Introduction

Increases in the global demand for energy are driving advances in natural gas extraction techniques such as hydraulic fracturing and horizontal drilling (Kennedy, 2007). These two technologies make it economically feasible to recover unconventional oil and gas resources from coal beds, shale formations, and tight sand reservoirs. Although hydraulic fracturing has received recent attention, the technology has been in commercial use in the United States (U.S.) for exploration and extraction of crude oil since the 1940s (STRONGER, 2011). Hydraulic fracturing is a technology that relies on the high pressure injection of water mixed with a combination of chemicals and sand formulated to physically fracture subsurface reservoirs for the purpose of extracting oil and gas. Depending upon the type of geological formation and the depth associated with horizontal drilling, fracturing activities can take place anywhere from several hundred feet to several miles below the surface (ALL Consulting, 2009).

Public concerns have been expressed about drinking water contamination from migration of chemicals used during the hydraulic fracturing process, as well as from the escape of methane from fractured rock and well casings (Dammel et al., 2011; Groat and Grimshaw, 2012; Osborn et al., 2011; Rozell and Reaven, 2012; USEPA, 2011). However, strong scientific evidence to support these concerns is lacking. To our knowledge, only one study has been published in the scientific literature evaluating the potential for groundwater contamination with methane (Osborn et al., 2011). These authors reported methane contamination of aquifers overlying the Marcellus Shale formation and noted that the contamination accompanied gas-well drilling and hydraulic fracturing activities in the area. However, the authors concluded that more research was still needed to clearly understand the mechanism of contamination (Osborn et al., 2011).

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