Geophysical Appraisal of the Aquifer Geomaterials of Ugep and Environs, Southeastern, Nigeria Using Resistivity Data

Abstract

Hydrogeophysical characterization of the aquifers of Ugep and environs, Southeastern Nigeria was carried out to delineate the aquifers of the study area, evaluate their geometrical potentials and to assess their level of vulnerability to pollution from surface contaminants. Forty (40) Schlumberger Vertical Electrical Soundings (VES) were acquired within the various parts of the study area with a maximum half current electrode separation (AB/2) of 500 metres using the digital terrameter, SAS 4000 model. Seven (7) out of these soundings were parametric soundings carried out at the exact positions of existing boreholes for the purpose of correlation and comparison. The VES data were processed using a combination of curve matching and computer iterative modeling techniques. Layer parameters interpreted from the VES data together with the available well data were used to assess the vulnerability of the shallow aquifers using the DRASTIC model. Results of the study revealed the presence of 3-8 geoelectric layers with the KH curve type being dominant. Information extracted from iso-resistivity models of the study area revealed a distinct hydrogeological divide in line with the geology of the study area. Depth to the water table ranges from 12.4 m to 147 m with a mean value of 67.96 m. The aquifers of the study area are variable in thickness with values ranging from 5.7 m to 123 m with a mean value of 47.3 m.The Dar Zarrouk parameters which gave insights into the hydrogeological condition of the study area revealed that longitudinal conductance values varies from 0.0053 Sm (VES 28) to 0.0053 Sm (VES 40), with a mean value of 0.2848 Sm while the transverse resistance ranges from 1106.33 Ωm2 to 84992 Ωm2 with a mean value of 19819.3 Ωm2. Result of the groundwater vulnerability assessment revealed that 2.5% of the study area falls within the low vulnerability zone, whereas about 55% of the study area is of moderate vulnerability to groundwater contamination with aquifer vulnerability index ranges of 108 to 133. The pattern of spatial variation of vulnerability is believed to be as the result of the variation in depth to water table from east to west. It was also revealed that about 42.5% of the study area falls within the high aquifer vulnerability zone with the DRASTIC index value ranging from 141 to 161.