Conflict of interest information is provided below for the authors of this paper.

  • Chesapeake Energy Corporation (Chesapeake) funded the authors of this paper through their organizations of employment and, in the case of the senior author, privately, to do basic research to evaluate this very large data set and prepare the paper. Data were collected on behalf of Chesapeake by paid third-party consultants to comply with regulatory programs. The analyses and interpretations, and report writing, were done by the authors of the paper. The decision to submit the paper was that of the authors. The opinions and conclusions expressed in this paper are those of the authors and do not necessarily reflect those of Chesapeake.

  • During the preparation of this paper, all authors worked for the organizations noted in authorship. Mark Hollingsworth is a current employee of Chesapeake, having worked there from February 2011 to the present. Prior to Mr. Hollingsworth’s employment by Chesapeake, he worked for TestAmerica Laboratories, Inc., which provided laboratory analytical consulting services to Chesapeake. Bert Smith is a former employee of Chesapeake, having worked there from May 2012 to September 2013, and has been employed by Enviro Clean Cardinal from November 2013 to the present. Enviro Clean Cardinal also does consulting work for Chesapeake. Prior to May 2013, Mr. Smith worked for Science Applications International Corporation, which did consulting work for Chesapeake. Elizabeth Perry works for AECOM, who provides energy consulting services to government and private industry, including Chesapeake. Rikka Bothun also worked for AECOM during most of the time this paper was under preparation but left AECOM in December 2014 and now works for a private consulting company that does not do consulting work for Chesapeake.

  • None of the following authors (Don Siegel, Bert Smith, Elizabeth Perry, or Rikka Bothun) have competing corporate financial interests exceeding guidelines presented by AAPG Environmental Geosciences Journal. Mark Hollingsworth is a current employee of Chesapeake and owns stock in the company in an amount in excess of $5000.

  • Donald Siegel is the lead author and contributor to the manuscript’s preparation, technical interpretations, and review of these data and the manuscript. Bert Smith contributed to the manuscript preparation, technical interpretations, and review of these data and the manuscript. Elizabeth Perry and Rikka Bothun contributed to the manuscript preparation, technical interpretations, and review. Mark Hollingsworth maintains the Chesapeake baseline data set and contributed to the manuscript preparation and review of these data and the manuscript.

  • Due to confidentiality agreements with landowners whose wells were sampled, latitude and longitude cannot be shown on maps.

We report the results of analysis and interpretation of 19,278 predrilling groundwater samples from water wells in the Appalachian Basin for dissolved methane collected from 2009 to 2012 (11,309 samples from northeastern Pennsylvania and 7969 samples from a western area that included north–central West Virginia, eastern Ohio, and southwestern Pennsylvania). We evaluate how concentrations of dissolved methane relate to geology, topographic position, well depth, groundwater circulation pathways, and hydrochemical facies. This data set, the most comprehensive to date for this part of the Appalachian Basin, shows 22.9% of predrilling samples naturally exceed the analytical detection limit for dissolved methane of 0.026 mg/L in northeast Pennsylvania, and 24.8% exceed the analytical detection limit of 0.026 mg/L in the western area. The concentration of dissolved methane broadly relates to the water-bearing geological unit penetrated by the water well, the hydrochemical facies (e.g., Na-Cl, Na-HCO3, or Ca-HCO3 groundwater type), whether the water well is found in a valley or an upland location, whether the water well intersects restricted or confined saline zones, and where the well is completed within the groundwater circulation pathways. Dissolved methane in shallow groundwater commonly occurs in sodium-dominated groundwater types (Na-Cl or Na-HCO3) and originates from the surrounding and underlying rocks or from deeper connate brines commonly found in valley settings at shallow depths in northeast Pennsylvania. A clear understanding of natural background conditions and natural variability must be considered when investigating water quality and stray gas complaints.

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