Estimation of cleanup time in layered soils by vapor extraction

Abstract

Soil vapor extraction (SVE) is a standard remediation technique for cleaning up soils contaminated by volatile organic compounds (VOCs). A key parameter for planning SVE operations is the time required to reach the desired cleanup standard. In this paper, an approximate analytical solution is developed, which allows the fast estimation of cleanup times in layered unsaturated zones. The contaminants are assumed to be dissolved in the pore water, sorbed on the soil matrix and mixed with the soil air. Liquid organic phase is absent. For partitioning between gas and water and water and solid, local-equilibrium is assumed. The analytical solution is based on the well mixed reservoir model and on the plug flow model. It is shown that, for a number of scenario cases, the results of the analytical solution are, for practical purposes, in reasonable agreement with a numerical solution of the partial differential equations for the local-equilibrium advection–dispersion model of mass transport in porous media by Fickian diffusion and Darcian air flow. The results are displayed in terms of Peclet number of molecular diffusion (PNMD). In the analytical solution three different approximations are used. The PNMD range is divided into three intervals, representing different transport regimes. At low PNMD, molecular diffusion dominates transport in both layers. In this interval cleanup time is estimated by the average of the plug flow time for one pore volume through the layer of higher permeability, and the cleanup time estimated by the mixed reservoir model. At intermediate PNMD values, advective transport dominates in the more permeable layer and molecular diffusion in the less permeable. Consequently, cleanup time is limited by diffusive mass transfer from the less to the more permeable layer. In this interval, the estimation of cleanup time is entirely based on the mixed reservoir model. At high PNMD values, transport is governed in both layers by advection. Here, cleanup time is estimated by applying the plug flow model to the layer of lower permeability. The range of soil permeability values, for which such high PNMD values can be achieved, as well as the range of the actual air flow rates, are identified. The limitations of the validity of local-equilibrium assumption at high PNMD values are discussed using the relationship between desorption rates and gas advection fluxes. Further, the sensitivity of cleanup time estimates with respect to the independent dimensionless parameters characterizing the SVE configuration considered, is studied.