Application of Electrical Resistivity Imaging in Investigating Groundwater Pollution in Sapele Area, Nigeria

Full text

Journal of Water Resource and Protection, 2014, 6, 1369-1379
Published Online October 2014 in SciRes. http://www.scirp.org/journal/jwarp
http://dx.doi.org/10.4236/jwarp.2014.614126
How to cite this paper: Uchegbulam, O. and Ayolabi, E.A. (2014) Application of Electrical Resistivity Imaging in Investigating
Groundwater Pollution in Sapele Area, Nigeria. Journal of Water Resource and Protection, 6, 1369-1379.
http://dx.doi.org/10.4236/jwarp.2014.614126
Application of Electrical Resistivity Imaging
in Investigating Groundwater Pollution in
Sapele Area, Nigeria
Okezie Uchegbulam, Elijah A. Ayolabi
Department of Geosciences, University of Lagos, Lagos, Nigeria
Email: okezie_uchegbulam@yahoo.com, eojelabi@yahoo.com
Received 13 August 2014; revised 8 September 2014; accepted 2 October 2014
Copyright © 2014 by authors and Scientific Research Publishing Inc.
This work is licensed under the Creative Commons Attribution International License (CC BY).
http://creativecommons.org/licenses/by/4.0/
Abstract
Sixty-four multi-electrode Lund imaging system coupled with ABEM SAS 4000 Terrameter was
used for the electrical imaging of the study area. Wenner and Gradient arrays with 2 m minimum
electrode spacing were employed which revealed resistivity changes in the vertical and horizontal
directions along the survey lines. Earth imager software was employed for the processing and the
iteration of the 2-D resistivity data. The subsurface is characterized with soil material with resis-
tivity ranging from 42 - 15,000 Ohm-m, reflective of varying degree of conductivity associated with
changing lithology and fluid type. Correlation with borehole data shows that the first 10 m is
composed of laterite. While sand materials occupy 10 to about 60 m beneath the surface, with
anomalously high resistivity ≤ 15,000 Ohm-m in most parts. These high resistivity formations can
be attributed to the presence of hydrocarbon within the subsurface, which is an indication that
shallow aquifer in the study area has been polluted. The water level in the study area is close to
the surface, between 4 - 5 m. As a result of the high resistivity formations in most parts, deep wells
of about 45 m are recommended after geophysical investigations.
Keywords
Electrical Resistivity, Imaging, Pollution, Traverse Lines
1. Introduction
A groundwater pollutant is any substance that makes the water unclean or otherwise unsuitable for a particular
purpose when it reaches an aquifer. Sometimes, the substance is a manufactured chemical, but it might be mi-

O. Uchegbulam, E. A. Ayolabi
1370
crobial contamination just as often. Contamination also can occur naturally from occurring mineral and metallic
deposits in rock and soil [1].
Hydrocarbon pollution and contaminants constitute serious problems wherever exploration and exploitation
activities are carried out [2]. Sources of contaminants include field brines and oil spillage. Reference [3] re-
ported over ten oil producing communities in the Niger Delta region that have experienced major oil spills and
its negative environmental consequences such as fire, destruction of aquatic lives, water pollution, soil pollution
and devastation of the ecosystem. According to a report by the Directorate of Petroleum Resources [4], over
6000 spills had been recorded in the 40 years of oil exploitation in Nigeria, with an average of 150 per year. In
the period 1976-1996, 647 incidents occurred resulting in the spillage of 2369407.04 barrels of crude oil. With
only 549060.38 barrels recovered. 1820410.50 barrels were lost to the ecosystem. Other human activities such
as felling of trees, indiscriminate disposal of chemicals and refuse, flooding caused by the blockage of water
ways, etc. have equally led to the devastation of the ecosystem in the study area.
Geophysical methods have been shown to be useful to the study of contaminated zones. These methods are
based on the contrasts in several physical properties that typically make up the different constituents of the af-
fected zone. In general, hydrocarbons have much lower electrical conductivity than water. This fact makes the
resistivity method especially suitable for hydrocarbon contamination delineation [5] [6]. Hence, 2-D Electrical
Resistivity Tomography (ERT) which provides a relatively low cost, noninvasive and rapid means of generating
spatial models of physical properties of the subsurface is employed in this study.
