Hydrogeophysical insight to delineate the potential zones for natural recharge in Basaltic aquifer of Central India

A number of recharge structures need to be proposed to develop minor irrigation in the Tapi River Basin, Central India. However, the best location for finding the conductive layer or weak zone of fault/fracture for maximum recharge is to be explored using geophysical techniques. The reason behind taking this task involves directing the water from Tapi river into a 25 km-wide valley like structure that runs between the river Tapi and the Satpuda mountain chains. The Electrical methods in delineating recharge zones sites for recharge structures in Tapi alluvial river basin were employed for the purpose. The geophysical investigation using Electrical Resistivity Tomography [ERT] was carried out for obtaining information upto depth of 200 meters and delineate the aquifer and aquiclude layers with their depth. Geologically the area is covered by both younger and older alluvium of quaternary to recent age. In Deccan Trap Basalt, groundwater occurs mostly in the upper weathered and fractured parts down to 20-25 m depth. At places potential zones are encountered at deeper levels in the fractures and inter-flow zones. The upper weathered and fractured parts form phreatic aquifer and groundwater occurs under water table (unconfined) conditions. At deeper levels, the groundwater occurs under semiconfined to confined conditions. The Tapi Alluvium can be subdivided into two sub-units, i.e., the upper younger alluvium extending down to 70-80 m depth and the deeper older alluvium attaining a maximum depth of 450 m. However, only upper 70-80 m of younger alluvium, having 2 to 5 layers of granular zones of sand and gravel ranging in thickness from 2 to 20 m, forms the potential aquifer. At deeper levels the alluvium is mostly clayey and does not form potential aquifer. Groundwater in alluvium occurs under water table, semi-confined and confined conditions. We obtained the resistivity values and inverted them for the pseudosection. Electrical Resistivity Tomography was carried out at 33 selected locations in and around Tapi Basin near Amravati River and sub tributaries. The continuity of structures are clearly correlated in the north and south part of the study area. The thick alluvium (proposed) which is correlated in the continuous profile length of Sites in the north part of the study area, is very thick and shallow. Similarly the southern part brings out the potential aquifer zone in the shallow region.