2 

DRINKING WATER EXPOSURE ASSESSMENT 

This memorandum addresses potential drinking water exposure related to pirimiphos-methyl 

existing uses. Pirimiphos-methyl post-harvest treatments to stored corn and sorghum grain 

commodities, eartag uses, and fogger treatments to Gladiola and Iris bulbs encompass all of the 

currently registered uses and are all being evaluated under Registration Review. 

For stored corn and sorghum grain commodities, drinking water exposure can occur as 

harvested grain treated with pirimiphos-methyl is re-planted back into the soil during the next 

growing season, usually 6 -7 months after treatment. Estimated drinking water concentrations 

(EDWCs) in surface water were previously provided and estimated using the Food Quality 

Protection Act (FQPA) Index Reservoir Screening Tool, aka the FIRST model (DP Barcode 

328151, Dated Feb. 22, 2007). These results for the stored grain uses are still applicable and 

provided in Table 1 below. However, EDWCs in ground water for pirmiphos-methyl stored 

grain uses are updated in this memorandum with the recent implementation of the PRZMGW 

model, and also provided in Table 1 below. Generally, very minimal exposures in both 

surface and ground water is expected associated with re-planted stored grain treated with 

pirimiphos-methyl.

Table 1. Summary of upper-bound EDWCs for surface water and ground water 

associated with the post-harvest stored grain treatments with pirimiphos-methyl. 

Surface Water (ppb) 

Acute1 Chronic and Cancer1

<0.01 <0.01 

Ground Water (ppb) 

Acute2 Chronic and Cancer2

<0.01 <0.01 1. Upper-bound EDWCs in surface water calculated from FIRST model (see memo DP Barcode 328151, dated 

Feb. 22, 2007 for exact results). EDWCs conservatively include TTRs furfural and its organophosphate demethylated degradate, produced by acidic-catalyzed hydrolysis, by assuming that 100 percent of residues are 

stable to hydrolysis (see Table A-2 in this assessment for information regarding the formation and decline of 

this degradate in the environment). 2

 Upper-bound EDWCs in ground water based on PRZM-GW FL Citrus scenario presented in this assessment 

(see Table B-1 and Table B-2 in this assessment). EDWCs include TTRs pirimiphos-methyl and its 

demethylated organosphosphate degradate (see Table A-2 in this assessment for information regarding the 

formation and decline of this degradate in the environment). 

For eartag uses, definitive EDWCs are not calculated in this memorandum due to uncertainties 

with pirimiphos-methyl loading in the environment resulting from the eartag use. With eartag 

uses, drinking water exposure can occur with washoff and subsequent runoff and leaching of 

pirimiphos-methyl residues with rainfall on treated cattle. The uncertainties stem from limited 

information on simultaneous cattle treated with pirimiphos-methyl eartags, and the maximum 

amount of pirimiphos-methyl available for washoff and runoff/leaching from treated cattle at use 

sites. Given these uncertainties, preliminary pirimiphos-methyl estimated exposure 
3 

concentrations1

 (EECs) in surface water are calculated using the FIRST model with results 

normalized to one-hundred percent of pirimiphos-methyl washing off from one head of cattle. 

The resulting preliminary normalized concentrations are then extrapolated to calculate the 

number of treated cattle causing concern based on a derived drinking water concentration level 

of concern (DWLOC). An acute DWLOC of 3,600 ppm (36,000,000 ppb) is calculated based on 

the acute rat LD50 = 2,400 mg/kg-bw (pirimiphos-methyl Reregistration Eligibility Decision 

document2

) assuming a standard assumption that a 15 kg child drinks 1L of water. Based on this 

DWLOC and preliminary normalized surface water concentrations from FIRST described above, 

the number of treated cattle expected to cause a toxicologically-relevant concern is presented in 

Table 2 below. Even with 100 percent of applied pirimiphos-methyl washing off, over 5 

trillion head of cattle treated simultaneously are necessary to result exceeding the 

DWLOC, which is well beyond any expected density of cattle at any pastureland use site 

nationwide3. Impacts to ground water were also evaluated using PRZMGW, but acute 

ground water concentrations indicate much smaller relative concentrations than estimated 

in surface water, and therefore an even higher amount of treated cattle would be expected 

to result in the exceedance DWLOC. 

Table 2. Head of Treated Cattle Causing Concern Given Washoff Fractions (number of cattle) for 

pirimiphos-methyl eartag uses. 

Drinking Water 

Exposure Source 

Pirimiphos

-Methyl 

DWLOC 

(g/L)1

100% 20% 10% 5% 1% 

Reservoir 

(Surface Water)2

36,000,000 

5.4 x 109

 2.7 x 1010 5.4 x 1010 1.0 x 1011 5.4 x 1011

Well 

(Ground Water)3 > 5.4 x 109

 > 2.7 x 1010 > 5.4 x 1010 > 1.0 x 1011 > 5.4 x 1011

1

 Calculated based on acute rat LD50 = 2,400 mg/kg-bw (pirimiphos-methyl Reregistration Eligibility Decision 

document) assuming a standard assumption that a 15 kg child drinks 1L of water. 

