Stable strontium (δ88/86Sr) and calcium (δ44/40Ca) isotope fractionation in coastal groundwater and its implications for the transport of dissolved cations to the ocean

Abstract

Coastal aquifers, a critical component of the land-ocean continuum, act as hotspots of fresh groundwater and seawater mixing while reactive processes further govern the elemental flux to the ocean in form of groundwater discharge. Here we report stable Sr (δ^88/86Sr) and Ca isotope (δ^44/40Ca) data for groundwater samples collected from multiple depths (14–333 mbgl) from two coastal aquifers in the Ganges River delta (Sundarbans). Significant variability is observed in δ^88/86Sr values (range 0.542 ‰) with shallow (up to 42 mbgl) aquifer samples showing high δ^88/86Sr values (up to 0.666 ‰ relative to NIST SRM987), which indicates pronounced interaction with seawater and fractionation induced by Sr removal during carbonate precipitation. The δ^44/40Ca values of these samples also show large variability (range 0.76 ‰) which loosely follows the δ^88/86Sr trend. The δ^88/86Sr-δ^44/40Ca correlation is relatively poor for shallow groundwater samples and reflects differences in fractionation mechanisms during carbonate precipitation for Sr and Ca isotopes. In contrast to the shallow aquifer samples, the deep aquifer samples display limited seawater influence. The depth-bound variability of molar Sr/Ca, δ^88/86Sr, and δ^44/40Ca suggests considerable removal of solute Sr and Ca due to secondary mineral formation, dominated by carbonates at shallower depths consistent with saturation index (SI) calculations. This study highlights that formation of secondary minerals in coastal aquifers, as inferred from stable Sr and Ca isotopes, can affect the transport of highly soluble elements like Sr and Ca from the continents to the oceans and has implications for solute geochemistry in coastal aquifers.