Spatiotemporal dynamics of trace elements and associated health risks in Phewa Lake, Nepal

Freshwater lakes are critical sources of drinking water worldwide, yet contamination by trace elements (TEs) presents significant health risks. Phewa Lake, Nepal, a Ramsar-listed wetland supporting many people, irrigation, fisheries, and tourism, was selected as a case study due to its socio-economic and ecological value. This study investigates the spatiotemporal distribution and health risks of 10 TEs across pre-monsoon and monsoon seasons. 50 lake water samples were collected, split evenly between pre-monsoon and monsoon. Results showed elevated concentrations of arsenic (As), chromium (Cr), copper (Cu), manganese (Mn), nickel (Ni), lead (Pb), and zinc (Zn) during the pre-monsoon, with values declining by approximately half during the monsoon due to rainfall dilution. Spatially, higher concentrations were observed near urban settlements and drainage points. Subsequent statistical analyses identified geogenic sources as predominant, with minor anthropogenic influence mapped to urban shorelines. Water quality was assessed using the Water Quality Index (WQI) and Metal Index (MI): scores were 9.66 and 0.09 pre-monsoon, and 2.86 and 0.03 during monsoon, all well within the World Health Organization (WHO) guideline limits. Hazard Index (HI) values for all TEs were below unity, with As posing the highest non-carcinogenic risk (HI_children = 0.115, HI_adults = 0.076). Cancer risk was low to medium for Pb, Cr, and As. Although water quality was generally acceptable with low risks, proactive measures, such as routine monitoring, regulated runoff, and improved wastewater treatment in alignment with Sustainable Development Goals (SDGs) 6, 13, and 15, are recommended. These findings can inform sustainable urban lake management in the Himalayas and comparable regions globally.