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INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES

Volume 3, No 1, 2012

© Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0

Research article ISSN 0976 – 4380

Submitted on May 2012 published on July 2012 30

Application of electrical resistivity techniques in the investigation of

groundwater contamination: A case study of Ile – Epo Dumpsite, Lagos,

Nigeria Ogungbe A.S

©, Onori E.O, Olaoye M.A

Department of Physics, Lagos State University, Ojo, Lagos, Nigeria

[email protected]

ABSTRACT

2D resistivity imaging and vertical electrical sounding (VES) surveys were integrated in the

study of the investigation of the level of groundwater contamination at Ile – Epo dumpsite in

Agbado Oke-Odo Local Council Development Area (LCDA), Lagos State, Southwestern,

Nigeria. A total of nine vertical electrical soundings and four 2D electrical imaging using

Wenner configurations was used for the study. Out of these, six VES and three 2D electrical

imaging were carried out on the dumpsite while three VES and one 2D imaging were carried

out in a school 100m away from the dumpsite in order to serve as control. The result of

interpretation of both data suggests that the study area is underlain by sand and clay

formations. Results of the resistivity Imaging delineated the contaminant plume as low

resistivity zones (0.5 – 48 Ωm). This was also correlated with the VES values of 0.1 to 31.6

Ωm which indicate contaminant plume. This was observed to have infiltrated to a depth of

about 50m which poses serious health risks to the inhabitants of that area.

Keywords: Contaminant plume, groundwater, leachate, dumpsite, aquifer, electrical

resistivity.

1. Introduction

Groundwater resources have been under rapidly increasing stress in large parts of the world

due to pollution. Pollution is primarily the result of irrigated agriculture, industrialization, and

urbanization, which generates diverse wastes, with the attendant impact on the ecosystem and

groundwater. Wastes may be loosely defined as any material that is considered to be of no

further use to the owner and is, hence discarded (Allen, 2001). Waste is generated universally

and is a direct consequence of all human activities. They are generally classified into solid,

liquid and gaseous.

The intensity of man’s activities has led to increasing volume of solid waste worldwide

despite the current level of technological advancement and industrialization. Explosive

population growth is one other major factor responsible for increased municipal solid waste

(MSW). Land filling of municipal solid waste is a common waste management practice and

one of the cheapest methods for organized waste management in many parts of the world (El-

Fadel et al., 1997; Dsakalopoulous et al., 1998, Jhamnani and Singh , 2009). In most low to

medium income developing nations, almost 100 per cent of MSW generated goes to landfills.

Landfill operations are most feasible in these countries as land is vastly available and

moderately inexpensive. Even in many developed countries where land is scarce and where

policies of reduction, reuse and diversion from landfills are strongly promoted, great

percentage of their generated MSW are still land filled. For instance, in 2006, out of the 251

million tons of MSW generated in the United States of America, 138.2 million tons

Application of electrical resistivity techniques in the investigation of groundwater contamination: A case

study of Ile – Epo Dumpsite, Lagos, Nigeria

Ogungbe A. S, Onori E.O, Olaoye M.A

International Journal of Geomatics and Geosciences

Volume 3 Issue 1, 2012 31

representing 55% was disposed of in landfills (USEPA, 2007). In England, out of the

29.1million tons of municipal solid waste generated between 2003 and 2004, 72% was land

filled (DEFRA, 2005). The scenario is similar in Northern Ireland and Scotland where 82.9%

and 85.4% of their generated MSW were land filled in 2005 and 2007 respectively (EHS,

2005; SEPA, 2007). Today, however, there is a progressive decrease in the volume of MSW

being land filled in these developed countries on a yearly basis as great efforts in solid waste

management are today directed towards waste reduction and recycling programmes which is

a real giant step in environmental improvements (USEPA, 2007, 2008). Disposal of refuse

occurs all over the world and proves to be a major problem. Careless dumping of refuse and

poor management can greatly affect one's health. Pollution from solid wastes always begins

with precipitates carrying the leachates into land surface and ends with the water reaching

surface water or groundwater. Precipitate on the refuse dumpsite will either infiltrate the

refuse or run off over as land flow. During the vertical percolation process (with rain water)

the water leaches both organic and inorganic constituents from refuse. The leachates become

part of the ground water flow system immediately they reach the water table. The extent of

pollution is greater in high rainfall areas than less humid and arid areas. Permeable soil

permits rapid movement of leachates unlike in less permeable zone.

