" in our study, we did several things. First, we documented the level of methane in the drinking-water wells. Second, we analyzed the isotopes in methane. The geochemical isotope is like a fingerprint in which you identify the source of the methane. And basically you can have two major sources of methane: one, thermogenic, is the methane exploited from deep shale gas and found in [drinking-water] wells. There is another type of methane that is called biogenic, which is formed in much more shallow depths."