Organic Carbon Inputs from Lakes and Septic Systems Increase Manganese Exposure Risk in Lakeside Private Wells

Abstract Groundwater manganese (Mn) contamination is a growing global health concern increasingly associated with adverse neurotoxic effects. We investigated the processes controlling Mn release into groundwater in two lake catchments spanning sewered and unsewered residential developments using a multitracer approach integrating hydrogeochemistry, stable isotopes (δ18O, δ2H), and artificial sweeteners as wastewater tracers in 50 private wells. Manganese exceeded the 0.12 mg L–1 health standard in 50% of wells, nearly double the regional average, and in almost all wells (92%) under suboxic–reducing conditions. Elevated Mn concentrations strongly correlated with dissolved organic carbon (DOC), supporting DOC-driven Mn mobilization. Isotopic signatures in nearshore wells indicated lake–groundwater mixing, suggesting a role for lake-derived carbon in Mn mobilization, while littoral sediment porewaters represent a potential groundwater Mn source. Using a novel application of artificial sweeteners, we fingerprint wastewater-derived organic carbon as an overlooked anthropogenic driver of Mn release to groundwater. Artificial sweeteners were detected in 85% of wells, particularly in unsewered areas, and positively correlated with Mn and DOC. Collectively, these findings reveal coupled hydrogeological, biogeochemical, and anthropogenic controls on groundwater quality and underscore the need to account for domestic wastewater inputs and lake water mixing as drivers of Mn contamination in vulnerable residential developments.