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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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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 |
A Mass-Balance Approach for Assessing PCB Movement During Remediation of a PCB-Contaminated Deposit on the Fox River, Wisconsin |
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Sampling methodsWater-column samples collected before, during, and after dredging operations were analyzed to support calculations of the mass transport of PCBs. The samples were collected from four discrete sites along an upstream transect and five sites on a downstream transect (fig. 3). Site spacing was closest in areas of focused flow (as determined by a portable Doppler flow meter). The southeast side of the channel is the deeper part and contains most of the flow; therefore, sample-collection sites are skewed toward that side. At each site, water was collected from two depths, at 20 percent and 80 percent of the total water depth. For a given transect (upstream or downstream), water from the transect sites was composited throughout the day to provide a representative PCB concentration for each transect. During the dredging operation, each transect (upstream and downstream) was sampled 23 times per sample day, resulting in 36 sample (daily composite) pairs. In addition to transect composites, discrete total suspended solids (TSS) samples, and water-quality field measurements (temperature, turbidity, pH, and specific conductance) were obtained from each site. At the original downstream site A, it appeared that dredging could produce a contamination plume that could remain close to shore and not be collected. Therefore, section A was divided to create sites A1 and A2, with collection volumes halve.
Upstream and downstream composite samples were analyzed for 101 individual PCB congeners (dissolved and particulate), TSS, dissolved organic carbon (DOC), and total organic carbon (TOC). The dredging contractor also collected turbidity data continuously at several sites during most of the dredging operation (fig. 3). Composite water-column samples (80-L volume) were filtered through 0.7-µm glass fiber filters to determine particulate congener PCB concentrations. Filtrate was pumped through an absorbent resin column (that is, XAD-2) to concentrate PCBs for the operationally defined "dissolved" phase. Complete procedures for 80-L PCB water-column samples are described in the dredging-project quality assurance plan (Blasland, Bouck, and Lee, Inc., 1999). Total PCB concentrations were computed by summing the dissolved and particulate fractions; concentrations reported as being less than the laboratory detection limit were given a zero value. Daily mean river discharge was used in conjunction with the water-column concentration data to compute daily TSS and PCB loads. On certain days, the entire sampling process was repeated to produce sample duplicates for analysis of PCB (6 duplicates) and TSS (3 duplicates). The purpose of the sample duplicates was to assess the ability to detect real change in the environment (that is, isolate laboratory and sampling artifacts from environmental change). The mean relative percent differences between sample duplicates were 5 percent for total PCB samples (combined dissolved and particulate phases); 10 percent for composited TSS samples; and 12 percent for discrete TSS samples. These duplicate results are similar to those from previous sampling efforts (Fox River Remediation Advisory Team, 2000). U.S. Department of the Interior, U.S.
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