								

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON D.C., 20460

OFFICE  OF 

PREVENTION, PESTICIDES AND 

TOXIC SUBSTANCES

MEMORANDUM							DATE: 9 September 2009

									DP Barcode: 361314

									PC Code: 117403

SUBJECT:	Tier I Drinking Water Assessment for Proposed New Use of
Clopyralid on Swiss Chard, Bushberry Subgroup 13-07B and Annual
Strawberry

FROM:	Andrew Shelby

		Physical Scientist, Environmental Risk Branch II

		Environmental Fate and Effects Division (7507P)

		William P. Eckel, Ph.D.

		Senior Scientist, Environmental Risk Branch II

		Environmental Fate and Effects Division (7507P)

THRU:	Tom Bailey, Branch Chief

		Environmental Risk Branch II

		Environmental Fate and Effects Division (7507P)

TO:		Laura Nollen

		Registration Division (7505C)

This assessment contains estimated drinking water concentrations (EDWCs)
for both surface and ground water for clopyralid
(3,6-dichloro-2-pyridinecarboxylic acid; CAS Reg. No. 1702-17-6) on
Swiss chard, Bushberry subgroup 13-07B and annual strawberry.  Because
of the chemical’s persistence, only the parent compound is assessed. 
Contact Andrew Shelby at 703-347-0119 if there are any questions. 

EXECUTIVE SUMMARY

Clopyralid is an herbicide within the pryridinecarboxylic acid family
that mimics the plant growth hormone auxin.  This systemic hormone mimic
causes uncontrolled and disorganized plant growth that leads to plant
death.  Current agricultural uses include asparagus, barley, canola,
corn, crambe, cranberry, fallow land, flax seed, garden beet, grasses,
head and stem Brassica, hops, mustard greens, oats, pasture, rangeland,
rapeseed, sod farms, spinach, stone fruits, sugar beets, tree
plantations, turnip, and wheat.  Non-agricultural uses include
non-residential turfgrass and non-residential lawn.  The proposed uses
assessed here are Swiss chard, strawberry and Bushberry subgroup 13-07B.
 Bushberry subgroup 13-07B includes aronia berry, highbush blueberry,
lowbush blueberry, buffalo currant, Chilean guava, highbush cranberry,
black currant, red currant, European barberry, gooseberry, edible
honeysuckle, huckleberry, jostaberry, Juneberry, lingonberry, native
currant, salal and sea buckthorn.  

Screening Concentration in Ground Water (SCIGROW) and FQPA Index
Reservoir Screening Tool (FIRST) models were parameterized for maximum
application rates and used to determine Estimated Drinking Water
Concentrations (EDWCs).  The following table summarizes results for
hazard maximizing modeling scenarios.  

Table 1. Maximum Tier I Estimated Drinking Water Concentrations (EDWCs)
for drinking water assessment based on ground application of Stinger
Herbicide

DRINKING WATER SOURCE (MODEL USED)	USE (rate modeled)	MAXIMUM ESTIMATED
DRINKING WATER CONCENTRATION (EDWC) (ppb)

Groundwater (SCIGROW)	Bushberry (0.25 lbs a.i./A)	Acute and Chronic	0.39

Groundwater (monitoring)	NAWQA, Alabama	Acute	13



Surface water (FIRST)

	Bushberry (0.25 lbs a.i./A)	Acute	41.1

	Bushberry (0.25 lbs a.i./A)	Chronic	10.8

	Non-crop (0.5 lbs a.i./ A)	Acute	45.0

	Non-crop (0.5 lbs a.i./ A)	Chronic	11.9



PROBLEM FORMULATION

This is a Tier I drinking water assessment that uses modeling and
available monitoring data to estimate the ground water and surface water
concentrations of pesticides in drinking water source water
(pre-treatment) resulting from pesticide use on sites that are highly
vulnerable.  This initial tier screens out chemicals with low potential
risk and provides estimated exposure concentrations for the human health
dietary risk assessment.

Background

The proposed new use of clopyralid on Swiss chard, Bushberry subgroup
13-07B and annual strawberry is the first assessment on clopyralid since
multiple new uses and special local needs were assessed from 1999 to
2002, the last of which was dated June 26, 2002 for canola in Colorado. 
This assessment determined estimated environmental concentrations (EECs)
based on a maximum application rate of 0.1875 lbs a.i./A.

