Water Quality Reports

In 2023, we experienced an unusually wet year, with rainfall, river flows, and groundwater levels all significantly higher than average. It was the wettest year in over two decades. This substantial increase in precipitation had a major impact on various environmental measurements, standing in stark contrast to the previous year, which was exceptionally dry.

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

Teal: tributary sites

Water temperature is a vital factor for the well-being of aquatic ecosystems, especially amidst growing concerns over global warming. In 2023, our observations showed that precipitation and flow significantly influenced water temperatures. While warm rainwater contributed to temperature increases across all streams, high flows mitigated overheating by reducing the time water spent in impoundments. Despite ongoing monitoring, we have yet to detect a clear long-term warming trend in river water temperatures. This is partly due to the moderating effect of cool inflowing groundwater on stream and river temperatures. Additionally, our data indicates that several warm years were recorded in the early 1990s, marking the beginning of our monitoring efforts.

In 2023, conductivity levels in our waterways were notably lower compared to previous years, mainly due to increased dilution caused by heavy rainfall and elevated flows. Despite this decrease, certain areas, such as those located beneath wastewater treatment plants and major roads, still exhibited relatively higher conductivity levels, albeit mitigated by the effects of rainfall.

In New England, conductivity is a crucial indicator of road salt pollution since it is highly correlated with chloride levels. Our extensive monitoring efforts have revealed a consistent and concerning trend: conductivity levels across all sections of our rivers have been steadily increasing over the long term. This upward trend strongly suggests a corresponding rise in chloride levels, posing a significant threat to the overall ecological health of our waterways.

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

Teal: tributary sites

• Assabet Sites: Encouragingly, there’s a positive trend in DO levels thanks to improvements in wastewater treatment processes.
• Lower Sudbury: Despite experiencing higher levels during the 2022 drought, there has been a concerning downward trend in DO levels. Particularly during heavy flooding in 2023, DO levels plummeted to less than 1 mg/L for several months.
• Hop Brook, Sudbury: While historically maintaining very low DO levels, there’s promising improvement in its trend.
• Nashoba Brook below Warner’s Pond: Unfortunately, there’s a clear downward trend here, largely due to increased nutrients in the pond leading to eutrophication.
• Elizabeth Brook and Sudbury Headwaters: These areas are of particular interest due to chronic low DO and pH levels downstream of significant wetlands.

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

Teal: tributary sites

In 2023, pH levels across various locations experienced a notable decrease, indicating increased acidity primarily attributed to heavy rainfall. However, recent trend analysis presents an encouraging development: a discernible upward trajectory in pH levels along the Assabet River. This positive trend hints at potential benefits such as diminished eutrophication and reduced levels of aquatic respiration, stemming from sustained phosphorus reduction efforts over the long term.

Noteworthy pH readings emerged from these sites:

• Lower Sudbury: sites exhibited exceptionally low pH values falling below Class B standards, largely influenced by the combined effects of rainfall and decomposition in inundated floodplains.
• Upper Sudbury, SUD-293: demonstrated similar patterns of acidity, reflecting the impact of environmental factors akin to the lower Sudbury sites.
• Elizabeth Brook Site, ELZ-004: registered persistently low pH levels over the past four years, presenting a unique case warranting further investigation.

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

Teal: tributary sites

We’ve been closely monitoring Total Phosphorus (TP) levels in the Assabet River to track improvements in wastewater treatment plants. Analysis shows a significant decrease in TP until 2012, when the last plant upgrades were completed. Since then, TP levels in the mainstem have remained close to the target of 0.05 mg/l. However, high TP concentrations persist in Hop Brook downstream of the Marlborough Easterly Wastewater Treatment Plant (WWTP). Additionally, there’s a potential trend emerging of increasing TP levels in the Lower Sudbury and Concord Rivers over the past five years.

Our examination of TP loads reveals that heavy rainfall significantly influences phosphorus levels in the river system. For instance, wet years like 2023, 2021, and 2013 resulted in notably high TP loads across all river sections.

Despite substantial reductions in phosphorus discharge from WWTPs, there’s been a recent issue at the Hudson WWTP. This plant exceeded its permitted TP discharge limits for four months in 2022 and eight months in 2023, leading to phosphorus loads during the summer that were twice as high as permitted. We’re actively working to address this issue.

Nitrate levels downstream of all wastewater treatment plants (WWTPs) are significantly elevated, and our data indicates that concentrations and amounts of nitrate in rivers are rising over time. WWTP effluent is the main contributor to nitrate pollution in rivers, and currently, there are no regulations in place to control these discharges. This is a concern for tidal estuaries downstream of these rivers. Our findings demonstrate that a considerable amount of nitrate is being transferred downstream to the Merrimack River.

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

Teal: tributary sites

In the Lower Sudbury and Concord Rivers, we often find higher concentrations of Total Suspended Solids (TSS). This could be because these areas are popular for motorized boating, which can stir up sediment. However, in 2023, we noticed that TSS levels were not as high in these locations. This was likely due to heavy rainfall and increased water flow, which carried a lot of sediment downstream. Despite the large amount of sediment being transported, the high water flow helped dilute it, resulting in lower TSS concentrations compared to previous years. Overall, our data over the years show that TSS levels in all our rivers are improving.

Chlorophyll a serves as a marker for planktonic algae presence in water bodies and can signal the onset of eutrophication, a process where excessive nutrients lead to harmful algal blooms. In our study, we focus on measuring chlorophyll a in the Sudbury River only.

Our data analysis reveals an encouraging trend: over the years, we’ve observed a consistent decrease in chlorophyll a levels across all monitored sites. The year 2023 continued this trend, but this can likely be attributed to heavy precipitation and increased water flows, which likely restricted optimal growth conditions for algae.

E. coli bacteria serve as a crucial indicator of the health and safety of rivers, especially for those who use them recreationally. OARS began monitoring bacterial levels in rivers back in 2019. Over the past four years, the levels of bacteria followed a consistent pattern. For more detailed information, refer to our dedicated Bacteria Report, below.