Stray gas source determination using forensic geochemical data

There is a standard method for rigorously using geochemical data to identify the source of stray gas. “Stray gas” is defined as gaseous material of undetermined origin found in an area where the gas has impacted the shallow-subsurface, potable water supplies, or has the potential to present a threat to public health and safety. Stray gas is composed of hydrocarbons or other gases, such as CO2. Previous studies have used geochemical data, including molecular and isotopic gas compositions, to empirically constrain the source of stray gas present in samples collected from specific locations (sometimes called “receptors”). On a more granular level than presented previously, we have discussed the interpretive workflow that should be followed to interpret geochemical data in the context of a stray gas investigation. A key concept is that geochemical data should be used impartially to evaluate all potential sources of the stray gas and, where possible, identify the most likely source. Focusing on only a subset of potential gas sources can lead to erroneous conclusions regarding gas sources. Furthermore, to accurately identify the gas source, geochemical data must be interpreted in the context of the site-specific geology, including evidence of previous gas migration events in the area. We illustrate these points using examples from Parker County, Texas, and from Lycoming County, Pennsylvania.