On 24 September 2026, The New Bedford Light reported that the Massachusetts Division of Marine Fisheries has been running a rhodamine dye tracer through New Bedford's treated wastewater to map how effluent moves across Buzzards Bay, with shellfish growers pressing regulators to revisit the model that tightened harvest rules on their beds in 2024.
Key takeaways
- Researchers began releasing 100 gallons of EPA-approved red dye from a boat off Fort Taber at 2 a.m. on 12 September and monitored the resulting plume through 17 September.
- New Bedford's combined sewer system had already discharged 296 million gallons of sewage and stormwater into the harbor year-to-date in 2026, including 50 million gallons during the week of 13 September.
- The city has spent close to $500 million since the 1990s on combined sewer upgrades and reports an over 90% reduction in CSO discharge, with the remaining work estimated at around $1.2 billion.
- Leavitt's 47-acre Fairhaven oyster farm has already seen four harvest closures in 2026, one of which ran for nearly all of July.
- Neighboring Fairhaven is sizing upgrades to its own wastewater treatment facility at between $62 million and $71 million, with one round of work due to complete in October.
What happened
According to The New Bedford Light, the Massachusetts Division of Marine Fisheries released 100 gallons of EPA-approved rhodamine dye into New Bedford's treated wastewater starting at 2 a.m. on 12 September, aboard a boat off Fort Taber. By 8 a.m. the same morning a reddish-purple plume had formed, and researchers tracked it through 17 September. The fluorescent red tracer is designed for water tracing and does not harm the environment, taking more than two weeks to break down, which gives the team time to follow how treated effluent disperses through Buzzards Bay (The New Bedford Light).
The study is being watched closely by local shellfish growers. In 2024, eight area oyster farmers, including Dale Leavitt, were told their beds would be subject to stricter food-safety rules after a University of Massachusetts Dartmouth SMAST model concluded that New Bedford's wastewater discharge posed a contamination risk (The New Bedford Light). Leavitt, who has run a 47-acre oyster farm in Nasketucket Bay, Fairhaven since 2021, told the outlet he is hoping the new tracer work will give regulators harder data than the earlier model.
"At this point, it's really hard to speculate as to where [the water] is all going to go," Leavitt said. "The good news is that we now have harder data than what we had before" (The New Bedford Light).
The New Bedford Light reports that the dye traces treated sewage, which theoretically carries no pathogens, but during heavy rain the city's combined sewer overflows (CSOs) release untreated wastewater and stormwater into the harbor. When a CSO is declared, Leavitt's farm must shut harvesting for 21 days, or for seven days if testing proves the shellfish are free of harmful bacteria. The nearest FDA-approved lab is in Gloucester, and results can take up to 10 days to come back. The state's Marine Fisheries Advisory Commission announced in August that funding has been secured for two new full-time positions on the shellfish team, including a SMAST-based lab technician, with commissioner Tom O'Shea telling the meeting, "But those CSOs are way too routine" (The New Bedford Light).
On the engineering side, The New Bedford Light reports that since the 1990s New Bedford has invested almost $500 million in combined sewer and stormwater upgrades, achieving an over 90% reduction in CSO discharge. City engineer Shawn Syde said the remaining work is estimated at around $1.2 billion, with the city's over 50-year-old wastewater treatment facility and 27 CSO outfall pipes set to absorb a rising share of capital. Fairhaven's separate wastewater treatment facility upgrade is sized between $62 million and $71 million, with one phase expected to be completed in October (The New Bedford Light).
Scientists will finalize the dye study results next year. The output is intended to update maps of wastewater flow and inform emergency response if the treatment plant fails, and Leavitt and fellow shellfisherman Scott Soares have donated oysters so researchers can see how the plume moves through the animals themselves, since oysters filter coliform bacteria and other pathogens from the water column (The New Bedford Light).
