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Research Article

Noble gases identify the mechanisms of fugitive gas contamination in drinking-water wells overlying the Marcellus and Barnett Shales

Thomas H. Darrah, Avner Vengosh, Robert B. Jackson, Nathaniel R. Warner, and Robert J. Poreda
PNAS first published September 15, 2014; https://doi.org/10.1073/pnas.1322107111
Thomas H. Darrah
aDivision of Earth and Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, NC 27708;
bDivisions of Solid Earth Dynamics and Water, Climate and the Environment, School of Earth Sciences, The Ohio State University, Columbus, OH 43210;
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  • For correspondence: darrah.24@osu.edu
Avner Vengosh
aDivision of Earth and Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, NC 27708;
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Robert B. Jackson
aDivision of Earth and Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, NC 27708;
cDepartment of Environmental Earth System Science, School of Earth Sciences, Woods Institute for the Environment, and Precourt Institute for Energy, Stanford University, Stanford, CA 94305;
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Nathaniel R. Warner
aDivision of Earth and Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, NC 27708;
dDepartment of Earth Sciences, Dartmouth College, Hanover, NH 03755; and
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Robert J. Poreda
eDepartment of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627
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  1. Edited by Thure E. Cerling, University of Utah, Salt Lake City, UT, and approved August 12, 2014 (received for review November 27, 2013)

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Significance

Hydrocarbon production from unconventional sources is growing rapidly, accompanied by concerns about drinking-water contamination and other environmental risks. Using noble gas and hydrocarbon tracers, we distinguish natural sources of methane from anthropogenic contamination and evaluate the mechanisms that cause elevated hydrocarbon concentrations in drinking water near natural-gas wells. We document fugitive gases in eight clusters of domestic water wells overlying the Marcellus and Barnett Shales, including declining water quality through time over the Barnett. Gas geochemistry data implicate leaks through annulus cement (four cases), production casings (three cases), and underground well failure (one case) rather than gas migration induced by hydraulic fracturing deep underground. Determining the mechanisms of contamination will improve the safety and economics of shale-gas extraction.

Abstract

Horizontal drilling and hydraulic fracturing have enhanced energy production but raised concerns about drinking-water contamination and other environmental impacts. Identifying the sources and mechanisms of contamination can help improve the environmental and economic sustainability of shale-gas extraction. We analyzed 113 and 20 samples from drinking-water wells overlying the Marcellus and Barnett Shales, respectively, examining hydrocarbon abundance and isotopic compositions (e.g., C2H6/CH4, δ13C-CH4) and providing, to our knowledge, the first comprehensive analyses of noble gases and their isotopes (e.g., 4He, 20Ne, 36Ar) in groundwater near shale-gas wells. We addressed two questions. (i) Are elevated levels of hydrocarbon gases in drinking-water aquifers near gas wells natural or anthropogenic? (ii) If fugitive gas contamination exists, what mechanisms cause it? Against a backdrop of naturally occurring salt- and gas-rich groundwater, we identified eight discrete clusters of fugitive gas contamination, seven in Pennsylvania and one in Texas that showed increased contamination through time. Where fugitive gas contamination occurred, the relative proportions of thermogenic hydrocarbon gas (e.g., CH4, 4He) were significantly higher (P < 0.01) and the proportions of atmospheric gases (air-saturated water; e.g., N2, 36Ar) were significantly lower (P < 0.01) relative to background groundwater. Noble gas isotope and hydrocarbon data link four contamination clusters to gas leakage from intermediate-depth strata through failures of annulus cement, three to target production gases that seem to implicate faulty production casings, and one to an underground gas well failure. Noble gas data appear to rule out gas contamination by upward migration from depth through overlying geological strata triggered by horizontal drilling or hydraulic fracturing.

  • noble gas geochemistry
  • groundwater contamination
  • methane
  • water quality
  • isotopic tracers

Footnotes

  • ↵1To whom correspondence should be addressed. Email: darrah.24{at}osu.edu.
  • Author contributions: T.H.D., A.V., R.B.J., and R.J.P. designed research; T.H.D., A.V., R.B.J., N.R.W., and R.J.P. performed research; T.H.D., A.V., R.B.J., N.R.W., and R.J.P. analyzed data; and T.H.D., A.V., R.B.J., N.R.W., and R.J.P. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1322107111/-/DCSupplemental.

Freely available online through the PNAS open access option.

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Mechanisms of stray gas contamination
Thomas H. Darrah, Avner Vengosh, Robert B. Jackson, Nathaniel R. Warner, Robert J. Poreda
Proceedings of the National Academy of Sciences Sep 2014, 201322107; DOI: 10.1073/pnas.1322107111

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Mechanisms of stray gas contamination
Thomas H. Darrah, Avner Vengosh, Robert B. Jackson, Nathaniel R. Warner, Robert J. Poreda
Proceedings of the National Academy of Sciences Sep 2014, 201322107; DOI: 10.1073/pnas.1322107111
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