FIRST User Manual
(F)QPA (I)NDEX (R)ESERVOIR (S)CREENING (T)OOL
ENVIRONMENTAL FATE AND EFFECTS DIVISION
OFFICE OF PESTICIDE PROGRAMS
U.S. ENVIRONMENTAL PROTECTION AGENCY
TIER ONE SCREENING MODEL
FOR DRINKING WATER PESTICIDE EXPOSURE
March 26, 2008
On this page
- Background for Tiered Modeling System Approach
- Description of FIRST
- Effect of Kd On Concentration
- Application Methods
- Washout of Pesticide from the Reservoir
- Limitations of FIRST for Drinking Water Exposure Assessments
- Running FIRST
- Retrieving Output Files
- Using "FIRST" Results
Background for Tiered Modeling System Approach
When developing risk assessments under the Food Quality Protection Act (FQPA), the Environmental Fate and Effects Division (EFED) of USEPA's Office of Pesticide Programs (OPP) uses a tiered system of exposure modeling to estimate the concentrations of pesticides in drinking water. 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 only for those that have not passed the next lower tier. Each tier screens out a percentage of pesticides that are not expected to exceed a level of concern and that will not require a more rigorous review prior to registration or reregistration.
The first two tiers of drinking water exposure modeling are based upon the characteristics of a drinking water reservoir located in Shipman, Illinois (the Index Reservoir). This reservoir is representative of a number of reservoirs in the Midwest that are vulnerable to pesticide contamination. Thus, the index reservoir scenario represents an upper-end exposure site for pesticide residues found in drinking water derived from surface water. (Hetrick, et al, 1998). Pesticide concentration values estimated using this scenario should be exceeded only rarely in the source water at the intake pipe of a community water supply (CWS) systems in the United States. The first tier, which is based on a few basic chemical parameters and pesticide label application information, is designed as a coarse screen and provides conservative estimates of pesticide concentrations in drinking water. "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 assessment tier.
Description of FIRST
OPP uses the Tier 1 model, FQPA Index Reservoir Screening Tool (FIRST), to estimate both acute and chronic potential pesticide concentrations in untreated drinking water. The "FIRST" program is a metamodel of the more complex Tier II simulation using the linked PRZM3 (Carsel et al., 1997) and EXAMS 2.97.7 (Burns et al., 2000) models. Being a metamodel, FIRST requires less time and effort to complete. FIRST considers the most salient features used in the PRZM/EXAMS Tier II simulations including the percentage of the watershed that is cropped (percent cropped area - PCA), spraydrift, sorption of pesticide to field soil and to reservoir bottom sediment, incorporation of the pesticide at the time of application, degradation of pesticide in the soil, and degradation of pesticide within the water body.
Effect of Kd On Concentration
This Tier 1 program assumes that up to 8% of the pesticide applied to a 427-acre (172.8-hectare) watershed (i.e., the Shipman watershed) is washed into the reservoir by one large storm. 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). These parameters describe the equilibrium partitioning of a pesticide between the sorbed (to soil) phase and the dissolved (in water) phase.
The impact on the system of changing the partition coefficient was determined through sequentially increasing the Kd value within PRZM/EXAMS simulations and then recording the resulting instantaneous EXAMS reservoir concentration for each value. This procedure produced a series of dissolved pesticide concentration values as a function of Kd. A dissolved vs. sorbed 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 sorbed phases, but also determines the relative rate of approach to equilibrium. In a non-infinite system (like the EXAMS reservoir), as Kd increases, the rate that a pesticide approaches equilibrium between the sediment and the water column increases. For very high Kd values (1,000 mL/g or more), the equilibrium is essentially reached within the first day, while for lower values (1.0 mL/g or less), an equivalent relative uptake may take almost a year. Each Kd value is used to calculate a pseudo sediment-uptake rate. Sediment uptake within the reservoir occurs simultaneously with abiotic chemical and biological degradation.
Degradation in the reservoir is calculated in order to estimate annual average concentration values for chronic exposure assessment. Degradation over time is estimated by first-order degradation. The model considers aerobic aquatic metabolism, abiotic hydrolysis, and aquatic photolysis. Biological and abiotic degradation of the spray drift applications (see below) in the reservoir begins immediately. Degradation of the pesticide reaching the reservoir via runoff begins two days later (on the day it reaches the reservoir).
