It’s a match … again.
In May 2011, my colleagues at Duke’s Nicholas School of the Environment published a controversial paper on “Methane contamination of drinking water accompanying gas-well drilling and hydraulic fracturing.”(see following article) New information appears to corroborate the authors’ findings.
Hydraulic fracturing, a k a fracking, as you might well know by now, refers to the use of horizontal drilling and hydraulic fracturing to exploit shale and other low-permeability formations that hold natural gas. A vertical well is drilled to the depth of the target rock (which in the case of the Marcellus Shale is often more than a mile below the surface) and extended horizontally along the gas-bearing formation. Water, sand and a mix of chemicals are then injected at high pressure into the well to split the rock and liberate the trapped gas. (See graphic at side and video below.)
In last year’s paper the Duke scientists reported that the data they had collected from 68 drinking water wells in Pennsylvania and New York indicated that drinking water in the vicinity of fracking operations was more likely to have higher concentrations of methane gas — and a specific form of the gas that looked like the same gas being extracted via fracking from the Marcellus Shale about a mile or more beneath the surface. Among the data they had to tie the well water to the shale gas was isotopic information: the methane in the wells had an isotopic signature similar to that of nearby shale gas wells.
No way, said the gas companies. In fact one company, Cabot Oil and Gas, in an attempt to discredit the Duke study, published an analysis in the industry publication Oil & Gas Journal demonstrating that the specific isotopic composition of the shale gas they pump is distinct from the methane found in drinking water wells.
But whaddya know? The Environmental Protection Agency also has data on drinking water and methane levels from in and around Dimock, Pennsylvania, data the agency left uninterpreted because of a change in its investigation and the complexity of isotopic “fingerprints.” But through a freedom of information request, Rob Jackson, one of the Duke scientists who co-authored the study, and colleagues obtained the isotopic data collected and analyzed earlier this year by EPA. The methane in some of the wells matches the methane in the gas Cabot is producing.[toggle title=”Methane contamination of drinking water” height=”auto”]
Methane contamination of drinking water accompanying gas-well drilling and hydraulic fracturing.
+ Author Affiliations
aCenter on Global Change, Nicholas School of the Environment,
bDivision of Earth and Ocean Sciences, Nicholas School of the Environment, and
cBiology Department, Duke University, Durham, NC 27708
Edited* by William H. Schlesinger, Cary Institute of Ecosystem Studies, Millbrook, NY, and approved April 14, 2011 (received for review January 13, 2011)
Directional drilling and hydraulic-fracturing technologies are dramatically increasing natural-gas extraction. In aquifers overlying the Marcellus and Utica shale formations of northeastern Pennsylvania and upstate New York, we document systematic evidence for methane contamination of drinking water associated with shale-gas extraction. In active gas-extraction areas (one or more gas wells within 1 km), average and maximum methane concentrations in drinking-water wells increased with proximity to the nearest gas well and were 19.2 and 64 mg CH4 L-1 (n = 26), a potential explosion hazard; in contrast, dissolved methane samples in neighboring nonextraction sites (no gas wells within 1 km) within similar geologic formations and hydrogeologic regimes averaged only 1.1 mg L-1 (P < 0.05; n = 34). Average δ13C-CH4 values of dissolved methane in shallow groundwater were significantly less negative for active than for nonactive sites (-37 ± 7‰ and -54 ± 11‰, respectively; P < 0.0001). These δ13C-CH4 data, coupled with the ratios of methane-to-higher-chain hydrocarbons, and δ2H-CH4 values, are consistent with deeper thermogenic methane sources such as the Marcellus and Utica shales at the active sites and matched gas geochemistry from gas wells nearby. In contrast, lower-concentration samples from shallow groundwater at nonactive sites had isotopic signatures reflecting a more biogenic or mixed biogenic/thermogenic methane source. We found no evidence for contamination of drinking-water samples with deep saline brines or fracturing fluids. 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.
Author contributions: S.G.O., A.V., and R.B.J. designed research; S.G.O. and N.R.W. performed research; A.V. contributed new reagents/analytic tools; S.G.O., A.V., N.R.W., and R.B.J. analyzed data; and S.G.O., A.V., N.R.W., and R.B.J. wrote the paper.
The authors declare no conflict of interest.
*This Direct Submission article had a prearranged editor.
- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1100682108/-/DCSupplemental
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