water as a method of water purification

Coagulation The process of forming liquid into semisolid lumps

Coastal plain An are of flat, low-lying land adjacent to a coast (sea) and separated from the interior by other

features

Conductance The capacity to conduct electricity

Confined aquifer Confined aquifers are permeable rock units that are generally deeper than unconfined aquifers.

They are overlain by relative impermeable rock that limits groundwater movement into and out of

the aquifer.

Depressurisation Decreasing the pressure

Drawdown Lowering of groundwater or drop in level of water in the ground. It is typically due to pumping of

wells / bores.

Filtration A method of water purification where solids are separated from liquids

Floodplain Flat or nearly flat land adjacent to a stream or river that experiences occasional or periodic flooding

Granite A common and widespread intrusive, felsic, igneous rock with a medium to coarse texture. Granites

can be pinto to dark grey depending on their mineralogy

Geology The science and study of the solid matter that constitutes Earth

Groundwater flux The rate of flow / discharge of groundwater

Groundwater recharge The process of adding water to an aquifer

ix42/15610/98344 Mulgrave River Aquifer Feasibility StudyGroundwater Modelling

Hydraulic Device / operation that uses pressure or flow of water

Hydraulic conductivity A property of soil or rock that describes the ease with which water can move through pore spaces or

fractures. It depends on the intrinsic permeability of the material and the degree of saturation

Hydrogeological Geologic characteristics that influence the underground flow or movement of water

Hydrograph Graph of water table versus time

Hydrology The study of the movement, distribution and quality of water throughout Earth, addressing the

hydrological cycle and water resources

Infiltration The process of water on ground surface enters the soil

Lithology The study / description of rock composition

Metamorphic The term used to describe rocks that have been transformed by extreme heat and pressure

Pilot Something that serves as a model

Pilot trials A precursor / foundation to a full-scale study

Quaternary A Geological Period that began after the Neogene Period, approximately 1.8 million years ago, to

the present

Recharge A hydrological process where surface water moves to groundwater, often resulting in water table

fluxes

Runoff A term used to describe the movement/flow of water from rainfall or other sources over the land

Sedimentation The term used to describe the deposition by settling of suspended material

Sludge Residual semi-solid material left from a process

Storativity The volume of water an aquifer released

Tertiary A Geological Period that marks the beginning of the Cenozoic Era, extending from approximately 65

million years ago to 1.8 million years ago.

Throughflow Movement of water horizontally beneath land surface

Transmissivity The rate at which water moves / transmitted

Turbidity The cloudiness / haziness of a fluid caused by suspended solids that are too small to settle out.


1. Introduction

As part of the finalisation of its Water Supply Strategy, Cairns Water and Waste is embarking on aprogram of works to provide necessary information to identify and confirm future water supply source.This Water Supply Strategy includes an assessment of the feasibility of obtaining a sustainable watersupply from the Mulgrave River Aquifer system (Mulgrave River Aquifer Scheme). If developed, thegroundwater supply will be used to supplement existing sources of water to Cairns.

GHD has been engaged to undertake the Feasibility Study into the Mulgrave River Aquifer. The keycomponent of this Feasibility Study is the assessment of the two primary issues considering the aquiferas a supplementary groundwater supply.

Environmental impacts; and

Sustainable abstraction volume.

In relation to the above a number of potential impacts were identified that required specific assessmentduring the development of ground water model. The following key impacts of abstraction need to beassessed in conjunction with other investigations. This assessment will be able to determine asustainable abstraction volume.

Unacceptable drop in groundwater levels for existing groundwater users;

Unacceptable impact on environmental flows in the Mulgrave River and its tributaries;

Inducement of saltwater intrusion to the aquifer from coastal areas;

Impacts of land settlement;

Creation of potential contaminant groundwater migration towards bores extraction points;

Inducement of negative environmental impacts through changes in groundwater conditions in areasof acid sulphate soils; and

Any other negative impacts to natural resources and the environment.

This report details the outcome of the groundwater flow modelling of Mulgrave River Aquifer. It providesinput with respect to assessment of the sustainable extraction rate and potential impacts on existinggroundwater and surface water users, existing infrastructure and the environment.

1.1 Scope of WorkDevelopment and operation of the numerical groundwater model can be broken down into six maintasks:

1. Data review and analysis

2. Conceptual Hydrogeological Model Development

3. Numerical Model Design and Construction

4. Numerical Model Calibration

5. Predictive Modelling – Estimation of Sustainable Yield

6. Model Output and Reporting


The first two main tasks have been completed in earlier studies, and are documented in Mulgrave RiverFeasibility Study Hydrogeological Report (GHD Draft report 42/14087/01/7410, August 2006). Tasks 3 to6 are presented in this report.

1.2 Previous StudiesThere have been a number of earlier studies that have examined the potential of Mulgrave RiverAquifer’s groundwater as a water source. These include:

Muller,P.J., 1975: Mulgrave River Groundwater Investigations, Report on Exploratory Drilling. Rec.Geol. Surv. Qld. 1975/17.

Leach L.M. and Rose U.E., 1979: Groundwater Storage Behaviour, Mulgrave River Area. Qld WaterRes. Comm. Report.

Connell Wagner (1992). Russell Mulgrave River Overview Study Report Stages 1-3. (In associationwith Environment Science and Services). Cairns.

Dept. of Natural Resources, Qld Huxley W. and Bjornsson B., 1998a: A review of GroundwaterConditions and Opportunities for Further Development – Mulgrave River Alluvium. ResourceSciences Centre. Dept. of Natural Resources, Qld

In 1999 GHD was commissioned by the Department of Natural Resources to review the above reportswith respect as to the potential for abstraction from the Mulgrave River Aquifer, or river itself, as a watersupply.

GHD (1999) Mulgrave River Aquifer Study. Report on Abstraction to Supply Cairns City. Report forthe Department of Natural Resources. GHD Pty Ltd, Cairns.

Further to this report with increasing pressures on existing water supplies, Cairns Water engaged GHD in1994 to review the aquifer option to reflect the most current urban growth and available water supplyposition.

GHD (2004) Cairns Water Supply Source Options Review. Mulgrave River Aquifer Water SupplyScheme. Report for Cairns Water. GHD Pty Ltd, Cairns.

On the basis of previous studies extending back to 1975, the recommendation was made that the aquiferbe formally investigated as a supplementary water supply.

Previous reports had identified that yields of up to 41 ML/day may be feasible, based on information fromhydrogeological surveys of bore productivity (Mullger 1975), modelling of the groundwater characteristics(Leach and Rose, 1979), and preliminary modelling and assessment of bore productivity (DNR, 1998).However, the immediate short to medium term requirement to meet the growing urban demand wasestimated to be in the region of 12 to 17 ML/day.

The numerical modelling undertaken in this report is therefore based on two scenarios:

A Stage 1 (short to medium term) supply of 15 ML/day to meet the short to medium term demand fora supplementary supply; and

A Stage 2 supply of 40 ML/day, to meet medium to longer term projected demand.

These limits have been set by previous investigations as being feasible potential requirements of theaquifer groundwater supply.


1.3 Regional ConditionsIn a number of instances throughout Australia the development of groundwater supplies has beenundertaken in the absence of data to ascertain what the potential impacts of a groundwater developmentmay be. Over 100 years after the first bores were developed in the Great Artesian Basin, there is now aconcerted capping program undertaken in Queensland by the Queensland Department of NaturalResources and Water as the cumulative impacts of a century of abstraction are having quantifiableenvironmental impacts.

In more recent times, limited data interpretation and poor understanding of hydrogeological relationshipsof the abstraction from the groundwater has resulted in salinity and acid sulfate soil conditions beinggenerated in coastal areas. These conditions have impacted the economic and environmentalsustainability of these areas (for example, rising saline groundwater in the Bundaberg district).

When considering the model, it was is important to differentiate the unique conditions in relation to theMulgrave River aquifer from that of other coastal groundwater resources currently in use in Queensland.

An annual rainfall which exceeds 9 metres (Mt Bellenden Ker) in the upper catchment of theMulgrave River, conferring an extraordinary high potential for aquifer recharge unmatched inAustralia. This rainfall also ameliorates the risk of saltwater intrusion into the southern section of theaquifer that directly receives surface surface and groundwater recharge from the wettest area inAustralia.

Topographical position. By comparison with other Queensland coastal aquifers, the Mulgrave Riveraquifer is relatively sheltered from saline sea water influences. To the north the catchment of theaquifer is defined by a basaltic flow that minimises the marine influences of Trinity Inlet on theaquifer. To the south the aquifer tapers to the narrow estuary at Mutcheroo inlet. East and westuplifted and eroded mountain massifs directly constrain the aquifer to the valley of the MulgraveRiver and prevent general coastal seawater incursion into the aquifer.

Geological nature of substrate - the alluvium for the majority of the aquifer is extremely deep (over90 m), with high amounts of sands/silts and highly porous material overlying base bed rock ofgranite. The high transmissivity of the aquifer confers high recharge potential from the catchmentvia surface and groundwater water flow.

1.4 Models UsedA list of the codes (software) and the datasets used in defining and running the model are listed below.The overall modelling program used is MODFLOW 2000 (Harbaugh et al., 2000). MODFLOW is a finitedifference saturated groundwater flow model that has been comprehensively tested, widely utilised andaccepted, and is freely available and well documented. Groundwater Vistas was used as the graphicaluser interface for most of the model construction.

Additional supplementary software packages were run for other specific parameters of the overallMODFLOW model. These include:

PERFECT, A computer simulation model of Productivity, Erosion, Runoff Functions to EvaluateConservation Techniques (Littleboy et al 1989);

PEST Model-independent Parameter Estimation (Doherty 2002);


HYSEP: A Computer Program For Streamflow Hydrograph Separation And Analysis. (Slota et al1996);

Rosetta: A computer program for estimating soil hydraulic parameters with hierarchical pedotransferfunctions (Schapp et al 2001)

In addition to the formal models above, several mathematically specific formulae used for the following:

Calibration of base flow separation methods with streamflow conductivity.

Using groundwater levels to estimate recharge

The relation between the lowering of the piezometric surface and the rate and duration of dischargeof a well using groundwater storage.

A closed-form equation for predicting the hydraulic conductivity of unsaturated soils.

1.5 Nature and Limitations of this ReportThis is a technical document for review and consideration by those with experience in the assessment ofgroundwater modelling. While an attempt has been made to assist the general public in theunderstanding of this report, much of the terminology is specific to the language of the discipline, and hasno direct common interpretation. This report is not intended to be a standalone report, but is to beconsidered with reference to other investigations being undertaken.

An artifice in the model is the separation of the relative impacts of the scenarios modelled between theMulgrave River and Behana Creek. The quaternary alluvium representing the primary ground waterstorage area is not separated by any ground water divide between the Behana Creek and MulgraveRiver valley floor, i.e., there is only the one aquifer beneath both surface systems. At an early stage inthe modelling a request was made to attempt to differentiate the relative impacts on the modelledabstraction scenarios (15ML/day and 40 ML/day) on the surface water characteristics of both waterways. Within the limitations of the available data for surface water conditions at Behana Creek thismodelling has assumed conservative values (based on sensitivity analysis of the lacking data) for theestimation of impacts.

This report presents the results of a groundwater modelling investigation conducted for the purposes ofthis commission. It has been prepared specifically for the use of the client who commissioned the works.Reliance by other parties on this report is at their own risk.

Data (drill hole or test pit logs, laboratory tests, geophysical tests, etc.) gathered that has been performedand recorded by others is included and used as provided. The responsibility for the accuracy of suchdata remains with the issuing authority and not with GHD.

The advice tendered in this report is based on information obtained from the investigation locations, testpoints and sample points and is not warranted in respect to the conditions that may be encounteredacross the study area other than these locations. It is emphasised that the actual characteristics of thesubsurface and surface materials may vary significantly between adjacent test points and sampleintervals and at locations other than where observations, explorations and investigations have beenmade. Sub-surface conditions, including groundwater levels and quality can change in a limited time.This should be borne in mind when assessing the data. However, it is our opinion that the test pointschosen are representative of conditions for the study area. Should additional data be provided at a latertime, GHD reserves the right to amend this report to reflect the new information.


It should be noted that because of the inherent uncertainties in the sub-surface evaluations, changed orunanticipated sub-surface conditions may occur that could affect total project costs and/or execution.GHD does not accept responsibility for the consequences of significant variances in the conditions.

An understanding of the site conditions depends on the integration of many pieces of information,regional, site specific, structure-specific and experience based. Hence this report should not be altered,amended or abbreviated, issued in part or issued incomplete in any way without prior checking andapproval by GHD. GHD accepts no responsibility for any circumstances, which arise from the issue ofthe report, which has been modified other than by GHD.

The modelling investigation undertaken necessarily contains a large number of simplifications of theobserved site conditions and a number of assumptions in the development of the numerical model. Keyassumptions of the model are that groundwater flow can be represented by relatively simplistic equationsand that material properties are homogenous over broad areas.

These assumptions may not accurately reflect the actual site conditions and this may lead to variationsbetween the modelled results and field observations. The purpose of the modelling is to provide a tool toinvestigate the potential impact of various changes on the behaviour of the groundwater system. Theresults are intended to be relative rather than absolute. The results of the modelling should be viewed inthis context only.


2. Background

2.1 Study AreaThe Project Study Area comprises both the catchment of Mulgrave River itself, and that area of theMulgrave River valley underlain by Quaternary alluvium, referred to as the Mulgrave River aquifer.

It is important to differentiate these two areas, as they are the focus of different assessments used in theoverall Feasibility Study. The Mulgrave River Catchment per se, includes only a small proportion of theMulgrave River aquifer, however information related to catchment conditions, particularly as they relateto climate and surface and groundwater features, are critical in developing the numerical groundwatermodel upon which many of the predictions of impact of the abstraction will be made.

By contrast, the impacts of the project will be restricted to the environs of the Mulgrave River aquifersystem, which comprises less than ¼ the area of the total river catchment area. For the purposes of thisproject, the impacted area (the “aquifer area”) is defined as the Trinity Inlet catchment and the lowerMulgrave River valley below 20 m AHD and bounded by points at:

17° 14’ S, 145° 57’ E;

17° 14’ S; 145° 55’ E,

17° 02’ S, 145° 45’ E; and

17° 02’ S, 145° 50’ E.

Both the location of the catchment and the aquifer area are described below in further detail. The StudyArea is shown in Figure 1.

The impact assessment process used in this modelling report has used an all of catchment approach tomodelling the potential impacts of the abstraction on the aquifer area.

2.2 Location and Topography and DrainageA Digital Terrain Model of the area, representing land surface topography, is presented in Figure 2. TheMulgrave River valley runs in a roughly north-south line and is bordered to the east and west by rangesreaching as high as 1500 mAHD. The headwaters of the Mulgrave River and its tributaries are located inthese high ranges. The valley is narrow 100-1000 m) at the higher elevations, opening to a broad (~5-8 km wide) relatively flat valley around Gordonvale.

Valley floor elevation ranges from around 25 mAHD down to less than 5 mAHD. A minor topographicdivide occurs around Gordonvale. Elevation falls from approximately 20 mAHD around Gordonvale to~5 mAHD in both the northern and southern ends of the valley. This divide directs the Mulgrave River todrain to the valley’s southern outlet. Swamps and mangrove flats occur around both Trinity andMutchero Inlets (Figure 1). However as noted above the topographic divide near Gordonvale clearlydemarcates the influences of Trinity Inlet from the Mulgrave River valley.

The Mulgrave River valley is located on a narrow alluvial plain. It forms a flat to undulating floodplain,with river terraces east and south of Gordonvale at 23 m, 9 m and 6 m above the present river level(Muller, 1975).


To the west and east of the floodplain lie the rugged terrain of the Bellenden Ker and Malbon ThompsonRanges respectively. These granitic ranges have been deeply incised to form steep sided ranges fallingabruptly to the floodplain. The Bellenden Ker Range to the west of the plain rises to 1592 m at MountBellenden Ker Central Peak and 1622 m at Mount Bartle Frere further south. The Malbon ThompsonRange rises to 1026 m at Bell Peak North in the east.

To the northwest of Gordonvale metamorphic rocks form more subdued ranges rising to 1098 m. GreenHill, approximately 6 km northeast of Gordonvale, is a volcanic feature rising to 131 m elevation.

To the south of the Mulgrave River, the Russell River has a similar morphology.

2.3 DrainageThe Mulgrave River aquifer is contained within the valley of the Mulgrave River (‘the valley’). The aquiferextends over a length of around 40 km, from the divide just south of Trinity Inlet at Cairns in the north, toMutchero Inlet in the south, where the Mulgrave River meets the Russell River (Figure 1). Together,both rivers discharge to the ocean via Mutchero Inlet. There is comparably little surface drainage intoTrinity Inlet.

The Mulgrave River is one of the major coastal rivers in North Queensland. Covering an area ofapproximately 810 km2 and with a mean annual discharge of 770,000 ML, the Mulgrave River Catchmenthas one of the highest areas of mean annual runoff of any Australian catchment. The headwaters are inthe ranges to the west of the coastal floodplain, which the river enters at Gordonvale, meanderingeasterly then southerly across the floodplain to Mutchero Inlet.

River flows are highly seasonal; mean total flow in the dry season (August to December) is around161,000 ML (or ~14% of mean total annual discharge), and 849,797 ML (75% of mean total annualdischarge) during the wet season (January to May). The remaining 11% of total flow occurs in the periodJune-July.

The deeply incised ranges to the east and west of the floodplain generate the headwaters of a number ofstreams that form tributaries to the Mulgrave River. Behana Creek is the largest tributary on the westernside of the floodplain. A number of smaller streams flow from the ranges to the east, the other moresignificant streams being Fishery and Figtree Creeks. All the creeks carry significant flows during thewet season, but during the drier months may be reduced to intermittent flows dependent on rainfallevents in the upper catchment.

River terraces on the floodplain east of Gordonvale form a surface water divide, with the main surfacewater drainage heading from the divide north to Trinity Inlet, and on the southern side entering into theMulgrave River and hence into Mutchero Inlet. Mutchero Inlet is at the junction of the Mulgrave River(from the north), and the Russell River (from the south).

The clearing of land for sugar cane cultivation has extended to the top of the river banks and hasresulted in bank erosion and silting of the river as streams become wider and shallower (ConnellWagner, 1992). Local Landcare groups have undertaken limited revegetation of riverbanks in someareas. Riparian vegetation, however is generally fragmented, impacted by varying historical and ongoingland usages, and discontinuous along the waterways in the study area.

