CECEvaluation of Probability ofExceedances of ChemicalConcentrationsJeffrey Rosen and Alan RobersonPresentation to EPAJanuary 6, 2010CECWhy Did We Do This Study?• Atrazine is the most common herbicide• Atrazine MCL established in 1991- 3μg/l– Based on cancer endpoint-70 yr. exposure– Annual average of quarterly samples• New subchronic health endpoints– 90 day window might need more frequentmonitoring than quarterly• EPA could revise MCL or monitoringthrough six-year review processCECPurpose of the Study• The purpose of this analysis was to develop asampling protocol that will result in falsenegative rate which is less than 5%. i.e.Sampling indicates a mean less than 3.0μg/lbut in reality the mean is greater than 3.0μg/l• Similar question for TCT 90 day movingaverage level of health effect concern (37.5μg/l) - how many samples would be requiredin any one 90 day moving average• Evaluate strategies for sampling based on theprobabilities calculatedCECLow VariabilityCECCECIf your Goal is to sample for Peaks then.....• Apply Bootstrap analysis to find optimal samplingstrategy• Syngenta has done this. Conclusions high successwith Averages poor success with sampling for Maxima• However we hypothesize that there are relationshipsbetween application, rainfall and observedconcentrations in RAW WATER (not the casenecessarily with finished water)• So we could stratify sampling based on these factorsand improve our possibilities of finding peaks• Regulatory values would have to be adjusted upwardsto prevent finding everyone in violationCECEffects of Variability00.10.20.30.40.50.60.70.80.910 2 4 6 8

Mass Distribution FunctionValueMean 2.5 sd=0.75Mean ValueMean 2.5 sd = 0.553 ug/LCECArea Under the CurveHigh Variability00.10.20.30.40.50.60.70.80.910 2 4 6 8Value

Mass Distribution FunctionMean 2.5 sd=0.75Mean ValueMean 2.5 sd = 0.553 ug/LCECArea Under the Curve –Lower Variability00.10.20.30.40.50.60.70.80.910 2 4 6 8Value

Mass Distribution FunctionMean 2.5 sd=0.75Mean ValueMean 2.5 sd = 0.553 ug/LCECMethods• TCT calculated using EPA’s seasonal andannual regression equations• Standard summary statistics• Shapiro-Wilk Test of Normality• Log transformation approaches normality• Growing Season defined as April 1 – June 30• Used Excel functions to determine probabilityof exceeding a threshold value (TDISTFunction)CECThe Data Base• Collected through two programs: Atrazine MonitoringProgram and Volunteer Monitoring Program• Collected from 135 utilities from 10 states - IL,IN,IA, KS, KY, LA, MO, NC, OH, and TX• Samples collected January 2002 through December2005• Raw waters and finished waters• Methods used for sample analysis GC/MS, IA, andLC/MS methodsCECData0204060801001201402002 2003 2004 2005

Number of utilities .0100020003000400050002002 2003 2004 2005

Number of data points .Raw waterFinished water• Number of data points for raw water = 12,149• Number of data points for finished water = 12,088Note Well: These data are for the utilities of highest concern.They are not representative of all the utilities inthe Nation.CECData Set Summary Untransformed –All SeasonsParameterRaw WaterFinished WaterEndpoint Min. Average Max. Endpoint Min. Average MaxAtrazine(ug/L)Mean 0.05 1.46 11.06 Mean 0.05 0.87 6.65StandardDeviation 0.02 1.45 15.75StandardDeviation 0.00 0.90 11.20N 1 26 56 N 1 26 46TCT(ug/L)Mean 0.20 2.36 17.56 Mean 0.20 1.44 9.61StandardDeviation 0.06 2.11 21.90StandardDeviation 0.03 1.30 15.52N 1 26 56 N 1 26 46CECData Set Summary Log10Transformed – All SeasonsParameterRaw Water Finished WaterEndpoint Min. Average Max. Endpoint Min. Average Max.Atrazine(ug/L)Mean -1.30 -0.14 0.91 Mean -1.36 -0.40 0.57StandardDeviation 0.03 0.40 0.96StandardDeviation 0.00 0.38 0.95N 1 26 56 N 1 26 46TCT(ug/L)Mean -0.69 0.14 1.11 Mean -0.70 -0.07 0.77StandardDeviation 0.03 0.33 0.82StandardDeviation 0.02 0.30 0.78N 1 26 56 N 1 26 46CEC-1.50-1.00-0.500.000.501.00All seasons Non-growing season Growing season

Log10 atrazine (μg/L) .25%ileMinimumMeanMaximum95%ile75%ileLog 10 Atrazine Finished Water –Annual AverageAtrazine MCL 0.477μg/L (log10)CECLog10 Atrazine Finished Water– Annual Average-1.50-1.00-0.500.000.501.00Flowing Static Flowing Static Flowing StaticAll seasonsNon-growing seasonGrowing season

Atrazine (μg/L) .25%ileMinimumMeanMaximum95%ile75%ileAtrazine MCL 0.477μg/LCECLog10 TCT Finished Water– Annual Average-1.00-0.500.000.501.00Flowing Static Flowing Static Flowing StaticAll seasons Non-growing season Growing season

TCT ( μ g/L) .25%ileMinimumMeanMaximum95%ile75%ileTCT Level of concern 1.57μg/LCECProbability of exceeding a 3.0 ug/l Atrazine MCL0.00000.05000.10000.15000.20000.2500-1.50 -1.00 -0.50 0.00 0.50Mean of Log10(atrazine)

Probability of exceeding MCL4.05.06.07.0Number ofSamples

Mean observed finished water growing seasonProbability of Exceeding AtrazineStandard Deviationfixed at 0.50~1⁄2 MCL(.176)CECProbability of Exceeding TCTProbability of exceeding TheTCT MCL Raw Water0.000.050.100.150.200.250.300.35-1.0 -0.5 0.0 0.5 1.0 1.5 2.0Mean of Log10 (TCT conc) ug/l

Probability of exceeding MCL4567numberofsamples

Mean TCT Concentration Finished Water Growing seasonStandard Deviationfixed at 0.50CECSamples Needed - 95% Confident thatMean≤3.0μg/L AtrazineMean and Standard Deviation Boundaries for Atrazine00.20.40.60.811.2-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6Log 10 Mean concentration

Standard Deviation of the log 10 atrazine concentrationsn=4n=5n=6n=7n=8n=10n=15n=25n=30CECSample Size Summary forpreventing false negativesCECConclusions and Recommendations• Results suggest log normal distribution• Model allows exploration of sampling sizes toachieve 95% confidence that results will notexceed level of concern• Most utilities will be able to do 4-7 samples• Between 2% and 5% of the utilities willrequire larger sample sizes• Risk of exceeding level of concern function ofmean, sample size and standard deviationCECPossible Implications for ModifyingAtrazine regulation• Establish a sampling plan based on the prior yearorthe prior three years of data. Determine the mean andstandard deviation and then define the requiredsample size based on slide 15 in this presentation.– Variability is critical• Any sampling over 4 samples (quarterly) should bedone in the growing season• Improved sampling could be issued as samplingguidance• Need to simplify to a level easily established andunderstood by small systems.CECFood for Thought• What is the variability of other regulatedcontaminants?– Nitrate, disinfection by-products, arsenic, andothers• Nitrate is a particular concern-acute healthendpoint• Single sample for MCL compliance• How should we consider samplingfrequency from a holistic view?CECThank you for your attentionQuestions?