# Problem Formulation for the Ecological Risk Assessment
of Hexazinone

Environmental Fate and Effects Division Office of Pesticide Programs US Environmental Protection Agency

Prepared by: ERB V Secondary Revision: Valerie Woodard, Biologist EFED Name, Title José L. Meléndez, Chemist November 26, 2009 Mah T. Shamim, Ph.D., Chief

1Table of Contents

1. Purpose ........................................................................................................................ 3 2. Problem Formulation .................................................................................................. 3 2.1. Nature of Regulatory Action .................................................................................... 3 2.2. Previous Risk Assessments...................................................................................... 3 3. Stressor Source and Distribution ................................................................................ 4 3.1. Mechanism of Action ............................................................................................... 8 3.2. Overview of Pesticide Usage................................................................................... 8 3.3. Environmental Fate and Transport ......................................................................... 11 4. Receptors ....................................................................................................................... 20 4.1 Aquatic and Terrestrial Effects ............................................................................... 20 4.2 Incident Database Review ....................................................................................... 26 4.3 Ecosystems Potentially at Risk ............................................................................... 28 5. Assessment Endpoints .................................................................................................. 28 6. Conceptual Model ......................................................................................................... 29 6.1 Risk Hypothesis ...................................................................................................... 29 6.2 Conceptual Diagram ......................................................................................... 30 7. Analysis Plan ............................................................................................................... 31 7.1. Stressors of Concern .............................................................................................. 32 7.2. Measures of Exposure ............................................................................................ 32 7.3. Measures of Effect ................................................................................................. 33 7.4. Integration of Exposure and Effects ...................................................................... 34 7.5. Deterministic and Probabilistic Assessment Methods ........................................... 34 7.6. Endangered Species Assessments .......................................................................... 34 7.7. Drinking Water Assessment .................................................................................. 35 7.8 Preliminary Identification of Data Gaps ........................................................... 36 8. References ................................................................................................................... 42 Appendix A. SRRD data request justification tables ...................................................... 43

21. Purpose

The purpose of this problem formulation is to provide the foundation for the ecological risk assessment being conducted for hexazinone . As such, it articulates the purpose and objectives of the risk assessment, evaluates th e nature of the problem, and provides a plan for analyzing the data and char acterizing the risk (EPA, 1998).

2. Problem Formulation

2.1. Nature of Regulatory Action

This report summarizes the Environmental Fa te and Effects Division’s (EFED) Problem Formulation for the Registration Review of hexazinone. Previous risk assessments were completed for the uses of the chemical, th rough 2008, which serve as the basis for this
assessment.

2.2. Previous Risk Assessments

A Reregistration Eligibility D ecision (RED) that included an ecological risk assessment for fish, invertebrates, and plants in a quatic habitats, as well as birds, mammals, invertebrates and plants in terrestrial habitats, was issu ed in September of 1994 for hexazinone. The data were considered suffi cient to allow the Agency to assess the registered uses of hexazinone and to determ ine that hexazinone could be used without resulting in unreasonable advers e effects to humans and the environment. Due to the risk to non-target plants, reduction of the maximum application rate from 13.5 lb a.i. per acre to 8 lb a.i. per acre was required. The 13.5 lb a.i. per acre rate was cancelled July 2009, but there are still rates of 10.0 and 12.0 lbs a.i. per acre included on the label for selected non-agricultural uses. These rates ar e included in this assessment. That assessment also determined that expos ure of non-target organisms to hexazinone could result from direct application, spray drift from treated ar eas, and runoff from treated areas. Such exposure would be chronic as well as acute. Hexazinone exceeded the levels of concern (LOCs) for terrestrial and a quatic plants, at all application rates, using aerial and ground equipment. Contamination of aquatic sites adjacen t to treated areas could be of great ecological significance and may be exacerb ated by the persistence and mobility of hexazinone. Effects to aquatic plants expected from the use of hexazinone may alter aquatic ecosystems, the severity of which is dependent on the frequency of application and the nature of the receiving body of water. Hexazinone also exceeded the LOC for small mammals at several of the hi gher application rates. Hexazinone exceeded the endangered species LOCs for grass- and insect-eating mammals at use rates of 3.6 pounds active ingredient per acre (lb ai/acre) or greater. It also exceeded the LOCs for both aquatic and terrestrial pl ants at all use rates.

3An endangered species litigation assessment for the California Red-Legged Frog supported the RED results for no effect on aqua tic animals, but with potential effects on terrestrial animals and both aquatic and terr estrial plants, with refinements based on information specific to the CRLF. The asse ssment resulted in an overall “May Affect, Likely to Adversely Affect” determina tion (February 20, 2008). There is an LAA determination for direct effects to the CRL F using the bird as a surrogate consuming small herbivore mammals for noncrop, conifer re lease, blueberry and Christmas tree uses due to LOC exceedence resulting from th e T-Herps analysis. There is an LAA determination for indirect dietary eff ects based on nonagricultural ROW, non-crop, conifer release, Christmas tree, alfalfa and pasture for nonagricultural ROW, non-crop, conifer release and Christmas tree uses and m ode of action for non-vascular plants based on LOC exceedence and mode-of-action for he xazinone. Indirect ha bitat modification results from mode of action from use of hexazinone.

3. Stressor Source and Distribution

Hexazinone is a triazone systemic herbicide used to control weeds and woody plants. Its potency is a function of soil texture, soil or ganic matter and post-application weather as it is activated by moisture. It is toxic to some evergreen trees and most broadleaf trees. The labels suggest field-testing by the user for possible toxicity to evergreens for which the toxicity is uncertain. Based on the availa ble information, hexazinone appears to be persistent and mobile in soil and aquatic enviro nments and is considered as the stressor in this assessment ( Table 3.1 ). The mobility of hexazinone was demonstrated in batch equilibrium as well as field and forestry dissi pation data. It is stable to hydrolysis and photolysis and degrades only slowly by aerobic metabolism. Aerobic aquatic and aerobic soil half-life values are 60 and 216 days, re spectively, and field dissipation half-lives were 123 to 154 days in bare ground silt loam soil. Laboratory adsorption data show low water/soil partitioning for hexazinone (K OC = 37). These data indicate that hexazinone is highly mobile, thus having strong potential to leach to ground water systems, especially in soil systems such as loam and sand soils that are poor in organic matter.

The mode of action of hexazinone is through i nhibition of photosynthesis. Hexazinone is used against broadleaf weeds, grasses and so me woody plants. It is systemic and it is absorbed through the foliage & roots; furthe r, it is translocated through the xylem. Rainfall or irrigation after application is requir ed to activate the chemical. It may be used with a nonionic surfactant to improve its contact activity.

A summary of physicochemical propert ies of hexazinone is included in Table 3.1

.

Table 3.1. Summary of physicochemical properties of hexazinone.

PARAMETER VALUE(S) (units) SOURCE

CAS Chemical Name 3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine2,4(1 H ,3 H )-dione Hexazinone data sheet: http://www.alanwood.net/pe

4Table 3.1. Summary of physicochemical properties of hexazinone.

PARAMETER VALUE(S) (units) SOURCE

IUPAC Chemical Name 3-cyclohexyl-6-dimethylamino-1-methyl-1,3,5-triazine2,4(1 H ,3 H )-dione

CAS Reg. No. 51235-04-2 sticides/index_cn_frame.ht ml Accessed 09/30/09

Molecular Formula C 12 H 20 N 4 O 2 HSDB

SMILES Code O=C(N=C(N(C1(=O))C)N©C)N1C(CCCC2)C2 EPI Suite TM

Chemical Structure (from chemical’s data sheet)

Molecular Weight 252.36 g/mol HSDB

Solubility ( 25 o C) 33,000 ppm; highly soluble (FAO 2000) Product Chemistry

Vapor Pressure (25 o C) 2 x 10 -7 mmHg; Non-volatile under field conditions (USEPA 2008) Product Chemistry

Henry’s Law Constant 2 x 10 -12 atm-m 3 /mol Product Chemistry

Octanol/ Water Partition Coefficient (log K OW and K OW ) 1.2 and 15, respectively [1.85 and 71, respectively] Product Chemistry [BioByte (1995), cited by EPI SuiteTM]

Octanol/ Air Partition

Coefficient (25 o C) (K OA ) K OA = 15 x 8.205746 m 3 atm/K mole x 298K = 2 x 10 16

2 x 10 -12 atm-m 3 /mol (7.7 x 10 11 ) Constant Law sHenry' RTK
K K K OW

AW (EPI SuiteTM estimate) OW OA ==

Biomagnification Potential Presumption*: If log K OA > 5, log K OW >2 and the rate of chemical transformation is low, the chemical may biomagnify in terrestrial food chains. This presumption is hereby used as a general guide since the EFED has not adopted a guide to determ ine the biomagnification potential. For hexazinone, log K OA =12-16 & rate of transformation is slow, however, log K OW =1.85; it appears that hexazinone may biomagnify, but possibl y not substantially, in terrestrial food chains. *Gobas et al. 2003 and Armitage & Gobas, 2007 support this presumption.

Hexazinone has a moderately high molecular we ight of 252.36 g/mol. With a very high solubility (33,000 ppm) and a low oc tanol/ water partition coefficient (K OW = 15), it appears that hexazinone has a very low potentia l to bioaccumulate/ bioconcentrate. With a relatively low vapor pressure (2 x 10 -7 mmHg), a very low Henry’s Law Constant (2 x 10 -12 atm-m 3 /mol), and with its very high solubility (33, 000 ppm ), hexazinone does not appear to volatilize subs tantially from wet surfaces. For hexazinone, log K OA =12-16 & rate of transformation is slow, however, log K OW =1.85; it appears that hexazinone may biomagnify, but possibly not substantially, in terrestrial food chains.

5Even though various degradates were observed in the laboratory studies ( e.g. 1, A, A-1 and B, etc.), it was found that they were gene rally present at relativ ely low concentrations due to the high persistence of hexazinone. Table 3.2 shows the chemical structures of hexazinone’s major metabolites. This asse ssment is based on the parent because hexazinone it is so persistent in the fi eld, therefore consideration of toxicity of degradates will not change the result of this assessment. At this time, they are not considered as
stressors. Table 3.2. Major Transformation Products of Hexazinone Common

Name Chemical Name Structure

A 3-(4-hydroxycyclohexyl)-6- (dimethylamino)-1-methyl-1,3,5- triazine-2,4 (1H,3H)-dione

N N

N O O

N CH 3

CH 3 CH 3 OH

S

A-1 3-hydroxycyclohexyl-6- dimethylamino)-1-methyl-1,3,5- triazine-2,4 (1H,3H)-dione
N N

N O O

N CH 3

CH 3 CH 3 OH

B 3-cyclohexyl-6-(methylamino)-1- methyl-1,3,5-triazine-2,4 (1H,3H)-dione

N N

N O O

NH

CH 3 CH 3 C 3-(4-hydroxycyclohexyl-6-methylamino)-1-methyl-1,3,5-triazine2,4 (1H,3H)-dione

N N

N O O

NH

CH 3 CH 3 OH

S

6Table 3.2. Major Transformation Products of Hexazinone Common

Name Chemical Name Structure

D 3-cyclohexyl-1-methyl-1,3,5-triazine2,4,6-(1H,3H,5H)-trione

N NH

N O O

O

CH 3 E 3-(4-hydroxycyclohexyl)-1-methyl1,3,5-triazine-2,4,6 (1H,3H,5H)-trione

N NH

N O O

O

CH 3 OH

S

G 3-cyclohexyl-6-(methylamino)-1,3,5- triazine-2,4 (1H,3H)-dione

N NH

N O O

CH 3 NH

CH 3 1 3-(4-ketocyclohexyl)-6- (dimethylamino)-1-methyl-1,3,5- triazine-2,4 (1H,3H)-dione O

N N

N O O

N CH 3

CH 3 CH 3

7Table 3.2. Major Transformation Products of Hexazinone Common

Name Chemical Name Structure

2 3-(2-ketocyclohexyl)-6- (dimethylamino)-1-methyl-1,3,5- triazine-2,4 (1H,3H)-dione
N N

N O O

N CH 3

CH 3 CH 3 O

3.1. Mechanism of Action

Tu, et al., 2001 indicate the following “Hexazinone is a systemic herbicide that inhibits photosynthesis in susceptible plants, diverting highly reac tive molecules into a chain reaction that destroys chlor oplast and cell membranes, and other vital compounds. It is usually applied as a pre-emergent herbicid e, and soils must be moist (by rain or irrigation) to activate hex azinone. Hexazinone works by binding to a protein component of the photosystem II complex, which blocks electron transport. The result is a chain reaction in which triplet-stat e chlorophyll reacts with oxygen (O 2 ) to form singlet oxygen (O), and both the chlorophyll and si nglet oxygen strip hydrogen ions (H + ) from unsaturated lipids in cell and organelle me mbranes, producing lipid radicals. The lipid radicals in turn attack and oxidize other lipid s and proteins, resulti ng in the loss of cell and organelle membrane integrity, loss of ch lorophyll and carotenoids, leakage of cellular contents, cell death, and ultimately death of the plant (WSSA, 1994).”

