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 (δ88/86Sr) and Ca isotope (δ44/40Ca) 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 δ88/86Sr values (range 0.542 ‰) with shallow (up to 42 mbgl) aquifer samples showing high δ88/86Sr 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 δ44/40Ca values of these samples also show large variability (range 0.76 ‰) which loosely follows the δ88/86Sr trend. The δ88/86Sr-δ44/40Ca 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, δ88/86Sr, and δ44/40Ca 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. © 2025 Elsevier B.V.
