Number range CHAPTERFIRST: A Screening Model to Estimate Pesticide

Concentrations in Drinking Water

Version 1.1.1

March 26, 2008

After the passage of the Food Quality Protection Act (FQPA) in August
1996, the Environmental Protection Agency / Office of Pesticide Programs
(EPA/OPP) worked diligently to develop new methods and tools for
estimating human exposure to pesticides for use in FQPA risk
assessments. One of the tools, which OPP developed in response to this
mandate, was the [F]{.underline}QPA [I]{.underline}ndex
[R]{.underline}eservoir [S]{.underline}creening [T]{.underline}ool
(FIRST), a Tier I model for assessing exposure to pesticides in drinking
water. This paper describes the development and use of the *FIRST* in
EPA's pesticide exposure assessments.

Background

Modeling methods and tools used by EFED for estimating pesticide
concentrations in drinking water have evolved from those used for
estimating pesticide concentration in surface water used for ecological
exposure/risk assessment. As part of the ecological risk assessment
process, EPA's Office of Pesticide Programs (OPP) has routinely
conducted aquatic exposure assessments, which provide estimated
pesticide concentrations in water. Initially, these assessments were
carried out through computer modeling of a standard agricultural field /
farm pond scenario and through use of field-monitored pesticide
concentration values. This "standard" scenario was appropriate not only
for predicting pesticide concentrations in the generic farm pond, but
was also shown to be a good predictor of upper level pesticide
concentrations in small but ecologically important upland streams
(Effland et al., 1999).

For Tier 1 modeling, OPP uses the surface water model GENEEC2 (Parker,
et. al., 1995) for aquatic exposure assessments and has developed the
surface water model FIRST for drinking water assessments based on the
method used in developing GENEEC2. The linked PRZM-3 (Carsel et.al.,
1997) and EXAMS II (Burns et.al., 1998) models (PRZM-EXAMS), which
better accommodate the specific characteristics of the chemical stressor
and which include more site specific information, are used to conduct
more refined Tier II surface water assessments.

The tier 1, GENEEC2 model is a meta-model of these more sophisticated,
electronically- linked PRZM-EXAMS surface water models, but requires
fewer inputs and less time and effort to use. It is based on a standard
pond which has a 20,000 cubic meter water volume and is 2 meters deep.
Using a few basic chemical characteristics and pesticide label use and
application information, GENEEC provides conservative (upper-level)
exposure values for both acute and chronic risk assessment of pesticides
in surface water.

For Tier 2 surface water assessments, OPP uses PRZM-EXAMS. The values
estimated by this linked model are still characterized as
screening-level concentration values. For ecological aquatic
assessments, OPP uses the standard pond scenario with PRZM-EXAMS (as it
does with GENEEC). When performing screening-level drinking-water
assessments with PRZM-EXAMS, OPP uses maximum pesticide application
rates and frequencies. The simulations also account for the impact of
daily weather on the treated agricultural field over a period of
thirty-six years. During this period of time, the model assumes that
pesticide is washed off the field into the water body by twenty to forty
rainfall/runoff events per year. Each new addition of pesticide(s) to
the water body adds to the amount of pesticide that arrived earlier
either through previous runoff events or through spray-drift. Pesticides
can degrade in the field as well as in the water. In contrast to
PRZM-EXAMS, the Tier 1 model GENEEC is a single event model, which
assumes that one single large rainfall/runoff event occurs and removes a
large quantity of pesticide from the field to the water at one time.

Both the Tier 1 model GENEEC and the linked Tier 2 PRZM/EXAMS model use
a standard field-pond scenario and form the first two tiers of an
ecological exposure assessment. This tiered system is designed to
minimize the amount of analysis that is required to evaluate any given
chemical. Each of the tiers is designed to screen out pesticides by
requiring higher, more complex levels of investigation for those that
have not passed the previous tier. Each tier screens out a percentage of
pesticides that will not have to undergo a more rigorous review prior to
registration or reregistration. Passing a given assessment tier
indicates that there is a low possibility of risk to human health.
Failing an assessment tier, however, does not mean the chemical is
likely to cause health problems, but that the assessment should continue
on to the next higher tier.

