The U.S. Geological Survey (USGS) said it found naturally occurring dissolved methane at several household wells used for drinking water in Sullivan County, PA, during pre-drill baseline testing at sites not near existing oil and gas drilling in the Marcellus Shale.

In a 38-page report released Tuesday, the USGS said it had randomly selected 20 water wells in the county and took water samples from them last August and September. The agency said seven of the 20 wells contained dissolved methane, and two of the seven tested above one milligram per liter (mg/L), considered an elevated concentration warranting an isotopic analysis.

The USGS said one positive well tested above 28 mg/L, a potentially explosive situation.

Two state agencies, the Department of Conservation and Natural Resources and the Bureau of Topographic and Geologic Survey, collaborated with the USGS for the study.

Ronald Sloto, the USGS scientist who led the study, said there was a lack of pre-drilling water quality data in the tested area, which is the central and southern portions of Sullivan County.

“Without baseline water quality data, it would not be possible to determine whether a relationship exists between gas production activities and the well water chemistry in the area,” Sloto said. “Although the number of water samples was small, the analytical results show the presence of naturally occurring methane in some private drinking water wells.”

The USGS added that since it had only tested 20 wells, a small sample, “additional sampling would be necessary to provide a broader picture of naturally occurring methane in the region.”

Besides methane and other dissolved gases, scientists analyzed the water samples for 47 constituents and properties, including nutrients, major ions, metals and trace elements. They were also tested for radioactivity.

The report showed the water wells’ depth ranged from 46 to 500 feet, and the wells had casing lengths of 12 to 268 feet. The water yielded from the wells also varied, from 1.5 to 30 gallons per minute.

The two wells that tested for high levels of dissolved methane, SU-151 (4.1 mg/L) and SU-154 (51.1 mg/L), were drilled into the Catskill and Huntley Mountain formations, respectively. The USGS said the SU-154 well had “a very strong odor of hydrogen sulfide” during the collection of water samples.

“The water sample from well SU-151 had the highest concentrations of arsenic, bromide, chloride, lithium, molybdenum and sodium; the second highest concentrations of barium, boron and strontium; and the third highest concentrations of fluoride of the 20 wells sampled,” the report said. Meanwhile, SU-154 “had the highest concentrations of boron and fluoride; the second highest concentration of bromide and sodium; and the third highest concentration of arsenic, lithium, molybdenum and strontium of the 20 wells sampled.”

Kathryn Klaber, CEO of the Marcellus Shale Coalition, told NGI’s Shale Daily that the oil and gas industry continues to take steps to ensure groundwater quality and the environment are protected.

“Private water well quality and construction, as well as methane migration, is a longstanding public health issue in Pennsylvania, dating back decades before Marcellus Shale development began,” Klaber said Thursday. “This independent study, like many others, further underscores the serious nature of this chronic issue and importantly demonstrates that high, ‘potentially explosive or flammable quantities’ of naturally occurring methane are indeed present in private water wells in areas with absolutely no Marcellus development.”

According to data from the Pennsylvania Public Utility Commission, there are 66 horizontal wells drilled in Sullivan County eligible for Act 13 impact fee funds for 2012. The county and 13 municipalities were to collectively receive about $460 million.