In 2024, the SuAsCo watershed experienced a notably warmer year with rainfall patterns closer to the long-term average—drier than 2023 but with episodic storms. Lower overall river flows and stream levels throughout much of the season influenced several key water quality indicators. These conditions offer important contrast to 2023’s extreme wetness and continue to inform our understanding of climate impacts on water quality.

Blue: mainstem sites, listed by river and in river mile order

Teal: tributary sites

Water temperature plays a critical role in river health, influencing the survival of aquatic life. In 2024, temperatures remained variable across sites, influenced by summer rainfall events and flow conditions. No clear long-term warming trend has emerged in our river temperature data. The continuing presence of cool groundwater inflows—especially in tributary streams—has helped buffer our rivers against warming, though elevated temperatures during low-flow periods remain a concern.

Conductivity levels rebounded slightly in 2024 from the unusually low values seen in 2023, driven by the return to drier conditions and reduced dilution from rainfall. While year-to-year variability is expected, the long-term upward trend in conductivity remains troubling. This is particularly true in winter and spring, when conductivity levels spike from road salt runoff. High conductivity remains most pronounced below WWTPs and major transportation corridors.

Blue: mainstem sites, listed by river and in river mile order

Teal: tributary sites

Dissolved oxygen is a critical measure of stream health. In 2024, DO levels were strongly influenced by site-specific factors:

• Assabet River: Continued strong performance due to improved wastewater treatment processes.
• Lower Sudbury River: Remained problematic, with chronic low DO levels, particularly in floodplain zones during warmer months.
• Hop Brook (Sudbury): Sustained modest improvements, though DO levels remain below standards in some reaches.
• Nashoba Brook below Warner’s Pond: Continued decline in DO, likely tied to pond eutrophication.
• Elizabeth Brook and Sudbury Headwaters: Persistently low DO and pH, indicating stress from surrounding wetlands and organic decomposition.
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Blue: mainstem sites, listed by river and in river mile order

Teal: tributary sites

pH values were more stable in 2024 but still showed patterns of concern in certain tributaries. While most sites remained within Class B standards, the Lower Sudbury and Elizabeth Brook sites continued to show lower pH levels, especially following rain events. Encouragingly, the long-term trend in the Assabet River remains upward, suggesting sustained benefits from phosphorus controls and reduced biological demand.

Blue: mainstem sites, listed by river and in river mile order

Teal: tributary sites

Phosphorus levels continue to serve as a key indicator of nutrient pollution. In 2024, total phosphorus levels remained close to the target of 0.05 mg/L at mainstem Assabet River sites. However:

• Hop Brook: Still experiences high TP loads from the Marlborough Easterly WWTP.
• Lower Sudbury and Concord Rivers: Show early signs of rising TP levels.
• Hudson WWTP: Exceeded its permitted phosphorus limits for eight months in 2024, continuing a troubling trend that began in 2022. Summer TP loads were double the allowable limits, presenting serious concerns for downstream water quality.

Elevated nitrate concentrations remain a consistent problem, particularly downstream of wastewater treatment plants. With no regulatory caps on nitrate discharge from WWTPs, levels continue to rise over time. This trend has implications for the health of the SuAsCo watershed and for nutrient loading in the Merrimack River estuary.

Blue: mainstem sites, listed by river and in river mile order

Teal: tributary sites

TSS concentrations stayed moderate in 2024 due to fewer major storms and relatively lower flows compared to 2023. While Lower Sudbury and Concord River sites are prone to resuspended sediment from recreational activity, 2024 saw fewer spikes. Long-term trends suggest improving TSS conditions across the watershed.

Measured in the Sudbury River as a proxy for planktonic algae, chlorophyll a levels remained low in 2024. This is likely due to cooler water temperatures and continued phosphorus management, reducing the risk of harmful algal blooms.

E. coli monitoring continued in 2024, with results published in a separate Bacteria Report. Patterns remain consistent with previous years: elevated bacteria levels are most common after rainfall and in areas with poor stormwater infrastructure. Despite these challenges, some sites—especially along the Upper Concord—now meet recreational water quality standards.