**FIRST Development**

For drinking water assessments that are part of OPP's human health
exposure assessments, OPP developed a consistent, parallel modeling
system called FIRST to replace GENEEC. Recognizing that few people drink
water derived from small farm ponds or from small, upland streams, EFED
developed an index agricultural watershed-drinking water reservoir
(index reservoir) scenario to replace the standard field-pond scenario.
In addition, OPP recognized that most watersheds are not entirely
planted in only one crop and developed Percent Cropped Area (PCA)
factors, which account for the percent of a watershed that is planted
with specific crops. Both the index reservoir scenario and the PCA were
intended to improve the quality and accuracy of EPA\'s modeling of
drinking water exposure to pesticides.

Using a few basic chemical parameters (e.g., half-life in soil) and
pesticide label application information, FIRST estimates peak values
(acute) and long-term average concentrations (chronic) of pesticides in
drinking water. Like GENEEC2, it is based on the linked PRZM and EXAMS
models and is a single-event process model. However, it is different
from GENEEC2 in several aspects. The FIRST program is designed to mimic
a more complex simulation and uses an index reservoir watershed and PCA
factors to provide more realistic estimations of pesticides in drinking
water. Similar to FIRST, the Tier 2 PRZM-EXAMS models use the index
reservoir watershed scenario and PCA factors to estimate pesticide
concentrations in drinking water.

**Index Drinking Water Reservoir.** In July 1998, OPP presented to the
FIFRA Scientific Advisory Panel (SAP) the proposed index reservoir
scenario to replace the field-pond scenario, which had been used as an
interim scenario for estimating pesticide concentrations in drinking
water. The index reservoir and its associated characteristics were
chosen to represent the standard set of conditions by which EPA could
judge the potential of a pesticide to contaminate drinking water derived
from surface water. The index reservoir, which was selected from a group
of community water system reservoirs throughout the country, represented
a particular reservoir that has characteristics associated with a high
potential for pesticide contamination of surface water. Because the
index reservoir models real world characteristics, it is likely to
produce more realistic estimates of pesticide concentrations in surface
water than models that rely on the standard pond scenario. In addition,
a model using the index reservoir is likely to be protective of other
drinking water sources that are less vulnerable to contamination.

The actual index reservoir that OPP chose for its models is a small
drinking water reservoir located in Shipman, Illinios. Shipman City Lake
is 13 acres in area, 9 feet deep, and has a watershed area of 427 acres.
Its ratio of drainage area to capacity (volume of water in the lake) is
approximately 12. As a comparison, the field pond scenario which is used
in for ecological assessments has a ratio of 5. Shipman City Lake is one
of a number of Midwestern reservoirs that are small and shallow with
frequent Safe Drinking Water Act (SDWA) compliance problems, involving
some widely used corn herbicides. The index drinking water reservoir
characteristics have been incorporated into the linked PRZM/EXAMS and
FIRST models and are implemented in conjunction with percent cropped
area (PCA) adjustment factors (FIFRA SAP, 1999). While estimates of
pesticide concentrations based on a Midwestern index drinking water
reservoir may not be representative of residue levels in drinking water
sources in other parts of the country, this scenario provides an
effective screening tool to determine if a more refined assessment is
needed. The modeling scenarios currently account for region-specific
rainfall, soil, and hydrologic/runoff factors.

