Heavy Metals Analysis and Sediment Quality Values in Urban Lakes

Problem statement: The objective of this research was to evaluate the degree of heavy metal contamination in lakes and the extent to which the sediment quality of the lakes of Bangalore city has deteriorated. Approach: In this study, heavy metals such as Cd, Co, Cu, Cr, Mn, Pb, Ni and Zn in lake bed sediments were analyzed using comparative sediment quality guidelines from various derived criteria. The selection of sampling points was based upon inflow and outflow regions of the lakes; geographical proximity of industrial units in relation to their effluent discharges; proximity of residential sites located on the banks of the wetland systems; drainage patterns and accessibility towards the lakes. Digestion and analysis of the samples were done by microwave-assisted digestion and atomic absorption spectrophotometry respectively. Results: The extent of sediment quality deterioration was more pronounced in Cu (203.50 ppm) and Ni (97.64 ppm) followed by Pb (206.0 ppm) and Cd (8.38 ppm). Cr (96.70 ppm) failed a single sediment quality guideline while Zn (220.0 ppm), Mn (176.0 ppm) and Co (47.7 ppm) remained within the safety levels of sediment quality guidelines prescribed for the study. The Sediment Geo-accumulation Index showed that Co, Cu and Pb showed moderate levels of pollution while the Pollution Load Index (PLI) between heavy metals in the lakes produced the following outputs: Ni > Pb > Cd > Cu > Cr > Co > Zn > Mn. Conclusion: 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. This prolonged presence in excessive levels of the studied heavy metals in the bed sediments casts doubt on the choice and effectiveness of the any mitigation measures in the long run.