Occurrence and risk assessment of total mercury, methylmercury and other selected trace metals in the surface sediments of the Cochin Estuary, Southwest Coast of India

This study assesses the ecological risk and impact of anthropogenic activities in the Cochin estuary – a Ramsar site – by trace metal indices in surface sediment collected from thirteen sampling stations in three seasons. The trace metals analyzed are THg, MeHg, Pb, Cu, Cd, Zn, Ni, Co, Mn, Cr, Fe, and Al in the surface sediments and their spatio-temporal variation is recorded. Pollution indices (i.e., Enrichment factor, Index of geoaccumulation, sediment quality guidelines, pollution load index, and potential ecological risk index) were applied to ascertain the sediment quality. The distribution pattern of Cr, Zn, and Cd revealed higher concentrations in the northern arm of the estuary and near the industrial regions indicating non-geogenic inputs. The concentration of THg and MeHg exhibited spatial and temporal variations and were higher than the background values. The results showed that higher levels of trace metals were found in the northern part of the Cochin Estuary, which could be attributed to the weak flow and significant input of industrial effluents.