Understanding groundwater systems and their functioning through the study of stable water isotopes in a hard-rock aquifer (Maheshwaram watershed, India)

► Stable-isotopes from hard-rock aquifer in India defined a dynamic pictures. ► Stable isotopes in monsoon cycles allowed defining two geographical monsoon signals. ► The recharge isotopic signature in the watershed fluctuates between the two monsoons. ► Individual open-end wells shows a different functioning depending on recharge and use. Groundwater degradation through abstraction, contamination, etc., shows a world-wide increase and has been of growing concern for the past decades. In this light, the stable isotopes of the water molecule (δ 18O and δ 2H) from a hard-rock aquifer in the Maheshwaram watershed (Andhra Pradesh, India) were studied. This small watershed (53 km 2) underlain by granite, is endorheic and representative of agricultural land-use in India, with more than 700 bore wells in use. In such a watershed, the effect of overpumping can be severe and the environmental effects of water abstraction and contamination are of vital importance. A detailed and dynamic understanding of groundwater sources and flow paths in this watershed thus is a major issue for both researchers and water managers, especially with regards to water quality as well as the delimitation of resources and long-term sustainability. To this end, the input from monsoon-precipitation was monitored over two cycles, as well as measuring spatial and temporal variations in δ 18O and δ 2H in the groundwater and in precipitation. Individual recharge from the two monsoon periods was identified, leading to identification of periods during which evaporation affects groundwater quality through a higher concentration of salts and stable isotopes in the return flow. Such evaporation is further affected by land-use, rice paddies having the strongest evapotranspiration.