UNITED STATES ENVIRONMENTAL PROTECTION AGENCY WASHINGTON D. C., 20460 OFFICE OF PREVENTION, PESTICIDES AND TOXIC SUBSTANCES March 22, 2006 DP Barcode: D292331 PC Code: 121601 MEMORANDUM Subject: Revised Drinking Water Exposure Assessment for Acetochlor To: Felecia Fort, Chemical Review Manager Reregistration Branch 3 ( RB3) Special Review and Reregistration Division ( 7508C)

Donald Stubbs, Chief Herbicide Branch Registration Division ( 7505C)

From: Michael R. Barrett, Ph. D., Senior Chemist Ronald Parker, Ph. D., Senior Environmental Engineer Environmental Risk Branch V Environmental Fate and Effects Division ( 7507C)

Through: Mah Shamim, Ph. D., Chief Environmental Risk Branch V Environmental Fate and Effects Division ( 7507C)

Steven P. Bradbury, Ph. D. Director Environmental Fate and Effects Division ( 7507C)

Attached is the revised drinking water exposure assessment for acetochlor. It has been updated to include the EFED response to comments from the Acetochlor Registration Partnership ( ARP).

Emphasis in this assessment is placed on acetochlor parent because of its important in the dietary risk assessment; however, data on exposure to acetochlor degradates are also reviewed in this assessment. Note that this assessment covers only the one major field use site ( corn for grain or forage) that is currently registered; registration applications for other major field uses of acetochlor are pending; additional field uses for acetochlor will require significant additions and revisions to the exposure assessment.  2 [ This page left intentionally blank]  3

Table of Contents 1. List of Tables ...................................................................................................................... 6 2. Table of Figures .................................................................................................................. 8 3. Abbreviations.................................................................................................................... 10 4. Executive Summary .......................................................................................................... 11 4.1. Purpose.............................................................................................................. 11 4.2. Acetochlor Usage, Monitoring Locations, and the Exposure Assessment ........... 11 4.3. Time Weighted Annualized Means and 95th Percentile Calculations .................. 12 4.4. Mitigation/ Cancellation Endpoints..................................................................... 12 4.4.1. ARP Surface Water Monitoring Endpoints ............................................. 12 4.4.2. ARP Ground Water Monitoring Endpoints   PGW Study....................... 13 4.4.3. ARP Ground Water Monitoring Endpoints   SGW Study....................... 13 4.5. Exposure Summary............................................................................................ 13 5. Introduction....................................................................................................................... 14 5.1. Overview of ARP Monitoring Programs ............................................................ 15 5.1.1. Surface Water Monitoring ( SDWS)........................................................ 15 <IP_ADDRESS>. Scope of the SDWS ................................................................ 15 <IP_ADDRESS>. Site Selection for Surface Drinking Water Sites ...................... 17 5.1.2. Prospective Ground Water ( PGW) Studies ............................................. 19 5.1.3. " State Ground Water" ( SGW) Monitoring Program................................ 22 5.2. Design and Scope of Exposure Assessment........................................................ 23 5.3. Documents and Data.......................................................................................... 24 5.4. Data Gaps .......................................................................................................... 24 5.5. Uncertainties in the Drinking Water Assessment................................................ 24 5.6. Environmental Fate............................................................................................ 25 5.6.1. Degradation Pathways ............................................................................ 25 5.6.2. Soil Mobility.......................................................................................... 26 5.6.3. Dissipation Pathways ............................................................................. 26 5.7. Acetochlor Usage .............................................................................................. 26 5.7.1. Summary of Registered / Proposed Uses ................................................ 26 5.7.2. Usage Targets in the Original Acetochlor Conditional Registration ........ 27 5.7.3. Geographic Patterns of Acetochlor Use .................................................. 28 5.8. Overview of Exposure Assessment .................................................................... 30 6. Residues  Acetochlor Parent............................................................................................. 31 6.1. Data Files Used.................................................................................................. 31 6.2. Time Weighted Annualized Means and 95th Percentile Calculations .................. 32 6.2.1. ARP Weighting Methodology ................................................................ 32 6.2.2. USGS WARP Beta Model Weighting Methodology............................... 33 6.2.3. 95th Percentile Calculations .................................................................... 34 6.3. Surface Water .................................................................................................... 34 6.3.1. ARP Data............................................................................................... 34 <IP_ADDRESS>. Regulatory Action Endpoints .................................................. 34 <IP_ADDRESS>. Acute Exposure....................................................................... 37 <IP_ADDRESS>. Acute and Chronic Exposure Distribution by Population......... 39 <IP_ADDRESS>. Chronic Exposure Distribution by System............................... 41  4

6.3.2. Surface Water Factorial Analysis ........................................................... 42 6.3.3. Characterization of Exposure to Surface Water....................................... 44 6.3.4. Comparison of ARP and WARP beta Model Results.............................. 48 6.3.5. Summary Results of National Water Quality Assessment ( NAWQA) Results 48 6.3.6. USGS / EPA Pilot Reservoir Monitoring Program.................................. 50 6.4. Ground Water .................................................................................................... 51 6.4.1. PGW Leaching Summary....................................................................... 52 6.4.2. Comparison of PGW Results to the Acetochlor Regulatory Action Endpoints ............................................................................................... 52 6.4.3. SGW Summary ...................................................................................... 54 <IP_ADDRESS>. Comparison of SGW Results to Regulatory Action Endpoints. 55 <IP_ADDRESS>. SGW Acute Exposure ............................................................. 55 <IP_ADDRESS>. SGW Annual Means ............................................................... 56 <IP_ADDRESS>. Ground Water Factor Analysis ................................................ 57 7. Residues  Acetochlor Degradates ..................................................................................... 57 7.1. Surface Water .................................................................................................... 57 7.1.1. Acute Exposure Distributions by SDWS Sites ........................................ 57 7.1.2. Chronic Exposure Distributions.............................................................. 59 7.2. Ground Water .................................................................................................... 60 7.2.1. Comparison of Ground Water Degradate Monitoring results to Cancellation / Mitigation Endpoints. ...................................................... 60 7.2.2. PGW Acute Exposure by Site................................................................. 61 7.2.3. Chronic Exposure................................................................................... 65 7.3. Summary Assessment of Exposure to Acetochlor Degradates ............................ 65 8. Other Chemicals................................................................................................................ 71 9. Conclusions....................................................................................................................... 75 9.1. Parent Acetochlor .............................................................................................. 75 9.2. Acetochlor Degradates....................................................................................... 76 10. References......................................................................................................................... 78 11. Bibliography ..................................................................................................................... 79 12. Appendices ....................................................................................................................... 93 12.1. Chemical Names and Structures......................................................................... 93 12.2. Acetochlor Registration Agreement   Cancellation / Mitigation Endpoints ........ 96 12.3. Acetochlor Usage   Detailed Summary............................................................ 101 12.4. Locations of Monitoring Sites for the ARP SDWS Study................................. 102 12.5. Site Selection for ARP Monitoring Studies ...................................................... 121 12.5.1. Surface Drinking Water Site Selection ( SDWS Study) ......................... 121 12.5.2. Site Selection   SGW Study................................................................. 134 12.5.3. PGW Study Site Selection and Characterization................................... 136 12.6. Analytical Method Summary Descriptions for the ARP Monitoring Programs .144 12.7. Statistical Analyses for the ARP monitoring Studies ........................................ 147 12.7.1. SDWS 147 <IP_ADDRESS>. Pearson Product Moment Correlation Coefficients for Raw vs. Finished Water Samples ........................................................... 147 <IP_ADDRESS>. Analysis of Factors Related to Occurrence of Acetochlor.... 150  5

12.7.2. CORRELATION MATRICES ( r2) FOR FACTORS RELATED TO THE OCCURRENCE OF ACETOCHLOR IN SURFACE DRINKING WATER SUPPLIES............................................................................. 151 Time weighted means over time........................................................... 155 12.7.3. MAXIMUM OBSERVED CONCENTRATIONS ( PER YEAR) OVER TIME FOR THE PGW STUDIES. ....................................................... 156 12.7.4. TIME WEIGHTED ANNUALIZED MEANS OVER TIME FOR THE PGW STUDIES. .................................................................................. 158 12.7.5. CORRELATION MATRICES ( r2) FOR FACTORS RELATED TO ACETOCHLOR ACUTE EXPOSURE IN THE PROSPECTIVE GROUND WATER STUDIES............................................................. 160 12.8. Data Tables for the ARP Monitoring Studies Related to Mitigation Endpoints .161  6

1. LIST OF TABLES Table 1. Summary presentation of chronic exposure to parent acetochlor: Time weighted annualized mean concentrations ( ppb) in surface and ground water from the ARP monitoring program ( based on maximum TWAM values observed at each site by calendar year) along with WARP model predictions for streams and rivers. .. 14 Table 2. Number of community drinking water supply sites sampled for the parent acetochlor in each year.................................................................................. 16 Table 3. PGW sites: Selected soil and aquifer characteristics........................................ 20 Table 4. Well characteristic summary for the SGM program. Values expressed in meters below ground surface ( bgs). .......................................................................... 23 Table 5. Key data files submitted by the ARP used in this assessment.......................... 32 Table 6. Frequency of time weighted annualized mean concentrations ( ppb) for the parent acetochlor herbicide in raw and finished water drinking water. ..................... 35 Table 7. Frequency of occurrence for all instantaneous parent acetochlor concentrations ( ppb) in raw and finished water drinking water. ............................................ 36 Table 8. Ten highest raw ( untreated) water concentrations of parent acetochlor at community water system ( CWS) intake locations. ........................................ 45 Table 9. Ten highest finished ( treated) water concentrations of parent acetochlor at community water system ( CWS) outflow locations....................................... 45 Table 10. Watershed characteristics for the ten highest finished ( treated) water concentrations of parent acetochlor at community water system ( CWS) outflow locations. ...................................................................................................... 46 Table 11. Highest raw ( untreated) water concentrations of parent acetochlor at community water system ( CWS) intake locations in each state ( sorted by Max TWAM). 47 Table 12. Highest finished ( treated) water concentrations of parent acetochlor at community water system ( CWS) outflow locations in each state. .................................... 47 Table 13. Top ten highest raw water concentrations ( ppb) of parent acetochlor modeled by WARP multi compound regression model and measured by ARP at community water system ( CWS) intakes. ........................................................................ 48 Table 14. Acetochlor monitoring concentrations at NAWQA study unit locations........ 50 Table 15. Maximum acetochlor concentration values in pilot reservoir monitoring study ( Bloomquist et al., 2001)............................................................................... 51 Table 16. PGW Sites exceeding 0.1 ppb at 9 feet depth ( exceedences only occurred at 1 of the 8 sites). ................................................................................................... 52 Table 17. Concentrations of AC observed in 3 foot lysimeters from the eight prospective ground water studies. .................................................................................... 53 Table 18. Concentrations of AC observed in 9 foot lysimeters from the eight prospective ground water studies. .................................................................................... 53 Table 19. Concentrations of AC observed in Shallow ground water from the eight prospective ground water studies. ................................................................. 53 Table 20. Concentrations of AC observed in Deep ground water from the eight prospective ground water studies. .................................................................................... 54 Table 21. Concentrations of ESA observed in 9 foot lysimeters from the eight prospective ground water studies. .................................................................................... 62  7

Table 22. Concentrations of ESA observed in shallow ground water from the eight prospective ground water studies. ................................................................. 62 Table 23. Comparison of Acetochlor ESA and bromide breakthrough in 9 foot lysimeters at the eight prospective ground water monitoring sites: Normalized concentrations. 1 ..................................................................................................................... 63 Table 24. Comparison of Acetochlor ESA and bromide breakthrough ( with months after treatment) in shallow ground water at the eight prospective ground water monitoring sites: Normalized concentrations. 1 .............................................. 63 Table 25. Concentrations of acetochlor OXA observed in 9 foot lysimeters from the eight prospective ground water studies. ................................................................. 64 Table 26. . Concentrations of acetochlor OXA observed in shallow ground water from the eight prospective ground water studies.......................................................... 64 Table 27. Summary presentation of time weighted annualized mean concentrations ( ppb) for the combined residues of acetochlor ( parent + ESA and OXA degradates) in surface and ground water ( based on maximum TWAM values observed at each site by calendar year). ................................................................................... 67 Table 28. Acute Exposure to ESA. Summary presentation of acute concentrations ( ppb) for the residues of acetochlor ethanesulfonic acid in surface and ground water. .. 68 Table 29. Acute Exposure to OXA. Summary presentation of acute concentrations ( ppb) for the residues of acetochlor oxanilic acid in surface and ground water....... 69 Table 30. Chronic Exposure to ESA. Summary presentation of time weighted annualized mean concentrations ( ppb) for the residues of acetochlor ethanesulfonic acid in surface and ground water ( based on maximum TWAM values observed at each site by calendar year). ................................................................................... 69 Table 31. Chronic Exposure to OXA. Summary presentation of time weighted annualized mean concentrations ( ppb) for the residues of acetochlor oxanilic acid in surface and ground water ( based on maximum TWAM values observed at each site by calendar year). .............................................................................................. 70 Table 32. Occurrence (%) of TWAMs in Finished Drinking Water at Various Concentrations by Sampling Stratum............................................................ 71 Table 33. Summary of the Distribution of Degradate Residues for ARP SGW Analytes   2001 Data only. ............................................................................................ 74 Table 34. Summary of the Distribution of Parent Residues for ARP SGW Analytes   2001 data only. ...................................................................................................... 74 Table 35. Pearson product moment correlation coefficients for raw versus finished water parent acetochlor concentrations observed at the P 1 sites in the SDWS monitoring data set. ...................................................................................... 147 Table 36. Paired t test for raw versus finished water samples. ....................................... 149 Table 37. SGW acetochlor numeric response samples exceeding 0.1 ppb for detection of " pattern of movement".................................................................................. 161 Table 38. Acetochlor degradate samples exceeding 0.1 ppb in the state monitoring program. ....................................................................................................... 163 Table 39. Acetochlor and degradate detections in PGW studies greater than 1.0 ppb at nine foot lysimter depth consistent with three and six foot lysimeters in that cluster as defined by " pattern of movement" criteria. MAT = months after treatment. .. 178  8

