Dept. of Environmental Science and Engineering
Oregon Graduate Institute of Science & Technology
Mass transfer between ground water and the unsaturated zone plays an important role in many subsurface processes. In this study, the mechanism(s) responsible for the movement of a dissolved, volatile organic compound (VOC) from ground water to the unsaturated zone were identified and quantitated. The study consisted of large-scale mass-transfer experiments, small-scale diffusion experiments, and numerical modeling. The mass-transfer experiments were conducted in a large physical model of the subsurface (1.0-m long, 1.0-m deep, and 0.75-m wide). Ground water containing dissolved trichloroethylene (TCE) flowed through the model and transport from ground water to the unsaturated zone was monitored as a function of the soil-moisture profile and water-table position. The diffusion experiments were conducted on discrete sections of gravity-drained sand columns in which the soil-moisture content ranged from field capacity to saturation. These experiments provided values for the effective diffusion coefficient of TCE through a zone representative of the interface between the saturated and unsaturated zones and were used to evaluate a mathematical expression for the effective diffusion coefficient [Millington, 1959]. Two numerical models were developed to simulate mass exchange between the saturated and unsaturated zones. The first was a two-dimensional particle-tracking model that simulates longitudinal and vertical transport due to advection and molecular diffusion. A one-dimensional finite-difference model that simulates vertical transport due to molecular diffusion and mechanical dispersion was also developed. The models were evaluated with experimental data and were used to simulate a variety of subsurface scenarios. Data from the physical experiments and numerical simulations showed that molecular diffusion was the primary vertical transport process responsible for mass exchange between the saturated and unsaturated zones and that vertical mechanical dispersion was negligible. As a result, even slight changes in soil properties that result in changes in soil-moisture content were shown to alter concentration profiles by three orders of magnitude. Finally, comparisons of results from the one- and two- dimensional numerical models showed that a one-dimensional approximation of vertical transport in the subsurface can be useful when conditions are appropriate.
McCarthy, Kathleen Ann, "The transport of volatile compounds across the capillary fringe" (1992). Scholar Archive. 287.