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Progress Report of the Ecological Committee on FIFRA Risk Assessment Methods: III Aquatic Exposure Assessment

Paul Hendley1, James Baker2, Lawrence Burns3, David Farrar4, Alan Hosmer5, David Jones6,Walton Low7, Mark Russell8, Mari Stavanja9, Martin Williams10 and James Wolf 11

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The Ecological Committee on FIFRA Risk Assessment Methods (ECOFRAM) was formed in June 1997. The Committee's purpose is to develop tools and processes within the FIFRA framework for predicting the magnitude and probabilities of adverse effects to non-target aquatic and terrestrial species resulting from the introduction of pesticides into their environment. An Aquatic Exposure Subgroup was formed to identify and discuss probabilistic methods for aquatic exposure assessments and develop recommendations for future use by EPA. In addition, we are identifying information that must be developed in order to validate the proposed methods in order to ensure that the proposed assessment process, if adopted by EPA, supports environmental decisions that are scientifically defensible.

This poster describes the conceptual model the Aquatic Exposure Subgroup has developed along with associated tables listing key factors. In addition, initial recommendations on the current Tier I exposure model (GENEEC) and a list of improvements to the background environmental fate FIFRA "subpart N" studies needed to support aquatic exposure estimates are also presented.

The subgroup recognizes that one of the fundamental steps to success will be the way in which the Aquatic Exposure and Effects subgroups manage to combine their recommendations into an integral Aquatic Risk Assessment approach. In an accompanying poster, the ECOFRAM Aquatic Exposure and Effects Subgroups will present a joint view on how an aquatic risk assessment framework may be generated within ECOFRAM by coordinating the activities of the two subgroups. Draft decision trees will presented from both groups along with key questions to prompt participation from SETAC attendees; the goal of the Aquatic decision tree is to serve as a primary tool in helping to make regulatory aquatic exposure assessments more predictable.

The items to be presented are "works in progress" and the subgroup is requesting feedback from conference attendees with participation in the poster session to help improve the concepts.

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The individuals listed above as authors represent an effective blend of knowledge, experience and skills. As the ECOFRAM Aquatic Exposure Subgroup, they have been developing a team effort to address the project's goals. During its original meeting, this subgroup developed some goals:

The group has decided that the factors and issues below should be considered during the process to achieve these goals :

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Conceptual Model of Aquatic Exposure

The draft conceptual model developed by the group to describe the exposure of non-target aquatic systems to pesticides is currently organized as follows:

The poster covers the highlights of the conceptual model; more detailed draft texts prepared by members of the group have been prepared for some sections and these will be made available at the SETAC conference. Please note that this is a developing framework (especially the graphics!!); other posters will outline the plan for future ECOFRAM communications.

The group is currently trying to rank the most significant factors in each section of the conceptual model and also the variabilities and uncertainties associated with each. In addition, it is thought that the relative time scales of pesticide residue presence in the water bodies compared with relevant biological processes merits further consideration.

The ECOFRAM Aquatic Exposure Subgroup encourages all who attend the poster session to provide comments. Comments should be addressed to Exit EPA Disclaimer Paul Hendley [(510) 231 1499 or Paul.Hendley@agna.zeneca.com] or any of the authors named above.

diagram of a cross-section of land with many transport routes including:  regional transport, dry deposition, precipitation, evaporation, spray drift, wind erosion, waste water, runoff, withdrawal from wells, seepage, recharge from streams, ground water discharge to streams, and entry through wells.

Figure 1
(courtesy of USGS NAWQA)
takes a broad view of potential pesticide transport routes.

Agricultural chemicals are essential for effective food production but may pose potential risks to humans and the environment; EPA OPP has the responsibility to address this dilemma under the FIFRA statute. While it is often assumed that pesticide contamination is an phenomena associated with agricultural areas, recent research shows that urban areas can contribute extensively to pesticide residues in urban streams. Therefore an assessment of the probabilities of non-target aquatic exposure to pesticides must take a wide view of pesticide use.

Figure 1 (courtesy of USGS NAWQA) takes a broad view of potential pesticide transport routes. Once the pesticide has been applied, one of the most significant routes for potential risk to non-target organisms, ecosystems and humans, is via subsequent contamination of the hydrologic system.

