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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

 PC Code: 061601 

 DP Barcode: 399985 

MEMORANDUM Date: April 18, 2012

Subject: Response to comments from Syngenta on paraquat dichloride preliminary 

problem formulation 

To: Molly Clayton, Risk Manager 

Pesticide Re-Evaluation Division 

Office of Pesticide Programs 

From: Kristina Garber, Senior Biologist 

 James Lin, Environmental Engineer 

 Environmental Risk Branch 2 

Environmental Fate and Effects Division 

 Office of Pesticide Programs 

Reviewed by: R. David Jones, Ph.D., Senior Agronomist 

 Donna Judkins, Ph.D., Biologist 

 Environmental Risk Branch 2 

Through: Brian Anderson, Chief 

 Environmental Risk Branch 2 

 Environmental Fate and Effects Division 

 Office of Pesticide Programs 

On February 21, 2012, Syngenta Crop Protection, LLC provided comments on the preliminary 

problem formulation for paraquat dichloride. Those comments are provided below. EFED has 

reviewed those comments and has prepared responses for your consideration. The structure of 

this memo is as follows: first, EPA’s original statement, as provided by Syngenta; second, 

Syngenta’s comment; third, EFED’s response to Syngenta’s comment. 

1) EPA statement (as taken from Syngenta’s comment document): A non-guideline – Drinking 

water treatability study is included as a data gap. EFED notes that submission of a drinking 

water treatability study (i.e., Jar Test) would be of high value to drinking water assessments. 

With paraquat’s high adsorptive properties, traditional water treatment processes are 

expected to remove almost all paraquat in the water. EFED is requesting the treatability study 

from the registrant to confirm this property. 

Syngenta’s comment (p. 4-5): “The required study has recently (November 2011) been 

submitted (Moore, 2006). This two-part study was conducted to evaluate the effects of 
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traditional water treatment processes (including flocculation and sedimentation) on removing 

paraquat residues from water and to also determine the mobility potential of paraquat through 

soil filtration columns. The results from this test demonstrate that paraquat was effectively 

removed by typical water treatment processes. The results of the soil column filtration tests 

demonstrated that paraquat applied to agricultural fields, even under a worst case scenario, is 

unlikely to leach into the groundwater due to the intrinsic property of paraquat to adhere/bind 

to soil particles. Instead of leaching, paraquat remains bound tightly to soil particles in the 

top surface layer even under saturated and flooding conditions.” 

EFED’s response: Based on the review of the submitted study, entitled “Review of Jar Test 

Results for Drinking Water Assessment Purpose (D396402)”, EFED has concluded that 

paraquat in water was effectively removed by a combination of typical water treatment 

processes conducted on a laboratory-scale: the “laboratory jar test” (coagulation using alum 

with either lime or soda ash, flocculation and sedimentation), followed by duel media 

filtration (anthracite atop of filtering sand). The combination process was able to reduce the 

level of 14C-paraquat to approximate or below the limit of detection of about 0.15μg/L (ppb). 

The limit of detection of 0.15 μg/L (ppb) will be used for drinking water exposure 

assessment. 

2) EPA statement (as taken from Syngenta’ comment document): The Agency invites 

submission of water quality data for this pesticide. (0002/10). Any available monitoring data 

from the registrants and public organizations such as USEPA, USGS, Department of 

Pesticide Regulation (CDPR), and others will be considered in this review process. (0003/16) 

The drinking water assessment will also include available surface and ground water 

monitoring data with consideration of changes in use patterns that may have occurred. States 

are encouraged to submit monitoring data for review. (0003/31) The drinking water 

assessment will also include available surface and ground water monitoring data with 

consideration of changes in use patterns that may have occurred. (0004/5,6) 

Syngenta comment (p. 5-6): “Two water monitoring studies have recently (November 2011) 

been submitted by Syngenta demonstrating that there is minimal potential for the occurrence 

of detectable paraquat residues in drinking water and that if there were to be any such 

detection in raw water, typical water treatment processes effectively remove any such 

paraquat residues. The first of these studies (Peters, 2007a) monitored for paraquat residues 

in rural communities biases to include areas of high paraquat usage. A total of 1,092 water 

samples were collected in this study and analyzed for residues of paraquat. There were no 

quantifiable detections of paraquat from any of the surface water sources evaluated based on 

the analytical method’s LOQ of 0.10 ppb. 

