6.5 Tier IV - Leachate Risk Assessment

6.5 Tier IV - Leachate Risk Assessment

6.5.1 Evaluation

Tier IV is intended to answer whatever specific, well-defined technical

questions may remain unanswered after thorough evaluation in earlier tiers. If

earlier tiers are used properly, Tier IV should rarely be necessary.

By the nature of the tiered evaluation approach, any technical questions that

remain unresolved after Tier III can best be answered by a detailed, case-specific

evaluation. By their very nature, detailed case-specific evaluations are not

amenable to the kind of generic guidance that can be presented in a national

manual. They require individual design to address unique technical questions

under site-specific conditions.

The best approach for Tier IV is usually a case-specific risk assessment.

Detailed guidance for conducting risk assessments for CDFs in Tier IV can be

found in Cura, Wickwire, and McArlde (in preparation). The information

generated in Tiers I through III should be used to the maximum extent technically

justified throughout the Tier IV risk assessment.

6.5.2 Tier IV Leachate Decision

After consideration of the Tier IV leachate evaluation results, all relevant

information is available and no further evaluation is possible. One of the

following conclusions is reached.

1. No management actions are required.

2. Management actions should be considered. A decision to implement

management actions for leachate, such as lining or operational

modification, may require more detailed information prior to design of

such actions.

6.6 Leachate Management Actions

If evaluation of the leachate pathway indicates leachate is of concern after

consideration of attenuation, appropriate actions to manage leachate may be

considered. These may include modification of the operation (e.g., encapsulating

the contaminated dredged material between cleaner layers of materials), liners and

leachate collection systems, and low permeability cover systems, among other

approaches. Additional information on management actions and references for

detailed guidance on such actions is found in Chapter 10 of this manual.

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Chapter 6

Leachate to Groundwater