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PC Code: 128931

 DP Barcode: 404138, 404806, 405887, 410802, 411382 

May 20, 2013 

MEMORANDUM 

SUBJECT: Addendum to the Environmental Fate and Ecological Risk Assessment for the 

Section 3 New Use of on Dicamba-Tolerant Soybean 

TO: Kathryn Montague, Risk Manager (RM 23) 

 Daniel Kenny, Branch Chief, Herbicides Branch 

Registration Division (7505P) 

FROM: Elizabeth Donovan, Biologist 

 Reuben Baris, Environmental Scientist 

Environmental Risk Branch VI 

Environmental Fate and Effects Division (7507P) 

 

THRU: Mark Corbin, Branch Chief 

Environmental Risk Branch VI 

Environmental Fate and Effects Division (7507P) 

REVIEWED Charles Peck, Physical Scientist

BY: Faruque Khan, Ph.D., Physical Scientist 

 Environmental Fate and Effects Division (7507P) 

The Environmental Fate and Effects Division (EFED) Ecological Risk Assessment for Dicamba 

and its Degradate, 3,6-dichlorosalicyclic acid (DCSA), for the Proposed New Use on DicambaTolerant Soybean (MON 87708) (DP Barcode 378444; dated March 8, 2011) did not include 

complete evaluation of risks to terrestrial non-target organisms exposed to dicamba through 

spray drift and vapor drift. This analysis was not included because there was incomplete 

information on the drift and volatility of the diglycolamine salt of dicamba (DGA) relative to the 

acid as well as a lack of information on the toxicity of vapor phase dicamba (DGA or acid) to 

terrestrial plants. Without this information EFED was not able to conduct a quantitative risk 

assessment for plants exposed to dicamba spray drift or vapor drift. Recent data submitted to the 

Agency by the registrant, Monsanto Company (Monsanto), provided sufficient information to 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460 
Page 2 of 13

support a more thorough analysis of spray drift and vapor drift exposure to plants. While this 

additional information does not allow for a full characterization of off-field movement of 

dicamba, conservative assumptions can be made for vapor emitted from the application site as 

well as assumptions for spray drift to estimate ecological risk from both of these exposure routes. 

Available information and additional analyses are presented below. 

In addition, as discussed in the Addendum to the Data Evaluation Report on the Toxicity of 

Clarity 4.0 SL (AI: Dicamba) to Terrestrial Vascular Plants: Vegetative Vigor (MRID 47815102; 

D411301; dated April 25, 2013), use of the EC05 (0.000013 lb a.e./A) is not appropriate for the 

listed species risk assessment because a NOAEC value of 0.000261 lb a.e./A is available. Based 

on this information, the terrestrial plant assessment has been updated to reflect the most current 

endpoints. Additionally, the terrestrial invertebrate assessment has been updated based on 

changes in Agency policy. 

Based on the weight of evidence analysis included in this addendum the dominant route of offfield exposure to non-target terrestrial and aquatic organisms is more likely to be a result of spray 

drift and runoff than the volatilized mass of dicamba from a treated field. This does not mean 

that volatility is not a concern; however, spray drift and run-off are more dominant routes of 

exposure. The first tier estimated distances where effects are predicted for non-target organisms 

from the treated field (0.5 lb a.e./A) are 210 and 475 feet for the non-listed and listed terrestrial 

plant endpoints, respectively (410 and 890 ft for the 1.0 lb a.e./A application, non-listed and 

listed species, respectively). Conclusions are based on the analysis of coarse droplet spectra data 

from the Spray Drift Task Force (SDTF) and the estimated deposition off field above the nonlisted and listed terrestrial plant endpoints. A different 100 ft buffer distance is proposed by 

Monsanto based on an alternative method for estimating distance to no effect off-field (see 

analysis below for spray drift analysis). However, there is uncertainty about how specific spray 

drift reduction strategies (e.g., DRT and nozzle/product specific labeling language) impact the 

distance of effects off-field. This is largely due to a lack of data. 

Additional analyses were conducted with available data and additional submissions provided by 

Monsanto aimed at refining the initial estimates of buffer distances; however, the following 

uncertainties persist: 

 Product and nozzle specific drift curves are not available.

 The Theoretical Shape Profile (TSP) method study submitted by Monsanto provided a 

line of evidence about the volatility of dicamba, however it is uncertain how this 

compares to standard field volatility studies (OCSPP guideline number 835.8100).

