Exploring Approaches to Pesticide Aquatic 

Ecological Exposure Assessment: 

Issues in Evaluating Risk Across 

the National Landscape

Exposure Modeling Public Meeting

September 20, 2011

Ronald Parker, Ph.D.
Mission of EPA Office of Pesticide Programs

• Protect Human Health and the 

Environment from Unreasonable 

Adverse Effects from the Use of 

Pesticides
Key Federal Pesticide Laws:

FIFRA and FFDCA

• Federal Insecticide, Fungicide, and 

Rodenticide Act (FIFRA)

 Review and Register New / Older Pesticides

 Risk / Benefit Balancing

• Federal Food, Drug, and Cosmetic Act 

(FFDCA) 

 Establishes Tolerances (Maximum Residue 

Limits) for Pesticides on Food and Feed

 Tolerances Apply to Domestic and Imported 

Foods

 Monitoring for Tolerances Falls to FDA
Key Federal Pesticide Laws: 

FQPA

• Food Quality Protection Act of 1996

 Amended FIFRA and FFDCA

 Reassess All Tolerances Within 10 Years

 The Agency Must Complete the Registration Review of Each New 

Pesticide Active Ingredient within 15 Years of its Initial Registration

 Aggregate Exposures from Diet, Drinking Water, Residential and 

other Non-occupational Sources

 Cumulative Effects and “Common Mechanism of Toxicity” e.g. 

Organophosphates

 Current Registration Review Program Is Replacing EPA's 

Pesticide Reregistration and Tolerance Reassessment Programs 

 Operates Continuously, Encompassing All Registered Pesticides
Key Federal Pesticide Laws:

Endangered Species Act 

• ESA Passed in 1973 and Amended in ’78’, 

’82’, & ’88’

• Section 7(a)(2) of the ESA Requires Federal 

Agencies (“Action Agencies”) in Consultation 

with the Services to Ensure that Any Action 

They Authorize, Fund or Carry Out Is Not 

Likely to Jeopardize the Continued Existence 

of a Listed Species or Result in Destruction 

or Adverse Modification of Habitat 

Designated by the Services as Critical

 Prohibits Against “Harming, Harassing, 

Pursuing, Hunting, Shooting, Wounding, Killing, 

Trapping, Capturing and Collecting any Listed 

Threatened or Endangered Species”
Delineating the Scope of US

Aquatic Landscape

• Habitat: Extensive & Complex US Aquatic Landscape

• 9,000,000 Ponds (Renwick, et al., 2005) 

• 3,660,000 Miles of Rivers, Streams and Canals (USEPA)

• Perennial: 1,300,000 Miles

• Non-perennial: 1,600,000 Miles

• Canals & Ditches: 110,000 Miles

• 123,439 Lakes > 10 Acres (USEPA)

• 82,642 Reservoirs (National Inventory of Dams)

• 102 Estuaries (USEPA)

• Extensive & Complex Community Water System 

(CWS) Network

• 11,671 Surface Water CWS Serve 204.1 Million People (USEPA)

• 40,301 Ground Water CWS Serve 88.1 Million People (USEPA)

• Extensive & Complex Agricultural Landscape

• 2,080,000 Working Farms (USDA)

• Extensive & Complex Urban Landscape

• 19,000+ Municipal Governments

• 275 Cities Over 100,000 Population
Spatial Aquatic Model (SAM) Pilot Project

• “Proof of Concept” Pilot SAM Project Began in May

• Scope of SAM Project

• Developing Geographic Components for Spatial 

Modeling

• Updating PRZM Routines and Links to Geographic 

Components

• Take Advantage of Storage Capacity for Spatial Model 

Inputs and Outputs – “Super Scenarios”

• Automate Linear Routing from Soil Map Units to Water

• Test Run the Linked Models 

• Produce a Prototype Demonstration Model

• Develop Recommendations for Production Model
Purpose of New OPP Aquatic 

Exposure Modeling

• Ecological Risk Assessment (ERA) Is a Process that 

Evaluates the Likelihood that Adverse Ecological 

Effects May Occur or Are Occurring as a Result of 

Exposure to One or More Pesticides 

• Pesticide Aquatic Exposure Assessment Provides 

the Exposure Component of the Risk Assessment 

Equation

• Main Purpose of Pesticide Risk Assessment Is to 

Inform Risk Management Decisions

• Needs of Risk Managers Are the Main Motivation 

behind New Spatial Aquatic Modeling Project
9

Current OPP Modeling Configuration 

for Ecological Exposure Assessment:

