On December 8, 2011, the U.S. Environmental Protection Agency (EPA) issued a draft report on its investigation of groundwater contamination near the town of Pavillion, Wyoming. This CRS report provides a synopsis of the statutory authority for EPA's investigation under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), a summary of the primary findings in the EPA Draft Report, and a brief discussion of issues raised subsequent to the release of the draft report by proponents and opponents of the use of hydraulic fracturing for natural gas development. EPA had extended the public comment period on its Draft Report through September 30, 2013, but on June 20, 2013, EPA announced that it no longer plans to finalize the report. EPA indicated that it would defer to the state of Wyoming to assume the lead role in investigating drinking water quality in the area, and that the continuing role of EPA would focus on providing technical support and input to the state. The scope of the investigation by the state would seek to address water quality concerns by evaluating the water quality of certain domestic water wells, the integrity of certain oil and gas wells, and the historic use of waste disposal pits in the Pavillion area. In its announcement, EPA noted that the state intends to conclude its investigation and release a final report by September 30, 2014. Although the EPA Draft Report focused on one specific region where hydraulic fracturing was employed to enhance the production of natural gas, it has raised concerns about hydraulic fracturing practices in general, and whether EPA's findings at Pavillion are more broadly applicable to other regions of the country. The EPA Draft Report stated that the agency received complaints from domestic well owners in the vicinity of Pavillion in 2008 who observed objectionable taste and odor in their well water. In response to these citizen complaints, the EPA Region 8 Office initiated an investigation of possible contamination of the drinking water aquifer underlying the town, using the authorities of CERCLA delegated to EPA by Executive Order 12580. Section 104(a) of the statute authorizes EPA to respond to a release or substantial threat of a release of a hazardous substance, or a pollutant or contaminant that may present an imminent and substantial danger to the public health or welfare. This authority is available in such situations if EPA deems that response actions would be necessary to protect public health or welfare, or the environment, subject to the availability of appropriations under the Superfund program to carry out such actions. The federal regulations of the National Oil and Hazardous Substances Pollution Contingency Plan (often referred to as the National Contingency Plan or NCP for short) establish the procedures under which EPA may evaluate a site to determine where federal response actions may be warranted under Section 104(a) of CERCLA. State or local officials often are responsible for elevating sites to EPA for evaluation under CERCLA, in their capacity as first responders under the NCP. Section 105(d) also establishes a mechanism under which citizens may request that EPA perform a preliminary assessment to determine whether response actions may be warranted at a site, if EPA has not previously assessed the site. This provision provides the authority for any person who is, or may be, affected by a release or threatened release of a hazardous substance, pollutant, or contaminant to petition EPA to assess the potential hazards to public health and the environment that may arise from such a release or threatened release. EPA is required either to complete a preliminary assessment of a site within 12 months of the submission of a citizen petition, or to provide an explanation of why an assessment may not be appropriate. As specified in the NCP, a citizen petition under CERCLA should be addressed to the EPA Regional Administrator in the region in which the site is located and should identify the location of the release, how the petitioner is or may be affected by the release, and to the extent available, what types of substances were or may be released and the nature of the activities that have occurred where the release is located. Petitions also should indicate whether state and local officials have been contacted about the release. EPA would use such information to determine whether a site assessment may be warranted at the federal level under CERCLA. The EPA Draft Report for the Pavillion site indicated that the EPA Region 8 Office determined an assessment of the groundwater underlying the site was appropriate based on the observations about domestic well water quality cited in the citizen complaints that the agency received in 2008. See the Appendix at the end of this report for further discussion of EPA's response authorities under CERCLA. To help inform its investigation of the groundwater underlying the Pavillion site, EPA requested that the Agency for Toxic Substances and Disease Registry (ATSDR), an agency of the U.S. Department of Health and Human Services, examine the potential health hazards that may be associated with contaminants found specifically in private residential well water, but not other portions of the aquifer. Section 104(i)(4) of CERCLA authorizes EPA (or state or local officials) to request that the ATSDR provide consultations on potential health