Two letters by Saba and Orzechowski (1) and Schon (2) address our research linking elevated methane and ethane concentrations to shale-gas drilling and hydraulic fracturing (3). We respond briefly here and point readers to a supplementary document for more details (4).
An assertion, and misconception, in both letters is that, because we found small amounts of mixed biogenic and thermogenic gas in 85% of groundwater samples, the thermogenic gas we observed near shale-gas wells occurred naturally. What we showed instead (figures 3 and 4 of ref. 3) was that drinking water was more likely to have high methane and ethane concentrations when homeowners lived within 1 km of a gas well. We also showed that the isotopic signatures for both δ13C and δ2H of methane found in high concentrations in private water wells closely matched the signatures of methane coming out of gas wells, and that the ratios of methane to ethane and propane were different [figure 4b (3)]. Furthermore, the methane present in high concentrations in water wells was more thermogenic in both its 13C and 2H signatures than background values more than 1 km from a gas well. There are indeed low concentrations of thermogenic methane found across the region. That methane does not, however, look like the methane found in drinking water near gas wells.