Soil gas methane at petroleum contaminated sites: Forensic determination of origin and source

Hydrocarbon vapors associated with spilled petroleum products arouse regulatory concern and can pose a significant health and safety risk. While petroleum products do not contain a significant amount of methane (CH4), high CH4 contents in soil gas near petroleum spills have been reported. While CH4 is nontoxic, its accumulation in shallow soil gas represents a potential explosion and asphyxiation hazard, especially in confined spaces. Identifying the source and origin of shallow CH4 accumulations is an important part of evaluating potential exposure pathways, selecting appropriate remedial measures, and determining environmental liability. This paper discusses the potential nature and anthropogenic sources for shallow CH4 and how integration of geological, geochemical, and land use data can be used to determine its origin and identify its source. Two case studies are presented, one where CH4 associated with a gasoline spill is shown to be derived from a natural source rather than the gasoline, and a second where CH4 associated with spilled crude oil is shown to be produced in the vadose zone by biodegradation of the oil. (C) 2000 AEHS.