UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

MEMORANDUM 

DATE: 

SUBJECT: 

TO: 

AND: 

FROM: 

APPROVED BY: 

Apri l 23, 2015 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

Estimated Drinking Waters Concentrations for the Registration Review of 

Hexazinone (PC Code 107201 ; DP Barcode 0424532) 

Dana Friedman, Chemical Review Manager 

Tom Myers, Team Leader 

Kevin Costello, Branch Chief 

Risk Management and Implementation Branch II 

Pesticide Re-evaluation Division (7508P) 

Shei la Piper, Chemist 

Donna Davis, Acting Branch Chief 

Risk Assessment Branch VI 

Health Effects Division (7509P) 

Jose Melendez, Chemist Qt4.e"' ~;,.J- /J"kb;;,4_ Environmental Risk Branch V t/' 25 

Environmental Fate and Effects Division 

~ 

(7507P) 

Mah T. Shamim, Ph.D., Branch Chief 

Environmental Risk Branch V 

Environmental Fate and Effects Division (7507P) 

This memorandum presents the Tier II Estimated Drinking Water Concentrations (EDWCs), 

calculated using the Tier II surface waters model SWCC, the Tier I/TI groundwater model 

PRZM-GW (in the Tier I mode), and the Tier J groundwater model SCI-GROW for use in the 

human health risk assessment. for the Registration Review of hexazinone (CAS No. 51235-04-

2). 

For the hexazinone total toxic residues (i. e., hexazinone plus its transformation products), the 

results of this Drinking Waters Assessment (DWA) show that for surface water, the maximum 

acute EDWC is 502 ppb of hexazinone, which is associated with the use on sugarcane in HI; the 

chronic value is 206 ppb; and the cancer/chronic value is 77.3 ppb. The maximum groundwater 

screening EDWC for acute exposure is 3090 ppb; and the chronic value is 2260 ppb, associated 

with the use on rights-of-way (Table la). 
Additionally, EFED calculated upperbound estimates of the EDWCs associated to hexazinone 

total toxic residues, including the unextracted residues (UERs) observed in the aerobic soil 

metabolism study. The results show that for surface water, the maximum acute EDWC is 502

ppb of hexazinone TTRs + UERs, which is associated with the use on sugarcane in HI; the 

chronic value is 207 ppb; and the cancer/chronic value is 77.6 ppb. The maximum groundwater

screening EDWC for acute exposure is 3710 ppb; and the chronic value is 2600 ppb, associated

with the use on rights-of-ways (Table 1b).

Finally, as a refinement, EFED calculated groundwater EDWCs associated to hexazinone total 

toxic residues, applied on pastures/rangeland. As explained in more detail below, the results 

presented in Table 1c represent applications on sandy loam or sandy soils (which are the soils in 

the six standard PRZM scenarios in PRZM-GW), for which the labeled application rate is 

smaller than for other types of soils. The maximum groundwater screening EDWC for acute 

exposure is 773 ppb; and the chronic value is 564 ppb, associated with the use on 

pastures/rangeland.

A more definitive assessment may be performed, using the Tier II aquatic model SWCC with 

regional PCAs, using additional refinement strategies with PRZM-GW (groundwater), or using

typical application rates. There are no major data gaps for hexazinone; however, certain 

uncertainties have been identified in the database as shown and detailed below. Should any 

questions arise, please contact José Meléndez at melendez.jose@epa.gov.

EXECUTIVE SUMMARY 

Hexazinone [CAS Name: 3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-

dione, IUPAC Name: 3-cyclohexyl-6-dimethylamino-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione, 

CAS Number of 51235-04-2 and a PC Code of 107201], is a triazinone herbicide. According to 

the sample label for Velpar L, hexazinone is an “herbicide...applied as a spray for weed control 

in certain crops, Christmas trees, forestry site preparation and release areas, and industrial areas. 

It may also be applied undiluted as a basal soil treatment for brush control in reforestation areas, 

rangeland, pastures and noncrop areas, or by stem injection for brush control.” There are also 

agricultural uses such like sugarcane, alfalfa, pineapple and blueberries. Hexazinone “is an 

effective general herbicide providing both contact and residual control of many annual, biennial 

and perennial weeds and woody plants.” It appears that hexazinone is systemic since it “can 

absorb...through roots extending into treated areas.” According to the Herbicide Resistance 

Action Committee (HRAC), hexazinone belongs to the HRAC Group C1 with a site of action 

inhibition of photosynthesis at photosystem II. The herbicide belongs to the triazinone chemical 

family1

.

1 http://www.hracglobal.com/Education/ClassificationofHerbicideSiteofAction.aspx (accessed 12/04/2014).

2

 
O

N

C H 3

N

C H 3

C H 3

O

N

N

Figure 1. Structure of Hexazinone

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. Exposure 

to surface water is possible through surface water runoff, soil erosion and/or off-target spray 

drift. Hexazinone has a high solubility, has a low propensity to volatilize, and will remain in the 

aqueous phase based on the low Henry's Law Constant. In laboratory tests, hexazinone was 

resistant to hydrolysis at all pHs tested and to photolysis in water and on soil. Hexazinone also 

appears to be relatively resistant to aerobic and anaerobic degradation in soils and aquatic 

environments with half-lives greater than 200 days in soils and in water.

The Health Effects Division’s (HED) Metabolism Assessment Review Committee (MARC) met 

on 01/29/2002 and then again on 03/12/2002 to consider residues of hexazinone and metabolites 

in/on plant and animal commodities and in water. The committee decided that hexazinone, the 

degradate G3170, and all metabolites with conjoined cyclohexyl and triazine rings should be 

included in the expression of the total toxic residues (TTRs) (memorandum from Sherrie Kinard 

to Christine Olinger dated April 25, 2002, DP Barcode D282111).

Table 8 lists chemical names and provides structures for the degradates, as they were observed 

in the submitted environmental fate studies (further information about the degradates is also 

provided in Appendix B). The table shows the chemical structures of hexazinone’s

transformation products: IN-G3170 (Met. G), IN-JS472 (Met. 1), IN-B2838 (Met. D), IN-G3453 

(Met. A-1), IN-A3928 (Met. B), IN-JT677 (Met. 2), and IN-T3937 (Met. A). These 

transformation products are structurally similar to the parent compound (except for G3170 or 

Met. G, which contains only one ring). They have essentially the same backbone structure with 

slight variations in the ring moieties. All the degradation products are considered stressors and 

the total residue (TR) approach was used to model the total toxic residues (TTRs). This 

modeling strategy requires an assumption that all residues of concern have similar physical, 

chemical, and partitioning characteristics. Application rates for the parent pesticide are used to 

represent the total mass loading of pesticide and its degradation product. This modeling approach 

does not consider temporal occurrence of degradation products.

This DWA is mainly based on modeling results. In this DWA, EDWCs were calculated for the 

uses with the maximum application rates: for SWCC, the use on sugarcane resulted in the 

highest EDWCs; further, for groundwater, the use of PRZM-GW was implemented and EDWCs 

are reported relative to the use on rights-of-ways. Results obtained from the models that yielded 

the highest EDWCs are summarized in Tables 1a through 1c; furthermore, results from all 

3
models are provided later in this review. A more definitive assessment could be explored, for 

example, using Tier II aquatic models SWCC with regional PCAs, performing additional 

refinements with PRZM-GW (groundwater), or using typical application rates.

Table 1a. Maximum Estimated Drinking Water Concentrations for the Use of Hexazinone

Total Toxic Residues (TTRs, excluding the unextracted residues)

DRINKING 

WATER SOURCE 

(MODEL USED) 

SCENARIO (USE) (rate modeled per year) MAXIMUM ESTIMATED DRINKING WATER 

CONCENTRATION (EDWC) ( ppb)

Groundwater 

(PRZM-GW)

NC coast1 (rights-of-way) (8 lb a.i.) Acute 3090

WI sand1 (rights-of-way) (8 lb a.i.) Chronic 2260

Surface water

(SWCC)

LA sugarcane with HI meteorological file2

(sugarcane in HI) (3.6 lb a.i.)

Acute 502

Chronic 206

Cancer/Chronic 77.3

1 Scenario was used as a surrogate and run with the maximum application rate for rights-of-ways.

2 Represents applications of hexazinone on sugarcane in Hawaii.

Table 1b. Maximum Estimated Drinking Water Concentrations for the Use of Hexazinone

Total Toxic Residues (TTRs), including the unextracted residues (UERs) (these EDWCs

provide an upperbound estimate of the TTRs)

DRINKING 

WATER SOURCE 

(MODEL USED) 

SCENARIO (USE) (rate modeled per year) MAXIMUM ESTIMATED DRINKING WATER 

CONCENTRATION (EDWC) ( ppb)

Groundwater 

(PRZM-GW) NC coast1 (rights-of-way) (8 lb a.i.) Acute 3,710

Chronic 2,600

Surface water

(SWCC)

LA sugarcane with HI meteorological file2

(sugarcane in HI) (3.6 lb a.i.)

Acute 502

Chronic 207

Cancer/Chronic 77.6

1 Scenario was used as a surrogate and run with the maximum application rate for rights-of-ways.

2 Represents applications of hexazinone on sugarcane in Hawaii.

Table 1c. Refined Estimated Groundwater Drinking Water Concentrations for the Use of 

Hexazinone, for the TTRs

DRINKING 

WATER SOURCE 

(MODEL USED)

SCENARIO (USE) (rate modeled per year) MAXIMUM ESTIMATED DRINKING WATER 

CONCENTRATION (EDWC) ( ppb)

Groundwater 

(PRZM-GW)

NC coast1 (pastures/rangeland) (2 lb a.i.) Acute 773

WI sand1 (pastures/rangeland) (2 lb a.i.) Chronic 564

1 Scenario was used as a surrogate and run with the prescribed application rate for sandy loam and loamy sand soils 

on pasture/rangeland.

Uncertainties

Even though the database for hexazinone is substantially complete, one major limitation of the

aerobic soil metabolism studies is that they showed high levels of unextracted radioactivity. 

