Analytical Solutions for Steady‐State Gas Flow in Layered Soils with Field Applications

Abstract

AbstractSoil vapor extraction (SVE) is widely used to remove volatile organic compounds from the vadose zone. Design of SVE systems rely largely upon vacuum responses and limited vapor concentration data measured during short‐term soil gas extraction tests performed in single extraction wells. Interpretation of such vacuum data is often simply a rule of thumb as most field sites have layering complexity negating applicability of existing analytical models. This paper provides the derivation of an analytical model for steady, axisymmetric gas flow in heterogeneous (layered) soils from a single well. A general, variable flow boundary condition along the well screen represents actual conditions more closely than a uniform flow or uniform well pressure condition. Each soil layer is assumed homogeneous with anisotropic gas permeability. The solution is derived using the generalized integral transform technique and includes expressions for vacuum, velocities, and streamlines. The model is applied to the interpretation of multiple well tests at a field site and uses linear superposition to extend the flow model to multi‐well extraction. The demonstration site included an array of vacuum monitoring data collected during nine individual well flow tests. A method of normalizing the vacuum data is illustrated that allowed the full data set to be employed in a single calibration effort. The test site also included a surface cap with an apparent vertical permeability two to three orders of magnitude smaller than the sands of the vadose zone. This large permeability contrast posed no difficulties in evaluating the solution.