The U.S. Geological Survey (USGS) said domestic water wells in two Arkansas counties showed no evidence of contamination from oil and natural gas drilling in the Fayetteville Shale, but it cautioned that additional testing would be necessary to provide an adequate comparative analysis.

According to a 42-page report released Tuesday, the USGS collected samples from 127 wells in Faulkner and Van Buren counties. The samples were then analyzed for concentrations of methane and chloride, as well as for major ions, trace metals and carbon isotopic composition.

“None of the data that we have looked at as part of this study suggests that any groundwater contamination is resulting from natural gas production activities,” USGS hydrologist Tim Kresse told the Arkansas Times. “However, this study does not speak to other wells that were not sampled, every chemical used during the hydraulic fracturing process, or water quality changes that might take longer to occur. It does provide a baseline to use to evaluate any possible changes in the future.”

The USGS added that the test results “represent a time frame relatively early in the gas production life cycle, and any contaminants released during production activities may not have had sufficient time to reach the sampled wells. As such, groundwater quality data from this study describe current conditions at the date of sampling and do not address potential legacy problems, if any should occur in the future.

“However, [this data provides] a baseline range and variation of geochemistry for groundwater in the study area, which can be used to assess future potential changes to groundwater quality in the area of gas production from the Fayetteville Shale.”

The USGS tested 51 of the 127 wells for methane. At 32 of those wells, methane occurred above a detection limit of 0.0002 milligram per liter (mg/L). Methane concentrations ranged up to 28.5 mg/L, and at seven wells registered greater than or equal to 0.5 mg/L. The agency said the carbon isotopic composition of the higher concentration samples, including the highest concentration of 28.5 mg/L, showed that the methane “was likely biogenic in origin with carbon isotope ratio values ranging from -57.6 to -74.7 per mil.”

Meanwhile the agency said chloride concentrations from all 127 wells tested ranged from about 1.0 to 70.0 mg/L, with a median concentration of 3.7 mg/L. By comparison, the maximum and median concentrations from historical data were 378.0 mg/L and 20.0 mg/L, respectively.

“Major ions and trace metals additionally had lower concentrations in data gathered for this study than in the historical analyses,” the USGS said. The agency added that there was “no statistical difference” in chloride concentrations at 94 water wells within two miles of a shale gas well, or the remaining 33 wells that were located two miles or more from shale gas wells.

Drilling activity has been declining in the Fayetteville Shale. According to NGI‘s Shale Daily Unconventional Rig Count, the number of rigs active in the play fell 37% in one year for the week ending Jan. 4, from 27 to 17 rigs.

In 2011, regulators with the Arkansas Oil and Gas Commission (AOGC) set a moratorium on wastewater disposal wells in an area of the Fayetteville after a series of small earthquakes there. Two injection wells in Faulkner County were shut down before the moratorium took effect (see Shale Daily, July 29, 2011; March 8, 2011).

Collaborating on the USGS report were the Arkansas Natural Resources Commission, the AOGC, Duke University, Faulkner County, the Shirley Community Development Corp., the University of Arkansas at Fayetteville and the USGS’ Groundwater Resources Program.