Nature of arsenic pollutants in groundwater of Bengal basin - A case study from Baruipur area, West Bengal, India

High incidence of arsenic in groundwater occurs as several small patches surrounded by zones with low arsenic in vast areas of West Bengal, India and adjoining Bangladesh. From the low- and high-arsenic groundwater zones, subsurface sediments have been analysed for mineral and chemical variations, in Baruipur, the worst arsenic-affected area in West Bengal. The Holocene sedimentary succession, in this part of Gangetic alluvium, shows a fining upward sequence from medium to fine sand, silt and finally to clay with occasional peat layer. The clay layer (As = 2–31 mg/kg) and peat layer (As = 9–40 mg/kg) contain higher concentration of a rsenic compared to the silt layer (As = 2–6 mg/kg) and the sand layer (As = 1–7 mg/kg). It is inferred that the arsenic contamination in groundwater is related more with the components of sandy layer than others. Within the sandy layer, the components like (i) coated iron oxy hydroxides with residual magnetite and ilmenite (As = 14–112 mg/kg), (ii) illite (As = 10–40 mg/kg), (iii) iron hydroxide-coated sand grains (As = 30 mg/kg), (iv) chlorite (As = 5–31 mg/kg), (v) biotite (As = 9 mg/kg), and (vi) siderite concretions (As = 7–9 mg/kg) store most of the arsenic and are identified as the arsenic pollutants for groundwater in this part of Bengal basin. Arsenic is introduced into the aquifer sediments in soluble state and get adsorbed on iron -rich clastic grains and on authigenic siderite concretions. Adsor ption of the element on the sites and its co nsequent desorption to the groundwater are interpreted to be controlled by microbial activity in the aquifer sediments. THE problem of arsenic contamination in groundwater in large areas of West Bengal and Bangladesh has been receiving wide attention because groundwater is the major source of drinking water in this part of the world. This vast area appears to be contaminated and also locally polluted and a large number of people in this densely populated region are likely to be affected by the menace.