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concentrations occurred in September/October 1991, corresponding to the flooding and scouring of highly contaminated PCB sediments from an old water intake structure just downstream from the GE facility at Hudson Falls (Rhea et al., 1997). Subsequent peaks in dissolved PCB concentrations occurred in the following years and are consistent with elevated PCB loads that are believed to be associated with seepage of PCB oil from the GE Hudson Falls plant site and the remobilization of deposited PCBs from Upper Hudson sediments, particularly during high flow events. For example, the large peak in dissolved PCB concentrations in January 1998 is associated with a large flow event that occurred in the Hudson River.

In contrast to Km 207 (RM 128.5), dissolved PCB concentrations 113 km (70 mi) downstream at Km 94 (RM 58.5) show a relatively smooth and slow decline of PCBs with time and no apparent variation corresponding to changing loads from the Upper Hudson. Although this could be interpreted as a large loss of PCBs (for example, by volatilization) during downstream transport, PCB responses at Km 94 (RM 58.5) are largely the result of the buffering capacity of sediments. In this case, the continuous interaction of the overlying water with sediments (through settling, resuspension, and pore water exchange) and the large capacity of the sediments to sorb PCBs work together to dampen the PCB responses downstream. This is demonstrated in the bottom panel of Figure 25.5 which shows PCB surface sediment concentrations at Km 207 (RM 128.5) increasing in the early 1990s in response to increased loads from the Upper Hudson. Later in the mid-to-late 1990s (as PCB loads from the Upper Hudson were reduced), PCBs stored in these sediments were slowly released to the overlying water and transported downstream. Although difficult to discern from Fig. 25.5, this downstream transport ofPCBs was typicallyhigher during spring high flow due the larger volume of water and suspended solids being transported downstream. The overall effect of trapping and subsequently releasing PCBs from sediments is to slow PCB transport down the river, and to smear down-streamresponses of PCBs to changes in Upper Hudson loads.

bioaccumulation

Dissolved and phytoplankton-bound PCB homologue concentrations from the transport and fate model calculations were used as exposure concentrations insubsequentbioaccumulationmodel calculations for zooplankton, small fish, white perch, and striped bass. Since little or no data were available for PCB accumulation in zooplankton

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