Abstract: Testing a Field-Based Model for Hydrocarbon Vapor Migration

RIRIE, G., Unocal, Brea, CA; ROBERT SWEENEY; SETH DAUGHERTY, Orange County Health Care Agency, Santa Ana, CA; PETER PEURON, Environmental Geoscience Services, Long Beach, CA

To better evaluate risk associated with hydrocarbon vapor transport, vertical soil gas profiles were collected at several hydrocarbon-impacted sites. A consistent pattern was found in the profiles of soil vapor composition in shallow uncontaminated soil. Generally, oxygen concentrations decreased from atmospheric levels at the surface to near zero just above the contamination. Hydrocarbon gas concentrations are inversely related to the oxygen. For soils with oxygen concentrations above 5%, hydrocarbon gases are generally near background detection levels. For most profiles, hydrocarbon gas concentration is highest within the contaminated zone, decreasing rapidly upwards in the first 3 to 5 feet. Depth to contaminated soil is usually the most critical factor controlling whether hydrocarbon vapors will reach the surface.

Many different models are in use to screen sites for vapor risk. They have not, however, been calibrated with field data. One such model (Orange County) that is currently being used in California contains a provision for vapor attenuation by a concrete slab. This model is easy to apply, is well documented, and has been accepted by other agencies, however, the model predictions usually do not match the field data. This can be partly attributed to the model's attenuation factor.

In this paper, an approach is presented which modifies the 'vapor attenuation' input for the Orange County model to include contribution from biodegradation in addition to the concrete slab. Case studies are shown to illustrate how field data have been used in the Orange County model to evaluate potential benzene health risk due to the vapor pathway.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California