Manganese (Mn) contamination poses a widespread risk to private well users, where limited monitoring can result in prolonged exposure. Effective management requires understanding how hydrogeochemical processes and human behavior jointly influence water quality and treatment outcomes. We investigated fifty household wells in Québec’s Eastern Townships, analyzing groundwater for 51 geochemical parameters and dissolved organic carbon to evaluate controls on Mn and co-occurring metal(loid)s. Half of wells exceeded the federal guideline for Mn (0.12 mg/L), with elevated chromium, arsenic, and uranium, including the first report of Cr up to 63 μg/L in local drinking water. Evaulation of residential treatment systems revealed limited uranium removal and, in some cases, increased Mn concentrations. Laboratory experiments showed that interactions between Mn oxides and dissolved organic matter mobilize Mn via reductive dissolution and promote formation of stabilized C–Mn colloids, with stability governed by formation mechanism and solution chemistry. Increasing C:Mn ratios enhanced colloid formation and stability, increasing “aqueous” Mn (<450 nm) by 56% and colloidal Mn (3–450 nm) by up to 32% relative to Mn-only systems. Natural surface and groundwater samples contained substantial fractions of Mn in colloidal phases (up to 19.2% and 27.2%, respectively). Divalent metals such as Cd²⁺ and Zn²⁺ destabilized colloids, altering Mn and co-contaminant mobility. These results highlight the important role of organic carbon–Mn oxide interactions and colloid dynamics in controlling Mn transport, reveal limitations of conventional treatment strategies, and underscore the importance of integrating hydrobiogeochemistry, colloid chemistry, and human behavior for sustainable private well water management.