1. Changes in Phosphorus Concentrations and Loads in the Assabet River Following Mandated Reductions in Wastewater Treatment Plant Discharges U.S. Geological Survey New England Water Science Center in cooperation with Massachusetts Dept. of Environmental Protection Division of Watershed Management US Department of Interior US Geological Survey U.S. Department of the Interior U.S. Geological Survey

2. Background: Municipal wastewater discharge to the river TMDL-mandated reductions in total-P concentrations in effluent: Apr-Oct – Reduced 0.75 mg/L 0.10 mg/L Nov-Mar – 1.0 mg/L (no change) Previous studies include field investigations of phosphorus flux from bed sediment, aquatic plant distribution, laboratory column experiments and computer modeling

3. DRAFT Problem Complex system- before and after upgrades would be difficult with conventional monthly sampling Solution Automated, flow- proportional composite sampling and continuous streamflow to accurately estimate of concentrations and loads

4. Objectives: Oct 2008 - Apr 2014 measure changes in phosphorus concentrations and loads in response to mandated reductions in WWTP effluent concentrations. Evaluate the effects of seasonal changes in phosphorus levels. Assess the effects of reservoirs on in-stream fluxes of total and dissolved phosphorus. DRAFT

5. Maynard 01097000 AMVETS 01096840 Port Street 01096835 Stow 01096885 Assabet River monitoring stations Westborough WWTP Marlborough Westerly WWTP Hudson WWTP DRAFT

6. Total-P concentrations at WWTPs decreased significantly following completion of all treatment- plant upgrades in early 2012. Median total-P concentrations in WWTP effluent approached 0.1 mg/L. Similar decreases were observed in concentrations measured at each of the monitoring stations, with the exception of Port Street. DRAFT Annual decrease in TP concentrations

7. Median reported total-P loads decreased significantly following implementation of WWTP upgrades. Median reported post-TMDL WWTP loads were less than 2 kg/day. Statistically significant (p < 0.05) decreases in median total-P loads were observed at all four in-stream monitoring stations. DRAFT Pre- and Post upgrade loads:

8. DRAFT Westboro + Marlboro + Hudson WWTP TP load (kg/day) PrePostChange Maximum15251- 101 Minimum30.5- 2.5 Median236-17 Stow TP load (kg/day) Maynard TP load (kg/day) PrePostChange PrePostChange Maximum402206-196474255- 219 Minimum11012+1 Median2710-17 3113- 18 The change in median reported load, pre- to post- TMDL implementation, for the combined WWTPs was 17 kg/day. Similar changes were observed at the Stow (17 kg/day ) and Maynard (18 kg/day ) monitoring stations. Pre- and Post upgrade loads:

9. Conclusions: Significant decreases in total-P concentrations and loads in WWTP effluent were matched by similar decreases at all four in-stream monitoring stations. No significant seasonal differences were observed after the upgrades. Effects of impoundments on P transport are variable. Continuous monitoring can greatly increase the ability to see the effects of management actions in highly varying systems. DRAFT