Figure 1. Contaminant migration pathway: leachate seepage

ERDC TN-DOER-C16

July 2000

and groundwater modeling have commonly been required to predict leachate quality (Myers and

Brannon 1991). Laboratory testing for leachate quality and groundwater modeling can be both

expensive and time-consuming and may be unnecessary for all but the most contaminated dredged

materials. Conservative screening procedures could be employed to identify scenarios when testing

for some or all classes of contaminants would not be needed. This technical note presents the

framework for developing such screening procedures.

Theoretical Basis for Leachate Quality Evaluations. Contaminant migration via leachate

seepage is a porous medium contaminant transport problem (Figure 2). Leaching is defined as

interphase transfer of contaminants from dredged material solids to the pore water surrounding the

solids and the subsequent transport of these contaminants by pore water seepage. The interphase

transfer is the first step or source of contaminated leachate generation. Interphase mass transfer

during dredged material leaching is a complicated interaction of many elementary processes and

factors. A complete description of all these processes, factors, and interactions is not presently

possible. Instead, a lumped parameter, the distribution coefficient, is used to describe the distribu-

tion of contaminant between aqueous and solid phases.

The contaminants in the aqueous phase are convected with pore water in the dredged material as

leachate. As leachate is transported through porous media, redistribution of the contaminants

between the advected pore water (leachate) and the new solids encountered (the surrounding porous

media) occurs, and a new equilibrium between the leachate and the solids is reached. This