Presentation on theme: "Estimating Allowable Phosphorus Load to Chatfield Reservoir Jim Saunders WQCD Standards Unit 10 April 2008."— Presentation transcript:
Estimating Allowable Phosphorus Load to Chatfield Reservoir Jim Saunders WQCD Standards Unit 10 April 2008
Roadmap for Technical Review MonthTopic Sep-07Technical comparison of existing control regulations Oct-07Existing chlorophyll target, incl magnitude, frequency, duration Nov-07Evaluation and discussion of concentration translator Dec-07Water budget and appropriate concentrations for each flow source as precursor to common set of phosphorus loads Jan-08Phosphorus load estimates; produce common set by source Feb-08Evaluation and discussion of load translator Mar-08Hydrologic considerations for TMAL Apr-08Discuss chlorophyll-phosphorus-load linkages as basis for proposal Jun-08WQCD to finalize proposal and circulate Jul-08Notice due Nov-08WQCC RMH
Today Components and problems Basis for modeling Estimates of allowable load Options for standards
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?
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
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
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?
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….
Deterministic Modeling Approach watershedlakealgae Phosphorus LoadPhosphorus ConcChlorophyll Outcomes111 Datum
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
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)
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
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
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