Measuring and Estimating Concentrations of 

Pesticides in Drinking Water: 

A Historical Perspective

Presentation to the

Exposure Modeling Public Meeting 

March 20, 2012
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Outline

 Introduction

 Monitoring versus Standard Risk Assessment

 Most Vulnerable Watersheds

 Historical Approaches to Risk Assessments

 Potential for Refined Risk Assessments

March 20, 2012
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Introduction

 The Food Quality Protection Act (1996) requires 

risk assessments to include the contribution of 

pesticide residues in drinking water to dietary 

assessments. 

 Triggered work by EFED to define both modeling 

and monitoring assessment procedures

By the late 1990’s EPA had developed (sometimes 

informal) guidelines for performing screening 

assessments and conducting monitoring studies.

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Introduction

 While drinking water assessments cover both 

ground and surface water, this presentation will 

address only surface water.

 The initial screening assessments were 

intentionally conservative.

 Unfavorable screening assessments created the 

need for refined assessments and monitoring.

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Introduction

 Industry initiated drinking water monitoring studies 

to provide realistic exposure estimates for risk 

assessment.

 Initial designs modified as a result of ILSI and 

WARP projects 

 EPA also refined drinking water exposure 

assessment procedures.

 Index reservoir and PCA (percent crop area) 

introduced

 Probabilistic assessments based on daily values 

estimated using 30 years of weather data 

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Introduction

 The drinking water exposure assessment 

procedures have not changed significantly in the 

last 10 years. 

 The large difference between the predictions from 

standard assessments and monitoring results 

demonstrates the overly conservative nature of 

drinking water assessments.

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Monitoring Versus Standard Assessment

 The following slides illustrate the differences 

between the EPA standard assessment and 

drinking water monitoring studies (from surface 

water) conducted by Bayer CropScience.

 Monitoring study designs

 Highest use watersheds

 Biweekly or weekly monitoring

 Three year study (two years for bromoxynil)

 Finished samples if residues above LOQ in raw 

water

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Monitoring Versus Standard Assessment

 Table definitions

 Maximum concentration:

Monitoring: highest concentration observed in any 

sample (all sites and all years)

Assessment: 1 in 10 year annual maximum value

 Max TWA (finished) 

Monitoring: maximum annual time weighted 

concentration using finished values when available 

(all sites and all years)

Assessment: assuming no losses during treatment

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Monitoring Versus Standard Assessment

 Table definitions (continued)

 Max TWA (raw): maximum time weighted 

concentration over the three year study period 

using only concentrations from raw samples (all 

sites)-iprodione only

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Monitoring Versus Standard Assessment

Concentrations (ppb)

Maximum Max TWA

Raw Finished Finished

Aldicarb (28 sites, 2426 samples)

 Assessment 0.95-17 - 0.17-5.8

 Monitoring 0.68 0.24 0.007

Bromoxynil (16 sites, 543 samples)

 Assessment 11 - 0.2

 Monitoring 0.38 0.11 0.01

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Monitoring Versus Standard Assessment

Concentrations (ppb)

Maximum Max TWA

Raw Finished Finished

Carbaryl (20 sites, 2369 samples)

 Assessment 47-745 - 1.9-31

 Monitoring 0.040 0.16 0.005

Ethoprophos (5 sites, 448 samples)

 Assessment 15-127 - 2.6-13

 Monitoring 0.012 ND(<0.003) (<0.003)

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Monitoring Versus Standard Assessment

Concentrations (ppb)

Maximum Max TWA

Raw Finished Finished

Oxadiazon turf (3 sites, 693 samples)

 Assessment 52 - 19

 Monitoring 0.17 0.13 0.025

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Monitoring Versus Standard Assessment

Concentrations (ppb)

Maximum Max TWA

Raw Finished Raw

Iprodione turf (3 sites, 587 samples)

 Assessment 361 - 1.6

 Monitoring 0.60 0.16 0.037

3,5-DCA (iprodione metabolite)

 Assessment 153 - 36

 Monitoring <0.032 <0.035 (<0.025)

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Missed the Peak

 A common response to the difference between 

standard assessment results and monitoring 

results is that the sampling was not frequent 

enough to observe the peak concentrations.

 Potentially true in some cases

 In most cases, the concentrations do not match 

over a significant portion of the concentration 

distributions.

This is shown with the distribution of the results from 

each sampling interval from the carbaryl study and 

the distribution of daily modeling values.

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Comparison of Distributions

 Carbaryl Concentration (ppb)

 at Specified Percentile 

Percentile 50 70 90 95 99 Max

Assessment

oranges <0.001 0.018 20.4 44.6 140 745

apples 0.002 0.040 3.2 7.7 15 66

Monitoring Study

raw ND ND ND 0.003 0.017 0.04

raw and ND ND ND ND 0.005 0.16

 finished

March 20, 2012
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Special Cases

 Assessments for rice and cranberries result in 

especially conservative assessments.

 Assume direct spray to paddy or bog water

 Water in the rice paddy or cranberry bog is 

essentially considered as drinking water.

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Most Vulnerable Watersheds

 The WARP research (discussed later) identified 

use intensity (mass of active ingredient/area of 

watershed) as the most important factor in its 

regression equations.

 High use intensity occurs in watersheds where the 

product is applied to most of the watershed area.

Highest use intensity often occurs in small 

watersheds composed mainly of agricultural fields.

