Modelling LNAPL Plume Breakthrough and Saltwater …AU J.T. 14(2): 119-130 (Oct. 2010) Technical...

AU J.T. 14(2): 119-130 (Oct. 2010)

Technical Report 119

Modelling LNAPL Plume Breakthrough and Saltwater Intrusion for a Coastal Site in the South-Western Nigeria

Oluwapelumi O. Ojuri and Samuel A. Ola

Department of Civil Engineering, Federal University of Technology Akure, Ondo State, Nigeria

E-mail:

Abstract

The study area has been subjected to various forms of soil, surface water and groundwater contamination, resulting from both the onshore and offshore crude oil exploitation activities.

Two-dimensional cross-sections where taken for three different orientations within the study area (model domain), to study the subsurface groundwater flow, density-dependent seawater intrusion and solute transport for the contaminants of concern, identified for the study area. Subsoil profile observed from existing borehole logs reveals a thick surficial clayey aquitard overlying the confined aquifer. The model was used to observe the breakthrough of both benzene and naphthalene (components of crude oil) at the underlying sandy aquifer, in different simulations. For the benzene, the concentration at the observation point reached 50% (0.5) relative concentration (0.5C/C0) within 10,000 days. This is the time it will take the advective front of the plume to impact the underlying aquifer if there was no effect of dispersion or sorption. The concentration of benzene at the observation point, reached 100% (1.0) relative concentration, within 4.0 × 108 days, due to the effect of hydrodynamic dispersion and sorption. It took the naphthalene plume, two orders of magnitude longer period to breakthrough due to retardation effects.

Keywords: Contamination, plume, aquifer, breakthrough, saltwater intrusion.

1. Introduction 1.1. Geomorphologic and Geological setting

The Niger-Delta plain consists of a series of old beach sand ridges with interdune depressions, both of which run parallel to the coastline. The beach sands are a product of the erosion, transportation and deposition of sediments by long shore drift. Results from ecological studies and borehole log, suggest an interbedding of sediments from continental fluviatile, brackish and marine environment. Generally the most important formations characterizing the sedimentary suite of the subsurface in the riverine areas and in the Niger-Delta are the Coastal alluvium, Coastal Plain sands (Benin Formations), Agbada formations and the very deep Akata formations. Generally the Niger-Delta, Quaternary deposits

are unconsolidated sediments saturated with near surface groundwater (Durotoye 1983).

Deltaic deposits of the tertiary age up to 12,000m thick in some places underlie the delta. It is still building even though accelerated erosion and flooding are taking place in many places (Ebisemiju 1985). 1.2. Environmental Fate of Organic Chemicals

Environmental Organic Chemicals are chemicals released into the environment as a result of human activities that affect human and ecosystem health at very low concentrations (i.e., ppm concentrations or lower); or natural (biogenic) organic substances that are useful as molecular markers of environmental processes. It is pertinent to understand and predict processes which govern the behavior and fate (phase transfer and reaction) of organic chemicals in the environment. The most

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important issues to study in environmental organic chemistry are (USEPA 1986):

• sources, sinks, exposure, transport and transformation;

• physicochemical properties of organic compounds;

• the holistic environmental distribution of organic chemicals using simple models.

1.3. Light Non-aquaeous Phase Liquids (LNAPL)

LNAPL is a convenient label for petroleum liquids in soils and groundwater. The acronym stands for Light Non-aqueous Phase Liquid. “Light” highlights the fact that petroleum liquids are (with a few minor exceptions) less dense than water. “Non-aqueous” highlights the fact that petroleum liquids do not mix with water (Fig. 1).

Fig. 1. Light non-aqueous phase liquid (LNAPL).

In more detail, LNAPLs are derived from crude oil. Common LNAPLs include fuels, lubricants, and chemical feed stock for manufacturing.

From an environmental perspective, key features of LNAPL include:

1) LNAPLs are typically found at the top of groundwater zones. The buoyancy of LNAPL in water inhibits LNAPL migration into the groundwater zone.

2) When combined, LNAPL and water do not mix. They are immiscible. The net result is that subsurface LNAPL and water share pore space in soils and rock impacted by LNAPL. This “sharing of pore space” limits the mobility of LNAPL and complicates its recovery.

Recognizing LNAPL releases as a problem involving multiple fluid phases in pore space is essential to developing effective solutions for LNAPL releases.