2. TABLE OF FIGURES Figure 1. States involved in the Surface Water Monitoring Program for Acetochlor. .................................................................................................. 16 Figure 2. Locations of ARP surface water monitoring sites ( blue circles) in relation to maximum observed concentrations of acetochlor in surface water ( SW) by county based on available NAWQA data.............................. 17 Figure 3. States with prospective ground water ( PGW) studies for acetochlor............... 20 Figure 4. States involved in the " SGW" ground water monitoring program for acetochlor and locations of wells. Source: De Guzman et al. ( 2005). ............ 23 Figure 5. Corn production intensity ( 2002 Census of Agriculture data) and general locations of drinking water intakes sampled in the ARP's SDWS monitoring program ( white = no reported corn acreage, green = lowest intensity category, red = highest corn intensity category). ................. 29 Figure 6. USGS estimated acetochlor use for 1997. ...................................................... 30 Figure 7. Cumulative frequency distribution for all acetochlor observations for each study. PGW data represented include separate distributions for the 9 foot depth lysimeter data and the shallow groundwater wells for each study. ................................................................................................... 37 Figure 8. Cumulative frequency distribution for acetochlor acute exposure in all ARP studies, based on the maximum observed concentration at each site. PGW data maximum exposures are provided for each lysimeter and each depth. ............................................................................................ 38 Figure 9. Distribution of acute exposure, based on the maximum observed concentration at each site. PGW data maximum exposures are provided for each lysimeter and each depth................................................................. 38 Figure 10. Acute acetochlor exposure distribution by population served for raw ( A) and finished ( B) water samples. ............................................................. 39 Figure 11. Chronic exposure to parent acetochlor in raw surface drinking water ( SDWS) using the average time weighted mean at each site......................... 40 Figure 12. Cumulative frequency distribution for parent acetochlor chronic exposure, based on the highest time weighted annual mean at each site. PGW chronic exposures are the maximum TWAM for each cluster at 9 foot depth for lysimeters and shallow monitoring wells for ground water............................................................................................................ 41 Figure 13. Percent reduction in acetochlor from pre treatment ( raw) to post treatment ( finished) sample. ......................................................................... 43 Figure 14. Distribution of maximum acetochlor concentrations observed at each site in the state ground water program. ......................................................... 56 Figure 15. Distribution of all time weighted annualized means for sites in the state ground water ( SGW) monitoring program............................................ 56 Figure 16. Maximum observed concentrations ( acute) of the two acetochlor degradates and Total Combined residues ( parent + ESA + OXA) in raw ( dashed) and finished ( solid) surface drinking water samples. ...................... 58 Figure 17. Percent reduction of acetochlor degradates in surface drinking water supplies........................................................................................................ 59  9

Figure 18. Chronic exposure distribution for acetochlor degradates ( ESA and OXA) in surface drinking water supplies using the maximum timeweighted annualized mean for each site. Summary of USGS monitoring results for acetochlor degradates ................................................ 60 Figure 19. Chronic exposure distribution for acetochlor degradates ( ESA and OXA) in the state ground water monitoring program using the maximum time weighted annualized mean for each site. .............................. 65 Figure 20. Box plot of annualized mean concentrations ( AMCs) of parent herbicides in finished drinking water from the SDWS study ( Hackett et al., 2005)...................................................................................................... 72 Figure 21. Co occurrence of ( a) sulfonic acid ( ESA) degradate residues and ( b) oxanilic acid ( OXA) degradate residues for acetochlor, alachlor, and metolachlor in the SGW study. Values reflect the number of SGW wells with observed residues ( Minimum detection limit was 0.2 ppb for the sulfonic acid degradates and 0.1 ppb for the oxanilic acid degradates). No ESA soil degradate residues were observed in 49 of the 182 wells and no OXA soil degradate residues were observed in 110 of the 182 wells. Source: de Guzman et al. ( 2005). ................................ 73  10

3. ABBREVIATIONS AC Acetochlor Ac_ ESA Acetochlor ethanesulfonic acid Ac_ OXA Acetochlor oxanilic acid ARP Acetochlor Registration Partnership BEAD/ OPP Biological and Economic Analysis Division of OPP CDF Cumulative frequency distribution CWS Community water system ESA Acetochlor ethanesulfonic acid EFED/ OPP Environmental Fate and Effects Division of OPP FIFRA Federal Insecticide, Fungicide and Rodenticide Act FQPA Food Quality Protection Act of 1996 GAC Granular activated carbon treatment GWM Ground Water Monitoring ( ARP Reference)

NAWQA National Water Quality Assessment Program of USGS NCFAP National Center for Food and Agricultural Policy ( pesticide usage monitoring program)

OPP Office of Pesticide Programs of USEPA OXA Acetochlor oxanilic acid PAC Powdered activated carbon treatment PGW Prospective ground water study ( monitoring program)

SDWS Surface drinking water supply ( monitoring program)

SGW State ground water ( monitoring program)

SWM Surface water monitoring ( ARP Reference)

TWAM Time weighted annualized mean USEPA United States Environmental Protection Agency USGS United States Geological Survey WARP Watershed Regression for Pesticides   USGS Pesticide Concentration Model  11

4. EXECUTIVE SUMMARY 4.1. Purpose In March of 1994, USEPA and ARP entered into a conditional registration agreement ( USEPA, 1994) for the chemical acetochlor ( 2 chloro N( ethoxymethyl) N( 2 ethyl 6 methylphenyl) acetamide), the active ingredient in a number of herbicides used on corn ( Zea maize) crops to control annual grasses and certain broadleaf weeds ( Hackett et al., 2004). Under the agreement, USEPA required ARP to conduct several acetochlor monitoring programs and specified a number of cancellation or mitigation endpoints aimed at protecting the environment and limiting potential risks to human health ( USEPA, 1994). The purpose of this assessment is, therefore, to estimate exposure to acetochlor ( in support of a human health dietary risk assessment) and to summarize the status of mitigation / cancellation endpoints encapsulated in the acetochlor conditional registration agreement.

The primary source data for both this exposure assessment and evaluation of the registration agreement compliance are the three major acetochlor data sets generated by the Acetochlor Registration Partnership ( ARP). Two data sets are ground water source based and include the " State" Ground Water ( SGW) monitoring program and the Prospective Ground Water studies ( PGW) and one is surface water source based referred to as Surface Drinking Water Supplies ( SDWS) monitoring program. The ARP provided a fourth acetochlor data set that consists of incident investigation of ground water primarily around pesticide dealer and storage facilities. The incident data are only indirectly related to impacts from registered uses of acetochlor therefore these data have not been explicitly included in the direct exposure assessment. When relevant to the exposure assessment, additional publicly available water monitoring data for acetochlor are discussed in this document.

4.2. Acetochlor Usage, Monitoring Locations, and the Exposure Assessment Acetochlor is now registered in 42 states as well as the District of Columbia ( Hackett et al., 2004). It is also used in corn growing areas of several countries including China, Europe, and Argentina. Presently, roughly 80% of the total use of acetochlor in the United States occurs in the Midwest. Detailed countylevel sales maps for acetochlor from 1994 to 2003 are provided in Appendix section 12.3 Acetochlor Usage   Detailed Summary. These sales data have been provided separately by members of the acetochlor registration partnership ( ARP) as confidential information and cannot be shared with unauthorized individuals.

The sales data are presented as a surrogate for the location of acetochlor usage. Pesticide sales data may not be a consistent estimator of usage in any particular watershed because usage may not occur near the location of purchase. The maps of acetochlor sales data and surface water monitoring locations show that the set of monitoring sites selected does not coincide well with all of the areas where high sales have been reported. A lower rate of utilization of surface water sources by drinking water facilities and lower overall numbers of CWS' utilizing surface water in some high acetochlor use regions appears to be a factor in the paucity of sites in these regions that were eventually selected for monitoring in the SDWS. For these reasons of facility location and sampling design, it is possible therefore that the drinking water intake locations that were monitored do not fully represent the sites where highest concentrations in then current or potential surface source drinking water occurred.

The lack of monitoring in some of the high acetochlor use areas is especially problematic for broader interpretation of the SDWS monitoring results where the lack of sampling of raw ( pre facility treatment)  12

water at most locations makes it difficult to isolate the effects of site specific usage, vulnerability factors, and water treatment processes on the observed residue levels.

4.3. Time Weighted Annualized Means and 95th Percentile Calculations Time weighted annualized means ( TWAMs) were calculated for each site in the three major monitoring programs ( SDWS, SGW, and PGW). Two separate weighting methods were implemented using a custom built TWAM computer program to verify the TWAMs computed by the ARP. The weighting method used by the ARP ( described later in this report) was cross checked with a slightly different method implemented in the WARP beta model developed by the USGS ( USGS 2004). Both weighting methods assign a weight to each discrete sample observation based upon the fraction of the time during a year that each sample represents. Weighted concentrations were then summed to provide an annualized mean.

4.4. Mitigation/ Cancellation Endpoints In addition to providing a drinking water exposure assessment for application to a dietary risk assessment for acetochlor, this document also addresses the endpoints or triggers for regulatory action incorporated into the acetochlor registration agreement are provided in Appendix 1 ( see USEPA, 1994; for a full copy of the agreement). These endpoints are directly tied to each of the major monitoring programs required of the ARP in the Acetochlor Registration Agreement; the reader may need to refer to the Appendix for a complete understanding of the reasons for the way in which these endpoints are discussed in this document. The triggers varied between monitoring programs, the following is a comparison of the results to the triggers for each program. Discussion of both parent and degradate occurrence and their relation to the triggers is separately provided in this document, however, only parent residues are clearly classified as residues of concern for which the triggers for mitigation measures in the Registration Agreement apply. At the time acetochlor was registered monitoring data for the major degradation products of acetochlor was virtually non existent and specific toxicity studies had not yet been conducted. Consequently, no informed determination could have been made at the time of the registration agreement about whether any significant risk could potentially arise from exposure to acetochlor ESA or OXA in water.

4.4.1. ARP Surface Water Monitoring Endpoints Acetochlor was detected above 8.0 ppb trigger for individual detections in 2 samples in the surface drinking water supply ( SDWS) monitoring program. Two finished ( treated water) samples were detected above 8.0 ppb, however the twelve month time weighted annualized mean did not exceed the 2.0 ppb regulatory action trigger for these or any of the other water supply systems included in the SDWS. No raw ( untreated) concentrations were detected above 8.0 ppb. For both raw and finished surface drinking water, roughly 99% of the time weighted  13

annualized means were below 0.5 ppb. Maximum acetochlor instantaneous concentrations, 95th percentiles, and time weighted annualized means were observed in Illinois.

4.4.2. ARP Ground Water Monitoring Endpoints   PGW Study For the PGW, the triggers for regulatory action were tied to both soil pore water ( lysimeter) and ground water detections. Acetochlor was detected above 0.1 ppb at only one site in nine foot lysimeters in the prospective ground water ( PGW) studies. The maximum concentration of acetochlor in soil pore water was 3.2 ppb observed in the nine foot lysimeters in Iowa. The maximum residue detected in ground water wells was 0.06 ppb observed in Iowa. The acetochlor degradates ethanesulfonic acid ( Ac_ ESA) and oxanilic acid ( Ac_ OXA) were generally detected more frequently than parent acetochlor. In the PGW studies for example, ESA demonstrated a pattern of movement as defined by concentrations greater than or equal to 1.0 ppb at three, six, and nine foot lysimeter depths. In 293 instances ESA was detected above 1.0 ppb at all three lysimeter depths. These exceedences occurred in seven out of the eight states.

4.4.3. ARP Ground Water Monitoring Endpoints   SGW Study For the SGW, the trigger for regulatory action was a pattern of detections in 20 or more wells at or above 0.10 ppb " followed by two subsequent detections of at least 0.10 ppb in monthly sampling of each of those wells, conducted over a period of six months" ( this language did not anticipate the impact of a large number of missing samples as in the reduced sample collection regime resulting in a maximum of four samples per well per year being collected during the last two years of the monitoring program). See Appendix 12.2 for details. Parent acetochlor exhibited a pattern of detection in the required number of samples in seven wells, or thirteen wells short of the trigger for regulatory action based on SGW results. Residues of acetochlor degradates were much more widespread in the SGW wells, but these compounds have not been deemed residues of concern.

Aproximately 10% of the site maximum instantaneous concentrations in the SGW wells were above 0.5 ppb and 15% of all time weighted annualized means were greater than or equal to 0.03 ppb ( i. e., the minimum detection limit). If the degradates are included in an exposure calculation, then the number of wells with a pattern of detections increases to approximately 36 ( requires a modification of the " pattern of movement" definition to 2 of 3 consecutive detections greater than 0.1 ppb ( 0.2 ppb for ESA since the detection limit was 0.2 ppb) since sampling of degradates never occurred more frequently than a quarterly basis.

4.5. Exposure Summary Acetochlor parent residue exposure is generally higher and more widespread through surface water sources than ground water ( Table 1). Available data indicate that water treatment involving the use of activated carbon may reduce exposure by close to 50% on average; however limitations on the data preclude generalizing this as a predictable effect of water treatment. In particular, no data are available that match the same water in raw and finished water, the ARP SDWS dataset did not measure samples in intake water from those systems using other types of water treatment, and most of the highest concentrations observed in the SDWS study occurred in  14

finished ( not raw) samples. EFED is aware that while specific matching of raw and finished water is not available for the ARP study, other studies of treatment effects are available such as that by Gustafson et al. ( 2003).

