Reply to Davies: Hydraulic fracturing remains a possible mechanism for observed methane contamination of drinking water

Davies (1) agrees that methane contamination of drinking water has occurred in aquifers overlying the Marcellus formation but asserts that we prematurely ascribed its cause to hydraulic fracturing (2). We respond briefly, noting that we carefully avoided ascribing any mechanism and suggested some additional research (2) for the important need that Davies (1) identifies—to understand the mechanism of contamination better. Comments about sampling procedures and methane seeps are in refs. 3 and 4.

Our paper discussed three mechanisms for stray gas migration (2). One was physical displacement of gas-rich water up from the shale formation, which we dismissed as "unlikely" (2). The other two mechanisms were leaky gas well casings and the possibility that hydraulic fracturing might generate new or enlarge existing fractures above the target formation, increasing connectivity. Of these two mechanisms, we wrote that "methane migration through the 1- to 2-km-thick geological formations that overlie the Marcellus and Utica shales is less likely as a mechanism for methane contamination than leaky well casings" (2).