3) LNAPLs are composed of mixtures of organic molecules that are slightly soluble in water. Where LNAPL comes in contact with groundwater, trace to low percent concentrations of the organic compounds dissolve into it. This often results in exceedances of water quality standards close to releases. A benefit of low solubility is that loading to the environment is typically small and natural processes often attenuate contaminants of concern over small distances. A disadvantage of low solubility is that LNAPL can persist as a source of groundwater contamination for extended periods.

2. Development of Site Conceptual Model 2.1. Site Conceptual Model

The layout of the study area showing proximity to Agbabu (oil sand deposit), the traversing River Oluwa, the shoreline, Igbokoda (terminal harbor and the cross-section locations is in Fig. 2. It can be expected that hydrocarbon from Oil sand sediments though the river can impact the groundwater in the study area. Indiscriminate dumping of waste oil from the terminal harbor together with the influence of seawater is also a concern.

Fig. 2. Site layout (Ilaje/Ese-Odo LGA).

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Figures 3 and 4 show the simplified borehole logs on cross-section showing stratigraphy along A-A (NN-SS) and C-C (NW-SE) for the study area. The simplified borehole logs showing the profile of the porous media material properties were compiled from existing Ondo State Water Corporation borehole logs at, Aiyetoro, Agadagba, Ugbonla, Zion Pepe, and Bolowo Zion. The cross-sections reveal a less permeable stratum of siltyclay overlying a more permeable sandy formation.

Fig. 3. Simplified borehole logs on cross-section showing stratigraphy along A-A for the study area NN-SS (Source: ODSWC 1991).

Fig. 4. Simplified borehole logs on cross-section showing stratigraphy along C-C for the study area NW-SE (Source: ODSWC 1991).

2.2. Contaminants of Concern/Potential Receptors

Water quality standards reflect the level of concern about specific contaminants: some, like iron (Fe) are only a nuisance; whereas, others are toxic or carcinogenic. In addition, the properties of other chemicals, which are of little concern from a water quality perspective, may be useful in defining pathways for contaminant movement. Benzene, toluene, ethlybenzene and xylene (BTEX) are the organic (hydrocarbon) priority pollutants from crude oil and its products (Bekins et al. 2002).

The solubility of benzene, 1,780 mg/L, is much more than the established drinking water limit (DWL) of 5 µg/L. Naphthalene, a polycyclic aromatic hydrocarbon (PAH), is also a component of crude oil considered to be of concern. Naphthalene, though of lower solubility than benzene and non-carcinogenic, is a major constituent of oil sands (bitumen). Acute toxicity is rarely reported in humans, fish or wildlife, as a result of exposure to low levels of a single (PAH) compound. PAHs in general are more frequently associated with chronic risks. These risks include cancer and often are a result of exposures to complex mixtures of chronic-risk aromatics (such as PAHs, alkyl PAHs, benzenes, and alkyl benzenes) rather than exposure to low levels of a single compound (Irwin et al. 1997). The high chloride and total dissolved solids concentration in borehole and surface waters due to the impact of saline waters is also a cause of concern.

Communities along the shoreline of the study area are potential receptors of surface and groundwater contaminants. During the dry season, people have to travel by canoes to many kilometres away from the shoreline to collect water in areas not polluted (ODSWC 1991). Water supply wells also exist at Agadagba, Ugbonla, Aiyetoro, Bolowo Zion, Zion Pepe, Agerige and Atijere. Water supply wells at Aiyetoro, Agerige, and Atijere were abandoned due to salinity and excessive iron compound contents (ODSWC 1991).

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2.3. Principles of Migration and Fate of Contaminants

Contamination can come from a variety of sources identified in section 2.1 above. It is important to evaluate the factors that control the migration and fate of these contaminants in the subsurface. The important processes are:

• fluid flow (advection) • dispersion and diffusion (hydrodynamic

dispersion) • adsorption/desorption • precipitation/dissolution • chemical and microbiological

transformation. The processes of hydrodynamic

dispersion, sorption, precipitation and chemical/microbiological transformation usually serve as sinks (or attenuation processes) for dissolved contaminants as it migrates with flowing groundwater.

