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

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0.000 0.010 0.020 0.030 0.040

Phosphorus, mg/L

Ch

loro

ph

yll,

mg

/L

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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

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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

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1.0

Re

sp

on

se

Ra

tio

0.00

0.10

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0.50

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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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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