Drinking Water Exposure Assessment for the Section 3 IR-4 Proposed New
Uses of 

S-Dimethenamid on Squash, Pumpkin, Rutabaga, Turnip, Radish and Hops 

 

						

						

S-Dimethenamid

(2-chloro-N-(1-methyl-2-methoxy)ethyl)-N-(2,4-dimethyl-thien-3-yl)
acetamide

PC Code 120051, CAS No. 163515-14-8

Prepared by:

	Paul Mastradone, Chemist

Registration Division Registration Support Branch

	U. S. Environmental Protection Agency

Office of Pesticide Programs

Registration Division 

Registration Support Branch 

And 

Environmental Fate and Effects Division

Environmental Risk Branch IV

1200 Pennsylvania Ave., NW

Mail Code 7507C

Washington, DC 20460

Reviewed by:

Keara Moore, Chemist

Stephanie Syslo, RAPL

Environmental Risk Branch III

Environmental Fate and Effects Division

	



I.	EXECUTIVE SUMMARY

IR-4 has submitted several new use requests for S-Dimethenamid and  a
request for an updated drinking water assessment was submitted to the
ARIA team.  The new uses included separate labels for ground application
for weed control in Winter Squash and pumpkin (both restricted to OR and
WA) , labels for turnip and rutabaga , radish (entire US) and for hops (
restricted to ID , OR ,and WA).  All uses appear to propose a single
application of S-Dimethenamid at 0.98lbs/acre.  

This document reports the Tier 2 drinking water exposure assessment for 
                      S-Dimethenamid conducted to support the human
health risk assessment for the proposed new IR-4 uses. For the purposes
of this assessment these uses were broken into three groups.  The first
group included the uses for squash and pumpkin in OR and WA.  The second
group is made up of the uses for turnip, rutabaga and radish. The final
group is solely made up of hops.  The previous EFED drinking water
assessment for                     S-Dimethenamid was conducted for
addition of uses on green onion, grass grown for seed ( at
0.98lbs.a.i./A) and ornamentals ( at 1.5 lbs. a.i./A).  This assessment
also included a complete quantitative environmental risk assessment as
well as a complete qualitative environmental fate assessment.

Table 1.  EDWAC’s (ppb) for proposed new IR-4 uses of S-Dimetheneamid

USE	Modeled Scenario)	PCA	Surface Water Acutea	Surface Water Chronica
Surface Water Cancer

Chronica

Squash and Pumpkin	CAonion	0.56	2.11	0.71	0.52

Radish, Rutabaga,  Turnip	FL Carrot	0.87	59.02	6.45	3.87

Hops	OR hops	0.62	7.93	2.61	1.22



Generated using Tier2 linked simulation modelsPRZM (3.12beta)/Exams
2.98.04)

II.	PROBLEM FORMULATION

This drinking water assessment uses modeling to estimate surface water
concentrations of pesticides in drinking water source water
(pre-treatment) resulting from pesticide use on sites that are highly
vunerable.  Surface water exposure is estimated using Tier 2 linked
models PRZM and EXAMS.  Teir 1 modeling is intended to screen out
chemicals with low potential risk and provides estimated exposure
concentrations for human health dietary risk assessment.  Tier 2
exposure values, although more refined than Tier 1, are still screening
values.

Monitoring data are not considered in this assessment.  Additionally,
this assessment only considers exposure from the proposed new uses of
S-Dimethenamid and does not account for exposure from previously
registered uses.

 

  SEQ CHAPTER \h \r 1 Table 2.  Summary use information for
S-Dimethenamid based submitted new IR-4 uses.

USE	SINGLE  APP. RATE             (lbs. a.i./A)	NUMBER OF APPS.	SEASONAL
APP. RATE (lbs. a.i./A)	APP. METHOD	INCORPORATION DEPTH (inches)

Squash and Pumpkin	0.98	1	0.98a	Ground spray	0

Radish  Rutabaga  Turnip	0.98	1	0.98	Ground spray	0

Hops	0.98	1	0.98	Ground Spray	0



a Rates calculated from presented labels do not fully agree with rates
presented in section B of the application.  Calculated maximum rate from
label was 0.84 lbs. a.i./A . Section B of the application states that
the maximum application rate is 0.98 lbs. a.i./A.  All calculations made
in this review were made using the higher rate

A.	Use Characterization

The active ingredient s-dimethenamid is the herbicidally active isomer
of r/s-dimethenamid.  It is a chloroacetamide herbicide (protein
synthesis inhibitor), used for control of certain annual grasses and
annual broadleaf weeds and sedges. A root and shoot growth inhibitor, it
is effective on susceptible germinating seedlings before or soon after
they emerge from the soil.  The racemic mixture of r/s-dimethenamid is
currently registered in other end use products (PC Code 129051).