Table 1. Summary presentation of chronic exposure to parent acetochlor: Timeweighted annualized mean concentrations ( ppb) in surface and ground water from the ARP monitoring program ( based on maximum TWAM values observed at each site by calendar year) along with WARP model predictions for streams and rivers.

Study N Maximum 95th Percentile Median Surface Water  SDWS raw 44 0.591 0.355 0.042 Surface Water  SDWS finished 189 1.428 0.347 0.032 Surface Water  WARP model ( raw) a 470 0.812 0.435 0.042 Ground Water ( shallow)  PGW site averages 8 < 0.03 < 0.03 < 0.03 Ground Water ( shallow)  PGW cluster maximums 8 < 0.03 < 0.03 < 0.03 Ground Water  SGW 182 0.520 0.039 < 0.03 a Includes TWAMs calculated by the WARP model. The WARP results are provided for comparison to the ARP monitoring results and include WARP results only for states where ARP also had surface water monitoring stations. N = total number of sites included in the statistics.

Should a toxicological concern arise from exposure anywhere near these levels ( up to 3x the levels reported in Table 1), a refined exposure assessment can be done adjusting the ARP exposure values for any disparity between usage intensity at the ARP monitoring sites and other watersheds with surface water serving as drinking water sources with higher use intensities. A requirement for this would be acquisition from the ARP or independent calculation by EPA of acetochlor usage by watershed based on the overlap of county and watershed boundaries ( the best available data representing acetochlor spatially have all been reported at the county level).

Finally, the results of this monitoring analysis only apply to acetochlor use on field corn ( significant new field uses are currently under review by EPA).

5. INTRODUCTION Pesticide substances in the United States are regulated under the Federal Insecticide, Fungicide and Rodenticide Act ( FIFRA), later amended by the Food Quality Protection Act of 1996 ( FQPA). Under FIFRA, any pesticide, be it a single active ingredient or a mixture, must be registered for use as a pesticide before a person may distribute or sell the product. Pesticides are also regulated at the state level ( usually by U. S. Department of Agriculture); however, state regulations must be at least as stringent as federal regulations. In order to register a pesticide in  15

the US, the USEPA must ensure that the pesticide, when used according to the product label, will not pose an unreasonable risk to human health or the environment. Under FQPA, regulators must also consider threats to human health through food residues and via pesticides in drinking water. The latter requirement has created a need to monitor and estimate pesticides in drinking water supplies, including both surface water and ground water sources.

In March of 1994, USEPA and ARP entered into a conditional registration agreement ( USEPA, 1994) for the chemical acetochlor ( 2 chloro N( ethoxymethyl) N( 2 ethyl 6 methylphenyl) acetamide), the active ingredient in a number of herbicides used on corn ( Zea maize) crops to control annual grasses and certain broadleaf weeds ( Hackett et al., 2004). Under the agreement, USEPA required ARP to conduct several acetochlor monitoring programs and specified a number of cancellation or mitigation endpoints aimed at limiting potential risks to human health and endangered species 5.1. Overview of ARP Monitoring Programs As part of the conditional registration agreement, the USEPA mandated ARP to develop an " early warning" detection system that would alert health officials if acetochlor is found migrating toward surface or ground water resources or may have the potential to migrate to receiving waters. This early warning system consists of rigorous surface and ground water monitoring programs, specifically: ( 1) Surface Drinking Water Supplies ( SDWS) as measured at water supply intakes for roughly 175 sites, ( 2) Prospective Ground Water ( PGW) studies at eight sites in eight states that are geographically diverse and generally representative of U. S. corn production regions, and ( 3) State Ground Water ( SGW) studies that included monitoring approximately 175 ground water wells located near treated cornfields. Appendices 0, 12.5, and 12.6 provide further details on the monitoring locations, site selection procedures and site descriptions, and the analytical methods used in the ARP programs.

5.1.1. Surface Water Monitoring ( SDWS)

<IP_ADDRESS>. Scope of the SDWS The surface drinking water supply ( SDWS) program is intended to detect the presence of acetochlor or any of its degradates of toxicological concern in surface water bodies that may be used for community drinking water supplies. The program is funded by ARP and is focused on states that were anticipated to be major use areas ( Figure 1). Specific details regarding the program are provided in Hackett et al. ( 2004). States involved in the surface water monitoring program are shown in Figure 1. In general, " finished" ( or treated) water samples were collected from approximately 175 sampling stations each year at biweekly to monthly intervals ( roughly 14 samples per year), although some sites were dropped and replaced by others in some years resulting in a total of 189 individual stations sampled over the seven year period ( Table 2). " Raw" or untreated samples were also collected for a total of 44 stations and ranged from 26 to 38 individual stations per year ( Table 2). Similar to finished water samples, some stations were dropped and others added throughout the seven year monitoring period. Specific details can be found in ARP annual reports as well as Hacket et al. ( 2005). Under the conditional registration agreement ( USEPA 1994), the need for monitoring is reassessed every five years. Concentrations of acetochlor ( and, potentially, acetochlor degradates, which were monitored for  16

only from 1999 to 2001) from drinking water intakes are then compared to target levels to determine if mitigation or cancellation actions are required.

Figure 1. States involved in the Surface Water Monitoring Program for Acetochlor.

Table 2. Number of community drinking water supply sites sampled for the parent acetochlor in each year.

YEAR # Raw Water Sites # Finished Water Sites 1995 26 175 1996 32 175 1997 35 175 1998 37 175 1999 38 175 2000 33 156 2001 37 152 TOTAL 44 a 189b a Total number of individual sites sampled. Raw water ( untreated surface water) samples were collected from all community water systems ( CWSs) that use granular activated carbon ( GAC), and from several systems that use powdered activated carbon ( PAC) ( Hacket et al. 2005). b Total number of individual sites sampled. Some sites were added in subsequent years while others were dropped. " The total number of CWSs was kept at 175 for the first five years with fewer than three sites requiring replacement in any year. Sites were always replaced by CWSs from the same or a higher vulnerability stratum. Several CWSs chose not to continue when the monitoring was extended for a final two years, dropping the number of sites to 156 in 2000, and to 152 in 2001" ( Hacket et al, 2005).  17

Figure 1 shows the locations of ARP surface water monitoring locations overlaid on the maximum concentration observed by county based on NAWQA data. The blue circles represent locations of community water supply intakes where ARP sampled finished ( treated) water and in some locations raw water samples were additionally sampled.

Figure 2. Locations of ARP surface water monitoring sites ( blue circles) in relation to maximum observed concentrations of acetochlor in surface water ( SW) by county based on available NAWQA data.

Figure 2 shows the locations of ARP surface water monitoring locations overlaid on the maximum concentration observed by county based on NAWQA data. The blue circles represent locations of community water supply intakes where ARP sampled finished ( treated) water and in some locations raw water samples were additionally sampled.

<IP_ADDRESS>. Site Selection for Surface Drinking Water Sites A particularly important issue in the assessment of exposure to parent acetochlor is how well the ARP SDWS study assesses the most vulnerable watersheds to acetochlor exposure ( parent exposure levels in ground water sources were generally significantly lower). Included here is a summary of the SDWS site selection procedures, and, additionally, an excerpt from the ARP report describing the SDWS site selection process in more detail is provided in Appendix 12.5.

A site selection process was conducted to identify 175 CWSs in 12 states. Data regarding population and CWS source( s) were collected, and watershed areas and corn intensities were determined. Each of the 175 systems was visited, inspected, and data confirmed. Watersheds for the 175 systems were mapped. The selected CWSs represent a broad spectrum based on geographic diversity, general size and corn intensity of the watersheds. The data for the selected  18

systems demonstrate the extensive diversity of the ARP surface water monitoring program. The watersheds are representative of the key acetochlor use states, with a few extending into numerous states not included in the program. The CWSs are supplied by surface water from a variety of sources including small rivers and lakes, larger rivers and lakes, and reservoirs, and employ a wide variety of treatment methods. The selected watersheds span a large range of watershed area, and serve a large range of populations.

A total of 175 CWSs in nine mid western and three mid Atlantic states were selected for the program. The selection process was designed to include a wide array of CWSs with watersheds in areas of corn production, with an emphasis on including worst case watersheds i. e., smaller watersheds ( not on the Great Lakes and Continental Rivers) in areas of high corn production. These watersheds are expected to have higher concentrations of acetochlor after runoff events than larger watersheds which drain areas of both high and low corn production, because dilution would be greater for CWSs taking water from the Great Lakes and Continental Rivers. Data were collected to characterize each community water system included in the program.

The steps for the CWS selection and characterization process are summarized below:

1) Identification of all public CWSs that use surface water in the following 12 states: Illinois, Indiana, Iowa, Minnesota, Nebraska, Kansas, Wisconsin, Ohio, Missouri, Pennsylvania, Maryland, and Delaware.

2) Identification of all CWSs that belong to the target population.

Target Population  All CWSs in the 12 states that:

  use only surface water, or can discretely sample surface water,   are willing to cooperate and   have a corn intensity ( for smaller watersheds that do not have an intake on a Great Lake or Continental River) greater than or equal to 5%, where corn intensity is the ratio of acreage of harvested corn to total acreage in the upstream watershed.

3) Separation of the target population of CWSs into disjoint ( non overlapping) strata based on the size of the watershed, the corn intensity ( for smaller watersheds), and State that the system is in:   State   size of watershed ( three major subdivisions) o Great Lakes o Continental Rivers ( Missouri, Mississippi, Ohio Rivers) o Smaller Rivers and Lakes   corn intensity (% corn planted in total area of watershed) ( three major subdivisions) o 5 10% CI o 11 20% CI o > 20% CI  19

4) Determination of the number of CWSs to be selected from each stratum. The focus was on strata containing CWS watersheds which are expected to have higher levels of acetochlor after runoff events, based on the size of the watershed and its corn intensity. A higher percentage of CWSs from these strata were chosen.

5) Random selection ( using random number generation) of the appropriate number of CWSs from each stratum. All CWSs meeting the target population criteria were selected from the identified strata ( for example, the > 20% corn intensity, smaller watershed strata). A total of 175 CWSs were required for the study.

6) Collection of information for each selected CWS regarding intake location, sources of water, treatment, customer information, point of finished water sampling, soil types, and corn intensity of the watershed( s) for that system.

7) Removal of systems that did not meet target population criteria based on additional data collected. Systems were replaced in the same stratum and state, if possible, by additional random selection from the stratum. If there were no systems available in the same stratum, then a system was randomly selected from another stratum with available CWSs.

8) Generation of maps of watersheds for each CWS. Data entry into a Geographical Information System ( GIS).

The highest percentage of CWSs, 100% of the available CWSs, was selected from the > 20% corn intensity strata, 66% were selected from the 11 20% corn intensity strata, 49% from the 5 10% corn intensity strata, 43% from the Continental River strata, and 14% from the Great Lakes strata. Almost 50% of the sites were selected from smaller watersheds with > 20% corn intensity, the watersheds expected to have the highest concentrations of acetochlor after runoff events. The focus on more vulnerable watersheds with higher corn intensity combined with the diversity of watersheds selected for this study will allow us to obtain both a worst case and representative evaluation of the impact of acetochlor and other corn herbicide usage on surface drinking water in significant corn growing areas of the United States.

5.1.2. Prospective Ground Water ( PGW) Studies ARP was also required to conduct eight Prospective Ground Water ( PGW) studies according to the protocol approved for other herbicides in order to determine the potential for pesticide transport, or a " pattern of movement". Specific details regarding the program are provided by Newcombe et al. ( 2005). In general, sites were geographically located based on representative product label uses, or " in accordance with widespread and commonly recognized practice, including vulnerable and typical use situations," as outlined in the registration agreement. Sites were required to be located on a wide variety of soil textures as per the product label, and an effort was made to include a broad geographical representation. Test sites were located in the following states: Wisconsin, Ohio, Minnesota, Nebraska, Iowa, Indiana, Pennsylvania, and Delaware ( Figure 3). Specifics of the study design are provided in Table 4. Newcombe et al., 2005 cites that these areas corresponded to areas of significant acetochlor use. Further details  20

regarding the geographic distribution of acetochlor can be found in the " Acetochlor Usage" section of this report.

Figure 3. States with prospective ground water ( PGW) studies for acetochlor.

Table 3. PGW sites: Selected soil and aquifer characteristics.

PGW Study Location NRCS Soil Series;

On site surface soil % O. M. & pH Subsoil Textures1 Avg. Hydraulic Conductivity2 ( mm/ hr) Aquifer soil textures determined1 Depth to ground water 3 ( m) Pore water velocity ( m/ day) 4 Wisconsin Richford loamy sand OM = 1.6% pH = 6.4 Loamy Sand Sand Sandy loam 0 1.2 m 1.2 2.4 m 2.4 3.6 m 3.6 4.8 m > 4.8 m 177 358 810 1482 776 Loamy sand Sandy loam Sand 7.6 10 1.9 x 10 3 Ohio Genessee silt loam Fox silt loam OM = 2.9% pH = 7.7 Clay loam Loam Sandy loam 0 1.2 m 1.2 2.4 m 2.4 3.6 m 3.6 4.8 m > 4.8 m 293 153 NA NA NA Sandy loam Loamy sand 0.6 5.2 0.8 x 10 1 Minnesota Estherville sandy loam OM = 3.5% pH = 6.3 Sandy loam Loamy sand Sand 0 1.2 m 1.2 2.4 m 2.4 3.6 m 3.6 4.8 m > 4.8 m 180 331 NA NA NA Sand Loamy sand Sandy loam 4.8 6.4 0.4 x 10 1 Nebraska Kenesaw silt loam Coly Kenesaw silt loam OM = 1.8% pH = 5.7 Loam Silt loam 0 1.2 m 1.2 2.4 m 2.4 3.6 m 3.6 4.8 m > 4.8 m 75 45 28 18 84 Silt loam Loam Sandy loam 7.0 9.7 0.4 x 10 2 Iowa Marshall silty Silty 0 1.2 m 207 Sand 1.2 8.5 0.9 x 10 1  21

Table 3. PGW sites: Selected soil and aquifer characteristics.