2

 Calculated based on FIRST acute EDWC of 0.0066 ppb per head of cattle at 100 percent of applied washed off. 

Results extrapolated to 20, 10, 5, and 1 percent washoff fraction via linear proportional scaling with application 

rate.

3 Non-definitive values due to very low EDWCs per head of cattle <<0.01 ppb including levels up to 100 percent 

residues washed off. Values for head of treated cattle causing concern expected greater than found for surface 

water exposure scenario. 

No drinking water exposure is anticipated with indoor fogger treatments of Gladiola and 

Iris bulbs. 

 

1

 EDWCs include TTRs pirimiphos-methyl and its de-methylated organophosphate degradate 2(diethylamino)-4-

hydroxy-6-methyl pyrimidine (see Table A-2 in this assessment for information regarding the formation and decline 

of this degradate in the environment). 

2 Available on-line at: http://www.epa.gov/pesticides/reregistration/REDs/pirimiphos_methyl_red.pdf 3 Based on release from a relatively small 25-acre or 10 hectare pasture field site. However, pirimiphos-methyl loading is 

expected to be similar per unit area for larger pastures possessing larger cattle populations. 
4 

Complete and detailed descriptions on the Use Characterization and Environmental Fate of 

pirimiphos-methyl is provided in Appendix A. Details on the complete exposure assessment 

and a description of the analysis supporting the above results is provided in Appendix B. 

REFERENCES 

Kentucky (State of), University of Kentucky. Horn Flies and Cattle. Available online at 

http://www2.ca.uky.edu/entomology/entfacts/ef509.asp Accessed December 2015. 

Mississippi (State of), Mississippi State University. Horn Flies on Cattle: Biology and 

Management. Available on-line at 

http://store.msuextension.org/publications/AgandNaturalResources/MT200912AG.pdf

Accessed December 2015. 

Nebraska (State of), University of Nebraska. Controlling Flies on Pastured Cattle in Nebraska. 

Available on-line: http://newsroom.unl.edu/announce/beef/3199/18423 Accessed 2015. 

 
5 

Appendix A. Pirimiphos-Methyl Use and Environmental Fate 

Characterization 

Use Characterization 

Pirimiphos-methyl uses encompass six active registrations. The labels by FIFRA classification 

are summarized below: 

 Four Section 3 labels, and 

 Two Special Local Needs (Section 24c) labels 

Uses for the Section 3 pirimiphos-methyl registrations include cattle eartag uses (for lactating and 

beef cattle) and post-harvest stored grain seed treatments for corn and sorghum seeds. Eartags 

contain up to 3.84 grams a.i. per animal (or 1.92 grams a.i. per eartag). There are no restrictions 

related to the quantity of cattle treated or the frequency of application per use site. For post-harvest 

seed treatments, up to 0.48 lbs a.i. per 300 tons of grain may be applied. There are no restrictions 

for where pirimiphos-methyl can be used nationwide. 

Special Local Needs uses include indoor fogger treatments using pirimiphos-methyl for 

ornamental bulbs in Washington State and Michigan. Up to 0.224 lbs. a.i. per 1,000 cubic feet of 

space may be applied for these uses. 

For eartag uses, the labels are not restrictive of pirimiphos-methyl’s use at any given use site. 

However, given pirimiphos-methyl’s intended use to control horn flies and face flies, it is most 

likely to be used exclusively use sites such as pasture lands and range land. Infestations are most 

prevalent at sites such as these since these insects thrive with excrement on ground surfaces 

which are not subject to disturbance and agitation. Other sites containing cattle such as feedlots 

are not likely to encumber infestations of horn flies and face flies since highly concentrated cattle 

populations likely provide ample agitation of the ground, thus limiting suitable living conditions 

for these insects to thrive. 

Pasture land use sites with pirimiphos-methyl are anticipated to contain much less inventories of 

cattle than at feedlots, but may be more variable in size. However, pirimiphos-methyl loading 

due to washoff and runoff/leaching processes are not anticipated to be higher than concentrated 

feeding operations at feedlots. This information is conveyed by multiple state agricultural 

extension services (e.g., Alabama, Mississippi, Kentucky, and Nebraska) where eartag usage is 

common or where feedlots are present. 

Environmental Fate and Transport Characterization 

Registrant-submitted data defining the physical, chemical, fate and transport characteristics 

associated with pirimiphos-methyl are summarized in Table A-1. As part of registration review, 

available environmental fate studies for pirimiphos-methyl have been reevaluated. The fate and 
6 

transport of pirimiphos-methyl in the environment is discussed below. 

Table A-1. General physical-chemical and environmental fate properties of pirimiphosmethyl.

