Transboundary Water Quality Issues in the Mekong River...

Mekong River Commission

Transboundary Water Quality Issuesin the Mekong River Basin

Barry T. Hart

Water Studies Centre, Monash University

Melbourne, Australia

Michael J. Jones & Gabrielle Pistone

NSR Environmental Consultants

Melbourne, Australia

November 2001

Mekong River Commission

Transboundary Water Quality Issues in theMekong River Basin

November 2001975_1_v2

Prepared by:

Water Studies Centre

In association with:

NSR Environmental Consultants Pty Ltd

Monash University 124 Camberwell Road

Wellington Rd, Clayton Victoria 3800 Hawthorn East, Victoria 3123

Australia Australia

Tel: 61 3 9905 4070 Fax: 61 3 9905 4196 Tel: 61-3-9882 3555 Fax: 61-3-9882 3533

e-mail: [email protected] e-mail: [email protected]

Transboundary water quality issues in Mekong River i

Water Studies Centre, Monash University NSR Environmental Consultants Pty Ltd

Executive Summary

Deteriorating water quality in the lower Mekong River basin has been identified as a prioritytransboundary issue by each of the four member countries. Three particular transboundarywater quality issues are considered in this report:

• the potential effects of municipal and industrial wastewater from Phnom Penh on bothdownstream Vietnam and fish migration in Tonle Sap River;

• the potential effects of municipal and industrial wastewater from Vientiane on bothneighbouring Thailand and fish migration in the Mekong River;

• the influence of upstream water on the degraded water quality in the Mekong Delta.

A risk assessment framework has been used to assess these issues. In particular, the risk ofadverse effects on three key values of the Mekong River – ecosystem health (characterised byeutrophication, toxicity due to dissolved oxygen and toxicants, and ecosystem processes), fishmigration and human health (drinking, recreation) - have been assessed. The risk to irrigationwater quality was also assessed for the third transboundary issue above.

Risk assessment is concerned with estimating the likelihood or probability of an undesiredevent occurring and the consequences if that event does occur. The risk assessment processseeks to:

• identify the key (ecological) issues and key stressors;

• identify the linkages between the key stressors (drivers) and each ecological consequence(conceptual model or quantitative ecological model), and from this provide information onwhich drivers are most sensitive to management or controls;

• assess the risks associated with each issue as quantitatively as possible (it is important hereto identify measurable end points for each issue);

• identify (and where possible quantify) all major uncertainties so the decision maker candecide on the confidence that should be placed on the final assessment;

• assist in establishing a robust monitoring & assessment program;

• identify the key knowledge gaps.

Recommendation 1: that MRC adopt the risk-based approach to assess transboundaryand basin-wide environmental and human health issues, and toprioritise the management actions required to reduce the risk due toeach important issue.

Unfortunately, the data currently available is inadequate to fully assess the risk oftransboundary water quality issues in the Mekong River basin. Assessment of the currentdatabase identified the following deficiencies:

• Physico-chemical data – the Mekong water quality network is similar to many other suchnetworks around the world in that it is collecting inadequate data. For example, many ofthe indicators currently being measured are inappropriate and should be replaced withmore appropriate indicators. Additionally, samples are being collecting at inappropriatesite locations and, for a number of indicators, at inadequate frequency. In all cases, the

Transboundary water quality issues in Mekong River ii


sampling design was such that there was essentially no statistical power in the data todetect any significant transboundary changes.

• Toxicant data – the pesticide and heavy metal data were either non-existent or insufficientto be used to assess transboundary or basin-wide toxicity issues.

• Biological data – there is no on-going biological monitoring program for the MekongRiver. In the time available we were able to access only a small amount of biological datarelevant to the Mekong, which would include fish, macroinvertebrates, algae andecosystem processes. Efforts should be made to collect all published and unpublishedinformation on the biology and ecology of the Mekong River and its tributaries, and toprepare a synthesis of this information that summarises current knowledge in this area.

• Urban contaminant loads – the loads of contaminants discharged from the urban centresof Vientiane and Phnom Penh are poorly known. We estimated likely loads in order tomake a preliminary assessment of the transboundary risks due to these discharges. Toimprove on the very preliminary risk assessment reported here, a more detailedunderstanding of the wastewater systems in each city needs to be developed, and both thequantity and quality of the wastewater discharges needs to be determined.

Recommendation 2: that the current review of the physico-chemical monitoring networkconsider in particular the optimum location of sampling sites, thefrequency of sampling, the need for depth sampling in some cases, theindicators being analysed and the power of the data collected to detectchanges.

Recommendation 3: that MRC undertake a preliminary risk assessment to identify possibletransboundary or basin-wide toxicity or bioaccumulation problems dueto organic contaminants and/or heavy metals, and if problems areidentified, the type of investigations (including monitoring) that shouldbe undertaken to better characterise the risk.

Recommendation 4: that the MRC establish a project to assess the feasibility of establishinga biological monitoring program for the Mekong River basin. Thefollowing biota should be considered – fish, macroinvertebrates, algaeand macrophytes.

Recommendation 5: that MRC collect all published and unpublished information on thebiology and ecology of the Mekong River and its tributaries, andprepare a synthesis of this information that summarises currentknowledge in this area.

Recommendation 6: that MRC obtain a more detailed understanding of the wastewatersystems (including information on the quantity and quality of thewastewater discharges) in the two major urban centres – Vientiane andPhnom Penh.

A summary of the preliminary risk assessment for the three transboundary water quality issuesis given in the table below. The present risks are low in all cases where they could beassessed. It was possible to undertake a reasonably quantitative assessment to assess the risksfrom eutrophication and the adverse effects of low dissolved oxygen concentrations caused bywastewater discharges from Phnom Penh. However, for the other effects we were forced to

Transboundary water quality issues in Mekong River iii


assess risk on the basis of either a comparison of the loads of contaminants discharged fromPhnom Penh and Vientiane with those transported “naturally” by the Mekong River, or thedegree of dilution achieved on discharge of the wastewater.

Issue Effect Issue 1

(Phnom Penh)

Issue 2

(Vientiane)

Issue 3

(Delta)

Ecological

Eutrophication

Toxic effects2

Ecosystem function3

Algal blooms

Fish/invertebrate kills

To be determined

Low-moderate risk1

Low risk

Not assessed

Low risk

Low risk

Not assessed

Low risk

Low risk

Not assessed

Fish migration4 Adverse effects on fishmovement upstream,downstream or ontofloodplains

Uncertain Uncertain,likely to be low

risk

Uncertain

Human health5

Drinking water

Recreation

Microbial contaminationcausing sickness

Uncertain

Uncertain

Uncertain

Uncertain

Uncertain

Uncertain

Agriculture

Irrigation Increased salinity Low risk1. More likely low risk since only nutrient concentration were used in the assessment; high turbidity and high flow

would also reduce the chance of algal problems.2. Risks based on toxic effects due to low dissolved oxygen concentrations. It was not possible to assess toxicity due to

toxicants (heavy metals, pesticides) because of the lack of data.3. No information is available at present, but should be developed in the future.4. Lack of data to make assessment, present water quality sampling network cannot provide the required information.5. Lack of data to make assessment. Risk likely to be low-moderate due to large dilution (also expect significant

microbial die-off during transport to Vietnam in case of Issues 1 & 3).

Recommendation 7: that MRC establish a project to undertake a more detailed assessmentof the transboundary ecological and human health risks due to thedischarge of wastewater from both Phnom Penh and Vientiane. Such aproject would provide an ideal opportunity to “train” relevant NationalMekong Committee members in the risk assessment methodology.

Recommendation 8: that MRC establish a project to investigate the key ecological processesoccurring in the Mekong River basin, including those associated withdeep pools in the Mekong River mainstream. The objective of thisproject should be to develop a number of sensitive ecosystem processindicators that can be used to assess the ecological “health” of theMekong River.

Another potential transboundary issue not covered in the objectives of this report, but whichappears to require assessment, is the apparent higher salinity (conductivity) in the river NamMun that drains the extensive agricultural region of northern Thailand.

While the assessment reported herein indicates that the transboundary risks due to waterquality are low, this is not the case for local effects. For Phnom Penh in particular, ourpreliminary assessment suggests that there are moderate to high risks of adverse ecological

Transboundary water quality issues in Mekong River iv


and human health effects in Chaktomuk, Tonle Sap River and the upper reaches of the BassacRiver. The relevant Cambodian authorities may wish to further investigate this situation.

Analysis of the present water quality monitoring network showed that it is unable to detectany transboundary changes due to the discharge of wastewater from either Vientiane orPhnom Penh. The network design has insufficient statistical power to detect realistic changesin physico-chemical water quality. Additionally, since no biological indicators are measured,there is no possibility of detecting transboundary or basin-wide changes in ecosystem health.

Recommendation 9: that MRC develop a new and more robust environmental assessmentprogram designed to identify and assess the risks from a broaderrange of current and future transboundary and basin-wide issues.The process to achieve this new assessment program should be donein collaboration with the member countries and would involve:

• using the (ecological) risk assessment technique to underpin theprocess, with the first task being to scope the full range of existingand possible future transboundary issues (and their priority);

• running a number of workshops (involving each country) todevelop the conceptual models and decide upon the target areas,the assessment endpoints and the best indicators to measure;

• undertaking a program of short-term, targeted investigations toprovide essential information on specific aspects of the system thatwill enable the main program to be better designed;

• preparing a full program proposal, obtaining funding andimplementing the program.

Transboundary water quality issues in Mekong River 1


EXECUTIVE SUMMARY .................................................................................................i

1. Introduction.................................................................................................................4

2. Project objectives ........................................................................................................6

3. Risk assessment approach...........................................................................................7

3.1 RISK ASSESSMENT FRAMEWORK ................................................................................................... 7

3.2 UNCERTAINTY............................................................................................................................... 10

4. Mekong River system and potential transboundary water quality issues............... 12

4.1 MEKONG RIVER ECOSYSTEM ....................................................................................................... 12

4.2 ENVIRONMENTAL VALUES ........................................................................................................... 16

4.3 KEY TRANSBOUNDARY ISSUES AND MAJOR STRESSORS............................................................ 16

5. Analysis of present environmental monitoring data ................................................ 20

5.1 PHYSICO-CHEMICAL WATER QUALITY MONITORING .................................................................. 20

5.2 TOXICANT (PESTICIDES, HEAVY METALS) MONITORING ............................................................ 28

5.3 BIOLOGICAL MONITORING............................................................................................................ 29

5.4 URBAN WASTEWATER LOADS ...................................................................................................... 30

5.5 USE OF EXISTING DATA TO ASSESS TRANSBOUNDARY ISSUES................................................... 31

6. Assessment of transboundary water quality issues .................................................. 33

6.1 PHNOM PENH ................................................................................................................................ 33

6.2 VIENTIANE..................................................................................................................................... 45

6.3 WATER QUALITY IN THE MEKONG DELTA .................................................................................. 48

7. Monitoring network required to assess transboundary issues................................. 51

7.1 ASSESSMENT TOOLS..................................................................................................................... 51

7.2 MONITORING NETWORK DESIGN.................................................................................................. 51

7.3 TOWARDS A NEW ENVIRONMENTAL ASSESSMENT PROGRAM ................................................... 53

7.4 RECOMMENDATIONS .................................................................................................................... 56

8. Conclusions & Recommendations ............................................................................ 58

9. References.................................................................................................................. 62



Tables

Table 1 Estimated contaminant loads (tonne/year) discharged from Vientiane and PhnomPenh (see Appendix A for details). In brackets is the contribution of each city as apercentage of the total load transported by the Mekong River at that point.......... 31

Table 2a Summary of the transboundary risks from Phnom Penh wastewater discharges onthe Vietnam Delta region .................................................................................... 36

Table 2b Summary of the transboundary risks from Vientiane wastewater discharges onThailand near Vientiane and downstream............................................................. 37

Table 2c Summary of the transboundary risks from upstream degraded water on the VietnamDelta region ........................................................................................................ 38

Table 3 Trigger values for the concentrations of filterable reactive phosphorus (FRP) thatcould cause algal problems in the Mekong River, and the likelihood that these willbe exceeded in the vicinity of Phnom Penh .......................................................... 40

Table 4 Trigger values for toxic effects due to low DO concentrations in the Mekong River,and the likelihood that these will be exceeded in the vicinity of Phnom Penh........ 41

Table 5 Logic of environmental decisions. Monitoring programs are generally established toconclude that projects or activities have or have not had an impact. ..................... 51

Table 6 Analysis of the power of the present water quality monitoring network to detectchanges of greater that 25% in conductivity, SPM and Total-P concentrations dueto wastewater inputs from Phnom Penh............................................................... 52

Figures

Figure 1 Location map of the lower Mekong River catchment............................................. 5

Figure 2 Schematic of the ERA process. ............................................................................ 8

Figure 3 Risk assessment curve showing the cumulative frequency of dissolved oxygenconcentrations in the Mekong River at Phnom Penh and the Tonle Sap River atPhnom Penh Port (all data and dry season data shown). Also shown on the graphare the low (1.5 mg/L) and high (6.0 mg/L) trigger values for adverse effects. .... 10

Figure 4 Mean monthly flows in the Mekong River at Vientiane and Phnom Penh. ............ 13

Figure 5 Conceptual models showing the potential transboundary water quality issues causedby wastewater discharges to the Mekong River from (a) Phnom Penh and (b)Vientiane, and (c) in the Delta region. ................................................................. 18

Figure 6 Box plots of the key water quality indicators over the length of the lower MekongRiver. Data for Lao PDR, Thailand and Vietnam collected in period 1985-2000,for Cambodia between 1993-2000....................................................................... 21

Figure 7 Conductivity vs time for the Mekong River at Tan Chau. .................................... 25

Figure 8 Plot of the monthly and annual SPM loads transported by the Mekong River atVientiane, Pakse, Kratie and Phnom Penh. .......................................................... 26

Figure 9 Plots of nutrient concentrations in the vicinity of Phnom Penh (a) mean (s.d.) FRP& Total-P concentrations (µg/L), (b) mean (s.d.) NOx-N & NH4-N concentrations(µg/L), (c) distribution of dry season FRP concentrations at Phnom Penh, TaKhmao and Chau Doc (mg/L). ............................................................................ 39

Figure 10 Plot of the distribution of dry season DO concentrations at Phnom Penh, Ta Khmaoand Chau Doc. .................................................................................................... 42



Figure 11 Sample design to test effect of wastewater discharged from Phnom Penh on theBassac River and downstream at the Vietnam Border. ................................................ 55

Appendices

Appendix A Stormwater and wastewater pollutant load estimates for Phnom Penh andVientiane



1. IntroductionThe lower Mekong basin (Figure 1) has a population of 60 million people (MRC, 1997) whoexist mainly by subsistence agriculture based on rice and wild-caught fish. Since the late1950s the four countries that make up the lower basin (Cambodia, Lao PDR, Thailand,Vietnam) recognised the need to cooperate to develop the basin’s resources sustainably ifthey were to avoid future conflict. Prior to 1995, the Mekong River Commission (MRC)managed a number of projects focused largely on water resources development within thebasin. Following the 1995 Agreement of Cooperation for Sustainable Development of theMekong Basin, the focus shifted to address basin-wide and transboundary issues through aprogram approach.

The MRC has three core programs – the Water Utilization Program (WUP), the BasinDevelopment Program (BDP) and Environment Program (EP) (MRC, 1999). The WUP,supported by the World Bank through the Global Environment Facility, has established threeworking groups within which member countries can develop solutions to basin-wideproblems. The working groups are addressing modeling, rules and protocols andtransboundary issues. This last working group identified water quantity and quality as two oftheir three highest priority transboundary issues. Fish production, river bank erosion andsedimentation were also identified as priority issues (Sukhsri, MRC, pers. comm.).

Water quality monitoring is undertaken throughout the basin under the Environment Programof the Mekong River Commission (MRC, 1998). The program was initiated in 1985 in LaoPDR, Thailand and Vietnam, and in 1993 in Cambodia. This program was not establishedspecifically to deal with transboundary issues and is now undergoing a major review in orderto revise and refocus the entire program. Additionally, there is a dearth of information onwhat the transboundary water quality issues are (or could be), especially as the main stem ofthe Mekong remains in generally good condition.

Water quality issues identified by the member countries as being of transboundary concern,and which form the basis of this report, include:

• potential effects of municipal and industrial wastes from Phnom Penh (the capital city ofCambodia);

• potential effects of municipal and industrial wastes from Vientiane (the capital city of LaoPDR);

• whether the degraded water quality in the Mekong Delta is caused by transboundarytransfers of poor quality water.

In the case of Phnom Penh and Vientiane, the presumed threat is not supported by anyanalysis of existing data that has drawn a definitive conclusion. For the Delta region, anumber of studies have shown that significant water quality degradation now occurs,particularly during the dry and early wet seasons (Tin & Wilander, 1995; Minh et al., 1997;Joy et al., 1999), but this may be more a product of local land use and seasonally lowtransboundary river flows than due to degraded water from upstream.

