Water and sediment geochemistry of an urban lake: Implications to weathering and anthropogenic activity

Lakes are sensitive ecosystems and respond quickly to any natural or anthropogenic contamination, particularly in urban areas due to poor planning and mismanagement in urban settlement, encroachment, and anthropogenic pollution. In the current study, water and sediment geochemistry are used along with available time series of geospatial data sets to assess: a) provenance and sources of pollution in the lake sediment and water, b) health risk using various sediment and water quality indices, and c) the key drivers of lake shrinkage. Chemical index of alteration (CIA) of lake sediment is higher (>85) with an atypical cluster of samples (CIA = 50–75) towards the south and west of the lake. Further, samples in this cluster show granodioritic provenance which is contrary to the overall mafic provenance of lake sediment. The unusual decrease in CIA in the southern and western side of the lake is attributed to anthropogenically induced fresh sediment in the lake. The results indicate that the sediment is highly polluted by cadmium (Cd) and moderately polluted by copper (Cu), molybdenum (Mo), lead (Pb), and boron (B). Higher Metal Index (MI) values for most of the water samples (>70%) and Hazard Index (HI) for chromium (Cr), nickel (Ni), and cadmium (Cd) towards west and south of the lake reflect the deterioration of water quality in the lake. Moreover, land use change detection attained from available time series of geospatial data sets indicate a drastic transformation of the open water area into other land use classes particularly human settlement, shrinking the lake open water area by 13 km² since 1859. The current study will help policy makers and managers to restore and conserve the lake.