United States Prevention, Pesticides EPA712-C-08-020
Environmental Protection And Toxic Substances October 2008
Agency (7101)
&EPA Fate, Transport and
Transformation Test
Guidelines
OPPTS 835.6100
Terrestrial Field
Dissipation
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INTRODUCTION
This guideline is one of a series of test guidelines that have been
developed by the Office of Prevention, Pesticides and Toxic Substances
(OPPTS), United States Environmental Protection Agency for use in the testing
of pesticides and toxic substances, and the development of test data to meet the
data requirements of the Agency under the Toxic Substances Control Act (TSCA)
(15 U.S.C. 2601), the Federal Insecticide, Fungicide and Rodenticide Act
(FIFRA) (7 U.S.C. 136, et seq.), and section 408 of the Federal Food, Drug and
Cosmetic (FFDCA) (21 U.S.C. 346a).
OPPTS developed this guideline through a process of harmonization of
the testing guidance and requirements that existed for the Office of Pollution
Prevention and Toxics (OPPT) in Title 40, Chapter I, Subchapter R of the Code
of Federal Regulations (CFR), the Office of Pesticide Programs (OPP) in
publications of the National Technical Information Service (NTIS) and in the
guidelines published by the Organization for Economic Cooperation and
Development (OECD).
For additional information about OPPTS harmonized guidelines and to
access this and other guidelines, please go to http://www.epa.gov/oppts and
select "Test Methods & Guidelines" on the left side menu.
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OPPTS 835.6100 Terrestrial field dissipation.
(a) Scope—(1) Applicability. This guideline is intended for use in meeting testing
requirements of the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) (7 U.S.C. 136, et
seq.). It describes procedures that, if followed, would result in data that would generally be of
scientific merit for the purposes described in paragraph (b) of this guideline.
(2) Background. The source materials used in developing this OPPTS test guideline are
OPP 164-1 Field dissipation studies for terrestrial uses, OPP 160-4 General test standards, and OPP
160-5 Reporting and evaluation of data (Pesticide Assessment Guidelines, Subdivision N -
Chemistry: Environmental Fate, EPA report 540/9-82-021, October 1982) and NAFTA Guidance
Document for Conducting Terrestrial Field Dissipation Studies, US Environmental Protection
Agency and Health Canada, Pest Management Regulatory Agency, March 31, 2006.
(b) Purpose. The purpose of terrestrial field dissipation studies is to determine the extent of
pesticide residue dissipation under actual use conditions While the laboratory studies are designed
to address one dissipation process at a time, terrestrial field dissipation studies address pesticide loss
as combined result of chemical and biological processes (e.g., hydrolysis, photolysis, microbial
transformation) and physical migration (e.g., volatilization, leaching, plant uptake). Pesticide
dissipation may proceed at different rates under field conditions and therefore may result in
formation of degradates at levels different from those observed in laboratory studies.
(c) Definitions.
50% dissipation time (DT50) is the amount of time required for 50% of the initial pesticide
concentration to dissipate. Unlike the half-life, the dissipation time does not assume a specific
degradation model (e.g., a first-order degradation).
75% dissipation time (DT75) is the amount of time required for 75% of the initial pesticide
concentration to dissipate. Unlike the half-life, the dissipation time does not assume a specific
degradation model (e.g., a first-order degradation).
Dissipation is the overall process leading to the eventual disappearance of substances from
the site of its application or an environmental compartment. Dissipation comprises two main types of
processes: transport processes, such as volatilization, leaching, plant uptake, runoff or erosion that
transfer substances to different environmental compartments; and transformation processes such as
microbial degradation, hydrolysis and/or phototransformation that produce transformation products.
First-order kinetics is a model that assumes that the rate of degradation/dissipation is
proportional to the concentration of the reactant and remains constant during the reaction time
period. The single first-order model is derived from the differential equation:
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Equation 1
dM
—— = - kM M(0) = Mo
C-f /
with
M= mass of the compound
MO = initial mass of the compound
k = rate constant for the compound
The integrated form of the above equation is a simple exponential equation with two
parameters (Mand k):
Equation 2
M = M»e-kt
with
M= mass of the compound at time t.
Half-life ft 1/2) is the time required for a concentration of a pesticide to be reduced (i.e.,
degrade, metabolize or otherwise dissipate) to one-half. With each half-life period, half of the
remaining concentration of pesticide will disappear from the system.
Half-life versus 50% dissipation time means when the reaction follows first-order
degradation kinetics, the half-life will be equivalent to the 50% dissipation time. In this case, the
reaction rate is proportional to the reactant concentration and constant over time. However, when the
degradation rate is not first-order, the half-life and the 50% dissipation time will differ. In this
situation, the half-life is usually greater than the 50% dissipation time. Discrepancies between the ti/2
and the DT50 may suggest that pesticide degradation follows something other than a first-order
reaction model.
Ideal application and planting techniques means the use of specially adapted application
machinery to accurately apply a pesticide in small plot field trials in a manner approximating field
methods.
Major transformation products are degradation products/metabolites of the parent compound
that are observed at any time in the laboratory or field studies at a level equal to or greater than 10%
of the initial concentration of the parent compound. In addition, major transformation products may
include other compounds of toxicological significance.
Plot: is a single experimental unit, e.g., a control plot, a treated plot.
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Replicate plot is one of two or more plots treated in an identical manner at one site.
Site is the exact geographical location of a study.
(d) General considerations—(1) Test data use. Terrestrial field dissipation data support
the registration of an end-use product intended for outdoor uses and to support each application to
register a technical grade active ingredient and manufacturing-use product used to make such an
end-use product.
(2) Conceptual Model. Central to this guidance is the development of a conceptual model,
using assumptions derived from environmental fate laboratory data along with the intended use
pattern and physiochemical properties of the pesticide. As such, the conceptual model is a
prediction or working hypothesis for the terrestrial field dissipation study and can focus the study on
the major routes of dissipation. Although laboratory data is the foundation for the hypothesis and
the basis for the conceptual model approach, the terrestrial field dissipation study provides the
primary mechanism for testing and refining the hypothesis for the transformation, fate and transport
of a pesticide under actual use conditions. Additional background is described in (j)(l) of this
guideline. A dissipation route should be included in the study design if it is expected to explain, in
part, the observed rate of chemical dissipation from the surface soil. One way to approach the study
design is to consider each route of dissipation as a potential study module. Using the conceptual
model, the study sponsor can determine which modules are needed to adequately characterize the
active routes of dissipation in the field (see paragraph (g)(l) of this guideline)).
(3) Suite of dissipation studies. Each terrestrial field dissipation study should be designed in
the context of a suite of terrestrial field dissipation studies that identify the route(s) and rate(s) of
dissipation of the active ingredient and major degradates/transformation products when a typical
formulation/end-use product is applied under field conditions representative of the significant area(s)
where pesticides are used. The studies should quantify the pathways of transformation and transport
as well as the distribution of the parent compound and its major transformation products in each
environmental compartment. In short, the studies should address the dissipation and fate of the
active ingredient and major transformation products in the environment.
(4) Endangered species. Field studies should not be conducted in critical habitats or areas
containing or suspected to contain endangered or threatened plants or animals that may be threatened
by the test to be conducted.
(5) Environmental chemistry methods. Procedures and validity elements for independent
laboratory validation of environmental chemistry methods used to generate data associated with this
study can be found in 850.6100. Elements of the original addendum as referenced in 40 CFR
158.1300 for this purpose are now contained in 850.6100. These procedures, if followed, would
result in data that would generally be of scientific merit for the purposes described in 40 CFR
158.1300.
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(e) Background. It may not be feasible or desirable to study each of the routes of dissipation,
as identified by the pesticide-specific conceptual model (see paragraph (g)(l) of this guideline), at
one field site. For example, testing conditions for the evaluation of pesticide runoff would not be
appropriate for an assessment of leaching. In this case, a modular approach is recommended in
which concurrent dissipation pathways are studied at one site, while non-concurrent pathways are
evaluated in separate studies (either field or laboratory, as appropriate). The suite of field dissipation
studies may be conducted in an iterative fashion until the results:
(1) Provide an integrated qualitative and quantitative environmental fate assessment that
characterizes the relative importance of each route of dissipation for the parent compound and major
transformation products (greater than 10% of applied) and/or lexicologically significant amounts of
parent and transformation products. The study design should acknowledge the relative importance of
each route may be different depending on use pattern, formulation type and climatic conditions.
(2) Determine whether potential routes of dissipation identified in the laboratory are
consistent with field results.
(3) Characterize the dissipation rates of the parent compound and formation product as well
as decline of the major and/or lexicologically significant transformation products under field
conditions.
(4) Characterize the rates and relative importance of the different transport processes,
including leaching, runoff and volatilization.
(5) Establish the distribution of the parent compound and the maj or transformation products
in the soil profile.
(6) Characterize the persistence of the parent compound and major transformation products
in soil, including retention and residue carryover in the soil to the following crop season.
(7) Characterize foliar dissipation, if the compound is applied to plants.
(8) Characterize the effect(s) of different typical pesticide formulation categories, where
applicable.
(f) Test method—(1) Test substance—(i) Typical end-use product. The test substance
should be a typical end-use product. If the manufacturing-use product is usually formulated into
end-use products from two or more major formulation categories, separate studies should be
performed with a typical end-use product for each category (e.g., wettable powder, emulsifiable
concentrate, granular).
(ii) Radiolabeling. Non-radiolabeled or radiolabeled substances can be used for the test,
although non-radiolabeled substances are preferred. The application of radiolabeled substances to
field environments is subject to pertinent national and local regulations.
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(iii) Analytical method. An appropriate analytical method of known accuracy, precision
and sensitivity for the quantification of the active ingredient and major transformation products
should also be included in the study. In most cases, "cold" (i.e., non-radiolabeled) analytical
methods that are sufficiently sensitive to detect and monitor pesticide residues in the field are used.
In order to be useful for terrestrial exposure assessments, the limit of quantitation (LOQ) of the
chosen procedure should be between one and two orders of magnitude less than the expected
concentrations and should ideally be less than the important endpoints for non-target organisms. The
analytical methods are subject to independent laboratory validation (see paragraph (j)(3) of this
guideline). This reference contains a description of environmental chemistry information for use in
validating analytical methods used in conducting field dissipation studies.
(iv) Formulation. The terrestrial field dissipation study should address the effect of
pesticide formulation on dissipation. Different formulations are expected to change the fate or
transport properties of the pesticide. For example, granular or microencapsulated formulations may
release the active ingredient more slowly than emulsifiable concentrate formulations. For this
reason, separate studies should be performed on at least one representative formulation from each of
the applicable formulation groups listed below. If the various commercial formulations of a given
pesticide are not expected to change the fate of the active ingredient, the applicant should provide
data in support of this assumption within the body of the study report. In general, it may be possible
to compare a field study conducted using water soluble liquids/water soluble powders/emulsifiable
concentrates with water dispersible liquids/wettable powders/water dispersible granules. The
behavior of microencapsulated and granular formulations should be addressed in separate field
studies. The recommended groupings of pesticide formulations are as follows:
(A) Water soluble liquids, water soluble powders and emulsifiable concentrates. The
release of an active ingredient into the environment is controlled by the formulation type and the
site-specific environmental conditions. Water soluble liquids and powders form true solutions when
mixed with water, and emulsifiable concentrates consist of oil soluble pesticides and emulsifiers.
