There was another sewage spill in the Hudson, from a broken sewage pipe at Ossining, New York.  Fortunately, this time the volume was about one hundred times less than the North River Wastewater Treatment Plant spill back in July.  Nevertheless, Hudson River beaches in Westchester and Rockland Counties have been closed.  [Note that actual bacterial measurements from the Hudson Riverkeeper are in, as of Sunday, showing low risk for swimmers and fishing from pathogens, likely due to simple dilution, but possibly also due to high amounts of chlorine added at the source.  We are back to normal conditions, where rainfall events are the primary reason for sewage overflows].

Here is the pathogen forecast for Saturday, covering the area’s waterways, based on the Stevens Institute of Technology’s sECOM ocean model, used for the New York Harbor Observing and Prediction System (NYHOPS).

Saturday's maximum pathogen concentrations in surface waters, relative to a source of 1.0. A full movie of the 3-day simulation is linked from the next figure, below.

The simulation shows how dilution and mortality reduce the concentration of pathogens from an unknown source concentration (assumed to be 1.0) in the Hudson near Ossining.

The main conclusions I draw from the simulation are:

• Maximum (relative) concentrations in Westchester County from Croton Point southward to Yonkers are high, close to the source concentration
• Transport northward is minimal — maximum concentrations at shorelines along Haverstraw Bay (north of Croton Point) are ~1/100 the source concentration. They are even lower beyond Indian Point, below 1/1000 around Peekskill.
• Maximum concentrations off Yonkers and Rockland County (west side of the Hudson) are ~1/10 the source concentration – potentially dangerous*!
• Maximum concentrations off northern Manhattan (Hudson and Harlem Rivers) are ~1/100 the source concentration, possibly (but unlikely) of danger for swimming or fishing*
• Maximum concentrations off southern Manhattan are ~1/1000 the source concentration
• Maximum concentrations at Staten Island and Brooklyn beaches are <<1/10000 the source concentration, definitely negligible

*The various counties and cities Departments of Health are in charge of making decisions on public safety, and is taking observations and running detailed microbiological models in place that should be more useful for determining where beach closures are necessary.  So one should consult the Westchester County DOH website , Rockland County DOH website, and NYC Dept of Health website for questions of water safety.

Here is a full 3-day movie of the entire simulation:

Click on the figure to go to YouTube for a full movie of the 3-day simulation

The simulation and sewage spill starts at 12:00 AM, Thursday August 11th.  The sewage spill ends at 5:00 PM on Friday, and the simulation ends at 11:55 PM on Saturday August 13th.  The time the sewage spill began is currently unknown, but we assume it started at midnight on the 11th.  The model simulation simulates transport in all three spatial dimensions, though the forecast maps above are for near-surface waters.  The pathogens have a standard (published) die-off rate, with temperature and salinity dependence.  The effects of particle settling and sunlight (phytotoxicity) were not included in this model run for lack of detailed information, but would both act to further reduce the concentrations.