Arsenic pollution in groundwater of West Bengal, India - An insight into the problem by subsurface sediment analysis

Arsenic values in groundwater above the maximum permissible limit of 0.05 mg/l have been reported from several areas of the lower delta region of the Ganga-Padma river system. It is confined to areas east of Hooghly River and to the shallow aquifer (<150 m below ground level). The aquifer sediments are channel fill sand and over-bank levee mud, deposited during late Holocene and Recent period. The present study at Chakdah in Nadia district and at Baruipur in 24-Parganas (S) district, West Bengal, demonstrates that tube wells yielding arsenious water occupy small, isolated clusters surrounded by safe water yielding tube wells. This is inferred to be the general pattern for the entire delta. Arsenic values in sub-surface sediments from arsenious water zones are significantly higher than values from safe water zones. The major part of the sediments, consisting of non-magnetic and some feebly magnetic minerals, is arsenic free. Some iron-rich clastic minerals, like illite, biotite, chlorite, Fe-coated grains, and also the authigenic siderite concretions, which together constitute only a small part of the sediments, are carriers of arsenic and contribute the element to contaminate groundwater. Illite, biotite and chlorite degenerate to produce iron oxide/hydroxide coating on their surface and also on the surface of some other sand particles. Arsenic is adsorbed on the iron hydroxide coating creating one kind of sink for the element. Bacteria induced growth of siderite concretions grow initially on these clastic minerals and after attaining bigger size fall off as individual globules. Arsenic gets adsorbed on these concretions forming a second kind of sink for the element. Arsenic carrying river water inundates the lower delta at the time of seasonal flood. Arsenic percolates with the water downwards to the shallow aquifer. It gets fixed in the aquifer sediments in the two ways described and is retained in the traps thus formed. Locally developed reducing condition causes dissolution of iron hydroxide coating on the surface of clastic arsenic traps and also of the siderite concretions. This leads to increase in arsenic level in groundwater at these sites. A currently active process of creation and periodic enrichment of the arsenic traps followed by their subsequent depletion through desorption and dissolution is suggested to be the reason for arsenic contamination of groundwater in this part of the delta.