Substantial submarine groundwater discharge in the estuaries of the east coast of India and its impact on marine strontium budget

Abstract

The concentration and isotopic compositions of dissolved Sr (87Sr/86Sr) were measured in four major estuaries of the east coast of India, including those of the Ganga (Hooghly), Mahanadi, Godavari, and the Krishna during their peak discharge period, July-September 2013, along with particulate matters in some of them. Dissolved Sr displays strong conservative behaviour in all four estuaries within the analytical uncertainties, emphasizing simple mixing between river water and seawater. Sr isotopic composition, however, behaved non-conservatively in all four estuaries. Dissolved 87Sr/86Sr of many samples in these estuaries deviate significantly from the theoretical mixing line defined by seawater and river water, indicating some additional sources of radiogenic Sr in all four estuaries. Such deviation in the Ganga estuary could result from a minor dissolution of suspended material with very radiogenic Sr in the turbid zone of the estuary. However, several lines of evidence support submarine groundwater discharge as an important source of Sr in these estuaries. We used an inverse modelling approach to characterize the significance of submarine groundwater discharge and quantified its fluxes from the four estuaries based on 87Sr/86Sr in this study, highlighting its potential to estimate groundwater contribution to the water budget in estuaries. The fluxes of submarine groundwater discharge from the estuaries of the east coast of India is estimated to be ∼ 11000 ± 900 m3s−1, dominated by the Ganga-Brahmaputra system, and delivers ∼ 1.1 ± 0.06 × 109 moles of Sr annually with quite radiogenic 87Sr/86Sr, 0.7228, comparable to their riverine fluxes. These estuaries contribute 0.3 – 0.7% to the global submarine groundwater discharge derived Sr. This study reveals a large supply of radiogenic Sr from submarine groundwater discharge which further increases the requirement of less radiogenic Sr to balance the continental radiogenic Sr in the global marine Sr budget.