INTEGRATED ASSESSMENTS OF POSSIBLE EFFECTS OF HYDROCARBON AND SALT WATER INTRUSION ON THE GROUNDWATER OF IGANMU AREA OF LAGOS METROPOLIS, SOUTHWESTERN NIGERIA

Abstract

Vertical electrical sounding (VES) and Induced Polarisation (IP) methods of geophysical survey were incorporated with physiochemical analysis of well water samples to determine vertical extent of petroleum-product contamination in subsurface soils and groundwater from bulk-fuel storage and distribution terminals in Iganmu area of Lagos. Interpreted results of VES and IP revealed four geoelectric layers. Clay with resistivity and IP values ranging from 1.5-14 Qm and 50 - 400mV/V respectively was encountered at the last layer penetrated by the survey except in four VES stations where the clay horizon was delineated at the third layer. This implies that subsurface aquifer is sealed by impervious layer which possibly prevents it from being contaminated by hydrocarbon and other refuse materials from the surface. Borehole log and electrical resistivity survey from a control site within the area were also incorporated with the geophysical measurements and these confirm lithologic similarity and the presence of a sealant above the aquifer layer. In addition to this, the results of the physical and geochemical analyses carried out on groundwater samples from shallow wells within the pack show very negligible level of hydrocarbon contamination which has no serious environmental implications on subsurface water in the area. However, electrical conductivity, salinity and TDS values obtained show high level of dissolved minerals (salts) making the water highly saline and unsuitable for drinking being far above recommended values for drinking water. We thus inferred that Lagos lagoon must have invaded the aquifer in some places leading to high salinity observed.

Full text

EARTH SCIENCES
RESEARCH JOURNAL
Earth Sci. Res. J. Vol. 14, No. 1 (June 2010): 100-110
INTEGRATED ASSESSMENTS OF POSSIBLE EFFECTS
OF HYDROCARBON AND SALT WATER INTRUSION
ON THE GROUNDWATER OF IGANMU AREA OF LAGOS
METROPOLIS, SOUTHWESTERN NIGERIA
Ayolabi, Elijah A., Folorunso, Adetayo F. And Obende Phillips W.
Department of Geosciences, University of Lagos, Lagos, Nigeria
Corresponding e-mails: eojelabi@yahoo.com, detayof@yahoo.com
ABSTRACT
Vertical electrical sounding (VES) and Induced Polarisation (IP) methods of geophysical survey were incorporated with
physiochemical analysis of well water samples to determine vertical extent of petroleum-product contamination in subsurface
soils and groundwater from bulk-fuel storage and distribution terminals in Iganmu area of Lagos. Interpreted results of VES
and IP revealed four geoelectric layers. Clay with resistivity and IP values ranging from 1.5 – 14 Wm and 50 – 400mV/V re-
spectively was encountered at the last layer penetrated by the survey except in four VES stations where the clay horizon was
delineated at the third layer. This implies that subsurface aquifer is sealed by impervious layer which possibly prevents it from
being contaminated by hydrocarbon and other refuse materials from the surface. Borehole log and electrical resistivity survey
from a control site within the area were also incorporated with the geophysical measurements and these confirm lithologic sim-
ilarity and the presence of a sealant above the aquifer layer. In addition to this, the results of the physical and geochemical anal-
yses carried out on groundwater samples from shallow wells within the pack show very negligible level of hydrocarbon
contamination which has no serious environmental implications on subsurface water in the area. However, electrical conduc-
tivity, salinity and TDS values obtained show high level of dissolved minerals (salts) making the water highly saline and un-
suitable for drinking being far above recommended values for drinking water. We thus inferred that Lagos lagoon must have
invaded the aquifer in some places leading to high salinity observed.
Key words:contamination, hydrocarbon, electrical resistivity, physiochemical, IP sounding.