3.2. Overview of Pesticide Usage

Hexazinone is a systemic herbicide used to control weeds and woody plants. Its potency is a function of soil texture, soil organic matter and post-application weather as it is activated by moisture. It is toxic to some evergreen trees and most broadleaf trees. The labels suggest field-testing by the user for possible toxicity to evergreens for which the toxicity is uncertain. Both liquid and gra nular formulations of hexazinone may be applied by aircraft as well as ground spraye r. While the specific equipment for ground application varies between the liquid a nd granular formulations, both types of formulations are applied such that the herbic ide sprayer for liquid or suspended granules or other equipment for granules is carrie d by backpack or some other appropriate container. Usually, hexazinone is applied dire ctly to the soil rather than sprayed on the vegetation; however, sometimes, directed foliar applications are used. In soil applications, the hexazinone is applie d in spots using a defined pattern. Analysis of labeled use information is the cr itical first step in evaluating the federal action. The current label for hexazinone represents the FIFRA regulatory action; therefore, labeled use and app lication rates specified on the label form the basis of this

8assessment. The assessment of use information is critical to the deve lopment of the action area and selection of appropriate modeling scenarios and inputs. Nationally, hexazinone is an herbicide used to control a broad spectrum of weeds including undesirable woody plants in al falfa, rangeland and pasture, woodland, pineapples, sugarcane and blueberr ies. It is also used on or namental plants, forest trees and other non-crop areas. Hexazinone is regi stered for pre-emergent, post-emergence, layby, directed spray and basal soil applications . It is used as a non-selective herbicide in non-cropland areas and as a se lective herbicide in refore station practices. Hexazinone products are formulated as pellet s/tablets, emulsifiable concen trates, ready-to-use liquids, soluble concentrates/solids and a technical grade active ingredient. Products are applied using aerial or ground equipment or by hand, or using a hand-held, boom, knapsack or power sprayer. There are no indoor uses. National (Section 3) uses for hexazinone are presented in Table 3.3 with label maximum application rates. Although th e maximum number of applications from the label for pasture, alfalfa and Christmas tree uses is one, other uses do not specify a maximum rate or maximum number of applications per year . For this assessment, one application per year for those other uses is based on additional informati on from the label. Blueberry uses are modeled at a maximum of one appli cation per year due to no application within 450 days of harvest. Pineapple is modeled as one application per ye ar due to the growing season, which is nearly a year in length. N on-crop uses and all gr anular uses, non-crop, forest site preparation and ra ngeland, are modeled as one app lication per year due to the requirement for rain to activate hexazinone and because maximum effects are achieved after 12-24 months. Hexazinone may be applied in the spring after ground thaws, or in the fall before snowfall.

Table 3.3. Summary of Hexazinone Uses 1 and Application Information

Crop Max Appl Rate (lb a.i./A) Sewage disposal areas (non-granular) 8 Agricultural rights-of-way/ fencerows/ hedgerows (non-granular) 8 Agricultural uncultivated areas (non-granular) 8 Airports/ landing fields (granular) 8 Conifer release (non-granular) 4 Sugarcane (non-granular) 3.6 Pineapple (non-granular) 3.6 Blueberry (non-granular) 3 Rangeland (granular) 3 Christmas tree plantations (non-granular) 2 Alfalfa (non-granular) 1.5 Pastures (non-granular) 1.1 1. Uses assessed based on LUIS report Table 1 and EFED label data report.
A summary from the Screening Level Usag e Analysis (SLUA) for hexazinone is provided in Table 3.4 . According to the SLUA, the major use for hexazinone is alfalfa, followed by sugarcane and pastureland. For all crops, ≤ 10% of the crop is treated.

9Table 3.4. Summary of Screening-Level Estimates of Agricultural Uses of Hexazinone (SLUA, dated 06/16/09). Percent (%) Crop Treated Crop (alphabetical) lb a.i. Average Maximum Alfalfa 300,000 <2.5 5 Cotton + 1,000 <1 <2.5 Fallow, Summer <500 <1 <2.5 Hay, Other (NPUD ‘02)+ 4,000 N/C <1 Pasture 20,000 <1 <2.5 Pastureland 8,000 <1 <2.5 Sugarcane 20,000 5 10 All numbers rounded. ‘<500’ indicates less than 500 pounds of active ingredient. ‘<2.5’ indicates less than 2.5 percent of crop is treated. ‘<1’ indicates less than 1 percent of crop is treated.

SLUA data sources include: USDA-N ASS (United States Department of Agriculture’s National Agricultural Statistics Service); 2001-2007; Private Pesticide Market Research, 2001-2007; NPUD 2002 (National Pesticide Use Database) of the CropLife America Foundation and California DPR data. These results reflect amalgamated data developed by the Agency and are releasable to the public. N/C = Not Calculated + = These crops were not known to be listed on active end use product or emergency exemption (Section 18) registrations when this report was run.

The U.S. Geological Survey (USGS) pesticide use maps ( Fig. 3.1 ) show regional scale patterns in use intensity within the United St ates. The USGS pesticide maps are based on State level estimates of pesticide use ra tes for individual crops, which have been compiled by the National Center for Food and Agricultural Policy (NCFAP) for 1999 through 2004, and on the 2002 Census of Agricu lture for county crop acreage crop. In comparison to the SLUA, the major use of hexazinone is again alfalfa hay (~91% of the use), followed by pastureland (~ 7%) and blueberries (~2%).

1 0

### Fig. 3.1. Use of Hexazinone in 2002.

Note of Caution: The pesticide use maps available from the web ( http://water.usgs.gov/nawqa/pnsp/usage/maps/compound_listing.php?year=02 ) show the average annual pesticide use intensity expressed as average weight (in pounds) of a pesticide applied to each square mile of agricultural land in a county. The area of each map is based on statelevel estimates of pesticide use rates for individual crops that were compiled by the CropLife Foundation, Crop Protection Rese arch Institute during based on information collected during 1999 through 2004 and on 2002 Census of Agriculture county crop acreage. The maps do not represent a specific year, but rather show typical use patterns over the five year period 1999 through 2004. Use intensity rate s are expressed as the pounds applied per square mile of mapped agricultural land in a county. The area of mapped agricultural land for each count y was obtained from an enhanced version of the 1992 USGS National Land Cover Data (NLCD). The key limitations of the data used to produce thes e maps include the following: (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 states and counties, (2) pesticide use estimates are not for a specific year, but represent typical use patterns for the five year period, (3) state pesticide use coefficients may not have been available for all states where a pest icide may have been applied to agricultural land, and therefore, are not displayed on the maps, (4) the county crop acreage is based on the 2002 Ce nsus of Agriculture and may not represent all crop acreage because of Census nondisclosure rules, and (5) agricultural land area used t o calculate the pesticide use intensity and display the data was derived from 30-meter satellite remote sensing data that may over estimate or underestimate the actual agricultural land area. The maps are not intended for making local-scale estimates of pesticide use, such as estimates a t the county level. Please refer to Method for Estimating Pesticide Use for a detailed discussion of how the pesticide use data were developed.
3.3. Environmental Fate and Transport

Based on the available information, hexazinone appears to be persistent and mobile in soil and aquatic environments and is consid ered as the stressor in this assessment ( Table 3.5 ). The mobility of hexazinone was demonstrated in batch equilibrium as well as field

11and forestry dissipation studies . Hexazinone is stable to hydrolysis and photolysis and degrades only slowly by aerobic metabolism. Aerobic aquatic and aerobic soil half-life values are 60 and 216 days, respectively, and field dissipation half -lives were 123 to 154 days in bare ground silt loam, silty clay loam and sandy loam soils. Laboratory adsorption data show low water/ soil partitioning for hexazinone (K OC = 38-77 mL/g OC , mobile FAO 2000 mobility classifi cation). These data indicate that hexazinone is highly mobile, thus having strong potential to leach to ground water systems, especially in soil systems such as loam and sand soil s that are poor in organic matter. Hexazinone is very soluble in water at 20 ° C with a solubility of 33,000 mg/L. Based on its low vapor pressure (2.0 x 10 -7 mm Hg at 20 ° C) and Henry’s Law Constant (2.0 x 10 -12

atm

⋅ m 3 /mol at 25 ° C) volatilization loss from soil and water systems is expected to be insignificant. Degradates and transformation products are formed slowly and are also mobile and persistent. Based on labor atory bioaccumulation in rainbow trout, hexazinone is not expected to bioaccumulate in fish, which is in concurrence with the low K OW value of 15 (log K OW = 1.12). Hexazinone has been shown to accumulate in rotational crops. The major photodegradation product, on soil, was Metabolite B (~10% of applied). Minor photodegradation products were Metabolit es A and D. At termination of the aerobic study, Metabolites A-1, 1, B and D were a maximum of 18.7, 10.9, 2.3 and 4.8% of the applied, respectively (365 days). In anaerobic aquatic environment, metabolites A’ and 1, were 5.5 and 25.0% of the applied, resp ectively, at 365 days. The metabolite D was reported to be at least 24.0%, and the metabolite 2 was a maximum 36.5% (of the recovered) at 365 days, for a single sample . Metabolites A, A- 1, B, C and 1 were identified in soils in terrestrial field. Me tabolites A, B, C, D, E, G, and H were discernible in various matrices (soil, water, litter and/or ve getation) samples in forestry field samples. Metabolites D and 2 were ma jor degradates found in the anaerobic aquatic metabolism study; however, the field dissi pation and the small-scale prospective groundwater monitoring studies did not monito r these two metabolites. Therefore, the fate of Degradates D and 2 could not be assessed under natural environment; the registrant believed that both field studies were mostly aerobic and the metabolites were unlikely to be observed. The field dissipat ion studies did not m onitor the fate of Metabolite G-3170 the degradate de tected at the highest concen trations in the small-scale prospective groundwater monitoring study. Th is metabolite was not observed in the aerobic soil metabolism study. A summary of en vironmental fate/ transport properties of hexazinone is included in Table 3.5

.

Table 3.5. Summary of Environmental Fate and Transport Properties of Hexazinone

PARAMETER VALUE(S) (units) SOURCE

Hydrolysis Half-life pH 5, 7, 9; (25 o C) Stable at all pHs. Hexazinone comprised from 99-100% of the total radioactivity in all samples. MRID 41587301 Acceptable

Aqueous Photolysis - Half-life (pH 5) Relatively stable, with similar hexazinone recoveries for the light irradiated and dark control samples. MRID 41300801 Acceptable

1 2Table 3.5. Summary of Environmental Fate and Transport Properties of Hexazinone

PARAMETER VALUE(S) (units) SOURCE

Soil Photolysis - Halflife SL soil from Modesto, CA (pH 6.5) – environmental half-life of 228 days. Hexazinone did not degrade significantly in the dark control samples. Major degradate was meta bolite B at 9.5-10.1% at 30 days. MRID 41300802 Acceptable

Aerobic Soil Metabolism Half-life Hexazinone’s half-life was 216 when applied to a SL soil (pH 6.5) from Modesto, CA, at 25oC. Meta bolites A-1, 1, B and D were a maximum of 18.7, 10.9, 2.3 and 4.8% of the applied, respectively, at 365 days (last interval). MRID 41807401; 42635001 Acceptable

Anaerobic Soil Metabolism - Half-life Not Available Not Available

Aerobic Aquatic Metabolism - Half-life The half-life for hexazinone in a sa ndy loam sediment (pH 7.8, %OM = 0.7) and water (pH 8.2) from Ma dera, CA system was >60 days. The minor metabolites 1, B, A-1 and D accounted for a maximum of 6.5, 5.3, 5.1 and 1.3% of the appl ied for individual replicates, at 28, 28, 14 and 56 days, respectively. MRID 41811801 Supplemental( 1 )

Anaerobic Aquatic Metabolism - Half-life In a loam (pH 8.5, %OC = 1.24) and pond water (pH 7.83) anaerobic system, the half-life was 230 days. The metabolites A’ and 1, were 5.5 and 25.0% of the applied, respectively, at 365 days. The metabolite D was reported to be at least 24.0%, and the metabolite 2

was also a major transformation product (maximum 36.5% of the recovered at 365 days, for a single sample). MRID 41807402 Acceptable

Organic Carbon Partition Coefficient for Parent (K OC ) 38, 75, 77 & 40 mL/g OC , for Tama SiL, Chino L, Trevino SL & Sassafras SL Mobile (FAO 2000)

Soil Partition Coefficient for Parent (K d ) 0.53, 0.84, 0.59 & 0.21 mL/g, for Tama SiL, Chino L, Trevino SL & Sassafras SL 1/N range 0.86-0.93 10.8, 0.24, 0.45 & 1.03 mL/g, for Madera L, Hanford SL, Cecil SL & Flanagan SiL Supplemental data provided by one of the studies, indicate that the metabolites A, B, D, H, 1, 2 and A-1 have similar mobility than the parent compound. MRID 41528101, 43621501 Acceptable, Supplemental

Terrestrial Field Dissipation
Half-life Study on a silt loam in Newark, DE and on a silty clay loam in Greenville, MS. The respective hal f-lives were 123 and 134 days in bareground plots at 12 lb a.i./A. Th e metabolites A, A-1, B, C and 1 were identified in soils at maximum concentrations of 0.04, 0.21, 0.31,

1.23 and 0.71 ppm, respectively. For hexazinone >95% of residue confined to the upper 30 cm of soil at the Newark site. Hexazinone did move to a depth of 60-75 cm at the Greenville site. The hexazinone metabolites (particularly C in Newark) leached to a depth of 75 cm of soil during the testing period.

