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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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Postdredging PCB concentrations and loadsDredging was discontinued on December 15, 1999. Low temperatures and freezing water in pipelines, process equipment, and the river surface required too many operating adjustments in all aspects of the hydraulic-dredging, water-treatment, and dewatering processes for the operation to continue. Three sets of water-column samples at the two transects were collected after the termination of dredging. Net daily PCB transport decreased (range was 12 g/d to 2 g/d) from that during active dredging. The dissolved-phase PCB concentration, however, still increased substantially from upstream to downstream (fig. 14) due to the deposit (a 1521 percent increase). On two of the sampled postdredging days, the particulate PCB concentration decreased (as did TSS concentration) at the downstream site. Apparently, the dredged area may be functioning, at least temporarily, as a depositional area. Daily mean flows during the postdredging sampling were moderate-less than 3,000 ft3/s (fig. 10)and also may have tended to promote deposition. Even though a new sediment layer is exposed-with greater PCB concentrations at the sediment surface than before the start of dredging (thus the observed increase in dissolved PCB)-the overall PCB concentration has decreased because of settling of particles in the dredged area. These data do not indicate how long this settling will continue, or at what rate of stream-flow the deposition will cease, or whether net scour of the exposed PCB sediment will occur. The dredging operation was planned to resume in summer or fall of 2000. The preceding observations were based on three sets of data points; more postremediation sampling would provide a stronger basis for conclusions. U.S. Department of the Interior, U.S.
Geological Survey |