RESUMEN
Los métodos de Sondeo Eléctrico Vertical (VES) y Polarización Inducida (IP) se incorporaron al análisis físico-químico de las
muestras de agua de pozo para determinar la extensión vertical de contaminación por petróleo en el subsuelo y las aguas
subterráneas desde los almacenamientos de combustible y terminales de distribución en Iganmu, en la zona de Lagos. La
interpretación de resultados de VES yIP revelaron cuatro capas geo-eléctricas. Arcilla con resistividad y valores IP que van
desde 1,5 hasta 14 Wm y 50 - 400 mV / V respectivamente, fue encontrada en la última capa alcanzada por el estudio, excepto
en cuatro estaciones de VES, donde se delineó el horizonte de arcilla en la tercera capa. Esto implica que los acuíferos del
100
Manuscript received: 05/03/2010
Accepted for publication: 28/05/2010
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subsuelo están sellados por una capa impermeable que posiblemente impide que sean contaminados por hidrocarburos y otros
materiales desechos de la superficie. Los registros de pozos y la resistividad eléctrica de un sitio de control dentro de la zona
fueron incorporados con las mediciones geofísicas y éstas confirman similitud litológica y la presencia de un sellador encima
de la capa acuífera. En adición, los resultados de los análisis geo-químicos y físicos llevados a cabo en muestras de agua
subterránea de pozos poco profundos dentro del área muestran un nivel insignificante de contaminación por hidrocarburos que
no tiene graves consecuencias medioambientales sobre el agua del subsuelo en la zona. Sin embargo, la conductividad
eléctrica, la salinidad y los valores TDS obtenidos muestran el alto grado de minerales disueltos (sales) haciendo que el agua
sea altamente salina y no apta para beber, ya que está muy por encima de los valores recomendados para ser agua potable. De
esta manera se infiere que la laguna de Lagos ha invadido el acuífero en algunos lugares dando paso a la elevada salinidad
observada.
Palabras clave: contaminación, hidrocarburos, resistividad eléctrica, físico-químico, sondeo por polarización inducida.
1. Introduction
Exploration geochemical methods may not be adequate in
environmental site characterizations. In fact, these methods,
developed and used by the petroleum industry have been de-
scribed as invaluable because most environmental consul-
tants and companies do not use them as the methods are
unfamiliar to environmental scientists and to most regula-
tory agencies (Agostino et al., 2002). Additional informa-
tion that must be integrated with analytical data to better
characterize a site of potential hydrocarbon contamination
include historical data, subsurface geology, hydrology and
geophysical data of environmental importance.
Historical data involve the history of activities that re-
late to the source of the contaminations; and a good under-
standing of the subsurface stratigraphy through which the
contaminants migrate. Hydrology is the scientific study of
water including occurrence, distribution in space and time
and its relation to people and the natural environment. Geo-
physical data sets would help to determine availability of
water in the subsurface strata; its occurrence, quality and
quantity.
Geophysical methods commonly use is the electrical re-
sistivity method which probes far to the subsurface giving
clue to the occurrence, or otherwise, of groundwater; and its
quality and quantity is determined from the measured resis-
tivity values. Though the determination is not in absolute
term but relative through the interpretation of inverted elec-
trical resistivity values compared with established resistivity
values of some common substances. Many authors attest to
the efficiency of vertical electrical sounding (VES) data not
only in exploration but also in the determination and map-
ping of groundwater quality, quantity and pollution
(Atekwana et al., 2003, Shevnin et al., 2005a & 2005b,
Ayolabi et al., 2009).
Additional electrical method employed in this type of
survey includes Induced Polarization (IP). The method is
based on the study of potential measurement observed in
geological formations when direct current is sent into them,
when the current is suddenly switched off, the potential dif-
ference observed between the measuring electrodes does not
vanished instantaneously but gradually dies down in course
of a few seconds or minute. This is the chargeability measure
in millisecond (ms) or in mV/V. For environmental study, it
is noted that saline water prevents IP because of its high con-
ductivity which does not allow for any ion accumulation
(Ayolabi et al., 2009). The data are subjected to qualitative
interpretation with reference to established chargeability of
various materials as shown in Table 1.
