A
study by Duke University researchers has found high levels of leaked methane in
well water collected near shale-gas drilling and hydrofracking sites. The scientists collected and analyzed water
samples from 68 private groundwater wells across five counties in northeastern
Pennsylvania and New York.
"At
least some of the homeowners who claim that their wells were contaminated by shale-gas
extraction appear to be right," says Robert B. Jackson, Nicholas Professor of
Global Environmental Change and director of Duke's Center on Global Change.
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.
The
study 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 peer-reviewed
study of well-water contamination from shale-gas drilling and hydrofracking appears
this week in the online Early Edition of the Proceedings of the National Academy of Sciences.
"We
found measurable amounts of methane in 85 percent of the samples, but levels were
17 times higher on average in wells located within a kilometer of active
hydrofracking sites," says Stephen Osborn, postdoctoral research associate at
Duke's Nicholas School of the Environment. 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.
"Methane
is CH4. By using carbon and hydrogen isotope tracers we could 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," says
Avner Vengosh, professor of geochemistry and water quality. Biogenic methane is
not associated with hydrofracking.
"Methane
in water wells within a kilometer had an isotopic composition similar to
thermogenic methane," Vengosh says. "Outside this active zone, it was mostly a mixture
of the two."
The
scientists confirmed their finding by comparing the dissolved gas chemistry of water
samples to the gas chemistry profiles of shale-gas wells in the region, using data
from the Pennsylvania Department of Environmental Protection. "Deep gas has a distinctive chemical
signature in its isotopes," Jackson says. "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 and
methane isn't regulated as a contaminant in public water systems under the
EPA's National Primary Drinking Water Regulations.
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.
Shale
gas comprises about 15 percent 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
study was funded by the Nicholas School and Duke's Center on Global Change. Nathaniel
R. Warner, a PhD student of Vengosh's, was a co-author.
Independent
of the PNAS study, Jackson and colleagues at the Center for Global Change, the
Nicholas School and Duke's Nicholas Institute for Environmental Policy
Solutions have issued a white paper on hydrofracking at www.nicholas.duke.edu/cgc. It includes recommendations
for monitoring and addressing potential environmental and human health risks.