Sand and gravel has been extracted from the bed of the Mulgrave River and Behana Creek, resulting inpools several metres deep in areas previously shallow in the dry season.


2.4 ClimateThe Mulgrave River catchment is one of the highest rainfall areas of Australia. Long-term averageannual (LTA) rainfall on the valley floor ranges from as high as 3800 mm near Mutchero Inlet in thesouth, to 2000 mm in the area extending from Gordonvale northwards to Cairns and Trinity Inlet (Figure3). This rainfall is highly seasonal, with distinct wet (January to May) and dry (August to December)seasons. The mean annual evaporation does not vary significantly across the area, with an annualmean of 2251 mm measured at Gordonvale.

Cyclones typically develop between January and April and result in heavy rainfall. Four to six tropicalcyclones are formed in the Coral Sea each year and an average of two cross the coast in any given year.These systems exert a strong influence on rainfall variability in the region due to their unpredictability.For example, during the 2006 wet season the far north had experienced well above average rainfallrecords. This was due to continued monsoonal activities through April 2006 with Severe TropicalCyclone Monica increasing monthly rainfall totals for the study area by greater than 3 times the average.Additionally, Severe Tropical Cyclone Larry crossed the coast near Innisfail on the 20th March 2006. Thiscyclone was a category 5 and the first of this magnitude to cross the coast since 1918, also at the samegeographical location.

Conversely, in 2002, Cairns recorded its lowest rainfall since records began in 1882. El Nino and LaNina have a strong impact on the wet tropics system and influence rainfall. Generally the region isgetting progressively drier with lower rainfall averages due to the significant increase in El Nino events(Weston and Goosem, 2004). Planning for urban water supplies in dry years has now become a realityfor historically deemed wetter areas in Australia.

Climate gauging station data are available at Cairns, Gordonvale, and Mt Sophia (Figure 6). Temporallyinfilled data for the Gordonvale and Mt Sophia gauges obtained from the Queensland Department ofNatural Resources, Mines and Water (DNRMW’s) SILO service were used in this modelling study. TheLTA rainfall distribution shown in Figure 3 was used to divide the area into two “climate zones”represented by the Mt Sophia and Gordonvale gauge SILO data.

2.4.1 Temperature and Humidity

Temperatures are quite uniform throughout the year with typical daytime min/max in mid summer rangingbetween 23/31oC and 18/26oC mid winter. Occasional cold snaps occur overnight in the winter monthsbut these rarely fall below 14oC in the catchment valley.

Relative humidity values are typically high for the region reaching an average of 79% during the monthsof February to March, and 68-70% during the cooler months, but may reach into the 90’s quite regularly.(BOM website).

2.5 Vegetation and Land UseThe hills and mountains bordering the Mulgrave River floodplain are densely vegetated with complex anddiverse rainforest communities. In some areas (such as Walsh’s Pyramid), lower rainfall and occasionalbushfires have allowed the development of eucalypt woodlands and open forests.

The floodplain has been almost completely cleared for sugar cane cultivation and it is only lower-lyingareas of poor drainage that may have some remnant vegetation. Sugar cane is also grown on some ofthe gentle slopes on the margins of the floodplain. Mangroves occur towards the coast at both TrinityInlet and the lower reaches of the Mulgrave River at Mutchero Inlet


Figure 1 Location Plan


Figure 2 Digital Terrain Model


Figure 3 Long Term Average Annual Rainfall


2.6 SoilsSoils in the Mulgrave River catchment have been surveyed and mapped down to soil series at 1:50,000scale (Murtha et al., 1996; Figure 4). The mapped soil series have been divided into seven broad groupsbased on parent material and drainage status. Five of which are the dominant soils in the Mulgrave Rivercatchment - soils of granitic rock origin, metamorphic rock origin, mangrove soils, and well- and poorly-drained soils formed on alluvium (Murtha et al., 1996). Soils of basic rock origin and those formed onbeach ridges are also found, although less extensively, in the catchment.

Parent material influence is most pronounced on the floodplain margins. This parent material originatingfrom granitic areas soils are coarse textured and dominantly sandy, uniform or gradational textured; whilethose that develop from metamorphic areas are fine textured and predominantly clayey (Willmott andStephenson, 1989). On the floodplain, soils range from little-developed uniform textured fine sandy soilson the younger terraces and levees to strongly structured uniform or gradational soils on well drainedalluvium and clays on poorly drained areas (Willmott and Stephenson, 1989).

In this modelling study, hydraulic properties of each major horizon were calculated using data presentedin Murtha et al., 1989. This is described in detail in Section 3.

2.6.1 Acid Sulfate Soils

Acid sulfate soils are predominantly soils associated with areas of Quaternary alluvium with high levels oforganic matter and sulphidic material present. In the majority of cases (though not all) these areas aretypically to be found in swampy and tidally influenced areas (including mangroves). Acid and toxicconcentrations of metals can be released into the environment when acid sulfate soils are exposed to airand become oxidised. The Queensland Department of Natural Resources and Water and QueenslandAcid Sulfate Soil Investigation Team (QASSIT) have mapped the likely occurrence of potential acid soils(PASS) throughout coastal Queensland and have mapped the lower section of the Mulgrave River asbeing PASS. These soils are in areas dominated by mangrove and melaleuca wetlands, and in mostinstances are tidally influenced.

Acid sulfate problems exist for some farmers about Mutchero Inlet where vegetation clearing and groundtilling (for sugar cane) has resulted in the generation of actual acid sulfate conditions and subsequentloss in agricultural productivity. The general areas mapped as PASS by DNRW and QASSITapproximate the extent of mangrove intertidal saline soils.

There are two distinct conditions associated with acid sulfate soils in the Mulgrave River area.

The first condition is that most typically found in Cairns area. That is, acid sulfate soils are generallyfound below 5 m Australian Height Datum (AHD) and in Holocene sediments (organic-rich sediments andsilts). They are usually associated with coastal lowlands and estuarine flood plains and contain pyritesand sulfides. Typically the areas around (and within) any of the mangrove/tidally influenced areas (lowerreaches of the Mulgrave River) should be considered to have acid sulfate soils. Under natural conditionsacid sulfate soils are usually located below the watertable.

When these low-lying areas are exposed through dewatering, excavated or drained, there is potential forthe acceleration of soil oxidation (of the pyrites) and subsequent acid leachate generation.

The second condition is that of naturally occurring acid soil conditions, where the acidity is generated asa result of organic acids. It should be noted that organic acids (e.g. humic/tannic acids generated in


Melaleuca swamps) are a common feature of tropical coastal ecosystems. These organic acids canproduce acid water and sediments with the pH of these usually around 4.5 - 5.5. These sediments donot have the ability to generate additional acid when exposed to air and therefore do not pose the samerisk as Potential or Actual Acid Sulfate Soils (PASS and ASS generated from exposure of acid sulfatesoils).

The lower reaches of the Mulgrave River are in a coastal environment with expansive areas of Melaleucaswamps capable of leaching organic acids with subsequent acid soil conditions. Cane farming in thelower Russell/Mulgrave River reaches, in the area about Mutchero Inlet, has exposed both types of acidsulfate soil conditions. Extensive areas of tea tree swamp has been cleared for farming, and subsequentexcavation for drainage works has exposed acid sulfate soils with the consequence of oxidation of PASSand generation of acid runoff which has adversely affected productivity in these areas.

2.7 GeologyA detailed description of catchment geology is provided in the Hydrogeological Report, which wasprepared by GHD in 2006.

The hydrogeological review of the Mulgrave River area identified the Quaternary alluvium as having thehighest potential for groundwater development (Table 1). Other aquifers have significantly lower potentialyield.

Table 1 Mulgrave River Groundwater Resource Potential

Unit Aquifer Type Groundwater Resource Potential

Quaternary Alluvium Porous media High

Atherton Basalt Fractured rock Low

Tertiary Alluvium Porous media Moderate?

Basement (granite / metamorphics) Fractured rock Low

Quaternary Alluvium is the most widespread unit in the valley and a review of drilling records indicatesthe generally sandy nature of the area, which provides the best potential for groundwater yield.Groundwater analyses also indicated that water in the Quaternary Alluvium is of generally potablequality.


Figure 4 Soils


Figure 5 Geology


Figure 5 (cont): Geologic Cross Section of the Mulgrave River Valley

2.8 Surface WaterThe Mulgrave River is one of the major coastal rivers in North Queensland. Covering an area ofapproximately 810 km2 and with a mean annual discharge of 1,136,165 ML (up-scaled from Peets Bridgegauge). The Mulgrave River Catchment is one of the areas of highest mean annual runoff of anyAustralian catchment. The headwaters are in the ranges to the west of the coastal floodplain, which theriver enters at Gordonvale, meandering easterly then southerly across the floodplain to Mutchero Inlet(refer to for drainage features).

River flows are highly seasonal; mean total flow in the dry season is around 161,000 ML (or ~14% ofmean total annual discharge), and 849797 ML (75% of mean total annual discharge). The remaining11% of total flow occurs in the period June to July. These reported flows are up-scaled from PeetsBridge gauge, based upon the total catchment area versus gauged area.

The deeply incised ranges to the east and west of the floodplain generate the headwaters of a number ofstreams that form tributaries to the Mulgrave River.

Behana Creek is the largest tributary on the western side of the floodplain. A number of smaller streamsflow from the ranges to the east. All the creeks carry significant flows during the wet season. Riverterraces on the floodplain east of Gordonvale form a surface water divide separating northward andsouthward drainage across the valley floor.

Several surface water flow gauges have been or are actively monitored in the catchment (GHD, 2006).Review and analysis of these data has revealed that they are of variable length and quality. This reviewis documented in Section 3.8.

2.8.1 Licensed Usage

Surface water allocation data for the Mulgrave River catchment were obtained from DNRMW. Licensedsurface water abstraction totals around 37,000 ML/year by volume (to 14 licenses), plus an additional927 ha licensed by irrigated area (to 55 licenses). Assuming an irrigation application of 9 ML/ha/monthover a 5-month dry season (Steve Bertocchi, DNRMW South Johnstone, pers. comm.), this equates to afurther 42,000 ML/year allocated volume. Therefore, the total surface allocation is 79,000 ML/year.


Around 20% of the total allocation by volume (16,000 ML/year) is licensed to Cairns Water for theirBehana Creek off take, and a further 25% (~20,000 ML/year) is allocated to the Mulgrave Mill atGordonvale, although this is largely non-consumptive (i.e. much of the abstracted water is returned to theriver). Furthermore, local knowledge suggests that cane is not irrigated in many years in this area, and ismost likely only required, on average, a couple of months of the dry season per year. Therefore theconsumptive surface water abstraction in most years is likely to be less than 50% of the total licensedvolume.


Figure 6 Hydrologic Monitoring


3. Hydrogeology

The following section provides a summary of the hydrogeological investigation and conceptualisationdetailed in previous report (GHD, 2006). This is to further develop the conceptual hydrogeological model,upon which the numerical model is based.

3.1 Field InvestigationsPotential sites for detailed field investigation and testing were chosen based on aquifer lithology andgroundwater recharge. The potential sites are also chosen based on the following area characteristics.

Sandy vertical profile where seasonal recharge potential from rainfall appears significant;

Where runoff from ranges or infiltration from headwaters of smaller streams occurs;

With significant aquifer thickness at depth to maximise bore yields;

Away from the low permeability Atherton Basalt; and

Within the valley where a thick section of aquifer appears to be overlain by semi-confining orconfining units that are likely to limit surface water – groundwater interaction.

Investigation drilling was completed at four sites, where two sites (Area 2 and Area 3) have pump boresfor extended aquifer tests. The extended aquifer tests are conducted to determine the sites’ response topumping and for the assessment of its parameters.

Table 2 Investigation Sites

Location Depth Drilled (m) Outcome

Area 1 115 Atherton Basalt intersected; no significantsands beneath basalt; bore not constructed

Area 2 94.5 Pump test site; aquifer 31 to 49 m; 3observation bores; pumped at 40 L/s. Aquiferthickness tested is estimated at ~20 m, with~10 m observed drawdown at the abstractionbore, and ~2 m in nearby observation bores.

Area 3 84 Pump test site; aquifer 41 to 64 m; 3observation bores; pumped at 30 L/s. Aquiferthickness tested is estimated at ~20 m, with~14 m observed drawdown at the abstractionbore, and ~1 to 3 m in nearby observationbores.

Area 4 97 Relatively clay rich sequence compared toArea 2 and 3; constructed as observation bore


3.2 Inferred HydrostratigraphyThe stratigraphy and variability in lithology across the valley was confirmed based on the results from thegeophysical logging of 21 existing government and private bores and the drilling of four bores. Aninventory of the geophysical logging undertaken is presented in Appendix A. The results indicated thatthe valley sediments could be broadly grouped into two layers, separated by a clay or sandy clay sectioncorresponding with the base of Atherton Basalt. This was only clear in the geophysical logs, where asignificant change in sedimentation was identified across most bores at depth. The two layers arereferred to as Layer 1 and Layer 2 throughout the remainder of this report, where Layer 1 overlies Layer2.

In general, Layer 2 is more clay-rich than Layer 1. In some deeper sections of the valley, TertiaryAlluvium was identified in Layer 2 while Quaternary Alluvium comprises Layer 1. Layer 2 may be thin orabsent at the valley margins. The Area 2 pump bore was constructed in Layer 1 while Area 3 was inLayer 2. Each was constructed to abstract from the sandiest portion of the profile at every site.

Geological surfaces were constructed for the groundwater model, representing the top of Layer 2 and thetop of the underlying bedrock (or base of Layer 2). The depth from ground surface to bedrock ispresented in Figure 7 and the top of Layer 2 is presented in Figure 8. It should be noted that datadefining the top of Layer 2 was limited largely to the geophysical logging undertaken on four new bores,and on 21 existing government and private bores. This was due to this stratigraphic horizon beingperceived only in geophysical logs. It was not clearly discernable in the geological logs. In areas ofsparse data, this geological surface was inferred utilising the better-defined bedrock surface, and spatialtrends in the top of Layer 2 in areas with available data.

The construction of the base of Layer 2 (or basement top) was largely based on earlier mapping byDNRMW (Dept. Natural Resources, QLD 1999). Although this was checked and adjusted in places usingthe latest available data:

A total of 129 bores were identified in the Queensland bore database with logs identifiyingintersection of the bedrock surface (Figure 6); and

The four bores drilled during this investigation.

The largest area of uncertainty in the construction of this surface is towards Cairns and Trinity Inlet,where the alluvial valley widens significantly and sparse data present intersects the bedrock. TheMulgrave Alluvium is mapped as being 100-200 m thick on average along the deepest line of sediments,thinning to the edges of the alluvium along the east and west valley sides.


Figure 7 Geological Surface Depth of Bedrock


Figure 8 Geological Surface – Top of Model Layer 2


3.3 Aquifer Tests and Hydraulic PropertiesAnalysis of 100-hour aquifer tests conducted in test Areas 2 and 3 indicated transmissivity and hydraulicconductivity values as shown in Table 3. These were used as initial values in the groundwater model,and the detailed data recorded in observation bores during the tests were used to calibrate thegroundwater model (Section 5). The test analysis suggests that the aquifer is a leaky, semi-confinedsystem in both areas. Further details on these tests and analysis can be found in the previousHydrogeological Report (GHD, 2006).

Slug testing was also completed on 18 existing bores as part of this investigation. However, the boresare constructed with gravel pack backfilled almost to land surface. This gives slug test datarepresentative of the gravel pack permeability rather than that of the aquifer. The slug test data weretherefore disregarded in the modelling.

Table 3 Aquifer Test Results

Area Transmissivity(m2/day)

Hydraulic Conductivity (m/d) Storage Coefficient

2 700 35 2.3 x 10-3

3 400 18 3.3 x 10-4

3.4 Groundwater LevelsDNRW maintains and regularly monitors a network of groundwater observation bores in the MulgraveAquifer (Figure 6). Groundwater levels in the active monitoring bores are measured on a monthly toquarterly frequency. Various statistics relating to the groundwater monitoring are presented in Figure 9.These were assessed in conjunction with climatic data to determine the best monitoring data to be used.The montiroing data will be used to create representative groundwater level contour maps of the area.These were also assessed to determine the most suitable period against which to calibrate the mode.The resulting groundwater level contour maps are presented in Figure 10, Figure 11, Figure 12, Figure13 and Figure 14.


Figure 9 Groundwater Level Hydrographs, 1973 –2004


Figure 10 Average June to July Groundwater Levels


Figure 11 Average Wet Season Groundwater Levels


Figure 12 Average Dry Season Groundwater Levels


Figure 13 Average Annual Groundwater Level Fluctuation


Figure 14 Average June to July Depth to Water Table


The contour maps show groundwater flow from the elevated areas of the west, particularly the upperMulgrave River and west of Edmonton, to the northeast and southeast. A groundwater flow divide isevident in the main irrigation and abstraction area, northeast of Gordonvale. Inspection of averageannual groundwater level fluctuations suggests that this is an area of relatively high groundwaterrecharge (Figure 13). The high groundwater recharge is due to its location in the north and west ofGordonvale, on the flanks of the alluvium. The fluctuations in the main abstraction area are related toincreased storage (via seasonal drawdown) and capacity of the aquifer to accept more recharge thanareas distal to major abstractions. The fluctuations on the western flanks of the alluvium are due to thecoarser, better-drained nature of the soils in this area, and therefore relatively high groundwaterrecharge. The high heads also observed on the western alluvial flanks are a result of a combination of:

Lower transmissivity, due to thinning of the aquifer along the western alluvial edges, bedrocktopography is much steeper beneath the alluvium on the eastern boundaries (Figure 15); and

Higher recharge on the (higher permeability) soils along parts of the western edge of the alluvium,mainly Soils of Metamorphic Rock Origin, and Well Drained Soils Formed on Alluvium (Figure 4).

The average groundwater depth below the land surface is presented in Figure 14. It shows that theaquifer is on average near saturation (groundwater generally less than 5 m below ground) across most ofits area. The only exceptions to this are typically around the valley margins where the sediments thin outand lap up onto the elevated areas of underlying bedrock. Groundwater levels are, on average, at orabove ground surface across large areas of the aquifer in the north and south of the catchment, it is alsoin the area where Behana Creek crosses onto the alluvium from the elevated bedrock areas. Thissuggests that Behana Creek may be a potential source of groundwater recharge or that the surroundingarea is a location of enhanced recharge. It also suggests that the aquifer in this vicinity is typicallyrecharged to its full capacity. Further discussion on this is presented in Section 3.8.