Chemical/Fate parameter Value Source (MRID) Study Acceptability 

Physical Chemical Properties 

Chemical Structure and SMILES 

Code 

CCN(CC)c1nc(C)cc(OP(=S) 

(OC)OC)n1 

Product 

Chemistry 

(MRID 

00129333 

N/A 

Molecular Weight (g/mol) 305 Product 

Chemistry 

(MRID 

00129333) 

N/A 

Vapor Pressure (torr at 30°C) 1.1 x 10-4 Product 

Chemistry 

(MRID 

00129333) 

N/A 

Octanol-water Partition Coefficient 

(Log KOW at 20°C) 

4.2 Product 

Chemistry 

(MRID 

92147003) 

N/A 

Octanol-air Partition Coefficient 

(Log KOA) 

8.743 EPI Suite 

(KOAWIN v. 1.1 

estimate) 

N/A 

Water Solubility (mg/L; at 20°C) 9.9 mg/L at pH 5.2 

8.6 mg/L at pH 7.3 

9.3 mg/L at pH 9.3 

Product 

Chemistry 

(MRID 

92147003) 

N/A 

Henry's Law Constant (atm-m3

mol-1) 

5.105 x 10-6 EPI Suite 

(HENRYWIN v. 

3.10) 

N/A 
7 

Table A-1. General physical-chemical and environmental fate properties of pirimiphosmethyl.

Chemical/Fate parameter Value Source (MRID) Study Acceptability 

Soil-water partition coefficients 

(Kd) 

England Soils5

: 

LS1: 15.6 mL/g (OC = 1.60%) 

LS2: 27.4 mL/g (OC = 1.69%) 

LS3: 32.7 mL/g (OC = 2.97%) 

S: 11.6 mL/g (OC = 0.29%) 

SiCL: 161.9 mL/g (OC = 

1.63%) 

SL: 45 mL/g (OC = 1.92%) 

MRID 48355601 Supplemental: 

 Significant 

degradation was 

observed in low pH 

soils (S, SiCL, SL) 

Abiotic Degradation1

Hydrolysis half=lives (days) 7.3 days at pH 5 

79 days at pH 7 

54 – 62 days at pH 92

MRID 42982401

MRID 43177601

Acceptable 

Aqueous photolysis half-life (days 

at 20°C) 3

0.2 days 

IUPAC Pesticide 

Properties 

Database6

N/A 

Photolysis in air half-life (hours at 

25°C) 

2.4 hours 

0.802 hours 

SRC Database 

EPI Suite 

(AopWIN v. 

1.92) 

N/A 

Soil Photolysis half-life (days) No Data Available - N/A 

Biotic Degradation1

Aerobic Soil Metabolism half-life 

(t1/2 in days) 

England Soils5: 

SL1: 33.6 days (OC = 3.70%,

 pH7.1) 

SL2: 28.7 days (OC = 1.00%, 

pH6.6) 

L: 42.7 days (OC = 6.0%, 

 pH7.5) 

P: 31.5 days (OC = 36.2%, 

 pH6.0) 

MRID 135358 Supplemental: 

Potential degradation 

associated with the 

extraction procedures 

not addressed.

Pertinent test 

conditions (e.g., 

temperature and 

moisture content) 

throughout the studies 

not reported.

Anaerobic Soil Metabolism halflife (days) 4

England Soils5

: 

SL1: 36.4 days 

(OC = 3.7%, pH7.1) 

L: 31.5 days 

(OC = 6.0%, pH7.5) 

Aerobic Aquatic Metabolism halflife (days) No Data Available - N/A 
8 

Table A-1. General physical-chemical and environmental fate properties of pirimiphosmethyl.

Chemical/Fate parameter Value Source (MRID) Study Acceptability 

Anaerobic Aquatic Metabolism 

half-life (days) 

No Data Available - N/A 

Field Studies

Terrestrial Field Dissipation No Data Available - N/A 

Bioaccumulation

BCF Fish 274X SRC Database 

EPI Suite 

(WSKOW v. 

1.42) 

N/A 

1

 Half-lives based on first order rate constant, k (t1/2 = ln2/k) derived from Time – Ln(Concentration) regression. 2

 Half-life range at pH9 among multiple replicates reported. 

3

 Estimated based on DT-50 value. 

4

 Half-life ranges based on replicates with flooded water soil atmosphere reported. 

5

 Soil texture identifiers: SLn: Sandy loam replicate, LSn: Loamy sand replicate, L: Loam, P: Peat, SiCL: Silty 

Clay Loam

6

 Available on-line: http://sitem.herts.ac.uk/aeru/iupac/

The measured soil-water partition coefficients (Kd) for pirimiphos-methyl residues range 

between 11.6 mL/g to 161.9 mL/g. These measured partition coefficients in soil are poorly 

correlated to organic carbon content (r2 = 0.012). The water solubility limit of pirimiphosmethyl ranges between 8.6 to 9.9 mg/L. Both pirimiphos-methyl’s values of solubility and 

sorption coefficients indicate that pirimiphos-methyl residues will partition both to the sorbed 

phase and dissolved phase in soil and aquatic sediment. Therefore, pirimiphos-methyl can be 

transported to offsite water bodies via runoff and erosion processes. Furthermore, pirimiphosmethyl has the potential for leaching into ground water given its high mobility. 

