Evaluation of processes controlling the geochemical constituents in deep groundwater in Bangladesh: spatial variability on arsenic and boron enrichment.

Forty-six deep groundwater samples from highly arsenic affected areas in Bangladesh were analyzed in order to evaluate the processes controlling geochemical constituents in the deep aquifer system. Spatial trends of solutes, geochemical modeling and principal component analysis indicate that carbonate dissolution, silicate weathering and ion exchange control the major-ion chemistry. The groundwater is dominantly of Na-Cl type brackish water. Approximately 17% of the examined groundwaters exhibit As concentrations higher than the maximum acceptable limit of 10 microg/L for drinking water. Strong correlation (R(2)=0.67) of Fe with dissolved organic carbon (DOC) and positive saturation index of siderite suggests that the reductive dissolution of Fe-oxyhydroxide in presence of organic matter is considered to be the dominant process to release high content of Fe (median 0.31 mg/L) in the deep aquifer. In contrast, As is not correlated with Fe and DOC. Boron concentration in the 26% samples exceeds the standard limit of 500 microg/L, for water intended for human consumption. Negative relationships of B/Cl ratio with Cl and boron with Na/Ca ratio demonstrate the boron in deep groundwater is accompanied by brackish water and cation exchange within the clayey sediments.

Hydrogeochemistry and arsenic contamination of groundwater in the Ganges Delta Plain, Bangladesh.

Geochemical composition and the level of Arsenic (As) contamination of groundwater in the Ganges Delta Plain, southwestern Bangladesh were elucidated. Hydrogeochemical data of tube well samples suggested that the groundwater is mostly Ca-Mg-HCO(3) type with bicarbonate (HCO(3)(-)) as the dominant anion, though other type waters are also observed. In contrast, the elevated EC, Cl(-) and high content of Na(+) relative to Ca(2+), Mg(2+) and K(+) in six groundwater samples suggest their saline origin. Low concentrations of NO(3)(-) and SO(4)(2-), and high concentrations of dissolved organic carbon (DOC), HCO(3)(-) and PO(4)(3-) indicate the reducing conditions of subsurface aquifer where sediments are deposited with abundant organic matter. The total As concentration in the analyzed samples is very high (0.0431-1.352 mg/L) along with high Fe (2.791-17.058 mg/L) and relatively low Mn (0.134-1.972 mg/L) at different depths. Distinct relationship of As with Fe and Mn, and strong correlation with DOC suggests that the biodegradation of organic matter and reductive dissolution of Fe-oxyhydroxide is considered to be the dominant processes to release As in aquifers. Moreover, negative correlation between As and SO(4)(2-) demonstrates the As may not be directly mobilized from sulfide minerals like arsenopyrite.