With FIRST, the user can also simulate application by aerial spray, air blast spray, ground spray, or broadcast application of granular material. For aerial application the program assumes that there is 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, FIRST assumes that 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. 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 value is a percentage of the application rate.
The program also accounts for those pesticides which 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.
Washout of Pesticide from the Reservoir
FIRST 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 volume 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 ml/g or less, the 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 ml/g with a 7% reduction of the peak concentration and a 35% reduction of the annual average concentration. Above a Kd of 10,000 ml/g, there is no reduction as a result of flow. This reduction is programmed into FIRST.
Limitations of 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 difficult to predict on a consistent basis because it can vary significantly from CWS to CWS and from day to day in the same CWS. If consistently high removal across most CWS systems has been documented for a specific pesticide, FIRST will overestimate concentrations in drinking water to the extent that removal by treatment is not estimated by the FIRST model.
FIRST is designed to yield concentration values which exceed those predicted by the higher-tiered, 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 which 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 FIRST values do exceed PRZM/EXAMS values, the exceedance is not expected to be 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. Allowing these few exceedances for extreme conditions makes FIRST a reasonable predictive tool for the rest of the country.
The following description provides the screen images that the FIRST model produces, with an "--->" at each point where the user needs to provide input. Instructions for entering data and using the program are shown in bold text.
Begin by creating a "FIRST" subdirectory at the desired location and copying the program "FIRST.EXE" to that subdirectory. The program can be run under any Windows environment. To run FIRST, go to the program and click on FIRST.EXE. This will lead to the following opening screen:
(F)QPA (I)NDEX (R)ESERVOIR (S)CREENING (T)OOL
ENVIRONMENTAL FATE AND EFFECTS DIVISION
OFFICE OF PESTICIDE PROGRAMS
U.S. ENVIRONMENTAL PROTECTION AGENCY
TIER ONE SCREENING MODEL
FOR DRINKING WATER PESTICIDE EXPOSURE
March 26, 2008
THIS IS A PROGRAM TO ESTIMATE TIER ONE, ACUTE AND CHRONIC
CONCENTRATION VALUES FOR PESTICIDES IN DRINKING WATER BASED
UPON AN INDEX RESERVOIR LOCATED IN SHIPMAN, ILLINOIS
THE PROGRAM IS USED TO ESTIMATE CONSERVATIVE EXPOSURE
VALUES FOR PESTICIDES IN ANY AREA OF THE UNITED STATES
PLEASE ENTER A RUN NUMBER TO CONTINUE --->
The run number is a bookkeeping aid to allow the user to keep track of multiple consecutive runs; any integer may be entered. This number will appear on the output screen and in the output file to identify each run of the program. The run number is not used for any calculation.
ENTER AN OUTPUT FILE NAME --->
The choice of a name for the output file is up to the user. Typical output files names would be: Metolachlor_Corn.TXT or CAPTAN_Tomato.TXT. If the same output file name is used in successive runs, the new output file will write over the old file and the old one will be lost. To save an older output file, give each new file a new, different name. The output file will be stored in the same directory in which the FIRST program is located. To insert a FIRST output file into a word-processed report (i.e. MSWord® or WordPerfect®), set the font to Courier to maintain the needed spacing between alphabetic and numeric characters.
PLEASE ENTER THE CHEMICAL NAME --->
Use either the common or chemical name. The structure is optional; use whatever is the most easily understood. This name will become part of the output file.
PLEASE ENTER THE CROP NAME --->
Enter the name of the crop. The name specified is used only for documentation purposes, and is not currently checked against a database of possible values.
THE PROGRAM ASSUMES THAT RAINFALL AND RESULTING RUNOFF ARE
SUFFICIENT TO REMOVE UP TO 8 % OF THE PESTICIDE
FROM THE PORTION OF THE 427 ACRES (172.8 HECTARES) OF
FIELDS IN THE RESERVOIR WATERSHED WHERE THE CROP IS GROWN
THE PORTION OF THE CHEMICAL THAT IS REMOVED FROM THE FIELDS
IN THIS WAY, FLOWS INTO THE RESERVOIR AND IS DISSOLVED IN
THE RESERVOIR WATER
THE CHEMICAL CONCENTRATION IN THE RESERVOIR REPRESENTS THE
PART THAT IS DISSOLVED AND NOT BOUND TO FIELD SOIL OR TO
RESERVOIR BOTTOM SEDIMENTS
THE FOLLOWING INFORMATION SHOULD BE TAKEN FROM THE MOST
CURRENT, ACCEPTED LABEL FOR THE USE SITE IN QUESTION
PLEASE ENTER APPLICATION RATE (IN POUNDS a.i. PER ACRE)--->
In most cases, this will be the maximum rate permitted by the label. Situations may also arise when it is desirable to use an average or typical application rate or some other rate less than the maximum rate. To use a rate in units of kilograms per hectare, enter a value of "zero" (0) at the prompt, and you will be asked for a metric rate.