“The second monitoring study (Peters, 2007b) was conducted in urban environments. The 

sites monitored in this study were biased to areas with potential paraquat exposure in surface 

water systems. Over the course of the 2 year monitoring period, water samples were collected 

at 21 scheduled intervals annually. A total of 1,056 raw water samples using immunoassay 

methodology were analyzed for the presence of paraquat; and with the exception of one raw 

water sample (with a residue of 0.252 ppb), there were no quantifiable residues at or above 

the LOQ (0.10 ppb). All samples corresponding to this single detection event including 
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duplicate replicate samples of both the raw and finished water were subsequently confirmed 

by LC/MS/MS. The average concentrations determined for the raw and finished water 

replicate samples were 0.373 ppb and <LOQ, respectively. This single raw water sample 

detection therefore did not result in any detectable residue in the corresponding finished 

drinking water sample. 

“These two studies clearly demonstrate that use of paraquat in the monitored communities 

did not result in the presence of detectable paraquat residues in drinking water. Syngenta 

requests that results from these monitoring studies be utilized in the drinking water 

assessments.” 

EFED’s response: EFED will consider these studies as part of the human health risk 

assessments for paraquat. Although EFED has not completed the review of the mentioned 

monitoring studies, as suggested in Question #1, the jar test results have demonstrated the 

removal of paraquat in drinking water due to its strong binding capacity with the solids, On 

this basis, EFED can assess the drinking water from surface water sources with confidence. 

3) EPA statement (as taken from Syngenta’ comment document): A list of the “non-agricultural 

use sites” included lawns as a potential use site. 

Syngenta comment (p. 7): “Paraquat is not approved for use in residential areas including 

lawns. In addition, Syngenta’s paraquat end-use product labels further minimize any 

potential for exposure to residential areas such as lawns by requiring that for 

 ‘Sensitive Areas – The pesticide must only be applied when the potential for drift to 

adjacent sensitive areas (e.g. residential areas, bodies of water, known habitat for 

threatened or endangered species, non-target crops) is minimal (e.g., when wind is 

blowing away from the sensitive areas).’ 

“In light of the prohibitions against use in residential areas, Syngenta respectfully requests 

that lawns be removed from the list of Non-Agricultural Use Sites on page 2 of docket item 

EPA-HQ-OPP-2011-0855-0006.” 

EFED’s response: The word “lawn” does not occur in the EFED’s Problem Formulation 

document. If the list of non-agricultural use sites includes “lawns” on page 2 of docket item 

EPA-HQ-OPP-2011-0855-006 - BEAD Chemical Profile for Registration Review: Paraquat 

Dichloride (Chemical Code 061610), then the use should be removed. 

4) EPA statement (as taken from Syngenta’ comment document): Based on an evaluation of 

available groundwater monitoring studies, the Environmental Fate and Effects Division 

(EFED) recommended use of monitoring data reported in Virginia (1.52 μg/L) for the 

drinking water exposure assessment, as a high-end, but not worst-case value for the acute and 

chronic analyses. The groundwater monitoring value was incorporated directly into the 

dietary assessments. 

Syngenta comment (p. 7-8): EFED has recommended use of monitoring data reported in 

Virginia (1.52 μg/L) for the drinking water exposure assessment, as a high-end, but not 
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worst-case value for the acute and chronic analyses. Syngenta, as described above, has 

provided new monitoring data (Peters 2007a and 2007b). Syngenta is also submitting a 

review of the Virginia monitoring data (Peters, 2006). As has been well documented 

(Roberts, 2002) paraquat is strongly adsorbed and considered immobile in soils making it 

unlikely that paraquat would be detected in surface water and even more unlikely to be 

detected in groundwater. These physiochemical properties that make paraquat unlikely to be 

found in drinking water supplies has been previously cited by the Agency (EPA, 2006) in the 

Paraquat Dichloride: Human Health Risk Assessment for Proposed Uses on Ginger and Okra 

and Amended Uses on Soybeans, Wheat, Cotton, Cucurbits, Onions, and Tanier which states: 

‘Because of its very low mobility and strong tendency to bind tightly to soils, paraquat 

contamination of drinking water supplies derived from groundwater is expected to be 

highly unlikely. In addition, the strong binding characteristics of paraquat are likely to 

render most residues in raw drinking water sources removable through sedimentation 

processes, which are typically included as part of standard drinking water treatments.’” 