These formulations are expected to have little effect on the transport of the pesticide in soil (see
paragraph (j)(4) of this guideline).
(B) Water dispersible liquids, wettable powders and water dispersible granules. Water
dispersible liquids, wettable powders, and dispersible granules consist of finely ground solids of
various dimensions. Various studies indicate that these formulations may affect the transport of
pesticides in soil (see paragraphs (j)(5), G)(6), GX7) of this guideline). For example, Ghodrati and
Jury (see paragraph (j)(5) of this guideline) showed wettable powder formulations may be more
resistant to preferential flow than emulsifiable concentrates and technical grade material dissolved in
water.
(C) Granules. After precipitation or irrigation, granual formulations release the active
ingredient gradually as a function of diffusion or leaching (see paragraph (j)(8) of this guideline).
Therefore, this formulation may have a significant effect on transport of the active ingredient if a
rain event or irrigation occurs after application.
(D) Microencapsulated pesticides. Microencapsulated/controlled- release formulations can
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reduce the potential of leaching through soil (see paragraph(j)(4) of this guideline) but may result in
higher surface losses of a chemical when compared to other formulations (see paragraph (j)(9) of
this guideline). Available literature on the effects of microencapsulated and controlled-release
formulations is inconsistent, and testing of this formulation type should be evaluated on a case-by-
case basis.
(2) Plot design—(i) Environmental fate processes An assessment of the fate of the
pesticide in the terrestrial environment should include all processes that can affect the fate of the
chemical, including transformation, leaching, volatilization, runoff, sorption to soil and plant uptake
(see paragraph (j)(10) of this guideline). Terrestrial field studies should be designed, conducted and
evaluated to assess the most probable routes and rates of pesticide dissipation under conditions
representative of actual use. The physicochemical properties of the pesticide, laboratory
environmental fate data, application techniques and site characteristics should be considered in
designing the study.
(ii) Modular approach. The basic field study design evaluates field dissipation in soil at a
bareground site. If the pesticide-specific conceptual model suggests that volatilization, leaching,
runoff or plant uptake are potentially important dissipation routes, then a modular approach is
recommended whereby dissipation pathways that can be studied concurrently at one site are
included, while those pathways that are incompatible are evaluated in separate studies.
(iii) Use practices and conditions. The study design should encompass the range of
practices and conditions that reflect the actual usage of the test substance. For all field dissipation
studies, non-cropped (bareground) plots should be included. If the proposed use pattern includes
application of a systemic pesticide on a standing crop and it is believed that uptake may be an
important route of removal from the field, the trial should be conducted with a cropped soil in
addition to the non-cropped (bareground) plots. Data generated from laboratory or greenhouse
studies may be used to supplement the field data. However, the use of laboratory or greenhouse data
will call for an explanation of the conditions under which the data were collected and how any
differences between conditions in the laboratory/greenhouse and the field study results and
laboratory hypothesis may influence the evaluation of the field results. The studies should also
include an untreated control plot. Because of field-scale variability, the experimental units in each
terrestrial field dissipation study should be replicated. Replication serves to provide an estimate of
experimental error; improve precision by reducing standard deviation of a mean; increase the scope
of inference of the experiment by selection and appropriate use of variable experimental units; effect
control of the error variance; and allow statistical comparisons of intra- and inter-site variability (see
paragraph (j)(H) of this guideline).
(3) Test site—(i) Field plot systems. (A) Plot size should be adequate to demonstrate the
transformation, mobility and fate of the test material in soil under controlled field conditions
representative of actual use. The decision concerning the plot size in field studies should be based on
factors such as application methods, crop and management factors, site characteristics and
anticipated total number of samples. For pesticides typically applied to cropped or conservation
tillage plots (e.g., with at least 30% crop residues on the surface), bareground pesticide-treated plots
are used to help distinguish dissipation pathways.
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(B) Large-scale studies (see paragraphs (j)(12). (j)(13), G)(14) of this guideline) are
conducted using normal agricultural practices (e.g., cultivation prior to planting, etc.) and
equipment. These studies may be used in combination with other field studies, such as crop residue
studies, provided the Terrestrial field dissipation studies are not disturbed. Small plots (see
paragraphs (j)(15) through (j)(19) of this guideline) are treated using research-plot application
techniques (e.g., hand-held or backpack sprayers) that, in some cases, may reduce the variability
seen in large-scale studies. These small-plot techniques can also limit the ability to interpret results
and obtain satisfactory pesticide dissipation curves. Large-scale and small-plot studies have the
following characteristics:
(1) Large-scale studies. Large-scale studies typically cover a treated area of 8 cropped rows
by 25 m, but may range up to an entire field of several hectares, depending on the design of the
experiment and the use for which the product is intended. Typical plot sizes range from 4 x 10 m to
10 x40m.
(2) Small-plot studies. Small plots (e.g., up to 2 m x 2-6 m or 4-12 m2 in area) are
preferable when pesticide dispersion is uneven and dissipation curves are difficult to generate or
interpret.
(C) Generally, cropped plots are not used in terrestrial field dissipation studies. However, if a
crop is expected to significantly influence the rate and/or route of pesticide dissipation (e.g., runoff
from turf, accumulation in the turf layer, accumulation into the crop, or abiotic degradation and
volatilization from leaf surfaces), then specific greenhouse or small-plot field studies (using the
same crop) are used to address these routes of dissipation. In some cases, though, the studies
conducted to satisfy other environmental fate or human health data requirements may be used. In the
case of foliarly applied pesticides that are systemic, the test substance should be applied to the
intended crop, as specified on the label, to characterize the influence of plant uptake and subsequent
foliar metabolism and to provide a complete picture of the dissipation of the pesticide from the
terrestrial system. The influence of plant uptake and subsequent dissipation should also be
characterized in the case of pre-plant and pre-emergent pesticides as well. When foliar processes
interfere with the characterization of soil dissipation processes, a bare study plot (i.e., not sown to
intended crops and maintained plant free) should be run in parallel to the cropped study. This
analysis can be conducted either within the field design or using suitable laboratory or greenhouse
data. However, the use of laboratory or greenhouse data will require an explanation of the conditions
under which the data were collected and how any differences between conditions in the
laboratory/greenhouse and the field study results and laboratory hypothesis may influence the
evaluation of the field results. While the bare plot study may be an artificial system, it is useful in
providing an interpretable pesticide dissipation curve in the soil.
(D) Cropped plot field studies are called for when plants are an important factor in
controlling field dissipation of the pesticide. Assessing the importance of plant processes in pesticide
dissipation calls for an examination of the mode of action of the pesticide, application timing relative
to canopy development, target crop or environment, and an evaluation of data from confined
rotational crop studies and foliar wash-off studies. Consideration of these factors calls for integration
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of the data into the overall hypothesis testing on probable routes of dissipation. Cropped plots should
be considered in the design of field studies when one or more of the following criteria have been
met:
(7) Systemic pesticides, which are designed to move into and through the plant, are used.
This type of pesticide is expected to become incorporated into the plant either through active or
passive uptake.
(2) Foliar-applied pesticides applied at half to full canopy of the plant are expected to be
predominantly deposited on leaf surfaces. Under these conditions, foliar dissipation is expected to be
the dominant process in the field, although washoff can lead to increased loadings to soil.
(3) Pesticides applied to pasture, foliage crop, grass and turf are expected to strongly
influence dissipation pathways of pesticides.
(4) Evidence of high foliar wash-off fractions or uptake of parent compounds (30-day
emergency crop rotation interval) from rotational crop studies indicate plant processes may control
pesticide dissipation. These studies should be conducted on the same crops as the terrestrial field
dissipation study crop(s).
(E) Execution of a study using a cropped plot should be conducted concurrently with a bare
ground study. Analyses of the data collected from the two plots should be similar except that plants
should be sampled and analyzed for pesticide residues in the cropped plots. The separate collection,
compositing and analyses of soil samples collected within and between the rows of the row crop(s)
may also be necessary. Whole plant residues should be designed to capture either dissipation or
accumulation in the plant. It is recommended that foliar wash-off half-lives, if available, and
potential plant accumulation rates be considered for designing sampling frequency. Crop residues
should be expressed as concentrations on both a dry weight and wet weight basis. Additionally, crop
yields, expressed as the total crop mass (g)/unit area (m2), should be determined at each sampling
time. Recording crop growth stage and area coverage can prove useful in the overall interpretation of
the results.
(ii) Site selection. Field study sites should be representative of the soil, climatic and
management factors under which the pesticide will be used.
(A) The following factors should be considered in selecting field study sites: number of
uses/crops; geographic extent and acreage of the crops/use patterns; soil characteristics;
climate (including temperature, amount and distribution of precipitation, solar exposure and
intensity); use and management practices; crop impacts on pesticide dissipation; pesticide
formulation; timing, frequency and method of pesticide application; and label restrictions regarding
usage, sites or conditions.
(B) Differences between the field study sites and the use patterns of one or more of these
factors could affect the fate properties and dissipation processes of the pesticide, thus reducing the
applicability of field study results beyond the conditions of the study. Geographic information
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system-based decision support models or vulnerability assessment tools that account for the critical
factors affecting pesticide dissipation can be used to determine the most appropriate field sites (see
paragraphs (j)(20), (j)(21) of this guideline). The GIS decision support model utilizes ecological
regions (e.g., the Ecological Regions of North America), geospatial soil and agricultural crops
databases, climatic information, and pesticide properties, including laboratory fate data. Comparable
field study area selection is based on environmental conditions and the conceptual pesticide
dissipation model developed from laboratory fate studies.
(C) The terrestrial field dissipation study should include multiple field sites, generally four to
six study sites. The actual number of sites needed depends on such factors as the number of
formulations, the geographical extent of the use pattern, the number of uses and management
practices as well as the range in soil and climatic conditions within the geographic extent of the uses.
If pesticide use is limited geographically and/or to minor crops, a reduced number of field studies
may be proposed.
(iii) Site characterization. Assessing pesticide dissipation calls for detailed description of
the site characteristics as well as characterization of "representative" soils at each test site. Ideally,
the site selected for the terrestrial field dissipation study should be represented by a single soil type
in order to reduce variability in the field. Such information is critical to assess in situ chemical and
physical properties of the test soil.
(A) Site Description. The study site should be described according to geographic coordinates
(e.g., latitude, longitude), location on a map (topographic map, aerial photograph or soil survey
map), location within the watershed, landforms, landscape position, land surface configuration (e.g.,
slope length and gradient, aspect and direction, micro-relief, roughness, shape, elevation) and depth
to groundwater. A suggested site description sheet can be found in Table 11 of paragraph (i)(5)(ii) of
this guideline.
(B) Soil characterization. (1) At each site, a representative soil pedon should be identified,
and a minimum of one soil profile should be described by soil horizons (preferably 2 m in depth)
using standard soil morphological properties (depth to and thickness of horizons or layers, Munsell
color, texture, structure, macroporosity, depth to a root restricting layer, etc.). Soil profiles will be
described and classified to family or series level according to an internationally recognized system
representative of the areas where the study is conducted. Examples of internationally recognized
systems are the United States Department of Agriculture (USD A) Natural Resources Conservation
Service (NRCS), Canadian or the Food and Agriculture Organization of the United Nations (F AO).
