The Environmental Protection Agency has doled out its annual report cards for the Charles, Mystic, and Neponset rivers, and for the most part the waterways scored moderately well, earning lots of B’s and even some A’s.
But despite largely decent grades, several stretches scored poorly, as pollution continues to threaten watersheds and local ecosystems.
Among the lower grades: a C-minus for the Muddy River tributary in Boston and Brookline; a D for Alewife Brook, which borders Cambridge and Arlington; and an F for Unquity Brook in Milton.
The assessments came earlier this month in the EPA’s Three Rivers Report Card — which analyzes the three major rivers that flow into Boston Harbor — in collaboration with the rivers’ watershed organizations. This year’s report involves data collection from the rivers from 2019 to 2021 and was published 50 years after the passage of the 1972 Clean Water Act, which called for US waterways to be “fishable and swimmable” by 1983.
Report card grades reflect what percentage of the time a waterway is safe for activities like boating and swimming. Those percentages are based on the amount of E. coli bacteria found in the rivers, weather data, and three-year water quality averages.
The report underscored the need for action to combat climate change, pointing to heavier rainfall events and extreme weather as contributors to more storm water runoff and combined-sewer overflows, which occur when combined sewer systems that contain waste, toxic materials, and storm water overflow and discharge into nearby bodies of water.
Throughout 2021, the watersheds experienced effects of climate change in the forms of heavier downpours, extreme heat, severe storms, and more frequent drought, the Mystic River Watershed Organization noted in their press release. The three watersheds experienced 52 inches of rainfall, 24 days above 90 degrees Fahrenheit, and several flash flooding events — all of which impacted the water quality of the rivers, the report said.
Hundreds of volunteers working with the Charles, Mystic, and Neponset watershed associations to obtain water samples from dozens of sites on a monthly basis, though volunteers only collect data from the Neponset from May through October. Data in each year’s reports is determined by averaging numbers across three years during both wet and dry weather.
The watersheds span hundreds of square miles, meaning different parts of the rivers vary widely in the grades they received. Here’s a closer look at each report card.
The Charles River Watershed
In line with grades from recent years, five of the Charles River Watershed’s six segments fell in the A to B range. The Muddy River, a Boston tributary, received a C-minus, an improvement from its D-minus rating in 2020.
Lisa Kumpf, the river science program manager for the Charles River Watershed Association, said the river has improved significantly since they first began testing in 1995.
The biggest issue in the Charles River at large right now is excess phosphorus, which occurs when there are too many nutrients that “throw the whole river ecosystem off balance,” Kumpf said. Phosphorus levels are impacted by events like sewage and storm water discharges and are less balanced in urban areas.
The Charles River experienced numerous combined-sewer overflows last summer, which decreased the water quality in the river’s lower basin from last year. During summer 2021, a record 35 inches of precipitation helped discharge over 126 million gallons of sewage and storm water into the river — the volume of 36 Olympic-size swimming pools, according to the report.
Kumpf said that the lower basin region of the Charles received a B-minus but would have been a B without the discharged sewage and storm water in 2021.
Water quality in the middle sections of the watershed — which received A ratings — is generally better than the upper and lower areas due to land use differences, Kumpf said. The middle watershed has more conserved areas with plants that filter out pollutants. The EPA report helps the organization know where to focus conservation and restoration efforts, she said.
“This report card is really based on the safety of water quality of the river for recreation,” Kumpf said. “So if people are going into the river, they should be aware that overall, the river has improved a lot since 1995, but there’s still work to be done.”
The Neponset River Watershed
Most bodies of water in the Neponset River Watershed received A’s and B’s, with the recreation-heavy main stem of the Neponset River receiving results in the B range and all of the rivers’ monitored ponds receiving A grades.
However, three areas face continued contamination with grades in the D to F range: Germany Brook, Unquity Brook, and Meadow Brook.
Sean McCanty, the river restoration director for the Neponset River Watershed, said past information indicates that the brooks’ particularly low ratings are probably caused by broken sewer pipes and pet waste.
The Neponset River’s biggest threat is polluted storm water runoff from streets, parking lots, and yards that can cause invasive species growth and harmful algal blooms when it rains, the report found. On average, water quality grades dropped 22 percent during wet weather.
While the Neponset River Watershed Association has only participated in the report for two years, McCanty said they plan to follow up on areas of concern with “hot spot monitoring,” which he said involves taking more rigorous samples in particular locations to identify what’s impacting the water.
The group can then plan targeted approaches, such as education campaigns on pet waste, to address areas with especially poor water quality.
“We do caution the people who use this data that just because something is an A grade doesn’t mean it’s always swimmable at every time of the year; it’s just a representative average,” McCanty said. “So we always tell people to trust their own judgment about whether something is safe.”
The Mystic River Watershed
Though the Mystic River itself and Mystic Lakes received B-plus to A-plus ratings, respectively, several smaller bodies of water in the watershed got failing grades on the report card.
The report said that Winn’s Brook in Belmont, Alewife Brook in Cambridge and Arlington, and Mill Creek in Chelsea showed “clear evidence of frequent contamination by waste water” and earned the lowest grades, similar to prior years.
Mystic River Watershed Association scientist Andy Hrycyna said that the main channels of the river are relatively clean for urban rivers as a result of “a lot of work” over the past several decades.
Small streams, however, tend to be “disproportionately negatively affected by continued sources of pollution,” he said, which include waste water contamination and excess nutrient pollution.
“A lot of investment has been put in to improve water quality, but it’s still not enough — the promise of the Clean Water Act has not been fully realized,” Hrycyna said.
Anjali Huynh was a Globe intern in 2022.Follow her on Twitter @anjalihuynh.