The Boston Public Health Commission reports all the rain has overloaded local sewer mains, to the point that one MWRA Outflow pipe, upstream of the North Washington Street Bridge, started pouring sewage-laden water into the inner harbor at 1:01 a.m, "creating a potential public health risk."
The sewage flow from MWRA Outflow MWR203 stopped at 3:02 a.m., but the commission reports that in the unlikely event you feel like jumping into the inner harbor for a bracing swim for the next couple of days: Don't. Also, pull up those fishing lines and consider not trying to row from one side of the harbor to the other.
The public is advised to avoid contact with affected water bodies for at least 48 hours after a sewage discharge or overflow, during rainstorms, and for 48 hours after rainstorms end, due to increased health risks from bacteria or other pollutants associated with urban stormwater runoff and discharges of untreated or partially treated wastewater.
Although the MWRA and the Boston Water and Sewer Commission have spent hundreds of millions of dollars over the past few decades reducing the amount of rainwater that gets into local sewers, overloading them and leading to raw-sewage discharges into the harbor and the rivers that feed it, intense bursts of rain can still lead to issues, whether through cracked pipes, gutter downspouts that still connect to sewers or illegal industrial connections. When this happens, sewage flows into "combined sewer overflow" pipes that pour into local waterways - designed to relieve pressure on the sewers and protect homes and businesses from sewage backups in such instances.
A combined sewer overflow occurs when a large storm overwhelms the combined sewerage system causing rainwater to mix with wastewater and discharge to a nearby water body. This prevents sewage backups into homes and businesses.
The MWRA says:
Most of the active CSOs in MWRA's service area receive treatment such as screening, disinfection and dechlorination.
MWRA map and status of its overflow pipes.
Map of overflow pipes owned by Boston and nearby communities.