Based on extensive sampling of drinking water wells around the US, 19% of all tested wells were contaminated with a substance such as arsenic, uranium, or radon at a concentration that poses a health risk, and about 10% of all wells have more than one such substance. That's according to a USGS report announced Aug. 30, 2011, based on data from 5,183 drinking water and monitoring wells.
- Trace Elements and Radon in Groundwater Across the United States, 1992-2003 ; Joseph Ayotte,  603-226-7810.
USGS tested public and private drinking water wells and monitoring wells. The substances tested for in this report – which complements other recent reports by USGS that looked at other well contaminants and situations – were 23 trace elements and radon. Trace elements are metals and semi-metallic elements at relatively low concentrations (though they can be elevated enough to cause health or aesthetic problems). Their source often is the surrounding geology, though they can also come from human-generated sources. Climate and land use also play an important role.
Drinking water wells that serve just a small number of people often are unregulated, and it's up to well owners to test their water and to respond to any elevated concentrations of hazardous substances. Many well owners aren't aware there may be a problem, or where responsibility for water quality lies.
Another important issue is that health standards and benchmarks continue to evolve, so test results from a few years ago may have new meaning now. For instance, the current arsenic standard is just 20% of what it was a few years ago, meaning that wells once considered safe could now be deemed very risky.
The report includes many maps that can help you determine if there may be a problem in your audience area. For instance, arsenic has been identified as a concern in parts of Maine, which is one of at least 33 states identified in this report as having one or more wells with a concentration above the current health standard. Arsenic contamination was the hook for an in-depth article Sept. 5, 2011, in the Kennebec (Maine) Journal.
- "Toxic Taps Part 1: High Amounts of Arsenic in Local Wells,"  Kennebec Journal, Sept. 5, 2011, by Mechele Cooper.
Many other substances, including radon, were present at risky concentrations in numerous locations around the country.