**Percent Crop Area (PCA) Adjustments.** The PCA is a generic adjustment
factor that accounts for the maximum percent of any watershed that is
planted to the crop or crops being modeled and that may potentially be
treated with the pesticide in question. PCA factors are generated from
Geographic Information System (GIS) overlays of cropping area and
watershed delineations and are applied to estimates of index reservoir
surface water pesticide concentration values from the PRZM/EXAMS and
FIRST models. The output generated by these models is multiplied by the
maximum decimal fraction of cropped area in any watershed generated for
the crop or crops of interest. To be effective as an adjustment to
screening model estimates, the PCA should result in estimated
concentrations that are closer to, but not less than, the actual
pesticide concentrations in vulnerable (prone to pesticide-laden runoff)
surface water sources. While it moves away from assuming that the entire
watershed is treated at the same time, the PCA is still expected to be a
screen because it represents the highest percentage of crop cover of any
large watershed in the U.S. and it assumes that the entire crop is being
treated. Current PCA values and a description of how the values are
derived and applied can be found in USEPA OPP (2000a). The PCA
adjustment is only applicable to pesticides applied to agricultural
crops and is [not]{.underline} applied to groundwater modeling
assessments.

As the index reservoir replaced the standard field pond, the FIRST model
replaced GENEEC as the surface water screen for FQPA pesticide exposure
assessments. FIRST represents a small drinking water reservoir
surrounded by a runoff-prone watershed, uses maximum pesticide
application rates, and assumes that no buffer exists between the
reservoir and the treated fields. As with GENEEC, FIRST assumes runoff
from a single, large rainfall event. It also considers adsorption of the
pesticide to soil or sediment, incorporation of the pesticide at
application, direct deposition of spray drift into the water body, and
degradation of the pesticide in soil before runoff and within the water
body. Because of the conservative nature of the assumptions in FIRST,
pesticide concentration values estimated using this model should be
exceeded only rarely in drinking water taken from most community water
supply (CWS) systems in the United States. Like the more complex and
resource-intensive Tier 2 PRZM and EXAMS models, which it mimics, FIRST
estimates pesticide concentrations in a vulnerable index reservoir and
includes Percent Cropped Area (PCA) factors that account for the percent
of a watershed that is planted with specific crops.

**Differences Between FIRST and GENEEC.** Differences between FIRST and
GENEEC include (1) the physical scenario, (2) the treatment of spray
drift, (3) the maximum percentage of applied pesticide that can reach
the water body, (4) the flow through the reservoir, (5) the use of a
PCA, and (6) the output values which are reported by the model.

**Scenario** The physical size of the watershed-reservoir system in
FIRST is much larger than the standard field-pond system in GENEEC. The
index reservoir scenario assumes a 172.8-hectare watershed channels
runoff to a 144,000-cubic meter water body. The standard field-pond
scenario assumes that a 10-hectare field provides runoff to a
20,000-cubic meter water body. The reservoir is 2.74 meters deep, while
the standard pond is 2.00 meters deep.

**Spray Drift.** Reservoir water concentrations of pesticides may
increase from deposition of spray drift into the feeding stream or
directly into the reservoir itself. Users of the FIRST model may
simulate application by aerial spray, air blast spray, ground spray, or
broadcast application of granular material. For aerial application, the
program assumes 16% spray drift directly into the reservoir with 95%
landing on the field (application efficiency). For air blast application
to orchards, groves and vineyards, FIRST assumes 6.3% spray drift
directly into the reservoir with 99% landing on the field. For ground
spray, 6.4% goes directly to the reservoir with 99% being deposited on
the field. For granular broadcast application, 100% application
efficiency is assumed with no pesticide drifting directly to the stream
or reservoir. Biological and abiotic degradation of the spray drift in
the reservoir begins immediately, and degradation of the pesticide
reaching the reservoir via runoff begins two days later (on the day it
reaches the reservoir). **NOTE: The application efficiency values and
the spray drift values do not add up to 100% because the application
efficiency is a percentage deposited on each hectare of the watershed
and the spray drift is a percentage of the application rate.** GENEEC
Version 2.0 uses a subroutine developed by the Spray Drift Task Force
(SDTF) which calculates spray drift based on the upper 90^th^ percentile
of expected drift percentages. The SDTF subroutine allows the user to
calculate a drift reduction based on the type of application and the
spray droplet size distribution. It also can simulate the impact of a
buffer between the treated field and the modeled water body.