The vapor pressure of 1.1 x 10-4 torr suggests that pirimiphos-methyl can exist in the vaporphase. However, the photolysis in air half-life of between 0.802 and 2.4 hours suggests that the 

residence time of pirimiphos-methyl vapors in air will be limited. Furthermore, no VOCs 

(including vapor-phase pirimiphos-methyl) were detected in any volatile trap in the aerobic and 

anaerobic soil metabolism studies summarized above in Table A-1. 

The major degradation pathway for pirimiphos-methyl O-(2-Diethylamino-6-methylpyrimidin-4-

yl) O,O-dimethyl phosphorothioate) is hydrolysis, especially in acidic environments. 

Hydrolysis half-lives from laboratory studies ranged from 7.3 days at pH 5, to 79 days at pH 7 

with a half-life of 54 – 62 days at pH 9. In acidic environments, the major degradate was 

2(diethylamino)-4-hydroxy-6-methyl pyrimidine (hereafter degradate no. 1). In the pH 5 
9 

solution, degradate no. 1 reached maximum levels up to 87.3 days post treatment of the overall 

material balance. In neutral and alkaline environments, degradate no. 1 was also found at major 

levels along with the degradate O-2 diethylamino-6-methylpyrimidin-4-yl o-methylphosphorothioate (hereafter degradate no. 2). In the pH 7 solution, degradate nos. 1 and 2 

reached maximum levels of 22.5 % and 26.5% of the overall material balance at 90 days posttreatment, respectively. In the pH 9 solution, degradate no. 1 and 2 reached levels of 21.0 and 

18.2%, respectively of the overall material balance. Degradate no. 1 did not retain the 

organophosphate (P=S bond) linkage, whereas degradate no. 2 did in a demethylated form. 

With degradate no. 2 retaining the organophosphate linkage, all exposure estimates 

presented in this assessment include this degradate as part of total toxic residues evaluated.

Pirimiphos-methyl degradation in soil occurs at similar rates and appears to be mediated by 

hydrolysis, with half-lives ranging between 28.7 – 42.7 days in four soils. The major metabolite 

from aerobic and anaerobic soil metabolism is degradate no. 1 (defined above) which formed at 

levels up to between 37 – 66% of applied. Major levels of unextractable residues consistently 

occurred throughout all tested soils, in the range of up to 28.2 – 66.3% of the applied material, 

and a nonpolar extraction solvent was not used to extract residues partitioned to soil. However, 

given pirimiphos-methyl’s high sorption to soil (see Section 3.1 above) and partially thorough 

extraction method utilizing acetone followed by methanol extraction solvents, residues appear to 

be bound to sediment. No Agency guideline studies have been conducted to determine the 

biodegradation of pirimiphos-methyl in water or its dissipation in the field. 

Degradate no. 1 did not maintain the organophosphate moiety (P=S bond) and is not expected to 

possess toxicity exceeding that of parent pirimiphos-methyl. However the organophosphate 

moiety was maintained with degradate no. 2. Therefore, the potential for toxicity exists with 

degradate no.2. Refer to Table A-2 for further information on the formation of degradates, 

A log Kow of 4.2 indicates that pirimiphos-methyl is sufficiently hydrophobic for binding to fatty 

tissue for aquatic organisms as well as available sediment. While there are no BCF in fish 

studies available, a regression Kow –based model suggests that the BCF value for pirimiphosmethyl is approximately 274X in aquatic organisms. This BCF further indicates the high 

potential for pirimiphos-methyl residues to build up in the tissues of aquatic biota. 

 
10 

Table A-2. Pirimiphos-methyl degradation products identified in environmental fate studies.

Degradate 

Identity Structure

Max% of 

Applied 

and Days 

Post 

Application

Comments

Hydrolysis (MRID Nos. 42982401 and 43177601) 

2(diethylamino)-4-

hydroxy-6-methyl 

pyrimidine 

(Non-OP degradate)

87.25%, 

30 days 

(pH5) 

22.51%, 

90 days 

(pH7) 

29.97%, 

60 days 

(pH9) 

pH5 study 

terminated at 30 

days. 

pH7 study 

terminated at 90 

days. 

pH9 study 

terminated at 60 

days. 

O-2 diethylamino-6-

methylpyrimidin-4-yl omethyl-phosphorothioate 

(OP Demethylated 

degradate)

4.99%, 

21 days 

(pH5) 

26.54%, 

90 days 

(pH7) 

18.17%, 

60 days 

(pH9) 

pH5 study 

terminated at 30 

days. 

pH7 study 

terminated at 90 

days. 

pH9 study 

terminated at 60 

days. 

Aerobic Soil Metabolism (MRID No. 135358) 

2(diethylamino)-4-

hydroxy-6-methyl 

pyrimidine 

(Non-OP degradate)

42.08%,, 

Peartree 

Sandy Loam 

(SL1) at 70 

days 

55.58%,, 

Frensham 

Sandy Loam 

(SL2) at 14 

days 

37.22%,, 

Gore Loam 

(L) at 14 days 

66.28 %,, 

Blackborough 

Peat (P) at 

210 days 

Studies in all soils 

terminated at 210 

days. 

N N

CH3

N

C

H2

C

H2

H3C CH3

HO

N N

CH3

N

C

H2

C

H2

H3C CH3

HO
11 

Table A-2. Pirimiphos-methyl degradation products identified in environmental fate studies.

