A new study has found that arsenic in drinking water from private wells, particularly dug wells, that were created during the first half of the 20th century may have contributed to an elevated risk of bladder cancer that has been observed in northern New England for decades.

The National Institutes of Health, in a news release Monday, said the elevated risk has been observed in Maine, New Hampshire and Vermont for more than 50 years. Other risk factors, such as smoking, did not explain the excess risk in the three-state region, it said.

Bladder cancer mortality rates also have been elevated in northern New England for more than half a century and are about 20 percent higher than the rest of the United States, the NIH said.

The study notes that a unique feature of northern New England is the high proportion of homes using private wells for their drinking water. Private wells are not regulated by the state, federal or local governments.

Such wells may contain arsenic, generally at low to moderate levels, the NIH said. Previous studies have shown that consumption of water containing high concentrations of arsenic increases the risk of bladder cancer.

There are two possible sources of arsenic in well water, according to the NIH. It can occur naturally in the earth. The other source is arsenic-based pesticides that were used extensively on crops such as blueberries, apples and potatoes in the period between 1920 and 1950.

The study was done by researchers at the National Cancer Institute; the Geisel School of Medicine at Dartmouth; the departments of health in Maine, New Hampshire, and Vermont; and the U.S. Geological Survey.

The study appeared Monday in the Journal of the National Cancer Institute.