Risk associated with spatio-temporal variations in trace metals and a metalloid in a major freshwater reservoir of Pakistan

Keenjhar Lake in Pakistan provides several ecosystems services including drinking water to millions of people. The seasonal variation (pre-, during, and post-monsoon) in Keenjhar Lake water quality was investigated during nine sampling rounds at ten locations. Water samples (n = 270) were analyzed for As, Cd, Cr(III), Cu, Fe, Hg, Mn, Ni, and Pb using atomic absorption spectrophotometry, Cr(VI) with a UV spectrophotometer, and water quality parameters (solids, oxygen, conductivity, and pH). Several elements exceeded WHO or Pakistan safe limits for unrestricted use including As (0.038 mg·L⁻¹, 95th upper confidence limit), Cr(III) (0.15 mg·L⁻¹), Cd (0.11 mg·L⁻¹), Fe (0.42 mg·L⁻¹), Ni (0.037 mg·L⁻¹), and Pb (0.04 mg·L⁻¹). Monsoon levels of chromium, Cu, Fe, Mn, and Pb were higher than other seasons, while As was highest during post-monsoon, and Ni was highest during pre and post-monsoon. There were significant spatial differences in metals concentrations in the lake. As, Cu, and Fe significantly declined in concentration with distance from the lake inlet. Using the lake as a source of drinking water presented unacceptable carcinogenic risk and non-carcinogenic hazards. This study suggests measures should be taken to limit discharge of pollution to the lake and future monitoring should consider seasonal monsoon effects on lake quality.