ENTER MAXIMUM NO. OF APPLICATIONS PERMITTED PER YEAR--->
The maximum number of applications or maximum pounds of pesticide is included in most labels. If the maximum annual mass is given on the label, divide this by the application rate to find the number of applications. A situation may also arise when it is desirable to use an average or typical number of applications. If the number of applications entered is greater than one, the following will be requested:
ENTER INTERVAL BETWEEN APPLICATIONS (DAYS) --->
Use the minimum interval permitted by the label or the typical interval in practice in the area of use, whichever is most appropriate. If not specified on the label, the interval may be available directly from the chemical manufacturer or from county extension agents in use areas. The source of the data should be documented.
THE AMOUNT OF PESTICIDE IN THE WATERSHED AVAILABLE FOR
WASHOFF BY RAINFALL INTO THE RESERVOIR IS DEPENDENT ON
ON EXTENT OF THE WATERSHED ON WHICH THE CROP IS GROWN.
THE PROGRAM REPRESENTS THIS AREA BY A PERCENT CROPPED AREA
(PCA) FACTOR FOR THE CROP AS FOUND IN THE FOLLOWING LIST:
PLEASE ENTER THE APPROPRIATE PCA FACTOR (DECIMAL) --->
Enter the decimal value corresponding to the percent of the area of the index watershed in which the crop is grown. This value is a maximum value from remotely sensed data.
The program assumes the pesticide degrades in the field by the aerobic soil metabolism rate after one application and between multiple applications. The actual amount of available pesticide that is washed from the field into the reservoir depends upon the number of applications. For a single application, it is the amount remaining after two days (zero days for pesticides for which the label specifies application prior to rainfall or irrigation). For more than one application, it is the amount in the field after the last application, assuming degradation of previous applications for the length of time each was in the field.
The next input requested is the equilibrium partition coefficient for the chemical:
THE DISSOLVED PESTICIDE CONCENTRATION IN THE RESERVOIR IS
CALCULATED BY SUBTRACTION OF THE PORTION OF THE CHEMICAL
THAT IS BOUND TO FIELD SOIL, TO FIELD ORGANIC MATTER, OR
TO RESERVOIR BOTTOM OR SUSPENDED SEDIMENT.
THIS BOUND FRACTION IS ESTIMATED BY USE OF THE SOIL/WATER
EQUILIBRIUM PARTITION COEFFICIENT (Kd) OR THE ORGANIC
CARBON NORMALIZED SOIL/WATER EQUILIBRIUM PARTITION
SEE THE FIRST PROGRAM USERS MANUAL FOR THE APPROPRIATE Kd
OR Koc VALUE TO USE.
TO USE A Kd VALUE, PLEASE ENTER IT HERE. TO USE A Koc
VALUE, PLEASE ENTER ZERO (0) --->
Sorption and desorption (pesticide binding to soil) tests are performed on soils of different textural classes, pH's, and organic matter contents. Either the soil/water partition coefficient (Kd) or the organic carbon normalized soil/water equilibrium partition coefficient (Koc) can be used to estimate the dissolved and the adsorbed fraction for any chemical. To use a Kd value, choose the lowest of the Kd values that was measured on a non-sand texture soil (not sand, coarse sand, fine sand, sandy loam or loamy sand). This will result in a realistic yet conservative water column exposure value. If an appropriate value for Kd is available, enter it here. If not, enter a value of zero (0) at the prompt and you will be asked for a Koc value:
PLEASE ENTER THE APPROPRIATE Koc VALUE --->
The organic carbon normalized, soil/water equilibrium partition coefficient (Koc) may also be used in lieu of the Kd value. The Koc of choice, when two or more are available, is the lowest Koc value measured on a non-sand texture soil (see above). When in question, the actual number to be used should be decided in discussion with EFED environmental fate scientists. The Kd / Koc conversion is based on an organic matter content of 2 % and an organic carbon content of 1.16 %. If neither the Kd nor the Koc is available, use 0.35 times the Kow value.
