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Page Title: D.2.2 Equilibrium-controlled desorption in a CDF
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(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.
D4
Appendix D Leachate Testing Procedures

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