In 1992, OAR started a testing water quality to evaluate the impact of wastewater treatment plant upgrades completed in the late 1980s. OAR volunteers have tested water quality every summer since 1992. Over the years, we have updated our methods, improved our data handling, and increased the number of sampling sites. In April 2000 the EPA approved OAR’s sampling methods as documented in the program’s Quality Assurance Program Plan (QAPP). In 2002, OAR added streamflow and water quality monitoring on the major tributaries to the Assabet River with the StreamWatch Project.

Today OAR tests water quality at 15 mainstem sites distributed from the headwaters of the Assabet River in Westborough to the end of the Concord River in Lowell. (View map) Water quality data and reports are available below and on the StreamWatch page (for each tributary stream).

During the last decade OAR's monitoring program has helped to raise awareness of the Assabet's nutrient problem, pointed to the need for stricter phosphorus limits in the wastewater treatment plant's NPDES permits, made a strong case for the Total Maximum Daily Loading (TMDL)  study, and indicated the need for a groundwater model of the Assabet watershed. Water quality and flow data collected under OAR's EPA-approved QAPP may be used by EPA and DEP in making regulatory decisions.

Nutrient Saturation in the Mainstem Assabet
High concentrations of both phosphorus and nitrogen compounds act like fertilizer in the river, contributing to the overgrowth of aquatic plants and algae. OAR's water quality data supports the conclusion that the mainstem Assabet is nutrient saturated - that neither phosphorus nor nitrogen concentrations limit the growth of aquatic plants in the river.

In the upper sections of the river (where dilution of the wastewater treatment plant effluent by natural flow is the least) nutrient concentrations are well above the thresholds for eutrophication for both phosphorus and nitrogen species. In the lower reaches of the river (below Rte 62 in Stow) nutrient concentrations, although still elevated, are lower than in the upstream sections. Downstream concentrations may be lower

because: (1) the proportion of natural flow to effluent is larger (more tributaries have joined the mainstem); (2) nutrients are taken up by aquatic plants during the growing season; (3) and particle-bound nutrients are deposited to the sediments in the slow-moving river sections.

To support fish and other aquatic life dissolved oxygen concentrations in the river need to be above 5.0 mg/L or 60% saturation (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 at sites all along the river fall below 5.0 mg/L at times during the summer.

Eutrophied Impoundments
The slow-moving river sections behind dams along the river (called "impoundments") show the effects of eutrophication more severely than free-flowing sections of the river. The impoundments have heavier rooted aquatic plant growth and duckweed accumulations, lower minimum daily DO concentrations, and larger daily changes in DO concentration.

Water quality measurements can be compared with:

• Water quality standards set by the state: Massachusetts DEP’s Class B Water Quality Standards (DEP site) and the Massachusetts 2002 Integrated List of Waters (DEP site).
• Data collected by US EPA in rivers in the same ecoregion; i.e. “reference conditions” (EPA site); nutrient concentrations less than the 25th percentiles, listed below, would be considered unimpaired.
• Water quality recommendations for maintaining healthy fish habitat (see the StreamWatch Stream Health Index. )