Submarine groundwater discharge to a tropical coastal lagoon (Chilika lagoon, India): An estimation using Sr isotopes

Abstract

Submarine groundwater discharge (SGD) is an important component of the marine 87Sr/86Sr budget, which is currently in an imbalance with a missing source. In this contribution, dissolved Sr concentrations and 87Sr/86Sr of the Chilika lagoon (India), the largest brackish-water lagoon in Asia, have been investigated for three different seasons (pre-monsoon (May 2017), monsoon (Aug., 2017) and post-monsoon (Jan., 2018)) to infer coastal behavior of Sr and estimate the SGD fluxes to the coastal ocean. Major source waters (groundwater, river and seawater) and suspended sediments from the lagoon system have also been analyzed for source characterization. Salinity and Sr concentrations of the Chilika samples show wide variations during pre-monsoon (0.2–35.8; 1–93 μmol/kg), monsoon (0.1–20.1; 0.8–55 μmol/kg) and post-monsoon (0.3–7.7; 1–20 μmol/kg) seasons. Despite of these variations, salinity and Sr concentrations of the lagoon co-vary linearly as expected for conservative mixing between river and seawater. In contrast, the mixing plot between 1/Sr and 87Sr/86Sr during the monsoon and pre-monsoon seasons deviate from the river-seawater mixing trend, indicating an additional source/sink of 87Sr to this lagoon. The non-conservative behavior of 87Sr/86Sr during monsoon has largely been restricted to low salinity (< ~2) regime, which could be attributed to subsurface ion-exchange process. During the pre-monsoon, the SGD can explain the non-conservative isotopic behavior that requires additional water supply with higher 87Sr/86Sr ratios. The SGD fluxes have been estimated using two separate approaches, (i) using an inverse model with fixed SGD composition and (ii) using a source-mixing computation using variable SGD compositions within the lagoon. These computations estimate that the SGD contributes ~20% of total water supplied to the Chilika lagoon during the pre-monsoon season. This SGD contribution corresponds to a flux of 1.51 × 106 m3/d to the lagoon. Data from this and earlier studies indicate that the 87Sr/86Sr ratios of the SGD to the western Bay of Bengal, which receives water from several large rivers from the Himalaya and Peninsular Indian regions, are relatively higher (~0.715) than the seawater value (0.7092). The SGD flux to the east coast of India, therefore, would not contribute in reducing the oceanic imbalance, which requires a less-radiogenic source.