D.2.3 Oxidation status of sediment

Figure D-3. Illustration of local equilibrium assumption for leaching in a CDF

D.2.3 Oxidation status of sediment

Neither hydraulic nor mechanical dredging adds sufficient oxygen to over-

come the sediment oxygen demand of fine-grained sediments. As a result, the

dredged material in a CDF remains 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 leaching procedure may be necessary if the full lift thickness is

dewatered prior to disposal of the next lift. Sequential batch leaching of aerobic,

aged sediment can be used to simulate leaching of the surface crust in a CDF

(Brannon, Myers, and Tardy 1994).

D.2.4 Ionic Strength

Sequential batch leaching of freshwater sediments usually yields desorption

isotherms such as shown in Figure D-4 (Brannon, Myers, and Tardy 1994).

This is what is known as a classical desorption isotherm. Its key feature is a

single distribution coefficient that is constant throughout the sequential leaching

procedure. A commonly observed feature of desorption isotherms for metals in

freshwater sediments is that they do not go through the origin but rather intercept

the ordinate at some other point. The intercept indicates the amount of metal in

geochemical phases that are resistant to aqueous leaching.

Appendix D Leachate Testing Procedures