Figure D-1. Model of dredged material leaching (from Hill, Myers, and Brannon 1998)

Figure D-1. Model of dredged material leaching (from Hill, Myers, and Brannon

1988)

potential just as electric current flows from a region of high electrical potential

to one of lower electrical potential. When chemical potentials are equal, the net

transfer of contaminant across the solid-water interface is zero, and the mass of

contaminant in each phase is constant, but not necessarily equal. The processes

shown in Figure D-2 control the rate at which equilibrium is reached and the

equilibrium distribution of contaminant between solid and aqueous phases.

Once equilibrium is reached, the ratio of contaminant mass in the solid phase to

the contaminant mass in the aqueous phases does not change.

In practice, a true equilibrium between dredged material solids and pore

water never exists because some of the processes shown in Figure D-2 have very

slow reaction rates. However, a pseudo steady state can be reached between

dredged material solids and water if the water is moving past the solids slowly

enough, as discussed in a following section. By assuming equilibrium between

solid and aqueous phases, the need for determining controlling processes and the

rate coefficients for these processes is eliminated. Without the equilibrium

assumption, laboratory testing and mathematical modeling would require

determination of controlling processes and investigation of the kinetics for these

processes. As is apparent from Figure D-2, predictive laboratory tests and

mathematical models based on chemical and mass transfer kinetics would be too

complicated for routine evaluation of dredged material leaching. Thus,

application of the equilibrium assumption is imperative for the development of

predictive techniques suitable for routine use.

Under equilibrium conditions, only the relative distribution of contaminant

between solid and aqueous phases is needed to predict leachate quality.

Kd = q / C

(D-1)

Appendix D Leachate Testing Procedures