Proposed new uses of s-dimethenamid are for control of weeds in Winter
Squash and pumpkins in OR and WA only, Turnip rudabega and radish
(entire US) and hops in ID< OR and WA..  The proposed end use products
is Outlook 97969-156).  It is a  liquid formulation consisting of 63.9%
active ingredient (6 lb a.i./gall).  

All uses allow for only a single application.  Maximum rate proposed is
0.98 lbs.A.I./ A.  However for two of the proposed uses a rate of 0.84
lbs. A.I./A  is the maximum calculated based on the proposed lavbel
instructions.  For all modeling the higher rate was used. 

B.	Environmental Fate and Transport Characterization

	1.	Fate and Transport Summary

General physical, chemical, and environmental fate properties of
dimethenamid are presented in Table 3.  This discussion of the
environmental fate of dimethenamid is based on laboratory studies of the
s-isomer when available.  Bridging studies show, however, that fate
properties of the racemic mixture are similar to those of the s-isomer
alone, and so data from studies of r,s-dimethenamid are included in this
discussion as well.  

Based on environmental fate properties measured in laboratory studies,
dimethenamid is moderately persistent in soil.  It has a high aqueous
solubility and limited partitioning to soil mineral or organic
fractions, leading to high mobility.  It therefore meets the criteria
for potential to leach to groundwater and it also may reach surface
water through runoff.  If the compound were to reach surface water, it
may persist for a significant period of time due to the fact that it is
stable to hydrolysis at pH 5, 7, and 9 and has limited photolysis.  The
major route of dissipation is through soil metabolism with an aerobic
soil half-life of 31 days (s-isomer) and an anaerobic soil half-life of
54 days (racemic mixture).  Terrestrial field studies found that
dimethenamid dissipated with half-lives of 4 to 43 days with no leaching
detected below 30 cm depth.

Volatilization from soil or water are not expected to be major
dissipation routes for dimethenamid.  Dimethenamid has low potential to
bioaccumulate.

Table 3.  Physical, chemical, and environmental fate properties of
s-dimethenamid.

PARAMETER	VALUE	SOURCE	COMMENT

Chemical Name	s-dimethenamid
(2-chloro-N-(1-methyl-2-methoxy)ethyl)-N-(2,4-dimethy-thien-3-yl)
acetamide

Molecular Weight	275	–	–

Aqueous Solubility (25 oC)	1,449 + 17 µg/mL	MRID:

 44332214	–

Vapor Pressure (25 oC)	1.88 x 10-5 mm Hg	MRID:

 44332215	–

Henry’s Law constant	8.52 x 10-8 atm-m3/mole	DP Barcode: D285445.
Measured on the R/S isomer mixture.  

pKa (25 oC)	No dissociation between pH 1 and pH 11	MRID:

41596510.	Measured on the R/S isomer mixture.  

Octanol-Water Partition Coefficient 

(KOW,  at 25 oC)	141 + 6	MRID:

41596511.	Measured on the R/S isomer mixture.  

Hydrolysis Half-life 

(pH 5, 7, 9; 25 oC)	Stable	MRID: 

44332258.	Stable at all pHs.

Aqueous Photolysis Half-life 

(pH 7)	50.4 days	MRID: 44332259.	Corrected to represent natural sunlight
in a 12 hour light/12 hour dark cycle.

Soil Photolysis Half-life	89.4 days	MRID: 44332260.	Corrected to
represent natural sunlight in a 12 hour light/12 hour dark cycle.

