The ability of aquatic plants to absorb, translocate and concentrate metals has led to the development of various plant-based treatment systems. The potential to accumulate chromium by Scirpus lacustris, Phragmites karka and Bacopa monnieri was assessed by subjecting them to different chromium concentrations under laboratory conditions. Plants showed the ability to accumulate substantial amounts of chromium during a short span of one week. When the plants were grown in tannery effluent and sludge containing 2.31 μg ml -1 and 214 mg kg -1 Cr, respectively, they caused significant reduction in chromium concentrations. While there was an increase in biomass, no visible phytotoxic symptoms were shown by treated plants. The plants can then be harvested easily and utilized for biogas production.Chromium is one ofthe toxic metals widely distributed in nature. Ofthe two forms found in the environment, trivalent and hexavalent, hexavalent chromium is the form considered to be the greatest threat because of its high solubility, its ability to penetrate cell membranes, and its strong oxidizing ability (1). The large-scale uses of chromium in metallurgical, pigment and dye, and in textile, and electroplating makes these industries potential sources of chromium pollution. The tanning industry is also a major contributor of chromium pollution of water resources. In India, according to a recent estimate, ca 2,000-3,200 tonnes of elemental chromium escape into the environment annually from the tanning industries alone (2). Chromium concentrations in effluents usually range between 2,000-5,000 pg ml" 1 compared to the recommended permissible limit of 2 pg ml 1 . The management of large amounts of effluent discharged by the tanneries has become a formidable task in developing countries. Contamination of water resources by these effluents is posing serious health hazards and is a threat to aquatic ecosystems. 274

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Plant roots influence the soil in which they grow in a variety of ways, making the soil more conducive for microbial growth and activity, as well as cometabolic reactions. Studies examining the fate of pesticides in plant-soil systems have revealed a marked decrease in persistence of these compounds. Additionally, similar observations have recently been made for oil residues, polycyclic aromatic hydrocarbons, trichloroethylene, and surfactants. These previous studies suggest that the root zone could potentially be managed to effect bioremediation of surface soils containing hazardous organic compounds through enhanced biochemical transformation of these organic constituents at waste sites by indigenous rhizosphere microorganisms. In order to further test this hypothesis, a study was initiated using soil and vegetation from an existing pesticide-contaminated site in Iowa. Initial characterization of this site revealed several herbicide-tolerant plant species including Kochia sp., knotweed (Polygonum sp.), and crabgrass (Digitaria sp.). Experiments testing the influence of these tolerant varieties on microbial degradation of a mixture of herbicides are reported. By facilitating microbial degradation of pesticide wastes, vegetation could provide a cost-effective and ecological approach to restoration of contaminated surface soils in situ.

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