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In its submission, DuPont states that environmental exposure estimates from single swath

application scenarios best represent railroad ROW applications; and, the AgDrift model and

data generated by the Spray Drift Task Force (SDTF) can be used to estimate drift from single

swath application practices as opposed to the default analysis that is based upon multiple

swath (20) exposure estimates.  DuPont proposed that other considerations such as release

height and other drift reduction technology be considered in developing exposure estimates for

the railroad ROW scenario.  Finally, based on proposed considerations, which are expected to

reduce drift and therefore required buffer zones, DuPont proposed a matrix for use on

sulfometuron methyl product labels that defined required buffer zones.  The matrix accounted

for variables such as: application method; number of swaths; and technical specifications (i.e.,

droplet size).  Below is a summary of the primary points made by DuPont in its submission, and

the Agency’s response, and where appropriate, a revised mitigation decision.

Agency Analysis of Proposal

DuPont proposed the Agency utilize the AgDrift Model’s capability to model single swath

applications, for both aerial and ground applications.  

Based upon information provided by the American Association of Railroads, the Agency agrees

that most pesticide applications made to railroad ROW’s are not made with 20 swaths (This

information was communicated via email and can be found in the docket under the entry AAR

to EPA. Railroad Herbicide Application Questions Answered (3.25.11)).  The Agency was able to

reproduce the modeling of single swath exposure scenarios, presented by DuPont, using the

AgDrift model and the currently validated data set.  The modeling verified that the expected

spray drift from a single swath application scenario is less than the 20‐swath application

scenario previously modeled by EPA.  EPA was able to estimate risk quotients from Tier 1, single

swath ground scenarios and compare them to risk quotients from previously modeled Tier 1

multiple swath ground scenarios.   

 Based upon revised environmental exposure estimates using single swath application

scenarios as compared to those reached in the sulfometuron methyl reregistration

eligibility decision, the Agency believes that the buffer zones for railroad ROW ground

based applications can be reduced from 50 feet to 25 feet.

DuPont proposed that drift data from the Canadian Drift Model data set be used to supplement

the Spray Drift Task Force data set to estimate potential drift from railroad ROW.  The Canadian

Drift Model data set reflects “drift reduction” technology (such as nozzle type) that yields

droplet spectrum and exposure estimates not captured in the AgDrift data base.  DuPont noted

that sulfometuron methyl is typically applied using droplets sizes that are much larger than that

which was modeled in the sulfometuron methyl registration eligibility decision (i.e., 500 volume

mean diameter (VMD) vs 1000 VMD).  DuPont proposed that these estimates (based on drift

reduction technology) can be applied to low boom (hand held applications) and high boom

(spray rig) application scenarios which will result in reduced buffer zones.  
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Currently, the AgDrift data set does not have many data points based upon “reduced risk”

technology.  Also, the Agency cannot rely on the Canadian Drift Model data to estimate

exposure and risk since this data set has not been validated.  In addition, parameters used in

DuPont’s analysis (such as percentiles) do not align with those used by EPA.  A small data set is

available in the AgDrift data base that demonstrates the potential drift reduction by applying

low‐drift technologies.   Analysis of the small data set revealed a notable reduction in drift.  

However, while these data are drawn from the AgDrift database, the Agency will not rely upon

these data either since the number of replicates is small and do not provide a confidence on

which to base a decision.  Regarding typical droplet size, the Agency agrees that larger droplet

sizes produce less drift.  However, the Agency cannot be certain all applicators of sulfometuron

methyl have access to the most recent drift technology that can produce droplet sizes ≥1000

VMD.  Consequently, the Agency must model potential exposure assuming the labeled

application parameters that have the potential to result in the greatest amount of

environmental exposure.   

 The Agency will not accept DuPont’s proposal to further reduce the buffer zone for

either ground or aerial (helicopter) application scenarios to railroad ROW (below 25

feet) based upon reduced drift technology, and the Canadian Drift Model data set.

EPA also examined the existing data sets to determine whether low boom (i.e., hand held)

application scenarios can be reduced.  EPA compared estimated exposure values from two

ground scenarios where all variables, except release height, remained the same.  The estimated

environmental exposure from a release height of 1.67 feet was 38% less than the estimated

exposure using a release height of 4.17 feet.   As discussed above, EPA also believes the data

indicates that when all other variables remain constant, fewer swaths results in lower potential

environmental exposure.  Based on these two points, (lower release height and fewer swaths

each lead to lower potential drift), the Agency believes it is reasonable then to assume that drift

from hand held application will be lower than (i) estimates associated with single swath

applications at 4.17 feet; and, (ii) less than exposure estimates associated with 20‐swath

applications at 1.67 feet release height

 Based upon DuPont’s proposal to apply drift reduction to low boom (i.e., hand held)

application scenarios, and EPA’s own analysis, EPA will reduce the buffer zone

required for all hand held applications of sulfometuron methyl from 30 feet to 15 feet.

DuPont proposed label language in the form of a matrix based upon several factors that affect

potential drift, including application method, number of swaths; and, technical specifications

(i.e., droplet size).

The Agency reviewed DuPont’s proposed label format that provides a matrix for buffer zone

requirements.  Based on internal discussions and discussions with State Lead Agency (SLA)

representatives, the Agency believes that the proposed format may not be easily understood by
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the user and will be difficult to enforce.  EPA believes label instructions should be both

understandable and enforceable.  

 Therefore, the Agency will not accept the proposed matrix format by DuPont to

convey buffer zone requirements on Sulfometuron Methyl labels.

Changes to Sulfometuron Methyl Labels

The Agency has considered new information provided by DuPont and has determined that the

buffer zones for certain application scenarios can be reduced while maintaining a similar

estimated risk that the Agency accepted in its sulfometuron methyl RED and Amended RED.  In

light of the information provided by the Association of American Railroads, (i.e., number of

miles of single, double, and multiple track rail in the U.S.), the Agency believes it is reasonable

to assume that the majority of sulfometuron methyl applications made to railroad ROW will

primarily be applied via single swath applications.  Therefore, modeling potential exposure

based upon a single swath application scenario is appropriate.  The Agency also believes that

the application scenario for railroad ROW is analogous to road‐side ROW, in that most roadside

ROW applications are also single swath applications.  Therefore, the Agency has applied

modifications being made to the railroad ROW use to roadside ROW use as well.  A summary of

required changes to buffer zones to be listed on sulfometuron methyl product labels is below.   

Summary of Revised Buffer Zone Requirements for Sulfometuron Methyl (Liquid Formulation)

Use Scenario App. Method Rate (lb

ai/A)

Amended

RED Buffer

Zones

Revised Buffer Zone

commercial forestry

(deciduous, conifer and

Christmas tree plantations)

aerial (rotary craft) 0.199 75 ft 75 ft

non‐crop, rights of way   aerial (rotary craft) 0.281 75 ft 75 ft

forestry ground equipment 0.199 50 ft 50 ft

railroad and roadside rights‐of‐

way

ground equipment 0.281 50 ft 25 ft

other than railroad or road side

rights of way

ground equipment 0.281 75 ft 50 ft

all uses handheld equipment 0.281 30 ft 15 ft