Table 1: Chargeability of Various Materials
Materials Chargeability
(MS)
Groundwater 0
Alluvium 1 – 4
Gravels 3 – 9
Precambrian Volcanics 8 – 20
Precambrian Gneisses 6 – 30
Schists 5 – 20
Sandstone 3 – 12
rightArgillites 3 – 10
Quartzite 5 - 12
Source: Murali and Patangay (2006)
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1.1 Background of the Study Area
Iganmu is located within Lagos metropolis. It is located be-
tween latitudes 6.475200to 6.475550N and longitudes
3.366400to 3.37020E (Fig 1). The pack is bulk-fuel storage
and distribution terminals where petroleum products busi-
ness transactions take place. Though a relatively small land-
mass, various activities that pose enormous environmental
challenges are carried out day in day out. These include stor-
ing fuel in surface hydrocarbon reservoir tanks, loading of
fuel by tankers (Lorries), rusting of abandon fuel trucks
(tankers), mixtures of black oil with other substances and
collection of refuse made of various materials dumped by
human being transacting one business or the other at the
pack (Fig 2).
Hydrocarbon pollution here occurred mainly at the sur-
face; however, this may find its way into the subsurface by
percolation through the soil profile in the absence of a seal-
ant such as clay or clayey layer. In addition, the site is
bounded by Lagos lagoon from where it derives salty water
intrusions. Residential and commercial properties are built
around the park. Drops of petroleum products mainly gaso-
line, kerosene, and heavy oil get to the surrounding build-
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AYOLABI, ELIJAH A., FOLORUNSO, ADETAYO F. AND OBENDE PHILLIPS W.Y
10°E
4°E
Niger Republic
Benin Republic
Atlantic Ocean
Lagos
FCT
Camerooms
N
6°N 14°N
2°30’E
2’30’E
0 20 km
6°30’N
Badagry
Atlantic Ocean
Ijanikin
Igando
Mushin
Surulere
Iganmu Badore
Lakowe
Lagos State
Akodo
Epe
Ikeja Ikorodu
Itaikin Eredo
N
Figure 1: Map Showing Iganmu within Lagos State
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ings from run-off during rain and from flow mechanism of
the products. These are widely observed along drainage
paths in the study area.
1.2 Geology
Lagos lies within the Dahomey basin. The basin extends
from the eastern part of Ghana through Togo and Benin Re-
public to the western margin of the Niger Delta (Fig 3). It is
actually the eastern part of the basin that falls within Nigeria
territory and is thus referred to as Eastern Dahomey Basin.
Many workers have described the stratigraphy of the basin
extensively. Jones and Hockey (1964) put the oldest unit as
Abeokuta group which are Maastrichtian in age. However,
Omatshola and Adegoke (1981) divided the group into three
lithologic units: Ise formation made of conglomerates and
grits at the base overlain by coarse to medium grained sands
with interbeded kaolinite; Afowo formation composed of
coarse to medium grained sandstone with interbeded shale,
siltstone and claystone with the sandy facies being tar-bear-
ing while the shale is organic-rich (Enu, 1990); and the Cre-
taceous sediment composed of fine to medium grained
sandstone at the base, overlies by shale, siltstone with
interbeded limestone, marl and lignite called Araromi for-
mation.
Overlying Abeokuta group is Ewekoro formation made
of shaly limestone unit (Adegoke, 1977) the limestone is
highly fossiliforous (Jones and Hockey, 1964) assigned with
Paleocene age. Akinbo formation overlies Ewekoro being
composed of shale and clay sequence (Ogbe, 1972) of
Paleocene to Eocene age. Oshoshun formation is the next
overlying Akinbo formation. It comprises pale greenish grey
laminated phosphate and glauconitic shale which is Eocene
in age. Ilaro formation conformably overlies Oshoshun for-
mation and composes of massive yellowish, poorly consoli-
dated cross-bedded sandstone while the youngest
stratigraphy in the basin is the Benin formation also known
as coastal plain sands (Jones and Hockey, 1964). It consists
of poorly sorted sands with lenses of clays. The age is
Oligocene to Recent.
The exposed rock unit in the study area is the coaster
plain sand or Benin formation consists of poorly sorted
sands with lenses of clays. The sands are in part cross bed-
ded and show transitional to continental characteristics.
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(a) (b)
(d)
(c)
Figure 2: Various Activities in the Pack (a) Surface Reservoir Tanks (b) Fuel Tankers loading the products (c) Mixtures of black oil with
several other materials with brackish water on the ground surface (d) Collection of Refuse in the polluted area.
