Geological controls on groundwater chemistry and arsenic mobilization: Hydrogeochemical study along an E–W transect in the Meghna basin, Bangladesh

Abstract

Summary Hydrogeochemical investigations along an E–W transect in the middle Meghna basin show groundwater chemistry and redox condition vary considerably with the change in geology. Groundwater in the Holocene shallow ( 10 μg/l) and salinity. On the other hand, groundwater from the Pliocene Dupi Tila sandy aquifer in the eastern part is fresh and low in As ( HCO 3 - , Fe and dissolved organic carbon (DOC), but generally low in SO 4 2 - and NO 3 - . High HCO 3 - concentrations (250–716 mg/l) together with high DOC concentrations (1.4–21.7 mg/l) in these aquifers reflect active sources of degradable natural organic matter that drives the biogeochemical process. There is generally de-coupling of As from other redox-sensitive elements. In contrast, the Pliocene aquifers are low in As, HCO 3 - and DOC. Molar ratio of HCO 3 - /H4SiO4 suggests that silicate weathering is dominant in the deeper Holocene aquifers and in the Pliocene aquifers. Molar ratios of Cl−/ HCO 3 - and Na+/Cl− suggest mixing of relict seawater with the fresh water as the origin of groundwater salinity. Speciation calculations show that saturation indices for siderite and rhodochrosite vary significantly between the Holocene and Pliocene aquifers. Stable isotopes (δ2H and δ18O) in groundwater indicate rapid infiltration without significant effects of evaporation. The isotopic data also indicates groundwater recharge from monsoonal precipitation with some impact of altitude effect at the base of the Tripura Hills in the east. The results of the study clearly indicate geological control (i.e. change in lithofacies) on groundwater chemistry and distribution of redox-sensitive elements such as As along the transect.