Abstract

Introduction

Description

Sampling methods

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

Lessons learned

References

Acknowledgments and Information

A Mass-Balance Approach for Assessing PCB Movement During Remediation of a PCB-Contaminated Deposit on the Fox River, Wisconsin

Description of the study area

In 1995, sediment mapping by the WDNR in the 7-mile reach of the Fox River between the De Pere dam and the river mouth revealed a nearly continuous mass of soft sediment deposits. SMU 56/57 is approximately midway between the De Pere Dam and Green Bay. A papermill is adjacent to SMU 56/57; its discharge pipe is upstream from the dredged site and the upstream water-column sampling transect. A permeable silt curtain fabric was deployed around the dredged area that allowed passage of water but reduced transport of sediment and protected the papermill water intake (fig. 3).

This area is a commonly used offloading area for coal ships. The offloading slip is immediately downstream from SMU 56/57, and the turning basin used by these deep-draft vessels is adjacent to the deposit area (fig. 3). Fifteen coal ships offloaded cargo during the 15-week dredging operation.

SMU 56/57 has a surface area of approximately 9 acres with overlying water depths of 2­14 ft. Maximum sediment thickness was 16 ft with an overall average PCB concentration of 53 ppm (parts per million). Maximum PCB concentration was 710 ppm, the highest concentrations being in the top 2­5 ft. Total PCB mass in the deposit was estimated to be between 2,090 and 3,000 kg (4,600­6,600 lb) (Montgomery Watson, 2000; Blasland, Bouch, and Lee, Inc., 1999).

The lower 7-mile reach of the Fox River has an ever-changing flow and depth oscillation commonly found in estuaries. Flow reversals (from Green Bay toward De Pere Dam) are common in this reach (fig. 4). A continuous streamflow record for the river at SMU 56/57 was based on stream-velocity data collected at 15-minute intervals with a double-path acoustic velocity meter located approximately 2.7 mi downstream from the deposit and 0.8 mi upstream from the river mouth at Green Bay (USGS site 040851385, fig. 2). Because the dredging-site location was upstream from the acoustic-velocity-meter site and the inflow point of the East River, the daily mean streamflow was adjusted by a factor of 0.98 to account for the basin area difference.

The average water depth in this reach of the river is a function of Lake Michigan and Green Bay water levels, wind speed and direction, and flow over the De Pere Dam (which depends on precipitation and control at nine upstream dams). During 1999, the river depth at the USGS acoustic-velocity-meter site varied by more than 6 ft; during the dredging period (September 1­ December 15, 1999) depth varied by more than 4.2 ft. River depth is important to sediment and PCB transport in that, for a given flow, water velocity increases as river depth decreases. An increase in water velocity results, in turn, in an increase to the fourth power for sediment resuspension (Jepsen and others, 1997).

U.S. Department of the Interior, U.S. Geological Survey
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Last Update: Mon 22 Dec 2001