Arsenic in Ground Water: Recent Research and Implications for Minnesota

The United States' federal drinking water standard for arsenic recently changed from 50 micrograms per liter (?g/l) to 10 ?g/l. Approximately 100 Minnesota public water supplies do not comply with the new rule. Additionally, results of recent private well sampling studies in Minnesota indicate that thousands of private wells have arsenic concentrations exceeding 10 ?g/l. Recent arsenic research provides regulatory agencies with results to support development of potential new rules and guidance concerning drilling wells in high arsenic areas, testing new wells for arsenic, and implementing low-cost compliance strategies. Arsenic contamination in upper Midwestern ground water is widespread, naturally occurring, and associated with the lateral extent of Des Moines lobe till. Although this till does not have particularly high arsenic concentrations, it does have specific physical characteristics (fine-grained matrix and entrained organic carbon) that create a geochemical environment favorable to regional scale mobilization of arsenic. In west-central Minnesota, private wells that have screens less than 8 feet long set within 4 feet of the upper confining till unit have an average arsenic concentration of 20 ?g/l, with 58% of wells exceeding 10 ?g/l. Private wells with longer screens set farther from the upper confining unit average only 12 ?g/l arsenic, and 40% of wells exceed 10 ?g/l. 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. Two procedures have been developed and tested for screening low-cost compliance options. A 'site investigation' evaluates the option of drilling a new well. A site investigation can identify different, low-arsenic aquifers at a different elevation and/or a different location. Sampling a well several times over a period of a few hours evaluates the feasibility of changing well operations. Changing well operations may be a viable compliance option for communities with arsenic concentrations that predictably fluctuate around 10 ?g/l.