SHALLOW GROUNDWATER QUALITY AND ISOTOPE GEOCHEMISTRY IN THE FAYETTEVILLE SHALE GAS PRODUCTION AREA

The late Mississippian Fayetteville Shale is a focus of unconventional natural gas production in north-central Arkansas. The gas-production area occupies approximately 3,000 square miles, and over 4,000 gas wells have been drilled to date. The gas-producing zone ranges in thickness from 50 to 550 feet and varies in depth from approximately 1,500 to 6,500 feet below ground surface, with average depths ranging from 2,500 to 3,500 feet. Numerous news articles and non-reviewed publications have commented on known and potential degradation of surface and groundwater quality thought to be associated with gas production. Most documented environmental problems have been found to be associated with surface activities, including inadequate sediment-control practices, leaking pipes, and overflowing holding ponds. Occurrences of environmental degradation arising from the subsurface--such as from leaking casing or from the hydraulic fracturing process--are less well documented and present a contentious issue in need of additional data. Most rural households in the area rely on shallow groundwater in fractured Pennsylvannian shales and sandstones for domestic supply; often, no other water source is available. Typical well depths range from 25 to 385 feet, with an average of approximately 85 feet. Concerns have been raised by local residents in the Fayetteville Shale production area about the potential threat of hydraulic fracturing to water quality and quantity local aquifers, and claims have been made relating gas production to problems of turbidity, taste, and overall degradation of the quality of water from domestic water wells. The USGS worked in cooperation with county, State and Federal agencies, private entities, and Duke University to assess shallow groundwater quality in 127 domestic wells in the Fayetteville Shale production area. All analyses were within the range of historically reported (before 1983) concentrations; chloride concentrations were similar between wells less than and greater than 2 miles from gas-production wells, and carbon isotopes demonstrated that detected methane was biogenic in origin. Assessment of general inorganic and isotope geochemistry indicated that water quality resulted from natural factors with no discernable effects from gas-production activities.