**Maximum Percentage of Pesticide Dissolved in the Reservoir.** The
FIRST model assumes that up to 8% of the pesticide applied to this
427-acre (172.8-hectare) watershed is washed into the reservoir by one
large storm. Eight percent was chosen in order to yield output values
that exceed both PRZM/EXAMS modeled values and upper level monitored
values in all but a few rare cases. The actual amount that appears as
the dissolved (bioavailable) concentration estimate is a function of the
equilibrium partition coefficient (K~d~) or the organic carbon
normalized equilibrium partition coefficient (K~OC~). In this
watershed-reservoir system, either of these parameters may be used to
partition the pesticide into two separate phases: a dissolved (in water)
phase and an adsorbed (to field soil or reservoir bottom sediments)
phase. Because a pesticide in the dissolved phase of the water column is
considered toxicologically available when consumed, only the mass of
dissolved pesticide is considered in calculating the concentration of
pesticide in the reservoir. The GENEEC model assumes that between 10%
and 0.1% of the pesticide applied to the field ends up in the water body
in a dissolved (bioavailable) form. The actual fraction is also a
function of the K~d~ or K~oc~.

**Flow Through the Reservoir.** FIRST also assumes that flow from the
watershed is sufficient for two full reservoir turnovers each year. In
other words, the annual flow through the reservoir is equal to twice the
reservoir volume of approximately 144,000 cubic meters. This is
equivalent to a flow of approximately 33 cubic meters per hour (EXAMS
parameter STFLO) through the reservoir. The reduction in pesticide
concentration as a result of this flow is related to the partition
coefficient (K~d~) of the chemical. At a K~d~ of 1.0 or less, PRZM
program simulation shows an approximate 2.5% reduction of the peak
concentration and a 30% reduction of the annual average concentration.
The reduction is greatest at a K~d~ of 10 with a 7% reduction of the
peak concentration and a 35% reduction of the annual average
concentration. Above a Kd of 10,000, there is no reduction as a result
of flow. This reduction in flow is programmed into FIRST but not in
GENEEC.

**Use of the Percent Cropped Area (PCA) Factor.** As reported above,
FIRST considers reduction of the area within the reservoir that is
planted to the modeled crop but assumes that pesticide is applied to the
total crop that is planted. GENEEC does not consider the non-cropped
area within the 10-hectare field simulated and therefore does not use a
PCA.

**Reporting of Output Values.** The FIRST model generates two
concentration values: the peak value, which occurs on the day of the
single large rainstorm, and the annual average value, which is the mean
of the peak and the values over the next 364 days. The peak value is
used for acute exposure assessments, and the annual average value is
used for chronic and cancer exposure assessments.

**Impact of Pesticide Partitioning.** The impact on the system of
changing the partition coefficient was determined through repetitively
increasing the K~d~ value within a PRZM/EXAMS simulation, running each
new simulation, and then recording the resulting instantaneous
concentration for each value. This process gave a series of dissolved
pesticide concentration values as a function of K~d,~ and a dissolved
versus adsorbed relationship to K~d~ was established and programmed into
FIRST. The value of the K~d~ parameter in the EXAMS program controls not
only the final equilibrium partitioning of the chemical between the
dissolved and adsorbed phases, but also determines the time it takes to
reach this equilibrium. The actual amount, which appears as the
dissolved concentration estimate, is a function of the equilibrium
partition coefficient (Kd) or the organic carbon normalized equilibrium
partition coefficient (K~OC~). This parameter is used to partition the
pesticide in the field/reservoir system into two separate phases: the
adsorbed (to soil) phase and the dissolved (in water) phase. A pesticide
in the dissolved phase in the water column is considered to be
toxicologically available when consumed, and only the mass of the
dissolved pesticide is used to calculate the concentration of the
pesticide in the reservoir. For very large K~d~ values, the binding
takes place largely within the first day, while for smaller values of
K~d~, the process may not be complete for almost a year. The K~d~ and
K~oc~ are calculated as follows in Equations 1 and 2 respectively:

