Residuals / Capping / Backfilling
Goal of the Standard
The residual standard is designed to detect and manage small amounts of contaminated sediments that may remain on the river bottom after dredging.
How the Standard Works
- Identification of “Certification Units” of approximately 5 acres in size (see Phase 2 Certification Units Map to Thompson Island Dam, at left).
- Collection of sediment cores from within a Certification Unit after dredging and analysis of sediment for PCB concentrations.
Possible Actions
- Lower concentrations might call for backfilling, where appropriate.
- Higher concentrations would call for actions such as redredging or constructing a cap.
Capping
Due to certain physical constraints in the river (e.g., bedrock, clay and shoreline stability considerations), concern for cultural resources, and other limitations on the ability to dredge near critical structures (e.g., bridges), some areas will require a cap to isolate the PCBs that remain after dredging occurs. Areas where it is not practical to continue dredging or areas which are physically restricted from dredging are not considered as Counted Areas.
Counted Areas are areas where dredging has occured but capping is still required due to (1) high residuals after dredging or (2) areas where "inventory" still remains. "Inventory" is considered to be significant PCB contamination below the top six inches of sediment after two dredging passes.
In 2011, dredging occurred between June 6-November 8. Phase 2 dredging will resume in spring 2012. See Table 1 below for 2011 capping limits and actual capping percentages.
| Table 1 | ||
| Certification units in which capping decision has been made | Area Dredged: 75 acres |
CU 9-16,19-25 |
| Counted Areas capped to date | Limit: 14.4 % | Actual: 3.05 % |
| Counted Areas with "Inventory" capped to date | Limit: 3.9 % | Actual: 0.0 % |
| Last Updated: November 22, 2011 | ||
The nodal capping index is used as the metric for determining compliance with the capping criteria of the residuals standard as stated above. This index provides more details than Table 1. Click here for a summary of the nodal capping index and more detail on capping limits for Phase 2 Year 1.