The Mekong River Commission commissioned the Water Studies Centre, MonashUniversity, in collaboration with NSR Environmental Consultants Pty Ltd, to undertake theTransboundary Water Quality Study Start-up Project. The aim of this project was to carryout a desk audit of existing information, together with discussions with local officials onfuture developments, and to advise Mekong River Commission on the three issues above.Further, if the desk audit indicated reason to be concerned, the Mekong River Commissionrequired that the specific studies needed in order to determine the magnitude of the risk beclearly identified.



Figure 1 Location map of the lower Mekong River catchment.



2. Project objectivesThe objectives of this study are to:

1. Review existing data and information to determine if real or potential downstream andtransboundary impacts exist from effluent discharges from the urban areas of PhnomPenh and Vientiane, and to determine if degraded water quality in the Mekong Delta iscaused by transboundary transfers of poor quality water.

2. Recommend specific studies or actions that should be carried out to make a definitiveassessment, if the present assessment remains inconclusive due to lack of sufficient dataor information.

3. Advise the Mekong River Commission on steps that could be taken to monitor real orpotential transboundary issues arising from this analysis.



3. Risk assessment approach3.1 Risk assessment frameworkA risk assessment framework has been used to assess the perceived water quality issues. Inparticular, the risk of adverse effects on three key uses or values of the Mekong River havebeen assessed, these being:

• ecosystem protection (specifically eutrophication and toxic effects on biota due to lowdissolved oxygen or chemical contaminants);

• fish migration;• human use (drinking, recreation).Risk assessment is a general term used to describe an array of methodologies and techniquesconcerned with estimating the likelihood and consequences of undesired events. Risk isoften defined as the product of the probability or likelihood of a hazard and the consequenceif that hazard occurs. Risks are characterised in terms of probability (the likelihood of someevent occurring), consequence (the severity if that event occurs) and the sensitivity tomanagement interventions.

More specifically, ecological risk assessment (ERA) is a relatively new technique that is nowavailable for assessing the level of risk to the health of river ecosystems posed by multiplestressors1. Ecological risk is defined as:

Ecological risk =likelihood of ecological effect x consequence of that effect.

To date, most applications of the ERA process in North America (Renner, 1996; USEPA,1998), Europe (Calow, 1995) and Australia (Hart et al., 2001) have focused on toxicchemicals. In these cases risk assessment includes a consideration of both the severity (orhazard) and frequency (or exposure) of the issue. For example, an extremely toxic chemical(e.g. mercury) may be a high or low risk, depending on its potential exposure to theecosystem. A less hazardous material, such as orthophosphate, may represent a low risk ifreleased in relatively small quantities, but can pose a high risk if released in quantities thatallow toxic cyanobacterial blooms to occur.

There are now a number of initiatives aimed at further developing the ERA technique toprovide a framework for considering a wider number of interacting stressors (e.g. nutrients,environmental flows, habitat, sediments, exotic species) within a catchment or river basincontext. Thus, the ERA process is an attractive tool that can assist in assessing the impactsof multiple stressors on complex ecosystems, the situation that exists within the MekongRiver system.

In this study, the focus of the ERA process is to identify the risks of transboundary waterquality issues occurring. Member countries may also find the process useful in assessing therisk of environmental problems occurring within each country.

Ecological risk assessments generally involve three steps discussed below (see alsoFigure 2).

Problem formulation

This is a planning and scoping process that establishes the goals, breadth and focus of therisk assessment. The end products of the problem formulation phase are:

1 Stressors or drivers are physical, chemical or biological factors influencing the system, for example,toxicants, nutrients, salinity, temperature, acidity, flow, habitat and exotic fish species.



• an outline of the assessment process that provides confidence that this process istransparent and credible;

• identification of the important ecological issues and key stressors;• identification of the appropriate spatial and temporal scales for evaluating the risks;• a conceptual model2 for each of the undesired issues, where the key stressors are linked

to the ecological effect;• identification of the assessment endpoints3.

Figure 2 Schematic of the ERA process.

Analysis and assessment

During this phase, information relevant to each key issue is gathered on the two riskcomponents - environmental exposure and severity of the effects.

The purpose of the likelihood characterisation is to predict or measure the spatial andtemporal distribution of the stressor(s) and the co-occurrence or contact with the ecologicalcomponents of concern. The purpose of the ecological effects characterisation is to identifyand quantify the effects caused by the stressor(s) and, to the extent possible, to evaluatecause-and-effect relationships.

Ideally, these assessments should be as quantitative as possible (Hart et al., 2001). However,with many systems, the Mekong included, it is often possible only to make semi-quantitative(e.g. low, medium, high) ratings of these two components of risk.

Risk characterisation

Here the likelihood and effects profiles are integrated to provide an estimate of the level ofrisk. In many circumstances, this step is reduced to assessing the risks associated with a listof hazards in which risks are ranked relative to one another.

2 These conceptual models form the basis for more quantitative ecological models in systems wherethere is both sufficient knowledge about the linkages and sufficient data to quantify these linkages.

3 These are explicit statements of the environmental values to be protected.



The final assessment of the level of risk should include some estimate of the uncertainty inthe predictions. For example, the final risk assessment might be that given a particular set ofconditions, there is a 60% likelihood of a blue-green algal bloom occurring in a waterbody inthe next three weeks. However, the manager would treat this result differently if he knewthat the upper and lower bounds to this prediction were, say, 50% and 70% respectively,compared with another situation where the prediction was less certain, say 20-80%, with themost likely being 60%.

The final risk assessment should include a summary of the assumptions used, the scientific(and other) uncertainties, and the strengths and weaknesses of the analyses.

Summary

In summary, the risk assessment process seeks to:

• identify the key ecological and human health issues and key stressors;

• identify the linkages between the key stressors (drivers) and each ecological consequence(conceptual model or quantitative ecological model), and from this provide informationon which drivers are most sensitive to management or controls;

• assess the risks associated with each issue (i.e. the likelihood that the issue will occur andthe consequences if it does occur) as quantitatively as possible;

• identify (and where possible quantify) all major uncertainties so the decision maker candecide on the confidence that should be placed on the information;

• assist in establishing performance monitoring & assessment programs;


The ERA process therefore is an ideal tool for assessing the risk of transboundary impacts onthe Mekong River from effluent discharges from the urban areas of Phnom Penh andVientiane, and within the Delta region.

Unfortunately, there is a dearth of information on the Mekong River that makes a detailedand quantitative risk assessment impossible at this stage. Thus, it has been necessary torevert largely to qualitative assessments, although these have been backed with quantitativedata where this was possible. Recommendations are made in Section 7 on the types ofinvestigations and data collection that are needed to make more quantitative transboundaryrisk assessments possible.

For the preliminary assessment considered in this report, the risk associated with each issuehas been assessed by combining information on the severity of the effect and the likelihoodthat the effect will occur. Each of these two components (severity and likelihood) will beseparated into a three scale rating scheme - low, moderate or high.

Where data was available, we have used the full data distribution to assess the probability(likelihood) that the system will be in excess of certain effect levels (or trigger values)4. Thisis shown in Figure 3 where the high and low severity trigger values for adverse effects toaquatic biota from low dissolved oxygen concentrations are marked on the cumulativefrequency distributions of the concentration data for a particular site. The points at whichthe cumulative curve crosses the trigger values can be used to assess the likelihood(probability) that low, moderate and high DO toxicity will occur at this site. For example,for the Mekong River at Phnom Penh, there is a ca. 7% probability of moderate effects dueto low DO (see Figure 3).

4 In a number of cases we used data for particular periods (e.g. dry season) when the problems aremore likely to occur.



For the issues assessed here, the trigger values were mostly obtained from relevant waterquality guidelines (e.g. USEPA, 1986a,b,c; 1989; 2000; CCME, 1999;ANZECC/ARMCANZ, 2000). Unfortunately, almost all of the available water qualityguidelines have been established for temperate regions, and may be less appropriate fortropical regions.

Figure 3 Risk assessment curve showing the cumulative frequency of dissolvedoxygen concentrations in the Mekong River at Phnom Penh and theTonle Sap River at Phnom Penh Port (all data and dry season datashown). Also shown on the graph are the low (1.5 mg/L) and high(6.0 mg/L) trigger values for adverse effects.

3.2 UncertaintyOne of the reasons why qualitative risk assessments fail is because they do not make thetreatment of uncertainty explicit. Risk assessments must deal with four types of uncertainty(Regan et al., 2001):

• Parameter uncertainty (e.g. measurement error or natural variation. This is the type ofuncertainty most commonly considered. Science deals with this kind of uncertainty byusing confidence intervals in statements such as “the size of the change is 54 with 95%confidence limits of 23”).

• Structural uncertainty (this is where an incorrect model for the system being studied isused).

• Shape uncertainty (this is uncertainty about the distribution of the data being considered).

• Dependency (relates to possible correlations between parameters).

For the Mekong, the main uncertainties are the lack of appropriate water quality andbiological data relevant to assessing transboundary issues, and a lack of understanding ofhow the system functions (e.g. fish spawning and migration). These are addressed in Section4.



Risk assessments also need to ensure that semantic uncertainties, including ambiguousstatements and vague definitions (e.g. concepts that permit borderline cases to occur), arekept to a minimum. An example of linguistic ambiguity is “there is a 70% chance of rain” –does this mean rain during 70% of the day, or over 70% of the area, or a 70% chance that itwill rain at a particular point (the weather station)? Equally, vague statements or definitionsare common in ecological risk assessments. For example, it is common to read statementsthat a certain hazard will pose a “low risk”. However, some hazards may be considereddefinitely low, whereas others may be low but tending towards moderate. Such vaguestatements can be strengthened if the definitions of low, moderate and high are quantified.



4. Mekong River system and potential transboundary waterquality issues

4.1 Mekong River ecosystem

General

The Mekong River rises in China (ca. 5,000 m altitude) and flows some 4,880 km throughfive other countries (Myanmar, Lao PDR, Thailand, Cambodia, Vietnam) before discharginginto the South China Sea (see Figure 1). The Mekong Basin is 795,000 km2 in area, and hasan annual flow of 475,000 million m3 (mean flow – 15,060 m3/s) which makes it the tenthlargest river in the world (MRC, 1997).

A good description of physiography of the Mekong basin is given in MRC (1997). North ofVientiane, the Mekong flows through a rather narrow valley in hilly and mountainouscountry. In the region of Vientiane, two large tributaries (Nam Ngum and Nam Lik) form abroad alluvial plain along the north bank of the Mekong, which then flows through a widevalley for around 560 km before entering the Khemarat rapids just south of the confluencewith the Se Bang Hieng. The river continues through a rocky gorge for a further 160 kmbefore emerging onto the plains above Pakse. The river Nam Mun (that drains the extensiveagricultural area of northern Thailand) also enters the Mekong approximately 40 km abovePakse at Khong Chiam (see Figure 1). From Pakse, the Mekong flows across lowlands tothe Khone Falls on the Cambodian border, located approximated 100 m above sea level.Below Khone Falls, the Mekong winds its way across the Cambodian lowlands to PhnomPenh, where it splits into three – the main Mekong channel, Bassac River and Tonle SapRiver. The behaviour of Tonle Sap River is described below. The Mekong and Bassac (ca.5-20% of the total flow) continue for 330 km through the highly agricultural Vietnam deltaregion before entering the South China Sea

The hydrological regime of the Mekong River is controlled by alternating wet and drymonsoon seasons. The annual flows in the Mekong are relatively predictable, with the wetseason extending from June-July to November, and the dry season from December to May(MRC, 1997). This pattern is well illustrated by the long-term flow regimes measured atVientiane and Phnom Penh (Figure 4). In general, the wet season accounts for around 85-90% of the total annual volume, with September being the peak flow month (contributing ca.20-30% of the annual flow). The Mekong’s flow increases with distance downstream due totributary inputs. For example, the mean annual flows increase from around4,500 m3/s at Vientiane to 10,100 m3/s at Pakse and 14,600 m3/s at Phnom Penh (MRC,1997).

Extensive areas of the lower Mekong Basin are flooded each year. In most years, over50,000 km2 is flooded, extending from around Kratie down to Phnom Penh, upstream toTonle Sap Great Lake and downstream to the delta region (see Figure 1). These floods arevery important in maintaining the high agricultural productivity of these lands by depositingalluvial sediments. Also, as discussed below, the annual floods are a key factor in the veryhigh fish productivity in the Mekong.

The population of the lower Mekong Basin is very poor and growing rapidly. Thepopulation in the lower Mekong has doubled in the past 30 years, and is predicted to increaseto around 108 million by 2020 (R. Corsel, MRC, pers. comm.). The current populationgrowth rate is 2.0% for the entire lower Mekong Basin, with higher rates (2.5%) in Lao PDRand Cambodia, and somewhat lower rates in Vietnam and Thailand (Kristensen, 2000).



Figure 4 Mean monthly flows in the Mekong River at Vientiane and Phnom Penh.

The Mekong and its tributaries are vital to the 73 million people living in the lower part ofthe catchment, who depend upon the river for water, food and transport. Most of thepopulation are engaged in agriculture and produce rice. It is estimated that food demandfrom the Mekong River basin will increase by 25-50% in the next 25 years, with acorresponding increase in water demand (Kristensen, 2000).

Rice and fish make up the main diet of the Mekong population, with fish being the singlemost important source of protein. People of the Mekong consume between 28 and 67 kgfish per capita annually (Jensen, 2000). The Mekong River is very rich in fish. The size ofthe inland fisheries is large with the total annual catch estimated at around 2 million tonne,distributed over more than 1,500 different fish species. This is supplemented by a rapidlygrowing aquaculture industry that currently produces around 230,000 tonne annually (MRC,2001).

MRC (1997) summarised the large number of land use changes and other activities that haveoccurred within the lower Mekong River basin in recent years, and the environmentalconcerns associated with these activities. Some of the most serious are:

• excessive logging in all countries, leading to deforestation, degradation of terrestrialecosystems and soil erosion, with consequent elevated sediment loads and sedimentation;

• clearing of important floodplain forests for agricultural land5, leading to a significant lossof important fish breeding areas;

• conversion of wetlands to rice farms or aquaculture, leading to loss of aquatic habitat;

• agricultural expansion and shifting cultivation;

• major irrigation developments in the Korat Plateau of northern Thailand, leading toincreased salinisation, sediment transport and water quality degradation;

• water quality and land degradation in the Delta region caused by agricultural activities onacid sulfate soils.

5 A large proportion of the 5,000 km2 of flooded forests around Tonle Sap Great Lake have beencleared.



Fisheries and fish migration

As noted above, caught fish are a very important food source for the people of the lowerMekong Basin. Studies in the lower Mekong have classified two groups of freshwater fish,based on their spawning behaviour and environmental tolerance (MRC, 1997). “White fish”(principal families Cyprinidae and Schilbeidae) migrate into the main channels during the dryand early wet season. They spawn in relatively sheltered waters after the peak of theinnundation. “Black Fish” (principal families Clariidae, Siluridae and Ophiocephalidae) aremore widely distributed than the “white fish”, more environmentally tolerant, are mainlybottom dwellers and have a broad range of spawning behaviours.

The biology and ecology of many of the fish species in the Mekong is poorly known (Poulsen& Valbo-Jorgensen, 2000), although the MRC Fisheries Program is seeking to address thissituation through its Assessment of Mekong Fisheries Component (AMFC).

From those studies that have been undertaken, it seems apparent that four aspects areimportant to the maintenance of viable fisheries in the Mekong:

• the Khone Falls region in southern Lao PDR;

• dry season refuges in the main channel;

• the extensive floodplain region further downstream of Khone Falls, and particularly TonleSap Great Lake and its floodplain;

• maintaining adequate water quality, environmental flows, habitat, fish migration patterns,fishing pressure and introduced species.

This report considers the risk of pollution from Phnom Penh and Vientiane adverselyaffecting fish migration patterns. However, it is clear that maintaining a viable fisheries in theMekong will involve more than just the management of urban pollution. Basin-widemanagement of water quality, habitat condition and flow regimes will also be needed.

Khone Falls

The Khone Falls region in southern Lao PDR has emerged as an important area for fish in theMekong, and not surprisingly has been the focus of a number of fish studies (Roberts &Warren, 1994; Roberts & Baird, 1995; Singanourvong et al., 1996a,b). Khone Falls does notappear to be a physical barrier for most of the described fish species (e.g. most species liveboth above and below the Falls), although the migratory patterns differ significantly belowand above the Falls (Poulsen & Valbo-Jorgensen, 2000). Many species migrate upstreamfrom southern regions of Vietnam and Cambodia to Khone Falls during the dry season, andthen migrate downstream with the onset of the wet season floods. Interestingly, many ofthese same species above the Falls have the opposite migratory behaviour, migratingupstream with the onset of the flood season. For most species, whether above or below theFalls, the timing of the migrations seems to coincide with the main spawning periods(Poulsen & Valbo-Jorgensen, 2000).

