ABSTRACT
ABSTRACT: Groundwater with geogenic arsenic enrichment is commonly encountered in the Holocene sedimentary aquifers of the Bengal Delta Plain (BDP). The present study was carried out in Brahmanbaria district, covering an area of 18 km2 in northeastern Bangladesh. The Chandina Formation is the main hydrostratigraphic unit of the area, which comprises silt and clay with high content of organic matter. Dissolved arsenic concentrations in groundwater are high, reaching 400g/L in some wells. Groundwater is reducing with general lack of detectable dissolved oxygen (DO) and contains low concentrations of nitrate and sulfate. Concentrations of dissolved Fe are high, which is in general in agreement with the reductive dissolution of ferric oxide and hydroxide hypothesis. Results of speciation modeling indicated the possibility of precipitation of siderite, and to less extent, vivianite for many samples. The log PCO2 values were extremely high (1.0 atm), suggesting production of CO2 in redox reactions involving the organic matter in the sediments. Redox potential values calculated on the basis of different redox couples and field Eh measurement indicated redox disequilibrium. Hierarchical cluster analysis (HCA) performed in paired groups mode using the program PAST indicated highest degree of similarity among redoxsensitive elements NO3, Mn, Fe, PO4, SO4, As, and pH. Na and Cl form a distinct group, which indicate the influence of sea water. Bicarbonate generated in several redox reactions and carbonate dissolution was linked to almost all parameters and this holds even more for the electrical conductivity (EC). Principal components analysis (PCA) yielded Principal Component 1 (PC1) corresponding to sea water, and Principal Component 2 (PC2) corresponding to redox reactions with generally high arsenic concentrations. In summary, combination of speciation modeling and multivariate statistics proved to be useful in testing of conceptual model of geochemical evolution of arsenicrich groundwater.