 A vapor phase toxicity endpoint is not available for terrestrial plants to compare to the 

estimates of vapor exposure. 

The body of evidence highlight the variability in the available data. Where the distances at the 

higher, more conservative, end may be overestimating distance off field to no-effect, and the 

distances at the lower end, less conservative, may be underestimating buffer distances. For the 

over-the-top 0.5 lb a.e./A application rate, a realistic distance from the application site to where 

no effects are observed ranges from 100-175 ft (assuming linearity, the 1.0 lb a.e./A rate would 

presumably yield roughly 2x greater distances). However, based on the weight of evidence for 
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the coarser droplet spectra, and a 0.5 lb a.e./A rate, this distance is 125 ft.*

 Distances for the 1.0 

lb a.e./A rate are roughly 2x the distance estimated for the 0.5 lb a.e./A rate assumptions are 

linear. 

Of paramount importance, product and nozzle specific drift curves based on empirical data are 

needed to address uncertainties with the distance off-field that effects are estimated for terrestrial 

plants. These type of data should be consistent with the Agency’s Drift Reduction Technology 

(DRT) program intending to improve the clarity and enforceability of product label use 

directions and drift restrictions and encourage the use of drift reducing application technologies 

and best management practices to minimize drift. Results from DRT studies can be incorporated 

into specific label use directions and drift restrictions that would better inform the spray drift risk 

assessment and would likely result in smaller estimated buffer distances between the treated field 

and non-target organisms. 

Updates to Conclusions from Previous Terrestrial Plant Assessment 

Dicamba exposure to terrestrial and semi-aquatic plants, estimated using the TerrPlant model 

(version 1.2.2), resulted in RQs that exceeded the listed and non-listed species level of concern 

(LOC = 1) for dicots in terrestrial areas due to spray drift and in semi-aquatic areas due to runoff 

and spray drift. RQs for monocots in terrestrial and semi-aquatic areas did not exceed the LOC. 

The EECs, toxicity endpoints, and resulting RQs for terrestrial and semi-aquatic plants for a 

single application of dicamba DGA at the maximum label rate for the proposed use on dicambatolerant soybeans are presented in Tables 1-3. 

Table 1. EECs for Terrestrial and Semi-Aquatic Plants Near Dicamba Use on DicambaTolerant Soybeans. 

Crop 

Single Max. 

Application 

Rate (lbs a.e./A) 

EECs (lbs a.e./A) Ground Spray 

Total Loading to 

Adjacent Dry Areas 

(sheet runoff + drift) 

Total Loading to SemiAquatic Areas (Channelized 

runoff + drift) 

Drift 

EEC 

DicambaTolerant 

Soybeans 

1.0 0.06 0.51 0.01 

Table 2. Plant survival and growth data used for RQ derivation. Units are in lb a.e./A. 

 Seedling Emergence Vegetative Vigor 

Plant type EC25 NOAEC EC25 NOAEC 

Monocot 1.68 0.64 0.427 0.137 

Dicot 0.123 0.0673 0.000513 0.000261 

Table 3. RQ values for plants in dry and semi-aquatic areas exposed to Diglycolamine Salt (DGA) 

through runoff and/or spray drift.* 

 

*

 Label language for the proposed Monsanto product includes application must use flat fan nozzles with “verycoarse to ultra-coarse” droplet distribution. 
Page 4 of 13

Plant Type Listed Status Dry Semi-Aquatic Spray Drift 

Monocot non-listed <0.1 0.30 <0.1 

Monocot listed <0.1 0.80 <0.1 

Dicot non-listed 0.49 4.15 19.49 

Dicot listed 0.89 7.58 38.31 

*If RQ > 1.0, the LOC is exceeded, resulting in potential for risk to that plant group. 

