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

Lessons learned

Commonly used techniques such as measurement of total suspended solids (TSS) and turbidity were inadequate to describe transport of PCBs during a dredging operation in the Fox River. Little or no measurable difference was found between the upstream and downstream TSS concentrations (or loads) over the length of the operation. However, neither turbidity nor TSS was sufficient to predict PCB transport because of increased PCB concentration on a particle and dissolved-phase PCB concentration. Approximately 35 percent of the PCB load at the downstream site was in the dissolved phase. Results of the study described here indicate that if chemical transport is to be quantified during a PCB remediation, then monitoring of TSS and turbidity alone is not adequate.

The study illustrates the importance of collecting water-column samples at numerous vertical and lateral locations to represent an entire transect concentration. The study found lateral concentration differences that would skew a sample if the entire cross-section was not adequately sampled. Additionally, in a dynamic situation (dredging operation), even in a large river, sampling over a prolonged interval is necessary to obtain a representative daily concentration of a constituent of interest.

Furthermore, the results of this study illustrate that a concentration-based approach to assessing remediation can be misleading. Clearly, the water-column PCB concentration increased as a result of dredging, but until this concentration is converted to a mass basis, comparisons such as the following cannot be made: that the PCB load into the water-column mass represented less than 2.5 percent of what was dredged from the deposit and approximately 9 percent of what was annually transported by the Fox River in 1994­95. The onshore-process effluent median PCB concentration of 509 ng/L may initially appear substantial, but when converted to a mass (0.147 kg), one can conclude that this is negligible compared to the mass of PCBs that was permanently removed from the deposit. Lastly, concentration-based approaches do not necessarily require a sample that represents an entire cross-section. Such sampling, if done only on the deposit side of the river (fig. 6), would have provided a biased data set.

Dredging ceased during arrival and departure of the coal vessels; ship movement apparently increased PCB transport in the area. On four sampling days, vessels moved in or out of the area shortly before or during sample collection (fig. 9). On the two days when ship movement coincided with sample collection, the PCB transport increase was more pronounced (400­600 g). The PCB loading increase due to vessel movement was probably not sustained throughout the entire day; thus, applying a daily mean flow to this concentration probably biased the resulting PCB load on the high side. The concentration increase during vessel movement (figs. 7 and 9), however, is substantially higher than the predredging days in August or the postdredging days (fig. 14). Vessel movement is a continuing PCB transport mechanism regardless of dredging operations.

In summary, hydraulic dredging, by means of a horizontal auger cutter head and permeable silt curtain, resulted in a net PCB load (Aroclor 1242) of 14.5 kg to the water column in the Fox River while 654 kg were permanently removed from the deposit. At the same time, less than 0.15 kg was discharged back to the water column from the onshore processing (fig. 1). This is compared to an annual load (congener summation) (1994­95) of 186 kg from the Fox River into Green Bay.

U.S. Department of the Interior, U.S. Geological Survey
http://wi.water.usgs.gov/
Maintainer: Webmaster, djsulliv@usgs.gov
Last Update: Mon 22 Dec 2001