Aquifer vulnerability assessment using Dar-Zarrouk parameter: A case study of Otofure and Ikhueniro dumpsites, Benin-city, South-South Nigeria

INTRODUCTION

The geoelectrical resistivity method has been successfully employed in the delineation of subsurface geological sequence, geological structures/features of interest, aquifer units, types, and depth extent in almost all geological terrains.[1-4] This is because of the significant resistivity contrasts that exist between different earth materials.[5] The resistivity method can therefore map interface along which a resistivity contrast exists. This interface may or may not coincide with geological boundary.[6] In addition, vertical electrical sounding (VES) has been widely used to evaluate groundwater potentials and area of high groundwater yield.[7-9] Geoelectrical methods are also used extensively in groundwater mapping for investigation of the vulnerability of shallow aquifers.[9]

The vulnerability of aquifers is largely dependent on the presence or absence of protective impermeable layer, usually clay. The earth medium acts as a natural filter to percolating fluid; its ability to retard and filter percolating fluid is a measure of its protective capacity.[10] Studies have shown that geoelectrical method is an invaluable tool in mapping aquifer vulnerability because of its capability to distinguish low- and high-resistive formations.[11] The concept of groundwater vulnerability is based on the assumption that the physical environment may provide some degree of protection to groundwater against natural impacts, especially with regard to contaminants entering the subsurface zone. Consequently, some land areas are more vulnerable to groundwater contamination than others. Henriet[12] showed that the combination of layer resistivity and thickness in the Dar Zarrouk parameters S (longitudinal conductance) and T (transverse resistance) may be of direct use in aquifer protection studies and for the evaluation of hydrologic properties of aquifer. The protective capacity is considered to be proportional to the longitudinal unit conductance in mhos.[1,10,13,14]

Atofure and Ikhueniro dumpsites are active old dumpsites located in Ovia North East and Ikpoba-Okha Local Government Areas (LGA), respectively, in Edo State, South-South, Nigeria. All classes of wastes are deposited on the dumpsites. Over the years, the wastes have gone through episodic decays that had generated leachates and definitely transported deep into the subsurface. It is, therefore, important to assess the vulnerability of the aquifers in these localities to leachate infiltration into the aquifer with a view to alerting the government and cautioning the residents of the areas from consuming contaminated groundwater in situations of vulnerability. In this study, aquifer vulnerability assessment using a Dar-Zarrouk parameter was carried out in Otofure and Ikhueniro areas with a view to preventing the exploitation and consumption of contaminated groundwater in the localities.

METHODOLOGY

Location and Geology

The study areas, Edo State falls within the Niger Delta Basin. The basin is an extensive continental margin basin situated in the Gulf of Guinea built out into the Central South Atlantic Ocean at the mouths of the Niger-Benue and Cross River systems during the Eocene [Figure 1]. It is a wave dominated and tidally influenced delta with sand bodies whose thickness may be influenced by growth faulting.[15]

Figure 1: Geological map of Niger Delta[17]

Otofure dumpsite area is located within Longitudes 005° 35’ 52.56” E - 005° 36’ 02.29” E, Latitudes 06° 27’ 40.48” N - 06° 27’ 48.99” N and elevation of 97–106 m while Ikhueniro dumpsite is located within Longitudes 005° 44’ 40.01” E - 005° 44’ 66.20” E, Latitudes 06° 19’ 15.45” N - 06° 19’ 38.99” N and elevation of 71–86 m. Otofure and Ikhueniro dumpsites are located in Ovia East Local Government Area and Ikpoba-Okha LGA, respectively, both in Benin-City, Edo State, Nigeria. The dumpsites are presumed to have existed for over 20 years and cover an area of 300–500 m² with refuse content consisting of various kinds of metallic, organic, and non-biodegradable materials. The areas occupy the Southern part of Edo State which is a sedimentary terrain and is underlain by sedimentary rocks of Paleocene to recent age [Figure 2]. The sedimentary rock contains about 90% of sand stone and shale intercalations.[16] Edo State is situated in south-southern part of Nigeria. It is an important sedimentary basin in Nigeria due to her closeness to the oil fields within the Niger-Delta region.