Specification read
Translating the reported CSO volumes into plant-scale units: 296 million gallons over the year-to-date works out to roughly 1,340 m³/day of combined sewage and stormwater discharged to receiving waters on a daily average, with a single wet week contributing about 230,000 m³/day averaged over seven days. That is a small-volume plant in absolute terms — order-of-magnitude 1–2 MLD — but it is being run on an aging combined sewer that mixes domestic sewage, industrial effluent and stormwater in the same conveyance, which is the worst hydraulic regime for a biological stage and is the reason secondary clarifiers and disinfection contact tanks downstream of an integrated sewage treatment train are sized with such large storm buffers.
The hydraulic story here is classic combined-sewer primary, biological and disinfection capacity: coarse and fine screening, grit removal, primary clarification, a biological stage (typically activated sludge with nutrient removal, or an MBR or SBR where footprint is tight), followed by tertiary filtration and disinfection before ocean discharge. The dye tracer is a diagnostic tool for the outfall and receiving-water end of that train — it tells operators nothing about BOD, COD or ammonia removal inside the plant, but it does tell the regulator whether the disinfection-stage effluent is actually being diluted and dispersed the way the permit assumes, and where shellfish beds sit relative to the plume during dry weather. The plant-side question this event raises is the secondary and disinfection stage's ability to hold residuals inside the pipework when stormwater inlets open, which is the engineering justification most municipal buyers cite when they ask for an MBBR or MBR retrofit on top of an existing aeration tank.
If your plant sits in the 1–2 MLD range, takes combined sewage, discharges to a tidal shellfish-growing water, and is being pressed by the regulator over coliform and CSO frequency, then the New Bedford project is the case study to watch: a city-sized combined interceptor upgrade priced near $1.2 billion, layered with a 50-year-old headworks that has to be rebuilt in parallel, and a neighboring municipal upgrade sized in the $60–70 million bracket for a single town. The procurement signal is that the next round of work is going to skew toward biological-stage intensification and stormwater attenuation rather than more outfall pipe — exactly the scope where an integrated packaged plant with built-in equalization earns its place. For comparison points, the same logic that drives a coastal CSO plant shows up in inland food and beverage sites with high organic loadings, where an MBR retrofit on an existing aeration basin is the typical path, and the sizing math runs through the same screening to disinfection chain documented in our MBR vs conventional activated sludge for food and beverage wastewater reference. For lower-flow, remote or rural sites facing similar effluent-quality pressure, the Australian cost guide in our rural sewage treatment library sets out the per-m³ benchmark for packaged plants in the same flow band.
FAQ
What is a combined sewer overflow, in plain terms? A combined sewer carries domestic sewage and stormwater in the same pipe. When heavy rain pushes flow past the treatment plant's capacity, the excess is discharged untreated through an outfall. The New Bedford Light reports that New Bedford operates 27 such outfall pipes and has spent close to $500 million since the 1990s cutting discharge volumes by more than 90%.
How much will New Bedford's remaining CSO work cost? City engineer Shawn Syde has put the outstanding upgrade at around $1.2 billion, with the over 50-year-old wastewater treatment facility due to take a growing share of capital in the next few years (The New Bedford Light).
What is the typical lead time and budget for a 1–2 MLD combined-sewer upgrade at a small municipal plant? The New Bedford Light puts a single-town upgrade of this class — Fairhaven's wastewater treatment facility — at $62–71 million, with one phase expected to be completed in October. For a packaged plant in the same flow band, general industry range, not from the sources: 6–12 months of design and build and a capital cost in the low single-digit US$ per litre of daily capacity.
How does a dye tracer like rhodamine change the permit picture for a coastal outfall? A tracer does not measure plant performance; it measures where the disinfected effluent actually goes. The New Bedford study will feed an updated map of wastewater flow and an emergency-response plan if the plant fails (The New Bedford Light), which is the same evidence base regulators usually request before loosening shellfish-bed closure rules.
