Diffusion Limited Soil Vapor Extraction: Geologic and Bed Thickness Controls

G. D. Beckett and D. A. Benson

Soil vapor extraction (SVE) can remove volatile contaminants from the subsurface environment. ln a heterogeneous geologic setting, SVE cleanup will progress rapidly through advective mass transfer in permeable sediments and primarily through slow diffusion in lower permeability soil. The contrast in rates of cleanup between high and low permeability soils is further increased by the associated soil moisture retention contrasts (i.e., capillarity) in the same soils. Low permeability soil generally has a higher soil suction capacity and moisture content than high permeability soil. This results in further diminishment of cleanup rate in fine-grained sediments in a heterogeneous environment.

This paper investigates how contrasts in soil type and bed thickness affect the rate of SVE diffusive cleanup. The numerical model VENT3D is used to simulate three heterogeneous geologic settings with differing soil contrasts. Within each geologic setting, four simulations are performed with varying bed thicknesses in each, effectively changing the diffusive half-length of the fine- grained soils while maintaining the total bulk percentages of fine- to coarse-grained material. Under these conditions, the bulk flow parameters measured during SVE field testing would be constant for each of the four simulations within a single geologic domain while the cleanup times would not.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California