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 

 1
Table 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 

2
1. 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 the nature of the problem, and provides a plan 

for analyzing the data and characterizing the risk (EPA, 1998). 

 

2. Problem Formulation 

2.1. Nature of Regulatory Action

This report summarizes the Environmental Fate and Effects Division’s (EFED) Problem 

Formulation for the Registration Review of hexazinone. Previous risk assessments were 

completed for the uses of the chemical, through 2008, which serve as the basis for this 

assessment. 

2.2. Previous Risk Assessments

A Reregistration Eligibility Decision (RED) that included an ecological risk assessment 

for fish, invertebrates, and plants in aquatic habitats, as well as birds, mammals, 

invertebrates and plants in terrestrial habitats, was issued in September of 1994 for 

hexazinone. The data were considered sufficient to allow the Agency to assess the 

registered uses of hexazinone and to determine that hexazinone could be used without 

resulting in unreasonable adverse 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 are included in this assessment. 

That assessment also determined that exposure of non-target organisms to hexazinone 

could result from direct application, spray drift from treated areas, and runoff from 

treated areas. Such exposure would be chronic as well as acute. Hexazinone exceeded the 

levels of concern (LOCs) for terrestrial and aquatic plants, at all application rates, using 

aerial and ground equipment. Contamination of aquatic sites adjacent to treated areas 

could be of great ecological significance and may be exacerbated 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 higher 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 plants at all use rates. 

 3
An endangered species litigation assessment for the California Red-Legged Frog 

supported the RED results for no effect on aquatic animals, but with potential effects on 

terrestrial animals and both aquatic and terrestrial plants, with refinements based on 

information specific to the CRLF. The assessment resulted in an overall “May Affect, 

Likely to Adversely Affect” determination (February 20, 2008). There is an LAA 

determination for direct effects to the CRLF using the bird as a surrogate consuming 

small herbivore mammals for noncrop, conifer release, blueberry and Christmas tree uses 

due to LOC exceedence resulting from the T-Herps analysis. There is an LAA 

determination for indirect dietary effects 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 mode of action for non-vascular plants based 

on LOC exceedence and mode-of-action for hexazinone. Indirect habitat 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 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. Based on the available information, hexazinone appears to be 

persistent and mobile in soil and aquatic environments 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 dissipation 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, respectively, 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 (KOC = 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 inhibition of photosynthesis. Hexazinone is 

used against broadleaf weeds, grasses and some woody plants. It is systemic and it is 

absorbed through the foliage & roots; further, it is translocated through the xylem. 

Rainfall or irrigation after application is required to activate the chemical. It may be used 

with a nonionic surfactant to improve its contact activity. 

A summary of physicochemical properties 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(1H,3H)-dione 

Hexazinone data sheet: 

http://www.alanwood.net/pe

4
Table 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(1H,3H)-dione 

CAS Reg. No. 51235-04-2 

sticides/index_cn_frame.ht

ml Accessed 09/30/09 

Molecular Formula C12H20N4O2 HSDB 

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

Chemical Structure 

(from chemical’s data 

sheet) 

Molecular Weight 252.36 g/mol HSDB 

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

Vapor Pressure (25oC) 2 x 10-7 mmHg; Non-volatile under field conditions 

(USEPA 2008) Product Chemistry 

Henry’s Law Constant 2 x 10-12 atm-m3

/mol Product Chemistry 

Octanol/ Water Partition 

Coefficient 

(log KOW and KOW) 

1.2 and 15, respectively 

[1.85 and 71, respectively] 

Product Chemistry 

[BioByte (1995), 

cited by EPI SuiteTM] 

Octanol/ Air Partition 

Coefficient (25oC) 

(KOA) 

KOA = 15 x 8.205746 m3

 atm/K mole x 298K = 2 x 1016

2 x 10-12 atm-m3

/mol 

(7.7 x 1011) 

Constant Law sHenry'

RTK

K

K K OW

AW

(EPI SuiteTM estimate) 

OW

OA ==

Biomagnification 

Potential 

Presumption*: If log KOA > 5, log KOW >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 determine the biomagnification 

potential. 

For hexazinone, log KOA=12-16 & rate of transformation is 

slow, however, log KOW=1.85; it appears that hexazinone 

may biomagnify, but possibly not substantially, in 

terrestrial food chains. 

*Gobas et al. 2003 and 

Armitage & Gobas, 2007 

support this presumption. 

Hexazinone has a moderately high molecular weight of 252.36 g/mol. With a very high 

solubility (33,000 ppm) and a low octanol/ water partition coefficient (KOW = 15), it 

appears that hexazinone has a very low potential 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-m3

/mol), and with its very high solubility (33,000 ppm), hexazinone does not

appear to volatilize substantially from wet surfaces. For hexazinone, log KOA=12-16 & 

rate of transformation is slow, however, log KOW=1.85; it appears that hexazinone may 

biomagnify, but possibly not substantially, in terrestrial food chains. 

 5
Even though various degradates were observed in the laboratory studies (e.g. 1, A, A-1 

and B, etc.), it was found that they were generally present at relatively low concentrations 

due to the high persistence of hexazinone. Table 3.2 shows the chemical structures of 

hexazinone’s major metabolites. This assessment is based on the parent because 

hexazinone it is so persistent in the field, 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

