Hydrogeophysical Evaluation and Vulnerability Assessment of Shallow Aquifers of the Upper Imo River Basin, Southeastern Nigeria

Abstract

Detailed hydrogeophysical study of the aquifers of the Upper Imo River Basin, Southeastern Nigeria was carried out to delineate the aquifers, evaluate their geometric characteristics and to assess their vulnerability of pollution from surface contaminants. Forty (40) Schlumberger Vertical Electrical Soundings (VES) were carried out in various parts of the study area with a maximum electrode separation of 1000 metres. Four parametric soundings were carried out at the exact positions of existing boreholes with available pumping test and electric log data for the purpose of comparison. The VES data were processed using a combination of curve matching techniques and computer iterative modeling. 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 4-7 geoelectric layers with a multi-aquifer system. The aquifers are variable in thickness with Umuekwule having the thickest aquifer of 108m while Ukomo Ibeku has the least aquifer thickness of 1.7m. Similarly, depth to water table varies from 0.6m at Ajata ibeku to 91.8m at Umukabia. Information from iso-resistivity model and diagnostic factor revealed a distinct hydrogeological divide in line with the geology of the study area. The aquifer vulnerability index assessment revealed that about 55% of the study area falls within the moderate vulnerability zones with DRASTIC index values ranging from 102 to 140. About 30% of the study area have high vulnerability index while the remaining 15% of the study area have low vulnerability index with DRASTIC index values of between 85 and 99.