Boston’s Fort Point Channel (left) and a USGS researcher collecting coastal sediment with a grab sampler (right), illustrating the waterway and sediment sampling central to studies of particle transport and pollution | (left) Wikimedia Commons, (right) USGS
A 1998 study of Boston’s Fort Point Channel found that the water in the narrow section of Boston Harbor could be replaced within roughly one to 2.7 days, while suspended particles that carry sediment-like material were removed from the water column at rates about an order of magnitude faster than laboratory settling tests had previously suggested. The researchers calculated effective deposition velocities of about 1.5 to 3.3 meters per day, and noted that these values were an order of magnitude faster than rates observed in laboratory settling columns. They released fluorescent tracers and followed their movement through the water rather than relying on observations alone.
The study was published in Estuarine, Coastal and Shelf Science , and used Rhodamine WT as a dissolved tracer to measure water movement. Fluorescent pigment particles were mixed with the dye to represent suspended material with characteristics that could be compared to sewage-associated particles. Water samples were collected across the channel for about a week after each tracer release. Boston’s Fort Point Channel (left) and a USGS researcher collecting coastal sediment with a grab sampler (right), illustrating the waterway and sediment sampling central to studies of particle transport and pollution | (left) Wikimedia Commons, (right) USGS This was the central part of the experiment. Rhodamine WT moved with the water and had little interaction with suspended sediment, and researchers could estimate how quickly water itself was being flushed from Fort Point Channel by following changes in its concentration. The fluorescent particles behaved differently, as their concentration declined relative to the dye, which indicated that some of the suspended material was being removed from the water rather than simply carried out with the moving water.
The researchers were trying to understand what happened to contaminated particles that entered the channel, which is an area that is affected by combined sewer overflows and historically polluted sediment.
The observations suggested that interactions at the sediment-water interface were important in determining the fate of suspended material. Fast disappearance of particles from the water does not necessarily mean that pollution has left the harbor, since a particle that settles onto the bottom may remain within the local environment rather than being transported into the outer harbor. The same report noted that the largest combined sewer overflow in Boston Harbor was located at the head of the channel at the time of the study. Combined sewer systems can carry both wastewater and stormwater, and overflows can discharge into receiving waters during wet weather. The MWRA now describes CSOs as occasional discharges of rainwater and sewage into Boston Harbor and nearby rivers, although decades of infrastructure work have greatly reduced the volume and frequency of those releases.
The result also fit with earlier observations from Salem Sound and with controlled laboratory work on particle aggregation at the sediment-water boundary.
