Aquifer types and the geoelectric/hydrogeologic characteristics of part of the central basement terrain of Nigeria (Niger State)

A study of lithological logs from forty (40) wells drilled in twelve (12) localities in parts of the Niger State of Nigeria and the interpretation results of the parametric vertical electrical soundings (VES) were carried out to identify the geoelectric/hydrogeologic characteristics of the basement complex area.

Five (5) aquifer types were identified. These include the weathered aquifer; the weathered/fractured (unconfined) aquifer; the weathered/fractured (confined) aquifer; the weathered/fractured (unconfined)/ fractured (confined) aquifer and the fractured (confined) aquifer. The mean groundwater yield for the aquifer types varies from 0.83 L/S for the weathered layer aquifer to 3.0 L/S for the weathered/fractured (unconfined)/ fractured (confined) aquifer.

The fracture frequency increases with depth and reaches a maximum at between 25–35 m for granite, gneiss and schist but decreaseswith further increase in depth. The cumulative fracture frequency is maximum in granite and minimum in schist.

The fracture thicknesses are maximum (greater than 3 m) and occur most frequently within the depths of 10–40 m. Minimum fracture thicknesses (≤ 1 m) occur most frequently between the depths of 50–70 m. Fractures rarely occur at depths greater than 90 m An optimum borehole depth for the study area is 60–70 m.