Spatiotemporal variability of physicochemical parameters, heavy metals and associated ecological and human health...

Rapti River (East) is a major Indo-Gangetic river in eastern Uttar Pradesh, India. Domestic sewage and industries along the river basin left the river polluted at numerous stretches. Water and sediment samples (n = 30) were collected across three seasons and analysed for heavy metal (HM) contamination, physicochemical properties, and associated risks. Pb exceeded permissible limits across multiple sites and seasons in water, while Ni exceeded the limit at S1 during monsoon; most HMs exceeded sediment quality guidelines (SQG), especially during monsoon. Zn and Cd were the highest and lowest in both compartments. Monsoonal flow altered sediment geochemistry, shifting Igeo status from unpolluted (Class 0) to highly polluted (Class 4) for Hg and Cd, with an extreme anomaly (Class > 6) for Hg at S5 during pre-monsoon. PLI and PERI showed low risk in water [max: 0.25; 31.17], and low to extremely high risk in sediment [PLI max: 4.10; PERI max: 1,030 during monsoon]. Non-carcinogenic risk (THI) was "low" in water and "high" in sediment; carcinogenic risk (TCR) exceeded safe thresholds in water during monsoon and was consistently high in sediment for children. PCA-based source apportionment identified lithogenic processes and upstream mining as primary sources of Cu, Zn, As, Cr, and Ni; industrial effluents contributed Pb and Cu at mid-to-downstream sites; and Hg reflected localized industrial inputs, with monsoonal flux as the primary basin-wide redistributor. Low DO and high BOD, TDS, and EC characterized urban and industrial stretches. Lead, arsenic, chromium, and nickel were the most concerning metals for long-term ecological and human health risks, underscoring the need for continuous monitoring and regulated management of HM contamination in the Rapti River Basin.