May 2022 Water Quality Report (CWMN)

Water quality looked promising with low E. Coli levels, high dissolved oxygen, and low phosphorus levels.


Neponset Reservoir, Foxborough

The 2022 CWMN season started off with promising results! Overall, water quality looked good with low E. Coli levels and high dissolved oxygen. Phosphorus levels were relatively low across the sites, with only a few locations reaching concerning levels.

On the morning of May 12th, forty-eight volunteers ventured through the foggy morning air to their sampling sites. Some samplers are returning to a site that they have been monitoring for over ten years, others were collecting their inaugural samples. Either way, every volunteer played an important role in collecting data and better understanding the water quality of the Neponset River Watershed. Here’s to five more months of successful water monitoring this season!

E. coli

E. Coli is a type of bacteria that lives in the intestines of healthy animals, including humans. Its presence in water is evidence of waste contamination. Typical sources of E. Coli include leaking sewer infrastructure and bird or dog feces. High levels of the bacteria associated with this waste can make people sick.

Low rainfall in May was reflected in our E. Coli data, with 34/41 sites (83%) remaining safe for swimming.  Only one site was found to be unsafe for boating.  This data shows how dry weather conditions can reduce E. Coli levels by reducing stormwater runoff.   In wet months, structural stormwater Best Management Practices (“BMPs”) are important to help treat stormwater and clean it before it discharges into the River.

Learn more about BMPs on the Neponset River Watershed Association website.

In this report, we use a boating standard for E. Coli that is different than the value used in past reports.  Rather than using the geometric mean maximum which represents the maximum level for E. Coli levels across multiple samples (630 CFU/100mL), we are now using the single sample maximum for E. Coli levels (1260 CFU/100mL).

The E. Coli results are summarized in the maps and graph below:



Total Phosphorus

Phosphorus is a key nutrient for proper plant growth. This means that fertilizers contain high levels of phosphorus and when it is washed off lawns via rain, the phosphorous can end up in streams and ponds. Phosphorus fertilizers are regulated within the Neponset River Watershed to help lessen these types of phosphorus loading. Too much phosphorus can lead to harmful algal blooms, cyanobacteria blooms, and fish kills.

Total phosphorus levels were relatively low in May and 60% of the sites tested had healthy phosphorus levels. Although most sampling sites had healthy levels of phosphorus in May, phosphorus pollution is a significant issue for most of the watershed.  Phosphorus levels typically increase from May to June, as the weather gets warmer. This increase is largely the result of high water temperatures and low dissolved oxygen, leading to a chemical reaction that releases phosphorus from lake sediments. Another factor is phosphorus-rich organic material such as leaves and pollen washing off paved surfaces and into storm drains.

You can help reduce phosphorus pollution by supporting investments in green stormwater infrastructure and stormwater utilities in your neighborhood and reducing your use of synthetic fertilizers.

The phosphorus results are summarized in the graph below:

Dissolved Oxygen

Dissolved oxygen is important for aquatic life because many underwater critters access the oxygen they need to respire, or breathe, through dissolved oxygen in the water. Certain animals, including trout, require more dissolved oxygen than others. Low dissolved oxygen levels are linked to drought conditions and sustained high temperatures and can lead to a reduction in the population sizes of the numerous aquatic species that are reliant on oxygen in the water.

The high levels of dissolved oxygen in May indicate that the waterways in the Neponset River Watershed offer a healthy habitat for plants and animals!

Almost 90% of the sites at which dissolved oxygen measurements were taken reached the recommended number of milligrams of oxygen per liter for cold-water fish. Only four of our measured sites had lower than recommended dissolved oxygen levels.

The dissolved oxygen results are summarized in the graph below:

All of this data has gone through preliminary quality control, however, note that a more extensive overview is given to the data before reporting it to DEP. If you have any questions or comments about the data, please reach out to NepRWA Environmental Fellow, Eleanor Yeomans, at


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