Hexazinone, at 12 lb a.i./A, had a di ssipation half-life of 140 days in a bareground sandy loam in Madera, CA. Metabolites A, A-1, B, C and

1 were identified in soils at maximum concentrations of 0.05, 0.04, 0.29, 0.62 and 0.20 ppm, respectively . The reported data indicate that

hexazinone leached through the 90 cm soil depth. The hexazinone metabolites were not reported below a depth of 30 cm of soil during the testing period. MRID 42377901, 42379201 Acceptable, Supplemental

13Table 3.5. Summary of Environmental Fate and Transport Properties of Hexazinone

PARAMETER VALUE(S) (units) SOURCE

Forestry Field Dissipation
Half-life Hexazinone dissipated with reported half-lives of 26 to 59 days and 19 to 36 days in plants for the Velpar ULW site and the Velpar L site, respectively. The chemical was applied to typical piedmont pine forest land at an application rate of6.72 lb a.i./A on separate forestland watersheds in Coosa County, AL. Similar half-lives of 55 to 77 days were reported for litter and soil (except litter covered soil) for the two sites. For litter covered soil, a half-life of 265 days was reported. By day 365 posttreatment, hexa zinone was discernible only in sediment samples. In a ddition, metabolites A, B, C, D, E, G, and H were discernible in various matrices (soil, water, litter and/or vegetation) samples. Hexazinone moved off-site through leaching and runoff: the chemical (0.01 to 0.04 ppm) was observed in soil at depths >30 cm; in addition, it was detected (80 to 140 ppb) in runoff water up to 6 months post-application. MRID 42336401 Acceptable

Biaccumulation in Fish (BCF) The studies showed that hexazin one did not have the tendency to bioaccumulate in fish . Based on the octanol/ water partition coefficient (15), hexazinone may not be expected to substantially accumulate in fish. Waived 00064265 (00078032)

Prospective Ground Water Monitoring Hexazinone [Velpar L] was applied at 0.75 lb a.i./A onto a field of alfalfa underlain with sandy soil in Merced County, California. The site is located within the recharge area for a shallow unconfined aquifer (OM of the soil was ≤ 0.7%, pH 7.5-8.9 in the upper 1.5 feet). Hexazinone dissipated from the upper 2 feet of soil with a calculated half-life of 25 days (r 2 = 0.9997; first order kinetics). Results indicated that hexazinone and its degradates are very mobile and persistent. They exhibited a cla ssic pattern of leaching through the soil profile and into the groundwater. Samples were analyzed for hexazinone and six transformation pr oducts: G3170, A, B, C, A1, and 1. Hexazinone concentrations above background (~0.05 ppb) were first detected in pore water at the 3-foot depth at 60 days posttreatment and in the 6- and 9-foot depths at 95 days. The breakthrough of hexazinone and degradate B into the shallow groundwater occurred at 152 days posttreatment. G3170 was the predominant degradate in the soil, soil pore water, and groundwater; breakthrough into the shallow groundwater occurred at 336 days. In the individual shallow groundwater wells (14-19 feet below ground surface), the maximum measured concentrations were as indicated in the table below.

Chemical Maximum Concentration in Groundwater (ppb) Maximum Concentration in Groundwater (ppb as parent equivalent) Parent 9.2 9.2 A-1 (G3453) 3 2.8 B (A3928) 7.2 7.6 C (T3935) 1.2 1.1 1 ((JS472) 2.1 2.0 G3170 12.9 19.1 MRID 45132801 (Other related studies, 44060101, 44082801, 44106601 and 44194001)

1. Previously considered acceptable, but upon reevaluation, it is considered supplemental du e to insufficient sampling periods.



### Abiotic Degradation

1 4Hexazinone was stable to hydrolys is in buffered solutions at pH 5, 7 and 9 at 25oC. The chemical comprised from 99-100% of the tota l radioactivity in all samples. When exposed to an artificial light source, hexazinone did not a ppear to degrade significantly in pH 7 buffered solutions. Similar hexazinone recoveries were observed for the light irradiated and dark control samples. No major degradates were identified in these samples. In a soil photode gradation study, using a sandy lo am soil from Modesto, CA (pH 6.5), hexazinone’s half-life was 82 days, equi valent to an envir onmental half-life of 228 days at latitude 30 to 50oN, a ssuming a 12-hour photoperiod. The major transformation product was Meta bolite B, which was 9.5-10.1% of the applied at 30 days (last test interval).



### Biotic Degradation

Hexazinone’s half-lives were 216 and 1440 days , when applied to viable and sterile (autoclave) sandy loam soil (pH 6.5) from Modesto, CA, and maintained under aerobic conditions in the dark at 25oC. Two majo r and two minor transformation products were identified. All of them reached a maximum at 365 days (last test interval). Metabolites A-1, 1, B and D were a maximum of 18.7, 10.9, 2.3 and 4.8% of the applied, respectively. The viable and sterile soil data indicate that degradation was mainly a result of microbial activity. Carbon dioxide account ed for 2.2 and 0.03% of the applied after 365 days for the viable and sterile soils, respectively.

Sediment/Water Systems

The half-life for hexazinone in an aer obic aquatic metabolism study, consisting of a sandy loam sediment (pH 7.8, %OM = 0.7) an d water (pH 8.2) from Madera, CA was >60 days, both in viable and sterile (autoc lave) systems. Hexazinone accounted for ~80- 84% and ~86-87% of the applied radioactivit y at the end of the study (56 days) in the viable and sterile samples, respectively. The minor metabolites 1, B, A-1 and D accounted for a maximum of 6.5, 5.3, 5.1 and 1.3% of the applied for individual replicates, at 28, 28, 14 and 56 days, respectively. The majority of the applied radioactivity was relate d to the water (as opposed to the sediment). At the beginning and end of the study, 88.3% and 76.4% of the applied radioactivity were related to the water, respectively. No anaerobic soil metabolism study was availabl e for review. In an anaerobic aquatic metabolism study, that consisted of a loam (pH 8.5, %OC = 1.24) and pond water (pH 7.83), the half-lives were 230 and >1,500 days for the viable and sterile (autoclave) systems. Hexazinone was an average of 95.6% and decreased to 29.0%, at times 0 and 365 days, respectively. The metabolites A’ and 1, were 5.5 and 25.0% of the applied, respectively, at 365 days. The metabolite D was reported to be at least 24.0% and the metabolite 2 was also a major transformati on product (maximum 36.5% of the recovered at 365 days).

Mobility

15In a Madera loam, Hanford sandy loam, Cecil sandy loam and Flanagan silty loam, the Freundlich K F values were 10.8, 0.24, 0.45 and 1.03 mL/g, respectively. Meanwhile, in a Tama silty loam, Chino L, Trevino sandy loam and Sassafras sandy loam, the Freundlich K F values were 0.53, 0.84, 0.59 and 0.21, respectivel y. For the latter soils, the Freundlich K FOC values were 38, 75, 77 and 40, respectively. The 1/N values were close to unity (0.93, 0.91, 0.91 and 0.86, respectively). Suppl emental data provided by the second study, indicate that the metabolites A, B, D, H, 1, 2 and A-1 have similar mobility than the parent compound.

Field Dissipation

One terrestrial field dissipation study was c onducted at two sites, on a silt loam in Newark, DE and on a silty clay loam in Gree nville, MS. The respective half-lives were 123 and 134 days in bare ground plots. The metabolites A, A-1, B, C and 1 were identified in soils at maximum con centrations of 0.04, 0.21, 0.31, 1.23 and 0.71 ppm, respectively. The reported data indicate that hexazinone, at 12 lb a.i./A, did not move beyond the top 30 cm of soil (>95% confined to the upper 30 cm of soil) at the Newark site. Hexazinone did move to a depth of 60-75 cm at the Greenville site. In addition, the hexazinone metabolites (particularly C in Newa rk) leached to a depth of 75 cm of soil during the testing period. Hexazinone, at 12 lb a.i./A, had a dissipati on half-life of 140 days in a bareground sandy loam in Madera, CA. Metabolites A, A-1, B, C and 1 were identified in soils at maximum concentrations of 0.05, 0.04, 0.29, 0.62 and 0.20 ppm, respectively. The reported data indicate that hexazinone leached through the 90 cm soil depth. The hexazinone metabolites were not reported be low a depth of 30 cm of soil during the testing period. The study was considered supplemental because the concentrations at time zero were too low, compared to the theo retical (or expected) value; however, the day 7 thru 60 post-treatment samples had con centrations of 40-51% of the theoretical maximum rate. Hexazinone dissipated with reported half-lives of 26 to 59 days and 19 to 36 days in plants for the Velpar ULW site and the Velpar L site, respectively, in a forestry field dissipation study. Hexazinone wa s applied to typical piedmont pine forestland; an application rate of 6.72 lb a.i./A was used on separate forestland watersheds in Coosa County, AL. Similar half-lives of 55 to 77 da ys were reported for litter and soil (except litter covered soil) for the two sites. For litter-covered soil, a half-life of 265 days was reported. By day 365 post-treatment, hexazi none was discernible only in sediment samples. In addition, metabolites A, B, C, D, E, G, and H were discernible in various matrices (soil, water, litter and/or vegeta tion) samples. However, metabolite G was not confirmed by MS. Hexazinone moved off-s ite through leaching and runoff. Hexazinone (0.01 to 0.04 ppm) was observed in soil at de pths >30 cm. In addition, it was detected (80 to 140 ppb) in runoff water up to 6 months post-application. In the forestry study, hexazinone was detect ed from stream samples from both sites (maximum concentration 473 ppb at 1150 hours pos t-treatment at the Velpar L site). Metabolites A, B and H were detectable in the peripheral drainages (1-3 ppb, <25 of 293 samples). Metabolites A, B, C, D and E were discernible in fern, dogwood, grass and

1 6blueberry plant tissues (both site s). All plant half-lives were <59 days. In litter samples, hexazinone half-lives were 55-56 days. The metabolites B and D were the most frequently detected (Velpar L site). Fina lly, in the soil samples, hexazinone and its metabolites were consistently observed thru 30 cm depth, with detections thru 75-90 cm soil depth. The metabolites A-1 and 1 were not analyzed for in the study. A prescripted burn of all three watersheds (Velpar ULW, Velpar L and Control) took place 6 months post-treatment.

Prospective Ground Water Monitoring

The registrant submitted a small-scale pr ospective groundwater monitoring study for hexazinone (MRID 45132801). Hexazinone [Velpar L, 2 lb a.i/gallon water-dispersible liquid] was broadcast applie d once at 0.75 lb a.i./A in January 1996 onto a field of alfalfa underlain with sandy soil in Merced County, Cali fornia. The site is located within the recharge area for a shallow unconfined aquife r. The treated area consisted of five 100 x 300 foot strips separated by irrigation berms, and the control area was 100 feet distant and upgradient from the treated area. The organic matter content of the soil was ≤ 0.7%, the pH was 7.5-8.9 in the upper 1.5 feet, and th ere were no continuous impeding layers. The first rainfall was received at 7 days and th e first irrigation event was at 75 days. The cumulative net daily water budget was <0.0 inches through 19 days, 0.10 inches at 21 days, and 0.93 inches at 21 days. The mont hly cumulative precipitation plus irrigation ranged from 111 to 195% of baseline. The dept h of the water table ranged from 8 to 15 feet below ground surface (bgs) during the study. Potassium bromide was used as a tracer; breakthrough into th e shallow groundwater occurr ed at about 152 days posttreatment. Soil (to 2 feet), soil pore wa ter (lysimeters at 3, 6, 9, and 12 feet), and groundwater (wells screened at 14-19 and 20-25 feet bgs) samples were collected at up to 1063 days post-treatment and analyzed using HPLC for hexazinone and six transformation products: G3170, A, B, C, A1, and 1. Hexazinone dissipated from the uppe r 2 feet of soil with a calcu lated half-life of 25 days (r 2 = 0.9997; first order kinetics). Hexazinone and its’ degradates exhibited a classic pattern of leaching th rough the soil profile and into the groundwater. Hexazinone concentrations above background (~0.05 ppb) were first detected in po re water at the 3- foot depth at 60 days post-treatment and in the 6- and 9-foot depths at 95 days. The breakthrough of hexazinone a nd degradate B into the sha llow groundwater occurred at 152 days post-treatment. G3170 was the pre dominant degradate in the soil, soil pore water, and groundwater; break through into the shallow groundwater occurred at 336 days. In the individual shallow groundw ater wells (14-19 feet bgs), the maximum measured concentrations were as indicated in the table above. In the individual deep groundwater wells (20-25 feet bgs), the ma ximum measured concentrations were 2.33 ppb for hexazinone, 2.61 ppb for G3170, 1.32 ppb for B, 0.952 ppb for A1, and 1.32 ppb for C. The average concentrations of all co mpounds of interest in the shallow and deep groundwater were below their Limit of Quantitation (LOQ 2.0 ppb except 4.0 ppb for C and G3170) in water, except once at 363 days when hexazinone averaged 2.02 ppb in the shallow groundwater. There was a distinct difference in the movement of hexazinone through soil in the western and eastern portions of the site. Hexazinone residues were

1 7detected infrequently and at later intervals in groundwater in the eastern portion of the site, where soils were less sandy with thicker soil lens.

Results indicated that hexazinone and its’ degradates are very mobile and persistent. The Degradates D and 2 (which were the major degradates found in the anaerobic aquatic metabolism study) were not monitored in the small-scale prospective groundwater monitoring studies. The following table (T able 3.6) provides a summary with the maximum concentrations of the parent and it s’ degradates, detected in the small-scale prospective groundwater monitori ng study. These concentrations were also expressed in parent equivalents.