K~d~ = C~S~ / C~aq~ Equation (1)

where:

K~d~ = adsorption coefficient (mL/g);

> C~S~ = content of the test substance adsorped on soil at adsorption
> equilibrium (μg/g); and
>
> C~aq~ = mass concentration of the substance adsorbed in the aqueous
> phase at adsorption equilibrium (μg/mL), and

Koc = (K~d~ / OC%)\*100 Equation (2)

where:

K~oc~ = organic-carbon normalized adsorption coefficient (mL/g);

K~d~ = adsorption coefficient (mL/g); and

OC% = percent organic carbon in the soil.^1^

In order to accurately mimic this EXAMS process in the FIRST model, an
empirical procedure was carried out to simulate a pseudo-binding rate as
a function of K~d~. This rate was determined by "turning off" all
degradation processes within the PRZM and EXAMS models and calculating
an apparent rate constant that would account for the continuous decline
in concentration values. A four parameter Morgan-Mercer-Flodin (1975 )
type function was then fit to calculate these apparent binding constants
as a function of K~d~. This ongoing adsorption within the reservoir was
then programmed to occur simultaneously with chemical and biological
degradation processes.

Other Processes Simulated

**Overall Photolysis Degradation Rate.** Degradation rates in the
reservoir are calculated in order to estimate the annual average
concentration values for chronic exposure assessments. The EXAMS
program, which is used to assess aquatic degradation, considers the
amount of light penetration in the simulated water body to calculate an
effective photolysis rate from the direct photolysis rate in clear
water. Based on an EXAMS simulation, the photolysis rate constant in the
GENEEC pond is 124 times slower than that in clear water; the reservoir
is assumed to follow this same reduction. The overall degradation rate
is calculated by summing the aerobic aquatic metabolism rate constant,
the abiotic hydrolysis rate constant, and 1/124^th^ of the aquatic
direct photolysis rate constant. This sum is the combined rate given in
the output table.

**Soil Incorporation.** The program also allows the user to account for
those pesticides that are incorporated at the time of application.
Incorporation reduces the mass of pesticide available to runoff by a
factor equal to the depth of incorporation in inches up to a maximum of
six inches.

**Degradation in the Field.** Some pesticides are designed to degrade
very quickly in the field, and in a short time, little residue remains.
In order to give "credit" to such pesticides, FIRST was designed to
allow a two-day degradation period for a pesticide in the treated
agricultural field prior to the single rainfall event, which washes the
pesticide into the stream feeding the reservoir and eventually into the
reservoir itself. The aerobic soil metabolism rate is used to simulate
the rate of decline during this two-day period as well as during the
period between multiple applications prior to the rainfall/runoff event.
(For pesticides whose label requires that the pesticide be 'wetted-in'
at the time of application either through irrigation or rainfall, the
two-day period is not used).

**Use of K~d~ versus K~oc~**

Sorption tests are performed on soils of several textural classes prior
to pesticide registration. The soil/water equilibrium partition
coefficient (K~d~) is defined as the ratio between the concentration in
soil and the concentration in water. The organic carbon normalized
soil/water equilibrium partition coefficient (K~oc~) may be preferred
for pesticides for which there is a strong positive correlation between
the K~d~ value and the organic carbon content of the soils on which the
adsorption tests were performed. If there is a correlation, the multiple
K~oc~ values will be less variable than the multiple K~d~ values, and
the K~oc~ is likely to be a more accurate estimator. If there is no
correlation, use of the K~d~ is preferable. In FIRST, a K~d~ / K~oc~
conversion is based on an assumed organic carbon content of 1.16%.

**Solubility.** There may be cases in which the amount of pesticide that
is washed off from the treated field as a result of a large rainstorm
exceeds the amount that can be dissolved in the water body. In this
case, a precipitate is formed and remains in that form until the
concentration in the overlying water column decreases to a point below
the solubility of the chemical and additional chemical can dissolve. In
the FIRST program, the solubility serves as an upper limit on the amount
that is dissolved in the water. It does not enter into program
calculations in any other way.

