Physical Processes Affecting Natural Depletion of Volatile Chemicals in Soil and Groundwater

Abstract

A Fickian model is described for dispersive vapor transport due to “pumping” induced by barometric pressure fluctuations and periodic water table fluctuations. The approach is appropriate for time scales that are large relative to the period of induced airflow variations. Comparisons of the magnitude of dispersive fluxes with those due solely to molecular diffusion indicated that dispersive vapor transport becomes increasingly important as soil porosity decreases and as the depth to groundwater and the contaminant source increases. For soils with low air-filled porosity, barometric pumping is likely to dominate transport even for shallow soils. Barometric pumping may predominate for soils with moderate to high air-filled porosity with deeper groundwater (>5–15 m). Water table pumping is predicted to predominate over diffusion only for high-frequency fluctuations, such as tidal conditions. A steady-state model for contaminant volatilization from groundwater is presented that considers diffusive and dispersive vapor transport, unsaturated zone aqueous phase advection, and dispersive mixing in groundwater, yielding an apparent first-order decay coefficient with respect to groundwater. Predicted volatilization coefficients for perchloroethene (PCE) range from <0.001 to >0.02 d−1 for various soil conditions and groundwater depths. Highest values are predicted for the most permeable soils. Volatilization rates are predicted to decrease with depth up to a point at which dispersive fluxes dominate over diffusion and then to increase to the extent that barometric pressure fluctuations propagate to greater depths. Vertical mixing within the saturated zone has a significant influence on volatilization from groundwater. Simple moving front and mixing cell models are presented to estimate depletion rates of soil contamination due to volatilization and leaching. Results indicate that natural depletion of residual soil NAPL may take many decades and is markedly influenced by soil conditions, hydraulic flux, and contaminant properties.