Evaluating heavy metal levels and their toxicity risks in an urban lake in Chennai, India

Abstract

Many urban water bodies in Chennai, India receive untreated sewage that pollutes their waters. An example is the Sembakkam lake, in which water reaches the Pallikaranai marshland, a proposed Ramsar site. In 2019, the city experienced the worst water crisis in 30 years, and many lakes were extremely dry, favoring peaks of heavy metals. Therefore, this study focuses on analysing heavy metal pollution and evaluating its potential effects on biota. In situ parameters were measured and water, sediment, and water hyacinth samples were collected during four campaigns. Al, As, Cr, Cu, Fe, Mn, Ni, Pb, and Zn were measured in all samples. Digestions for total metal content were performed in solid samples and acetic acid extractions only in sediments. The average pH (7.89–8.46) was neutral-alkaline and electrical conductivities (1559–2864 µS/cm) were high. In water, Pb (average: 2.59 µg/l) posed the highest toxicity risk according to the United Nations Economic Commission for Europe, followed by Cu and Cr. In sediment, Cu and Cr reached severe enrichment with respect to continental crust (averages: 19.46 and 13.65) followed by Ni and Zn with moderately severe enrichment. Ni produced the highest toxicity risk (average: 76.18 mg/kg), above the effects range-median, followed by Cr and Cu, between the effects range-low and effects range-median. The highest bioaccumulation factors in the water hyacinth were in the roots. Translocation factors showed similar concentrations in stems and leaves. Proper management of sewage is necessary to diminish the potential deleterious effects of metals on aquatic life and by extension, human health.

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