**Limitations on Using FIRST for Drinking Water Exposure Assessments**

The FIRST program does not consider the impact of water treatment
processes. Removal and transformation of most pesticides by water
treatment processes is highly variable from CWS to CWS and from day to
day in the same CWS and is therefore difficult to predict on a
consistent basis. If consistently high removal across most CWS systems
has been documented for a specific pesticide, FIRST will overestimate
concentrations in drinking water to that extent.

FIRST is designed to yield concentration values that exceed those
predicted by the linked PRZM and EXAMS models for all but the most
extreme sites, application patterns, and environmental fate properties.
PRZM/EXAMS predictions may exceed FIRST predictions under the following
circumstances:

\(1\) Applications to crops in managed environments known to produce
excessive runoff (e.g., crops grown over plastic mulch).

\(2\) Applications at sites with hydrologic group D soils that also
receive excessively high rainfall (e.g., EFED sweet potato scenario in
southern Louisiana).

\(3\) Multiple applications over a window of 30 days or longer in
exceptionally high rainfall areas (e.g., far southeastern U.S.). In each
of these cases, FIRST will usually exceed PRZM/ EXAMS estimated peak
concentrations values, but will not always exceed the annual average
concentration values. Even when it does exceed FIRST values, it is not
expected to be by more than a factor of 2.

\(4\) For applications of chemicals with half-life values of 5 days or
less at exceptionally high runoff sites, the PRZM/EXAMS concentrations
values may exceed both the FIRST peak and annual average values by a
factor of 2. Because FIRST allows these few exceedances for extreme
conditions, it is a more reasonable predictive tool for the rest of the
country.

**BIBLIOGRAPHY**

1.  Burns, L.A. 1991. Exposure Analysis Modeling System: Users Guide for
    EXAMS II version 2.94, Environmental Research Laboratory, Office of
    Research and Development, U.S. Environmental Protection Agency,
    Athens, GA.

2\. Burns, L.A. March 1997. Exposure Analysis Modeling System (EXAMSII)
Users Guide for Version 2.97.5, Environmental Research Laboratory,
Office of Research and Development, U.S. Environmental Protection
Agency, Athens, GA.

3\. Carsel, R.F., C.N. Smith, L.A. Mulkey, J.D. Dean and P. Jowise.
1984. Users manual for pesticide root zone model (PRZM): Release 1,
Rep.EPA-600/3-84-109, 219 pp. Environmental Research Laboratory, Office
of Research and Development, U.S. Environmental Protection Agency,
Athens, GA.

4\. Effland, W.R., Thurman, N.C., Kennedy, I. Proposed Methods For
Determining Watershed- Derived Percent Cropped Areas and Considerations
for Applying Crop Area Adjustments To Surface Water Screening Models;
USEPA Office of Pesticide Programs; Presentation To FIFRA Science
Advisory Panel, May 27, 1999.

5.  R.F. Carsel, J.C.Imhoff, P.R.Hummel, J.M.Cheplick and J.S.Donigian,
    Jr. 1997. PRZM-3, A Model for Predicting Pesticide and Nitrogen Fate
    in Crop Root and Unsaturated Soil Zones: Users Manual for Release
    3.0; Environmental Research Laboratory, Office of Research and
    Development, U.S. Environmental Protection Agency, Athens, GA.

6\. Morgan, P.H., L.P. Mercer and N.W. Flodin, 1975. A General Model for
Nutritional Responses of Higher Organisms. Proceedings of National
Academy of Sciences, USA; Vol.72 pp. 4327-4331.

7.  Parker, R.D., R.D. Jones and H.P. Nelson., 1995. GENEEC: A Screening
    Model for Pesticide Environmental Exposure Assessment.,in
    Proceedings of the International Exposure Symposium on Water Quality
    Modeling; American Society of Agricultural Engineers, pp. 485-490;
    Orlando, Florida.