- Division of Marine Fisheries
Shellfisheries state-wide are being impacted by increasing closures associated with wastewater discharges from both combined sewer overflows (CSOs) and wastewater treatment plant (WWTP) outfalls. These are two distinct wastewater contamination issues subject to different management approaches. Potential public health risks associated with wastewater discharges factor into shellfisheries management as a requirement in the National Shellfish Sanitation Program (NSSP). The NSSP is governed by the Interstate Shellfish Sanitation Conference which meets every two years to update regulatory requirements and guidance for the sanitary control of shellfish sold into interstate commerce based on the best available science at that time.
The NSSP has long-required, permanent shellfishing closures around WWTP outfalls to protect public health. In addition, a conditional safety zone beyond the permanently closed area is required that can be closed immediately in the event of a treatment plant performance upset. In 2015, the NSSP adopted new criteria for determining the size and configuration of these required safety zones. DMF has been working since then to evaluate and reclassify these Prohibited and Conditionally Approved areas while trying to minimize the impacts on shellfishing activities and the aquaculture industry. In so doing, we identified a critical need for hydrodynamic modeling and partnered with Dr. Changsheng Chen from the University of Massachusetts School for Marine Science and Technology for technical assistance in modeling WWTP effluent dilution and dispersion to inform decisions on shellfish growing area sanitary classification around WWTP outfalls.
In the event of a WWTP discharge of untreated or partially treated sewage, the NSSP requires DMF to consider factors such as dilution and dispersion of effluent, microbial quality of effluent, time of transport, and volume of flow to delineate appropriate Prohibited and Conditionally Approved areas. Guidance for the permanently closed area around a WWTP outfall recommends a minimum dilution of 1000:1 (clean water:wastewater), and 100,000:1 for the adjacent conditional area that may remain open provided the plant is operating as expected and effluent quality is meeting permitted standards. These ratios can be reduced if justified by data; however, the Prohibited area cannot be reduced below a dilution ratio of 320:1 which is the national standard used by the Environmental Protection Agency, U. S. Food and Drug Administration, and Centers for Disease Control. Although many States have chosen to utilize the 1000:1 dilution ratio for Prohibited areas, DMF is investing heavily in the available tools to collect data to justify smaller closed safety zones.
Advanced modeling and reclassification have already taken place for the Scituate WWTP discharging in the North and South Rivers in Scituate and Marshfield, and the New Bedford and Fairhaven WWTPs that discharge into Buzzards Bay. For Scituate and Marshfield, model results showed that effluent did not significantly travel upriver and achieved considerable dilution downriver once outside the embayment. Data collected on the Scituate WWTP’s fully treated effluent also showed that it is a high performing plant. Additionally, DMF has established reliable communication pathways with plant operators there to ensure a timely closure of the adjacent Conditionally Approved area in the event of a treatment process disruption. These factors justified a reduced size for the Prohibited area of 2,000 acres based on a 320:1 dilution ratio, rather than almost 5,000 acres if the 1000:1 dilution ratio had been used.
In New Bedford and Fairhaven, modeling similarly showed how environmental conditions affect the dispersion of WWTP effluent in Buzzards Bay. While model results showed extensive areas of Buzzards Bay potentially impacted by the WWTP effluent, including longstanding aquaculture operations, DMF committed to expanding our lab capacity and testing efforts to justify classifying those important aquaculture areas as Conditionally Approved rather than Prohibited.
Currently both the Ipswich and Dartmouth WWTPs are in the process of being re-evaluated with this advanced modeling. The modeling process is quite lengthy and has been ongoing for about a year at these plants. Preliminary results for both Ipswich and Dartmouth are expected by early 2025. Statewide, the Division is hoping to complete the re-assessments of the remaining WWTPs by 2030.
The other type of wastewater discharge that is causing considerable hardship for the shellfishing and aquaculture industry is CSO discharges which result in emergency closures as required by the NSSP. During rain events, stormwater runoff and municipal wastewater can combine in aging underground infrastructure and if the system is overwhelmed with high flows, portions of this combined effluent can release into the marine environment. Twenty-one-day closures are required when untreated or partially treated sewage is released, and the size of the closure depends on the fecal concentration and volume discharged.
To determine the size of the required closure, DMF must calculate the volume of receiving water needed to dilute the CSO discharge to the NSSP standard for shellfish harvested for direct human consumption. Tide and wind direction are also considered when evaluating whether an area is potentially impacted by CSO discharges. Closures can be lifted sooner than 21 days using male-specific coliphage (MSC) testing on shellfish, but those samples cannot be collected in the first seven days of the closure. MSC is an indicator of viral contamination.
Emergency CSO closures occurred in portions of Buzzards Bay 20 times due to CSO activations in New Bedford in 2024, for a total of 210 days lost to harvest for the year. DMF has performed MSC shellfish testing and successfully lifted the closure sooner than 21 days every time a closure was required. Fortunately, most other areas in the State potentially impacted by CSOs are in already large enough Prohibited areas such that DMF does not anticipate significant impacts to productive shellfishing areas outside of Buzzards Bay.
While there may be a reduction in shellfish acreage for some areas of the state, DMF is hopeful that improvements in other areas will help to minimize impacts on shellfishing overall. As we gather more data through advanced modeling, increased testing, and assessing the efficacy of novel, real-time sensors near important shellfish growing areas, we will continue our commitment to our dual mandate to protect public health while supporting the shellfish industry.
By DMF Shellfish Program Staff
