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)

Plant biomass: In 2005, OAR started monitoring the growth of aquatic plants in the river to be ready to measure changes (hopefully for the better) in the mass of plants in the river as upgrades are made to the watershed's wastewater treatment plants. This work is sponsored in-part by a grant from the Massachusetts Environmental Trust.

Water quality data and reports are available below and on the StreamWatch page (for each tributary stream).

What do we test?

OAR's monitoring program focuses on a set of interrelated chemical and physical parameters that can be directly measured in the river. These parameters are also among those used by the Mass DEP for setting water quality standards and regulating discharge permits for wastewater discharge (NPDES permits).

Parameters:

• Dissolved Oxygen
• Phosphorus (total and dissolved)
• Nitrogen (nitrate, ammonia, total Kjeldahl nitrogen)
• Solids (total suspended)
• pH
• Water temperature
• Streamflow

General Findings:

Nutrient Saturation in the Mainstems
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 Assabet (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. Nutrient concentrations at the three Concord River mainstem sites tested are generally lower than upstream concentrations, but still exceeded the thresholds for eutrophication. 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 dissolved oxygen concentrations, and larger daily changes in dissolved oxygen concentration. In 2005, OAR started a multi-year survey to measure the summertime growths of aquatic plants in the impoundments. With this information, we hope to measure reductions in the mass of aquatic plants as total phosphorus discharged to the river from the wastewater treatment plants is reduced as the treatment plants are upgraded.

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 were lower in the tributaries than in the Assabet or Concord 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.

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. )