Problem: Subsurface contamination due to the release(s) of gasoline on a service station property was remediated using pump and treat and vapor extraction systems. After several quarterly monitoring events during which no hydrocarbon contamination above action levels was encountered, phase separated hydrocarbons (PSH) were discovered in a monitoring well located off-site (south of the service station property).
Approach: A soil vapor survey was conducted on the service station property as well as off-site to determine the areal (horizontal) extent of petroleum product (gasoline) contamination in subsurface soils and groundwater. The soil vapor data were also utilized to determine if there was a "new" release or if the PSH were the result of a release(s) not previously delineated during the initial assessment (prior to remediation operations).
Results: Soil vapor analytical results and contoured plume maps indicated the presence of subsurface petroleum product contamination off-site as well as on the service station property. Extensive off-site migration of gasoline contamination was indicated beneath the major road located west of the station. All constituent plume maps are similar in areal extent and indicate the source of the subsurface contamination was a release (or releases) of gasoline on the service station. The iso-butane/normal butane (major volatile components of gasoline) plume map indicates the "freshest" contamination is located under the road. The soil vapor data do not indicate a "new" release; the data indicate the initial assessment was not comprehensive enough and did not identify the full extent of the plume, the majority of which is located off-site. Subsequent drilling and monitor well installations will be required to remediate the plume located off-site. Soil vapor plume maps will be used for the placement of additional monitor/recovery wells.
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