The areas in the north and south of the valley, where groundwater levels are also at or above groundsurface, indicate that these are sections of groundwater discharge. The majority of rainfall falling ontothese areas is likely to be shed as runoff, because the aquifer is typically saturated to its capacity.

The average June-July groundwater level map (Figure 10) will be used to calibrate the steady stategroundwater flow model. This period was selected because it straddles the wet - and dry seasons, and itrepresents the average annual aquifer condition.

Hydrographs (Appendix B) show that groundwater levels generally fluctuate seasonally by between 1 mand 7 m. Appendix B presents a selection of bores with hydrographs of sufficient temporal resolutions.These bores provide the most appropriate basis against which to calibrate the groundwater model.Figure 13 depicts the wet season to dry season difference in interpolated groundwater levels, and showsannual fluctuation up to 4 m over much of the aquifer. It should be noted that the wet - and dry seasongroundwater level maps were constructed using point data available at different times and locations.Therefore, some of the mapped fluctuations may be a source of data insufficiency in one season versusthe other in some locations, rather than the true average annual fluctuation. Lower hydrographfluctuations are observed in bores close to surface water bodies, and greatest fluctuation is observed inbores on the more elevated flanks of the alluvium, particularly in the west.

3.4.1 Inferences on River-Aquifer Connection

It is evident in all the constructed maps that Mulgrave River is the preferential path for the majority ofgroundwater discharge in this catchment. The maps clearly show that the river controls groundwater


levels over large areas, acting as a drain to the Mulgrave Alluvium. Therefore it is considered that theaquifer and river are in direct hydraulic connection in this catchment, and that the river is largely againing feature (i.e. is fed by baseflow from the aquifer much of the time, over most of the catchment).Further discussion is presented in Section 3.8.

Groundwater levels appear to become increasingly controlled by surface water features (Trinity Inlet andthe Mulgrave River) towards the northern and southern catchment outlets. In addition, it is generallyhigher than surface water stage heights throughout the catchment. Furthermore, the constructedgroundwater level maps, particularly for the average wet season, show a strong influence (loweredgroundwater levels) of surface water features over large areas. The depth to groundwater map alsosupports this conclusion (Figure 14).

Review of bore hydrograph data at locations close, but up-catchment of river flow gauges provide furtherevidence that the river is a gaining feature. Comparison of average groundwater levels at bore11100075 (14.6 mAHD), which is the closest bore with sufficient data located up-catchment of theGordonvale gauge, shows that groundwater levels are on average 6.7 m. This is higher than the averagegauged river height at Gordonvale. This bore is located only 1 km from the gauge, which suggests highhydraulic groundwater gradients (0.007 m/m) to the river from the aquifer upstream of Gordonvale.Assuming an average 20 m thickness of aquifer contributing baseflow to the river, and an averageaquifer hydraulic conductivity of 26.5 m/day (from the aquifer tests), the estimated baseflow to the river is7 ML/day per kilometre of river length (accounting for baseflow from either side of the river).

If the same assumptions will be applied to the whole rivers and streams crossing Mulgrave Alluvium, arough estimate of total baseflow from the main body of the alluvium is 497 ML/day, or 181456 ML/year.This will be used in the conceptual water balance presented in Section 3.9. The proposed abstractionscenarios are for 15 ML/day (3% of the groundwater baseflow in Stage 1) and up to 40 ML/day (8% ofthe groundwater baseflow in Stage 2).

3.5 Fresh/Saline Groundwater InterfacesThe location of the fresh/saline groundwater interfaces expected to exist at Trinity Inlet and MutcheroInlet are presented in the groundwater level maps (Figure 10, and Figure 12). Groundwater salinity datain DNRMW’s Groundwater Bore Database was used to define these boundaries. Sufficient data wereavailable to delineate the Trinity Inlet boundary. However, there was no real indication of the presence ofa fresh/saline interface in the data for Mutchero Inlet from the available monitoring bore data. Thissuggests that the interface does not extend a significant distance into the Mulgrave Alluvium from thecoast.

For Mutchero Inlet, a bore approximately 4 km north of Deeral (11100054) shows a single elevatedreading of 950 µS/cm electrical conductivity (EC) in 1977, whilst all other readings in this bore are wellbelow 100 µS/cm. EC measurements in other bores further to the south, nearby, and further north showsimilar low readings. This suggests that the fresh/saline interface (for groundwater, as opposed tosurface water) at Mutchero Inlet may not extend far at all up the Mulgrave Alluvium from the inlet, andmay exist very close to the coast, near to the mouth of the Mulgrave River with Mutchero Inlet.

For the purposes of impact assessment, bore 11100054 will be used conservatively to delineate thelocation of the saline/fresh groundwater (as opposed to the surface water) nterface at Mutchero Inlet.

Delineation of the interface at Trinity Inlet was made much easier with several bores with recorded EC ofaround 20000-40000 µS/cm. Other bores have readings exceeding 1000 µS/cm, whilst most bores in the


Mulgrave alluvium show recorded EC of around 50-200 µS/cm. The ground interface at Trinity Inletcorresponds closely with the extent of the mangrove flats.

3.6 Groundwater UsageLeach and Rose in 1979 recorded 17 irrigation bores in the area with a total allocation of 5,707 ML perannum. Actual usage in the irrigation season between August and November was thought to be lessthan 1,710 ML per year, or 30% of allocations. Application rates were thought to comprise three 75 mmapplications during the irrigation season.

In 1998, DNR reported total groundwater allocation of about 10,500 ML per annum, with the majority inthe northern section of the valley.

A search of the bores in the declared management area indicated that there are currently approximately90 bores licensed for groundwater extraction that have a total licensed volume of 22,000 ML/year. Thisincludes the Cairns Water application that has a total volume of 15,000 ML/year. These bores are shownin Figure 15 and Figure 16. The remaining 7,000 ML/year is licensed to private irrigators and smallertown water supplies. The average licence is in the order of 85 ML/year.

Despite significant increase in allocated groundwater since 1970s, there has been no observed long-termimpact on groundwater levels. This suggests that either actual usage has not increased over the yearsor that current long-term average abstractions remain less than long term average recharge.Furthermore, seasonal groundwater level fluctuations suggest that there is excess recharge available(i.e. insufficient aquifer storage to accept the recharge) in this catchment. Therefore there is a potentialfor greater amount of groundwater abstraction. This would also explain the lack of long-term observedabstraction impacts on groundwater levels.


Figure 15 Existing Groundwater Users – Primary Bore Purpose


Figure 16 Existing Groundwater Users – License Allocation


3.7 Groundwater RechargeAnalysis of bore hydrographs indicates that groundwater levels respond rapidly to seasonal rainfall andthe annual rainfall is in excess of the aquifer storage capacity (GHD, 2006). Groundwater levels appearto respond more to seasonal (and hence annual) rainfall than the long-term rainfall trend. This suggeststhat seasonal rainfall exceeds the recharge capacity of the aquifer. After an initial saturation of the soilprofile, additional rainfall in any single rainfall event in excess of the saturated infiltration rate cannot beaccepted and becomes runoff. Particularly dry years, where rainfall is significantly below annualaverage, have a much greater impact on groundwater levels than particularly wet years that exceedrecharge capacity. This inference is clear in:

The hydrographs in Appendix B, for the years 2002-2003 (around time 2350 on the hydrographs’ x-axes). In these hydrographs, water levels drop only slightly lower than their “base” level, but therecharge peaks are well below the typical peak level. In contrast, there are no extremely wet years(eg. 1999-2001, or time 1000-1700 on the hydrographs’ x-axes), in which the observed peakgroundwater levels are considerably higher than the surrounding years. The long-term hydrographtrends remain largely flat over the period of record, with the exception of recharge peaks, andsubsequent declines over the dry season, back to their “base” level; and

The observed depth to groundwater statistics presented in Figure 9, in which the minimum depth togroundwater is rarely more than 1 m below ground, and is typically at ground surface. In addition,the average depth to groundwater is rarely more than 5 m below ground surface. Given that theaquifer is generally 50 m to more than 100 m thick (Figure 7), this, in combination with water levelsgenerally remains within 5 m of land surface. This suggests that across the entire aquifer, for bothwet and dry seasons, it remains close to saturated. This is also clear in the depth to groundwatermap presented in Figure 14.

Following a dry period, recovery in groundwater levels occurs quickly when seasonal rainfall returns toaverage or better conditions.

Investigations suggest that recharge commences within hours of rainfall onset and may continue for aweek after rainfall events (Leach and Rose, 1979). It also appears, from a comparison of borehydrographs and rainfall records, that recharge capacity was met by a daily rainfall of 100 mm, withexcess rainfall becoming runoff (Leach and Rose, 1979). This is substantiated by the hydrograph andrainfall analysis presented by GHD (2006), and by the modelling reported in Section 5.3. Furthermore,the groundwater level analysis and discussion of aquifer saturation presented in Section 3.4 alsosupports this inference.

An analysis of potential recharge based on the hydrograph fluctuation method, indicates an estimatedaverage total annual gross recharge to the groundwater system of around 200 mm, within a range of 100and 400 mm. The hydrograph fluctuation method converts the annual variation in groundwater level to arecharge volume based on the storability of the aquifer system and the area of the aquifer. The analysisfurther indicates that between 10% and 30% of this gross recharge is lost to groundwater discharge fromthe aquifer, via processes including river baseflow, evapotranspiration, and groundwater abstraction.

Analysis identifies that there is sufficient annual recharge potential to allow recovery in groundwaterlevels following increased groundwater abstraction. However, in successive dry years, increasedabstraction may have an observable impact on groundwater levels.


It is considered that runoff recharge onto the alluvium from the bedrock outcrop flanking the valley islikely to be an insignificant component of aquifer recharge. This is because of the greater potentialavailable annual recharge than can be accommodated by the aquifer. However, runoff recharge may beimportant in occasional heavy rainfall events in dry periods.

3.8 Baseflow AnalysisSurface water flow and salinity data for the Mulgrave River and tributaries were collated and analysed inan effort to develop the understanding of groundwater / surface water interactions in the catchment. It isalso to provide calibration constraints to the groundwater model. A discussion of the data sources andquality, analysis methods and interpretation of results is included in Sections 3.8.1, 3.8.2, and 3.8.3below.

3.8.1 Data Sources and Quality

Mean daily flow and level data for the Mulgrave River (Fisheries, Peets Bridge and Gordonvale gauges)and Behana Creek at Aloomba were obtained from DNRMW. A gauge at Whites Falls on Behana Creekhas only a short record of level only; there are no flow data for this gauge. Daily records of streamconductivity (EC) were also obtained for the Peets Bridge gauge. The only gauges in the catchment witha useable record are Fisheries, Peets Bridge and Gordonvale gauges on the Mulgrave River, andAloomba gauge on Behana Creek. However, even some of these records are very limited.

Discussions with DNRMW hydrographer, Alan Hooper, suggested that the Gordonvale gauge data arequestionable This is primarily because during periods of low flows the gauge (which is in the MulgraveMill pump house) is bypassed. Low flows are therefore likely to be under-estimated by the Gordonvalegauge.

The best quality data are from the Peets Bridge gauge, which also has daily records of stream EC.Although this gauge is located in the upper (thin, narrow) reaches of the Mulgrave River alluvial aquifer, itdoes provide some insight into groundwater/surface water interactions in the area. This gauge was thefocus of the majority of the baseflow analysis.

It must be reinforced that the use of the data from a limited area of the aquifer infers a conservativestance to the ground water model. That is, the flow data at Peets Bridge will reflect a lower volume ofwater than would a gauging station, for example, record in the reach of the Mulgrave River at Aloomba.In order to counter some of the inherent conservatism in the model, an allowance has been made in themodel for additional flow into the aquifer below the upper limits. The mechanism of this allowance isdescribed in the following sections.

3.8.2 Methods

The Conductivity Mass Balance (CMB) (Stewart et al., 2007) and United States Geological Survey(USGS) local minima (Sloto and Crouse, 1996) approaches were used to estimate the amount ofbaseflow for each gauged data set.

The CMB method utilises the relationship between stream flow and stream electrical conductivity (EC;roughly analogous to salinity) to define the proportion of baseflow within the total stream flow. It requiresestimates of stream runoff EC and stream baseflow EC. These data were estimated from the gauged


data at 55 uS/cm and 27 uS/cm, respectively. The USGS methods are less physically based, and aresimple digital filters.

The CMB method was used to separate out baseflow for the Peets Bridge gauge, and the USGSmethods were used on the remaining gauges (where no stream salinity records were available). TheCMB analysis on the Peets Bridge gauge was used to calibrate the USGS method, the calibrationparameters for which were then applied to the remaining gauges.

The use of the CMB method was justified for this catchment given the high degree of correlation betweenstream flow and stream EC, and reasonable satisfaction of the assumptions inherent in the method (referto Stewart et al., 2007).

It should be noted that river flow data were not naturalised due to lack of information on actual surfacewater abstraction and discharge. For the river upstream of Peets Bridge gauge, from license allocations,it is estimated that a maximum of 98 ML annual dry season surface water abstraction occurs. Most ofwhich is abstracted between Peets Bridge and the Fisheries. Lack of accounting for these abstractionsproduces an underestimate of baseflow to the river, whilst lack of accounting for discharges to the riverresults in an overestimate of baseflow. This should be noted when reviewing the baseflow analysispresented below the analysis presented provides an underestimate of baseflow.

Figure 17 Measured Flow and Salinity Relationships


3.8.3 Results and Discussion

Time series records of baseflow for each of the gauging stations were successfully generated.

Table 4 summarises the results of the surface water analysis undertaken. The baseflow estimates arederived from the CMB analysis undertaken, and runoff was calculated as the difference between totalflow and estimated baseflow. The baseflow index (BFI) was calculated as the proportion of total flow thatwas estimated to be baseflow. Catchment yield is calculated as the total flow divided by the catchmentarea upstream of the gauge. The reported average baseflow gain or loss is calculated by subtracting thecalculated baseflow of a downstream gauge from that of the next upstream gauge.

The main conclusions from this are:

Baseflow into the Mulgrave River on the upper, thin and less extensive reaches of the alluvial aquiferis estimated to be around 50% (~1000 ML/d) of the total flow, as estimated from the Peets Bridgegauge. Data from the Fisheries gauge upstream suggests that as much as 25% of this (~280 ML/d)is derived from the alluvial aquifer.

Behana Creek is likely to be a sporadic recharge feature at least upstream of the Aloomba gauge,whereby runoff flowing into this creek during the wet season recharges the alluvial aquifer vialeakage from the creek. This is supported by the on-average saturated aquifer condition in this area(refer to Section 3.4). As a result, there is no, or very limited, perennial baseflow in this creek.However, during times of high flow and high groundwater levels, 50% (or ~174 ML/d) of total flowappears to be baseflow.

The lack of gauged and questionable quality of data downstream of the Mulgrave Mill at Gordonvaleprevents any further inferences being made with regard to surface water / groundwater interaction on themain alluvial deposits of the valley.

As suggested by the data, it is possible that the river “loses” water to the aquifer on average, it is unlikelygiven the high driving groundwater heads up catchment and apparent storage in most years. This is dueto the high rainfall and recharge rate. In this situation, the river would be expected to be, on average, a“gaining” stream on the main alluvial deposits.

Further analysis of river-groundwater interaction is presented in the numerical modelling undertaken anddocumented later in this report. This includes model calibration to groundwater level responses ingovernment observation bores located close to rivers and streams in the catchment (refer to Section 5)and detailed model calibration to the aquifer tests undertaken as part of this study. Specifically, on thislatter aspect, the Area 3 aquifer test monitored the groundwater response to pumping in a shallow borelocated close to the Mulgrave River. As expected, this bore (A3Ob2; Appendix C) responded in arestricted fashion, due the adjacent Mulgrave River acting as a recharge boundary under the stress ofaquifer testing. The model was successfully calibrated to the drawdown response in this bore.


Table 4 Surface Water Analysis Summary

Gauge Name(OrderedUpstream toDownstream)

UpstreamArea(km2)

BaseflowIndex(BFI)

CatchmentYield(mm/year)

MeanRunoff(ML/d)

MeanBaseflow(ML/d)

MeanBase flow(mm/year)

AverageGain orLoss(ML/d)

Comments

111005A

Mulgrave Riverat The Fisheries

357 0.5 1560 805 721 737 n/a Suggests reasonable level ofgroundwater discharge from the bedrockaquifers into the river.

111007A

Mulgrave Riverat Peets Bridge

520 0.5 1528 1174 1003 704 282 Suggests river gains from the Fisheriesgauge to Peets Bridge (mainly onbedrock, or thin alluvials), which possiblysuggests a small amount groundwaterstorage and discharge in the thin alluvialdeposits high up in the catchment.

111001A

Mulgrave Riverat Gordonvale

552 0.4 1323 1210 791 523 -213 Possibly suggests river loses from PeetsBridge to Gordonvale, howeverGordonvale gauge thought tounderestimate low flows, so this isprobably an underestimate of baseflow -groundwater/surface water interaction onthe main alluvial valley unable to beaccurately quantified with the availabledata

111003C

Behana Creekat Aloomba

86 0.4 2054 310 174 738 n/a Sporadic flows (regularly nil flow),suggesting runoff-dominated water body,which looses much of its flow to theunderlying alluvial aquifer. The very lowrelative catchment yield supports thisnotion.


3.9 Preliminary Catchment Water BalanceA conceptual water balance has been constructed for the Mulgrave River Alluvium. The purpose of thisis to provide an initial (pre-numerical modelling) assessment of the relative proportions of the variouswater balance components. The numerically modelled water balance can be compared to thisconceptual balance. This can provide a framework against which the hydrogeological conceptualisationcan be further assessed in light of information gleaned from the numerical modelling. The conceptualwater balance is summarised in Table 5 while the inputs and assumptions are detailed in Appendix D.

Table 5 Preliminary Water Balance – Mulgrave River Alluvium

IN (ML/year) OUT (ML/year) OUT (as % of IN)

Recharge 151574 - -

Inflow from up catchment areas 12606 - -

Stream leakage into aquifer 35533 - -

Groundwater Abstractions - 2100 1%

Baseflow - 181456 91%

Groundwater discharge to ocean - 11098 6%

TOTAL 199713 194655 97%

BALANCE DISCREPANCY (IN-OUT):

5059 ML/year (3% of estimated recharge)

The water balance for the alluvium stabilises quite well, despite being based upon simplifyingassumptions. The key piece of information obtained from constructing this water balance is the inferencethat river baseflow is the primary path for groundwater discharge in the catchment. This is common inareas of high rainfall with highly permeable aquifers, such as the Mulgrave River catchment, andsuggests that the river and aquifer are in strong connection with one another.

