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[AHW23-15] Sources of Shallow Groundwater Salinity in the Ganges Delta of Bangladesh
Keywords:Elevated salinity, Upper shallow groundwater, Electrical conductivity, Isotopes, Intertidal environment, Evaporite minerals
The upper shallow groundwater shows slightly acidic to alkaline in nature (6.1-8.5) over the delta. EC ranges from 210-29,400 microsimens/cm with lower salinity in fluvial area (210-2,000 microsimens/cm) than those in tidal domain (383-29,400 microsimens/cm). Cl-/Br- molar ratios are higher in tidal area (160-540,000) than those in fluvial setting (42-2,800). The cross plot between oxygen-18 and deuterium isotopes shows that groundwater data fall on or slightly below the local meteoric water line (Figure 1). Most of the brackish groundwater (n=43, EC>2,000-10,000 microsimens/cm) contains depleted isotope signatures. In EC vs oxygen-18 cross plot, EC mostly increases with almost constant isotopic signatures and this trend does not follow seawater mixing. Tritium concentrations are either low (0.1-2.9 TU, n=6) or below detection limit (<0.3 TU, n=9) in tidal domain, whereas around 57% of data show 0.2-7.2 TU in fluvial area. Radiocarbon (C-14) signatures are also higher in fluvial area (around 50-88 pMC) than those in tidal domain (around 36-86 pMC).
These results indicate that present-day seawater intrusion or connate seawater/estuarine water entrapped within sediments is not a major source of elevated salinity in groundwater. Current seawater intrusion is being occurred probably only in limited areas, i.e., near the coast. Also, it is hypothesized that groundwater salinity was possibly provided through the dissolution of soluble salts in paleosols by paleo-precipitation and concurrent infiltration to aquifers. During the transgression, intertidal environment was dominated in the study area, where evaporites and mud-cracks were likely to be developed during prolonged dry season. Subsequently, the monsoonal precipitation dissolved these evaporite minerals and vertically infiltrated into aquifers through mud-cracks or sediments. The future tasks should concentrate on analyzing whether these processes are occurring currently in the delta as well as reconstruction of paleo-geography for validating the hypothesis.