Tracing Deep-Seated Saturated Fractures in Depleted Shallow Aquifer Systems in a Granitic Terrain: An Integrated Hydro-geophysical Approach

Abstract

Groundwater is a vital renewable natural resource that largely supports the agriculture sector, especially in semi-arid climate of hard rock. However, the over-exploitation and inadequate recharge of groundwater in crystalline granitic terrains have depleted the shallow aquifer systems constraining the groundwater to be sporadically distributed in deep fractures. Therefore, tracing bedrock fractures becomes important, but the overlying thick pile of unsaturated saprolite layer presents a challenge to map them due to geophysical ambiguity. Currently, most studies have been done at laboratory scale, while bedrock fractures at natural field conditions are rarely attended as evidenced by numerous failures of borehole drillings in semi-arid hard rock terrain. To trace saturated bedrock fractures at natural field sites, we performed a multi-disciplinary experiment comprising hydro-geological insights, social information, remote sensing, gradient resistivity profile (GRP), vertical electrical sounding (VES) and electrical resistivity tomography (ERT) followed by exploratory borehole drillings, and hydro-chemical source speciation in a semi-arid, crystalline granitic terrain in southern India. The results showed (1) GRP as a precursor records the signatures of saturated bedrock fractures qualitatively, (2) least square inversion models of ERT demarcate distinct litho-units and saturated bedrock fractures, (3) exploratory borehole drilling shows saturated bedrock fractures at 49–54 m and 63–67 m depth designated with high yield (Q = 3382 lph), which compare well with electrical imaging results, and (4) hydro-chemical source speciation with dominated alkali-feldspar (albite) weathering confirmed groundwater from bedrock fractures, which supplemented the geophysical anomalies. These observations led to a practical step-by-step field guide for tracing deep-seated bedrock fractures in geologically similar semi-arid regions.