Mirror Lake Watershed Management Plan May 2012 Prepared For: Mirror Lake Protective Association P.O. Box 311 Mirror Lake, NH 03853 Prepared By: 289 Great Road, Suite 105 Acton, MA 01720 www.geosyntec.com Contents EXECUTIVE SUMMARY ...................................................................................................................... 3 1. INTRODUCTION ...................................................................................................................... 6 2. LITERATURE REVIEW AND EXISTING WATER QUALITY .......................................................... 8 2.1 Water Quality Data ................................................................................................................................... 8 2.2 Trophic Status Assessment ......................................................................................................................... 11 2.3 Cyanobacteria Data ................................................................................................................................. 14 2.4 Assimilative Capacity ................................................................................................................................ 15 2.5 Summary of Mirror Lake Water Quality .............................................................................................. 16 3. MIRROR LAKE PHOSPHORUS BUDGET ................................................................................ 17 3.1 Land-Use Based Pollutant Modeling ...................................................................................................... 17 3.2 Phosphorus Loading From Septic Systems ............................................................................................. 21 3.3 Internal Phosphorus Loading .................................................................................................................... 22 3.4 Atmospheric Deposition ............................................................................................................................. 22 3.5 Wolfeboro Wastewater Treatment Plant Spray Fields ..................................................................... 22 3.6 Future Conditions Analysis ........................................................................................................................ 25 3.7 Summary of the Mirror Lake Phosphorus Loading Budget ................................................................ 26 4. MIRROR LAKE HYDROLOGIC BUDGET ................................................................................. 28 5. MIRROR LAKE PHOSPHORUS CONCENTRATION MODELING ............................................. 30 5.1 Vollenweider Model .................................................................................................................................. 30 5.2 Nürnberg Model ........................................................................................................................................ 32 5.3 Monte Carlo Simulation ............................................................................................................................ 34 5.4 Nürnberg Modeling Scenarios and Water Quality Goal ................................................................. 37 5.5 Additional Modeling: Dynamic Mass Balance Model ......................................................................... 49 5.6 Summary of Phosphorus Concentration Modeling Results .................................................................. 54 6. ACTION PLAN FOR REDUCING PHOSPHORUS LOADING TO MIRROR LAKE ...................... 55 6.1 Storm Water Management ...................................................................................................................... 55 6.2 Potential Community Septic Systems Locations .................................................................................... 68 6.3 Land Conservation Measures ................................................................................................................... 72 6.4 Summary of Proposed Action Plan to Reduce Phosphorus Loading ................................................. 72 7. SUMMARY OF TECHNICAL AND FINANCIAL SUPPORT ....................................................... 75 7.1 Technical Support ....................................................................................................................................... 75 7.2 Financial Support ....................................................................................................................................... 75 8. PUBLIC INFORMATION AND EDUCATION ........................................................................... 78 9. SCHEDULE AND INTERIM MILESTONES ................................................................................ 79 10. EVALUATION CRITERIA AND MONITORING ....................................................................... 81 Funding for this project was provided by a grant from the New Hampshire Department of Environmental Services with funding from the United States Environmental Protection Agency under Section 319 of the Clean Water Act, the Tuftonboro Conservation Commission and the Mirror Lake Management Plan stakeholders. 1 FIGURES Figure 1: Mirror Lake Watershed Boundary .................................................................................................................................... 7 Figure 2: Historic Sampling Locations and NHDES 2010 Bathymetry .......................................................................................... 9 Figure 3: UNH LLMP Epilimnetic Total Phosphorus Data ................................................................................................................. 8 Figure 4: NHDES 2010 Epilimnetic Phosphorus Data .................................................................................................................... 10 Figure 5: Carlson Trophic State Index for Mirror Lake ................................................................................................................. 12 Figure 6: Summary of Mirror Lake Cyanobacteria Sampling, 2007-2010 .............................................................................. 15 Figure 7: NHDES OneStop Environmental Monitoring Database Epilimnetic Phosphorus Data ............................................. 15 Figure 8: Land Use - Mirror Lake Watershed ................................................................................................................................ 19 Figure 9: Impervious Surfaces - Mirror Lake Watershed ............................................................................................................. 20 Figure 10: Mirror Lake Phosphorus Budget ....................................................................................................................................... 27 Figure 11: Area-Discharge Relationship for New England USGS Stream Gages (<3000 acres) .......................................... 29 Figure 12: Nurnberg Model Results vs. Observed Epilimnion Data for Year 2011 .................................................................. 34 Figure 13: Box/Whisker Plots Representing Monte-Carlo Results for Current Conditions ........................................................ 36 Figure 14: Nürnberg pepi Monte Carlo Results Scatter-Plot and Linear Regression ................................................................ 36 Figure 15: Current Conditions and Year 2030 Buildout Conditions Box/Whisker Plots ........................................................... 37 Figure 16: Current and Future Conditions Compared to Water Quality Goal .......................................................................... 48 Figure 17: Modeled epilimnetic and hypolimnetic concentrations, 1970 to 2060, assuming spray field loads removed in 2010 and no new development in watershed .......................................................................................................... 51 Figure 18: Modeled epilimnetic and hypolimnetic concentrations for 2010, and observed epilimnetic and hypolimnetic Concentrations ............................................................................................................................................. 51 Figure 19: Modeled epilimnetic and hypolimnetic concentrations for 1990-2010, and observed epilimnetic concentrations ...................................................................................................................................................................... 52 Figure 20: Modeled epilimnetic and hypolimnetic concentrations, 1970 to 2060, assuming spray field loads removed in 2010 and future conditions buildout continues to 2030 ......................................................................................... 53 Figure 21: Modeled epilimnetic and hypolimnetic concentrations, 1970 to 2060, assuming spray field loads removed in 2010 and septic system loads removed in 2030 ..................................................................................................... 53 Figure 22: Locations of Proposed BMPs ............................................................................................................................................. 56 Figure 23: Potential Community Septic System Regions .................................................................................................................. 70 Figure 24: NRCS Hydrologic Soil Classification-Mirror Lake Watershed.....................................................