Lithologic characterisation of the basement aquifers of Awe and Akinmorin areas, Southwestern Nigeria

Abstract

Just like many communities in Nigeria, people living in Awe and Akinmorin areas rely on groundwater supply for their domestic water need. However, these areas are underlain by impermeable crystalline rocks that included migmatite, schist and quartzite that can only transmit water when fractured and where there is sufficient weathered regolith overlying the bedrocks. Therefore, a total of twenty-five vertical electrical soundings (VES) was conducted across the study area with the aim of investigating the lithological characteristics of the regolith and the underlying bedrocks for groundwater prospect. An aggregate of nine soundings were conducted on quartzite terrains and eight each across terrains underlain by migmatite and schist. From the VES results, the dominant curves across the study areas were the H and KH types. These geo-electric curves indicate that the middle (saprolite) layers are more conductive and the bedrocks are more resistive. The total regolith thickness ranged from 11.9 to 50.9m with an average of 26.4m on migmatite terrains, 11.7 to 28.8m and average of 19.4m on schist terrain and 8.4 to 41m with an average of 22.3m on quartzite terrain. From the lithologic characterisation, 70% of the saprolite layers are interpreted as fine grained with resistivities of less than 100Ωm, which is an indication of deep and intense weathering across the study area. The occurrences of fractured and fresh bedrocks are almost of the same proportions. However, there is more dominant of partially weathered bedrocks than those that are either fresh or fractured. Altogether, bedrocks resistivities of more than half of the surveyed area are <1500Ωm and were either fractured or slightly weathered. This increased groundwater prospect across the study area, particularly within terrains underlain by fractured bedrocks. Generally, the potential for groundwater development in the study area is reasonably credible. Although, the saprolite units are mostly fine grained and may not be the best groundwater bearing zone, they may act as confining units for areas underlain by fractured or partially weathered bedrocks. In addition, the fairly thick fine grained saprolite layer may reduce the risk of groundwater contamination from direct recharge.

KEYWORDS: Groundwater, Saprolite, Bedrock, Lithology