Page 1 of 13 PC Code: 128931

DP Barcode: 404138, 404806, 405887, 410802, 411382 May 20, 2013

MEMORANDUM

SUBJECT: Addendum to the Environmental Fate a nd 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 a nd Effects Division (7507P)

The Environmental Fate and Effects Division (E FED) Ecological Risk Assessment for Dicamba and its Degradate, 3,6-dichlor osalicyclic acid (DCSA), for th e 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 volatil ity of the diglycolamine salt of dicamba (DGA) relative to the

acid as well as a lack of information on the toxi city of vapor phase di camba (DGA or acid) to terrestrial plants. Without this information EF ED was not able to c onduct a quantitative risk assessment for plants exposed to dicamba spray drif t or vapor drift. Recent data submitted to the Agency by the registrant, Monsanto Company (M onsanto), provided sufficient information to OFFICE OF CHEMICAL SAFETY
AND POLLUTION PREVENTION UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460Page 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 ecol ogical 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 Repor t on the Toxicity of Clarity 4.0 SL (AI: Dicamba) to Terrestrial Vascular Plants: Vegeta tive Vigor (MRID 47815102; D411301; dated April 25, 2013), use of the EC 05 (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 assessme nt has been updated to re flect the most current endpoints. Additionally, the terrestrial invert ebrate assessment has been updated based on changes in Agency policy.

Based on the weight of evidence an alysis included in this adde ndum the dominant route of offfield exposure to non-target terrestri al and aquatic organisms is more likely to be a result of spray

drift and runoff than the volatili zed mass of dicamba from a trea ted field. This does not mean that volatility is not a concer n; however, spray drift and run-o ff are more dominant routes of exposure. The first tier estimated distances wher e effects are predicted for non-target organisms from the treated field (0.5 lb a.e./A) are 210 a nd 475 feet for the non-liste d and listed terrestrial plant endpoints, respectively (410 and 890 ft fo r the 1.0 lb a.e./A a pplication, 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 pl ant endpoints. A different 100 ft buffer distance is proposed by

Monsanto based on an alternative method for es timating 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 availa ble data and additional submissions provided by Monsanto aimed at refining the initial estimat es of buffer distances; however, the following uncertainties persist:

 Product and nozzle specific drif t curves are not available.

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

line of evidence about the vol atility 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 availabl e 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 overestim ating 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 applicatio n rate, a realistic distance from the application site to where no effects are observed ranges from 100-175 ft (assu ming linearity, the 1.0 lb a.e./A rate would presumably yield roughly 2x greater distances). However, based on the weight of evidence forPage 3 of 13 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 o ff-field that effects are estimated for terrestrial plants. These type of data should be consiste nt with the Agency’s Drift Reduction Technology (DRT) program intending to improve the clarit y 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 restric tions that would better inform the spray drift risk assessment and would likely result in smaller esti mated buffer distances between the treated field and non-target organisms.

Updates to Conclusions from Prev ious Terrestrial Plant Assessment

Dicamba exposure to terrestrial and semi-aquatic plants, estimated usi ng 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 terrestri al 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 DG A 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-Aqua tic 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 EC 25 NOAEC EC 25 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-aq uatic 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 terrestr ial invertebrates expos ed to the DGA salt of dicamba from the proposed use on dicamba-tol erant soy. Terrestrial EECs for the DGA were derived for the proposed use based on th e maximum labeled application rate ( i.e , 1.0 lb a.e./A application followed by two 0.5 lb a.e./A applicat ions at 6 day interval s). The foliar dissipation half-life of 35 days is used based on the T-RE X user guide. The dietary-based EEC calculated by T-REX for arthropods (units of μg a.e./g of b ee, or ppm) are used to estimate exposure to terrestrial invertebrates. The EECs are compared to the adjusted acute cont act toxicity data for bees in order to derive RQs. For dicamba, the available acute contact t oxicity 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 ho ney bee). In this case, the acute contact LD 50 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 poten tial 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, th e 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 vegetati ve growth and yield endpoints under field conditions. Soybean was selected as the te st species in part because available lab data suggest it is highly sensitive to dicamba during vegetative gr owth stages with NOAEC and EC 25 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 th ree 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 a nd plant responses werePage 5 of 13 measured weekly for nine week s. The study auth ors calculated EC x values for each week of measurement and determined that plant effects peaked three weeks afte r pesticide application. The most sensitive endpoint across the three sites was plant height, with the lowest EC 25 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 s oybean 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 poten tial for off-site movement of the DGA

