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Roadmap for Technical Review. Today. Components and problemsBasis for modelingEstimates of allowable loadOptions for standards. Some Assembly Required
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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. Deterministic Modeling Approach
12. 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