EFED’s response: After further review of the original reference used for drinking water 

assessment, EFED has determined that the ground water value of 1.52 μg/L is invalid; 

therefore, the drinking water assessment will be based on the accepted jar test results as 

discussed in the recent review of the jar test (D396402; January, 2012). 

5) EPA statement (as taken from Syngenta’ comment document): EFED recommends that PRD 

require acute and chronic toxicity data for one species of sediment-dwelling invertebrates for 

paraquat. Paraquat dichloride registrants must submit a study protocol for review by the 

Agency prior to initiation of the chronic toxicity study with sediment dwelling invertebrates. 

Syngenta Comment (p. 13): “Syngenta concurs with the fact that the half-life of paraquat in 

soil under aerobic conditions is >10 days. It is well established that paraquat binds very 

strongly to soil and sediment and is only released after reflux under acidic conditions 

(Roberts 2002). Paraquat bound to sediment would thus not be bioavailable for sedimentdwelling invertebrates as previously stated in the Paraquat RED: ‘Due to paraquat 

dichloride’s strong adsorption to soil which makes it biologically unavailable, chronic 

testing on estuarine and marine organisms is not required.’ (EPA 1997 pg 130). 

“The reduced bioavailability and toxicity of paraquat in the presence of sediments has been 

demonstrated in a number of lab studies, both with plants in the water column and with 

benthic invertebrates. In one example with the alga Navicula pelliculosa, the most sensitive 

algal species discussed by EPA in the preliminary problem formulation, alga cells were 

inoculated into test medium containing paraquat at concentrations ranging from 0.9-900 μg 

paraquat technical/L equivalent to 0.29-290 μg paraquat ion/L (Smyth and Shillabeer, 2000). 

Subsamples of this inoculated medium were then added to test vessels containing 3 g of a 

sandy loam sediment. The Navicula were thus exposed to the concentrations of paraquat 

before being added to the test vessel containing the sediment. The alga was cultured under 

standard test conditions for 120 hours and the effects on algal growth determined every 24 

hours during this period. After 96 hours, growth effectively stopped, likely due to the algal 

growth medium being exhausted. There were not significant effects on algal growth at any 
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concentration and so the EC50 of paraquat to Navicula pelliculosa in the presence of sediment 

was >290 μg paraquat ion/L, and the NOEC was 290 μg/L. This is approximately 725x 

greater than the 960h EC50 of 0.4 μg paraquat in the absence of sediment (Smyth et al., 

1992). 

“A study was conducted with the aquatic midge Chironomus riparius in which larvae were 

exposed to radiolabeled (14C) paraquat in sediment-water systems using a formulated 

sediment (Hamer, 1998). Test systems were prepared by applying paraquat to a nominal 

concentration of 100 mg paraquat ion/kg sediment (dry weight). First-instar C. riparius were 

introduced to the test systems and observations made for emerging adults. Results indicated 

no difference in the pattern of emergence between control and treated systems. Total 

emergence was 97 and 99% for the control and 100 ppm paraquat ion/kg sediments, 

respectively, with mean emergence times of 14.1 and 14.0 days. 

“The physicochemical properties of paraquat that result in very tight binding of paraquat to 

sediments render paraquat biologically unavailable to sediment-dwelling organisms and, as 

such, a chronic toxicity study with sediment dwelling invertebrates is of limited benefit.” 

EFED’s response: The sediment toxicity studies referenced by Syngenta have not been 

submitted to EPA. EFED recommends that PRD request that the registrant submit the studies 

involving exposures of the algae N. pelliculosa (Smyth and Shillabeer, 2000) and the midge 

C. riparius (Hamer 1998) so that they can be reviewed. Upon completion of these reviews, 

EFED can consider whether additional toxicity data for sediment-dwelling invertebrates is 

necessary for paraquat. The citations given by Syngenta for these studies are as follows: 

Hamer, MJ (1998) Sediment toxicity test with Chironomus riparius. Report RJ2649B. 

(Unpublished study from Zeneca Agrochemicals). 