Degradate 

Identity Structure

Max% of 

Applied 

and Days 

Post 

Application

Comments

Anaerobic Soil Metabolism (MRID No. 135358) 

2(diethylamino)-4-

hydroxy-6-methyl 

pyrimidine 

(Non-OP degradate)

61.4%,, 

Peartree 

Sandy Loam 

(SL1) at 14 

days 

37.22%,, 

Gore Loam 

(L) at 14 days 

Studies in all soils 

terminated at 210 

days. 

Findings from 

flooded water soil 

atmosphere 

reported. 

 

N N

CH3

N

C

H2

C

H2

H3C CH3

HO
12 

Appendix B. Detailed Exposure Analysis Description for Pirimiphos-Methyl 

Current Uses 

Post-harvest stored grain seed treatments

Surface water EDWCs using the FIRST model are estimated in the previous drinking water 

assessment for new use on post-harvest stored grain seed treatments. The maximum application 

rate of 0.48 lbs. a.i. per 300 tons of grain was evaluated. The analysis and results are provided in 

the previous drinking water memo (DP Barcode 328151, Dated Feb. 22, 2007). FIRST EDWCs 

in surface water for pirimiphos-methyl4 post-harvest stored grain treatments are very 

minimal at << 0.1 ppb. 

Ground water EDWCs are updated in this memo using the PRZMGW model (version 

1.07), dated . Table B-1 below shows the PRZMGW input parameters for seed treatment 

applications, and results are shown in Table B-2. PRZMGW EDWCs for the post-harvest 

grain treatment uses for pirimiphos-methyl4 are also very minimal at <<0.1 ppb, and is 

consistent with the previous EDWCs from SCIGROW provided in the previous drinking 

water memo. Sample documentation of the PRZMGW model simulations are provided in 

Appendix C. 

Table B-1. PRZM-GW input parameter values for physical chemical and fate 

properties used for pirimiphos-methyl post-harvest stored grain treatments surface 

water aquatic exposure estimation. 

PRZM/EXAMS Input 

Parameter 

Input Value and Unit Source 

Pesticide Properties

Hydrolysis at pH7 (t1/2) 198 days TTR half-life of pirimiphosmethyl and its demethylated 

OP degradate (degradate no. 

2) 

MRID Nos. 42982401 & 

43177601 

Surface soil metabolism (t1/2) at 

25°C 

39.1 days MRID 48604492 & 48604493 

EFED Input Parameter Guidance 

90th percentile on the upper 

confidence bound half-life from 

n=4 soils (refer to Table A-1) 

 

4

 EDWCs include TTRs pirimiphos-methyl and its de-methylated organophosphate degradate 2(diethylamino)-4-

hydroxy-6-methyl pyrimidine. (see Table A-2 in this assessment for information regarding the formation and 

decline of this degradate in the environment). 
13 

Table B-1. PRZM-GW input parameter values for physical chemical and fate 

properties used for pirimiphos-methyl post-harvest stored grain treatments surface 

water aquatic exposure estimation. 

PRZM/EXAMS Input 

Parameter 

Input Value and Unit Source 

Soil-Water Partition Coefficient 

(Koc) 

(mL/g) 

49.03 MRID 48355601 

EFED Model Input Parameter 

Guidance 

 

Average value from n=6 soils 

(refer to Table A-1) 

MRID 48355601 

Application Methods and Crop Management Input Parameters

Crop Scenarios, Application 

Date5

, and Weather 

WI Sands: Jul. 20, w14920 (La Crosse, WI) 

NC Coastal: Feb. 1, w13722 (Raleigh, NC) 

GA Coastal: June 15, w93805 (Tallahasse, FL) 

Delmarva Peninsula: July 10, w13781 (Wilmington, 

 DE) 

FL Citrus: Aug. 10, w12842 (Tampa, FL) 

FL Potato: Sept. 5, w13889 (Jacksonville, FL) 

Application Rate 8.96 x 10-5 kg/ha Equivalent maximum single 

application rate for seed 

treatment with corn and sorghum 

grain, maximum seasonal 

application rate at one 

application for seed treatment. 

No degradation of residues 

before planting assumed. 

Number of Applications and 

Retreatment Interval 

1 applications 

Application Methods Soil Incorporated 

at 4 inches 

Appropriate for seed treatment 

and information from product 

labels 
14 

Table B-2. PRZM-GW estimated upper bound concentrations in ground 

water (ppb) for pirimiphos-methyl1 post-harvest corn and sorghum grain 

treatment uses. 

Tier I Scenario 

Highest Daily Value 

(Acute) 

Post-Breakthrough 

Average 

(Chronic and Cancer) 

WI Sands 

<0.01 

NC Coastal 

GA Coastal 

FL Citrus 

FL Potato 

Delmarva Peninsula 1

 EDWCs include TTRs pirimiphos-methyl and its demethylated organosphosphate degradate (see Table A-2 in 

this assessment for information regarding the formation and decline of this degradate in the environment). 

