1. Barr-Milton Watershed Modeling Project - Workshop #4 David Pillard, Ph.D. – Project Manager, Ft. Collins, CO Ken Heim, Ph.D. – Lead Modeler, Westford, MA Ken Wagner, Ph.D. – Solutions Support – Willington, CT

2. Key Project Premises Barr Reservoir and Milton Lake experience periodic high pH (>9.0) that violate state standard (<9 for 85% of time) High pH is believed to be a consequence of algal productivity (removal of CO2 during photosynthesis) Algal productivity is controlled mainly by nutrients, light and temperature Nitrogen affects the types of algae present, but phosphorus tends to control the quantity of algae and is the most practical nutrient to control Available phosphorus levels are very high in both waterbodies

3. Derivation of Potentially Protective P Loads

5. Limitations of Empirical Modeling We don’t have site specific data for the low end of the scale We have a reasonable idea of “background” pH We can project how background would be approached with declining TP and the level of associated variability A TP value somewhere near 0.1 mg/L appears most appropriate, but is not a hard threshold

6. Important Things to Remember When Considering Modeling Biological factors are less amenable to modeling and induce variability/uncertainty in predictions Types of algae may matter almost as much as quantity of algae, especially diatom vs. bluegreen dominance Historical and current loads are high; predicting responses at lower loads requires reliance on other systems (prediction outside range of site data) Internal load is likely to compensate for reduced external load Water in Milton appears to be “pre-conditioned” – arrives with higher pH and less margin for compliance Actual data is “spotty” – exercise caution when comparing predictions and observations

7. Watershed Influences Major tributary InletsMajor tributary Inlets Sub-watershed inletsSub-watershed inlets Point SourcesPoint Sources ReservoirsReservoirs Withdrawals/TransfersWithdrawals/Transfers (not shown, very complicated pattern) (not shown, very complicated pattern)

8. Schematic Showing Steps Taken to Complete Watershed-Lake Modeling SWAT = Soil and Water Assessment Tool WASP = Water Analysis Simulation Program

9. Results of Watershed Model Barr Lake Predicted Volume Barr Lake Predicted P Concentration

10. Barr Lake Total Phosphorus Calibration Results

11. Barr Lake Total Chlorophyll Calibration Results

13. Quantify the effects of several management options on Barr Lake/Milton Reservoir water quality. 1.Wastewater – Reduced phosphorus loads from three major dischargers. 2.Refill – Alter the timing and sources of water for Barr Lake infill. 3.Land Use – Alter the effects of agriculture and urbanization. 4.Reservoir Treatment – Reduced internal phosphorus loading. Model Scenarios

14. Total Phosphorus Predictions for Barr Lake

15. Total Chlorophyll Predictions for Barr Lake

16. Total Phosphorus Predictions for Milton Reservoir

17. Total Chlorophyll Predictions for Milton Reservoir

18. Total Phosphorus Predictions for Barr Lake

19. Total Chlorophyll Predictions for Barr Lake

20. Total Phosphorus Predictions for Milton Reservoir

21. Total Chlorophyll Predictions for Milton Reservoir

22. Modeling Implications for Compliance Strategies The lower the WWTP output P level, the better for the reservoirs, but not necessarily proportionally Addressing P from Metro and Littleton/Englewood WWTP is essential Internal loading must be addressed Unless WWTP and internally loaded P are both addressed, compliance is unlikely to be achieved through P control There is enough loading from other sources to prevent achievement of “pristine” conditions Have to think in terms of distribution of values; not just mean/median, but shape of upper “tail”

23. In-Lake Strategies for Lowering pH P inactivation – reducing available P with aluminum sulfate additions to lakes (also directly lowers pH) Mixing/aeration – would avoid localized high pH with algal blooms, may disrupt blooms (esp. blue-greens), encourages CO2 transfer Biomanipulation – fosters zooplankton that minimize algal biomass; usually involves altering the fish community Algaecides – directly kills algae when too abundant, could limit maximum pH

24. Towards a TMDL and Management Plan A few more combination scenarios might be worthwhile (e.g., WWTP @ 1.0 ppm and 70+% internal load reduction, or other water management options) Pilot testing of mixing or reduced P in limnocorrals in the reservoirs might be useful to define direct pH response Defining goals in terms of use attainability would be appropriate, with comparison to WQ standards (e.g., when and why should the pH standard be met, based on water uses?)

25. Questions and Comments