 Other factors may be important in determining 

vulnerability between watersheds of similar use 

intensity

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Most Vulnerable Watersheds

 Variability of concentrations

 Concentrations in flowing water are more variable 

than in lakes and reservoirs.

As a result of the decreased variability, sampling can 

be less frequent for lakes and reservoirs.

 Smaller streams tend to be more variable than 

larger rivers.

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Most Vulnerable Watersheds

 Small streams are not a source of drinking water 

because of low flow during dry periods.

 Damming a small stream may provide enough 

storage to provide continuous drinking water

 Small reservoirs in watersheds composed largely 

of agricultural fields tend to be the most vulnerable 

watersheds.

 EPA’s Index Reservoir is an example of a 

vulnerable watershed. 

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Historical Approaches-Assessment

 Farm Ponds

 Index Reservoir

 SWMI (Surface Water Mobility Index)

 WARP (WAtershed Regressions for Pesticides)

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Farm Ponds and Index Reservoir

 At first EPA used the farm pond ecological 

scenario to estimate concentrations in drinking 

water.

 EPA changed the farm pond into a reservoir and 

incorporated a PCA (percent crop area) factor for 

crops.

 Not applied for rice and cranberries

 The meta-model FIRST was developed to provide 

an upper bound screening value.

March 20, 2012
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Farm Ponds and Index Reservoir

 EPA worked with USGS to conduct monitoring at a 

number of Index Reservoirs around the country.

 Resulting concentrations were much lower than 

predictions (probably because of use intensity).

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SWMI-Surface Water Mobility Index

 Wenlin Chen developed a regression procedure to 

predict concentrations in surface water.

 Concentration distributions were developed for a 

reference compound (atrazine) in reference 

watersheds as a function of use intensity.

 Concentrations of other compounds are estimated 

by multiplying by a surface water mobility index 

SWMI (function of half life and koc )

March 20, 2012

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WARP-Watershed Regressions for Pesticides

 USGS regression model for estimating 

concentration statistics in surface water

 Provides estimated concentrations for nine 

percentiles for a single compound 

95th, 90th, 80th, 75th, 50th, 25th, 15th, 10th and 5th

 A number of regression models have been 

developed

 Single compounds

 Multi-compound (based on SWMI)

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WARP

 Variables for multi-compound WARP

 Use intensity

 SWMI

 Vapor pressure factor

 Percent C and D soils in watershed

 Dunne overland flow 

 May and June rainfall

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WARP

 A key finding is use intensity explained 50-70 

percent of the variability in the measured data.

 USGS developed a procedure for estimating use 

intensity in watersheds used to supply drinking 

water 

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Implementation of WARP

 Scientists from four organizations (EPA, USGS, 

USDA, and CropLife America) worked on a project 

to develop procedures for implementing WARP in 

estimating drinking water residues for dietary risk 

assessments.

 Started in 2000 and continued through about 2006

 Accomplishments - Monitoring

 Conducted a monitoring program which helped 

assess sampling frequencies needed for flowing 

water streams

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Implementation of WARP

 Accomplishments - Assessment

 Developed two procedures for estimating watershed-specific daily concentrations with appropriate 

temporal associations suitable for use in dietary risk 

assessments

 Developed a procedure for conducting regional 

cumulative assessments involving multiple 

community water systems

 EPA was unable to continue development of 

WARP for drinking water assessments due to nontechnical reasons. 

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Potential for Refined Assessments

 Because standard assessments appear to be 

overly conservative when compared to monitoring 

data, refined approaches are necessary.

 Two approaches

 Methodology for acceptance of monitoring studies

 Refined assessment procedures

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Acceptance of Monitoring Studies

 Differences between standard assessments and 

targeted monitoring data can be greater than two 

orders of magnitude

 Monitoring now often rejected due to “missed the 

peak” argument

 Potential solution is the application of bias factors

This has been a subject of discussion in the atrazine 

SAP meetings

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Refined Assessments

 Potential approaches

 Use of refined use information

 Option to evaluate all relevant community water 

systems rather than a hypothetical index reservoir

This can be helpful for crops with limited geographical 

extent (for example, products used on rice and 

cranberries or on geographically limited pests).

 Flexibility to consider other factors when relevant

Other kinetics than single first order

Vegetative buffer strips when required by the label

Other case-specific factors 

March 20, 2012
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Refined Use Data

 As mentioned for WARP, use intensity is an 

important variable so a realistic assessment 

depends on having an accurate estimate of use.

 Use intensity is composed of three factors:

 PCA (percent cropped area)

 PCT (percent crop treated)

 Typical annual application rate

 All three factors have been used in cumulative 

assessments.

The use factors used in cumulative assessments 

have varied. 

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Refined Use Data

 Sales data is another potential source of 

information on use intensity 

 The difference between standard assessments 

and monitoring data narrows considerably when 

accurate use intensities are used.

 For example, in the iprodione turf example (sales 

data available for individual watersheds) the use 

intensity assumed in the standard assessment was 

a factor of 58 higher than in the U.S. watershed 

with the highest use intensity

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Conclusions

 Standard assessments of residues in drinking 

water are overly conservative as shown by 

comparisons with targeted monitoring data.

 Use of more realistic use intensities will provide 

more realistic, yet conservative exposure 

estimates.

 Use of bias factors and similar approaches can be 

used to quantify potential uncertainty in monitoring 

data to allow their use in higher tier assessments.

March 20, 2012