HYDROGEOCHEMICAL EVOLUTION AS A FUNCTION OF HYDROSTRATIGRAPHY ACROSS THE GANGES RIVER DELTA: INSIGHTS FROM MAJOR-ION CHEMISTRY AND STABLE ISOTOPIC SIGNATURES OF GROUNDWATER

The arsenic contamination in the groundwater of the Ganges-Brahmaputra-Meghna River delta has garnered significant scientific attention over the last few decades. However, the hydrogeochemical signatures across the delta remain less studied. Recent studies showed that hydrostratigraphically the Ganges River delta transitions from being a single unconfined [Type I] aquifer in the furthest inland to a transitional semiconfined and vertically segregated [Type II] aquifer system, and then to a nearly confined and multilayered [Type III] aquifer system along the delta mouth.

In our recent study, we attempt to characterize the varying major-ion chemistry and stable isotopic signatures of groundwater within the major aquifer sub-systems and thus, understand the hydrogeochemical evolution pathways across the Ganges River delta. Analyses of hydrogeochemical and stable isotopic data of groundwater from 306 groundwater wells sampled across the delta provide clear evidence of hydrogeochemical evolution from Ca²⁺ to Na⁺/K⁺-rich groundwater chemistry along the regional flow paths. The Type I aquifer is dominated by Ca²⁺-HCO₃⁻ type freshwaters across depth with relatively depleted δ¹⁸O values, indicating high meteoric recharge due to higher elevation and absence of large-scale surficial/intercalating confining units above/within the aquifer. On the contrary, the layered Type II and III aquifers exhibit variation in major hydrogeochemical and isotopic signatures of groundwater across depth classes [shallow: ≤70 m, intermediate: >70–150 m, deep: >150 m bgl]. The deep aquifers of Type II and III systems are dominated by Na⁺-HCO₃⁻ facies suggesting evolved groundwater. The intermediate aquifers of Type II and III systems and the shallow aquifers of Type III system are marked by Na⁺-Cl⁻ type brackish water and very high total dissolved solid concentrations, indicating a high influx from the Bay of Bengal. On the contrary, the shallow aquifers of the Type II system further inland are dominated by Ca²⁺-HCO₃⁻ type waters. These observations align with the hypothesis from previous findings, suggesting that the intercalating confining units within the Type II and III systems substantially restrict vertical connectivity within aquifers which results in heterogeneous hydrogeochemical signatures along the depth.