ARSENIC CONCENTRATION VARIABILITY IN NEWLY CONSTRUCTED DRINKING WATER WELLS IN MINNESOTA, USA

Arsenic is a naturally-occurring contaminant adversely affecting drinking water quality sourced from groundwater in geologically diverse aquifers in Asia, Europe, Africa, and North and South America. Minnesota has an estimated 125,000 domestic water well users with drinking water arsenic concentrations above 10 µ/L (elevated arsenic). The State of Minnesota revised the well code in 2008 to include testing all new potable wells for total arsenic (in addition to nitrate and bacteria), but 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 and sampling point; 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. Field parameters were also collected during each round.

Study results demonstrate that many aspects of groundwater sampling protocols affect, and potentially bias, measured arsenic concentration in well water. Filtration, sample collection point, and sample collection timing all affect measured arsenic concentrations in newly-constructed drinking water wells. For example, 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. Sample collection practices such as filtration, sample collection from plumbing, and delaying sample collection for several months after well construction all reduce variability in measured arsenic concentrations over time in newly constructed wells, thus improving the public health message to well owners.