Previous assessments found clopyralid is miscible in water (1 x 104
ppm), does not hydrolyze in water, is stable to photolysis, is
persistent under laboratory conditions in aerobic soil (DT90 greater
than 78 days) and is very mobile in soil (KOC = 0.4 mL/g o.c.).  Based
on these characteristics, previous assessments recommended a groundwater
advisory on the label:  

This chemical demonstrates the properties and characteristics associated
with chemicals detected in ground water.  The use of this chemical in
areas where soils are permeable, particularly where the water table is
shallow, may result in ground water contamination 

Use Characterization

The new use assessed here calls for a maximum application rate of 0.25
lbs a.i./A for Bushberry subgroup 13-07B and annual strawberry and
maximum application rate of 0.188 lbs a.i./A for Swiss chard.  Each new
proposed use would be applied via ground spray.  Bushberry subgroup
13-07B is the only use modeled in this assessment because it has the
highest single application rate and highest potential number of
applications.  These maximum single application rates are less than the
label maximum application rate of 0.5 lbs a.i./A for non-crop land. 
This maximum application rate is also modeled in this assessment and
resultant EDWCs should be used for assessing risk.  The proposed new
uses for Swiss chard and strawberry call for foliar, broadcast
applications at 30 days and 7 days prior to harvest respectively with
one application per year.  The proposed new use for Bushberry subgroup
13-07B calls for application to the soil from the plants to the row
middle without contacting the foliage or woody portions.  The label also
allows for one additional application with minimum interval of 14 days
between applications.  All above use information is summarized in the
following table:

Table 2: Summary of maximum use information for clopyralid, based on
Stinger Herbicide labels

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

Bushberry Subgroup	0.25	2	0.5	14	Ground Spray	0

Swiss Chard	0.25	1	0.25	N/A	Ground Spray	0

Annual Strawberry	0.188	1	0.188	N/A	Ground Spray	0



Fate and Transport Characterization

Clopyralid is non-volatile thereby limiting transport in air.  The
chemical also has a low rate of soil sorption (KOC = 0.4 mL/g o.c.)
restricting storage within soil.  The dominating environmental
compartment is water due to these characteristics and the chemical’s
miscibility.

Clopyralid is stable to photolysis and anaerobic metabolism and
moderately stable to aerobic metabolism.  Due to anaerobic metabolism
stability and high mobility, the chemical is likely to persist in
groundwater.  Aerobic metabolism is expected to be the primary pathway
of degradation for clopyralid in the environment.  

Though aerobic aquatic metabolism is the anticipated primary route of
degradation, it is currently a data gap for this chemical.  Modeling
guidance assumes aerobic aquatic metabolism half-life to be double that
of aerobic soil metabolism in the absence of data.  Although this is a
conservative assumption, it should be well noted that the primary route
of degradation for this chemical is not well-characterized.  

Clopyralid has been found to survive the composting process when yard
clippings are collected and composted (Washington State University). 
The chemical becomes concentrated in the final compost, which then may
be toxic to plants to which it is applied.  Plant material from treated
areas should therefore not be used as compost feedstocks.

Clopyralid degradates are not modeled in this assessment because the
aerobic soil metabolism study did not produce metabolites representing
at least 10% of the applied dose.  However, these degradates were not
identified and whether they are of toxicological concern is a source of
uncertainty.  Additionally, a different suite of metabolites potentially
present in larger concentrations could be produced from aerobic aquatic
metabolism relative to aerobic soil metabolism.  However, due to the
small size of the molecule, large variations in degradates is not
expected.

Though new studies are not available for this assessment, a more
accurate determination of aerobic soil metabolism half-life has been
made using second-order degradation kinetics. Degradation, especially
through metabolism, often does not follow strict first order kinetics
which is only dependent on the concentration of the parent compound. 
More nuanced models such as first-order multi-compartment (FOMC),
intermediate order rate equation (IORE) and second-order rate equation
take into account heterogeneous media and/or interactions from
degradates.  Data from a clopyralid aerobic soil metabolism study (MRID
43891403) was found to match most closely to a second-order rate
equation.  Though second-order degradation most accurately reflects the
data, FIRST and SCIGROW are based on first-order kinetics.  This
distinction requires that first-order degradation be used to derive
modeling parameters and second-order degradation only be used to better
characterize the chemical.  The following table outlines clopyralid
chemical characteristics.

Table 3: Summary of physical/chemical and environmental fate and
transport properties of clopyralid.

