Groundwater connectivity of a sheared gneiss aquifer in the Cauvery River basin, India

Connectivity of groundwater flow within crystalline-rock aquifers controls the sustainability of abstraction and baseflow to rivers, yet is often poorly constrained at a catchment scale. Here groundwater connectivity in a sheared gneiss aquifer is investigated by studying the intensively abstracted Berambadi catchment (84 km 2 ) in the Cauvery River Basin, southern India, with geological characterisation, aquifer properties testing, hydrograph analysis, hydrochemical tracers and a numerical groundwater flow model. The study indicates a well-connected system, both laterally and vertically, that has evolved with high abstraction from a laterally to a vertically dominated flow system. Likely as a result of shearing, a high degree of lateral connectivity remains at low groundwater levels. Because of their low storage and logarithmic reduction in hydraulic conductivity with depth, crystalline-rock aquifers in environments such as this, with high abstraction and variable seasonal recharge, constitute a highly variable water resource, meaning farmers must adapt to varying water availability. Importantly, this study indicates that abstraction is reducing baseflow to the river, which, if also occurring in other similar catchments, will have implications downstream in the Cauvery River Basin.