In 1998, researchers used fluorescent tracers in Boston's Fort Point Channel to track polluted sediment; water flushed in 1–2.7 days, but particle removal ran 10 times faster than lab tests
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 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 . 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.
The dye and particles did not behave the same way
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. That produced a replacement time of about 1 to 2.7 days, and the exact rate varied with tidal conditions and freshwater inflow, which meant that the channel did not have one fixed flushing time.
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 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.
A layer of material on the bottom may have trapped particles
The paper proposed that the suspended particles could have been scavenged by a soft bottom layer, and was described by the researchers as a bottom “fluff” layer. The material in that layer could interact with particles that moved through the water and remove them from suspension more quickly than ordinary settling would predict.
The observations suggested that interactions at the sediment-water interface were important in determining the fate of suspended material. The result also fit with earlier observations from Salem Sound and with controlled laboratory work on particle aggregation at the sediment-water boundary. 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.
Fort Point Channel was already a difficult place for pollution studies
A Massachusetts Water Resources Authority report on contaminated sediments describes Fort Point Channel as a restricted-flushing area with a major combined sewer overflow at its head. The report said that the channel was important for understanding how dissolved and particle-bound pollutants from combined sewer overflows moved between the inner and outer harbor. Fluorescent tracers were released near the head of the channel and monitored for roughly a week in the earlier tracer surveys. The channel's bridges and geometry also made repeated sampling possible.
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 study changed the picture of particle transport
Water in Fort Point Channel could be replaced within days, but particles could leave the water column much faster than expected from laboratory settling measurements. The researchers attributed the difference, at least in part, to interactions with material that had already accumulated on the channel floor. That meant the fate of contaminants depended not only on how quickly the channel flushed, but also on what happened when suspended particles encountered the sediment at the bottom.
A Massachusetts Water Resources Authority study later reported that more than half of contaminated particles entering Fort Point Channel from combined sewer overflows settled and were trapped within the channel rather than reaching the outer harbor.
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.
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. That produced a replacement time of about 1 to 2.7 days, and the exact rate varied with tidal conditions and freshwater inflow, which meant that the channel did not have one fixed flushing time.
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 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.
A layer of material on the bottom may have trapped particles
The paper proposed that the suspended particles could have been scavenged by a soft bottom layer, and was described by the researchers as a bottom “fluff” layer. The material in that layer could interact with particles that moved through the water and remove them from suspension more quickly than ordinary settling would predict.
The observations suggested that interactions at the sediment-water interface were important in determining the fate of suspended material. The result also fit with earlier observations from Salem Sound and with controlled laboratory work on particle aggregation at the sediment-water boundary. 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.
Fort Point Channel was already a difficult place for pollution studies
A Massachusetts Water Resources Authority report on contaminated sediments describes Fort Point Channel as a restricted-flushing area with a major combined sewer overflow at its head. The report said that the channel was important for understanding how dissolved and particle-bound pollutants from combined sewer overflows moved between the inner and outer harbor. Fluorescent tracers were released near the head of the channel and monitored for roughly a week in the earlier tracer surveys. The channel's bridges and geometry also made repeated sampling possible.
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.
Water in Fort Point Channel could be replaced within days, but particles could leave the water column much faster than expected from laboratory settling measurements. The researchers attributed the difference, at least in part, to interactions with material that had already accumulated on the channel floor. That meant the fate of contaminants depended not only on how quickly the channel flushed, but also on what happened when suspended particles encountered the sediment at the bottom.
A Massachusetts Water Resources Authority study later reported that more than half of contaminated particles entering Fort Point Channel from combined sewer overflows settled and were trapped within the channel rather than reaching the outer harbor.
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