PARAMETER	VALUE(S) (units)	SOURCE	COMMENT

Chemical Name	3,6-dichloro-2-pyridinecarboxylic acid



Molecular Weight	192



Solubility	Miscible



Vapor Pressure (250C)	1.26 x 10-6 mm Hg



pKa 	2.30		Senseman 2007

	Octanol-Water Partition Coefficient (KOW)	21.81	Senseman 2007

	Hydrolysis Half-life	Stable	MRID 42805701

	Soil Photolysis Half-life	Stable	MRID 43891402

	Aerobic Soil Metabolism Half-life	16.8 days x 3 = 50.4 days

DT50: 7 to 14 days

DT90 > 78 days	MRID 43891403	Three times first-order half-life used as
per 2002 Input Parameter Guidance

2nd Order Degradation; DT90/3.32 = 34.6 days

Anaerobic Aquatic Metabolism Half-life	Stable	MRID 43891405

	Aerobic Aquatic Metabolism Half-life	Data gap; used 100 days (2X
aerobic soil metabolism half-life)



Organic Carbon Partition Coefficient (KOC)	0.4 ml/g o.c.	MRID 40095701

	Terrestrial Field Dissipation Half-life	10.7 days – 25.2 days	MRID
43891406

	

Drinking Water Exposure Modeling

SCIGROW (Screening Concentration in Ground Water) is a regression model
used as a screening tool to estimate pesticide concentrations found in
ground water used as drinking water. SCIGROW was developed by fitting a
linear model to groundwater concentrations with the Relative Index of
Leaching Potential (RILP) as the independent variable.  Groundwater
concentrations were taken from 90-day average high concentrations from
Prospective Ground Water studies; the RILP is a function of aerobic soil
metabolism and the soil-water partition coefficient.  The output of
SCIGROW represents the concentrations that might be expected in shallow
unconfined aquifers under sandy soils, which is representative of the
ground water most vulnerable to pesticide contamination likely to serve
as a drinking water source.

FIRST (FQPA Index Reservoir Screening Tool) is a metamodel of PRZM and
EXAMS used as a screening tool to estimate pesticide concentrations
found in surface water used as drinking water.  FIRST was developed by
making multiple runs of PRZM using varying sorption coefficients and
determining the concentration in the EXAMS index reservoir scenario
after a two-inch single storm event.  (The Index Reservoir is a standard
water body used by the Office of Pesticide Programs to assess drinking
water exposure (Office of Pesticide Programs, 2002).  It is based on a
real reservoir (albeit not currently in active use as a drinking water
supply), Shipman City Lake in Illinois, that is known to be vulnerable
to pesticide contamination.)  The single runoff event moves a maximum of
8% of the applied pesticide into the reservoir.  This amount can be
reduced by degradation or effects of binding to soil in the field. 
Additionally, FIRST can account for spray drift and adjusts for the area
within a watershed that is planted with the modeled crop (Percent Crop
Area).  Spray drift (modeled as direct deposition of the pesticide into
the reservoir) is assumed to be 16% of the applied active ingredient for
aerial application, 6.3% for orchard air blast application, and 6.4% for
other group spray application.  Despite being a single event model,
FIRST can account for spray drift from multiple applications.  The
default agricultural Percent Cropped Area (PCA) is 87%.  The PRZM
scenario used for FIRST development was among the most vulnerable, and
thus resulting surface water concentrations represent the upper bound
values on the concentrations that might be found in drinking water from
the use of a pesticide.

Modeling Approach and Input Parameters

Input parameters were determined based on “Guidance for Selecting
Input Parameters in Modeling the Environmental Fate and Transport of
Pesticides” (Version II, February 28, 2002).  The following tables
display input parameters for SCIGROW and FIRST models respectively and
provide explanation for the determination of the parameter.  The same
modeling inputs were used for determining non-crop EDWCs with exceptions
for “Maximum Application Rate” (0.5) and “Number of Applications
per Year” (1).

Table 4: SCIGROW (v2.3) input parameters for Clopyralid use on Berry
subgroup 13-07B, Annual Strawberry and Swiss chard.

PARAMETER (units)	VALUE(s)	SOURCE/RATIONALE

Maximum Application Rate (lbs a.i./A)	0.25	Attached IR-4 documents;
Petition: 8E7481

Number of Applications per Year	2	Attached IR-4 documents; Range
including maximum possible applications in season



Organic Carbon Partition Coefficient (KOC;mL/g o.c.)	0.4	Minimum
non-sand value; MRID 40095701

Aerobic Soil Metabolism Half-life (days)	16.8	1st Order Degradation;
MRID 43891403, Acc. Nos. 099727 and 099728.  .



Table 5: FIRST (v1.1.0) input parameters for Clopyralid use on Berry
subgroup 13-07B, Annual Strawberry and Swiss chard.

PARAMETER (units)	VALUE(s)	SOURCE/RATIONALE

Application rate (pounds a.i. acre-1)	0.25	Attached IR-4 documents;
Petition: 8E7481

Number of applications (per year)	2	Attached IR-4 documents; Maximum
possible applications in season

Application interval (days)	14	Attached IR-4 documents

Percent Crop Area	0.87	Since no PCA has been determined, 0.87 is the
recommended value from Guidance

KOC	0.4	Minimum non-sand value; MRID 40095701

Aerobic soil t1/2 (days)	50.4	1st Order Degradation; MRID 43891403, Acc.
Nos. 099727 and 099728.  Half-life value (16.8 days) multiplied by
three.

