Abstract
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 km2) 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.
Highlights
Crystalline-rock aquifers are important in India, as crystalline rocks underlie two-thirds of the country (Mukherjee et al 2015) and groundwater is widely abstracted from crystalline aquifers for irrigation
The sustainability of abstraction and its impact on surface water is contested in Indian crystalline-rock aquifers: there is some evidence from groundwater monitoring boreholes and satellite measurement of Earth’s gravity that groundwater levels are not declining (Tiwari et al 2011) or that they are declining
Connectivity of groundwater flow within crystalline-rock aquifers controls the sustainability of abstraction, baseflow to rivers and the transport of solutes
Summary
British Geological Survey, The Lyell Centre, Edinburgh EH14 4AP, UK. Crystalline-rock aquifers play an important role in catchment hydrology (Ofterdinger et al 2019), capable of providing domestic water supplies (Chilton and Foster 1992) and larger supplies for irrigation or towns (Maurice et al 2019) as well as sustaining baseflow to rivers and aquatic ecosystems (Comte et al 2019). The sustainability of abstraction and its impact on surface water is contested in Indian crystalline-rock aquifers: there is some evidence from groundwater monitoring boreholes and satellite measurement of Earth’s gravity that groundwater levels are not declining (Tiwari et al 2011) or that they are declining. Loss of baseflow has been hypothesised as a major factor in reduced river flows such as in the Arkavathy River basin (4,253 km2), west of Bangalore, where the drying of the river can be attributed to neither precipitation nor potential evapotranspiration (Srinivasan et al 2015; Penny et al 2018)
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