PGW Study Location NRCS Soil Series;

On site surface soil % O. M. & pH Subsoil Textures1 Avg. Hydraulic Conductivity2 ( mm/ hr) Aquifer soil textures determined1 Depth to ground water 3 ( m) Pore water velocity ( m/ day) 4 clay loam Minden silty clay loam OM = 3.9% pH = 5.6 clay loam Silt loam 1.2 2.4 m 2.4 3.6 m 3.6 4.8 m > 4.8 m 84 172 87 1.0 Silt loam Loam Indiana Door loam Lydick loam OM = 3.0% pH = 6.7 Sandy clay loam Sandy loam Sand 0 1.2 m 1.2 2.4 m 2.4 3.6 m 3.6 4.8 m > 4.8 m 64 190 244 742 978 Sand 7 9.1 0.6 x 10 1 Pennsylvania Clarksburg silt loam Duffield silt loam OM = 2.7% pH = 6.3 Loam Sandy loam 0 1.2 m 1.2 2.4 m 2.4 3.6 m 3.6 4.8 m > 4.8 m NA 382 138 95 19 NA Sandy loam Loam 1.8  7.3 0.4 x 10 1 Delaware Sassafras sandy loam OM = 2.9% pH = 5.8 Sandy loam Loamy sand Sand 0 1.2 m 1.2 2.4 m 2.4 3.6 m 3.6 4.8 m > 4.8 m 30 86 30 129 NA Sand Sandy loam Loamy sand 3.3 6.1 0.6 x 10 2 1 Soil texture determined by 3 fraction analysis (% sand, silt, and clay) 2 Vertical saturated hydraulic conductivity determined by constant head permeability method 3 Depth to ground water listed is below ground surface, and the minimum and maximum values are of all measurements made in the test plot piezometers during the course of the study 4 Average value determined during the course of the study This table is modified from a more extended version by the ARP found in Newcombe et al. ( 2005).  22

5.1.3. " State Ground Water" ( SGW) Monitoring Program In addition to PGW studies, the ARP, as part of the " State Ground Water" Monitoring program, was required to monitor 25 ground water wells in each of the expected seven high use states ( WI, IL, IA, MN, IN, NE, KS) in Figure 4 all located adjacent to fields with contractually guaranteed use of acetochlor and located in areas representing a variety of use conditions based on soil characteristics, local hydrogeology, and climatic conditions. The monitoring data serve as an early indication that pesticide residues may be reaching ground water. Risk managers can then use this information to assess the potential threat to humans. Specific details regarding the program are provided in de Guzman et al., ( 2004), but a brief description of the program design follows here.

The SGW study was set up through the establishment of a network of 175 monitoring sites in regions of high corn production in each of the seven states chosen for this study. A site selection scheme for the SGW wells was set up using corn production data, soils database information, and consultations with state regulatory officials to obtain a set of wells representing a range of soil textures typical of corn agriculture in those regions. Soil classification was not a direct component of the site selection procedure. Soil survey data were collected for each site and are available in the documents submitted to the EPA by the ARP. The soil classification data are not available in a readily summarized form and are not presented here, but details can be found in the ARP Site Selection submission ( MRID 43899601).

In general, ground water monitoring wells were to be located down gradient of acetochlor use areas at a distance agreed upon by the states. States participating in the SGW are shown in Figure 4. Where technically feasible, ARP is required to provide assistance to water system operators in monitoring for acetochlor residues at drinking water wells.  23

Figure 4. States involved in the " SGW" ground water monitoring program for acetochlor and locations of wells. Source: De Guzman et al. ( 2005).

Table 4. Well characteristic summary for the SGW program. Values expressed in meters below ground surface ( bgs).

State Buffer distance2 T Screen length Depth to water Screening Depth Interval3  meters Minimums Maximums Mean DTW   DTS Illinois 9.1  45.7 4.6 3  22.8 2.7 4.6 7.3 9.1 2.5 Indiana 15.2  45.7 3 < 7.6  22.8 5.8 23.5 8.8 26.5 3.1 Iowa 9.1  45.7 4.6 1.5  15.2 1.5 7.0 6.1 11.6 6.5 Kansas 9.1  30.5 3 4.6  22.8 4.0 22.9 7.0 25.9 1.2 Minnesota 15.2  45.7 3 7.6  22.8 4.9 21.3 7.9 24.4 3.4 Nebraska 15.2  45.7 4.6 3  22.8 4.6 20.4 9.1 25.0 1.8 Wisconsin 9.1 3 < 7.6  15.2 1.2 14.9 4.3 18.0 1.4 1 Table is adapted from Newcombe et al. ( 2005); state average difference between average screening interval ( DTS) and average depth to ground water ( DTW) has been added. 2 Distance between wellhead and nearest point of the acetochlor treatment area. 3 Screening depth interval data were extracted from master ground water database submitted by ARP. The screening interval represents the positions of the top and bottom of the screen measured during installation. The first pair is the minimum top of screen and the maximum top of screen, the second pair is the minimum bottom of screen and the maximum bottom of screen.

5.2. Design and Scope of Exposure Assessment The purpose of this assessment is to evaluate exposure to acetochlor by reviewing results from the three major acetochlor data sets generated by the ARP as described above ( two ground water source based and one surface water source based) in the context of other available monitoring data. The assessment focuses on the status of acetochlor in ground and surface water with respect to specific endpoints triggering mandatory requirements for implementation of mitigation measures or cancellation of acetochlor uses ( detailed in the conditional registration agreement, USEPA 1994) and evaluation of the impact of acetochlor on drinking water sources in support of human health risk assessments. ARP provided a fourth acetochlor data set that consists of incident investigation of ground water primarily around pesticide dealer and storage facilities. The incident data are only indirectly related to impacts from registered uses of acetochlor therefore these data have not been included in the direct exposure assessment.

Primary focus of this exposure assessment is on the parent acetochlor, with secondary emphasis on acetochlor degradates in water   widespread occurrence. This section deals with exposure to acetochlor parent residues in water and serves as the basis of the current Drinking Water  24

Assessment. Conclusions made about exposure to the parent acetochlor apply to the parent chemical only. Although this assessment is focused on the parent acetochlor, exposure levels to degradates can be quite significant and has been characterized with secondary emphasis. Some of the ARP monitoring studies also contain data on the occurrence of other chloroacetanilide herbicides ( alachlor and metolachlor and / or other corn herbicides ( atrazine)   a limited discussion on these data and their utility for other exposure assessments will also follow this section. A portion of these data were reviewed for a previous drinking water assessment for another pesticide  atrazine ( Environmental Fate and Effects Division, OPP, EPA, 2001).

5.3. Documents and Data This assessment is based primarily on extensive surface water and ground water monitoring programs submitted in support of acetochlor registration and intended to provide a reasonably comprehensive portrait of exposure levels possible in ground and surface water. Discussion of the most relevant outside monitoring programs for acetochlor, most notably the NAWQA monitoring program by the USGS, is also provided. Since there were many hundreds of interim documents and reports submitted, only selected references ( but including all final reports) are included in the bibliography.

5.4. Data Gaps The ARP monitoring program was designed to assess exposure or exposure potential to acetochlor in the context of an evaluation of the compliance of the ongoing usage of acetochlor with exposure limits, and other regulatory requirements contained in the Acetochlor Registration Agreement ( USEPA, 1994).

5.5. Uncertainties in the Drinking Water Assessment A number of uncertainties must be recognized when interpreting this exposure assessment. These include the following:

  The surface drinking water supply ( SDWS) and state ground water ( SGW) monitoring programs were designed to focus on areas of high acetochlor use. The monitoring does not cover the entire geographic distribution of acetochlor use. Geographic analysis of the SDWS site locations and acetochlor use patterns seems to indicate that even a number of high acetochlor use areas were not monitored. Conclusions drawn in this report apply only to those areas monitored by the ARP and it may not be possible to generalize to all acetochlor usage areas. This is especially true for the SDWS where the lack of sampling of raw ( pre facility treatment) water at most locations makes it difficult to isolate the effects of site specific usage and vulnerability factors and water treatment processes on the observed residue levels.

  County level sales data submitted separately by members of the ARP from 1994  2003 is arguably some of the most extensive data available as a close approximation of acetochlor usage across the US. As such, it has been incorporated in this exposure assessment as a surrogate for acetochlor use in the mapping and statistical analyses. It is assumed that acetochlor sold in an individual county is, in general, also applied in the  25

same county and in the same watershed. However, the exposure characterization recognizes that inter county as well as inter watershed transfer of acetochlor does occur in some cases.

  Acute exposure in this risk assessment is defined as the overall maximum observed concentration at a site. The actual peak concentration, however, may have occurred between sampling times. Thus, the maximum observed concentrations reported in this study may underestimate the true maximum acute exposure.

5.6. Environmental Fate 5.6.1. Degradation Pathways Acetochlor persistence in a confined soil system appears to increase with coarser soil texture and increased application rate. The half lives in aerobic soils for the 3, 4.5, 10.5, 41, and 50 ppm application rates were 8 12, 14, 110 245, 55, and 300 days, respectively. However, the most representative aerobic soil half life is 8 14 days determined in the Monsanto study conducted in Ray silt loam ( 1.2 X OM), Drummer silty clay loam ( 3.4 % OM), and Spinks sandy loam ( 2.4 % OM) soils treated with 3 ppm (  2X label rate) of acetochlor. The 8 14 day half life represents the labeled application rate and the soils to be treated with acetochlor. The longer half lives were found only at exaggerated application rates ( 7.5 36X) labeled rates to coarse, low organic matter soils. The aerobic soil metabolism degradates oxanilic acid ( oxamic acid), sulfonic acid, and thioacetic acid sulfoxide degradates of acetochlor. These degradates are rearrangement products of one amino moiety of the acetochlor molecule.

Relevant to this discussion are label restrictions to which the ARP has previously implemented on all product labels to prohibit acetochlor use in certain areas with ground water that is highly vulnerable to contamination by pesticides such as acetochlor. Acetochlor product use is restricted on coarse textured, low organic matter soils where groundwater is within 30 feet of the surface.

The following language is included with all acetochlor product labels:

Acetochlor products may not be applied to the following soils, if depth to groundwater is 30 feet or less:

  Sands with less than 3% organic matter.   Loamy sands with less than 2% organic matter.   Sandy loams with less than 1% organic matter.

Acetochlor herbicides may be applied to the above soils if depth to groundwater is more than 30 feet.  26

5.6.2. Soil Mobility Parent acetochlor has a reported water solubility of 223 mg/ l and Kd values of 0.4 2.7 ml/ g in various soils texturally classified as sandy loam, loamy sand, silt loam and silty clay soils. Acetochlor also leached through soil columns.

The degradates are expected to have even higher mobility based on structural features. Kd values for the degradates were 0.15 to 0.97 for ESA, 0.13 to 0.86 for OXA, and 0.10 to 0.90 for a third degradate ( acetochlor thioacetic acid sulphoxide) not included in this exposure assessment because of the low levels detected in environmental samples in previous studies. Koc values were 21 to 68 in 9 of 10 soils tested and 430 in the other soil ( median = 57) for ESA and 17   124 ( median = 45) for OXA.

The results of studies submitted to support Subdivision N requirements for registration appear to be inconsistent with the laboratory data with respect to mobility. In at least one study, leaching of oxamic acid ( oxanilic acid) and sulfonic acid and thioacetic acid sulfoxide was observed to a depth of 18 inches in a silt loam soil in Illinois containing 1.7% organic matter. No leaching was detected in another silt loam soil in Mississippi containing only 0.5% organic matter.

5.6.3. Dissipation Pathways The major routes of dissipation for acetochlor appear to be microbially mediated degradation, runoff, and leaching. Although acetochlor generally degrades rapidly when applied to soil, in some field situations it can be relatively persistent ( e. g., field dissipation half lives were up to 36 days) and it has been found in ground water at numerous locations. There is variable evidence as to the persistence of acetochlor in subsoil horizons ( often persistence is increased substantially for organic pesticides that are subject to microbial degradation) with a published study by the registrant reporting only a modest increase in persistence from surface soils at two sites using in situ methods ( Mills et al., 2001). Lavy et al. ( 1996) have reported a much more substantial increase in persistence at two sites ( also in situ studies) for alachlor, a herbicide that is chemically related to acetochlor and tends to have a very similar environmental fate profile. Laboratory degradation data indicate that acetochlor does not degrade by abiotic processes ( hydrolysis and photolysis); this may be to the higher application rates than used in the Mills et al. study. While acetochlor has relatively short half lives in fine textured aerobic soil, it may be moderately persistent in coarser soils ( this may be related to the lower rate of microbial activity in sandy, low organic matter soils).

5.7. Acetochlor Usage 5.7.1. Summary of Registered / Proposed Uses Acetochlor is restricted for direct use only on field corn and corn grown only for silage or seed. Labels allowing direct application to pop corn should be amended to prevent such application until the registrant formally petitions for the use. Although several of the labels also allow for a  27

fall application to soybean stubble after crop harvest, replanting the following spring is restricted to field corn. Generally, acetochlor can be applied as either a single broadcast or banded application, preplant, preemergence, or early post emergence. Preplant applications can be made as either a single or split application that is either surface applied or incorporated. The early post emergence application is allowed only on corn up to eleven inches in height. For application, acetochlor may be either diluted with water or liquid fertilizers or impregnated onto dry bulk fertilizers. Only applications using ground equipment are allowed; applications through irrigation systems and using aerial equipment are prohibited.

Application rates for acetochlor are dependent on the soil type and the type of weeds to be controlled. However, the maximum single use rate for any soil type is 3.0 lb ai/ A, which is also the maximum seasonal use rate. Formulations containing only acetochlor do not specify pregrazing ( PGI) or preharvest intervals ( PHI); however, the multiple active ingredient formulations contain PGI and PHI restrictions that are based on the other active ingredients included in the formulation. Following application with acetochlor, the labels only allow for rotation to soybeans, corn ( all types), grain sorghum ( milo), wheat, or tobacco.