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1.3 Hydrology
The sand, sandy clay and clayey sand deposits of Oligocene
to Recent are the major aquifers in the study area. Individual
boreholes in residential places around the area serve as
sources of potable water for the citizens. Any unconfined
aquifer in the area could easily be infiltrated by the percolat-
ing solutions because of weak protective capacity of the
overlying layers. The area has a saline base whose source is
the Lagos lagoon that infiltrates into the nearby subsurface
strata. Unfortunately, with increasing activities of petroleum
dealers and indiscriminate dumping of refuse in the area, the
groundwater around the vicinity of the pack may be ren-
dered unsuitable for domestic purposes, except for deep
borehole overlain by impermeable layer.
2. Methodology Employed
A holistic method involving electrical resistivity and IP
soundings, study of well log of a drilled water borehole and
physicochemical analysis of two water samples from shal-
low wells within the study area was adopted. A control site
was also chosen. The site – National Theatre – is located on
latitudes 6.471730to 6.471780N and longitudes 3.370110to
3.370210E far away in southeastern side of the main study
area to ensure that it is free from any environmental chal-
lenges the study area might have been subjected to due to
various human activities. The idea was to enable compre-
hensive reports on the environmental condition of the
subsurface water, to ascertain if the hydrocarbon and other
anthropogenic activities at the surface have reflection on the
groundwater and for verification of results. Nine (9) VES
and IP soundings were established in the study area and ad-
ditional two (2) from the control site far from the study area
(Fig 4). The data were collected using ABEM SAS – 1000
Terrameter with schlumberger electrode configuration. A
maximum electrode separation, AB, of 300m was used ow-
ing to non-availability of space and the fact that the study fo-
cuses on shallow depth investigation of few meters below
the surface. For the IP sounding, it is a common practice to
measure it along with resistivity for correct interpretation of
field data (Murali and Patangay, 2006).
104
AYOLABI, ELIJAH A., FOLORUNSO, ADETAYO F. AND OBENDE PHILLIPS W.Y
Figure 3: Geological Map of Main Dahomey Basin (Modified after Billman, 1992).
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Borehole log of a deep well drilled in National Theatre was
obtained and studied alongside the two electrical soundings
conducted within the National Theatre compound for com-
parative study. The idea is to verify the lithologic description
from electrical resistivity and IP surveys since both the study
and control sites are from Iganmu area of Lagos metropolis.
In addition to these, two water samples were collected from
hand-dug wells within the area for physiochemical analysis
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Figure 4: Data Acquisition Map.
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to confirm, or otherwise, if the anthropogenic activities de-
scribed earlier have impacted negatively on water from both
shallow and deep (borehole) well of the area.
3. Results and Discussions
3.1 Electrical Resistivity and IP Results
Quantitative interpretation of the VES data was carried out
in stages: (1) Plotting and smoothing of the apparent resis-
tivity field data curve and removing the noise appropriately;
(2) curve matching the smooth curve on tracing paper using
two layer model master and the corresponding auxiliary
curves (Bhattacharya and Patra, 1968); (3) initial
geoelectrical model(thicknesses and resistivities) emerging
from the previous stage was prepared; and (4) entering the
geoelectrical model into the Vander Velpen (1988)
geoelectric modeling and inversion package. The iteration
was achieved using RESIST software at a minimum root
mean square error. Samples of the curves obtained after the
process are shown in Figure 5.
The IP data were equally plotted against the electrode
spacing and presented as sounding curves (Fig 5). Only
qualitative interpretation is needed here most especially to
differentiate between sand with saline water and clay forma-
tions which give the same signature in resistivity sounding
(Ayolabi et al., 2009)). Murali and Pantangay (2006) noted
that familiarity with the theoretical response over known
geometric bodies and searching for similar characteristics
features in the field data can help in detecting the location,
106
AYOLABI, ELIJAH A., FOLORUNSO, ADETAYO F. AND OBENDE PHILLIPS W.Y
Figure 5: Typical Curves for VES and IP in the Area.
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lateral extent and depth of causative body. This starts with
identifying the anomaly (usually of higher polarisability) on
the background of noise created by surfacial layers and other
formations.