The baseflow component of the water balance was initially extrapolated down-catchment from gaugedriver baseflow yield per unit up-catchment area at Peets Bridge gauge. However, in comparison with therecharge estimates derived from the data analysis, it was found that the baseflow estimated in thismanner was too high or that the estimated recharge was too low. It was decided that the baseflowestimate was more likely the source of error. This is due to the extrapolation of baseflow data from up-catchment areas to down-catchment areas on the alluvium, whereas the recharge was estimated frommonitoring data on the alluvial aquifer. Baseflow to the river upstream of Peets Bridge is driven by farlarger hydraulic gradients than it is on the lower, topographically flatter areas on the main body of theMulgrave Alluvium.

Furthermore, much of the catchment upstream of Peets Bridge is of differing geology and receives muchhigher rainfall than the lower parts of the catchment (i.e., the alluvium). As a result, the baseflow


estimate was re-visited using a different approach as detailed in Section 3.4.1 and Appendix D, whichprovided a far more reasonable estimate and smaller water balance error.

This conceptual water balance is compared with the water balance derived from the numerical modellingin Section 5.


4. Model Construction

4.1 Code SelectionThe selected code with which the numerical modelling was completed is MODFLOW 2000 (Harbaugh etal., 2000). MODFLOW is a finite difference saturated groundwater flow model that has beencomprehensively tested, widely utilised and accepted, and is freely available and well documented.Groundwater Vistas was used as the graphical user interface for most of the model construction.

4.2 Model Datasets and ExtentThe model has been constructed to extend along the entire Mulgrave River valley between Trinity Bayand the outlet of the river at Mutchero Inlet. The extent of the Mulgrave Alluvium mapped in the1:100000 scale geological map sheets was taken as the extent of the model (Figure 18).

The model grid was separated into 150 m square cells and rotated 25 to the west (from north), whichparallels the line of the Mulgrave valley and the predominant groundwater flow directions. The modelgrid does not entirely cover the mapped area of Mulgrave Alluvium, particularly the sediments extendingup the Little Mulgrave River. This is due to a lack of data and because the sediments in these areas areoften narrower than the model grid cell size. Their thickness and areal extent are severely limitedcompared to the extent and thickness of the remaining alluvial deposits. Consequently, this slightlylimited grid extent will have no significant impacts on the modelled water balance or estimation ofsustainable aquifer yield.

A second, refined model was extracted from the larger (regional) model for detailed calibration to theaquifer tests undertaken at Areas 2 and 3 outlined in Section 3.1 (Figure 19). This refined model gridwas separated into 10 m grid cells in the vicinity of the aquifer tests.

Temporally, the regional model is separated into monthly stress periods, each subdivided into ten timesteps. The regional model spans the period from June 1996 to January 2007, with 128 stress periods.The refined aquifer test calibration model spans the whole of January 2007. It is separated into variablelength stress periods depending on the groundwater abstraction regime, each subdivided into up to tentime steps.

Datasets from previous investigations, current study results and long term groundwater monitoring wereused in model construction, and include:

Digital Terrain Model, sourced from the United States Geological Survey’s Shuttle Radar TopographyMission (Figure 2)

Hydrologic monitoring (Figure 6)

Geology ()

Soils (Figure 4)

Depth to basement (Figure 7)

Top of Layer 2 (i.e. division of the Mulgrave Alluvium; Figure 8)

Observed groundwater levels (Figure 10, Figure 11, Figure 12 and Figure 14).


The extent of the regional model is shown in Figure 18, indicating the extent of Layers 1 and 2. Twolayers were used due to the apparent variability in aquifer parameters with depth as indicated from thegeophysical logging and recent investigation drilling. Layer 1 comprises the upper section of theQuaternary Alluvium and the Atherton Basalt north of the river. Layer 2 (Figure 8) consists of the lowersection of the Quaternary Alluvium and the Tertiary Alluvium in the deeper sections of the valley.

The extent of the aquifer test calibration model is presented in Figure 19. No structural changes (fromthe regional model) were made to this model, aside from assignation of General Head boundaryconditions to the north and south to allow groundwater flux into and out of the model.


Figure 18 Groundwater Model Grid and Boundaries


Figure 19 Aquifer Test Calibration Grid


4.3 Boundary ConditionsAt the northern (Trinity Inlet) and southern end (Mutchero Inlet) of the valley MODFLOW General HeadBoundaries (GHBs) were applied at mean sea level. GHBs allow groundwater flow into or out of themodel dependent on the modelled head in the aquifer, the head assigned to the GHBs, and the assignedboundary conductance.

Bedrock areas are poor aquifers, therefore these areas neither contribute nor accept any flow to or fromthe alluvial aquifer (Layers 1 or 2). The east and west model boundaries (alluvium/bedrock contact) weredesignated no flow boundaries.

The Mulgrave River and Behana Creek are represented using MODFLOW River Boundaries, whichsimulate interaction between surface water bodies and the underlying aquifer. The modelled rate of gainor loss from/to the river is proportional to the head difference between the river and aquifer and a userspecified riverbed conductance. The rate becomes constant and independent of aquifer head when thehead drops below the river stage. In such cases, the river loss/gain is dependent only on the assignedriverbed elevation, and the river stage (i.e. it becomes a constant rate). Riverbed conductance is largelycontrolled by riverbed permeability and given the difficulty of measuring this parameter in the field, it istypically estimated during model calibration.

Drain boundaries were assigned to the top of Layer 1. This was designed to allow natural groundwaterdischarge at times of high groundwater levels (i.e. when groundwater levels rise above the land surface),and to allow the rejection of the assigned recharge (from PERFECT; refer to Section 4.4 and Section3.9). The Drain boundaries were assigned a drain elevation equal to ground surface elevation, and asufficiently high conductance so as not to impede groundwater discharge from the model via theseboundaries. Use of Drain boundaries provides a means by which to assess the catchment water balancein more detail with the model. Particularly, with the rejection of groundwater recharge at times of highgroundwater levels, and the generation of runoff. This is discussed further in Section 5.

4.3.1 Representation of River-Aquifer Interconnection

River boundaries were assigned a (temporal) mean stage and riverbed height that was linearlyinterpolated between surveyed gauge heights and sea level at Mutchero Inlet, taking into account theDTM elevation. Minor tweaks were made to stage height during model calibration in order to provide abetter match between modelled and observed groundwater levels in bore located near river boundaries.Due to a lack of sufficient supporting data (i.e., gauged river flow over much of the alluvial aquifer), riverstage height was maintained at a constant level through all simulated time periods.

4.4 Near Surface Processes and Groundwater RechargeSeveral methods were used to quantify the proportion of annual rainfall that results in groundwaterrecharge (Section 3.7).

Potential recharge was modelled for each mapped soil type and climate zone using the PERFECT model(Littleboy et al., 1989). The PERFECT model is based on soil hydraulic properties and can representsimple or complex evapotranspiration and cropping. In this manner, spatially distributed recharge wasapplied over the entire groundwater model, depending on soil type and climate zone.


Soil hydraulic properties were derived from the particle size distribution data presented in the CSIROBabinda-Cairns soils mapping report (Murtha et al., 1996). These data were input to the Rosetta pedotransfer model (Schaap et al., 2001) to derive parameters used in the van Genuchten soil-moistureretention model. These data were used to describe the water-holding properties of a soil (vanGenuchten, 1980). This model was used to estimate the relevant hydraulic properties (residual moisturecontent, wilting point, field capacity, saturated water content) for input to PERFECT. The assignedhydraulic properties are presented in Appendix E.

The grain size data for each major soil horizon were weighted averaged (based upon the samplethickness relative to the horizon thickness) and adjusted for the presence of gravels. The derivedhydraulic parameters are in general agreement with those documented in the earlier NRM modelling.Other soil hydraulic properties required by PERFECT (eg runoff curve number) were taken directly fromthe default soil files in PERFECT for the relevant Great Soil Group (Appendix E).

Crop parameters were largely derived from the APSIM sugar cane module(http://www.apsim.info/apsim/Publish/apsim/sugar/docs/sugar_science.htm). Given the dominance ofcane cropping on the Mulgrave Alluvium, it was assumed to cover the entire area and was therefore theonly crop modelled in PERFECT. The large areas of mangrove/intertidal communities in the south andnorth of the catchment are groundwater discharge areas. This simplifying assumption regarding land usehas no impact on the groundwater model in these areas. Simple cropping and evapotranspiration wasmodelled in this case, rather than the detailed crop growth/ratoon model. Rooting depth was set to1.2 m, and no ratooning or replanting was simulated (i.e. constant evapotranspiration by cane). Cropfactors were taken from the earlier DNRMW modelling (DNRMW, 1999). Climate data were obtainedfrom DNRMW’s SILO database.

The PERFECT model does not account for shallow groundwater levels and saturated soils, constant freedrainage from the soil profile is assumed. These conditions result in overestimated groundwaterrecharge in PERFECT. Thus, the groundwater model was designed to allow for rejection of recharge bythe aquifer in the event that the aquifer became saturated and had no capacity to accept it. This wasachieved by assigning MODFLOW Drain cells to Layer 1, which allow groundwater flux out of the modelif the modelled head rises above ground surface.

In this manner, the groundwater model was used to provide quantification of groundwater recharge andthe generation of runoff in response to the aquifer having no capacity to accept further recharge (i.e.towards the end of most wet seasons). To clarify this, PERFECT models runoff generation in responseto rainfall magnitude, soil type, and soil moisture deficit. However, PERFECT does not account forsaturation of the soil profile from below (i.e., water table raising to surface) - the MODFLOW model wasused to model this aspect of runoff generation via Drain cells.

4.5 Groundwater and Surface Water AbstractionGroundwater and surface water abstraction information was sourced from DNRMW to identify thelocation of licensed users and annual abstraction volumes. This is detailed in Section 2.8.1 and 3.6.

Full annual groundwater allocation volumes were assigned to the model at the documented location ofeach bore. This is a conservative approach to assess the sustainable yield of the resource. Forirrigation licenses (i.e. most licenses), the annual allocation was distributed across the wet seasonaccording to the soil moisture deficits (and irrigation demand) modelled in PERFECT. This was achieved


using reported average seasonal variation in allowable soil moisture deficits reported in the earlierDNRMW modelling (DNRMW, 1999).

For all other types of licenses (aquaculture, industrial, etc), the full annual allocation was assumed,distributed evenly throughout the year.

Due to the simple representation of rivers in the catchment and the inadequate data to enable calibrationto river baseflow, surface water abstractions were not incorporated into the model.

4.6 Aquifer ParametersAquifer parameters were derived from the investigation bore pump testing completed as part of thisproject (Section 3.3). These parameters provided a starting point for the modelling, although werealtered during the model calibration process.

Final calibrated model parameters are presented in Figure 20, Figure 21, Figure 22 and Figure 23. Thehydraulic properties estimated from the aquifer tests are generally higher than the calibrated parametersin the vicinity of the aquifer tests. This is mos likely due to the tested abstraction bores screening andtesting discrete coarse sand layers. Whereas the model parameters represent much thicker sequencesof alluvium, much of which is likely to be of lower permeability than the discrete coarse layers targeted foraquifer testing. Further discussion of model parameterisation with particular reference to calibrationsensitivity is presented in Section 5.


Figure 20 Hydraulic Conductivity Layer 1


Figure 21 Hydraulic Conductivity Layer 2


Figure 22 Storage Coefficient Layer 1


Figure 23 Storage Coefficient Layer 2


5. Model Calibration

The model calibration process comprised of the following:

The automated calibration code PEST (Doherty, 2002) was used to refine aquifer parameters(vertical and horizontal hydraulic conductivity) in steady state against the monitored average June-July (1998) groundwater level distribution in DNRMW observation bores (35 bores have records overthis period). This steady state model was then updated to a transient model, for transient calibrationto groundwater level hydrographs.

A subset of the model was refined down to a local scale model for detailed transient calibration in thevicinity of the two aquifer tests undertaken at Area 2 and Area 3 (Figure 19). Note that this was thefirst step in a two-phase transient calibration – the model was also calibrated to transient water levelsfor the entire Mulgrave Alluvium (refer below). For the aquifer test calibration, General HeadBoundaries were used as local model boundaries, all other boundaries and properties weremaintained from the calibrated steady state model. Calibration was undertaken against groundwaterlevel drawdown monitored during aquifer tests at Areas 2 and 3 as a subset of the larger valleymodel. Pump test calibration parameters were input to the larger model. Detailed drawdown datafrom five monitoring bores constructed during this investigation were used to calibrate this refinedmodel (Appendix C).

Aquifer test and steady-state calibration parameters were checked in the main transient model (i.e.,covering the entire Mulgrave Alluvium) and refined manually as necessary to provide an adequatetransient model calibration for the whole alluvium. This model was calibrated against the monitoredgroundwater levels from DNRMW observation bores between June 1996 and December 2005 (16bores were found to have sufficient hydrograph data against which to calibrate over this period). Thiscalibration period was selected due to the following:

– The data available from observation bores over this period are of the highest spatial and temporalresolution over the period of all records, and therefore provide a firm base against which tocalibrate the model; and

– This time period covers particularly wet climatic years and in which the wet season has failed.This provides a robust basis from which to calibrate the model.

The calibration was achieved through altering hydraulic conductivity (vertical and horizontal), storage,river stage and riverbed elevation, and riverbed conductance (i.e., the degree to which the river andaquifer are interconnected).

For the steady state calibration, complete (unhindered) river-aquifer connection was initially assumed.During transient calibration, river boundary conductance was adjusted in areas where the modelled waterlevels at monitoring bore locations were sensitive to the modelled interconnection between the river andthe aquifer. In these areas, the effective riverbed hydraulic conductivity is 7*10-3 m/day (assuming anaverage 50 m wide river), a very low hydraulic conductivity for this environmental setting. In all otherareas (i.e. where there were no groundwater level observations clearly affected by river-groundwaterinteraction), complete river-aquifer interconnection was assumed, for lack of information indicatingotherwise. Predictive model sensitivity analysis is presented in Section 5, and calibration modelsensitivity analysis is discussed in Section 5.2.


Final calibrated parameters are presented in Figure 20, Figure 21, Figure 22 and Figure 23. Acomparison between observed and modelled head distribution for the steady state calibration ispresented in Figure 24. Transient calibration hydrographs are presented in Appendix B. Aquifer testcalibration hydrographs are presented in Appendix C, Appendix F and Appendix G respectively. Borelocation are shown in Figure 6.

5.1 Calibration Quality

5.1.1 Steady State Calibration

The normalised Root Mean Squared (nRMS) error of the steady state calibration is 6.6%, which is withinthe nominated 10% limit of acceptance. This error is caused by observation bore 92951, which shouldbe excluded from the calibration. This private bore which displays an unusually high measuredgroundwater level compared to other nearby bores.

Plots depicting calibration various residual statistics are presented in Appendix F for the steady statecalibration. The steady state plots show that the measured water level in bore 92951 is likely to beerroneous.

The modelled steady state and observed average June-July groundwater head distributions in Figure 24show generally good agreement. The only major exception to this being towards Mutchero Inlet, wherethe modelled 1 m head contour does not extend as far up the valley as it does in the mapped headdistribution. This could be related to tidal effects on measured groundwater levels in this part of thecatchment (i.e. water level observations may have been read at times of high tide, not mean sea level).

5.1.2 Transient Calibration

The nRMS error of the transient model calibration, incorporating all data from all bores, is 9.8% within the10% target acceptable level of error adopted in this study. Much of the error is due to discrepancybetween the DTM elevations and the surveyed elevations of monitoring bores.

The calibration hydrographs presented in Appendix B and Appendix C show a reasonable agreementbetween modelled and observed temporal and spatial variation in groundwater head and drawdown inthe aquifer tests. Seasonal variation in the major aquifer stresses – groundwater recharge andabstraction for irrigation – appear to be adequately represented in the model. The calibration toobserved drawdown in the aquifer tests conducted at Area 2 and Area 3 shows a good match. This iswith the exception of bore A2Ob2, in which the modelled drawdown is lower than that observed. There isno clear reason for this, but the calibration could not be improved for this bore without adversely affectingthat of the others. This suggests either a conceptual model error (e.g. model not of sufficient lithologicdetail) or systematic error in the testing and data gathering process (e.g. in instrumentation). The formerexplanation is the most likely.

Plots depicting calibration various residual statistics are presented in Appendix G for the transientcalibration. For the transient calibration, the plots show a greater scatter in the residual error at higherobserved groundwater levels. Further discussion on this is presented below.

In the identified area of greatest groundwater recharge (Section 3.4), many of the model hydrographpeaks do not match those of the corresponding observation bores in particularly wet years (1998-1999 to2000-2001, 2003-2004; days 3000 through 3900 on the x-axes of each chart). Upon detailed model


inspection, it was found that this is due to DTM used to represent the land surface for each 150 m by150 m model grid cell, the elevation values of which differ from the surveyed elevation of many of thebores. The DTM is typically up to 3 m higher than the surveyed bore elevation. This is an artefact of allDTM’s, which average the land surface elevation over an area and have an inherent degree of error withrespect to the true land surface elevation. The bore elevations on the other hand represent one pointwithin the land surface area represented by a model grid cell. This will differ from the averaged landsurface elevation for the broader surrounding area, as represented by the DTM. This means thatmodelled groundwater levels may rise higher than those observed in some of the observation bores. Themodelled groundwater levels may rise up to the elevation of the DTM. This is not considered tocompromise model predictions in any way, because at the scale of the model, the DTM provides a goodrepresentation of land surface topography. Despite this issue, the overall seasonal response torecharge, and lack of response in the particularly dry years, are adequately represented.

The calibration hydrographs for bores located in close proximity to the Mulgrave River and Behana Creek(e.g. bores 11100098, 11100043, 11100042, 11100049) show poorer agreement between modelled andobserved groundwater head. The modelled base level heads are too low and are controlled by the riverboundary in the model. At times of low flow it is evident that the lack of detailed data pertaining to stageheights and riverbed levels is inadequate to provide a detailed picture of the river stage variationsthrough the catchment. The calibration could potentially be improved in these areas through riverbedand stage surveys, and potentially the acquisition of a more detailed Digital Terrain Model for thefloodplain, using data such as LIDAR. However, it should be noted that these issues are unlikely tosignificantly affect model predictions. These predictions provide relative rather than absolute impact (i.e.drawdown) assessment. The benefit of undertaking detailed surveys and acquiring more detailedtopographic data was considered to be low for this assessment given the associated high costs.