In addition to the description of soil morphology, information on the soil parent material, vegetation,
erosion class, natural drainage class, surface runoff, infiltration and saturated hydraulic conductivity
should be reported. A suggested soil profile description can be found in Table 1.
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Table 1. Sample Description of the Soil Profile (USDA)
TAXONOMIC CLASS: Fine-loamy, mixed, thermic Aridic Paleustalfs; Amarillo Series
PEDON DESCRIPTION: Amarillo fine sandy loam—grassland. (Colors are for dry soil unless
otherwise stated.)
Designation
Description
A
0 to 11 inches; brown (7.SYR 4/4) fine sandy loam, dark brown (7.SYR 3/4) moist;
weak fine granular structure; hard, very friable; many fine roots; many fine and
medium pores; many wormcasts; mildly alkaline; clear smooth boundary. (5 to 19
inches thick)
Bt
11 to 27 inches; reddish brown (SYR 4/4) sandy clay loam, dark reddish brown
(SYR 3/4) moist; moderate coarse prismatic structure parting to weak medium
subangular blocky structure; very hard, friable; many fine and medium pores; thin
patchy clay films on faces of prisms; clay bridged sand grains throughout; common
wormcasts; mildly alkaline; gradual wavy boundary. (8 to 25 inches thick)
Btkl
27 to 38 inches; yellowish red (SYR 4/6) sandy clay loam, moist; weak medium
subangular blocky structure; hard, friable; clay bridged sand grains; common films
and threads of calcium carbonate on faces of peds; interiors of peds noncalcareous;
moderately alkaline; gradual wavy boundary. (8 to 30 inches thick)
Btk2
38 to 56 inches; pink (SYR 7/3) sandy clay loam, light reddish brown (SYR 6/3)
moist; weak medium subangular blocky structure; hard, friable; estimated 60
percent calcium carbonate equivalent; 30 percent by volume is concretions of
calcium carbonate less than 1 inch in diameter; calcareous, moderately alkaline;
gradual wavy boundary. (6 to 36 inches thick)
Btk3
56 to 85 inches; yellowish red (SYR 5/6) sandy clay loam, yellowish red (SYR 4/6)
moist; weak very coarse prismatic structure parting to weak medium subangular
blocky structure; slightly hard, friable; thin patchy clay films and clay bridging of
sand grains; few, mostly vertical stringers of soft bodies of calcium carbonate are
concentrated along faces of prisms; few calcium carbonate concretions less than 1
inch in diameter; calcareous, moderately alkaline; gradual wavy boundary. (8 to 50
inches thick)
Btk4
85 to 99 inches; light reddish brown (SYR 5/4) sandy clay loam, yellowish red
(SYR 4/5) moist; weak very coarse prismatic structure parting to weak medium
subangular blocky structure; hard, friable; thin patchy clay films and bridged sand
grains; few soft bodies of calcium carbonate are concentrated
(2) Soil samples from each horizon should be collected and characterized by determining the
physicochemical properties in the laboratory. The physical properties should include particle size
distribution (i.e., % sand, % silt and % clay, with size fractions specified), textural class (USDA),
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undisturbed bulk density, and soil moisture characteristic curve (0-15 bar) to help determine the soil
water balance throughout the study. The soil chemical properties should include pH, percentage of
organic carbon and cation exchange capacity. Standardized methods should be used and referenced
for the determination of these properties (see paragraphs (j)(22), G)(23) of this guideline for
examples). Depending on the chemical properties or use site, additional analyses, such as clay
mineralogy, specific surface area, and anion exchange capacity (especially in soils dominated by low
activity clays or derived from volcanic materials) of the surface soil layer or epipedon and the
subjacent horizon (layer), may be helpful for determining sorption potential at the field site. A
suggested format for reporting the soil properties is given in Table 12 of paragraph (i)(5)(ii) of this
guideline.
(3) Soil water balance. Soil water content can affect the mode of degradation, degree of
microbial activity, potential for volatilization, plant growth and potential for movement (i.e., up or
down in the soil profile). To interpret routes and patterns of dissipation of the test substance, the soil
water content should be measured on a regular basis to adequately determine the flux of soil water.
Continuous or daily measurements are preferred, but, at a minimum, readings should be collected at
each sampling time. Various methods of measuring soil water include tensiometers, time domain
reflectometry (TDR), neutron probes, gypsum blocks and direct measurement of the moisture
content of the soil samples (see paragraph (j)(22) of this guideline).
(4) Using tracers to track the potential depth of leaching. A conservative tracer can be
applied along with the test chemical to help determine the direction, depth and rate of soil water
movement through the vadose zone. Tracer selection should consider the chemistry of the tracer,
including potential sources of interference, background/baseline levels, analytical detection limits
and potential losses such as plant uptake. If a tracer is used, background concentrations should be
analyzed prior to the study.
(iv) Environmental conditions. Measurement of meteorological variables is necessary to
understand pesticide dissipation in the field. Daily records of maximum, minimum and mean
temperature (air and soil), total precipitation, mean wind speed and potential evapotranspiration are
recommended from five days prior to the first application of the pesticide through to the conclusion
of the study. When irrigation is used to supplement rainfall, timing and amounts of irrigation water
should also be reported. Historical climatological data should be obtained to help evaluate site data
with respect to long-term regional variation, and the source and location of the historical data should
be specified. Historical climatic information should include monthly average rainfall, average
monthly minimum and maximum temperatures, and the dates and the number of days in the average
annual frost-free period. A suggested format for reporting the historical meteorological conditions is
given in Table 13 of paragraph (i)(5)(iii) of this guideline.
(v) Management history. Information on the use of the study site, for example, crops
grown, pesticides and fertilizers used, should be provided for the previous three years. The site
selected should not have a history of the use of the study pesticide or other pesticides of similar
nature (chemical class, common nonvolatile transformation products, etc.) for at least three years
prior to the study. This is used to reduce analytical interferences and potential microbial adaptations
for the test. Management factors, such as tillage and cultivation methods, irrigation practices, etc.,
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should be described in detail (see Table 14 of paragraph (i)(5)(iv) of this guideline for an example
data sheet).
(4) Pesticide application—(i) Label rate. The pesticide product should be applied at the
maximum proposed use rate utilizing the same application method(s) as stated on the label. In
limited instances (e.g., for ultra-low application rates), the pesticide should be applied at a rate
greater than the maximum proposed use rate due to analytical detection limits.
(ii) Timing and number of applications. The timing and the number of applications should
be consistent with labeling. The pesticide application should:
(A) Occur at the typical time(s) of the year and stage(s) in crop development when it is
normally used.
(B) Be performed according to label instructions for the specific formulation (e.g., a granular
pesticide typically applied as a band should be applied as a band in the field dissipation
study).
(C) Be incorporated if the pesticide is typically incorporated.
(D) Be measured by spray cards or similar verification techniques and related to the target
application rate and measured concentration in the spray tank.
(iii) Multiple applications. Where multiple applications are allowed, an experimental
design that enables assessments of dissipation should be used. The study design should include
collection and analysis of samples prior to and after each application with a full set of samples
collected after the final application in order to estimate a dissipation rate. Replicated treatment plots
will evaluate both single and multiple applications. This guidance acknowledges that the use of
multiple applications can complicate the analysis of data generated during the course of the study.
However, a critical aspect of the terrestrial field dissipation study is that the conditions under which
it is conducted reflect actual use conditions for the pesticide as closely as possible. Also, the use of a
seasonal maximum amount of pesticide in a single application can alter the conditions of soil
microbial populations which may alter the results of the study. Given these factors, it is
recommended that the terrestrial field dissipation study be conducted using multiple applications at
the maximum allowable rate specified on the labels for that compound.
(iv) Application equipment. Recommended equipment for pesticide delivery in the
terrestrial field dissipation study should be of high precision, suited for the particular pesticide
formulation (some pesticides may have to be homogenized by a continuous mixing device in the
tank) and outfitted with a device to keep drift loss to a minimum.
(5) Test duration. The duration of the terrestrial field dissipation study, which has
historically taken up to two years to complete, should be sufficient to determine the DT75 of the
parent compound as well as the pattern of formation and decline of major transformation products in
the soil. In determining the decline of the major transformation products, the study duration should
12
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be sufficient to determine the time for major transformation products to dissipate to 25% of their
maximum detected values in the soil. A major transformation product is one accounting for >10% of
the applied amount at any time during the laboratory studies, or one that has been identified as being
of potential toxicological or ecological concern.
(6) Management. The management (e.g., fertilization, seed bed preparation, weed control,
sowing and tillage) of the field dissipation study site should be carried out in accordance with good
agricultural practices. Tillage practices (conventional tillage, conservation tillage or no-till) should
be typical of those used for the particular crop and label recommendations.
(7) Irrigation. The study design should include sufficient water to meet the crop need in
quantity and timing. If the use pattern includes irrigation to supplement the water requirements of
the plant, then the study should be conducted under irrigated conditions. In this case, the study
design should ensure appropriate timing and sufficient water to meet 110% to 120% of the crop
need. Also, in the case of bare plots, the site should receive sufficient water at the appropriate time
to meet the crop water need for the intended crop in that use pattern. In other words, a bare plot site
conducted for a corn use should receive 110% to 120% of the water need for corn in that use area.
Alternatively, if the use pattern does not involve irrigation, then the field studies do not necessarily
have to be conducted with supplemental irrigation. However, it may be necessary to prepare the site
for irrigation in case of drier than normal conditions. For non-irrigated sites, the study design should
ensure that 110% to 120% of normal monthly rainfall is delivered to the site.
(8) Soil sampling. Soil samples for residue analysis should be representative of each
replicate plot at each sampling time. Replicate plots can be defined as repetitive, homogeneous
sections of a field treated with the test pesticide in a similar manner to allow comparison between
treatments. Sampling procedures can have a major effect on variability of pesticide concentrations in
soil; accurate and consistent sampling is vital for meaningful results. Variables such as plot size, soil
variability, crop management practices, pesticide application method and existing knowledge of the
behavior of the pesticide in the environment should be considered in designing an appropriate
soil-sampling protocol.
(i) Sampling patterns. (A) Soil core holes should be marked after sampling. Plugging holes
with soil from untreated areas of the site will prevent the cross-contamination at greater depths and
subsequent anomalous results.
(B) A random or systematic soil sampling pattern (see paragraph (j)(24) of this guideline)
may be followed, depending on the type of pesticide application and other variables listed above. For
example, the soil may be sampled in-row only (e.g., seed furrow or band treatment) or by a random
pattern that covers the entire treatment area (i.e., broadcast application). Because it may be difficult
to obtain interpretable results using an in-row sampling pattern, extreme care should be taken in the
application and sampling procedures.
(C) In order to avoid variability resulting from possible under-coverage, drift or edge effects,
outside rows of treated areas should be excluded from sampling.
13
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(D) In small plots, systematic sampling is preferred to ensure that all treated sectors of the
plot are represented and to make it easier to avoid sampling in a previous core hole or in zones
where spray patterns in successive passes of the application equipment may have overlapped or
failed to cover the surface adequately.
