MINERALOGY AND WATER CHEMISTRY OF SHALLOW AQUIFER OF MATLAB SOUTH, CHANDPUR DISTRICT, BANGLADESH

The study was done in Matlab South Upazila in SE Bangladesh where the main source of drinking water is groundwater from shallow alluvial aquifers. Groundwater chemistry is linked to subsurface sediment characteristics like lithofacies distribution and mineral composition of aquifer sands. Study of these sedimentological properties along with hydrochemical analysis of groundwater samples have been carried out to find out the relation of sediment characteristics with groundwater chemistry. Lithofacies distribution has been delineated using grain size and sediment colors. In general two main lithofacies groups, channel fill (sandy) and over bank (silt-clay) deposits, have been identified. These deposits were further subdivided on the basis of color such as white, off-white, red and black. Red and off-white colors are the characteristics of oxidized/less reduced sediments whereas the black color indicates a more reducing condition. Mineral composition significantly varies with the sediment color and grain size. Red color sediments contain less metastable minerals (hornblende, actinolite, kyanite and pyroxenes etc.) than off-white and black sediments. Biotite and chlorite is relatively high in black and white sediments than red or off-white sediments. Mineral composition and sediment color also vary within different size fractions of the same sediment facies. Finer fractions have more metastable minerals than coarser fractions.

Hydrochemical analyses of groundwater samples collected from aquifers of respective color groups shows that the red and off-white sands yield Na–Ca–Cl–HCO₃ to Na-Ca–Cl or Na–Cl type water whereas black/white sands yield Ca-Mg-HCO₃ to Ca-Na-HCO₃ type water. Groundwater of red and off-white sediments contain low amount of dissolved arsenic (As) but high manganese (Mn) and black sediments shows very high concentration of As but relatively low Mn. The black group of sediments represent a reducing geochemical environment indicated by high concentrations of dissolved As, NH₄¯, PO₄³¯ and HCO₃¯, and very low SO₄²¯ in groundwater. Whereas, the groundwater from red, white/off-white group of sediments show low As, NH₄¯, PO₄³¯, and HCO₃¯ concentrations but relatively high SO₄²¯ indicating a less reducing/oxidizing environment.