5.1.3 Summary

The described calibration quality indicates that the model is sufficiently calibrated given the quality andspatial and temporal resolution of the available data. Model residual error is within the prescribed limits.If there is a poor match between modelled and observed data, logical reasoning can explain the sourcesof error. The model is therefore considered sufficiently calibrated for the purposes of the investigation,however uncertainty and sensitivity analysis will be used to indicate the ambiguity in model predictions.


Figure 24 Calibrated Steady State Watertable Contours


5.2 Calibration Sensitivity AnalysisThe results of a sensitivity analysis show that the horizontal hydraulic conductivity in particular areas arethe parameters to which the model calibration is most sensitive (Figure 20 and Figure 21). This isgenerally observed around the main abstraction and cane irrigation area (north of Gordonvale in modelLayer 1. In Figure 20 and Figure 21 the points represented by the largest circles are the parameters towhich the model is most sensitive. Conversely those with the smallest circles represent the parametersto which the model is least sensitive.

The points to which the calibrations was least sensitive were adjusted by PEST to their specifiedmaximum These values were later adjusted down to the average of the other pilot points (to which thecalibration was sensitive) in order to maintain a physically realistic parameterisation that is not skewed byparameter points to which the calibration is insensitive. The calibration was far less sensitive to verticalthan horizontal hydraulic conductivity.

Sensitivity analysis of the calibration to other key parameters – riverbed conductance and recharge, wasalso undertaken (Figure 25). It shows that (reduced) riverbed conductance is another significantparameter to the calibration. However increasing the conductance much higher than the calibratedvalues changes th model calibration very little. The This suggests that the river is almost entirelyconnected to the aquifer in the calibrated model. This theory is supported by the Hydrogeologicalconceptualisation and data analysis. Reduction in the degree of river-aquifer connection results in apoorer match between the modelled and observed results (Figure 26) Consequently the modelled degreeof connection in the calibrated model is considered provide an adequate reflection of the true connection.

As a result of this high degree of river-aquifer interconnection, model predictions are likely to be largelycontrolled by the river boundaries in the model. This is because river boundaries provide the primaryavenue of groundwater discharge from the alluvium (i.e., they largely control the groundwater headdistribution). Thus, any stress on the aquifer is likely to manifest itself as a change to inflows or outflowsacross the river boundaries.

Sensitivity analysis of model predictions to riverbed conductance will however be conducted to assessthe potential range in predicted extents of drawdown in the aquifer. This is in response to increasedgroundwater abstraction. It should be noted that the lowest value for conductance tested in thesensitivity analysis is the equivalent to a riverbed hydraulic conductivity of 7*10-5 m/day. This amount ofriverbed hydraulic conductivity is considered to be extremely low. This value is at the lowest end of thephysically possible levels of hydraulic conductivity and is not supported by the conceptualisation ormodel calibration. However, this was the level to which river-aquifer connection had to be dropped inorder to achieve a noticeable change in the modelled responses to groundwater abstraction (i.e.,drawdown and impact on river flows).

The sensitivity analysis of the model to recharge suggests that the model is more sensitive to reductionsin applied groundwater recharge than the degree of river-aquifer interconnection. The model is lesssensitive to increase in recharge. This supports the conclusion reached earlier in this report (Section 3.9and earlier reporting (GHD, 2006)) that aquifer storage is completely filled in most wet seasons and theexcess recharge is rejected by the aquifer as surface runoff. Lower rates of recharge rapidly reduce thequality of the calibration, suggesting that the modelled rates of recharge are a reasonable reflection ofthe true rates.


Further on the topic of river/groundwater interaction, during the transient model calibration, it was notedthat the calibration hydrograph for several bores located particularly close to the river were highlysensitive to river bed conductance. Initially, the conductance was set extremely high under theassumption that the river is in complete (unhindered) connection to the aquifer. Under this assumption,the bores located close to the river showed flat hydrographs (i.e. they were unresponsive to rechargeevents) with groundwater levels completely hinged to the model river stage, whilst the observed datasuggested otherwise. Typical annual river fluctuations (2-3 m) do not account for most of the observedbore hydrograph fluctuation in these areas (6-8 m). This suggests that it is more likely that river/aquiferconnectivity is the controlling parameter on bore hydrograph fluctuations, rather than river stagedynamics. The river conductance was thus lowered iteratively until the modelled response to rechargeevents matched those in the observed data.

This provides useful information on what the true river/aquifer connectivity may be in those areas wherebores are adjacent to the river. This also provides confidence in the model predictions in these areas,which are discussed below.

Figure 25 Calibration Sensitivity Analysis

5.3 Calibrated Model Water BalanceThe purpose of this Section is to use the output from the calibrated model to further refine the waterbalance for the Mulgrave Alluvium that was presented in Section 3.9. The model provides a moredetailed (numerical) assessment of the groundwater components of that water balance. The model is a3-dimensional, spatially distributed model, and directly links the recharge modelling with the groundwater


model, whereas the preliminary water balance was constructed at a more conceptual level. Themodelled water balance is summarised in Table 6.

Table 6 Mulgrave Alluvium Water Balance – Using Calibrated Groundwater Model

IN (ML/year) OUT (ML/year) OUT (as % of IN)

Recharge 179678 - -

Surface Water Leakage intoaquifer

29063 - -

Inflow into alluvium from upcatchment alluvia

14020 - -

GWABS - 4034 2%

Baseflow - 127947 57%

Rejected recharge anddischarged to surface

- 65326 29%

Through flow to ocean - 25516 11%

TOTAL 222761 222823 100%

BALANCE DISCREPANCY: -62 ML/year (0% of total annual recharge to the alluvial aquifer)

The recharge component of the water balance was estimated at 460 mm using the hydrographfluctuation method. This equates to ~151574 ML/year for the Mulgrave Alluvium, which compares wellwith modelled recharge rate of 179678 ML/year.

Other key differences between this calibrated water balance and the initial conceptual water balance are:

Modelled baseflow is lower than the initial estimate, with the model indicating 127947 ML/year of theoverall balance, whilst the estimated baseflow was 181456 ML/year;

The groundwater discharge to ocean via Trinity and Mutchero Inlets was modelled as being higherthan the initial estimate (25516 ML/year versus 11098 ML/year);

The modelled leakage from surface water into the alluvium is lower than the conceptual waterbalance estimate (29063 ML/year versus 35533 ML/year); and

The modelled groundwater abstractions are higher than the initial estimate purely throughconservatism – the full license allocation of each bore was applied to the modelling, whereas the bestestimate of actual usage was used in the initial water balance.

The largest modelled versus conceptual water balance relative discrepancies are baseflow andthroughflow to the ocean. These discrepancies are due to better (distributed, 3-dimensional)representation of groundwater head gradients, aquifer parameters, groundwater recharge, and aquiferboundary conditions in the numerical model. With respect to the baseflow discrepancy, this indicatesinaccuracy in the broad-brush approach that was used in the conceptual water balance, in the absenceof gauged river flow data on the main body of the alluvial aquifer.


5.4 Model LimitationsThe key issue is the lack of flow data in the middle reaches of the Mulgrave River, and limited data forBehana Creek. Consequently the model assumed that there is no surface or subsurface recharge ofthese systems below Peets Bridge (which is not the practical case) but as the Peets Bridge flow datawas the longest most reliable flow data, the model was calibrated against a far lower recharge thancertainly actually exists to avoid extrapolation beyond the sensitivity analysis conducted (as mentionedbelow).

The lack of flow data also skews interpretation of the modelled potential impacts on the sameconservative basis, i.e., by assuming no surface or subsurface recharge (particularly for Behana), themodelled impacts are disproportionate for the different scenarios between Behana Creek and theMulgrave River. With further data these modelled impacts could be greatly refined and would (mostlikely) show a far lesser proportional impact than is presented in the current model. Following is asummary of the identified limitations of the numerical model developed in this investigation, andassociated consequences. These limitations should not be uniformly regarded as negative attributes, butare the primary elements in the conservative outputs and assumptions derived for this model.

Lack of calibration to river baseflow due to insufficient gauged river data against which to calibrate.This results in uncertainty in the degree of river – aquifer interaction in the model, and has the flow-on effect of uncertainty in model predictions of drawdown and induced river leakage into the aquifer.This results in modelled impacts greater than actually exists. As described in earlier and latersections of this report, some of this uncertainty is minimised via sensitivity analysis, and confidencein model predictions is not affected significantly by the remaining uncertainty.

Lack of data on river and creek stage heights in the lower parts of the catchment. This would causea minor impact on the quality of the model calibration in the lower catchment. It does not significantlyaffect model predictions and again the impacts modelled err on the conservative side, adopting lowerconditions than would be normally expected in such a high rainfall area.

Lack of recorded groundwater usage data. In this investigation, the entire allocated volume has beenmodelled (as opposed to the actual usage which is a fraction of the actual legal licensed allocations).This means that existing groundwater abstraction is overestimated in the model, but as groundwaterusage is a small component of the water balance, thus results are insensitive;

Groundwater observation data are concentrated around the main groundwater abstraction / irrigationarea north of Gordonvale. This results in a model that is better-constrained in this area than inothers, particularly the far north and south of the Mulgrave Alluvium;

The existing groundwater observation data are not of sufficient temporal resolution to identify thetiming and magnitude of seasonal peaks in groundwater recharge. For average climatic years this isnot an issue, but does mean that for wetter than average climatic years groundwater recharge isunderestimated and does compromises the quality of the calibration, ie: assumes less recharge thanactually happens.

The model does not incorporate runoff-recharge events from the bedrock outcrop bordering the eastand west valley sides. The water balance assessments undertaken indicate that the majorcomponents of the water balance have been accounted for. Lack of representation of this processdoes not compromise the modelling outcomes, but does increase the conservatives of the modelthrough assuming less run off than is likely to occur.


6. Numerically Modelled Impact Assessment

6.1 Background and ApproachAssessment of the impacts of groundwater abstraction from the Mulgrave Alluvium has been undertakenthrough the simulation of two scenarios. Both of these scenarios have been derived from therecommendations of previous studies into the Mulgrave Aquifer.

1. Abstraction of 15000 ML/year (40 ML/day) from two bore fields located in the two areas found to bemost promising in terms of aquifer yield (“Area 2” – near Gordonvale, and “Area 3”- south of BehanaCreek).

2. Abstraction of 5475 ML/year (15 ML/day) from only the “Area 2” bore field. No abstraction wasmodelled in “Area 3”.

Both scenarios was assumed that pumping would be from Layer 1 in Area 2, and from Layer 2 in Area 3based upon the geology encountered during the drilling investigation, the conceptual hydrogeologicalmodel, and the test bores’ construction.

Scenario 2 (reduced abstraction localised to the “Area 2” bore field) was modelled subsequent to reviewof the predictions obtained from Scenario 1, which indicate potential impacts on surface water flows(refer to Section 6.3). In reviewing previous reports and recommendations and in discussion with CairnsWater it was proposed that a reduced abstraction scenario be the first phase implementation of thegroundwater supply. This will allow for environmental monitoring of impacts in response to theabstraction. Should no significant impacts be identified during this first phase groundwater supply, thesupply could be expanded to greater supply volumes with the collected data during the first stage. Thiswill provide technical support to such expansion.

Should no significant impacts be identified during this first phase groundwater supply, the supply couldbe expanded to greater supply volumes, with the collected data during the first stage providing technicalsupport to such an expansion. Monitoring data collected during this first stage could be used to refinethe groundwater model to provide stronger support for prediction of the impacts under expansion of thegroundwater supply.

The calibrated transient model was used as the basis for the predictive modelling. Therefore thepredictive models were run for the same period as the calibrated model, with the same climatic inputs.

The modelled drawdown in response to the abstraction for each model layer is presented in Figure 26and Figure 27 respectively.

The predictive model was also run with reduced river boundary conductance for sensitivity analysis ofmodel predictions to the degree of river-aquifer connection. The results of this predictive modelsensitivity analysis are presented in Figure 28 and Figure 29 for Scenarios 1 and 2, respectively.

The effects of existing abstractions and water level variations on modelled drawdown have beenremoved so that only the impact in response to the proposed abstraction is presented.

Particle tracking analyses were also conducted in the predictive modelling using MODPATH in steadystate mode. Such analyses provide an indication of all potential sources of water being abstracted fromeach bore. The particle tracking results can be used as a guide to the potential groundwater sourceareas within the valley, and potential water quality issues.


Figure 26 Modelled Drawdown Response Scenario 1


Figure 27 Modelled Drawdown Response Scenario 2


Figure 28 Scenario 1 Sensitivity Analysis: 1500 ML/year Abstractions, Modelled Dry Season Impacts, Minimal River/Aquifer Connection


Figure 29 Modelled Drawdown Following Successive Dry Years


6.2 Bore Field DesignThe initial modelled arrangement of the bore fields at “Area 2” and “Area 3” was iteratively derived viaspreadsheet modelling using Theis (1935) analysis. The Theis (1935) analysis suggested a minimumbore spacing of 400 m. The arrangement of the bore field was determined through:

Utilising the ideal minimum bore spacing;

The likely maximum total groundwater abstraction required (40 ML/day);

Likely achievable bore yields;

Local physical constraints such as land ownership (road reserve versus private land); and

Existing nearby groundwater users at each area.

Three iterations of the modelling were conducted to determine a final bore field arrangement thatminimised the drawdown impacts of abstraction. The resulting bore field arrangement is shown in Figure26 and Figure 27.

An ideal bore field arrangement is not constrained by land availability. Ideally, each bore should beevenly spaced from one another in a symmetrical spatial distribution in order to minimise drawdowninterference between bores. This is also to minimise the area of land required for the bore field.However, the model-predicted drawdown in response to abstraction (Section 6.5) suggests that the(physically-constrained) bore field arrangement does not cause any undue drawdown and boreinterference effects on any of the other bores in the bore field. The modelled drawdown distribution islargely controlled by transmissivity (thickness) variations in the alluvial aquifer (Figure 26 and Figure 27),particularly at Area 2. This where the drawdown cone is offset to the west of bore field centre towardsthe edge of the alluvial aquifer

The modelled drawdown within the Area 2 and Area 3 bore fields indicates that the proposed averagebore depths (50 m at Area 2 and 65 m at Area 3) are sufficient to provide the required availabledrawdown under the maximum proposed abstraction scenario (40 ML/day) This can be done with amaximum modelled drawdown of 5 m at Area 2 and 2 m at Area 3. The modelled bore field drawdown,particularly at Area 2, is offset to the west of bore field centres due to thinning of aquifer to the west(Figure 8). The aquifer thinning results in reduced aquifer transmissivity in this direction. Therefore, thebore fields have been located as far to the east as practicable.

6.3 Predicted River Baseflow Impacts

6.3.1 Introduction

Potential river baseflow impacts have been predicted based upon the outcomes of the groundwatermodel. The groundwater model is inherently limited in so far as that it must assume:

Pumping and drawdown is constant, that is, water is being continually abstracted from the aquifer.

Abstraction intensity is uniform, i.e, the same volume of water is being withdrawn constantly.

In the above for Stage 1, the model assumes that 5475 ML/year would be withdrawn at the rate of15 ML/day over the entire year, and similarly for Stage 2, that the maximum licensed allocation of 15000ML/year would be withdrawn uniformly at the rate of 40ML/day over the entire year.


Neither of these assumptions reflect the operational parameters under which the bore field wouldoperate. Both 15ML/day (Stage 1) and 40 ML/day (Stage 2) represent the uppermost daily limit ofabstraction, in accordance with the existing groundwater licence allocation of a maximum of 15000ML/year. However the timing of the abstraction would not be constant, nor the rate of abstraction. Theremay be prolonged periods of the year when water is not being abstracted from the aquifer and as thedaily limit would not be exceeded the actual volume of abstracted water may be significantly less that thetotal modelled abstraction volume. The proposed abstraction is a supplementary supply only, not amajor development in its own right. During the months January to April, for example, there may be noabstraction from the bore, therefore the estimation of impacts as modelled may exceed the actualoperational impacts of the bore field by 30%.

In this regard the predicted modelled impacts have been very conservative in assuming uniform pumpingand constant drawdown when in actuality the bore field may not be used for up to four months of theyear. The daily rates of pumping will not be exceeded and the net result would be an overall decrease inthe total volume and therefore total modelled impacts of the proposal.

In considering the modelled impacts, the model has been run over the entire duration for which climaticdata is available, ie: 60 years of quantifiable, reliable climatic data. When assessing withdrawal from theaquifer to the relationships with surface flows consideration must be given to the fact that these impactsmay take place over 60 years. The model water balance was based on the assumptions of constantdemand on the aquifer and assumed no further baseflow or runoff into surface waters below PeetsBridge. Subsequently the drawdown for each bore field (Figure 26 and Figure 27) show that thedrawdown for each bore field is largely controlled by the nearest river boundary, suggesting that theabstraction is inducing movement of water from the river into the aquifer

Further investigation into the model water balance suggests that the maximum abstraction regime of15000 ML/year (or 40 ML/d) induces 7 ML/day of leakage from the river into the aquifer, and a reductionin groundwater discharge into the river by 34 ML/day.

This is not an instantaneous relationship (modelled over 60 years), and as strongly indicated, does notreflect actual operation of the borefield. On average (over the duration of the modelled abstraction) all ofthe abstracted groundwater will impact surface flows via a combination of reduction of supply to thebaseflow of surface waters, and induced leakage into the aquifer from the surface waters. Thisrelationship is not uniform, and will depend on seasonal factors (wet seasons, prolonged dry seasons)and on bore field operational practices. Over the long term (the 60 years of the model) the ratio of thevolume of abstracted groundwater to the total volume of river flow is predicted to be 1:1. To put this intocontext, this has assumed uniform pumping, and no surface water recharge or base flow below PeetsBridge, both extremely unlikely scenarios. On a more seasonal, yearly basis the degree of river impactwill vary, but in no instance would there be a situation where the impact on the surface water wouldexceed that of the volume of abstracted groundwater.

In catchments with a highly connected groundwater and surface water system, the phenomenon of a 1:1abstraction to surface water impact is not unusual. In this catchment it is due to the Mulgrave Riverproviding the only major means of groundwater discharge out of the alluvial aquifer. The east and westsides of the the valley are bordered by the elevated and relatively impermeable bedrock. Also there areno other discharge avenues for groundwater other than Mutchero Inlet and Trinity Inlet. Trinity Inlet ishowever hydraulically separated from the Mulgrave River by the groundwater level divide north ofGordonvale (Figure 11). This hydraulic separation of the groundwater between Trinity Inlet and the


Mulgrave aquifer is the key parameter restricting the potential impacts of salinity intrusion into theMulgrave aquifer as a result of any lowering of the groundwater table.

