Heavy metal in urban wetlands

A study of metal contaminants in urban lakes in India using atomic absorption spectroscopy reveals that mitigation actions aimed at removing the pollution may not be a long-term remedy for the problem.

Aboud Jumbe and N. Nandini of the Department of Environmental Science, at Bangalore University, Jnana Bharathi Campus, India, point out that the lake bed sediments of urban lakes of Bangalore city are heavily contaminated with toxic metals, including cadmium, chromium, cobalt, copper, manganese, lead, nickel and zinc. They have now analysed these lake bed sediments using comparative sediment quality guidelines from various derived criteria.

The problems of heavy metals in waste and storm water drainage entering natural urban aquatic ecosystems are documented, Jumbi and Nandini say. Such pollution is of grave environmental concern given that it is non-degradable, toxic and persistent. With many people reliant on such wetlands for food and recreation, and whole ecosystems at stake understanding the effects of long-term industrial discharge, storm water runoff, and accidental spills is crucial. Perhaps even more pressing is to understand whether or not mitigation technologies that might be in place are working.

Bangalore occupies around 741 square kilometers and has a population of almost 6 million. "It is not hard to imagine the sheer quantity of unaccounted sewage that ends up in the fragile wetlands of the city causing chemical pollution as well as public discomfort," the researchers say.

The team has investigated 17 lake systems located in three major watershed zones of Bangalore. They chose 64 sampling points on the basis of different inflow and outflow regions of the lakes as well as geographical proximity to industrial units and effluent discharges. In addition, they also considered residential sites located on the banks of the wetlands, local drainage patterns and accessibility to the lakes.

Preparation of the samples was carried out using microwave-assisted digestion and they were then analysed using atomic absorption spectroscopy.

The results show that sediment "quality" was more markedly poor for copper (203.50 parts per million, ppm) and nickel (97.64 ppm), then lead (206.0 ppm) and cadmium (8.38 ppm). Chromium levels at 96.70 ppm failed sediment quality guidelines, although zinc (220.0 ppm), manganese (176.0 ppm) and cobalt (47.7 ppm) were actually within proscribed safety limits.

"This study proves that the level of sustained metal contamination of the fragile urban wetlands has not receded even after the recent urban wetlands rejuvenation works were completed," the team says, "The prolonged presence of excessive levels of heavy metals in the bed sediments casts doubt on the choice and effectiveness of the any mitigation measures in the long run."

They add that if current trends are allowed to continue unabated, it is mostly likely that the local food web complexes in these fragile wetlands may collapse under the strain of heavy metal contamination, with serious environmental, ecological and humanitarian consequences.