AU J.T. 14(2): 119-130 (Oct. 2010)
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: <ojurip@yahoo.com; samuelola41@yahoo.com>
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.
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. 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
Technical Report
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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AU J.T. 14(2): 119-130 (Oct. 2010)
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.
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.
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. “Nonaqueous” highlights the fact that petroleum
liquids do not mix with water (Fig. 1).
2. Development of Site Conceptual Model
2.1. Site Conceptual Model
Fig. 1. Light
(LNAPL).
non-aqueous
phase
The layout of the study area showing
proximity to Agbabu (oil sand deposit), the
traversing River Oluwa, the shoreline,
Igbokoda (terminal harbor and the crosssection 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.
liquid
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.
Technical Report
Fig. 2. Site layout (Ilaje/Ese-Odo LGA).
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AU J.T. 14(2): 119-130 (Oct. 2010)
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.
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).
Fig. 3. Simplified borehole logs on crosssection showing stratigraphy along A-A for the
study area NN-SS (Source: ODSWC 1991).
Fig. 4. Simplified borehole logs on crosssection showing stratigraphy along C-C for the
study area NW-SE (Source: ODSWC 1991).
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AU J.T. 14(2): 119-130 (Oct. 2010)
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.
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 threedimensional numerical model describing fullyintegrated 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, fullycoupled 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
Technical Report
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, densitydependent 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 crosssection 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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AU J.T. 14(2): 119-130 (Oct. 2010)
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.
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).
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 twodimensional 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.
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
Fig. 6. Finite element mesh for cross-section
X1 (A-A), showing finer mesh for the upper
layer.
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AU J.T. 14(2): 119-130 (Oct. 2010)
(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).
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. 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-ofmagnitude estimates of the hydraulic
conductivity.
Technical Report
Fig. 8. Cross-section X1: showing hydraulic
head and streamtrace.
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AU J.T. 14(2): 119-130 (Oct. 2010)
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.
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 densitydependent and solute transport).
Table 1. Initial estimates and calibrated values
for hydraulic conductivity.
1. Silty/Clayey
Sand(alluvial)
Initial Range
(m/day)
8.64 × 10-3 to
86.4
2. Sand
(alluvial)
8.64 × 10-2 to
864.0
Calibrated
Value (m/day)
Kxx = 8.64 × 10-1
Kyy = 8.64 × 10-2
Kzz = 1.0 × 10-20
Kxx = 86.4
Kyy = 8.64
Kzz = 1.0 × 10-20
4. Model Results and Analysis
Fig. 9. Pressure head distribution: crosssection 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.
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
Fig. 10. Material property zones: cross-section
X2.
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
Technical Report
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AU J.T. 14(2): 119-130 (Oct. 2010)
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 saltwaterfreshwater 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 crosssection. 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 saltwaterfreshwater interface. The freshwater discharge
zone is narrower than the case in Fig. 9, where
there is no saltwater wedge.
Fig. 12. Position of the chloride front - 60 days
(cross-section X2).
Fig. 13. Position of the chloride front - 600
days (cross-section X2).
Fig. 14. Position of the chloride front – 6,000
days (cross-section X2).
Fig. 11. Position of the chloride front - 6 days
(cross-section X2).
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AU J.T. 14(2): 119-130 (Oct. 2010)
Table 2. Extent of the saltwater-freshwater
interface (mixing zone) for the area.
Top
Middle
Bottom
X (m)
124.3
8877.8
8753.5
124.3
8916.5
8792.2
46.8
9536.4
9489.6
Y (m)
-11.6
-11.6
-101.8
-101.8
-220.8
-220.8
4.3. Contaminant Solute Transport
Fig. 15. Position of the chloride front - 600,000
days (cross-section X2).
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. 16. Position of the chloride front - 600,000
days, with minor vertical exaggeration (crosssection X2).
Fig. 17. Velocity vector pattern showing
freshwater discharge close to the shoreline.
Technical Report
Chloride
1.6
0.1
∆X
1.6
0.1
∆X
1.6
0.1
∆X
Fig. 18. Flow field for cross-section X3.
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AU J.T. 14(2): 119-130 (Oct. 2010)
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. 19. Material property zones for crosssection 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.
Fig. 21 (a & b). Six and 10,000 days of
benzene plume (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
Technical Report
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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AU J.T. 14(2): 119-130 (Oct. 2010)
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. 22 (a & b). 8 × 106 and 4 × 108 days of
benzene plume (cross-section X3).
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
Fig. 23 (a & b). Naphthalene plume snapshot
for 6 and 10,000 days (cross-section X3).
Technical Report
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AU J.T. 14(2): 119-130 (Oct. 2010)
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.
Ebisemiju, F.A. 1985. Human impact on
marine processes in the Western Niger Delta
Nigeria. Proc. Policy Seminar on
Environmental Issues and Management in
Nigerian Development. Benin City, Edo
State, Nigeria, 25-27 November 1985.
Freeze, R.A.; and Cherry, J.A. 1979.
Groundwater. Prentice-Hall, Englewood
Cliffs, NJ, USA.
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:
<http://www.nature.nps.gov/hazardssafety/t
oxic/naphthal.pdf>.
Mclaren, R.G. 2004. Grid Builder - A preprocessor for 2D, triangular element, finiteelement
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.
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.
References
Bekins, B.A.; Cozzarelli, I.M; Warren, E.; and
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.
Drabbe, J.; and Badon-Ghijben (Ghyben), W.
1889. Nota in verband met de voorgenomen
putboring nabij Amsterdam (Notes on the
probable results of well drilling near
Amsterdam). Tijdschrift van het Koninklijk
Institut van Ingenieurs 1888-1889: 8-22.
Durotoye, B. 1983. Geomorphology and
quaternary deposits of Nigeria. In: Tropical
soils of Nigeria in engineering practice. Ola,
S.A. (ed.). A.A. Balkema, Rotterdam, The
Netherlands, pp. 1-16.
Technical Report
130