The U.S. Environmental Protection Agency regulates just over 90 contaminants in public drinking water systems. They include manmade chemicals as well as naturally occurring contaminants from rocks and soil. A look at some of the contaminants that fall under federal or state regulations:
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ARSENIC: Enters drinking water through eroding natural deposits in the ground or mining and other industrial activity. Studies say excessive pumping of groundwater can increase arsenic levels. Can cause skin damage, circulatory problems and increased risk of cancer.
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CHROMIUM: A metallic element used to make stainless steel, dyes, paints and wood preservatives. While the EPA regulates chromium in general, there is no separate nationwide standard for chromium-6, which became widely known after the film "Erin Brockovich." Can cause various cancers.
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MICROCYSTIN: Comes from blue-green algae found in lakes and reservoirs. It was largely unregulated in public water systems until toxins contaminated the supply in Toledo, Ohio, last summer. The EPA released nationwide guidelines in May. Can cause vomiting, abdominal pain, liver and kidney damage. Can also kill pets, farm animals and wildlife that drink or swim in the water.
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NITRATE: Commonly found in farm fertilizers, but also comes from animal waste. Carried into drinking water sources by field runoff, leaking septic tanks and sewage. Poses risk to infants under 6 months who drink contaminated water. Can cause shortness of breath, vomiting and the potentially fatal "blue baby" syndrome.
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PERCHLORATE: Used to make rocket fuel, fireworks and other explosives. Contamination has been caused by improper disposal at rocket testing sites, military bases and chemical plants. While regulated in California since 2007, the EPA says it is working on a nationwide standard for drinking water. Can cause thyroid problems in pregnant women and young children.
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SELENIUM: A naturally occurring element that can make its way into water through mountaintop-removal mining or after being discharged by oil and metal refineries. Also found in coal ash left behind when coal is burned for electricity. Can cause damage to the kidneys, skin and nervous system.