High Cadmium Contamination at the Gateway to Sundarban Ecosystem Driven by Kolkata Metropolitan Sewage in India

Cr, Pb, Zn, Cd, Ni, Cu, Co and Fe content in the surface sediments was studied at three sites along a 20 km stretch on the Bidyadhari River (situated at the gateway to Indian Sundarban mangrove ecosystem) starting from the sewage outfall zone of the Kolkata metropolis (northern limit of Sundarban) during the dry (March) and wet (August) seasons in the year 2011. The concentration levels of the heavy metals (mg kg^-1) were in the following order: Fe (30399 ± 3679) > Cr (68.16 ± 39.28) > Zn (61.33 ± 10.47) > Ni (32.50 ± 4.79) > Cu (27.67 ± 6.59) > Pb (19.69 ± 5.20) > Co (15.23 ± 4.39) > Cd (1.70 ± 0.63). Computation of pollution load index comprising all the heavy metals was found higher during the wet season, mainly due to the enhanced surface run-off led by monsoonal rain. Concentrations of Cr and Co were found higher than the world average in a few cases (especially during wet season); however, computation of pollution indices like contamination factors, enrichment ratio and geoaccumulation index exhibited extremely high Cd contamination in all the sites and during both seasons.