| Summary: | Abstract Anthropogenic activities impact the major ion composition of fresh water sources. The inorganic species are likely to be influenced by mineral dissolution, mineral precipitation, redox and ion-exchange reactions in the contaminated lakes. Owing to paucity of research, this study examines the influence of bio-geochemical reactions on the major ion composition of sewage contaminated Bengaluru lakes. The selected lakes represent water bodies in the major valley systems of the city that are polluted by partly treated sewage and stormwater runoffs. Hydrogeochemical facies of the lake samples showed that enrichment of Mg2+, Na+, Cl− and SO4 2− ions from anthropogenic contamination altered the chemical type of the lake-water. Examination of processes influencing the major ion composition of surface waters indicated that evaporation than rock-weathering tends to influence the chemical composition of the Bengaluru lakes. Precipitation of carbonate minerals in the alkaline pH contributed to the deficiency of alkaline earth ions, while dissolution of anthropogenic gypsum enhanced the SO4 2− ion concentration of the lakes. Solute diffusion from lake water into the pore solutions of sediments and ion-exchange reactions between monovalent ions of sediments and divalent cations in lake water are additional pathways that influence the major ion composition of the contaminated lakes. Besides alterations in major ion composition, organic contamination, biochemical reactions associated with photosynthesis activity of algae and release of toxic Al3+ ions from mineral dissolution are consequences of the lake contamination.
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