salt under field conditions usi ng non-tolerant soybean as a bioassay. The study was conducted under varying field conditions to represent a range of applicat ion 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 pr evailing 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 an d shape of the experimental field. There were no controls in this study so statistic could not be verified us ing standard EFED approaches. Instead, mean noeffect distances were determined by fitting non-li near mixed effects model to the available plant height data and calculating an e ffective distance. For those trans ects that did not fit the non-linear mixed effects model, mean no-effect distances we re visually determined from scatterplots for

each transect. The overall mean no-effect distan ce based on reduction in plant height was less than 90 ft for all trials. This study forms the ba sis for the registrant’s recommendation of a 100 ft buffer for 0.5 lb a.e./A applications of the DGA salt to dicamba tolera nt soybean. The study was reviewed in conjunction with the Statisti cs 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 observati on at a distance along a transect at which the maximum plant height was greater than tw o times the minimum plant height”, were removed from the analysis to calculate no-e ffect 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, tran sect, nozzle type), wh ich would better assess potential sources of variability.

 The analysis reports the mean no effect di stance for each site and nozzle combination instead of the upper 90 th confidence interval, which w ould 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 result s 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 AnalysisPage 6 of 13 An analysis of the Coarse Droplet data from Spray Drift Task Force † (SDTF) showed that exceedances of the non-listed terrestrial plant en dpoint could occur up to 210 ft and 410 ft from the edge of field using max single rate a nd 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 asse ssment where the non-listed EC 25 vegetative vigor endpoint for soybeans was used (0.000513 lbs a.e. /A, MRID 47815102) as well as the 90 th percentile of the coarse droplet size distribution (DSD) for spra y drift. There was no exceedance for monocot plant species, only dicot plant speci es. 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, a nd 890 ft from the edge

of field for the 1.0 lb a.e./A rate . Calculations of distances of e ffect are based on the vegetative vigor endpoint for soybean (NOAEC = 0.000261 lb a.e./A) ( Table 4) . This analysis was completed using the 90 th percentile of the coarse DSD from the empirical data that forms the basis for the AgDRIFT model. The low-boom, co arse 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 90 th 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 perc entile 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 effect s off field are minimized, produc t- and nozzle-specific drift

curves are needed.

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

DSD from 90 th %-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, EC 25 = 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 50 th %-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, EC 25 = 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: 50 th %-tile DSD is a non-standard approach that is not typically us ed for spray drift modeling as it may under predict potential dri ftable

fines.

Characterization of Spray Drift Analysis

† Teske et al., 2001Page 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 in cludes 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 th e 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 standa rd conservative assumptions built into standard modeling approaches to characterize potential off-field exposure from spray drift. First, AgDRIFT SDTF data, which contain empirical da ta based on 4 swaths, were compared to

modeled data based on 1 sw ath. The result at the 90 th 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 li sted species , compared to 475 and 890 ft discussed above using a 4 swat h analysis (a 300 ft a nd almost 600 ft reduction if a single swath analysis is us ed). This approach accounts for th e 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 ava ilable 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 rounde d 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 togeth er 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 Biologi cal 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 in cludes additional study s ubmissions by Monsanto. These studies include an independe nt analyses of spray drift us ing the AgDRIFT Model and field investigations titled, Summary of Investigations of the Pote ntial for Off-Site Movement through the Air of the Herbicide MON 54140 Following Ground Applications (MRID 48876001) and Concordance of MON 54140 Buffer Distances Dete rmined using Field Spray Drift Studies and AgDRIFT (MRID 49022404). The purpose of these submi ssions was to present the results of eight field trials in the US and Argentina, desi gned 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 ar e 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 co nditions to represent a range of application scenarios including applications with and withou t 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 50 th %-tile DSD with an incorporation of an adjustment to the

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

combined with the submitted fiel d data, Monsanto concluded that a distance of approximately 100 ft would be needed to reduce potential effect s to non-target sensitive plants. Note Monsanto also completed a comparison of AgDRIFT model runs using the 90 th %-tile DSD resulting in an average of 40 ft greater no-effect distance for the 90 th %-tile DSD runs.

