Description
Soil vapor extraction (SVE) is a useful remedial technique to remove volatile contaminants from unsaturated soil. In order to design an effective SVE system, the controlling air flow parameters must be described. Prior work has suggested that vapor flow at contaminated sites is generally confined between overlying concrete and asphalt and the underlying water table; i.e., there is no vertical leakage into the subsurface from the atmosphere. Repeated SVE field tests exhibited data responses that were not consistent with a confined flow model. It appeared that overlying asphalt and concrete are not completely air-tight. Therefore, this paper explores and develops a transient soil vapor extraction (SVE) testing method that considers the effects of vertical air leakage. The method is a corollary to the Hantush-Jacob (1955) leaky-confined analytic flow model. In all cases considered, the confined flow model was less representative than the leaky condition. Analytic methodologies are developed that allow use of commonly available ground water flow type-curves to evaluate SVE test data. The study indicates that vertical air leakage causes a general increase in the time required to achieve cleanup of contamination, and decreases the radial effectiveness of SVE. The confined flow model was shown to overpredict soil permeability, resulting in further bias toward unreasonably rapid and radially efficacious cleanup estimates. Results of the study also imply that Darcy's Law is valid for most vapor flow conditions. This suggests that other subsurface conditions besides leakage can be evaluated with the appropriate flow model prescribed by site geologic conditions. For instance, anisotropic flow could be considered at sites where fractures or depositional setting suggest contrasts in permeability tensors. It is also suggested that numerical models may be used to simulate vapor flow in heterogeneous settings.