Dry season refuges

A small amount of evidence now exists pointing to the importance of deep pools in theMekong mainstream as dry season habitat for many fish species (Poulsen & Valbo-Jorgensen,2000). Deep pools exist both upstream and downstream of Khone Falls. In particular, theregion from the Falls to Kratie in Cambodia (see Figure 1) contains a number of deep pools.

We have no information on the existing water quality in the pools during either the dry orwet seasons. However, these deep pools would be vulnerable to upstream pollution and toreductions in river flow.



In view of the potential importance of the deep pools as dry season habitat, we recommendthat the MRC commission a study to determine their extent, importance and risk from futureland use changes and water resources management options.

Floodplains

The extensive areas of floodplain that dominate the lower Mekong Basin during the wetseason are the basis of the high fish productivity. Poulsen & Valbo-Jorgensen (2000)reported that all 50 priority fish species covered in their survey of fish migration patternsspent some stage of their lifecycle on flooded areas, this being particularly so for larval andjuvenile stages. It seems that many species time their longitudinal migrations so that theiroffspring can access the highly productive flooded areas, where they can capitalise on thefood resources associated with these floodplains.

The floodplains extend from upstream of Khone Falls to the Mekong Delta region inVietnam, some 900 km downstream (see Figure 1). However, there are major differencesbetween these floodplain systems. Upstream of Khone Falls in the middle Mekong, thefloodplains are mainly associated with tributaries, while downstream of the Falls they aredirectly connected to the main channel. The migratory behaviour of the fish species reflectthese differences in floodplain location.

Tonle Sap Great Lake, thought to be the area of highest fish production in the Mekong, is avery important part of the lower floodplain system. This large lake, situated ca. 125 kmnorth west of Phnom Penh (see Figure 1), varies dramatically in area from around 2,500 km2

in the dry season to 14,000 km2 in the wet season (MRC, 1997). During the wet season, thelake is filled by water from the Mekong main channel flowing up Tonle Sap River to the lake(Rainboth, 1996). Large numbers of young fish are brought into the lake with thesefloodwaters. During the dry season (December to February), the flow reverses and waterflows out of the lake back into the Mekong main channel. With these receding floodwatersthe fish migrate out of the lake back to their spawning grounds. During this annual migrationperiod, huge numbers of fish are caught in the set-bag-net fisheries (dai fisheries) in TonleSap River.

Fish migration

Fish migrations are an important feature of river ecology in most major tropical rivers, andare adaptations to life in running waters. Three types of migrations are observed:“longitudinal migration” that occurs within the main channel and larger tributaries, “lateralmigration” where fish move from the main channel to the floodplain and back again, and“larval drift” where, during the flood season, fish larvae can drift downstream from upstreamspawning areas to downstream nursery areas in the flood plain.

In their extensive study of fish migration patterns in the Mekong River, Poulsen & Valbo-Jorgensen (2000) separated the migrations into three time periods: the late wet to early dryseason (October to February), the late dry to wet season (May to September), and the dryseason (March to May). It is during these times of migrations that a substantial number ofthe fish are caught. For example, the dai fisheries in Cambodia (Lieng et al., 1995) and theKhone Falls fishery in Lao PDR both exploit times of major fish migration (Singanouvong etal., 1996a,b).

The fish of the Mekong are a transboundary resource. As such, any land use change, waterresource development or urban pollution that interferes with their migration patterns is atransboundary management issue. In this report, we cover one aspect of the wider issue, thepossible risks to fish migration due urban wastewater discharges from Phnom Penh andVientiane.



4.2 Environmental valuesMost water quality management strategies or plans require that the environmental values6 ofthe water resource should be explicitly determined in order that management of the resourcecan be properly focused to protect these values (Hart et al., 1999; ANZECC/ARMCANZ,2000). For example, the Australian National Water Quality Management Strategy(ANZECC/ARMCANZ, 1994) recommends that the following environmental values beconsidered: ecosystem protection, drinking water, water for agriculture (irrigation, livestockdrinking water, aquaculture), and water for recreation and aesthetics.

Somewhat surprisingly, we have not been able to find any MRC documents that provide anexplicit statement of a shared vision the four countries have for the Mekong River basin, norindeed any statement of the environmental values they wish to protect in this system.However, it seems clear that the four countries with the MRC are managing this river systemto protect the following values:

• the riverine and floodplain ecosystems;

• native fisheries production;

• water for drinking;

• irrigation (mainly rice);

• aquaculture;

• recreation and aesthetics.

This report is restricted to an assessment of the impact of transboundary water qualitychanges, focussing on three environmental values:


• fish migration;

• human health (drinking water & recreational use).

It should be noted that our focus on fish migration is only one component of what should bea much larger study to assess the risk to native fish production in the Mekong River system.This larger study would require the inclusion of information on at least three key stressors –water quality changes, habitat changes and possible reduction in flows and flooding.Changes in the total flows and in key components to the flow regime would be particularlyimportant, probably more important than water quality changes.

The MRC is currently considering a major study of the environmental flow requirements toensure that the ecological “health” of the Mekong River system is maintained into the future.It should be noted that determination of environmental flow regimes will require considerablymore knowledge about how this system functions ecologically, including what factors controlthe vitally important Mekong fisheries.

4.3 Key transboundary issues and major stressorsThe three transboundary water quality issues considered in this assessment are:

• potential effects of municipal and industrial wastes from Phnom Penh, both downstreamand on fish migration near the outfalls;

• potential effects of municipal and industrial wastes from Vientiane, both downstream andon fish migration near the discharge point;

6 Environmental values are also referred to as “beneficial uses” or “functions of water”.



• whether the degraded water quality in the Mekong Delta is caused by transboundarytransfers of poor quality water.

Below we outline a conceptual model for each situation in which the spatial scale isaddressed and the key stressors are identified. In each case we have tried to keep separatethe transboundary issues (i.e. adverse effect on downstream or adjacent country) from thosearising entirely within one country.

Phnom Penh

A conceptual model for the potential water quality issues resulting from municipal andindustrial wastes from Phnom Penh is shown in Figure 5a. The details are discussed inSection 6.1 and include:

• wastewater is discharged to the Tonle Sap and Bassac Rivers via a number of drains(with very little treatment);

• impacts are possible in the immediate vicinity of the outfalls and further downstream onentering Vietnam;

• there are two potential transboundary issues: (a) transport of contaminants to Vietnam,and (b) effects on fish migration both upstream and downstream of Phnom Penh;

• wastewater discharged to the Bassac River will have the greatest potential to reachVietnam7.

Vientiane

A conceptual model for the potential water quality issues resulting from municipal andindustrial wastes from Vientiane is shown in Figure 5b. The details are discussed in Section6.2 and include:

• wastewater is discharged to the Mekong via a wetland (hence some treatment);• impacts are possible in the immediate vicinity of the discharge and further downstream,• since the Mekong River near Vientiane is the border between Lao PDR and Thailand,

potential transboundary issues could presumably occur both in the immediate vicinity ofthe discharge and further downstream;

• the wastewater discharge could potentially interfere with the migration of fish upstreamand downstream in the Mekong River.

Mekong Delta

The conceptual model for the degraded water quality in the Mekong Delta is shown in Figure5c. The details are discussed in Section 6.3 and include three potential sources ofcontamination:

• transboundary transfers of poor quality water from upstream Phnom Penh;• pollution from the intensive agricultural areas within the Delta region;• intrusion of seawater into the Delta region8.

7 Transport of contaminants from Tonle Sap River is likely to occur only during periods when waterdrains from Tonle Sap Great Lake back into the Mekong (Dec-Feb).

8 This presently occurs during the dry season, and is predicted to increase if the major water resourcesdevelopments occur in upstream countries.



(Figure 5a)

(Figure 5b)

Figure 5 Conceptual models showing the potential transboundary water qualityissues caused by wastewater discharges to the Mekong River from (a)Phnom Penh and (b) Vientiane, and (c) in the Delta region.



(Figure 5c)

Figure 5 continued.



5. Analysis of present environmental monitoring data5.1 Physico-chemical water quality monitoringThe MRC supports an extensive physico-chemical monitoring network with 103 sitessampled monthly. This program commenced in 1985 (1993 in Cambodia) and was reviewedin 1997. Currently, the program is being reviewed again, this time with a focus on whetherthe existing network can adequately characterise transboundary and basin-wide water qualityissues (E. Ongley, MRC Consultant, pers. comm.).

The network consists of 16 stations along the mainstream, 35 along tributaries, 46 stations inthe Vietnam Mekong delta, and 6 stations in wetlands along the Mekong corridor. Eachsample is analysed for pH, DO, conductivity, SPM (TSS), Ca, Mg, Na, K, alkalinity, Cl, SO4,Tot-Fe, Tot-P, FRP, Tot-N, NOx-N, NH4-N, Si and COD.

We have analysed part of this water quality database in an attempt to provide information onthe three transboundary issues. We have used the data as provided and have not done anychecks for accuracy or quality; presumably, this will be done as part of the above mentionedreview.

We found that much of the available data was of limited value in addressing thesetransboundary issues. There were three main reasons for this:

• the sampling sites were not well located to address these issues;

• only surface samples are taken and then only at monthly intervals;

• many of the indicators measured regularly (e.g. Na, K, Ca, Mg, Cl) are of limited use inassessing water quality issues. Further, indicators we would like to have had informationon were not measured (e.g. Chlorophyll-a, toxic contaminants, depth-integrated SPMconcentrations, diurnal DO concentration changes, total and dissolved organic carbon).

We expect that the current review will recommend substantial changes to the present waterquality monitoring network. In brief, there was 15 years (1985-2000) of data for sites in LaoPDR, Thailand and Vietnam, and 7 years (1993-2000) of data for sites in Cambodia.

The following subset of the full database was used:

• SPM (TSS) indicator of land disturbance and erosion

• Conductivity (EC) indicator of salinity increases

• pH, alkalinity indicators of buffer capacity and possiblyautotrophic/heterotrophic status

• Tot-P, FRP, NOx-N, NH4-N (Tot-N) indicators of eutrophication potential

• DO, COD indicators of organic pollution.

The full water quality database is held by the Mekong River Commission. The subsetanalysed for this report is available from the authors on request.

General observations

Figure 6 contains box plots of the key indicators (EC, SPM, pH, alkalinity, DO, COD, Tot-P,Tot-N) over the length of the lower Mekong River from Chang Saen (at the Thai/LaoPDR/Myanmar border) to the Delta. The water quality of major tributaries (at Ubon, TonleSap River at Phnom Penh Port, Prek Kdam (that enters Bassac River just downstream ofPhnom Penh), and Bassac River at Ta Khmao, Chau Doc and Can Tho) are also included onFigure 6 (and are identified by an asterisk (*)). The interpretation of these plots is asfollows: the box spans the 25 to 75 percentile concentrations with the middle dash being themedian concentration; the whiskers span the 10 to 90 percentile concentrations.



(Figure 6a)

Figure 6 Box plots of the key water quality indicators over the length of the lowerMekong River. Data for Lao PDR, Thailand and Vietnam collected inperiod 1985-2000, for Cambodia between 1993-2000.



(Figure 6b)

Figure 6 continued.



(Figure 6c)

Figure 6 continued.



(Figure 6d)

Figure 6 continued.



Two general observations can be made:

• the water quality indicators are all subject to considerable variation, with typical relativestandard deviations (RSD) ranging from 20-30% for conductivity to over 100% forSPM. This variability has major implications for the number of samples required todetect statistically valid changes between locations and with time;

• a major change in most of the water quality indicators seems to occur between Pakse(869 km) and Kratie (545 km) – this may correspond to the large change in river gradientbetween these two locations (MRC, 1997).

Conductivity

The Mekong River contains low conductivity waters (Figure 6a). At all sites there was anobvious annual pattern with low conductivity recorded during the wet season andcorrespondingly high conductivity during the dry season. Figure 7 shows the record at TanChau and this is typical of all sites. Interestingly, the conductivity of the Mekong actuallydecreased with downstream distance, from a median of ca. 24 mS/m at Chang Sean to ca.14 mS/m at My Thuan (Figure 6a). Between Pakse and Kratie, the median conductivityappeared to decrease by around 20% and then remain relatively constant between Kratie andthe lower Delta. An explanation for this observation is not immediately obvious. It isapparent that higher conductivity water is entering the Mekong from the Nam Muncatchment in northern Thailand (see Ubon site in Figure 6a). This probably reflects theincreased salinity reportedly occurring in this highly agricultural catchment (MRC, 1997).There was no noticeable increase in conductivity at the two sites closest to the sea (My Than,Can Doc), as would have been expected had seawater intrusion reached these points.

Figure 7 Conductivity vs time for the Mekong River at Tan Chau.



Suspended particulate matter

The Mekong River transports high concentrations of suspended particulate matter9 (SPM,Figure 6a). The SPM concentrations are highly variable and are correlated with flow.Interestingly, the SPM concentration appears to reduce with distance downstream. Forexample, over the 1,500 km between Luang Prabanh (2010 km) and Kratie (545 km), themedian SPM concentration reduced from ca. 175 mg/L to 74 mg/L. There also appears tobe a significant decrease in SPM concentration between Pakse and Kratie (Figure 6a). Thismay be due to sedimentation of SPM during the wet season when much of the Mekong flowis dispersed over a very extensive floodplain area.

Considering both the high SPM concentrations and the very large flows, it is hardlysurprising that the Mekong River transports massive loads of SPM, with most of thetransport occurring during the wet season (Figure 8). Using the available (inadequate) data,it is estimated that the annual loads are in the range of 65-120 million tonne, but could be ashigh as 200 million tonne/year10. Figure 8 shows the monthly and annual load calculated atVientiane, Pakse, Kratie and Phnom Penh. It is tempting to suggest from these data thatthere is a large loss of SPM between Pakse and Kratie; however, the uncertainty in theseestimates (ca. 100%) makes such interpretation speculative at best.

Figure 8 Plot of the monthly and annual SPM loads transported by the MekongRiver at Vientiane, Pakse, Kratie and Phnom Penh.

These SPM load estimates are similar to those reported elsewhere. For example, Milliman &Meade (1983) and Milliman & Syvitski (1992) estimated that the Mekong discharges around180 million tonne/year, while a mean annual discharge rate of 120 million tonne is calculated

9 We have used SPM rather than TSS in this report.10 The SPM data base is inadequate because the SPM concentrations are only measured once per month

and then only at the surface. Reasonably accurate load estimates would require a significantlyincreased sampling frequency, particularly during the high flow period, and depth-integratedsampling instead of surface sampling.



using the mean SPM concentration of 250 mg/L reported by Wolanski et al. (1996) and amean flow of 15,060 m3/s (MRC, 1997).

To put these figures into perspective, the annual sediment yield (load) for the Mekong isaround 130 tonne/km2 (load = ca. 100 million tonne/year), making it comparable with theMississippi (120 tonne/km2, 210 million tonne/year) and the Amazon (190 tonne/km2, 1,200million tonne/year), but very much less than the Ganges/Brahmaputra (1,670 tonne/km2,2,180 million tonne/year) and the Fly River (1,500 tonne/km2, 116 million tonne/year)(Wolanski et al., 1996).

pH

The Mekong River waters are slightly alkaline (Figure 6b), with median pH values rangingfrom ca. 7.9 in the upstream reaches (Chang Sean - Nakhon Phanom) to ca. 7.5-7.6downstream of Kratie.

Alkalinity

The Mekong River has noticeably higher alkalinity water in the upstream reaches aroundVientiane (ca. 100 mg/L (as (HCO3), i.e. approx. 1.64 meq/L) than in the downstreamreaches below Kratie (ca. 60 mg/L, i.e. approx. 0.98 meq/L). Figure 6b also shows thesomewhat lower alkalinity waters entering the Mekong from the Nam Mun catchment(Ubon) and Prek Kdam (near Phnom Penh). The lower alkalinity water in the Bassac at TaKhmao probably reflects the influence of Prek Kdam which enters very close to this site.

Nutrients

Phosphorus – total phosphorus concentrations are plotted in Figure 6c. Except for the lowersites in Vietnam, the median concentrations are all reasonably low (<50 µg/L). The fourlower sites in the Delta region are all noticeably higher with median concentrations around100 µg/L. There are two possible explanations for these higher results, i.e. either they arecorrect and represent additional nutrient inputs in this region, or there is a difference in theanalytical procedures between the Vietnamese and Cambodian laboratories.

Nitrogen – total nitrogen concentrations are not determined at all sites. However, for thosesites where data are available (Figure 6c), the trend in median concentrations is similar tototal phosphorus, namely relatively low concentrations in upstream sites (ca. 450 µg/L) andslightly higher concentrations at the four lower sites (ca. 600 µg/L).

