Flow Sheet Logic and Decision Criteria for Defining an End Point to Dredging When Cleanup Goals Are Not Obtained

摘要

An environmental dredging project was completed on the St. Lawrence River near Massena,New York between April and December 2001.The project entailed the dredging of PCB-contaminated sediments from an area of 12 hectares,in water depths up to 8.2 m,adjacent to the main shipping channel for the St. Lawrence Seaway.The area was divided into a total of 268 dredge cells,each with a target depth of excavation.Sediment verification samples were collected after each dredge pass.PCB results from these samples determined whether the cells had been remediated or required additional dredging to meet the cleanup goal of 1 ppm PCBs. A key issue during the design involved the development of decision criteria for determining how much additional dredging to do when the cleanup goals were not attained after the second dredge pass.The primary objective was to retain control over the dredging process by defining the requirements for each cell prior to the start of dredging,establishing the exact conditions when redredging was required as well as those situations where any further redredging was at the discretion of the project team (e.g.,to avoid capping).This approach sought to avoid potential delays waiting on field decisions by oversight personnel as to whether additional dredging was needed. A dredging process flow sheet logic was developed that considered the magnitude of contamination in the verification sample and both analytical and matrix variability inherent in sediment PCB analyses.The logic involved a comparison of the verification sample result collected after the initial dredge pass to that collected after the cell was redredged.It assigned quantitative measures of progress based on the relative percent difference between samples collected before and after redredging,which in turn were tied to the PCB concentration ranges in the cells prior to the redredging.These measures worked both ways—if the sample showed that concentrations were decreasing by a specified percentage,another dredge pass would be conducted.Conversely,if the sample showed that concentrations were increasing by the same percentage(i.e., contamination was getting worse),then another dredge pass would also be conducted. The end points were reached either when the cell was remediated to less than 1 ppm PCBs or there was neither any measurable progress nor worsening of conditions after successive dredge passes.In the latter situation,the cell was classified as being “marked for further evaluation,”meaning it could be left as is,marked for capping,or undergo additional dredging—at the discretion of the project team–to satisfy the overall remediation requirements for the site.