Redox controls on methane formation, migration and fate in shallow aquifers

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dc.contributor.authorHumez, Pauline
dc.contributor.authorMayer, Bernhard
dc.contributor.authorNightingale, Michael
dc.contributor.authorBecker, Veith
dc.contributor.authorKingston, Andrew
dc.contributor.authorTaylor, Stephen
dc.contributor.authorBayegnak, Guy
dc.contributor.authorMillot, Romain
dc.contributor.authorKloppmann, Wolfram
dc.date.accessioned2022-09-02T21:55:28Z
dc.date.available2022-09-02T21:55:28Z
dc.date.issued2016-07-12
dc.description.abstractDevelopment of unconventional energy resources such as shale gas and coalbed methane has generated some public concern with regard to the protection of groundwater and surface water resources from leakage of stray gas from the deep subsurface. In terms of environmental impact to and risk assessment of shallow groundwater resources, the ultimate challenge is to distinguish (a) natural in situ production of biogenic methane, (b) biogenic or thermogenic methane migration into shallow aquifers due to natural causes, and (c) thermogenic methane migration from deep sources due to human activities associated with the exploitation of conventional or unconventional oil and gas resources. This study combines aqueous and gas (dissolved and free) geochemical and isotope data from 372 groundwater samples obtained from 186 monitoring wells of the provincial Groundwater Observation Well Network (GOWN) in Alberta (Canada), a province with a long record of conventional and unconventional hydrocarbon exploration. We investigated whether methane occurring in shallow groundwater formed in situ, or whether it migrated into the shallow aquifers from elsewhere in the stratigraphic column. It was found that methane is ubiquitous in groundwater in Alberta and is predominantly of biogenic origin. The highest concentrations of biogenic methane (>  0.01 mM or  >  0.2 mgL−1), characterized by δ13CCH4 values  <  −55 ‰, occurred in anoxic Na-Cl, Na-HCO3, and Na-HCO3-Cl type groundwaters with negligible concentrations of nitrate and sulfate suggesting that methane was formed in situ under methanogenic conditions for 39.1 % of the samples. In only a few cases (3.7 %) was methane of biogenic origin found in more oxidizing shallow aquifer portions suggesting limited upward migration from deeper methanogenic aquifers. Of the samples, 14.1 % contained methane with δ13CCH4 values  >  −54 ‰, potentially suggesting a thermogenic origin, but aqueous and isotope geochemistry data revealed that the elevated δ13CCH4 values were caused by microbial oxidation of biogenic methane or post-sampling degradation of low CH4 content samples rather than migration of deep thermogenic gas. A significant number of samples (39.2 %) contained methane with predominantly biogenic C isotope ratios (δ13CCH4 <  −55 ‰) accompanied by elevated concentrations of ethane and sometimes trace concentrations of propane. These gases, observed in 28.1 % of the samples, bearing both biogenic (δ13C) and thermogenic (presence of C3) characteristics, are most likely derived from shallow coal seams that are prevalent in the Cretaceous Horseshoe Canyon and neighboring formations in which some of the groundwater wells are completed. The remaining 3.7 % of samples were not assigned because of conflicting parameters in the data sets or between replicates samples. Hence, despite quite variable gas concentrations and a wide range of δ13CCH4 values in baseline groundwater samples, we found no conclusive evidence for deep thermogenic gas migration into shallow aquifers either naturally or via anthropogenically induced pathways in this baseline groundwater survey. This study shows that the combined interpretation of aqueous geochemistry data in concert with chemical and isotopic compositions of dissolved and/or free gas can yield unprecedented insights into formation and potential migration of methane in shallow groundwater. This enables the assessment of cross-formational methane migration and provides an understanding of alkane gas sources and pathways necessary for a stringent baseline definition in the context of current and future unconventional hydrocarbon exploration and exploitation.en_US
dc.description.grantingagencyNatural Sciences and Engineering Research Council (NSERC)en_US
dc.description.grantingagencyAlberta Innovates - Research Granten_US
dc.identifier.citationHumez, P., Mayer, B., Nightingale, M., Becker, V., Kingston, A., Taylor, S., Bayegnak, G., Millot, R., & Kloppmann, W. (2016). Redox controls on methane formation, migration and fate in shallow aquifers. Hydrology and Earth System Sciences, 20(7), 2759–2777. https://doi.org/10.5194/hess-20-2759-2016en_US
dc.identifier.doihttp://dx.doi.org/10.5194/hess-20-2759-2016en_US
dc.identifier.issn1027-5606
dc.identifier.urihttp://hdl.handle.net/1880/115160
dc.language.isoengen_US
dc.publisherCopernicus Publicationsen_US
dc.publisher.departmentGeoscienceen_US
dc.publisher.facultyScienceen_US
dc.publisher.hasversionpublishedVersionen_US
dc.publisher.institutionUniversity of Calgaryen_US
dc.publisher.policyhttps://www.hydrology-and-earth-system-sciences.net/policies/licence_and_copyright.htmlen_US
dc.rightsUnless otherwise indicated, this material is protected by copyright and has been made available with authorization from the copyright owner. You may use this material in any way that is permitted by the Copyright Act or through licensing that has been assigned to the document. For uses that are not allowable under copyright legislation or licensing, you are required to seek permission.en_US
dc.rights.urihttps://creativecommons.org/licenses/by/4.0en_US
dc.subjectEarth Sciencesen_US
dc.subjectGeochemistryen_US
dc.subjectSciences of the Universeen_US
dc.titleRedox controls on methane formation, migration and fate in shallow aquifersen_US
dc.typejournal articleen_US
ucalgary.item.requestcopyfalseen_US
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