Organic matter

Unfortunately, chemical oxygen demand (COD) is the only measure of organic matteravailable for the Mekong River. We would have preferred to have had data on biochemicaloxygen demand (BOD), which provides a better measure of the biologically available organicmatter, or total organic carbon (TOC). The median COD concentration approximatelydoubles between the upstream reaches and downstream of Phnom Penh (Figure 6d). AtUbon the COD concentrations are noticeably higher than in the Mekong, and this probablyreflects organic pollution either from the city of Ubon or more generally from the Nam Muncatchment in northern Thailand. Further downstream, the relatively high values in the twosites close to Phnom Penh (Prek Kdam – a tributary stream that drains the southern part ofPhnom Penh to the Bassac; Ta Khmao – a site in the Bassac just downstream of PhnomPenh) almost certainly reflect organic pollution from Phnom Penh.

Dissolved oxygen

The median dissolved oxygen concentrations decreased by around 20% between Chang Seanand My Thuan (Figure 6d). This change probably reflects the general increase in both theinputs of organic matter (see COD graph) and temperature over the length of the Mekong.



The lower median DO concentrations at Prek Kdam and Ta Khmao probably reflect the inputof stormwater and sewage from Phnom Penh.

5.2 Toxicant (pesticides, heavy metals) monitoring

Pesticides

MRC (1997) report that pesticides have been monitored in Lao PDR (5 stations) andVietnam (15 stations) since 1991 and in Thailand (11 stations) since 1994. At each station,fish (5 species) and water are sampled and analysed twice a year, once in the dry season andonce in the rainy season. Over 26 pesticides are analysed, including HCB, aldrin, dieldrin,DDT, parathion, dichlorvos and endosulphan. The databases for water samples and fishtissues provided by the MRC had significantly fewer analytical results than the abovesampling regime should have produced.

The database for pesticides in water shows that a total of 117-166 water samples wereanalysed by the three countries from a total of 35 sites11 in the period 1994-1997. Asexpected, pesticide concentrations were very low in those water samples that had detectableconcentrations, although on some occasions elevated levels of DDT (max – 220 µg/L),endosulphan (max – 180 µg/L) and diazinon (max – 1,160 µg/L) were measured. We haveno information on the quality of the pesticide analysis data because no quality assuranceinformation was provided.

Pesticide analysis in water samples from the Mekong Basin is questionable given the very lowlevels generally recorded, the difficulties in undertaking these types of analyses and the factthat it does not appear these analyses have been undertaken since 1997. We recommend thatthe MRC review the effectiveness of this part of the pesticide analysis program.

The fish tissue analysis database also contains considerably less data than suggested by theMRC’s information. Since 1994, there have been 580-800 fish tissue samples analysed forpesticide contamination. The numbers of samples analysed have varied significantly betweencountries, e.g. Cambodia - 3 sites, 14 samples; Lao PDR – 5 sites, 25 samples; Thailand – 12sites, 198 samples; Vietnam – 19 sites, 564 samples. As expected, more fish tissue sampleswere found to contain pesticide residues because of the potential for pesticides to beconcentrated in the fatty tissues of fish. For example, quite high concentrations of parathion(max – 100 µg/kg), DDT (150 µg/kg), dieldrin (22 µg/kg), endosulphan (74 µg/kg) anddiazinon (600 µg/kg) were recorded in the period between 1994 and 1997. Approximately10% of the fish tissues analysed for parathion (578 samples) and 15% of those analysed forDDT (801 samples) contained higher than 5 µg/kg of the pesticide. The highest tissue levelswere recorded in Vietnam, presumably reflecting the intensive agriculture occurring in theMekong Delta region. Again, we have no information on the quality of the pesticide analysisdata because no quality assurance information was provided.

Monirith et al. (1999) reported generally low concentrations of persistent organochlorinepesticide12 residues in 27 species of freshwater and marine fish from Cambodia. DDT and itsderivatives were detected in all the fish analysed (mean – ca. 10 µg/kg (wet wt), range – 0.3-25 µg/kg). The other organochlorines were present in very much lower concentration (mean– ca. 0.1 µg/kg). Kannan et al. (1995) also reported quite high concentrations of DDT andchlordanes in fish from Thailand (DDT: 0.48-19 µg/kg; chlordanes: 0.1-15 µg/kg) andVietnam (DDT: 3.9-76 µg/kg; chlordanes: <0.01-0.35 µg/kg).

11 Lao PDR – 5 sites sampled once only; Thailand – 11 sites sampled between 1994 and 1997; Vietnam– 19 sites sampled between 1994 and 1997.

12 Polychlorinated biphenyls (PCBs), DDT compounds, hexachlorocyclohexanes (HCH) and chlordanecompounds.



Unfortunately, it has not been possible in the time available to use the available pesticidedatabase to assess the transboundary risks from these toxicants in the Mekong Basin. Thiswould require a further dedicated study that could be undertaken if MRC wished.

Heavy metals

We have been unable to obtain any data on heavy metal concentrations in water, sediments orbiota in the Mekong Basin. Typical sources of heavy metals are mining activities, certainmanufacturing industries and urban areas. Currently, there are relatively small numbers ofthese activities in the lower Mekong basin.

Thus, the risk of toxic effects from excessive heavy metal concentrations is likely to be low,except perhaps for localised effects close to poorly managed mining activities.

5.3 Biological monitoring

Benthic macroinvertebrates

Benthic macroinvertebrates are being used in a number of countries to assess river “health”(Bailey et al., 1998; Norris et al, 1995, 2001; Rosenberg & Resh, 1993; Simpson & Norris,2000; Wright et al., 2000).

According to MRC (1997), benthic macroinvertebrates are sampled seasonally in Lao PDR(9 stations), Cambodia (9 stations) and Vietnam (unknown number of stations). However,we were able to locate only one report (Smith, 1988) on macroinvertebrate sampling. Thisreports a major study to sample the benthic invertebrates in the period April to June 1988,and also some less intense sampling in 1987. Apparently, another sampling run wasundertaken in 1989 or 1990 but we have not been able to locate this report.

In April to June 1988, benthic macroinvertebrates were sorted from sediment samplescollected at 11 sites in the Mekong main channel, 17 sites in main tributaries and 10 in thePlain of Reeds in the Mekong Delta region. This study reported difficulty in being able tosample consistent habitat type over the length of the Mekong. Much of the sediment in themain stream consists of sand and silt, a poor habitat for benthic invertebrates. As aconsequence, only 4-8 species were present, the dominant fauna being oligochaete wormsand chironomids. A larger number (ca. 30) of species was found in the lower Delta region ofthe Mekong. Smith (1988) suggested that the four main channel locations in the Delta regionwere suitable, but the upstream main channel was unsuitable.

Given the major advances that have been made over the past decade in the use of benthicmacroinvertebrates to assess the biological condition of rivers, we recommend the MRCestablish a project to assess the potential use of these biological indicators in the MekongRiver.

Fish

Fish have also been extensively used as indicators of the biological condition of rivers. Wehave no information on the use of fish as a biological (or biodiversity) indicator in theMekong River. However, the large number of fish species known to be present in thissystem suggests there is great potential to use these organisms to provide a robust measureof river condition.

We recommend the MRC establish a project to assess the potential use of fish as biologicalindicators in the Mekong River. The investigations that would be necessary in such a projectwould also provide excellent information on the ecology of this important resource.

Algae

Member countries have identified eutrophication (excessive growth of aquatic plants) as apotential problem in the Mekong Basin. Given the rather poor management of stormwater



and sewage effluent in urban regions, and the equally poor management of agriculturalactivities, it seems highly likely that many aquatic systems within the basin are, or willbecome, eutrophic. However, as indicated in Section 6, we have assessed the risk oftransboundary eutrophication problems as low.

With the present information it is possible to provide only a preliminary assessment of thepotential for eutrophic conditions to exist, both within each country and at the borders. Thecurrent monitoring program does not collect any information on algal species composition orbiomass (e.g. Chlorophyll-a concentrations).

We recommend the MRC consider including algal monitoring as part of a biologicalmonitoring network for the lower Mekong River.

Ecological processes

As noted above, biological monitoring programs based on macroinvertebrates, fish and algaehave been introduced in a number of countries over the past decade. These make use ofchanges in the pattern of these communities, using indicators such as abundance, richness andspecies composition compared with known reference systems. For example, Australia now hasa national program based on the AUSRIVAS system (AUStralian RIVer ASessment System)(Norris et al., 1995; 2001; Simpson & Norris, 2000).

However, Bunn & Davies (2000) have argued that because changes in pattern do not alwaysequate to changes in ecological integrity, measures of key ecological processes should also beincluded in programs to assess the health and integrity of ecosystems. Ecosystem processmeasures that are now starting to be used include benthic metabolism, gross primaryproduction, respiration, nitrification and denitrification. Over the next few years we should seethe development of more robust ecosystem process measures that can be incorporated intoexisting biological monitoring programs.

We recommend that the MRC consider establishing a project to investigate a range ofecological processes in the Mekong River. The ultimate objective of this project should beto develop a number of ecosystem process indicators that can be used to assess theecological “health” of the Mekong.

5.4 Urban wastewater loadsA key objective of this project was to assess whether real or potential transboundary impactsresult from the wastewater discharges from the urban areas of Phnom Penh and Vientiane.An essential component of such an assessment is good information on the quantity andquality of these wastewater discharges.

It seems that there is very little routine monitoring of the wastewater from either city, mainlybecause of a lack of resources. We were able to meet the relevant officials in Phnom Penhwho were very cooperative and provided a great deal of very useful information on thegeneral wastewater collection and treatment system (Section 6.1). We were also able toobtain similar information for Vientiane, although we did not visit that city (Section 6.2).However, even with this information we still lack detailed knowledge on the collectionsystems, treatment facilities, discharge locations and the magnitude of the contaminant loadsentering the Mekong.

This information must be obtained if MRC is to better assess the transboundary risks fromthese urban areas. We recommend that MRC obtain considerably better information on thewastewater systems in the two major urban centres – Vientiane and Phnom Penh.

Because it was not possible to obtain sufficient good quality information on the total load ofwastewater and contaminants discharged from each city in the time available, Assoc. Prof.Tony Wong (Ecological Engineering Pty Ltd, Melbourne) was commissioned to provide an



estimate of the likely loads of SPM, BOD, Total-P and Total-N from each city (see AppendixA for report).

The estimates of loads of sediment, organic matter and nutrients from both stormwater andsewage for both cities are listed in Table 1. The various assumptions are listed in thefootnote of the table. These estimates are likely to be accurate to an order of magnitudeonly, given the lack of knowledge about the generation of stormwater and sewage, and theeffectiveness of treatment. Also shown in Table 1 is the relative annual load from each cityas a percentage of that transported by the Mekong at that location. This latter figure allowsan estimate of the relative contribution of contaminants to the river from the two cities.

On an annual basis, each city contributes relatively small amounts (0.1-5%) of sediment,organic matter and nutrients to the river. However, the relative contribution at particulartimes of the year (e.g. dry season, low flow) could be higher than this.

Table 1 Estimated contaminant loads (tonne/year) discharged from Vientiane and PhnomPenh (see Appendix A for details). In brackets is the contribution of each city asa percentage of the total load transported by the Mekong River at that point.

City SPM BOD TP TN

Vientiane 24,000

(0.1%)

6,500

(4%)

300

(4%)

1000

(1%)

Phnom Penh 82,000

(0.1%)

15,000

(2%)

600

(5%)

2,000

(2%)Assume urban area: Vientiane = 60 km2; Phnom Penh = 375 km2

Assume populations: Vientiane = 580,000; Phnom Penh = 1,000,000

5.5 Use of existing data to assess transboundary issuesThis section reviews the capacity of the existing data set (summarised in the above sections)to assess transboundary issues.

Physico-chemical water quality data

Despite the fact that the MRC has 7-15 years of water quality data at 16 sites along the mainMekong channel, these are of limited use in assessing transboundary issues. The mainlimitations in the database include:

Inappropriate indicators – a wide range of traditional indicators are measured (seeSection 5.1). However, many of these are inappropriate for assessing important ecologicaland human health changes in the system. We have focused on 11 indicators - SPM (TSS),conductivity, pH, alkalinity, Tot-P, FRP, NOx-N, NH4-N, (Tot-N where available), DO andCOD. Additionally, we have identified important indicators (e.g. Chlorophyll-a, TOC, DOC)that are not currently measured but should be.

Inappropriate site locations – when the water quality network was designed it seemsobvious that assessment of transboundary issues was not one of the objectives for which thedata was being collected. The sites are located considerable distances apart (see Figure 1)and at best can be used to assess broad overall changes in water quality over the length of theMekong. The situation for Vientiane illustrates the poor network design for transboundaryissues. Wastewater is discharged approximately 15 km downstream of Vientiane and couldaffect environmental values in Thailand which shares the river with Lao PDR in this region.



However, there are no sample locations immediately downstream of the effluent discharge.Below Vientiane, the next sample location is some 300 km downstream at Nakhon Phanom.

Inadequate sample frequency – current sampling occurs monthly and is inappropriate to pickup changes that occur more rapidly than this, e.g. algal growth. Dissolved oxygenfluctuation will occur over a day, and taking a single measurement at one time once a monthis unlikely to detect any major changes in this important indicator.

Surface samples – only surface water samples are taken at present. This is inappropriate forindicators that vary with depth (e.g. SPM, DO). Additionally, the dissolved oxygen status ofdeep pools in the main channel cannot be assessed with surface samples.

Statistical power – the present sampling design has very poor statistical power to detect anysignificant changes (see Section 7).

Toxicants (pesticides, heavy metals)

A small amount of pesticide data exists, but no heavy metal data. These data are insufficientto assess possible transboundary toxicity issues.

Biological

Biological assessment is increasingly being used to assess the condition of rivers worldwide.Unfortunately, there is no on-going biological monitoring program for the Mekong.Additionally, in the time available we were able to access only a small amount of biologicaldata relevant for the Mekong. We are told (J. Lacoursiere, Univ Lund, pers. comm.) that aconsiderable amount of relevant biological information does exist. A large number ofinternational (e.g. WHO, FAO, WWF, IUCN) studies have been funded over the years, andthese have been complemented by studies undertaken by national institutions in all membercountries. However, reports that do exist have not been collected in any central location(e.g. MRC) and the information has not been well synthesised.

We recommend that MRC make efforts to collect all published and unpublished informationon the biology and ecology of the Mekong River and its tributaries, and prepare a synthesisof this information that summarises current knowledge in this area.

Fluxes and loads

In view of the deficiencies in the water quality database, we have sought to obtaininformation on the amounts or loads of contaminants discharged into the Mekong,particularly from Vientiane and Phnom Penh. Comparison of these human-generated loadswith those transported “naturally” by the Mekong provides a first broad assessment of thepossible impacts on the system.

These load estimates still have considerable uncertainties. For example, it was difficult toobtain daily flows for some sites to use in calculating daily loads. Additionally, we hadlimited concentration data (monthly values) and had to make assumptions about what washappening over the rest of the time when samples were not available.

Ecosystem processes

A number of contaminants can undergo significant transformations after they are discharged.For example, organic matter is decomposed by the natural bacterial populations andphosphorus can be adsorbed by particulate matter. We have no information on suchprocesses. Such information will only be obtained by undertaking special studies aimed atanswering specific questions.



6. Assessment of transboundary water quality issues6.1 Phnom Penh

Background

The current population of Phnom Penh is ca. 1 million, with approximately 57% living in theurban areas of the city and 43% living in semi-rural areas on the outskirts of the city13

(General Population Census of Cambodia, 1998).

The central part of Phnom Penh has a combined sewage system (domestic, industrial,stormwater), but this is in poor condition having been poorly maintained since the 1980s14.The remainder of the city has no sewerage system, with the majority of the population inthese areas using latrines or septic tanks.

A network of street drains conveys water and sewage by gravity to retention ponds wheresome “treatment” of the wastewater occurs. Wastewater from these ponds is either pumpedover embankments directly into the Bassac or Tonle Sap Rivers, or into a system of naturalponds and canals that eventually drain into these rivers. A total of 14 sewers discharge tothese two rivers, but we have no information on the relative pollution loads from thesesewers.

There are a small number of industries in Phnom Penh (e.g. dye, galvanising & tanningfactories, paper mill, beer production), but few of these (8 of 148) undertake any treatmentbefore discharging their wastewater directly into the sewerage system or to nearby creeks.

Unfortunately, there is little monitoring of either the quantity or quality of Phnom Penh’swastewater. A recent survey of the effluents from a small number of factories in PhnomPenh found these discharges had quite high BOD concentrations (100-600 mg/L; ChrinSokha, Cambodian Ministry for Environment, internal report).

