As many 2.1 million Americans using household water wells may be at risk of harmful exposure to arsenic according to a recent national study. The finding underscores the importance of testing well water for contaminants of local concern, according to the National Ground Water Association (NGWA).

“Our standard advice to private well owners is to test water for anything of local concern,” says Cliff Treyens, NGWA’s director of public outreach. “Contamination threats can vary from locale to locale with some caused by human activity and others such as arsenic occurring naturally in the environment.

The study by the U.S. Geological Survey (USGS)and the CDC’s National Center for Environmental Health shows areas potentially affected by arsenic levels greater than 10 parts per billion — the federally established “maximum contaminant level” at which regulated public water systems are required to take corrective action. Private well water quality, however, is not regulated by the government on an ongoing basis. Private well owners are responsible for testing their water and taking action to keep it safe.

Treyens says a good place to check on local groundwater contamination threats is the county health department. Well owners can look up their county health department on

The study’s top 10 states for people at risk for harmful arsenic exposure based on the size of the at-risk population are:

  1. Michigan: 192,747
  2. Ohio: 189,191
  3. Indiana: 150,858
  4. North Carolina: 119,633
  5. California: 115,823
  6. Maine: 102,452
  7. Texas: 95,455
  8. Pennsylvania: 80,729
  9. Minnesota: 80,353
  10. Wisconsin: 72,670

The study may shed new light on areas not known to have arsenic issues in the groundwater.

“We identify areas of high and low potential exposure in areas of limited arsenic data. These areas may be viewed as potential areas to investigate further or to compare to more detailed local information,” the study states.

To help well owners address the issue of arsenic in well water, NGWA has developed educational tools on testing water and treatment if necessary. The tools on, include:

  • An arsenic Web page
  • A short, online lesson
  • State lists of certified drinking water testing labs
  • An online water test interpretation tool

NGWA is a nonprofit that supports responsible development, management and use of water resources. It is comprised of groundwater professionals ranging from contractors to equipment manufacturers to scientists and engineers. For more information, visit