Due to concerns about ground water contamination, at the time of the original registration on corn the ARP volunteered to prohibit use in certain areas with ground water that is highly vulnerable to contamination by pesticides such as acetochlor. Acetochlor product use was and is restricted on coarse textured, low organic matter soils where groundwater is within 30 feet of the surface. The following language is included with all acetochlor product labels:

Acetochlor products may not be applied to the following soils, if depth to groundwater is 30 feet or less:

° Sands with less than 3% organic matter. ° Loamy sands with less than 2% organic matter. ° Sandy loams with less than 1% organic matter. ° Acetochlor herbicides may be applied to the above soils if depth to groundwater is more than 30 feet.

Acetochlor is now registered in 42 states as well as the District of Columbia ( Hackett et al., 2004). It is also used in corn growing areas of several countries including China, Europe, and Argentina. In the United States, the ARP has submitted petitions for other direct field uses of acetochlor including application to sweet corn and sorghum for grain or silage / forage.

5.7.2. Usage Targets in the Original Acetochlor Conditional Registration Acetochlor is effective on a broad spectrum of weeds in corn fields and it therefore was expected that reductions in overall corn herbicides would occur. As such, the conditional registration agreement mandated a 33 percent reduction in the aggregate use of the selected corn herbicides ( alachlor, metolachlor, atrazine, EPTC, butylate, and 2,4 D) over a five year period. Cancellation of the conditional registration agreement would be triggered if any one of the following usage target levels were not met:  28

1. At the end of 18 months from the date of registration, a net cumulative reduction of the six corn herbicides by 4 million pounds ( 4 M lbs) from 1992 levels, adjusted for planted acreage differences; or 2. At the end of three years from the date of registration, a net cumulative reduction of the six corn herbicides of 22.6 M lbs from 1992 levels, adjusted for planted acreage differences; or 3. At the end of five years from the date of registration, a net cumulative reduction of the six corn herbicides of 66.3 M lbs from 1992 levels, adjusted for planted acreage differences.

Based on OPPs Biological and Economic Analysis Division's analysis, it appears that increased use of acetochlor did result in a decrease in the combined use of the six selected herbicides. OPP/ BEAD's review of the usage data submitted by ARP concluded that the 18 month, three year, and five year target reductions were achieved. The cumulative net reduction for the three year target ( 22.6 M lbs) was exceeded by 1996, and the five year target ( 66.3 M lbs) was exceeded by 4 M lbs in 1998. Overall, the proportion of acetochlor used relative to other pesticides steadily increased based on USDA and Doane Marketing Research surveys. These research surveys indicate that the percent of field corn treated specifically with acetochlor increased from 7 percent in 1994 to 24 percent in 1997. This increase amounted to an increase in total acetochlor use ( based on surveys of 39 states) from 7.4 M lbs in 1994 to 31.8 M lbs by 1998.

At the time of its registration, the increase in acetochlor use was of particular concern since it was listed as a probable human carcinogen ( a classification which it still maintains), and therefore exposure to acetochlor in drinking water contamination may pose a human health risk. In anticipation of its widespread use, several use restrictions were implemented as preventative measures. Specifically, acetochlor may only be applied by certified applicators. It may not be applied to coarse soils ( e. g., sands with less than 3% organic matter) where depth to ground water is less than 30 feet. Acetochlor cannot be applied through any irrigation system ( including flood irrigation), nor via aerial application. Acetochlor may not be applied directly to water or areas where surface water is present. In addition, acetochlor must not be mixed or loaded within 50 feet of surface water or wells, unless proper containment and disposal measures are in place. Each of these measures is intended to prevent acetochlor from migrating to ground water and/ or surface water resources.

5.7.3. Geographic Patterns of Acetochlor Use This summary only presents selected data on acetochlor usage from non confidential sources. Presently, roughly 80% of the total use of acetochlor in the United States occurs in the Midwest. The usage areas generally mirror the production areas for field corn ( Figure 5 and Figure 6 ).

Detailed annual county level sales maps for 1994 to 2003 are provided in Appendix section 12.3 Acetochlor Usage   Detailed Summary. These sales data have been provided separately by members of the acetochlor registration partnership ( ARP) as confidential information and cannot be shared with unauthorized individuals. These sales data are presented as a surrogate for the location of acetochlor usage. It should be noted that pesticide sales data may not be a consistent  29

estimator of usage in any particular watershed because usage does not necessarily occur near the location of purchase and watershed boundaries do not coincide with county boundaries ( all sales data were reported by county units).

The following language is included with all acetochlor product labels:

Acetochlor products may not be applied to the following soils, if depth to groundwater is 30 feet or less:

  Sands with less than 3% organic matter.   Loamy sands with less than 2% organic matter.   Sandy loams with less than 1% organic matter.

Acetochlor herbicides may be applied to the above soils if depth to groundwater is more than 30 feet.

Figure 5. Corn production intensity ( 2002 Census of Agriculture data) and general locations of drinking water intakes sampled in the ARP's SDWS monitoring program ( white = no reported corn acreage, green = lowest intensity category, red = highest corn intensity category).

Figure 5 shows corn production intensity and the generalized locations of the ARP SWDS monitoring locations ( 2002 Census of Agriculture data, see Appendix B for maps based on 1992 and 1997 Census of Agriculture data). Figure 6 provides the USGS estimate of acetochlor usage  30

in the United States for 1997. Note that this map is a coarse estimate and should not be used for decision making at the county level. The USGS provides the following caveat with the data: " The pesticide use map shows regional scale patterns of use intensity within the United States and [ is] not intended for making local scale estimates of pesticide use, such as for individual counties. The USGS maps are based on state level estimates of pesticide use rates for individual crops, which have been compiled by the National Center for Food and Agricultural Policy ( NCFAP) for 1995 1998, and on 1997 Census of Agriculture county crop acreage. Key limitations include: ( 1) state use coefficients represent an average for the entire state and consequently do not reflect the local variability of pesticide management practices found within many states and counties, and ( 2) the county level acreage are based on the 1997 Census of Agriculture and may not represent all crop acreage due to Census non disclosure rules.

Figure 6. USGS estimated acetochlor use for 1997.

5.8. Overview of Exposure Assessment This exposure assessment is based primarily on an extensive monitoring program submitted by the ARP as a requirement for registration of acetochlor. Other monitoring data and modeling results are also discussed in order to provide a more complete picture of exposure to acetochlor. Section 6 deals with parent acetochlor exposure, Section 7 evaluates exposure to degradates of acetochlor, and Section 8 provides an overview of the extensive body of monitoring data for other herbicides and herbicide degradates ( including parent atrazine, and parent + degradates of  31

alachlor and metolachlor) that the ARP compiled in the course of conducting some major surface water and ground water studies to support the acetochlor registration.

Uniquely relevant to OPP's exposure assessment for acetochlor is an evaluation of the detection rates and amounts in the ARP monitoring studies relevant to endpoints identified in the original Acetochlor Registration Agreement which could trigger requirements for mitigation or cancellation of uses should the endpoints be exceeded. There are unique endpoints identified for each of the three major ARP monitoring programs ( SDWS, PGW, and SGW) as well as for outside monitoring; these are discussed separately for each of these monitoring programs. At this time, only acetochlor parent residues have been identified as relevant to the regulatory triggers.

Precedence in the review of the monitoring data is given to acetochlor parent based upon a presumption that the current risk assessment will focus on exposure to acetochlor parent. The Health Effects Division ( HED) of the Office of Pesticide Programs has evaluated currently available toxicity and carcinogenicity data and determined that the dietary drinking water risk assessment should be based upon parent acetochlor alone ( HED, 2004).

Although not anticipated to be included in the current drinking water risk assessment the degradate data are also included in this exposure assessment in a separate section of this document. The primary reason for this is to document the data submitted by the ARP which show exposure levels to acetochlor degradates that are frequently higher than acetochlor and many other pesticide residues and are widespread ( Table 27 and Table 1); see below for a complete characterization). Some of the ARP monitoring studies also contain data on the occurrence of other chloroacetanilide herbicides ( alachlor and metolachlor) and / or other corn herbicides ( atrazine)   a limited discussion on these data and their utility for other exposure assessments also follows this section ( See " Other Chemicals" section of this document). A portion of these data were reviewed for a previous drinking water assessment for atrazine ( Environmental Fate and Effects Division, OPP, EPA, 2001).

6. RESIDUES  ACETOCHLOR PARENT The focus of the current risk assessment is on the parent acetochlor, with secondary emphasis on acetochlor degradates in water and their widespread occurrence. The assessment focuses on the status of acetochlor in ground and surface water with respect to specific endpoints triggering mandatory requirements for implementation of mitigation measures or cancellation of acetochlor uses ( detailed in the conditional registration agreement, USEPA 1994) and to evaluate the impact of acetochlor on drinking water sources in support of human health risk assessments. The following sections present time weighted annualized means an 95th percentile values, as well as the methodology implemented to compute these values. Assessments of acute and chronic exposure are also provided for each of the three surfaces and ground water monitoring programs.

6.1. Data Files Used  32

Table 5 lists the data files used for computing annualized means and summary statistics for each data set.

Table 5. Key data files submitted by the ARP used in this assessment Data set File Name Modified SDWS Data: Surface water monitoring concentrations File Name: swm_ conc. xls Datswm_ anc. xls cws population served. xls 2/ 10/ 2003 2/ 10/ 2003 9/ 17/ 04 SGW master SGM reporting dbase. mdb; Table " tblGWM_ all" 6/ 25/ 2002 PGW http:// www. arpinfo. com/ download/ pgw/ PGW_ NUM_ FINAL. TXT ( All observed concentrations in the PGW studies) http:// www. arpinfo. com/ download/ pgw/ pgw_ uncensored. xls ( Uncensored data provided ARP for computation of TWAMs and Percentiles) 4/ 22/ 04 10/ 2/ 04 6.2. Time Weighted Annualized Means and 95th Percentile Calculations Time weighted annualized means ( TWAMs) were calculated for each site in the three major monitoring programs ( SDWS, SGW, and PGW). Two separate weighting methods were implemented using a custom built TWAM computer program to verify the TWAMs computed by the ARP. The weighting method used by the ARP ( described below) was cross checked with a slightly different method implemented in the WARP beta model developed by the USGS ( USGS 2004). Both weighting methods assign a weight to each discrete sample observation based upon the fraction of the time during a year that each sample represents. Weighted concentrations are then summed to provide an annualized mean.

6.2.1. ARP Weighting Methodology The weighting method implemented by the ARP ( equation 1) calculates annual means based on the calendar year. January 1  December 31). Separate time weighted annualized means are computed for each combination of analyte, site id, sample type, and year. Weighted concentrations are computed based on a two step process. First, an average concentration is calculated as the sum of a value and the previous value divided by two. A weighting factor is then calculated as the time interval between a value and the previous value, divided by the time in 1 year. The final weighted concentration is the product of the average concentration and the corresponding weight factor.  33

Equation 1 :

TWAM= ( c1+ c0) ( t1 t0)/ 2 + ( c2+ c1) ( t2 t1)/ 2 +  .+( cn+ cn 1) ( tn tn 1)/ 2 +( cn) ( tf tn) Daysyr Where " c" is the observed concentration, " t" is the sample date, " Daysyr" is the total number of days in the given year ( accounts for leap years), and " n" is the total number of observations in the given year. The subscripts represent the observation number where " 0" is Jan 1st at 0 hours, and " f" is December 31st at 2400 hrs, note that this is slightly different than ARP's code, which does not include the time from 0 hrs to 2400 hrs on December 31st. Each annualized mean begins January 1st. Therefore, for each new year, a January 1st concentration must be calculated. This is done based on linear interpolation between the last record of the previous year and the first record of next consecutive year. For the first record in the set, there is no previous year for the first sample of a new site. The first concentration is used as the mean concentration from January 1st to the first observation at that site. Similarly, for the last year in record in a data set (" Cn" and " tf"), the last concentration is used as the mean concentration through the end of the year.

6.2.2. USGS WARP Beta Model Weighting Methodology The USGS beta model weighting method ( equation 2) also calculates time weighted annualized means based on the calendar year ( January 1  December 31). This method is different from the ARP method, as individual weights are computed as " the amount of time extending from onehalf the time interval between a value and the preceding value and one half the time interval extending from the value to the subsequent value, divided by the total time in 1 year.... The annual mean concentration is simply the sum of the sample weight times the sample concentrations" ( USGS 2004).

Equation 2:

TWAM= ( c1) [( t1 t0) +( t2 t1)/ 2] + ( c2) ([( t3 t2 ) + ( t4 t3)] / 2) +  .+ ( cn) [( tn tn 1 )/ 2 + ( tf tn)] Daysyr Where " c" is the observed concentration, " t" is the sample date, " Daysyr" is the total number of days in the given year ( accounts for leap years), and " n" is the total number of observations in the given year. The subscripts represent the observation number where " 0" is Jan 1st at 0 hours, and " f" is December 31st at 2400 hrs. Each annualized mean begins January 1.

This method requires special conditions to handle leap years as well as the first and last records of a subset ( e. g., unique combination of site, type, and year). The Visual Basic for Applications ( VBA) workbook developed for the TWAM calculations automatically accounts for leap years using a custom built visual basic procedure. In cases of leap years, the weighting factors are divided by 366 rather than 365. Additionally, for the first record of a year, the weighting factor is calculated as the time interval between a value and January 1st of the corresponding year, and one half the time interval extending from the value to the subsequent value, divided by the total time in 1 year. For the last record of a year in a subset the weighting factor is calculated as one  34

half the time interval between a value and the preceding value plus the time interval extending from the value to December 31st of the corresponding year, divided by the total time in 1 year.