We have estimated that 75 million m3 of sewage and 470 million m3 of stormwater isdischarged annually to the Tonle Sap and Bassac Rivers (assume approximately equalamounts to each river). The loads of sediment, organic matter and nutrients are listed inTable 1. These loads comprise a relatively small proportion of the total loads transported bythe Mekong near Phnom Penh. However, this comparison is hardly relevant because thesepollutants are not discharged to the main Mekong River channel, but to the Tonle Sap andBassac Rivers (see Figure 5a).

The potential local impacts from these discharges are likely to be greatest during the dryseason when flows in these rivers are at their lowest, and during no-flow periods in TonleSap River. Flow data collected during 2000 as part of the Chaktomuk Project (2001),provides a useful picture of the changing flow conditions around Phnom Penh during theperiod August to November. In this year, the Mekong flow increased rapidly upstream ofPhnom Penh around late July (flow increased from 5,000 m3/s to 30,000 m3/s in a few days).Before this rapid increase, the flow in both Tonle Sap and the Bassac was essentially zero.With the high flow in the Mekong, flows increased to around 3,000 m3/s in the Bassac and6,000 m3/s in Tonle Sap for most of August and September. By the end of September theflow north in Tonle Sap had started to decrease and by the end of October had actuallychanged direction so that it was flowing out of Tonle Sap at around 5,000 m3/s. At this timethe flow in the Bassac River had increased to around 8,000 m3/s.

13 The population increase in Phnom Penh has been quite rapid, e.g. in 606,800 in 1989, 718,400 in1993, and 999,800 in 1998.

14 Mr Chrin Sokha (Chief, Office of Water & Soil Quality Management, Cambodian Ministry ofEnvironment) assisted us in assessing the sources of pollution from Phnom Penh.



There is some evidence (DO, COD, NOx, FRP concentrations – see Figure 6) that thedischarges to the Bassac River (and Prek Kdam) are having a local effect with higherconcentrations of COD and nutrients and lower concentrations of DO measured at times oflower flows. Additional information on water quality in Tonle Sap River (at Phnom PenhPort) also shows that, at certain times of the year, and particularly when the flow is very low,the water quality is degraded (see Figure 6).

Conceptual model

To assess the risk of transboundary water quality issues arising because of pollution fromPhnom Penh, we have adopted the following conceptual model (see Figure 5a):

• wastewater from Phnom Penh is discharged to the Tonle Sap and Bassac Rivers via anumber of discharge points - we have no information on how much of this wastewaterenters the main channel of the Mekong, but have assumed it is very low;

• wastewater discharged to the Bassac River will be transported downstream, while thatdischarged to Tonle Sap will be either transported downstream (Dec-Mar), upstream(Jul-October) or remain in the immediate vicinity of the outfalls (remainder of the year);

• the quality of this wastewater will range from very poor (during the dry season and thefirst flush of the wet season) to poor, since it receives essentially no treatment;

• the urban regions in Phnom Penh have little paving, which means that the SPM loads willbe high, particularly during the wet season;

• there is very poor information on both the quantity and quality of this wastewater, sincethere is no regular monitoring program;

• adverse effects due to these wastewater discharges may occur -

(a) in the immediate vicinity of the discharges (Tonle Sap, Bassac),

(b) further upstream in Tonle Sap River (and possibly even in Tonle Sap Great Lake),

(c) further downstream on entering Vietnam (transboundary issues).

Issues

As noted in Section 4.2, assessment to the possible transboundary water quality effects willfocus on three environmental values:


• fish migration;


For the first environmental value, ecosystem protection, we have attempted to assess the riskof adverse changes occurring to three key biological components:

• eutrophication or algal blooms due to excessive nutrient concentrations;

• toxicity to biota from either low dissolved oxygen concentrations or toxic compounds;

• ecosystem processes (e.g. changes to the light regime due to increased SPMconcentrations, with consequent changes to primary productivity).

This transboundary assessment focuses on two regions:

• in the immediate vicinity of Phnom Penh where pollution may interfere with fishmigration in the Bassac and Tonle Sap Rivers;

• in the Delta region of the Mekong, where pollution from Phnom Penh could result inunacceptable risks to the aquatic ecology and to human health.



Local issues in the Tonle Sap and Bassac Rivers, such as the impact on the ecology or humanhealth during the dry season, are not considered to be transboundary issues and are thereforenot covered in this report.

Risk assessment

Table 2 contains a summary of the information provided below for each issue considered.Where appropriate, three lines of evidence have been used to assess each issue.

• The existing water quality data both in the vicinity of Phnom Penh and at the border wascompared with appropriate trigger values to determine the proportion of time thesetrigger values were exceeded.

• The existing water quality data was used to discern any influence of Phnom Penh on thedelta region. A typical upstream/downstream experimental design was adopted, with themajor influence being the wastewater discharges from Phnom Penh (see Figure 5). Thus,we compared the water quality data from the two (three) upstream sites – Kratie,Kompong Chan and Phnom Penh – with the downstream sites in both the Mekong (TanChau, My Than) and the Bassac (Ta Khmao, Chau Doc, Can Tho). As discussed inSection 7, the available data are too sparse and too variable to show statistically whetherPhnom Penh is having an effect in Vietnam.

• The loads of key contaminants likely to be discharged to the system from Phnom Penhwere compared with those transported “naturally” by the Mekong. Somewhat arbitarily,we have assumed the following categories of risk of adverse impacts:

Low risk If ratio of load from Phnom Penh to natural load <5%

Moderate risk If ratio of load from Phnom Penh to natural load 5-50%

High risk If ratio of load from Phnom Penh to natural load >50%

The following points are relevant in making the assessments below:

• we have assumed that the bulk of Phnom Penh wastewater transported to the Delta willoccur via the Bassac River;

• the distance from Phnom Penh to the Vietnam border is ca. 110 km;

• the water temperature is high (ca. 29oC) meaning that the rate of decomposition of anyorganic matter discharged from Phnom Penh will be relatively rapidly broken down.

Ecosystem protection

Eutrophication

Algal growth is primarily influenced by three factors:

• nutrients - available nutrient concentrations are likely to be greatest in the dry season;

• light - in view of the high SPM concentrations, the Mekong system is likely to be lightlimited for much of the time, although during the dry season Mekong water tends tobecome clear, particularly in the upstream reaches;

• flow conditions - it seems likely that the flows in the main Mekong channel will be toogreat to sustain high phytoplankton biomass, even in the dry season. We have insufficientinformation to assess whether the conditions in wetlands associated with the mainchannel are conducive to phytoplankton growth, but this is more likely to occur.



Table 2a Summary of the transboundary risks from Phnom Penh wastewater discharges on the Vietnam Delta regionIssue Effect Effect rating Likelihood

assessmentLikelihood rating Transboundary risk

Ecological

Eutrophication 1 Algal blooms High – [FRP] >40 ug/LMod – [FRP] 10-40 ug/LLow – [FRP] <10 ug/L

that FRP trigger valueis exceeded during dryseason

• 10% likelihood higheffect• 80% likelihood moderateeffect

Low-moderate 2

Toxicity – DO 3 Toxicity to fish andbenthic invertebrates

High – [DO] <1.5 mg/LMod – [DO] 1.5-6.0 mg/LLow – [DO] >6.0 mg/L

that DO trigger value isexceeded during dryseason

• 95% likelihood of loweffect• 5% likelihood ofmoderate effect

Low

Toxicity – chemicals 4 Toxicity to fish andbenthic invertebrates

Insufficient information Risk not assessed

Ecosystem processes 5 To be determined Insufficient information Risk not assessedFish migration 6

Adverse effect on fishmovementupstream,downstream,or ontofloodplains

Insufficient information Insufficient information Risk not assessed

Human healthDrinking water 7 Increased microbial

contamination causingsickness

High – > 0 coliforms/100mlLow – < 0 coliforms/100ml

that coliform triggervalue is exceededduring any season

Insufficient information Risk not assessed 8

Recreation 7 Microbialcontamination causingsickness due toprimary contact

High – >150 faecalcoliforms/100mLLow – <150 faecalcoliforms/100mL

that faecal coliformtrigger value isexceeded during dryseason

Insufficient information Risk not assessed 8

1. Would prefer to use Chlorophyll-a rather than TP as the indicator, but no data.2. More likely low risk since only nutrient concentration used; high turbidity and high flow would also reduce chance of algal problems.3. DO measured at one time during the day, need more measurements during dry season and with depth in deep pools, special surveys needed.4. Cannot assess risk because no data on heavy metals or toxic organics.5. No information to assess risk in the main Mekong River, risk likely to be higher in off-channel wetlands and Tonle Sap Great Lake.6. No data, present water quality sampling network will not provide the required information, special survey needed.7. Apparently some data on total coliform levels (no data on E. coli), but could not obtain in time available.8. Risk likely to be low-moderate, due to large dilution and microbial die-off during transport to Vietnam.



Table 2b Summary of the transboundary risks from Vientiane wastewater discharges on Thailand near Vientiane and downstreamIssue Effect Effect rating Likelihood

assessmentLikelihood rating1 Transboundary risk

Ecological

Eutrophication Algal blooms High – [FRP] >40 ug/L

Mod – [FRP] 10-40 ug/L

Low – [FRP] <10 ug/L

that FRP trigger valueis exceeded during dryseason

Insufficient information Low2

Toxicity – DO Toxicity to fish andbenthic invertebrates

High – [DO] <1.5 mg/L

Mod – [DO] 1.5-6.0 mg/L

Low – [DO] >6.0 mg/L

that DO trigger value isexceeded during dryseason


Toxicity – chemicals 3 Toxicity to fish andbenthic invertebrates


Ecosystem processes To be determined Insufficient information Insufficient information Risk not assessed

Fish migration 4

Adverse effect on fishmovementupstream,downstream,or ontofloodplains


Human health

Drinking water Increased microbialcontamination causingsickness

High – > 0 coliforms/100ml

Low – < 0 coliforms/100ml

that coliform triggervalue is exceededduring any season


Recreation Microbialcontamination causingsickness due toprimary contact

High – >150 faecalcoliforms/100mL

Low – <150 faecalcoliforms/100mL

that faecal coliformtrigger value isexceeded during dryseason


1. Existing water quality data inadequate to assess the risk in either the immediate location of the wastewater discharge or further downstream, location of water quality sample locations inappropriate –one site upstream (Vientiane, H011910), one about 300 km downstream (Nakhom Phanom (H013101) – leads to a flawed “experimental design” that has no power to detect impacts due to Vientiane’swastewater discharge.

2. The tentative assessment of low risk is based on the high dilution expected when Vientiane’s wastewater enters the Mekong River – however we recommend MRC investigate further.3. Cannot assess risk because no data on heavy metals or toxic organics.4. No data, present water quality sampling network will not provide the required information, special survey needed.



Table 2c Summary of the transboundary risks from upstream degraded water on the Vietnam Delta region

Issue Effect Transboundary risk

Ecological 1

EutrophicationToxicity

Ecosystem function

Algal blooms

Fish/invertebrate kills

To be determined

Low

Low

Not determined

Human health 2

Drinking waterRecreation

Increased microbialcontamination causingsickness

Risk not assessed 3

Risk not assessed 3

Agriculture 4

Irrigation Increased salinity Low1. Used the analysis for effects of Phnom Penh (see above).2. Used the analysis for effects of Phnom Penh (see above).3. Risk likely to be low-moderate, due to large dilution and microbial die-off during transport to Vietnam.

4. Assessed on the basis of changes in conductivity.



In this preliminary assessment of the risk that high algal biomass will be stimulated inVietnam by nutrients released from Phnom Penh, we have focused on the existing nutrientconcentrations. Figure 9 shows the mean nutrient concentrations in the Mekong, Bassac andPrek Thnot Rivers over the period 1995 to 2000. The high variability (RSD approx. 50-60%) in the data precludes any meaningful statistical analysis.

Generally, low concentrations of available phosphorus (FRP) were observed (Figure 9a).For example, mean FRP concentrations ranged from 13 to 37 µg/L, and made up 15-65% ofthe Total P concentration. The generally higher FRP concentrations observed in Prek ThnotRiver probably reflect the relatively high proportion of wastewater received by this river.The considerably higher Total-P concentrations noted at the lower four sample locations inVietnam (see Figure 6) may be real, but equally may indicate a difference in the analyticalprocedures between Vietnam and Cambodia. Most of the available phosphorus dischargedfrom Phnom Penh would be expected to become associated with SPM, and hence essentiallyunavailable for algal growth, by the time it was transported downstream to Vietnam.

Available nitrogen was mostly present as NOx-N (Figure 9b). The mean NOx-Nconcentration appeared to be elevated in the Bassac River downstream of Phnom Penh (ca.230 µg/L cf 160 µg/L); this effect was more noticeable during the dry season.

The range of DIN15/FRP ratios (11-44 mol/mol) suggests that the system is mostly P limited,a not surprising result given the likelihood that available phosphorus would associate with thelarge number of suspended particles known to be present in this system.

(Figure 9a) (Figure 9b) (Figure 9c)

Figure 9 Plots of nutrient concentrations in the vicinity of Phnom Penh (a) mean(s.d.) FRP & Total-P concentrations (µµµµg/L), (b) mean (s.d.) NOx-N &NH4-N concentrations (µµµµg/L), (c) distribution of dry season FRPconcentrations at Phnom Penh, Ta Khmao and Chau Doc (mg/L).

Table 3 below shows the dry season (Dec-May) FRP concentration ranges adopted toindicate the potential for low, moderate and high algal biomass to form (assuming that thelight and flow conditions are not limiting phytoplankton growth). Also shown in Table 3 isthe likelihood that these triggers will be exceeded at Phnom Penh and Chau Doc (latter

15 DIN = dissolved inorganic nitrogen = [NOx-N] + [NH4-N]



selected as most vunerable transboundary downstream site). Figure 9c contains the datadistributions used to make these assessments.

Table 3 Trigger values for the concentrations of filterable reactive phosphorus (FRP)that could cause algal problems in the Mekong River, and the likelihood thatthese will be exceeded in the vicinity of Phnom Penh

Algal growthpotential

FRP trigger* Likelihood of trigger being exceeded

Phnom Penh Chau Doc

Low < 10 µg/L 20% 10%

Moderate 10-40 µg/L 75% 80%

High > 40 µg/L 5% 10%* derived using the method outlined in ANZECC/ARMCANZ (2000).

This analysis suggests there is a slightly higher probability that conditions at Chau Doc couldsupport moderate to high algal growth compared with the situation at Phnom Penh, but thisprobability is still very low.

In summary, our preliminary assessment suggests that on the basis of FRP concentrationsonly, there is a low risk that nutrients released from Phnom Penh would cause excessive algalgrowth downstream in Vietnam. When one factors in the high turbidity and the generallyhigh river flows, the risk is negligible.

Nutrient loads have also been used to estimate the potential transboundary issue due tonutrients discharged from Phnom Penh. In assessing the relative impact of nutrient loadsdischarged from Phnom Penh we have assumed:

• the total annual load transported to Vietnam (via the Bassac River) is 75% of the totalload discharged from Phnom Penh;

• there is no reduction in the load during transportation from Phnom Penh to Vietnam;

• there are no additional nutrient inputs downstream of Phnom Penh;

• the annual flow in the Bassac River is 15% of Mekong at Phnom Penh (i.e. 1,800 m3/s).

With these assumptions, it is estimated that the contribution of nutrients from Phnom Penhadds less that 1% to the loads of Total-P and Total-N transported naturally by the BassacRiver16.

This suggests a very low risk of transboundary nutrient (algal) problems from Phnom Penh,which is a similar assessment to that derived above on the basis of concentration data.

Toxicity due to low dissolved oxygen concentrations

Low dissolved oxygen (DO) concentration has an adverse effect on many aquatic organisms(e.g. fish, invertebrates and microorganisms) that depend upon oxygen dissolved in the waterfor efficient functioning. It can also cause reducing conditions in sediments, with theconsequent release of previously-bound nutrients and toxicants to the water column wherethey may add to existing stresses.

16 Tot-P – Phnom Penh contributes 340 t/y compared with Bassac’s 41,000 t/y; Tot-N (actually DINused in calculation) – Phnom Penh contributes 2000 t/y compared with Bassac’s 270,000 t/y.



The concentration of DO is highly dependent on temperature, salinity, biological activity(microbial, primary production) and rate of transfer from the atmosphere. Under naturalconditions, DO will change, sometimes considerably, over a daily (or diurnal) period, andhighly productive systems (e.g. tropical wetlands, dune lakes and estuaries) can becomeseverely depleted in DO, particularly when these systems are stratified.

Of greater concern is the significant decrease in DO that can occur when organic matter isadded (e.g. from sewage effluent or dead plant material). The depletion of DO depends on theload of biodegradable organic material and microbial activity, and re-aeration mechanismsoperating. A number of predictive computer models now exist for estimating the DOdepletion in a particular ecosystem type, and so it should be possible to estimate sustainableloads of biodegradable organic matter for most situations.

