Elemental profile of abiotic components of the East Calcutta Wetlands, a Ramsar site in India

摘要

Beyond the eastern edge of the city of Kolkata (previously 'Calcutta'), there is a vast wetland area. This cluster of marshlands, known as the East Calcutta Wetlands (ECW) is a Ramsar site (no. 1208) and a Wetland International Site (no. 2IN013) in India. The ECW receives composite industrial effluents, mixed with city sewage (about 600 million litres a day). Consequently, various heavy metals are transported into the area throughout the year. This composite wastewater is observed to play an important role in the ECW ecosystem, as discharges are being productively utilized in aquaculture after stabilization, and are also being used to irrigate adjoining farmland, producing around 10 915 metric tonnes offish and nearly 370 650 kg/ha of vegetables annually. The present investigation was carried out to study the distribution of elements, namely Ca, Cr, Mn, Fe, Zn, Cu and Pb, in the various abiotic components of the wetlands, using atomic absorption spectrophotometry. For this purpose, water, bottom sediments and marginal bank soils were collected from wastewater-fed fishponds, agricultural soils and selected sites on a wastewater-carrying canal along a stretch of 40 km from the source point to the final confluence with the Kultigong river. Samples were also collected from a selected control wetland area, which was apparently uncontaminated by industrial effluent, and were compared with data collected from the study site. Variations were found in the concentration levels of various metals in abiotic components. Cr, a major constituent of tannery effluent, was found to be present in the highest concentrations, at 3.8 ± 0.31 mg L~(-1) and 16 495.9 ± 1480.99 mg kg~(-1) dw in water and bottom sediment respectively from Site 1 of the wastewater-carrying canal. The concentration of Ca was very high (279.1 ± 6.24 mg L~(-1)) in the wastewater-fed pond-water; however, Pb was not detected in the same sample. Interestingly, a considerable decrease in the concentrations of various elements in water and sediment along the wastewater-carrying canal was recorded, indicating that the wetlands have a natural ameliorative capacity.