Evaluation of hydrogeochemical processes, toxic trace element mobilization and health risk in urban aquifer of Mymensingh, Bangladesh

Mymensingh City is a groundwater-dependent area with more than one million residents, and groundwater is contaminated by geogenic and anthropogenic sources. This study integrates hydrogeochemical analysis, spatial mapping, multivariate statistics, Water Quality Index (WQI), and health risk assessment to evaluate the contamination dynamics in this rapidly urbanizing Quaternary aquifer system. Thirty groundwater samples were collected from shallow (< 50 m), moderate (50–100 m), and deep (> 100 m) tubewells during the pre-monsoon season in 2022 and analyzed for physicochemical parameters, major ions, and trace metals using standard analytical methods. Results showed that elevated concentrations of As (3–72 µg/L), Fe (0.10–4.59 mg/L), and Mn (0.10–1.72 mg/L) were detected in a substantial proportion of samples, with 70% of As concentrations exceeding WHO drinking water standards [ 1 ]. In contrast, Pb, Cd, Ni, and Cu remained below the adopted standards. Exploratory spatial mapping identified localized areas of comparatively elevated EC (255–862 µS/cm) and TDS (185–551 mg/L), As, Fe, and Mn concentrations near the Old Brahmaputra River and in densely urbanized areas, indicating potential anthropogenic influence. The dominance of the Ca 2+ –Mg 2+ –HCO 3 - facies reflects a common geogenic source associated with active mineral dissolution and interactions with the aquifer matrix. Multivariate analyses and saturation indices revealed multiple sources of contamination, with undersaturation of carbonate and sulfate minerals enhancing solute mobility. The observed associations among As, Fe, Mn, and redox-related parameters support the possibility of redox-mediated mobilization, while major-ion chemistry, hydrochemical facies, and saturation indices further implicate water–rock interaction and mineral dissolution in shaping groundwater composition. WQI assessment classified 57% of samples as poor, 3% as very poor, and 40% as good; no sample was rated excellent or unsuitable for drinking. Health risk assessments indicated that children are 2–3 times more vulnerable to elevated As-related carcinogenic risks (CR ≤ 7.2 × 10⁻ ³) and severe non-carcinogenic hazard indices (HI ≤ 16.36). A sensitivity analysis using the current EPA oral slope factor for inorganic As demonstrated by the effect of toxicity-value selection. Mitigation measures include protecting recharge zones, regulating effluent discharge, and implementing filtration systems. These findings provide a local baseline for sustainable groundwater management in rapidly urbanizing areas.