Updates to Conclusions from Previous Terrestrial Invertebrate Assessment 

T-REX is used to calculate EECs for terrestrial invertebrates exposed to the DGA salt of 

dicamba from the proposed use on dicamba-tolerant soy. Terrestrial EECs for the DGA were 

derived for the proposed use based on the maximum labeled application rate (i.e, 1.0 lb a.e./A 

application followed by two 0.5 lb a.e./A applications at 6 day intervals). The foliar dissipation 

half-life of 35 days is used based on the T-REX user guide. The dietary-based EEC calculated by 

T-REX for arthropods (units of μg a.e./g of bee, or ppm) are used to estimate exposure to 

terrestrial invertebrates. The EECs are compared to the adjusted acute contact toxicity data for 

bees in order to derive RQs. For dicamba, the available acute contact toxicity endpoint for bees 

exposed to dicamba (in units of μg a.e./bee), is converted to μg a.e.,/g (of bee) by multiplying 1 

bee by 0.128 g (the average weight of an adult honey bee). In this case, the acute contact LD50 is 

> 91 μg a.e./bee for the honey bee (Apis mellifera; MRID 00036935), which results in an 

adjusted toxicity value of >711 μg a.e./g of bee. 

While RQs cannot be calculated for terrestrial invertebrates because of the non-definitive toxicity 

endpoint, EECs generated for the arthropod can be compared to the available toxicity data to 

determine whether there is potential for risk. The arthropod EEC for dicamba is 162.85 μg a.e./g 

of bee, which represents 23% of the highest dose tested in the acute contact study. To further put 

potential exposures in context, the highest concentration tested in the available acute contact 

study, which did not result in mortality or signs of overt toxicity, is already below the Agency’s 

interim LOC (0.4). Based on this information, the risks to listed and non-listed terrestrial 

invertebrates from the proposed use of dicamba is low. 

Additional Analyses 

Field Studies 

The registrant submitted additional information in support of their request for registration of the 

DGA salt for use on Dicamba-tolerant soybean (MON 87708). 

The first study (MRID 48892301) measured the effects of small amounts of MON 54140, a 

technical end use product with the DGA salt, on soybean vegetative growth and yield endpoints 

under field conditions. Soybean was selected as the test species in part because available lab data 

suggest it is highly sensitive to dicamba during vegetative growth stages with NOAEC and EC25

values of 0.000261 and 0.000513 lb a.e./A, respectively (MRID 47815102; Acceptable). During 

the study, MON 54140 was applied to plants at three field sites within the major soybean 

growing region (AR1 located in Proctor, Arkansas, IL1 located in Carlyle, IL, and IL2 located in 

Wyoming, IL). Six spray application rates plus a control were used and plant responses were 
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measured weekly for nine weeks. The study authors calculated ECx values for each week of 

measurement and determined that plant effects peaked three weeks after pesticide application. 

The most sensitive endpoint across the three sites was plant height, with the lowest EC25 and 

associated NOAEC values of 0.0008 lb a.e./A and less than 0.0006 lb a.e./A, respectively. 

Results of this study support the contention that soybean in the field showed similar sensitivity to 

dicamba as soybean in the lab and can thus be used as a field bioassay. 

The second study (MRID 48876001) addressed the potential for off-site movement of the DGA 

salt under field conditions using non-tolerant soybean as a bioassay. The study was conducted 

under varying field conditions to represent a range of application scenarios possible for the 

proposed new use on dicamba tolerant soybean. MON 54140 was applied at a rate of 0.5 lb 

a.e./A to the eight field sites using TeeJet AIXR 11004, TTI 11004, or AIC 11003 nozzles. 

Applications were made perpendicular to the prevailing wind direction to maximize the potential 

for spray drift. Plant heights were measured at regular intervals up to 328 ft downwind of the 

spray areas depending on the size and shape of the experimental field. There were no controls in 

this study so statistic could not be verified using standard EFED approaches. Instead, mean noeffect distances were determined by fitting non-linear mixed effects model to the available plant 

height data and calculating an effective distance. For those transects that did not fit the non-linear 

mixed effects model, mean no-effect distances were visually determined from scatterplots for 

each transect. The overall mean no-effect distance based on reduction in plant height was less 

than 90 ft for all trials. This study forms the basis for the registrant’s recommendation of a 100 ft 

buffer for 0.5 lb a.e./A applications of the DGA salt to dicamba tolerant soybean. The study was 

reviewed in conjunction with the Statistics Technical Team (STT) and the following 

uncertainties were noted: 

 Control plants were not used in this study, necessitating use of non-standard statistical 

methods. 

 Outliers in the data, defined as “an observation at a distance along a transect at which the 

maximum plant height was greater than two times the minimum plant height”, were 

removed from the analysis to calculate no-effect distances. These outliers may represent 

sensitive plants and may have an impact on the calculated no-effect distances. 