6.2.3. 95th Percentile Calculations The 95th percentile concentration was also computed for each calendar year of observations at a site. In this analysis, a given percentile represents the fraction of the year that the concentration was at or below the given percentile of the distribution of concentration values. This method is based on the method implemented in the USGS WARP beta model ( USGS 2004). In general, the 95th percentile indicates that 95% of the time the value was at or below the given concentration. Percentiles were calculated in several steps. First, the observed concentrations within a year for a given site and sample type are ranked from low to high. The corresponding weighting factors ( calculated as the fraction of the year the individual concentration represents based on one of the weighting methods described above) are then summed to obtain a cumulative distribution function ( CDF), the sum of which equals 1. The concentration for each percentile is then obtained by matching the percentile values to the CDF. If a percentile falls between two values in the CDF, the corresponding weight and concentration is then linearly interpolated. In some cases, the weight corresponding to the lowest observed concentration is greater than a desired percentile; in these cases, exact percentiles could not be calculated. Specific details for each weighting method are described in the following sections.

6.3. Surface Water 6.3.1. ARP Data Time weighted annualized means were calculated using the method described in the prior section ( implemented by TWAM Tool version 2.0). The weighting method used was the same method as that used by the ARP in the data submission to USEPA, with the exception of one modification in the code to account for the last day of the year in each site subset. As described earlier, Table 3 lists the files used in computing TWAMs. For the surface drinking water supplies ( SDWS), separate TWAMs were computed for " finished", " raw" sample types. Finished ( or treated) water samples were sampled post treatment and the water treatment system outflow. Raw ( or untreated) samples were collected prior to treatment at the treatment system intake. A total of 189 individual drinking water supplies were monitored ( Table 2).

<IP_ADDRESS>. Regulatory Action Endpoints The conditional registration agreement includes a number of regulatory action endpoints that, if exceeded by acetochlor or its related degradates of toxicological concern, would trigger mitigation measures or the cancellation of acetochlor registration ( USEPA 1994). These endpoints are discussed in detail in the " Regulatory History" report provided in an earlier  35

deliverable. A brief list of the cancellation triggers and the results of the ARP monitoring program are provided below.

In addition to mitigation/ cancellation endpoints, acute and chronic exposure to acetochlor and its degradates in surface drinking water was also of concern. For the purposes of this analysis, acute exposure was defined as the overall maximum instantaneous concentration observed at a site. This approach may underestimate actual acute exposure since typically only 14 samples were collected each year ( generally bi weekly samples collected during late winter to late fall) and it is unlikely that the sampling times coincided with peak annual acetochlor concentrations. Chronic exposure was defined using both the maximum time weighted average and mean time weighted annual average for a site.

<IP_ADDRESS>.1. Endpoint 1: 2.0 ppb TWAM The conditional registration agreement states that " If one ( 1) community water supply system, that derives its water primarily from surface water, detects an annual time weighted mean concentration of 2.0 ppb, then the use of acetochlor in the related watershed will be prohibited....... or; the ARP will absorb 100% of the costs required to restore the community water supply system to compliance." Cancellation would automatically occur if two large community water supply systems or ten community water supply systems of any size observed time weighted mean concentrations of 2.0 ppb or were out of compliance.

No time weighted annualized means for acetochlor exceeded 2.0 ppb ( Table 6). For both raw and finished surface drinking water, roughly 99% of the time weighted annualized means were below 0.5 ppb.

Table 6. Frequency of time weighted annualized mean concentrations ( ppb) for the parent acetochlor herbicide in raw and finished water drinking water.

Raw Finished Bin Frequency Cumulative % Frequency Cumulative %

0 0 0.00% 12 1.01%

0   0.003 33 13.87% 395 34.40%

0.003   0.005 48 34.03% 115 44.13%

0.005   0.01 34 48.32% 138 55.79%

0.0   0.05 63 74.79% 333 83.94%

0.05   0.1 25 85.29% 93 91.80%

0.1   0.5 34 99.58% 94 99.75%

0.5   1.0 1 100.00% 2 99.92%

1.0  2.0 0 100.00% 1 100.00%

> 2 0 100.00% 0 100.00%

Total 238 1183 <IP_ADDRESS>.2. Endpoint 2: 8.0 ppb Instantaneous Concentration  36

The conditional registration agreement ( USEPA 2004) also states that " If any community water supply system that derives its water primarily from surface water detects a single peak acetochlor concentration of 8.0 ppb, the ARP will make biweekly sampling of that system throughout the following 12 months to determine whether the 2.0 ppb annual time weighted mean concentration has been exceeded." Acetochlor was detected above 8.0 ppb in 2 cases for the finished water samples ( Table 7), however, the twelve month annualized mean did not exceed 2.0 ppb ( Table 6) since none of the sites exceeded an acetochlor TWAM of 2.0 ppb. Acetochlor concentrations in SDWS were the highest of all three studies, followed by raw surface water samples, state ground water samples, and PGW studies as indicated by the cumulative frequency distribution ( CDF) for all sample observations ( Figure 7). The lines on the CDF represent the percent of samples ( frequency) that were detected at or below the corresponding concentration. For example, roughly 80% of all raw ( untreated) water samples in the SDWS drinking water program were less than or equal to 0.05 ppb.

Table 7. Frequency of occurrence for all instantaneous parent acetochlor concentrations ( ppb) in raw and finished water drinking water.

Finished Water RAW Water Bin Frequency Cumulative % Frequency Cumulative %

0 4297 26.00% 530 15.94%

0.0   0.003 3107 44.80% 538 32.12%

0.003  0.005 1502 53.88% 325 41.89%

0.005  0.01 2084 66.49% 536 58.02%

0.01  0.05 3279 86.33% 808 0 0.05  0.1 832 91.37% 188 87.97%

0.1  0.5 1096 98.00% 279 96.36%

0.5  1 183 99.10% 71 98.50%

1  4 136 99.93% 47 99.91%

4  8 10 99.99% 3 100.00%

> 8 2 100.00% 0 100.00%

Total 16528 3325  37

Cumulative Frequency of Acetochlor Observations 0% 20% 40% 60% 80% 100% 120%

0 0.003 0.005 0.01 0.05 0.1 0.5 1 2 4 8 > 8 Concentration ( ppb) Cumulative Frequency PGW Shallow Wells PGW 9ft Lysimeter SGW SDWS finished SDWS Raw Figure 7. Cumulative frequency distribution for all acetochlor observations for each study. PGW data represented include separate distributions for the 9 foot depth lysimeter data and the shallow groundwater wells for each study.

<IP_ADDRESS>. Acute Exposure Maximum exposures for acetochlor parent were generally higher at the SDWS sites than the PGW and SGW sites ( Figure 8). Roughly 85% of the SDWS maximum overall peak finished observations for each site were below 2.0 ppb and 80% of maximum overall peak raw water observations for each site were below 2.0 ppb. Approximately 99% of the PGW and SGW peak observations were below 2.0 ppb. Median values were 0.3 for SDWS raw water, 0.25 for SDWS finished water, 0.02 for SGW ground water, and 0.004 for PGW ground water studies. The majority of overall maximum peak acetochlor concentrations for each site in the state ground water ( SGW) program were less than 0.05 ppb.  38

Cumulative Acute Exposure Based on Maximum Acetochlor Concentrations 0.0% 20.0% 40.0% 60.0% 80.0% 100.0% 120.0%

0 0.003 0.005 0.01 0.05 0.1 0.5 1 2 4 8 > 8 Concentration ( ppb) Cumulative Frequency PGW SGW SDWS Finished SDWS Raw Figure 8. Cumulative frequency distribution for acetochlor acute exposure in all ARP studies, based on the maximum observed concentration at each site. PGW data maximum exposures are provided for each lysimeter and each depth.

Acute Exposure Based on Maximum Acetochlor Concentrations 0 20 40 60 80 100 120 0 0.003 0.005 0.01 0.05 0.1 0.5 1 2 4 8 > 8 Concentration ( ppb) Frequency SDWS Finished SDWS Raw SGW PGW Figure 9. Distribution of acute exposure, based on the maximum observed concentration at each site. PGW data maximum exposures are provided for each lysimeter and each depth.  39

<IP_ADDRESS>. Acute and Chronic Exposure Distribution by Population.

Acute Exposure by Population Served ( Finished Water)

214 GI IL ( 18.2) 157 MA IL ( 7.9)

455 MO OH ( 11.1) 330 LO IN ( 7.4)

0 5 10 15 20 0.2 1.0 2.0 3.2 5.0 8.0 16.0 36.5 82.0 Population Served ( thousands)

Maximum Concent ration ( ppb) Acute Exposure by Population Served ( Raw Water)

168 PA IL 7.2 228 SA IL 7.1 0 1 2 3 4 5 6 7 8 0.4 1.7 3.6 4.5 10.0 12.0 18.0 32.0 50.0 76.0 125.0 Population Served ( thousands) A B Figure 10. Acute acetochlor exposure distribution by population served for raw ( A) and finished ( B) water samples.  40

Chronic Exposure by Population Served ( Raw Water)

0 0.05 0.1 0.15 0.2 0.25 0.3 0.2 1.0 1.7 3.0 4.5 6.5 10.0 20.0 42.5 89.5 Population Served ( thousands)

Ave rage Time We ighted Annual iz ed Mean ( ppb) B Chronic Exposure by Population Served ( Raw Water)

345 RI IN 0.14 168 PA IL 0.19 0 0.05 0.1 0.15 0.2 0.4 1.7 3.6 4.5 10.0 12.0 18.0 32.0 50.0 76.0 125.0 Population Served ( thousands) A Figure 11. Chronic exposure to parent acetochlor in raw surface drinking water ( SDWS) using the average time weighted mean at each site.  41

<IP_ADDRESS>. Chronic Exposure Distribution by System Figure 12. Cumulative frequency distribution for parent acetochlor chronic exposure, based on the highest timeweighted annual mean at each site. PGW chronic exposures are the maximum TWAM for each cluster at 9 foot depth for lysimeters and shallow monitoring wells for ground water. Cumulative Chronic Exposure Based on Maximum Acetochlor TWAMs 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

0 0.003 0.005 0.01 0.05 0.1 0.5 1 2 4 8 > 8 TWAM ( ppb) Cumulative Frequency PGW Shallow Ground Water PGW 9 ft lysimeters SDWS Finished SDWS Raw SGW Maximum Observed TWAMs: PGW, 9 ft. = 0.178 PGW, gw = 0.008 SDWS, fin. = 1.428 SDWS, raw = 0.591 SGW = 0.520  42

6.3.2. Surface Water Factorial Analysis For parent acetochlor, the most toxic of residues, surface water is the dominant medium of exposure. Consequently, the focus of statistical analysis was on factors related to occurrence in surface drinking water supplies. Statistical analyses examined environmental variables that could potentially explain the spatial variability among sites ( e. g., watershed size, corn intensity, etc.). In addition, the relationship between raw and finished samples was examined to determine the effects of water treatment.

Appendix 12.7 presents correlation matrices for surface drinking water sites, individually for raw and finished water samples. A number of hypothesized explanatory variables were examined including watershed area, average watershed sales ( 1994 2003), the mean sales ( 1994) for the county with the overall highest sales in each watershed, watershed runoff, watershed corn intensity, 30 yr average precipitation, and 30 yr average spring precipitation ( April   June). In general, the ancillary variables that were available were unable to explain a significant amount of the variability in maximum observed concentrations ( acute exposure), average TWAMS, and maximum TWAMs ( chronic exposure). It was originally expected that acetochlor acute and chronic exposure would be moderately to strongly correlated with the variability in acetochlor sales in the associated watersheds, however sales were only weakly correlated ( r < 0.5).

Some associations were observed between ancillary variables as expected. For example, watershed corn intensity was moderately to strongly correlated with the watershed runoff curve number ( RCN) with correlation coefficients ( r) ranging from 0.78 for all sites where raw water samples were collected to 0.82 for only those sites where finished water samples were collected. The correlation between runoff curve number and watershed corn intensity is not surprising, since land cover is a factor in generating the curve number.

Statistical analysis of time weighted means revealed no significant increase or decrease in annualized mean concentrations for acetochlor over time, nor did ARP's analysis detect a change in annualized means over the seven year monitoring period. Scatter plots for raw and finished time weighted means can be found in Appendix section 12.7.

Raw water concentrations in the SDWS program were significantly ( p < 0.05) greater than treated water concentrations. A paired two sample t test for means was performed on those sites and sample dates that had both raw and finished water observations. Results of the t test are provided in the Appendix section entitled Statistical Analyses for the ARP monitoring Studies. Statistical analysis indicates that water treatment plants that use granulated activated carbon ( GAC) or powdered activated carbon ( PAC) significantly reduce acetochlor concentrations in drinking water ( p < 0.001)

In nearly half the cases ( 43%), finished water samples were moderately to strongly associated ( r >= 0.75) with observed raw water concentrations, suggesting that finished water samples are moderately predictive of raw water concentrations. Raw water concentrations explained at least 75% of the variability in finished water concentrations for 30% of the sites. Raw water concentrations explained at least 50 % ( r2 >= 0.5) of the variability in finished water concentrations using a simple linear model. In general increasing the sample size ( N) did not result in an increase in correlation between raw and finished water concentrations. Lack of  43

correspondence for some sites may be partially a result of differences in sampling times for raw and finished samples and the uncertainty in residence time for each of the water treatment facilities. Because there is a time lag from when water enters the intake ( raw water) to when the treatment processes in completed ( finished water) it is unlikely that raw and finished samples were taken from the same volume of water.

Percent reduction due to treatment was also calculated to assess the relative success of treatment. Percent reduction was computed for those observations that had non zero raw values using the following formula:

Percent Reduction = (( Raw Finished)/ Raw)* 100.

