A study by Duke University researchers has found evidence of methane contamination of drinking water associated with shale-gas extraction via directional drilling and hydraulic-fracturing technologies.
In their analysis of water samples from 68 private groundwater wells across five counties in northeastern Pennsylvania and New York, the team found high levels of methane in well water collected near shale-gas drilling and hydrofracking sites.
However, they found no evidence of contamination from chemical-laden fracking fluids, which are injected into gas wells to help break up shale deposits, or from “produced water,” wastewater that is extracted back out of the wells after the shale has been fractured.
The study appeared in the online Early Edition of the Proceedings of the National Academy of Sciences. It is the first peer-reviewed study to measure well-water contamination from shale-gas drilling and hydrofracking.
Methane concentrations were detected generally in 51 of 60 drinking-water wells (85%) across the region, regardless of gas industry operations, but concentrations were substantially higher closer to natural-gas wells. Methane concentrations were 17-times higher on average (19.2 mg CH4 L-1) in shallow wells from active drilling and extraction areas than in wells from nonactive areas (1.1 mg L-1 on average; P < 0.05). The average methane concentration in shallow groundwater in active drilling areas fell within the defined action level (10–28 mg L-1) for hazard mitigation recommended by the US Office of the Interior, and our maximum observed value of 64 mg L-1 is well above this hazard level. Understanding the origin of this methane, whether it is shallower biogenic or deeper thermogenic gas, is therefore important for identifying the source of contamination in shallow groundwater systems.—Osborn et al.
The contamination was observed primarily in Bradford and Susquehanna counties in Pennsylvania. Water wells farther from the gas wells contained lower levels of methane and had a different isotopic fingerprint.
By using carbon and hydrogen isotope tracers we can distinguish between thermogenic methane, which is formed at high temperatures deep underground and is captured in gas wells during hydrofracking, and biogenic methane, which is produced at shallower depths and lower temperatures. Methane in water wells within a kilometer had an isotopic composition similar to thermogenic methane. Outside this active zone, it was mostly a mixture of the two.—Avner Vengosh, professor of geochemistry and water quality
Biogenic methane is not associated with hydrofracking.
The researchers also compared the dissolved gas chemistry of water samples to the gas chemistry profiles of shale-gas wells in the region, using data released publicly by the Pennsylvania Department of Environmental Protection. Deep gas has a distinctive chemical signature in its isotopes, says Robert B. Jackson, Nicholas Professor of Global Environmental Change and director of Duke’s Center on Global Change. “When we compared the dissolved gas chemistry in well water to methane from local gas wells, the signatures matched.”
Methane is flammable and poses a risk of explosion. In very high concentrations, it can cause asphyxiation. Little research has been conducted on the health effects of drinking methane-contaminated water. Methane isn’t regulated as a contaminant in public water systems under the EPA’s National Primary Drinking Water Regulations.
Hydraulic fracturing, also called hydrofracking or fracking, involves pumping water, sand and chemicals deep underground into horizontal gas wells at high pressure to crack open hydrocarbon-rich shale and extract natural gas. Shale gas comprises about 15% of natural gas produced in the United States today. The Energy Information Administration estimates it will make up almost half of the nation’s production by 2035.
The Duke team collected samples from counties overlying the Marcellus shale formation. Accelerated gas drilling and hydrofracking in the region in recent years has fueled concerns about well-water contamination by methane, produced water and fracking fluids, which contain a proprietary mix of chemicals that companies often don’t disclose.
We conclude that greater stewardship, data, and—possibly—regulation are needed to ensure the sustainable future of shale-gas extraction and to improve public confidence in its use.—Osborn et al.
All funding for the study came from the Nicholas School and Center on Global Change. Nathaniel R. Warner, a PhD student of Vengosh’s, co-authored the study.
Independent of the PNAS study, Jackson and colleagues at the Center on Global Change, Nicholas School and Nicholas Institute for Environmental Policy Solutions have issued a white paper on hydrofracking, including recommendations for monitoring and addressing potential environmental and human health risks.
Osborn, SG, A Vengosh, NR Warner, RB Jackson. (2011) Methane contamination of drinking water accompanying gas-well drilling and hydraulic fracturing. Proceedings of the National Academy of Sciences doi: 10.1073/pnas.1100682108
Jackson RB, B Rainey Pearson, SG Osborn, NR Warner, A Vengosh (2011) Research and policy recommendations for hydraulic fracturing and shale-gas extraction. Center on Global Change, Duke University, Durham, NC.