The U.S. Environmental Protection Agency said Friday it would not take any action in response to tests of 16 more drinking-water wells in the embattled natural gas-drilling town of Dimock, Pa., and one resident whose well showed elevated levels of carcinogenic arsenic declined the agency's offer for alternative water.

The test results largely reinforced findings the EPA released recently on its tests of 31 other residential water wells in the Susquehanna County township, where opponents and supporters of Marcellus Shale natural gas development have clashed. The EPA intervened in Dimock in January after some residents expressed doubt about a state finding of improved water quality in the town.

But the tests found that one well had more than 90 parts per billion of arsenic, nine times higher than the "maximum contaminant level" — the allowable limit for a public water supply. EPA rules do not apply to private wells.

"We have visited the resident several times and explained the data results," Roy Seneca, an EPA spokesman in Philadelphia, said in an e-mail. "The resident has chosen to decline our offer to provide alternate water." It is not clear whether the well is the resident's primary drinking-water supplier.

The EPA did not indicate whether the arsenic is related to natural gas drilling.

Arsenic, whose long-term exposure is linked to bladder, lung, and skin cancer, can come from natural deposits in the earth or "runoff from orchards, runoff from glass and electronic production wastes," according to the EPA's website. It can be removed with filtration devices.

Based on EPA data, the Natural Resources Defense Council in 2000 estimated that 34 million Americans were drinking tap water from systems that contained arsenic levels that posed "unacceptable cancer risks."

Natural gas drilling companies have contended that most contaminants found in rural private water wells occur naturally or existed before shale-gas development.

Only half of Pennsylvania's million private water wells have ever been tested, and about half of those tested have at least one water-quality problem, according to Pennsylvania State University's College of Agricultural Sciences.

For more information on the sampling results, visit: http://www.epa.gov/aboutepa/states/pa.html.

Contact Andrew Maykuth at 215-854-2947 or amaykuth@phillynews.com, or follow on Twitter @Maykuth.