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Progress Report of the Ecological Committee on FIFRA Risk Assessment Methods: VI. Terrestrial Exposure Assessment

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ECOFRAM Terrestrial Exposure Team Members

Member NameAssociation
Ed Fite(Chair) U.S. Environmental Protection Agency
Larry Brewer Ecotoxicology and Biosystems Analysis, Inc.
Kristin Brugger Dupont Agricultural Products
David Fischer Bayer Corporation
Andy Hart Ministry of Agriculture, United Kingdom
Dennis Laskowski DowElanco
Paul Mastradone U.S. Environmental Protection Agency
Robert Menzer U.S. Environmental Protection Agency
Daryl Moorhead Texas Tech. University
Henry Nelson U.S. Environmental Protection Agency
Ron Parker U.S. Environmental Protection Agency
Carolyn Raffensperger Science and Environmental Health Network
Duane Wolf University of Arkansas

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The Ecological Committee on FIFRA Risk Assessment Methods(ECOFRAM) was formed in June 1997. The committee's purpose is to developing tools and processes within the FIFRA framework for predicting the magnitude and probabilities of adverse effects to nontarget aquatic and terrestrial species resulting from the introduction of pesticides into their environment. The Committee is divided into two workgroups, Aquatic and Terrestrial, which are both subdivided into two subgroups, Effects and Exposure. Working closely with the Terrestrial Effects Subgroup, the Terrestrial Exposure Subgroup's task is to identify and/or develop probabilistic methods for terrestrial exposure assessments and develop recommendations for future use by EPA. Initial discussions of the Terrestrial Exposure Subgroup focused on defining the questions the Workgroup could address given time and resource constraints. It also examined various tools and methods available to address magnitude and probability of exposure of terrestrial species. While the Workgroup recognized the significance of indirect effects, given the state of the art and time and resource constraints, the Terrestrial Workgroup initial effort will focus on direct effects. The Terrestrial Exposure Group's initial objective is to develop a set of tools to predict the distribution (probability and magnitude) of pesticide doses to relevant terrestrial species within agroecosystems, provide guidance on the tools' general principals and use, and provide recommendations for additional work to develop further and validate probabilistic tools for estimating the dose to nontarget terrestrial species. This poster outlines the overall approach the Terrestrial Exposure Subgroup has developed and the initial progress the Subgroup has made in identifying major variables that influence exposure levels of terrestrial species. In addition, the initial efforts to define or estimate the distributions for these variables are presented.

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Steps to Produce Terrestrial Exposure End Products

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Terrestrial Exposure Conceptual Model

Compartments and Components

diagram with arrows connecting 6 compartments:  air, plants, vertebrates, soil, invertebrates, and water

A few of the more important connections (pesticide transfers) among these compartments are indicated by arrows.

The terrestrial exposure modeling group identified 6 distinct compartments which need to be considered to estimate distribution of doses to terrestrial wildlife species.

Compartments and Connections

From the Conceptual Model, we devised an interaction matrix that identified transfers of pesticides among pools that were likely to be important in influencing the dose to terrestrial wildlife species.

Definition of Subscripts and Representative Compartments
Subscript Compartment
3Soil Solution
4Soil Solids
5Free Water
 X1 X2X3 X4 X5 X6 X7
X2 ab  ab a ab  
X3 a a  a a  
X4  a a  a  
X5 ab ab a a   
X6 b ababab ab  b
X7babababab b  

a = Abiotic Links, b = Biotic Links

Operational Model

diagram of operational model with input to sytem at the top and loss from system at the bottom and compartments of air, plant, and soil, in the middle.  Arrows showing directional flow connect all of these items.

Operational Model with some Standard Features

A general compartment model has some standard features:

Example Mass Balance on Environmental Compartment X2

The value of X2 after time Δt if given by:

X2,( t+Δt) = X2,t + (a2,1 + b2,1) X1,t + y2 - (a3,2 + b3,2) X2,t - z2

... or in standard matrix form:

x(t + Δt) = (A + B) xt + y - z

... where x is the vector of state variables (pools), A and B are matrices for transfer coefficients, and y and z are vectors of inputs and losses from the system, respectively.

In this simplified approach we assumed the transfer coefficients, inputs and losses were constant.

  1. they are likely to vary over time and space,
  2. they are likely to vary among systems, and
  3. our ability to measure fluxes is limited.

Thus, model uncertainty and natural variation must enter the effort at this point.

