Estimating Allowable Phosphorus Load to Chatfield Reservoir

1. Estimating Allowable Phosphorus Load to Chatfield Reservoir Jim Saunders WQCD Standards Unit 10 April 2008

2. Roadmap for Technical Review

3. Today • Components and problems • Basis for modeling • Estimates of allowable load • Options for standards

4. Some Assembly Required… • Standards and goals • Concentration translator (chl-phos) • Load translator • Input concentration • Retention coefficient • Hydrologic scenario

5. What’s “broken”? • Chlorophyll goal met consistently • Phosphorus standard is not • They’re supposed to be hard-wired • Is phosphorus irrelevant or is the expectation (straight line) wrong?

6. Real Issue: too Great Expectations • We expect phosphorus to be a perfect predictor of chlorophyll – to explain all variation in chlorophyll • Based on capacity to explain variation among lakes • It doesn’t – much variation is not explained by phosphorus alone • In one lake, variation among years comes from many factors

7. Big Picture for Phosphorus

8. Phosphorus in Lakes • Nutrient enrichment causes excessive algal abundance • Chlorophyll-phosphorus data from many lakes show strong pattern • In most lake restorations, reducing phosphorus reduces chlorophyll • In case of non-attainment, focus on phosphorus simplifies implementation • TMDL development • WQBEL determination

9. Back to Chatfield Data:Distillate or Stew? • Begin with all data • Extract essence of chlorophyll-phosphorus relationship with linear regression (ignore unexplained variation) • Or, throw all data in the pot and stir well. Assume that any sample equally representative (retain all variation) • How strong are predictors?

10. Defining What Is Known • Responsiveness of algae to phosphorus captured in each sample (chl:TP) • Retention coefficient measured each year • Create set of all values observed in Chatfield • Assume each measured value equally likely to occur next year or years after….

11. Datum watershed lake algae Phosphorus Load Phosphorus Conc Chlorophyll Retention Deterministic Modeling Approach

12. watershed lake algae Phosphorus Load Phosphorus Conc Chlorophyll Retention Probabilistic Modeling Approach

13. Probabilistic Model • 1 hydrologic scenario • 14 input conc.; random draw • 14 retention coeff; random draw • Yields 196 “years” of in-lake summer TP conc [=input*(1-R)] • Draw 6 response ratios from set of 83 and take average (millions) • Match summer TP with ratio at random, 10,000 times • Examine distribution of chlorophyll • Adjust input concentration and repeat

14. What’s the Allowable Load?Assume 1-in-5-yr exceedance frequency • Option 1: 13,655 lbs/y at median inflow • Retain the existing phosphorus standard (0.027 mg/L) • Reduce chlorophyll standard (11 ug/L) • Defend existing water quality conditions • Option 2: 21,438 lbs/y at median inflow • Preserve existing chlorophyll standard (17 ug/L) • Accept a relaxed phosphorus standard (0.042 mg/L)

15. Additional Changes and Clarifications in Regulation 38 • At least 3 samples from summer months (Jul-Sep) • Samples must be representative of the mixed layer • The allowable exceedance frequency is once in 5 years • The intent of the phosphorus standard is to ensure attainment of the chlorophyll standard

16. Tasks to be Addressed Later,If Commission adopts Division proposal • Partitioning of allowable load between the two main basins (South Platte and Plum Creek • Allocation of loads within each basin according to the usual format of TMDLs = LA+WLA+MOS • Review of WLAs as appropriate

17. Next Steps • Discuss relative merits of regulatory options; select one for proposal • Continue discussing technical issues • Meet with Board to outline process and progress • Circulate draft proposal