Four (4) geoelectric layers were delineated from the in-
terpreted VES data. Results of the nine VES and IP
soundings were used complimentarily to arrive at the
lithological description inferred in this work. The top soil re-
sistivity and IP values vary from 18 – 244 Wm and 1 – 10
mV/V respectively with a thickness of 0.5 – 0.9 m. This
layer is very sandy having very low IP values allowing per-
colation of liquid or solution. The envisaged pollutant in the
area is a composite one from diverse sources and substances:
hydrocarbon products, refuse comprising waste, metallic
and nonmetallic substances, and other anthropogenically de-
rived materials. Hydrocarbon is an insulator but when occur
with saline water will have resistivity much lower than its
original resistivity (Halliburton, 2001). Hence the resistivity
measured in this work is not on the high side reflecting the
composite nature of the pollutant even though the fuel prod-
ucts littered the surface as shown in Fig 2 previously. Thus,
high resistivity values (133 and 244 Wm) of top soil in VES
stations 3 and 5 could be due to the hydrocarbon pollutant as
observed during the field survey.
The second layer is made of clayey sand having resistiv-
ity and IP values of 50 – 66 Wm and 10 – 100 mV/V respec-
tively, with a thickness range of 1.2 – 2.0 m; and sand bed
containing brackish water with resistivity range of 30 – 66
Wm and IP values of <20 mV/V. It is not unlikely that traces
of hydrocarbon and other pollutants are also encountered in
this layer being overlain by permeable (sandy) formation of
relatively thin thickness but the effect is not prominent from
the electrical resistivity sounding results, however, VES 8
shows traces of hydrocarbon with the resistivity value of 139
Wm. The third layer is interpreted as clay (1.5 – 14 Wm) and
sand formation containing brackish or saline water (4.6 – 9.5
Wm) with IP values of >60 mV/V and < 20 mV/V respec-
tively. The clay horizon in this layer is found to be sealant
for the underlying formations (VES 1,2,3,6 and 7, Fig 6) thus
preventing the aquifer in the area from surface contami-
nants, but may not prevent saline water from the nearby
Lagos lagoon, which flow into the surrounding subsurface
layers by osmosis rather than by percolation.
The last geoelectric layer comprises clay with resistivity
range of 3 – 9 Wm and >50 mV/V IP value and clean sand
bed containing fresh water having a resistivity range of 40 –
189 Wm and IP values ranging from 0 – 40 mV/V. Notewor-
thy, the VES stations where clean sand formation was en-
countered have clay overlying it thus preventing any
possible contamination from the surface, while other VES
stations have clay as the last layer penetrated by this survey.
It thus means that there may not be any significant contami-
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INTEGRATED ASSESSMENTS OF POSSIBLE EFFECTS OF HYDROCARBON AND SALT WATER INTRUSION
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NW
Key
Depth (meter)
Top soil
Sand
Clay
Resistivity in ohm-m
ves 1 ves 3 ves 4 ves 9 ves 8 SE
Depth (meter)
Key
Top soil
Sand
Clay
Resistivity in ohm-m
NE ves 2 ves 3 ves 4 ves 5 ves 6 SW
Figure 6: The Geoelectric Cross-Section from the Area
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nation of groundwater in aquifer deeper than 50 m (maxi-
mum depth probed in this study) in the area.
3.2 VES, IP and Borehole Log from
the Control Site
Two VES and IP data obtained from the control site was inter-
preted and juxtaposed with the well log of a producing water
borehole within the premises of the National Theatre. Four
geoelectric layers of relatively similar lithology were also de-
lineated here. It is worthy to note that the third layer is also
clay (1.7 – 3.5 Wm) which acts as a seal with high value of IP
(>70 mV/V). This is equally underlain by a sand of 44 – 125
Wm and <20 mV/V resistivity and IP values respectively. The
borehole log reveals that aquifer here is overlain by an imper-
vious clay layer confirming the findings of electrical sound-
ing. However, the borehole is deeper (above 160m) than the
depth probed by the resistivity soundings (Fig 7).
