Water Quality

High concentrations of phosphorus and nitrogen compounds can act as fertilizers in rivers, leading to the overgrowth of aquatic plants and algae, a phenomenon known as eutrophication. This has been a persistent issue in the Assabet River. However, our data indicates a significant improvement since the wastewater treatment plants along the Assabet upgraded their systems to reduce summer phosphorus discharge to 0.1 mg/L. As a result, the total phosphorus concentrations in the water have decreased to nearly the levels expected in clean rivers. Despite this progress, nitrate concentrations remain above recommended levels, and, due to legacy phosphorus in the sediments of the Assabet impoundments continues to contribute to aquatic plant and algae overgrowth in the slower-moving sections of the river.

In contrast, the absence of point-source wastewater treatment plants discharging into the Sudbury River has helped maintain nutrient concentrations at or below eutrophication thresholds.

Dissolved oxygen (DO) concentrations are crucial for supporting fish and other aquatic life. The state’s warm water Class B standard requires DO levels to be above 5.0 mg/L and below approximately 170% saturation. Aquatic plants generate oxygen through photosynthesis during the day and consume it during respiration at night, causing significant fluctuations in DO levels. In most sections of the Assabet and Concord rivers, DO concentrations are satisfactory. However, in the impounded areas of the Assabet River, DO levels can drop below 5.0 mg/L during the summer. Similarly, summer DO levels in the lower Sudbury River often fall below the state’s Class B aquatic life standard minimum of 3.0 mg/L. This condition may be natural for the area below the marshy Great Meadows in Wayland/Sudbury, but nutrient inputs are also suspected to contribute to the low DO levels.

Overall, while significant improvements have been made, ongoing monitoring and management are essential to address the remaining water quality issues in these rivers.

Water quality in the headwaters and tributary streams of the Assabet and Sudbury rivers is generally better than in their main channels. Median phosphorus concentrations in these tributaries are typically lower than in the mainstems, and DO levels are generally healthy. While total nitrogen concentrations in the tributaries are lower than in the Assabet mainstem, they range from healthy to somewhat elevated.

The nutrients found in the tributaries primarily come from non-point sources, such as stormwater runoff from roads and lawns, or failing septic systems. However, there is an exception: Hop Brook (also known as Landham Brook) in Marlborough, Sudbury, and Wayland. This brook receives treated effluent from the Marlborough Easterly wastewater treatment plant and faces nutrient concentrations and eutrophication issues similar to those the upper Assabet River experienced before its wastewater treatment plant upgrades. Although upgrades to the Marlborough Easterly wastewater treatment plant were completed in the spring of 2015, significant amounts of legacy phosphorus remain in the brook’s impounded sections.