O N

O

N

CH3

CH3 CH3

OH

S

A-1 3-hydroxycyclohexyl-6-

dimethylamino)-1-methyl-1,3,5-

triazine-2,4 (1H,3H)-dione 

N N

O N

O

N

CH3

CH3 CH3

OH

B 3-cyclohexyl-6-(methylamino)-1-

methyl-1,3,5-triazine-2,4 (1H,3H)-dione 

N N

O N

O

NH

CH3 CH3

C 3-(4-hydroxycyclohexyl-6-

methylamino)-1-methyl-1,3,5-triazine2,4 (1H,3H)-dione 

N N

O N

O

NH

CH3 CH3

OH

S

6
Table 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

O N

O

O

CH3

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

N NH

O N

O

O

CH3

OH

S

G 3-cyclohexyl-6-(methylamino)-1,3,5-

triazine-2,4 (1H,3H)-dione 

N NH

O N

O

CH3

NH

CH3

1 3-(4-ketocyclohexyl)-6-

(dimethylamino)-1-methyl-1,3,5-

triazine-2,4 (1H,3H)-dione 

O

N N

O N

O

N

CH3

CH3

CH3

7
Table 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

O N

O

N

CH3

CH3 CH3

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 reactive molecules into a chain 

reaction that destroys chloroplast and cell membranes, and other vital compounds. It is 

usually applied as a pre-emergent herbicide, and soils must be moist (by rain or 

irrigation) to activate hexazinone. 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-state chlorophyll reacts with oxygen (O2) to form singlet oxygen 

(O), and both the chlorophyll and singlet oxygen strip hydrogen ions (H+) from 

unsaturated lipids in cell and organelle membranes, producing lipid radicals. The lipid 

radicals in turn attack and oxidize other lipids and proteins, resulting in the loss of cell 

and organelle membrane integrity, loss of chlorophyll 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 granular formulations of hexazinone may be 

applied by aircraft as well as ground sprayer. While the specific equipment for ground 

application varies between the liquid and granular formulations, both types of 

formulations are applied such that the herbicide sprayer for liquid or suspended granules 

or other equipment for granules is carried by backpack or some other appropriate 

container. Usually, hexazinone is applied directly to the soil rather than sprayed on the 

vegetation; however, sometimes, directed foliar applications are used. In soil 

applications, the hexazinone is applied in spots using a defined pattern. 

Analysis of labeled use information is the critical first step in evaluating the federal 

action. The current label for hexazinone represents the FIFRA regulatory action; 

therefore, labeled use and application rates specified on the label form the basis of this 

 8
assessment. The assessment of use information is critical to the development 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 alfalfa, rangeland and pasture, woodland, 

pineapples, sugarcane and blueberries. It is also used on ornamental plants, forest trees 

and other non-crop areas. Hexazinone is registered 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 selective herbicide in reforestation practices. Hexazinone 

products are formulated as pellets/tablets, emulsifiable concentrates, 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 the 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 information from the label. Blueberry 

uses are modeled at a maximum of one application per year due to no application within 

450 days of harvest. Pineapple is modeled as one application per year due to the growing 

season, which is nearly a year in length. Non-crop uses and all granular uses, non-crop, 

forest site preparation and rangeland, are modeled as one application 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 Uses1 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 Usage 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. 

9
Table 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-NASS (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 States. The USGS pesticide maps are based on 

State level estimates of pesticide use rates for individual crops, which have been 

compiled by the National Center for Food and Agricultural Policy (NCFAP) for 1999 

through 2004, and on the 2002 Census of Agriculture 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%). 

 10
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 Research 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 rates are expressed as 

the pounds applied per square mile of mapped agricultural land in a county. The area of mapped agricultural land for each county was obtained 

from an enhanced version of the 1992 USGS National Land Cover Data (NLCD). The key limitations of the data used to produce these 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 pesticide 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 Census of 

Agriculture and may not represent all crop acreage because of Census nondisclosure rules, and (5) agricultural land area used to 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 at 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 considered as the stressor in this assessment (Table 

3.5). The mobility of hexazinone was demonstrated in batch equilibrium as well as field 

 11
and 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 (KOC = 38-77 mL/gOC, 

mobile FAO 2000 mobility classification). 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. 

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⋅m3/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 laboratory bioaccumulation in rainbow trout, 

hexazinone is not expected to bioaccumulate in fish, which is in concurrence with the low 

KOW value of 15 (log KOW = 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 Metabolites 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, respectively, 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. Metabolites A, B, C, D, E, G, and H were 

discernible in various matrices (soil, water, litter and/or vegetation) samples in forestry 

field samples. Metabolites D and 2 were major degradates found in the anaerobic aquatic 

metabolism study; however, the field dissipation and the small-scale prospective 

groundwater monitoring studies did not monitor 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 dissipation studies did not monitor the fate of 

Metabolite G-3170 the degradate detected at the highest concentrations in the small-scale 

prospective groundwater monitoring study. This metabolite was not observed in the 

aerobic soil metabolism study. A summary of environmental 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; (25oC) 

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 

 12
Table 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 metabolite 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. Metabolites 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 sandy loam sediment (pH 7.8, %OM 

= 0.7) and water (pH 8.2) from Madera, 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 applied 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 (KOC) 

38, 75, 77 & 40 mL/gOC, 

for Tama SiL, Chino L, Trevino SL & Sassafras SL

Mobile (FAO 2000) 

Soil Partition 

Coefficient for Parent 

(Kd) 

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 half-lives were 123 and 134 days in 

bareground plots at 12 lb a.i./A. The 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 dissipation 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 

 13
Table 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 of 6.72 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, hexazinone 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 

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 hexazinone 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 (r2

 = 0.9997; first order kinetics). Results 

indicated that hexazinone and its degradates are very mobile and 

persistent. They exhibited a classic pattern of leaching through the 

soil profile and into the groundwater. Samples were analyzed for 

hexazinone and six transformation products: 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 due to insufficient sampling 

periods. 

