Equilibrium-Controlled Desorption in a CDF

ERDC TN-DOER-C16

July 2000

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 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 technical note refers to the composite effect of the elementary interphase

transfer processes (release of contaminants from the solid phase) shown in Figure 3.

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 4. 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 indistin-

guishable 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 for desorption; that is, predictions based on the equilibrium assumption will overesti-

mate leachate contaminant concentrations for dredged material. Additionally, the solubility of

contaminant in the solids may also further limit the contaminant concentration in the leachate. The

equilibrium assumption is conservative because interphase transfer is from the dredged material

solids to the pore water. At equilibrium 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. Equilibrium is not a conservative approximation for the adsorption of

contaminants in the clean soils below the contaminated dredged material. As such, adsorption is

often neglected during transport of leachate in the predominantly coarse-grained materials of an

aquifer.

Partitioning Factors

Oxidation status of sediment. Neither hydraulic nor mechanical dredging adds sufficient oxygen

to overcome the sediment oxygen demand of polluted sediments. As a result, the dredged material

in a CDF is anaerobic except for a surface crust that may develop if the CDF dewaters by evaporation

and seepage. Such an oxidized crust may eventually be several feet thick, but seldom represents a

significant portion of the vertical profile for the typically fine-grained material in CDFs. An aerobic