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 nonresidential 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 104ppm), 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 HalflifeStable MRID 42805701

Soil Photolysis 

Half-life

Stable MRID 43891402

Aerobic Soil 

Metabolism Halflife

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

2

nd Order Degradation; 

DT90/3.32 = 34.6 days

Anaerobic Aquatic 

Metabolism Halflife

Stable MRID 43891405

Aerobic Aquatic 

Metabolism Halflife

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 Halflife

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 Halflife (days)16.8 1

st 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 1

st Order Degradation; MRID 43891403, Acc. 

Nos. 099727 and 099728. Half-life value (16.8 

days) multiplied by three.

Is the pesticide wettedin?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 http://water.usgs.gov/nawqa/

Washington State University, Clopyralid Contaminated Compost FACT SHEET 

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