(E) Larger diameter cores are expected to reduce variability in the field. Typically, a core of
one to two inches in diameter has been used in terrestrial field dissipation studies, but use of larger
diameter cores should be considered in the field design.
(ii) Depth of soil sampling. (A) In order to fully demonstrate the fate and transport of the
pesticide under study, soil should be collected from a depth sufficient to encompass the vertical
distribution of the pesticide and its maj or transformation products at each sampling time. Data from
laboratory studies (physicochemical properties, mobility and transformation) can be used in
conjunction with water recharge estimates (e.g., average rainfall data and expected irrigation
coupled with evapotranspiration estimates) and soil permeability properties to establish appropriate
core depths. Soil sampling should proceed to at least a depth of one meter, particularly for pesticides
with laboratory fate characteristics that indicate leaching is an important route of dissipation.
(B) The maj or transformation processes usually occur within the "biologically active" zone
of the soil. For sampling purposes, this zone can be defined as the maximum depth of tillage, rooting
depth of agronomic plants or the depth of an impermeable soil layer, whichever is deepest. If the
laboratory studies indicate a low potential of a pesticide to leach, the emphasis of soil sampling
designs should be placed on this zone of soil rather than subsoils. The "biologically active" soil zone
concept will allow flexibility in experimental design because of different agronomic practices, types
of soil and site characteristics.
(C) For most studies, soil cores should be collected to 1 m in depth and divided into six or
more depth increments for analysis (e.g., 15 cm, 15 cm, 15 cm, 15 cm, 20 cm and 20 cm). For low
application rate pesticides or where the results of the laboratory studies indicate very low mobility of
the parent chemical and its maj or transformation products in soil, core depths could be sectioned into
shorter increments to circumvent dilution of the chemical residues with excess soil. In all cases,
analysis of the sectioned cores should clearly define the extent of leaching of the parent chemical
and its major transformation products in the soil profile.
(D) Soils should be sampled to a sufficient depth such that the lowest section of the sampled
cores does not contain detectable amounts of the active ingredient or maj or transformation products.
In the absence of rainfall or irrigation, the initial or zero time samples can be taken to at least one
sample increment below the depth of incorporation. For example, a pesticide incorporated to 3
inches (7.6 cm) below the surface should be sampled from 0 to 6 inches (0-15 cm) and from 6 to 12
inches (15-30 cm), assuming a 6-inch (15 cm) interval.
(iii) Times of soil sampling. (A) Soil sampling should be carried out prior to treatment,
immediately after treatment (zero time) and at increasing intervals (daily, weekly, monthly) between
sampling times. If more than one application is made, then soil sampling should be done before and
immediately after each application and then at increasing intervals after last application. Time
14
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intervals should be based on the results of laboratory studies and other field studies, if available.
Sampling frequency should consider laboratory half-life estimates with increased frequency of
sampling for shorter half-life compounds. Other factors that may affect sampling frequency include
compound mobility and site-specific environmental conditions (e.g., rainfall and micro-climate). The
frequency of sampling should be concentrated after each application time to characterize the
dissipation of the test substance. However, the number and distribution of sample times should also
be sufficient to adequately characterize the formation and decline of the transformation products.
(B) The dissipation of a product used in multiple applications over a season should be studied
through a full cycle of applications (see paragraph (j)(18) of this guideline).
(C) Residue data should be obtained until at least 75% of the pesticide and/or its major
transformation products have dissipated from the soil profile or the pattern of dissipation has been
clearly delineated (see paragraphs Q)(25), (j)(26) of this guideline). The study sponsor should
determine the DT50 and DT75 from the initial concentration because the dissipation rate constant
often decreases with time (i.e., the half-life is not constant as in first-order kinetics). If 75%
dissipation is not reached by the time it freezes in the fall, the study should be continued in the
following year(s).
(D) The plot should be sampled at the end of the growing season to determine residue
carryover to the next season; sampling in subsequent years may be necessary. Long-term studies
may be recommended if dissipation is slow to occur. This is particularly important for persistent,
low mobility pesticides or for those chemicals that show pesticidal activities at low concentrations.
(E) If a control plot is included in the study design, then soil sampling can be conducted
during the early stages of the study with a limited number of samples. The intention of the control
sample is to ensure that the pesticide is not present prior to application and to provide information
concerning potential loss of the pesticide from drift.
(iv) Time zero sampling. (A) The time zero concentration lays the foundation for all
subsequent sampling and is used to build confidence that the pesticide was applied uniformly and
accurately. The following points should be considered in developing a time zero sampling protocol
for a single application on bare ground:
(1) Availability of an appropriate analytical method with limits of quantitation low enough to
detect the parent and key degradates at relevant concentrations.
(2) Handling of all fortification samples in the same manner as soil samples.
(3) Testing of verification devices before use to provide confidence in compatibility with the
test substance.
(4) Application of reasonable correction factors provided they are within 10% to 20%,
although correction is not necessary.
15
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(5) Verification of the actual rate applied.
(6) Calculation of an expected concentration in the field.
(7) Comparison of time zero concentrations with the expected concentration.
(B) For multiple applications, each application should be treated as time zero, and
concentrations prior to and immediately after application should be determined.
(C) For cropped plots, the time zero sampling strategy should be modified to measure the
portion of pesticide reaching foliage as well as the portion reaching the ground surface.
(D) The initial concentration in the soil immediately after treatment ("time zero") is a crucial
benchmark value. Time zero sampling is recommended to verify residue concentrations reaching the
target and confirm uniformity of its distribution. The pesticide residues in all subsequent soil
samples are evaluated in relation to this value. An initial residue value that is significantly lower
than the value found for a subsequent sample may jeopardize the utility of the study by rendering
estimation of dissipation times (DT50 and DT75) meaningless. It cannot be emphasized enough how
critical the accurate delivery and accounting of a pesticide at time zero are for the evaluation of the
study results. Ideally, a study should utilize techniques that maximize the delivery of the pesticide to
the field at the target rate and keep corrected time zero (time zero concentration after applying
corrections related to delivery efficiency and field monitoring results) results within 10% of that
rate. However, it is recognized that this is a "goal" and that may not always be obtainable.
(E) Determination of time zero concentration involves the following steps: analysis of the
spray tank mixture before and after application; use of collection devices such as filter paper, spray
cards, etc.; and soil sampling immediately after application
(F) Preferably, time zero sampling is conducted in duplicate, and the two sets of soil samples
are processed separately to provide two estimates of the mean time zero concentration. Time zero
sampling data should be used to confirm that the pesticide was uniformly applied to each plot at the
intended rate. Techniques used and any deficiencies associated with the delivery of the pesticide to
the field should be described and accounted for when analyzing the study results.
(G) Although not routine, there may be instances where a cropped plot should be sampled
concurrently with a bare soil plot. In this case, the following factors should be considered in the
sampling strategy of a well-designed protocol: time zero samples; types of sample (i.e., soil versus
plant) and sampling frequency; sample locations (e.g., between rows, under rows); accounting for
plant uptake versus foliar dissipation; residues in roots; chemical factors such as formulation and
application method; and crop characteristics
(v) Number and pooling of samples. (A) The purpose of soil sampling in the terrestrial field
dissipation study is to measure the mean concentration of the pesticide (and its degradates or
transformation products) so that dissipation may be followed quantitatively over time. In order to
16
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generate a reliable estimate of the mean concentration that represents the entire treated plot, a
sufficient number of core samples should be taken to achieve acceptable variability in the
concentration across the plot. The number of cores to estimate the mean concentration (statistically,
the sample size) will then depend on the desired precision (i.e., standard deviation around the mean)
and the variability of the pesticide concentration in the field (the field or population variability).
(B) The statistics of this estimation were developed several years ago (see paragraph (j)(27)
of this guideline) and have been made into a calculator (DQO-PRO) by EPA's Superfund program to
support the development of Data Quality Objectives (DQOs). The DQO process is an experimental
design exercise intended to quantitatively define the sampling effort, given the data precision
necessary to support decision-making (see paragraph (j)(28) of this guideline).
(C) In the case of the terrestrial field dissipation study, the maj or DQO is measurement of the
mean concentration at each sampling time with a small enough error (standard deviation) that the
means at different sampling times over the course of the study can be used to calculate a statistically
significant rate of dissipation from the soil. Data presented by Jones et al. in 2004 at the 227th
American Chemical Society National Meeting (see paragraph (j)(29) of this guideline) suggest that
the standard deviation among 16 samples individually analyzed from a variety of terrestrial field
dissipation studies is about 110%. (This analysis provides an estimate of field or population
variability.) Further analysis by industry (I. van Weesenbeck, 2004 (see paragraph (j)(30) of this
guideline)) suggests that the variability in calculated half-lives is less than the variability of the mean
concentrations from which they are calculated (assuming constant variability of the soil means over
the course of the study), and that standard deviations of up to 100% in the mean concentration result
in tolerable error for half-lives up to one year in length.
(D) Table 2, based on calculations with DQO-PRO, provides the number of individually
analyzed cores needed to estimate, at a 95% confidence level, the mean concentration at any time,
within a known error (standard deviation), given various assumptions about the population
variability. Using the assumption of 110% for a population, a sample size of 15 or 16 cores is
expected to estimate the mean concentration to within 60% standard deviation. (The actual number
of cores calculated by DQO-PRO was 16, but 15 facilitates the use of 3 replicate subplots.)
17
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Table 2. Tolerable Error in Estimation of Mean Concentration
Population
(field)
%CV
200
190
180
170 '
160
150
140
130
120
110
100
90
80
. 70 , '.
60
50
40
30
20
Number of Non-composited Cores Needed to Estimate Mean to
Within Tolerable Error at 95% Confidence
100%
18
17
15
14
13
12
11
10
9
8
8
—
—
—
—
—
—
—
—
90%
22
20
18
17
15
14
13
11
10
9
9
8
—
—
—
—
—
—
—
80%
27
25
22
20
18
16
15
13
12
11
9
9
8
—
—
—
—
—
—
70%
34
31
28
26
23
21
18
16
14
12
11
10
8
8
—
—
—
—
—
60%
46
41
38
34
30
27
24
21
18
;ftS«5;5;8;8;;
14
12
10
9
8
—
—
—
—
50%
64
58
53
47
42
38
33
29
25
22
18
15
13
11
9
8
—
—
—
40%
99
90
81
72
64
57
50
44
38
32
27
22
18
15
12
9
8
—
—
30%
174
157
141
126
112
99
87
75
64
55
46
38
30
24
18
14
10
8
—
20%
387
350
314
280
249
219
191
165
141
119
99
81
64
50
38
27
18
12
8
10%
1540
1390
1248
1113
986
867
756
652
556
468
387
314
249
191
141
99
64
38
18
(E) The study sponsor may use 15 cores per sampling episode as a minimum, although this
minimum number implies that the mean concentration calculated from the 15 cores is no more
precise than the 60% coefficient of variation (CV). If the cores are composited before analysis, the
standard deviation of the results should be corrected for compositing. (Note: If the standard
deviation calculated from the core analysis is greater than the expected 60%, the assumption of
110% CV in the field population may not be valid, and a larger number of cores should be
collected.)
