Integrated Geophysical and Hydro-chemical Investigations of Impact of the Ijemikin Waste Dump Site in Akure, Southwestern Nigeria, on Groundwater Quality

Background. Improper waste disposal can negatively impact the ecosystem and constitutes a major human health risk. Objectives. The present study evaluated the environmental impact on groundwater quality of an open-air waste dump in Akure, southwestern Nigeria, using an integrated geophysical survey and hydro-chemical analysis of water samples. Methods. The geophysical survey involved three dipole-dipole 2-D imaging profiles and seven vertical electrical soundings (VES) along three traverses. The dipole-dipole data were inverted using the Dipro for Windows software, while the VES data were quantitatively interpreted using partial curve matching and computer assisted 1-D forward modeling with the WINResist software. The VES interpretation results were used to generate geoelectric sections. For the hydro-chemical analysis, samples were taken from five hand-dug wells at various distances from the dumpsite. The samples were analyzed for temperature, pH, conductivity, total dissolved solids (TDS), and some major elements (calcium ion (Ca2+), magnesium ion (Mg2+), chloride ion (Cl−), nitrate (NO3−), sulfate (SO42−)) whose concentration values were compared with World Health Organization (WHO) and Nigerian Industrial Standard (NIS) standards for assessment of groundwater quality. Results. The VES curves revealed three distinct geoelectric/geologic layers with thicknesses and resistivities in the range of 0.7 - 2.0 m and 31 - 55 Ωm for topsoil, 6.2 - 14.6 m and 13 - 114 Ωm for the weathered layer, and a fresh basement with resistivity values ranging from 344 - ∞ Ωm. In the hydro-chemical analysis, pH values ranged between 7.57 - 7.8, electrical conductivity ranged from 884 - 1510 μS/cm, and TDS ranged between 588 - 1008 mg/l. Concentration values of Ca2+ and Mg2+ ranged between 78 - 132 mg/l and 1.8 - 19 mg/l, respectively. Conclusions. The results from the combined electrical resistivity methods showed relatively low resistivity values at the topsoil and weathered layers and the hydro-chemical assessment of water samples indicated that the topsoil and groundwater within the dumpsite may have been polluted by leachate. Competing Interests. The authors declare no competing financial interests.


Introduction
Improper waste disposal and management have led to the microbial and chemical contamination of the environment and water supply sources globally. There have been a number of successful investigations of the impact of leachate plume migration from waste dumps into the environment using various geophysical and hydrochemical methods. 1, 2, 3, 4 The present study used integrated geophysical and hydro-chemical investigation methods to delineate the subsurface layers, thicknesses, and resistivity values, as well as the hydro-chemical characteristics of Background. Improper waste disposal can negatively impact the ecosystem and constitutes a major human health risk.
Objectives. The present study evaluated the environmental impact on groundwater quality of an open-air waste dump in Akure, southwestern Nigeria, using an integrated geophysical survey and hydro-chemical analysis of water samples.
Methods. The geophysical survey involved three dipole-dipole 2-D imaging profiles and seven vertical electrical soundings (VES) along three traverses. The dipole-dipole data were inverted using the Dipro for Windows software, while the VES data were quantitatively interpreted using partial curve matching and computer assisted 1-D forward modeling with the WINResist software. The VES interpretation results were used to generate geoelectric sections. For the hydro-chemical analysis, samples were taken from five hand-dug wells at various distances from the dumpsite. The samples were analyzed for temperature, pH, conductivity, total dissolved solids (TDS), and some major elements (calcium ion (Ca 2+ ), magnesium ion (Mg 2+ ), chloride ion (Cl -), nitrate (NO 3 -), sulfate (SO 4 2-)) whose concentration values were compared with World Health Organization (WHO) and Nigerian Industrial Standard (NIS) standards for assessment of groundwater quality.

Conclusions.
The results from the combined electrical resistivity methods showed relatively low resistivity values at the topsoil and weathered layers and the hydro-chemical assessment of water samples indicated that the topsoil and groundwater within the dumpsite may have been polluted by leachate.
well water samples to assess the impact of leachate from the Ijemikin market waste dumpsite in Akure, southwestern Nigeria, on groundwater quality.

Methods
The present study utilized integrated geophysical methods and a hydrochemical analysis of water samples from wells around the Ijemikin waste Research dumpsite to investigate possible contamination of groundwater by leachate migration from the waste dump. The geophysical method employed in the present study was the electrical resistivity method involving the vertical electrical soundings (VES) technique with the Schlumberger array and 2-D imaging with the dipoledipole array.

