Woods Hole Coastal and Marine Science Center

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Motivation for Realistic Simulations

A major sewage treatment project is currently underway in Boston, principal components of which include upgrade to secondary treatment and relocation of sewage outfall pipes from Boston Harbor to a site 9 miles offshore (hereafter referred to as the future outfall site). Discharge at the future outfall site is currently estimated to begin in 1997. The impact of the new treatment project on Massachusetts Bay is of critical concern to the Massachusetts Water Resources Authority (MWRA), which is responsible for the project, and to the citizens of Massachusetts. Selection of the future outfall location was based in part on steady, two-dimensional (depth-averaged) modeling results that showed improved dilution of effluent at the offshore location. Lacking in this model, however, was the ability to simulate flow under stratified and time-variable conditions (although attempts were made to parameterize these effects). With the pressing need to better understand and simulate transport processes in Massachusetts Bay, the region was chosen by the USGS as a testbed for developing a prototype transport model.

To address water-column effects of the future outfall, the MWRA contracted HydroQual, Inc in the Spring of 1992 to develop a three-dimensional water quality model of the bay. To avoid duplication of effort, since three-dimensional circulation is required by both sediment transport and water quality models, the USGS agreed (as part of a cooperatively-funded set of studies with MWRA) to supply HydroQual with the necessary hydrodynamic calculations for their water quality model.

October 1, 1989 - July 1, 1991 was selected for testing and calibration of the circulation and water quality models due to a large data collection effort during this period by the Massachusetts Bays Program (Geyer, 1992). By the fall of 1992, the USGS had made significant progress on hydrodynamic modeling. As water quality results were not yet available, comparative dilution simulations were made to assess the difference between the existing and future outfall locations. Using a conservative tracer, the results suggested that the future outfall would result in a significantly smaller region affected by high concentrations of effluent. These results played a significant role in the EPA environmental assessment of the impact of the future outfall on endangered species on Stellwagen Bank (EPA, 1993). Although the impact of the outfall is now more realistically represented by the water quality model which includes biochemical processes (HydroQual and Battelle, 1995), the comparative dilution simulations still provide important insight into the transport and dispersion of material in Western Massachusetts Bay, and are thus presented in this report.

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Next: Organization Up: Introduction Previous: Physical Setting

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