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(F) If greater precision is desired to support more quantitative uses of terrestrial field
dissipation study results, or if greater than 110% variability in the field is expected, then the study
sponsor should use the table provided in Table 2 or the DQO-PRO calculator to find the number of
cores necessary to achieve the desired level of precision.
(G) In Table 2 in paragraph (f)(8)(v)(D) of this guideline, the tabular values indicate the
tolerable error that can be achieved in estimating the mean concentration (top row) when 30 to 45
cores are taken from fields having variability in concentration indicated by CV figures in the left-
most column (population). These calculations assume that samples are analyzed individually or that
a correction for compositing is made. For example, if a 200% population (field) CV is assumed, 34
cores will estimate the mean concentration to within 70% error, and 46 cores will estimate it to
within 60% error, after correction for compositing. If a 100% population (field) CV is assumed, then
46 cores will estimate the mean concentration to within 30% error, after correction for compositing.
(H) Finally, certain principles apply to the study design, regardless of the intended use of the
study results. The number and diameter (typically 3 to 12 cm) of soil cores should be based on the
size of the plot, the type of soil and the amount of soil necessary for analysis. Corresponding depths
of soil cores from a single replicate plot can be pooled and mixed thoroughly to produce one
representative composite sample that can be analyzed. An adequate number of cores per plot should
be collected at each sampling time to ensure the sample is representative of the plot. For example, a
composite sample from a 2-m x 1-m small plot may consist of 15 soil cores (3-cm diameter) per
sampling time over a period of one year (see paragraphs (j)(15), (j)(16), (j)(31), (j)(32) of this
guideline). In large plots, cores of greater diameter are usual (see paragraphs (j)(12), G)(24) of this
guideline). The variability within a large plot is typically greater than in a small plot because of less
uniform pesticide application and soil spatial variability. For field studies of longer duration with
small plots, the plot area should be increased to accommodate collection of a greater number of
cores, resulting from an increased number of sampling times. If a large-scale plot contains areas of
different types of soil, soil organic matter content, etc., or knolls/depressions, then representative
cores from areas of different soil types should be pooled and analyzed separately from other
samples.
(vi) Handling of samples. Soil samples should be handled in the following manner:
(A) Soil samples should be frozen if they cannot be extracted immediately.
(B) Air-drying of soil samples before extraction is not recommended because of possible loss
of chemical residues from the samples via volatilization.
(C) To check the stability of pesticide residues during storage, untreated soil samples should
be fortified in the field with analytical standards (parent chemical and major transformation
products), stored, and then extracted and analyzed within the same time period and in the same
manner as samples from treated field plots (see paragraph (j)(17) of this guideline). Recovery results
from field-fortified samples are preferred to recovery data from more conventional storage stability
studies such as laboratory-fortified samples.
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(9) Sampling of other media. Measuring pesticide residues in soil overtime provides direct
information on a limited number of dissipation routes, e.g., transformation, sorption and leaching.
Other dissipation routes that often play major roles in the environmental fate of a compound include
accumulation and metabolism in plants; volatilization from soil, water and/or plant surfaces; soil
binding; runoff; and spray drift. To meet the objectives of the terrestrial field dissipation study and
to determine where the pesticide goes in the environment, the study sponsor should design the
sampling scheme to account for routes of dissipation that cannot be accounted for through soil core
sampling alone.
(i) Sampling plants and foliage. (A) When the pesticide is applied to cropped plots, plant
material should be sampled. The sampling scheme should be designed to track the decline in
pesticide residues from foliage with time, and foliage sampling should include a time zero residue
level. Pesticide residues may also be affected by abiotic degradation (hydrolysis and/or photolysis),
be translocated into plant foliage and volatilize from foliage more readily than from soil. If any of
these processes from foliage are a likely route of dissipation, the study design should ensure that
appropriate measurements are made. In contrast to soil sampling times, foliage samples should be
collected more frequently at the beginning of the study in order to adequately characterize foliar
dissipation.
(B) It may be appropriate to use existing laboratory and/or greenhouse plant studies as a
substitute for a full scale field sampling of plant material. However, when relying on
laboratory/greenhouse data to support a route of dissipation in the field study, the registrant should
characterize any differences between the conditions under which the laboratory/greenhouse studies
were conducted relative to the field dissipation study. These laboratory/greenhouse studies should be
conducted using similar conditions as those present in the field study, e.g., plants, application,
treatment, etc., if possible. The registrants should consider collecting a set of benchmark samples
from the field study to determine how much of the pesticide was removed by the crop and for
comparison with the laboratory/greenhouse studies.
(ii) Air sampling. Monitoring studies have found pesticide residues in the atmosphere,
demonstrating that some pesticides have the potential to volatilize from the field (see paragraph
(j)(33) of this guideline). Many pesticides are soil-incorporated, though, to retard volatilization and
enhance efficacy. In cases where the vapor pressure and Henry's law constant of the pesticide or
site-specific environmental conditions (e.g., warm temperatures, windy conditions) suggest potential
volatilization, the terrestrial field dissipation study should provide meaningful data on volatilization
losses from the field. In this case, air sampling, with methods that measure pesticide residues in the
vapor phase, may be needed to determine whether volatilization is a route of dissipation. Air samples
should be collected more frequently at the beginning of the study to adequately characterize the
volatilization of the test substance.
(iii) Sampling for pesticide residues in runoff. Laboratory studies may indicate the
potential for pesticide residues to move offsite dissolved in runoff water or through erosion.
Typically, the terrestrial field dissipation study is conducted on a site that is essentially flat.
However, if the use pattern suggests that the pesticide will be used in areas of significant slope (e.g.,
orchard uses) or that there are significant risks associated with aquatic exposures from runoff, then a
20
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runoff component may be necessary (see paragraph (j)(34) of this guideline).
(10) Sampling strategies to increase sensitivity. Strategies that could be used to increase
the detection sensitivity of pesticides in terrestrial field dissipation study samples include the
following: decreasing the thickness of sample soil depth (thinner increments); increasing the area of
soil or foliage samples; increasing the volume of runoff water or air samples; increasing the
application rates; increasing the number of replications; refining/improving analytical methods for
parent and major transformation products; and improving recovery efficiencies.
(g) Conceptual model—(1) General considerations (i) Well-designed terrestrial field
dissipation studies answer the risk assessor's basic question: "Where did the pesticide go when
applied in the field?" By using a conceptual model in the study design phase, the study sponsor can
address this question by determining which routes of dissipation should be evaluated in order to
adequately characterize the behavior of a pesticide in the field under actual use conditions. The study
sponsor should consider the use pattern and ensure that the overall study design accounts for
potential formulation effects. Different designs may be necessary for multiple formulation types,
such as granular and emulsifiable concentrate.
(ii) Before conducting a study, the study sponsor should carefully consider all potential
processes and routes of dissipation as well as determine which of these are critical to answering the
risk assessor's basic question (see Figure 1). A conceptual model, based on pesticide properties,
laboratory environmental fate results, formulation type and anticipated use patterns, can focus the
studies on the major routes of dissipation. A dissipation route should be included in the study design
if it is expected to explain, in part, the observed rate of chemical dissipation from the surface soil.
Figure 1: Conceptual Model of the Factors Affecting the Field Dissipation of a Chemical
Volatilization &
Pliototransformatidn
Applied
Pesticide
Foliar
Interception
&
(iii) One way to approach the study design is to consider each route of dissipation as a
21
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potential study module. Using the conceptual model, the study sponsor can determine which
modules are needed to adequately characterize the active routes of dissipation in the field. An
advantage of this approach is that it offers flexibility in addressing data needs by including modules
either concurrent with or separate from the basic field study. With this approach, not all modules
have to be performed in the same study. For example, runoff experiments may be conducted in
small-plot studies, and volatility experiments may be conducted as separate experiments in either the
field or the laboratory. Ultimately, the decision regarding when to include a module rests with the
study sponsor.
(iv) Before initiating a terrestrial field dissipation study, the study sponsor should develop a
working hypothesis of the pesticide-specific conceptual model. This working hypothesis can form
the foundation for optional consultations with the Agency and can be included as a section in the
final report. The working hypothesis is the foundation for the pesticide-specific conceptual model
and forms the basis for determining how well the study design captures the fate of the pesticide in
the field under actual use conditions. The working hypothesis should include the following
parameters:
(A) Estimates for each module's contribution to the dissipation process (quantitative and/or
qualitative) based on laboratory physicochemical and fate properties.
(B) Basic study modules:
(1) Soil abiotic/biotic transformation.
(2) Leaching.
(C) Additional modules:
(1) Volatilization.
(2) Runoff.
(3) Plant uptake.
(4) Deep leaching.
(5) Others.
(v) The conceptual model described above should then be modified based on the anticipated
conditions in the individual terrestrial field dissipation study sites. Modifications of the laboratory
estimated contribution to dissipation for each module (both quantitative and/or qualitative) should be
described for both the basic study modules and any additional modules that are necessary based
upon a review of laboratory data. The modifications to the conceptual model should be based on
field soil properties compared to soils used in laboratory studies, weather data, water balance,
formulation type, mode of delivery, crop influence (if any), agronomic practices and other factors.
22
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(vi) The study sponsor should consider the following when determining if a module should
be included or excluded:
(A) Only those routes of dissipation that are included in the field study or measured by an
acceptable guideline study can be claimed to "significantly" affect the fate of a pesticide and/or its
degradates in the field.
(B) Additional modules should not be excluded from the study when data indicate that
associated processes may contribute to "significant" pesticide dissipation or result in any pesticide
dissipation of toxicological concern. (See paragraph (g)(2) of this guideline for a discussion of
indicators that are used to determine inclusion of additional modules.)
(C) Ideally, when all modules are chosen, total dissipation attributed to excluded modules
should not exceed 10-20%.
(D) Because drift modules are not included in the study, special equipment should be used to
minimize any loss due to spray drift.
(vii) Ultimately, it is the responsibility of the study sponsor to establish a hypothesis of the
routes of dissipation (i.e., the conceptual model) that will affect the outcome of the terrestrial field
dissipation study. The terrestrial field dissipation study should test the established hypothesis, and
the final report should include the hypothesis and the results analyzed in order to confirm or modify
the hypothesis (see Figure 2)
23
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.Figure 2—Iterative process for evaluation of terrestrial field dissipation results relative to the
presticide-specific conceptual model.
Iterative Process of Evaluating the Conceptual Model of
Terrestrial Field Dissipation
Required
Studies
Use in Models to
Estimate Soil
Dissipation
Compare
Develop Field
Dissipation
Kinetics
Risk
Assessment
Higher Tier Studies
As Needed
(2) Additional study modules (i) The basic terrestrial field dissipation study focuses on
pesticide dissipation from the soil surface layer in a bareground study; it can be used to estimate
field degradation only when other major routes of dissipation (e.g., sorption and binding, leaching,
volatilization, runoff and plant uptake) are quantified and shown to be negligible. In addition to the
guidance described in this document, the Agency may use other dissipation studies to answer
specific risk assessment questions. In deciding if an additional study module should be formed as
part of a field study, the study sponsor should ask the following questions:
(A) What is the potential for dissipation of the parent compound and its major transformation
products by a given route (e.g., volatilization, leaching, runoff, plant uptake, etc.)?
(B) Is the potential great enough to warrant measurement under field conditions
representative of actual use?
