6.4.5 Tier III Leachate Quality Decision

the flow of contaminants provides valuable alternatives for design and operation

of CDFs.

The HELPQ model predicts the concentration of contaminants in the CDF

and vadose (unsaturated) zone below the CDF. Concentrations are predicted in

the pore water and associated with the solid materials as a function of time.

Additionally, the model predicts the leachate flow rate and contaminant mass flux.

Saturated Zone Models. Modeling contaminant transport beyond the vadose

zone and to the receptor requires use of additional models such as the

MULTIMED model, the Multimedia Environmental Pollutant Assessment System

(MEPAS) (http://mepas.pnl.gov:2080/) or the Department of Defense

Groundwater Modeling System (GMS) (http://chl.wes.army.mil/software/gms/).

Similarly, for CDF sites where groundwater flows directly into the dredged

material (such as nearshore CDFs), more complex modeling operations using the

GMS may be needed to predict the movement and concentration of contaminants

at the CDF boundaries. Flow of anaerobic leachate through oxic dikes is another

complicated situation potentially requiring complex modeling to predict

contaminant concentrations.

6.4.5 Tier III Leachate Quality Decision

After consideration of the Tier III leachate quality information based on test

data and modeling, one of the following conclusions is reached (Figure 6-2):

1. Information is sufficient to reach a decision. In this case either:

a. Standards applicable to the intended use of the groundwater (Section

6.1.1) exist for all COC and are met for all COC after consideration

of attenuation. No further leachate evaluation is necessary.

b. Standards applicable to the intended use of the groundwater (Section

6.1.1) are exceeded for one or more COC after consideration of

attenuation, and management actions should be considered.

2. Information is not sufficient to reach a decision, which includes cases

where there are no standards applicable to the intended use of the

groundwater (Section 6.1.1). The case-specific risk from leachate should

be determined in Tier IV, or management actions as an alternative to

further evaluation should be considered. A decision to implement

management actions for leachate, such as design modification or leachate

collection, may require more detailed information prior to design of such

actions. If management actions are selected, no further leachate

evaluation is necessary.

6-15

Chapter 6

Leachate to Groundwater