Table 3.6. Max. Concs. in Small-Scale Pros pective Groundwater Monitoring Study Chemical Parent A-1 B C I G3170 ppb 9.2 3 7.2 1.2 2.1 12.9 ppb (parent equivalents) 9.2 2.8 7.6 1.1 2.0 19.1 PE = Parent Equivalents

Bioaccumulation

Based on laboratory bioaccumulation in rainbow trout, hexazinone is not expected to bioaccumulate in fish, which is in concu rrence with the low octanol/ water partition coefficient, K OW value of 15 (log K OW = 1.12). Hexazinone has been shown to accumulate in rotational crops. The fate and transport characterization al so includes the vari ous degradation and transformation products formed by each process in the studies reviewed. Table 3.7 provides a summary of thes e transformation products.

Table 3.7. Summary of Degradate Format ion from Degradation of Hexazinone

Maximum Degradate Concentration (% of applied) and Time (days) to Maximum Concentration in Study: Degradates Analyzed in Study:

Metabolite Soil Photolysis Aerobic Soil Aerobic Aquatic Anaerobic Aquatic Field Dissip. Ground Water
A (T3937) 5.5% (365d) yes yes
A-1 (G3453) 18.7% (365d) <10.0% (56d) yes yes

B (A3928) 10.1% (30d) 2.3% (365d) <10.0% (56d) yes yes
C (T3935) yes yes
D (B2838) 4.8% (365d) <10.0% (56d) 24.0% (365d)

1 ((JS472) 10.9% (365d) <10.0% (56d) yes yes

1 8Table 3.7. Summary of Degradate Format ion from Degradation of Hexazinone

Maximum Degradate Concentration (% of applied) and Time Degradates (days) to Maximum Concentration in Study: Analyzed in Study:

Metabolite Soil Photolysis Aerobic Soil Aerobic Aquatic Anaerobic Aquatic Field Ground Dissip. Water
2 (JT677) 25.0% (365d)

G3170 yes

Metabolites D and 2 were the major tran sformation products found in the anaerobic aquatic metabolism study; however, the field di ssipation and the small-scale prospective groundwater monitoring studies did not monito r these two metabolites. Therefore, the fate of Metabolites D and 2 could not be assessed under natural environment; the registrant believed that both field studies were mostly aerobic and degradates were unlikely to be observed. The field dissipati on studies did not mon itor the fate of the degradate detected at the highest concen trations in the small-scale prospective groundwater monitoring study, Degradate G- 3170, however, this degradate was not observed in the aerobic soil metabolism study.

Table 3.8. Mobility of Vari ous Hexazinone Metabolites

Adsorption (Mobility) Studies K d K OC Source (MRID)

Metabolite 1 0.20 122

Metabolite 2 0.15 45

Metabolite A 0.69 176

Metabolite A-1 0.03 27

Metabolite B 0.44 90

Metabolite D 0.34 59

Metabolite H 0.31 51 43621501 (Supplemental portion of the study)

Potential transport mechanisms include pest icide surface water runoff and spray drift. The magnitude of pesticide transport via secondary drift depend s on the pesticide’s ability to be mobilized into air and its eventual removal through wet and dry deposition of gases/particles and photochemi cal reactions in the atmosphere.

1 94. Receptors

4.1 Aquatic and Terrestrial Effects

The receptor is the biological entity that is exposed to the stresso r (EPA, 1998). Due to the outdoor uses of hexazinone, the types of recept ors that may be exposed include both aquatic and terrestrial receptor s, such as birds, reptiles, mammals and freshwater and estuarine/ marine fish, non-targ et invertebrates and terrestrial and aquatic plants. This list may not be comprehensive. Spray drift and runoff exposures are expected for both ground and aerial applications of hexazinone. Consistent with the process described in the Overview Document (EPA, 2004), this ri sk assessment uses a surrogate species approach in its evaluation of hexazinone. Toxicological data generated from surrogate test species, which are intende d to be representative of broad taxonomic groups, are used to extrapolate to potential eff ects on a variety of species (r eceptors) included under these taxonomic groupings.

Table 4.1 provides a summary of the taxonomic groups and the surrogate species tested to help understand potential eco logical effects of pesticides to these non-ta rget taxonomic groups. In addition, the table provides a preliminary overview of the potential acute toxicity of hexazinone by providing the acute toxicity classifications.

4.1.1 Terrestrial 

### Species

Birds: Acute Exposure (Mortality) Studies Acute oral toxicity data are available for quail (LD 50 = 2,258 mg/kg MRID 00073988) using 98% a.i. The results indicate that hexa zinone is practically nontoxic to birds. This result will be used in this assessment. Dietary acute toxicity studi es resulted in an LC50 > 5,000 mg/kg for mallard duck (MRID 0010498), and bobwhite qua il (MRID 00107878 and MRID 00072663) indicating that hexazinone is practically nontoxic to birds. These studies are not used quantitatively in this assessment because no mo rtality was observed at the highest test concentration.



### Birds: Chronic Exposure (Growth, Reproduction) Studies

Chronic reproductive toxicity data is av ailable for the bobwhite quail (NOAEC = 300 mg/kg MRID 417649-01). The endpoint was a statistically signifi cant increase in food consumption for survivors from day 14 through test termination. This endpoint will be used in this assessment. Chronic reproductive toxicity data is av ailable for the mallard duck (NOAEC >1,000 mg/kg, MRID 41764902). However, this is not the most sensitive endpoint reported, therefore this value will not be used in the assessment.



### Mammals: Acute Exposu re (Mortality) Studies

2 0A registrant submitted rat acute oral (LD 50 =1,200 mg/kg, MRID 41235004) was reported. The results indicate that hexazinone is practically nontoxic to mammals. There is an open literature endpoint that is more sensitive than the registrant submitted data. An acute oral toxicity study is available for rat LD 50 = 530 mg/kg (Kennedy 1984). The results indicate that hexazinone is pract ically nontoxic to mamma ls. This result will be used in this assessment.



### Mammals: Chronic (Growth, Reproduction) Studies

An acceptable two-generation rat study (M RID 42066501) resulted in a systemic and reproductive NOAC=200 mg/kg. This value will be used in this assessment to calculate potential effects.

Toxicity to Terrestrial Invertebrates

Terrestrial invertebrate toxicity data are used to assess potenti al direct and indirect effects of hexazinone to non-target species, as well as to endangered species. Effects to terrestrial invertebrates result ing from exposure to hexazinone could indirectly affect endangered species via reduc tion in available food. An acceptable honeybee study (LD50 > 100 μg/bee, MRID 41216502 ) indicates that hexazinone is practically nontoxic to terrestrial invertebrates. This value will be used qualitatively in the risk description. 4.1.2 Aquatic Species

Both acute and chronic studies provided inform ation on fish and aqua tic invertebrates. A summary of acute and chroni c data is provided below.

Fish: Acute Exposure (Mortality) Studies

With respect to technical grade he xazinone, the reported acute 96-hour LC 50 values were >320 mg/l for rainbow trout (MRID 00104980), and 274 mg/l for Fathead minnow (MRID 00104980). Two studies were availa ble for bluegill sunfish, 96-hour LC 50 >370 mg/L and 96-hour LC 50 = 505 mg/L (MRID 00104980 and 00076959). The results of these tests indicate that hex azinone is practically nontoxic to fish. The fathead minnow LC 50 = 274 mg/l will be used to assess the chroni c effects for the use of hexazinone.

Fish: Chronic Exposure (G rowth/Reproduction) Studies

A fish life cycle test with fathead minnow was submitted for hexazinone (MRID 41406001). A chronic NOAEC=17 mg/L was reported for juvenile survival (day 39), length and weight. This value will be used in this assessment.

Toxicity to Aquatic Invertebr ates: Acute Exposure Studies

21Acute toxicity data for hexazinone are avai lable for the preferred test species, Daphnia magna. A 48-hour EC 50 =151.6 mg/l (MRID 00116269) indica tes that hexazinone is practically nontoxic to a quatic invertebrates. This value will be used in this assessment. Three estuarine/marine species were tested to determine effects from the use of hexazinone (MRID 00047164). The results from the three species will be used to describe a species range. A 48 h EC 50 > 320 mg/L for the Eastern oyster was estimated in this toxicity test, as well as th e grass shrimp with a 96 h LC 50 =78 mg/L and the fiddler crab with a 96 h LC 50 > 1000 mg/L. These studies demonstr ate that hexazinone ranges from slightly to practically nontoxic to mo llusks and slightly toxic to crustaceans.

Aquatic Invertebrates: Ch ronic Exposure Studies

Chronic toxicity data are available for daphnia magna. Two supplemental studies for daphnia resulted in NOEACs of 20 mg/L for reproduction (MRID 00078041) and 29 mg a.i./L for daphnid survival (MRID 41406002). The reproductive NOEAC will be used in this assessment as the most sensitive endpoint.



### Terrestrial Plants

Plant toxicity data from both registrant-subm itted studies and studies in the scientific literature were reviewed for this assessment. Registrant-submitted studies are conducted under conditions and with species defined in EPA toxicity test gui delines. Sub-lethal endpoints such as plant growt h, dry weight, and biomass are evaluated for both monocots and dicots are evaluated at both seedling emer gence and vegetative life stages. Guideline studies generally evaluate toxic ity to ten crop species. A drawback to these tests is that they are conducted on herbaceous crop species only, and extrapolation of effects to other species, such as the woody shrubs and trees and wild herbaceous species, contributes uncertainty to risk conclusions. Commercial crop species have b een selectively bred, and may be more or less resistant to particular stressors than wild herbs and forb s. The direction of this uncertainty for specific plants and stressors, including hexazinone, is larg ely unknown. Homogenous test plant seed lots also lack the genetic vari ation that occurs in na tural populations, so the range of effects seen from tests is likely to be smaller than would be expected from wild populations.

The results from the studies are used to eval uate the effects on non-ta rget plants and are used to evaluate direct and indirect effects for endangered species. Th e results of the Tier II seedling emergence and vegetative vigor to xicity tests on non-target plants are summarized in Table 4.1.

Toxicity data are available for seedling emergence and vegetative vigor for terrestrial plant exposure to hexazinone. The results from the seedling emergence test study resulted in an EC 25 of 0.0064 lbs/A for tomato. The monocot EC 25 result for sorghum was 0.019, lbs/A.

2 2The vegetative vigor EC25 for dicots was 0.011 for rape. The EC 25 for emergence for monocots was 0.020 for wheat. These values wi ll be used in this assessment.



### Aquatic Plants

Aquatic plant toxicity studies are also used to estimate potential non-target effects and endangered species direct and in direct effects to evaluate whether hexazinone may affect primary production and the availability of aquatic plants as food. An acceptable study is available for vascular aquatic plants using Lemna gibba with frond count as an endpoint. The EC 50 =0.0374 mg/L (MRID 43225101) was used in this
assessment. Four studies for nonvascular aquatic plants were available. The acceptable nonvascular aquatic plant study with Anabaena flos-aquae resulted in an EC 50 =0.210 mg/L (MRID 43302701). An acceptable study is available for Selenastrum capricornutum . The EC 50 =0.007 mg/L with a NOAEC=0.004 mg/L (MRID 41287001) were based on nominal concentrations. The most sensitive value for (formerly Selenastrum capricornutum) will be used to determine an LAA/NLAA effect. The acceptable study with the freshwater diatom Navicula pelliculosa resulted in an EC 50 =0.012 mg/L (MRID 43302701). The acceptable study with the estuarine/marine diatom Skeletennoma

costatum resulted in an EC 50 =0.012 mg/L (MRID 43225102).

Table 4.1. Test Species Evaluated for Assessi ng Potential Ecological Effects of Hexazinone and the Associated Acute Toxicity Classification

Taxonomic Group Example(s) of Surrogate Species Acute Toxicity Classification

8d LC 50 > 5000 mg/kg (MRID 00107878 and 00072663) Practically non-toxic Bobwhite ( Colinus virginianus )

Avian Reproduction NOAEC=300 mg/kg

(MRID 41764901)

14d LD 50 =2258 mg/kg-bw (MRID 00073988) Practically non-toxic Birds 1

Mallard ( Anas platyrhynchos )

Mallard LC 50 > 5000 mg/kg (MRID 00104981) Practically Non-toxic

23Table 4.1. Test Species Evaluated for Assessi ng Potential Ecological Effects of Hexazinone and the Associated Acute Toxicity Classification

Taxonomic Group Example(s) of Surrogate Species Acute Toxicity Classification

Avian Reproduction NOAEC>1000 mg/kg

(MRID 41764902)

LD 50 = 530 mg/kg-bw

(Kennedy, 1984) Practically Non- Toxic LD 50 = 1200 mg/kg-bw

(MRID 41235004) Practically Non- Toxic Mammals Laboratory rat ( Rattus norvegicus )

Rat Reproduction NOAEL=200 mk/kg-bw

Insects Honey bee ( Apis mellifera L. ) 48 h LD 50 >100 μg/bee for (MRID 41216502) Practically non-toxic

Bluegill LC 50 =505 mg/L (MRID 00076959) Practically nontoxic BlueGill ( Lepomis macrochirus ) BluegillLC 50 >370 mg/L (MRID 00104980) Practically non-toxic

Rainbow trout ( Oncorhynchus mykiss ) Rainbow trout LC 50 >320 mg/L (MRID 00104980) Practically non-toxic

Fathead Minnow LC 50 =274 mg/L (MRID 00104980) Practically non-toxic Freshwater Fish 2

Fathead minnow ( Pimephales promelas ) NOAEC = 17 mg/L (MRID 41406001) Juvenile survival, length and weight

Acute EC 50 =151.6 mg ai/L (MRID 00116269) Practically non-toxic Aquatic Invertebrates Daphnia magna