Lagos state as an industrialized city generates very high amounts of waste annually but does

not have a technologically advanced means of waste disposal. Iyana Ipaja is one of the

busiest parts of Lagos. It can be described as one of the most populated areas in Lagos

metropolis hence contributes significantly to the amount of waste generated in the state. The

means of waste disposal like that of many industrialized towns in developing countries is the

use of open dump landfills. This is an improper solid waste management system which is

globally accepted as the major cause of environmental pollution in the world.

The electrical resistivity method is a unique geophysical tool used in groundwater and landfill

studies (Zohdy, 1964, Dahlin and Zhou, 2002). The resistivity method is used for electrical

sounding and imaging. The electrical sounding provides information about vertical changes

in subsurface electrical properties and thus, it is useful in the determination of hydrogeologic

conditions such as the depth to water table, depth to bedrock, and thickness of soil (Zohdy,

1964). The electrical resistivity imaging maps ground water contaminant leachate plumes,

contaminant source, migration paths, and depth (Griffiths and Barkers, 1993).

This study is driven by the desire to investigate the geologic structure of the aquifer with a

view to assessing the pollutant risk of the aquifer to the seepage of contaminants near Ile-

Epo landfill in the Agbado Oke – Odo Local Council Development Area. This became

necessary as the inhabitants in the study area depend mostly on groundwater for their water

supply needs.

1.1 Site location and accessibility

Ile-Epo dumpsite is located at Abule-Egba in Agbado Oke - Odo Local Council Development

Area of Lagos State (Figure 1). It is bounded by Lagos - Abeokuta expressway in the East

and other regions by residential buildings, filling stations, offices, and Ile-Epo markets. Its

geographical location is 6.870N, 3.38

0E. The dumpsite is located on a 21-hectare of land and

was opened in 1982. The waste at this site is dumped in one area adjacent to the only

vehicular entrance to the site. Due to lack of proper equipment, the waste has been “pushed”

only so far inwards as the equipment on site can go. As a result, the waste elevation is much

higher near the entrance compared to further back. With no soil cover, heavy burnings of the






waste has been occurring over time and continued random burning was observed during the

field work at his site. The site is accessible by tarred road. Like most dumpsites in Lagos

State, Ile- Epo dumpsite is surrounded by residential, commercial and industrial set-ups and

the waste stream is made up of domestic, market, commercial, industrial and institutional

origins. The wastes are of different types, ranging from organic to inorganic, hazardous and

non-hazardous.

Plate 1: Ile – Epo dumpsite.

1.2 Hydrogeology and geology of the study area

Two principal climatic seasons can be easily distinguishable; the dry season which is usually

from November to March and the wet season which starts from April and ends in October,

with a short dry spell in August. Average annual precipitation is put at about 1,700m3

and

serves as a major source of groundwater recharge (Jeje, 1983).

Lagos is basically a sedimentary area located within the Western Nigeria coastal zone, a zone

of coastal creeks and lagoons developed by barrier beaches associated with sand deposition.

The subsurface geology reveals two basic lithologies; clay and sand deposits. These deposits

may be interbedded in places with sandy clay or clayey sand and occasionally with vegetable

remains and peat (Ayolabi et al., 2005). The entire area is covered by mangrove forest, raffia

palm trees, scrub, scattered cultivated areas, and stilt rooted trees. The tall grasses and woody

plants cover the sandy plains. The main relief features of the area is the low lying nature of

its terrain, more than half of the entire area has an elevation of between 3 and 6 meters high

above the sea level (Odewunmi, 1995). It is identified that the geology is made up of

sedimentary rock mostly of alluvial deposits. These consist of loose and light grey sand

mixed variously with varying proportion of vegetation matter on the lowland, while the

reddish and brown loamy soil exists in the upland.






