FIRST User's 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
August 1, 2001
On this Page
- Introduction TO FIRST
- Limitations on Using FIRST for Drinking Water Exposure Assessments
- Running "FIRST"
- Retrieving Output Files
- Using "FIRST" results
Introduction TO FIRST
The Environmental Fate and Effects Division (EFED) of USEPA's Office of Pesticide Programs (OPP) uses a tiered system of exposure modeling to evaluate the risk posed to human health by the presence of pesticides in drinking water for assessments conducted for the Food Quality Protection Act (FQPA). This tiered system is designed to minimize the amount of analysis which is required to evaluate any given chemical. Each of the tiers is designed to screen out pesticides by requiring higher, more complex levels of investigationonly for those that have not passed the next lower tier. Each tier screens outa percentage of pesticides from having to undergo a more rigorous review prior to registration or reregistration.
The first two tiers are based upon an "Index Reservoir" located in Shipman, Illinois. This scenario is used to represent an upper-end exposure site for pesticide residues found in drinking water derived from surface water. 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 program does not consider the impact of water treatment processes. The first tier is designed as a coarse screen and estimates concentrations from a few basic chemical parameters and pesticide label application information. "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.
The Tier 1 model described here, the FQPA Index Reservoir Screening Tool (FIRST), is a program to estimate both acute and chronic potential pesticide concentrations in untreated drinking water. The program considers reductions in dissolved concentration due to the percentage of the watershed which is cropped (percent cropped area - PCA), due to adsorption of pesticide to field soil and to reservoir bottom sediment, due to incorporation of the pesticide at the time of application, due to degradation in soil before washoff to the reservoir, and due to degradation of the pesticide within the water body. Reservoir water concentrations may be increased due to deposition of spray drift into the feeding stream or directly into the reservoir itself. The "FIRST" program is designed to mimic a more complex simulation using the linked PRZM3 (Carsel et. al., 1997) and EXAMS 2.97.7 (Burns et.al., 2000) models but requires less time and effort to complete.
This Tier 1 program assumes that up to eight percent of the pesticide applied to this 427 acre (172.8 hectare) 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). This parameter is used within the program to partition the pesticide in this field/reservoir system into two separate phases: a dissolved (in water) phase and an adsorbed (to soil) phase. It is pesticide in the dissolved phase in the water column which is considered to be toxicologically available when consumed. It is therefore only the mass of dissolved pesticide which enters into the calculation of the concentration in the reservoir.
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 gave a series of dissolved pesticide concentration values as a function of Kd. A dissolved vs. adsorbed relationship to Kd was established and programed 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. For very high Kd values (1,000 or more) the binding takes place largely within the first day while for lower values (1.0 or less), the process may not be complete for almost a year. Each Kd value is used to calculate a pseudo binding rate. Adsorption within the reservoir occurs simultaneously with abiotic chemical and biological degradation.
Calculating degradation in the reservoir is for the purpose of estimating annual average concentration values for chronic exposure assessment. Degradation in the reservoir considers aerobic aquatic metabolism, abiotic hydrolysis and aquatic photolysis.
The user may 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 sixteen percent spray drift directly into the reservoir with 95 percent landing on the field (application efficiency). For air blast application to orchards, groves and vineyards, FIRST assumes 6.3 percent spray drift directly into the reservoir with 99 percent landing on the field. For ground spray, 6.4 percent goes directly to the reservoir with 99 percent being deposited on the field. For granular broadcast application, 100 percent 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. 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 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 incorporation 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.
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 is programmed into FIRST.
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 which exceed those predicted by the linked EPA 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 (eg. crops grown over plastic mulch).
Applications at sites with hydrologic group D soils which also receive excessively high rainfall (eg. EFED sweet potato scenario in southern Louisiana).
Multiple applications over a window of 30 days or longer in exceptionally high rainfall areas (eg. far southeastern US). In each of these cases, FIRST will exceed PRZM/ EXAMS estimated peak concentrations values, but not always the annual average concentration values. Even then PRZM/EXAMS would not be expected to exceed the FIRST values 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. Allowing these few exceedences for extreme conditions makes FIRST a more reasonable predictive tool for the rest of the country.
The following description provides the screen images which the FIRST model produces, with an "--->" at each point where the user needs to provide input. The bold text incorporated throughout is instructions on entering data and using the program.
