Arsenic route to groundwater mapped
doi:10.1038/nindia.2008.214 Published online 1 June 2008
An Indo-German team of researchers has found out how exactly arsenic is released into groundwater, an insight that will help devise future strategies to combat the toxic element. Probing the sediments in an arsenic-affected village of West Bengal, the team has found that a special type of mineral harbours and releases arsenic into groundwater.
The team carried out lithological, mineralogical and geochemical studies in some boreholes of the Chakdaha block of Nadia district in West Bengal. The lithological character of the sediments was categorized into sand (fine, medium), sandy clay, silty clay and topsoil. The total arsenic content of aquifer sediments varied between 1 and 22 milligrams (mg) per kg with a mean of about 6.9 mg per kg. The study found that the phyllosilicates (silicate minerals) and carbonate mineral phases act as a source of arsenic in the sediments. With chemical changes, these minerals release arsenic into groundwater.
The geochemical data shows that grain size plays a role in controlling solid phase distribution of arsenic. The mean arsenic and iron concentrations in the fine-grained sediment samples was about three times higher than that from coarse-grained sediments. In abandoned channels, seasonal flooding formed layers of silt containing fine-grained material with high solid phase arsenic, manganese and iron. Given right chemical conditions and microbial activity, in such channels, a huge amount of arsenic was liberated from adsorbed solid phase to groundwater.
"Besides the natural processes, certain human factors like land-use pattern also release arsenic into groundwater," says lead researcher Bibhash Nath. "In West Bengal, high arsenic can be present up to a depth of 100 m. Beyond 100 m depth arsenic-free water can be tapped," says Nath.
- Nath, B. et al. Mobility of arsenic in West Bengal aquifers conducting low and high groundwater arsenic. Part II: Comparative geochemical profile and leaching study. Appl. Geochem. 23, 996-1011 (2008)