Study area
The study area, Ijemikin Market waste dumpsite, lies within latitude 7°14'48" N to 7°14'58" N and longitude 5°11'40" E and 5°11'46" E ( Figure 1). The area is underlain by the migmatite gneiss rock of the Precambrian basement complex rocks of southwestern Nigeria. 5 The surface drainage shown in Figure 1 is a public concrete gutter system that runs through the community as a channel for storm water runoff.
Seven VES stations were investigated, while 2-D imaging was carried out along three traverses: two trending north-south and one trending eastwest (Figure 1). A dipole length of 5 m and expansion factor n varying from 1 to 5 was adopted.

Geophysical methods
The VES data were processed by plotting the apparent resistivity values against electrode spacings (AB/2) on a bi-logarithmic paper as sounding curves. The curves were interpreted by partial curve matching and computer assisted 1-D forward modelling with WinRESIST version 1.0 software. 6 The dipole-dipole data were inverted using the DIPRO software into the 2-D pseudosections and resistivity structures.
For the hydro-chemical test, five water samples collected from unlined hand-dug wells (without lining rings) within a radius of 60 m from the dumpsite were securely corked and stored in sample bottles which were thoroughly rinsed three times with the sample water on site before samples were taken and then transported to the laboratory for analysis within 24 hours. Physical parameters such as pH, temperature, and electrical conductivity were determined in situ using a portable digital conductivity and pH meter, while major cations (calcium ion (Ca 2+ ), magnesium ion (Mg 2+ )), and anions (chloride ion (Cl -), nitrate (NO 3 -), sulfate (SO 4 2-)) were determined in the laboratory using standard methods. Calcium was determined using 0. 05

Geophysical methods
Geo-electric sections were developed by correlating the various VES points along the three traverses to reveal the subsurface layers, their thicknesses and resistivity values (Figures 2 and 3).

Vertical electrical sounding
Three distinct geoelectric/geological layers (topsoil, weathered layer and fresh basement) were delineated. 9

Dipole-dipole imaging
The 2-dimensional pseudosections and resistivity structures (Figures 4,  5 and 6) along the three traverses indicated showed variation in resistivity and thickness of the overburden above the presumably fresh basement.

Hydro-chemical results
A summary of the water quality results from the hydro-chemical analysis on the five groundwater samples taken from wells without lining rings within the study area are presented in Table 1.

Geophysical methods
The combined geophysical methods revealed three distinctive geo-electric layers of topsoil, weathered layer and fresh basement, and identified low resistivity zones in the subsoil, which were indicative of leachate plume migration into the bedrock. 3

Vertical electrical sounding
The geo-electric section along traverse 1 ( Figure 2) showed evidence of low resistive topsoil with a resistivity value range of 31-37 Ωm and a weathered layer with resistivity values between 13 Ωm -40 Ωm above a highly resistive basement complex with resistivity values ranging between 344-∞ Ωm. The geo-electric section along traverse 2 ( Figure 3) revealed three geoelectric layers consisting of topsoil with low resistivity values ranging between 31-55 Ωm, and a weathered layer with resistivity values between 25-114 Ωm overlying a resistive basement complex with resistivity values ranging between 345-1521 Ωm. The relatively low resistivity values of less than 100 Ωm at the topsoil and weathered layer are indications of conductive materials which are suspected to be fluids and leachates from the waste dump in the subsoil.

Dipole-dipole imaging
Three resistivity structures along the three traverses shown in Figure 4c, 5c and 6c revealed 2-D subsurface imagery consisting of low resistive topsoil and weathered layers of resistivity values below 45 Ωm at a shallow depth of 5 m and a highly resistive underlying basement complex with a resistivity value above 123 Ωm. The very low resistivity areas around the dumpsite (Figure 5c) are possible indications of leachate or conductive fluids infiltrating into the subsurface from the overlying dumpsite.

Hydro-chemical results
As shown in the hydro-chemical analysis results in Table 1

Table 1 -Hydro-chemical Parameters of Sampled Well Water Compared to WHO and NIS Recommended Standards
Abbreviations: ng, no guideline; n/a, not available.
relatively high concentration values for electrical conductivity and TDS could be indicative of leachate pollution. The cation Ca 2+ value for all wells ranged between 78 -132 mg/l, which was higher than the WHO threshold of 75 mg/l.

Conclusions
Hydro-chemical analysis of well water samples around the waste dump also examined environmental water quality and hydro-chemical parameter concentrations compared to established water quality standards. 7, 8 Threshold values for drinking water quality revealed that the groundwater had been impacted by leachate from the dumpsite. 4 The relatively low soil/subsoil resistivity values and concentrations of some hydrochemical elements above permissible levels in the water samples indicate pollution of the soil/subsoil and groundwater by leachate from the investigated waste dump site.