This leaves only Mutchero Inlet for groundwater discharge in the southern part of the catchment. Thispart of the catchment is of very limited capacity in terms of discharge due to its narrow nature, distancedown catchment, and vast volumes of recharge entering the alluvial aquifer up-catchment each year.Therefore, groundwater levels in the alluvium rise towards ground surface under this restricted aquiferdischarge capacity, and groundwater is forced to discharge into surface water features as baseflow. Asa result, any significant volume of water removed from the aquifer will result in a direct reduction in thenet groundwater discharge to surface water. These surface flow impacts have been derived over theduration of the model, i.e., over a period of 60 years assuming constant withdrawal from the aquifer, sothese impacts are neither immediate, nor irreversible. Operational requirements of the bore field indicatethat the bore field (as a supplementary water supply) may not be utilised for number of months in eachyear (the length being seasonally dependent) so the degree of impact will be temporal and as previouslyiterated the maximum case only has been modelled.

The surface water flow impacts are predicted to affect both the Mulgrave River and Behana Creek.Figure 30 graphically presents the relative impacts upon each of these surface water features forabstraction rates varying from 15 ML/day (5450 ML/year) up to 40 ML/day (15000 ML/year). Under thehigher abstraction scenarios, most (~two thirds) of the impact is on the Mulgrave River, whilst at thelowest modelled abstraction rate the impacts on Behana Creek and the Mulgrave River are roughlyequal.

Figure 30 also shows that in the average dry season, most of the impact on surface water is the result ofreduced baseflow in response to drawdown in the aquifer under the modelled abstraction, particularly forthe Mulgrave River. For the Mulgrave River, under very dry conditions such as those in November 2003,the impacts tend towards to be more a result of induced leakage from the river rather than reductions inbaseflow. This is because groundwater levels fall below the river stage under such dry conditions,hydraulically encouraging water to flow from the river into the aquifer. For Behana Creek under suchconditions, the relative impact derived from baseflow reductions and induced river leakage remainsroughly the same as in average climatic conditions. This is due to the rapid rise of the land surface upBehana Creek towards the edge of the alluvium. Consequently, Behana Creek’s stage height risesabove groundwater elevations up-catchment. This suggests that Behana Creek under both average anddry conditions, much of its upper reaches (that section still within the Mulgrave alluvium) naturally “lose”water to the underlying aquifer. This is because the creek stage height in this area is generally higherthan the surrounding groundwater levels. Subsequently as Behana Creek further rises the basement ofthe creek becomes the parent granite material of the Bellenden Ker range that intersects the alluvium ofthe Mulgrave River west of the Bruce Highway. .As the creek basement changes to granite, and riseswith elevation, the potential for withdrawal from the Mulgrave alluvium to affect surface or base flows ofBehana Creek on the granite basement is nil. Abstraction from the Mulgrave alluvial aquifer cannotinduce a surface or subsurface response from that elevated section of Behana Creek which has agranite basement. That area of granite bedrock is within the World Heritage Area, and surface flows(there is no subsurface flow through the granite) of Behana Creek within the World Heritage Areascannot be affected.


6.4 Predicted Surface Water Impacts in ContextIn an average dry season (1998), mean total river flow is in the order 866 ML/day at the Peets Bridgegauge. Conservatively assuming that the river gains no further baseflow or runoff downstream of thislocation (unlikely, and certainly not the case in the calibrated model), it could be estimated that theproposed abstraction of 15000 ML/year (40ML/day) may reduce low flows by up to 5%. In particularlydry periods1, the reduction in river flow may be as high as 34% of total river flow as a result of themodelled abstraction. Note that this is predicted to be a 1 in 60 year event.

Prior to such a level of abstraction (Stage 2) data would be obtained from gauging stations to beestablished on the mid-reaches of the Mulgrave River (within the likely drawdown area) and on BehanaCreek (also in the drawdown area). This data would be used to refine the model, and provide furtherdetailed information for the necessary detailed assessment that would be required prior to Stage 2 beingimplemented.

For implementation of Stage 1 (5450 ML/year), the volume is small by comparison with the rechargepotential of the aquifer and surface/subsurface flows of the Mulgrave and Behana systems. Theconservatism of the model assumes a constant withdrawal rate modelled over 60 years( that would notreflect actual operation which may be 30% less in duration than modelled) and assumes no net baseflowor recharge below Peets Bridge (which does not happen, but as the gauging data from Peets Bridge wasthe most reliable data then this conservative point was adopted). The model has identified that even withthese conservative approaches incorporated that impacts on river baseflow in an average seasonal year,over 60 years, would be less than 1%.

6.5 Predicted Drawdown ImpactsThe predicted extent of drawdown is limited to the north and east of Aloomba by the Mulgrave River asshown in Figure 26 and Figure 27. These model predictions have been made using the calibratedmodel. To the west drawdown extends to the margin of the valley and into the lower section of theBehana Creek valley. To the south, drawdown extends to the Meerawa area. The 0.5 m drawdowncontour extends to a distance of ~10 km from Mutchero Inlet. Consequently, there is minimal potential forextraction to alter the current groundwater – seawater interface, which is thought to occur at a maximumof 4-5 km north of Deeral, based on extremely limited data (refer Section 3.5). The predicted drawdownis highly unlikely to induce saline intrusion from the south of the model (around Mutchero Inlet), andcertainly not from the north (Trinity Inlet).

Because the drawdown resulting from abstraction is predicted to induce leakage to the aquifer from theupper-most tidal section of the Mulgrave River (around the junction with Behana Creek; Section 6.3), it ispossible that this will result in deterioration of groundwater quality within the aquifer in these areas. Thisis likely to be minimal however, because the average salinity of surface water is likely to decline withdistance up-catchment. In addition, the volume of water derived from the Mulgrave River relative to thetotal abstracted volume is on average / minimal (Figure 30). Thus, the water quality in the Mulgrave Riveris not predicted to have any significant impact on the quality of abstracted groundwater.

The drawdown does not extend to the sensitive wetland vegetation communities and acid sulfate soilsaround Mutchero Inlet, not northwards towards those around Trinity Inlet (shown in Figure 26 and Figure

1 The end of the historical dry climatic years of June 2001 to November 2003 (a 1 in 60 year climatic event) was used as thereference (i.e., November 2003)


27). The draw down is localised to the bore field and locality, with the extent of drawdown confirmed foran extended period of dry years in the sensitivity analysis presented in Section 6.6.

In order to assess the existing users that may be affected by drawdown in the aquifer from theabstraction a threshold of 0.5m modelled drawdown was specified (this being the minimum modelled limitof the drawdown. Figure 26 identifies that four irrigation/domestic supply bores (78268, 92900, 109373and 109846) may be affected (>0.5 m drawdown), with a combined total annual allocation of 258 ML.Details of these bores are presented in Table 7.

Table 7 Existing Groundwater Users Potentially Affected by the Proposed Abstraction

BoreID License LicenseVolume(ML/year)

Bore Use Bore Depth(m)

ModelledDrawdown (m)

% Reductionin AvailableDrawdown

109373 173970 1.5

Irrigation,DomesticSupply 18 1.2 11%

92900 92900K 2 Irrigation 24 1.1 8%

109846 182129 6 Irrigation 39 1.7 6%

78268 78268K 248 Irrigation 18 (estimated) 0.5 5%

NOTES:

a. To estimate the depth of bores without recorded depths, it was assumed that bores were drilled to 10 m belowthe average mapped groundwater elevation at the bore location.

b. The modelled drawdown presented in this table is derived from a particularly dry (1 in 60 year) dry season(November 2003 was used as the reference)

In terms of induced land subsidence, in the worst case, the modelling predicts up to 0.26 m in theimmediate vicinity of abstraction points, and up to 0.09 m over the broader area (Appendix H). Anysubsidence will be gradual over several years and is anticipated to be relatively uniform over the area. Itis not expected to affect infrastructure or drainage. Settlement within the sands is expected to occursoon after groundwater drawdown and aquifer depressurisation (“immediate settlement”), whilst thesettlement within clays will take a long period of time to complete. On average, approximately one thirdof total settlement will occur immediately in response to groundwater drawdown, while the residualsettlement will occur over a long period of time. The results indicate that there will be minimalsubsidence impacts in the area.

The particle tracking results for Scenario 1 in Figure 26 and Figure 27 (40 ML/day) identifies that there islittle potential for abstraction of contaminated groundwater. This could happen if it is assumed that thenearest potential contaminant point source is Gordonvale (waste water treatment plant and Mulgravemill) on the northern side of the Mulgrave River, or south of Aloomba (capped and sealed landfill), ornear Babinda (Babinda waste water treatment plant). All of these areas are well away from the modelledsource zone of both abstraction points.

However, the simulated abstraction is predicted to induce leakage from the river into the aquifer. Thus,contaminated surface water poses a potential risk to groundwater quality. The most likely source ofsurface water contamination is the Gordonvale sewage treatment plant (STP). Gordonvale STPdischarges a waste stream into the river upstream of the proposed abstraction area (and the identifiedarea of drawdown/river flow impact). However, as already described, and shown in Figure 29, therelative proportion of abstracted groundwater modelled as being derived from the Mulgrave River is, on


average / minimal. Therefore, mixing of surface water with the large volume of water within the aquiferwill reduce any potential contaminant concentrations. The particle tracking analyses undertaken in thisinvestigation suggest the shortest groundwater travel times between the river and the bore field arearound 1 year in the calibrated model for the Area 3 bore field, and 18 years for the Area 2 bore field.These predicted travel times suggest that the aquifer will have significant time and capacity to naturallyattenuate any contaminants that may enter the aquifer from the river under induced river leakage into theaquifer.


Figure 30 Abstraction Volume and Baseflow Impacts – Sensitivity Analysis


6.6 Predictive Model Sensitivity AnalysisThe calibrated model was altered to investigate the effects on model predictions of the minimum level ofriver-aquifer connection. The results of this sensitivity analysis with respect to river boundaryconductance are presented in Figure 28. The river boundary conductance in this case was lowered to anequivalent riverbed hydraulic conductivity of 7*10-3 m/day (assuming an average river width of 50 m).Thereby reducing the degree of river-aquifer interconnection to the lowest, but still realisticallyparameterised level. This provides a conservative indication of the maximum likely drawdown extent inresponse to the proposed abstraction. In contrast, the equivalent riverbed hydraulic conductivity of thecalibrated model is 0.07 m/day.

As expected, the predicted maximum extent of drawdown in this case covers a larger area than thehigher river conductance model. The 0.5 km drawdown contour extends a further 2 km southwards,beyond Meerawa, to the south of the abstraction area. Whereas, to the north of the abstraction area, the0.5 m drawdown contour extends a further 3 km to the northeast, beneath the Mulgrave River, towardsthe edge of the thinning alluvium.

The model was also used to assess the impacts of a period of successive dry years, with the lowestlikely level of river - aquifer interaction. June 2001 to November 2003 was a period of two particularly dryconsecutive years, equivalent to a 1 in 60 year dry climatic event. The modelled drawdown in November2003 is presented in Figure 29. This indicates that the drawdown induced by continuous abstraction of15000 ML/year under such conditions would extend from northeast of Gordonvale by ~15 km to thesouth, to ~4 km southeast of Meerawa. The drawdown in this case extends towards the mainabstraction area north of Gordonvale. It impacts an additional nine existing groundwater users (Table 8),but does not extend sufficiently far south to affect the saline/fresh groundwater interface north of Deeral.

Table 8 Existing Groundwater Users Potentially Affected by the Proposed AbstractionAssuming the Lowest Level of River Aquifer Connection

BoreID License LicenseVolume(ML/year)

Bore Use Bore Depth(m)

ModelledDrawdown (m)

% Reductionin AvailableDrawdown

109373 173970 1.5

Irrigation,DomesticSupply 18 6.1 59%


92900 92900K 2 Irrigation 24 3.2 23%

109846 182129 6 Irrigation 39 4.2 15%


45975 45975K 10 Irrigation 33.5 1.1 6%

45845 45845K 12 Irrigation, Stock 29.5 1.0 5%

45312 45030K 203 Irrigation 21.9 0.6 4%

45145 45145K 160 Irrigation 40.3 1.1 4%

45029 45029K 170 Irrigation 27.4 0.6 3%

109675 45030K 203 Irrigation 29.7 0.6 3%


45126 45126K 380 Irrigation 33.8 0.6 2%

109325 178518 40 Irrigation 42 0.5 2%

NOTES:

1. To estimate the depth of bores without recorded depths, it was assumed that bores were drilled to 10 m below theaverage mapped groundwater elevation at the bore location.

2. The modelled drawdown presented in this table is derived from a particularly dry (1 in 60 year) dry season (November2003 was used as the reference)

This sensitivity analysis of river conductance shows that river baseflow impacts resulting from15000 ML/year of groundwater abstraction are reduced with decreasing river-aquifer connection (Figure31).

The sensitivity analysis also supports the above conclusions of Section 6.5. Predicted drawdown doesnot extend to the southern maximum extent of the saline/fresh groundwater interface, nor to theprotected ecological communities or acid sulphate soils around either Mutchero Inlet or Trinity Inlet.Despite increased drawdown extent, there are unlikely to be significant impacts in terms of landsubsidence, saline intrusion into the aquifer, or induced contamination of the groundwater supply fromGordonvale, or any other urban areas.

Figure 31 Modelled Drawdown following successive dry years

6.7 Potential Impacts of Climate ChangeGiven the large proportion of the annual water balance shed as rejected recharge (i.e. runoff, Section 3.9and Section 4.4), it is considered unnecessary to explicitly model the recharge and groundwater flow


model impacts of climate change scenarios. Instead, a water balance impact approach has been utilisedto show that the net excess recharge available in this catchment each year is larger than the potentialimpacts of climate change. This assessment is based upon information presented by the CSIRO report“Climate Change in Queensland under Enhanced Greenhouse Conditions” (Walsh et al., 2002). Theassessment is as follows:

The CSIRO report that net annual impact on soil moisture is estimated at a maximum of –40 mm perdegree of warming in this region of Queensland (Walsh et al., 2002);

The predicted maximum dry season temperature increase over this area of Queensland is between~1 and ~4 C by 2050 (mean annual is between ~0.5 and ~3 C) (Walsh et al., 2002);

In the worst case (~4 C warming by 2050), a worst-case negative impact of 160 mm on soil moisturecan be assumed. This is highly conservative because the calculation assumes the worst-casewarming across the entire year (i.e. does not consider seasonal variation in warming);

In the study area, assuming that this is a consistent change across the entire Mulgrave Alluvium, thisequates to an extra ~6327 ML/year increase in available aquifer or soil moisture storage, or around8% of the model-predicted potential recharge that is, under the current climate, rejected by aseasonally fully-saturated aquifer. This assumes a soil porosity of 0.12 after Leach and Rose, 1979.

In summary, when very conservatively considering climate change in the worst case, there is predicted toremain an excess potential groundwater recharge in this catchment. If it were assumed that the increasein soil moisture deficit under climate change occurs wholly in the dry season, this would result in a slightdelay in the initial groundwater recharge response than would otherwise have occurred. The rechargemodelling undertaken in this study indicates that the soil moisture deficit is largely replenished within30 days of the wet season onset. The groundwater model operates on a monthly time step, andtherefore the impacts of climate change will not be evident in the model. This also indicates that anychanges in groundwater levels in response to climate change at the onset of the wet season are likely tobe unnoticeable.

Although this is a simple water balance approach to the problem of climate change, it can be reasonablyconcluded that the worst-case predictions of climate change will result in minimal net losses to theaquifer water balance. This is due to the current net lack of aquifer storage with respect to the potentialgroundwater recharge in this catchment.


7. Conclusions

7.1 ResultsThe hydrogeological data analysis, conceptualisation presented and other supporting investigations inthis report were used to construct and calibrate a numerical groundwater flow model of the MulgraveRiver Alluvium. The model is considered sufficiently calibrated for the purposes of the investigation,however uncertainty and sensitivity analyses have been carried out to produce a range of possibleoutcomes.

The calibrated transient model was used as the basis for simulating the following two scenarios:

1. 15,000 ML (40 ML/day) constant annual abstraction from the two areas found to be most promisingin terms of aquifer yield (“Area 2” – near Gordonvale, and “Area 3”- south of Behana Creek); and

2. 5450 ML/year (15 ML/day) constant annual abstraction from Area 2 only.

The latter was modelled to provide the basis for a staged implementation of the proposed abstraction. Ithas a view to expand the groundwater abstraction up from 15 ML/day in the future, dependent on theoutcomes of environmental impact monitoring during the first stage.

Several bore field arrangements were modelled to provide the least impact, yet practicable spatial borefield arrangement.

In terms of aquifer drawdown impacts, summary results for an average climatic year (1997-98) andfollowing a particularly dry period (June 2001 to November 2003) are presented in Table 9 and Table 10,respectively. In terms of river flow impacts, results for the same climatic periods are presented in Table11 and Table 12. All reported impacts refer to dry season drawdown and river flow impacts, forconservatism.

7.2 Model SummaryA range of model predictive runs has been undertaken using the calibrated model and assuming ‘highriver-aquifer connection’ and ‘low river-aquifer connection’ interaction between the aquifer and theMulgrave River. The calibrated model provides the ‘best estimate’ basis for predicting impacts ofgroundwater abstraction, given the currently available data. The ‘high river-aquifer connection’ scenarioassumes direct connection between the aquifer and the river effectively maximising the volume ofabstracted water derived from the river. Conversely, the ‘low river-aquifer connection’ case assumes avery low riverbed conductivity of 7*10-3 m/day (assuming an average river width of 50 m) therebyminimising abstraction-induced leakage from the river to the aquifer. This represents the lowest,physically realistic level of river-aquifer connection.

The modelled arrangement of the bore fields at Area 2 and Area 3 was iteratively derived viaspreadsheet modelling using Theis (1935) analysis, which suggested a minimum bore spacing of 400 m.Three iteration of modelling were conducted to determine a final bore field arrangement that minimisedthe drawdown impacts of abstraction. The resulting bore field arrangement is shown in Figure 26 andFigure 27.

The model-predicted drawdown in response to abstraction (Section 6.5) suggests that the proposed borefield arrangement has minimum drawdown and bore interference effects on other bores in the area with


potentially only 4 bores impacted at 40ML/day Stage 2 abstraction and only 1, marginally 2 at most,being affected at a Stage 1 abstraction of 15 ML/day. In all instances the potentially affected bores areshallow bores and are not withdrawing from the main quaternary alluvial field, but from shallower lensesin the upper reaches of the aquifer. Compensatory measures may be as simple as extending the depthof the existing bores.

