Department of
Environmental Quality

Arsenic is a naturally occurring element found in the earth's crust that is found most everywhere. It occurs naturally in rocks and soil, water, air, and plants and animals. There are trace amounts of it in all living matter.

Approximately 90% of industrial arsenic in the U.S. is used as a wood preservative. Arsenic is a well-known poison used in the manufacture of agricultural chemicals such as pesticides, weed killers, and rodenticides. It is also used in the production of paints, dyes, metals, drugs, soaps, and semi-conductors.

Arsenic can be released into the environment through natural activities such as volcanic action, erosion of rocks, and forest fires, or through human activities such as pesticide application, improper disposal of arsenic-containing waste chemicals, agricultural applications, mining, and smelting.

The Arsenic Standard

Concern over the potential effects of long-term, chronic exposure to arsenic in drinking water prompted EPA to reduce the drinking water standard for arsenic from 50 parts per billion (ppb) to 10 ppb, effective January 26, 2006. The standard applies to all 750 Community Water Systems (CWS) and to 245 Non-transient Non-community Water Systems (NTNCWS) in Idaho that exceed 10 ppb of arsenic.

Arsenic and Drinking Water

Most arsenic in drinking water comes from natural rock formations. Water that encounters rock formations can dissolve arsenic and carry it into underground aquifers, streams, and rivers that may be used as drinking water supplies. Arsenic deposited on the ground from industrial or agricultural uses tends to remain in the top few feet of soil for a long time and is not likely to have a significant impact on most aquifers. When dissolved in water, arsenic has no smell, taste, or color, even at high concentrations.

Health Impacts of Exposure

Arsenic has been reported to cause more than 30 different adverse health effects including cardiovascular disease, diabetes mellitus, skin changes, nervous system damage, and various forms of cancer. Although a very high dose (60,000 micrograms) of arsenic can be lethal, the amount of arsenic in drinking water is very small by comparison and any health effects are the result of prolonged exposure over a period of years.

The more people are exposed to arsenic over time, the higher the risk becomes for experiencing health effects. Different people may have different responses to the same exposure to arsenic, depending on dose, duration, general health, age and other factors, so there is no way to know exactly what may happen in any given case. Reducing the amount of arsenic allowed in drinking water will lessen  exposure and reduce risk of adverse health effects.

Incidence of Arsenic

Compared to the rest of the United States, western states have higher arsenic levels (levels greater than 10 ppb). Parts of the Midwest and New England have some areas where arsenic levels are greater than 10 ppb. While many areas may not have detected arsenic in their drinking water above 10 ppb, there may be geographic "hot spots” with higher levels of arsenic than in surrounding areas.

• Map of Arsenic Levels in the U.S.

Arsenic is a problem in some parts of Idaho. Data compiled by the Idaho Department of Water Resources (IDWR) show that concentrations of arsenic in ground water are highest in the southwestern counties of Elmore, Gem, Owyhee, and Washington; in Kootenai County in northern Idaho; and Jefferson County in eastern Idaho. Other counties have moderate or only trace amounts of arsenic in ground water samples.

• Map of Arsenic Levels in Idaho

All community water systems (those that serve at least 15 service connections or 25 people year-round in their primary residences) are required to include health information and arsenic concentrations in their annual drinking water consumer confidence report (CCR) to DEQ for water that exceeds half the new standard (5 ppb). The CCR is available from your water system.

Private well owners are advised to have their well water tested by a certified drinking water laboratory.