3.3 Hydrogeochemical Study
Two water samples collected from two hand-dug shallow
wells labelled A and B were analysed and the results shown
in Table 3. The pH is a measure of acidity and alkality of
water. The values obtained for the two wells fall very close
to the maximum permicile by WHO (2006). According to
Ezeigbo (1989), the water could be classified as alkaline
water. The electrical conductivity (EC) is a reflection of the
degree of dissolved matters in water. Chemically pure wa-
ter has a very low EC (Montgomery, 2002). Very high val-
ues of EC and chloride content from the two wells attest to
its high salinity and finally confirm saline water intrussion
from the nearby lagoon as delineated from electrical sound-
ing results.
The total dissolved solute (TDS) and hardness values
obtained were too high far above the minimum and maxi-
mum permicible levels proposed by WHO (2006). It also
suggests high level of dissolved solids and falls within saline
and very hard classes proposed by Todd (1980). Total hy-
drocarbon content (THC), oil and grease values observed
further butress the fact that there is moderate infilteration of
hydrocarbon into the shallow wells. However, it should also
be noted that these values are not above the permisible WHO
level. Worthy to note is the fact that THC, oil and grease con-
tents of well A are more than that of well B. This is possiblly
108
AYOLABI, ELIJAH A., FOLORUNSO, ADETAYO F. AND OBENDE PHILLIPS W.Y
Depth (meter)
Depth (meter)
Geoelectric Section
Borehole Log
Key
Top soil
Sand
Clay
Resistivity in ohm-m
Key
Sand (fine)
Sandy clay
Clay
Sand+Gravel
Sand (coarse)
ves 10 ves 11
Figure 7: Geoelectric Section and Borehole Log from the Control Site
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due to the proximity of well A to the pollution plume.
Whereas, pH, EC and TDS are higher in well B perhaps due
to it closeness to the lagoon. Thus, the result of
physico-chemical analysis also agrees with electrical
soundings that there is no significant hydrocarbon contami-
nation of groundwater in the area.
4. Conclusions
Geophysical and hydrogeochemical methods were used to
determine vertical extent of petroleum-product contamina-
tion in subsurface soils and groundwater from bulk-fuel
storage and distribution terminals in Iganmu area of Lagos
Metropolis. Electrical resistivity (VES) and IP results were
used to delineate four geoelectric layers. Clay layer with re-
sistivity and IP values ranging from 1.5 – 14 Wm and 50 –
400mV/V respectively was encountered either in the third or
fourth layer in all the VES points. This implies that
subsurface aquifer is sealed by impervious layer which pos-
sibly prevents it from being contaminated by hydrocarbon
and other refuse materials from the surface. Borehole log
and electrical resistivity survey from a control site within
Iganmu area also confirms lithologic similarity and the pres-
ence of a sealant above aquifer layer. A further step was
taken to ascertain if groundwater from shallow wells around
the pack has been affected in any way. Thus, physical and
chemical characterization of the water from two hand-dug
wells did indicate low to moderate -level of hydrocarbon, oil
and grease contents which has no significant environmental
implications. However, electrical conductivity, salinity and
TDS values obtained show high level of dissolved mineral
(salts) making the water highly saline. Thus, it is concluded
that the water found in the subsurface of the investigated
area are not suitable for human consumption.
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INTEGRATED ASSESSMENTS OF POSSIBLE EFFECTS OF HYDROCARBON AND SALT WATER INTRUSION
ON THE GROUNDWATER OF IGANMU AREA OF LAGOS METROPOLIS, SOUTHWESTERN NIGERIA
Table 3. Result of Physicochemical Analysis Water Samples from Well in the Area
Measured
Parameters
Values from Present Study WHO (2006) Standard
Sample A Sample B Min. Perm. Level Max. Perm. Level
pH 9.2 9.4 6.5(mg/L) 9.5(mg/l)
TDS (mg/l) 1720 1845 500(mg/L) 1500(mg/l)
EC (µS/cm) 3715 3975 400 1500
Hardness(mg/l) 900 200 30 200
Salinity 1.41 1.41 - -
Oil and Grease 0.08 0.01 - -
THC 0.01 0.004 - 0.1
Cl- (mg/l) 800 800 200(mg/L) 400(mg/L)
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AYOLABI, ELIJAH A., FOLORUNSO, ADETAYO F. AND OBENDE PHILLIPS W.Y
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