However, for the aquatic metabolism studies, the levels of unextracted radioactivity were very 

low. Upperbound estimates of the EDWCs were calculated using the half-lives derived for the 

TTRs and the UERs. Furthermore, for some of the studies, the duration was too short compared 

to the derived representative half-lives. Uncertainty arises in these half-life estimates. Other 

4
uncertainties of the aquatic models used in this DWA (i.e., SWCC v.1.106 and PRZM-GW

v.1.07), are described in the characterization section.

PROBLEM FORMULATION

The purpose of this DWA is to assess the existing uses for hexazinone and update EDWCs 

accordingly. Previous assessments have been conducted for this chemical and this DWA builds 

upon their results. EDWCs were obtained based on modeling, as opposed to monitoring. Even 

though there is a prospective groundwater monitoring study, it represented the use of alfalfa at 

0.75 lb a.i./A (maximum application rate for hexazinone on alfalfa is up to 1.5 lb a.i./A). For 

hexazinone, the maximum rate for the chemical is 8 lb a.i./A on rights-of-ways. Monitoring data 

was used qualitatively. Hexazinone may reach surface drinking waters via runoff or drift. It 

may also reach groundwater through leaching.

Background

A previous DWA has been conducted for hexazinone (DP Barcode D215026, dated 04/16/2002). 

The surface water concentrations, for hexazinone residues were as follows: acute (peak) value,

130 ppb, and the chronic annual average value, 47 ppb, based on the application of hexazinone 

on alfalfa, which is the major food crop for the chemical. The groundwater screening 

concentration for hexazinone residues was 41.8 ppb, which were based on monitoring. Since the 

latest DWA the model SWCC has replaced PRZM/EXAMS and the groundwater PRZM-GW 

has been approved for use in EFED, in addition to SCI-GROW. This DWA reflects those 

changes. Furthermore, there is a fair amount of new environmental fate data included in this 

assessment.

Use Characterization

Use Information

Nationally, hexazinone is an herbicide used to control a broad spectrum of weeds 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 preemergent, post-emergence, layby, directed spray and basal soil applications. It is used as a nonselective herbicide in non-cropland areas and as a selective herbicide in reforestation practices. 

Hexazinone products are formulated as pellets/tablets, emulsifiable concentrates, soluble 

concentrates/solids and a technical grade active ingredient. Products are applied using aerial or 

ground equipment, 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 2 with label maximum 

application rates. Although the maximum number of applications from the label for some of the 

uses is one (1 application per season or year), for certain uses the maximum number of 

applications per year is not specified. For this assessment, one application per year for those 

other uses is based on additional information from the label. For example, blueberry uses may 

be modeled at a maximum of one application per year due to no application within 450 days of 

5
harvest. Pineapple is modeled as one application per year due to the growing season, which is 

nearly a year long. Non-crop uses and 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 is usually applied in the spring 

after ground thaws, or in the fall before snowfall (see Table 2).

Table 2. Summary of Hexazinone Uses and Application Information

Available 

SWCC Scenario Crop

Methods of 

Application

Max 

Application

Rate

(lb a.i./A)

Timing of 

Application

Non-granular applications

NC alfalfa Alfalfa (grown for hay or seed production),

Alfalfa (MT, ND, SD, WY)

Seed alfalfa (CA, ID, MT, NV, OR, UT, WA),

Seed alfalfa (Walla Walla, WA)

Ground, aerial, 

chemigation, or 

impregnated on 

dry bulk fertilizer 

(except CA & 

AZ)

1.5 (PHI =

30 d),

1,

1.5,

1.5

Most western

states winter;

eastern states 

spring

LA sugarcane, 

(using a HI 

meteorological file

to represent 

sugarcane in HI)

Sugarcane (except in FL) Ground;

or aerial (for preemergence only)

HI 3.6 (PHI= 

180 d);

LA 0.9 and 

1.5/yr (PHI= 

234 d);

PR 0.5 and 

1/yr (PHI= 

288 d);

TX 1.75 and 

1.75/yr (PHI= 

234 d)

Pre and postemergence;

Spring and fall;

Pre- and postemergence;

Pre-emergence & 

layby (60-day 

interval)

No available 

scenario, not 

modeled

Pineapple

Field edges and roadsides

Ground;

or aerial

1.75/A/app,

3.6/A/year

(PHI=181 d);

3.6

Intercrop period, 

post-mulch preplant, post-plant, 

post-emergence 

(3-10 months after 

planting)

CA wine grape Blueberry (low bush);

Blueberry (high bush)

Ground (avoid 

contact with 

leaves), or 

impregnated on 

dry bulk fertilizer

3 (PHI 450 d);

2 (PHI 90 d)

Spring

6
Available 

SWCC Scenario Crop

Methods of 

Application

Max 

Application

Rate

(lb a.i./A)

Timing of 

Application

CA rights-of-way Non-crop sites (weed and brush control):

Uncultivated nonagricultural areas (such as, airports, 

highway, railroad and utility right-of way, sewage disposal 

areas);

Uncultivated agricultural areas (non-crop producing, which 

includes: farmyards, fuel storage areas, fence rows, barrier 

strips); industrial sites (outdoor, such as, lumberyards, 

pipeline and tank farms);

Non-crop brush control.

Basal (soil) single stem treatment

Ground or aerial 

(helicopter only);

Ground or aerial 

(helicopter only);

Handgun 

applicator

8 ground and

6 for aerial 

apps;

8 ground and

6 for aerial 

apps;

2-4 mL3 per 

ea. stem in 

diameter, do 

not exceed 8 

lb a.i./A/yr

Pre-emergence or 

post-emergence 

when weeds are 

actively growing

Late winter 

through summer

CA forestry (see 

below)

Forestry site preparation

Eastern and Lakes states;

Western US

Grid application (applied directly to the soil surface)

Basal (soil) single stem treatment

Injection

Ground or aerial 

(helicopter only)

Handgun 

applicator

Handgun 

applicator

Tree injector

5;

4;

5;

2-4 mL3 per 

ea. stem in 

diameter;

1 mL3 through 

the bark of 

undesirable 

trees

Early spring to 

early summer;

Rain belt late 

winter or spring 

and snow belt fall;

From hardwood 

bud break to early 

summer;

NS;

Summer

CA forestry (using 

a MI meteorological scenario to 

represent eastern 

forests)

Conifer release

Eastern and Lakes states;

Western US

Grid application (applied directly to the soil surface)

Basal (soil) single stem treatment

Injection

Ground, aerial 

(helicopter only), 

or impregnated on 

dry bulk fertilizer

Handgun 

applicator

Handgun 

applicator

Tree injector

4;

3;

4;

2-4 mL3 per 

ea. stem in 

diameter;

1 mL3 through 

the bark of 

undesirable 

trees

Early spring to 

early summer;

Rain belt late 

winter or spring 

and snow belt fall;

From hardwood 

bud break to early 

summer;

NS;

Summer

OR Christmas 

trees

Christmas tree plantations (except in 22 states listed in 

label)

Ground or aerial First year 

plantings 1.5;

Established 

trees 2 lb 

a.i./A

East spring;

West >20 in 

rainfall spring;

West <20 in 

rainfall fall

7
Available 

SWCC Scenario Crop

Methods of 

Application

Max 

Application

Rate

(lb a.i./A)

Timing of 

Application

No available 

scenario, not 

modeled

Yellow poplar plantations Ground or aerial 2 Spring

CA rangeland hay 

(using a NY 

meteorological file

to represent eastern 

pastures)

Pastures/Rangeland

Brush control: Eastern and lakes states,

Western US;

Basal (soil) single stem treatment

Ground or aerial 

(helicopter only);

Handgun 

applicator

4,

3;

2-4 mL3 per 

ea. stem in 

diameter,

do not exceed 

0.8 lb a.i./yr

Late winter 

through summer

PA turf Bermudagrass/Bahiagrass;

Industrial turfgrass

Ground 1.125 When weeds are 

actively growing

Granular applications

CA rangeland hay Rangeland (pellets) By hand or in a 

grid pattern for 

area control

600 pellets/A= 

0.63 lb a.i./A2

Late Winter to 

early Summer

CA forestry Conifer release (granular)

Conifer release (pellets)

NS (assumed 

ground)

By hand or in a 

grid pattern for 

area control

3;

3800 

pellets/A=

4.0 lb a.i./A2

Late Winter to 

early Summer

CA forestry Forestry site preparation (granular)

Forestry site preparation (pellets)

NS (assumed 

ground)

By hand or in a 

grid pattern for 

area control

4.7 (Eastern),

4 (Western);

5800 

pellets/A=

6.1 lb a.i./A2

Spring

Spring or Fall

CA rights-of-way Non-crop sites (granular)

Uncultivated, nonfood producing agricultural areas: 

farmyards, fuel storage areas, fence rows, rights-of-way, 

fallow land, barrier strips;

Uncultivated, outdoor nonagricultural areas: airports, 

highway rights-of-way, railroad rights-of-way, utility rightsof-way (pipelines), sewage disposal areas; Industrial sites 

(lumberyards, tank farms), etc.

Non-crop sites (pellets)

Noncropland areas such as utility, highway, pipeline and 

railroad rights-of-way, petroleum tank farms, lumberyards, 

storage areas, industrial plant sites, the berm of drainage 

ditches (do not apply to the inside of ditch banks or to the 

inside of banks along waterways) and other similar areas.

Ground, except 

for military 

installations 

(aerial allowed)

By hand or in a 

grid pattern for 

area control

8

7000 

pellets/A= 7.4 

lb a.i./A2

Late Winter to 

Summer

NS

1 Uses assessed based on LUIS report Table 1, EFED label data report, and inspection of labels.

2 Based on the number of pellets in a 5.5 lb container ~3900, and 75% A.I. in formulation (source of number of 

pellets is Pro-Serve Inc.’s website (registrant’s website), at http://www.pro-serveinc.com/powerpellet.html, 

accessed 11/21/14).

3 Application rates in mL are representative of Velpar L labels. They vary for other products, but the rate is the 

same.

Environmental Hazard Statements

The following statements appear in the example label for Velpar L Herbicide:

8
“Do not apply directly to water, or to areas where surface water is present or to intertidal 

areas below the mean high water mark. Do not contaminate water when disposing of 

equipment wash waters.