The 1992 Australian water quality guidelines (ANZECC, 1992) recommend that dissolvedoxygen should not normally be permitted to fall below 6 mg/L (or 80−90% saturation),determined over at least one diurnal cycle. Also, the USEPA (2000) has recently reportedDO guidelines for protecting aquatic life (but in seawater). These guidelines recommend thatDO should be above 4.8 mg/L to provide long term protection and above 2.3 mg/L forjuvenile and adult fish survival.

We have used these data to calculate the toxic effects levels associated with DOconcentrations in the Mekong River. The upper and lower limits (or trigger values) havebeen taken as the concentrations equivalent to 80% saturation and 20% saturation of waterat ca. 30oC during the dry season (Dec-May) when it is more likely that low DOconcentrations will exist. These are shown in Table 4. Also shown in Table 4 is thelikelihood that these triggers will be exceeded at Phnom Penh, Ta Khmao and Chau Doc (seeFigure 10 for the data distributions used).

This analysis suggests that there is a low probability that adverse effects due to low DOwould exist during the dry season at either Phnom Penh or Chau Doc (Table 4).Interestingly, there is a higher probability that moderately toxic conditions exist during thedry season at Ta Khmao (as discussed below).

Table 4 Trigger values for toxic effects due to low DO concentrations in the MekongRiver, and the likelihood that these will be exceeded in the vicinity of PhnomPenh

Potentialadverse effect

DO trigger Likelihood of trigger being exceeded

Phnom Penh Ta Khmao Chau Doc

Low >6.0 mg/L (80%satn)

95% 25% 95%

Moderate in range 1.5-6.0 mg/L 5% 75% 5%

High <1.5 mg/L (20%satn)

0% 0% 0%



Figure 10 Plot of the distribution of dry season DO concentrations at Phnom Penh,Ta Khmao and Chau Doc.

In summary, this preliminary assessment suggests that there is a low risk that pollution fromPhnom Penh would cause toxicity problems in downstream Vietnam (at Chau Doc) due toreduced DO concentrations.

On the other hand, the analysis suggests a higher probability of low DO toxicity problemsmore locally around Phnom Penh. Low DO values have been regularly measured in theBassac River at Ta Khmao, in Prek Kdam and in the Tonle Sap River at Phnom Penh Port(see Figure 6d). The low DO concentrations are likely to have been caused by organicpollution from Phnom Penh, since the COD concentrations at both Prek Kdam and TaKhmao (and Phnom Penh Port) are noticeably higher than the COD concentrations at themore upstream Mekong River sites (e.g. Kompong Chan, Kratie) (see Figure 6d). This resultis not in conflict with the lower mean COD concentration measured at Chau Doc (Figure6d), since it is quite possible that this organic matter was partially decomposed in the severaldays it would take the water to reach Chau Doc during the low flow period.

Toxicity due to excessive concentrations of heavy metal and organic contaminants

It was not possible to assess the risk to aquatic organisms due to possible toxic heavy metaland organic contaminants released to the Mekong River from Phnom Penh, becauseinsufficient water quality information was available concerning toxicants.

If the MRC wishes to assess this risk it will be necessary to collect information on keytoxicants (see Section 7).

Ecosystem processes

Very few river health programs make any attempt to measure changes to key ecosystemprocesses, such as primary production, metabolism, respiration or denitrification. This isbecause the methods available to measure these ecosystem indicators are not commonlyavailable and the interpretation of the results is presently limited by our knowledge of howunmodified river systems behave. Except for upland streams, it is often difficult to findunmodified rivers to serve as reference systems.



However, given that the Mekong River still appears to be in quite good condition, it isappropriate that baseline ecosystem process information be collected now. This would thenprovide the basis for future assessment of possible changes in the system with the plannedfurther developments.

Fish migrationIt is well known that very extensive migration of fish occurs up and down the Tonle SapRiver, with upstream migration occurring during July to September and downstreamoccurring during October to February. We have no information on fish migration in theBassac River.

Wastewater discharges from Phnom Penh to the Tonle Sap River could potentially interferewith both the upstream and downstream migration of fish. At this stage, there is insufficientwater quality data to assess either of these conditions in detail. However, it seems likely thatthe greatest risk would be during the dry season, and particularly around March and April,when the greatest impacts on water quality would occur. The available information suggeststhat minimal fish migration occurs at this time.

Water quality in Tonle Sap River is measured at the Phnom Penh Port site, located close tothe city. These data have been used to make a very preliminary assessment of the risk to fishmigration. However, it should be recognised that these data are totally insufficient to allow aproper risk assessment. Deficiencies include: surface samples only are taken, there is nosampling across the river or with depth, and no toxicant concentrations are measured.

The data summarised in Figure 6 suggest that the water quality in the Tonle Sap River nearPhnom Penh is slightly degraded (low DO, high COD concentrations). We also haveevidence that the dissolved oxygen concentration during the dry season is considerablyreduced (see Figure 3). These reductions in the surface water DO are unlikely to besufficient to cause interference to fish migration, based on the earlier defined DO effectconcentrations. However, it is possible that the DO concentrations at depth could be evenlower and that the situation at times is much worse than suggested by the monthlymonitoring data.

In summary, there are insufficient data to be able to assess the risk that wastewaterdischarges from Phnom Penh are causing problems with fish migration in the Tonle SapRiver. However, the available data show that water quality in this river near Phnom Penh isdegraded, particularly during the dry season.

We recommend that a more comprehensive assessment of the risk to fish migration beundertaken. Almost certainly, this will require a specific study designed to answer thisquestion. It will never be possible to satisfactorily assess this risk with water quality datapresently being collected.

Human health

Drinking water

There is little information available on the amount of water taken directly from the MekongRiver in downstream Vietnam for drinking. However, for this preliminary risk assessment ithas been assumed that both the Bassac and Mekong channels are used for drinking water. Itis not possible to complete a detailed risk assessment without more detailed information onthis potential use.

Microbial contamination of drinking water presents the greatest risk to human health(NH&MRC/ARMCANZ, 1996), with the microorganisms of concern including bacteria,protozoa, toxic algae and viruses. Most guidelines require that safe drinking waters contain



no coliform bacteria (including faecal coliform bacteria or E. coli) per 100 mL sample(WHO, 1984; NH&MRC/ARMCANZ, 1996).

We have been unable to obtain any relevant bacterial water quality data for the MekongRiver in the time available. Therefore, it has not been possible to complete a quantitativeassessment of the risks to human health from drinking water from the river (withouttreatment).

However, it is possible to provide a qualitative assessment of the transboundary risks ofhuman health problems in downstream Vietnam due to faecal contamination from PhnomPenh. Our assessment is that the risk is likely to be low to moderate. It is expected that veryhigh bacterial levels will be present in wastewater from Phnom Penh since it receivesessentially no treatment. Two factors, the relative large dilution of any discharges fromPhnom Penh (estimated to be a minimum of ca. 800:1) and the likely dieoff would act toreduce these bacterial levels during transport downstream to Vietnam (estimated to bearound 3-5 days during low flow periods),

It is recommended that MRC collate all available data relating to bacterial levels in theMekong River at Phnom Penh, Chau Doc and Tan Chau, so that a quantitative riskassessment can be made. If there are insufficient data or these data are suspect, MRCshould commission a special study designed to specifically assess the risk the bacterialcontamination from Phnom Penh is adversely affecting people drinking Mekong water whenit enters Vietnam (specifically at Chau Doc and Tan Chau).

Recreational water

The Mekong River and its tributaries are used extensively for swimming, bathing andwashing. Many countries have developed water quality guidelines aimed at protecting suchprotect primary contact activities (WHO, 2001; ANZECC/ARMCANZ, 2000). Most requirethat such waters should be sufficiently free from faecal contamination, pathogenic organismsand other hazards (e.g. poor visibility or toxic chemicals) to protect the health and safety ofthe user.

The new Australian and New Zealand guidelines (ANZECC/ARMCANZ, 2000), for example,recommend that the median bacterial content in samples of water taken over the bathingseason should not exceed:

• 150 faecal coliform organisms/100 mL (minimum of five samples taken at regularintervals not exceeding one month, with four out of five samples containing less than600 organisms/100 mL);

• 35 enterococci organisms/100 mL (maximum number in any one sample: 60–100 organisms/100 mL).

Additionally, these guidelines recommend that recreational waters with temperatures in excessof 24°C should also be free from pathogenic free-living protozoans.

We have been unable to obtain any relevant bacterial water quality data for the MekongRiver in the time available. Therefore, it has not been possible to complete a quantitativeassessment of the risks to human health from primary contact use of the river.

If MRC decide to go ahead with a more detailed risk assessment related to human health werecommend that the most recent WHO document on Bathing Water Quality and HumanHealth (WHO, 2001) be used for guidance on the risk-based approach that might be used.



6.2 Vientiane

Background

Vientiane has a population of 583,000 people distributed over an urban area of 3,920 km2

(Lao National Statistics Book, 2000).

Vientiane does not have an effective sewerage system, a fact of considerable relevance to thisstudy. All sewage and stormwater is collected either in small open concrete or dirt channelsthat exist throughout most of the city, or in the newly-constructed combined stormwater-sewage pipe network that now runs through the city center and along the main roads. Thispipe network connects to three main drainage channels that flow into the That Luang-Salakham wetland complex (see Figure 5b - J. Lacoursiere, Univ Lund, pers. comm.).

Some of the sewage and stormwater which gathers in the large and deep channel nowrunning through Nong Chan wetland (behind the Morning Market/Bus station) is pumped toa complex of six oxidation ponds built some years ago by the European Community. Theseeach have a treatment capacity of 30,000 equivalent persons. The output from these pondsalso flows to That Luang wetland (J. Lacoursiere, Univ Lund, pers. comm.).

That Luang is a large wetland system (ca. 13 km long) located to the east of Vientiane. Itnow receives the entire wastewater load from the city since all direct outflows from the cityto the Mekong River ceased around 1990. Outflow from the wetland drains to the Mekongvia a 50 km long small river.

Since 1990, the size of the wetland has been reduced by drainage and agricultural expansion,seriously affecting its capacity to treat the continually increasing wastewater load fromVientiane (J. Lacoursiere, Univ Lund, pers. comm.). Additionally, the quality of thewastewater entering the wetland has deteriorated over the years as the channels thattransport the wastewater to the wetland have had in-channel vegetation removed and beenconcrete-lined17. During the dry season, the wastewater is almost pure sewage, but is morediluted in the wet season.

We have estimated that the total volume of stormwater and sewage discharged annually tothe Mekong from Vientiane is around 120 million m3; the volume of stormwater isapproximately double that of sewage. This compares with an annual volume of ca. 140,000million m3 transported by the Mekong River at Vientiane, giving an annual average dilutionof over 1000:1. Even during the dry season when flows in the Mekong are much lower (e.g.mean flow at Vientiane in March = 1,190 m3/s), the dilution of the sewage is still around800:1 (assuming most of the wastewater flow is sewage with little stormwater).

Conceptual model

To assess the risk of potential transboundary water quality issues arising because of pollutionfrom Vientiane, we have adopted the following conceptual model (see Figure 5b):

• wastewater from Vientiane is discharged downstream of the city to the Mekong via awetland and passage through ca. 50 km of a small river. Thus, this wastewater receivesvarying levels of treatment depending upon the flow. Unfortunately, we were not able toobtain any detailed information on the quality of the wastewater from Vientiane in thetime available;

• the urban regions in Vientiane have little paving, which means that the SPM loads will behigh, particularly during the wet season;

17 Apparently, the Water Quality Laboratory of the Lao Dept of Irrigation (Director: Mr Bon Suk) hasbeen monitoring the wastewater outflows since early 1990, but we have not been able to obtain anyinformation in the time available.



• adverse effects due to this wastewater discharges may occur:

(a) in the immediate vicinity of the discharge (i.e. in the Mekong River downstream ofVientiane),

(b) further downstream in both Thailand and Cambodia.

• since the Mekong River between Vientiane and Khong Chiam (see Figure 1) is the borderbetween Lao PDR and Thailand, potential transboundary issues could presumably occurboth in the immediate vicinity of the discharge and further downstream;

• contaminants discharged into the Mekong River by the city of Vientiane could alsopotentially cause problems in Cambodia (see Figure 1).

Issues

As noted in Section 4.2, assessment to the possible transboundary water quality effects willfocus on three environmental values:


• fish migration;


For the first value, ecosystem protection, we have attempted to assess the risk of adversechanges occurring to three key biological components:

• eutrophication or algal blooms;

• toxicity to biota from either low dissolved oxygen concentrations or toxic compounds;

• ecosystem processes (e.g. changes to light regime with consequent changes to primaryproductivity).

The focus of this transboundary assessment is in the region in the immediate vicinity of thewastewater discharge and further downstream.

Risk assessment

Table 2 contains a summary of the information provided below for each issue considered.

For Phnom Penh (Section 6.1), we were able to use two lines of evidence to provideinformation for the assessment of each issue: (a) existing water quality data and comparisonwith trigger values to discern any influence of Phnom Penh on the Delta region, and (b) acomparison of the estimated loads of key contaminants discharged from Phnom Penh withthose transported “naturally” by the Mekong.

Unfortunately, the existing water quality data is of little use in assessing the risk in either theimmediate location of the wastewater discharge or further downstream. The location ofwater quality sample locations – one site upstream (Vientiane, H011910) and one about 300km downstream (Nakhom Phanom (H013101) - leads to a flawed “experimental design” andone that has no statistical power to detect impacts due to Vientiane’s wastewater discharge.

To assess the transboundary risks from this wastewater discharge, it would be necessary for acompletely different experiment design to be developed, probably involving several sitesupstream coupled with several located downstream of the discharge point (see Section 7).

It has been possible to provide a qualitative assessment of risk using the comparative loads ofkey contaminants likely to be discharged to the Mekong from Vientiane. Somewhatarbitarily, we have assumed the following categories of risk of adverse impacts:

Low risk If ratio of load from Vientiane to natural load <5%

Moderate risk If ratio of load from Vientiane to natural load 5-50%



High risk If ratio of load from Vientiane to natural load >50%

The following points are also relevant in making the assessments below:

• the distance from Vientiane to the Cambodian border is ca. 600 km;

• the water temperature is high (ca. 21-34oC) meaning that organic matter discharged fromVientiane will be relatively rapidly broken down.

Background comments on each of the environmental values made in the previous section willnot be repeated.


Eutrophication

As noted above, there were no statistically meaningful differences in any of the nutrientconcentrations between Vientiane and Nakhom Phanom (see Figure 6c).

Using the estimated nutrient loads discharged from Vientiane (see Table 1), we have beenable to make a qualitative assessment of possible adverse effects. On an annual basis,Vientiane adds around 1-4% to the loads of Total-P and Total-N transported naturally by theMekong River18. Even during the dry season when flows in the Mekong are much lower(e.g. mean flow at Vientiane in March = 1,190 m3/s), dilution of the wastewater would stillbe around 800:1 (assuming most of the wastewater flow is sewage with little stormwater).

This suggests a low risk of transboundary nutrient (algal) problems from Vientianewastewater discharges.

Toxicity due to low dissolved oxygen concentrations

As with nutrients, there is inadequate DO data to be able to assess the possible risks fromlow DO toxicity problems. However, given the very high dilution ratios (>800:1), it isreasonable to predict that the transboundary risk from low dissolved oxygen concentrationswill be very low.

Toxicity due to excessive concentrations of heavy metal and organic contaminants

It was not possible to assess the risk to aquatic organisms due to possible toxic heavy metaland organic contaminants released to the Mekong River from Vientiane, because insufficientwater quality information was available on toxicants. However, given the relatively smallamount of industrialisation in this city, it is reasonable to assume that very small loads oftoxicants are likely to be released.

If the MRC wishes to assess this risk it will be necessary to collect information on keytoxicants (See Section 7).

Ecosystem processes

There is no quantitative information on ecosystem processes occurring in the upper reachesof the Mekong around Vientiane.

The MRC should investigate the possible introduction of innovative ecosystem processmeasures for the Mekong River system.

Fish migrationAn assessment of fish migration in the Mekong River by Poulsen & Valbo-Jorgensen (2000)found that some fish migrate past Vientiane, the direction of travel depending upon the flowconditions. It is possible, therefore, that wastewater discharges from Vientiane could

18 Tot-P – Vientiane contributes 300 t/y compared with Mekong’s 7,000 t/y; Tot-N (actually DIN usedin calculation) – Vientiane contributes 1,000 t/y compared with Mekong’s 70,000 t/y.



potentially interfere with both the upstream and downstream migration of fish in the MekongRiver.

