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Why the Outfall?

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  1. Why the Outfall?

  2. Protecting the Harbor/Bays Ecosystem 1. Cleaner effluent through source reduction and secondary treatment 2.Dilution 3. Monitoring and Contingency Planning

  3. Outfall Siting Siting process began in 1986. Minimum requirements were set by EPA: • Initial dilution of 50 • More than one tidal cycle away from shoreline • Avoid sensitive and unique resources • Harbor site ruled out

  4. Outfall Siting Process: Siting Criteria Siting criteria developed by Facilities Planning Citizens Advisory Committee: • Attain compliance with state and federal water quality criteria • Protection of commercial on-the-water activities • Maintenance and enhancement of aesthetics • Avoidance of areas of important habitat

  5. Outfall Siting Process: Technical Analysis 7 potential sites evaluated independently by EPA and MWRA: Broad Sound to a location 10 miles offshore • Siting studies in 1987 and 1988 • Engineering and modeling • 2 years of oceanographic sampling including biological, physical and chemical studies

  6. Outfall Siting Process: Site Selection After extensive regulatory review and public comment, 9.5-mile site selected. FEIS determined that: • Water depth and current patterns promote effective dilution • Least likely to affect sensitive resources • Feasible to construct

  7. Outfall Design: Tunnel and Risers • 380 feet drop shaft from plant • 9.5 mile tunnel, bored through bedrock • Diffuser system (55 risers each with 8 ports) along final 1.25 miles provide maximum dilution

  8. Outfall design

  9. Outfall Design: Scale Model • Roberts scale model of outfall and diffuser system used to test different port configurations. • 8 ports gave most effective, rapid dispersion.

  10. Outfall Design: Diffuser Head Diffuser head ready for installation

  11. Dilution: Harbor Location • Average depth about 30 feet • Available dilution is about 14 to 1. • With onshore winds, effluent can reach the shoreline. • Effluent plumes reach the surface and are visible. • Effluent is flushed to Massachusetts Bay in a surface plume.

  12. Dilution: Model Harbor Outfalls USGS-Hydroqual model • 77 square miles <200-fold • Lower dilution contours extend along shoreline south of Boston • Parts of CCB shoreline 600-1000.

  13. Dilution: Bay Outfall • Average depth about 100 feet • 1.25 mile diffuser system provides effective dispersion. • Available dilution is about 150 to 1. • Effluent is more than one tidal cycle away from shoreline. • Circulation is greater and more variable than in Harbor, providing better mixing.

  14. Dilution: Model Bay Outfall USGS-Hydroqual model • 3 square miles <200 • Harbor and South Shore 400-600. • Most of MB and all of CCB >1000

  15. Bay Outfall: Mixing in Winter • Water column is well-mixed. • Freshwater effluent plume rises rapidly, and mixes to same density as seawater within a few tens of meters.

  16. Bay Outfall: Mixing and Stratification in Summer • Water column stratification traps effluent below pycnocline (about 15m below the surface). • Nutrients less available to phytoplankton at surface, where there is the most light.

  17. Outfall Benefits: Less Chlorine Use • Chlorine disinfection necessary to protect public health; minimizing chlorine use is environmentally beneficial. • Disinfection effectiveness, K, depends on concentration of chlorine, C, and duration of exposure, t : K = Ct. • Length of outfall increases t, allowing less chlorine use.

  18. CSO/System Master Plan andWet Weather Hydraulic Flow Capacity • Size of treatment plant, interceptors, pumping, CSO facilities integrated in MWRA’s CSO/System Master Plan. • Maximum plant pumping (and new outfall) capacity = 1270 MGD. • New outfall is necessary to achieve levels of CSO control in Plan.

  19. CSO/System Master Plan andWet Weather Hydraulic Flow Capacity • Two existing major Harbor outfalls maximum capacity = 400 MGD. (MWRA average = 370 MGD) • Flows >400 MGD activate two shoreline emergency outfalls in Winthrop Bay; total capacity ~1000 MGD. • At flows >1000 MGD headworks must restrict flow, increasing combined sewer overflows. • New outfall will provide hydraulic capacity of 1270 MGD.

  20. Protecting Resources in the Harbor/Bays Ecosystem • Outfall-diffuser system designed to maximize dilution and mixing. • Benefits Harbor and Bay ecosystem. • Monitoring and Contingency Planning to detect outfall-related impacts and link findings to action plan.

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