A review of physical, chemical, and hydrogeologic characteristics of stray gas migration: Implications for investigation and remediation

Highlights: • Stray gas exhibits distinct physiochemical characteristics from other contaminants. • The acute risk of stray gas differs from the long-term health effects of many compounds. • Persistence of stray gas largely depends on hydrogeologic and redox conditions. • Response actions should be commensurate with the beneficial use of an aquifer. • Unique characteristics of stray gas inform investigation and management strategies. Releases of natural gas into groundwater from oil and gas exploration, production, or storage (i.e., ^stray gas⁾ can pose a risk to groundwater users and landowners in the form of a fire or explosive hazard. The acute nature of stray gas risk differs from the long-term health risks posed by the ingestion or inhalation of other petroleum hydrocarbons (e.g., benzene). Stray gas also exhibits different fate and transport behaviors in the environment from other hydrocarbon contaminants, including the potential for rapid and extensive transport of free-phase gas through preferential pathways, and the resulting variable and discontinuous spatial distribution of free and dissolved gas phases. While there is extensive guidance on response actions for releases of other hydrocarbons such as benzene, there are relatively few examples available in the technical literature that discuss appropriate response measures for the investigation and remediation of stray gas impacts. This paper describes key considerations in the physical, chemical, and hydrogeological characteristics of stray gas releases and implications for the improved investigation and mitigation of associated risks.