FAULKNER COUNTY, Ark.—In January, the USGS announced results from its year-long groundwater study in the central portion of Arkansas' Fayetteville shale.
The Mississippian Fayetteville shale extends across north-central Arkansas. Drilling began in the play in 2004, and as of April 2012 approximately 4,000 producing gas wells have been completed in the region. Low gas prices have slowed down some of the activity for the state, but the reservoir remains prolific, relatively easy to produce, and maintains the interest of several operators that continue to develop more production.
"Two or 3 years ago when the Fayetteville shale picked up activity, several local groups wanted to know what we knew about the new development and any effects it might have on the local environment and water quality specifically," said Hydrologic Investigations Program Chief Jim Petersen, with the USGS Arkansas Water Science Center (WCS).
Both Faulkner and Van Buren counties have been active drilling environments through the early development of the Fayetteville. Van Buren County's Shirley Community Development Corp. (CDC), a local non-profit organization, was the first to approach USGS about conducting the study. The problem, according to Petersen, was how to fund such a project. "The WSC is funded by the USGS, but nearly 70% of its budget comes through working with other agencies," he explained.
The Shirley CDC had previously hired a private Little Rock, Ark.-based firm to conduct 18 surface water tests. "I was appointed by the Van Buren County Court to serve on the County Gas Advisory Board in 2011," said Thomas Kimmons, Shirley CDC. "I met several of the hydrologists at USGS in the course of that testing," he added.
The goal at the time was to educate the court and the public on the condition of groundwater resources in the county. Van Buren County contributed $7,000 to the initial study. Neighboring Faulkner County also took an interest in the effort.
"As a county trying to balance the industry with its citizens' concerns, our interest was spurred by the development of this study," said Faulkner County Judge Preston Scroggin. "We received a lot of calls concerning well water in the few years prior to this study," he added. Participating alongside Van Buren County, Faulkner County's government contributed to the water study, believing its findings would help address the concerns of the local population. "Sound science was the answer, and the USGS was a great partner in this effort," Scroggin said. Faulkner County's government supplied $25,000 to conduct the testing. Other contributors included the University of Arkansas, Arkansas Natural Resources Commission, the Arkansas Oil and Gas Commission, Duke University, and the USGS Groundwater Resources Program.
The subsequent study examined water quality in 127 shallow domestic wells in the Fayetteville shale production area in both counties. Ultimately, it found no groundwater contamination associated with gas production. The USGS focused on chloride and methane concentrations as well as carbon isotope ratios from a subsample of 51 wells. "The selection of 51 out of 127 tested for methane was based partly on funding," Petersen said.
These samples were singled out on several criteria including the proximity to ongoing gas production; specific geology; to provide a consistent geographical distribution across the area; and to specifically test water where landowners and community members had reported a significant change in water conditions (typically characterized by smell and/or discoloration). "Water is important to land owners and to the local community, and the aquifer in this region is not prolific," Petersen said.
The study determined that groundwater chemistry in the shallow aquifer system within the study area was a result of natural processes. Chloride concentrations were not higher in the 2011 samples than in samples from nearby areas collected from 1951 through 1983. The study also determined that chloride concentrations found in water wells within 2 miles of a gas-producing well were similar to concentrations from wells outside of this radius. Methane concentrations and carbon isotope ratios indicated that almost all methane present in groundwater samples were naturally occurring as a result of biological processes in shallow shale formations used as a source of water for domestic purposes. These concentrations did not originate from the Fayetteville shale.
"As groundwater moves through shallow aquifers it changes as it comes in contact with the shale and sandstone in the aquifers," Petersen explained. As the groundwater migrates through natural fractures in the reservoir, it picks up chloride and other dissolved solids—longer contact with the rock equals higher concentrations. Oxygen in younger recharge water is consumed through oxidation of inorganic compounds by biochemical processes; lack of oxidation leads to a variety of reductive processes, of which production of methane is the last part of this series.
"This is known as biogenic methane, and it presents a different isotopic signature," Petersen explained. "Methane produced under heat and pressure forms a thermogenic signature, which is relative to the gas being produced from the Fayetteville shale," he added. According to the USGS study, these signatures are consistent in the region and can be used to easily identify the source of methane occurring in groundwater samples. Of those samples tested in this study, a biogenic signature was present. "If the methane detected in these samples carried a thermogenic isotopic signature, it could signify that the gas presence in groundwater occurred from recent shale gas development," Petersen said.
The study results show that groundwater resources in Van Buren and Faulkner counties are unaffected by the most recent wave of drilling activity. Although, the report admits that data limitations exist in all environmental studies; the use of independent comparative methods implies that groundwater quality remains unchanged from shale operations and that groundwater chemistry is attributed to natural processes.
The report findings also make clear that results from this study represent a timeframe relatively early in the gas-production life cycle. Any contaminants released during production activities may not have had sufficient time to reach the sampled wells. Groundwater-quality data from this study describe current conditions at the date of sampling and do not address potential legacy problems.
"The results have been well received in Faulkner County," Scroggin said. "People feel good that we now have a study in place and data to use for a baseline reference for future studies." Judge Scroggin has proposed that Faulkner County follow up on this study in another 2 years.