This is also where important and innovative work is under
way to correct a problem that dates to the late 1800s: ridding the landscape of
combined sewer overflow (CSO) outfalls that impair water quality in the Alewife
City Engineer Owen O’Riordan and Engineering Project
Coordinator Catherine Woodbury of the Cambridge Department of Public Works gave
a presentation on the city’s sewer separation project at the Mystic River
Watershed Association’s Joint Committee Meeting on January 7.
Ideally, stormwater and sanitary sewer systems should
function independently. But many older cities have combined systems that
collect rainwater runoff, domestic sewage and industrial wastewater in the same
pipe. Most of the time, these systems transport all of their wastewater to a
sewage treatment plant, where it is treated and then discharged to a water
During big storms, however, the volume in a combined sewer
system can exceed capacity. In such instances, combined sewer systems are
designed to overflow, which discharges excess wastewater directly to nearby
water bodies. These discharges help prevent sewage backups into homes,
businesses and streets, but they impair water quality, often to dangerous levels.
In Cambridge, the city and the Massachusetts Water Resources
Authority are collaborating on six projects to significantly reduce CSO
discharges into Alewife Brook, including eliminating three of the eight
original CSO outfall locations.
In addition, the amount of stormwater that enters the
combined system is being reduced, which lessens the likelihood of combined
outfalls. This is being accomplished in part by installing new storm drains in
three neighborhoods, and by separating manholes that were common to the storm
drain and sewer systems (effectively functioning as CSOs).
Several “green infrastructure” projects are part of the
project, most notably the creation of a 3.5-acre wetland basin near the Alewife
MBTA Station. The wetland, completed
last October, collects the stormwater flows removed from the combined sewer
system, and provides a level of natural water quality treatment before the
water drains into the Little River and Alewife Brook. It also will reduce flooding,
a long-time problem in the area, and improve the health of adjacent natural
The marshy basin features 115,000 new wetland plants, 3,800
upland plants, paths and boardwalks, benches and bike racks, interpretive
signs, overlooks, and a seating area designed for school group visits. Other
techniques the city is implementing to reduce stormwater runoff include
planting more than 400 street trees, building rain gardens, and replacing
impervious pavement with porous pavement.
When the project is completed next year, officials expect the
annual amount of untreated sewage released to Alewife Brook will be 85% below 1996
levels – dropping from 50 million gallons to 7.3 million gallons – with average
annual CSO discharges falling from 63 to seven.
These initiatives will significantly improve the Alewife
Subwatershed water quality, help restore the health of the Mystic River, and
make the Alewife and Mystic River reservations safer and more inviting for
More information about the Cambridge sewer separation
project is available on the city’s stormwater
management web page.
The next Mystic River Watershed Association Joint Committee
Meeting will take place at 7 p.m. on Tuesday, February 4, in the Rabb Room of
the Lincoln Filene Center at Tufts University.
At that meeting, the Massachusetts Water Resources Authority will give a
presentation about its climate adaptation plans.
The public is welcome at all Joint Committee Meetings.
For more information on the Mystic River Watershed
Association, visit www.mysticriver.org.