Smyth, D, Shillabeer N. (2000) Paraquat Dichloride: Toxicity to the freshwater diatom 

Navicular pelliculosa in the presence of sediment: Lab Project Number: BL6800/B 

6) EPA statement (as taken from Syngenta’ comment document): EFED recommends that PRD 

require acute and chronic toxicity data for one species of sediment-dwelling invertebrates for 

paraquat. Paraquat dichloride registrants must submit a study protocol for review by the 

Agency prior to initiation of the chronic toxicity study with sediment dwelling invertebrates. 

Syngenta Comment (p. 13): “The EPA aquatic risk assessment does not take into account the 

effect of adsorption in reducing the bioavailability, and thus the potential toxicity of 

paraquat. This extreme adsorption property of paraquat is a critical factor that should be 

considered in the environmental exposure assessments.” 

EFED’s response: When assessing risks of a pesticide in sediment to benthic invertebrates, 

EFED uses PRZM/EXAMS to estimate concentrations of the pesticide in pore water. This 

approach uses the soil water partitioning coefficient (Kd) to determine the pesticide 

concentration in pore water. As a cation, paraquat binds electrostatically to the negatively 
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charged surfaces of clay particles in soil. PRZM only considers organic adsorption, this type 

of cation binding is not considered. 

7) EPA statement (as taken from Syngenta’ comment document): EPA anticipates requiring the 

following data for use in conducting an ecological risk assessment for the registration review 

of paraquat dichloride: 835:4400 – anaerobic aquatic metabolism 

Syngenta Comment (p. 14): “Syngenta suggests that conducting an anaerobic aquatic 

metabolism study is unlikely to provide additional information that would be useful in an 

ecological risk assessment for the following reasons: 

- Paraquat is known to be stable to degradation in anaerobic soils (Vickers, J.; Hurt, A.; 

Bewick, D. (1989) 

- Paraquat is hydrolytically stable from pH 5 – 9 at 40o

C (Upton, B.; Hendly, P.; Skidmore, 

M., 1988) 

- Paraquat does not degrade photolytically in aqueous solutions (EPA 1997; Parker, S.; 

Leahey, J. 1988) 

“In addition, one study (Coats, et al. 1964) conducted on a simulated pond system (soil:water 

in a plastic pool) showed that paraquat was removed from the water column with a ‘resident 

half-life of <2 weeks’ which suggests that paraquat would be readily removed from the water 

of anaerobic sediment system by rapidly binding to the sediment. For these reasons, it is 

unlikely that new degradate(s) would be formed in any significant amount or that additional 

information would be gained from what is already known about paraquat by conducting an 

anaerobic aquatic metabolism study.” 

EFED’s response: The first comment that by combining the study results of anaerobic soils, 

hydrolysis and aquatic photolysis will yield the results for anaerobic aquatic metabolism is 

not correct. Otherwise, the guideline requirement will not be needed if the three mentioned 

studies have been fulfilled. 

As for the second comment that paraquat will be removed from the water into the sediment 

and no need to investigate further, EFED is concerned about any possible degradates formed 

in the sediment layer, the study referenced above has not addressed this adequately. 

Therefore, the anaerobic aquatic metabolism study is needed. 

8) Syngenta Comment (p. 15): “Syngenta is preparing scientific comments to the CRLF 

[California Red-Legged Frog] assessment which will be submitted to EPA shortly.” 

EFED’s response: The EFED effects determination for exposures of the CRLF to paraquat 

was completed in June 2009. Syngenta is welcome to submit comments on the assessment. 

9) EPA statement (as taken from Syngenta’ comment document): Based on use patterns of 

paraquat dichloride, its fate and transport potential, and the sensitivity of non-target aquatic 

and terrestrial species, paraquat has the potential to reduce survival, reproduction, and/or 

growth in non-target terrestrial and aquatic organisms when used in accordance with labels. 
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These non-target organisms include Federally-listed threatened and endangered species as 

well as non-listed species. 

Syngenta Comment (p. 15): “The transport potential for paraquat is limited due to the 

extremely tight binding of the paraquat cation to soil and other particulates. Bound particles, 

even if available for offsite movement with suspended soil particles, are not biologically 

available based upon the nearly irreversible binding of paraquat to soil/particulate matrices. 