It is likely that the Subgroup will concentrate mostly on refining an understanding of the impact of spray drift and runoff routes of entry on the probabilities of aquatic exposure in non-target water bodies. A major issue that the ECOFRAM process is likely to accentuate is how regulators, the regulated community and society at large can better understand which water bodies need to be protected and to what degree. One corollary to that debate is the recommendation of appropriate modeling scenarios (e.g. edge of field concentrations, concentrations in farm ponds or reservoir residues) for the various "tiers" of an aquatic risk assessment.

diagram of interactions with agricultural landscape and aquatic ecosystem due to pesticide application

Figure 2
how the agricultural landscape and the aquatic ecosystem interact to influence exposure

Figure 2 shows in more detail how the agricultural landscape and the aquatic ecosystem interact to influence exposure. While most of the themes are developed in detail in later sections of the model, a key point is the range of spatial scales involved for both lentic and lotic aquatic systems and the way this will tend to parallel various durations of exposure. Hand in hand with the increasing duration over which a water body might experience pesticide exposure is the increasing dilution phenomena that come into play. For example, ponds have more depth and overflow potential than wetlands; reservoirs not only have even more depth than ponds, they also tend to collect water from larger areas and so not all the runoff entering the water body will be treated. From a spray drift perspective, as one moves to progressively larger water bodies the chance for even and high level spray drift entry in more than a few margins tend to decrease. Similar trends are seen with lotic systems where flow dilution adds an additional complicating factor.

Table A shows the factors that the subgroup have identified as potentially influencing the fate of the pesticide in the field after application and the subsequent probability of transport to a non-target aquatic system. The work group has been prioritizing the list further regarding which factors may be most significant in order to help focus future efforts on the most worthwhile parameters.

Table A: Parameters Influencing Edge-of-Field Chemical Runoff and Erosion from Agricultural Fields

Chemical / Physical

  • Molecular weight
  • Solubility
  • PKa
  • Vapor pressure


  • Koc


  • Hydrolysis half-life
  • Aqueous photolysis half-life
  • Soil photolysis half-life
  • Aerobic soil degradation half-life
  • Anaerobic soil degradation half-life
  • Field soil degradation half-life
  • Canopy volatilization half-life
  • Canopy degradation half-life
  • Canopy washoff rate

Formulation Issues??

  • Incorporation depth

Time-Invariant Factors

  • Organic matter
  • pH
  • Texture
  • Hydrologic group
  • Field capacity

Time-Variant Factors

  • Bulk density / compaction
  • Field capacity
  • Wilt point
  • Tillage
  • Surface sealing / infiltration

Field slope and length

Structure of complex slopes


  • Type of ground cover
  • Relative area and shape
  • Sediment removal efficiency
  • Infiltration capacity

Wetlands (mitigation)

Buffer strips(mitigation)

Landscape factor co-occurence


Air temperature

Relative humidity

Wind speed

Solar radiation

Antecedent Moisture content


Crop Type

Crop Growth Rate

Rotational pattern

Tillage practices

Conservation management practices

Application method

  • Air
  • Ground
  • Air Blast
  • Nozzles
  • Incorporation







Tile drainage


Canopy washoff





Tile drainage

Runoff Mixing Zone

flow diagram of aquatic system and pesticide exposure through the atmosphere, the limnetic zone, and the benthic zone.

Figure 3
Conceptual Model of Pesticide Exposure in Aquatic Ecosystems

Figure 3 represents the aquatic system using the ecological circuit language designed by H.T. Odum. The arrows to ground represent degradative processes while the tanks represent storage compartments. The model represents direct entry to a static water body (spray drift), runoff entry, interflow entry as well as the potential for buffer area mitigation of runoff entry. In addition, the model covers potential chemical, physicochemical and biological adsorption, transformation, transport and impacts.

Table B provides the supporting list of factors significant for determining the fate of a chemical in the aquatic system. This list is also being prioritized and assessed for contributions to variability and uncertainty.

Table B: Parameters Influencing Chemical Fate in Aquatic Systems
Chemical ParametersFlow / Geometry ParametersEnvironmental Parameters

Molecular weight

Henry's law constant


Vapor pressure

Sediment part. coef.

Organic carbon partition coefficient

Octanol water partition coefficient

Water col bact. rate

Benthic bacteria rate

Direct photol rate

Hydrolysis rate constant

Horizontal discretization

Longitudinal discretization

Vertical discretization

Bed slope

Bed friction

Water depth



Sediment scour

Sediment deposition

Direct precipitation

Pan evaporation

Relative humidity

Solar radiation

Optical path length to vertical depth

Mean monthly cloud cover

Water temperature

Wind shear / atmospheric turb

Suspended sediment

Plankton population

Submerged aquatic plant biomass

Benthic bacteria

Benthic biomass

Fraction of organic carbon

Bulk density

Sediment porosity

Anion exchange capacity

Cation exchange capacity

Dissolved oxygen

Dissolved organic carbon



diagram of agricultural landscape of 3 crops (A, B, and C) and components including slope, ditch, VFS buffer strip, marginal vegetation, tall trees, prevailing wind, separation between crop and water body, and depth

Figure 4
Figurative diagram of an Agricultural Landscape indicating some Factors from Table C

Figure 4 and Table C both reflect factors in the Agricultural Landscape that need to be included in detailed assessments of aquatic exposure arising from pesticide use. For example, the percentage of the crop of interest in the watershed, the proximity of that crop to the water itself, the percentage of the crop that is treated and the spatial relationship of the crop and water body are all critical determinants of the potential exposure. The co-occurrence of sensitive variables is an issue that the group plans to incorporate into subsequent deliberations.

