GEOCHEMICAL EVOLUTION OF GROUNDWATER AFTER DRINKING WATER WELL INSTALLATION: EFFECTS ON GEOGENIC ARSENIC MOBILIZATION

Geogenic arsenic can adversely affect drinking water quality sourced from geologically diverse aquifers around the world. Geochemical conditions that facilitate arsenic mobilization are diverse, and the effect that well construction has on local aquifer geochemistry and arsenic mobilization is not well understood. Elevated arsenic concentrations (>10 µ/L) are common across the northern U.S. In Minnesota (MN), ~130,000 domestic water well users have drinking water with elevated arsenic concentrations. In 2008 the State of MN began requiring testing all new potable wells for total arsenic. To better quantify geochemical evolution of groundwater after well installation and its relation to arsenic concentrations, we conducted a comprehensive study of ~250 newly constructed wells in MN. Wells were chosen from three geologically distinct regions with elevated arsenic concentrations. Samples were collected within one month, 3-6 months after, and 12+ months after well construction. Field parameters of pH, ORP, and dissolved oxygen were measured, and laboratory analyses included total and aqueous arsenic, arsenate, arsenite, nitrate, manganese, iron, and sulfate.

Arsenic mobilization and concentration is sensitive to redox conditions and pH. Geochemical conditions between the sampling periods are significantly different, indicating well construction likely contributed geochemical disturbance to the aquifer. Initial sample results show no regional patterns or relations. However, over the year of monitoring, groundwater geochemistry evolved to distinct regional patterns and geochemical regimes. Arsenic, iron, and sulfate concentrations combined with ORP and pH indicate three primary arsenic mobilization mechanisms: a) reductive dissolution of iron; b) rapid oxidation of iron sulfides and re-precipitation of Fe as (oxyhydr)oxides; and c) incongruent dissolution of iron sulfides (rapid oxidation and re-precipitation of iron as oxides). In new wells, arsenate comprised the largest percentage of arsenic, contrary to earlier findings of predominantly arsenite species in established wells (in-place for years). If arsenic species change from arsenate to arsenite over time, there are public health implications because arsenite is more difficult to remove with typical home water treatment systems.