Stable isotopic source apportionment of shallow groundwater in Southwest coast of India

Abstract

This study examines the spatio-temporal variation in the stable isotopic composition of oxygen (δ¹⁸O) and hydrogen (δD) in groundwater from alluvial and lateritic aquifers in Northern Kerala, India, in relation to seasonal rainfall patterns and hydrogeological settings. The isotopic composition of rainwater in the study area exhibits higher variability during the monsoon seasons, attributed to the influence of cyclonic activity and isotopic fractionation during precipitation events. Groundwater in both alluvial and lateritic aquifers of the study area exhibits distinct seasonal variations in δ¹⁸O and δD values between pre-monsoon and post-monsoon periods, reflecting a shift from evaporation-influenced conditions during the pre-monsoon to dominant meteoric recharge in the post-monsoon season. Negative isotopic separation (Δδ) values and regression characteristics indicate that Southwest Monsoon (SWM) rainfall is the primary source of groundwater recharge, with post-monsoon depletion reflecting seasonal mixing rather than dominant North East Monsoon (NEM) influence. Deuterium excess in groundwater serves as an effective proxy for recharge dynamics, and its relationship with Total Dissolved Solids (TDS) highlights the influence of infiltration rates, evaporation intensity, and aquifer permeability on groundwater recharge processes. A mass balance approach estimates that rainwater contributes 35.7% of recharge in alluvial aquifers and 32% in lateritic aquifers, with alluvial aquifers receiving more direct recharge due to their shallow water table. These findings provide crucial insights for sustainable groundwater management in the region.