OPTIMUM HYDRAULIC CONDUCTIVITY TO LIMIT CONTAMINANT FLUX THROUGH CUTOFF WALLS

Cutoff walls are becoming increasingly attractive options for the cont rol of solute migration from long-term sources of contamination. The m ain advantage of low permeability enclosures is that they restrict adv ective transport of solutes away from the source. However, with high c oncentration source zones surrounded by cutoff walls, there exists the potential for notable mass fluxes outward due to diffusive transport. This paper shows, through the use of the steady-state flux equations, that there is an optimal range of hydraulic conductivities for barrie r materials which permit the outward diffusive flux to be counter bala nced by an inward advective and dispersive flux. This concept of desig ning optimum contaminant containment using an inward advective flux to counter the outward diffusive flux is valid for sealable joint sheet pile walls, bentonite-slurry walls and clay liners, but not synthetic membrane materials with extremely low hydraulic conductivities. The ef fective diffusion coefficient for the common chlorinated organic solve nts such as TCE in water-saturated clayey materials is approximately 1 x 10(-6) cm(2)/sec, resulting in an optimum hydraulic conductivity ra nging from 1 x 10(-6) to 1 x 10(-8) cm/sec. This,range in hydraulic co nductivity is within the range of common barrier materials but not the lowest achievable. The steady-state concentration profile in a slurry cutoff wall can result in a substantial amount of contaminant mass st ored within the wall which will need to be considered over the long te rm or dealt with during site remediation. Large inward advective fluxe s reduce the total chemical mass stored within the low permeability ba rrier material.