Some homeowners living near shale gas wells appear to be at higher risk of drinking water contamination from stray gases, according to a new Duke University-led study.
The scientists analyzed 141 drinking water samples from private water wells across northeastern Pennsylvania's gas-rich Marcellus Shale basin.
They found that, on average, methane concentrations were six times higher and ethane concentrations were 23 times higher at homes within a kilometer of a shale gas well.Propane was detected in 10 samples, all of them from homes within a kilometer of drilling.
"The methane, ethane and propane data, and new evidence from hydrocarbon and helium content, all suggest that drilling has affected some homeowners' water," said Robert B. Jackson, a professor of environmental sciences at Duke's Nicholas School of the Environment."In a minority of cases the gas even looks Marcellus-like, probably caused by poor well construction."
The ethane and propane data are "particularly interesting," he noted, "since there is no biological source of ethane and propane in the region and Marcellus gas is high in both, and higher in concentration than Upper Devonian gases" found in formations overlying the Marcellus shale.
The scientists examined which factors might explain their results, including topography, distance to gas wells and distance to geologic features."Distance to gas wells was, by far, the most significant factor influencing gases in the drinking water we sampled," said Jackson.
The team published its peer-reviewed findings this week in the online Early Edition of the Proceedings of the National Academy of Sciences.
Shale gas extraction -- a process that includes horizontal drilling and hydraulic fracturing -- has fueled concerns in recent years about contamination of nearby drinking water supplies.
Two previous Duke-led studies found direct evidence of methane contamination in water wells near shale-gas drilling in northeastern Pennsylvania, as well as possible hydraulic connectivity between deep brines and shallow aquifers.A third study, conducted with U.S. Geological Survey scientists, found no evidence of drinking water contamination from shale gas production in Arkansas.None of the studies found evidence of current contamination by hydraulic fracturing fluids.
The new study is the first to offer direct evidence of ethane and propane contamination.
"Our studies demonstrate that the integrity of gas wells, as well as variations in local and regional geology, play major roles in determining the possible risk of groundwater impacts from shale gas development.As such, they must be taken into consideration before drilling begins," said Avner Vengosh, professor of geochemistry and water quality at Duke's Nicholas School.
"The new data reinforces our earlier observations that stray gases contaminate drinking water wells in some areas of the Marcellus shale.The question is what is happening in other shale gas basins," Vengosh said.
"The helium data in this study are the first in a new tool kit we've developed for identifying contamination using noble gas geochemistry," said Thomas H. Darrah, a research scientist in geology, also at Duke's Nicholas School."These new tools allow us to identify and trace contaminants with a high degree of certainty through multiple lines of evidence."
Co-authors of the new study are Nathaniel Warner, Adrian Down, Kaiguang Zhao and Jonathan Karr, all of Duke; Robert Poreda of the University of Rochester; and Stephen Osborn of California State Polytechnic University. Duke's Nicholas School of the Environment and the Duke Center on Global Change funded the research.
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