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      Impact of an historic underground gas well blowout on the current methane chemistry in a shallow groundwater system

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          Significance

          The rapid increase in shale gas production in recent years has led to increased attention to its potential negative environmental effects, including the risks of contaminating groundwater with methane and other substances. In this context, the uncontrolled gas migration that is triggered during well blowouts is an understudied environmental hazard. We show that the methane chemistry in shallow groundwater overlying the site of a catastrophic underground blowout continues to be impacted 50 y later. The occurrence of anaerobic methane oxidation limits the spatial extent to which the dissolved thermogenic methane plume could be observed and discerned from local biogenic methane sources. However, it also highlights the requirement to carry out monitoring in close proximity to potential gas leakage sources.

          Abstract

          Blowouts present a small but genuine risk when drilling into the deep subsurface and can have an immediate and significant impact on the surrounding environment. Nevertheless, studies that document their long-term impact are scarce. In 1965, a catastrophic underground blowout occurred during the drilling of a gas well in The Netherlands, which led to the uncontrolled release of large amounts of natural gas from the reservoir to the surface. In this study, the remaining impact on methane chemistry in the overlying aquifers was investigated. Methane concentrations higher than 10 mg/L ( n = 12) were all found to have δ 13C-CH 4 values larger than −30‰, typical of a thermogenic origin. Both δ 13C-CH 4 and δD-CH 4 correspond to the isotopic composition of the gas reservoir. Based on analysis of local groundwater flow conditions, this methane is not a remnant but most likely the result of ongoing leakage from the reservoir as a result of the blowout. Progressive enrichment of both δ 13C-CH 4 and δD-CH 4 is observed with increasing distance and decreasing methane concentrations. The calculated isotopic fractionation factors of ε C = 3 and ε D = 54 suggest anaerobic methane oxidation is partly responsible for the observed decrease in concentrations. Elevated dissolved iron and manganese concentrations at the fringe of the methane plume show that oxidation is primarily mediated by the reduction of iron and manganese oxides. Combined, the data reveal the long-term impact that underground gas well blowouts may have on groundwater chemistry, as well as the important role of anaerobic oxidation in controlling the fate of dissolved methane.

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          Most cited references34

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          Methane contamination of drinking water accompanying gas-well drilling and hydraulic fracturing.

          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 CH(4) 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 δ(13)C-CH(4) 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 δ(13)C-CH(4) data, coupled with the ratios of methane-to-higher-chain hydrocarbons, and δ(2)H-CH(4) 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.
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            Fractionation of carbon and hydrogen isotopes by methane-oxidizing bacteria

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              Increased stray gas abundance in a subset of drinking water wells near Marcellus shale gas extraction.

              Horizontal drilling and hydraulic fracturing are transforming energy production, but their potential environmental effects remain controversial. We analyzed 141 drinking water wells across the Appalachian Plateaus physiographic province of northeastern Pennsylvania, examining natural gas concentrations and isotopic signatures with proximity to shale gas wells. Methane was detected in 82% of drinking water samples, with average concentrations six times higher for homes <1 km from natural gas wells (P = 0.0006). Ethane was 23 times higher in homes <1 km from gas wells (P = 0.0013); propane was detected in 10 water wells, all within approximately 1 km distance (P = 0.01). Of three factors previously proposed to influence gas concentrations in shallow groundwater (distances to gas wells, valley bottoms, and the Appalachian Structural Front, a proxy for tectonic deformation), distance to gas wells was highly significant for methane concentrations (P = 0.007; multiple regression), whereas distances to valley bottoms and the Appalachian Structural Front were not significant (P = 0.27 and P = 0.11, respectively). Distance to gas wells was also the most significant factor for Pearson and Spearman correlation analyses (P < 0.01). For ethane concentrations, distance to gas wells was the only statistically significant factor (P < 0.005). Isotopic signatures (δ(13)C-CH4, δ(13)C-C2H6, and δ(2)H-CH4), hydrocarbon ratios (methane to ethane and propane), and the ratio of the noble gas (4)He to CH4 in groundwater were characteristic of a thermally postmature Marcellus-like source in some cases. Overall, our data suggest that some homeowners living <1 km from gas wells have drinking water contaminated with stray gases.
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                Author and article information

                Journal
                Proc Natl Acad Sci U S A
                Proc. Natl. Acad. Sci. U.S.A
                pnas
                pnas
                PNAS
                Proceedings of the National Academy of Sciences of the United States of America
                National Academy of Sciences
                0027-8424
                1091-6490
                9 January 2018
                26 December 2017
                26 December 2017
                : 115
                : 2
                : 296-301
                Affiliations
                [1] aCopernicus Institute of Sustainable Development, Utrecht University , 3584 CS Utrecht, The Netherlands;
                [2] bEarth Sciences Department, Utrecht University , 3584 CD Utrecht, The Netherlands;
                [3] cGeohydrology Unit, KWR Water Cycle Research Institute , 3433 PE Nieuwegein, The Netherlands;
                [4] dGeological Survey of the Netherlands, Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek (TNO), 3584 CB Utrecht, The Netherlands
                Author notes
                1To whom correspondence should be addressed. Email: g.schout@ 123456uu.nl .

                Edited by Susan L. Brantley, Pennsylvania State University, University Park, PA, and approved November 27, 2017 (received for review June 27, 2017)

                Author contributions: G.S., N.H., S.M.H., and J.G. designed research; G.S. performed research; G.S., N.H., S.M.H., and J.G. analyzed data; and G.S., N.H., and J.G. wrote the paper.

                Article
                201711472
                10.1073/pnas.1711472115
                5777038
                29279383
                a2a09eb4-9bc2-4c3e-a3b5-92bbffcaa891
                Copyright © 2018 the Author(s). Published by PNAS.

                This open access article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).

                History
                Page count
                Pages: 6
                Funding
                Funded by: Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) 501100003246
                Award ID: 859.14.001
                Categories
                Physical Sciences
                Environmental Sciences

                groundwater contamination,well blowouts,methane,isotopic fingerprinting,anaerobic methane oxidation

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