Aerobic Soil Metabolism Half-life	31 days (s-isomer)

30 days (r/s mixture)

38 days (r/s mixture)	MRIDs: 

44332261

41596532

	Anaerobic Soil

Metabolism Half-life	54 days	MRID: 41706801.	Measured on the R/S isomer
mixture.  

Anaerobic Aquatic Metabolism Half-life	36 days	MRID: 42367210.	Measured
on the R/S isomer mixture.  

Soil Partition Coefficient (Kd)	1.4 to 13.5 mL/g	MRID: 44332263.	Tested
on 5 American soils and 5 foreign soils.

Soil Partition Coefficient (Koc)	90 to 474 mL/goc	MRID: 44332263.	Tested
on 5 American soils and 5 foreign soils

Terrestrial Field Dissipation		8 days 	(Missouri)

	8 days 	(North Carolina)

	4 days 	(Indiana)

	43 days 	(Minnesota)	MRIDs: 42266202

42266203

42266204

42266205	Measured on the R/S isomer mixture.  Studies acceptable but not
representative of worst-case conditions.



Transformation Products

In laboratory studies, one major degradate, oxalamide (M23), was formed
through aerobic soil metabolism at up to 14.8% of the applied parent
compound.  The major degradates M3 (dechlorinated parent), M10 (methyl
sulfone derivative of M3), and M13 (methyl sulfoxide derivative of M3)
were formed through anaerobic aquatic metabolism at levels up to 20.6%,
9.8%, and 12.4% of the applied, respectively.  The structures and
complete chemical names of these compounds are provided in Appendix A.  

Multiple minor degradates (<10% of applied) are seen in laboratory
studies.  These include PL3688 (methylthio derivative of the
decholorinated parent); M9
[N-(2,4-dimethyl-3-thienyl)-5-methyl-3-morpholine]; M11
[N-(2,4-dimethyl-3-thienyl)-2-hydroxy-N-(2-methoxy-1-methylethyl)-acetam
ide]; M30 (STLA); M31 [STGA; thioglycolic acid sulfoxide]; M32 [TGA;
thioglycolic acid conjugate;
N-(2,4-dimethyl-3-thienyl)-N-(2-methoxy-1-methylethyl)-carboxymethylene
thionyl-acetamide]; and Fr4 (sulfoxide of the thioglycolic acid of
dimethenamid).  Structures and names of some of these minor degradation
products are shown in Appendix A.  It should be noted that although no
transformation products were included in the modeling scenarios used for
this assessment, based on the environmental fate profile it is unlikely
that even if the degradates were considered as parent, it is unlikely
there would significant effect on the EDWAC’s produced by this
modeling.

C.	Drinking Water Exposure Modeling

	1.	Models

Estimated drinking water concentrations (EDWCs) of s-dimethenamid from
the proposed new uses were generated using the Tier 1 model SCIGROW for
groundwater exposure and the Tier 2 linked screening simulation models
PRZM and EXAMS for surface water exposure.  Models and additional
information are available at the EPA’s water modeling website,
http://www.epa.gov/oppefed1/models/water/.

The Tier 2 models Pesticide Root Zone Model (PRZM v3.12beta, May 24,
2001) and Exposure Analysis Modeling System (EXAMS v2.98.04, Aug. 18,
2002) were coupled with the input shell pe4v01.pl (Aug.8, 2003) to
generate EDWCs of dimethenamid that may occur in surface water.  The
PRZM model simulates pesticide movement and transformation from crop
application through soil residue processes.  It has two major
components, hydrology and chemical transport, and considers factors such
as rainfall and plant transpiration of water, as well as how and when
the pesticide is applied.  The EXAMS model simulates the fate of
pesticides that reach an adjacent water body through runoff, erosion,
and spray drift.  It accounts for volatilization, sorption, hydrolysis,
biodegradation, and photolysis of the pesticide.  EXAMS is a
steady-state model and so the water bodies are modeled as having
constant volume.  The simulated watershed is based on an Index Reservoir
(IR) scenario, and a percent cropped area (PCA) adjustment factor is
used to adjust for the area within the watershed that is planted to the
modeled crop (OPP, 2000).  Crop-specific scenarios are generally
developed to represent environmental conditions at sites at roughly the
upper 90th percentile of runoff-vulnerability for pesticide transport to
surface waters from a particular crop.  Using appropriate scenarios
linked with local meteorological data, the models are run for 30 years
with the reported EDWCs representing the values that are expected once
every ten years, based on the thirty years of daily values generated
during the simulation.