The active ingredient, hexazinone, in this product is known to leach through soil into 

ground water under certain conditions as a result of agricultural use. Use of this chemical 

in areas where soils are permeable, particularly where the water table is shallow, may 

result in groundwater contamination.”

Usage

A summary from the Screening Level Usage Analysis (SLUA, provided by BEAD) for 

hexazinone is provided in Table 3. According to the SLUA, the major use for hexazinone by far 

is alfalfa, followed by sugarcane and pastureland. For all crops, ≤10% of the crop is treated. It 

should be pointed out that the SLUA does not provide information about non-agricultural use 

patterns of hexazinone.

Table 3. Summary of Screening-Level Estimates of Agricultural Uses of Hexazinone 

(SLUA, dated 06/16/2009)

Crop (alphabetical) lb a.i.

Percent (%) Crop Treated

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 map (Figure 2) shows regional scale patterns 

in use intensity within the United States. In comparison to the SLUA, the major use of 

hexazinone in 2009-11 is pastures and hay (~90% of the use), followed by other (~10%). It is 

noted that the USGS website provides additional usage maps for 2010 and 2011; however, it is 

indicated that they are preliminary at this time (and thus, not provided here). It is apparent from 

the map, that hexazinone was more frequently used in the western half of the continental U.S. in 

2009. Additionally, it was used frequently in FL, areas of the MS Delta, and the PA/NJ region.

9
Figure 2. Hexazinone Estimated Agricultural Use (2009) and Use by Year and Crop

Conceptual Model 

The conceptual model is a written description and visual representation of potential routes of offsite movement of the pesticide or degradates of concern into surface or ground water that may be 

a source of drinking water. This is a drinking water assessment that uses modeling data to 

estimate the ground water and surface water concentrations of pesticides in drinking water 

10
source water (pre-treatment) resulting from pesticide use on sites that are highly vulnerable. 

This initial tier DWA, using Tier II tools, screens out chemicals with low potential risk and 

provides estimated exposure concentrations for the human health dietary risk assessment. For 

hexazinone, the following drinking water hypothesis is being employed for this assessment:

Hexazinone use in accordance with the label, results in potential 

contamination of, both, surface and ground water resources by the parent 

compound and its metabolites of concern.

The conceptual site model is depicted in Figure 3. Through a preliminary iterative process of 

examining available data, the conceptual model (i.e., the drinking water hypothesis) was refined 

to reflect the likely exposure pathways that are most relevant and applicable to this assessment.

Figure 3. General Conceptual Model for Pesticide Transport for Hexazinone

Hexazinone is applied to the field (on the surface of the soil, foliarly, or as pellets); aerial and 

ground applications are allowed on several of the use sites, such as rights-of-ways. Crop 

absorption (crop uptake) is likely when there is foliage since this chemical is systemic. Spray 

drift is possible due to the methods of application, except for pellet applications, resulting in 

possible contamination of nearby bodies of water. As the field may be kept irrigated, large 

runoff events may be a factor in horizontal movement due to the high tendency of the chemical 

to dissolve and low potential to partition with the soils. Since hexazinone and its residues are

persistent, they may be available for very long periods of time in the field (on the order of many 

months to years, depending on the environmental conditions). Further, they can also be applied 

at very high rates of up to 8 lb a.i./A. Vertical movement to subsurfaces (i.e., leaching) is 

expected to be an important component for hexazinone in vulnerable sites, due to its high 

mobility, combined with its persistence of the chemical and its residues in most environments, 

and its high application rates. Volatilization is not expected to be an important route of 

dissipation for the active ingredient due to its relatively low Henry’s Law Constant. Several 

degradates were observed in the laboratory environmental fate studies (refer to Table 8). Per the 

MARC recommendations, in this assessment they were considered stressors.

11
The ground water conceptual model (PRZM-GW), is based on a rural drinking water well 

beneath an agricultural field (a high pesticide use area) drawing from an unconfined, high watertable aquifer. The depth of the well is site specific (i.e., scenario specific). The well extends into 

a shallow unconfined aquifer and has a well-screen that starts at the top and extends down into 

the aquifer2. The length of the well-screen represents the region of the aquifer where drinking 

water is collected. The well-screen length is well-specific and can be adjusted. Processes included 

in the conceptual model that influence pesticide transport through the soil profile include water flow, 

chemical specific dissipation and transportation parameters (i.e., degradation and sorption), and crop 

specific factors, including transpiration, pesticide interception and management practices.

Analysis Plan

Evaluation of submitted labels for the new uses indicates that the highest seasonal rate is for 

rights-of-ways, at 8 lb a.i./A (for ground applications). Therefore, primary focus on this use will 

be placed in SWCC (v.1.106), SCI-GROW (v.2.3), and PRZM-GW (v.1.07). Additionally, other 

uses of hexazinone will also be modeled in the SWCC. In all models, the maximum application

rate indicated in the label will be used. The timing of application will be the one typical for the 

crop modeled (according to the label directions).

Even though the database for hexazinone is essentially complete, one uncertainty arises related to 

the levels of unextracted radioactivity in the aerobic soil metabolism studies. In order to clear 

this uncertainty, for the crops yielding the highest EDWCs, special runs will be performed using 

the half-lives for the total residues, including the unextracted radioactivity. For more details, see 

the Characterization section.

Exposure output of interest to HED will be primarily point estimates and, if necessary in future 

modeling, distributions will be provided. Acute and chronic EDWCs will be provided.

ANALYSIS

Fate and Transport Characterization

The chemical profile of hexazinone is provided in Table 4. Environmental fate characteristics of 

the parent compound hexazinone are listed in Tables 5 and 6 and are discussed in greater detail 

in the following sections.

Hexazinone has a moderately high molecular weight of 252.3 g/mol. With a very high solubility 

(29800 ppm) and a low octanol/water partition coefficient (KOW = 15). Based on these 

properties, it appears that hexazinone has a very low potential to bioaccumulate/bioconcentrate. 

With a very high solubility, a relatively low vapor pressure (1.9 x 10-7 mmHg), and a very low 

Henry’s Law Constant (2 x 10-12 atm-m3/mol), hexazinone does not appear to volatilize 

substantially from wet/moist surfaces.

2 A well screen is a filtering device that allows groundwater from unconsolidated and semi-consolidated aquifers to 

enter the well while at the same time keeping the majority of sand and gravel out of the well.

12

 
Table 4. Chemical Profile of Hexazinone

Property Value Reference 

and Comments

Chemical Structure

O

N

C H 3

N

C H 3

C H 3

O

N

N

Product Chemistry

Pesticide type Herbicide

Product Chemistry

Chemical class Triazinone

Molecular Formula C12H20N4O2

Molecular Weight 252.3 g/mol

CAS No. 51235-04-2

CAS name 3-Cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine2,4(1H,3H)-dione

IUPAC name 3-Cyclohexyl-6-dimethylamino-1-methyl-1,3,5-triazine2,4(1H,3H)-dione

Water solubility 29800 ppm at 25°C

MRID 41203201 (reported 

Vapor pressure in DP Barcode D279324)

1.9 x 10-7 torr at 25°C

[‘Non-volatile under field conditions’ from dry nonadsorbing surfaces(1)]

Henry’s Law Constant 2 x 10-12 atm-m3/mol; at 25°C Calculated from WS and VP

Octanol/Water Partition 15 at pH 7.0 (temperature unspecified) MRID 41203201 (reported 

in DP Barcode D279324)

(1) For classification scheme, see USEPA, 2008. See also “Guidance for Reporting on the Environmental Fate and 

Transport of the Stressors of Concern in Problem Formulations,” available at 

http://www.epa.gov/pesticides/science/efed/policy_guidance/team_authors/endangered_species_reregistration_work

group/esa_reporting_fate.htm#guidance (accessed 12/04/2014).

Based on the available information, hexazinone residues (hexazinone and various metabolites) 

appear to be persistent and mobile in soil and aquatic environments and considered as the 

stressor(s) in this assessment (note that the half-lives for the total toxic residues or TTRs of 

hexazinone are presented in the next to the last column in Table 5, before the study 

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

terrestrial field, forestry dissipation, and a prospective groundwater monitoring studies (Table 6). 

Hexazinone is stable to hydrolysis and photolysis and degrades at moderately slow rates under 

aerobic soil metabolism conditions. Aerobic and anaerobic aquatic metabolism and anaerobic 

soil metabolism studies show slow degradation for hexazinone. Consistent with this persistence, 

field dissipation half-lives were 123 to 154 days for the parent compound 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 according to FAO 2000 mobility 

classification). These data indicate that hexazinone is mobile, thus, combined with its 

persistence, having strong potential to leach to ground water systems, especially in soil systems 

such as loamy and sandy soils that are poor in organic matter. Based on laboratory 

13
bioaccumulation in rainbow trout, hexazinone is not expected to bioaccumulate substantially in 

fish tissue, which is consistent with the low KOW value of 15 (i.e., log KOW = 1.1).