During more than 40 years of use, in over 100 countries covering many and varied 

agronomic practices, there have been no reports of the reactivation of adsorbed paraquat 

residues due to desorption (Roberts, 2002). Paraquat is 99.99% soil-bound, does not come off 

the soil unless refluxed in hot acid and is therefore biologically unavailable (Roberts, 2002). 

Any residues of paraquat in soil that may be present from prior use will be very strongly 

bound and not be bioavailable and thus do not have the potential to reduce survival, 

reproduction, and/or growth in non-target terrestrial and aquatic organisms.” 

EFED’s response: EFED accepts the point that once paraquat is bounded to soil, it will not be 

bioavailable through contact exposure. However, organisms that consume soil may be 

exposed. 

10) EPA statement (as taken from Syngenta’ comment document): The conceptual model 

presented in figure 1 indicates further exposure once paraquat reaches soil. 

Syngenta Comment (p. 16): “The post-soil exposure component in the model should be 

modified to reflect that upon contact with soil, paraquat residues are no longer bioavailable. 

The transport and exposure potential for paraquat is extremely limited due to the extremely 

tight binding of the paraquat cation to soil and other particulates. The soil bound particles, 

even if available for offsite movement, are not biologically available based upon the nearly 

irreversible binding of paraquat to soil/particulate matrices. The conceptual model for 

paraquat (Figure 1) should be modified for soil exposure in a similar manner as Figure 2 

which indicates through the dotted lines that represent transport pathways that are assumed to 

have a low likelihood of occurring and/or of contributing to ecological risk.” 

EFED’s response: EFED concurs with Syngenta’s comments that paraquat does bind tightly 

with the soil particles, and therefore is not generally bioavailable via contact exposure. 

However, organisms that consume soil and sediment may be exposed. The soil exposure 

pathway in the conceptual model will be modified. 

11) EPA statement (as taken from Syngenta’ comment document): Paraquat dichloride is 

generally applied as a flowable solution. As a result, paraquat dichloride is already 

dissociated into its cation, paraquat. Therefore, assessments involving paraquat dichloride 

will focus on the paraquat cation as the stressor of concern. All EECs and toxicity values will 

be reported in terms of the paraquat cation. 

Syngenta Comment (p. 17): “Paraquat dichloride (technical) is registered in the form of a 

concentrate (Paraquat Concentrate ES, EPA Reg. No. 100-1067). Paraquat Concentrate ES 
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contains 45.6% (w/w) paraquat dichloride equivalent to 33.0% (w/w) paraquat cation. This 

product is considered the “technical grade active ingredient” (TGAI) and should be 

considered as the test substance for testing where the TGAI is required. For risk assessment 

purposes, Syngenta agrees that stressor of concern should be reported in terms of the 

paraquat cation.” 

EFED’s response: EFED appreciates the clarification of the nature of TGAI paraquat. 

12) EPA statement (as taken from Syngenta’ comment document): EPA lists a number of 

aquatic ecotoxicology data gaps in Table 11, on page 37 of the EFED Preliminary Problem 

Formulation. 

Syngenta Comment (p. 17): “850:1400 Saltwater and freshwater early life-stage fish tests are 

identified as data gaps. 

“Chronic aquatic exposure to paraquat is very unlikely due to the uses and physiochemical 

characteristics of this herbicide. In addition, the acute aquatic toxicity to animals is generally 

slight to moderate. Syngenta suggests that a single freshwater ELS study could be used with 

the acute fish data that will be available to estimate a saltwater NOEC using the EPA’s Acute 

to chronic Ratio (ACR) method.” 

EFED’s response: If the registrant submits acceptable acute (guideline 850.1075) and chronic 

(guideline 850.1400, early life stage test) studies with the same freshwater fish species 

exposed to TGAI paraquat, EFED will consider using an ACR for that freshwater fish test 

species to estimate a chronic endpoint for a saltwater fish species. 

13) EPA statement (as taken from Syngenta’ comment document): EPA lists a number of 

aquatic ecotoxicology data gaps in Table 11, on page 37 of the EFED Preliminary Problem 

Formulation. 

Syngenta Comment (p. 17): “850.1735 and non-guideline freshwater invertebrate acute and 

chronic sediment toxicity tests. 

