Hydrogeophysical evaluation of aquifer hydraulic characteristics using surface geophysical data: a case study of Umuahia and environs, Southeastern Nigeria

The determination of aquifer hydraulic characteristics of Umuahia area from Dar-Zurrouk parameters was carried out using 30 vertical electrical sounding (VES) data and information from ten well-log data. The Schlumberger electrode configuration was employed for the field data acquisition with a maximum half current electrode separation (AB/2) of 500 m. The VES data were interpreted using the conventional partial curve matching technique to obtain initial model parameters which were used as input for computer iterative modeling using the OFFIX software. Aquifer apparent resistivity in the area varies from 109 to 850 a"broken vertical bar m with a mean value of 253.2 a"broken vertical bar m. The depth to water table varies from 0.7 m around Ajata Ibeku to 82.67 m at Mater Dei Umuahia. The aquifers of the study area are highly variable in thickness with Ajata Ibeku having the least thickness of 1.2 m. The hydraulic conductivity across the area ranges from 0.64 m/day to about 10.22 m/day, while the transmissivity have mean values of 308.16 and 6.257 m(2)/day for zones A and B, respectively. The mean storativity values of 1.92 x 10(-4) and 1.74 x 10(-5) for zones A and B, respectively, suggest that zone A has a fairly productive aquifer unlike zone B which revealed very poor hydraulic properties. Information extracted from iso-resistivity models, geo-electric cross-sections, litho logs, and electric logs revealed a sandy clay lithology at very shallow depths with a thick layer of shale/clay extending to over 300 m within zone B. These potential shallow aquiferous units are however perched, partially saturated and may not have sufficient yield.