Tracing urban lake water quality through uranium: sources, reactive process controls, and ecological risks for...

Lakes in rapidly growing megacities are increasingly stressed by wastewater inputs, altered hydrology, and road runoff, yet controls on dissolved metal concentrations remain poorly constrained. We investigated dissolved metal dynamics across ten urban lakes in Bengaluru, India, spanning minimally impacted systems to lakes receiving untreated/treated wastewater. From six sampling campaigns during the 2023 monsoon, we quantified major ions, trace metals, and dissolved organic carbon (DOC) to identify sources and governing geochemical processes. Uranium emerged as a key contaminant in wastewater-impacted lakes, reaching 16.47 µg/L in Jakkur Lake (receives treated wastewater), ~15 times higher than Yelahanka Lake (upstream, no direct/treated wastewater input). Uranium concentrations correlated with chloride, nitrate, dissolved oxygen, and redox-sensitive metals (Fe, Cd), indicating that although U is geogenic (from groundwater), elevated levels in lakes arise from the combined effects of groundwater overuse and return flow, wastewater treatment (redox alterations), and diverse DOC pools. Therefore, we hypothesise that altered in-lake redox transformations and organic complexations enhance uranium mobility in lakes where U is otherwise found naturally low. Furthermore, geogenic metals (Li, Rb, Sr, Ba) showed strong correlations with NO₃^- and PO₄^3-, indicating contributions from agricultural wastewater and/or groundwater return flow. Road runoff was enriched in V, Cr, Cd, As, and Pb relative to rainfall, indicating contributions from road dust and soil leaching, vehicular emissions, and brake/tire wear, and corroborated by elevated concentrations in lakes adjacent to dense road networks. Integrating ionic tracers with process-sensitive metal relationships provides a robust framework for disentangling source and process controls in managed urban systems, highlighting the need for targeted monitoring in wastewater-reuse-driven urbanscapes holding implications for One Health.