Table 5. Summary of Environmental Fate Properties of Hexazinone Related to 

Degradation and Metabolism

Study

System Name/ 

Characteristics

Value for Hexazinone1

(Kinetic Model if Applicable)

Half-life Value 

Used to Derive 

Model Input 

Parameters TTRs2

Reference, 

(Study 

DT50 (days) DT90 (days) Classification*)

Abiotic 

Hydrolysis

pH 5, 250C Stable Stable

pH 7, 250C Stable Stable Stable (0) 41587301 (A)

pH 9, 250C Stable

Atmospheric 

Degradation3 Hydroxyl radical3 0.119 (estimated) EPI Suite v.4.11

Aqueous 

Photolysis pH 7, 250C Stable Stable Stable (0) 41300801 (A)

Soil 

Photolysis

Sandy loam from CA, pH 

6.5, 0.6% OM, 200C

82 (light 

exposed);

stable (dark 

control)

41300802 (A)

Aerobic Soil 

Metabolism

Sandy loam (CA), pH 

6.5, 0.6% OM, 250C 216 604 days (SFO) 41807401/ 

42635001 (A)

Audrain clay loam soil 

(MO), pH 6.6, 4.1% OM, 

20°C

45.2 (IORE) 180 (IORE) 208 days (SFO)

48898001 (S) Sassafras sandy loam 

(MD), pH 5.4, 2.0% OM, 

20°C

DFOP slow 

t1/2 = 143 days 416 days (SFO)

Porterville loam (CA), pH 

7.7, 1.11% OM, 20°C 115 (SFO) 381 (SFO) 534 days (SFO)

Input value for modeling 

with the SWCC, at 250C. 

from the four above 

mentioned systems

90th percentile confidence 

bound on the mean half-life 

value: four values are 

available: 208, 416 and 534 (at 

20°C), and 604 (at 25°C) days;

the corrected half-lives at 

25°C, are 147, 294, 378, and 

604 days; mean = 355.75 days; 

std. dev. = 191.06 days, n = 4, 

t90,n-1 = 1.638.

n

t s

t t 90,n-1

input = 1/2 +

= 512.2 days

Anaerobic 

Soil 

Metabolism

Raymondville sandy clay 

loam (TX), pH 8.0, 

0.83% OM, 20oC

389 (SFO) 1292 (SFO)

48898002 (S)

Mexico clay loam (MO), 

pH 6.6, 4.1% OM, 20°C 997 (SFO) 3311 (SFO)

Sassafras sandy loam 

(MD), pH 5.4, 2.0% OM, 

20°C

309 (SFO) 1026 (SFO)

Porterville loam (CA), pH 

7.7, 1.11% OM, 20°C 1,165 (SFO) 3872 (SFO)

14
Study

System Name/ 

Characteristics

Value for Hexazinone1

(Kinetic Model if Applicable)

Half-life Value 

Used to Derive 

Model Input 

Parameters TTRs2

Reference, 

(Study 

DT50 (days) DT90 (days) Classification*)

Aerobic 

Aquatic

Water:sandy loam 

sediment (CA), water pH 

7.9-8.04

, sediment pH not 

reported, 0.7% OM, 25°C

>2 months 1046 days (SFO) 41811801 (A)

Wabasha Water:sandy

loam sediment (MN), 

water pH 7.2, sediment 

pH 6.0, 7.1% OM, 20°C

528 (SFO) 1755 (SFO) 1274 days (SFO)

49109901 (S) Tift Water:sandy loam 

sediment GA, water pH 

7.7, sediment pH 7.5, 

1.2% OM, 20°C

719 (SFO) 2389 (SFO) 1900 days (SFO)

Input value for modeling 

with the SWCC, at 250C. 

from the three above 

mentioned systems

90th percentile confidence 

bound on the mean half-life 

value: three values are 

available: 1274 and 1900 days 

(at 20°C), and 1046 days 

(25°C); the corrected half-lives 

at 25°C, are 901, 1344, and 

1046 days; mean = 1097 days; 

std. dev. = 225.86 days, n = 3, 

t90,n-1 = 1.886.

n

t s

t t 90,n-1

input = 1/2 +

= 1343 days

Anaerobic 

Aquatic

Water:loam sediment 

(CA), water pH 7.83, 

sediment pH 8.5, 1.24% 

OC, 25°C

230 1837 days (SFO) 41807402 (A)

Wabasha Water: sandy 

loam sediment (MN), 

water pH 7.2, sediment 

pH 6.0, 7.1% OM 20°C

482 (SFO) 1602 (SFO) 1395 days (SFO)

49109902 (S) Tift Water:sandy loam 

sediment (GA), water pH 

7.7, sediment pH 7.5, 

1.2% OM, 20°C

961 (SFO) 3192 (SFO) 1343 days (SFO)

Input value for modeling 

with the SWCC, at 250C. 

from the three above 

mentioned systems

90th percentile confidence 

bound on the mean half-life 

value: three values are 

available: 1395 and 1383 days 

(at 20°C), and 1837 days 

(25°C); the corrected half-lives 

at 25°C, are 986, 978, and 1837 

days; mean = 1267 days; std. 

dev. = 493.65 days, n = 3, t90,n-1

= 1.886.

n

t s

t t 90,n-1

input = 1/2 +

= 1805 days

*(A) = Acceptable; (S) = Supplemental

Bolded values represent input values in the aquatic modeling in SWCC and/or PRZM-GW.

1 DT50 and DT90 values were calculated using nonlinear regression and single first-order (SFO), double first- order 

in parallel (DFOP), or indeterminate-order equation (IORE). For DFOP, the overall DT50 and DT90 are reported 

when available. Otherwise, a first and second DT50 are reported. The equations can be found in the document, 

Standard Operating Procedure for Using the NAFTA Guidance to Calculate Representative Half-life Values and 

15
Characterize Pesticide Degradation, U.S. Environmental Protection Agency dated November 30, 2012 and 

approved December 17, 2012.

2 The value used to estimate a model input value is the calculated SFO DT50, tR IORE, or the 2nd DT50 from the DFOP 

equation. The model chosen is consistent with that recommended using the, Guidance for Evaluating and 

Calculating Degradation Kinetics in Environmental Media, Health Canada, U.S. Environmental Protection Agency, 

December 21, 2012.

3 Estimated value. In order to make this estimate, there are certain assumptions made by the module of EPISUITE, 

which represent a typical hydroxyl reaction in the atmosphere (refer to AOPWIN module at

http://www.epa.gov/opptintr/exposure/pubs/episuite.htm accessed 12/09/14). There is no ozone reaction 

calculation available in EPI Suite v.4.1 for this chemical.

4 Measured in the first test interval.

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 Met. B, which was 9.5-10.1% of the applied 

at 30 days (last test interval).

Hexazinone’s half-lives were 216 and 1440 days, when applied to viable and sterile (autoclaved) 

sandy loam soil (pH 6.5) from Modesto, CA, and maintained under aerobic conditions in the 

dark at 25oC. 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. The aerobic transformation of hexazinone was also 

studied in three U.S. soils in a separate study: a sandy loam from MD (Sassafras, pH 5.4), a clay 

loam soil from Missouri (Audrain, pH 6.6), and a loam soil from CA (Porterville, pH 7.7) for up 

to 150 days at 20°C. Representative IORE half-life (tR IORE) was 54.3 days in the clay loam soil; 

slow t1/2 DFOP was 143 days in the sandy loam soil; the SFO half-life was 115 days in the loam 

soil. Evolved 14CO2 totalled a maximum of 2.8-9.7% of the applied at study termination. 

Unextracted radioactivity was significant and increased to a maximum of 25.6% of the applied at 

150 days (for the Audrain clay loam soil). A transformation pathway was provided by the study 

author (Figure 4). 

16
Figure 4. Transformation Pathway Proposed by the Registrant (source: MRID 48898001)

The anaerobic soil metabolism of hexazinone was studied in four U.S. soils: a sandy clay loam 

soil from TX (Raymondville, pH 8.0), a clay loam soil from MO (Mexico, referred to as 

Audrain; pH 6.6), a sandy loam soil from MD (Sassafras, pH 5.4), and a loam soil from CA

(Porterville, pH 7.7) for 31 days under aerobic conditions, followed by 149 days under anaerobic 

conditions in darkness at 20°C. Half-lives were 389 days in the Raymondville sandy clay loam 

soil, 997 days in the Audrain clay loam soil, 309 days in the Sassafras sandy loam soil, and 1,165

days in the Porterville loam soil (SFO in all instances). Unextracted radioactivity increased to a 

maximum of 28.2% of the applied at 31 days posttreatment (0 day post-flood), and was 12.6% at 

180 days (149 days post-flood) in the clay loam soil. Total evolved 14CO2 was ≤1.3% of the 

applied in all four soils.

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 majority of the applied radioactivity was related to the water (as opposed to 

17
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. The aerobic aquatic metabolism of 

hexazinone was also studied in a MN water:sandy loam sediment system (Wabasha; water pH 

7.2, sediment pH 6.0 and OC 4.1%) and a GA water:sandy loam sediment system (Tift; water pH 

7.7, sediment pH 7.5, and OC 0.70%) in closed systems at 20°C. Hexazinone dissipated with 

half-lives of 528 days in the Wabasha water:sandy loam sediment system and 719 days in the 

Tift water:sandy loam system. Unextracted radioactivity was ≤4.9%. 14CO2 was a maximum of 

0.1% of the applied at 100 days posttreatment.

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 

(autoclaved) systems. Hexazinone was an average of 95.6% and decreased to 29.0%, at times 0 

and 365 days, respectively. In another study, the anaerobic aquatic transformation of hexazinone 

was studied in a MN water:sandy loam sediment system (Wabasha; water pH 7.2, sediment pH 

6.0 and OC 4.1%) and a GA water:sandy loam sediment system (Tift; water pH 7.7, sediment pH 

7.5, and OC 0.70%) in closed systems at 20°C. Hexazinone dissipated with half-lives of 482

days in the Wabasha water:sandy loam sediment system and 961 days in the Tift water:sandy 

loam system. Unextracted radioactivity was ≤4.8% of the applied and 14CO2 was not detected 

during the study.

Table 6. Summary of Other Environmental Fate Properties of Hexazinone (Mobility, Field 

Dissipation and Fish BCF)

Study

System Name/ 

Characteristics Value (units if applicable)

Reference, 

(Study Classification*);

Comments

Mobility for 

Hexazinone

Soil\Property KF (mL/g) KFOC (mL/gOC)

MRID 43621501 (A);

KOC model describes the 

mobility better than the Kd

model. For KOC the 

coefficient of variation is 

smaller than for Kd.

Sassafras sandy loam 0.21 40

Trevino sandy loam 0.59 77

Chino loam 0.84 75

Tama silt loam 0.53 38

Mean Value 57.5 Mobile1

Additional notes 1/n range 0.86-0.93 for the adsorption isotherms. 

Desorption step done at a single concentration.

Terrestrial 

Field 

Dissipation

Bareground silt loam 

in Newark, DE

123 days; >95% confined to the upper 30 cm of 

soil

42377901 (A); Mets A, A-1, 

B, C, and 1 were identified in 

one or more sites at max. of 

0.04, 0.21, 0.31, 1.23, and 

0.71 ppm, respect.

Bareground silty clay 

loam in Greenville, 

MS

154 days; hexazinone did move to a depth of 60-

75 cm

Bareground sandy 

loam soil in Madera, 

CA

140 days; hexazinone leached to the 90 cm depth 

level of soil.

