Drinking Water Indicator:   MCL violations and mean concentration

Measure : Arsenic MCL violations and mean concentrations in Community Water Systems in Oregon

Arsenic is a naturally occurring element widely distributed in soil and minerals. It may enter water from runoff and leaching; and enter air, water and land from wind-blown dust. Many common arsenic compounds can dissolve in water.  Arsenic is used in the manufacturing of many products, including as a wood preservative, to make pressure treated lumber, and other industrial applications. Organic arsenic compounds are also used as pesticides, primarily on cotton fields and in orchards.

Arsenic has been identified as a human carcinogen by the International Agency for Research on Cancer. Exposure to arsenic at a concentration of hundreds of micrograms per liter (mcg/L) in the drinking water of Taiwan, Chile, Argentina, Mexico, Bangladesh and India has been associated with adverse health effects including lung, bladder, liver and skin cancers. Other adverse health effects include nausea, cardiovascular disease, developmental and reproductive effects, diabetes, and skin keratosis and hyperpigmentation.

In 2006 the U.S. Environmental Protection Agency (EPA) strengthened the regulatory drinking water standard on the basis of bladder and lung cancer risks and reduced the Maximum Contaminant Level (MCL) of arsenic allowed in drinking water from 50 mcg/L to 10 mcg/L. The EPA also has set limits on the amount of arsenic that industrial sources can release into the environment and has restricted or stopped many of the uses of arsenic in pesticides.

The arsenic measures below provide simple estimates of the number of community water supplies with high levels of arsenic that could potentially cause adverse health effects, as well as the number of people potentially exposed to water that does not meet regulatory limits.

From 2002 to 2007 less than 1 percent of Oregonians who received water from community water systems were exposed to water not meeting the EPA’s current arsenic drinking water standard of 10 mcg/L.  In addition, arsenic levels above the 10 mcg/L MCL rare and the water delivered to more than 95 percent of public water recipients had average arsenic levels lower than 30 percent of the MCL, i.e., less than 3 mcg/L.

Table 1:  Annual percentage and number of CWSs with arsenic MCL violations, and number of people served by CWSs with arsenic MCL violations (using the current standard of 10 mcg/L)    

Table 2:  Three-year compliance period mean arsenic concentrations across CWSs and number of people served by mean arsenic concentrations   

Graph 1:  Distribution of number of people by mean arsenic concentrations for compliance periods 2002–2004 and 2005–2007   

Graph 2:  Distribution of number of Community Water Systems by mean arsenic concentrations for compliance periods 2002–2004 and 2005–2007  

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Table 1: Annual percentage and number of CWSs with arsenic MCL violations, and number of people served by CWSs with arsenic MCL violations (using the current standard of 10 mcg/L)  

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Table 2: Three-year compliance period mean arsenic concentrations across CWSs and number of people served by mean arsenic concentrations

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Graph 1: Distribution of number of people by mean arsenic concentrations for compliance periods 2002–2004 and 2005–2007

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Graph 2: Distribution of number of Community Water Systems by mean arsenic concentrations for compliance periods 2002–2004 and 2005–2007

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