Arsenic Concentration Variability in Newly Constructed Drinking Water Wells in Minnesota, USA

The State of Minnesota revised the well code in 2008 to require testing all new potable wells for arsenic (in addition to nitrate and bacteria). The well code requires testing at a certified laboratory before the well is used as a potable water supply. However the code does not require any particular sample collection method or sampling point. To better understand the influence that sample collection methods, sampling point, and timing have on measured arsenic concentration, arsenic concentrations were measured in 250 newly constructed wells over one year in several counties known to have prevalent elevated arsenic concentrations in groundwater. Study samples were collected in the following ways: 1) total arsenic samples (unfiltered) collected by well drillers in each respective driller’s common practice (from the drill rig or from plumbing); 2) initial total and dissolved (filtered) arsenic samples by MDH staff replicating driller sample timing, sampling point, and method; and 3) total and dissolved arsenic samples by MDH staff 3-6 months after well construction; and 4) total and dissolved arsenic samples by MDH staff 12 months after well construction. Initial total arsenic concentrations varied significantly from later sample concentrations, both total and dissolved. In contrast, initial dissolved sample concentration varied much less over time. Over one year, total initial arsenic samples switched between categories of above or below the 10 µg/L drinking water standard in more than 13% of wells. Dissolved arsenic samples switched between categories in only 7% of wells. Filtering initial arsenic samples reduces concentration variability over time.