Arsenic in Minnesota groundwater and its impact on the drinking water supply

Arsenic contamination in upper Midwestern ground water is widespread, naturally occurring, and associated with the lateral extent of northwest source Late Wisconsin (Des Moines lobe) till. Arsenic concentration in ground water is not directly related to arsenic concentration in sediment. In west-central Minnesota, private wells that have relatively short screens set close to the upper confining unit are more likely to have elevated arsenic concentrations than otherwise comparable private wells. The variability of arsenic concentrations over time in newly constructed wells is similar to concentration variability observed in older wells; there is no temporal trend. Reductive desorption is the mechanism proposed to explain observed important temporal changes in water quality in two Minnesota public water supply wells. Two procedures have been developed and tested for screening low-cost compliance options for public water systems with elevated arsenic. A 'site investigation' evaluates the option of drilling a new well. The procedure can identify low-arsenic aquifers at different elevations and/or different locations. Sampling a well several times over a period of a few hours provides the necessary information to evaluate the option of changing well operations. Changing well operations may be a viable option for communities with arsenic concentrations that predictably fluctuate around 10 ?g/l.