Geophysical Investigation of Anomalous Groundwater Seasonal Behaviour of a Well at Ayegbaju-Ekiti, Southwestern Nigeria

Abstract

Groundwater is the essential criterion for population driven growth and civilization of rural areas which is often accompanied by constructional activities of various type. Anomalous groundwater behaviour of a well at Ayegbaju-Ekiti has been studied with the use of 2D horizontal profiling and 1D vertical electrical sounding (VES) techniques of geophysical prospecting. Two traverses of 100 m length were established with progressive electrode spacings of 5 m, 10 m, 15 m, 20 m, and 25 m using Wenner array. Between station 20 and 30 on traverse 1, anomalous low resistivity (<111 Ωm) zone was observed at a depth up to 15 m. Five (5) VES sounding points were established on the traverses and A, HA, KH and HKH-type curves were identified. The geoelectric sections showed three (3) to five (5) lithologic layers which includes the topsoil, sandy clay, weathered/fractured basement and fresh basement. VES 1 and the well on the geoelectric section were both underlain by fractured basements which revealed that the well is rightly located within the study area. Correlation of 2-D resistivity structures with geoelectric sections revealed that the well is underlain by an aquifer system but abnormal seasonal variation in groundwater supply of the well could have been caused by the delayed recharge or block infiltration during peak rainy season by low resistivity zone which might be a thick column of clay.  Borehole with full casing could be preferred instead of well in the study area at a location far from the clayey formation.