Modeling Documentation for the Lake Memphremagog TMDL 4-26-17 i Contents 1 Introduction .............................................................................................................................................................1 2.1 Sources for measured tributary inflows to Lake Memphremagog ..................................................................2 2 Estimating Flow for Tributaries and direct drainages to Lake Memphremagog. ....................................................2 2.2 Modeled or estimated flows ............................................................................................................................2 3 Bathymetry ..............................................................................................................................................................8 4 Estimating Chloride loading to Lake Memphremagog. ........................................................................................ 11 4.1 Sources for chloride loading to Lake Memphremagog ................................................................................. 11 4.2 Castle and Cherry Rivers (5.2 % of watershed) ............................................................................................. 13 4.3 Chloride from Small Quebec Tributaries (8.6% of watershed) ...................................................................... 13 4.4 Unmonitored watersheds (14.5% watershed) .............................................................................................. 14 4.5 Precipitation and Evaporation (5.5% watershed) .......................................................................................... 18 4.6 In-lake Chloride data ..................................................................................................................................... 18 4.7 Chloride loading calibration with Inner Fitch Bay and South Bay. ................................................................ 20 5 Estimating Phosphorus for Tributaries and direct drainages to Lake Memphremagog. ..................................... 23 5.1 Sources for Phosphorus loading to Lake Memphremagog ........................................................................... 23 5.2 Vermont minor tributary loading estimates.................................................................................................. 25 6 Land use phosphorus export model ..................................................................................................................... 27 6.1 Land use export model updates .................................................................................................................... 30 6.2 Septic loading. ............................................................................................................................................... 31 6.3 WWTF loading ............................................................................................................................................... 31 6.4 Stream Channel Instability Phosphorus Load Estimation .............................................................................. 32 6.5 Updating the model land use layer. .............................................................................................................. 34 6.5.1 Roads ...................................................................................................................................................... 34 6.5.2 Developed lands ..................................................................................................................................... 35 6.5.3 Farmstead areas. .................................................................................................................................... 35 6.5.4 Hay and Pasture lands ............................................................................................................................ 36 6.6 Watershed model calibration ........................................................................................................................ 37 6.6.1 Calibration using minor Memphremagog Tributaries ............................................................................ 38 6.6.2 Calibration results .................................................................................................................................. 43 7 Lake Phosphorus data .......................................................................................................................................... 47 7.1 South Bay phosphorus concentrations .......................................................................................................... 49 7.2 Precipitation and Evaporation (5.5% watershed) .......................................................................................... 50 8 Lake Modeling Approach ...................................................................................................................................... 51 8.1 Chloride Exchange between lake segments .................................................................................................. 51 8.2 Phosphorus sedimentation............................................................................................................................ 53 8.3 Model Validation ........................................................................................................................................... 57 9 Scenario tool ......................................................................................................................................................... 58 9.1 Phosphorus estimate from stormwater from future growth ........................................................................ 62 10 References .......................................................................................................................................................... 63 Appendix A. Smi Lake Memphremagog phosphorus export model technical corrections and updates and Lake Retention calculations. ............................................................................................................................................ 65 Appendix B. Quebec Tributary loading estimates .................................................................................................. 74 Appendix C. BMP phosphorus reduction efficiencies used in the Lake Memphremagog Scenario tool ............... 77 ii Appendix D Lake Phosphorus data from MDDEFP transformed to VT lab values (ug/l) ......................................... 82 Appendix E. Source GIS Layers and Excel files used in modeling ........................................................................... 83 List of Figures Figure 1. Regressions between hydrotel discharge and Coaticook or Missisquoi Gauged discharge.......................3 Figure 2. Comparison of measured Johns Discharge vs discharge modeled through hydrotel model or based on drainage area ratio with the Black River. ...................................................................................................................4 Figure 3. Methodology for estimating discharge for tributaries, direct drainages and surface of Lake Memphremagog .........................................................................................................................................................7 Figure 4. Lake Memphremagog lake segmentation used in modeling ......................................................................9 Figure 5. Relationship between chloride concentration and discharge on the Barton River from 2010-2013. ..... 11 Figure 6. Chloride residual on the Barton River based on relationship Cl = 14.603x Q-0.246 and the applied seasonal adjustment factor. ................................................................................................................................................... 12 Figure 7. Estimated chloride levels on the Barton River based on relationship Cl = 14.603x Q-0.246 along with the estimated concentrations with the applied seasonal adjustment factor and measured concentrations. ............. 12 Figure 8. Seasonal adjustment factors applied for the Black, Barton, Clyde and Johns Rivers along with an average which was applied to other tributaries in the watershed. ........................................................................ 13 Figure 9. Mean Chloride Concentration vs Km of paved road per Km2 of watershed showing with Vermont and Quebec watersheds and watersheds seperated between those with ski area development. ............................... 15 Figure 10. Mean Chloride Concentration vs Km of paved road per Km2 of watershed with all watersheds together. .................................................................................................................................................................. 15 Figure 11. Methodology for estimating Chloride for tributaries, direct drainages and surface of Lake Memphremagog ...................................................................................................................................................... 17 Figure 12. Water quality sampling sites in Lake Memphremagog. ........................................................................ 20 Figure 13 A. Measured and modeled chloride concentrations in South and Fitch Bays. ....................................... 21 Figure 13 B. Measured and modeled chloride concentrations in South and Fitch Bays with an adjustment