“The sorption of paraquat to sediments is nearly irreversible, and the herbicide is not 

bioavailable in sediments, as discussed above. A chronic Chironomus riparius study (Hamer, 

1998) was conducted at 100 mg paraquat ion/L, and demonstrated no effects on survival or 

emergence. Syngenta suggests that additional sediment toxicity testing will not provide 

additional information for risk assessment. If sediment testing is required, then Syngenta 

suggests that both acute and chronic studies are not necessary.” 

EFED’s response: As noted above, the sediment toxicity study referenced by Syngenta has 

not been submitted to EPA. EFED recommends that PRD request that the registrant submit 

the study involving the midge C. riparius (Hamer 1998) so that it can be reviewed. Upon 

completion of these reviews, EFED can consider whether additional toxicity data for 

sediment-dwelling invertebrates is necessary for paraquat. 
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14) EPA statement (as taken from Syngenta’ comment document): Table 4 presents the 

Environmental Fate Parameters of Paraquat including the parameter, value cited and 

reference for the cited value. The following Syngenta comments refer to table 4. 

Syngenta Comment (p. 18): “Table 4 presents the log octanol-water partition coefficient (log 

Kow) as 2.44 citing Platford (1983). The octanol/water partition coeffieint for paraquat 

dichloride in the Platford publication is reported as -4.22±0.05. The -4.2 value is consistent 

with other published values for the octanol-water coefficient for paraquat including in the -

4.5 value cited in the RED (EPA, 1997 pg 91) and the FAO Specifications for paraquat 

(FAO, 2003). Please verify the log octanol-water partition coefficient for paraquat.” 

EFED’s response: EFED will revise the value of log Kow as needed. 

15) EPA statement (as taken from Syngenta’ comment document): Table 4 presents the 

Environmental Fate Parameters of Paraquat including the parameter, value cited and 

reference for the cited value. The following Syngenta comments refer to table 4. 

Syngenta Comment (p. 18): “The hydrolysis study by Upton, et al does not have an MRID 

number. However, inclusion of the study in the reference section (section 12) should be 

considered. 

“Upton, B.; Hendly, P.; Skidmore, M. (1985) Paraquat: Hydrolytic Stability in Water at 

pH 5, 7 and 9, Laboratory Project RJ0436B. Unpublished study prepared by ICI Plant 

Protection Division, 20 p.” 

EFED’s response: The mentioned study will be considered for hydrolysis study. 

16) EPA statement (as taken from Syngenta’ comment document): Table 4 presents the 

Environmental Fate Parameters of Paraquat including the parameter, value cited and 

reference for the cited value. The following Syngenta comments refer to table 4. 

Syngenta Comment (p. 18): “In the row presenting the aqueous photolysis half-life, the 

review by Jordan, 2/14/1986 is not included in the references. Inclusion of the review in the 

reference section (section 12) should be considered. 

EFED’s response: EFED will consider the above mentioned reference. 

17) EPA statement (as taken from Syngenta’ comment document): Table 4 presents the 

Environmental Fate Parameters of Paraquat including the parameter, value cited and 

reference for the cited value. The following Syngenta comments refer to table 4. 

Syngenta Comment (p. 18): In the row presenting the adsorption/desorption study; the 

conclusions are referenced by MRID 40762701 however this report is not included in the 

reference section (Section 12). Please add the reference as: 
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MRID 40762701 Robbins, A., Lane, M., Riley, D. (1988) Paraquat: Adsorption and 

Desorption Equilibrium in Soils, Laboratory Project RJ0662B. Unpublished study prepared 

by ICI Plant Protection Division, 15 p.” 

EFED’s response: EFED will consider the above mentioned reference. 

18) EPA statement (as taken from Syngenta’ comment document): Paraquat did not 

photodegrade when mixed with a sterile soil solution and exposed to natural sunlight for 85 

weeks (Pack, 1982). 

Syngenta Comment (p. 19): “The sampling periods for the Pack (1982) study were 0, 6, 12, 

18 and 24 months as cited in Table 3 of the study. A sampling period of 85 weeks was 

mentioned in the summary of the Pack study but that was citing a sampling period of an 

earlier study (Day, S.; Hemingway, R., 1981). Therefore, Syngenta suggests changing the 

sentence to read ‘paraquat did not photodegrade when mixed with a sterile soil solution and 

exposed to natural sunlight for 2 years (Pack, 1982).’ ” 

EFED’s response: EFED will revise this sentence if after re-evaluation of the mentioned 

study confirms the results. 