Figure 13 summarizes the percent reduction in acetochlor parent in surface drinking water supplies sampled. Values on the x axis represent percent reduction; a value of 100% indicates that all of the acetochlor was eliminated. A negative value means that the concentration went up between pre and post treatment. Based on the chart roughly 35% of the non zero samples had complete elimination of Acetochlor, another 15% had about an 80% reduction, another 10% had about a 60% reduction and so forth. About 12.5% of surface water samples had concentrations that increased after treatment.

Percent Reduction in Acetochlor Due to Treatment 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45  10 0  80  60  40  20 0 20 40 60 80 100 Acetochlor Percent Reduction Relative Frequency Figure 13. Percent reduction in acetochlor from pre treatment ( raw) to post treatment ( finished) sample.  44

6.3.3. Characterization of Exposure to Surface Water Exposure to acetochlor parent was significantly higher in the surface water monitoring sites than the ground water monitoring sites ( see Figure 7, Figure 8, and Figure 12). Only the surface water monitoring samples represented water from existing drinking water intakes.

While most of the factors for selection for the ground water sites would tend to make these sites susceptible to higher levels of contamination than occurring in samples from actual drinking water wells, this is not an assumption which is directly verifiable. The preponderance of the evidence does indicate that it is proper to base the parent exposure assessment on the surface water monitoring results, nonetheless, there are still some unknowns with regard to the relative conservativeness ( i. e., degree of tendency to overestimate exposure) of the ground water monitoring studies by the ARP. For example, a major limitation in a monitoring survey for a new pesticide is that the full impact of the use of the pesticide on ground water quality may not be observed for a number of years. The number of years required for residues to reach ground water at each SGW sampling site is not knowable. Data from the PGW studies show, that even with higher than average rainfall supplemented by irrigation it can take several years for some residues to reach shallow ground water, witness the Nebraska PGW site where residues of the acetochlor ethanesulfonic acid degradates were still moving through the soil pore water at a 9 foot depth when the study was terminated more than seven years after the only acetochlor application.

The ten overall highest single acetochlor concentrations, time weighted annualized means, and 95th percentiles for the community water systems were determined for both raw ( Table 8) and finished ( Table 9) water samples. These systems generally draw water from watersheds with high corn crop intensity ( ca. 20 to 35%) and include both reservoir and river water sources ( Table 10). Acetochlor concentrations were highest in Illinois. Eight of the ten highest raw water concentrations, time weighted annualized means, and 95th percentiles were observed in Illinois, including the overall highest raw water concentration ( 7.19 ppb), TWAM ( 0.59 ppb), and 95th percentile ( 3.31 ppb) were observed at a single site ( 168 PA IL) in Illinois. Similarly, six of the ten highest finished water concentrations were observed in Illinois, including the overall highest finished water concentration ( 18.21 ppb), TWAM ( 1.43 ppb), and 95th percentile ( 6.97 ppb) observed at 214 GI IL. Most of the ten highest observations occurred between the years 1996 1998.

Statistical analysis of raw versus finished water concentrations indicates that treatment does indeed on average significantly decrease acetochlor concentrations, the top ten finished ( treated) water concentrations exceed the top ten raw ( untreated) concentrations suggesting that paired sampling of raw and finished water at all sites would have provided a much more reliable indication of the level of impact of acetochlor residues on surface waters and the degree of mitigation provided by treatment systems with and without carbon filtration ( that is, in general, with the recognition that it is generally not possible to exactly match the water sampled prior to and post treatment). Without these data, we have no possibility of determining, for example, whether the treatment systems that did not use carbon filtration reduced exposure and we have an inadequate ability to determine the impact on acetochlor residue levels of treatment systems that include GAC ( because most of these CWS also did not sample pre treatment water). In fact, the SDWS was set up with paired raw and finished water sampling at only about 25 ( initially in  45

1995) to 44 ( overall, for one or more years) of the up to 175 sites sampled each year during this seven year study.

To further isolate treatment effects in the SDWS it would have been worthwhile to attempt to match, to the extent it can be determined and is feasible, the volumes of water that are sampled for the raw and finished water analyses at each sampling date. Future studies of this nature should detail results of the investigation of water handling practices and a determination on a site by site basis of the degree to which this is feasible. If attempts to more closely pair the raw and finished water are deemed to be futile, the reasons for this determination should at least be specifically documented.

Table 8. Ten highest raw ( untreated) water concentrations of parent acetochlor at community water system ( CWS) intake locations.

Maximum Single Concentration ( ppb) Maximum TWAM ( ppb) Maximum 95th % tile Concentration ( ppb) a CWS Name Value Year CWS Name Value Year CWS Name Value Year 168 PA IL 7.19 1998 168 PA IL 0.59 1998 168 PA IL 3.31 1998 228 SA IL 7.09 1996 168 PA IL 0.43 1996 222 HI IL 2.10 1998 168 PA IL 5.89 1996 228 SA IL 0.40 1996 1070 WY MO 1.81 1997 228 SA IL 3.45 1998 222 HI IL 0.36 1998 228 SA IL 1.56 1996 606 KA IL 2.88 1998 1070 WY MO 0.32 1997 259 SP IL 1.44 1998 1070 WY MO 2.50 1997 345 RI IN 0.30 1997 168 PA IL 1.39 1996 222 HI IL 2.36 1998 222 HI IL 0.26 1996 225 CE IL 1.38 1998 345 RI IN 2.27 1997 259 SP IL 0.25 1998 228 SA IL 1.37 1998 259 SP IL 2.22 1998 606 KA IL 0.23 1996 603 BL IL 1.28 1995 225 CE IL 2.01 1999 603 BL IL 0.23 1995 557 DM IA 1.13 2001 a Max 95% tile indicates that 95% of the time the value was less than or equal to the specified value ( USGS 2004).

Table 9. Ten highest finished ( treated) water concentrations of parent acetochlor at community water system ( CWS) outflow locations.

Maximum Single Concentration ( ppb) Maximum TWAM ( ppb) Maximum 95th % tile Concentration ( ppb) a CWS Name Value Year CWS Name Value Year CWS Name Value Year 214 GI IL 18.21 1996 214 GI IL 1.43 1996 214 GI IL 6.97 1996 455 MO OH 11.14 1997 455 MO OH 0.58 1997 168 PA IL 3.01 1998 157 MA IL 7.93 1996 166 NE IL 0.53 1996 340 NV IN 2.87 1996 330 LO IN 7.35 1997 214 GI IL 0.49 1998 182 GE IL 2.70 1998 168 PA IL 5.43 1998 168 PA IL 0.48 1998 166 NE IL 2.67 1996 455 MO OH 5.17 1996 157 MA IL 0.46 1996 143 SO IL 2.65 1998 340 NV IN 4.31 1996 330 LO IN 0.42 1997 214 GI IL 2.21 1998 214 GI IL 4.28 1998 182 GE IL 0.39 1998 518 US OH 2.03 1996 537 WM OH 4.16 2000 518 US OH 0.37 1996 330 LO IN 1.94 1996 168 PA IL 4.14 1996 340 NV IN 0.37 1996 242 CO IL 1.71 1996 a Max 95% tile indicates that 95% of the time the value was less than or equal to the specified value ( USGS 2004).  46

Table 10. Watershed characteristics for the ten highest finished ( treated) water concentrations of parent acetochlor at community water system ( CWS) outflow locations. CWS Name Watershed Area, acres Type Reservoir Volume or Area2 Chronic Rank1 Acute Rank1 % Corn Intensity 214 GI IL 2996 Reservoir 250 mg 1,4 1, 8 25.0 455 MOOH 138245 River NA 2 2, 6 18.7 157 MA IL 11916 Reservoir 900 mg 6 3 34.5 330 LO IN 524144 River NA 7 4 28.3 340 NV IN 68241 River NA 10 7 21.0 537 WMOH 427302 River NA NC 9 28.3 168 PA IL 11733 Reservoir 900 mg 5 5, 10 38.7 166 NE IL 34849 Reservoir NA 3 NC 33.5 182 GE IL 724 Reservoir 50 acres, 9 ft deep 8 NC 29.1 518 USOH 894 Reservoir 90 mg 9 NC 23.8 1 Ranking by TWAM ( Chronic) or single highest concentration ( acute) as presented in Table 9. NC = Not in top ten. 2 mg = million gallons, NA = not available or unknown.

The highest overall maximum TWAM, single concentration, and 95th percentile was observed in Illinois, followed by Missouri at nearly half the maximum concentrations observed in Illinois ( Table 11 and Table 12). The top ten raw ( untreated) water TWAMs, peak concentrations, and peak 95th percentile concentrations ranged from ranged from 0.007 ppb in WI to 0.591 ppb in IL, 0.044 ppb in OH to 7.186 ppb in IL, 0.019 ppb in PA to 3.313 ppb in IL, respectively. Peak finished ( treated) water concentrations were again sometimes higher than pre treated water samples. The top ten treated water TWAMs, peak concentrations, and peak 95th percentile concentrations ranged from 0.004 in MD to 1.428 ppb in IL, 0.034 ppb in MD to 18.21 ppb in IL, 0.011 in DE to 6.973 in IL, respectively. The highest maximum TWAM for MD occurred three times at two different sites ( Table 12).  47

Table 11. Highest raw ( untreated) water concentrations of parent acetochlor at community water system ( CWS) intake locations in each state ( sorted by Max TWAM). Max TWAM ( ppb) Maximum Single Concentration ( ppb) Max. 95th % tile Concentration ( ppb) a State Value Year CWS Name State Value Year CWS Name State Value Year CWS Name IL 0.591 1998 168 PA IL IL 7.186 1998 168 PA IL IL 3.313 1998 168 PA IL MO 0.317 1997 1070 WY MO MO 2.504 1997 1070 WY MO MO 1.807 1997 1070 WY MO IN 0.304 1997 345 RI IN IN 2.265 1997 345 RI IN IN 1.118 1997 345 RI IN IA 0.217 1996 574 OS IA IA 1.762 2001 557 DM IA IA 1.129 2001 557 DM IA KS 0.085 1999 89 MI KS KS 0.426 1999 89 MI KS KS 0.272 1999 89 MI KS NE 0.045 2001 301 BL NE NE 0.161 2001 301 BL NE NE 0.131 2001 301 BL NE MN 0.019 1999 296 SC MN MN 0.251 1999 296 SC MN MN 0.066 1999 296 SC MN OH 0.010 2001 452 MC OH OH 0.044 2001 452 MC OH OH 0.033 2001 452 MC OH PA 0.010 1996 737 AW PA PA 0.241 1996 737 AW PA PA 0.019 1998 737 AW PA WI 0.007 1996 13 AP WI WI 0.046 1996 18 OK WI WI 0.024 1996 18 OK WI a Max 95% tile indicates that 95% of the time the value was less than or equal to the specified value ( USGS 2004).

Table 12. Highest finished ( treated) water concentrations of parent acetochlor at community water system ( CWS) outflow locations in each state.

Max TWAM ( ppb) Maximum Single Concentration ( ppb) Max. 95th % tile Concentration ( ppb)

State Value Year CWS Name State Value Year CWS Name State Value Year CWS Name IL 1.428 1996 214 GI IL IL 18.21 1996 214 GI IL IL 6.973 1996 214 GI IL OH 0.584 1997 455 MO OH OH 11.14 1997 455 MO OH OH 2.03 1996 518 US OH IN 0.416 1997 330 LO IN IN 7.353 1997 330 LO IN IN 2.872 1996 340 NV IN MO 0.258 1998 1098 GE MO MO 1.289 1997 1070 WY MO MO 1.114 1998 1098 GE MO IA 0.207 1996 570 MO IA IA 2.328 1998 572 MP IA IA 1.402 1998 572 MP IA KS 0.133 2001 125 TO KS KS 1.88 1999 25 AT KS KS 0.983 1999 71 KC KS PA 0.092 1995 729 PH PA PA 2.34 1995 729 PH PA PA 0.045 1995 769 RE PA NE 0.088 1999 301 BL NE NE 1.116 1995 303 OM NE NE 0.288 2001 303 OM NE WI 0.039 1997 17 ME WI WI 0.192 1997 17 ME WI WI 0.167 1997 17 ME WI DE 0.025 1998 652 WI DE DE 0.598 1998 652 WI DE DE 0.011 1998 651 NE DE MN 0.006 1999 296 SC MN MN 0.043 1997 277 MI MN MN 0.03 1999 296 SC MN MD** 0.004 2001 702 LA MD MD 0.034 1996 699 HG MD MD 0.012 1996 699 HG MD a Max 95% tile indicates that 95% of the time the value was less than or equal to the specified value ( USGS 2004).

** There are also two other Maryland sites with concentration values of 0.004 ppb  48

6.3.4. Comparison of ARP and WARP beta Model Results Table 13 presents the highest acetochlor concentrations estimated by the WARP beta model ( USGS 2004) for states where ARP also had surface water monitoring locations. For comparison, the top ten peak raw water concentrations measured by the ARP for community water supply systems are also provided. In both data sets, the majority of the top ten peak concentrations were located in Illinois. In general, the maximum time weighted annualized means measured by the ARP are close to those modeled by WARP, as are the 95% tile values. Recall that the 95th percentile values represent the fraction of the year ( e. g., 95 percent of the time) that the concentration was equal to or less than the listed value ( USGS 2004).

The modeling results are based on use estimates provided by the National Center for Food and Agricultural Policy ( http:// www. ncfap. org/ database/ default. php ) These data are different than the annual sales data provided by the ARP. The USGS modeling is based on nationally available hydrologic and soils data.

Table 13. Top ten highest raw water concentrations ( ppb) of parent acetochlor modeled by WARP multicompound regression model and measured by ARP at community water system ( CWS) intakes.

WARP results ( Beta version, results supplied by USGS) are only for states where ARP also had surface water monitoring locations. WARP data are ranked by maximum 95% tile and measured results by ARP are ranked separately by maximum time weighted mean and 95% tile.