Figure 1: Geological Map of Lagos State

2. Materials and method

A total of nine (9) vertical electrical soundings and four 2D electrical imaging (Wenner) were

used for the study. Out of these, six vertical electrical soundings and three 2D electrical

imaging were carried out on the dumpsite while three VES and one 2D electrical imaging

were carried out in a school 100m away from the dumpsite in order to serve as control for

other traverses. The PASI terrameter model (16GL) was used for data acquisition. A

maximum electrode expansion (AB/2) of 180m and (MN/2) of 5m were utilized in the survey

using the Schlumberger electrode array for the VES. The measured resistance values were

converted into apparent resistivity (ρa) by multiplying with a geometric factor (K), such that

( ) ( )MN

RMNAB

a

−

=

22

22π

ρ (1)

= K.R

The apparent resistivity measurements at each station were plotted against half current

electrode spacing (AB/2) on bi-logarithmic graph sheets, using a transparent tracing paper

superimposed on the log-log paper. The curves obtained were matched using master curves

and auxilliarry curves and were inspected to determine the number and nature of the layers.

The results of the curve matching (layer resistivity and thickness) were fed into the computer

as a starting model in an iterative forward modeling technique using WinRESIST version 1.0.

This is to vindicate the correlation of the field curve and the theoretical curve. From the

interpreted results (layer resistivity and thickness); geoelectric sections of each of the VES

points were drawn based on the interpretation of the resistivity data using AutoCAD software.






The 2D resistivity imaging was accomplished using a four – electrode system with equal

electrode spacing “a” ranging from 10 – 60m for successive measurements covering a

distance of 160 -180m. The apparent resistivity values were calculated from the field

resistance values using the equation

aRa πρ 2= (2)

Where a = electrode spacing

R = field resistance value

The values of the apparent resistivities were inverted using DIPRO Software which operates

mathematically based upon the smoothness - constrained least squares method ( Loke and

Barker, 1996).

3. Results and discussion

The results of the interpretation of the vertical electrical sounding data revealed the different

geoelectric layers in terms of their resistivities and depths in the study area. A total three (3)

to six (6) geoelectric layers were delineated with varying curve types, reflecting the

lithological variations with depth. The nature of the curve types suggests that the measured

resistivities vary with depth of investigation in the study site.

Figure 3: VES 1

Figure 4: VES 2






Figure 3-5 show VES 1 to 3 with 5 to 6 geoelectric layers. These consist of topsoil, sand and

clay with varying resistivities and thickness. The topsoil in these VES is characterized by

resistivity values ranging from 20.2 to 168.6Ωm and layer thickness of 0.3 to 0.7m. The

second and third layer represent sand and clay respectively with resistivity ranging from 4.6

to 54.8 Ωm and thickness of 0.4 to 4.9m while the fourth and fifth layers in VES 1 are also

sand but with very low resistivity values ranging from 5.2 to 13.1 Ωm. The same was also

observed in VES 2 and 3 at the second and fourth layers. These are zones of probable risks to

groundwater contamination because these values indicate leachate contaminant plumes.

Figure 6-8 depict VES 4 to 6 with four (4) to five (5) geoelectric layers with different

resistivity values and layer thickness. In both VES is topsoil with resistivity values between

9.9 and 22.5 Ωm and to a depth of 1.1m. Beneath this is layer with resistivity values ranging

from 2.5 – 13.7 Ωm and to a depth of 6.4m which is likely to be clay formations. The last

layer in all the VES here is probably sand with resistivity ranging from 7.8 – 176 Ωm but that

of VES 4 is contaminated because of the very low value of resistivity.

Figure 9-11 show VES 7 to 9 which were taken in a school compound which is 100m away

from the dumpsite. Here, 3 - 5 geoelectric layers were delineated. The topsoil which is laterite

clay with resistivity values of between 53.8 – 94.6 Ωm and to a depth of 1m was delineated,

while the last layer in the VES indicates sand of resistivity between 1731 – 5199 Ωm. These

high resistivity values are an indication that the area is not contaminated.

