Elevated arsenic and manganese in groundwaters of Murshidabad, West Bengal, India

Analyses of core samples (2 in each of the blocks) drilled to a depth of 45 m indicate that there is no significant variation in bulk As (5-20 mg/kg) between the Holocene and Pleistocene sediments, indicating that favorable subsurface redox conditions conducive to mobilization are responsible for the release of As. The same applies to Mn, but concentrations vary more widely (20-2000 mg/kg). Sequential extraction of Holocene sediments showed As to be associated with 'specifically sorbed-phosphate-extractable' phases (10-15%) and with 'amorphous and well crystalline Fe-oxyhydroxide' phases (around 37%) at As-contaminated well depths, suggesting that the main As release mechanisms could be either competitive ion exchange with PO43 −, or the dissolution of Fe oxyhydroxides. In the Pleistocene sediments Mn is predominantly found in the easily exchangeable fraction.