Electrical Resistivity Tomography has been successfully applied by several researchers in the detection of
pollution [7] [8].
A serious environmental concern is the movement of pollutants to the water table and subsequent contamina-
tion of drinking water resources.
The behavior of the electrical resistivity of contaminants, with respect to the host environment, depends on
several factors, such as the host lithology, the moisture and the solubility of the contaminants in the groundwater.
They make the 2-D ERT effective in delineating the contaminated zones due to resistivity contrasts. Fresh or-
ganic compounds in the water saturated soils usually have high electrical resistivity values.
1.1. Background of the Study Area.
Sapele, located in Delta State (Figure 1), Western Niger Delta, Nigeria, lies between longitude 5˚E - 5˚45'E and
latitude 5˚30'N - 6˚N in geographic coordinate. The city hosts a flow station and an oil rig owned by one of the
oil and gas companies. Figure 1 shows the map of Delta State while Figure 2 shows the picture of a fared gas
taken during the survey.
Figure 1. Map of Delta state, showing Urhboland and major Rivers of Western Niger Delta: (map drawn by
Prof. Francis Odermerho, Southern Illinois University, Edwardsville; USA. Copyright: Urhobo Historical So-
ciety, 2008).

O. Uchegbulam, E. A. Ayolabi
1371
Figure 2. View of gas being flared at a flow station near the survey site in Sapele
(Field Survey, October 2012).
Oil and gas production is usually accompanied by substantial discharge of wastewater in the form of brines.
Constituents of brines include sodium, calcium, ammonia, boron, trace metals, and high total dissolved solids
(TDS). Oil spillage is a result of leakages of hydrocarbon from the pipes, and to an extent, poor maintenance of
oil pipelines and poor monitoring of pressure regimes of the fluids with respect to the strength of the pipe,
equipment failure, operators error, corrosion, pigging operations, flow line replacement, flow station upgrade
and the activities of vandals in search of crude and refined petroleum products [9].
As a consequent of the environmental problems highlighted above, 2-dimensional electrical resistivity imag-
ing was carried out in a residential area close to the flow station to see if contaminants have actually infiltrated
into the groundwater resources in the study area.
1.2. Site Geology and Hydrogeology
The Niger Delta is a large curve shaped delta which is located in Southern Nigeria like some other deltaic envi-
ronments in the world. It occupies an area lying between longitude 4˚E - 9˚E and latitude 4˚N - 6˚N. It is
bounded in the west by the Calabar flank, in the north by the Anambra platform and in the south by the Atlantic
Ocean under which it extends Figure 1 and Figure 3. Both marine and mixed continental depositional environ-
ment characterize the Niger Delta of Nigeria [10]. The Niger Delta covers an area of about 75,000 km2 (28,957
mi2) in southern Nigeria, where the Niger Delta discharges its water into the Atlantic Ocean through a series of
distributaries.
From the Eocene to the present, the Delta has prograded Southwest ward, forming depobelts that represent the
most active portion of the Delta at each stage of its development [11]. These depobelts form one of the largest
regressive deltas in the world with an area of some 300,000 km2 [12] a sediment volume of 500,000 km3 [13]
and a sediment thickness of over 10 km in the basin depocenter.
The Niger Delta consists of three main tertiary stratigraphic units overlain by Quaternary deposit [14] shown
in Table 1. These three subsurface stratigraphic units in the Niger Delta are Benin, Agbada and Akata forma-
tions. The base is the Akata formation comprising mainly of marine shale and sand beds consisting of dark grey
sandy, silty shale with plant remains at the top. It is over 4000 ft thick. The underlying Agbada formation is a
sequence of sandstones and shales [15]. It consists of an upper predominantly sandy unit with minor shale inter-
calations and a lower shale unit which is thicker than the upper sandy unit. It is over 10,000 ft thick. The aquifer
in the Benin formation is largely phreatic. These formations are overlain by various types of Quaternary deposits
[15]. These areas are made up of top soil, red laterite, clay, fine sand, medium sand and coarse sand in form of

O. Uchegbulam, E. A. Ayolabi
1372
Figure 3. The Niger Delta coastline of West Africa (Short and Stauble, 1967).
