Naturally-occurring arsenic in groundwater in Missoula County

Arsenic occurs naturally in some rocks and sediments. The Health Department has compiled data on tests of private wells in Missoula County, and has identified three areas where arsenic is more likely to be elevated: The Blackfoot Valley, the Nine-Mile/Huson area, and the Lolo Creek area. Not all well water in these areas is impacted, so testing individual wells is required to determine if there is a problem with your water. In addition, Researchers from the Montana Bureau of Mines and Geology (MBMG) and University of Montana analyzed water samples taken from domestic wells in Missoula County. Results from these samples and public water system monitoring samples also indicated that there are some private and public wells in Missoula County that have exceeded an arsenic concentration of 10 ppb. 15 of the 153 samples collected by MBMG exceeded a concentration of 5 ppb, and 5, or approximately 3%, exceeded the standard of 10.0 ppb. None of these were collected in areas known to be contaminated with mining waste, so it is presumed to be from naturally-occurring sources. Although the drinking water standard is set at 10 ppb, there is significant increased cancer risk for those drinking water between 5 ppb and 10 ppb, so you should consider treatment or bottled water if your water is in this range. The Environmental Protection Agency considered setting the standard at 5 ppb in 2006 when they revised the arsenic standard down form 50 ppb, but settled on 10 ppb due to implementation cost concerns.

Sample results indicate that almost all public systems in Missoula County contain water that meets the drinking water standard for arsenic. If you have concerns about the quality of water from your public system, contact your water system operator and ask for a copy of the "Consumer Confidence Report". Privately-owned wells are not required to meet water quality standards, but we recommend that all well owners test their water for arsenic at least once or twice (in addition to annually testing for bacterial contamination and possibly other contaminants), to rule out elevated arsenic levels. There are effective treatment methods for removing arsenic from drinking water, if you determine that your water contains elevated arsenic levels.