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

Geogenic arsenic can adversely affect drinking water quality sourced from geologically diverse aquifers around the world. However, effects of well installation and operation on arsenic concentrations and mobilization are not well understood. This knowledge gap impacts both reliable detection of arsenic in drinking water and effective public health recommendations to reduce exposure to arsenic. High arsenic concentrations (exceeding 10 µg/L) are common across the northern U.S. In Minnesota, ~130,000 domestic water well users tap drinking water aquifers with high arsenic concentrations, and more than 500,000 well users have detectable arsenic. The Minnesota Department of Health recommends that well owners consider water treatment if arsenic is detected.

This study examines changes in arsenic and redox geochemistry over one year following installation of 254 new domestic water wells in three regions of Minnesota that commonly have elevated arsenic concentrations. Our regions' geologic settings have common physical characteristics of high-arsenic aquifers globally: igneous bedrock aquifers; or late Pleistocene-age glacial sand and gravel aquifers interbedded with aquitards. Over the study, arsenic concentrations increased by 16% or more in 25% of wells in glacial aquifer regions, and the redox conditions changed towards more reducing. In wells in the bedrock region, there was no significant change in arsenic concentrations, and redox conditions changed towards more oxidizing. Our findings illustrate the importance of understanding short- to moderate-term impacts of well installation and operation on arsenic and aqueous chemistry, as it relates to human exposure. Our study informs water quality sampling requirements, which currently do not consider the implications of sampling timing with respect to well installation.

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. Evaluating arsenic concentrations in samples from new wells in the context of general regional pH and redox conditions can provide information regarding the degree of disequilibrium created by well drilling. Our analysis approach may be transferable and scalable to other aquifer settings.