Abiotic Degradation 

 14
Hexazinone was stable to hydrolysis in buffered solutions at pH 5, 7 and 9 at 25oC. The 

chemical comprised from 99-100% of the total radioactivity in all samples. When 

exposed to an artificial light source, hexazinone did not appear 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 photodegradation study, using a sandy loam soil from Modesto, CA 

(pH 6.5), hexazinone’s half-life was 82 days, equivalent to an environmental half-life of 

228 days at latitude 30 to 50oN, assuming a 12-hour photoperiod. The major 

transformation product was Metabolite 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 major 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 accounted 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 aerobic aquatic metabolism study, consisting of a 

sandy loam sediment (pH 7.8, %OM = 0.7) and water (pH 8.2) from Madera, CA was 

>60 days, both in viable and sterile (autoclave) systems. Hexazinone accounted for ~80-

84% and ~86-87% of the applied radioactivity 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 related 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 available 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 transformation product (maximum 36.5% of the recovered 

at 365 days). 

Mobility 

 15
In a Madera loam, Hanford sandy loam, Cecil sandy loam and Flanagan silty loam, the 

Freundlich KF 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 

KF values were 0.53, 0.84, 0.59 and 0.21, respectively. For the latter soils, the Freundlich 

KFOC 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). Supplemental 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 conducted at two sites, on a silt loam in 

Newark, DE and on a silty clay loam in Greenville, 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 concentrations 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 Newark) leached to a depth of 75 cm of soil 

during the testing period. 

Hexazinone, at 12 lb a.i./A, had a dissipation 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. The study was considered supplemental because the concentrations at 

time zero were too low, compared to the theoretical (or expected) value; however, the day 

7 thru 60 post-treatment samples had concentrations 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 was 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 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 post-treatment, hexazinone 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 vegetation) samples. However, metabolite G was not 

confirmed by MS. Hexazinone moved off-site through leaching and runoff. Hexazinone 

(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. 

In the forestry study, hexazinone was detected from stream samples from both sites 

(maximum concentration 473 ppb at 1150 hours post-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 

 16
blueberry plant tissues (both sites). 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). Finally, 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 prospective groundwater monitoring study for 

hexazinone (MRID 45132801). Hexazinone [Velpar L, 2 lb a.i/gallon water-dispersible 

liquid] was broadcast applied once at 0.75 lb a.i./A in January 1996 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. 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 there were no continuous impeding layers. 

The first rainfall was received at 7 days and the 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 monthly cumulative precipitation plus irrigation 

ranged from 111 to 195% of baseline. The depth 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 the shallow groundwater occurred at about 152 days posttreatment. Soil (to 2 feet), soil pore water (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 upper 2 feet of soil with a calculated half-life of 25 days 

(r2

 = 0.9997; first order kinetics). Hexazinone and its’ degradates exhibited a classic 

pattern of leaching through the soil profile and into the groundwater. Hexazinone 

concentrations above background (~0.05 ppb) were first detected in pore 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 and degradate B into the shallow groundwater occurred at 

152 days post-treatment. 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 bgs), the maximum 

measured concentrations were as indicated in the table above. In the individual deep 

groundwater wells (20-25 feet bgs), the maximum 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 compounds 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 

 17
detected 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 (Table 3.6) provides a summary with the 

maximum concentrations of the parent and its’ degradates, detected in the small-scale 

prospective groundwater monitoring study. These concentrations were also expressed in 

parent equivalents. 

Table 3.6. Max. Concs. in Small-Scale Prospective 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 concurrence with the low octanol/ water partition 

coefficient, KOW value of 15 (log KOW = 1.12). Hexazinone has been shown to 

accumulate in rotational crops. 

The fate and transport characterization also includes the various degradation and 

transformation products formed by each process in the studies reviewed. Table 3.7

provides a summary of these transformation products. 

Table 3.7. Summary of Degradate Formation 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 

 18
Table 3.7. Summary of Degradate Formation 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 transformation products found in the anaerobic 

aquatic metabolism study; however, the field dissipation and the small-scale prospective 

groundwater monitoring studies did not monitor 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 dissipation studies did not monitor the fate of the 

degradate detected at the highest concentrations 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 Various Hexazinone Metabolites 

Adsorption (Mobility) Studies Kd KOC 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 pesticide surface water runoff and spray drift. 

The magnitude of pesticide transport via secondary drift depends on the pesticide’s 

ability to be mobilized into air and its eventual removal through wet and dry deposition 

of gases/particles and photochemical reactions in the atmosphere. 

 19
4. Receptors 

4.1 Aquatic and Terrestrial Effects

The receptor is the biological entity that is exposed to the stressor (EPA, 1998). Due to 

the outdoor uses of hexazinone, the types of receptors that may be exposed include both 

aquatic and terrestrial receptors, such as birds, reptiles, mammals and freshwater and 

estuarine/ marine fish, non-target 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 risk assessment uses a surrogate species

approach in its evaluation of hexazinone. Toxicological data generated from surrogate 

test species, which are intended to be representative of broad taxonomic groups, are used 

to extrapolate to potential effects on a variety of species (receptors) included under these 

taxonomic groupings. 

Table 4.1 provides a summary of the taxonomic groups and the surrogate species tested 

to help understand potential ecological effects of pesticides to these non-target 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 (LD50 = 2,258 mg/kg MRID 00073988) 

using 98% a.i. The results indicate that hexazinone is practically nontoxic to birds. This 

result will be used in this assessment. 

Dietary acute toxicity studies resulted in an LC50 > 5,000 mg/kg for mallard duck 

(MRID 0010498), and bobwhite quail (MRID 00107878 and MRID 00072663) 

indicating that hexazinone is practically nontoxic to birds. These studies are not used 

quantitatively in this assessment because no mortality was observed at the highest test 

concentration. 

Birds: Chronic Exposure (Growth, Reproduction) Studies 

Chronic reproductive toxicity data is available for the bobwhite quail (NOAEC = 300 

mg/kg MRID 417649-01). The endpoint was a statistically significant 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 available 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 Exposure (Mortality) Studies 

 20
A registrant submitted rat acute oral (LD50=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 LD50 = 530 mg/kg (Kennedy 1984). 

