estimating allowable phosphorus load to chatfield reservoir
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Estimating Allowable Phosphorus Load to Chatfield Reservoir. Jim Saunders WQCD Standards Unit 10 April 2008. Roadmap for Technical Review. Today. Components and problems Basis for modeling Estimates of allowable load Options for standards. Some Assembly Required…. Standards and goals - PowerPoint PPT PresentationTRANSCRIPT
Estimating Allowable Phosphorus Load to Chatfield Reservoir
Jim SaundersWQCD Standards Unit10 April 2008
Roadmap for Technical Review
Month Topic
Sep-07 Technical comparison of existing control regulations
Oct-07 Existing chlorophyll target, incl magnitude, frequency, duration
Nov-07 Evaluation and discussion of concentration translator
Dec-07 Water budget and appropriate concentrations for each flow source as precursor to common set of phosphorus loads
Jan-08 Phosphorus load estimates; produce common set by source
Feb-08 Evaluation and discussion of load translator
Mar-08 Hydrologic considerations for TMAL
Apr-08 Discuss chlorophyll-phosphorus-load linkages as basis for proposal
Jun-08 WQCD to finalize proposal and circulate
Jul-08 Notice due
Nov-08 WQCC 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
0
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0.000 0.010 0.020 0.030 0.040
Phosphorus, mg/L
Ch
loro
ph
yll,
mg
/L
0
5
10
15
20
25
30
0.000 0.020 0.040 0.060 0.080 0.100 0.120
Phosphorus, mg/L
Ch
loro
ph
yll,
mg
/L
Big Picture for Phosphorus
0
5
10
15
20
25
30
0.000 0.020 0.040 0.060 0.080 0.100 0.120
Phosphorus, mg/L
Ch
loro
ph
yll,
mg
/L
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….
0.0
0.1
0.2
0.3
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0.5
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0.8
0.9
1.0
Re
sp
on
se
Ra
tio
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
Ph
os
ph
oru
s R
ete
nti
on
Deterministic Modeling Approach
watershed lake algae
Phosphorus Load Phosphorus Conc Chlorophyll
Inflow Volume
Inpu
t C
on
c
Hydraulics
P R
ete
ntio
n
Phosphorus
Ch
loro
ph
yll
Outcomes 1 1 1
Chlorophyll
Freq
uenc
y
Datum
Probabilistic Modeling Approach
Outcomes 14 14*14=196 millions
watershed lake algae
Phosphorus Load Phosphorus Conc Chlorophyll
Inflow Volume
Inpu
t C
onc
Hydraulics
P R
ete
ntio
n
Phosphorus
Ch
loro
phyl
l
Chlorophyll
Fre
quen
cy
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)
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5
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Option 1 Option 2
Ch
loro
ph
yll,
ug
/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