Document ID: EPA-HQ-OPP-2011-0569-0008
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2012-09-19T04:00Z

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
                               WASHINGTON D.C., 20460
                                          
                                          
                                          
                                                                    OFFICE  OF 
                                                                    PREVENTION, PESTICIDES AND 
                                                                    TOXIC SUBSTANCES
						
                               December 19, 2011
                                       
                                                                PC Code: 117403
                                                 MEMORANDUM 	DP Barcode: 391398

      
      SUBJECT:	Tier I Drinking Water Assessment for Proposed New Use of Clopyralid on Apple, Leafy Brassica Greens Subgroup 5B, and Rapeseed Subgroup 20A, except Camelina (Gold of Pleasure)
      
      FROM:		Wm. J. Shaughnessy, PhD, Environmental Scientist
      			Environmental Risk Branch II
                  Environmental Fate and Effects Division (7507P)
            
      REVIEWED BY:	James Lin, Environmental Scientist
                  Environmental Risk Branch II
                  Environmental Fate and Effects Division (7507P)
      		
      THRU:		Brian Anderson, Branch Chief
      			Jean Holmes, DVM, RAPL
                  Environmental Risk Branch II
      			Environmental Fate and Effects Division (7507P)
      
      TO:			Barbara Madden, Risk Manager
                  Laura Nollen, Risk Reviewer
      			RIMUERB
                  Registration Division (7505C)
      
      The Environmental Fate and Effects Division (EFED) has conducted a Tier I Drinking Water Assessment (DWA) of the herbicide clopyralid (3,6-dichloro-2-pyridinecarboxylic acid); CAS Reg. No. 1702-17-6).  IR-4 and the registrant Dow AgroSciences are proposing new tolerances (Petition No. 1E7882) for apple, leafy brassica greens subgroup 5B, and rapeseed subgroup 20A, except camelina (gold of pleasure).  This assessment contains estimated drinking water concentrations (EDWCs) for both surface and ground water from the use of clopyralid on apple.
   
      Clopyralid is currently registered for use on numerous crops, including: brassica (cole), leafy vegetables, asparagus, canola, field corn, garden beet, grasses grown for seed, mint, sweet and popcorn, spinach, stone fruits, sugar beet, turnip, barley, oats, wheat, conservation reserve program (CRP) acres, tree plantations, fallow and non-cropland, rangeland and permanent grass pastures.
   
      In September 2009, EFED completed a drinking water assessment of clopyralid for a proposed new use on Swiss chard, Bushberry subgroup 13-07B and annual strawberry (see attachment).  EDWCs in surface and ground water for the proposed new use were generated by the FIRST and SCI-GROW models, respectively.  
   
      EFED notes that there was some uncertainty in the data used to model the EDWCs.  Aerobic aquatic metabolism is considered to be the primary route of degradation.  However, it was considered a data gap for this chemical.  Therefore, for EDWC modeling purposes, the aerobic aquatic metabolism half-life was estimated to be double that of aerobic soil metabolism half-life value (per model input guidance).  This may have introduced some conservatism in the modeling results.
      
      Modeling results from the 2009 DWA are presented in Table 1 (copied from the 2009 DWA report).  The EDWCs were based on ground applications of clopyralid and two applications per year (at 14 day intervals) of 0.25 lbs a.i./A for bushberry and a single application of 0.5 lbs a.i./A for non-food crops.  

      Table 1. 2009 DWA - Maximum Tier I Estimated Drinking Water Concentrations (EDWCs) for drinking water assessment based on ground application of Stinger Herbicide (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

Surface water (FIRST)

Bushberry (0.25 lbs a.i./A @2 apps./yr)
Acute
41.1

Bushberry (0.25 lbs a.i./A@2 apps./yr)
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
      Source: EFED 2009 Drinking Water Assessment

   In the current assessment EFED is evaluating the use of clopyralid on apple, leafy brassica, greens subgroup 5B, and rapeseed subgroup 20A.  Table 2 summarizes the application information for each of the specified uses.   EFED notes that the maximum application rate for clopyralid use on apple is 0.25 lbs a.i./A, which is half the applied rate of clopyralid on bushberry used in the 2009 DWA.
     
