Water chemistry and sediment-water interaction responsible for mobilization of arsenic in groundwater in Bengal delta plain

摘要

Recent development of groundwater in the Bengal Delta Plain (West Bengal, India and Bangladesh) has been envisaged as a problem of global concern due to the occurrence of arsenic much above the drinking water standards of international (10 μg 1~(-1)) as well as national levels (50 μg 1~(-1)). The current situation is a threat to human health. Understanding the geochemical processes leading to high As concentrations may provide suitable guidelines to mitigate the problem. Examination of water quality reveals that the groundwater is anoxic with high arsenic, dissolved iron (II), manganese (II), phosphate and bicarbonate as well as low nitrate and sulphate. Water chemistry reflects a positive correlation of arsenic with dissolved iron, phosphate and alkalinity as well as a negative correlation with dissolve oxygen and nitrate. Arsenic is derived as a consequences of desorption and reductive dissolution of Fe-oxyhydroxide occurring as coating on sedimentary grains. Reduction of the iron is regulated by microbial processes with the help of sedimentary organic matter. They are the possible electron donors in the reduced zone of the aquifer rich in fine grained sediments. Evidence of arsenic release mechanisms provides for the delineation of a contaminated zone and for future drilling of new waterwells in a safer zone to supply safe drinking water.