FIRST: A Screening Model to Estimate Pesticide Concentrations in Drinking Water
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 FQPA Index Reservoir Screening Tool (FIRST), a Tier I model for assessing exposure to pesticides in drinking water. This document describes the development and use of the FIRST in EPA's pesticide exposure assessments.
On this page
- FIRST Development
- Index Drinking Water Reservoir
- Percent Crop Area (PCA) Adjustments
- Differences between FIRST and GENEEC
- Impact of Pesticide Partitioning
- Other Processes Simulated
- Use of Kd versus Koc
- Limitations on Using FIRST for Drinking Water Exposure Assessments
Modeling methods and tools used by OPP 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 36 years. During this period of time, the model assumes that pesticide is washed off the field into the water body by 20-40 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 the 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.
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, OPP 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 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
- the physical scenario
- the treatment of spray drift
- the maximum percentage of applied pesticide that can reach the water body
- the flow through the reservoir
- the use of a PCA
- the output values which are reported by the model
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 that 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.
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 90th 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 (Kd) or the organic carbon normalized equilibrium partition coefficient (KOC). 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 Kd or Koc.
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 (Kd) of the chemical. At a Kd 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 Kd 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 Kd 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 Kd, and a dissolved versus adsorbed relationship to Kd was established and programmed into FIRST. The value of the Kd 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 (KOC). 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 Kd values, the binding takes place largely within the first day, while for smaller values of Kd, the process may not be complete for almost a year. The Kd and Koc are calculated as follows in Equations 1 and 2 respectively:
Kd = CS / Caq
Kd = adsorption coefficient (mL/g);
CS = content of the test substance adsorped on soil at adsorption equilibrium (µg/g); and
Caq = mass concentration of the substance adsorbed in the aqueous phase at adsorption equilibrium (µg/mL), and
Koc = (Kd / OC%) * 100
Koc = organic-carbon normalized adsorption coefficient (mL/g);
Kd = adsorption coefficient (mL/g); and
OC% = percent organic carbon in the soil.
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 Kd. 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 Kd. 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 1/124th of the aquatic direct photolysis rate constant. This sum is the combined rate given in the output table.
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 for 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 Kd versus Koc
Sorption tests are performed on soils of several textural classes prior to pesticide registration. The soil/water equilibrium partition coefficient (Kd) is defined as the ratio between the concentration in soil and the concentration in water. The organic carbon normalized soil/water equilibrium partition coefficient (Koc) may be preferred for pesticides for which there is a strong positive correlation between the Kd value and the organic carbon content of the soils on which the adsorption tests were performed. If there is a correlation, the multiple Koc values will be less variable than the multiple Kd values, and the Koc is likely to be a more accurate estimator. If there is no correlation, use of the Kd is preferable. In FIRST, a Kd / Koc conversion is based on an assumed organic carbon content of 1.16%.
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:
Applications to crops in managed environments known to produce excessive runoff (e.g., crops grown over plastic mulch).
Applications at sites with hydrologic group D soils that also receive excessively high rainfall (e.g., EFED sweet potato scenario in southern Louisiana).
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.
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.
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