Is the pesticide wetted-in?	No	Section 3 label

Method of application	B	Ground (all uses), 6.4% drift

Depth of incorporation (inches)	0	Section 3 label

Solubility (mg/L)	10,000	Miscible

Aerobic aquatic t1/2 (days)	100.8	2 x (aerobic soil metabolism
parameter)

Photolysis Half-life (days)	0	Stable. Acc. Nos. 099727 and 099728



Modeling Results

SCIGROW concentration (ppb) represents the groundwater concentration
that might be expected in shallow unconfined aquifers under sandy soils.
 Output is used for both acute and chronic endpoints.

FIRST concentrations (ppb) represent untreated surface water
concentrations.  The one-in-10-year peak day concentration is used for
acute endpoints and the one-in-10-year average concentration is used for
chronic endpoints.

The estimated concentrations provided in this assessment are
conservative estimates of concentrations in drinking water.  If dietary
risks require refinement, higher tiered crop-specific models and
modeling scenarios can be used.

Table 6: Maximum Tier I Estimated Drinking Water Concentrations (EDWCs)
for drinking water risk assessment based on ground application of
Clopyralid. 

DRINKING WATER SOURCE (MODEL USED)	USE (rate modeled)	MAXIMUM ESTIMATED
DRINKING WATER CONCENTRATION (EDWC) (ppb)

Groundwater (SCIGROW)	Bushberry (0.25 lbs a.i./A)	Acute and Chronic	0.39

Groundwater (monitoring)	NAWQA, Alabama	Acute	13

Surface water (FIRST)	Bushberry (0.25 lbs a.i./A)	Acute	41.1

	Bushberry (0.25 lbs a.i./A)	Chronic	10.8

Surface water (FIRST	Non-crop (0.5 lbs a.i./ A)	Acute	45.0

	Non-crop (0.5 lbs a.i./ A)	Chronic	11.9



Monitoring Data

Monitoring data provide different kinds of information than modeling
estimates.  For example, monitoring data consist of actual information
from the field, reflecting current use pattern and usually
underestimating frequency of occurrence.  Monitoring data does not
always include peak values, and inputs for monitoring cannot be adjusted
as modeled ones can.  In addition, monitoring is often conducted for
purposes other than characterizing exposure from a particular pesticide,
and as a consequence is used to complement modeling rather than to
refine it.  In general, a useful interpretation of monitoring values
requires in-depth assessment of the data, which is beyond the scope of a
Tier I assessment. 

	Several incidences of monitoring concentrations exceeding EDWCs have
occurred.  Groundwater concentrations as high as 13 ppb have been
detected in Alabama and surface water concentrations of 41 and 42 ppb
have been detected in North Carolina, Illinois and Ohio (NAWQA).  Peak
groundwater concentrations greatly exceed the groundwater EDWC and peak
surface water concentrations slightly exceed the associated EDWC.  It
cannot be assumed that these monitoring data represent peak clopyralid
concentrations.  Considering this, use of EDWCs in any assessment should
be qualified with these monitoring data to indicate uncertainty.

Drinking Water Treatment

No data are available to indicate the effectiveness of drinking water
treatment on the degradation of clopyralid.  

CONCLUSIONS

Clopyralid is a persistent chemical that partitions to water. 
Degradation is driven by aerobic aquatic metabolism though this pathway
is not directly characterized through a guideline study.  Further,
degradation behavior best fits second-order kinetics though first-order
kinetics are used to derive and parameterize FIRST and SCIGROW. 
Second-order kinetics, in this case, provides a substantially larger
half-life estimate than first-order kinetics.  Finally, monitoring data
indicate concentrations exceeding EDWCs and it can be assumed that this
is an underestimate of peak concentrations in the environment.  

Though FIRST and SCIGROW, by definition, provide conservative estimates
of surface water and groundwater concentrations, the preceding
qualifiers should be considered when using these EDWCs in any
assessment.

	REFERENCES

	Senseman, S. S., ed. 2007. Herbicide Handbook. 9th ed. Lawrence, KS:
Weed

Science Society of America

	USGS National Water Quality Assessment Data Warehouse   HYPERLINK
"http://water.usgs.gov/nawqa/"  http://water.usgs.gov/nawqa/ 

	Washington State University, Clopyralid Contaminated Compost FACT SHEET
	

December 27, 2001, http://cahenews.wsu.edu/clopyralid.htm

