Occurrence and geochemical controls of manganese, arsenic, and iron in groundwater in Alberta, Canada

Groundwater quality in Alberta, Canada, is influenced by geogenic constituents such as manganese (Mn), arsenic (As), and iron (Fe), which can pose health risks when present above drinking water standards. This study investigates the geochemical controls on Mn, As, and Fe occurrence and mobility using over 10,600 groundwater samples (depths <250 m) for Mn and Fe, and 1700 samples for As. Guideline exceedances were observed in 18% of samples for Mn, 11% for As, and 21% for Fe (aesthetic objective). Elevated concentrations in bedrock units were most common in marine and transitional formations, where exceedance frequencies surpassed 20% for all three elements. In surficial sediment aquifers, Mn and As concentrations exceeded drinking water guidelines in 48% and 22% of the samples, respectively, while the Fe aesthetic objective was exceeded in 46% of the samples, highlighting a potential risk to untreated groundwater users and demonstrating the impact of regional lithology on aqueous geochemistry. Post-oxic conditions, characterized by nitrate depletion, exhibited the highest median concentrations of Mn, As, and Fe. Reductive dissolution of Fe(III) oxides facilitates the release of Fe and As, with As primarily occurring as arsenite [As(III)], a more mobile and toxic species. Arsenic mobility was further enhanced under alkaline, Na-HCO 3 -rich conditions. Manganese release was linked to both oxide and carbonate dissolution under suboxic to anoxic conditions. Overall, redox zonation and aquifer mineralogy strongly control Mn, As, and Fe distributions in groundwater. These findings support improved groundwater resource management in sedimentary basin settings.