Groundwater recharge in semi-arid karst context using chloride and stable water isotopes

Abstract

The karst aquifers of India are an indispensable source of water to millions of people but the current globalization and climate change have altered the hydrological and other processes and thus have threatened the sustainability of the groundwater reserves in both quantity and quality. The study aims to conceptualize the recharge processes and estimate the groundwater recharge in a karst aquifer located in semi-arid southern India. For this reason, we used stable isotopes of rain and groundwater to understand aquifer dynamics and estimate the recharge volume as well as gain knowledge about different factors that are affecting the aquifers. The isotope data show that the southwest-monsoon rainwater has a weighted mean (WM) δ2H, δ18O, and chloride concentration of −10.5‰, −1.03‰, and 0.81 mg/L, respectively. The rainfall of other seasons is highly depleted in heavy isotopes. The δ2H vs. δ18O plots of rainwater and groundwater suggest that recharge is highly variable in space and time and depends on climatic factors as well. A mixing model based on isotopes and chloride values shows that recharge around highly karstified areas of the aquifer is about 64% and largely infiltrates as fast allogenic type. High hydraulic conductivity and fast transfer velocity make such areas very prone to contamination. Irrigation return-flow contributes about 53% of the aquifer storage in the paddy cultivated areas, which along with several other factors contaminate the groundwater. This study provides an estimated annual replenishable groundwater volume of the Narji Limestone aquifer of 1031 ± 180 MCM. The chemical and isotope modeling enables a better understanding of water resources in fractured karst aquifer and the study encourages their use to investigate the karst hydrodynamics in India and elsewhere as well.