 The analysis used PROC NLMIXED in SAS to fit the nonlinear regression model for 

each transect rather than running a single nonlinear mixed effects model accounting for 

all of the effects of the study (e.g., site, transect, nozzle type), which would better assess 

potential sources of variability. 

 The analysis reports the mean no effect distance for each site and nozzle combination 

instead of the upper 90th confidence interval, which would better represent possible 

exposures. 

The STT expressed reservations about the overall study design (e.g., the lack of control data) and 

were uncertain whether the results are meaningful given the amount of variability inherent in 

terrestrial plant field studies. It is the team’s opinion that results of the study should be 

interpreted with caution. 

Drift Analysis 
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An analysis of the Coarse Droplet data from Spray Drift Task Force†

 (SDTF) showed that 

exceedances of the non-listed terrestrial plant endpoint could occur up to 210 ft and 410 ft from 

the edge of field using max single rate and max in-crop app rate of 0.5 and 1.0 a.e./A, 

respectively. These distances were calculated using an approved method for estimating spray 

drift for Agency risk assessment where the non-listed EC25 vegetative vigor endpoint for 

soybeans was used (0.000513 lbs a.e./A, MRID 47815102) as well as the 90th percentile of the 

coarse droplet size distribution (DSD) for spray drift. There was no exceedance for monocot 

plant species, only dicot plant species. For listed plants, there are exceedances up to 475 ft from 

the edge of field for the 0.5 lb a.e./A maximum single application rate, and 890 ft from the edge 

of field for the 1.0 lb a.e./A rate. Calculations of distances of effect are based on the vegetative 

vigor endpoint for soybean (NOAEC = 0.000261 lb a.e./A) (Table 4). This analysis was 

completed using the 90th percentile of the coarse DSD from the empirical data that forms the 

basis for the AgDRIFT model. The low-boom, coarse DSD was extracted and each of the 4 

swaths were plotted. For each run, values were ranked and the 90th percentile was selected for 

this analysis. The 90th percentile deposition with distance was plotted and estimated distances 

where off field effect are likely to be minimized were calculated based on the fraction of droplet 

deposition. 

Additional characterization of the potential spray drift using the 50th percentile deposition curve 

would result in buffer distances of approximately 125 and 250 ft for the 0.5 lb a.e./A rate and the 

non-listed and the listed endpoint, respectively (Table 5). In order to further refine these 

modeled distances where effects off field are minimized, product- and nozzle-specific drift 

curves are needed.

Table 4. Estimated distance off field effects for non-target species observed based on Coarse 

DSD from 90th %-tile SDTF data (4-swaths). 

Vegetative Vigor Endpoint Application Rate 

0.5 lbs a.e./A 

(max single app in crop) 

1.0 lbs a.e./A (total in crop) 

Dicot, soy (non-listed, EC25 = 

0.000513 lbs a.e./A) 

210 ft 410 ft 

Dicot, soy (listed, NOAEC = 

0.000261 lbs a.e./A) 

475 ft 890 ft 

Table 5. Estimated distance off field effects for non-target species observed based on Coarse 

DSD from 50th %-tile*

 SDTF data (4-swaths).

Vegetative Vigor Endpoint Application Rate 

0.5 lbs a.e./A 

(max single app in crop) 

1.0 lbs a.e./A (total in crop) 

Dicot, soy (non-listed, EC25 = 

0.000513 lbs a.e./A) 

125 ft 250 ft 

Dicot, soy (listed, NOAEC = 

0.000261 lbs a.e./A) 

250 ft 425 ft 

*

 Note: 50th%-tile DSD is a non-standard approach that is not typically used for spray drift modeling as it may under predict potential driftable 

fines. 

Characterization of Spray Drift Analysis 

 

†

 Teske et al., 2001 
Page 7 of 13

The Agency continues to reevaluate approaches to estimating spray drift off-field from the 

treatment site. In the case of this DGA assessment, a weight of evidence approach was used to 

refine standard AgDRIFT estimates of spray drift to calculate distance to no-effect for listed and 

non-listed dicot plant species. Appendix A includes a tabulation of the available data 

(submissions) and individual model estimates of buffer distances, which are described below. 

The buffer distance based on the analysis of the weight of evidence for the 0.5 lb a.e./A 

application rate is 125 ft.‡

 However, product and nozzle-specific drift curves based on empirical 

data that are incorporated into specific label use directions and buffer restrictions and would 

better inform the spray drift risk assessment and would likely result in smaller buffer distances 

between the treated field and non-target organisms. 