Table 1. Geologic units of the Niger Delta (Short and Stauble, 1967).
Geologic Unit Lithology Age
Alluvium (general) Gravel, sand, clay, silt
Quaternary
Freshwater backswamp and meander belt Sand, clay, some silt and gravel
Mangrove and salt water/backswamps Medium-fine sand, clay and some silt
Sombreiro-warri deltaic plain Sand, clay and some silt
Benin formation (coastal plain sand) Coarse to medium sand with subordinate silt and clay lenses Miocene
Agbada formation Mixture of sand, clay and silt Eocene
Akata formation Clay Paleocene
pebbles. The thickness is variable but generally exceeds 6000 ft [16].
The deposits of the Freshwater Swamps and the Sombreiro-Warri Deltaic Plain are universally considered to
be recent expressions of and a continuation of the Benin Formation. They result from the sediment laden dis-
charges of the River Niger that is spread on the delta by its various tributaries. The sediment is generally an ad-
mixture of medium to coarse-grained sands, sandy clays, silts and clays that eventually settle in fluvial/tidal
channel, tidal flat and mangrove swamp environments, a process that has been ongoing since the late Quaternary
and is related to interglacial marine transgressions [17]-[19]. The Niger Delta is one of the most hydrocar-
bon-rich regions in the world. Exploration and exploitation of hydrocarbons has been going on in the region
since 1956, when oil was discovered there [20]. The oil and gas production and a rapidly growing population
have resulted in environmental degradation of the Delta [21].
The study area consists of Fresh water swamp, Coastal Plain Sands, Mangrove swamps, and Sombreiro-Warri
plains [22]. The water table in the study area is approximately 4 to 5 m beneath the surface, and the direction of

O. Uchegbulam, E. A. Ayolabi
1373
flow is towards River Ethiope which drains into the Atlantic ocean through the Benin River (Figure 1).
1.3. Electrical Resistivity and Hydrocarbon Pollution
2-dimensional model, where the resistivity changes in the vertical direction, as well as in the horizontal direction
along the survey line was employed in the study area to image the subsurface. In this case, it is assumed that re-
sistivity does not change in the direction that is perpendicular to the survey line.
Geophysical methods are frequently used to study subsoil contamination caused by industrial residues of dif-
ferent nature. The effectiveness of electrical methods for the characterization of oil contaminated subsoil has
been reported by several researchers [23]-[26].
Geoelectric method (Resistivity method) has been used by several researchers to study environmental prob-
lems and groundwater studies. Reference [27] used Geoelectric method in the evaluation of Olushosun landfill
site Southwest Nigeria and its implication on Groundwater. Geophysical and hydrochemical assessment of
groundwater pollution due to a dumpsite in Lagos State was also conducted by [28].
References [9] [29] also found out that high hydrocarbon content of soils has been known to affect soil phy-
siochemical properties, which in turn affect the agricultural potentials of such soils. Hence, this work attempts to
find out if hydrocarbon has infiltrated into the groundwater in the study area.
Hydrocarbon pollution is very complex. Fresh spilled hydrocarbon gives different result from aged spills. The
following results from some researchers give clue on what to expect from the investigation in the study area.
Reference [30] in their work on “Two-dimensional electrical imaging for detection of hydrocarbon contami-
nants” pointed out the usefulness and the pitfalls of electrical tomography in the characterization of underground
leakage of hydrocarbons. Experimental evidence, obtained from a joint geochemical and geophysical investiga-
tion approach, indicated that subsoil which has been saturated with diesel oil for a long period (>20 years) exhi-
bits an increased conductivity. It suggests that electrical tomography could be useful for monitoring the effects
of induced biodegradation (bioremediation) through the repetition of the survey at different times, in order to
observe any changes in the resistivity due to the increase of free ions resulting from hydrocarbon degradation.
