Influence of Tropical Cyclone Remal on Porewater Toxic Solute Displacement in a Tropical Estuary Adjacent to the Bay of Bengal

This periodic field-based study investigates pre- and post-cyclonic [Days (D): 0, 15, 70] hydrochemical and isotopic (222Rn, δ18O, and δ2H) variations in porewater (PW) as it is a bridge in the land–sea hydraulic connection. Results revealed that cyclone-associated decline in atmospheric pressure and storm surge caused transient offshore displacement of the seawater–groundwater (SW–GW) interface, enhancing fresh groundwater discharge, as indicated by the depleted δ18O (Day-0: −3.14 ± 0.13‰, Day-15: −3.04 ± 0.06‰, and Day-70: −4 ± 0.1‰), elevated 222Rn (Day-0: 845 ± 375 Bq/m3, Day-15: 1920 ± 250 Bq/m3, Day-70: 238 ± 60 Bq/m3), and reduced salinity (Day-0: 28 ± 0.31 PSU, Day-15: 32.41 ± 1.17 PSU, Day-70: 32.75 ± 0.45 PSU). Post-cyclone recovery promoted landward interface migration and partial geochemical stabilization. Uranium concentrations increased plausibly under oxidizing, carbonate-buffered conditions without speciation change, while fluoride mobilization was potentially controlled by coupled carbonate–fluorite equilibria and divalent cation availability, and lithium concentration declined due to dilution. Principal component and hierarchical cluster analysis revealed systematic shifts from marine-dominated pre-cyclone conditions to groundwater-dominated post-cyclone regimes. Thus, the study highlights PW as a sensitive tracer of cyclone-related SW–GW interactions and demonstrates how extreme weather events rapidly reorganize solute transport and geochemical processes in coastal aquifers.