Boundary Conditions for Modeling Transport in Vertical Barriers

Abstract

One-dimensional advective-dispersive-reactive models are often used to simulate the performance of vertical subsurface barriers. However, a review of the literature reveals a lack of consistency in the treatment of boundary conditions (BCs), particularly for field applications. Commonly used simplified BCs are shown to result in nonconservative predictions of contaminant flux when applied to the low flow scenarios expected in the field. Consequently, the constant concentration entrance BC and zero concentration exit BC are recommended as the starting point for the design of slurry walls. Generalized “mixing zone” conditions are also proposed and evaluated for modeling both field installations and laboratory columns. For field scenarios, the exit mixing zone provides flexibility in adjusting between the limiting cases of zero concentration and semiinfinite BCs. For laboratory columns, mixing zones may be used to represent the influence of porous materials placed at the column ends to promote lateral distribution of the permeating fluid.