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| U.S. Department of the Interior | U.S. Geological Survey |
| USGS Water-Resources Investigations Report 00-4245 | December 2000 |
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Suspended-solids transport during dredging PCB concentration changes during dredging PCB loading in the Fox River due to the dredging operation PCB transport back into the river from the onshore-processing operation Postdredging PCB concentration and loads Adjusting water-column PCB concentrations to allow comparison with onshore-sample PCB data |
A Mass-Balance Approach for Assessing PCB Movement During Remediation of a PCB-Contaminated Deposit on the Fox River, Wisconsin |
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PCB concentration changes during dredging
After dredging started on September 1, a consistent PCB concentration increase was evident at the downstream site (fig. 8A). The mean upstream concentration of PCB was 50.7 ng/L and the downstream PCB concentration was 92.0 ng/L. The paired upstream-downstream samples had a mean relative percent difference of 59 percent, substantially larger than the 5-percent difference between sample duplicates. Initially, it seems contradictory that PCB concentration increased while suspended solids loading remained the same or decreased (because of settling) during the dredging operation. However, material exposed to or resuspended into the water column during dredging increased the dissolved PCB concentration (fig. 8B), as well as the PCB concentration on a given particle (fig. 8C). Therefore, even though the overall mass of particles transported downstream did not increase, the PCB in solution and transported on the particles did increase. The TSS and PCB comparison (downstream minus upstream) illustrates that TSS is not a reliable indicator of PCB transport during a dredging operation. For example, from September 1 to October 6, a period of negative TSS loading (less at the downstream than at the upstream site), the PCB loading was positive. Thus, if one is to monitor PCB transport during a remediation operation, sole reliance on turbidity or TSS measurements is inadequate. One must also directly measure the concentration of the contaminant of interest because exposed layers of contaminated sediment and exposed concentrated pore waters can contribute to particle- and dissolved-phase PCB concentrations in downstream waters. Concentration data, however, do not form a complete picture of the effects of dredging; the mass of transported PCBs also must be taken into account. U.S. Department of the Interior, U.S.
Geological Survey |