THE DISSOLVED PESTICIDE CONCENTRATION IS ALSO REDUCED
BY DEGRADATION IN THE FIELD PRIOR TO A RAINFALL/RUNOFF
EVENT - THE PROGRAM ASSUMES DEGRADATION BY AEROBIC
METABOLISM FOR TWO DAYS AFTER THE FINAL APPLICATION
(IF STABLE TO AEROBIC METABOLISM OR IF DATA IS
UNAVAILABLE, PLEASE ENTER ZERO (0))
ENTER SOIL AEROBIC METABOLIC HALF-LIFE (IN DAYS) --->
There are often multiple half-life values from tests performed on a variety of soil types. In order not to underestimate the actual value, EFED policy is to use an upper 90% confidence limit on the mean value. The aerobic metabolic half-life value used in FIRST is calculated by the following formula:
if n = 1: single value × 3
n = number of half-life values
if n ≥ 2: µ + t90 × σ / √n, where
µ = mean of the half-life values
t90 single tail student's t, α = 0.1 (Table 1 below, n-1 d.o.f.)
σ = standard deviation of the n half-life values, and
|Number of Half-life Values (n)||t90 Single Tail Student's "t"|
(n-1 degrees of freedom)
SOME PESTICIDE LABELS REQUIRE THAT THE PESTICIDE BE
WETTED-IN EITHER THROUGH RAINFALL OR IRRIGATION:
IN THIS CASE, RUNOFF TO THE RESERVOIR IS ASSUMED TO
OCCUR IMMEDIATELY RATHER THAN AFTER TWO DAYS
IS THIS PESTICIDE TO BE WETTED-IN ? (Y or N) --->
Choose the appropriate response based on the label. Some pesticides must be activated by water. If so, this will be specified on the label. In this case, the pesticide enters the pond on the day of the final application rather than two days after the final application.
In the next section of the model, spray drift is added to the stream and reservoir.
THE DISSOLVED PESTICIDE CONCENTRATION MAY BE INCREASED BY
DEPOSITION OF SPRAY DRIFT EITHER DIRECTLY INTO THE RESERVOIR
OR INTO THE STREAMS THAT FLOW INTO THE RESERVOIR
PLEASE ENTER A, B, C, or D TO SELECT METHOD OF APPLICATION:
A: AERIAL SPRAY (DRIFT=16.0%; APPLICATION EFFICIENCY=95%)
B: GROUND SPRAY (DRIFT=6.4%; APPLICATION EFFICIENCY=99%)
C: ORCHARD-VINYARD AIRBLAST SPRAY (DRIFT=6.3%; APP EFF=99%)
D: GRANULAR (DRIFT=0.0%; APPL. EFFICIENCY=100%) --->
Pesticide applied as a granular product or as a ground spray may be incorporated into the soil by various types of equipment driven or pulled through the field. The following section allows the model user to simulate the impact of physical incorporation on the amount of pesticide that runs off the field and is dissolved in the water-body: If the Method = B (Ground spray) or D (Granular (Non-spray)), then the following will be asked:
THE DISSOLVED PESTICIDE CONCENTRATION MAY BE REDUCED BY
INCORPORATION OF THE PESTICIDE AT THE TIME OF APPLICATION
IN THIS CASE, THE FOLLOWING DEPTHS ARE SUGGESTED:
|METHOD OF APPLICATION||INCORPORATION DEPTH (IN)|
|DISKED IN AFTER BROADCAST||4.0|
|CHISEL PLOWED AFTER BROADCAST||6.0|
|BANDED - INCORPORATED||1.2|
|T - BANDED||1.5|
|AERIAL or AIRBLAST SPRAY||0.0|
|GROUND SPRAY||DEPENDS ON METHOD|
PLEASE ENTER DEPTH OF INCORPORATION (IN INCHES) --->
The method of application will be specified on the label. Enter one of the suggested depths based on label instructions. The program limits the depth of incorporation to six inches. If more than one application method is possible, use the one which calls for the most shallow depth of incorporation.
