**UNITED STA TES ENVIRONMENTA**  **L PROTECTION A GENCY**  WA SHINGTON D.C., 20460

OFFICE OF PREVENTION, PESTICIDES AND TOXIC SUBSTANCES December 1 9 , 2011 PC Code: 117403 **MEMORANDUM** DP Barcode: 391398 SUBJECT: Tier I Drinking Water Assessment for Proposed New Use of Clop yr alid on

Apple , L eafy

Brassica Greens Subgroup 5B, and Rapeseed Subgroup 20A, except Camelina (Gold of Pleasure) FROM: Wm. J. Shaughnessy , PhD, Environmental Scientist

Environmental Risk Branch I I 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 W ater Assessment (DWA) of the herbicide clop yralid (3,6
- dichloro
- 2
- py ridinecarboxylic 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 clopy ralid on apple
. Clopy ralid is currently

re gistered 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 clopy ralid for aproposed new use on Swiss chard, Bushberry subgroup 13
- 07B and annual strawberry ( see attachment)
. 1 EDWCs in surface and ground water for the proposed new use were generated b y the FIRST and SCI
- GROW models, respectively
. EFED notes that there was some uncertainty in the data used to model the EDWCs. A erobic 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 conservati sm in the modeling results. Modeling re sults from the 2009 DWA are presented in Table 1 (copied from the 2009 DWA report)
. The EDWCs were based on ground applications of clop yralid and two application s per ye ar (at 14 day

intervals) of 0.25 lbs a.i./A fo r bushberry and a single application of 0.5 lbs a.i./A for non - food crops. **Table 1. 2009 DWA**  **- Maximum Tier I Estim**  **ated 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) Ac ute and Chronic 0.39 Surface water (FIRST) Bushberry (0.25 lbs a.i./A @2 apps./yr) Ac ute 41.1 Bushberry (0.25 lbs a.i./A @2 apps./yr) Chronic 10.8 Non
- crop (0.5 lbs a.i./ A) Ac ute 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 clop yralid 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 applic ation rate for clop yralid use on apple is 0.25 lbs a.i./A , which is half the applied rate of clop yralid 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 Pleasur e) 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
1 Memorandum from Andrew Shelby and William Eckel, ERB2, EFED to Laura Nollen, RD, subj: Tier I Drinking Water Assessment for Proposed New Use of Clopyralid on Swiss Chard, Bushberry Subgroup 13
- 07B and Annual Straw berry , DP Barcode 361314, dtd: September 9, 2009.clopy

ralid 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) inpu t 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 param eters 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 t 1/2 (day s) 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 t 1/2 (day s) 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 Estim ated 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) Ac ute 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) Ac ute (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 Ac ute 42 1 Source: 2009 DWA.In c omparing T ables 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
. SCI GROW concentration (ppb) represents the groundwater concentration that might be expected in shallow unconfined aquifers under sandy soils. SCI GROW o utput is used for both acute and chronic endpoints. FIRST concentrations (ppb) represent untreated surface water concentrations. The one
- in
10
- ye ar 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 clopy ralid in surface and ground water are presented in Table 6
. These values are higher than the EDWCs generated b y the FIRST and SCIGROW models f or clop yralid 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 STA TES ENVIRONMENTA**  **L PROTECTION A GENCY**  WA SHINGTON 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**  **Environme ntal 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 clop yr alid (3,6
- dichloro
- 2
- py ridinecarbox yl ic acid; CAS Reg. No. 1702
- 17
- 6) on Swiss chard, Bushberry subgroup 13
- 07B and annual strawberry . Because of the chemical’s persistence, onl y the parent compound is assessed. Contact Andrew Shelb y at 703
- 347
- 0119 if there are any questions. **EXECUTIVE SUMMARY** Clopy ralid is an herbicide within the pry ridinecarboxylic acid famil y that mimics the plant growth hormone auxin. This sy stemic 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 frui ts, 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 honey suckle, 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 SOURC**  **E (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 highl y vulnerable. This initial tier screens out chemicals with low p otential risk and provides estimated exposure concentrations for the human health dietary risk assessment. **Background** The proposed new use of clopy ralid on Swiss chard, Bushberry subgroup 13
- 07B and annual strawberry

