A Grand Day on the Alewife Brook Measuring Sediment Depth
The Environmental Protection Agency’s mention of dredging the Alewife Brook in its Response to the Massachusetts Water Resources Authority’s April 2022 Draft Scope for the new CSO Control Plan has revived the discussion about the role removing sediment from the Alewife Brook can play in restoration and flood prevention efforts. EPA recommends MWRA include dredging the Alewife Brook as part of the Alternatives Analysis for the new Control Plan.
Elimination of Combined Sewer Overflows and sewage pollution in the brook must be achieved through continued sewer separation in Cambridge and Somerville. But sewer separation means removing stormwater from the sanitary sewer system. Once it’s removed, that stormwater has to go somewhere, and that somewhere is the Alewife Brook. Increasing the amount of stormwater flowing to the Alewife Brook could increase the threat of area flooding. Therefore, flood mitigation measures must be taken alongside sewer separation. EPA suggests that one way to accommodate an increase in stormwater would be to increase the Alewife Brook’s storage and flow capacity by dredging the channelized portion of the brook.
The Time Has Come to Dredge the Alewife Brook to Increase Capacity and Prevent Flooding
In its Response letter, EPA quotes a 2005 United States Geological Survey study that estimated sediment volume in the Alewife Brook at approximately half a million cubic feet. In 1988, one of many moments when the idea of dredging the Alewife was floated, the Metropolitan District Commission (MDC – precursor of both MWRA and the Department of Conservation & Recreation) commissioned a study to determine what was in the sediment. This was necessary to determine how to dispose of the dredged material. When testing indicated that the material was toxic enough to require disposal at a Certified Waste Disposal Site, the MDC’s enthusiasm for dredging faded. If EPA are now endorsing consideration of dredging, then it may be an idea whose time has finally come.
Our Measurements Suggest a Doubling of Sediment Since 1988
On a fine summer day in July, three members of Save the Alewife Brook set out to measure the depth of sediment at the 1988 testing sites. Equipped with a precision hand-crafted measuring tool – a 5-foot steel rod with a ruled scale ground onto it – and a hand-built canoe. We dropped the boat in at John Wald Park in Cambridge.
It’s no surprise that the amount of sediment has increased since 1988, given three decades of sewage discharges, and infrequent removal of branches and trash. At the center of the Little River opposite MWRA’s CSO MWR003 (‘CCSO’ on the map), our tester measured a sediment depth of approximately 36 inches. That’s double the 18 inches recorded in 1988. Further upstream, adjacent to 20 Acorn Drive (‘AB03’), the sediment measurement was 48 inches. That’s an increase of 18 inches over the 1988 measurement.
We also measured sediment at two sites in the Alewife Brook not found on the 1988 map. Under the Pedestrian/Bikepath Bridge over the brook, sediment was 36 inches deep. At the canoe launch in Arlington, adjacent to Lafayette Street (the overlook marked by the boulder with the carved Snapping Turtle image), the sediment depth was 48 inches – that’s 4 feet of sediment in the narrow concrete channel!
We didn’t measure the site marked on our 1988 map just downstream of the Mass. Ave. Bridge (AB06A). None of the crew felt like dragging the canoe past the growing channel obstructions or measuring the sediment depth immediately downstream of two of Cambridge’s active CSO outfalls (CAM002 & CAM401B), what with the 1.5 million gallons of sewage pollution they dumped in this spot last year.
The stench emanating from this portion of our poor debris-strewn channel on that hot summer day undercut the (wildly misleading) claim of an 85% reduction in the volume of sewage discharged to the Alewife Brook since 1988 (the year our sampling map was made). In 1988 the sediment depth at AB06A was 10 inches. Eyeballing it from the banks, it’s clear that the sediment now catching and trapping branches and other debris is a great deal deeper.
Now is the time to consider dredging the Alewife Brook. Removing the sediment could provide immediate benefits in terms of capacity, flow, and improved water quality. Federal Infrastructure Law funds can get this done.
References:
EPA’s May 11 2022 Response to MWRA’s Draft Scope of Work
MWRA Updated CSO Control Plan – Draft Scope of Work and Schedule 04/01/2022
2005 USGS Sediment Study of Rivers and Lakes
1988 Little River / Alewife Brook Sediment Survey