The modelled drawdown within the Area 2 and Area 3 bore fields suggests that the proposed averagebore depths (50 m at Area 2 and 65 m at Area 3) are sufficient to provide the required availabledrawdown under the maximum proposed abstraction scenario (40ML/day).

7.3 Potential Impacts

7.3.1 Interaction with Surface Flows

Over the period of the modelling (based on 60 years of data) in the longer term the maximum abstractionregime of 15000 ML/year (or 40 ML/d) induces 7 ML/day of leakage from the river into the aquifer. It alsosuggests that a reduction in groundwater discharge into the river is by 34 ML/day. This leakage andreduction is not immediate, and the output figures are averaged over the 60 life of the model. Thesefigures are the maximum scenario, based on the assumptions of continual abstraction with no allowancefor operational aspects, and the model assumed no surface water recharge or base flow entering thesurface water system downstream of Peets Bridge. This was assumed owing to the paucity of gaugederived flow data within the immediate alluvial area, and consequently imparts a high level ofconservatism. Surface discharges into the Mulgrave River downstream of Peets Bridge include surfaceand subsurface run off from numerous features but as the data is not present for these features, theirinput into the model was limited.

On average, all of the abstracted groundwater will impact the Mulgrave River and Behana Creek by acombination of combination of reduction in baseflow to the river and creek, and by induced leakage fromthe river and creek back into the aquifer. The degree of impact will vary seasonally and upon theoperational requirements of the bore field, however in the long term the ratio of the volume of abstractedgroundwater to the total volume of river flow is predicted to be a one to one ratio.

The surface water flow impacts will affect both the Mulgrave River and Behana Creek. Most (~twothirds) of the impact of abstracting 40 ML/day is on the Mulgrave River, whilst at an abstraction rate of15 ML/day the impacts on Behana Creek and the Mulgrave River are roughly equal. Care must beexercised in adopting these figures as the final impact regime. As has been strongly reinforced, noaccounting has been made of surface recharge and subsurface flow in the model downstream of PeetsBridge (a conservative approach adopted owing to the limited data for the middle reaches of theMulgrave River and Behana Creek) and of the regime of the bore field operation which may not pump forup to 4 months of the year (recognising the bore field is a supplementary supply). Therefore the likelypractical (as opposed to modelled) impacts on Behana Creek (and Mulgrave River) will be significantlyless than the model prediction.

The installation of gauging stations in the middle reaches of the Mulgrave River (within the affecteddrawdown area) and on Behana Creek will supply essential data to refine the model for a more detailedassessment prior to the implementation of Stage 2. For Stage 1 (15 ML/day) the abstraction, asmodelled, will result in a less than 1% impact on surface flows. In consideration of the conservative limits


as outlined previously, the practical impacts of the abstraction on the surface flows of Mulgrave andBehana will be less than that modelled.

The model does identify that when comparing the predicted river flow impacts to gauged flow at PeetsBridge, the 40 ML/day of river flow impact represents a relatively small percentage of the estimated long-term average and peak river flows. However, under the simulated abstraction in the climatic case of theparticularly dry 2003 dry season, model predictions suggest flows may be reduced by up to 34% (c.f.daily gauged mean total flow at Peets Bridge of 122 ML/day on November 23rd, 2003). Note that this isan unusually dry (1-in-60 year) climatic event. In the average dry season (November 1998), this impactis predicted to be in the order of 5% of the gauged mean total flow.

7.3.2 Drawdown Impacts

The predicted drawdown may extend up to 10 km from north to south in dry periods, centred on theproposed abstraction area. The predicted maximum aquifer drawdown in the immediate vicinity of theabstraction points is 5 m. However, this is predicted to be 2 m over the broader area, within 2 to 3 km ofeither the Area 2 or Area 3 bore field.

The modelled drawdown has the potential to affect up to four existing groundwater users, with acombined total annual allocation of more than 258 ML. The maximum predicted reduction in any of thebores’ available drawdown is less than 11%. From the information supplied by NRW on the groundwaterbores, these groundwater users can be compensated by the extension of the depths of their bores.

The model has not identified that drawdown will significantly affect the saline groundwater interface, andthere is therefore a low risk of inducing saline intrusion into the aquifer. The predicted drawdown doesnot extend to areas of acid sulphate soils or the significant ecological communities at either Trinity Inletor Mutchero Inlet.

In terms of induced land subsidence, in the worst case, the modelling predicts up to 0.26 m in theimmediate vicinity of abstraction points, and up to 0.09 m over the broader area. Any subsidence will begradual over several years and is anticipated to be relatively uniform over the area and is not expected toaffect infrastructure or drainage. The results indicate that there will be minimal subsidence impacts in thearea.

7.3.3 Groundwater Contamination

There is little potential for abstraction of contaminated groundwater if it is assumed that the nearestpotential contaminant point source is Gordonvale (waste water treatment plant and Mulgrave mill) on thenorthern side of the Mulgrave River, or south of Aloomba (capped and sealed landfill), or near Babinda(Babinda waste water treatment plant). All of these areas are well away from the modelled source zoneof both abstraction points.

However, the simulated abstraction is predicted to induce leakage from the river into the aquifer, andtherefore contaminated surface water poses a potential risk to groundwater quality. The most likelysource of surface water contamination is the Gordonvale sewage treatment plant (STP). The GordonvaleSTP discharges a waste stream into the river upstream of the proposed abstraction area (and theidentified area of drawdown/river flow impact). However, the relative proportion of abstractedgroundwater modelled as being derived from the Mulgrave River is, on average / minimal. Therefore, themixing of surface water with groundwater will reduce any potential contaminant concentrations. Theparticle tracking analyses undertaken in this investigation suggest the shortest groundwater travel times


between the river and the bore field are around 1 year in the calibrated model for the Area 3 bore field.Whilst Area 2 bore field has 18 years of groundwater travel time. These predicted travel times suggestthat the aquifer will have significant time and capacity to naturally attenuate any contaminants that mayenter the aquifer from the river under induced river leakage into the aquifer.

7.3.4 Summary of Model Predicted Impacts

The following figures represent the raw outcomes of models constructed for this assessment.

These figures are not absolutes.

The figures are the result of modelling which has assumed a number of conservative approaches owingto limited data available for some attributes. These approaches have included assumptions of constantpumping and drawdown of the aquifer (which may be up to 30% in excess of potential operationalrequirements), and no surface recharge or subsurface flow downstream of the most reliable gauging data(Peets Bridge, upstream of the aquifer). Therefore the modelled has undervalued the contribution of theinputs from other surface features as data was not available.

In particular, this has had a very strong bearing on the modelled figures of potential river flow impact forBehana Creek. Behana Creek has limited flow data, and in accordance with the model parameters,assumed no recharge at all of Behana by additional subsurface and surface flows. This has resulted incomparative impacts between the Mulgrave River and Behana Creek being highly skewed, with BehanaCreek showing a disproportionate river flow impact by comparison with the Mulgrave River. As data fromthe recommended gauging station on Behana Creekbecomes available and is able to be included intothe model, the model then will be able to assume subsurface and surface water inputs into BehanaCreek from its catchment. It is then highly likely that the propotional impacts between the figures in thetables below will significantly alter, with Behana Creek surface flowing absorbing a much small proportionof the river flow impact than is illustrated below.

Table 9 Model Predicted Impacts – Dry Season, Average Climatic Year (1997-98)

Impact Under High River-Aquifer Connection

Impact Under Low River-Aquifer Connection

Best Estimate Impact

Maximum Drawdown (m) 9 10 5

Drawdown Extent (to0.5 m) (km)

7 10 8

Number of ExistingUsers Affected

2 5 4

Total Licensed Volumeof Affected Users(ML/year)

8 268 258

River Flow Impacts(ML/day)

41 (27 Mulgrave River, 14Behana Creek)



Subsidence (m) 0.12 (0.04 over broader area) 0.13 (0.04 over broader area) 0.12 (0.04 over broader area)

Drawdown at SalineInterface (m)

0 0 0


Table 10 Model Predicted Impacts – After Several Dry Climatic Years (June 2001 to November2003)




Maximum Drawdown (m) 5 10 5

Drawdown Extent (to0.5 m) (km)

10 16 10

Number of ExistingUsers Affected

4 11 (13 bores) 4

Total Licensed Volumeof Affected Users(ML/year)

258 1233 258

River Flow Impacts(ML/day)




Subsidence inimmediate vicinity ofbores (m)

0.12 (0.04 over broader area) 0.26 (0.09 over broader area) 0.12 (0.04 over broader area)

Drawdown at SalineInterface (m)

0 0.1 0

Table 11 Model Predicted River Flow Impacts – Dry Season, Average Climatic Year (1997-98)




Total MulgraveRiver

BehanaCreek

Total MulgraveRiver

BehanaCreek

Total MulgraveRiver

BehanaCreek

Induced Leakage fromRiver (ML/day)

15 11 4 0 0 0 7 2 5

Reduced GroundwaterDischarge to River(ML/day)

25 16 10 22 16 6 34 23 10

Total Predicted RiverFlow Impact (ML/day)

41 27 14 22 16 6 41 26 15

NOTES:

1. The best estimate reduction in groundwater discharge to the river is higher than that in the high river-aquifer connectioncase because the reduced river-aquifer connection in the best estimate case causes a greater pumping-induced drawdownextent in the aquifer. This therefore leads to a larger area over which groundwater discharge to the river is reduced.

2. These numbers have been rounded to the nearest integer. Therefore minor discrepancies between the totals and theconstituents may occur.


Table 12 Model Predicted River Flow Impacts – Aquifer Several Dry Climatic Years (June 2001to November 2003)




Total MulgraveRiver

BehanaCreek

Total MulgraveRiver

BehanaCreek

Total

MulgraveRiver

BehanaCreek

Induced Leakage fromRiver (ML/day)

28 21 7 16 13 3 23 15 8

Reduced GroundwaterDischarge to River(ML/day)

13 6 7 8 7 1 18 10 8

Total Predicted RiverFlow Impact (ML/day)

41 27 14 24 20 4 41 25 16

NOTES:

1. The best estimate reduction in groundwater discharge to the river is higher than in the high river-aquifer connection casebecause the reduced river-aquifer connection in the best estimate case causes a greater pumping-induced drawdownextent in the aquifer. This therefore leads to a larger area over which groundwater discharge to the river is reduced.

2. These numbers have been rounded to the nearest integer. Therefore minor discrepancies between the totals and theconstituents may occur.


8. Recommendations

The following recommendations should be undertaken, should the proposed groundwater supply fromthe Mulgrave Alluvium will commenced:

The groundwater supply is initiated via a staged approach, with an initial stage of abstraction at15 ML/day. The supply could then be gradually expanded to greater volumes. This will depend onthe identification of any impacts associated with the operating groundwater supply.

The bore field layout presented in this report should be used to plan any drilling works. This layoutcould be further improved if private land were sought to place the bore field, rather than beingrestricted to public land.

Detailed monitoring of river flows and drawdown in the aquifer during the first stage of supplyoperation. These data can be used to provide support to future expansion of the groundwatersupply. They may also be used to provide a detailed data set against which the groundwater modelmay be re-calibrated, and used to provide better-supported model predictions of potential impacts ofthe future expansion of the supply.

Confidence in the numerical model and the resulting impact predictions presented above could beimproved through:

The installation of permanent river flow gauges in the lower catchment and regular spot gauging ofdry season flows along the length of the river and key tributaries, such as Behana Creek.Specifically:

– Gauge installation and daily monitoring of Mulgrave River flows at the tidal limit (high priority),and at the catchment outlet if possible (Mutchero Inlet) (lowest priority).

– Rehabilitation / replacement of the Gordonvale gauge so that it measures total flow at all times(high priority).

– Regular spot flow gauging at times of low flows, for the purposes of flow accretion profiling. Thisshould be completed at:

o Several (say more than 5) locations along the Mulgrave River between Peets Bridge and theCatchment outlet (high priority);

o Several locations along the alluvial deposits of Behana Creek (medium priority); and

o Along any other major tributaries to the Mulgrave River (lowest priority).

Detailed assessment of river flow data once sufficient data are collected in order to better understandriver-aquifer interaction in the area.

It is recommended that drilling investigations be undertaken around Mutchero Inlet to confirm thelocation of the saline/fresh groundwater interface.


9. References

Cook P.G. and Herczeg A.L. (1998) Groundwater Chemical Methods for Recharge Studies. Part 2 of L.Zhang and G. Walker (ed.) The Basics of Recharge and Discharge. CSIRO Publishing.

Cook, P.G, Healy R.W., 2002. Using Groundwater Levels to Estimate Recharge. Hydrogeology Journal10: pp. 91-109.

Dept. Natural Resources, QLD 1999: Report on Mulgrave Groundwater Model

Doherty, J. E., 2002. PEST Model-independent Parameter Estimation, Users Guide. WatermarkNumerical Computing.

GHD, 2006. Mulgrave River Feasibility Study Hydrogeological Report (GHD Draft report42/14087/01/7410, August 2006)

Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, M.G., 2000. MODFLOW-2000, the U.S.Geological Survey modular ground-water model – user guide to modularization concepts and the ground-water flow process, U.S. Geological Survey Open-File Report 00-92.

Leach L.M. and Rose U.E., 1979: Groundwater Storage Behaviour, Mulgrave River Area. Qld WaterRes. Comm. Report.

Littleboy, M., Silburn, D.M., Freebairn, D.M., Woodruff, D.R. and Hammer, G.L. 1989. PERFECT, Acomputer simulation model of Productivity, Erosion, Runoff Functions to Evaluate ConservationTechniques. Queensland Department of Primary Industries, Bulletin QB89005, 119 pp.

Muller,P.J., 1975: Mulgrave River Groundwater Investigations, Report on Exploratory Drilling. Rec. Geol.Surv. Qld. 1975/17.

Murtha G., Cannon M. and Smith C. 1996: Soils of the Babinda - Cairns Area, North Queensland. CSIROAust. Div. Soils, Divl Rpt. No. 123.

Schapp, M.G., Leij, F.J., and van Genuchten, M.Th. (2001). "Rosetta: A computer program for estimatingsoil hydraulic parameters with hierarchical pedotransfer functions". Journal of Hydrology 251 (3): 163–176

Sloto, R.A., Crouse, M.Y., 1996. HYSEP: A Computer Program For Streamflow Hydrograph SeparationAnd Analysis. U.S. Geological Survey Water-Resources Investigations Report 96-4040

Stewart, M., Cimino, J., and Ross, M. 2007. Calibration of Base Flow Separation Methods withStreamflow Conductivity. Ground Water Vol. 45, No. 1, January–February 2007, pp 17–27.

Theis, C.V., 1935. The relation between the lowering of the piezometric surface and the rate andduration of discharge of a well using groundwater storage. Trans. Amer. Geophys. Union 2, pp. 519-524.

van Genuchten M.Th. A closed-form equation for predicting the hydraulic conductivity of unsaturatedsoils. Soil Sci. Soc. Am. J. 1980;44:892-898.

Walsh, K., Cai, W., Hennessy, K., Jones, R.,

McInnes, K., Nguyen, K., Page C., and Whetton, P., 2002. Climate Change in Queensland underEnhanced Greenhouse Conditions Final Report, 2002. Report on research undertaken for Queensland


Departments of State Development, Main Roads, Health, Transport, Mines and Energy, Treasury, PublicWorks, Primary Industries, and Natural Resources. CSIRO 2002.

Western, A., McKenzie, N., 2006. Soil hydrologic Properties of Australia. Rev. 1.01. CooperativeResearch Centre for Catchment Hydrology, University of Melbourne, Victoria, Australia.

Willmott, W.F. and Stephenson, P.J. 1989. Rocks and landscapes of the Cairns district. QueenslandDept. of Mines.


Appendix A

Geophysical Logging Inventory


Bore Status

11100002 Not monitored for many years, not located, suspect bore is abandoned







11100009 Okay, gamma and induction

11100010 Obs bore for 0009

11100011 Obs bore for 0009



11100014 No records

11100015 No records



11100018 Equipped with auto recorder, no log possible


11100020 No records



11100023 No records



11100026 Obs bore for 0025

11100027 Obs bore for 0025




11100031 No records



11100034 Obs bore for 0033


Bore Status

11100035 Relined, new pipe too small to fit logging probes, no logging possible

11100036 No records

11100037 No records




11100041 No records





11100046 Obs bore for 0045

11100047 Obs bore for 0045




11100051 No records

11100052 No records








11100060 No records

11100061 No records

11100062 No records

11100063 No records

11100064 No records

11100065 No records

11100066 No records

11100067 No records


Bore Status

11100068 No records

11100069 No records









11100078 Blocked/kinked at 5m, no logging done


11100080 No records


11100082 No records

11100083 No records

11100084 No records

11100085 No records

11100086 No records

11100087 No records

11100088 No records

11100089 No records

11100090 No records

11100091 No records

11100092 No records


Appendix B

Observed and Modelled Groundwater LevelHydrographs

GHD www.ghd.com.auTel. (03) 8687 8000 Fax. (03) 8687 8111180 Lonsdale Street Melbourne Vic 3000

Enter Client/Project Name HereEnter Spreadsheet Name Here

Appendix CG:\42\14087\MODFLOW\02\02e\output\Appendix D\Appendix C.xls28/05/2007 5:53 PM Page 1 of 2

http://www.ghd.com.au


The hydrographs on this page are from bores located in the south of the catchment. It is evident that river stage dynamics and the degree of river-aquiferinterconnection play an important role in controlling groundwater levels and fluctuations. The model is unable to represent these dynamics in its currentstate (uncalibrated to river baseflow due to a lack of data), and therefore the calibration quality deteriorates down catchment towards Mutchero Inlet.