24
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(ii) In most cases, using the suggested criteria found in paragraph (g)(3) or a lines-of-
evidence approach based on physicochemical properties and laboratory fate data is the best way to
answer these questions and to determine if an additional module(s) should be included in the
terrestrial field dissipation study.
(3) Module selection—(i) Field study indicators. In deciding what modules to incorporate
into a field study, the study sponsor should ask the following questions:
(A) What is the potential for dissipation of the parent compound and maj or transformation by
a given route (e.g., volatilization, leaching, runoff, etc.)?
(B) Is the potential great enough to warrant measurement under field conditions
representative of actual use?
In many cases, several criteria or a weight-of-evidence approach based on physicochemical
properties of the test substance and laboratory studies is the best way to answer these questions. No
single laboratory study by itself can absolutely predict transformation, transport or dissipation in the
field. Laboratory data can, however, provide quantitative or semi-quantitative indices of the inherent
persistence and mobility under field conditions.
(ii) Volatilization potential. Important physicochemical properties influencing
volatilization are vapor pressure and solubility in water. The partitioning of a chemical between air
and water is described by Henry's law and can increase or decrease the volatilization potential.
Adsorption to soil is an important process that reduces volatilization. Volatilization from soil and
water may be specially studied under laboratory conditions to gain additional knowledge.
Quantification of trapped volatile organics in standard laboratory studies of
biotransformation/metabolism in soil and aquatic systems also addresses volatilization of the parent
compound and transformation products. Other factors that may be considered include method of
application (e.g., foliar versus soil surface versus soil incorporated, injected and watered-in),
temperature, soil moisture content, soil organic carbon content, soil texture, soil porosity, residue
persistence and leaching.
(iii) Vapor pressure. The measured vapor pressure of a chemical compound is a guide to its
volatility and to the probability of its movement into the atmosphere. A volatility classification
based solely on vapor pressure is best suited to dry, non-adsorbing surfaces. In general, pesticides
with vapor pressures < 1 x 10"6 mmHg (1.33 x 10"4Pa= 1.33 x 10"1 mPa) are considered relatively
non-volatile under field conditions, whereas pesticides with vapor pressures > 3.9 x 10"5 mm Hg
(5.20 x 10"3 Pa = 5.2 mPa) are considered to be of intermediate to high volatility under field
conditions (see paragraph (j)(35) of this guideline). Thus, a vapor pressure > 3.9 x 10"5 mm Hg or
5.2 mPa at 25°C raises concern regarding potential volatilization and vapor drift of the active
ingredient.
(iv) Henry's law. Henry's law addresses the partitioning of a compound between water and
air, a process that can increase or decrease the overall volatilization of the compound from a water or
moist surface (see the following Table 3).
25
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A unitless water/air distribution ratio can be calculated by the following equation (see
paragraphs (j)(36), Q))(37) of this guideline):
Equation 3:
Cwater =rSx 1x82.08x760^
C.ir I PxGMWxlO6 J
where:
s
T
82.08
760
P
GMW
concentration of the compound in water [|ig/mL]
concentration of the compound in air [|ig/mL]
the solubility of the compound [jig/mL]
absolute temperature [°K = °C + 273.15]
gas constant, R, [(mL x atm) / (°K x mol)]
mm/atm
vapor pressure [Torr] of the compound
gram molecular weight of the compound [g/mol]
Table 3. Volatility classification from a water surface based on Cwater/C
air-
C /C
^water* ^
<102
102-103
103-105
>105
Volatility Class
Rapidly lost from a water surface
Volatile from a water surface
Slightly volatile from a water surface
Non-volatile
(v) Soil adsorption effects. Because adsorption to soil can significantly reduce
volatilization, volatilization from a moist soil is assumed to be volatilization from water modified by
adsorption. The distribution ratio between wet soil and air can be calculated by the following
equation (see paragraphs (j)(36), (j))(37) of this guideline):
26
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Equation 4:
C C
water+soil water
, v
-- r IS.,
air
-'water + soil
-water
= concentration of the compound in wet soil (w/w on a dry weight
basis)
= concentration of the compound in water (w/v),
= concentration of the compound in air (w/v),
= (weight of soil)/(weight of water), and
= linear adsorption coefficient
Although no generic classification of volatility from moist soil was presented by Burkhard
and Guth and EPA (see paragraphs (j)(36), Q)(37) of this guideline), several non-fumigant
compounds were categorized as volatile, slightly volatile and non-volatile from moist soil, based on
their wet soil/air distribution ratios and assuming a standard soil containing 2% organic carbon and a
value of 6 for r, the soil/water weight ratio (see the following Table 4)
Table 4. Estimated Tendency of Compounds to Volatilize from Water and Moist Soil
Compound
Vapor Pressure
(mm/Hg)
(mPa)
Solubility
in Water
(ug/mL) .
^ water* '>-* air
Kda
C/jp b
water + soil * ^air
Fumigants
cis-l,3-D
tmns-l,3-D
EDB
DBCP
2.5 x 10
1.85 x 10
7.7 x 10'1
5.8 x 10"1
2700
2800
3370
1230
1.77 x 10
2.49 x 10
4.33 x 10
1.67 x 102
0.51
0.56
0.65
2.58
1.2 x 10
1.81 x 10
3.54 x 10
4.59 x 102
Volatile from moist soil
chloroneb
EPTC
dichlobenil
3.0 x 10'3
1.97 x 10"2
5.5 x 10"4
2.62 x 103
7.32 x 10
8
370
18
2.35 x 102
1.84 x 103
3.48 x 103
23.2
5.66
3.28
5.49 x 103
1.07 x 104
1.2 x 104
Slightly volatile from moist soil
27
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Compound
disulfoton
diazinon
Gamma-HCH
isazophos
DDT
Vapor Pressure
(mm/Hg)
1.8 x ID'4
3.2 x 10"5
—
1.9 x 10"7
(mPa)
9.71
4.26
4.26
2.53 x 10"2
Solubility
in Water
(ug/mL) .
15
40
10
150
0.0012
^ water* '>-* air
5.55 x 103
3.29 x 104
1.96 x 104
2.73 x 105
3.26 x 102
:«<."
42.6
10
26.8
2.06
4860
C/jp b
water + soil * ^air
2.37 x 105
3.34 x 105
5.29 x 105
6.08 xlO5
1.58 x 106
Non-volatile from moist soil
parathion
metolachlor
chlorpropham
atrazine
methidathion
monuron
metalaxyl
3.8 x 10"5
—
1.0 x ID'5
8.9 x lO'7
—
5.0 x 10'7
—
5.05
1.73
1.33 x 10'1
6.65 x 10'2
2.93 x 10'1
20
530
88
33
240
230
7100
3.3 x 104
2.63 x 106
8.0 x 105
3.2 x 106
1.45 x 107
4.2 x 107
2.11 x 108
209
2.73
11.8
3.44
3.71
1.66
0.75
6.9 x 106
7.62 x 106
1.0 x 107
1.2 x 107
5.62 x 107
7.67 x 107
1.93 x 108
asoil adsorption coefficient corrected for a standard soil containing 2% organic carbon
bsoil to soil water (w/w) = 6; soil water to soil air (v/v) = 1
Considering the values calculated for Cwater + soii/Cair in the previous table, the following
categorization seems reasonable for volatilization from moist soil with 2% organic carbon and a soil
to water ratio (w/w) of 6 (see the following Table 5).
Table 5. Volatility classification from moist soil based on Cwater + soii/Cair
ter+soil'^air
3
< 1 x 10
1 x 103-1.5x 104
1.5 x 10 - 1 x
1 x 105-2x 106
>2x 106
Volatility from Moist Soila
Rapidly lost from moist soil
Volatile from moist soil
Intermediately volatile from moist soil
Slightly volatile to non-volatile from moist soil
Non-volatile from moist soil
a2% soil organic carbon, soil to soil water (w/w) = 6 and soil water to soil air (v/v) = 1
Based on the above data and categorization, volatilization of chemicals from soil under
laboratory conditions should be investigated for chemicals with a volatility (Csoii + water/Cair)-value <
28
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106. Furthermore, values <105 indicate that volatility studies are called for under field conditions.
(vi) Leaching potential. The movement of a chemical through soil is dependent on several
factors including rainfall and irrigation and the properties of the chemical and the soil. In general,
leaching is faster and more extensive in coarse-textured soils and soils that have low organic matter
and clay content. An assessment of leaching potential at sites in specific use areas should also
consider the likelihood of potential preferential flow through relatively large soil voids, e.g., cracks,
root channels and Karst topography. A mobility classification based on soil column leaching was
developed by Guth and Hermann (see paragraph (j)(38) of this guideline). Monuron has been
proposed as the reference compound.
Relative mobility factors (RMF) from soil column leaching studies and corresponding
mobility classes for a variety of pesticides are presented in Table 6, adapted from Guth and Hermann
(see paragraph (j)(38) of this guideline). The relative mobility factor is calculated as follows:
Equation 5
RMF =
peaching distance of test compound (cm) 1
I leaching distance of reference compound (cm)J
Adsorption of a chemical to soil, expressed as the adsorption coefficients, Kd and KOC, is a
major determinant of leaching potential. The following mobility classification of Me Call et al. (see
paragraph (j)(39) of this guideline) is based on the soil organic carbon adsorption coefficient, KOC,
and is best suited to non-ionic chemicals (see the following Table 6).
Table 6. Relative mobility factors.
RMF— -Range
<0.15
0.15-0.8
0.8-1.3
1.3-2.5
2.5-5.0
>5.0
Compound (RMF)
fluorodifen (< 0.15), parathion (< 0. 15)
profenophos (0.18), propiconazole
(0.23), diazinon (0.28), diuron (0.38),
terbuthylazin (0.52), methidathion
(0.56), prometryn (0.59), alachlor
(0.66), metolachlor (0.68)
monuron (1.00), atrazine (1.03),
simazin (1.04), fluometuron (1.18)
prometron (1.67), cyanazin (1.85),
bromacil (1.91), karbutilate (1.998)
dioxacarb (4.33)
monocrotophos (> 5.0), dicrotophos (>
5.0)
Mobility Class
I immobile
II slightly mobile
III moderately mobile
IV fairly mobile
V mobile
VI very mobile
29
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The following Table 7 describes the classification of soil mobility potential of chemicals
based on HPLC retention times (see paragraph (j)(39) of this guideline).
Table 7. Classification of soil mobility potential of chemicals based on HPLC retention times
0-50
50-150
150-500
500-2000
2000-5000
>5000
Mobility Class
Very high
High
Medium
Low
Slight
Immobile
Leaching potential is indicated by a mobility classification of medium to very high.
Dissociation of ionic compounds in response to the ambient soil pH affects adsorption and,
therefore, mobility in soil. Anionic species that have a negative charge at ambient soil pH are likely
to have a very high leaching potential. The effects of soil pH on the adsorption of acids and bases by
soil is summarized by Tinsley (see paragraph (j)(40) of this guideline). (See the following Table 8.)