issues that may be associated with the release of a hazardous substance at a specific site. In response to EPA's request under this authority, the ATSDR issued a Health Consultation for Pavillion in August 2010. The ATSDR concluded that exposure to some of the contaminants found in the private residential well water were at levels that could lead to certain health effects, based on the potential for exposure relative to the health screening criteria that the ATSDR applied, and that some of the contaminants (such as methane) could present potential explosive hazards in residences under certain conditions. The ATSDR recommended that residents use alternate or treated water supplies, and recommended certain other measures to address potential explosive hazards, such as ventilating bathrooms while showering. It should be emphasized that the ATSDR's study focused specifically on potential hazards associated with the private residential well water, whereas the scope of EPA's site investigation was broader in terms of identifying and characterizing contaminants across the aquifer more widely and at greater depths. The ATSDR's finding of the presence of potential hazards was limited to the private residential well water itself, at shallower depths common to most domestic wells, and not the greater depths of natural gas production wells. The distinction between chemical constituents found at shallow depths in the aquifer and those found in deeper portions is discussed below. The Pavillion gas field lies within the Wind River Basin, a deep sedimentary basin extending across a large area of central Wyoming and bounded on the north and southwest by upfolded and faulted mountain ranges. (See Figure 1 .) The Wind River Formation, an accumulation of sandstone, conglomerate, shale, and mudstone, is the major source of drinking water for domestic and public-supply uses in the Wind River Basin. The Wind River Formation varies in thickness, and extends from the ground surface to as deep as 3,400 feet in the Pavillion gas field area. Natural gas is produced from wells drilled into the Wind River Formation, and from deeper wells drilled into the Fort Union Formation, which lies directly underneath the Wind River Formation. The most productive zone of natural gas extraction is from the bottom of the Wind River Formation, although hydraulic fracturing to enhance gas production has occurred at locations as shallow as 1,220 feet below ground surface, according to the EPA Draft Report. The EPA sampled residential wells, stock wells, shallow monitoring wells, and two municipal wells. The domestic wells range in depth from approximately 20 feet to nearly 800 feet, and the two municipal wells are 505 and 515 feet deep. The shallow monitoring wells were approximately 15 feet deep. According to the EPA Draft Report, the early phases of the investigation detected the presence of methane and diesel-range organic chemicals in some of the deeper domestic wells, which prompted EPA to install two deep monitoring wells in June 2010. EPA stated that the purpose of installing two deep monitoring wells—one at 785 feet and the second at 980 feet—was to differentiate potentially deep sources from potentially shallow sources of contamination. Shallow sources of contamination were thought to be related to leakage from surface pits used for storage and disposal of drilling wastes and produced and flowback water. Potential deeper sources were thought to be related to gas production, which would include drilling and hydraulic fracturing, as well as actual gas production. Detecting and distinguishing between potentially shallow and potentially deep sources of groundwater contamination lies at the heart of the primary findings in the EPA Draft Report. Whether the report clearly links groundwater contamination to drilling or hydraulic fracturing activities at depth has been the source of relatively heated commentary by proponents and opponents of the use of hydraulic fracturing for natural gas development. The primary findings in the report and examples of reactions and commentary by stakeholders are discussed below. According to the EPA Draft Report, the objective of the EPA investigation was to determine the presence of groundwater contamination above the Pavillion gas field, and to the extent possible identify the source of the contamination. The investigation identified a suite of contaminants in samples from shallow monitoring wells—wells that monitor the upper portions of the Wind River aquifer. The contaminants identified in the shallow portions of the aquifer included benzene, xylenes, gasoline-range organics (GROs), and diesel-range organics (DROs). According to the report, at least 33 surface pits were likely sources for the contaminants detected in shallow groundwater: "detection [of these contaminants] in ground water samples from shallow monitoring wells near pits indicates that pits are a source of shallow ground water contamination in the area of investigation." The pits were used for disposal of drilling cuttings, hydraulic fracturing flowback, and water produced from the formation. The Draft Report further noted that EPA is a member of a stakeholder group working with the gas field operator—Encana Oil & Gas Inc., a subsidiary of the Canadian Encana Corporation—to "determine the areal and vertical extent of shallow ground water contamination caused by these pits."