The results indicate that hexazinone is practically nontoxic to mammals. This result will 

be used in this assessment. 

Mammals: Chronic (Growth, Reproduction) Studies 

An acceptable two-generation rat study (MRID 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 potential direct and indirect effects 

of hexazinone to non-target species, as well as to endangered species. Effects to 

terrestrial invertebrates resulting from exposure to hexazinone could indirectly affect 

endangered species via reduction 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 information on fish and aquatic invertebrates. A 

summary of acute and chronic data is provided below. 

Fish: Acute Exposure (Mortality) Studies 

With respect to technical grade hexazinone, the reported acute 96-hour LC50 values were 

>320 mg/l for rainbow trout (MRID 00104980), and 274 mg/l for Fathead minnow 

(MRID 00104980). Two studies were available for bluegill sunfish, 96-hour LC50 >370 

mg/L and 96-hour LC50 = 505 mg/L (MRID 00104980 and 00076959). The results of 

these tests indicate that hexazinone is practically nontoxic to fish. The fathead minnow 

LC50 = 274 mg/l will be used to assess the chronic effects for the use of hexazinone. 

Fish: Chronic Exposure (Growth/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 Invertebrates: Acute Exposure Studies 

 21
Acute toxicity data for hexazinone are available for the preferred test species, Daphnia 

magna. A 48-hour EC50=151.6 mg/l (MRID 00116269) indicates that hexazinone is 

practically nontoxic to aquatic 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 EC50 > 320 mg/L for the Eastern oyster was estimated in this 

toxicity test, as well as the grass shrimp with a 96 h LC50=78 mg/L and the fiddler crab 

with a 96 h LC50 > 1000 mg/L. These studies demonstrate that hexazinone ranges from 

slightly to practically nontoxic to mollusks and slightly toxic to crustaceans.

Aquatic Invertebrates: Chronic 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-submitted 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 guidelines. Sub-lethal 

endpoints such as plant growth, dry weight, and biomass are evaluated for both monocots 

and dicots are evaluated at both seedling emergence and vegetative life stages. Guideline 

studies generally evaluate toxicity 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 been selectively bred, and may be more or less resistant to 

particular stressors than wild herbs and forbs. The direction of this uncertainty for 

specific plants and stressors, including hexazinone, is largely unknown. Homogenous 

test plant seed lots also lack the genetic variation that occurs in natural 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 evaluate the effects on non-target plants and are 

used to evaluate direct and indirect effects for endangered species. The results of the Tier 

II seedling emergence and vegetative vigor toxicity 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 EC25 of 0.0064 lbs/A for tomato. The monocot EC25 result for sorghum was 

0.019, lbs/A. 

 22
The vegetative vigor EC25 for dicots was 0.011 for rape. The EC25 for emergence for 

monocots was 0.020 for wheat. These values will 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 indirect 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 EC50=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 EC50=0.210 mg/L (MRID 

43302701). An acceptable study is available for Selenastrum capricornutum. The 

EC50=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 EC50=0.012 mg/L (MRID

43302701). The acceptable study with the estuarine/marine diatom Skeletennoma

costatum resulted in an EC50=0.012 mg/L (MRID 43225102). 

Table 4.1. Test Species Evaluated for Assessing Potential Ecological Effects of Hexazinone 

and the Associated Acute Toxicity Classification 

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

8d LC50 > 5000 mg/kg 

 (MRID 00107878 and 00072663) 

Bobwhite (Colinus virginianus Practically non-toxic ) 

Avian Reproduction 

NOAEC=300 mg/kg 

(MRID 41764901) 

14d LD50=2258 mg/kg-bw 

(MRID 00073988) 

Practically non-toxic 

Birds1

Mallard (Anas platyrhynchos) 

Mallard LC50 > 5000 mg/kg 

 (MRID 00104981) 

Practically Non-toxic 

 23
Table 4.1. Test Species Evaluated for Assessing 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) 

LD50 = 530 mg/kg-bw 

 (Kennedy, 1984) 

Practically Non- Toxic

LD50 = 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 LD50 >100 μg/bee for 

(MRID 41216502) 

Practically non-toxic

Bluegill LC50=505 mg/L 

(MRID 00076959) 

Practically nontoxic 

BlueGill (Lepomis macrochirus) 

BluegillLC50>370 mg/L 

(MRID 00104980) 

Practically non-toxic 

Rainbow trout (Oncorhynchus mykiss) 

Rainbow trout LC50>320 mg/L 

(MRID 00104980) 

Practically non-toxic 

Fathead Minnow LC50=274 mg/L 

(MRID 00104980) 

Practically non-toxic 

Freshwater Fish2

Fathead minnow (Pimephales 

promelas) NOAEC = 17 mg/L 

(MRID 41406001) 

Juvenile survival, length and 

weight 

Acute EC50=151.6 mg ai/L 

(MRID 00116269) 

Practically non-toxic 

Aquatic Invertebrates Daphnia magna 

TEP: Acute EC50=339.9 mg/l 

(MRID 41235003) 

Practically non-toxic 

 24
Table 4.1. Test Species Evaluated for Assessing 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 EC25=0.011 lb/A 

Vegetative Vigor (MRID 43162501)

Monocots – Wheat (Tritum gestivum) 

Dicots – Rape (Brassica napur) 

VV: Wheat 21 d 

EC25=0.020 lb/A 

(MRID 43162501) 

SE: Tomato 21 d 

EC25=0.0064 lb/A 

(MRID 43162501) 

Terrestrial plants3

Seedling Emegence 

Monocots – Sorghum (Sorghum 

bicolor) 

Dicots – Tomato (Lycopersicon 

esculentum) 

Wheat 21 D 

EC25=0.0019 lb/A 

(MRID 43162501) 