4237920 (S); Mets A, A-1, 

B, C, and 1 were identified in 

soils at max. of 0.05, 0.04, 

0.29, 0.62, and 0.20 ppm, 

respect.

18
Study

System Name/ 

Characteristics Value (units if applicable)

Reference, 

(Study Classification*);

Comments

Forestry 

Field 

Dissipation

Forest-land

watersheds (loams)

in Coosa county, AL

Hexazinone, at 6.72 lb a.i./A, dissipated with 

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. Half-lives of 55 to 77 days 

were reported for litter and soil. For litter 

covered soil, a half-life of 265 days was reported. 

Hexazinone was observed in soil at depths >30 

cm (0.01 to 0.04 ppm). Hexazinone was detected 

in runoff water up to 6 months post-application 

(80 to 140 ppb). Hexazinone was also detected 

in stream samples (both sites).

42336401 (A); Mets A, B, C, 

D, E, G, and B were 

discernible in various 

matrices (soil, water, litter 

and/or vegetation) samples. 

Hexazinone and metabolites 

were consistently discernible 

in soil samples taken to a 

depth of 15 to 30 cm from 

both sites.

Prospective 

groundwater 

monitoring

Velpar L was applied 

at 0.75 lb a.i./A in 

Merced County, CA

sand planted with 

alfalfa

The leaching potential of hexazinone and INT3937 (Met. A), IN-A3928 (Met. B), IN-T3935 

(Met. C), IN-G3453 (Met. A-1), IN-JS472 (Met. 

1), and IN-G3170 (Met. G3170) was studied in a 

vulnerable site for up to 35 months. Soil, 

groundwater, and soil pore-water samples were 

analyzed for hexazinone and metabolites. 

Hexazinone dissipated from the upper 2 feet of 

soil with a calculated half-life of 25 days. 

Results indicated that hexazinone and its 

degradates are very mobile and persistent. They 

leached 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 

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.

45132801 (S); Hexazinone 

and all metabolites except 

Met. G moved at the same 

rate through the unsaturated 

zone and into groundwater as 

the bromide tracer, while 

Met. G movement was 

somewhat retarded. Met. G 

was the predominant 

degradate in the soil, soil 

pore water, and groundwater; 

its breakthrough into the 

shallow groundwater 

occurred at 336 days.

Fish BCF Rainbow trout

The studies showed that hexazinone did not have 

the tendency to bioaccumulate in fish. Based on 

its low octanol/water partition coefficient (15 at 

pH 7.0, temperature unspecified), hexazinone is

not be expected to substantially accumulate in 

fish.

00064265 (00078032); 

41203201 (reported in DP 

Barcode D279324) (W)

*(A) = Acceptable; (S) = Supplemental; (W) = Waived

 Bolded values represent input values in the aquatic modeling in SWCC and/or PRZM-GW.

1. Mobility classification per FAO 2000.

Mobility

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

adsorption Kd 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 relatively close to unity (0.93, 0.91, 

0.91 and 0.86, respectively). Additional 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 (Table 7).

19
Table 7. Mobility of Hexazinone Metabolites

Adsorption (Mobility) Studies Kd KOC Source (MRID)

Metabolite 1 0.20 122

43621501 (supplemental portion 

of the study)

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

Field Studies

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 substantially beyond the top 30 cm of soil (>95% 

confined to the upper 30 cm of soil) at the Newark site. However, 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.

Forestry Field Study

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 for the two sites (except litter covered soil). For littercovered soil, a half-life of 265 days was reported. By day 365 post-treatment, hexazinone was 

discernible only in sediment samples. 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 study, 

hexazinone was detected from stream samples from both sites (maximum concentration 473 ppb 

at 1150 hours post-treatment at the Velpar L site).

In addition, metabolites A, B, C, D, E, G, and H were discernible in various matrices (soil, water, 

litter and/or vegetation) samples. Metabolite G was not confirmed by MS. Metabolites A, B 

20
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 blueberry plant tissues (both sites). 

B and D were the most frequently detected metabolites (Velpar L site). 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 plots) took place 6 

months post-treatment.

Small Scale Prospective Groundwater Monitoring Study

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, 

CA. 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 site ran

a slight water deficit through January 29, 1996 (18 days posttreatment). The cumulative water 

budget was ca. 1.5 inches by 1 month and 2 inches by 2 and 3 months posttreatment. During the 

length of the study, the total rainfall received was 41.62 inches and the total irrigation received 

was 133.7 inches for a combined water input of ca. 175 inches, which was reported as 111% of 

the normal water input. The cumulative water budget through the study was positive, as the sum 

of precipitation plus irrigation remained greater than evapotranspiration. However, the 

cumulative water budget decreased in the summer when evapotranspiration was high, despite the 

irrigation, while the water budget increased in the winter when evapotranspiration was low. No 

substantial runoff events were reported.

Bromide concentrations in soil pore-water and groundwater showed that the total water input 

was sufficient to facilitate leaching to groundwater during the study period. 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 post-treatment. Soil (measured 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 (~35 months) post-treatment and analyzed using HPLC for hexazinone and six 

metabolites: 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 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 (or Met. G) was the 

predominant degradate in the soil, soil pore water, and groundwater; G3170 breakthrough into 

the shallow groundwater occurred at 336 days. Hexazinone and Mets. G3170 (or G), B, and A-1 

21
were the major analytes detected when the residues reached groundwater at 336 days 

posttreatment. Following 824 days and at the end of the study period, Met. G3170 was the 

dominant analyte. 

Average background hexazinone concentrations in shallow monitoring wells were 0.05-0.18 ppb 

from pre-application to 214 days, increasing to 0.43 ppb at 271 days, 0.71 ppb at 336 days, and 

2.02 ppb at 363 days. The first detections in groundwater were observed between 125 and 152 

days in Clusters 1 and 4, with higher detections observed beginning at 271 days. Met. G3170 

first increased above background levels to an average of 1.27 ppb at 363 days, and was a 

maximum average concentration of 2.95 ppb in shallow wells at 424 days. Met. B first increased 

above background levels at 363 days, and was an average maximum concentration of 1.31 ppb at 

424 days. Met. A-1 first increased above background levels at 363 days, and was an average 

maximum concentration of 0.71 ppb at 424 days. Met. 1 first increased above background levels 

at 363 days, and was an average maximum concentration of 0.42 ppb at 424 days. Metabolite C 

was first observed consistently above background levels at 641 days, and was an average 

maximum concentration of 0.54 ppb at 851 days. Met. A was not observed above background 

levels. 

Hexazinone was first detected in deep monitoring wells above background levels in Cluster 1 at 

214 days (0.258 ppb), in Cluster 4 at 487 days (0.113 ppb), in Cluster 6 at 522 days (0.212 ppb), 

in Cluster 2 at 578 days (0.408 ppb), in Cluster 3 at 673 days (0.142 ppb), in Cluster 7 at 673 

days (0.115 ppb), and was not observed in deep wells at Clusters 5 and 8, which were located in 

the eastern portion of the test plot (refer to next paragraph).

There was a distinct difference in the movement of hexazinone through soil in the western and 

eastern portions of the site. Hexazinone residues were 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 

Mets. D and 2 were not monitored in the small-scale prospective groundwater monitoring study.

Bioconcentration in Fish

Based on laboratory bioaccumulation in rainbow trout, hexazinone is not expected to 

bioaccumulate in fish substantially, which is consistent with the low octanol/water partition 

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

rotational crops.

Transformation Products

Transformation products of hexazinone are generally formed slowly and are also mobile and 

persistent (Table 8). The major photodegradation product on soil, was Met. B (10.1% of 

applied). Under aerobic soil and aquatic conditions, major metabolites included G, 1, and A-1, 

with various other minor metabolites observed. In anaerobic soil and aquatic conditions, Mets. 

G, 1, D, and 2 were among the major degradates. Mets. A, A-1, B, C and 1 were identified in 

soils in terrestrial field. Mets. A, B, C, D, E, G, and H were discernible in various matrices (soil, 

water, litter and/or vegetation) samples in forestry field samples. Mets. D and 2 were major 

22
degradates found in anaerobic conditions; however, the field dissipation and the small-scale 

prospective groundwater monitoring studies did not monitor these two metabolites. The fate of 

Mets. 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 Met. G3170 (G); however, it was found to be at the 

highest concentrations in the small-scale prospective groundwater monitoring study. This 

metabolite was found to increase with time in three aerobic soils in a 150-day long aerobic soil 

metabolism study. Further information about the degradates of hexazinone can be found in 

Appendix B.

Unextracted Residues

It was observed that in the aerobic soil and anaerobic soil metabolism studies high levels of 

unextracted radioactivity were formed, by using only simple extraction techniques. In this 

assessment, to clear the uncertainty relative to the unextracted residues (UERs), EDWCs were 

calculated using both the total toxic residues and the total toxic residues plus the UERs, to 

provide an upperbound level for the EDWCs.