Electrical resistivity models for oil contamination in some area showed high resistivity [31] and low resistivi-
ty [32] [33]. Recent oil pollution shows a high resistivity anomaly, while mature oil pollution produces a low re-
sistivity anomaly [32]. Months after spill, a low resistivity anomaly is developed in the contaminated zone, with
a strength that depends on the geological characteristics of the subsoil [32] [34]. The formation processes of
such low resistivity anomaly are related to chemical reactions and to variations in the physical characteristics of
the oil contaminated zone. The low resistivity anomaly is caused by an increase in the total dissolved solids
(TDS), due to bacterial degradation of hydrocarbons in the lower part of the vadose zone. Reference [32] found
that aged contamination appears as a low resistivity horizon slightly above groundwater table (GWT).
2. Materials and Method
64 multi-electrode Terrameter was used for the electrical imaging with 2 m minimum electrode spacing. Each
traverse covered a lateral distance of 126 m. Three traverses were taken proximal to the oil rig, flow station and
oil pipelines (Figure 4). We could not get too close to the pipelines and other installations due to restrictions by
the oil companies and the presence of security forces monitoring the installations. The areas we took the tra-
verses were the only allowed areas.
Environmental geophysical surveys are concerned with near surface, typically to depths of less than 30 m. So
a small electrode spacing of 2 m was adopted, in order to be able to provide considerable details of any plumes
related to leakages from the underground pipes and flow stations. It is well known that the Wenner array is rela-
tively sensitive to vertical changes of resistivity below the centre of the array, and Gradient array exploit the ad-
vantages of Schlumberger and Wenner arrays. So, Wenner array and Gradient methods were chosen in the ac-
quisition of data because of good vertical resolution, less sensitivity to noise and better lateral coverage.
Computer iteration was carried out on the data obtained, using the Earth Imager software. This software si-
mulates the values of the apparent resistivity and that of the current electrode spacing to obtain a two dimen-
sional (2D) layered model. Consequently, resistivities and the depths of the layers were estimated. Borehole logs
(Figure 5 and Figure 6) of the study area collected from The Federal Ministry of Water Resources, Asaba were
used as an aid in the interpretation of the electrical resistivity tomography results. The elevation of the study area
is about 3 - 4 m.

O. Uchegbulam, E. A. Ayolabi
1374
Figure 4. Map of Sapele showing the traverse lines.
Figure 5. Borehole 1: lithologic description of the study area (Courtesy: Federal Ministry of Water Resources, Asaba).

O. Uchegbulam, E. A. Ayolabi
1375
Figure 6. Borehole 2: lithologic description of the study area (Courtesy: Federal Ministry of Water Resources, Asaba).
3. Results and Discussion
The results and discussion of the Electrical Resistivity Tomography of the study area is given below.
3.1. Traverse 1
Traverse 1 reveals the resistivity image along the profile. Figure 7(a) and Figure 7(b) show the Gradient and
Wenner array maps respectively.
Integration of the two 2-D ERT array results show that subsurface is characterized with soil material with re-
sistivity ranging from 42 - 15,000 Ω∙m reflective of varying degree of conductivity associated with varying li-
thology and fluid type. Figure 5 and Figure 6 are the borehole data used for comparing the interpretation of the
2-D ERT section.
The ERT result shows that the subsurface is composed predominantly of sand material from the surface to a
depth of about 60 m beneath the surface. The 2-D electrical resistivity imaging result (Figure 7(A)) show the

O. Uchegbulam, E. A. Ayolabi
1376
Figure 7. (a) Sapele (traverse 1) Gradient array; (b) Sapele (traverse 1) Wenner array.
resistivity distribution over a lateral distance of 126 m from the surface to a depth of about 21 m beneath the
surface. The 2-D section revealed an anomalously high resistivity (1500 - 15,000 Ω∙m) structure within a lateral
distance of 2 - 32 m; 36 - 40 m; 40 - 52 m; 58 - 63 m; 76 - 86 m; and 114 - 118 m at a depth of 0.5 - 9 m; 0.1 - 1
m; 5 - 18 m; 7 - 11 m; 8 - 10 m; and 0 - 2 m respectively. These anomalously high resistivity structures are at-
tributed to the presence of hydrocarbon within the subsurface which may be due to leakages from various pipe-
lines within the study area or possible activity of the vandals.