TEP: Acute EC 50 =339.9 mg/l (MRID 41235003) Practically non-toxic

2 4Table 4.1. Test Species Evaluated for Assessi ng Potential Ecological Effects of Hexazinone and the Associated Acute Toxicity Classification

Taxonomic Group Example(s) of Surrogate Species Acute Toxicity Classification

Lifecycle Daphnia NOAEC=20 mg/L Reproduction (MRID 00078041) Supplemental Lifecycle Daphnia NOAEC=29 mg/L Daphnid survival. (MRID 41406002) Supplemental

Estuarine/marine fish Sheepshead minnow (Cyprinodon variegatues) Waived based on low FW fish toxicity

Mysid shrimp (Americamysis bahia) Acute estuarine/marine toxicity based on MRID 00047164 shrimp results. Estuarine/marine invertebrates Eastern oyster ( Crassostrea virginica) Acute oyster 96h EC50>320 mg/L (MRID 00047164) Practically nontoxic VV: Rape 121 d EC 25 =0.011 lb/A (MRID 43162501) Vegetative Vigor Monocots – Wheat ( Tritum gestivum )

Dicots – Rape ( Brassica napur ) VV: Wheat 21 d EC 25 =0.020 lb/A (MRID 43162501)

SE: Tomato 21 d EC 25 =0.0064 lb/A (MRID 43162501) Terrestrial plants 3 Seedling Emegence Monocots – Sorghum ( Sorghum bicolor ) Dicots – Tomato ( Lycopersicon esculentum ) Wheat 21 D EC 25 =0.0019 lb/A (MRID 43162501)
Duckweed ( Lemna gibba ) EC 50 =0.0374 mg/L (MRID 43225101)

Green algae (Pseudokirchneriella subcapitata ) 96h EC 50 =0.007 mg/L (MRID 41287001)

Blue green Algae (Anabaena flosaquae ) 96 h EC 50 =0.210 mg/l (MRID 43302701 Acceptable
Freshwater Diatom ( Navicula pelliculosa) EC 50 =.012 mg/L (MRID 43302701) Aquatic plants and algae

Marine Diatom ( Skeletonema costatum ) EC 50 =0.012 mg/L (MRID 43225102)

25Table 4.1. Test Species Evaluated for Assessi ng Potential Ecological Effects of Hexazinone and the Associated Acute Toxicity Classification

Taxonomic Group Example(s) of Surrogate Species Acute Toxicity Classification

1 Birds represent surrogates for terrest rial-phase amphibians and reptiles.-
2 Freshwater fish may be surroga tes for aquatic-phase amphibians.
3 Four species of two families of monocots, of which one is corn; six species of at least four dicot families, of which one is soybeans.

4.2 Incident Database Review

A preliminary review on October 26, 2009 of th e Ecological Incident Information System (EIIS, version 2.1), maintained by the Agency’s Office of Pesticide Programs, and the Avian Monitoring Information System (AIM S), maintained by the American Bird Conservancy, indicates nine re ported ecological incident as sociated with the use of hexazinone. These incidents are summarized below in Sections 4.2.1 through 4.2.3 . This total excludes incidents classified as ‘un likely’ and only includes those incidents with certainty categories of ‘highly probable’, and ‘probable’ a nd ‘possible’ (for EIIS) and ‘certain’, ‘highly likely’, ‘likel y’, ‘probable’, and ‘possible’ (f or AIMS). In the EIIS and AIMS databases, the “unlikely” category is used when a chemical is not likely to be responsible for the incident. For example, an ‘unlikely’ classification might be applied in situations where a given chemical is practic ally nontoxic to the category of organism killed and/ or there is evidence that anot her pesticide or stressor likely caused the incident. Incidents classified as ‘unlikely’ for hexazinone will not be included in this Problem Formulation or the ecological ri sk assessment conducted for Registration Review. Incident reports for non-target organisms t ypically provide information only on mortality events and plant damage incidents. Except for phytotoxic effects in te rrestrial plants, sublethal effects for organisms such as reduced growth or impaired reproduction are rarely reported. EPA’s changes in the registrant reporting requirements for incidents in 1998 may account for a reduced number of reporte d incidents. Registrants are now only required to submit detailed information on ‘maj or’ fish, wildlife, and plant incidents. Minor fish, wildlife, and plant incidents, as well as all other non-ta rget incidents, are generally reported aggregately a nd are not included in EIIS. In addition, there have been changes in state monitoring efforts due to lack of resources.

4.2.1 Terrestrial Animal Incidents

No terrestrial animal incidents were repor ted in the EIIS database for hexazinone.

4.2.2 Plant Incidents

Nine hexazinone incidents have been reporte d in the EIIS database for terrestrial

2 6plants. One incident, I000548-001 reporting non -target plant exposure, was classified as highly probable. Velpar ULW, a produc t with hexazinone, was applied using an aerial application method on a forest site in Florida. No application rate or total magnitude was reported. Five reports were classified as possible. I016312-001: Westar and Velpar, products containing hexazinone, were applied on a forest site in Oregon (2005). Aerial and ground methods were used to apply 2 lbs/A with a total magnitude of 300 acres. The caller reported th e appearance of reddening on Ponderosa Pine needle growth. Damage c ould be due to the application of multiple products. I016680-001: Velpar, a product containing hexazinone, was applied on a right-of-way site in Oregon (2005). Thirteen acres of vineyards were damaged through spray in a ground application. No application rate wa s reported. Damage could be due to the application of multiple products. I000611-001: Velpar L, a pr oduct containing hexazinone, was applied in Georgia (1993). No report of use site, total magnitude , or application rate was provided. The report cited foliar damage (mottling and yello w-spotting) to the trees. Grasses were not affected. I014407-036: A product containi ng hexazinone was applied on an alfalfa site in Washington (1994). No report of total magn itude, or application rate was provided. The damage to yield was to be a ssessed after the crop was harvested. I015265-001: Outstar, a product containing hexazinone, was applied at a forest product site in Texas (2004). The loss of si xty-five percent of 129 acres of loblolly pine seedlings was reported, but no app lication rate or method was provided. Damage could be due to the appl ication of multiple products. Two incidents reported were classified as misuse, I005822-001 reporting damage to flora and I007984-008 reporting damage to a tree. A report of trees dying, I0012233- 001, was classified as unlikely. None of these incidents were us ed in this risk
assessment.

4.2.3 Aquatic Animal Incidents

No aquatic animal incidents were report ed in the EIIS database for hexazinone

. A preliminary review of the Avian M onitoring Information System (AIMS), maintained by the American Bird Cons ervancy, was completed on October 26, 2009. No incidents were reported, but this in formation will be updated for the risk
assessment. Although registrants are only re quired to provide detailed information on major fish, wildlife and plant incidents recorded in the EIIS data base, minor incidents are reported aggregately. Aggregate incidents are categorized as “minor fish and wildlife” (W-B), “minor plan t” (P-B) and “other non-targ et” (ONT) incidents and are

2 7recorded per product per quarter. Three minor fish and wildlife incidents and sixteen minor plant incidents were reported for hexazinone. In the absence of additional information on these incidents, it is assume d that the incidents are representative of currently registered uses for this assessment. In the risk assessment, the incidents will be further evaluated to determine if they represent current patterns of use for hexazinone. Examples of additional considerations are mitigation ( e.g ., reduced application rates), product cancellations, and changes in use patterns that have o ccurred since the da te of the reported incidents. Hexazinone is among the constituents monitored by the National Water Quality Assessment Data Warehouse (NAWQA), mainta ined by the U.S. Geological Survey. According to the summary report, hexazinone was found in 8.6% of filtered water samples and 0% of solid samples (on a dry weight, in the Potomac River Basin, Kent County, MD, ground water and vadose zone samples, total samples: 22).

4.3 Ecosystems Potentially at Risk

The ecosystems at risk may be extensive in scope because hexazinone may be applied on a wide range of sites, and as a result it may not be possible to identify specific ecosystems during the development of a baseline risk as sessment. In general terms, terrestrial ecosystems potentially at risk due to the use of hexazinone, could include the treated field and areas immediately adjacent to the treated fi eld that may receive drift or runoff. Areas adjacent to the treated field could include cultivated fields, fencerows and hedgerows, meadows, fallow fields or grasslands, woodlands, riparian habitats and other uncultivated areas. Due to the persistence of hexazinone, it is expected to drift and/ or runoff due to application to food crops, resu lting in possible exposure to aquatic ecosystems. Aquatic ecosystems potentially at risk due to the use of hexazinone include wa ter bodies adjacent to, or down stream from, the treated fiel d and might include impounded bodies such as ponds, lakes and reservoirs, or flowing waterway s such as streams or rivers. For uses in coastal areas, aquatic habitat also includes marine ecosystems, including estuaries.

5. Assessment Endpoints

Assessment endpoints are defined as “explicit expressions of the ac tual environmental value that is to be protected.” Defining an assessment endpoint involves two steps: 1) identifying the valued attributes of the envir onment that are considered to be at risk; and 2) operationally defining the assessment endpoint in terms of an ecolo gical entity (i.e., a community of fish and aquatic invertebrate s) and its attributes (i.e., survival and reproduction). Therefore, selection of th e assessment endpoints is based on valued entities (i.e., ecological recep tors), the ecosystems potential ly at risk, the migration pathways of pesticides, and the routes by which ecological receptors are exposed to pesticide-related contamination. The selecti on of clearly defined assessment endpoints is important because they provide direction a nd boundaries in the ri sk assessment for addressing risk management issues of c oncern. Changes to assessment endpoints are typically estimated from the available toxicity studies, which are used as the measures of

2 8effects to characterize potentia l ecological risks associated with exposure to pesticides, such as hexazinone. To estimate exposure concentra tions, the ecological risk as sessment considers a single application at the maximum application rate to fields that have vul nerable soils. The most sensitive toxicity endpoints are used from surrogate test species to estimate treatment-related direct eff ects on acute mortality and chr onic reproductive, growth and survival assessment endpoints. Toxicity te sts are intended to determine effects of pesticide exposure on birds, mammals, fish, te rrestrial and aquatic invertebrates, and plants. These tests include short-term acu te, sub-acute, and repr oduction studies and are typically arranged in a hierarchical or tiered system that progresse s from basic laboratory tests to applied field studies. The toxicity st udies are used to evaluate the potential of a pesticide to cause adverse effects, to determ ine whether further tes ting is required, and to determine the need for precautionary label st atements to minimize the potential adverse effects to non-target animals and plants. An open literature search will be conducted to determine any relevant endpoints. The search will focus on survival, growth and repr oductive effects for a quatic and terrestrial effects of hexazinone. More sensitive e ndpoints from acceptable open literature studies will be included in this risk assessment.

6. Conceptual Model

For a pesticide to pose an ecological risk, it must reach ecological receptors in biologically significant concen trations. An exposure pathway is the means by which a pesticide moves in the environment from a source to an ecological receptor. For an ecological pathway to be complete, it must have a source, a release mechanism, an environmental transport medium, a point of exposure for ecological receptors, and a feasible route of exposure. A conceptual model provides a written desc ription and visual re presentation of the predicted relationships between hexazinone , potential routes of exposure, and the predicted effects for the assessment endpoint. A conceptual model consists of two major components: risk hypothesis and a conceptual diagram (EPA, 1998).

6.1 Risk Hypothesis

Risk hypotheses are specific assumptions about potential adverse effects (i.e., changes in assessment endpoints) and may be based on theory and logic, empirical data, mathematical models, or probability models (E PA, 1998). For this assessment, the risk is stressor-initiated, where the stressor is the re lease of hexazinone into the environment. The following risk hypothesis is presumed for this screening-level assessment: Given the uses of hexazinone and its envi ronmental fate prop erties, there is a likelihood of exposure to non-target terrestrial and aquatic organisms.

When used in accordance with the label, hexazinone results in potential adverse

2 9effects upon the survival, growth, and repr oduction of non-target terrestrial and aquatic organisms. The conceptual model is a generic graphic de piction of the risk hypothesis. It includes the potential pesticide or stressor (hexazi none), the source of the pesticide and/or transport pathways, exposure media, exposur e point, biological receptor types, and attribute changes.

