Seasonal dynamics, sources, and health risks of trace and heavy metals in the tropical critical zone of the Western Ghats, India.

Monsoon-driven hydrological variability and anthropogenic pressures in tropical critical zones pose significant threats to water quality and public health. This study investigates the spatial and temporal distribution, sources, and human health risks of trace and heavy metals at the Munnar Critical Zone Observatory in the Western Ghats, India. Through integration of hydrogeochemical analyses, land use-land cover data, and health risk frameworks, revealed pronounced seasonal contrasts: monsoon rains drive two-threefold increases in Al, Fe, Pb, and Zn in surface waters due to intensified erosion of hornblende-biotite gneiss and agrochemical runoff from tea and eucalyptus plantations. Groundwater chemistry reflects aquifer lithology, while Cd and Pb enrichment during monsoon recharge is significantly influenced by anthropogenic sources. Multivariate statistical analyses attribute (PCA, HCA) 60-75% of metal variability to geogenic weathering (PC1: Al, Fe, Ti) and 15-25% to anthropogenic sources (PC3: Cd, Pb from agrochemicals). Pollution indices (WPI, HPI, HEI) classify contamination as low-to-moderate overall, yet Fe exceeds thresholds in rivulets during monsoon (notably at S1 and S6). Health risk assessments prioritise Co, Mn, and Pb as non-carcinogenic threats (HI < 1), while Cr and Cd pose significant carcinogenic risks, with children's ingestion risk 3-4 times higher than adults. Concentration-discharge relationships indicate consistent chemodynamic metal transport in river systems, contrasting with seasonally-variable behaviour in rivulets, where chemostasis occurs for specific elements during pre-monsoon. These findings underscore the need for regulating agrochemical use in erosion-prone catchments and prioritising Cr and Cd monitoring in vulnerable sites. This provides a holistic framework for managing water security in climate-sensitive, human-altered landscapes globally.