Correlation with the borehole data (Figure 5 and Figure 6) shows that the subsurface is characterized predo-
minantly with sand, and may compose of clayey sand/clay in some locations. The 2-D ERT result shows that the
resistivity within the depth of investigation ranges from 45 - 15,000 Ohm-m. Since the formation in the study
area is compose of sand with water table between 4 - 5 m beneath the surface, the high resistivity (≥1500 Ω∙m)
is a possible indication that the aquifer within this area may have been polluted by hydrocarbon.
3.2. Traverse 2
Figure 8(a) and Figure 8(b) show the Gradient and Wenner array 2D resistivity structures. The two resistivity
structures also show that the subsurface is equally compose of varying degrees of resistivity as can be seen from
the resistivity values (43 - 15,000 Ω∙m), revealing varying degree of conductivity associated with lithology and
fluid type.
The 2-D section also revealed an anomalously high resistivity (2000 - 15,000 Ω∙m) structure within a varying
lateral and vertical location distributed over the entire traverse. At a lateral distance of 2 - 58 m, possible hydro-
carbon pollution were noticed at a depth of 0.5 - 6 m, while at a lateral distance of 60 - 126 m the depth of pollu-
tion vary from 0.5 - 21 m beneath the surface.
Integration of the two array maps and correlation with the borehole data (Figure 5 and Figure 6) show that
the resistivity of the sand layer within the depth of investigation of the 2-D ERT ranges from 45 - 15,000 Ω∙m
and may compose of fine to coarse sand/clay in some location.

O. Uchegbulam, E. A. Ayolabi
1377
Figure 8. (a) Sapele (traverse 2) Gradient array; (b) Sapele (traverse 2) Wenner array.
3.3. Traverse 3
Traverse three reveal the resistivity image along the survey line. Figure 9(a) and Figure 9(b) show the Gradient
and Wenner array structures respectively.
Integration of the 2-D ERT array results show that the subsurface has soil materials with resistivity ranging
from 42.5 - 15,000 Ω∙m indicative of changing degree of resistivity associated with varying lithology and fluid
type. The 2-D section revealed an anomalously high resistivity (1500 - 15,000 Ω∙m) structure within a lateral
distance 2 - 16 m within a depth of 0.1 - 6 m reflective of near surface hydrocarbon pollution. Similar occur-
rence was noticed within a lateral distance of 62 - 126 at a depth of 0.1 to 4 m; however within a lateral distance
of 20 - 70 m and within 86 - 100 m the hydrocarbon pollution seems to have migrated to a depth of about 21 m
beneath the surface. This may be a clear evidence of groundwater pollution by the hydrocarbon since the water
table lies within 4 - 5 m within the study area.
4. Conclusion
The 2-D Electrical Resistivity Tomography results of the study area show that the subsurface is characterized
with soil material with resistivity ranging from 42 - 15,000 Ω∙m, reflective of varying degree of conductivity
associated with changing lithology and fluid type. Correlation with borehole data shows that the subsurface is
predominantly composed of sand material from the surface to a depth of about 60 m beneath the surface, with
anomalously high resistivity ≥1500 Ω∙m in most parts. This low conductivity formations can be attributed to hy-
drocarbon which is an indication that shallow aquifer in the study area has been polluted. It is recommended that
water from hand dug well should not be used for drinking and certain domestic work. Moreover, geophysical
investigation should be carried out in the study area before sinking boreholes.

O. Uchegbulam, E. A. Ayolabi
1378
Figure 9. (a) Sapele (traverse 3) Gradient array; (b) Sapele (traverse 2) Wenner array.
Acknowledgements
We thank the Federal Ministry of Water Resources, Asaba for providing us the borehole data of the study area.
