Evaluation of Probability o

Exceedances of Chemical 

Concentrations

Jeffrey Rosen and Alan Roberson

Presentation to EPA

January 6, 2010
Why Did We Do This Stud

• Atrazine is the most common herbic• Atrazine MCL established in 1991- 3 

– Based on cancer endpoint-70 yr. expo– Annual average of quarterly samples

• New subchronic health endpoints

– 90 day window might need more frequ

monitoring than quarterly

• EPA could revise MCL or monitoring 

through six-year review process
Purpose of the Study

• The purpose of this analysis was to dev

sampling protocol that will result in falsnegative rate which is less than 5%. i.eSampling indicates a mean less than 3.0 

but in reality the mean is greater than 3.

• Similar question for TCT 90 day movin

average level of health effect concern (3μg/l) - how many samples would be reqin any one 90 day moving average

• Evaluate strategies for sampling based o

probabilities calculated
Low Variability

If your Goal is to sample for Peaks th

• Apply Bootstrap analysis to find optimal sam

strategy

• Syngenta has done this. Conclusions high s

with Averages poor success with sampling f

• However we hypothesize that there are relat

between application, rainfall and observed concentrations in RAW WATER (not the casnecessarily with finished water)

• So we could stratify sampling based on thes

and improve our possibilities of finding peak

• Regulatory values would have to be adjuste

to prevent finding everyone in violation
Effects of Variability

0

0.10.20.30.40.50.60.70.80.9

1

0 2 4 6 8

Mass Distribution Function

Value

Mean Mean Mean 

3 ug/L
Area Under the Curve 

High Variability

0

0.10.20.30.40.50.60.70.80.9

1

0 2 4 6 8

Value

Mass Distribution Function Mean 2.5 

Mean ValMean 2.5 

3 ug/L
Area Under the Curve –

Lower Variability

0

0.10.20.30.40.50.60.70.80.9

1

0 2 4 6 8

Value

Mass Distribution Function Mean 2.5 

Mean VaMean 2.5 

3 ug/L
Methods

• TCT calculated using EPA’s seasonal a

annual regression equations

• Standard summary statistics• Shapiro-Wilk Test of Normality• Log transformation approaches normali• Growing Season defined as April 1 – Ju• Used Excel functions to determine prob

of exceeding a threshold value (TDIST Function)
The Data Base

• Collected through two programs: Atrazine M

Program and Volunteer Monitoring Program

• Collected from 135 utilities from 10 states - 

IA, KS, KY, LA, MO, NC, OH, and TX

• Samples collected January 2002 through De

2005

• Raw waters and finished waters• Methods used for sample analysis GC/MS, 

LC/MS methods
Data

0

20406080

100120140

2002 2003 2004 2005

Number of utilities .

0

10002000300040005000

2002 2003 2004 

Number of data points .

Raw waterFinished water

• Number of data points for raw water = 12,1• Number of data points for finished water = 

Note Well: These data are for the utilities of highest cThey are not representative of all the utilities in the N
Data Set Summary Untransform

All Seasons

Parameter

Raw Water Finished W

Endpoint Min. Average Max. Endpoint Min. 

Atrazine

(ug/L)

Mean 0.05 1.46 11.06 Mean 0.05 StandardDeviation 0.02 1.45 15.75

StandardDeviation 0.00 

N 1 26 56 N 1 

TCT 

(ug/L)

Mean 0.20 2.36 17.56 Mean 0.20 StandardDeviation 0.06 2.11 21.90

StandardDeviation 0.03 

N 1 26 56 N 1 
Data Set Summary Log10 

Transformed – All Season

Parameter

Raw Water Finished Wate

Endpoint Min. Average Max. Endpoint Min. A

Atrazine

(ug/L)

Mean -1.30 -0.14 0.91 Mean -1.36 StandardDeviation 0.03 0.40 0.96

StandardDeviation 0.00 

N 1 26 56 N 1 

TCT

(ug/L)

Mean -0.69 0.14 1.11 Mean -0.70 StandardDeviation 0.03 0.33 0.82

StandardDeviation 0.02 

N 1 26 56 N 1 
-1.50-1.00-0.50

0.000.501.00

All seasons Non-growing season Growing 

Log10 atrazine (μg/L) .

25%ile Minimum Mean Maximum 95%il

Log 10 Atrazine Finished Wate

Annual Average

Atrazine MCL 0.477 μg/L (log10)
Log10 Atrazine Finished Wat

– Annual Average

-1.50-1.00-0.50

0.000.501.00

Flowing Static Flowing Static Flowing 

All seasons Non-growing season Growing 

Atrazine (μg/L) .

25%ile Minimum Mean Maximum 95%ile 75%ile

Atrazine MCL 0.477 

μg/L
Log10 TCT Finished Water 

– Annual Average

-1.00-0.50

0.000.501.00

Flowing Static Flowing Static Flowing 

All seasons Non-growing season Growing 

TCT (μg/L) .

25%ile Minimum Mean Maximum 95%ile 75%ile

TCT Level of concern 1.57 

μg/L
Probability of exceeding a 3.0 ug/l Atrazine MCL

0.00000.05000.10000.15000.20000.2500

-1.50 -1.00 -0.50 0.00 0.50

Mean of Log10

(atrazine)

Probability of exceeding MCL 

S

Mean observed finished 

water growing season

Probability of Exceeding Atra

Standard D

fixed at 0

~1⁄2 MCL (.176)
Probability of Exceeding TC

Probability of exceeding TheTCT MCL Raw Water

0.000.050.100.150.200.250.300.35

-1.0 -0.5 0.0 0.5 1.0 1.5 

Mean of Log10 (TCT conc) ug/l

Probability of exceeding MCL

Mean TCT Concentration 

Finished Water Growing season

Standa

fixe
Samples Needed - 95% Confiden

Mean 

≤ 3.0 

μg/L Atrazine

Mean and Standard Deviation Boundaries for Atrazine

0

0.20.40.60.8

1

1.2

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 

Log 10 Mean concentration

Standard Deviation of the log 10 

atrazine concentrations
Sample Size Summary f

preventing false negative
Conclusions and Recommendat

• Results suggest log normal distribution• Model allows exploration of sampling s

achieve 95% confidence that results wilexceed level of concern

• Most utilities will be able to do 4-7 sam• Between 2% and 5% of the utilities will 

require larger sample sizes

• Risk of exceeding level of concern func

mean, sample size and standard deviatio
Possible Implications for Modif

Atrazine regulation

• Establish a sampling plan based on the prior y

the prior three years of data. Determine the mstandard deviation and then define the requiresample size based on slide 15 in this presenta

– Variability is critical

• Any sampling over 4 samples (quarterly) shou

done in the growing season

• Improved sampling could be issued as sampli

guidance

• Need to simplify to a level easily established 

understood by small systems.
Food for Thought

• What is the variability of other regula

contaminants?

– Nitrate, disinfection by-products, arsen

others

• Nitrate is a particular concern-acute health 

endpoint

• Single sample for MCL compliance

• How should we consider sampling 

frequency from a holistic view?
Thank you for your attent

Questions?