Sr and ⁸⁷Sr/⁸⁶Sr in estuaries of western India: impact of submarine groundwater discharge

Abstract

Dissolved Sr and 87Sr/86Sr are measured in the Narmada, Tapi and the Mandovi estuaries linked to the eastern Arabian Sea. The concentration of dissolved Sr and ⁸⁷Sr/⁸⁶Sr in the river water endmembers show significant differences reflecting the lithologies they drain. The distribution of Sr in all these estuaries shows a near perfect two endmember mixing between river water and seawater suggesting that there is no discernible net addition/removal of Sr from the estuarine waters. In contrast, ⁸⁷Sr/⁸⁶Sr shows non-conservative behaviour in all these estuaries, its distribution exhibits significant departure from the theoretical mixing lines. A likely mechanism for this difference in the behaviour between dissolved Sr and its ⁸⁷Sr/⁸⁶Sr is the discharge of submarine groundwater (SGD) which can modify the ⁸⁷Sr/⁸⁶Sr of the estuarine waters by exchange with sediments without causing measurable changes in Sr concentration. The impact of such an exchange process on the ⁸⁷Sr/⁸⁶Sr of the estuaries and therefore on the Sr isotope composition of dissolved Sr entering the Arabian Sea differs among the three estuaries and also between seasons in the Narmada. The non-conservative behaviour of ⁸⁷Sr/⁸⁶Sr provides a handle to estimate the quantum of SGD to these estuaries. The Sr concentration, ⁸⁷Sr/⁸⁶Sr ratio and salinity of the submarine groundwater and estimate of its fluxes to the Narmada estuary have been made using inverse model calculations. The model derived SGD flow rates are ∼5 and 280 cm/day during pre-monsoon and monsoon, respectively. The more radiogenic Sr isotope composition of SGD relative to the seawater suggests that SGD acts as an additional source of 87Sr to the Arabian Sea.