6.2 Conceptual Diagram

The conceptual site model is a generic gr aphic depiction of the risk hypothesis, and assumes that the herbicide he xazinone, having outdoor uses , is capable of affecting aquatic and terrestrial animals provided that environmental concentrations are sufficiently elevated as a result of proposed label us es. Based on an examination of the physicochemical properties of hexazinone, the fate a nd disposition in the e nvironment, and mode of application, a conceptual model was developed that represents the possible relationships between the stressors, ecologica l receptors, and the assessment endpoints. Through a preliminary iterative process of examining available data, the conceptual model (i.e., the representation of the risk hypot hesis) may be refined to reflect the likely exposure pathways and the organisms that ar e most relevant and applicable to this assessment (see Figure 6.1 , below). It includes the potential pesticide or stressor (hexazinone), the sources and/ or transpor t pathways, exposure media, exposure points, biological receptor types and attribute changes. In order for a chemical to pose an ecologica l risk, it must reach ecological receptors in biologically significant concen trations. An exposure pathway is the means by which a pesticide moves in the environment from a source to an ecological receptor. For an ecological exposure pathway to be complete, it must have a source, a release mechanism, an environmental transport medium, a point of exposure for ecologi cal receptors, and a feasible route of exposure. In addition, the potential m echanisms of degradation/ transformation (i.e., which degradation/ transformation products may form in the environment, in which media, and how much ) must be understood, especially if for the chemical, its metabolites/ tran sformation products are of gr eater toxicological concern than the parent compound. The assessment of ecological exposure pathways, therefore, includes an examination of the source and pot ential migration pathways for constituents, and the determination of potential exposure routes. Under the possible uses of hexazinone, the sources and mechanisms of release of the compound are from ground or aerial spray applic ations. Surface runoff from the areas of application is assumed to depend on factors such as topography, irrigation, and rainfall events. Direct deposition may result in contamination of food items that may be consumed by terrestrial organisms. Spray drift results in contaminated adjacent areas, including bodies of water. For aquatic receptors, the major point of expos ure is through direct contact with the water column, sediment, and pore water (gill/ integum ent) contaminated with spray drift (from spray application) and/ or runoff from treated areas. Indirect effects to aquatic organisms (especially fish) can also occur through impact to various food chains. The representative

3 0aquatic receptors are certain fr eshwater and estuarine/ marine fish, invertebrates, and, in certain cases, aquatic plants. The major poi nt of exposure for terrestrial animals is consumption of food contaminated with residues such as grass, foliage, and small insects. For plants, the point of exposure is direct contact or root uptake . The representative terrestrial receptors are mammals , birds, and, in certain case s, terrestrial plants. The attribute changes used to assess risk to terr estrial receptors depend on the type of test (e.g., reduced survival, growth, or reproducti on for animals and seedling emergence and vegetative vigor for plants). It should be noted, that these species do not cover all the possible species in the animal and plant kingdoms; certain taxa are considered as surrogates for other taxa. For example, fishare considered surroga tes for aquatic phase amphibians.

7. Analysis Plan

In order to address the ri sk hypothesis, the potential for adverse effects on the environment is estimated. Usage, environm ental fate and trans port, and ecological effects of hexazinone are characterized and in tegrated to assess the risks. This is accomplished using a risk quotient (ratio of exposure concentration to effects concentration) approach. Although risk is often defined as the likelihood and magnitude of adverse ecological effects, the risk quotient-based approach does not provide a quantitative estimate of likeli hood and/or magnitude of an adverse effect. However, as

31outlined in the Overview Document (USE PA, 2004), the likelihood of effects to individual organisms from par ticular uses of a chemical is estimated using the probit dose-response slope and either the level of concern (discussed below) or the actual calculated risk quotient value. This analysis plan will be revisited and may be revised depending upon the data available in the open literature and the information s ubmitted by the public in response to the opening of the Registration Review docket.

7.1. Stressors of Concern

The focus of this assessment is on the parent material, hexazinone. However, the Agency will review open literature to identify metabolite(s) of poten tial toxicological concern. Various degradates of hexazinone, such as A, A-1, 1 and G3170, exist; however, they are also persistent and mobile. Toxicity data for environmental mixtures of hexazinone with other pesticides (those mixtures occurring in the environment followi ng application), if available, may be presented as part of the ecological risk assessmen t. It is expected th at the toxic effect of hexazinone, in combination with other pesticides used in the environment, is likely to be a function of many factors in cluding, but not necessarily limited to: (1) the exposed species, (2) the co-contaminants in the mixt ure, (3) the ratio ofhexazinone and cocontaminant concentrations, (4) differences in the pattern and duration of exposure among contaminants, and (5) the differentia l effects of other physical/ chemical characteristics of the receiving waters (e .g. organic matter present in sediment and suspended water). Quantitatively predicting th e combined effects of all these variables on mixture toxicity to any gi ven taxa with confidence is beyond the capabilities of the available data and methodologies. However, a qualitative discussion of implications of the available pesticide mixture effects da ta on the confidence of risk assessment conclusions will be addressed as part of the uncertainty analysis.

7.2. Measures of Exposure

In order to estimate risks of hexazinone expos ures in aquatic and terrestrial environments, all exposure modeling and resulting risk c onclusions will be made based on maximum application rates for the current use patterns. Measures of exposure are based on aquatic and terrestrial models that predict estimated environmental concentra tions of hexazinone using maximum-labeled application rates and methods, as well as any mitigation measures specifically indicated on the label. The models used to predict aquatic EECs are the Pesticide Root Zone Model coupled with the Exposure Analysis Model System (PRZM/EXAMS). The model used to predict te rrestrial exposure is T-REX. The model used to derive EECs relevant to terrestrial and wetland plants is TerrPlant. These models are parameterized using relevant reviewed registrant-submitted environmental fate and transport data. PRZM (v.3.12.2, May 2005) and EXAMS (v.2.98.4.6, April 2005) are screening simulation models coupled with the input sh ell PE5.pl (August, 2007) to generate daily exposures and 1-in-10 year EECs of hexazi none, that may occur from spray drift and

3 2runoff to surface water bodies adjacent to app lication sites. PRZM simulates pesticide application, movement and transformation on an agricultural fiel d and the resultant pesticide loadings to a recei ving water body via runoff, erosion and spray drift. The EXAMS model simulates the fate of the pesticide and resulting concentrations in the water body. The standard scenario used fo r ecological pesticide assessments assumes application to a 10-hectare agri cultural field that drains in to an adjacent 1-hectare water body that is 2 meters deep (20,000 m 3 volume) with no outlet. PRZM/ EXAMS is used to estimate screening-level exposure of aquatic organisms to hexazinone and/ or its transformation products, should they be of c oncern. The measure of exposure for aquatic species is the 1-in-10 year re turn peak or rolling mean c oncentration. The 1-in-10 year peak is used for estimating acute exposures of direct effects to aquatic organisms. The 1- in-10-year 60-day mean is used for assessi ng chronic exposure to fi sh and aquatic-phase amphibians. The 1-in-10-year 21-day mean is used for assessing aquatic invertebrate chronic exposure. Exposure estimates for terrestrial animals assumed to be in the targ et area are derived using the T-REX model (version 1.3.1, 12/07/200 6). For granular pesticides, this includes the amount of pesticid e per square foot, used in LD50 per square foot risk assessment calculations. EECs for terrestrial plants inhabiting dr y and wetland areas are derived using TerrPlant (version 1.2.2, 12/26/ 2006). This model uses estimates of pesticides in runoff and in spray drift (assumed to be 0% for granulars) to calculate EECs. EECs are based upon solubility, application rate and minimum incorporation depth.

7.3. Measures of Effect

Ecological effects data are used as measures of direct and indirect effects to biological receptors. Data are typically obtained from registrant-submitted studies or from literature studies identified by ECOTOX. The ECOTOX da tabase provides more ecological effects data in an attempt to bridge existing data gaps. ECOTOX is a source for locating single chemical toxicity data and potential chemi cal mixture toxicity data for aquatic life, terrestrial plants, and wildlife. ECOTOX wa s created and is maintained by the USEPA, Office of Research and Devel opment, and the National Health and Environmental Effects Research Laboratory’s Mid- Continent Ecology Division. Information on the potential effects of hexazi none on non-target animals is also collected from the Ecological Incident Information Sy stem (EIIS). The EIIS is a database containing adverse effect (typically mortality) repor ts on non-target organisms where such effects have been associated with the use of pesticides. Where available, sub-lethal effects observed in both registrant-submitted and open literature studies will be evaluated qualitat ively. Such effects may include behavioral changes (e.g., lethargy and changes in colora tion). Quantitative assessments of risks, though, are limited to those endpoints that ca n be directly linked to the Agency’s assessment endpoints of impaired survival, growth and reproduction.

The assessment of risk for direct effects to non-target organisms makes the assumption that the toxicity of hexazinone to birds is similar to terrestrial-phase amphibians and reptiles. The same assumption is made for fish and aquatic-phase amphibians.

33The acute measures of effect used fo r animals in this assessment are the LD 50 , LC 50 and EC 50 . LD stands for “Lethal Dose”, and LD 50 is the amount of a material, given all at once, that is estimated to cause the death of 50% of the test organisms. LC stands for “Lethal Concentration” and LC 50 is the concentration of a chemical that is estimated to kill 50% of the test organisms. EC stands for “Effective Concentration” and the EC 50 is the concentration of a chemical that is estim ated to produce a specific effect in 50% of the test organisms. Endpoints for chronic measures of exposure for listed and non-listedanimals are the NOAEL/NOAEC and NOEC. NOAEL stands for “No ObservedAdverse-Effect-Level” and refers to the highest tested dose of a subs tance that has beenreported to have no harmful (adverse) effects on test organisms. The NOAEC (i.e., “NoObserved-Adverse-Effect-Concentration”) is th e highest test concen tration at which none of the observed effects were statistically diffe rent from the control. The NOEC is the NoObserved-Effects-Concentration. For non-li sted plants, only acute exposures are assessed (i.e., EC 25 for terrestrial plants and EC 50 for aquatic plants); for listed plants either the NOAEC or EC 05 is used.

7.4. Integration of Exposure and Effects

Risk characterization is the integration of exposure and ecological e ffects characterization to determine the potential ecologi cal risk from the use of pe sticides and the likelihood of direct and indirect effects to non-target organisms in aquatic and terrestrial habitats. The exposure and toxicity effects data are integrated in order to evaluate the risks of adverse ecological effects on non-target species. For the assessment of risks, the risk quotient (RQ) method is used to compare exposure and measured toxicity values. EECs are divided by acute and chronic toxicity values. The resulting RQs are then compared to the Agency’s Levels of Concern (LOCs) (USEPA , 2004). These criteria will be used to indicate when hexazinone’s uses , as directed on the label, have the potential to cause adverse direct or indirect eff ects to non-target organisms. In addition, incident data from the EIIS will be considered as part of the risk characterization.

7.5. Deterministic and Probabilistic Assessment Methods

The quantitative assessment of risk will primarily depend on the deterministic pointestimate based approach described in the risk a ssessment. An effort may also be made to further qualitatively describe ri sk using probabilistic tools th at the Agency has developed. These tools have been reviewed by FIFRA Scientific Advisory Panels ( http://www.epa.gov/scipoly/sap/index.htm ) and have been deemed as appropriate means of refining assessments where deterministic approaches have identified risks.

7.6. Endangered Species Assessments

Consistent with the Agency’s responsibility under the Endangered Species Act (ESA), the Agency will evaluate risks to Federally-listed threatened and/or endangered (listed) species from registered uses of hexazinone. This assessment will be conducted in accordance with the Overview Document (USE PA, 2004), provisions of the ESA, and the Services’ Endangered Species C onsultation Handbook (USFWS/NMFS, 1998).

3 4The assessment of effects associated with th e registration of hex azinone is based on an action area. The action area is considered to be the area directly or indirectly affected by the federal action, as indicated by the exceedan ce of Agency Levels of Concern (LOCs) used to evaluate direct or indirect effect s. For the nationwide risk assessment of hexazinone, the action area will encompass the en tire United States and its territories. This ensures that the initial area of cons ideration encompasses all areas where the pesticide may be used now and in the futu re, and includes the potential for off-site transport via spray drift and downstream dilution. It also takes into account the potential for direct and indirect effect s and any modification to criti cal habitat based on ecological effect measures associated with reduction in survival, growth and reproduction as well as sub-lethal effects. The nationwide action area does not imply that di rect and/or indirect effects and critical habitat modification are expected to or are likely to occur over the full extant of the action area, but rather to identify all listed sp ecies and critical habitat that may potentially be affected by the action. The agency will use more rigorous analyses, including consideration of available la nd cover data, toxicity data and exposure information to determine areas where individual listed species and designated cr itical may be affected or modified via endpoints a ssociated with survival, growth or reproduction. The Agency’s approach to defining the acti on area under the provisi ons of the Overview Document (USEPA 2004) consider s the results of the risk assessment process to establish boundaries for that action area with the understa nding that exposures below the Agency’s defined LOCs constitute a no-effect threshol d. For the purposes of this assessment, attention will be focused on the footprint of the action (i.e., th e area where hexazinone application occurs), plus all areas where offs ite transport may result in potential exposure that exceeds the Agency’s LOCs. Specific meas ures of ecological eff ect that define the action area for listed species include any dire ct and indirect eff ects and/ or potential modification of its critical habitat, in cluding reduction in survival, growth, and reproduction as well as the full suite of sub- lethal effects available in the effects literature. Therefore, the action area extends to a point where environmental exposures are below any measured lethal or sub-lethal effect threshold for any biological entity at the whole organism, organ, tissue, and/ or cellular level of organization.

7.7. Drinking Water Assessment

A drinking water assessment will be conducte d to support future human health risk assessments of hexazinone should it be re quired by HED. The drinking water assessment will incorporate model estimates of he xazinone in surface and ground waters. Concentrations in surface waters will be estimated using FQPA Index Reservoir Screening Tool (FIRST, v.1.1.1, 12/18/07) (or su bsequently using PRZM/ EXAMS, with the Index Reservoir - see description above , if refinements are required). Ground water estimates of concentrations will be estimat ed using the Screening Concentration in Ground Water (SCI-GROW) model (v.2.3, July 2003). The drinking water assessment will also include a summary of availabl e surface and ground water monitoring data.

357.8 Preliminary Identification of Data Gaps

7.8.1. Fate

The environmental fate database for hexazinone is largely complete and consists mostly of acceptable studies. For registration review , the following are the data gaps for the chemical:

835.4100 Aerobic Soil Metabolism – One study, conducted on a sandy loam (pH 6.5) is available. Studies conducted on three soils are required, to address the rate of reaction. According to the OPPTS Guideline 835.4100 & 835.4200 , “ The types of soils tested should be representative of the environmenta l conditions where release or use will occur. Aerobic and anaerobic studies with one so il type are generally sufficient for the evaluation of transformation pathways. Rates of transformation should be determined in at least three additional soils...” Even though hexazinone appeared to be relatively stable to hydrolysis at all pHs teste d, the profile of degradation of the chemical is known to be dependent on the pH. A range of soil pHs should be used.

