Effective Gas-Phase Diffusion Coefficients in Soils at Varying Water Content Measured Using a One-Flow Sorbent-Based Technique

Abstract

This paper focuses on techniques used to measure and predict effective gas-phase diffusion coefficients for volatile organic compounds in soils. Large differences found among the existing correlations for the effective diffusion coefficient, laboratory tests, and field experiments indicate the need for accurate measurements of diffusion parameters in soils. The theory, techniques, and experimental issues involved in laboratory measurements are summarized. A new one-flow sorbent-based laboratory experimental system is developed. The system maintains a constant concentration gradient across a soil column using a test gas flow at one side of the column and a high-capacity sorbent at the other. The diffusive flux and the effective diffusion coefficient are estimated using the difference between inlet and outlet concentrations. Mixing factors account for concentration gradients at column ends. A sequence of tests is used to quantify diffusion coefficients for trichloroethylene in laboratory-prepared soils at soil water contents from 0 to 80% of saturation (0−16% by weight). Results obtained are generally equivalent to measurements from a conventional two-flow experimental system. A curvilinear relationship is found between the air-filled porosity and the effective diffusion coefficient. Measured gas-phase diffusion coefficients at intermediate and high soil water contents are significantly larger than values estimated using literature correlations, although the correlations cover a large range. The new technique provides precision comparable to that of existing experimental techniques but offers greater convenience, flexibility, and control.