Table 8. Transformation Products of Hexazinone

Common Name Chemical Name Structure

IN-G3170; Met. G 

or G3170

IUPAC: 1-Methyl-6-(methylamino)-1,3,5-triazine2,4-dione

Formula: C5H8N4O2

MW: 156.14 g/mol

SMILES: [H]n1c(=O)nc(n(c1=O)C)N([H])C

H

N

O N

C H 3

N

C H 3

H

N

O

IN-JS472; Met. 1 IUPAC: 6-(Dimethylamino)-1-methyl-3-(4-

oxocyclohexyl)-1,3,5-triazine-2,4-dione

Formula: C12H18N4O3

MW: 266.3 g/mol

SMILES: 

Cn1c(nc(=O)n(c1=O)C2CCC(=O)CC2)N(C)C

O

N N

N

C H 3

C H

N

C H 3

O

O

23
Common Name Chemical Name Structure

IN-B2838; Met. D IUPAC: 1-Cyclohexyl-3-methyl-1,3,5-triazinane2,4,6-trione

Formula: C10H15N3O3

MW: 225.2 g/mol

SMILES: Cn1c(=O)[nH]c(=O)n(c1=O)C2CCCCC2

N

O N

C H 3

O

N

O

IN-G3453; Met. A-1 IUPAC: 6-(Dimethylamino)-3-(2-

hydroxycyclohexyl)-1-methyl-1,3,5-triazine-2,4-dione

Formula: C12H20N4O3

MW: 268.3 g/mol

SMILES: Cn1c(nc(=O)n(c1=O)C2CCCCC2O)N(C)C

O H

O

N

N

C H 3

N

C H

C H 3

N

O

IN-A3928; Met. B IUPAC: 3-Cyclohexyl-1-methyl-6-(methylamino)-

1,3,5-triazine-2,4-dione

Formula: C11H18N4O2

MW: 238.28 g/mol

SMILES: [H]N(C)c1nc(=O)n(c(=O)n1C)C2CCCCC2 N

O N

C H 3

N

H

C H 3

N

O

24
Common Name Chemical Name Structure

IN-JT677; Met. 2 IUPAC: 6-(Dimethylamino)-1-methyl-3-(2-

oxocyclohexyl)-1,3,5-triazine-2,4-dione

Formula: C12H18N4O3

MW: 266.3 g/mol

SMILES: 

Cn1c(nc(=O)n(c1=O)C2CCCCC2=O)N(C)C

O

N

O N

C H 3

N

C H 3

C H 3

N

O

IN-T3937; Met. A IUPAC: 6-(Dimethylamino)-3-(4-

hydroxycyclohexyl)-1-methyl-1,3,5-triazine-2,4-dione

Formula: C12H20N4O3

MW: 268.3 g/mol

SMILES: 

Cn1c(nc(=O)n(c1=O)C2CCC(CC2)O)N(C)C

O

N N

N

C H 3

C H 3

N

C H 3

O

H O

Carbon dioxide IUPAC: Carbon dioxide

Formula: CO2

MW: 44 g/mol

SMILES: C(=O)=O

O C O

Drinking Water Exposure Modeling

Surface Water Models

SWCC

For agricultural and certain non-agricultural uses, exposure concentrations for surface 

drinking waters assessments are estimated based on EFED’s Tier II aquatic model 

PRZM/VVWM. A graphical user interface Surface Water Concentration Calculator (SWCC 

v.1.106, May 6, 2014), developed by the EPA, was used to facilitate inputting chemical 

and use specific parameters into the appropriate input files and chemical files, replacing 

the older PE5 shell (November 2006), which used PRZM3 and EXAMS. The SWCC 

estimates pesticide concentrations in water bodies that result from pesticide 

applications to land. The calculator was designed for regulatory applications as applied 

in the Office of Pesticide Programs, USEPA, as well as PMRA, Health Canada. The 

calculator uses the Pesticide Root Zone Model (PRZM v5.0+, May 6, 2014) and the 

Varying Volume Water Body Model (VVWM, May 6, 2014, a replacement for the older 

EXAMS model). The PRZM field or orchard crop scenario is the basic file which 

describes the local or regional climatological information, soil hydrology, soil 

characteristics, crop characteristics, and the pesticide properties necessary to determine 

pesticide loadings to surface water. This basic file constitutes the first part of the 

“exposure scenario.” It simulates the processes in the agricultural field, such as runoff 

and erosion, on a daily time step. The runoff and erosion flux output data from PRZM 

are used as chemical loadings to the VVWM, which simulates surface water, in order to 

25
predict the EDWCs. The VVWM is conceptually similar to the current EXAMS-based 

model in that both are 2-compartment models driven by first-order processes. It

simulates the USEPA standard water bodies (i.e., farm pond and/or index reservoir) but 

with greater efficiency and flexibility for incorporation into a user interface. The VVWM 

also allows for variations in water body volume on a daily basis due to runoff, 

precipitation, and evaporation. Temperature, wind speeds, and pesticide dissipation 

processes are also allowed to vary daily. EDWCs were determined using the Index 

Reservoir scenario which describes a vulnerable surface drinking waters scenario for the 

VVWM component of the modeling exercise. For further information see 

http://www.epa.gov/oppefed1/models/water/index.htm#swcc (accessed 12/15/2014).

Ground Water Models

PRZM-GW

Tier 1 groundwater estimated drinking water concentrations (EDWCs) for a chemical, 

were derived with PRZM-GW (Pesticide Root Zone Model for Groundwater, version 1.07, 

March 6, 2014), using the GW-GUI (Graphical User Interface, version 1.07, March 6, 

2014). PRZM-GW is a one-dimensional, finite-difference model that estimates the 

concentrations of pesticides in groundwater. It accounts for pesticide fate in the crop 

root zone by simulating pesticide transport and degradation through the soil profile 

after a pesticide is applied to an agricultural field. PRZM-GW permits the assessment of 

multiple years of pesticide application (up to 100 years) on a single site. Six standard 

scenarios, each representing a different region known to be vulnerable to groundwater 

contaminations, are available for use with PRZM-GW for risk assessment purposes. In 

PRZM-GW simulations, each of these standard scenarios was used. PRZM-GW output 

values represent pesticide concentrations in a vulnerable groundwater supply that is 

located directly beneath a rural agricultural field. See

http://www.epa.gov/oppefed1/models/water/index.htm#przm_gw (accessed 

12/15/2014).

SCI-GROW

SCI-GROW (Screening Concentration In GRound Water version 2.3) is a regression 

model used as a screening tool to estimate pesticide concentrations found in ground 

water used as drinking water. SCI-GROW was a model developed by fitting a linear 

model to groundwater concentrations with the Relative Index of Leaching Potential 

(RILP) as the independent variable. Groundwater concentrations were taken from 90-

day average high concentrations from Prospective Ground Water studies; the RILP is a 

function of aerobic soil metabolism and the soil-water partition coefficient. The output 

of SCI-GROW represents the concentrations that might be expected in shallow 

unconfined aquifers under sandy soils, which is representative of the ground water most 

vulnerable to pesticide contamination likely to serve as a drinking water source. More 

information at http://www.epa.gov/oppefed1/models/water/#scigrow (accessed 

12/15/2014).

Scenario Selection

Three agricultural crops were modeled: alfalfa, sugarcane and blueberries. For alfalfa, the NC 

alfalfa scenario was selected. For sugarcane, it was noted that the highest application rate 

applies only to sugarcane in Hawaii. Since there is no available scenario for HI sugarcane, the 

LA sugarcane was used as a surrogate, with a HI weather scenario. For blueberries, the CA wine 

grape scenario was used as a surrogate for the crop.

26
For the non-agricultural crops, the CA rights-of-way scenario was used to model both aerial and 

ground applications (the rights-of-way has the maximum allowed application rate among all 

uses). Additionally, other crops were run as shown in Table 9. To the extent possible, crops 

from diverse regions from across the U.S. were selected.

Model Inputs

Tables 9 and 10 show the input values used in SWCC. It should also be noted that three nonagricultural uses may be applied aerially, but only by means of helicopter. The agricultural 

module of AgDRIFT was used, using the Wasp Helicopter as the aircraft, to estimate the spray 

drift. At this time, EFED does not recommend to use the forestry module of AgDRIFT; thus the 

agricultural module was to estimate forestry spray drift.

Table 9. Hexazinone Uses, Scenarios, and Application Information Used for Aquatic 

Exposure Inputs in Drinking Water Exposure Assessment

Run NO, Scenario (bold 

font)/ Uses Represented

Method

of App.

Drift/

App 

Efficiency

Single 

App Rate

kg a.i./A

(lb a.i./A)

Max 

No. of 

Apps 

at Max 

Rate

Min. 

Interval 

Between 

Apps 

(days)

Day of 

App 

(daymonth)

CAM IPSCND

Agricultural Use Patterns

01 NC alfalfa/Alfalfa Aerial 0.135/0.95 1.68 (1.5) 1 N/A 01-04 Foliar Surface

applied

02 LA sugarcane (HI meteorological file 22521)/Sugarcane

in HI

Aerial 0.135/0.95 4.03 (3.6) 1 N/A 10-01 Foliar Surface

applied

03 CA wine grape/Blueberries Ground 0.066/0.99 2.24 (2) 1 N/A 01-04 Ground Surface

applied

Non-Agricultural Use Patterns

04 CA rights-of-way/CA 

impervious/ Non-crop sites 

(weed and brush control)

Aerial 

helicopter

0.124/0.95 6.72 (6) 1 N/A 01-04 Foliar Surface

applied

05 CA rights-of-way/CA 

impervious/ Non-crop sites 

(weed and brush control)

Ground 0.066/0.99 8.97 (8) 1 N/A 01-04 Foliar Surface

applied

06 CA forestry (MI 

meteorological file 

14850)/Conifer release in 

Eastern and Lakes states

Aerial 

helicopter

0.124/0.95 4.48 (4) 1 N/A 01-05 Foliar Surface

applied

07 OR Christmas trees/ 

Christmas trees plantations 

(except in 22 states listed in 

label)

Aerial 0.135/0.95 2.24 (2) 1 N/A 01-04 Foliar Surface

applied

08 CA rangeland hay (NY 

meteorological file 14860)/ 

Pastures/Rangeland in Eastern 

U.S.

Aerial 

helicopter

0.124/0.95 4.48 (4) 1 N/A 01-03 Foliar Surface

applied

09 PA turf/ Bermudagrass/ 

Bahiagrass; Industrial turfgrass

Ground 0.066/0.99 1.26 

(1.125)

1 N/A 01-03 Foliar Surface

applied

27
The chemical input parameters are summarized in Table 10. Note that two aerobic soil 

metabolism half-lives are included in the table. One is for the total toxic residues (TTRs), which 

are hexazinone plus its metabolites of concern. The other one includes the TTRs plus the 

unextracted residues (UERs). For the aerobic and anaerobic aquatic metabolism studies, only 

one half-life for the TTRs is reported since the levels of unextracted radioactivity were very low 

in these studies.

Table 10. Environmental fate data used for aquatic exposure inputs in Hexazinone aquatic 

risk assessment in the SWCC1

Parameter (units) Value(s) Sources Comments

Chemical Name Hexazinone Labels Label

Application Rate

(Kg a.i./ha)

Refer to Table 9 Highest single 

application 

rates according 

to the labels

---

Number of 

Applications 

Allowed

Refer to Table 9 Labels Maximum allowed by the label, at the 

maximum rate.

Interval between 

Applications (days)

Refer to Table 9 Labels Minimum allowed by the label.