      Table 2.  Clopyralid Application Rates
Crop Use
Maximum yearly/seasonal application rate (lbs. a.i./A)
Method of Application
Apple
0.25
Ground
Brassica (cole), Leafy Vegetables, Crop Group 5
0.187
Ground
Rapeseed, Subgroup 20A (except Gold of Pleasure)
0.187
Ground
   
   The FIRST and SCI-GROW models are again used to generate EDWCs for the new use on apple.  Model input parameters, other than the application rate, are the same as those used in the 2009 DWA (Tables 3 and 4).  Model results are presented in Table 5.
   
   EFED completed a search of the NAWQA database (12/21/2011) for detectable levels of clopyralid in ground water.  The peak observed value was 0.5288 ppb (Oregon) with a nationwide mean value of 0.065 ppb (n=107).  These values along with surface water monitoring data noted in the 2009 DWA are also presented in Table 5. 
   
   Table 3: SCIGROW (v2.3) input parameters for Clopyralid use on apple.
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
                                       1
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
1[st] Order Degradation; MRID 43891403, Acc. Nos. 099727 and 099728.  .
      
      
   Table 4: FIRST (v1.1.0) input parameters for Clopyralid use on apple.
                               PARAMETER (units)
                                   VALUE(s)
                               SOURCE/RATIONALE
Application rate (pounds a.i. acre[-1])
                                     0.25
IR-4 documents; Petition: 1E7882
Number of applications (per year)
                                       1
IR-4 documents; Maximum possible applications in season
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 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

   Table 5. 2011 DWA - Maximum Tier I Estimated Drinking Water Concentrations (EDWCs) for drinking water assessment based on ground application of Stinger Herbicide (clopyralid)
DRINKING WATER SOURCE (MODEL USED)
USE (rate modeled)
MAXIMUM ESTIMATED/OBSERVED DRINKING WATER CONCENTRATION (ppb)
Groundwater (SCIGROW)
Apple (0.25 lbs a.i./A)
Acute and Chronic
0.20
Groundwater (monitoring)
NAWQA (Oregan)
Peak value 
0.5288

NAWQA (nationwide)
Mean (n=107)
0.0646

Surface water (FIRST)
Apple (0.25 lbs a.i./A)
Acute (one-in-10-year peak daily concentration)
22.5

Apple (0.25 lbs a.i./A)
Chronic (one-in-10-year average concentration)
5.9
Surface water (monitoring)[1]
NAWQA, North Carolina, Illinois and Ohio
Acute
42
   [1] Source: 2009 DWA.
   
      In comparing Tables 1 and 5 it is apparent that the current model results are about half the 2009 DWA results and reflect the 50% difference in application rates used in the two studies.  SCIGROW concentration (ppb) represents the groundwater concentration that might be expected in shallow unconfined aquifers under sandy soils.  SCIGROW output is used for both acute and chronic endpoints.

      FIRST concentrations (ppb) represent untreated surface water concentrations.  The one-in-10-year peak concentration is used for acute endpoints and the one-in-10-year average concentration is used for chronic endpoints.
   
      Overall, for purposes of assessing risk, the highest observed/estimated concentrations of clopyralid in surface and ground water are presented in Table 6.  These values are higher than the EDWCs generated by the FIRST and SCIGROW models for clopyralid use on apple and reflect the 50% higher application rate.

      Table 6.  Highest Observed or Modeled Clopyralid Concentration for Surface and Ground Water Environments
                                     Media
                            Max Concentration (ppb)
                                    Source
Ground water 
0.5288
NAWQA (Oregon)
Surface water (acute)
45
FIRST (non-food crop  -  1 applications per year @ 0.5 lb a.i./A)
Surface water (chronic)
11.9
FIRST (non-food crop  -  1 applications per year @ 0.5 lb a.i./A)

Attachment

								
                                       
                    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, 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 10[4] 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 (25[0]C)
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

2[nd] 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
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 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 http://water.usgs.gov/nawqa/

	Washington State University, Clopyralid Contaminated Compost FACT SHEET 	
      December 27, 2001, http://cahenews.wsu.edu/clopyralid.htm