	2.	Modeling Approach and Input Parameters

For all modeling, input parameters were selected based on EFED input
guidance (EFED, 2002).  Maximum application rates were taken from the
proposed supplemental labels for the end-use product Outlook™ and the
proposed label.  Physical, chemical, and environmental fate properties
represent the parent compound only.  Modeled EDWCs do not directly
account for potential exposure to transformation products of
s-dimethenamid. However,  based on  the environmental fate profile of
the compound it is unlikely that the inclusion of these products, even
if modeled as parent, would have any significant impact on the EDWAC”s
proiduced.

		b.	Surface Water

Surface water EDWCs were estimated using Tier 2 linked models
PRZM/EXAMS.  Tier 2 modeling accounts for crop type, use locations, and
application date through the use of crop-specific scenarios linked to
local meteorological data.  The scenarios and application dates modeled
for the proposed new uses are presented in Table 5.  The proposed label
for Rutabaga, radish and turnip is a national label . Therefore  these
uses of dimethenamid includes a broad range of plant types and
geographic areas. Although other areas were modeled, the Florida carrots
scenario was selected to represent the most vulnerable sites nationally.
 The Winter Squash and pumpkin uses are proposed for CA and OR only,
therefore only these areas were considered when selecting a scenario for
modeling. The California onion scenario was considered to represent the
most vulnerable site and use for this type of application  For the
highly specialized hops uses in OR, WA and ID are represented by the
hops scenario.  there is a modeled to capture this range  The hops
modeling was highly affected by the timing of the application, where a
change of the application timing by one week made as nuch as a 3 fold
diffence in the EDWAC. For the purposes of this evaluation the timing
that producedMore detail about these scenarios can be found in the
scenario metadata available on the modeling website 

Application dates are not scenario specific.  An application window was
determined based on label recommendations and crop emergence dates. 
From within this window, the application date when the scenario had the
highest precipitation was chosen to provide a conservative estimate of
potential runoff.

TABLE 5. Summary of Crop Scenarios Used in Estimating Surface Water
EDWCs

USE	CROP SCENARIO

(Location)	PRIMARY SOIL TYPE	MET FILE	APPLICATION DATE

Rutabaga, Turnips and Radish 	FL carrots (Palm Beach County)	Riviera
sand	W12842.dvf	February 15

	GA onion

(Toombs County)	Tifton loamy sand	W03822.dvf	June 15







Squash and Pumpkin	CA onion

(San Joaquin County)	Ciervo clay 	W23155.dvf	March 15

Hops	Oregon hops (Marion County)	Woodburn silt loam	W24232.dvf	February
1









Modeling input parameters for PRZM/EXAMS are presented in Table 6. 
Ground spray only modeled for a single application at 0.98 lbs. A.I. /A.

Table 6. PRZM (3.12beta) / EXAMS (2.98.04) modeling inputs for proposed
new uses.

PARAMETER	VALUE	COMMENT	SOURCE

Application Rate per Event

lb a.i./A (kg a.i./ha)	Squash, Pumpkin, rutabaga, turnip, radish and
hops  0.98  (1.1)





Reg. No. 7969-156



No. of Applications

 (Interval)

		Squash, Pumpkin, rutabaga, turnip, radish and hops :	1





Reg. No. 7969-156



CAM

(Chemical App. Method)	1	Pre-emergence spray without crop canopy.

	Depth of Incorporation	0 cm



Aerobic Soil Metabolism t1/(	37 days	Estimated upper 90th percentile
based on 3 studiesa (2 with R/S mixture as the test substanceb)	MRID
44332216

MRID 41596532

Spray Drift Fraction / Application Efficiency	Ground spray:	0.064 / 0.99

Aerial spray:	0.16 / 0.95

EFED Input Guidancea

Aerobic Aquatic Degradation  t1/(  	74 days	No data available; used
one-half aerobic soil metabolism rate	EFED Input Guidancea

