Woods Hole Coastal and Marine Science Center

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Simulation of chlorination breakdown at the existing and future outfall locations

  One of the design considerations for the future outfall site was that it should be more than one tidal excursion offshore so that in the event of a chlorination failure, untreated effluent would not be swept into Boston area beaches on the incoming tide. Signell et al. (1992) showed that even at spring tide, the future outfall location is well offshore of the region from which Boston Harbor draws water on flood tide. To quantify the expected benefit of the offshore locations, however, chlorination failures were simulated in ECOM-si and the bacteria concentrations were monitored at beaches throughout Massachusetts Bay. Chlorination failure was modeled as a breakdown that releases concentrations of 5,000,000 cells/100 ml for a period of 1 day. A e-folding decay timescale of 12 hours was used to simulate die-off of the bacteria.

Since currents are variable, chlorination failures were simulated every 10 days over a one year period from October 1989 to October 1990, and the observed concentration levels were monitored at 15 different beaches and harbors along the coast (Table 5.1).

 Table of chlorination failures

Two runs were conducted, one for the existing outfalls at Nut and Deer Islands, and one for the future outfall near the Boston Buoy. For each location, the number of events where the concentration (counts/100 ml) exceeded various values were calculated, and are shown in Tables 5.2 and 5.3. Events were only counted if they occurred at least 5 days after the previous event.

 TAble of bacteria concentrations

 table of future bacvterial concentrations with the proposed plant

The future outfall location obviously has a large beneficial effect, as the the total number of events with concentrations greater than 5 counts/100 ml dropping from 136 to 6 and the number of events greater than 10 counts/100 ml dropping from 130 to 0. The existing outfall caused 12 events greater than 1000 counts/100 ml, whereas the highest concentration for the future outfall was less than 10 counts/100 ml.

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