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Organization for the Assabet River
Damonmill Square
Concord, MA 01742

Tel: 978-369-3956
Email: oar@assabetriver.org



Water Quality

Assabet River near the Aqueduct in Northborough
Reports What do we test General Findings How to interpret the data
OAR's Water Quality Monitoring Program

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.

Bench-top chemistry - 1992

WQ Program Reports
Right click to download OAR's complete water quality reports (in pdf format): 

1992 Report (100KB)   1992 - 1997 Data (76KB)
1999 Report (500KB) 1999 Data Appendix (86KB)
2000 Report (500KB)  2000 Data Appendix (114KB)
2001 Report(2.0 MB) 2001 Data Appendix (2.3MB)
2002 Report (2.2MB) 2002 Data Appendix (96KB)
2003 Report (1.3 MB) 2003 Data Appendix II (68 KB)
2003 Data Appendix III (75 KB)

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).
Dissolved Oxygen
Phosphorus (total and ortho-)
Nitrogen (NO3, NH3, and TKN)
Solids (total suspended)
Water temperature

General Findings:
Stream-side chemistry - 2001

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.

Sampling in the Powdermill Impoundment (don't try this at home)
Healthier Tributaries
Water quality, as measured in 2002, in the Assabet headwater (upstream of the first wastewater treatment plant discharge) and in tributary streams of the watershed was generally better than in the mainstem. Phosphorus concentrations were lower than in the mainstem, and dissolved oxygen levels were mainly healthy. The only problems that we observed in 2002 were high phosphorus and low dissolved oxygen concentrations measured at two sites just downstream of beaver dams. 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.

How to interpret the data

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. )
Massachusetts Standards
Parameter  Standard
Dissolved oxygen* >= 5.0 mg/l and 60% saturation for warm water fisheries
>= 6.0 mg/l and 75% saturation for cold water fisheries
pH* 6.5-8.3 for inland waters
Nutrients* "control cultural eutrophication"
Temperature* <= 28.3 C and less than 2.8 C deviation for warm water fisheries
<= 20.0 C and less than 1.7 C deviation for cold water fisheries
Suspended Solids**  Aquatic life: 25 mg/L maximum and less than 10 mg/l increase due to a discharge
Primary or secondarily contact recreational use: no nuisance organisms that render the water aesthetically objectionable or unusable, "best professional judgment"; cover of macrophytes < 50% within any portion of the lake area at maximum extent of growth.
Fecal coliforms** Primary contact recreational use -- Dry weather guidance: (<5 samples taken) <= 400 cfu/100ml. Wet weather guidance: dry weather samples meet and wet samples <=2000 cfu/100ml.
Secondary contact recreational use -- Dry weather guidance (< 5 samples taken) <=2000 cfu/100ml. Wet weather guidance: dry weather samples meet and wet samples <= 4000 cfu/100ml.
*MADEP 1997 Massachusetts Surface Water Quality Standards - 314 CMR 4.00 1997.
** MADEP 2002 Massachusetts Year 2002 Integrated List of Waters, Part 1.

Reference Conditions for Ecoregion XIV Subregion 59 Streams*
Reference condition
(25th percentile of June - September data)
Total Phosphorus (mg/L)
Total Nitrogen (calculated) (mg/L)
NO2 + NO3 (mg/L)
TKN (mg/L)
* adapted from USEPA. 2000. Ambient Water Quality Criteria Recommendations: Rivers and Streams in Nutrient Ecoregion XVI. EPA 822-B-00-022. United States Environmental Protection Agency: Office of Water, Office of Science and Technology, Health and Ecological Criteria Division. Washington, D.C. December 2000.


Page updated: January 2005 (send comments to sflint@assabetriver.org)