Excerpt: We appreciate the opportunity to respond to the letter by Goldstein et al.¹ The letter mentions a few issues that Goldstein et al consider to be major flaws of our study. Unfortunately, it appears that Goldstein et al. did not fully understand the databases we used or our analytical approach. We welcome the opportunity to further clarify these issues.

Our analysis focused on all wells drilled in Pennsylvania between 1990 and 2009. The Pennsylvania Department of Environmental Protection (PDEP) database does not specifically identify hydraulic fractured (HF) wells. The variables, “Marcellus Shale (yes/no),” well type (oil, gas, or combined oil and gas), and “Horizontal Well (yes/no),” were available in the PDEP database at the time we downloaded data. In our analyses, we considered horizontal gas wells to most likely be HF wells. Goldstein et al state that they do not find the same number of wells in the PDEP database as we did. They fail to recognize that the PDEP database is a “living” database, and that it is constantly being updated and corrected (Roger Dietz, IT specialist in the Pennsylvania DEP Office of Oil & Gas Management, personal communication, August 13, 2013). In addition, the database was redesigned in April 2012 as a result of Pennsylvania House Bill 1950.² Act 13 required the PDEP to keep track of “unconventional gas development.” Therefore, the categories of wells had to be changed. The variable “Marcellus Shale” was replaced with “Unconventional well.” The Marcellus Shale is an unconventional formation, but so are 9 other formations in Pennsylvania. Furthermore, an unconventional well is one that is drilled on an unconventional formation, not necessarily how it is drilled (vertical or horizontal). If one chooses to select unconventional wells for a report from the updated PDEP database, all horizontal and vertical wells drilled on these 10 formations will be selected. Goldstein et al chose all unconventional wells by year, which is why they got different numbers of wells than we did.