Isotope mass balance estimation of groundwater recharge in a hard rock tropical river basin in Kerala, India

To better assess the groundwater recharge pattern of the Kuttiyadi River Basin (KRB) situated in Northern Kerala, the oxygen (delta O-18 %) and hydrogen (delta D%) isotopic compositions of river water, reservoir water and groundwater were determined. Isotopic characterization of the waters of the basin showed that river water experienced altitude effect from lowland (elevation: <7 m) to midland (elevation: 7-75 m) and highland (elevation: >75 m). Isotopic enrichment of river water near the mouth during premonsoon and post-monsoon can be due to the influence of sea water ingress especially during tidal events. Additionally, correlation of O-18 and electrical conductivity (EC) further highlights the role of evaporation in isotopic modification of surface water. The regression plot of delta O-18 and delta D of groundwater with LMWL (Local Meteoric Water Line) and surface water samples clearly showed the meteoric origin of groundwater. Using oxygen isotope mass balance method it is estimated that surface water contribution to groundwater was up to 17% in the lowland physiographic zone and it was up to 14% in the midland zone and only 7% surface water is contributing to groundwater regime in highland zone of the KRB. This study demonstrates the usefulness of stable isotopic ratios for baseline assessments of water resources of a river basin or watershed.