3. Numerical Flow/Solute Transport Model Development 3.1. Code Selection and Description

3.1.1. HydroGeoSphere: The computer program package used for the numerical model is HydroGeoSphere. It is a groundwater flow and solute transport simulation package developed by the Groundwater Simulations Group, Waterloo Centre for Groundwater Research, University of Waterloo (Therrien et al. 2004). HydroGeoSphere is a three-dimensional numerical model describing fully-integrated subsurface and surface flow and solute transport. Hydrosphere is a unique and ideal tool to simulate the movement of water and solutes within watersheds in a realistic, physically-based manner. The HydroGeoSphere code was designed to handle more complex field problems in an efficient and robust manner. It’s finite element numerical approximation technique, fully-coupled analysis and compatibility with advanced visualization tools gives it several advantages over FRAC3DVS, Visual MODFLOW Pro and MODFLOW(USGS), similar software from WHI and USGS, respectively. The choice of the code

HydroGeoSphere was also necessitated by the need to simulate density-dependent seawater intrusion in the area of case study. HydroGeoSphere is based on a rigorous conceptualization of the hydrologic system, comprising surface and subsurface flow regimes with the interactions of the ground and surface waters. The model is designed to take into account all key components of the hydrologic cycle. For each time step, the model solves surface and subsurface flow and mass transport equations simultaneously and provides complete water balance and solute budgets.

The HydroGeoSphere code was used to model groundwater flow, density-dependent seawater intrusion and transport of organic contaminants [benzene (BTEX) and naphthalene (PAH)] in the subsurface for the area of case study. The numerical model was used to study the conceptual model of the study area. The boundaries of the numerical model coincide with natural hydrogeologic boundaries to minimize the influence of artificial model boundaries on simulation results. Model calibration involved adjusting model parameters to obtain a reasonable match between observed and simulated output variables. Patterns of groundwater flow and contaminant flux were predicted by the model with a view to integrating these results into a remedial feasibility design. 3.2. Model Discretization and Grid Generation

Two-dimensional cross-sections where taken for three different orientations within study area (model domain), to study the subsurface groundwater flow, density-dependent seawater intrusion and solute transport for the contaminants of concern, identified for the study area (section 2.2). The numerical model is essentially to test the site conceptual model. The layout of the cross-section locations can be seen in Fig. 5.

Numerical modelling offers the ability to quantify the flux of groundwater and contaminants as it travels between its various source areas and ultimate points of discharge, and to potentially explore a variety of alternative scenarios for future remediation or

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land-use changes. The overall representation of this area in the numerical model derives directly from the geologic and hydrologic characterization presented in the conceptual model report, and thus the numerical results will be reliable only to the same degree that the conceptual model is accurate.

Fig. 5. Location of cross-sections on study area.

Figure 6 illustrates the model grid for cross-section X1 (A-A), used for the groundwater flow analysis. The two-dimensional grid is approximately 8.5 km2, with a depth of about 230metres. The first step in the modelling exercise was to discretize the area using triangular elements. An automatic mesh generation scheme, ‘Grid Builder’ developed by Mclaren (2004) was used to produce the two-dimensional finite element mesh shown in Fig. 6.

Fig. 6. Finite element mesh for cross-section X1 (A-A), showing finer mesh for the upper layer.

For each of the nodal points in the mesh, the model requires values of initial hydraulic head, initial saturated thickness, recharge rate and hydraulic conductivity. Cross-sections were taken from a digital elevation model (DEM), developed for the area, from spot height elevation data for forty points, extracted from topographic maps of the area. The initial hydraulic head distribution was estimated from the river elevations and was essentially a plane, which sloped from a height of 12, 33 and 10.5 metres above sea level (MASL) at the north end (upland) to 0 at the south (Atlantic Ocean), respectively for A-A, B-B, and C-C respectively.

Cross-section X1 (A-A) contains 46,430 elements and 23,662 nodes. Grid generation for cross-sections X2 (B-B) and X3 (C-C) were also done in a similar manner. Cross-section X1 was used to simulate steady state groundwater flow, X2 was used to simulate transient density-dependent seawater intrusion and X3 was used to simulate solute transport for benzene (BTEX) and naphthalene (PAH). 3.3. Boundary Conditions

Nodes on the outer boundary require a boundary condition, which can be a specified hydraulic head, specified groundwater flux (recharge or discharge) or a specified concentration/mass flux (solute transport), which will remain constant thereafter.

