Proceedings - Teaching and Learning Centre - Simon Fraser ...

Fraser River Action Plan 3rd Research Workshop
pulp and paper mill effluent discharge rate to derive contaminant loadings in units of grams per day.
These loadings were used to conduct model simulations.
• Reported concentrations of 2,3,7,8-tetrachlorodibenzo-p-dioxin and 2,3,7,8-tetrachlorodibenzofuran in
water, sediments and fish of the Fraser and Thompson rivers for the period of 1989 to 1993 were
compiled. These data were used to verify model predictions and to derive model uncertainty.
• An initial model verification was conducted by comparing available observed and predicted
concentrations for the time period from 1989 to 1993. The results were compiled and are graphically
illustrated in Figures 1 and 2.
Figure 1 illustrates the agreement between observed and predicted concentrations of TCDF in the
organic carbon of sediments at various locations in the Fraser and Thompson rivers. The quality of
agreement between observed and predicted TCDF concentrations in river bed sediments can be
expressed by the mean ratio of the observed and predicted concentrations of TCDF in the sediments,
which is 1.33, suggesting the model slightly under-predicts the observed concentrations. The 95%
confidence limits of the model predictions range by a factor of 1.7 of the mean, suggesting that 95% of
the observed sediment concentrations are approximately within a factor of 1.7 of the predicted
concentrations. The uncertainty introduced in the model due to year-to-year variability in monthly river
flow rates, river compartment widths and depths, concentration in suspended sediments, organic carbon
content of suspended sediments and water temperature is represented in Figure 1 by the error bars which
are 95% confidence limits of the mean derived through Monte Carlo Simulation. The 95% confidence
limits range between a factor of 1.0 to 2.3 of the mean among the various compartments in the Fraser
River and between a factor of 2.2 to 4.5 in the Thompson River. This means that year-to-year variability
in the above mentioned model parameters is expected to cause variations in TCDF concentrations in the
sediments that range between a factor of 2.0 to 4.5 depending on the location in the rivers. With the
exception of the TCDF concentration observed in the Fraser River estuary (i.e., Compartment 18), all
observed TCDF concentrations in the sediments of the Fraser and Thompson rivers are within the 95%
confidence limits of the predicted concentrations. The higher TCDF concentration in the Fraser River
estuary may be explained by local sources of TCDF, which are ignored in this study, as the Fraser River
estuary is located in a highly industrialized part of Vancouver.
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