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Evidence of sulfate-dependent anaerobic methane oxidation within an area impacted by coalbed methane-related gas migration Amy L. Wolfe, and Richard Thomas Wilkin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03709 • Publication Date (Web): 28 Dec 2016 Downloaded from http://pubs.acs.org on December 28, 2016
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Environmental Science & Technology
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Evidence of sulfate-dependent anaerobic methane
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oxidation within an area impacted by coalbed
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methane-related gas migration
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Amy L. Wolfe† and Richard T. Wilkin*
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U.S. Environmental Protection Agency, National Risk Management Research Laboratory,
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Ground Water and Ecosystems Restoration Division, 919 Kerr Research Drive, Ada, Oklahoma
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74820, United States
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*
Corresponding author. E-mail:
[email protected]; Tel: 580-436-8874.
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†
Present Address: Department of Geology and Environmental Earth Science, Miami University,
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Oxford, Ohio 45056, United States
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ABSTRACT: We evaluated water quality characteristics in the northern Raton Basin of
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Colorado and documented the response of the Poison Canyon aquifer system several years after
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upward migration of methane gas occurred from the deeper Vermejo Formation coalbed
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production zone. Results show persistent secondary water quality impacts related to the
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biodegradation of methane. We identify four distinct characteristics of groundwater methane
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attenuation in the Poison Canyon aquifer: (i) consumption of methane and sulfate and production
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of sulfide and bicarbonate, (ii) methane loss coupled to production of higher-molecular-weight
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(C2+) gaseous hydrocarbons, (iii) patterns of
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dissolved inorganic carbon, and (iv) a systematic shift in sulfur and oxygen isotope ratios of
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sulfate, indicative of microbial sulfate reduction. We also show that the biogeochemical response
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of the aquifer system has not mobilized naturally occurring trace metals, including arsenic,
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chromium, cobalt, nickel, and lead, likely due to the microbial production of hydrogen sulfide
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which favors stabilization of metals in aquifer solids.
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INTRODUCTION
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C enrichment and depletion in methane and
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Unconventional gas resources, such as natural gas produced from shales and coals, have
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become a strategic alternative to conventional resources: it is abundant, efficient, and cleaner
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burning than other fossil fuels, making it an attractive fuel source in countries where
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governments are implementing policies to reduce greenhouse gas emissions. In the United States,
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advances in drilling technologies (i.e., horizontal drilling, hydraulic fracturing) have stimulated
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large-scale development of coalbed methane (CBM) and shale gas resources. Estimates for total
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U.S. technically recoverable natural gas resources have nearly doubled over the past decade and
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natural gas production within the U.S. is expected to grow by an average rate of 1.4%/year from
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2012 to 2040.1-2 The International Energy Agency (IEA) asserts that the development of
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unconventional gas resources, such as shale gas and CBM, will account for close to half of the
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increase in global gas production by 2040, and projects that approximately 15% of the total
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increase in world gas production will be met by the United States.3
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The rapid expansion of unconventional gas resources has been accompanied by an increase
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in research on the potential effects of drilling operations and resource extraction on drinking
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water sources.4-9 Reports have focused on incidents of blowouts [e.g., Converse County, WY;
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Lawrence Township, PA; Aliso Canyon, CA], home/property explosions [e.g., Bainbridge
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Township, OH; Dimock, PA; Huerfano County, CO] and water contamination caused by
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methane migration in drinking water wells located within areas of hydraulic fracturing
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activities.4,6,9-16 Groundwater movement is an important mechanism by which methane migrates
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from its source, in solution, and accumulates in existing traps.17 The presence of methane in
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water presents a risk of explosion if the gas comes out of solution and accumulates in spaces
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which are not ventilated [lower explosive limit (LEL) = 5 to 15%];18 explosive hazard exists if
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the partial pressure of methane is greater than 0.05 bars.17,19 Although much attention has been
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given to the direct effects of methane migration in groundwater,6,7,20,21 there are also secondary
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water quality impacts stemming from methane migration.22, 23 This study reports on secondary
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water quality impacts, specifically the production of hydrogen sulfide, related to methane
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contamination of drinking water associated with CBM extraction.
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The Raton Basin (RB) is one of the most productive CBM–basins in the United States;
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coalbeds in the Raton Formation (Late Cretaceous to Tertiary) and the Vermejo Formation
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(Cretaceous) are the primary sources of methane. Over 500 billion cubic feet (Bcf) of gas has
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been produced in the Colorado portion of the Raton Basin since initiation of production in the
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1980s; however, major exploration began in the mid-1990s with the development of
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infrastructure to transport the gas out of the basin.24 Annual production of methane from
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coalbeds in Las Animas and Huerfano counties averaged about 103 billion cubic feet during
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2007–2013, approximately 20% of Colorado’s total natural gas production.25 Significant
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commercial levels of CBM have been obtained, almost exclusively, from development and
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production of resources within Las Animas County. Minimal commercial quantities of CBM
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(
