Environmental risk assessment of the surface sediments based on trace elements analysis from the largest freshwater lake in the southern slope of the Himalaya, Nepal

Freshwater ecosystems, including high-altitude lakes, can be affected by trace metal pollution derived from a mix of natural sources and anthropogenic activities. These pollutants often collect in surface sediments, with notable concentrations in the deeper areas of lakes. To evaluate the environmental risk associated with metal contaminated sediment in Rara Lake, southern Himalaya, surface sediment samples were systematically collected in November 2018, with a subsequent specific emphasis on determinations of trace element concentrations. Subsequent analysis revealed nine elements exhibiting a descending mean concentrations order: iron (Fe) > manganese (Mn) > chromium (Cr) > rubidium (Rb) > nickel (Ni) > strontium (Sr) > cobalt (Co) > copper (Cu) > cadmium (Cd), of 7205.55 mg kg^-1, 2290.34 mg kg^-1, 176.29 mg kg^-1, 153.78 mg kg^-1, 51.86 mg kg^-1, 44.61 mg kg^-1, 38.89 mg kg^-1, 29.11 mg kg^-1, and 0.10 mg kg^-1, respectively. Comparisons to sediment quality guidelines highlight that Mn, Cr, Cu, and Cd as significant threats to the aquatic ecosystem in Rara Lake. To assess the impact of metal pollution, enrichment factor (EF), geo-accumulation index (I_geo), pollution load index (PLI), and contamination factor (CF) were computed. All metals (except Cd) had I_geo value exceeding 5, displaying strong contamination. EF values for Mn, Cr, Co, and Ni metals were > 10, indicating severe effects of anthropogenic influences. CF and PLI values also indicated significant pollution for most of the investigated sites. Elevated trace element concentrations have the potential to adversely affect water, sediment, and aquatic life, also potentially impacting nutrient cycling and microbial activity. This study enhances our understanding of the metal compositions within Rara Lake sediments and provides a basis for more effective lake management and pollution control strategies. Urgent action by regional governing bodies is crucial to address the early stages of metals pollution, including identification and controlling of pollution sources, by appropriate regulations, optimizing industrial practices, and remediating existing pollution to prevent further contamination and protect the lake ecosystem.