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Missoula Valley Water Quality District
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Missoula Valley Water Quality District

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Water Quality District - Wells: Arsenic


Naturally-occurring arsenic in groundwater in Missoula County

The Environmental Protection Agency and the State of Montana set water quality standards for public drinking water systems. Recently, the EPA changed the allowable concentration of arsenic in drinking water from 50 parts per billion (ppb) to 10 ppb. Evidence indicated that the old standard did not adequately protect public health. This new, stricter standard took effect in January 2006.

Arsenic occurs naturally in some rocks and sediments. 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 indicate 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 new 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. Based on this information, it is possible that approximately 3% of wells in Missoula County contain arsenic exceeding the drinking water standard. Additional studies are under way to better determine the extent of arsenic occurrence in our area

Sample results indicate that almost all public systems in Missoula County contain water well below the new standard. 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.

For a limited time, the Missoula City-County Health Dept. is offering free arsenic tests for private well users in Missoula County. Click here for information.

Click here for general information about testing private wells.

Visit the MSU Extension Well Educated Program for more information, or call the Water Quality District at 258-4890.

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