Figure 2: Geologic map of the study areas showing the Otofure and Ikhueniro dumpsites

Data Acquisition and Processing

Geophysical investigation

Twenty VES data were acquired from both locations, with seven VES data from each location at different points and three VES data acquired 100 m away from each dumpsite as control data points. The acquired apparent resistivity data were processed both quantitatively and qualitatively. The quantitative interpretation of the depth sounding curves was carried out using the partial curve matching technique.[18] The combination of two or more interpreted VES results along a profile was used to generate a geoelectric section along the profile [Figure 3].

Figure 3: Field basemap of the study areas showing the Otofure and Ikhueniro dumpsites

Dar-Zarrouk parameters

Dar-Zarrouk (D-Z) parameters were defined by Maillet.[19] T is the resistance normal to the face and S is the conductance parallel to the face for a unit cross-section area, which plays an important role in resistivity soundings. D-Z parameters are sufficient for computing the distribution of surface potential and hence an electrical resistivity graph.[12]

Suppose that a section consists of N fine layers with thickness h₁, h₂,………., h_n and resistivity ρ₁, ρ₂, ρ₃,……..,ρ_n for a block of unit square area and thickness

These values of S and T are set equal to those for an anisotropic block with unit square area. So that:

Longitudinal Unit Conductance S,

Transverse Unit Resistance T,

Longitudinal Resistivity R_S,

Transverse Resistivity, R_T

In this study; however, only the Longitudinal Unit Conductance S in mhos (Equation 2) was considered as it is found to be proportional to the protective capacity of the overburden.[1,10,13,14]

RESULTS AND DISCUSSION

Otofure

The VES curves around the Otofure dumpsite are presented in Figure 4a-j. The seven VES data acquired within the dumpsite (i.e., VES 4-10) delineates five geoelectric layers which are indicative of the topsoil, clay/sandy clay, clayey sand, leachate, and sand [Figure 4d-j]. The control VES (1–3) delineate 4–6 geoelectric layers which are only topsoil and sand units (with varying degree of moisture content). There are indications of leachate at VES 6–10 at a depth ranging from 2.9 to 4 m in Otofure dumpsite area [Figure 4d-j].

Figure 4a-f: VES curves around Otofure dumpsite

Figure 4g-j: VES curves around Otofure dumpsite

Evaluation of Aquifer Protective Capacity

Aquifer protective capacity (APC) is the ability of the overlying layers of rock (i.e., the overburden) above the aquifer unit to impede, slow-down, filter and contains percolating ground surface contaminating fluids and run-offs. The second order geoelectric parameter – longitudinal conductance (which is a Dar Zarrouk parameter) was evaluated from the first-order parameters (thickness and resistivity) of the geoelectric layers which were used in the classification of the APC of the area. Highly impervious materials such as clay and shale usually have high longitudinal conductance values (resulting from their low resistivity values) while pervious materials such as sand and gravels have low longitudinal conductance values (resulting from their high resistivity values). While high longitudinal conductance value corresponds to excellent and good APC, low longitudinal conductance values are associated with poor and weak APC [Tables 1 and 2].

Table 1: Longitudinal conductance/protective capacity rating[12]

Table 2: Modified longitudinal conductance/protective capacity rating[1]

Otofure Aquifer Protective Capacity Map

Figure 5 presents the aquifer protective capacity map around Otofure. Western part of Otofure from the north to the south vary in protective rating from 0.2 to 1.4 mhos indicating moderate to good rating.[1,12] In the eastern part of Otofure, from the north to south, the protective capacity ranges from 1.8 to 4.8 mhos [Figure 5] which indicates good protective capacity rating.[1]

