EFFECTIVE GAS-PHASE DIFFUSION-COEFFICIENTS IN SOILS AT VARYING WATER-CONTENT MEASURED USING A ONE-FLOW SORBENT BASED TECHNIQUE

This paper focuses on techniques used to measure and predict effective gas-phase diffusion coefficients for volatile organic compounds in so ils. Large differences found among the existing correlations for the e ffective diffusion coefficient, laboratory tests, and field experiment s 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 l aboratory experimental system is developed. The system maintains a con stant concentration gradient across a soil column using a test gas flo w at one side of the column and a high-capacity sorbent at the other. The diffusive flux and the effective diffusion coefficient are estimat ed using the difference between inlet and outlet concentrations. Mixin g factors account for concentration gradients at column ends. A sequen ce of tests is used to quantify diffusion coefficients for trichloroet hylene in laboratory-prepared soils at soil water contents from 0 to 8 0% of saturation (0-16% by weight). Results obtained are generally equ ivalent to measurements from a conventional two-flow experimental syst em. A curvilinear relationship is found between the air-filled porosit y and the effective diffusion coefficient. Measured gas-phase diffusio n coefficients at intermediate and high soil water contents are signif icantly larger than values estimated using literature correlations, al though the correlations cover a large range. The new technique provide s precision comparable to that of existing experimental techniques but offers greater convenience, flexibility, and control.