Eartag Uses

Since runoff and leaching of pirimiphos-methyl is being evaluated, drinking water exposure can 

be estimated using routine fate and transport models to evaluate the amount of pirimiphos-methyl 

applied that would be required to runoff and leaching in sufficient quantities to trigger adverse 

impacts. 

 Surface Water

To evaluate impact on surface water resources, the screening Tier I drinking water exposure model 

FIRST (one generic upper-bound watershed exposure scenario, version 1.1.1) is used for this 

evaluation given that it addresses contaminant runoff in a watershed feeding into a reservoir. An 

adverse impacts analysis then incorporates the FIRST estimated drinking water concentrations 

(EDWCs) normalized to one head of treated cattle and available acute toxicity data and exposure 

dose drinking scenario (e.g., a 15 kg child drinks 1L of water for human exposure scenario) to 

determine the number of treated cattle exceeds the resulting exposure dose level of concern for 

drinking water. 

The exposure analysis for feedlot and pasture pirimiphos-methyl applications will evaluate the 

potential exposure which occurs from heads of cattle treated simultaneously. It will be assumed 

that each head of cattle possesses two eartags containing 1.92 grams a.i. each for a total of 3.84 

grams (8.47 x 10-3 lbs.) total of pirimiphos-methyl as permitted by the label. From this scenario, 

an application rate can be calculated with the initial assumption that the total amount of pirimiphosmethyl washes off from the treated cattle, is available for runoff, and that the resulting residue will 

be evenly distributed with the 10-hectare treated pastures. From these assumptions, an equivalent 

application rate normalized to one head of treated cattle can be calculated considering the 3.84 

grams (8.47 x 10-3 lbs.) of pirimiphos load with each head of cattle and the 10-ha treated area. The 

resulting equivalent application rate is 3.43 x 10-4 lbs./A per head of cattle [8.47 x 10-3 lbs. ÷ (10 

ha x 2.47 A/ha)]. Pasture land and ranches larger than 10 hectares are not evaluated since the 
15 

loading of pirimiphos-methyl is expected to the same per unit area with larger cattle populations 

residing at larger properties. Thus, the effective application rate should not be impacted. 

However, the major uncertainty is the fraction of pirimiphos-methyl that could potentially wash 

off from treated cattle. The 3.43 x 10-4 lbs./A value is a high-end application rate assuming 100% 

of pirimiphos-methyl residues washes off. Therefore, this drinking water exposure analysis will 

use a bounding technique, where the equivalent application rate is linearly proportional to the 

amount that washes off. Accordingly, the application rates for each of the washoff fractions 

(evaluated for 100%, 20%, 10%, 5%, and 1%) from treated cattle are shown in Table B-4.

The remaining parameterization for FIRST is composed of the physical chemical and fate 

properties of pirimiphos-methyl utilizing EFED Model Input Parameter Guidance5

. All input 

parameter values for the aquatic exposure assessment for pirimiphos-methyl cattle eartags are 

shown in Table B-3. 

Table B-3. Input parameter values used in FIRST for pirimiphos-methyl adverse 

impacts analysis for cattle ear tag uses at feedlot and pasture use sites.

FIRST Input Parameter Input Value and 

Unit Comment Source 

Application Rate 

(lbs. a.i./A per head of treated cattle) 

See Table B-4 Based on 3.84 grams 

of pirimiphosmethyl per head of 

treated cattle and 10-

hectare feedlot size 

for specified 

washoff fraction 

Product label and 

standard runoff 

exposure scenario 

evaluated 

Application Method Broadcast1 No Spray Drift, 

Incorporated to 

Surface Only 

Consistent with washoff 

scenario for eartag use 

Hydrolysis (t1/2) 198 days TTR half-life of 

pirimiphos-methyl and 

its demethylated OP 

degradate (degradate 

no. 2) 

MRID Nos. 42982401 

& 43177601 

Aerobic soil metabolism (t1/2) 39.1 days 90th percentile on the 

upper confidence 

bound half-life from 

n=4 soils (refer to 

Table A-1) 

MRID 135358 

EFED Model Input 

Parameter Guidance 

 

5 Available on-line: http://www.epa.gov/oppefed1/models/water/input_parameter_guidance.htm
16 

Table B-3. Input parameter values used in FIRST for pirimiphos-methyl adverse 

impacts analysis for cattle ear tag uses at feedlot and pasture use sites.

FIRST Input Parameter Input Value and 

Unit Comment Source 

Aerobic aquatic metabolism (t1/2) Stable Conservative 

assumption 

- 

Solubility in Water at 20oC 8.6 mg/L Product Chemistry Product Chemistry 

(MRID 92147003) 

Soil-Water Partition Coefficient (Kd) 49.03 mL/g Average value from 

n=6 soils (refer to 

Table A-1) 

MRID 48355601 

EFED Model Input 

Parameter Guidance 

Aqueous Photolysis (t1/2) 0.2 days DT-50 value IUPAC Pesticide 

Properties Database2

1

 Broadcast application method consistent with chemical distribution associated washoff scenario evaluated. 2

 Available on-line: http://sitem.herts.ac.uk/aeru/iupac/

Table B-4. Pirimiphos-methyl surface water EDWCs at equivalent application rates 

normalized to one head of treated cattle for certain washoff fractions.