Table C: Parameters Relevant for Considering Impacts of Landscape Level Effects
Physical AspectsAgronomic aspects Water Body FactorsWeather Variables Spatial FactorsModel Issues

Land area

Land area/water area


Basin Geometry

Range of distances from treated land to water

Homogeneity of soil textures

Homogeneity of soil OM%, pH etc

Range of slopes

Uniformity of slopes within watershed

Complexity of slopes and related depressions within fields (micro-relief)

Presence of ditches or rills to transport runoff

Complexity of drainage network [if scale medium to large]

Area in agriculture, urban development etc

Area in crop of interest

Ag area/water area

Crop area/water area

Presence and width of Buffers

Composition of buffers

Requirement for & width of set backs

Extent of "pesticide of interest" usage

Use of same pesticide for other use patterns (e.g. urban lawns)

Adoption of conservation tillage practices

Presence of "engineering controls" (e.g. terracing)

Extent of chanellization in rills and water body entry points

Presence of tile drainage

Relative spatial positioning of crop and water body (e.g. relative to wind)

Crop vigor and density

Crop planting date & growth rate





Flow in/out (controls)

Return flow

Bank Storage

No of RO entry points

Representativeness within region

Marginal vegetation

Natural or man-made pond, lake or reservoir

Self sustaining or manipulated (catfish pond)

Range of species represented

Stream order/pond class

Tile drainage entry??

Prevailing wind direction and speed

Range of wind speeds and directions

Storm frequency

Storm intensity

Storm hyetograph (typical hydrograph)

Temperature change with time

Relative positioning of crop of interest and water body

Do all entries deliver from treated areas?

Extent of differences between regions

Suitability of watershed/water body for existing models

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GENEEC Status and Plans

In the current regulatory tiered process for aquatic risk assessment; GENEEC, a meta-model of PRZM-EXAMS output is used as the regulatory touchstone to estimate exposures for comparison with "worst case" aquatic toxicity values to determine whether further risk characterization effort is warranted. The ECOFRAM Aquatic Exposure Subgroup is seeking to rationalize the tier system but during debates decided that GENEEC might serve as an interim first tier "worst-case assumption" model in the new system. Accordingly the group decided to summarize its thoughts on GENEEC.

General Advice

Recommendations for how to develop/maintain GENEEC

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More / Improved Data Needed to Support Exposure Modeling

In 1993, the FIFRA Exposure Modeling Workgroup (FIFRA EMWG) developed a list of environmental fate studies that needed study design improvement or inclusion in the FIFRA requirements in order to supply the information needed for exposure modeling. The ECOFRAM Aquatic exposure Subgroup has endorsed that list and added on or two points.

Suggested changes

The subgroup has suggested that, in addition to the suggested changes, some of the studies in subpart N could effectively be placed into tiers I order to concentrate effort on those aspects of those compounds that merit the intense scrutiny.

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  1. Zeneca Ag. Products, 1200 South 47th Street - Box 4023, Richmond, CA 94804-0023

  2. Iowa State University, Department of Agricultural and Biosystems Engineering, Davidson Hall, Ames, IA 50011

  3. US EPA-ORD/NERL, 960 College Station Road, Athens, GA 30605-2700

  4. US EPA-OPP, Environmental Fate and Effects Division (7507C), 401 M Street, SW, Washington, D.C. 20460

  5. Novartis, Ecological Toxicology, PO Box 18300, Greensboro, NC 27419-8300

  6. US EPA-OPP, Environmental Fate and Effects Division (7507C), 401 M Street, SW, Washington, D.C. 20460

  7. USGS, MS413, 12201 Sunrise Valley Dr., Reston VA 20192-0001

  8. DuPont Ag. Products, Barley Mill Plaza, Building 15/1118, Rt. 48 and Rt. 141, Wilmington, DE 19805

  9. Scientific Evaluation Section, Bureau of Pesticides, 3125 Conner Boulevard, Tallahassee, FL 32399-1650

  10. Waterborne Environmental, 897-B Harrison Street, S.E., Leesburg, VA 20175

  11. US EPA-OPP, Environmental Fate and Effects Division (7507C), 401 M Street, SW, Washington, D.C. 20460

The ECOFRAM Aquatic Exposure Group would also like to acknowledge contributions made by Ron Parker (US EPA-OPP) and Paul Mastrodone (US EPA-OPP).

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