Abstract

Hydrocarbon vapors associated with spilled petroleum products can arouse regulatory concern and pose a significant health and safety risk. Identifying the source and origin of these vapors is an important part of evaluating potential exposure pathways, selecting appropriate remedial measures, and determining environmental liability. Although gasoline does not contain a significant amount of methane, high methane contents in soil gas near petroleum spills have been reported.

An investigation of the distribution and origin of soil gas methane was conducted at the location of a large gasoline spill in an urban area. During routine monitoring of the soil vapor recovery system, methane was observed to comprise a significant component of the influent vapor stream. The methane was originally interpreted to have formed by bacterial degradation of the spilled gasoline. However, the persistence of the methane over time raised new questions about its origin and lead to the current investigation.

A survey of the composition of subsurface vapors showed high methane and carbon dioxide concentrations both within and well beyond the limits of the gasoline spill. Site investigations also found that spilled gasoline was not the only subsurface carbon source present. Lake sediments and substantial quantities of wood and sawdust were also encountered. Stable carbon and hydrogen isotope ratios indicate that the methane is biogenic in origin and formed by the acetate fermentation pathway. However, these data did not allow identification of the source of the methane.

Carbon-14 (¹⁴C) analysis of onsite and offsite methane samples provided the strongest indication of the methane source. Petroleum and its derivative products contain no ¹⁴C and are said to have a ¹⁴C concentration of 0 percent modern carbon (pMC). Consequently, methane derived from degradation of gasoline also has a ¹⁴C concentration of 0 pMC. Methane in soil gas samples from beneath and adjacent to the site had ¹⁴C concentrations much higher than 0 pMC. ¹⁴C concentration averaged 86 pMC, demonstrating that the spilled gasoline is, at most, a minor contributor to the elevated concentration of methane in the subsurface. It is likely that the underlying lake sediments and/or wood debris are instead the predominant source of the methane. The findings of this investigation lead to a substantial reduction in site liability and costs and show that methane in association with spilled petroleum products is not necessarily derived from degradation of those products.