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Abstract
VLEACH is a simple one-dimensional vadose zone leaching model capable of simulating volatile chemical transport. There is no explicit dispersion coefficient in VLEACH. Solutions computed with it show dispersion due to diffusion in the gaseous phase and its interaction with the liquid phase and also due to the discretization of the problem domain. By turning off gaseous diffusion and comparing VLEACH results to the standard convection dispersion equation, this Computer Note illustrates that the effective dispersivity due to the discretization is equal to half of the model cell size.
Introduction
VLEACH is a simple one-dimensional vadose zone leaching model that simulates volatile chemical transport (Rosenbloom et al. 1993; U.S. EPA 1996). As one of the individuals involved in the early development of VLEACH (along with Peter Lawson, Fritz Carlson, and primary author Jake Turin), I have found it rewarding to see its continued use and development. The recent release of a Windows version may make the program accessible to even more users. However, I was and continue to be concerned about the model's misapplication and misinterpretation of its output, particularly with respect to dispersion. Modelers, regulators, and other consumers of VLEACH results need to be aware of the significance of dispersion and its implicit nature in VLEACH and similar models. Dispersion is controlled by the model cell size.
VLEACH is fundamentally a mixing cell model in which mixing in every cell is instantaneous and complete at each time step (Bear 1972). VLEACH has no option for specification of dispersivity (at). Nevertheless, breakthrough curves generated with it-even when all explicit diffusive processes (diffusion in the gas phase) are turned off-clearly show dispersive behavior.
Impact of Dispersion
It should be well understood by modelers that the dispersivity, which is scale-dependent and notoriously difficult to estimate a...