To provide additional support for the 100 ft bu ffer 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 Agen cy’s analysis of th e 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 correc ting the driftable fractions to match the driftable fines used in AgDRIFT results in greate r no-effect distances. Furt her, the assumption of

driftable ratio correcti on 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) we re consistent across all spectra, the correction approach to the application rate in AgDR IFT 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 Ag DRIFT 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 re sulting 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 overestim ating distance off field to no-effect, and the distances at the lower end, less c onservative, may be underestimati ng buffer distances. A realistic distance from the application s ite to where no effects are obser ved ranges from 100-175 ft. Based on the weight of evidence fo r the coarser droplet spectra , this distance is 125 ft. §

Again, it is important to note th at product and nozzle specific drift curves based on empirical data that are incorporated into specific label us e directions would better inform the spray drift risk assessment and would likely result in smalle r 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 cont ribution that va por phase drift would impact the terrestrial risk assessment. Th e 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 th e 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 regardi ng the amount of dicamba that vol atilizes from a field treated

with the dicamba DGA salt formulation. Uncerta inties associated with converting air concentrations to a plant deposit ion would be greatly reduced by the submission of a terrestrial plant vapor phase toxicity end poi nt that measures air exposure concentrations. Without these data the Agency used a number of tools to estimate exposure and conve rt 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 predic t flux based on the work by Woodrow et al. (1997) and a model developed by the Office of Solid waste and Emergency Respons e 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 emissi ons 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 th e Woodrow papers to estimate the 90 th percentile upper confidence limit around the slope an d 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 applie d the air concentration to calculate an approximation of deposition in order to estimate wh at the air concentration would need to be at the edge of the treated field in order to cause an effect (see discus sion below). The equation

below is a modification of the Woodrow et al. (1997) equation based on a se ries of field trials for volatile and semi-volatile compound s used to estimate the flux from a field for foliar applied compounds.

Modified Woodrow equation for Plants:

݂݈ݔݑ ൌ ݁ ሺ଴.଼ଶ଺଼ ୪୬ ሺ ௏௉ ሻ ାଵଶ.଴଼ଵ ൊ 3600

Where:

** http://www.epa.gov/ttn/scram/dispersion_prefrec.htmPage 10 of 13 VP = vapor pressure in Pa flux = mass of vapor emitted from the field per unit area per second (μg/m 2 •s -1 )

flux = 0.566 μg/m 2 •s -1

In order to estimate the amount of mass potentially drifted as vapor from th e field off-site, EFED adapted the Office of Solid wa ste and Emergency Response USEP A (2005) tool for estimating vapor phase exposure to non-target plants to develop an aerial va por transfer concentration from the air concentration; termed Aboveground Produc t Concentration Due to Air-to-Plant Transfer (equation 5-18 of USEPA, 2005). Conceptually the pl ant concentration (depos ition) is calculated from the aboveground produce concentration do to air- to-plant transfer (va por transfer). The airto-plant transfer was developed to determin e 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 (A ERSCREEN) to determine the air concentration at the edge of

field.

∙ ݒܨ ∙ ܳ ൌ ݒܲ ܽ݃ܩܸ ∙ agݒܤ ∙ ݒݕܥ

ܲܽ

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) ‡‡

F

v = Fraction of COPC air concentr ation in vapor phase (unitle ss) (conservatively assume 1 meaning 100% of compound in vapor phase) Cyv = Unitized yearly average air concentration from vapor phase (μg-s/g-m 3 )

Bv ag = COPC air-to-plant biot ransfer factor (unitless) §§

VG ag = Empirical corrector fact or for aboveground produce (a ssumed 1 for compounds with Log kow <4)

ρ a = Density of air (g/m 3 ) = 1,200 g/m 3

The air concentration determined using AERSCREEN (C yv ) was used as an input to the OSWER model. The resulting deposition ( Pv ) was compared to the soybean EC 25 , converted to air-toplant DW concentrations assuming one ton of gr ass (dry weight) per acr e using equations A-2-19 and A-2-20 (USEPA, 2005, appendix A). Based on th is calculation the Agency concludes that the resulting deposition at the edge of field from volatile drift of dicamba is less than the EC 25 . In

order to exceed the EC 25 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/m 3 . 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/m 2 •s -1 using the modified Woodrow equation (see above) the maximum 1-hour average

†† For the purposes of a screening model, EC 25 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 fo r an 80 acre field.

