Inverse modeling to obtain air permeabilities from multiple well field tests in unsaturated weathered till

Abstract

Soil vapor extraction (SVE) to remove volatile contaminants from contaminated soil in the unsaturated zone has been successful in sand and gravel formations. In less permeable glacial till, the usefulness of SVE is unclear;A field experiment in the loam till of central Iowa (U.S.A.) was performed to determine air permeabilities in-situ at an uncontaminated site. Air permeability is a key design parameter for SVE systems and is one indicator of its feasibility. Vertical wells and tensiometers were installed in the vadose zone. The wells were installed at distances up to 100 cm from a central air extraction/injection well. The wells were screened over various vertical depths so that vertical as well as horizontal pressure variations could be observed. The water table varied between 150 and 300 cm below the ground surface (BGS). The pressure response due to various air extraction and injection rates was investigated during 1991 and 1992. In all tests, steady state was reached in seconds but due to barometric pressure changes, this would better be called a quasi steady state for a test lasting more than several minutes. Typically, with an air discharge rate of 4000 cm[superscript]3/sec (8.5 cubic feet per minute), the vacuum recorded at 100 cm was 0.5 cm of water;A two-dimensional anisotropic, heterogeneous compressible flow model coupled with an optimization routine solved the inverse problem by using the data to estimate air permeabilities. The air permeability was found to vary little for different flowrates and soil conditions so long as the water table was between 225 and 300 cm BGS. Under these conditions, the bottom no flow boundary in the modeling was placed at 200 cm BGS and the flow domain was split into four layers, each 50 cm thick. Anisotropy within a layer was not nearly as profound as the inhomogeneity between different layers. Thus, an isotropic, four layer description of the deposit was a good representation of the field site. The air permeability for each 50 cm thick layer beginning at the soil surface and working downward was 1 x 10[superscript]-5, 3 x 10[superscript]-6, 9 x 10[superscript]-7, and 4 x 10[superscript]-8 cm[superscript]2