As noted above, there is no water quality data available near the wastewater discharge pointand so it is not possible to assess the condition of the river at this point. This informationwill need to be collected if MRC wishes to make an assessment of the risk to fish migration.

It is possible, however, to make a preliminary assessment that the risk is likely to be verylow, based on the high level of dilution (>800:1) of the wastewater even under dry seasonlower flow conditions.

In summary, there are no water quality data available to undertake an assessment of the riskthat wastewater discharges from Vientiane are causing problems with fish migration in theMekong River. However, on the basis of the very high dilution of this wastewater onentering the river, it seems likely that this risk would be very low. A comprehensiveassessment of the risk to fish migration would require a specific study designed to answerthis question. It will never be possible to satisfactorily assess this risk with water quality datapresently being collected.

Human health

Drinking water

Little information is available on the amount of water taken directly for drinking from theMekong River in the vicinity and downstream of Vientiane’s wastewater discharge.Additionally, we were unable to obtain any relevant bacterial water quality data for theMekong River in this region in the time available.

Therefore, it has not been possible to complete a quantitative assessment of the risks tohuman health from drinking water from the river (without treatment).

It is recommended that MRC commission a survey of the behaviour of wastewaterdischarges from Vientiane and the possible human health (and ecological) risks in thevicinity and downstream of the discharge point.

Recreational water

We have assumed that the Mekong River in the vicinity of the wastewater discharge is usedfor swimming, bathing and washing. Unfortunately, we were unable to obtain any relevantbacterial water quality data for the Mekong River in this region in the time available.

Therefore, it has not been possible to complete a quantitative assessment of the risks tohuman health from recreational use of the river. See above for recommendation.

6.3 Water quality in the Mekong Delta

Background

The MRC required that this report “assess the degree to which degraded water in MekongDelta can be attributed to transboundary transport of poor quality water from upstream”.

The Mekong Delta region of Vietnam is intensively used for agriculture, and has a wide-spread and well-recognised problem with acid sulfate soils (Minh et al., 1997; MRC, 1997).Over 40% of the region has these acid sulfate soils, which are of marine origin and containhigh levels of pyrite (FeS2). During the dry season, the soils dry out and crack, which allowsair to penetrate and the pyrite to oxidise. The products of this oxidation, i.e. low pH waterand high concentrations of iron, aluminium and sulfate, are then leached from the soil profilewith the first heavy rains of the wet season (Tin & Willander, 1995), and the acidic andmetal-polluted water enters the irrigation canals and subsequently causes significantdetrimental effect on crops.



An additional water quality issue in the delta region occurs during the dry season when lowflows in the Mekong allow high salinity water to penetrate further up the rivers, and preventirrigation. There is considerable concern that the possible construction of hydroelectricityand irrigation dams further up the Mekong could increase this saline intrusion, and adverselyaffect even larger areas of irrigated agriculture (MRC, 1997).

This section is concerned with assessing the risk from a possible third water quality issue,that caused by degraded upstream water reaching the Delta.

Conceptual model

To assess the risk of potential transboundary water quality see (Figure 5c):

• wastewater discharged from Phnom Penh, mainly via the Bassac River, could potentiallycause water quality problems in the Delta region;

• agricultural activities in the Mekong catchment upstream of Phnom Penh are unlikely toinfluence the quality of the river sufficiently to result in any impacts in the Delta region;

• however, catchment activities that influenced the Mekong’s flow (e.g. building reservoirsfor hydropower generation and irrigation) could have a major impact on water quantityand quality in the Delta region;

• previous studies have shown that serious pollution (acid sulphate soils) is occurring withinthe intensive agricultural areas of the Delta region (although this is not a transboundaryissue).

Issues

Assessment of the possible transboundary water quality effects in the Delta region will focuson three environmental values:


• human health (drinking water & recreational use);

• agricultural use of the water.

Risk assessment

Table 2 contains a summary of the information provided below for each issue considered.


Assessment of risks to the ecology in the Delta caused by degraded upstream water isequivalent to the assessment that has already been undertaken for Phnom Penh (see Section6.1). This assessment showed that, despite a deficiency of good data, the discharge ofwastewater from Phnom Penh would most likely result in a low risk of adverse ecologicaleffects in the Delta region.

Human health

The assessment of risks to human health (drinking water, recreation) in the Delta caused bydegraded upstream water has already been undertaken in Section 6.1. This assessmentshowed that, despite a deficiency of good data, the discharge of wastewater from PhnomPenh would most likely result in a low risk of human health problems.

Agriculture

The main agricultural use of water in the delta region is for irrigation of rice fields. Problemscould arise for this agricultural use if the water from upstream was polluted, the most likelyproblem being increases in salinity (conductivity). The long-term water quality data (seeFigure 6a) indicate that the Mekong is low in conductivity and shows no trend towards anincrease in conductivity towards the Delta region.



The risk to agricultural water quality from upstream activities is assessed as being very low.This of course is not the case for pollution from within the Delta region (see below).

The other water quality indicator that could feasibly affect agricultural production in theDelta region over the long term is the amount of sediment that is deposited on the deltafloodplains. This annual replenishment is needed to continue the sustained high productivityof these areas. The data available (e.g. SPM loads) allows calculation of the loads ofmaterial transported to the floodplain annually, but not of the amount deposited with eachflow event. Equally, the data available are not good enough to indicate whether the loads ofSPM transported to the Mekong Delta have increased or decreased over the years.

This preliminary assessment indicates that the risk of transboundary water quality issues inthe Delta region due to degraded upstream water (particularly wastewater discharges fromPhnom Penh) is low.

Other issuesAs indicated above, there are water quality problems in the agricultural areas of the MekongDelta. However, these arise more from issues within the Delta (e.g. acid sulphate soils) thanfrom upstream.

Also, as noted above in the conceptual model for the Delta region, changes to the Mekongflows such as could occur by the building of reservoirs for hydropower generation andirrigation as has been planned (MRC, 1997), could result in a number of problems. Thesewould include a major impact on the flooding/drying cycle, with consequent increase inacidity and aluminium toxicity, and on the ingress of saline water from the South China Sea(MRC, 1997; Joy et al., 1999).



7. Monitoring network required to assess transboundaryissues

7.1 Assessment toolsThe tools for risk assessment are a little different from those of ordinary scientific methods,such as null-hypothesis testing, because natural resource management concerns itself with thecosts of two kinds of error (Table 5). First, decision-makers are keen to avoid declaringthere is an impact when there is none. Second, they are also keen to avoid declaring aproposal is safe when, in fact, it leads to an unacceptable environmental impact. Typically,the judgement is based on a statistical test of a null-hypothesis of no impact, which focuseson just one of these errors, the Type I error rate which, by convention, is set at 0.05.However, a problem with this approach is that it fails to account for the probability of theother kind of error, a false negative or Type II error (Mapstone, 1995; Johnson, 1999).

Quantitative ecological risk assessments seek to make an explicit treatment of both kinds oferrors. These include confusion matrices (Table 5 is an example) in which the false positiveand false negative rates are specified explicitly. These tables can then be generalised intoreceiver operator curves (Swets et al., 2000) that may be used to evaluate the consequencesof adjusting decision thresholds, and to make risk-weighted decisions that account for therelative costs of false positive and false negative outcomes. Unfortunately, it is not possibleat this stage to use these tools in assessing the risk of adverse ecological effects in theMekong River because of a lack of relevant data.

Table 5 Logic of environmental decisions. Monitoring programs are generallyestablished to conclude that projects or activities have or have not had animpact.

True response Measured response (outcome of test)

Positive result Negative result

Positive result

(impact occurs)

Correct decisionProbability of reaching correctdecision = 1- α

Wrong decisionType II error

Error risk = β

Negative result

(no impact occurs)

Wrong decisionType I error

Error risk = α (significance level)

Correct decisionProbability of reaching correctdecision = 1- β

7.2 Monitoring network designMany water quality monitoring networks are poorly designed and have little statistical powerto detect likely changes in the indicators that are measured. Common deficiencies include alack of a clear objective(s) and, for variable systems, insufficient sample sites and samplenumbers to detect the expected changes.

A well-designed monitoring program should be able to answer the specific question forwhich it is established. For example, below we have sought to test the effectiveness of thepresent monitoring network around Phnom Penh to answer the question “is there an impact(25% change) on downstream water quality due to wastewater discharged from PhnomPenh”?



The most sensible design to answer the question of interest to the MRC (are thereunacceptable transboundary impacts due to wastewater discharges from Vientiane andPhnom Penh?) would have sample locations upstream and downstream (U/D) of thedischarge point(s). For detailed discussions on experimental design the reader is referred toMapstone (1995), Underwood (2000) and Quinn & Keough (2001).

Assessment of the present water quality network near Vientiane clearly shows that it isunable to detect any downstream changes due to wastewater discharges from Vientiane.There is one sample point upstream at Vientiane and one 300 km downstream, by which timeany changes would have been diluted out or other changes would have occurred to mask anyeffects from Vientiane.

The present network is a little better designed to detect changes due to Phnom Penh. Forexample, there are 3 sample sites upstream of Phnom Penh (Kratie, Kompong Cham, PhnomPenh) and potentially 5 sites downstream (Neak Luong & Tan Chau on the Mekong, TaKhmao, Koh Khel & Chau Doc on the Bassac). In designs such as this, the monitoring sitesare effectively the replicates for testing the effect of Phnom Penh (Keough & Mapstone,1995). Hence, the number of sites has the greatest effect on the capacity of the monitoringprogram to detect an effect due to the city, and there would be little advantage in increasingthe number of samples taken at each site.

Power analyses of the test of the effect of Phnom Penh on conductivity, SPM and Total-P arepresented in Table 6. We have used data from 3 sites upstream of Phnom Penh and 3 sites inthe Bassac downstream to estimate the variation seen among sites. For this analysis it wasassumed that the Bassac River transports most of the wastewater from Phnom Penh.

Table 6 Analysis of the power of the present water quality monitoring network to detectchanges of greater that 25% in conductivity, SPM and Total-P concentrations dueto wastewater inputs from Phnom Penh.

Indicator No.upstream

sites

Meanupstream

No.downstream

sites

Meandownstream

Power

(1-β)

No. samplesrequired for0.80 power

Conductivity 3 14.9 3 11.2 0.64 8

SPM 3 109 3 82 0.25 20

Tot-P 3 26 3 20 0.06 618

Two types of results are presented. First, the power of the current design to detect a 25%change in each of the three indicators is presented. Statistical power is the chance ofdetecting the specified effect given the design, and is calculated as 1 minus the Type II errorrate (1-β). Second, the total number of sites that would be required to achieve a statisticalpower of 80% is presented. Implicit in this latter calculation is that an equal number of siteswould exist upstream and downstream of the city. The calculations were performed usingthe shareware computer program G-Power (Faul & Erdfelder, 1992). The Type I error rate(α) was set at 0.05 for all calculations.

The analyses (Table 6) show that for conductivity there is a 64% chance that the specifiedeffect will be detected (i.e. that a change of >25% will not be missed). For SPM, the poweris considerably less with only a 25% probability that such changes will be detected, orconversely a 75% chance that these changes will be missed. For Total-P, the sample designhas essentially no power (6%) to detect changes of 25% in this indicator.



In order to achieve 80% statistical power for conductivity, SPM and Total-P, the respectivetotal number of sites that would be required are 8, 20 and 618. In other word, to ensure an80% probability that a 25% change in the Total-P concentrations between upstream anddownstream locations was detected, a total of 618 sites (309 upstream and 309 downstream)would be needed. The differences in the number of sample sites required for the differentindicators are the result of the different variability of these indicators. In this example,conductivity had the least inter-site variation, while Total-P had the greatest.

It is clear from the above statistical analysis that the present water quality network has verylittle power to detect relatively large (25% or more) changes in water quality indicators(with the possible exception of conductivity) due to the discharge of wastewater from eitherVientiane or Phnom Penh.

7.3 Towards a new environmental assessment programA key management objective of the MRC is to ensure that the environmental values of theMekong River are not degraded by activities in upstream countries (i.e. that unacceptabletransboundary issues do not arise). This report recommends that the Mekong RiverCommission should manage this river system to protect the following environmental values:


• native fisheries production;

• drinking water;

• irrigation (mainly for rice);

• aquaculture;

• recreation and aesthetics.

This report has focused on three key environmental values – riverine and floodplainecosystem protection, fish migration and human health – in the belief that if these values areadequately protected so will the others.

A monitoring program that concentrates solely on physico-chemical indicators (even if itwere better designed) is inadequate for assessing the quality of aquatic ecosystems and thequality of water for human uses. Clearly, the MRC should consider developing andimplementing a new environmental assessment program.

Approach

A possible approach to develop a new environmental assessment program focused ontransboundary issues would involve the following steps:

• define all transboundary issues (this report has covered three such issues, but there aremore) – this task must be done in collaboration with member countries;

• for each transboundary issue (or those given top priority), develop a conceptual modellinking the stressors or drivers and the environmental and human health effects;

• using these conceptual models, design a statistically robust investigation and monitoringprogram to determine each effect (this may require that preliminary investigations becarried out to provide relevant information to assist in designing the monitoringprogram);

• put in place a regular (3-5 yearly) review of each program to ensure its effectiveness;

• initiate a targeted training program to ensure each member country has trained personalto carry out the required monitoring and investigation tasks.



What to measure?

The indicators selected to provide information on a particular transboundary issue shouldreflect as directly as possible the effect. For example, if the effect being investigated iseutrophication then direct measures of algae (species composition, biomass, primaryproduction) are preferred to measuring surrogates such as nutrient concentrations. Equally,if the objective is to protect ecosystem “health”, then direct measures of key biota (e.g.macroinvertebrates, fish, algae, bacteria) or ecosystem processes (metabolism, gross primaryproduction, respiration) should be targeted. That is, measures of both ecosystem structureand processes should be developed.

An objective method for selecting ecological health indicators that could potentially be usedfor the Mekong is the recently completed Stage 3 of the Southeast Queensland RegionalWater Quality Management Strategy (Smith & Storey, 2001). They tested each of thepotential indicators for their sensitivity to a disturbance gradient, in this case to a land usegradient that ranged from forested, through grazing, cropping, horticulture and urban. Thoseindicators that were sensitive to the disturbance gradient were kept and the others werediscarded.

A preliminary list of appropriate indicators for assessing transboundary issues in the MekongRiver basin would include:

• daily flow;

• physico-chemical indicators (conductivity, SPM, pH, alkalinity, nutrients (TP, FRP, TN,NOx-N, NH4-N), DO, TOC, DOC);

• biological (it would be desirable if MRC initiated a project to determine possiblebiological indicators – this should be done in close collaboration with member countries);

• toxicants (a possible cost-effective program would be to monitor fish tissue forpesticides, say once every 2 years);

• monitoring both the flow and quality of wastewater discharges from Phnom Penh andVientiane.

This list of indicators should be further developed once the full list of transboundary issueshas been determined.

Where to measure?

The main channel of the Mekong River has been the focus of this preliminary risk assessmentof the three transboundary issues. However, if the MRC is to establish a new environmentalassessment program concerned with a broader range of basin-wide and transboundary issues,it is likely that other waterways (particularly floodplain and wetlands) will need to beincluded, as well as specific activities within the catchment.

Typical study design

In this section, we provide an example of a possible network design that could be introducedto investigate the downstream effects of wastewater discharges from Phnom Penh andVientiane. While these experimental designs focus on investigating physico-chemicalindicators, the principles would also be applicable for other indicators (e.g. biological,microbiological).

Phnom Penh

For Phnom Penh the question to be tested is: Is there a measurable effect of Phnom Penhwastewater in the Bassac River and does this effect continue to the Vietnam border?



The design we favour would have sites nested within three locations of anUpstream/Downstream (UD) factor. The locations would be (a) upstream of the city (US),(b) downstream of the city (DS), and (c) at the border (B) (Figure 11). The sites should belocated relatively close to the area of interest to prevent effects of spurious influences,perhaps within 20 km of each other for each of the three locations.

Figure 11 shows a possible monitoring program with 6 sites within each location of the UDdesign. The above power analysis suggests that this would be the likely numbers of sitesrequired (at least for conductivity and SPM). It is the number of sites sampled that largelycontrols the power of the analyses. Taking more than 12 samples per year, and more thanone sample each sampling time, could reduce the variance among sites within the samelocation of the UD design, and increase the power, but only slightly.

Figure 11 Sample design to test effect of wastewater discharged from Phnom Penhon the Bassac River and downstream at the Vietnam Border.

Two planned comparisons would then be performed. First, a test of US v DS would establishwhether the city was having an effect. Second, a follow-up test of US v B would establishwhether there was an effect at the border. A significant result for both comparisons would beexplained by an effect of the city that persists to the border19. The logical progression of thestatistical tests is shown below.

