Water Quality

How's the river?
OARS, with lots of help from our volunteers, has been monitoring water quality since 1992. OARS' water quality reports are available on the reports page. More about the details of our monitoring programs are under: Our Work/Monitoring.

Under the Clean Water Act, states must develop a list of waterbodies that are not expected to meet water quality standards – they are unable to support designated uses such as fishing, recreation, drinking water supply, or aquatic life support. Every two years, states identify all such impaired stream/river segments and lakes and prioritize the development of pollutant loading studies (TMDLs) based on the severity of the pollution and the sensitivity of the uses to be made of the waters. The TMDL study is to identify the sources of pollution and corrective measures.

The resulting Integrated List of Waters (the “303d List”) in Massachusetts includes segments of the Assabet, Sudbury and Concord Rivers (see: Category 5, Concord (SuAsCo)). The main impairments caused by pollutants are: eutrophication (Assabet), dissolved oxygen (Assabet), bacteria (Assabet and Concord), phosphorus (Assabet and Concord), algae (Assabet and Concord), and mercury in fish (Sudbury and Concord). Invasive plants are a problem for all three rivers but are not directly caused by pollutants. Many ponds and lakes in the watershed are also listed for various pollutants. A TMDL has been completed for the Assabet River.

General Water Quality Findings
Nutrients in the Assabet and Concord Mainstems
High concentrations of phosphorus and nitrogen compounds can act like fertilizer in the rivers, contributing to the overgrowth of aquatic plants and algae (eutrophication). Excess total phosphorus and nitrogen had been a long-standing problem in the Assabet. OARS' data shows that, since the wastewater treatment plants along the Assabet River finished upgrades to reduce phosphorus going to the river to 0.1mg/L in the summer, the total phosphorus concentrations in the water column have decreased to levels that would be expected in clean rivers. Nitrate concentrations are still above recommended levels in both the Assabet and Concord Rivers. Aquatic plant and algae overgrowth is still a problem in the slower-moving impoundments of the Assabet behind the dams in Hudson, Stow, and Maynard.

Nutrient concentrations at the Subury River sites tested (from Saxonville to Concord) are generally at or below thresholds for eutrophication.

To support fish and other aquatic life dissolved oxygen concentrations in the river need to be above 5.0 mg/L (the state's warm water Class B standard) and below about 170% saturation. Plants generate oxygen as a by-product of photosynthesis during the day, and take oxygen back out of the water column as they respire at night. So, when there are heavy growths of aquatic plants, dissolved oxygen concentrations can change dramatically over the day. Dissolved oxygen (DO) concentrations in most sections of the Assabet and Concord are good, although DO in impounded areas along the Assabet River still falls below 5.0 mg/L at times during the summer.

Summer dissolved oxygen (DO) levels in the Sudbury River were low (below 3.0 mg/L) in 2009 but have mostly met minimum criteria in the years since then. Low DO levels may be a natural condition for this section of the river, immediately below the long marshy section of the Great Meadows in Wayland/Sudbury.

Eutrophic Impoundments
The slow-moving river sections behind dams along the Assabet River show the effects of eutrophication more severely than free-flowing sections of the river. The impoundments still have heavier rooted aquatic plant and algal growth, duckweed accumulations, lower minimum daily dissolved oxygen concentrations, and larger daily changes in dissolved oxygen concentration. In 2005, OARS started a multi-year survey to measure the summertime growths of aquatic plants in the Assabet impoundments. With this information, we hope to measure reductions in the mass of aquatic plants since the total phosphorus discharged to the river from the wastewater treatment plants has been reduced. Slow-moving sections of the lower Sudbury River (Fairhaven Bay) and Concord River (North Billerica impoundment) do not have as heavy algal and duckweed growth as in the Assabet, but have sections that are overgrown with invasive water chestnut. Water chestnut has also invaded sections of the Assabet River.

Healthier Tributaries
Water quality in the Assabet headwater (upstream of the first wastewater treatment plant discharge) and in tributary streams of the watershed is generally better than in the mainstem. Median phosphorus concentrations in the tributaries were similar to concentrations in the mainstems, and dissolved oxygen levels are mainly healthy. Total nitrogen concentrations in the tributaries, although lower than in the mainstem, ranged from healthy to somewhat elevated (> 0.75mg/L). Nutrients in the tributaries are mainly from non-point sources such as stormwater runoff from roads and lawns or failing septic systems. The exception is Hop Brook (aka Landham Brook) in Marlborough/Sudbury/ Wayland; this book receives treated effluent from the Marlborough Easterly wastewater treatment plant and has nutrient concentrations and eutrophication problems like the upper Assabet River had before the wastewater treatment plant upgrades. Upgrades to the Marlborough Easterly wastewater treatment plant that discharges to Hop Brook were completed in spring 2015.