Formatted Title:

Post-Dredge Evaluation: Practical Application of a Decision Tree

Background/Objectives:

After years of data collection/analysis, remedial design, and planning followed by agency approval, dredging at a large Superfund sediment site in Michigan was finally completed. During this process, a decision tree was created to provide clear guidance for both the responding parties and the agencies when faced with field decisions regarding the number and depth of dredge passes required. The intention was to offer flexibility while considering location-specific conditions such as the total depth of dredging and thickness of cover/backfill.

Good questions at this point are, “How successful were we in implementing the decision tree?”, “Did we get the flexibility for practical infield decisions when needed?”, and “Was sound science used to make decisions around “gray” areas that did not fall precisely into a predetermined category?”. After completing dredging, WSP offers a clear-eyed review of the practical application of a dredging decision tree.

Approach/Activities:

Based on the results of a pilot study, a decision tree was developed that identified required sample analysis under varying conditions and allowed for 1) placement of a 6- or 12-inch cover/backfill depending upon the dredge surface concentrations or 2) performing a re-dredge pass. Re-dredge passes were limited to three, provided that undredged inventory was not present.

The initial dredge cut was extended by 1 foot for cores in the dredge management units (DMUs) only where depth to clean material was not clearly defined prior to dredging. No depth contingency was added to the DMUs where depth to clean material was clearly defined, other than the contractor-allowed over-dredge of 6 inches. To confirm that sufficient sediment was removed, confirmation cores were collected to depths well below the surface of the dredge cut. Analytical results from 0 to 6 and 6 to 12 inches were compared to the decision tree criteria for cover and/or re-dredge.

Results/Lessons Learned:

The decision tree provided a good framework and allowed quick decisions on some DMUs. Results confirmed that application of a safety factor to the dredge cut was not necessary. However, reality in the field is never as neat as a flowchart decision tree would make it appear. Several areas of potential improvement were identified during dredging: 1) allowing the use of professional judgment to perform targeted dredging rather than a full second pass, 2) clearer definition of and adherence to a process for dealing with high subgrade/coarse river bottom, 3) more flexibility in the use of increased cover thickness to manage residuals, and 4) emphasis on risk reduction rather than volume/depth removed.

Jeshua Hansen

Affiliation

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(Group 1, Poster Board #76) Post-Dredge Evaluation: Practical Application of a Decision Tree