THE SOLUBILITY OF A PESTICIDE IN WATER IS A MEASURE OF THE
MAXIMUM AMOUNT OF THE CHEMICAL THAT CAN BE DISSOLVED
THUS, THE AQUEOUS CONCENTRATION IN A WATER BODY
CANNOT EXCEED THE SOLUBILITY OF THE CHEMICAL.
PLEASE ENTER THE SOLUBILITY (IN PPM) --->
This number does not actually affect the exposure estimate unless for some reason the estimated exposure concentration is greater than this value. If so, the EEC will be automatically reduced to the maximum solubility.
THE CHRONIC DRINKING WATER PESTICIDE CONCENTRATION VALUE
IS ESTIMATED USING A DEGRADATION RATE THAT IS CALCULATED
BY SUMMING THE INDIVIDUAL AQUATIC DEGRADATION RATES (THE AEROBIC
AQUATIC METABOLIC RATE IS ASSUMED TO INCLUDE HYDROLYSIS)
ENTER ANY OR ALL OF THE FOLLOWING THAT ARE AVAILABLE:
(PLEASE ENTER ZERO (0) FOR ANY THAT ARE STABLE OR
FOR WHICH VALUES ARE UNAVAILABLE)
ENTER AEROBIC AQUATIC METABOLIC HALF-LIFE IN DAYS (IF
UNAVAILABLE, THE RECOMMENDED EFED DEFAULT IS 2 TIMES
THE AEROBIC SOIL INPUT VALUE) --->
If there is a range of half-lives, the half-life entered here is usually the longest one that would result in the highest exposure. If this value is not available or if the test was performed in a water/sediment system rather than a water-only system, the appropriate value is twice the aerobic soil half-life.
PLEASE ENTER pH 7 HYDROLYSIS HALF-LIFE (DAYS) --->
The program requests the hydrolysis half-life value of pH 7 if the aerobic aquatic half-life value entered is zero. Otherwise the program assumes the aerobic aquatic value includes the hydrolysis value. If there is a range, the half-life entered here is usually the longest one that would result in the highest exposure.
ENTER PHOTOLYSIS HALF-LIFE (DAYS) --->
The half-life entered here is usually the longest one, which would result in the highest exposure. The effective photolysis half-life will be 124 times longer than the one entered in the program due to light attenuation in the reservoir.
Check over the output on the screen to ensure that all the values are those that you wished to enter. The "output" will be saved to the designated output file name as it appears on the screen.
At the bottom of the output screen is the following question:
DO YOU WANT TO DO ANOTHER RUN (Y OR N) --->
If you wish to perform another simulation before leaving the program, answer "Y" to this question. If you continue to perform additional simulations, results will be added to the end of the output file named in the first run. You will be prompted to enter a new run for each additional simulation:
PLEASE ENTER A NEW RUN NUMBER --->
As above, the run number is a bookkeeping aid to allow the user to keep track of multiple consecutive runs; any whole number may be entered. This number will appear on the output screen and in the output file to identify each run of the program. The run number is not used for any calculation.
If you do not wish to perform another simulation before leaving the program, answer "N" to the previous question.
After entering a new run number, the program will start over and will ask for the chemical name, etc. These results will be added to the bottom of the existing output file. None of the inputs from the previous run are saved and therefore must be entered again for subsequent runs.
Retrieving Output Files
Once you have finished running FIRST and have exited the program, the output is automatically saved under the file name you specified in your FIRST subdirectory.
This file may be retrieved and printed with any text editor or word processing program. To insert the FIRST output file into a MSWord® or WordPerfect® report, set the font to Courier to maintain the needed spacing between alphabetic and numeric characters. Once retrieved, it may be inserted directly into reviews and science chapters or printed and included as an attachment.
Using "FIRST" Results
The program yields two values as output: The peak day concentration value is an upper-end, conservative estimate for use in an acute exposure assessment. The annual average concentration value is used as an upper-end, conservative estimate for use in a chronic (including cancer) exposure assessment.
Burns, L.A. 2000. Exposure Analysis Modeling System (EXAMS), Environmental Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Athens, GA.
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.
J. Hetrick, 1998. Scientific Advisory Panel (SAP): Final Report for the July 1998 Meeting: Part 1; http://www.epa.gov/scipoly/sap/meetings/1998/july/final1.pdf