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

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

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

Big Picture for Phosphorus

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 Modeling Approach Outcomes1414*14=196millions watershedlakealgae Phosphorus LoadPhosphorus ConcChlorophyll

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