835.4200 Anaerobic Soil Metabolism – No study is availabl e. A study is required. 835.4300 Aerobic Aquatic Metabolism – According to OPPTS Guideline 835.4300 and 835.4400 , “Two sediments are normally used for the aerobic studies... The two sediments selected should differ with respect to organic carbon content and texture. One sediment should have a high organic carbon co ntent (2.5-7.5%) and a fine texture, the other sediment should have a low organic carb on content (0.5-2.5%) and a coarse texture. The difference between the organic carbon c ontents should normally be at least 2%. “Fine texture” is defined as a (clay + silt) conten t of >50% and “coarse texture” is defined as a (clay + silt) cont ent of <50%. (Clay + silt) is the mineral fraction of the sediment with particle size of < 50 μ m. The difference in (clay + silt) content for the two sediments should normally be at least 20%. In cases, where a chemical may also reach marine waters, at least one of the water-sedi ment systems should be of marine origin.” A study is available in only one sediment system (a sandy loam, pH 7.8). Furthermore, the available study was conducted for only 56 days . Hexazinone’s transformation profile is known to be susceptible to pHs. The new te st system should have a near neutral or slightly acidic pH. 835.4400 Anaerobic Aquatic Metabolism – According to OPPTS Guideline 835.4300 and 835.4400, “For the strictly anaerobic study, two sediments (including their associated waters) should be sampled from the anaerobic zones of surface water bodies...” A study is available in only one sediment system (l oam, pH 8.5). Data from another water/ sediment system is required. Hexazinone’s transformation profile is known to be susceptible to pHs. The new test system shoul d have a near neutral or slightly acidic pH. Environmental Chemistry Methods (ECMs) , and Independent Laboratory Validations (ILVs), for Soil and Water. The requirements for ECMs associated with the Terrestrial Field Dissipation study (Field diss ipation §158.1300; OPPTS guidelines 835.6100, 835.6200, and 835.6300), along with successful confir matory method trials (validation) by an independent laboratory (i.e. ILVs), are placed in reserve at this time . If there is risk

3 6concern for a given taxon, ECMs should be av ailable for the environmental media in which organisms of the taxon reside. These ECMs should have limits of quantization for the residues of concern that are lower than the relevant toxico logical levels of concern. According to the 1994 RED, hexazinone exceeds the level of concern for aquatic plants and endangered aquatic plants. Furthermore, a life time drinking water Health Advisory has been established by the USEPA at 400 ppb (September 1996). Therefore, ECM for water is required in addition to the ECM fo r soil. The ECMs should include parent and those residues found in the laboratory studies th at exceeded 10% of the applied or that are considered of concern. The following app ear to be the residues of concern for each media: Soil – parent and its metabolites of concern Water – parent and its metabolites of concern The registrant is encouraged to submit stat e-of-the-art environmental chemistry methods; further, multi-residue methods (MRMs) for soil and water are prefer red. The registrant submitted one method for soil (MRID 42379201, LOQ 0.03 μg/g, LOD 0.01 μg/g), published in the Agency’s EC M website, and evaluated by BEAD’s Analytical Chemistry Branch (memo dated 06/18/96). The method c ould quantify hexazinone in soil but is not useful to quantify the hex azinone’s metabolites. Accord ing to the memo, “An early review of the method showed the five metabo lites to be very poorly recovered with RSDs well above 30 percent.” A full description of the method validation procedures performed by an independent laboratory should be submitted. Independent laboratory validation reports have been located (MRID#s 45132802 for soil and 45132803 for water, both dated 1999), but it appears that they have not been evaluated. Th erefore, these reports will be evaluated by the EFED. They should include the following in formation: (a) Recovery level(s) of the test compound(s) from the soil and water (sub strates) at various re levant fortification level(s) using the residue analytical me thodology; (b) a validated method sensitivity level; (c) results of the study and statistical test applied, including a stepwise presentation of the procedure for calculating percent rec overy from the raw data; (d) all the data/ information necessary to independently verify the results; (e) summary of the results; and, (f) discussion and conclusions of the results.

Table 7.1 lists the status of the environmenta l fate data requirements for hexazinone

.

Table 7.1. Summary of Environmental Fa te Data Requirements for Hexazinone

Study Identification Use Pattern 1 Does EPA Have Data To Satisfy This Requirement? Bibliographic Citation Must Additional Data Be Submitted Under FIFRA

3©(2)(B)?

§158.1300 ENVIRONMENTAL FATE

Degradation Studies-Lab :
835.2120 Hydrolysis 1,2,3,4 Yes 41587301 No

3 7Table 7.1. Summary of Environmental Fa te Data Requirements for Hexazinone

Must Additional Data Be Submitted Does EPA Have

Study Identification Use Bibliographic Under FIFRA Data To Satisfy Pattern 1 This Requirement? Citation 3©(2)(B)?

835.2240 Photodegradation in Water 1,2,3,4 Yes 41300801 No

835.2410 Photodegradation on Soil 1,2,3,4 Yes 41300802 No

835.2370 Photodegradation in Air 1,2,3,4 No Not Available Waived 2

Metabolism Studies-Lab :

835.4100 Aerobic Soil 1,2,3,4 Partially 41807401, 42635001 Yes 3

835.4200 Anaerobic Soil 1,2,3,4 No Not Available Yes
835.4300 Aerobic Aquatic 1,2,3,4 Partially 41811801 Yes 4

835.4400 Anaerobic Aquatic 1,2,3,4 Partially 41807402 Yes 4

Mobility Studies :
835.1230 Mobility – Adsorption/ Desorption 1,2,3,4 Yes 41528101, 43621501 No

835.1410 Volatility (Lab) 1,2,3,4 No Not Available Waived 2

835.8100 Volatility (Field) 1,2,3,4 No Not Available Waived 2

Dissipation Studies-Field :

835.6100 Terrestrial Field Dissipation 1,2,3,4 Yes 42377901, 42379201 No

835.6200 Aquatic Field Dissipation 1,2,3,4 No N/A Reserved

835.6300 Forestry Dissipation 1,2,3,4 Yes 42336401 No

Ground Water Monitoring Studies :

835.7100 Ground Water Monitoring 1,2,3,4 Yes 45132801 (also 44060101, 44082801, 44106601 and

44194001) No

§158.1100 SPRAY DRIFT 201-1 Droplet Size Spectrum 1,2,3,4 No 2 NA No 5

202-1 Drift Field Evaluation 1,2,3,4 No 2 NA No 5

3 8Table 7.1. Summary of Environmental Fa te Data Requirements for Hexazinone

Must Additional Data Be Submitted Does EPA Have

Study Identification Use Bibliographic Under FIFRA Data To Satisfy Pattern 1 This Requirement? Citation 3©(2)(B)?

Other Studies :

ECM and ILV 1,2,3,4 Partially 42379201, 45132802, 45132803 Reserved 6

1. Use Patterns: 1=Terrestrial/Food; 2=Terrestria l/Feed; 3=Terrestrial Non-Food; 4=Forestry. 2. Hexazinone has low vapor pressure and Henry’s Law Constant, data requirement was waived. 3. Studies are required in three soils, for the assessment of the rate of reaction only. 4. One study is required in only one water/sediment system. 5. Data requirement covered by submission of the Spray Drift Task Force. 6. Available data will be evaluated by the Agency to verify its suitability.

7.8.2 Effects

Table 8 lists required ecological studies for he xazinone. Studies will provide information to reduce uncertainty for non-target and e ndangered species assessments. Based on new data requirements, a passerine acute oral toxicity te st is required. Some crop species from the submitted plant studies were deemed invalid and should be repeated. The following pl ant studies are needed:

123-1: Tier II - Terrestrial Plant Growth - Vegetative vigor for cucumber is needed based on another pesticide used on the seeds.

123-2: Tier II – Seedling Emergence for cucumber is needed based on another pesticide used on the seeds. New studies are needed for onion and pea based on damage to the plants observed from the submitted study (MRID 43162501).

Table 7.2 Ecological Effects Da ta Requirements for Hexazinone

Data Requirement Use Pattern 1 Does EPA Have Data To Satisfy This Requirement?

(Yes, No, or

Partially) Bibliographic Citation Must Additional

Data Be Submitted Under FIFRA

3©(2)(B)?

§158.490 WILDLIFE AND AQUATIC ORGANISMS
71-1(a) Acute Avian Oral, Quail or Duck 1,2 Yes 00073988 No
71-1(a) Acute Avian Oral, Passerine 1,2 No Yes

3 9Table 7.2 Ecological Effects Da ta Requirements for Hexazinone

Does EPA Have Must

Data Requirement Use Pattern 1 Data To Satisfy Additional This Data Be Requirement? Submitted

(Yes, No, or

Partially) Bibliographic Under FIFRA Citation 3©(2)(B)?
71-2(a) Acute Avian Diet, Quail 1,2 Yes 00107878,00072663 No
71-2(b) Acute Avian Diet, Duck 1,2 Yes 0010498 No
71-3 Wild Mammal Toxicity 1,2 NA
71-4(a) Avian Reproduction Quail 1,2 Yes 41764901 No
71-4(b) Avian Reproduction Duck 1,2 Yes 41764902 No
71-5(a) Simulated Terrestrial Field Study 1,2 NA
71-5(b) Actual Terrestrial Field Study 1,2 NA
72-1(a) Acute Fish Toxicity Bluegill 1,2 Yes 00076959, 00104980 No
72-1(b) Acute Fish Toxicity (TEP) 1,2 NA
72-1© Acute Fish Toxicity Rainbow Trout 1,2 Yes 00104980 No
72-1(d) Acute Fish Toxicity Rainbow Trout (TEP) 1,2 NA

72-2(a) Acute Aquatic Invertebrate 1,2 Yes 00116269 No
72-2(b) Acute Aquatic Invertebrate (TEP) 1,2 Yes 41235003 No
72-3(a) Acute Est/Mar Toxicity Fish 1,2 Waived No

72-3(b) Acute Est/Mar Toxicity Mollusk 1,2 Yes 00047164 No
72-3© Acute Est/Mar Toxicity Shrimp 1,2 Yes 00047164 No
72-3(d) Acute Est/Mar Toxicity Fish (TEP) 1,2 NA
72-3(e) Acute Est/Mar Toxicity Mollusk (TEP) 1,2 NA

72-3(f) Acute Est/Mar Toxicity Shrimp (TEP) 1,2 NA

72-4(a) Early Life Stage Fish 1,2 Yes 41406001 No
72-4(b) Life Cycle Aquatic Invertebrate 1,2 Yes 00078041, 41406002, 458221-01 No

72-5 Life Cycle Fish 1,2 NA

72-6 Aquatic Organism Accumulation 1,2 NA

4 0Table 7.2 Ecological Effects Da ta Requirements for Hexazinone

Does EPA Have Must

Data Requirement Use Pattern 1 Data To Satisfy Additional This Data Be Requirement? Submitted

(Yes, No, or

Partially) Bibliographic Under FIFRA Citation 3©(2)(B)?
72-7(1) Simulated Aquatic Field Study 1,2 NA
72-7(b) Actual Aquatic Field Study 1,2 NA

§158.540 PLANT PROTECTION
122-1(a) Seed Germ, Seedling Emergence 1,2 Partial 431625-01 Yes

122-2 Aquatic Plant Growth 1,2 Yes 432251-01 No
122-1(b) Vegetative Vigor 1,2 Partial 431625-01 Yes

123-2 Aquatic Plant Growth 1,2 Yes 412870-01

433027-01

432251-02 No

124-1 Terrestrial Field Study 1,2 NA
124-2 Aquatic Field Study 1,2 NA

§158.490 NONTARGET INSECT TESTING

141-1 Honey Bee Acute Contact 1,2 Yes 422084-11 No
141-2 Honey Bee Residue on Foliage 1,2 NA

141-5 Field Test for Pollinators 1,2 NA

§158.630 AQUATIC ORGANISMS TESTING

850.1730 Bioaccumulation in Fish 1,2,3 No NA Waived 2

1. Use Patterns:1=Terrestrial/Food; 2=Terrestrial/Feed; Forestry

2. With a K OW of only 15, the data requirement was waived . Hexazinone is unlikely to bioconcentrate.

418. References BioByte. 1995. ClogP for Windows Program ( v1.0). BioByte Corp., Claremont, CA. A reference cited by EPI SuiteTM v.4.0. Food and Agriculture Organization of the United Nations. FAO PESTICIDE DISPOSAL SERIES 8. Assessing Soil Contaminatio n: A Reference Manual. Appendix 2. Parameters of pesticides th at influence processes in th e soil. Editorial Group, FAO Information Division: Rome, 2000. http://www.fao.org/DOCREP/003/X2570E/X2570E00.htm U.S. Environmental Protection Agency. 1998. Guidelines for Ecological Risk Assessment. Risk Assessment Forum, Office of Research and Development, Washington, D.C. EPA/630/R-95/002F. April 1998. U.S. Environmental Protection Agency. 2000. Risk Characterization Handbook. Science Policy Council, U.S. Environmenta l Protection Agency, Washington, D.C. 20460. EPA 100-B-00-002. December 2000. U.S. Environmental Protection Agency. 2004. Overview of the Ecological Risk Assessment Process in the Office of Pesticid e Programs, U.S. Environmental Protection Agency. Endangered and Threatened Spec ies Effects Determinations. Office of Prevention, Pesticides and Toxic Substances , Office of Pesticide Programs, Washington, D.C. January 23, 2004. Tu, M., C. Hurd & J.M. Randall. 2001. Weed Control Methods Handbook, The Nature Conservancy, http://tncweeds.ucdavis.edu

, version: April 2001 p.7f.2. U.S. Environmental Protection Agency. 2005. Generic Format and Guidance for the Level I Screening Ecological Risk Assessm ents Conducted in the Environmental Fate and Effects Division. Office of Pesticide Programs, Washington, D.C. January 24, 2005. U.S. Environmental Protection Agenc y. 2008. OPPTS 835.6100 Terrestrial Field Dissipation. EPA 712-C-08-020. Oc tober 2008. Office of Prevention, Pesticides, and Toxic Substances. United States Envi ronmental Protection Agency. Available at http://www.epa.gov/opptsfr s/publications/835_6100.pdf (Accessed July 30, 2009). U.S. Environmental Protection Agency. 2009. EPI Suite (Version 4.0): United States Environmental Protection Agency. Available at http://www.epa.gov/oppt/expos ure/pubs/episuitedl.htm (Accessed July 8, 2009). U.S. Fish and Wildlife Service (USFWS) and National Marine Fisheries Service (NMFS). 1998. Endangered Species Cons ultation Handbook: Procedures for Conducting Consultation and Conference Activ ities Under Section 7 of the Endangered Species Act. Final Draft. March 1998.