Input/Output Variable Washoff Fraction 

100% 20% 10% 5% 1% 

Equivalent Application Rate 

(lbs. a.i./A per head of treated 

cattle)a,b

3.43 x 10-4 6.85 x 10-5 3.43 x 10-5 1.71 x 10-5 3.43 x 10-6

Preliminary FIRST Surface 

Water EDWCs 

(μg/L per head of treated 

cattle) 

0.0066 0.0013 0.0006 0.0003 0.00006 

a For calculation of the equivalent application rate, please refer to the discussion above Table B-4. b

 Based on 10-hectare sized pasture. 

The surface water EDWCs normalized to one head of treated cattle for each washoff fraction, 

shown in Table B-4, are used along with a DWLOC to compute the number of treated cattle 

leading to adverse impacts resulting from eartag use. The acute DWLOC of 3,600 ppm 

(36,000,000 ppb) is calculated based on the acute rat LD50 = 2,400 mg/kg-bw (pirimiphos-methyl 

Reregistration Eligibility Decision document6

) assuming a standard assumption that a 15 kg child 

drinks 1L of water. Based on this DWLOC and preliminary normalized surface water 

concentrations from FIRST (generic upper-bound watershed exposure scenario) described above, 

the number of treated cattle expected to cause a toxicologically-relevant concern is presented in 

Table B-5 below. Sample documentation of the FIRST model simulations are provided in 

Appendix C. 

 

6 Available on-line at: http://www.epa.gov/pesticides/reregistration/REDs/pirimiphos_methyl_red.pdf
17 

Table B-5. Head of Treated Cattle Causing Concern due to Drinking Water Exposure 

from Surface Water Given Washoff Fractions (number of cattle) based on FIRST 

model results (generic upper-bound watershed exposure scenario) and DWLOC.2,3

PirimiphosMethyl 

DWLOC 

(g/L)1

100% 20% 10% 5% 1% 

36,000,000 5.4 x 109 2.7 x 1010 5.4 x 1010 1.0 x 1011 5.4 x 1011

1

 Calculated based on acute rat LD50 = 2,400 mg/kg-bw (pirimiphos-methyl Reregistration Eligibility Decision 

document) assuming a standard assumption that a 15 kg child drinks 1L of water. 

2

 Calculated based on FIRST acute EDWC of 0.0066 ppb per head of cattle at 100 percent of applied washed off. 

Results extrapolated to 20, 10, 5, and 1 percent washoff fraction via linear proportional scaling with application 

rate. 

3 Population size on 10-hectare lot assumed. 

 Ground Water 

A similar approach can be used to evaluate impact on ground water resources, except using the 

PRZMGW model (version 1.07). PRZMGW input parameter values shown in Table B-1 for 

post-harvest stored grain uses can be used is applicable to the analysis for eartag uses except for 

1.) The equivalent broadcast pesticide application method appropriate to evaluate the washoff 

scenario and 2.) The equivalent application rate to evaluate the various washoff scenarios. Refer 

to Table B-3 regarding the pesticide application method simulated and Table B-4 regarding the 

equivalent application rates modeled for the different washoff fractions. The number of treated 

cattle expected to cause a toxicologically-relevant concern for ground water exposure is 

presented in Table B-6 below for each PRZMGW screening scenario. The FL Potato scenario 

possesses the most vulnerable results; modeling simulations with other scenarios indicate that a 

much larger amount of treated cattle which would lead to toxicologically-relevant concerns. The 

results for other scenarios are non-definitive due to EDWCs per head of cattle <<0.01 ppb. 

Sample documentation of the PRZMGW model simulations are provided in Appendix C. 

 
18 

Table B-6. Head of Treated Cattle Causing Concern due to Drinking Water Exposure from Ground 

Water Given Washoff Fractions (number of cattle) based on PRZMGW model results (six screening 

scenarios) and DWLOC.2,3

PirimiphosMethyl 

DWLOC 

(g/L)1

PRZMGW 

Scenario 

100% 20% 10% 5% 1% 

36,000,000 

FL Potato 

>5.4 x 109 >2.7 x 1010 >5.4 x 1010 >1.0 x 1011 >5.4 x 1011

FL Citrus 

WI Sands 

Georgia Coastal 

North Carolina 

Coastal 

Delmarva 

1

 Calculated based on acute rat LD50 = 2,400 mg/kg-bw (pirimiphos-methyl Reregistration Eligibility Decision 

document) assuming a standard assumption that a 15 kg child drinks 1L of water. 

2 The results for other PRZMGW scenarios are non-definitive due to EDWCs per head of cattle <<0.01 ppb 

including levels up to 100 percent residues washed off. Values for head of treated cattle causing concern expected 

greater than found for surface water exposure scenario. 3

 Population size on 10-hectare lot assumed. 

 
19 

Appendix C. Drinking Water Exposure Model Simulation Documentation 

Figure C-1. Sample PRZMGW model simulation file for postharvest corn and sorghum 

grain treatment use. 