Figure 5: Otofure protective capacity map

Ikhueniro

Figure 6a-j present the VES curves around the Ikhueniro dumpsite. All the seven VES acquired within the dumpsite delineate five geoelectric layers which are the topsoil, clay, clayey sand, leachate, and sand [Figure 6d-j]. The control VES data acquired away from the dumpsite (i.e. VES 1-3) delineates five geoelectric layers which are topsoil and sand [Figure 5a and c]. The sands are suspected to be characterized with varying degree of groundwater saturations and similar type-curve which are very dissimilar from the type-curves of VES data acquired within the dumpsite. VES 4–10 in this location gives both near surface and deeper indications of the presence of leachate migration at a depth range of 0.6–13.6 m [Figure 6d-j].

Figure 6a-f: VES curves around Ikhueniro dumpsite

Figure 6g-j: VES curves around Ikhueniro dumpsite

Ikhueniro Aquifer Protective Capacity Map

Aquifer protective capacity map in Ikhueniro is shown in Figure 7. The northwest of Ikhueniro varies in protective rating from 0.15 to 0.45 mhos [Figure 7]. This indicates a weak to moderate rating.[1,12] The central part of Ikhueniro varies in protective rating from 0.5 to 0.75 mhos thus indicating moderate to good protective rating while the eastern to south eastern part of Ikhueniro varies in the protective rating values from 0.8 to 1.1 mhos [Figure 7 and Tables 3 and 4] and thus implying a good protective capacity rating in the study area.[1]

Figure 7: Ikhueniro protective capacity map

Table 3: Protective capacity rating in Otofure area

Table 4: Protective capacity rating in Ikhueniro area

CONCLUSION

Due to growing urbanization around Otofure and Ikhueniro dumpsites and the age-long existence of the dumpsites which has the high possibility of contaminating the regional aquifer with the leachates they have generated over the years, an assessment of the aquifer vulnerability using Longitudinal Unit Conductance, S - a Dar-Zarrouk parameter was carried out in both Otofure and Ikhueniro with a view to prevent the exploitation and consumption of contaminated groundwater in the localities.

Ten VES data were acquired from each of Otofure and Ikhueniro, respectively: Seven VES data from each location and three VES data acquired 100 m away from each dumpsite as control data points. The VES results in Otofure and Ikhueniro delineate five geoelectric layers which are indicative of the topsoil, clay/sandy clay, clayey sand, leachate, and sand. The control VES delineate 4–6 geoelectric layers which are only topsoil and sand units (with varying degree of moisture content). The leachate occurs at a depth of from 2.9 to 4 m and 0.6 to 13.6 m in in Otofure and Ikhueniro, respectively.

In Otofure, the Total Longitudinal Unit Conductance, S varies from 0.0258 to 0.0915 mhos away from the dumpsite, while S varies from 0.6438 to 1.1562 mhos around Otofure dumpsite. These show that away from the dumpsite, the aquifer protective capacity is poor while in the immediate vicinity of dumpsite of the dumpsite, the protective capacity varies from moderate to good, implying that aquifers that are away from the dumpsite are vulnerable to contamination while aquifers in the immediate vicinity of the dumpsite are well protected from contamination either from the migrating leachate from the dumpsite or from other sources of contamination. In Ikhueniro, the Total Longitudinal Unit Conductance, S varies from 0.0237 to 0.0273 mhos away from the dumpsite, while S varies from 0.4119 to 4.4206 mhos around Otofure dumpsite. These values indicate that away from Ikhueniro dumpsite, the aquifer protective capacity is poor while around the dumpsite, the protective capacity varies from moderate to good, implying that aquifers that are away from the dumpsite are vulnerable to contamination while aquifers in the immediate vicinity of the dumpsite are protected from contamination similarly to Otofure.

The study has shown that aquifers in the vicinity of both Otofure and Ikhueniro dumpsites have good protective capacity against migrating sources of contamination while those away have very poor protective capacity against migrating sources of contamination. The study has further revealed the importance of deliberate selection of suitable locations for sitting dumpsites and landfills as revealed in the results.