Post-depositional memory record of mercury in sediment near the effluent disposal site of a chlor-alkali plant in Thane Creek-Mumbai Harbour, India.

A mercury-cell chlor-alkali plant, operating for 40 years at Airoli (eastern bank of Thane Creek), India, has caused widespread contamination of the surrounding environment. Untreated wastewater from the plant was discharged to Thane Creek for several years. Thane Creek joins the Ulhas Estuary by a narrow arm and opens to the Arabian Sea through Mumbai Harbour. The Ulhas Estuary is impacted by mercury (Hg) released from several industries including two chlor-alkali industries. In order to understand the historical record of anthropogenic Hg and its association to Al, Fe and total organic carbon (TOC), estimation of Hg, Al, Fe and TOC was made in surface sediments and cores from Thane Creek-Mumbai Harbour and the adjacent coastal area. Though 70% of the chlor-alkali plant has been changed to membrane cell technology, surficial sediment in the vicinity of effluent release contains a high concentration (up to 1.19 microg g(-1) dry wt) of Hg compared with its background value (0.10 microg g(-1) dry wt). The contaminated creek sediments are prone to current-driven resuspension and are acting as a strong source of Hg to the sediment of the coastal region. Several rocks and sediments from the catchment area were analysed to find out the natural background level of Hg. A high suspended load transported from the catchment region provides natural dilution to the Hg-contaminated sediment of the Bay. Lithogenic and anthropogenic Hg buried in marine sediments is quantified based on normalization with Al, Fe and TOC, and inter-comparisons of the results indicate comparable values obtained by using Al and Fe, while discernible deviations are found when normalization is carried out using TOC. The Hg profile in the core, from the effluent release site for which sedimentation rate has been established, is discussed in terms of the progressive removal of Hg from the effluent after the mid-1970s and partial changeover of the manufacturing process from Hg cell to membrane cell production subsequent to 1992. Based on the reported sedimentation rate in the locality, a maximum concentration (49.19 microg g(-1) dry wt) of Hg corresponds to the year 1967, when the chlor-alkali plant started discharging its untreated effluent to the creek. Results indicate that more than 80% of Hg settles in the vicinity of its discharge and, once deposited in the sediment, it is not affected to any substantial degree by diagenesis.