Transient Soil Vapor Extraction from a Pressure-Controlled Well.

New semianalytical solutions for transient soil vapor extraction are developed for P (pressure) or P² linearizations of the governing air flow equation. Our method treats flow rate and wellhead pressure as two distinct variables instead of customarily coupling them together. An extraction well can be either pressure or flow rate controlled with uniform applied pressure along the screen and nonuniform radial flux. The two linearizations yield different solutions for subsurface pressures; the difference increases with applied wellhead pressure. This increasing difference serves as a constraint on the magnitude of the wellhead pressure used in a test. The P equation is preferred for simulating the vadose zone pressure because it allows for more flexible implementation of boundary conditions. The solutions cover the common field test condition of unrestricted flow of atmospheric air into the vadose zone and can be readily modified for a leaky confining layer or time-varying pressure at the ground surface. Analysis of field test data showed that the new model can closely simulate transient subsurface pressure distribution and revealed mutual dependence of porosity θ, and permeability k_r and k_z for the problem analyzed. The estimated k_z/ k_r was greater than 1, higher than would be expected for natural soil in the absence of secondary effects on permeability, such as vertical cracks. [ABSTRACT FROM AUTHOR]