River stage height and the degree of river-aquifer interconnection is poorly representedin this (down-river) area of the model

Appendix CG:\42\14087\MODFLOW\02\02e\output\Appendix D\Appendix C.xls28/05/2007 5:53 PM Page 2 of 2



Appendix C

Observed and Modelled Aquifer TestDrawdowns


Enter Client/Project Name HereEnter Spreadsheet Name Here

Appendix DG:\42\14087\MODFLOW\03\03b\OUTPUT\Appendix_D_03b_Drawdowns.xls28/05/2007 5:51 PM Page 1 of 1



Appendix D

Detailed Water Balance Calculations –Mulgrave River Alluvium


Cairns Water / Mulgrave River Aquifer Groundwater ModelMulgrave River Alluvium Water Balance - Detailed Calculations

WATER BALANCE INPUTS Units BALANCE COMMENTS

RECHARGEArea of Mulgrave Alluvium 329508824 m2Recharge 460.0 mm From hydrogrpah fluctuation methodTotal Recharge 151574 ML IN

Stream leakage into aquifer

Leakage 35533 ML INAssume 2% of Behana creek annual flow (176660ML) recharges aquifer. This is thecomponent used to balance the water balance

Groundwater Inflows From Upstream Alluvials

Behana Creek Upstream Alluvials 994 ML

Assuming average hydraulic conductivity of 26.5m/d (from aquifer tests), gradients estimatedfrom water level mapping (10m/800m), cross sectional areas from constructed geologicalsurfaces (depth to bedrock; 10m thickness, 300m width)

Mulgrave River Upstream Alluvials 11612 ML

Assuming average hydraulic conductivity of 26.5m/d (from aquifer tests), gradients estimatedfrom water level mapping, cross sectional areas from constructed geological surfaces (depth tobedrock)

Total Inflows 12606 ML IN

Groundwater AbstractionsTotal Allocations 22000 MLUnused (Cairns Water) 15000 ML

Used Allocation 2100 ML OUT Assume 30% of allocation is used (Leach and Rose, 1979) and most are irrigation licenses

BASEFLOW

Hydraulic gradient to river 0.007Estimated average gradient in vicinity of average stage height at Gordonvale gauge, andaverage groundwater level in bore 11100075, located 1km upstream of this gauge

Aquifer thickness 20 m Estimated thickness of aquifer contributing to baseflow67000 m Estimated total major river/stream length draining the alluvium

Hydraulic conductivity 26.5 m/d Average hydraulic conductivity from aquifer testsBaseflow 181456 ML OUT Detailed discussion of this calculation in Section 3.4.1 of report

GROUNDWATER THROUGHFLOWTrinity Inlet cross-sectional area 402268 m2 From modelled geological surfacesTrinity Inlet hydraulic gradient 0.002 m/m From mapped groundwater levels (average June-July)Trinity Inlet hydraulic conductivity 26.5 m/d From aquifer tests

Throughflow via Trinity Inlet 8979 MLMutchero Inlet cross-sectional area 85455 m2 From modelled geological surfacesMutchero Inlet hydraulic gradient 0.003 m/m From mapped groundwater levels (average June-July)Mutchero Inlet hydraulic conductivity 26.5 m/d From aquifer tests

Throughflow via Mutchero Inlet 2119 MLTOTAL Throughflow 11098 ML OUT

Balance - alluv only (2)G:\42\14087\Tech\Water balance\water balance.xls11/07/2007 1:30 PM Page 1 of 1


Appendix E

PERFECT Model – Assigned SoilProperties


Soil Horizon Sa(%)

Si(%)

Cl(%)

DepthTo(m)

AdjustedThetaR

AdjustedThetaS

AdjustedKsat(cm/d)

AdjustedWiltingPoint

AdjustedFieldCapacity

BulkDensity

CONA U GreatGroup

Bg_Bulgun A 31.8 22.3 45.9 0.3 0.09 0.45 9.60 0.17 0.37 1.1 4 9.5 Euchrozem

Bg_Bulgun B 27.6 25.3 47.1 1.2 0.09 0.46 11.33 0.17 0.38 1.1 4 9 Euchrozem

Cn_Canoe A 65.6 14.3 20.1 0.3 0.06 0.38 21.09 0.10 0.27 1.2 3.75 8.5 Earth

Cn_Canoe B 74.3 9.9 15.8 1.2 0.06 0.38 34.18 0.08 0.24 1.2 3.5 6.75 Earth

Cn_Canoe C 81.0 8.0 11.0 1.5 0.05 0.38 77.08 0.06 0.20 1.2 3.5 6.75 Earth

Ct_Clifton A 70.8 17.0 12.2 0.4 0.05 0.37 41.32 0.07 0.23 1.2 3.5 6.75 Earth

Ct_Clifton B 61.3 20.6 18.1 1.1 0.04 0.30 17.18 0.07 0.21 1.3 3.75 8.5 Earth

Ct_Clifton C 61.8 22.0 16.2 1.7 0.05 0.37 24.65 0.08 0.26 1.2 3.75 8.5 Earth

Et_Edmonton A 23.5 44.3 32.3 0.1 0.08 0.45 12.11 0.12 0.38 1.1 4 9 Euchrozem

Et_Edmonton B 30.5 41.1 28.4 1.1 0.08 0.43 11.34 0.11 0.36 1.1 4 9 Euchrozem

Il_Inlet A 43.0 20.8 36.2 0.2 0.08 0.42 6.34 0.14 0.33 1.1 4 9.5 Euchrozem

Il_Inlet B 36.8 24.5 38.7 1.6 0.09 0.44 5.34 0.15 0.35 1.1 4 9.5 Euchrozem

Il_Inlet D 16.1 32.8 51.2 2.8 0.10 0.49 17.44 0.18 0.41 1.1 4 9 Euchrozem

In_Innisfail A 32.5 25.4 42.1 0.4 0.09 0.44 6.70 0.16 0.36 1.1 4 9.5 Euchrozem

In_Innisfail B 44.1 25.8 30.2 1.5 0.08 0.41 6.64 0.12 0.32 1.1 4 9 Euchrozem

In_Innisfail C 67.5 15.8 16.8 2.1 0.05 0.38 27.49 0.09 0.26 1.2 3.75 8.5 Euchrozem

Km_Kirrama A 64.2 5.5 30.4 0.5 0.06 0.35 12.62 0.12 0.27 1.2 4 9 Earth

Km_Kirrama B 52.9 2.3 44.7 1.8 0.07 0.33 15.05 0.15 0.27 1.3 4 9.5 Earth

Li_Liverpool A 64.8 13.9 21.4 0.2 0.06 0.38 19.28 0.10 0.27 1.2 3.75 8.5 Earth


Soil Horizon Sa(%)

Si(%)

Cl(%)

DepthTo(m)

AdjustedThetaR

AdjustedThetaS

AdjustedKsat(cm/d)



BulkDensity

CONA U GreatGroup

Li_Liverpool B 69.7 12.6 17.7 1.2 0.06 0.38 26.51 0.09 0.26 1.2 3.75 8.5 Earth

Li_Liverpool D 76.0 11.0 13.0 1.5 0.05 0.38 48.66 0.07 0.23 1.2 3.5 6.75 Earth

Mb_Malbon A 54.0 20.0 26.0 0.2 0.06 0.36 10.40 0.11 0.27 1.2 4 9 Earth

Mb_Malbon B 64.0 13.2 22.8 1.0 0.05 0.29 12.75 0.08 0.22 1.3 3.75 8.5 Earth

Mb_Malbon D 20.1 36.4 43.5 1.9 0.09 0.47 12.41 0.16 0.39 1.1 4 9.5 Earth

Ms_Mission A 68.0 12.0 20.0 0.1 0.09 0.44 11.69 0.15 0.37 1.1 3.75 8.5 Earth

Ms_Mission B 70.9 10.8 18.3 0.8 0.06 0.33 8.63 0.11 0.27 1.2 3.75 8.5 Earth

Ms_Mission C 79.6 11.1 9.3 1.6 0.05 0.27 7.22 0.09 0.23 1.3 3.5 6.75 Earth

Mn_Mangrove A 50.5 24.8 24.8 0.1 0.09 0.47 12.44 0.16 0.39 1.0 3.75 8.5 Euchrozem

Mn_Mangrove B 40.0 25.5 34.5 0.3 0.09 0.47 12.44 0.16 0.39 1.1 4 9 Euchrozem

Mn_Mangrove D 37.0 27.0 36.0 0.6 0.09 0.47 12.44 0.16 0.39 1.1 4 9.5 Euchrozem

Pg_Pin_Gin A 12.0 35.5 52.5 0.3 0.09 0.47 12.32 0.15 0.39 1.1 4 9 Euchrozem

Pg_Pin_Gin B 9.5 36.9 53.6 1.7 0.09 0.47 12.30 0.15 0.39 1.1 4 9 Euchrozem

Th_Thorpe A 69.9 11.0 19.1 0.4 0.06 0.31 8.15 0.10 0.26 1.3 3.75 8.5 Earth

Th_Thorpe B 71.6 8.8 19.6 1.0 0.06 0.29 7.57 0.09 0.24 1.3 3.5 6.75 Earth

Th_Thorpe C 82.8 7.9 9.4 1.6 0.04 0.22 5.91 0.07 0.19 1.4 3.5 6.75 Earth

Th_Thorpe D 72.5 15.8 11.8 1.8 0.09 0.47 12.32 0.15 0.39 1.0 3.5 6.75 Earth

Ti_Timara A 8.5 61.3 30.3 0.2 0.09 0.47 12.44 0.16 0.39 1.1 4 9 Euchrozem

Ti_Timara B 6.0 58.3 35.7 0.7 0.09 0.47 12.44 0.16 0.39 1.1 4 9 Euchrozem


Soil Horizon Sa(%)

Si(%)

Cl(%)

DepthTo(m)

AdjustedThetaR

AdjustedThetaS

AdjustedKsat(cm/d)



BulkDensity

CONA U GreatGroup

Ti_Timara 2.0 4.6 50.2 45.2 1.0 0.09 0.47 12.44 0.16 0.39 1.1 4 9.5 Euchrozem

Ti_Timara D 29.4 48.6 22.0 1.4 0.09 0.47 12.44 0.16 0.39 1.0 3.75 8.5 Euchrozem

Vi_Virgil A 61.2 15.1 23.6 0.3 0.09 0.47 12.32 0.15 0.39 1.0 3.75 8.5 Earth

Vi_Virgil B 59.3 19.9 20.8 2.2 0.09 0.47 12.29 0.15 0.39 1.0 3.75 8.5 Earth


Appendix F

Residual Statistics – Steady StateCalibration


Appendix G

Residual Statistics – Transient Calibration


Appendix H

Land Subsidence Analysis


IntroductionAquifer depressurisation induced land subsidence is the gradual settling of ground surface due toreduction in water pressure and a corresponding increase in effective stresses in the ground.Subsidence is commonly caused by the compression of soils and rock in and around areas of l arge scalegroundwater pumping. Land subsidence due to groundwater withdrawal has been recorded in Asia,Europe, South America, North America and Australia.

At Mulgrave River, aquifer depressurisation during groundwater extraction operation is not expected toresult in significant land subsidence.

Land Settlement MechanismsThe physical process that links fluid withdrawal to settlment is fundamental and essentially means thatfor a confined aquifer system the two physical processes are chained together. Given the areal extent ofthe aquifers at Mulgrave River means that vertical compression of the aquifer systems at depth will resultdirectly in settlement in land surface without significant three-dimensional (i.e. differential) effects.

Initially the weight of overburden (soil and water) above an aquifer is in equilibrium being carried bysupport forces consisting of water pressure and sediment grain-to-grain stress. As water is removedfrom the aquifer, the fluid pressure decreases. Because the weight above the aquifer does not changewith time this weight must continue to be carried by the aquifer system. The portion of overburdenweight that was initially supported by the water decreases and an increasing portion is carried by the soilstructure. The skeletal structure of the soil becomes more densely packed to achieve a new equilibriumresistance to the overburden load. The result is a decrease in porosity within the aquifer system andcorresponding settlement of the land surface.

In addition, the slow draining, low permeability clay members of an aquifer system are often found to bemore compressible than sands. This results in a time lapse between changes in water pressures andcumulative compression of the entire system. Although settlement of sand units is relatively fast andoccurs quickly, volume changes within the clay soils are delayed and occur over a long period of time.

The settlement behaviour of clay soils is usually dependant on its stress history and can be expressed inengineering terms as being in either one of two states:

1. Normally consolidated: Where the in-situ stress state of the soil has seen little or no increase inoverburden pressure in the past. This is usually expressed as the preconsolidation pressure (Pc)being equal to the current stress state (Po). (i.e. Pc=Po).

2. Overconsolidated: Where the in-situ stress state of the soil is presently less than in the past, due toremoval of overburden pressure. This is usually expressed as the preconsolidation pressure (Pc)being greater than the current stress state (Po). (i.e. Pc>Po).

The compressibility behaviour of clay soils between normally and overconsolidated states is usually oneto two orders of magnitude apart.

Soils that are defined as normally consolidated are considered to be more susceptible to settlement thanoverconsolidated soils and have a correspondingly higher compressibility index value.

The degree of over consolidation greatly reduces the compressibility index of the soil and is directlyreflected by the how much the preconsolidation pressure (Pc) exceeds the current overburden pressure(Po). Subsequently, the effective stress range within sediment sequence in relation to its


preconsolidation pressure (Pc) will have a major bearing on the magnitude of land subsidence that willoccur in response to a given reduction in groundwater levels.

Settlement Model

In this study settlement is assumed to occur in vertical direction only in response to changes in aquiferpressure due to the operation of the production bores.

The process of consolidation settlement has a log-linear relationship. The differential equation thatdescribes the process of consolidation of horizontal clay beds presented below is widely used in thenumerical simulation of subsidence.

)(

)()(3 log

io

iioi p

ppHCRS when Po + P > Pc

)(

)()(3

)(2 loglog

io

iioi

ioi p

ppHCR

pPcHCRS when Po + P < Pc

Where: S = the sum of primary consolidation settlement

CR2 = Compression Ratiooe

Cc1

CR3 = Compression Ratiooe

Cs1

Cc = compression Index

Cs = Swell Index

Hi = Initial thickness of sub-layer i

Po(i) initial average effective overburden pressure for sub-layer i

P = increase in effective stress in each sub-layer of soil analysed

Pc = maximum preconsolidation pressure

In applying the settlement model to the Mulgrave River site the adopted stratigraphic soil profile wassimplified at two selected locations, as follows:

Area 2 Clay 0 to 9 m depth;

Sand 9 to 14 m depth;

Clay 14 to 20 m depth;



Weathered Granite 65m to 92 m depth

Granite Fresh >92 m depth

Area 3 Clay 0 to 5 m depth;






Weathered Granite 65m to 90 m depth

Granite fresh >90 m depth

The settlement discussed considers Quaternary and Tertiary units only. Settlement within the underlyingbasement rock units is anticipated to be negligible in the context of the magnitudes assessed(ie, settlements within rock units are likely to be insignificant compared with those in Quaternary andTertiary).

Groundwater levels are 9 m and 6 m below ground surface in areas 2 and 3 respectively at the start ofgroundwater extraction from the aquifers.

All soil units were divided into 1 m thick sub-layers for subsidence modelling purposes.

Secondary consolidation settlements (ie, creep) have not been included in this assessment ofsubsidence and their contribution will depend on the degree of overconsolidation present if any.

Overconsolidation Ratio

General

At this time we have no information at our disposal to indicate if the soil strata at the Mulgrave River siteis normally consolidated or over consolidated.

Sea Level ChangesMajor sea level changes occurred during the Late Pliocene (128ka to 10ka BP) to Holocene (10ka to 0kaBP), which have influenced the geological process within the Mulgrave River site including depositionaland erosional cycles. Sea level changes during this time were caused by the most recent global ice age,which resulted in widespread glaciation of the northern and southern hemispheres. Worldwide studies ofsea level changes in response to glacio-eustatic events during the late Quaternary and Holocene periodshave been well studied. The worldwide fluctuations in mean sea levels since the last interglacial periodare generally based on studies completed in this area by Chappel and Shackelton (1986), Pirazzoli(1991) and others. These studies indicate that around 128ka before present (BP) the sea levels wereclose to today’s levels and over the period from 117ka BP to 17ka BP gradually fell by approximately120 m coinciding with glacial maximum period of 17ka to 24ka BP. After this event sea levels gradually


rose returning to present day levels at around 7ka BP. It is understood that approximately 20 suchcycles may have occurred during the Quaternary period which begun 1 800ka BP.

Adopted Approach

Based on published papers on similar sites around the world a range of possible overconsolidationconditions have been adopted to gauge the likely range of subsidence that may be triggered bywithdrawal of groundwater.

The largest magnitudes of land settlement will occur when the soil strata being affected is normallyconsolidated. At the other end of the scale the minimum subsidence will occur when this strata is heavilyoverconsolidated. The most likely outcome is expected to be somewhere in the middle.

Land Settlement EstimateSections below detail subsidence calculations including the total long term settlements as well asindication of short-term settlement component.

Soil ParametersPhysical soil parameters presented in Table 13 have been adopted for analyses of aquiferdepressurisation induced settlement at Mulgrave River.

Table 13 Soil Parameters for Land Settlement Calculations

Geological Unit CR2 (kPa)

Po to Pc

CR3 (kPa)

>Pc

ExcessPreconsolidationPressure (kPa)*

Predominant MaterialType

Upper Clay 0.03 0.240 100 Clay

Sand Units 0.013 0.100 100 Sand

Middle and Lower ClayUnits

0.03 0.240 100 Clay

Weathered Granite 0.006 0.050 1000 Clay

Note: * - The following upper bound and lower bound estimates for Excess Preconsolidation pressure of 0 kPa and 300 kPa

respectively have also been adopted.

Groundwater LevelsHydrogeological modelling of extraction from the Mulgrave River aquifer indicates development of adrawdown cone centred on the Aloomba – Charinga area. For settlement estimates we have adoptedthe 30-year drawdown contours.

Average drawdowns across the area resulting from groundwater extraction have been estimated fromthe numerical modelling. These have been interpreted at 25 m and 35 m below ground surface for Areas2 and 3 respectively.

Estimated Total Settlement

Table 14 summarises the best estimate and possible ranges of total settlement at the two locations.


Table 14 Calculated Settlement (m)

Settlement (m)

Range

InitialWater

Level (bgl)

Drawdown(m)

BestEstimate

Minimum Maximum

Area 2 9 3 0.03 0.03 0.26

Area 3 6 3 0.05 0.05 0.39

Note: Minimum and maximum estimates are based on over consolidation of 0 kPa and 300 kParespectively.

The results indicate that under the most likely scenario there will be minimal subsidence impacts in thearea. Any subsidence will be gradual over several years and is anticipated to be relatively uniform overthe area. It is not expected to effect infrastructure or drainage. Settlement at the maximum calculatedcould affect hydraulic gradients in the Mulgrave River and may influence local flooding, for example.

The settlement within the sands is expected to occur soon after groundwater drawdown and aquiferdepressurisation (“immediate settlement”), whilst the settlement within clays will take a long period oftime to complete. On average approximately one third of total settlement will occur immediately inresponse to groundwater drawdown, while the residual settlement will occur over a long period of time.


Appendix I

Groundwater Quality Analysis

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