Deuterium Excess of Groundwater as a Proxy for Recharge in an Evaporative Environment of a Granitic Aquifer, South India

The deuterium excess (d-excess) is a function of the composition of stable isotopes, oxygen (δ18O), and deuterium (δD) in water. It contains information about initial moisture source, evaporation effects during monsoon, and recirculation of moisture from large inland water. To meet the objectives of this study, a total of seventy-two groundwater samples were collected from bore wells during the pre and post-monsoon seasons. d-excess values vary from — 52.81 to 2.29 ‰ with a mean of −9.90 ‰ and −60.89 to 9.44 ‰ with a mean of −0.7‰ in the pre and post-monsoon seasons respectively. Based on the δ18O concentration groundwater samples are classified into three groups. Group I and II samples having high d-excess with respect to δ18O, which indicates the dry conditions in continental local water air and their source of water vapor. Most of the deep groundwater wells (> 20 m bgl) fall under these categories, which indicates low degree of evaporation. Group III samples having low d-excess values and enriched with δ18O, indicates that these waters have undergone evaporation to different extents before recharge. Most of the shallow (< 10 m bgl) and moderate groundwater wells (10–20 m bgl) fall under this category, indicating high degree of evapotranspiration originated either from unsaturated or saturated zones. The climatic water balance studies also indicate that the annual average potential evapotranspiration (PET) is higher (2131 mm) than the annual average rainfall (662 mm) in the study period. This study reveals that the diffuse rainfall recharge flows are more dominated to recharge the aquifer during the monsoon season and the past/rainfed recharge flows through preferential pathways along the gradients in the fractured zone are dominated to recharge the aquifer in the post-monsoon season. These processes play a significant role to keep the hydrological balance in areas of high evaporation.