Duckweed (Lemna gibba) EC50=0.0374 mg/L 

 (MRID 43225101) 

Green algae (Pseudokirchneriella 

subcapitata) 

96h EC50=0.007 mg/L 

 (MRID 41287001) 

Blue green Algae (Anabaena flosaquae) 

96 h EC50=0.210 mg/l 

(MRID 43302701 

Acceptable 

Freshwater Diatom (Navicula 

pelliculosa)

EC50=.012 mg/L 

(MRID 43302701) 

Aquatic plants and algae 

Marine Diatom (Skeletonema costatum) 

EC50=0.012 mg/L 

(MRID 43225102) 

 25
Table 4.1. Test Species Evaluated for Assessing 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 terrestrial-phase amphibians and reptiles.- 2

 Freshwater fish may be surrogates 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 the Ecological Incident Information System 

(EIIS, version 2.1), maintained by the Agency’s Office of Pesticide Programs, and the 

Avian Monitoring Information System (AIMS), maintained by the American Bird 

Conservancy, indicates nine reported ecological incident associated 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 ‘unlikely’ and only includes those incidents with 

certainty categories of ‘highly probable’, and ‘probable’ and ‘possible’ (for EIIS) and 

‘certain’, ‘highly likely’, ‘likely’, ‘probable’, and ‘possible’ (for 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 practically nontoxic to the category of organism 

killed and/ or there is evidence that another pesticide or stressor likely caused the 

incident. Incidents classified as ‘unlikely’ for hexazinone will not be included in this

Problem Formulation or the ecological risk assessment conducted for Registration 

Review. 

Incident reports for non-target organisms typically provide information only on mortality 

events and plant damage incidents. Except for phytotoxic effects in terrestrial 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 reported incidents. Registrants are now only 

required to submit detailed information on ‘major’ fish, wildlife, and plant incidents. 

Minor fish, wildlife, and plant incidents, as well as all other non-target incidents, are 

generally reported aggregately and 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 reported in the EIIS database for hexazinone. 

4.2.2 Plant Incidents 

Nine hexazinone incidents have been reported in the EIIS database for terrestrial 

 26
plants. One incident, I000548-001 reporting non-target plant exposure, was classified 

as highly probable. Velpar ULW, a product 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 the appearance of reddening 

on Ponderosa Pine needle growth. Damage could 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 was reported. Damage could be due to the 

application of multiple products. 

I000611-001: Velpar L, a product 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 yellow-spotting) to the trees. Grasses were 

not affected. 

I014407-036: A product containing hexazinone was applied on an alfalfa site in 

Washington (1994). No report of total magnitude, or application rate was provided. 

The damage to yield was to be assessed 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 sixty-five percent of 129 acres of loblolly 

pine seedlings was reported, but no application rate or method was provided. 

Damage could be due to the application 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 used in this risk 

assessment. 

4.2.3 Aquatic Animal Incidents 

No aquatic animal incidents were reported in the EIIS database for hexazinone. 

A preliminary review of the Avian Monitoring Information System (AIMS), 

maintained by the American Bird Conservancy, was completed on October 26, 2009. 

No incidents were reported, but this information will be updated for the risk 

assessment. 

Although registrants are only required to provide detailed information on major fish, 

wildlife and plant incidents recorded in the EIIS database, minor incidents are 

reported aggregately. Aggregate incidents are categorized as “minor fish and 

wildlife” (W-B), “minor plant” (P-B) and “other non-target” (ONT) incidents and are 

 27
recorded 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 assumed 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 occurred since the date of the reported 

incidents. 

Hexazinone is among the constituents monitored by the National Water Quality 

Assessment Data Warehouse (NAWQA), maintained 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 assessment. 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 field 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, resulting in possible exposure to aquatic ecosystems. Aquatic 

ecosystems potentially at risk due to the use of hexazinone include water bodies adjacent 

to, or down stream from, the treated field and might include impounded bodies such as 

ponds, lakes and reservoirs, or flowing waterways 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 actual environmental 

value that is to be protected.” Defining an assessment endpoint involves two steps: 1) 

identifying the valued attributes of the environment that are considered to be at risk; and 

2) operationally defining the assessment endpoint in terms of an ecological entity (i.e., a 

community of fish and aquatic invertebrates) and its attributes (i.e., survival and 

reproduction). Therefore, selection of the assessment endpoints is based on valued 

entities (i.e., ecological receptors), the ecosystems potentially at risk, the migration 

pathways of pesticides, and the routes by which ecological receptors are exposed to 

pesticide-related contamination. The selection of clearly defined assessment endpoints is 

important because they provide direction and boundaries in the risk assessment for 

addressing risk management issues of concern. Changes to assessment endpoints are 

typically estimated from the available toxicity studies, which are used as the measures of 

 28
effects to characterize potential ecological risks associated with exposure to pesticides, 

such as hexazinone. 

To estimate exposure concentrations, the ecological risk assessment considers a single 

application at the maximum application rate to fields that have vulnerable soils. The 

most sensitive toxicity endpoints are used from surrogate test species to estimate 

treatment-related direct effects on acute mortality and chronic reproductive, growth and 

survival assessment endpoints. Toxicity tests are intended to determine effects of 

pesticide exposure on birds, mammals, fish, terrestrial and aquatic invertebrates, and 

plants. These tests include short-term acute, sub-acute, and reproduction studies and are 

typically arranged in a hierarchical or tiered system that progresses from basic laboratory 

tests to applied field studies. The toxicity studies are used to evaluate the potential of a 

pesticide to cause adverse effects, to determine whether further testing is required, and to 

determine the need for precautionary label statements 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 reproductive effects for aquatic and terrestrial 

effects of hexazinone. More sensitive endpoints 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 concentrations. 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 description and visual representation 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 (EPA, 1998). For this assessment, the risk is 

stressor-initiated, where the stressor is the release of hexazinone into the environment. 

