OARS River Log | By Suzanne Flint | Published May, 2008

In July 1999, the Commonwealth of Massachusetts and the US Army Corps of Engineers funded ENSR, a private consulting company, to carry out a nutrient TMDL study of the Assabet. A TMDL study is designed to assess how much pollution a water body can assimilate without exceeding state water quality standards. In 1999 and 2000, ENSR conducted intensive field sampling of the river to better define the river’s eutrophication problem. The data collected was then used by ENSR to develop a watershed and water quality model of the Assabet River using the HSPF v10 application. After calibration and validation, the model evaluated multiple future growth and remediation scenarios with varying point and non-point source inputs.

In 2004, the Massachusetts Department of Environmental Protection (MassDEP) issued the final TMDL report (also available through Mass.gov report number MA82B-01-2004-01) concluding that the Assabet is “nutrient-saturated” and that significant decreases in the concentrations of nutrients in the river would be needed to improve water quality. The findings confirm that the majority of the nutrients entering the river come from the publicly-operated wastewater treatment works (POTWs) that discharge treated effluent into the river. In particular, treatment plants are the primary source of ortho-phosphorus (the bioavailable form of phosphorus) throughout the year. Non-point sources (such as stormwater and sediment) contribute nutrients but significantly less than the point sources over the growing season. The report concludes that the principal non-point source during the growing season is phosphorus flux from the sediments.

Nutrient Loading From Point Sources

* point sources, the four major WWTPs: Westborough, Marlborough, Hudson. Adapted from ENSR 2001.

With these findings in mind, the MassDEP’s final TMDL report proposes a two-phased adaptive management plan for the Assabet River:

• • “Phase 1 will establish POTW effluent total phosphorus limits of 0.1 mg/l at all major POTWs discharging to the Assabet River and allow the communities sufficient time to fund and implement a detailed evaluation of impoundment sediment as a potential alternative to lower permit limits. DEP believes that some sediment and/or dam removal options will allow the Assabet River to achieve water quality standards faster and, possibly, be more cost-effective than establishing lower POTW total phosphorus limits and waiting for the system to respond over time.

• • Requirements were incorporated into the NPDES permits developed and issued in 2004. Phase 1 required that all POTWs be upgraded to achieve 0.1 mg/l of effluent phosphorus by April 2009 and that the design be consistent with adding new technology in the future to achieve further reductions if deemed necessary. Based on the modeling results, current permitted flows will be allowed. However, any request to increase a discharge beyond currently permitted volumes would require supporting documentation satisfying DEP’s Antidegradation Policy that no other feasible alternative exists, including, but not limited to, the discharge of additional treated effluent to groundwater to help restore tributary flows. Phosphorus limits will be seasonal. DEP and EPA jointly developed an implementation strategy in the Spring of 2008 to decide if, when, and to what level additional upgrades were needed based on the results and recommendations of the sediment evaluation.

• • Phase 2 limitations will be established in permits to be reissued in 2009 if sediment remediation, based upon the sediment/dam evaluation results, is not pursued and/or new phosphorus criteria that may be developed in the interim by DEP and US EPA are applicable. If the communities pursue sediment remediation alternatives, a revised schedule and work plan will be negotiated in the summer and fall of 2008. If the communities choose not to pursue sediment remediation alternatives, they must complete phase 2 improvements during the second 5-year permit cycle, begin operating by April 2013, and achieve the new limits by April 2014.

• • In the interim, prior to facility upgrades in 2009, the POTWs will be required to continue optimizing the seasonal removal of total phosphorus in their effluents to meet the 2000 interim NPDES permit limits of 0.75 mg/l.

Long-term monitoring of the Assabet River is essential to determining the efficacy of the adaptive management controls as they are implemented, whether water quality standards have been achieved, and whether additional source controls will be required. EPA and DEP will develop a detailed monitoring plan prior to implementing Phase 1 upgrades. The agencies or their agents will implement the plan with assistance from the Assabet communities to evaluate and document water quality improvements and environmental indicators after POTW upgrades are completed during Phase 1.

This TMDL can be achieved through the continued cooperation, effort, and oversight of federal, state, and municipal agencies and the watershed stakeholders.”

UPDATES:

• 2012: As of 2012, all four municipal wastewater treatment plants that discharge to the Assabet River completed upgrades to meet the total phosphorus effluent limits set out in Phase 1.