• Agricultural Field Scale

• Field Area (10 ha)

• 100% Treated

• Pesticide Carried Off the 

Field by:

• Runoff Water

• Eroding Soil

• Spray Drift 

• Adjacent Pond

• Static (No Inflow or Outflow of 

Surface Water or Groundwater)

• Site Specific Soil and Weather

• Fixed Water Quality - Georgia 

Farm Pond

• Pond Surface Area (1 Hectare)

• Pond Depth (2 Meters) 

• Pond Volume (20x106 Liters)

10 ha (25 Acre ) Field

100% Pesticide Treated

1 ha (2.5 Acre) 

x 2 m Pond
10

Current EFED Field / Pond Configuration:

Empirically Also Serves as Conservative Screen for Rivers and 

Streams Receiving Runoff from Multiple Agricultural Fields
11

Current Aquatic Screening Approach

Pass

Fail

Characterize 

Certainties and 

Uncertainties

Management Decisions Based on 

One (or a Few) Conservative 

“Standard” Scenarios for Each 

Treated Crop
12

Advantages / Disadvantages of 

Current OPP Modeling Configuration:

• Advantages:

• Easy to Describe and Understand

• Valid for Comparisons among 

Pesticides

• Provides Estimates of Temporal 

(Day-to-Day, Year-to-Year) 

Variability

• All Applied Pesticide Retained 

within the Assessment System

• Disadvantages:

• This Combination of Land-Water 

Configuration and Site 

Assumptions Actually Exists Only 

Rarely

• Does Not “Explicitly” Represent 

Flowing Water or Estuaries

• Cannot Locate High-exposure 

“Hot-spots” for Mitigation

25 Acre (10 ha) Field

100% Pesticide Treated

2.5 Acre (1ha)

x 2 m Pond
13

Potential Transition to Spatially 

Explicit Assessment

Pass

Fail

Current Aquatic Screening Process

Spatially Explicit Assessment

Stressor 

distribution

Receptor 

range

Area exceeding level of concern
14

Soils, Hydrology, Elevation, Drainage 

Patterns, Weather, Land Use, Crop and 

Management Practices, Growing Season

Spatial Representation of 

Pesticide Concentrations 

Across the Pesticide Use Area

Gather Spatial 

Data

Develop/ Adapt 

Models

Determine 

Outputs

Hydrology, Pesticide 

Fate and Transport

Arrows 

Represent 

Key Model 

Linkages that 

Allow Us to 

Process Data 

and Models 

Spatially

Spatial Aquatic Modeling
Conceptual Aquatic Exposure Model

Watershed 

With 

Pesticide 

Applications

(PRZM) Receiving 

Water Body

(EXAMS)

Pesticide Fate 

and Transport

Pesticide

Loading

Soil, Site, Crop,

Hydrology

Weather 

(Rainfall, Temp.,

Irrigation)

Registrant

Studies

Fate/

Transport

Properties

Use area

Site-Specific

Conditions

App. Rates,

Frequency,

Timing

Labels Runoff

Spray Drift

Soil Erosion

Comparison 

with Toxicity 

Endpoints

- Aquatic 

Organisms

- Human Health

Aquatic

Exposures

- Concentration

- Duration

- Frequency 
“Super Scenario” Development

Use GIS Coverages for Soil, 

Catchment Hydrology, Slope, 

Crops and Land Use
PRZM Modeling Units and NHDplus

Framework

NHDplus Catchments

• Initial modeling will be aggregated 

within NHD+ catchments (shown as 

red lines)

• Catchments are linked to stream 

reaches for hydrologic routing, 

processing

NHDplus Hydrology

• Hydrologic framework

• Each stream reach (dark 

blue lines) serves as the 

“water body”