Begin by creating a subdirectory called "FIRST" on your hard drive and copy the program "FIRST.EXE" to that subdirectory. The program can be run either from a Windows or a DOS environment. To run FIRST under Windows, go to the program and click on FIRST.EXE. To run it under DOS, go to the "FIRST" subdirectory where the program is located, type "FIRST" and press "Enter". Either procedure will lead you 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
AUG 1, 2001
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, but must be in standard DOS format with eight (8) or fewer alphameric characters, followed by a dot "." and then three (3) alphameric characters. Examples of output file names might be: CAPTAN1.OUT or CAPTAN2.OUT. 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 FIRST subdirectory. If the subdirectory and program are on the C:\ drive, the output file will be C:\FIRST\CAPTAN1.OUT. If you run the program again, and use the same output file name, it will write directly over the old file, and any previous results will be lost unless they have been retrieved and stored elsewhere. See below for retrieving output files with WordPerfect®. Once retrieved and saved as a WordPerfect® file to another subdirectory, the "output" file in FIRST can be safely overwritten.
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 and will printout when the output file is printed out (using either a DOS printer, or WordPerfect®.)
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 EIGHT PERCENT 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 WHICH 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 WHICH 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 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 TO BE
WASHED OFF 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:
|Crop Name||PCA factor||Crop Name||PCA factor||Crop Name||PCA factor|
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 pesticide that is available in the field to be washed into the reservoir depends upon the number of applications. For a single application, it is the amount remaining there after two days (zero days for pesticides for which the label specifies application just prior to rainfall or irrigation). For more than one application, it is the amount in the field just 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
WHICH 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) --->
Adsorption 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 which 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 scientists. The Kd / Koc conversion is based on an organic matter content of 2 percent and an organic carbon content of 1.16 percent. 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 HALFLIFE (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 halflife value to use in FIRST is calculated by the following formula:
if n=1: model input value = laboratory value × 3
n = number of half-life values
if n ≥ 2: model input value = µ + t90 × s /
µ = mean of the n laboratory half-life values
t90 single tail student's "t", α = 0.1 (Table 1 below, n-1 d.o.f.)
s = sample standard deviation of the n laboratory half-life values.
|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 WHICH 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 which runs off of 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 ALSO BE
REDUCED BY INCORPORATION OF THE PESTICIDE AT THE TIME
OF APPLICATION - 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 six10 inches. If more than one application method is possible, use the one which calls for the shallowest depth of incorporation.
THE SOLUBILITY OF A PESTICIDE IN WATER IS A MEASURE OF THE
MAXIMUM AMOUNT OF THE CHEMICAL WHICH CAN BE DISSOLVED
THE DISSOLVED PESTICIDE CONCENTRATION IN A WATER BODY
THEREFORE 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 WHICH IS CALCULATED BY
BY SUMMING THE INDIVIDUAL AQUATIC DEGRADATION RATES (THE AEROBIC
AQUATIC METABOLIC RATE IS ASSUMED TO INCLUDE HYDROLYSIS)
ENTER ANY OR ALL OF THE FOLLOWING WHICH ARE AVAILABLE:
(PLEASE ENTER ZERO (0) FOR ANY WHICH ARE STABLE OR
FOR WHICH VALUES ARE UNAVAILABLE)
ENTER AEROBIC AQUATIC METABOLIC HALFLIFE - DAYS (IF UNAVAILABLE,
RECOMMENDED EFED DEFAULT IS 2x AEROBIC SOIL INPUT VALUE) --->
If there is range of halflives, the halflife entered here is usually the longest one, which 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 HALFLIFE (DAYS) --->
The program requests the hydrolysis halflife value only if the aerobic aquatic halflife value entered is zero. Otherwise the program assumes the aerobic aquatic value includes the hydrolysis value. If there is range, the halflife entered here is usually the longest one, which would result in the highest exposure. However, the actual number used should be decided in discussion with the EFED scientists.
ENTER PHOTOLYSIS HALFLIFE (DAYS) --->
The halflife entered here is usually the longest one, which would result in the highest exposure. However, the actual number used should be decided in discussion with the EFED scientists. The effective photolysis halflife 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 asking 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 inset the FIRST output file into a WordPerfect® report, set the font to Courier to maintain the needed spacing between alphabetic and numeric characters. Once retrieved with WordPerfect®, 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.
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.