References
[1] Boulding, J.R. (1995) Practical Handbook of Soil, Vadose Zone, and Ground-water Contamination: Assessment, Pre-
vention, and Remediation. Lewis Publishers, Boca Raton.
[2] Atakpo, E.A. and Ayolabi, E.A. (2008) Evaluation of Aquifer Vulnerability and the Protective Capacity in Some Oil
Producing Communities of Western Niger Delta, Nigeria. The Environmentalist.
http://dx.doi.org/10.1007/s10669-008-9191-3
[3] Ekoriko, M. and Egwu, E. (1995) Neglect of Oil Communities Rage of the People. Newswatch Magazine, 18 Decem-
ber 1995, 13.
[4] DPR (1997) Department of Petroleum Resources. Annual Reports, Abuja, 191 p.
[5] Fetter, C.W. (1993) Contaminant Hydrogeology. Prentice-Hall, Inc., Upper Saddle River.
[6] Reynolds, J. (1998) An Introduction to Applied and Environmental Geophysics. John Wiley & Sons Ed., New York.
[7] Daily, W., Ramirez, A. and Johnson, R. (1998) Electrical Impedance Tomography of a Perchloroethelyne Release.
Journal of Environmental and Engineering Geophysics, 2, 189-201.
[8] Goes, B.J.M. and Meekes, J.A.C. (2004) An Effective Electrode Configuration for the Detection of DNAPLs with
Electrical Resistivity Tomography. Journal of Environmental and Engineering Geophysics, 9, 127-141.
http://dx.doi.org/10.4133/JEEG9.3.127
[9] Ezebuiro, P.E. (2004) A Review of Effect of Oil Pollution in West African Environment. Science and Nature, 5, 14-18.
[10] Uko, E.D., Ekine, A.S., Ebeniro, J.O. and Ofoegbu, C.O. (1992) Weathering Structure of the East-Central Niger Delta,
Nigeria. Geophysics, 57, 1228-1233. http://dx.doi.org/10.1190/1.1443338
[11] Doust, H. and Omatsola, E. (1990) Niger-Delta. In: Edwards, J.D. and Santogrossi, P.A., Eds., Divergent/Passive Mar-
gin Basins, AAPG Memoir 48, American Association of Petroleum Geologists, Tulsa, 239-248.
[12] Kulke, H. (1995) Nigeria. In: Kulke, H., Ed., Regional Petroleum Geology of the World, Part II, Africa, America, Aus-

O. Uchegbulam, E. A. Ayolabi
1379
tralia and Antarctica, Gebruder Borntraeger, Berlin, 143-172.
[13] Hospers, J. (1965) Gravity Field and Structure of the Niger-Delta, Nigeria, West Africa. Geological Society of America
Bulletin, 76, 407-422. http://dx.doi.org/10.1130/0016-7606(1965)76[407:GFASOT]2.0.CO;2
[14] Short, K.C. and Stauble, A.J. (1967) Outline of the Geology of Niger Delta. American Association of Petroleum Geol-
ogists Bulletin, 51, 761-779.
[15] Merki, P.J. (1970) Structural Geology of the Cenozoic Niger Delta. African Geology, University of Ibadan Press, Iba-
dan, 251-268.
[16] Egbai, J.C. (2011) Resistivity Method: A Tool for Identification of Areas of Corrosive Groundwater in Agbor, Delta
State, Nigeria. Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS), 2, 226-230.
[17] Allen, J.R.L. (1965) Late Quaternary Niger Delta and Adjacent Areas: Sedimentary Environments and Lithofacies.
American Association of Petroleum Geologists Bulletin, 49, 547-600.
[18] Oomkens, E. (1974) Lithofacies Relations in the Late Quaternary Niger Delta Complex. Sedimentology, 21, 195-222.
http://dx.doi.org/10.1111/j.1365-3091.1974.tb02056.x
[19] Durotoye, B. (1975) Quaternary Sediments in Nigeria. In: Kogbe, C.A., Ed., Geology of Nigeria, Rockview, Jos,
431-444.