Crop Names and 

Days of Application 

(day-month)

Refer to Table 9 Labels

KOC (ml/gOC) 57.5 43621501 Sorption is better correlated with organic 

carbon content. Represents the mean of 

the following four values:

38 mL/gOC Tama silt loam

75 mL/gOC Chino loam

77 mL/gOC Trevino sandy loam

40 mL/gOC Sassafras sandy loam

Aerobic Soil 

Metabolism (days)

25°C = 512.2 for the TTRs

41807401/ 

42635001;

48898001

90th percentile confidence bound on the 

mean half-life value: four values are 

available: 208, 416 and 534 (at 20°C), 

and 604 (at 25°C) days; the corrected 

half-lives at 25°C, are 147, 294, 378, and 

604 days; mean = 355.75 days; std. dev. 

= 191.06 days, n = 4, t90,n-1 = 1.638.

= 1093 for the TTRs + 

UERs

41807401/ 

42635001;

48898001

90th percentile confidence bound on the 

mean half-life value: four values are 

available: 611, 1078 and 1604 (at 20°C), 

and 1023 (at 25°C) days; the corrected 

half-lives at 25°C, are 432, 762, 1134, 

and 1023 days; mean = 837.75 days; std. 

dev. = 312.22 days, n = 4, t90,n-1 = 1.638.

Method of 

Application

Refer to Table 9 Labels Labels

Buffer zone (ft) 0 Labels No buffer restrictions.

Application

Efficiency (fraction)

Refer to Table 9 Labels ---

Spray Drift (fraction) Refer to Table 9 Labels ---

Solubility in Water 

(ppm) (25°C)

29800 41203201 DP Barcode D279324.

t t

t s

n input

n

= + −

1 2

90 1

/

, ( )

t t

t s

n input

n

= + −

1 2

90 1

/

, ( )

28
Parameter (units) Value(s) Sources Comments

Aerobic Aquatic 

Metabolism (days)

25°C = 1343 for the TTRs2

41811801;

49109901

90th percentile confidence bound on the 

mean half-life value: three values are 

available: 1274 and 1900 days (at 20°C), 

and 1046 days (25°C); the corrected halflives at 25°C, are 901, 1344, and 1046 

days; mean = 1097 days; std. dev. = 

225.86 days, n = 3, t90,n-1 = 1.886.

Anaerobic Aquatic 

Metabolism (days)

25°C = 1805 for the TTRs2

41807402;

49109902

90th percentile confidence bound on the 

mean half-life value: three values are 

available: 1395 and 1383 days (at 20°C), 

and 1837 days (25°C); the corrected halflives at 25°C, are 986, 978, and 1837 

days; mean = 1267 days; std. dev. = 

493.65 days, n = 3, t90,n-1 = 1.886.

Hydrolysis at pH 7 0 41587301 Stable at pH 5-9.

Aqueous Photolysis 

(days)

0 41300801 Stable under the testing conditions. For 

the modeling, assume 40° latitude.

Molecular Weight 

(g/mol)

252.32 EPI Suite 

v.4.11

---

Vapor Pressure 

(25°C) (torr)

1.9 x 10-7 41203201 DP Barcode D279324.

Henry’s Law 

Constant (25°C)

(atm-m3/mol)

~2 x 10-12 VP/WS This value is calculated by the SWCC.

EXAMS 

Environment

EPA Reservoir and EPA 

Standard Pond

--- Default for index reservoir and standard 

pond. In this instance, the applicable 

Field Size Index Reservoir and EPA results are for the Index Reservoir.

Pond

---

PRZM Scenario and 

Metfile

As suggested by the shell 

SWCC, with exceptions 

noted in Table 9.

SWCC shell Exceptions include LA sugarcane (run 

with a HI met file), and CA forestry (run 

with a MI met file). See Table 9 for 

selected met file.

Incorporation Depth 

(cm)

0.0 Labels Proposed label

CAM, Chemical 

Application Method

Refer to Table 9 Labels ---

IPSCND, Condition 

for disposition of 

pesticide remaining 

on foliage after 

harvest)

Surface applied Labels ---

FEXTRC, Foliar 

extraction

0.5 Default value ---

PLDKRT, Decay 

rate on foliage

0 Default value ---

PLVKRT, 

Volatilization rate 

from foliage

0 Default value ---

UPTKF, Uptake 

factor

0 Default value ---

1 Data were selected according to EFED Guidance on Input Parameters v.2.1, dated 10/22/2009 and approved 

11/10/2009 (USEPA 2009).

2 In the aerobic and anaerobic aquatic metabolism studies the levels of UERs were very low at all test intervals.

t t

t s

n input

n

= + −

1 2

90 1

/

, ( )

t t

t s

n input

n

= + −

1 2

90 1

/

, ( )

29
The input parameter values in PRZM-GW are as shown in Table 11. For PRZM-GW, only 

rights-of-way were modeled, since they involve the maximum application rate. Again, two halflives for the TTRs and TTRs + UERs were modeled (see above).

Table 11. Hexazinone PRZM-GW Input Parameters for Modeling Applications of the 

Chemical on Rights-of-Ways

PARAMETER (units) VALUE1 SOURCE COMMENT

Application date (day/month)

15-03

According to 

the label, 

applications 

occur late 

winter to 

summer.

Delmarva emergence 10-04, crop maturation 

30-05, harvest 15-06

15-03 FL citrus crop emergence 01-01, crop 

maturation 01-02, harvest 31-12

15-03 FL potatoes emergence 15-02, crop maturation 

01-05, harvest 01-06

15-03 GA coast emergence 01-05, crop maturation 01-

08, harvest 01-10

15-03 NC coast emergence 15-05, crop maturation 01-

08, harvest 01-11

15-03 WI sand emergence 01-05, crop maturation 21-

07, harvest 20-10

Maximum Application Rate 

(kg a.i./ha) 8.97

Number of Applications per Label Maximum application rate is 8 lb a.i./A.

Year 1

Retreatment Intervals (days) N/A

Chemical Application 

Method (CAM) 2 Label Foliar

Incorporation Depth (cm) 0.0 ---

Air diffusion coefficient 

(cm2/day) 0.0 N/A Assume no volatilization.

Vaporization enthalpy 

(kcal/mol) 0.0 N/A Assume no volatilization.

Henry’s Law Constant (atmm3

/mol) 0.0 N/A Assume no volatilization.

Partition Coefficient

(KOC; mL/gOC) 57.5 43621501

Sorption is better correlated with organic carbon 

content. Represents the mean of the following 

four values:

38 mL/gOC Tama silt loam

75 mL/gOC Chino loam

77 mL/gOC Trevino sandy loam

40 mL/gOC Sassafras sandy loam

Aerobic Soil Metabolism 

Half-life (days) 512.2

(TTRs)

41807401/ 

42635001;

48898001

90th percentile confidence bound on the mean 

half-life value: four values are available: 208, 

416 and 534 (at 20°C), and 604 (at 25°C) days;

the corrected half-lives at 25°C, are 147, 294, 

378, and 604 days; mean = 355.75 days; std. 

dev. = 191.06 days, n = 4, t90,n-1 = 1.638.

t t

t s

n input

n

= + −

1 2

90 1

/

, ( )

30
PARAMETER (units) VALUE1 SOURCE COMMENT

1093

(TTRs + 

UERs)

41807401/ 

42635001;

48898001

90th percentile confidence bound on the mean 

half-life value: four values are available: 611, 

1078, and 1604 (at 20°C), and 1023 (at 25°C)

days; the corrected half-lives at 25°C, are 432, 

762, 1134, and 1023 days; mean = 837.75 days; 

std. dev. = 312.22 days, n = 4,

t90,n-1 = 1.638.

Hydrolysis Half-life (days) 0 41587301 Stable at pH 5-9.

1 Parameters are selected as per “Guidance for Selecting Input Parameters for Modeling Pesticide Concentrations in 

Groundwater Using the Pesticide Root Zone Model” v.1.0, dated October 15, 2012.

The input parameter values in SCI-GROW are as shown in Table 12. In SCI-GROW, like for 

the PRZM-GW and SWCC, two half-lives for the TTRs and TTRs + UERs were modeled.

Table 12. SCI-GROW (v 2.3) Input Parameter Values for Hexazinone TTRs on Rights-ofWays1.

PARAMETER (units) VALUE(S) SOURCE COMMENT

Output File Name HexazinonePRA2.txt

Hexazinone_PRA.txt N/A Output file names for the runs for the TTRs and TTRs + UERs, respectively

Maximum Application 

Rate (lb a.i./A) 8 Label Represents most-conservative scenario in 

which the total maximum rate per year is 

applied in a single application. Number of 

Applications per Year 1 Label

Organic Carbon 

Partition Coefficient 

(KOC; mL/gOC)

57.5 43621501

Represents the median of the following four 

values: 38 mL/gOC Tama silt loam; 40 mL/gOC

Sassafras sandy loam; 75 mL/gOC Chino loam; 

77 mL/gOC Trevino sandy loam. There is less 

than a three-fold variation.

Aerobic Soil 

Metabolism Half-life 

(days)

475 (TTRs)

41807401/ 

42635001;

48898001

There are four or more half-lives available, 

therefore, use the median of the following 

values: 208, 416, 534, and 604 days. There is 

less than a five-fold difference.

1051 ( for the TTRs 

+ UERs)

41807401/ 

42635001;

48898001

There are four or more half-lives available, 

therefore, use the median of the following 

values: 611, 1023, 1078, and 1604 days. There 

is less than a five-fold difference.

1 Data were selected according to the Guidance for Selecting Input Parameters in Modeling the Environmental Fate 

and Transport of Pesticides (Version 2.1, dated October 22, 2009).

Model Results /EDWCs for the Use of Hexazinone

Tables 13 and 14 show summaries of the EDWCs obtained in SWCC. For the TTRs + UERs, 

only the results of the crop that yielded the highest and lowest EDWCs were reported. It was 

noted that when the UERs were included, the peak EDWCs were the same than for the TTRs 

without the UERs.

31
Table 13. Tier II Estimated Drinking Water Concentrations (EDWCs) for Surface Water 

Based on Application of Hexazinone Products Modeled in the SWCC, for the TTRs 

(Excluding the UERs)

Scenario App. 

