Model Results and Discussion

pcb transport and fate

Transport and fate model calculations for five PCB homologues (di- through hexa-CB) were performed for a simulation period beginning in January 1987. The model was tested by comparing computed results to observed 1992 PCB homologue concentrations in surface sediments. For this evaluation, partitioning of PCBs to suspended solids was initially specified as a direct function of octanol-water partition coefficients (K<j = foc Kow). Although these results provided a reasonable descriptionfor tetra-, penta-,andhexa-CB, the model underestimated observed concentrations of di- and tri-CB in surface sediments. A more generalized partitioning relationship (previously discussed) was then used to describe PCB partitioning to suspended solids. The coefficients aphyto and kg/ku were adjusted to values of 6.3 (dimension-less) and 6 x 10-7 (kg C L-1), respectively, to obtain the final model calibration.

Comparisons of model results to observed 1992 surface sediment concentration are shown in Figure 25.4 for the tri- and penta-CB. In this figure, homologue concentrations in surface sediments are shown as a function of distance down the Hudson River from Troy (Km 246; RM 153) past Kingston (Km 143; RM 89) to the Battery inNewYork City (Km 0; RM 0) and out into NewYork Harbor and the NewYork Bight. Data points shown by the open symbols in Figure 25.4 were not considered in these

Figure 25.4. Model-field data comparisons fortri- andpenta-CB concentrations in surface sediments (presented as organic carbon normalized concentrations) for 1992. (Data are from 1992 EPA Region 2 high resolution sediment cores (TAMS/Gradient 1995).)

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