4 2Appendix A. SRRD data reque st justification tables

The following proposed Data Call-in tables include rationales for requiring the data requested in this problem formulation, expl anations of the util ity of the data, and explanations for how the data might impact risk assessment, following the format provided by SRRD.

Guideline Number: 835.4100 Study Title: Aerobic Soil Metabolism

Guideline Number: 835.4200 Study Title: Anaerobic Soil Metabolism Rationale for Requiring the Data According to 40 CFR Part 158, Subpart N (Environmental Fate) §158.1300 (Environmental fate data requirements table), these data are re quired for all terrestrial use patterns.

A new aerobic soil metabolism study is required, utilizing three soil systems, which are preferably domestic. Studies conducted on three soils are required, to address the rate of reaction of hexazinone. One study, conducted on a sandy loam (pH 6.5) is available, which addresses the transformation pathway in an acidic soil. According to the OPPTS Guideline 835.4100 & 835.4200 , “The types of soils tested should be re presentative of the environmental conditions where release or use will occur. Aerobic and anaerobic studies with one soil type are generally sufficient for the evaluation of transformation pathways. Rates of transformation should be determined in at least three additional soils...” Even though hexazinone app eared to be relatively stable to hydrolysis at all pHs tested, the profile of degradation of the chemical is known to be dependent on the pH. A range of soil pHs should be used.

There is no anaerobic soil metabolism study availa ble at this time. A study is required. Practical Utility of the Data How did the Agency make its registr ation decision without these data? The previous assessments were based on a singl e soil for the aerobic soil metabolism. Furthermore, the data from the anaerobic aquatic metabolism study was utilized in lieu of the anaerobic soil metabolism study.

How will the data be used? Hexazinone is a triazone herbicide with high toxi city towards aquatic and terrestrial plants. Knowledge and accurate characterization of its pers istence in terrestrial (soil) environments is important. Available data suggest that the chemic al is persistent in the environment. Further aerobic soil metabolism data are required to address more accurately the rate of reaction. The rate of reaction and transformation pathway in anaerobic soil environments should also be addressed and assessed. The results of this study will be used in the environmental fate assessment, and later on, in the ecological risk characterization.

In addition, the data will be used to derive the ra te of reaction input parameter to be used in the aquatic models GENEEC2, PRZM/ EXAMS and FIR ST. The results from these models are used for ecological risk estimation and in drinking water exposure assessments.

How could the data impact the Agency’s future decision-making? Without these data, the Agency would have to make certain conservative assumptions ( e.g. use a three times the aerobic soil metabolism half-life as the input parameter in the aquatic models).

43These conservative assumptions could lead to higher estimated exposure concentrations or estimated drinking water concentrations (EEC s and EDWCs), according to the models, and consequently, higher risk quotients and drinking water exposure concentrations. Also, there would be a higher degree of uncertainty in the environmental fate assessment. Knowledge of these dissipation pathways in detail is essential to perform a better and accurate environmental fate assessment and risk assessment.

The lack of these data will limit the flexibility th e Agency and registrants have in coming into compliance with the Endangered Species Act and c ould result in use restrictions for hexazinone that may be unnecessary.

4 4Guideline Number: 835.4300 Study Title: Aerobic Aquatic Metabolism

Guideline Number: 835.4400 Study Title: Anaerobic Aquatic Metabolism Rationale for Requiring the Data According to 40 CFR Part 158, Subpart N (Environmental Fate) §158.1300 (Environmental fate data requirements table), these data are re quired for all terrestrial use patterns.

A new aerobic aquatic metabolism study is required, utilizing one water/ sediment system, which is preferably domestic. One study, conducted in a sandy loam sediment (pH 7.8) and water (pH 8.2) from Madera, CA, is available, which addresses the transformation pathway in a basic water/ sediment system. Furthermore, the available study was conducted for only 56 days. According to OPPTS Guideline 835.4300 & 835.4400 , “Two sediments are normally used for the aerobic studies... The two sediments selected should differ with respect to organic carbon content and texture. One sediment should have a high organi c carbon content (2.5-7.5%) and a fine texture, the other sediment should have a low organic carbon content (0.5-2.5%) and a coarse texture. The difference between the organic carbon contents shoul d normally be at least 2%. “Fine texture” is defined as a (clay + silt) content of >50% and “coar se texture” is defined as a (clay + silt) content of <50%. (Clay + silt) is the mineral fraction of the sediment with particle size of < 50 μ m. The difference in (clay + silt) content for the two sed iments should normally be at least 20%. In cases, where a chemical may also reach marine wa ters, at least one of the water-sediment systems should be of marine origin.” Hexazinone’s transformation profile is known to be susceptible to pHs. The new test system should have a near ne utral or slightly acidic pH and be of marine origin.

A new anaerobic aquatic metabolism study is re quired, utilizing one water/ sediment system, which is preferably domestic. One study, conducte d in a loam sediment (pH 8.5) is available, which addresses the transformation pathway in a b asic water/ sediment system. According to OPPTS Guideline 835.4300 & 835.4400 , “For the strictly anaerobic study, two sediments (including their associated waters) should be sampled from the anaerobic zones of surface water bodies...” Data from another water/ sediment syst em is required. Hexazinone’s transformation profile is known to be susceptible to pHs. The new test system should have a near neutral or slightly acidic pH and be of marine origin. Practical Utility of the Data How did the Agency make its registr ation decision without these data? The previous assessments were based on aerobic aquatic metabolism and anaerobic aquatic metabolism studies conducted with a single water/ sediment system instead of the required two.

How will the data be used? Hexazinone is a triazone herbicide with high toxici ty towards aquatic and terrestrial plants. In addition a lifetime drinking water Health Advisory has been set for hexazinone. Knowledge of its persistence in aquatic environmen ts is important. The results of these studies will be used in the environmental fate assessment, and later on, in the ecological risk characterization.

In addition, the data will be used to derive the ra te of reaction input parameter to be used in the aquatic models GENEEC2, PRZM/ EXAMS and FIR ST. The results from these models are used for ecological risk estimation and in drinking water exposure assessments.

45How could the data impact the Agency’s future decision-making? Without these data, the Agency would have to make certain conservative assumptions ( e.g. use a three times the aerobic aquatic metabolism half- life as the input value for the aerobic aquatic metabolism or three times the anaerobic aquatic metabolism half-life for the anaerobic aquatic metabolism in the aquatic models). These conservative assumptions could lead to higher estimated exposure concentrations or estimated drinking water concentrations (EECs and EDWCs), according to the models, and conseque ntly, higher risk quotients or drinking water exposure values. Also, there would be a higher de gree of uncertainty in the environmental fate assessment. Knowledge of these dissipation pathwa ys in detail is essential to perform a better and accurate environmental fate assessment and risk assessment.

The lack of these data will limit the flexibility th e Agency and registrants have in coming into compliance with the Endangered Species Act and c ould result in use restrictions for hexazinone that may be unnecessary.

4 6Guideline Number: 850-2100 Study Title: Passerine Acute Avian Oral Rationale for Requiring the Data Although an acute avian oral study was received the Agency updated its data requirements in 40 CFR Part 158 (October 26, 2007) to include an acute oral toxicity study for both a passerine species and either a waterfowl or an upland game species. Prior to starting toxicity testing, a protocol will need to be provided for review.

o Many passerine species utili ze agricultural fields, fo rests, residential areas and surrounding areas, and, therefore, ha ve the potential to be exposed to pesticides used in agricultural, forest, and residential settings

 It is likely that, for most pesticid e use patterns, passerines are more likely to be exposed to pesticid es than upland game species and waterfowl

o Passerines are smaller and have faster metabolisms than the waterfowl and upland game bird species traditionally used in avian toxicity tests which could impact their sensitivity to chemicals

Practical Utility of the Data How did the Agency make its registr ation decision without these data? Since this is a new requirement, EPA was usi ng the results from the toxicity tests for the waterfowl and the upland game bird.

How will the data be used? Data from passerine toxicity studies will be used to estimate potential risks to birds associated with uses of hexazinone. The data will reduce uncertainties associated w ith the current risk assessment for passerine species and will improve our understanding of the potential effects of hexazinone.

How could the data impact the Agency’s future decision-making? Because birds significantly contribute to overall environmental quality, a solid understanding of the potential risks to birds, including passerine species, is essential for sound environmental management. Without acceptable data for hexazinone, the Agency cannot determine the levels of hexazinone that result in effects to passerine species. If the data indicates that registered hexazinone usage may pose a risk of adverse effect s to non-target birds above the Agency Level of Concern, the Agency may explore decision options to mitigate this risk. The lack of these data will limit the flexibility the Agency and registra nts have in coming into compliance with the Endangered Species Act, and could result in use r estrictions for hexazinone which may otherwise be avoided, or which are unnecessarily severe.

4 7Guideline Number: 850.4225 Study Title: Seedling Emergence, Tier II

Rationale for Requiring the Data

Due to the use of another pesticide on the cucu mber seeds, no acceptable toxicity data are currently available to assess the risk of hexazinone to cucumber plants. Due to the damage to the onion and pea plants during the submitted SE study (MRID 43162501), anothe r study is needed.

Since hexazinone has residential outdoor uses, seedling emergence studies are required. These phytotoxicity data are needed to evaluate the le vel of pesticide exposure to non-target terrestrial and endangered and threatened plants. Practical Utility of the Data

How did the Agency make its re-regis tration decision without these data? Since EPA was unable to evaluate the potential ri sks to terrestrial plants associated with the proposed uses of hexazinone, risks were presumed for seedling emergence endpoints for cucumber, onion and pea plants.

How will the data be used? Data from plant toxicity studies will be used to estimate potential risks to plants associated with uses of hexazinone. The data will reduce uncertain ties associated with the current risk assessment and will improve our understanding of the pot ential effects of hexazinone on plants.

How could the data impact the Agency’s future decision-making? Because plants form the basis of most habitats and significantly contribute to overall environmental quality, a solid understanding of the potential risks to terrestrial plants is essential for sound environmental management. Without acceptable plant growth data for hexazinone, the Agency cannot determine the levels of hexazinone that result in effects to plants. If the data indicates that registered hexazinone usage may pose a risk of adverse effects to non-target terrestrial plants above the Agency Level of Con cern, the Agency may explore decision options to mitigate this risk. The lack of these data will lim it the flexibility the Agency and registrants have in coming into compliance with the Endangered Sp ecies Act, and could result in use restrictions for hexazinone which may otherwise be avoided, or which are unnecessarily severe.

4 84 9 Guideline Number: 850.4250 Study Title: Vegetative vigor, Tier II

Rationale for Requiring the Data

Due to the use of another pesticide on the cucu mber seeds, no acceptable toxicity data are currently available to assess the risk of hexazinone to cucumber plants.

Since hexazinone has residential outdoor uses, vegetative vigor studies are required. These phytotoxicity data are needed to evaluate the le vel of pesticide exposure to non-target terrestrial endangered and threatened plants. Practical Utility of the Data

How did the Agency make its re-regis tration decision without these data? Since EPA was unable to evaluate the potential ri sks to terrestrial plants associated with the proposed uses of hexazinone, risks were presume d for vegetative vigor endpoints for cucumber plants.

How will the data be used? Data from cucumber plant toxicity studies will be used to estimate potential risks to plants associated with uses of hexazinone. The data w ill reduce uncertainties associ ated with the current risk assessment and will improve our understandi ng of the potential effects of hexazinone on cucumber plants.

How could the data impact the Agency’s future decision-making? Because plants form the basis of most habitats and significantly contribute to overall environmental quality, a solid understanding of the potential risks to terrestrial plants is essential for sound environmental management. Without acceptable plant growth data for hexazinone, the Agency cannot determine the levels of hexazinone th at result in effects to cucumber plants. If the data indicates that registered hexazinone usage ma y pose a risk of adverse effects to non-target cucumber plants above the Agency Level of Con cern, the Agency may explore decision options to mitigate this risk. The lack of these data will lim it the flexibility the Agency and registrants have in coming into compliance with the Endangered Sp ecies Act, and could result in use restrictions for hexazinone which may otherwise be avoided, or which are unnecessarily severe.