A rigorous solution is developed from first principles to guide the preliminary design of cutoff walls installed to contain the migration of contaminants from source zones. The full analytic solution is used to develop a criterion for determining the configuration and hydraulics of optimal wall designs. The solution is used to demonstrate the interaction between the properties of the wall, the Darcy flux, and the concentration of contaminants at the outside face of the well. For a particular wall design, the containment criterion can be used to estimate the long-term concentration that will develop at the outside face of the wall. Alternatively, for a given concentration on the outside face of the cutoff wall, the containment criterion can be used to estimate the Darcy flux required to balance the outward diffusion of contaminants. The results of numerical simulations are presented to evaluate the analytic approach. The numerical results confirm that for a wall with known transport properties, a specified Darcy flux is associated with a unique outside contaminant concentration.
BIOSCREEN is used extensively for screening-level evaluation of the transport of dissolved contaminants in ground water. The code has an effective graphical user interface that makes it ideal for use in both professional practice and as a teaching aid. BIOSCREEN implements the approximate transport solution of Domenico (1987). This note describes an enhanced version of the program, BIOSCREEN-AT, which supplements the Domenico solution with an exact analytical solution. The exact analytical solution has been integrated seamlessly within the BIOSCREEN interface and provides a simple and direct way to calculate an exact solution to the transport equation and, if desired, to assess the significance of the errors introduced by the Domenico (1987) solution for site-specific applications. The modified version of BIOSCREEN is designated BIOSCREEN-AT and can be downloaded free of charge from http://www.sspa.com/./software/BIOSCREEN.htm.
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