A new study by the U.S. Geological Survey and Centers for Disease Control and Prevention estimates about 2.1 million people in the U.S. may be getting drinking water from private domestic wells with high concentrations of arsenic, presumed to be from natural sources.

“About 44 million people in the lower 48 states use water from domestic wells,” said Joe Ayotte, a USGS hydrologist and lead author of the study. “While we’re confident our research will help well owners understand if they live in an area of higher risk for arsenic, the only way for them to be certain of what’s in their water is to have it tested.”

Using a standard of 10 micrograms of arsenic per liter — the maximum contaminant level allowed for public water supplies — the researchers developed maps of the contiguous U.S. showing locations where there are likely higher levels of arsenic in groundwater and how many people may be using it.

Nearly all of the arsenic in the groundwater tested for this study and used to map probabilities is likely from natural sources and is presumed to be coming primarily from rocks and minerals through which the water flows.

The findings highlight the importance of private well owners working with their local and state officials to determine the best way to test and, if necessary, treat their water supplies.

“Fortunately, in most areas of the country and with appropriate safeguards, the majority of homeowners can get good-quality drinking water from private wells,” Ayotte says. “But this study is a good reminder that prudent, routine testing of the water, including its interaction with the water supply system, is an essential first step so homeowners and their families can confidently drink water from their faucets.”

Using water samples from more than 20,000 domestic wells, the researchers developed a statistical model that estimates the probability of having high arsenic in domestic wells in a specific area. They used that model in combination with information on the U.S. domestic well population to estimate the population in each county of the continental United States with potentially high concentrations of arsenic in domestic wells.

“One of our study’s basic assumptions is that the probability of high arsenic can be estimated by a statistical model. We also assume that the domestic water-use population is represented by census information used in the study,” Ayotte says.

Some of the locations where it’s estimated that the most people may have high levels of arsenic in private domestic well water include much of the West, parts of the Northeast and Midwest and some Atlantic coastal states in the Southeast.

“Although high-arsenic wells can occur in all 48 contiguous states, it is more prevalent in some states than in others,” Ayotte says. “The study did not include Alaska and Hawaii.”

The researcher provided a cautionary note that while the study provides state and county estimates, they are not intended to take the place of more detailed or local information that may already be available in some areas.

Long-term exposure to arsenic in domestic wells may cause health-related problems, including an increased risk of cancer. Testing and, if necessary, treating the water is an effective way of reducing or eliminating the concern. A CDC fact sheet provides more information as does the CDC’s Agency for Toxic Substances and Disease Registry.

“Ultimately, this study should be helpful not only in assessing the likelihood of people being exposed to arsenic in domestic well water, but the results of the study may assist other researchers evaluate situations where adverse health outcomes, such as cancers or adverse birth outcomes, may be related to environmental factors,” Ayotte says.

Public water supplies are regulated by the U.S. EPA, but maintenance, testing, and treatment of private water supplies are the sole responsibility of the homeowner. About 44 million people in the U.S. get their drinking water from private wells, yet surveys indicate many homeowners are unaware of some basic testing that should be done to help ensure safe drinking water in the home.

The study, “Estimating the high-arsenic domestic-well population in the conterminous United States” by J.D. Ayotte, L. Medalie, S.L. Qi, L.C. Backer, and N. T. Nolan is available online in Environmental Science and Technology.