The following risk hypothesis is presumed for this screening-level assessment: 

Given the uses of hexazinone and its environmental fate properties, 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 

 29
effects upon the survival, growth, and reproduction of non-target terrestrial and 

aquatic organisms. 

The conceptual model is a generic graphic depiction of the risk hypothesis. It includes 

the potential pesticide or stressor (hexazinone), the source of the pesticide and/or 

transport pathways, exposure media, exposure point, biological receptor types, and 

attribute changes. 

6.2 Conceptual Diagram

The conceptual site model is a generic graphic depiction of the risk hypothesis, and 

assumes that the herbicide hexazinone, having outdoor uses, is capable of affecting 

aquatic and terrestrial animals provided that environmental concentrations are sufficiently 

elevated as a result of proposed label uses. Based on an examination of the physicochemical properties of hexazinone, the fate and disposition in the environment, and mode 

of application, a conceptual model was developed that represents the possible 

relationships between the stressors, ecological receptors, and the assessment endpoints. 

Through a preliminary iterative process of examining available data, the conceptual 

model (i.e., the representation of the risk hypothesis) may be refined to reflect the likely 

exposure pathways and the organisms that are most relevant and applicable to this 

assessment (see Figure 6.1, below). It includes the potential pesticide or stressor 

(hexazinone), the sources and/ or transport pathways, exposure media, exposure points, 

biological receptor types and attribute changes. 

In order for a chemical to pose an ecological risk, it must reach ecological receptors in 

biologically significant concentrations. 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 ecological receptors, and a 

feasible route of exposure. In addition, the potential mechanisms 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/ transformation products are of greater toxicological concern 

than the parent compound. The assessment of ecological exposure pathways, therefore, 

includes an examination of the source and potential 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 applications. 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 exposure is through direct contact with the water 

column, sediment, and pore water (gill/ integument) 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 

 30
aquatic receptors are certain freshwater and estuarine/ marine fish, invertebrates, and, in 

certain cases, aquatic plants. The major point 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 cases, terrestrial plants. The 

attribute changes used to assess risk to terrestrial receptors depend on the type of test 

(e.g., reduced survival, growth, or reproduction 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, fish are considered surrogates for aquatic phase 

amphibians. 

7. Analysis Plan 

In order to address the risk hypothesis, the potential for adverse effects on the 

environment is estimated. Usage, environmental fate and transport, and ecological 

effects of hexazinone are characterized and integrated 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 likelihood and/or magnitude of an adverse effect. However, as 

 31
outlined in the Overview Document (USEPA, 2004), the likelihood of effects to 

individual organisms from particular 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 submitted 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 potential 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 following application), if available, may be 

presented as part of the ecological risk assessment. It is expected that the toxic effect of 

hexazinone, in combination with other pesticides used in the environment, is likely to be 

a function of many factors including, but not necessarily limited to: (1) the exposed 

species, (2) the co-contaminants in the mixture, (3) the ratio of hexazinone and cocontaminant concentrations, (4) differences in the pattern and duration of exposure 

among contaminants, and (5) the differential effects of other physical/ chemical 

characteristics of the receiving waters (e.g. organic matter present in sediment and 

suspended water). Quantitatively predicting the combined effects of all these variables 

on mixture toxicity to any given 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 data 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 exposures in aquatic and terrestrial environments, 

all exposure modeling and resulting risk conclusions 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 concentrations 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 terrestrial 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 shell PE5.pl (August, 2007) to generate daily 

exposures and 1-in-10 year EECs of hexazinone, that may occur from spray drift and 

 32
runoff to surface water bodies adjacent to application sites. PRZM simulates pesticide 

application, movement and transformation on an agricultural field and the resultant 

pesticide loadings to a receiving 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 for ecological pesticide assessments assumes 

application to a 10-hectare agricultural field that drains into an adjacent 1-hectare water 

body that is 2 meters deep (20,000 m3

 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 concern. The measure of exposure for aquatic 

species is the 1-in-10 year return peak or rolling mean concentration. 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 assessing chronic exposure to fish 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 target area are derived 

using the T-REX model (version 1.3.1, 12/07/2006). For granular pesticides, this 

includes the amount of pesticide per square foot, used in LD50 per square foot risk 

assessment calculations. EECs for terrestrial plants inhabiting dry 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 database 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 chemical mixture toxicity data for aquatic life, 

terrestrial plants, and wildlife. ECOTOX was created and is maintained by the USEPA, 

Office of Research and Development, and the National Health and Environmental Effects 

Research Laboratory’s Mid-Continent Ecology Division. 

Information on the potential effects of hexazinone on non-target animals is also collected 

from the Ecological Incident Information System (EIIS). The EIIS is a database 

containing adverse effect (typically mortality) reports 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 qualitatively. Such effects may include behavioral 

changes (e.g., lethargy and changes in coloration). Quantitative assessments of risks, 

though, are limited to those endpoints that can 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. 

 33
The acute measures of effect used for animals in this assessment are the LD50, LC50 and 

EC50. LD stands for “Lethal Dose”, and LD50 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 LC50 is the concentration of a chemical that is estimated to 

kill 50% of the test organisms. EC stands for “Effective Concentration” and the EC50 is 

the concentration of a chemical that is estimated to produce a specific effect in 50% of 

the test organisms. Endpoints for chronic measures of exposure for listed and non-listed 

animals are the NOAEL/NOAEC and NOEC. NOAEL stands for “No ObservedAdverse-Effect-Level” and refers to the highest tested dose of a substance that has been 

reported to have no harmful (adverse) effects on test organisms. The NOAEC (i.e., “NoObserved-Adverse-Effect-Concentration”) is the highest test concentration at which none

of the observed effects were statistically different from the control. The NOEC is the NoObserved-Effects-Concentration. For non-listed plants, only acute exposures are 

assessed (i.e., EC25 for terrestrial plants and EC50 for aquatic plants); for listed plants 

either the NOAEC or EC05 is used. 