§§ Based on correlation of Log kow and HLC (A-2-20 of USEPA, 2005).

*** Assumptions of flux, application rate,...etcPage 11 of 13 concentration from AERSCREEN at th e edge of the field is 283 ug/m 3 less than the air concentration required to exceed the EC 25 (as predicted by the OSWER tool) but within the same order of magnitude

.

Similarly, the volatile flux data (Theoreti cal Profile Shape (TPS) method) submitted by Monsanto in March, 2013 (MRID 49022501) provid ed a 6 hour average flux rate of 0.0004 μg/m 2 •s -1 , greater than 4 orders of magnitude lower than the Woodrow et al estimated flux rate. The air concentration and resulting deposition ra te 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 bo und 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 depositi on algorithm. Similar to the AERSCREEN Tier I analysis above, the air co ncentration at the edge of th e field is approximately 283 ug/m 3
however using the AERMOD deposition algorithm th e distance off field where no effect would be observed would be 1500m ( i.e., the distance where the deposition value is less than the EC 25 ). 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 EC 25 at the edge of the

field.

Uncertainties associated with c onverting air concentrations to a plant deposition would be greatly reduced by the submission of a terrestrial plant va por phase toxicity end poi nt 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 runo ff. However, there are uncertainties associated with the analysis that would be tter clarify the potential for va por phase exposure to dicamba.Page 12 of 13 References:

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49:3841-3846Page13of13Appendix A. Tabulated Estimates byMethod for Distances to No-EffectStudy/AnalysesMethod#SwathsDSDDistance(ft)0.5lbae/A(non‐listed)Distance(ft)0.5lbae/A(listed)SourceCommentsSDTF(Coarse)90th%4C210475EPAStandardEFEDapproachfordriftmodelingoutsideofdefaultassumptions(4‐20swaths,90th%‐tiledistribution).SDTF(Coarse)50th%4C125250EPASDTF(Coarse)90th%1C85175EPASDTF(Coarse)50th%1C‐‐ ‐‐EPAField+AgDRIFT50th%4VC/UC100100MonsantoMonsantoproposedapproachinMRID49022404Field+AgDRIFT90th%4VC/UC140140MonsantoPlantheight(NOAEC)‐‐  ‐‐VC/UC9090MonsantoMonsantoanalysisincludedinMRID48876001AgDrift+PMRAEADmodel‐‐  ‐‐VC/UC175175Monsanto‐‐SDTF(Malathiononly)VC‐‐  ‐‐VC/UC8181MonsantoNote:theonlysubsetofdatathatcontainedvc/coarseDSD.Log‐Logtransformation.Field(Clarity)‐‐3(120ft)UC6262BASF/MonsantoFieldstudies(providedinpowerpointpresentation–Dec.17,2013)Field(Engenia)‐‐3(120ft)XC106106BASF/MonsantoFieldstudies(providedinpowerpointpresentation–Dec.17,2013)CLAdataBasedonfitcurvesandln‐lntransformedsupportingdata1VC/Lowboom8787EPA50th%‐tile1VC/Lowboom9292EPA90th%‐tile*4VC/Lowboom230230EPA90th%‐tile*1C‐VC/HighBoom116116EPA90th%‐tile*4C‐VC/HighBoom210210EPA90th%‐tile*20C‐VC/HighBoom375375EPA90th%‐tileAve.(all)142.75172.75AllDSDAve.(coarse)124.08124.08DSD>VCAve.(xc)107.71107.71DSD>XC*Note:no difference in ln-ln vs curve fit estimates.