Saturday, 15 July 2006

Metal Contamination in Urban Soil-Water Environment and Remediation Strategies.

Palanisamy Singaram, K Lalsuna, and Santiago Mahimairaja. Tamil Nadu Agricultural University, Department of Environmental Sciences, Coimbatore, India

Rapid urbanization and industrialization created a great pressure on delicate ecosystem in and around urban areas. Aerial deposition and/or indiscriminate disposal of wastes from a variety of industries result in the contamination of urban soils with metals (As, Cr, Cu, Hg, Pb, Zn etc.). Persistence of these metals in soil and water poses a serious risk to environmental and human health. Information on metal distribution in urban environment may help to develop appropriate strategies for managing the uncontrolled dumping of industrial wastes and creating safer environment for urban populations. In this context, the metal contamination in Coimbatore urban environment, which is the second largest industrial city in Tamil Nadu, India was examined. Apart from textiles, hundreds of small scale industries like dyeing, foundry, metallurgy, smelting and chemicals are in operation in and around the city. Geographical Information System (GIS) was used to identify the ‘hot spot' of the metal contamination. Through a laboratory closed incubation experiment the biotransformation of lead (Pb), a dominant metal in the urban soil, was examined to predict its possible groundwater contamination.

The relative proportion of metal distribution in Coimbatore urban soils followed: Pb>Cr>Zn>Cu>Cd. The contamination of Pb and Cr was widespread in the urban soils and in many locations the concentrations exceeded the maximum permissible limits. Metal contamination in urban water showed greater variability as marked difference was observed between samples. Though the soils have shown varied degrees of metal contamination, ground waters remained relatively uncontaminated, possibly be due to the geological formation existing in the urban environment that would have effectively prevented the leaching of metals. However, water samples from ponds, ditches and drainage channels have had high concentration of metals which followed: Cr>Pb>Zn>Cu>Cd and the concentrations mostly exceeded the maximum permissible limit of the WHO. Plants sampled around the ‘hot-spots' reflected the severity of the metal contamination in the urban soils. Fractionation study has indicated that Pb and Cr mostly existed as organic followed by iron-oxide bound species in soils. However, significant increase in the concentration of water soluble and exchangeable fractions was also observed during incubation which is of concern as it has environmental significance. Remedial measures including phytoremediation with a ‘clean and green' concept are recommended to either manage and/or remediate the contaminated sites within the city. Implementing stringent regulatory measures for the disposal of solid and liquid wastes from industries is urgently needed for sustaining the urban environmental health.

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