WARP ARP Max TWAM Max 95% tile Conc. a Site TWAM 95% tile Conc. Max Single Conc. Value CWS Name Value CWS Name KASKASKIA RIVER ( E. FORK) FARINA, IL 0.81 3.77 7.19 0.59 168 PA IL 3.31 168 PA IL LITTLE WABASH RIVER FLORA, IL 0.56 2.64 7.09 0.43 168 PA IL 2.10 222 HI IL LITTLE WABASH RIVER CLAY CITY, IL 0.55 2.60 5.89 0.40 228 SA IL 1.81 1070 WY MO LITTLE WABASH RIVER FAIRFIELD, IL 0.54 2.59 3.45 0.36 222 HI IL 1.56 228 SA IL KASKASKIA RIVER ( E. FORK) FARINA, IL 0.54 2.50 2.88 0.32 1070 WY MO 1.44 259 SP IL WILDCAT CREEK KOKOMO, IN 0.52 2.31 2.50 0.30 345 RI IN 1.39 168 PA IL WHITE RIVER NORTH INDIANAPOLIS, IN 0.50 2.28 2.36 0.26 222 HI IL 1.38 225 CE IL KASKASKIA RIVER EVANSVILLE, IL 0.48 2.24 2.27 0.25 259 SP IL 1.37 228 SA IL KASKASKIA RIVER FAYETTEVILLE TWP, IL 0.48 2.22 2.22 0.23 606 KA IL 1.28 603 BL IL KASKASKIA RIVER NEW ATHENS TWP, IL 0.47 2.22 2.01 0.23 603 BL IL 1.13 557 DM IA a Max 95% tile indicates that 95% of the time the value was less than or equal to the specified value ( USGS 2004).

6.3.5. Summary Results of National Water Quality Assessment ( NAWQA) Results NAWQA data ( NAWQA a long term multi faceted monitoring program being conducted by the USGS) have included serial monitoring for a large schedule of pesticides, including acetochlor in  49

multiple study areas across the United States. NAWQA monitoring sites are not selected to represent the locations of drinking water intakes nor are they directly selected to represent sites at which specific pesticides are used. They do, however, represent ambient pesticide concentrations in the environment, include many watersheds where agriculture is the documented dominant land use, and may be an indication of vulnerability of sites to runoff of acetochlor ( These data are used as an indication of the occurrence pattern and concentration of pesticides in surface source water). Table 14 summarizes acetochlor monitoring concentrations measured in NAWQA study unit locations. The two columns of data represent overall ( non time weighted) mean and maximum concentration data at all sites for which the maximum concentration value is above 1.00 ppb.  50

Table 14. Acetochlor monitoring concentrations at NAWQA study unit locations. ( Notes: ` Mean Conc' values are not Time Weighted Annual Mean ( TWAM) concentration values; ` Max Conc' values occurred, for the most part, at different times [ dates and years] during the monitoring period which also varied from site to site.)

Site Name Mean Max Conc Conc ( ppb) ( ppb)

MAPLE CREEK NEAR NICKERSON, NE 1.359 61.00 SUGAR CREEK AT MILFORD, IL 0.993 35.90 ELKHORN RIVER AT WATERLOO, NE 1.304 31.00 PLATTE R AT LOUISVILLE NE 0.416 14.20 LA MOINE RIVER AT COLMAR, IL 0.532 11.60 LITTLE COBB RIVER NEAR BEAUFORD, MN 0.361 10.70 MAUMEE RIVER AT WATERVILLE OH 0.596 10.60 SKUNK RIVER AT AUGUSTA, IA 0.768 10.60 SANGAMON RIVER AT MONTICELLO, IL 0.510 9.71 MAUMEE RI AT NEWHAVEN IN 0.710 8.88 OLD MANS CREEK NEAR IOWA CITY, IA 0.636 8.16 WEST FORK CEDAR RIVER AT FINCHFORD, IA 0.366 7.62 SUGAR CREEK AT CO RD 400 S AT NEW PALESTINE, IN 0.238 7.17 CEDAR RIVER NEAR CONESVILLE, IA 0.376 7.10 ST JOSEPH RIVER NEAR NEWVILLE IN 0.373 5.61 AUGLAIZE RIVER NEAR FORT JENNINGS OH 0.515 5.13 LITTLE BUCK CREEK NEAR INDIANAPOLIS, IN 0.053 4.20 MAD RIVER AT ST PARIS PIKE AT EAGLE CITY OH 0.065 4.02 BLACK RIVER NR JEDDO MI 0.292 3.80 CLEAR CK NR SANGER, TX 0.115 3.59 WHITE RIVER AT HAZLETON, IN 0.194 3.56 IOWA RIVER NEAR ROWAN, IA 0.116 3.50 WAPSIPINICON RIVER NEAR TRIPOLI, IA 0.188 3.09 DUCK CREEK AT SEMINARY ROAD NEAR ONEIDA, WI 0.124 2.90 IOWA RIVER AT WAPELLO, IA 0.159 2.89 FLOOD CREEK NEAR POWERSVILLE, IA 0.164 2.56 WAPSIPINICON RIVER NEAR DE WITT, IA 0.360 2.30 BOGUE PHALIA NR LELAND, MS 0.068 2.28 ENGLISH RIVER AT RIVERSIDE, IA 0.117 2.23 ILLINOIS RIVER AT VALLEY CITY 0.240 2.01 ILLINOIS RIVER AT OTTAWA, IL 0.260 2.00 BIG SUNFLOWER RIVER NR ANGUILLA, MS 0.104 1.68 CEDAR RIVER AT GILBERTVILLE, IA 0.111 1.66 IOWA RIVER AT MARENGO, IA 0.184 1.50 MINNESOTA RIVER NEAR JORDAN, MN 0.143 1.50 YAZOO RIVER BL STEELE BAYOU NR LONG LAKE, MS 0.036 1.45 PLATTE RIVER NEAR GRAND ISLAND, NEBR. 0.133 1.40 SOUTH FORK IOWA RIVER NE OF NEW PROVIDENCE, IA 0.090 1.36 WOLF CREEK NEAR DYSART, IA 0.121 1.25 6.3.6. USGS / EPA Pilot Reservoir Monitoring Program The highest levels of chronic exposure to acetochlor parent most often occur in reservoirs ( compared to streams and rivers and to ground water, Table 10), and a significant source of  51

additional monitoring for a number of pesticides in settings of high vulnerability is provided in Bloomquist et al. ( 2001). Each sampling site included in the USGS reservoir monitoring study consisted of both a reservoir ( raw water sample source) and a Community Water System ( finished water source). The study focused on small drinking water supply reservoirs in areas with high pesticide use ( not necessarily high acetochlor use areas). The program was implemented with a NAWQA design structure and strong consideration in site selection was given to sites within existing NAWQA Study Units. One drinking water reservoir was chosen in each of 12 states: California, Indiana, Ohio, Oklahoma, Louisiana, Missouri, South Carolina, South Dakota, New York, North Carolina, North Carolina, Pennsylvania and Texas. No samples were taken in Illinois, a high acetochlor usage state.

Table 15. Maximum acetochlor concentration values in pilot reservoir monitoring study ( Blomquist et al., 2001).

The maximum concentration in the intake water at the Mitchell, South Dakota site was 0.334 ppb and the concentrations in both the outflow from the CWS treatment facility and the reservoir were 0.395 ppb. The highest 67 concentration values were all found at the South Dakota, Ohio and Indiana sites.

6.4. Ground Water  52

6.4.1. PGW Leaching Summary Two separate data files were used in this analysis ( Table 5). One file contained all the concentration values observed in the PGW studies provided by the ARP, while the second contained raw uncensored concentrations also provided by the ARP. Because of the overwhelming number of censored values ( defined in this context as values that were not reported numerically   generally because of the precision and accuracy limitations of the analytical method for low residue levels), the PGW uncensored file was used to compute timeweighted annualized means and percentiles. The underlying assumption here is that the uncensored data represent the best available estimates of unmeasured values ( any substitution method for nondetects would be arbitrary).

6.4.2. Comparison of PGW Results to the Acetochlor Regulatory Action Endpoints The conditional registration agreement states that automatic cancellation of acetochlor will occur if " out of the eight sites, 4 sites in a variety of geographic, and climatic conditions under both vulnerable and general use conditions ( as determined by EPA) in corn growing states indicate a pattern of movement of acetochlor toward ground water" ( USEPA 1994). ". In the PGW studies, one indication of a pattern of movement was defined as the detection of acetochlor greater than or equal to 1.0 ppb at nine foot lysimeter depth as well as corresponding three and 6 foot depths in that cluster. Table 16 indicates that only one site ( Iowa) had detections greater than 1.0 ppb, and moreover it was the only site to have detected concentrations greater than 0.1 ppb in the nine foot lysimeters.

Peak concentrations of the parent acetochlor were determined for each state and are presented separately for three foot lysimeters ( Table 17), nine foot lysimeters ( Table 18), shallow ground water ( Table 19), and deep ground water ( Table 20). The maximum soil pore water residue for parent acetochlor was measured as 3.2 ppb, which was observed in the 9 ft ( 2.7 m) lysimeters in Iowa ( Table 18). According to the pgw_ num_ final. txt file provided by the registrant, the maximum residue observed in ground water was 0.06 ppb, observed in Iowa. Concentrations in the deep ground water monitoring wells ( Table 20) were only slightly lower than concentrations in the shallow ground water wells.

Table 16. PGW Sites exceeding 0.1 ppb at 9 feet depth ( exceedences only occurred at 1 of the 8 sites).

MAT DATEa STATE DEVICE DEPTH CLUSTER RAW CONCENTRATION 0.5 6/ 24/ 1996 IA LY 9 1 2.6 1 7/ 9/ 1996 IA LY 9 1 0.195 0.5 6/ 24/ 1996 IA LY 9 3 3.2 1 7/ 9/ 1996 IA LY 9 3 0.628 1.5 7/ 24/ 1996 IA LY 9 3 0.208 2 8/ 8/ 1996 IA LY 9 3 0.102 0.5 6/ 24/ 1996 IA LY 9 4 0.132 0.5 6/ 24/ 1996 IA LY 9 6 0.365 a Date was not provided, but was approximated using the initial treatment date, months after treatment, and assuming average of 30 days per month.  53

Table 17. Concentrations of AC observed in 3 foot lysimeters from the eight prospective ground water studies.

Max TWAM Max 95 % tile Max Concentration STATE Cluster Average Single Cluster Cluster Average Single Cluster Cluster Average Single Cluster Max Moving Average DE 0.004 0.009 0.008 0.019 0.009 0.024 0.012 IA 0.000 0.000 0.000 0.000 0.000 0.003 0.000 IN 0.006 0.045 0.028 0.195 0.047 0.330 0.228 MN 0.001 0.001 0.000 0.000 0.001 0.001 NA NE 0.004 0.007 0.010 0.017 0.011 0.018 0.011 OH NA NA NA NA NA 0.025 NA PA 0.009 0.068 0.016 0.129 0.020 0.156 0.118 WI NA NA NA NA NA 0.003 NA Single Cluster = Statistic applies to all observed values for the given depth; Cluster Average = Statistic applies to concentrations averaged across all cluster for a given date for the depth listed; Max TWAM = Maximum timeweighted average observed based on uncensored data file; Max 95% tile = Represents the amount of time during the calendar year the concentration was below the listed value; Maximum Moving Average = The single highest average for 3 consecutive values ( separately by cluster and depth) across all clusters; NA = Insufficient uncensored concentrations to compute the value Table 18. Concentrations of AC observed in 9 foot lysimeters from the eight prospective ground water studies.

Max TWAM Max 95 % tile Max Concentration STATE Cluster Average Single Cluster Cluster Average Single Cluster Cluster Average Single Cluster Max Moving Average DE 0.004 0.006 0.006 0.013 0.006 0.014 0.009 IA 0.458 1.639 0.820 2.931 0.900 3.200 1.345 IN 0.000 0.000 0.000 0.000 0.000 0.003 0.000 MN 0.000 0.001 0.000 0.000 0.000 0.003 NA NE 0.003 0.007 0.006 0.018 0.010 0.018 0.011 OH NA NA NA NA 0.003 0.003 NA PA 0.003 0.013 0.010 0.061 0.015 0.072 0.062 WI NA NA NA NA 0.003 0.003 NA Single Cluster = Statistic applies to all observed values for the given depth; Cluster Average = Statistic applies to concentrations averaged across all cluster for a given date for the depth listed; Max TWAM = Maximum timeweighted average observed; Max 95% tile = Represents the amount of time during the calendar year the concentration was below the listed value; Maximum Moving Average = The single highest average for 3 consecutive values ( separately by cluster and depth) across all clusters; the ARP provided no uncensored data implying the data was all below detection ; NA = Insufficient uncensored concentrations to compute the value Table 19. Concentrations of AC observed in Shallow ground water from the eight prospective ground water studies.

Max TWAM Max 95 % tile Max Concentration  54

STATE Cluster Average Single Cluster Cluster Average Single Cluster Cluster Average Single Cluster Max Moving Average DE 0.002 0.003 0.005 0.008 0.005 0.008 0.004 IA 0.000 0.000 0.000 0.000 0.000 0.025 0.002 IN 0.001 0.000 0.000 0.000 0.003 0.005 0.002 MN NA NA NA NA 0.003 0.003 NA NE 0.003 0.008 0.006 0.014 0.007 0.027 0.011 OH NA NA NA NA NA 0.003 NA PA 0.002 0.005 0.003 0.009 0.004 0.011 0.005 WI NA NA NA NA NA 0.003 NA Single Cluster = Statistic applies to all observed values for the given depth; Cluster Average = Statistic applies to concentrations averaged across wells for a given date for the depth listed; Max TWAM = Maximum time weighted average observed; Max 95% tile = Represents the amount of time during the calendar year the concentration was below the listed value; Maximum Moving Average = The single highest average for 3 consecutive values ( separately by cluster and depth) across all clusters; NA = Insufficient uncensored concentrations to compute the value Table 20. Concentrations of AC observed in Deep ground water from the eight prospective ground water studies.