EFED explored several refinements to the standard conservative assumptions built into standard 

modeling approaches to characterize potential off-field exposure from spray drift. First, 

AgDRIFT SDTF data, which contain empirical data based on 4 swaths, were compared to 

modeled data based on 1 swath. The result at the 90th percentile DSD for the 0.5 lb a.e./A is 175 

ft and 300 ft for the 1.0 lb a.e./A from the treated field to no-effect for listed species , compared 

to 475 and 890 ft discussed above using a 4 swath analysis (a 300 ft and almost 600 ft reduction 

if a single swath analysis is used). This approach accounts for the impact of a single pass within 

the field and the resulting estimated drift off field, however a single swath scenario is not 

realistic given the application practices in soy agriculture. 

Second, a crude approach looks at all of the available data and averages all distances together, 

equaling approximately 175 ft as the distance beyond which effects to listed plant species are not 

expected. Extracting just the coarse, extra-coarse, and ultra-coarse values from this table the 

average is reduced to 124 ft (which is rounded to 125 ft). Additionally, examining the extracoarse and ultra-coarse data points exclusively in the weight of evidence result in an estimated 

average distance of 107 ft. All of the methods for estimating spray drift and distances to noeffect use different assumptions and thus aggregating them together should be done with caution. 

However, this analysis highlights the range in potential variability (62-475 ft) when estimating 

the distance off field below the where deposition is less than the NOAEC. Again to address the 

identified uncertainties a study that evaluates the product and nozzle specific recommendations 

according to the American Society of Biological and Agricultural Engineers (ASABE) DSD 

could be submitted to inform the distance to no-effect, in all likelihood reducing the potential 

buffer distance. 

Part of the weight of evidence approach includes additional study submissions by Monsanto. 

These studies include an independent analyses of spray drift using the AgDRIFT Model and field 

investigations titled, Summary of Investigations of the Potential for Off-Site Movement through 

the Air of the Herbicide MON 54140 Following Ground Applications (MRID 48876001) and 

Concordance of MON 54140 Buffer Distances Determined using Field Spray Drift Studies and 

AgDRIFT (MRID 49022404). The purpose of these submissions was to present the results of 

eight field trials in the US and Argentina, designed to evaluate off-site movement of dicamba 

from fields treated with the DGA formulation and compare those results to AgDRIFT modeling. 

 

‡

 Modeled estimates for 1.0 lb a.e./A are approximately 2x the 0.5 lb a.e./A rate, therefore it is reasonable to assume 

the weight of evidence distance is 250 ft. 
Page 8 of 13

The trials were conducted under varying field conditions to represent a range of application 

scenarios including applications with and without a glyphosate formulation. Spray solutions were 

applied using either TeeJet AIXR 11004, TTI 11004, or AIC 11003 nozzles at nominal dicamba 

rates of 0.5 lb a.e./A to plots of soybeans or corn contained within larger soybean fields. In the 

study submitted by Monsanto (MRID 48876001) results of field trials were compared with 

AgDRIFT model runs using the 50th%-tile DSD with an incorporation of an adjustment to the 

driftable fines fraction obtained from wind tunnel test. Based on results from this analysis, 

combined with the submitted field data, Monsanto concluded that a distance of approximately 

100 ft would be needed to reduce potential effects to non-target sensitive plants. Note Monsanto 

also completed a comparison of AgDRIFT model runs using the 90th%-tile DSD resulting in an 

average of 40 ft greater no-effect distance for the 90th%-tile DSD runs. 

To provide additional support for the 100 ft buffer recommended by the various field trials, 

Monsanto submitted an analysis using the PMRA Buffer Zone Workbook and the underlying 

data supporting the tool (D405887). A review of these data were completed and were included in 

the weight of evidence approach (USEPA, 2013). 

