Localized or diffusive: insights into the source and geochemical dynamics of dissolved mercury contamination in groundwater of the mid-Gangetic Plains

Dissolved mercury (Hg) in groundwater poses underrecognized risks in industrial regions of South Asia. We assessed Hg occurrence, sources, and dynamics, with co-occurring metals, in the mid-Gangetic Plains (Kanpur), India. Hg was detected in all groundwater samples (n = 39; 0.02–3.8 µgL⁻¹), with 20% exceeding 1 µgL⁻¹ guideline, reflecting complex hydrological-geogenic–anthropogenic interactions, whereas river water samples (n = 11) showed no exceedances. Ca²⁺-Mg²⁺-SO₄²⁻ type water indicates sulfur dominance linked to anthropogenic inputs (industrial emissions or coal combustion), creating sulfate-rich aquifer conditions under which Hg enrichment is observed. Hydrochemical patterns, elevated EC/TDS, and the proximity to irrigated land suggest that irrigation return-flow mediated evaporation-based enrichment may act as an important driver, contributing to Hg concentrations. Eh-pH analysis indicates aquifer redox conditions conducive to the stability of reduced dissolved Hg species. Probability exceedance shows a sharp decline in higher Hg levels, suggesting anthropogenic inputs, primarily atmospheric deposition. Health risk assessment indicates significant non-carcinogenic risks, especially for children (HQ > 2.5), dominated by arsenic (As) in river water. Co-contaminants [As, U (uranium), Cr (chromium), and Fe (iron)], and altered multivariate ionic clustering warrant further investigation. This study underscores the need for targeted regulation informed by hydrochemical diagnostics and public health risk frameworks.