19 This latter could also be explained by a new effect near the border after the other one had dissipated.



US v DS

US v B

US & DS v B

Significant

Non-Significant

Significant

Non-Significant

Significant

Non-Significant

Persistenteffect of city

Non-persistenteffect of city

Effect occurringafter city

No effect

Vientiane

The issues surrounding potential pollution of the Mekong by Vientiane are slightly differentto those around Phnom Penh, but the questions could be answered with a similar samplingdesign. Because the Mekong near Vientiane is the border between Thailand and Lao PDR,any effect of that city on the river would be transborder pollution. A reasonable expectationmight be that any effects of Vientiane’s wastewater discharge are non-detectable a certaindistance downstream of the discharge point20. This distance would not be easily defined (aswas the case with the border of Cambodia and Vietnam), but would have to be based onexpert opinions and negotiations between governments.

If this is acceptable (i.e. that effects from Vientiane’s wastewater should not be detectable xkm downstream from the discharge point), a design fundamentally identical to thatrecommended above for Phnom Penh could be employed. Groups of sites (generally around6) would be located immediately upstream of the city, immediately downstream of thewastewater discharge point, and at the agreed “no effect” distance downstream. Thenumbers of sites required would probably be the same, and the analysis would proceed viathe same design and logical progression outlined above.

We have noted above that the magnitude of flows in the Mekong River is such that there is alow risk of major water quality effects due to Vientiane. Hence, we recommend that theMRC first seek to answer the question: “Is there any effect of the city immediatelydownstream of the discharge point?”, before progressing to examine sites furtherdownstream to look for a reduction of this “effect”. Such a preliminary study would requireonly two thirds of the sites (US and DS only). If an effect of the city were noted, the extrasites (B) would then be set up, and the experiment undertaken again.

7.4 RecommendationsWe recommend that MRC develop a new and more robust environmental assessmentprogram designed to identify and assess the risks from a broader range of current and futuretransboundary and basin-wide issues. The process to achieve this new assessment programshould be undertaken in collaboration with the member countries and would involve:

• using the (ecological) risk assessment technique to underpin this process, with the firsttask being to scope the full range of existing and possible future transboundary issues(and their priority);

• running a number of workshops (involving each country) to develop the conceptualmodels and decide upon the target areas, the assessment endpoints and the best indicatorsto measure;

20 At this stage we have no information on the behaviour of the wastewater plume when it enters theMekong. This will need to be established before a sensible experimental design can be established.



• undertaking a program of short-term, targeted investigations to provide essentialinformation on specific aspects of the system that will enable the main program to bebetter designed;

• preparing a full program proposal, obtaining funding and implementing the program.



8. Conclusions & RecommendationsDeteriorating water quality in the lower Mekong River basin has been identified as a prioritytransboundary issue by each of the four member countries. Three particular transboundarywater quality issues are considered in this report:

1. The potential effects of municipal and industrial wastewater from Phnom Penh on bothdownstream Vietnam and fish migration in Tonle Sap River.

2. The potential effects of municipal and industrial wastewater from Vientiane on bothneighbouring Thailand and fish migration in the Mekong River.

3. The influence of upstream water on the degraded water quality in the Mekong Delta.

A risk assessment framework has been used to assess these issues. In particular, the risk ofadverse effects on three key values of the Mekong River – ecosystem health (characterised byeutrophication, toxicity due to dissolved oxygen and toxicants, and ecosystem processes), fishmigration and human health (drinking, recreation) - have been assessed. The risk to irrigationwater quality was also assessed for the third transboundary issue above.

Risk assessment is concerned with estimating the likelihood or probability of an undesiredevent occurring and the consequences if that event does occur. The risk assessment processseeks to:

• identify the key (ecological) issues and key stressors;

• identify the linkages between the key stressors (drivers) and each ecological consequence(conceptual model or quantitative ecological model), and from this provide informationon which drivers are most sensitive to management or controls;

• assess the risks associated with each issue as quantitatively as possible (it is importanthere to identify measurable end points for each issue);

• identify (and where possible quantify) all major uncertainties so the decision maker candecide on the confidence that should be placed on the final assessment;

• assist in establishing a robust monitoring & assessment program;


Unfortunately, the data currently available is inadequate to fully assess the risk oftransboundary water quality issues in the Mekong River basin. Assessment of the currentdatabase identified the following deficiencies:

• Physico-chemical data – the Mekong water quality network is similar to many other suchnetworks around the world in that it is collecting inadequate data. For example, many ofthe indicators currently being measured are inappropriate and should be replaced withmore appropriate indicators. Additionally, samples are being collecting at inappropriatesite locations and, for a number of indicators, at inadequate frequency. In all cases, thesampling design was such that there was essentially no statistical power in the data todetect any significant transboundary changes.

• Toxicant data – the pesticide and heavy metal data were either non-existent orinsufficient to be used to assess transboundary or basin-wide toxicity issues.

• Biological data – there is no on-going biological monitoring program for the MekongRiver. In the time available we were able to access only a small amount of biological datarelevant to the Mekong, which would include fish, macroinvertebrates, algae andecosystem processes. Efforts should be made to collect all published and unpublishedinformation on the biology and ecology of the Mekong River and its tributaries, and toprepare a synthesis of this information that summarises current knowledge in this area.



• Urban contaminant loads – the loads of contaminants discharged from the urban centresof Vientiane and Phnom Penh are poorly known. We estimated likely loads in order tomake a preliminary assessment of the transboundary risks due to discharges. To improveon the very preliminary risk assessment reported here, a more detailed understanding ofthe wastewater systems in each city needs to be developed, and both the quantity andquality of the wastewater discharges needs to be determined.

A summary of the preliminary risk assessment for the three transboundary water qualityissues is given in the table below. The present risks are low in all cases where they could beassessed. It was possible to undertake a reasonably quantitative assessment to assess therisks from eutrophication and the adverse effects of low dissolved oxygen concentrationscaused by wastewater discharges from Phnom Penh. However, for the other effects we wereforced to assess risk on the basis of either a comparison of the loads of contaminantsdischarged from Phnom Penh and Vientiane with those transported “naturally” by theMekong River, or the degree of dilution achieved on discharge of the wastewater.

Issue Effect Issue 1(Phnom Penh)

Issue 2(Vientiane)

Issue 3(Delta)

Ecological

Eutrophication

Toxic effects2

Ecosystem function3

Algal bloomsFish/invertebrate killsTo be determined

Low-moderate risk1

Low risk

Not assessed

Low risk

Low risk

Not assessed

Low risk

Low risk

Not assessed

Fish migration4 Adverse effects on fishmovement upstream,downstream or ontofloodplains

Uncertain Uncertain, likelyto be low risk

Uncertain

Human health5

Drinking water

Recreation

Microbialcontamination causingsickness

Uncertain

Uncertain

Uncertain

Uncertain

Uncertain

Uncertain

Agriculture

Irrigation Increased salinity Low risk

1. More likely low risk since only nutrient concentration were used in the assessment; high turbidity and high flowwould also reduce the chance of algal problems.

2. Risks based on toxic effects due to low dissolved oxygen concentrations. It was not possible to assess toxicity due totoxicants (heavy metals, pesticides) because of the lack of data.

3. No information is available at present, but should be developed in the future.

4. Lack of data to make assessment, present water quality sampling network cannot provide the required information.

5. Lack of data to make assessment. Risk likely to be low-moderate due to large dilution (also expect significantmicrobial die-off during transport to Vietnam in case of Issues 1 & 3).

Another potential transboundary issue not covered in the objectives of this report, but whichappears to require assessment, is the apparent higher salinity (conductivity) in the river NamMun that drains the extensive agricultural region of northern Thailand.

While the assessment reported herein indicates that the transboundary risks due to waterquality are low, this is not the case for local effects. For Phnom Penh in particular, ourpreliminary assessment suggests that there are moderate to high risks of adverse ecologicaland human health effects in Chaktomuk, Tonle Sap River and the upper reaches of the



Bassac River. The relevant Cambodian authorities may wish to further investigate thissituation.

Analysis of the present water quality monitoring network showed that it is unable to detectany transboundary changes due to the discharge of wastewater from either Vientiane orPhnom Penh. The network design has insufficient statistical power to detect realistic changesin physico-chemical water quality. Additionally, since no biological indicators are measured,there is no possibility of detecting transboundary or basin-wide changes in ecosystem health.

Recommendations1: that MRC adopt the risk-based approach to assess transboundary and basin-wide

environmental and human health issues, and to prioritise the management actionsrequired to reduce the risk due to each important issue.

2: that the current review of the physico-chemical monitoring network consider inparticular the optimum location of sampling sites, the frequency of sampling, the needfor depth sampling in some cases, the indicators being analysed and the power of thedata collected to detect changes.

3: that MRC undertake a preliminary risk assessment to identify possible transboundaryor basin-wide toxicity or bioaccumulation problems due to organic contaminantsand/or heavy metals, and if problems are identified, the type of investigations(including monitoring) should be undertaken to better characterise the risk.

4: that the MRC establish a project to assess the feasibility of establishing a biologicalmonitoring program for the Mekong River basin. The following biota should beconsidered – fish, macroinvertebrates, algae and macrophytes.

5: that MRC collect all published and unpublished information on the biology andecology of the Mekong River and its tributaries, and prepare a synthesis of thisinformation that summarises current knowledge in this area.

6: that MRC obtain a more detailed understanding of the wastewater systems (includinginformation on the quantity and quality of the wastewater discharges) in the twomajor urban centres – Vientiane and Phnom Penh.

7: that MRC establish a project to undertake a more detailed assessment of thetransboundary ecological and human health risks due to the discharge of wastewaterfrom both Phnom Penh and Vientiane. Such a project would provide an idealopportunity to “train” relevant National Mekong Committee members in the riskassessment methodology.

8: that MRC establish a project to investigate the key ecological processes occurring inthe Mekong River basin, including those associated with deep pools in the MekongRiver mainstream. The objective of this project should be to develop a number ofsensitive ecosystem process indicators that can be used to assess the ecological“health” of the Mekong River.

9: that MRC develop a new and more robust environmental assessment programdesigned to identify and assess the risks from a broader range of current and futuretransboundary and basin-wide issues. The process to achieve this new assessmentprogram should be done in collaboration with the member countries and wouldinvolve:



• using the (ecological) risk assessment process to underpin the process, with thefirst task being to scope the full range of existing and possible futuretransboundary issues (and their priority);

• running a number of workshops (involving each country) to develop the conceptualmodels and decide upon the target areas, the assessment endpoints and the bestindicators to measure;

• undertaking a program of short-term, targeted investigations to provide essentialinformation on specific aspects of the system that will enable the main program tobe better designed;

• preparing a full program proposal, obtaining funding and implementing theprogram.



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1

Appendix A:

Stormwater and Wastewater Pollutant Load Estimates for PhnomPenh and Vientiane

Associate Professor Tony H F WongDepartment of Civil EngineeringMonash University, Melbourne, Australia

IntroductionEstimates of the mean annual pollutant loads of TSS, TP, TN and BOD derived fromstormwater and wastewater were made for the cities of Phnom Penh and Vientiane.

Pollutant loads generated from urban stormwater runoff were estimated using a continuoussimulation model operating in a daily time step using daily rainfall records for the two cities(Pochentong – 1985 to 1994; Vientiane – 1990). In addition to the utilisation of local data,representative daily rainfall distribution for the two cities were selected from reference citiesin Australia (for which long term daily rainfall records are available) and adjusted for theirdifferences in mean annual rainfalls.

Local rainfall and the adjusted “Australian Reference City” long term rainfall records werethen applied to the urban stormwater quality model MUSIC (Model for Urban StormwaterImprovement Conceptualisation). The log-normal probability distributions of Event MeanConcentrations (EMC) of TSS, TP, TN and BOD derived from analysis of world data wereadopted in MUSIC and corresponding EMCs generated stochastically for each daily stormevent.

Pollutant loads generated from wastewater were estimated from published data normalised topopulation.

WastewaterWastewater contributions to the pollutant load discharge from the cities of Phnom Penh andVientiane were estimated from published data1 on loads per capita per day as follows:

• TSS = 700 mg/L/person with a range of 300 to 1200• TP = 12 mg/L/person with a range of 5 to 20• TN = 40 mg/L/person with a range of 15 to 90• BOD = 250 mg/L/person with a range of 100 to 40

The expected volume of wastewater generated daily per capital in Australian cities is between150 to 200 L/person/day and corresponding figure for USA cities is approximately300 L/person/day. For Phnom Penh and Vientiane, a wastewater rate of 200 L/person/day isrecommended in this report.

1 Sundstrom & Klei (1979), Wastewater Treatment, Prentice-Hall, NY.

2

StormwaterPollutant LoadsTSS, TP, TN and BOD concentrations in stormwater used in the estimation of stormwaterpollutant loads from the Phnom Penh and Vientiane were based on a comprehensive review ofstormwater quality in urban catchments undertaken by Duncan (1999)2. Analysis by Duncan(1999) found event mean concentrations of these water quality constituents to beapproximately log-normally distributed. Table 1 shows the collated Event MeanConcentrations (EMC) for a range of stormwater quality constituents and their standarddeviations (in log domain).

Table 1: Mean ± one standard deviation EMC values from analyses of worldwide databy Duncan (1999) – analyses carried out in the log domain

Water quality parameter Unit Number of data points and mean EMC (and lowerand upper value of first standard deviation EMC)

All data Rainfall < 550 mm

Total Suspended Solids (TSS) mg/L 247 150 (51 – 460) 19 420 (130 – 1300)

Total Phosphorus (TP) mg/L 206 0.35 (0.15 – 0.84) 13 0.61 (0.28 – 1.3)

Total Nitrogen (TN) mg/L 139 2.6 (1.4 – 5.1) 13 4.8 (3.2 – 7.4)

Chemical Oxygen Demand (COD) mg/L 165 80 (36 – 180) 16 170 (92 – 300)

Biological Oxygen Demand (BOD) mg/L 127 14 (7.2 – 26)

Rainfall Characteristics

Monthly rainfall records for Phnom Penh (1985 to 1993) and Vientiane (1950 to 2000) wereavailable and the computed mean annual rainfalls in Phnom Penh and Vientiane are 1301 mmand 1635 mm respectively. The mean annual rainfall in Vientiane corresponding to the period1985 to 1993 (ie. period of record for Phnom Penh) was 1444 mm (compared to the long-termmean annual rainfall of 1635 mm) suggesting that the long-term mean annual rainfall forPhnom Penh, estimated from the 1985 to 1993 record, may be under-estimated by 10% to15%.

As indicated above, daily rainfall records (Pochentong) for a 10 year period between 1985 to1994 (mean annual rainfall of 1295 mm) were available for pollutant load estimation fromPhnom Penh. For Vientiane, daily rainfall for 1990 alone, corresponding to an annual total of1498 mm, was available for pollutant loading export modelling.

2 Duncan, H.P. (1999), Urban Stormwater Quality: A Statistical Overview, Report 99/3, CooperativeResearch Centre for Catchment Hydrology, February 1999.

3

Comparisons of the mean monthly rainfall distributions for Phnom Penh and Vientiane withthat for capital cities in Australia found the most appropriate reference cities to be Darwin andPerth respectively. Figures 1 and 2 show the normalised monthly rainfall distributionsderived.

Figure 1: Cumulative distribution of Mean Monthly Rainfall (Vientiane Vs Perth)

Figure 2: Cumulative distribution of Mean Monthly Rainfall (Phnom Vs “Translated”Darwin)

4

Generated Stormwater Pollutant Loads

Continuous simulations were undertaken to generated urban stormwater runoff and associatedpollutant loads for a 1 km2 representative urban catchment with an assumed fractionimperviousness of 0.6. This is considered typical of Asian cities.

A stochastic routine was used to assign stormwater pollutant EMCs for each storm event suchthat the average and standard deviation of the EMCs over the simulation period is similar tothat listed in Table 1. The results from the use of local rainfall data as well as the “Australianrepresentative cities” were found to be of similar magnitude and a single set of mean annualloads was considered sufficient to represent both cities. Table 2 listed the mean annualpollutant loads expected from a 1 km2 catchment. Figures 3, 4, 5 and 6 show the cumulativefrequency distributions of daily pollutant loads estimated.

The estimated mean annual stormwater runoff volume for a 1 km2 urban catchment inVientiane and Phnom Penh approximately 1250 ML/km2/yr.

Table 2: Estimated Pollutant Loads from Urban Stormwater

Water Quality Constituents Mean Annual Load (kg/km2/yr)

TSS 300,000

TP 605

TN 3,900

BOD 24,000

5

Figure 3: Cumulative Probability Curve of Generated TSS daily load (kg/km2)

Figure 4: Cumulative Probability Curve of Generated TP daily load (kg/km2)

6

Figure 5: Cumulative Probability Curve of Generated TN daily load (kg/km2)

Figure 6: Cumulative Probability Curve of Generated BOD daily load (kg/km2)

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