Figure 5: VES 3

Figure 6: VES 4






Figure 7: VES 5

Figure 8: VES 6

Figure 9: VES 7






Figure 10: VES 8

Figure 11: VES 9

The 2D resistivity imaging taken at this site runs through the VES profiles. A depth of 50m

was probed with resistivity value ranging from 0.5Ωm to 23.6Ωm (figure 12). In this profile,

the leachate effect is mapped out in the regions with relatively low resistivity (0.5 to 6.6Ωm)

which is mapped out in deep blue colour code. The low resistivity variation is indicative of

the degree of decomposition of the refuse materials and are indications of saturated zones

starting from the ground surface. Below the first layer, the subsurface is characterized by

materials with low resistivity (1.0 – 13. 6Ωm) within the lateral distance of 110 – 160m.

Within the lateral distance of 0 – 115m, the subsurface indicates low resistivity value of

between 1 – 13Ωm. This indicates the migration of leachate into the subsurface which is

mapped within the depth of 20 – 50m on the section, an indication that the water table in this

area may have been contaminated.






In figure 13, the presence of leachate could be seen from a spread of 40 to 80m along the

surface of the profile. Below this region, within the lateral distance of 80 – 120m, the

subsurface is characterized with low resistivity (about 2.0 – 4.1Ωm). The arrows indicate

regions that have been polluted by the migrating leachate to a depth of 50m. They also

indicate the possible medium and direction of migration, which is usually through sand.

For the 2D resistivity structure in figure 14, the topsoil is characterized with distribution of

contaminants with resistivity values from 3Ωm to 9Ωm. The layer beneath the horizontal

profile indicates segments having leachate effect which is found within a depth of about 20m

with resistivity ranging from 3.6 – 4.0Ωm. The subsurface is characterized with relatively

low resistivity at 140m spread showing regions of pollution as a result of infiltrated leachate.

The lateral distance of 0 – 110m and the depth of 20 – 30m signify clay which does not

permit infiltration by leachate effect.

Figure 15 is the 2D profile carried out at 100m from the dumpsite. It was carried out at Oke -

Odo High School with resistivity values ranging from 7 to 800 Ωm. The high resistivity

values indicate that the site is not contaminated. The low resistivity values observed on the

lateral distance of 95 – 130m and to a depth of 5.7m could likely be due to clay formations.

Figure 12: Profile 1 (2-D Resistivity structure and Pseudosections)






Figure 13: Profile 2 (2-D Resistivity structure and Pseudosections)

Figure 14: Profile 3 (2-D resistivity structure and Pseudosections)






Figure 15: Profile 4 (2-D resistivity structure and Pseudosections)

4. Conclusion

The results of both VES and the 2D resistivity imaging revealed the various lithological units

in the study site based on the respective depth of investigation. The geologic interpretation of

the VES showed that the site is dominantly underlain by clay and sandy formations of

varying grain sizes which constitute the hydrogeologic unit of the area. This study showed

that parts of the dumpsite had been contaminated. This was evident in the attendant low

resistivity values of 0.1 – 48 Ωm. This contamination was also observed to have infiltrated to

a depth of about 50m in the dumpsite. This could pose serious health risk to the inhabitants of

that area who depend largely on wells and shallow boreholes for their drinking water supply.

5. References

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engineering geology, 60, pp 3-19.

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assessment of the impact of solid waste on the groundwater at Okealfa refuse

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3. Dahlin, T, and B Zhou, (2002), Gradient and Mid-point referred measurements for

multichannel 2-D resistivity imaging, Procs. 8th

meeting of the environmental and

engineering geophysics, Aveiro, Portugal 8-12 September 2002, pp 157-160.






4. Daskalopoulos, E., O. Badr and S. D. Probert., (1998), An integrated approach to

municipal solid waste management. Res. Cons. Rec., 24 (1) pp 33 – 50

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waste management strategy. http://www.ehsni.gov.uk/pubs/publications/towardsre

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9. Jeje, L.K., (1993), Aspects of geomorphology in Nigeria. In a geography of Nigeria.

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14. SEPA, (2007), EU waste statistics regulation Scotland Report 2004.

http://www.sepa.org.uk/pdf/publications/wds/wdd_5.pdf, accessed during May 2012.

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