Many private ground water wells in New Hampshire and Maine may have arsenic at concentrations close to or above federal safety standards for public water supplies. A recently released study by the U.S. Geological Survey (USGS) shows the likely locations of elevated arsenic. Bedrock aquifer wells - often known as rock, deep, or artesian wells - are the most common type of well installed for homes in the region and it is the bedrock aquifer that is the primary source of arsenic in the locations where it is elevated, according to the findings.

"Our study shows where the highest probability of having arsenic in wells occurs," USGS hydrologist Joseph Ayotte says. "We knew from previous studies that arsenic is a regional problem in New England. The information is intended to assist planners and health officials. It also is intended to help owners in deciding whether to test their well."

The study, published in the journal Environmental Science and Technology, identifies factors that may contribute to high arsenic in wells, and confirms findings from previous studies. Private wells supply drinking water for more than 40 percent of the population of northern New England (20 percent of all of New England) and are not regulated by state and federal agencies. Officials recommend that all private well users test their wells for arsenic.

The collaborative study between the USGS, the National Cancer Institute, part of the National Institutes of Health, Dartmouth Medical School and the departments of health in the states of Maine, New Hampshire and Vermont, concluded that geology was the most significant factor related to arsenic in wells. Other factors include the chemistry of the ground water and characteristics of local aquifers.

The current federal standard for arsenic in public water supplies is 10 micrograms per liter. In New England, 12 percent of the area studied has a greater than 50 percent chance of having wells with arsenic concentrations above 5 micrograms per liter. Nearly one-quarter of the combined area studied in Maine and New Hampshire has a greater than 50 percent chance of having wells with arsenic at or above 5 micrograms per liter.

"Arsenic in ground water used for private or a public water supply is a significant public health concern," says Robert W. Varney, regional administrator of the U.S. Environmental Protection Agency´s (EPA) New England office. "To protect families, EPA recommends that private well owners routinely test their drinking water for arsenic. As of January 2006, public water suppliers are required to meet a new drinking water standard of 10 micrograms per liter."

Arsenic is probably better known as causing acute illness at higher doses. But health effects from long-term exposure at low levels, such as those found in this study, are unclear.

According to Ayotte, this new study is consistent with other, recent studies that have suggested that arsenic is predominantly naturally occurring and related to the geology of the area. "Although human sources may contribute arsenic to ground water, our results suggest that arsenic used as an agricultural pesticide over the past century is not a major source of arsenic in ground water today," Ayotte says.

The complete findings are available at