Some 2 million people in the U.S. get their drinking water from wells which may be infused with naturally-occurring arsenic, according to a new study published by the American Chemical Society.

The estimates indicate that the Americans who rely on personal wells should check to see if the geology of their homes has caused a problem at their taps, report the scientists from the U.S. Geological Survey and the Centers for Disease Control and Prevention.

“The major results of this study are the estimates of the total population in the conterminous U.S. potentially exposed to high arsenic, based on a model of arsenic probability for domestic wells,” they write in the study published this morning in the ACS journal Environmental Science and Technology. “Arsenic in groundwater reflects geologic sources, aquifer geochemistry, and national-to-local scale processes, such as climatic, physiochemical, and geochemical variation.”

More than 20,000 domestic wells served as the source of arsenic concentration readings. Those data were used to develop a logistic regression model based on geology, soil chemistry, hydrology, and climate conditions, among other natural factors.

That data was overlaid with demographics for where the some 44 million people relying on wells lived.

The scientists looked for target levels of where there would be arsenic readings higher than 10 micrograms per liter, the safety threshold as determined by the U.S. Environmental Protection Agency.

The greatest numbers of well users who may have high arsenic in their wells:  Michigan (nearly 200,000), Ohio (about 190,000), Indiana (150,000), North Carolina (120,000), and California (115,000).

The “hot spots” of potential arsenic contamination were spread across the continental U.S. For instance, a swath of New England from Maine to New Hampshire is likely to have a high percentage of wells with elevated arsenic. The band in the upper Midwest includes the big cluster from Michigan and Ohio. The southwest bunches include parts of Nevada, southern Arizona, southern and central California, and isolated regions all over the west. An extent of southern Texas also includes major densities of arsenic-contaminated wells, they add.

The health effects of drinking the inorganic arsenic could include elevated cancer risks, developmental problems in the young, as well as cardiovascular disease, neurotoxic effects, and even diabetes, according to health agencies.