Anaerobic Aquatic Degradation  t1/(	108 days	3 x single reported valuea

(R/S mixture as the test substance  b)	MRID 42367201

Aqueous Photolysis t1/(	50 days	Dark corrected; 

12 hour light/12 hour dark cycle	MRID 44332259

Hydrolysis t1/(	Stable	pH 5, 7, 9	MRID 44332258

Soil Partition Coefficient (Kd)	1.4  mL/g	Lowest non-sand Kd	MRID
44332263

Molecular Weight	275 g/mole

Product Chemistry

Water Solubility @ 25°C	1,449 mg/L

MRID 44332214

Vapor Pressure	1.88 x 10-5 torr

MRID 44332215

Percent Cropped Area	Green onions &

Grass for seed:	0.87

Ornamentals:	1.0

The default PCA of 0.87 is only appropriate for agricultural crops. 

(OPP, 2000)

a   HYPERLINK
"http://www.epa.gov/oppefed1/models/water/input_guidance2_28_02.htm" 
http://www.epa.gov/oppefed1/models/water/input_guidance2_28_02.htm 

b These data were measured on the R/S isomer mixture.  No data are
available for the S isomer alone.

	

Modeling Results

PRZM/EXAMS results, presented in Table 8, represent untreated surface
water concentrations. The one-in-10-year peak day concentration is used
for acute endpoints and the one-in-10-year annual average concentration
is used for chronic endpoints.  When available, multiple scenarios are
used to characterize potential regional variation, the most conservative
values (in bold) are most appropriate for estimating effects at a
national level.  The model output files are attached in Appendix C.

Table 8.  Tier 2 EDWCs in SURFACE WATER from proposed new uses of 

s-dimethenamid.

APPLICATION METHOD	USE	MODELED SCENARIO	MAXIMUM EDWC  ( ppb)



	Acute

(Peak)	Chronic

(Annual Avg.)

Ground Spray	Squash and Pumpkin	CA onion	2.11	0.71

	Rutabaga, Turnip and Radish	Fl Carrot	59.02	6.45

	Hops	OR hops	7.93	2.61



D.	Monitoring Data

Monitoring data were not considered in this assessment of proposed new
uses.  A search for available monitoring data was not conducted.  

E.	Drinking Water Treatment

There is no available information on the drinking water treatment
effects on dimethenamid.

IV.	CONCLUSIONS

The estimated concentrations provided in this assessment are were
generated from Tier I (groundwater) and Tier 2 (surface water) models
and are conservative estimates of s-dimethenamid concentrations in
untreated drinking water.  They represent the maximum use patterns on
squash, pumpkin, turnip, rutabaga, radish, and hops

Tier 1 modeling is based on sites that are highly vulnerable to runoff
or leaching and includes protective assumptions about environmental
conditions and fate properties.  The presented Tier 2 modeling uses
crop-specific scenarios to represent vulnerable sites in areas where the
treated crops are grown, but otherwise uses the same protective
assumptions about fate properties.  Tier 2 modeling is more refined than
Tier 1 modeling, but Tier 2 estimated exposure values are still screens,
although finer screens than Tier 1 models.  A standard set of
recommended crop PCA’s were used in these analyses. Due to the limited
labeling for squash and pumpkin (restricted to OR and WA) and hops
(restricted to OR, WA and ID) regional PCA were used in the calculations
of the EDWAC’s.  These were 0.56 for squash and pumpkin and 0.62 for
hops.  Because Rutabaga, turnips and radish were more broadly labeled
(national label) a generalized PCA of 0.87 was used. 

V.	REFERENCES

Environmental Fate and Effects Division.  2002a.  Guidance for Selecting
Input Parameters in Modeling the Environmental Fate and Transport of
Pesticides, Version II. U. S. Environmental Protection Agency.
Washington, D.C.    HYPERLINK
"http://www.epa.gov/oppefed1/models/water/input_guidance2_28_02.htm/" 
http://www.epa.gov/oppefed1/models/water/input_guidance2_28_02.htm/ 

Office of Pesticide Programs. 2000.  Drinking Water Screening Level
Assessments.     HYPERLINK
"http://www.epa.gov/oppfead1/trac/science/reservoir.pdf" 
http://www.epa.gov/oppfead1/trac/science/reservoir.pdf 



			