• Downstream routing and/ or 

dilution follows NHD+ reaches

SSURGO Soil Map Units

• Soil mapping units (shown in various color 

tones) within each NHD+ catchment will 

serve as the basic PRZM modeling unit

• Model inputs, outputs will be aggregated at 

the map unit scale

A1

A2

A3

A4

A5

B1

B2

B3

B4

A1 – Basic Model Unit

(soil map unit x catchment)
Moving Outputs from Soil Map Units 

to the Stream Reach

• Once PRZM Pesticide Loads Arrive at the Stream Reach, 

NHD+ Can Be Used to Move them into Downstream 

Reaches to Drinking Water Intake Locations or into 

Drinking Water Reservoirs

• One Challenge Is Moving the Outputs from Irregular Soil 

Mapping Units in the Catchment to Downslope Stream 

Reaches and then Estimating Downstream Dilution

• We Have Been Looking to See How other Modelers Have 

Handled that Problem

• Checked SWAT, BASINS, AGNPS, Recent University 

Publications and Industry Watershed Modeling

• Focused on Using NHD+ Flow Grid Network

18
Super PRZM Development

• Following Agency Guidance Regarding Data Quality and 

Best Practices for Model Development

• Take Advantage of Modern Programming & Hardware

• Using Parallel Processing, Reducing Redundancies, 

Storing Calculations for Later Use

• Current PRZM: Hydrology, Crops, and Chemistry Are 

Largely Separate and Occur in Overlapping Loops

• Super PRZM: Chemistry and Application Are Completely 

Separate from Hydrology, Crops, Soil and Weather

• Hydrology/Crop Pre-calculations Are the Super 

Scenarios

• Super Scenarios Are Stored in Unformatted Binary 

Form for Extra-fast Input / Output Performance 19
Addressing Data Quality Issues

• Each PRZM Routine Was Reviewed for:

• Availability of Science References

• Application of Sound Science

• Code Verification

• Coding Inefficiencies

• Coding Duplications

• Issues Still Under Discussion

• Spatial Extent of “Super-Scenarios”

• Acceptable Levels of Simplification

• Extent, Spatial and Temporal Variability of Application 

within Each Catchment 

• Addressing Temporal Variability of “Hot-spots”
Displaying Outputs: Catchment Option

Summarize Results at 

the NHD+ Catchment 

Scale

• Map Shows the 

Results of AreaWeighting Averages 

by Catchment (No 

Downstream Flow)

• Would Incorporate 

Simple Downstream 

Flow Routing to 

Account for 

Contributions from 

Upstream Areas
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Displaying Outputs - Option 2:

Integrating Downstream Exposure

The Ideal Situation: Aquatic Exposure 

Estimates by Catchment (Left) Would Be 

Integrated Downstream with Information on 

Species Habitat (Above). This Approach 

Would Allow for Temporal Considerations
Continuous Interaction with Risk Managers 

and Other End Users

• Main Purpose of Pesticide Risk Assessment Is to 

Inform Risk Management Decisions

• Risk Managers Will Be Involved throughout the Model 

Development Process to Assure Optimal Usefulness of 

the Model in Guiding Risk Management Decision

• Consultations Will Be Conducted through Questionnaires 

and Frequent Face-to-Face Meetings

• Types of Decisions Needed Include:

• Optimal Representation of Spatial and Temporal Model Outputs

• Ancillary Data for Output Values

• Characterization and Immediate Causes of “Hotspot” Occurances

• Estimating Uncertainties

23
Types of Questions for Risk Managers & 

Other End Users (1)

• Optimal Numbers and Locations of Output Concentration 

Values; Use of Temporal Ranges, Aerial Percentages, etc.

• Most Useful Presentations of Output (Tabular, Maps, other) 

• Spatial Scale of Concentration Values of Most Use in Risk 

Management (Field, County, State, other)

• Characterization of High Exposure Locations

• Differences In Number and Sizes of Treated Fields and Receiving 

Water Bodies 

• Impact of Land to Water Ratios on Results

• Impact of Flow on Results 

• Necessary Level of Validation (Comparison with Monitored 

Values)

• Acceptable Levels of Uncertainty

24
Types of Questions for Risk Managers & 

Other End Users (2)

• When Should Averaging, if Necessary, Occur?

• How Much and What Types of Simplifications Are 

Acceptable? 

• What Types of Risk Mitigation (Exposure Reduction) 

Options Can Be Accurately Modeled

• On-site Options for Runoff Reduction by Reduced Tillage, 

Incorporation, Grassed Buffers, etc., 

• On-site Options for Spray Drift Reduction by Separation 

Buffers

• Use of Flow Reduction Buffers

• Impact of Application Methods, Rates, Timings, Number, 

Intervals, Equipment, etc.

25
Status as of 9/20

• Conceptual Model Framework Has Been Built

• Spatial Framework Still under Development

• Basic Modeling Units & Flow Routing Concept Established

• Data Development for “Super Scenarios” Underway

• Developing Programming for Scenario Development

• Super PRZM Has Been Compiled

• Separate Hydrology and Pesticide Models Established

• Awaiting Testing for Spatial Linkages

• Considering Options for Processing & Displaying 

Outputs

• Scales for Drinking Water and Ecological Assessments

• Developing Questions for Risk Managers & End Users 26