7.4. Integration of Exposure and Effects

Risk characterization is the integration of exposure and ecological effects characterization 

to determine the potential ecological risk from the use of pesticides 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 effects 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 assessment. An effort may also be made to 

further qualitatively describe risk using probabilistic tools that 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 (USEPA, 2004), provisions of the ESA, and the 

Services’ Endangered Species Consultation Handbook (USFWS/NMFS, 1998). 

 34
The assessment of effects associated with the registration of hexazinone 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 exceedance of Agency Levels of Concern (LOCs) 

used to evaluate direct or indirect effects. For the nationwide risk assessment of 

hexazinone, the action area will encompass the entire United States and its territories. 

This ensures that the initial area of consideration encompasses all areas where the 

pesticide may be used now and in the future, 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 effects and any modification to critical 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 direct 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 species and critical habitat that may potentially 

be affected by the action. The agency will use more rigorous analyses, including 

consideration of available land cover data, toxicity data and exposure information to 

determine areas where individual listed species and designated critical may be affected or 

modified via endpoints associated with survival, growth or reproduction. 

The Agency’s approach to defining the action area under the provisions of the Overview 

Document (USEPA 2004) considers the results of the risk assessment process to establish 

boundaries for that action area with the understanding that exposures below the Agency’s 

defined LOCs constitute a no-effect threshold. For the purposes of this assessment, 

attention will be focused on the footprint of the action (i.e., the area where hexazinone 

application occurs), plus all areas where offsite transport may result in potential exposure 

that exceeds the Agency’s LOCs. Specific measures of ecological effect that define the 

action area for listed species include any direct and indirect effects and/ or potential 

modification of its critical habitat, including 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 conducted to support future human health risk 

assessments of hexazinone should it be required by HED. The drinking water assessment 

will incorporate model estimates of hexazinone 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 subsequently using PRZM/ EXAMS, with 

the Index Reservoir - see description above, if refinements are required). Ground water 

estimates of concentrations will be estimated 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 available surface and ground water monitoring data. 

 35
7.8 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 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 appeared 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. 

835.4200 Anaerobic Soil Metabolism – No study is available. 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 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 should normally be at least 2%. 

“Fine texture” is defined as a (clay + silt) content of >50% and “coarse texture” is 

defined as a (clay + silt) content of <50%. (Clay + silt) is the mineral fraction of the 

sediment with particle size of < 50 μm. 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-sediment 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 test 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 (loam, 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 should 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 dissipation §158.1300; OPPTS guidelines 835.6100, 

835.6200, and 835.6300), along with successful confirmatory method trials (validation) 

by an independent laboratory (i.e. ILVs), are placed in reserve at this time. If there is risk 

 36
concern for a given taxon, ECMs should be available 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 toxicological levels of concern. 

According to the 1994 RED, hexazinone exceeds the level of concern for aquatic plants 

and endangered aquatic plants. Furthermore, a lifetime 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 for soil. The ECMs should include parent and 

those residues found in the laboratory studies that exceeded 10% of the applied or that are 

considered of concern. The following appear 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 state-of-the-art environmental chemistry methods; 

further, multi-residue methods (MRMs) for soil and water are preferred. The registrant 

submitted one method for soil (MRID 42379201, LOQ 0.03 μg/g, LOD 0.01 μg/g), 

published in the Agency’s ECM website, and evaluated by BEAD’s Analytical Chemistry 

Branch (memo dated 06/18/96). The method could quantify hexazinone in soil but is not 

useful to quantify the hexazinone’s metabolites. According to the memo, “An early 

review of the method showed the five metabolites 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. Therefore, these reports will be evaluated by 

the EFED. They should include the following information: (a) Recovery level(s) of the 

test compound(s) from the soil and water (substrates) at various relevant fortification 

level(s) using the residue analytical methodology; (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 recovery 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 environmental fate data requirements for hexazinone. 

Table 7.1. Summary of Environmental Fate Data Requirements for Hexazinone

Study Identification 

Use 

Pattern1

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

 37
Table 7.1. Summary of Environmental Fate Data Requirements for Hexazinone

Must Additional 

Does EPA Have Data Be Submitted 

Study Identification 

Use Data To Satisfy Bibliographic Under FIFRA 

Pattern1

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 Waived2

Metabolism Studies-Lab:

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

42635001 

Yes3

 835.4200 Anaerobic Soil 1,2,3,4 No Not Available Yes 

 835.4300 Aerobic Aquatic 1,2,3,4 Partially 41811801 Yes4

 835.4400 Anaerobic Aquatic 1,2,3,4 Partially 41807402 Yes4 

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 Waived2

 835.8100 Volatility (Field) 1,2,3,4 No Not Available Waived2

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 No2 NA No5 

 202-1 Drift Field Evaluation 1,2,3,4 No2 NA No5

 38
Table 7.1. Summary of Environmental Fate Data Requirements for Hexazinone

Must Additional 

Does EPA Have Data Be Submitted 

Study Identification 

Use Data To Satisfy Bibliographic Under FIFRA 

Pattern1

This Requirement? Citation 3©(2)(B)? 

Other Studies: 

 ECM and ILV 1,2,3,4 Partially 42379201, 

45132802, 

45132803 

Reserved6

1. Use Patterns: 1=Terrestrial/Food; 2=Terrestrial/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 hexazinone. Studies will provide information 

to reduce uncertainty for non-target and endangered species assessments. Based on new 

data requirements, a passerine acute oral toxicity test is required. 

