Interaction of shallow and deep groundwater with a tropical ocean: Insights from radiogenic (87Sr/86Sr) and stable isotope cycling and fluxes

Coastal groundwater is susceptible to physico-chemical modification from interaction with seawater and other surface waters. Surface water-groundwater (SW-GW) interaction can alter the Sr concentration and radiogenic 87Sr/86Sr signature of both seawater and groundwater from multi-depth aquifers. In this study, we document such an interaction between a tropical ocean (Bay of Bengal [BoB]) and the coastal aquifers of a large mega-deltaic system formed by the Himalayan-sourced Ganges River, at shallow (10–50 m below ground level [bgl]), and deeper (115 and 333 m bgl) depths, using radiogenic strontium isotopes (87Sr/86Sr), stable isotope ratios (δ18O and δD), salinity and dissolved solutes. The mean 87Sr/86Sr for shallow coastal aquifers (10–50 m bgl: 0.71094) suggests that seawater mixes with the terrestrial-sourced shallow groundwater, modifying them to brackish water. This is further supported by the stable isotope signatures (14–25 m bgl: −3.63 to −0.7 ‰ and 30–50 m bgl: −3.5 to −1.2 ‰ δ18O). The radiogenic 87Sr/86Sr (115 m bgl: 0.71681 and 333 m bgl: 0.71995) and depleted δ18O (115 m bgl: −5.04 to −1.61 ‰ and 333 m bgl: −4.43 to −2.38 ‰) suggest relatively less to negligible mixing between seawater and terrestrial-sourced resident groundwater at greater depths. The mixing process is additionally characterized by a significant Sr flux discharged from these coastal aquifers to the BoB, which ranges between 7.7 × 104 and 12 × 105 mol/year for shallow aquifers, and between 1.78 × 104 and 8.26 × 104 mol/year for deep aquifers, respectively. The overall contribution of Sr from old groundwater of deep aquifers is 1.43 % (115 m bgl) and 0.66 % (333 m bgl), whereas shallow aquifers show a higher contribution, ranging from 6.18 to 9.57 % of BoB Sr budget. This study suggests that the discharge of recirculated brackish water to the BoB from the shallow aquifers contributes more than 5 times higher Sr to the oceanic budget than the deep aquifer, contributing as an essential component of the global oceanic budget of Sr.