Insight into surface water-groundwater interaction derived solute mobilization using stable isotopes in Sundarbans delta front aquifer: An implication to drinking water sustainability

Safe drinking water in the Sundarbans, a UNESCO world heritage site, is increasingly at risk as coastal aquifers are repeatedly impacted by salinity intrusion, tidal influence, cyclones, and anthropogenic stresses such as population growth, urbanization, and over-extraction. Consequentyly, this ecologically vulnerable delta-front region is experiencing growing groundwater stress, threatening about 4.5 million people. Therefore, this study aimed to assess groundwater vulnerability in the Sundarbans using stable water isotopes (δ²H and δ¹⁸O) and hydrochemistry to apprehend SW-GW mixing processes and salinity pathways from coastal shoreline to further inland across four zones (I, II, III, and IV). Results revealed that despite an increasing rainfall trend, groundwater levels (GWL) and salinity (Cl⁻) have shown a decreasing trend over the past decade. Groundwater salinity and δ¹⁸O values varied widely in Zone I (salinity: 0.7–4 PSU, δ¹⁸O: −2.3 to +0.5‰) and Zone II (0.7–9 PSU, δ¹⁸O: −2.8 to −0.5‰), while Zones III and IV exhibited narrow ranges (Zone III: salinity 0.8–1.2 PSU, δ¹⁸O: −2 to −0.6‰; Zone IV: salinity 0.7–1.2 PSU, δ¹⁸O: −1.6 to −1.1‰). Groundwater in zones I and IV closely aligns with the Global and Local Meteoric Water Line (GMWL and LMWL), indicating direct meteoric recharge. In contrast, groundwater zones II and III slightly deviates, suggesting evaporative enrichment prior to recharge. This is further supported the by average d-excess in zones I, II, III, and IV are 4.5 ± 3‰, -2. ± 1.7‰, -2 ± 1‰, and 5 ± 6‰, respectively. River water, with high salinity (10 and 24 PSU in Zones I and II, respectively), appears to be a major source of saline intrusion, and seawater near coast (salinity: 33 PSU, zone IV)  elevating groundwater salinity suggesting the potential pathways of SW-GW interaction and solute mobilization contributing to groundwater vulnerability in the study area. Consequently, it is evident that the inland groundwater is more depleted indicating monsoonal rainfall recharge with very less maritime influence while the delta-front groundwater near shoreline suggest enriched isotopic signature indicates possible vertical mixing which raise concern for water security. Therefore, this study emphasizes for implying immediate and effective groundwater management strategies for sustainable drinking-water management in the Sundarbans.

Keywords: Groundwater; Coastal aquifers; Stable isotopes; δ²H; δ¹⁸O; Sundarbans; Salinity intrusion; Drinking water security