The rivers form a natural boundary condition for the model, and each boundary node is assigned a constant head equal to the river elevation at that point as determined from topographic maps of the area. The left hand side of the cross-section model representing the oceanfront was assigned a constant head of zero value, and right hand side (RHS) boundary was assigned a value of head equal to the river top elevation, which forms the RHS boundary. The base of the cross-section taken to be relatively impermeable clay is taken to be a no-flow boundary. For the density-dependent transport (cross-section X2) a specified relative chloride concentration of 1.8 (representing 1800mg/l chloride concentration) was assigned to the oceanfront, and a relative chloride concentration of 0 was assigned to RHS

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(Upland) boundary. An initial chloride concentration (1.8) was assigned to elements within the Ghyben-Herzberg, zone of saltwater based on the Ghyben-Herzberg approximation of the position of the freshwater-saltwater interface (Fig. 7).

Fig. 7. Cross-section X1: showing material property zones.

The boundary condition for the solute transport simulation using cross-section X3, form Igbokoda area to the Atlantic ocean, was a specified relative concentration of 1.0 at the RHS (upland) boundary, for BTEX and naphthalene. 3.4. Hydrostratigraphy and Hydraulic Parameters

Material property zones within the model domain were delineated based on conceptual stratigraphy in Figs. 3 and 4. A more permeable alluvial sand is sandwiched between less permeable silty and clayey sand as shown in Fig. 7. The base is relatively impermeable clay. The model requires input of hydraulic parameters such as the hydraulic conductivity, infiltration or recharge rate and hydraulic head. Hydraulic conductivity and hydraulic head values were inputted into the model. General material types provided the basis for the estimates of hydraulic conductivity values. There were no data of sufficient quantity or quality to allow estimates based on measured values. Literature for similar material types (Freeze and Cherry 1979) provided order-of-magnitude estimates of the hydraulic conductivity.

The initial estimate for each of the three stratigraphic units considered was determined based on the porous media material property. An initial estimate of 0.0864 m/day (isotopic) was used for the silty sand and 8.64 m/day (isotropic) was used for the sand (Freeze and Cherry 1979). Material properties used for cross-sections X2 (B-B) and X3 (C-C), include the incorporation of the hydrodynamic dispersion parameters, transverse and longitudinal dispersion coefficients. Values of the dispersion coefficients are from field-scale dispersion experiments for similar material types, after Gelhar et al. (1992). 3.5. Model Calibration

By assuming that surface water bodies were expressions of the water table, and utilizing the available depth to water table map, approximate hydraulic head contours for the study area was developed (Fig. 8). This was used as a target for model calibration. Model calibration entails adjusting model parameters to obtain a reasonable match between observed and simulated output variables. The adjustable input variables are termed calibration parameters and the output variables that are compared to observed data are termed calibration targets. The calibration parameter for the study area is essentially the hydraulic conductivity values. The model calibration used an iterative process that begins with a relatively broad range of possible hydraulic conductivity values.

Fig. 8. Cross-section X1: showing hydraulic head and streamtrace.

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This range, given in Table 1, is derived from qualitative information (Freeze and Cherry 1979). The model was first run to steady-state for each of the simulations for the three cross-sections, and then the final hydraulic heads distribution were used as the initial condition for a second run. It was essential to get satisfactory groundwater flow solution, compatible with observed values before running the transient mode of the simulations for X2 and X3 (used for density-dependent and solute transport). Table 1. Initial estimates and calibrated values for hydraulic conductivity. Initial Range

(m/day) Calibrated Value (m/day)

1. Silty/Clayey Sand(alluvial)

8.64 × 10-3 to 86.4

Kxx = 8.64 × 10-1 Kyy = 8.64 × 10-2 Kzz = 1.0 × 10-20

2. Sand (alluvial)

8.64 × 10-2 to 864.0

Kxx = 86.4 Kyy = 8.64 Kzz = 1.0 × 10-20

4. Model Results and Analysis 4.1. Groundwater Flow

The pattern of groundwater flow suggested by the conceptual model (section 2), is a flow towards the Atlantic Ocean in the southern part of the study area, and also a flow towards the south eastern low elevation around Awoye and Jirinwo within the study area. The groundwater flow simulation for cross-section X1 (A-A), with the assigned boundary conditions has yielded a flow/hydraulic head pattern illustrated in Fig. 8.

The stream traces show major discharging of the groundwater towards the ocean and minor discharge at the rivers. It also reveals middle material zone (Fig. 7) as a preferred path for the groundwater flow. The hydraulic head distribution varies from a value of 0 at the oceanfront to 12 metres at the upland boundary for the cross-section. 4.2. Density-dependent Transport

The seawater intrusion was simulated using cross-section X2, which passes through the lowest elevation area in the study area, adjacent to the Atlantic Ocean. Pressure head

input was used in the input ‘grok’ file, for the Hydrosphere code in order to be able to simulate density-dependent seawater intrusion. The pressure head distribution for the initial steady-state run is shown in Fig. 9. This distribution was then used as an initial condition for the transient seawater intrusion modeling.