The Agency’s conclusions and approaches to estimating buffer distances are different than 

Monsanto’s. Ultimately the Agency disagrees with the application of the correction factor for 

driftable fines based on wind tunnel data. The Agency’s analysis of the driftable fraction (% 

<150μm) that underlies the AgDRIFT model is 9.5% compared to the 15.63% and 14.64% used 

by Monsanto to calculate the ratio of driftable fines for the MON54140 and the 

MON54140+MON79789 mixes, respectively. By correcting the driftable fractions to match the 

driftable fines used in AgDRIFT results in greater no-effect distances. Further, the assumption of 

driftable ratio correction is not consistent for other DSD spectra (e.g., very fine to fines). If the 

assumptions for coarser droplet spectra (i.e., lower driftable fraction) were consistent across all 

spectra, the correction approach to the application rate in AgDRIFT for driftable fraction would 

be a reliable method; however, this is not the case. Therefore the Agency used the Coarse DSD 

analysis as a refinement to the standard AgDRIFT modeling to estimate no-effect distances. 

At the first refinement level, the difference between the Monsanto estimate and the Agency 

estimate of no-effect distances resulting from drift at the 0.5 lb a.e./A rate is approximately 100 

ft for non-listed (100 ft vs 200 ft) and 375 ft for listed species (100 ft vs 475 ft). 

The body of evidence highlight the variability in the available data. Where the distances at the 

higher, more conservative, end may be overestimating distance off field to no-effect, and the 

distances at the lower end, less conservative, may be underestimating buffer distances. A realistic 

distance from the application site to where no effects are observed ranges from 100-175 ft. Based 

on the weight of evidence for the coarser droplet spectra, this distance is 125 ft.§

 

Again, it is important to note that product and nozzle specific drift curves based on empirical 

data that are incorporated into specific label use directions would better inform the spray drift 

risk assessment and would likely result in smaller buffer distances between the treated field and 

non-target organisms. 

 

§

 Label language for the proposed Monsanto product includes application must use flat fan nozzles with “verycoarse to ultra-coarse” droplet distribution. 
Page 9 of 13

Vapor Analysis 

Additional analyses were completed to determine the potential contribution that vapor phase drift 

would impact the terrestrial risk assessment. The analyses are based on the potential for a semivolatile compound such as dicamba acid to volatilize from the treated site and drift off-field and 

redeposit in sensitive, non-target areas and cause an effect. Data were gathered for dicamba acid, 

dicamba DGA, and dicamba DMA (in the case of volatile flux data). This was done so that the 

Agency is able to provide multiple lines of evidence to support risk conclusions. Based on these 

multiple lines of evidence and characterization of the potential for off-field drift due to 

volatilized material, the Agency concluded that the dominant route of off-field exposure is more 

likely to be a result of spray drift and runoff based on the analysis below. However there are 

associated uncertainties regarding the amount of dicamba that volatilizes from a field treated 

with the dicamba DGA salt formulation. Uncertainties associated with converting air 

concentrations to a plant deposition would be greatly reduced by the submission of a terrestrial 

plant vapor phase toxicity end point that measures air exposure concentrations. Without these 

data the Agency used a number of tools to estimate exposure and convert the vapor phase air 

concentration to a deposition value to calculate risk to sensitive non-target plants. 

As part of the multiple lines of evidence approach, multiple screening-level tools (models and 

data) were used to characterize and support the Agency’s conclusions. The screening tool used 

for characterization of non-target plant risk from volatile mass utilizes physical and chemical 

properties to predict flux based on the work by Woodrow et al. (1997) and a model developed by 

the Office of Solid waste and Emergency Response that estimates vapor phase exposure to nontarget plants. Woodrow et al. (1997) and Woodrow et al. (2001) developed a linear regression 

between the natural logarithm of a chemical’s physical and chemical properties to the natural 

logarithm of the amount of chemical emitted from the soil or foliar surface of a plant. However, 

like all linear relationships, half of the emissions were underestimated when compared to the 

measured values used to derive the relationship. For the purposes of characterizing volatile drift 

EFED used the data provided in the Woodrow papers to estimate the 90th percentile upper 

confidence limit around the slope and intercept so that 90% of the estimated flux rates would 

exceed the measured values, i.e., a conservative estimate of flux. The estimated flux along with 

the AERSCREEN** model estimates of air concentrations at different distances from a treated 

field can be calculated, however EFED applied the air concentration to calculate an 

approximation of deposition in order to estimate what the air concentration would need to be at 

the edge of the treated field in order to cause an effect (see discussion below). The equation 

below is a modification of the Woodrow et al. (1997) equation based on a series of field trials for 

volatile and semi-volatile compounds used to estimate the flux from a field for foliar applied 

compounds. 