Some crop species from the submitted plant studies were deemed invalid and should be 

repeated. The following plant 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 Data Requirements for Hexazinone

Data Requirement 

Use 

Pattern1

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 

 39
Table 7.2 Ecological Effects Data Requirements for Hexazinone

Does EPA Have Must 

Data Requirement 

Use 

Pattern1

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 

 40
Table 7.2 Ecological Effects Data Requirements for Hexazinone

Does EPA Have Must 

Data Requirement 

Use 

Pattern1

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 Waived2 

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

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

 41
8. 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 Contamination: A Reference Manual. Appendix 2. 

Parameters of pesticides that influence processes in the 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. Environmental 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 Pesticide Programs, U.S. Environmental Protection 

Agency. Endangered and Threatened Species 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 Assessments Conducted in the Environmental Fate 

and Effects Division. Office of Pesticide Programs, Washington, D.C. January 24, 2005. 

U.S. Environmental Protection Agency. 2008. OPPTS 835.6100 Terrestrial Field 

Dissipation. EPA 712-C-08-020. October 2008. Office of Prevention, Pesticides, and 

Toxic Substances. United States Environmental Protection Agency. Available at

http://www.epa.gov/opptsfrs/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/exposure/pubs/episuitedl.htm (Accessed July 8, 2009). 

U.S. Fish and Wildlife Service (USFWS) and National Marine Fisheries Service 

(NMFS). 1998. Endangered Species Consultation Handbook: Procedures for 

Conducting Consultation and Conference Activities Under Section 7 of the Endangered 

Species Act. Final Draft. March 1998. 

 42
Appendix A. SRRD data request justification tables 

The following proposed Data Call-in tables include rationales for requiring the data 

requested in this problem formulation, explanations of the utility 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 required 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 representative 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 appeared 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 available at this time. A study is required. 

Practical Utility of the Data 

How did the Agency make its registration decision without these data? 

The previous assessments were based on a single 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 toxicity towards aquatic and terrestrial plants. 

Knowledge and accurate characterization of its persistence in terrestrial (soil) environments is 

important. Available data suggest that the chemical 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 rate of reaction input parameter to be used in the 

aquatic models GENEEC2, PRZM/ EXAMS and FIRST. 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). 

 43
These conservative assumptions could lead to higher estimated exposure concentrations or 

estimated drinking water concentrations (EECs 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 the Agency and registrants have in coming into 

compliance with the Endangered Species Act and could result in use restrictions for hexazinone 

that may be unnecessary. 

 44
Guideline 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 required 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 organic 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 should normally be at least 2%. “Fine texture” is 

defined as a (clay + silt) content of >50% and “coarse texture” is defined as a (clay + silt) content 

of <50%. (Clay + silt) is the mineral fraction of the sediment with particle size of < 50 μm. 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-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 neutral or slightly acidic pH and be of marine 

origin. 

A new anaerobic aquatic metabolism study is required, utilizing one water/ sediment system, 

which is preferably domestic. One study, conducted in a loam sediment (pH 8.5) is available, 

which addresses the transformation pathway in a basic 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 system 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 registration 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 toxicity towards aquatic and terrestrial plants. In 

addition a lifetime drinking water Health Advisory has been set for hexazinone. Knowledge of its 

persistence in aquatic environments 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 rate of reaction input parameter to be used in the 

aquatic models GENEEC2, PRZM/ EXAMS and FIRST. The results from these models are used 

for ecological risk estimation and in drinking water exposure assessments. 

 45
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 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 consequently, higher risk quotients or drinking water 

exposure values. 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 the Agency and registrants have in coming into 

compliance with the Endangered Species Act and could result in use restrictions for hexazinone 

that may be unnecessary. 

 46
Guideline 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 utilize agricultural fields, forests, residential areas 

and surrounding areas, and, therefore, have the potential to be exposed to 

pesticides used in agricultural, forest, and residential settings 

 It is likely that, for most pesticide use patterns, passerines are more 

likely to be exposed to pesticides 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 registration decision without these data? 

Since this is a new requirement, EPA was using 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 with 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 effects 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 registrants have in coming into compliance with the 

Endangered Species Act, and could result in use restrictions for hexazinone which may otherwise 

be avoided, or which are unnecessarily severe. 

 47
Guideline Number: 850.4225 

Study Title: Seedling Emergence, Tier II 

Rationale for Requiring the Data 

Due to the use of another pesticide on the cucumber 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), another study is needed. 

Since hexazinone has residential outdoor uses, seedling emergence studies are required. These 

phytotoxicity data are needed to evaluate the level of pesticide exposure to non-target terrestrial 

and endangered and threatened plants. 

Practical Utility of the Data 

How did the Agency make its re-registration decision without these data? 

Since EPA was unable to evaluate the potential risks 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 uncertainties associated with the current risk assessment 

and will improve our understanding of the potential 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 Concern, the Agency may explore decision options to 

mitigate this risk. The lack of these data will limit the flexibility the Agency and registrants have 

in coming into compliance with the Endangered Species Act, and could result in use restrictions 

for hexazinone which may otherwise be avoided, or which are unnecessarily severe. 

 48
 49

Guideline Number: 850.4250 

Study Title: Vegetative vigor, Tier II 

Rationale for Requiring the Data 

Due to the use of another pesticide on the cucumber 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 level of pesticide exposure to non-target terrestrial 

endangered and threatened plants. 

Practical Utility of the Data 

How did the Agency make its re-registration decision without these data? 

Since EPA was unable to evaluate the potential risks to terrestrial plants associated with the 

proposed uses of hexazinone, risks were presumed 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 will reduce uncertainties associated with the current 

risk assessment and will improve our understanding 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 that result in effects to cucumber plants. If the 

data indicates that registered hexazinone usage may pose a risk of adverse effects to non-target 

cucumber plants 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 registrants have 

in coming into compliance with the Endangered Species Act, and could result in use restrictions 

for hexazinone which may otherwise be avoided, or which are unnecessarily severe.