is the first assessment on clopy ralid since multiple new uses and special local needs were assessed from 1999 to 2002, the last of which was dated June 26, 2 002 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 clopy ralid is miscible in water (1 x 10 4 ppm), does not hydrol yz e in water, is stable to photolysis, is persistent under laboratory conditions in aerobic soil (DT90 greater than 78 day s) and is very mobile in soil (K OC = 0.4 mL/go.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 subgroup13
- 07B is the onl y 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 day s and 7 day s prior to harvest respectivel y 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 with out contacting the foliage or woody portions. The label also allows for one additional application with minimum interval of 14 day s between applications. All above use information is summarized in the following table: **Table 2: Summary of maximum use inf ormation 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** Clopy ralid is non
- volatile thereb y limiting transport in air. The chemical also has a low rate of soil sorption (KOC = 0.4 mL/go.c.) restricting storage within soil. The dominating environmental compartment is water due to these characteristics and the chemical’s miscibility
. Clopy ralid is stable to photoly sis 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 p rimary pathway of degradation for clop yr alid in the environment. Though aerobic aquatic metabolism is the anticipated primary route of degradation, it is currentl y 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. Clopy ralid 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. P lant material from treated areas should therefore not be used as compost feedstocks. Clopy ralid degradates are not modeled in this assessment because the aerobic soil metabolism study did not produce metabolites representing at least 10% of the applied do se. However, these degradates were not identified and whether they are of toxicological concern is a source ofuncertainty. 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 metab olism 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 s uch 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 clopy ralid aerobic soil metabolism study (MRID 43891 403 ) was found to match most closely

to a second
- order rate equation. Though second
- order degradation most accuratel y reflects the data, FIRST and SCIGROW are based on first
- order kinetics. This distinction requires that first
- order degradation be used t o derive modeling parameters and second
- order degradation only be used to better characterize the chemical. The following table outlines clopy

ralid chemical characteristics. **Table 3: Summary of physical/chemical and environmental fate and transport prope rties of**  **clopyralid.** PARAMETER VALUE(S) (units) SOURCE COMMENT Chemical Name 3,6
- dichloro
- 2
-

py ridinecarbox yl ic 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 Hydroly sis Half

life Stable MRID 42805701 Soil Photolysis Half
- life Stable MRID 43891402 Aerobic Soil Metabolism Half

life 16.8 day s x 3 = 50.4

day s DT50: 7 to 14 day s DT90 > 78 day s MRID 43891403 Three times first
- order half
- life used as per 2002 Input Parameter Guidance 2 nd Order Degradation; DT90/3.32 = 34.6 day s Anaerobic Aquatic Metabolism Half

life Stable MRID 43891405 Aerobic Aquatic Metabolism Half

life Data gap; used 100

day s (2X aerobi c soil metabolism half
- life)Organic Carbon

Partition Coefficient (KOC ) 0.4 ml/g o.c. MRID 40095701 Terrestrial Field Dissipation Half

life 10.7 day s – 25.2

day s MRID 43891406 **Drinking Water Exposure Modeling** SCI GROW (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. SCI GROW 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 SCI GROW 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 sourc e. 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 b y 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 b y the Office of Pesticide Programs to assess drinking water exposure (O ffice of Pesticide Programs, 2002). It is based on a real reservoir (albeit not currentl y in active use as a drinking water suppl y), Shipman City L ake 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 o ther 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 SCI GROW and FIRST models respectivel y 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 u**  **se on Berry s**  **ubgroup 13 -**  **07B,**

**Annual Strawberry and Swiss c**  **hard. 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 (day s) 16.8 1 st Order Degradation; MRID 43891403, Acc. No s. 099727 and 099728. . **Table 5**  **: FIRST (v1.1.0) input parameters for Clopyralid u**  **se on Berry s**  **ubgroup 13 -**  **07B ,**  **Annual Strawberry and Swiss c hard.**  **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 ye ar) 2 Attached IR
- 4 documents; Maximum possible applications in season Application interval (day s) 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 t 1/2 (days) 50.4 1 st Order Degradation; MRID 43891403, Acc. Nos. 099727 and 099728. Half
- life value (16.8 day s) 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 t 1/2 (day s) 100.8 2 x (aerobic soil metabolism parameter) Photolysis Half

- life (day s) 0 Stable. Acc. Nos. 099727 and 099728 **Modeling Results** SCI GROW 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
-

ye ar peak day

concentration is used for acute endpoints and the one
- in
- 10
- ye ar 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 W ater 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 alway s 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 clop yralid concentrations. Considering this, use of EDWCs in any assessment shoul d 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 clopy ralid. **CONCLUSIONS**Clopy

ralid 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 exceedin g 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 shoul d 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, Clopy ralid Contaminated Compost FACT SHEET December 27, 2001, http://cahenews.wsu.edu/clopyralid.htm