Table 8. Effect of pH on Adsorption of Acids and Bases by Soils
Compound
Strong acid
Weak acid
Strong base
Weak base
Polar molecule
Non-polar molecule
Molecular/Ionic
LowpH
Anion
Neutral molecule
Cation
Cation
Neutral molecule
Neutral molecule
Species
ffighpH
Anion
Anion
Cation
Neutral
Neutral molecule
Neutral molecule
pH Effect
Small
Large effect: less
adsorption at
pH > pKa
Decrease at very low pH
Increasing adsorption to
PH = pKa,
decreasing with pH < pKa
Small effect
Little effect
Other factors, such as the compound's persistence, affect its leaching potential. Cohen et al.
(see paragraph (j)(41) of this guideline) summarized the various physicochemical, transformation
and mobility characteristics of a chemical that has the potential to leach under standard soil
conditions: solubility in water > 30 mg/L; Kd < 5 and usually < 1 or 2; Koc < 300 to 500; Henry's
law constant < 10"2 atm-mVmol; negatively charged (either fully or partially) at ambient pH;
hydrolysis half-life > 25 wk; photolysis half-life > 1 wk; and half-life in soil > 2 to 3 wk. Note that
all of these criteria should be considered together, not individually, in the assessment of leaching
30
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potential. Gustafson (see paragraph (j)(42) of this guideline) developed the following leaching
potential index, based on persistence in soil and adsorption:
Equation 6
= log10(t1/2soiiX4-log10(Koc))
where: t/2 soii = 50% decline time in soil under field conditions
Koc = soil organic carbon adsorption coefficient
This index is best suited for non-ionic compounds. More importantly, it is better to use
laboratory soil metabolism /biotransformation values for t/2 soii, as field values include decline via
leaching (which is what is being assessed). In any case, based on the calculated GUS score, the
leaching potential of compounds can be as follows:
Table 9. Classification system based on calculated GUS scores (Gustafson, 1989)
GUS
>2.8
>1.8and<2.8
<1.8
Leaching Potential
leacher
borderline leacher
non-leacher
The leaching potential of compounds with GUS scores > 1.8 should be investigated further.
(vii) Using this approach, the following modules should be considered in all phases of the
study design:
(A) Leaching. Laboratory studies on adsorption/desorption, column leaching, solubility and
persistence can predict the possibility of leaching beneath the root zone. The basic terrestrial field
dissipation study has traditionally incorporated a leaching component and calls for analyses of soil
cores extending below the surface (generally considered as 6 in. or 15 cm) to a given depth . If
neither the parent nor degradates of concern are detected in all cores below a given depth (see
paragraphs (j)(10), (j)(43) and )(j)(44) of this guideline), analysis of deeper cores is usually not
necessary. However, if leaching mechanisms other than flow through a porous medium are
suspected for the site in question (preferential flow or karst topography), then all soil core depths
should be analyzed. A conservative tracer, such as bromide ion, should be applied to the test plot to
verify the depth of water leaching over the course of the study.
(B) Runoff. Runoff is possible for both weakly adsorbed, highly soluble chemicals and
31
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strongly adsorbed, slightly soluble chemicals. The former may run off in the dissolved phase, and
the latter adsorbed on the particulate phase. However, the potential for runoff depends more on the
type of formulation, cover crop, mode of application (e.g., surface application versus soil
incorporation) and site factors (e.g., slope, type of soil, infiltration capacity and rainfall intensity)
than on the chemical properties of the active ingredient(s) and transformation product(s). Depending
on the conditions of the particular field dissipation study site, loss due to runoff may be a significant
or insignificant component of pesticide dissipation from the surface. A simple runoff collector at the
downslope edge of the field may be adequate to adjust for the amount of pesticide loss due to runoff
from an unanticipated event (i.e., storm).
(C) Volatility. Volatilization of an applied chemical is a function of partitioning of the
chemical into solid, liquid and gaseous phases in the soil environment as well as other factors (e.g.,
wind speed, temperature and humidity). However, the application method of the chemical (e.g., soil-
incorporation and watering-in) may serve to suppress volatilization. For example, soil-incorporation
and watering-in are used to limit chemical losses to volatilization.
(D) Plant Uptake and Translocation. For systemic pesticides and transformation products
whose mode of action involves uptake through plant tissues (roots, leaves, etc.), this pathway may be
a significant route of dissipation. The study sponsor can characterize this route by conducting a
cropped-plot study in the field or by greenhouse studies on the same crop.
(viii) In summary, the process of selecting modules to include in the suite of terrestrial field
dissipation studies is the responsibility of the study sponsor. The study design should anticipate the
needs of the risk assessor who will rely on a clear explanation of the assumptions used in the
development of the study design. Although not essential, the study sponsor may consult with the risk
assessor and the risk manager on the design of the pesticide-specific conceptual model early in the
process. Early consultation will give the study sponsor time to assess the needs of the risk assessor
and conserve time and resources. A well-developed pesticide-specific conceptual model should be
prepared and used as the basis for such consultation.
(ix) As noted above, the terrestrial field dissipation study is a keystone study, in that it
provides the primary means for testing the hypothesis of environmental transformation/degradation,
transport and fate developed during the problem formulation phase of a risk assessment. The current
guidance has been developed to provide the risk assessor with a better understanding of the
assumptions and limitations inherent in the data, an improved perspective on the estimate of error in
the study results and, ultimately, better confidence in the data generated. The guidance has been
written to provide maximum flexibility for the study design while increasing confidence in the data.
Therefore, the study designer should look to the overall hypothesis of pesticide fate based on a
combination of data, including laboratory studies and physicochemical properties as well as climate,
soil, agronomic and site characteristics. Once a hypothesis is developed, the study design may
include additional modules as needed. The modules may be run concurrently with the basic soil
study or may be "plugged in" using other data, as long as the data are scientifically valid and
appropriate. One of the most important points to remember when designing this study is that the
results of the study describe the pesticide's major routes of dissipation in the environment.
32
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(4) Use of terrestrial field dissipation study results (i) The results of the terrestrial field
dissipation study are used to validate and/or refine the established hypothesis that the pesticide
dissipates in accordance with the pesticide-specific conceptual model. Differences between field
study findings and the established pesticide-specific conceptual model may suggest the need for
revision of the pesticide-specific conceptual model and possibly the need for additional laboratory
and/or field studies (see Figure 2 of this guideline).
(ii) While this section provides examples of where the terrestrial field dissipation study
results may be used quantitatively, the value of this study in qualitative assessments should not be
overlooked. A critical component of all risk assessments is the characterization of risk, in which the
assumptions, limitations and uncertainties inherent in the risk assessment are captured and the
potential effect of these factors on overall risk are explained. The terrestrial field dissipation study
results have been and will continue to be a critical element of the risk characterization component of
the risk assessment; it is the only avenue by which the laboratory-based hypothesis of field behavior
can be tested.
(iii) Results of field dissipation studies are used to estimate the field persistence of parent
compound, formation and decline of transformation products, residue carryover, and leaching
potential under representative actual use conditions. When other modules are included in the study,
results of these tests may provide important information on major dissipation routes such as
transformation, transport, volatilization, plant uptake and runoff. Although not specifically relevant
to this technical guidance document, a brief discussion of how the terrestrial field dissipation study
results can be used in risk assessments deserves consideration. In addition to its value in
characterizing the dissipation of a pesticide in an actual field environment, field dissipation study
results can be used to evaluate the algorithms and input data for environmental fate models, and the
results can be used to develop more refined ecological risk assessments. The following sections
discuss some of the potential uses and limitations of using terrestrial field dissipation results
quantitatively.
(5) Model evaluation. The results of terrestrial field dissipation studies can be compared
with pesticide estimations generated by the Pesticide Root Zone Model (PRZM) to evaluate how
well the model is performing. Although the current field study does not always track specific routes
of dissipation and identify reasons for discrepancies, field dissipation studies can be designed to test
hypotheses regarding routes of dissipation predicted by environmental fate models such as PRZM.
Not only can modeling efforts be used to focus and interpret the results of field dissipation studies,
but the study results can also be used to evaluate the model.
(6) Input for environmental fate and transport models (i) Currently, the Agency does not
routinely use dissipation rates determined in the field as degradation inputs for fate and transport
modeling, such as the coupled PRZM and EXAMS (Exposure Analysis Modeling System). Such an
application is misleading when reported dissipation half-lives (often DTso (time for 50% dissipation
of the parent chemical) values and not true half-lives) include the combined routes of dissipation
(degradation/transformation and transport) from the surface. A rapid field dissipation rate may be
due to degradation, movement out of the surface soil, or both. Thus, the reviewer would expect a
persistent, highly mobile chemical to have a short half-life (ti/2) in the surface (provided rainfall or
33
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irrigation occur) because it would move out of the surface.
(ii) Current models use inputs that represent the individual routes of dissipation (degradation
half-life values, rate constants, sorption/partitioning coefficients) to simulate overall dissipation.
Thus, substitution of a persistence half-life for a dissipation half-life would effectively treat
movement out of the surface (and potentially into the compartment of interest, i.e., surface water or
groundwater) as if it were degradation. It may be possible, in some instances, to replace the
route-specific model inputs with a combined dissipation rate determined in a field study under the
following conditions:
(A) The sole focus of the modeling effort is to simulate runoff into a water body, and thus an
estimate of the amount of chemical that is available at the surface and subject to runoff over time
may be provided.
(B) The weight of laboratory and field evidence indicates that dissipation in the field can be
confidently attributed solely to degradation/transformation (i.e., negligible loss by the other
dissipation routes, such as leaching, runoff, volatilization and plant uptake).
(7) Terrestrial exposure assessment. Although terrestrial field study results can be used to
determine the potential for pesticide residues to remain in the soil from year to year, most of these
studies do not provide adequate information on plant residue concentrations or residue
concentrations in other food sources, such as seeds or insects, in a manner that can be used in refined
terrestrial exposure assessments. However, when data are collected from foliage/food sources, they
can provide estimates of residue concentrations overtime under actual use conditions in refined risk
assessments. In these cases, study results have been used to calculate estimated exposure
concentrations (EECs) in soil for buffer zone determinations in terrestrial habitats. Finally, results
from terrestrial field dissipation studies can be used to evaluate the potential for carry-over of
residues (both parent and degradates) from one crop season to the following. This is particularly
important for persistent pesticides used in colder climates where the potential for persistence is
greatest. Evidence of from Terrestrial field dissipation studies will have implications for long-term
exposure to non-target organisms and may trigger additional studies (e.g., soil accumulation).
(8) Refined risk assessments (RRAs). Refined risk assessments produce a range or
distribution of values instead of one fixed value produced in a deterministic approach. Current
research is focused on refining risk assessment through the implementation of advanced probabilistic
models that look at multiple pathways for exposure and allow for sensitivity analysis to determine
the significance of exposures to overall risk. Well-designed Terrestrial field dissipation studies can
provide results that are useful for interpreting and providing feedback on model assumptions and
results, and may even be considered as possible inputs for Monte Carlo analysis.
(h) Data analysis and interpretation—(1) Statistical analysis Data gathered from the
study should be analyzed by statistical methods that describe the pesticide's rate of dissipation.
Methods should be specified and consistent with the study design; goodness of fit of the data to the
statistical analysis should be provided. Analysis should emphasize the dissipation of the pesticide
34
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from the upper soil layer to which the pesticide is applied, as well as comparisons of within-site and
among-site variability.