[20] Oloibiri (2014) The Community that Hosted Nigeria’s First Oil Well in 1956. http://www.premiumtimesng.com
[21] Obasi, R.A. and Balogun, O. (2001) Water Quality and Environmental Impact Assessment of Water Resources in Ni-
geria. African Journal of Environment Studies, 2, 228-231.
[22] Omo-Irabor, O.O. and Oduyemi, K. (2006) A Hybrid Image Classification Approach for the Systematic Analysis of
Land Cover (LC) Changes in the Niger Delta Region. Proceedings of the 6th Int’l Conference on Earth Observation
and Geoinformation Sciences in Support of Africa’s Development, Cairo, 30 October-2 November 2006.
[23] Vanhala, H., Soininen, H. and Kukkonen, I. (1992) Detecting Organic Chemical Contaminants by Spectral Induced
Polarization Method in Glacial Till Environment. Geophysics, 57, 1014-1017. http://dx.doi.org/10.1190/1.1443312
[24] Atekwana, E.A., Sauck, W.A. and Werkemer, D.D. (2000) Investigations of Geophysical Signatures at a Hydrocarbon
Contaminated Sites. Journal of Applied Geophysics, 44, 167-180. http://dx.doi.org/10.1016/S0926-9851(98)00033-0
[25] Atekwana, E.A., Sauck, W.A., Abdel Aal, G.Z. and Werkema Jr., D.D. (2002) Geophysical Investigation of Vadose
Zone Conductivity Anomalies at a Hydrocarbon Contaminated Site: Implications for the Assessment of Intrinsic Bio-
remediation. Journal of Environmental & Engineering Geophysics, 7, 103-110. http://dx.doi.org/10.4133/JEEG7.3.103
[26] Osella, A., de la Vega, M. and Lascano, E. (2002) Characterization of a Contaminant Plume Due to a Hydrocarbon
Spill Using Geoelectrical Methods. Journal of Environmental & Engineering Geophysics, 7, 78-87.
[27] Ayolabi, E.A. (2005) Geoelectric Evaluation of Olushosun Landfill Site Southwest Nigeria and Its Implication on
Groundwater. Journal of Geological Society of India, 66, 318-322.
[28] Ayolabi, E.A. and Folashade, J.O. (2005) Geophysical and Hydrochemical Assessment of Groundwater Pollution Due
to Dumpsite in Lagos State, Nigeria. Journal of Geological Society of India, 66, 617-622.
[29] Amadi, A., Dickson, A. and Maate, G.O. (1993) Effect of Organic and Inorganic Nutrient Supplements on the Perfor-
mance of Maize (Zea may L). Water, Air, and Soil Pollution, 66, 59-76.
[30] Godio, A. and Naldi, M. (2003) Two-Dimensional Electrical Imaging for Detection of Hydrocarbon Contaminants.
Near Surface Geophysics, 2003, 131-137.
[31] Osella, A., de la Vega, M. and Lascano, E. (2002) Characterization of a Contaminant Plume Due to a Hydrocarbon
Spill Using Geoelectrical Methods. Journal of Environmental & Engineering Geophysics, 7, 78-87.
http://dx.doi.org/10.4133/JEEG7.2.78
[32] Sauck, W.A. (2000) A Model for the Resistivity Structure of LNAPL Plumes and Their Environs in Sandy Sediments.
Journal of Applied Geophysics, 44, 151-165. http://dx.doi.org/10.1016/S0926-9851(99)00021-X
[33] Cassidy, D.P., Werkema, D.D., Sauck, W.A., Atekwana, E.A., Rossbach, S. and Duris, J. (2001) The Effects of
LNAPL Biodegradation Products on Electrical Conductivity Measurements. Journal of Environmental & Engineering
Geophysics, 6, 47-52. http://dx.doi.org/10.4133/JEEG6.1.47
[34] Omar, D.R., Vladimir, S., Jesús, O.V. and Albert, R. (2006) Using Electrical Techniques for Planning the Remediation
Process in a Hydrocarbon Contaminated Site. Revista Internacional de Contaminación Ambiental, 22, 157-163.