Groundwater Analysis for Pirimiphos-methyl and the FLORIDA - 

Potato - Met File (13889.Dvf) Jacksonville Scenario

Estimated groundwater concentrations and breakthrough times for Pirimiphos-methyl are 

presented in Table 1 for the FLORIDA - potato - Met File (13889.dvf) Jacksonville 

groundwater scenario. A graphical presentation of the daily concentrations in the aquifer is 

presented in Figure 1. These values were generated with the PRZM-GW (Version 1.07). 

Critical input values for the model are summarized in Tables 2 and 3. 

Table 1. Groundwater Results for Pirimiphos-methyl and the FLORIDA - potato - Met 

File (13889.dvf) Jacksonville Scenario. 

Peak Concentration (ppb) 1.12E-13 

Post-Breakthrough Mean 

Concentration (ppb) 

Incomplete 

Entire Simulation Mean 

Concentration (ppb) 

1.85E-14 

Average Breakthrough 

Time (days) 

Incomplete 

Throughputs 0.04941383 

Table 2. Chemical Properties for Groundwater Modeling of Pirimiphos-methyl. 

Kd (ml/g) 49.03 

Surface Soil Half Life (days) 39.1 

Hydrolysis Half Life (days) 198 

Diffusion Coefficint Air (cm2/day) 0.0 

Henry's Constant 0.0 

Enthalpy (kcal/mol) 0.0 
20 

Table 3. Pesticide application scheme used for Pirimiphos-methyl. This application 

scheme was applied every year of the simulation. 

Application Date 

(Month/Day) 

Application Method Application Rate 

(kg/ha) 

09/05 All pesticide placed at 10.16 cm 

below surface 

1.12 

Figure 1. Aquifer Breakthrough Curve for Pirimiphos-methyl and the FLORIDA - 

Potato - Met File (13889.Dvf) Jacksonville Scenario 

* Simulation of scenario with highest EDWC shown 

Notes: 

1. Results scaled to equivalent maximum single application rate for seed treatment with corn and sorghum 

grain, as shown in Table B-1. No degradation of residues before planting assumed.

2. Pirimiphos-methyl and its demethylated OP degradate included as TTRs in this analysis.

 
21 

Figure C-2. FIRST model simulation for eartag uses. 

Notes:

1. First-guess unit application rate of 1 lbs. a.i./A shown here. Results in Table B-4 scaled using equivalent 

application rate considering initial loading as per mass of a.i. on eartag along with washoff fraction. 

2. Granular application type equivalent to broadcast application method which mimics washoff scenario. 

3. Pirimiphos-methyl and its demethylated OP degradate included as TTRs in this analysis. 

 
22 

Figure C-3. Sample PRZMGW model simulation file for eartag use. 

Groundwater Analysis for Pirimiphos-methyl and the 

FLORIDA - Potato - Met File (13889.Dvf) Jacksonville 

Scenario

Estimated groundwater concentrations and breakthrough times for Pirimiphos-methyl are 

presented in Table 1 for the FLORIDA - potato - Met File (13889.dvf) Jacksonville 

groundwater scenario. A graphical presentation of the daily concentrations in the aquifer 

is presented in Figure 1. These values were generated with the PRZM-GW (Version 

1.07). Critical input values for the model are summarized in Tables 2 and 3. 

Table 1. Groundwater Results for Pirimiphos-methyl and the FLORIDA - potato - 

Met File (13889.dvf) Jacksonville Scenario. 

Peak Concentration (ppb) 7.32E-19 

Post-Breakthrough Mean 

Concentration (ppb) 

Incomplete 

Entire Simulation Mean 

Concentration (ppb) 

1.12E-19 

Average Breakthrough 

Time (days) 

Incomplete 

Throughputs 0.04941383 

Table 2. Chemical Properties for Groundwater Modeling of Pirimiphos-methyl. 

Kd (ml/g) 49.03 

Surface Soil Half Life (days) 39.1 

Hydrolysis Half Life (days) 198 

Diffusion Coefficint Air (cm2/day) 0.0 

Henry's Constant 0.0 

Enthalpy (kcal/mol) 0.0 
23 

Table 3. Pesticide application scheme used for Pirimiphos-methyl. This application 

scheme was applied every year of the simulation. 

Application Date 

(Month/Day) 

Application Method Application Rate 

(kg/ha) 

09/05 Ground application with mass 

distribution increasing 

proportionally with depth to 4 cm 

1.12 

Figure 1. Aquifer Breakthrough Curve for Pirimiphos-methyl and the FLORIDA - 

Potato - Met File (13889.Dvf) Jacksonville Scenario 

* Simulation of scenario with highest EDWC shown 

Notes:

1. Results using first-guess unit application rate of 1 lbs. a.i./A shown here. Results in Table B-4 arrived at by 

scaling results above with the equivalent application rate (also shown in Table B-4) as determined by the 

amount of a.i. per eartag, and washoff fraction. Equivalent application rate shown in Table B-4 depicts 

release from 25 acre or 10-ha area. 

2. Pirimiphos-methyl and its demethylated OP degradate included as TTRs in this analysis.