Fig. 9. Pressure head distribution: cross-section X2.

The distribution of the pressure head shows a decrease in pressure head down gradient towards the ocean, with intermediate discharge points at the rivers before reaching the ocean. Low pressure head values were used for the simulation, which is representative of the dry season hydraulic head distribution for the area. The problem of seawater intrusion is usually at a climax during the dry season in the study area. Figure 10 shows the material property zones used for cross-section X2.

Fig. 10. Material property zones: cross-section X2.

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The result of the transient run for the simulation (density-dependent transport) for 6 days is in Fig. 11 showing the commencement of movement of the chloride front, from the ocean to the fresh water zone. A specified relative chloride concentration of 1.8, for the seawater can be seen interfacing with the fresh water, based on an initial position prescribed after Drabbe and Badon-Ghyben (1889) and Herzberg (1901). The model was run to equilibrium for 600,000 days to determine the position of the saltwater freshwater interface, and extent of the mixing (brackish) zone. Figures 12 to 14 show the advancement of the chloride front from 60 to 6000 days. Figure 15 shows the 600,000 days simulation, which reveals equilibrium position of saltwater-freshwater interface and the extent of the mixing zone. Removing the effect of vertical exaggeration the real pattern and position of the interface can be seen (Fig. 16).

From the analysis in Table 2 to measure the width of the mixing zone, a value of about 8.8 km for the top and middle portion of the cross-section. A value of about 9.5 km was obtained for the bottom portion of the cross-section. Figure 17 shows the pattern of the velocity vectors within the flow field for density-dependent transport. Freshwater can be seen discharging, close to the saltwater-freshwater interface. The freshwater discharge zone is narrower than the case in Fig. 9, where there is no saltwater wedge.

Fig. 11. Position of the chloride front - 6 days (cross-section X2).



Fig. 14. Position of the chloride front – 6,000 days (cross-section X2).

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Fig. 15. Position of the chloride front - 600,000 days (cross-section X2).

Fig. 16. Position of the chloride front - 600,000 days, with minor vertical exaggeration (cross-section X2).

Fig. 17. Velocity vector pattern showing freshwater discharge close to the shoreline.

Table 2. Extent of the saltwater-freshwater interface (mixing zone) for the area.

Chloride X (m) Y (m) 1.6 124.3 -11.6 0.1 8877.8 -11.6

Top

∆X 8753.5 1.6 124.3 -101.8 0.1 8916.5 -101.8

Middle

∆X 8792.2 1.6 46.8 -220.8 0.1 9536.4 -220.8

Bottom

∆X 9489.6 4.3. Contaminant Solute Transport

Cross-section X3 (C-C) (Fig. 5) from Igbokoda area to the oceanfront was used to simulate the dissolution of organic contaminants sitting as a pool or entrapped in river bottom sediments. Benzene (representing BTEX group) and naphthalene (representing PAH group) were used for the simulation. The steady-state flow field for cross-section X3 and the material Property zones are in Figs. 18 and 19. Essentially the simulation is to model the movement of the contaminants through the covering less permeable strata to the underlying more permeable formation which is conceived to be a groundwater supply source. A specified relative concentration of 1.0 was emplaced, as a source on a portion of the upper boundary, and the model was used to observe the breakthrough of both benzene and naphthalene at the underlying sandy aquifer, in different simulations.

Fig. 18. Flow field for cross-section X3.

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Fig. 19. Material property zones for cross-section X3.

Chemical parameters inputted into the model were, the diffusion coefficients (Dd) and the distribution coefficients (Kd) derived based on the fraction of organic carbon (foc) of the soil. Figure 20 shows the velocity vector plot for the flow field in Fig. 18.

Fig. 20. Velocity Vector Plot for the flow field in cross-Section X3.

The result of the simulation for benzene with a lower distribution coefficient than naphthalene for 6 to 10,000 days is shown in Fig. 21.

Observation point was created in the model at a point 30 metres below ground surface for the plume. Figure 22 shows the plume for 8 × 106 days and 4 × 108 days.

For the benzene, the concentration at the observation point reached 50% (0.5) relative

concentration (0.5C/C0) within 10,000 days. This the time it will take the advective front of the plume to impact the underlying aquifer if there was no effect of dispersion or sorption (Freeze and Cherry 1979), where: C - observed concentration at time ‘t’ or in the output; and C0 - concentration at time ‘t0’ (at the input).