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Defining/Estimating the Distribution of Major Variables

EXPOSURE (Dose) = Residues + Biology

To facilitate defining/estimating the distribution of major variables, the Exposure Modeling Group categorized the compartments of the interactive matrix into either residues or biology.

Residues Team Responsibility

Biological Team Responsibility

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Residue Modeling Team Progress

Graphical conceptual models have been developed depicting the source, transformation and transport of pesticides between and within environmental compartments. Based upon the graphical conceptual models, matrices have been developed which could serve as a mathematical foundation for the development of computer models for estimating residue concentrations in various environmental compartments over time and location. In addition,

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Biology Teams Progress


It is apparent that some pesticide have the potential to move from the soil to soil invertebrates and accumulate at concentrations which present significant risk to terrestrial vertebrates. It has been shown select organochlorine compounds can accumulate in earthworms to levels which present a risk to robins (Gish, 1970). Pesticide transfer and accumulation could result from direct ingestion of soil pore water and soil particles or via direct contact with contaminated soils and subsequent absorption of the chemical. Beyer et al., (1994) demonstrated the potential importance of this exposure route by showing that soil can comprise <2 % to 30% of the diet in some bird species. Models are available which could estimate the accumulation of pesticide concentrations in earthworms and other soil invertebrates (Ross et. al., 1989; Connell and Markwell, 1990; Trapp, 1995)

This subgroup will perform three tasks:

  1. assess the status of current data on soil invertebrates in relation to the transfer of pesticides from soil solution and particles to soil invertebrates.

  2. evaluate the availability and applicability of current soil to invertebrate transfer models.

  3. offer an estimate of the overall importance of this exposure route in general and, if necessary, recommend methods for incorporation into probabilistic models.


A modification of equation 1 of Pasterock et. al. (1996, Human & Ecol. Risk Assess., 2:449) is being investigated to estimate dietary doses of terrestrial vertebrates.

(Dietary Dose) DD (mg/kg b.w./time) = I [(FIR) (AV) (PDi) (PTi) (Ci) (DWFi) / (W)]


FIR = Food Ingestion Rate (kg food on dry weight basis per unit time), a species specific value which may be obtained from Nagy's (1987) allometric equations,

AV = Avoidance factor: Fraction of normal FIR that birds exhibit when provided only pesticide-treated food.

PDi = Proportion of food type I in the diet.

PTi = Proportion of food type I obtained in treated area (i.e., with pesticide residues).

Ci = concentration (mg/kg) of pesticide in food type I.

DWFi = Dry to Wet Factor: ratio of fresh weight to dry weight.

W = Weight (kg) of the species under consideration.


Diet is usually considered to be the predominant route of exposure to pesticides for birds and mammals. Consequently, exposure by inhalation or dermal contact is rarely considered in risk assessment. However, there is evidence that dermal and inhalation exposure may be important under some conditions. For bobwhite quail foraging in a simulated cotton crop, inhalation was the most important route of exposure in the first hour after spraying with methyl-parathion. Eight to 48 hours after spraying the dermal route was most important, while dietary exposure accounted for only 10-20% (Driver et al, 1991). Furthermore, dermal exposures to OP pesticides inhibit plasma cholinesterases for much longer than dietary exposures, increasing the potential for birds to accumulate a lethal dose (Henderson et al., 1994).

ECOFRAM will draw on these and other relevant studies to assess the feasibility of including non-dietary routes in risk assessment. We plan to construct a simple model, then use sensitivity analysis to identify the conditions under which dietary and inhalation routes are important. The results will be used to decide when and if these routes should be considered, and to guide the further development of assessment methods.


This subgroup is working on the development of a variable residue - behavior modified foraging exposure scenario. The underlying assumption of the model is that dose is related to mass (M) of the daily food intake and the pesticide concentration on the food items (C) divided by the body weight of the animal.

diagram of a grid of a simulated field with shaded boxes indicating randomly designated edge, and treated field.  Also with X in boxes indicating random exposure of vertebrate.

Simulated Field

Dose = (Mtime, location Ctime, location) / Body Weight

The following assumptions are made:

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Challenges to developing probabilistic exposure methods

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Assessment Endpoints

ECOFRAM recognized the need to consider endpoints up to the system-level. However, due to time constraints and the lack of easily adaptable community and system-level models, the primary assessment endpoints will be survival, reproduction and persistence of valued ecological entities.




(*)Primary endpoint to be considered by ECOFRAM

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Terrestrial Exposure Assessment Endpoint

Estimation of the distribution of pesticide doses to relevant terrestrial species within Agroecosystems.

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