Water Quality Report 2022

OARS compiles a science-based water quality report every one or two years to report the data from our monitoring and to analyze trends in the data over the full history of our monitoring. The latest report was issued in April 2023 for the 2022 field season. All previous reports can be found in our online technical reports repository.

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Following are some of the high-level findings of the latest report:

The year 2022 was noteworthy as a very dry year, with precipitation, flows, and groundwater all well below average – the second lowest in the past 20 years. This factor had a major effect on almost all of the parameters monitored and is in contrast with the previous year which was extremely wet.
Water Temperature is an important characteristic for aquatic life and is particularly important to watch considering concerns of global warming. In 2022, water temperatures did not reach the concerning levels that we saw in 2020, the most recent drought year, though tributaries were still generally above the Cold Water standard.
Conductivity levels are very high in the Upper Assabet and some tributaries, especially downstream of wastewater treatment plant discharges and roadway runoff. Since conductivity in New England is highly correlated with Chloride, it is an indicator of road salt pollution. Our long-term conductivity data show a clear and statistically significant upward trend in conductivity for all sections of our rivers.
pH readings in 2022 were generally elevated (less acidic) due to the drought and lack of rainwater. Trend analysis continues to show a clear upward trend in pH in the Assabet River, which may be a positive sign of reduced eutrophication and lower levels of aquatic respiration, driven by long-term phosphorus reductions.
Dissolved Oxygen (DO) continues to show a positive upward trend in the Assabet sites as a result of the WWTP improvements that have been made there. In the Lower Sudbury the trend has been downward, but DO levels in 2022 were much higher than previous years, possibly related to high chlorophyll levels that were documented at the same times.
Total Phosphorus (TP) is the primary indicator that we watch as improvements are made to the wastewater treatment plants on the Assabet. Trend analysis shows a dramatic reduction in TP through 2012, when the final upgrades were implemented. Since 2012, TP levels have been relatively stable. The treatment plants are generally meeting their NPDES discharge permit limits, but our rivers are hovering at or above the targeted 0.05 mg/l and we still have consistently high TP concentrations in Hop Brook downstream of the Marlborough Easterly WWTP.
Nitrate levels are very high downstream of all WWTPs, and trends show that river concentrations and loads are increasing over time. The WWTPs are the primary source of nitrate in the rivers, and nitrate discharges are currently not regulated.
Total Suspended Solids have consistently been highest in the Lower Sudbury and Concord Rivers, possibly driven by motorized boating that is common in these sections. In 2022 there were some higher TSS concentrations related to low waters during drought, but our long-term data show an improving trend in all of our rivers.
Chlorophyll a is a measure of planktonic algae in the water and can be an indicator of eutrophication. High nutrient levels could result in algal blooms. We are measuring chlorophyll a in only the Sudbury River. Our year-on-year chlorophyll a data show a fairly strong downward (improving) trend for all sites combined, but 2022 results were much higher than previous years. This spike was probably a result of the drought conditions and reduced flow.
E. coli bacteria are an indicator of the health safety of the rivers for recreational users. OARS started monitoring the rivers for bacteria in 2019. Bacteria levels in all four years since have generally followed a similar pattern. Details can be found in our separate Bacteria Report.