Fig. 21 (a & b). Six and 10,000 days of benzene plume (cross-section X3).

The concentration of benzene at the observation point, reached 100% (0.1) relative concentration, within 4 × 108 days, due to the effect of hydrodynamic dispersion and sorption. The result of the simulation for naphthalene, with a higher distribution coefficient for 6 and 10,000 days is presented in Fig. 23.

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Fig. 22 (a & b). 8 × 106 and 4 × 108 days of benzene plume (cross-section X3).

Fig. 23 (a & b). Naphthalene plume snapshot for 6 and 10,000 days (cross-section X3).

Fig. 24 shows the simulation results for 8 × 106 days and 4 × 1010 days. It has taken the naphthalene plume 600,000 days to reach 50% relative concentration at the observation point, and 4 × 1010 days to reach 100% relative concentration. It took the naphthalene plume, two orders of magnitude longer period to breakthrough due to retardation effects.

Fig. 24 (a & b). Naphthalene plume snapshot for 8 × 106 and 4 × 1010 days (cross-section X3).

5. Conclusion

From the results of the simulation for the fate and transport of organic contaminants, it can be suggested that the deep groundwater sources within the study area are relatively protected from the impact of open pool or entrapped hydrocarbon from the rivers/tidal rise. Naphthalene with low mobility and higher distribution coefficient than benzene is not supposed to be a treat. Benzene, however, which is carcinogenic can be a concern, since it has taken 10,000 days (27 years) for the advective front to break through the aquifer. This is because there are possibilities of holes and fractures in the overlying less permeable strata at certain points which can increase the

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pace of migration of the contaminant. It should also be noted that biodegradation can indeed attenuate the plumes of these organic contaminants.

The extent of the saltwater-freshwater interface (mixing zone) for the area was determined from the simulation of the saltwater intrusion (density-dependent transport) with the HydroGeoSphere computer code. From the analysis to measure the width of the mixing zone, a value of about 8.8 km for the top and middle portion of the cross-section, and about 9.5 km was obtained for the bottom portion of the cross-section.

Acknowledgements

The authors would like to acknowledge the funding by the Federal University of Technology, Akure, Ondo State, Nigeria, and the Department of Earth Sciences, University of Waterloo, Ontario, Canada, for the provision of computer facilities, software and a six months fellowship opportunity.

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Godsy, E.M. 2002. Microbial ecology of a crude oil contaminated aquifer. In: Groundwater Quality: Natural and Enhanced Restoration of Groundwater Pollution. Thornton, S.F.; and Oswald, S.E. (eds.). Proc. of the Groundwater Quality 2001 Conference, Sheffield, UK, 18-21 June 2001, pp. 57-63. International Association of Hydrological Sciences (IAHS) Publ. no. 275.2002, Wallingford, Oxfordshire, UK.

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Gelhar, L.W.; Welty, C.; and Rehfeldt, K.R. 1992. A critical review of data on field-scale dispersion in aquifers. Water Resources Research 28(7): 1,955-74.

Herzberg, A. 1901. Die Wasserversorgung einiger Nordseebäder. Journal für Gasbeleuchtung und Wasserversorgung 44: 815-9; 842-4.

Irwin, R.J.; van Mouwerik, M.; Stevens, L.; Seese, M.D.; and Basham, W. 1997. Environmental contaminants encyclopaedia, Naphthalene Entry. National Park Service, Water Resources Division, Water Operations Branch, Fort Collins, CO, USA. Available: .

Mclaren, R.G. 2004. Grid Builder - A pre-processor for 2D, triangular element, finite-element programs. Groundwater Simulations Group, Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario, Canada.

ODSWC. 1991. Water supply to the riverine areas of Ondo State. Ondo State Water Corporation (ODSWC), Almasol, Nigeria.

Therrien, R.; Panday, S.M.; Mclaren, R.G.; Sudicky, E.A.; Demarco, D.T.; Matanga, G.B.; and Huyakorn, P.S. 2004. HydroGeoSphere: A three-dimensional numerical model describing fully-integrated subsurface and surface flow and solute transport. Groundwater Simulations Group, Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario, Canada.

USEPA. 1986. United States Environmental Protection Agency, Superfund Public Health Evaluation Manual, EPA/5401/1-86/060, US Environmental Protection Agency, Washington, DC, USA.

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