ARSENIC CONCENTRATIONS AFTER DRINKING WATER WELL INSTALLATION: TIME-VARYING GEOCHEMICAL EFFECTS ON ARSENIC MOBILIZATION (Invited Presentation)

Chronic exposure to geogenic arsenic via drinking water is a worldwide health concern. Effects of geochemical changes after well installation and operation on arsenic concentrations are not well understood. We examine changes in arsenic and geochemistry over one year in water from 250 new domestic water wells in three regions of Minnesota, north-central USA, that commonly have elevated arsenic concentrations. Like other aquifers with high arsenic, our aquifers are late Quaternary-age glacial aquifers and fractured crystalline bedrock aquifers. During the study, arsenic concentrations increased in wells in glacial aquifers, and redox conditions changed toward more reducing. In bedrock aquifer wells, there was no significant change in arsenic concentrations, and conditions became more oxic. The arsenic concentration variability we identified has important implications for water treatment, and for programs that require testing of new wells, such as in Minnesota and New Jersey. Information on how and why concentrations of arsenic vary at new wells provides context on what constitutes a representative sample in situations where testing is required or desired. Measurement and mechanistic characterization of human-induced geochemical changes associated with drilling, installing, pumping, and sampling of drinking water wells can improve guidance to well owners and policy makers on when to sample wells.

Our findings show that short to moderate-term impacts of drilling and well operation on geochemical conditions and arsenic concentrations can potentially affect human exposure. Prior characterization and exposure studies have not considered temporal and geochemical equilibrium aspects of water sampling. Our findings suggest that sampling within 30 days of well installation may underestimate arsenic concentrations. We suggest that rules, laws, and other legislation that require sampling and testing may benefit from consideration of potential changes in arsenic concentrations over time. Furthermore, although a well may reach geochemical equilibrium over time and concentrations of arsenic may become relatively stable, any changes in land use, other contamination, or potential climate variability that might alter geochemical equilibrium would likely result in changing arsenic concentrations.