(2) Data interpretation and quantitative assessment An evaluation of the data collected
in the field dissipation study and interpretation of the results should include the following
considerations:
(i) Half-life and times for 50% and 75% dissipation of the parent chemical (DT50 and DT75,
respectively) under field conditions, determined from the residue data.
(ii) Dissipation parameters of the major transformation products (e.g., quantities and rates of
formation and decline, including DT50).
(iii) Mobility of the parent compound and the major transformation products under field
conditions.
(iv) A comparison of the dissipation and mobility parameters from the field studies with
corresponding results from laboratory studies and predictions based on the pesticide's
physical/chemical properties (e.g., solubility in water, vapor pressure, Henry's law constant,
dissociation constant and w-octanol-water partition coefficient).
(v) Plant uptake of pesticide residues in the field compared with that under laboratory or
greenhouse conditions, within the context of the experimental parameters at the field site, e.g.,
application, climatic (precipitation and temperature), edaphic (soil properties and moisture
conditions) and cropping parameters.
(vi) Identification and discussion of discrepancies between the results of field studies and
laboratory studies.
(3) Mass accounting considerations. The residue data for the parent chemical, each of the
maj or transformation products and the total maj or chemical residues should be expressed in terms of
equivalent amounts of parent chemical on a dry-weight basis, and then as percentages of the 0-day
concentration. These percentages can then be summed for the sampled environmental compartments
(e.g., soil depths, air, water, plants) and plotted versus time to estimate an overall mass account. If
the overall mass accounting is unexpectedly low, major route(s) of dissipation were possibly not
adequately addressed in the field study design.
(i) Reporting and evaluation of data. The study report should be clear and succinct with
definitive conclusions regarding the environmental fate and transport of the pesticide after field
application. Soil samples results should always be reported on a dry-weight basis along with percent
moisture. The study conclusion should be discussed both in terms of the data developed in the field
study and in terms of the expected route(s) of dissipation suggested by the laboratory studies.
Discussion of how the study compares with other field studies of this active ingredient should be
included. The report should clearly identify those aspects of the study having a direct bearing on the
author's conclusions and the validity of the study results.
35
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(1) Units. Reporting units should be in the metric system, but the English system may be
used, in addition. The systems should not be mixed (e.g., kilograms/acre).
(2) Test method. A full description of the experimental design and procedures. Any
protocol deviations or modifications should be described.
(3) Test substance and relevant transformation products, (i) The test substance should be
identified including: (An example data sheet is shown in Table 10):
(A) Chemical name and percentage of active ingredient,
(B) Molecular structure of the active ingredient,
(C) Qualitative and quantitative description of the chemical composition, and
(D) Names and quantities of known contaminants and impurities;
(E) Limits of analytical detection/quantification;
(F) Physiochemical and environmental fate properties, and specific activity, and
(G) Labeling positions (if appropriate).
(ii) Manufacturer and lot and sample numbers of the test substances.
(iii) Properties of the test substance, including:
(A) Physical state, pH, and stability.
(B) Solubility in water (See paragraphs (j)(10), GX44X GX45)and (j)(46) of this guideline.).
(C) Vapor pressure (See paragraphs (j)(10), (j)(44X GX4^) and (j)(46) of this guideline.).
(D) Henry's law constant.
(E) w-octanol-water partition coefficient (See paragraphs (j)(10), (JX44X GX4^) and GX46)
of this guideline).
(F) Dissociation constant in water, reported as pKa or pKb (See paragraphs G)(10), G)(4^)
and G)(46) of this guideline).
(G) Hydrolysis as a function of pH (See paragraphs 0)(10), (JX44X G)(45) and GX4?) of this
guideline).
(H) Photolysis on soil and in water (See paragraphs G)0°)> GX44X GX45) and G)(4§) and
36
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(j)(49) of this guideline).
(I) Soil aerobic biotransformation. (See paragraphs G)(10), G)(44), G)(48) and G)(50) of this
guideline).
(J) Soil anaerobic biotransformation. (See paragraphs G)(10)> G)(44),and G)(48) of this
guideline).
(K) Adsorption/desorption coefficient (See paragraphs G)(10), G)(44) and G)(48) of this
guideline).
The following Table 10 contains the physicochemical properties of the test substance and the
environmental fate laboratory studies necessary to design a terrestrial field dissipation study.
Table 10. Test substance properties.
Property/lab study
Solubility (mg/L)
Vapor pressure (Pa)
Henry's law constant (atm-m3/mol)
Dissociation constant (pKa or pKb)
w-octanol-water partition coefficient
C^ow)
Hydrolysis (half-life)
Major transformation products
Soil photolysis (half-life)
Major transformation products
Soil aerobic biotransformation (half-life
and persistence)
Major transformation products
Soil anaerobic biotransformation (half-
life and persistence)
Major transformation products
Adsorption/desorption
(^dand^oc)
Mobility class
Others
Values
Classification
Reference
(4) Test equipment. A description of the test equipment used, and photographs or detailed
descriptions of nonstandard equipment.
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(5) Field study site.
(i) Data and information should include:
(A) Location.
(B) Climatic conditions and history.
(C) Soil taxonomic classification and properties with depth.
(D) Hydrologic setting.
(E) Grade (slope).
(F) Size and configuration of the treatment and control plots.
(G) Crop, management and pesticide-use history.
(H) Depth to the water table.
(ii) Tables 11 and 12 are examples of data sheets which can be used for characterization of
the site and soil at the test location.
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Table 11. Site characterization data sheet.
Parameter
Geographic coordinates
Latitude
Longitude
Data Source
FIPS Code for State, County
Location within watershed
Landforms
Landscape position
Land surface
Slope gradient
Slope length
Direction
Micro-relief
Roughness
Elevation
Data source(s)
Depth to groundwater
Average rainfall (yearly /monthly)
Average air temperature
(daily /weekly /monthly)
Minimum
Maximum
Average soil temperature
(daily /weekly /monthly)
Minimum
Maximum
Average annual frost-free period
Dates
Number of days
Others
Site Description
Information Source
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Table 12. Physicochemical properties of soil.
Property
Depth
Texture
% sand
% silt
% clay
Textural class (USD A)
Bulk density
Soil moisture characteristic
Obar
0.1 bar
0.33 bar
Ibar
5 bars
10 bars
15 bars
pH
Organic carbon (%)
Cation exchange capacity
(meq/100 g)
Base saturation (%)
Clay mineralogy
Specific surface
Anion exchange capacity
Others
Horizon
Ht
.. Ha-
ft
H4
' Hs
Method
(iii) The following Table 13 can be used to describe the pertinent meteorological factors
that can influence the dissipation of the test substance in terrestrial environments.
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Table 13. Meteorological history data sheet.
Parameter
Site Description
Information Source
Average monthly rainfall
January
February
March
April
May
June
July
August
September
October
November
December
Average minimum/maximum air
temperature
January
February
March
April
May
June
July
August
September
October
November
December
Average annual frost-free period
Dates
Number of days
Others
(iv) The following Table 14 can be used to describe the site use and management history
of the site for the past three years.
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Table 14. Site use and management history for the previous three years.
• . • Use •. . • • • •
Crops grown
Pesticide and fertilizer use
Cultivation methods
Tillage
Irrigation practices
Others
Previous Year
Previous 2nd Year
Previous 3rd Year
(6) Application of the test substance. Include:
(i) Time(s) of application.
(ii) Rate(s) of application.
(iii) Method of application.
(iv) Confirmation of application rate.
(v) Field condition at the time of application.
(vi) Meteorological conditions at the time of application.
(7) Use of tracers. Include type of tracer(s) (if any) and rate and method of application.
(8) Maintenance activities. Include type of vegetation agricultural practices (date of
seeding, time of harvest, yields, etc.); and weed control.
(9) Environmental conditions.
(i) Data and information should include:
(A) Daily air temperature (minimum, maximum).
(B) Daily precipitation and irrigation (reporting of single rainfall events), intensity and
duration
(C) Irrigation technique.
(D) Weekly and monthly sums of precipitation and irrigation.
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(E) Weekly mean soil temperature.
(F) Soil water content.
(G) Daily evapotranspiration or pan evaporation.
(H) Movement of tracers (if used).
(ii) The following data and information should be recorded daily at the study site:
(A) Precipitation.
(B) Mean air temperature.
(C) Potential evapotranspiration or pan evaporation (can be determined from a nearby site, or
evapotranspiration may be calculated from other environmental data).
(D) Hours of sunshine and intensity of solar radiation; mean soil temperature; and soil
moisture content.
(10) Residues in soil. Data should include:
(A) Residues (as mg/kg dry weight and % of applied amount) at each sample interval,
concentration of test substance in each soil depth.
(B) Concentration of transformation products in each soil depth.
(C) Concentration of extractable radioactivity in each soil depth, if applicable.
(D) Concentration of non-extractable radioactivity in each soil depth, if applicable.
(E) Total amounts of test substance, transformation products, other unidentified extractable
residues and non-extractable radioactivity, if appropriate.
(11) Residues on and in plants. Data should include residues(in mg/kg fresh weight and %
of applied amount) at each sample interval, if appropriate. In addition, plant residues should be
reported based on how much of the pesticide was removed from a unit-area of the field in order to be
useful in mass accounting.
(12) Residues detected via other avenues of dissipation. Residues detected by other
avenues (e.g., volatility, runoff, leaching), if appropriate should be reported.
(13) Mass accounting. Report the recovered percentage of applied test substance at each
sample interval.
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(14) Protocol deviations. Report protocol deviations and amendments (however, see
paragraph (i)(2) of this guideline).
(15) Statistical analysis. Statistical analysis of the collected data should be described.
(j) References. The following references should be consulted for additional information on
this guideline.
(1) Mastradone, P.J., J. Breithaupt, PJ. Hannan, J.A. Hetrick, A.W. Jones, R.D. Jones,
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guidelines. In M.L. Leng, E.M.K. Leovey and P.J. Zubkoff (eds.). Agrochemical Environmental
Fate Studies: State of the Art. Lewis Publishers, Boca Raton, FL. p. 93-98.
(2) U.S. Environmental Protection Agency, (2008). OPPTS 850.6100 Independent
Laboratory Validation of Environmental Chemistry Methods.
(3) Marlow, D.A., D.D. McDaniel, A.E. Dupuy, Jr., and E.M. Leovey. 1995. Data
Reporting Guideline for Environmental Chemistry Methods. Subdivisions N, E, and K. EPA
Office of Pesticide Programs, Washington, DC.
(4) Flury, M. 1996. Experimental evidence of transport of pesticides through field soils -
a review. J. Environ. Qual. 25: 25-45.
(5) Ghodrati, M., and W.A. Jury. 1992. A field study of the effects of soil structure and
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11:101-125.
(6) Hurto, K., and M. Prinster. 1993. Dissipation of turfgrass foliar dislodgeable residues
of chlorpyrifos, DCPA, diazinon, isofenphos, and pendimethalin. In: K. Racke and A. Leslie
(eds.). Pesticides in urban environments: fate and significance. American Chemical Society,
Washington, DC. pp. 86-99.
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agricultural fields: II. Effects of formulation on atrazine runoff. J. Environ. Qual. 16(3):212-216.
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soil and vegetable crops following treatment of the soil with the insecticide.
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pesticide teachability. Environ. Toxicol. Chem. 8: 339-357.
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