No shale gas production issue may be more fraught with partisan rhetoric than the possibility of methane in drinking water. Flaming faucets make for great imagery no matter the actual frequency of occurrence.
Well water contamination is personal and frightening. Carbon dioxide in the air barely registers on the average personal anxiety scale, but the quality of well water makes for avid reading and activism. In the case of shale gas, the industry response has been sweeping in denial. Both sides are playing fast and loose with the English language.
Shale gas operations can contaminate aquifers in potentially two ways. One would be through the leakage of chemicals used in fracturing (i.e., a liquid contaminant). The second would be the infiltration of aquifers by produced methane, a gaseous contaminant. If present, a portion may be released as a gas, as spectacularly depicted in the well-publicized documentary Gasland. Natural occurrences such as the Eternal Flame Waterfall in the Shale Creek Preserve in New York demonstrate methane intrusion into a freshwater source. The name follows from the fact that the gas remains lit with a visible flame under the rock overhang of the waterfall.
Methane is odorless and colorless, so it is hard to identify. It leaves no taste in the water, but can be hazardous if it collects in an enclosed space. When used in commerce, the industry deliberately adds mercaptans, a smelly substance (travelers on the New Jersey Turnpike near refineries know the smell well) for added safety. Gas leaks in a commercially supplied setting, such as a kitchen or furnace, are therefore detectable by smell. Leaks from a well would not be detectable by smell.
Natural contamination is either from relatively shallow biogenic methane or from thermogenic gas from deep deposits escaping upward along faults and fissures. The latter are generally caused by tectonic activity. The two types of gas leave fairly different fingerprints and often can be distinguished on that basis. Good oil and gas exploitation practitioners will avoid producing in areas with significant vertical leak paths because they impair normal sealing mechanisms.
Despite the seemingly sound scientific fingerprint techniques available, identifying the source of gas is subject to interpretation. This is particularly the case when, within the class of thermogenic origin, efforts are made to identify the age of the rock from which the leakage took place. In principle, both the isotope signature and the natural gas liquids (NGL) content have relationships with rock age. These relationships are not always well behaved enough to be unequivocal.