Woods Hole Coastal and Marine Science Center

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Comparison of model results with observations indicated that ECOM-si simulations are appropriate for studying the continuous release of effluent in western Massachusetts Bay, as they reproduce the most important features of the observed stratification and current regime in this region. The seasonal development and breakdown of the thermocline occur within a few weeks of the correct time, the vertical structure has the right top-to-bottom density difference and about the right vertical profile (though the modeled pycnocline is not quite as sharp as reality), and the currents responsible for transporting material have about the right mean flow and level of variability. The fact that the best comparisons are found on the western side of Massachusetts Bay reflects the strong effect of local wind driving in this region, an effect that can be directly modeled rather than parameterized by boundary conditions. Further offshore, near Stellwagen Bank for example, the currents and water properties are largely determined by Gulf of Maine conditions, and the model does not compare as well. Although significant improvements to salinity boundary conditions were obtained by including an adjunct model to model the western Gulf of Maine, boundary conditions remain one of the largest roadblocks to obtaining more realistic simulations. In the future, improvement of boundary condition information may be obtained from a time-varying coarser scale model of the entire Gulf of Maine or possibly from data collected from sensors at strategic locations and assimilated into model runs.

The effluent simulations show that with either the existing or future outfall, the region of Massachusetts Bay that experiences relatively high effluent concentration (1 part effluent: 200 parts sea water) is relatively small and confined to western Massachusetts Bay. The region of high concentration (1:50) that covers much of Boston Harbor with the existing outfall is completely eliminated with the future outfall scenario. This is consistent with the results of the two-dimensional modeling conducted as part of the outfall siting process (MWRA, 1988). The subsurface areal extent of the bottom trapped summertime plume is about the same size as the winter plume for the future outfall scenario. Although the initial vertical mixing of the effluent is confined to the lower half of column during the summer, this effect is offset by stronger currents and shears during the summer that act to increase dispersion. The total amount of effluent in the bay obtained from the future outfall is within 10% of that obtained from the existing outfall, and the time series track each other closely. Effluent levels build up over the winter, reaching their peak in March before being greatly reduced by the increased flushing of the system in April.

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