D.2.2 Equilibrium-controlled desorption in a CDF

(sediment oxidation status, pH, and ionic strength). Varying these factors during

leaching can shift the equilibrium position of the system and change Kd.

D.2.2 Equilibrium-controlled desorption in a CDF

The assumption of equilibrium-controlled desorption in a CDF is based on

two arguments: (a) the intuitive argument that the interphase transfer rates

affecting leachate quality are fast relative to the volumetric flux of pore water in

CDFs and (b) the argument that equilibrium-controlled desorption provides

conservative predictions of leachate quality. This section discusses these

arguments. The term "desorption" as used here and in the remainder of the

leachate discussion refers to the composite effect of the elementary interphase

transfer processes shown in Figure D-2.

Contaminated dredged materials are usually fine-grained and have hydraulic

conductivities in the range of 10-8 to 10-5 cm/sec. When the hydraulic

conductivity is this low, pore water velocity is also low for the gradients

normally encountered in CDFs. Consolidation with excess pore pressure can

yield greater localized gradients at the bottom. For gradients near 1, pore water

velocities approximate hydraulic conductivities; that is, the water moves very

slowly at velocities of 10-8 to 10-5 cm/sec.

When the rate at which water moves is slow relative to the rate at which

equilibrium is approached, a local chemical equilibrium exists between the pore

water and the sediment solids. The local equilibrium concept is illustrated in

Figure D-3. The local equilibrium assumption implies that as a parcel of water

passes a parcel of dredged material solids, the water and solids come to chemical

equilibrium before the parcel of water moves to contact the next parcel of

dredged material solids. Leachate quality at the surface of a CDF will differ

from leachate quality at the bottom of a CDF, while leachate in both locations

will be in equilibrium with the dredged material solids. In reality, equilibrium-

controlled desorption requires an infinitely fast desorption rate. However, if the

critical interphase transfer rates are sufficiently fast, the equilibrium assumption

can yield results indistinguishable from full kinetic modeling (Jennings and

Kirkner 1984; Valocchi 1985; Bahr and Rubin 1987).

In addition to being a good approximation, the assumption of equilibrium-

controlled desorption is conservative; that is, predictions based on the equi-

librium assumption will overestimate leachate contaminant concentrations for

dredged material where contaminant desorption is occurring. However, the

equilibrium assumption is not conservative in the foundation soils where con-

taminant adsorption, retardation, and diffusion occurs, because less contaminants

would be removed from the leachate as it passes through the foundation soils

than would be removed if equilibrium were achieved. The equilibrium

assumption is conservative because interphase transfer is from the dredged

material solids to the pore water, and equilibrium means that all of the

desorption that can occur has occurred. Thus, for clean water entering the

dredged material, pore water contaminant concentrations cannot be higher than

the equilibrium value.

Appendix D Leachate Testing Procedures