Modified Woodrow equation for Plants: 

݂݈ݔݑ ൌ ݁ሺ଴.଼ଶ଺଼ ୪୬ሺ௏௉ሻାଵଶ.଴଼ଵ ൊ 3600

Where: 

 

** http://www.epa.gov/ttn/scram/dispersion_prefrec.htm 
Page 10 of 13

VP = vapor pressure in Pa

flux = mass of vapor emitted from the field per unit area per second (μg/m2

•s-1)

flux = 0.566 μg/m2•s-1

In order to estimate the amount of mass potentially drifted as vapor from the field off-site, EFED 

adapted the Office of Solid waste and Emergency Response USEPA (2005) tool for estimating 

vapor phase exposure to non-target plants to develop an aerial vapor transfer concentration from 

the air concentration; termed Aboveground Product Concentration Due to Air-to-Plant Transfer 

(equation 5-18 of USEPA, 2005). Conceptually the plant concentration (deposition) is calculated 

from the aboveground produce concentration do to air-to-plant transfer (vapor transfer). The airto-plant transfer was developed to determine the exposure of plants from point source 

contaminant release sites that were in the vapor phase. This equation was adapted, and used with 

the inhalation screening tool (AERSCREEN) to determine the air concentration at the edge of 

field. 

ܽ݃ܩܸ ∙ agݒܤ ∙ ݒݕܥ ∙ ݒܨ ∙ ܳ ൌ ݒܲ

ܲܽ

Where: 

Pv = Concentration of compound of potential concern (COPC) in the plant resulting from air-toplant transfer (μg COPC/g DW)††

Q = COPC emission rate (g/s) (assumed to be 0.1833 g/s)‡‡

Fv = Fraction of COPC air concentration in vapor phase (unitless) (conservatively assume 1 

meaning 100% of compound in vapor phase) 

Cyv = Unitized yearly average air concentration from vapor phase (μg-s/g-m3) 

Bvag = COPC air-to-plant biotransfer factor (unitless)§§

VGag = Empirical corrector factor for aboveground produce (assumed 1 for compounds with 

Logkow <4)

ρa = Density of air (g/m3

) = 1,200 g/m3 

The air concentration determined using AERSCREEN (Cyv) was used as an input to the OSWER 

model. The resulting deposition (Pv) was compared to the soybean EC25, converted to air-toplant DW concentrations assuming one ton of grass (dry weight) per acre using equations A-2-19 

and A-2-20 (USEPA, 2005, appendix A). Based on this calculation the Agency concludes that 

the resulting deposition at the edge of field from volatile drift of dicamba is less than the EC25. In 

order to exceed the EC25 at the edge of field (i.e., the air concentration results in a deposition 

greater than 0.0005 lb a.i./A) the air concentration (Cyv) would have to be greater than 721

μg/m3. These comparisons of the air concentration values calculated from the OSWER tool to 

the air concentration estimated from AERSCREEN*** show that with a calculated flux rate of 

0.566 μg/m2•s-1 using the modified Woodrow equation (see above) the maximum 1-hour average 

 

†† For the purposes of a screening model, EC25 plant deposition is effectively converted to air-to-plant vapor dry 

weight concentration assuming one ton of grass (dry weight) per acre (USEPA, 2005; Appendix A, equations A-2-

19 & A-2-20). 

‡‡ Converting calculated flux rate from modified Woodrow et al. equation for an 80 acre field. 

§§ Based on correlation of Logkow and HLC (A-2-20 of USEPA, 2005). *** Assumptions of flux, application rate,...etc 
Page 11 of 13

concentration from AERSCREEN at the edge of the field is 283 ug/m3

 less than the air 

concentration required to exceed the EC25 (as predicted by the OSWER tool) but within the same 

order of magnitude. 

Similarly, the volatile flux data (Theoretical Profile Shape (TPS) method) submitted by 

Monsanto in March, 2013 (MRID 49022501) provided a 6 hour average flux rate of 0.0004 

μg/m2•s-1, greater than 4 orders of magnitude lower than the Woodrow et al estimated flux rate. 

The air concentration and resulting deposition rate is directly proportional to the flux rate. 

Therefore since there is uncertainty about what the actual flux rate is, this value can be used as a 

lower bound estimate of flux. Using the lower bound estimate of flux from the treated field 

would result in low exposure concern off-field due to volatile drift. 