19) EPA statement (as taken from Syngenta’ comment document): The LD50 for northern 

bobwhite (Colinus virginianus) is listed as 124 mg cation/kg-bw. 

Syngenta Comment (p. 19): “In the avian acute oral toxicity study, the LD50 for northern 

bobwhite (Colinus virginianus) is listed as 124 mg cation/kg-bw (Fink, R.; Beavers, J.B.; 

Grimes, J.; et al. (1979). According to the DER (Hopkins, 1980), the AI-corrected LD50 is 

164 mg paraquat dichloride/kg-bw. This is equivalent to 127 mg cation/kg-bw rather than 

124 mg cation/kg-bw as reported in Table 6. Syngenta requests that the LD50 for northern 

bobwhite (Colinus virginianus) be verified.” 

EFED’s response: EFED recently re-reviewed this study and verified the LD50 as 124 mg 

cation/kg-bw. An addendum to the original DER for this study (MRID 29001) was included 

in a memo to PRD that was sent on December 9, 2011 (DP barcode 392076). 

20) EPA statement (as taken from Syngenta’ comment document): Table 6 [of the Problem 

Formulation] cites a rat acute oral toxicity study (Rittenhouse, MRID 00054573) with an 

LD50 of 91 mg cation/kg bw. 

Syngenta Comment (p. 20): “A more recent acute rat oral toxicity study by L. Duerden 

(MRID 43685001) was submitted, and a DER was prepared by EPA-HED (Hurley, 1996). 

The LD50 for female Wistar rats was calculated to be 283 mg/kg equivalent to 206 mg/kg 

cation. According to the DER, this study was classified as acceptable and satisfied the 

guidline (81-1) requirement. The study also cited as the relevant endpoints for acute oral 

toxicity in the Paraquat RED for the Acute oral LD50 (rat) as well as in the April 19, 2000 
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report of the Hazard Identification Assessment Review Committee. (EPA, 2000) This study 

should be the basis of the acute oral LD50 for small mammals.” 

EFED’s response: EFED will consider both acute oral toxicity studies when selecting the 

appropriate endpoint for mammals. EFED will use the most sensitive endpoint from the 

available scientifically valid studies. 

21) EPA statement (as taken from Syngenta’ comment document): Table 6 [of the Problem 

Formulation] cites and acute inhalation rat study (MRID 00046105) by Gage (1968). The 

reported LC50 from this study is 0.001 mg/L. 

Syngenta Comment (p. 20): “A more recent acute inhalation rat study (Rattray, 2005) is 

available and will be submitted under a separate cover for the Agency’s review. This study 

was conducted according to OECD guideline reference 403 (1981) and United States 

Environmental Protection Agency, Health Effects Test Guidelines, OPPTS 870.1300 (1998) 

for Acute Inhalation Toxicity. The results of this study were that nose-only exposure for 4 

hours to paraquat ion at concentrations of 0.36, 1.58 or 2.49 μg/L resulted in 0, 1, and 5 

deaths respectively. It is concluded that the median lethal concentration of paraquat ion was 

1.79 μg/L for females only (equivalent to 2.47 μg paraquat dichloride/L or 5.36 μg paraquat 

dichloride technical material). In this study since males were only exposed to the lowest 

atmospheric concentration (0.3 μg paraquat ion/L) to demonstrate equivalence with the 

females, the MLC for males was not calculated. Syngenta recommends the data from this 

study be considered in determining the most relevant acute inhalation endpoints for 

paraquat.” 

EFED’s response: If the registrant submits the more recent acute inhalation study with the rat 

(Rattray, 2005), these data can be reviewed by OPP and potentially considered for use in risk 

assessments of paraquat. 

22) EPA statement (as taken from Syngenta’ comment document): Table 6 [of the Problem 

Formulation] cites a honey bee (Apis millifera) acute contact and oral toxicity study (MRID 

43942603) by Bull and Wilkinson (1987). The reported contact LD50 for honey bees is given 

as 52 μg cation/bee. 

Syngenta Comment (p. 21): “The original study report and DER (Vaughan, 1996) reported 

the contact LD50 as 51 μg cation/bee. Syngenta requests that the correct value be verified.” 

EFED’s response: Syngenta is correct that the contact LD50 from MRID 43942603 is 51 μg 

cation/bee. 

