A comparison of direct measurement and model simulation of total flux of volatile organic compounds from the subsurface to the atmosphere under natural field conditions

[1] Accurate quantification of natural attenuation of volatile organic compounds (VOCs) in groundwater requires an accounting of all VOC mass-loss processes, including volatilization into soil gas with ultimate transport to land surface and the atmosphere. This paper compares two methods of estimating VOC flux to the atmosphere. The first method measures VOC flux at land surface using a vertical flux chamber (VFC). The VFC has been shown in a previous laboratory study to reliably measure organic vapor fluxes from soil to the atmosphere that are caused by either gas phase diffusion or a combination of advection plus diffusion [Tillman, 2003]. The second method simulates VOC flux using a transient, one-dimensional gas flow and transport model incorporating the effects of gas phase diffusion, equilibrium air-water partitioning of organic vapors, and unsaturated zone airflow caused by atmospheric pressure changes (i.e., barometric pumping). Flux chamber measurements were made concurrently with field data measurements required for the flow and transport model at a trichloroethene (TCE)-contaminated field site. The two methods of VOC flux estimation provided comparable results in nine sets of field comparisons with the flux chamber estimating a lower flux during most of the time periods. Sensitivity analyses of model input parameters produce upper and lower bounds on best estimate flux simulations. Average VFC measurements fall within these upper and lower bounds of combined parameter values for all nine comparison events. The ease of use, lack of a priori site knowledge and accuracy of the VFC compared with the data-intensive flow and transport model suggests that direct measurement of VOC flux from the subsurface to the atmosphere at contaminated sites could become routine practice, providing important information to decision makers about the progress of monitored natural attenuation.