The last screening-level tool that was used to estimate off-field exposure from volatile drift is the 

AERSCREEN model using the AERMOD deposition algorithm. Similar to the AERSCREEN 

Tier I analysis above, the air concentration at the edge of the field is approximately 283 ug/m3

however using the AERMOD deposition algorithm the distance off field where no effect would 

be observed would be 1500m (i.e., the distance where the deposition value is less than the EC25). 

These estimates are based on the estimated flux using the Woodrow et al. equation as the upper 

bound. The lower bound TPS flux value results in no exceedance of the EC25 at the edge of the 

field. 

Uncertainties associated with converting air concentrations to a plant deposition would be greatly 

reduced by the submission of a terrestrial plant vapor phase toxicity end point that measures air 

exposure concentrations. 

Based on this multiple lines of evidence approach, the primary route of off-field exposure is 

more likely to be a result of spray drift and runoff. However, there are uncertainties associated 

with the analysis that would better clarify the potential for vapor phase exposure to dicamba. 

 
Page 12 of 13

References: 

Teske, M.E., Bird, S.L., Esterly, D.M., Ray, S.L., Perry, S.G. 2001. A User’s Guide for 

AgDRIFT® 2.01: A Tiered Approach for the Assessmetn of Spray Drift of Pesticides (Regulatory 

Version). Spray Drift Task Force, Macon, MS. CDI Report No. 01-02. 

USEPA 2005. Human Health Risk Assessment Protocol for Hazardous Waste Combustion 

Facilities. EPA530-R-05-006. United Stated Environmental Protection Agency, Office of Solid 

Waste and Emergency Response, Washington, DC. 

USEPA 2013. Data Evaluation Report for EPA MRID No. 49135901. ARCADIS. 2010. 

Generation of Empirical Curves for Spray Drift D~ta. Report prepared by ARCADIS U.S. Inc., 

Chelmsford, MA, sponsored and submitted by CropLife America, Washington, DC; 62 pages. 

Final report issued June 2, 2010. 

Woodrow, J., Seiber, J., Baker, L. 1997. Correlation Techniques for Estimating Pesticide 

Volatilization Flux and Downwind Concentrations. Environ. Sci. Technol., 31:523-529 

Woodrow, J., Seiber, J., Dary, C. 2001. Predicting Pesticide Emissions and Downwind 

concentrations Using Correlations with Estimated Vapor Pressures. J. Agric. Food Chem. 

49:3841-3846 
Page 13 of 13

Appendix A. Tabulated Estimates by Method for Distances to No-Effect

Study/Analyses Method # Swaths DSD Distance (ft)

0.5 lb ae/A

(non‐listed)

Distance (ft)0.5 lb ae/A

(listed)

Source Comm

SDTF (Coarse) 90th% 4 C 210 475 EPA Standa

model

assum

distrib

SDTF (Coarse) 50th% 4 C 125 250 EPASDTF (Coarse) 90th% 1 C 85 175 EPASDTF (Coarse) 50th% 1 C ‐‐ ‐‐  EPAField + AgDRIFT 50th% 4 VC/UC 100 100 Monsanto Monsa

MRID 

Field + AgDRIFT 90th% 4 VC/UC 140 140 MonsantoPlant height (NOAEC)  ‐‐  ‐‐ VC/UC 90 90 Monsanto Monsa

48876

AgDrift + PMRA EAD model  ‐‐  ‐‐ VC/UC 175 175 Monsanto ‐‐SDTF (Malathion only) VC  ‐‐  ‐‐ VC/UC 81 81 Monsanto Note: 

contai

Log‐Lo

Field (Clarity)   ‐‐  3 (120 ft) UC 62 62 BASF/Monsanto Field s

presen

Field (Engenia)   ‐‐  3 (120 ft) XC 106 106 BASF/Monsanto Field s

presen

CLA data Based on fit

curves and ln‐

lntransformedsupportingdata

1 VC/Low

boom

87 87 EPA 50th%‐

1 VC/Low

boom

92 92 EPA 90th%‐

4 VC/Low

boom

230 230 EPA 90th%‐

1 C‐VC/High

Boom

116 116 EPA 90th%

4 C‐VC/High

Boom

210 210 EPA 90th%

20 C‐VC/High

Boom

375 375 EPA 90th%

Ave. (all) 142.75 172.75 All DSDAve.(coarse) 124.08 124.08 DSD>VAve. (xc) 107.71 107.71 DSD>X

*Note: no difference in ln-ln vs curve fit estimates.