23) EPA statement (as taken from Syngenta’ comment document): In Table 7 [of the Problem 

Formulation] the seedling emergence NOAEC and EC25 values for cocklebur (Xanthium sp) 

are given as 0.37 and 0.67 lb cation/A for the cited study (MRID 42639601). 

Syngenta Comment (p. 21): “Those values provided in Table 7 are not consistent with the 

endpoints reported in the DER (Mossler, 1993) for this study. The reviewer calculated the 
Page 12 of 13 

NOEL and EC25 as 0.59 and 1.17 lb ai/A, respectively. Syngenta respectfully requests that 

these values be verified.” 

EFED’s response: EFED reviewed the endpoints for the vegetative vigor study involving 

cocklebur and confirms that the NOAEC and EC25 values should be 0.59 and 1.17 lb a.i./A, 

respectively. 

24) EPA statement (as taken from Syngenta’ comment document): Table 7 [of the Problem 

Formulation] cites a vegetative vigor study (Canning, L. and J.S. White., 1992) with NOAEC 

and EC25 values of 0.064 and 0.16 lb cation/A, respectively, for corn (Zea mays). 

Syngenta Comment (p. 21): “These NOAEC and EC25 values are not consistent with the 

endpoints in the study report (Canning, L. and J.S. White. 1992). The reported NOEL for 

corn is 100 g ai/ha (0.0892 lb ai/A), and the EC25 is 274 g ai/ha (0.244 lb ai/A) based on dry 

weight. The NOAEC and EC25 for cocklebur (Xanthium sp) from the same study were 

calculated to be ~5 g ai/ha (0.0044 lb ai/A) and 19.6 g ai/ha (0.0174 lb ai/A), respectively, 

based on dry weight. The field rates (lb ai/A) were extrapolated from Tables 23-24 of the 

study report and are not consistent with the values reported in Table 7 (EPA-HQ-OPP-2011-

0855). Syngenta requests that the indicated values be verified.” 

EFED’s response: EFED recently re-reviewed this study (MRID 42601001) and verified the 

endpoints, which are provided in the table below. An addendum to the original DER for this 

study was included in a memo to PRD that was sent on December 9, 2011 (DP barcode 

392076). 

Updated endpoints – Dry weight [lbs cation/A] 

Sci. name Common name EC25 NOAEC* 

Xanthium strumarium Cocklebur 0.014 (95% CI 0.01-0.019) 0.0065 

Zea mays Corn 0.16 (95% CI 0.073-0.36) 0.064 

( ) 95% Confidence Interval 

* For Cockelbur an EC05 was calculated and used in place of a NOAEC 

25) EPA statement (as taken from Syngenta’ comment document): 1) The dietary exposure 

assessment is adequate based on current methodologies but may need to be revised to 

incorporate changes to toxicity endpoints and/or changes to the drinking water assessment. 

2) The drinking water exposure assessment needs to be revised based on current 

methodologies. 3) The occupational exposure assessment needs to be updated to reflect 

current policy. Use of chemical specific biomonitoring data should be reevaluated. 4) The 

aggregate risk assessment is adequate based on existing uses and methodologies but may 

need to be revised to incorporate changes to the dietary and/or drinking water assessment, 

Syngenta Comment (p. 22): “As the Agency intends to conduct updated exposure 

assessments for dietary, residential, aggregate and occupational exposure scenarios, Syngenta 

requests EPA to use the best available data to produce refined, realistic risk assessments such 

as the recently submitted water treatment data (Moore, 2006) and water monitoring data 

(Peters, 2007a and 2007b). 
Page 13 of 13 

EFED’s response: Based on the review of the submitted study, entitled “Review of Jar Test 

Results for Drinking Water Assessment Purpose (D396402), EFED has concluded that 

paraquat in water was effectively removed by a combination of typical water treatment 

processes conducted on a laboratory-scale: the “laboratory jar test” (coagulation using alum 

with either lime or soda ash, flocculation and sedimentation), followed by duel media 

filtration (anthracite atop of filtering sand). The combination process was able to reduce the 

level of 14C-paraquat to approximate or below the limit of detection of about 0.15μg/L (ppb). 

The limit of detection of 0.15 μg/L (ppb) will be used for drinking water exposure 

assessment.