Winnicut River Final Report August 2017 Watershed Restoration and Management Plan Prepared by: Prepared for: Acknowledgements This project was funded, in part, by NOAA’s Office for Coastal Management under the Coastal Zone Management Act in conjunction with the NH Department of Environmental Services Coastal Program with additional funding provided through a Watershed Assistance Grant with Clean Water Act Section 319 Funds from the U.S. Environmental Protection Agency, and from the generous contributions from members of the New Hampshire Rivers Council. i Winnicut River Watershed Restoration and Management Plan Contents Introduction....………….….……...………………………………………………………………………….…………………1 Section 1. Winnicut River Water Quality Data .......................................................................................... 3 1.1 Sampling Data and Locations ....................................................................................................... 3 1.2 Dissolved Oxygen ......................................................................................................................... 5 1.3 Bacteria ......................................................................................................................................... 7 1.4 Nutrients ........................................................................................................................................ 9 1.5 Watershed Impervious Surfaces ................................................................................................. 11 1.5 Summary of Water Quality Impairments ..................................................................................... 13 1.6 Tributary Data Review ................................................................................................................ 15 Section 2. Water Quality Goals .............................................................................................................. 16 2.1 Dissolved Oxygen ....................................................................................................................... 16 2.2 Nutrients ...................................................................................................................................... 16 2.2.1 Nitrogen ............................................................................................................................. 17 2.2.2 Phosphorus ........................................................................................................................ 18 2.3 Establishment of Water Quality Goals ........................................................................................ 20 Section 3. Nutrient Load Modeling ........................................................................................................ 22 3.1 Modeling Overview ..................................................................................................................... 22 3.1.1 Subject Area ...................................................................................................................... 22 3.2 Stormwater Load ......................................................................................................................... 24 3.2.1 Hydrologic Response Units ............................................................................................... 24 3.2.2 Impervious Surface Disconnection .................................................................................... 25 3.2.3 Unattenuated Stormwater Load ......................................................................................... 27 3.2.4 Attenuated Stormwater Load ............................................................................................. 2 8 3.3 Groundwater Nitrogen Load ....................................................................................................... 29 3.3.1 Unattenuated Groundwater Nitrogen Load ........................................................................ 29 3.3.2 Attenuated Groundwater Load........................................................................................... 30 3.4 Baseline Total Nitrogen Load Estimates .................................................................................... 31 3.5 Subwatershed Loads .................................................................................................................. 32 3.5.1 Land Use ............................................................................................................................ 34 3.5.2 Total Nitrogen .................................................................................................................... 35 ii 3.5.3 Total Phosphorus ............................................................................................................... 37 3.6 Pristine Load ............................................................................................................................... 39 3.7 Future Impacts ............................................................................................................................ 41 3.7.1 Historic Population and Land Use Trends ......................................................................... 41 3.7.2 Future Population Projections ............................................................................................ 42 3.7.3 Future Developed Land Projections .................................................................................. 43 3.7.4 Projected Future Stormwater Load .................................................................................... 47 Section 4. Watershed Management ....................................................................................................... 49 4.1 Structural Best Management Practices ...................................................................................... 49 4.1.1 Field Watershed Investigation ........................................................................................... 49 4.1.2 Linear Optimization Model for Structural Stormwater Management Practices .................. 80 4.2 Wastewater Management ........................................................................................................... 81 4.2.1 Background ........................................................................................................................ 81 4.2.2 Alternative Treatment Systems.......................................................................................... 83 4.2.3 Recommendations for Wastewater Management ............................................................. 87 4.3 Non-Structural Best Management Practices .............................................................................. 88 4.3.1 Public Information and Education ...................................................................................... 88 4.3.2 Land Conservation ............................................................................................................. 90 4.3.3 Regulatory Tools ................................................................................................................ 94 4.3.4 Institutional Practices and Programs ................................................................................. 98 4.4 Recommended BMP Summary Tables .................................................................................... 100 Section 5. Summary of Technical and Financial Support ................................................................. 104 5.1 Technical Support ..................................................................................................................... 104 5.2 Financial Support ...................................................................................................................... 105 Section 6. Schedule and Interim Milestones ...................................................................................... 108 Section 7. Evaluation Criteria and Monitoring .................................................................................... 110 7.1 Evaluation Criteria ..................................................................................................................... 110 7.2 Monitoring ................................................................................................................................. 112 7.3 Adaptive Management .............................................................................................................. 112 Section 8. References ........................................................................................................................... 113 Appendices A. Structural BMP Cost and Pollutant Reduction Estimates (Stormwater and Culvert Improvements) B. Linear Optimization Model for Cost-Effective Structural Stormwater Management Practices (Task 17), Winnicut River Watershed Restoration and Management Plan iii Figures Figure 1. NHDES OneStop Environmental Monitoring Database - Winnicut River Sampling Locations .... 4 Figure 2. Monthly Average Dissolved Oxygen Concentrations, Winnicut River Nontidal Reach, 2002- 2015 .............................................................................................................................................. 5 Figure 3. Dissolved Oxygen Concentrations, Winnicut River Nontidal Reach, 2001-2015 .......................... 6 Figure 4. Percent of Summer (June-Sept.) DO Measurements Below Class B WQS (5 mg/L), 2001-2015 .....................................................................................................................................................