Method

App. 

Rate

(lb a.i./A)

Date of 

First 

Application

(dd-mm)

No. of 

Apps

App. 

Interval 

(days)

National 

PCA1

(fraction)

Surface Waters EDWCs (μg/L)

Acute

(1-in-10-

year)

Chronic

(1-in-10-

year)

Cancer 

30-year 

Daily 

Average

NC alfalfa/Alfalfa Aerial 1.5 01-04 1 N/A 1.002 35.5 11.2 5.81

LA sugarcane (HI met 

file)/Sugarcane in HI

Aerial 3.6 10-01 1 N/A 1.002 502 206 77.3

CA wine grape/ 

Blueberries

Ground 2 01-04 1 N/A 1.002 20.9 12.7 8.58

CA rights-of-way/CA 

impervious/Non-crop 

sites (weed and brush 

control)

Aerial 

helicopter

6 01-04 1 N/A 1.003 204 88.0 45.2

CA rights-of-way/CA 

impervious/Non-crop 

sites (weed and brush 

control)

Ground 8 01-04 1 N/A 1.003 260 112 52.5

CA forestry (MI met 

file)/Conifer release in 

Eastern and Lakes states

Aerial 

helicopter

4 01-05 1 N/A 1.003 103 84.9 64.6

OR Christmas trees/ 

Christmas trees

Aerial 2 01-04 1 N/A 1.003 28.9 21.8 18.8

CA rangeland hay (NY 

met file)/Pastures/ 

Rangeland in Eastern US

Aerial 

helicopter

4 01-03 1 N/A 1.003 159 71.9 36.5

PA turf/Bermudagrass/ 

Bahiagrass; Industrial 

turfgrass

Ground 1.125 01-03 1 N/A 1.002 10.4 8.24 6.05

Bolded values represent the highest EDWCs in surface waters.

1 Guidance on Development and Use of Community Water System Drinking Water Intake Percent Cropped Area 

Adjustment Factors for use in Drinking Water Exposure Assessments: 2014 Update, dated 09/09/2014 and approved 

09/12/2014.

2 PCA for Turf-All Agriculture.

3 No PCAs have been issued for these non-agricultural uses.

Table 14. Tier II Estimated Drinking Water Concentrations (EDWCs) for Surface Water 

Based on Application of Hexazinone Products Modeled in the SWCC, for TTRs + UERs1

Scenario App. 

Method

App. 

Rate

(lb a.i./A)

Date of 

First 

Application

(dd-mm)

No. of 

Apps

App. 

Interval 

(days)

National 

PCA2

(fraction)

Surface Waters EDWCs (μg/L)

Acute

(1-in-10-

year)

Chronic

(1-in-10-

year)

Cancer 

30-year 

Daily 

Average

LA sugarcane (HI met 

file)/Sugarcane in HI

Aerial 3.6 10-01 1 N/A 1.003 502 207 77.6

PA turf/ Bermudagrass/ 

Bahiagrass; Industrial 

turfgrass

Ground 1.125 01-03 1 N/A 1.003 10.4 8.25 6.06

Bolded values represent the highest EDWCs in surface waters.

1 Only the crops that yielded the highest and lowest EDWCs were presented, calculating the TTRs + UERs.

2 Guidance on Development and Use of Community Water System Drinking Water Intake Percent Cropped Area 

Adjustment Factors for use in Drinking Water Exposure Assessments: 2014 Update, dated 09/09/2014 and approved 

09/12/2014.

3 PCA for Turf-All Agriculture.

32
The PRZM-GW results obtained for each scenario for the use of hexazinone on rights-of-ways

are summarized in Tables 15 and 16. It should be noted that each of the PRZM-GW scenarios 

were used as surrogates for rights-of-ways.

Table 15. Hexazinone Groundwater Estimated Drinking Water Concentrations (EDWCs) 

Resulting from the Use of the Chemical on Rights-of-Ways, for the TTRs (Excluding the 

UERs)

Crop/Scenario

Highest Daily 

Value

(μg/L)

Post Breakthrough 

Average

(μg/L)

Average Simulation 

Breakthrough Time

(days)

Rights-of-way/Tier I Delmarva 2470 1920 1752

Rights-of-way/Tier I FL citrus 2210 1840 1294

Rights-of-way/Tier I FL potatoes 1080 951 1800

Rights-of-way/Tier I GA coast 944 765 2166

Rights-of-way/Tier I NC coast 3090 2210 2485

Rights-of-way/Tier I WI sand 2530 2260 3141

Bolded values represent the highest groundwater EDWCs.

Table 16. Hexazinone Groundwater Estimated Drinking Water Concentrations (EDWCs) 

Resulting from the Use of the Chemical on Rights-of-Ways, for the TTRs + UERs

Crop/Scenario

Highest Daily 

Value

(μg/L)

Post Breakthrough 

Average

(μg/L)

Average Simulation 

Breakthrough Time

(days)

Rights-of-way/Tier I Delmarva 2760 2110 1752

Rights-of-way/Tier I FL citrus 2390 1990 1294

Rights-of-way/Tier I FL potatoes 1530 1250 1800

Rights-of-way/Tier I GA coast 1060 852 2166

Rights-of-way/Tier I NC coast 3710 2600 2485

Rights-of-way/Tier I WI sand 2680 2390 3141

Bolded values represent the highest groundwater EDWCs.

Table 17 gives EDWCs for the same use (rights-of-ways), using SCI-GROW.

Table 17. Tier I EDWCs for Surface Water Based on Application of Hexazinone on 

Rights-of-Ways

Drinking Water 

Source (Model Used)

Use Scenario

(rate modeled)

Estimated Drinking Water Concentrations (EDWCs)

(ppb)

Acute and Chronic

Groundwater

(SCI-GROW)

Rights-of-ways (8 lb a.i./A)

TTRs 180

Rights-of-ways (8 lb a.i./A)

TTRs + UERs 527

33
Additional Refinements of Groundwater EDWCs

EFED shared with HED preliminary EDWCs (as shown in Tables 15 to 17). HED indicated that 

for chronic exposure, the EDWCs exceeded the maximum recommended level for the rights-ofway scenarios using the results of PRZM-GW. Therefore, EFED pursued additional refinements 

in the PRZM-GW modeling results.

Inspection of the labels for hexazinone indicates that the application rate range for the chemical 

on rights-of-way is wide (range is 2–8 lb a.i./A); furthermore, it depends on the soil texture and 

pest to be treated. For example, the recommended range is 2–5 lb a.i./A for certain pests, and 6–

8 lb a.i./A for others. Further, the maximum rate is higher for the ground (8 lb a.i./A) than aerial

(6 lb a.i./A) applications. The label further instructs the applicator to...“Use lower rate on coarse 

textured soils (sand to sandy loam). Use the higher rate on fine textured soils (clay loam to clay) 

and on soils high in organic matter.”

EFED conducted in 2006 an analysis of the percentage of land that is covered by rights-of-ways 

in the Chesapeake Bay area. The analysis can be found in Section 3.2.1.2 of the atrazine 

endangered species assessment located at: 

http://www.epa.gov/oppfead1/endanger/litstatus/effects/atrazine/2007/determination-cheasp.pdf

(accessed 03/05/2015) (excerpts are reproduced in Appendix C). According to this analysis, 

approximately 10% of the surface area (in the evaluated area) is rights-of-way (refer to the 

appendix). Therefore, if such a factor is applied to the rights-of-way uses, the equivalent 

application rate range is reduced from 2–8 lb a.i./A to 0.2–0.8 lb a.i./A.

Given this reduced equivalent application rate for rights-of-ways, other uses of hexazinone 

would have higher application rates than rights-of-way (i.e., higher than 0.2–0.8 lb a.i./A); 

therefore, EFED explored the uses with the next highest application rate for hexazinone, which

are pasture/rangeland and forestry. For the pasture/rangeland use (which among the 

conventional agricultural uses is second in pounds active ingredient applied, refer to the Usage 

section), the maximum application rate is 4 lb a.i./A, but for loamy sand and sandy soils the 

application rate range is 1–2 lb a.i./A. An application rate range for different soil textures is 

common for other crops as well for hexazinone as explained above.

EFED verified the soil textures of the six standard PRZM scenarios in PRZM-GW. Table 18

shows the percent sand, clay and organic carbon, and soil textures for the top soils (0–0.4 m 

depth) in the standard PRZM-GW scenarios. Lower soil layers (below 0.4 m depth), were 

excluded from this analysis. Inspection of the table reveals that all the PRZM-GW scenarios 

have sandy, sandy loam or loamy sand textures in the top layers (0-0.4 m depth), with the 

exception of the lower (0.2-0.4 m) layer of the NC coast scenario. In addition, the majority of 

the scenarios have a low level of organic carbon (<1%). The exceptions are the top layer (0-0.1 

m depth) of the FL potato scenario (4.19%), and the lower (0.2-0.4 m) layer of the FL citrus 

scenario.

Table 18. Percent Organic Carbon, Sand, and Clay, and Soil Textures for the Top Soils in 

the Standard PRZM-GW Scenarios

Scenario/depth (m) Organic Carbon (%) Sand (%) Clay (%) Soil texture

Delmarva (0-0.1) 0.52 92.3 2.4 Sand

34
Scenario/depth (m) Organic Carbon (%) Sand (%) Clay (%) Soil texture

Delmarva (0.1-0.2) 0.52 92.3 2.4 Sand

Delmarva (0.2-0.4) 0.20 91.5 2.7 Sand

FL citrus (0-0.1) 0.144 96 2.4 Sand

FL citrus (0.1-0.2) 0.144 95 2.8 Sand

FL citrus (0.2-0.4) 1.44 95 2.8 Sand

FL potato (0-0.1) 4.19 93 0.001 Sand

FL potato (0.1-0.2) 0.78 95 0.2 Sand

FL potato (0.2-0.4) 0.18 92 0.6 Sand

GA coast (0-0.1) 0.75 66 3.7 Sandy loam

GA coast (0.1-0.2) 0.75 66 3.7 Sandy loam

GA coast (0.2-0.4) 0.25 67 12.2 Sandy loam

NC coast (0-0.1) 0.61 76 2.4 Loamy sand

NC coast (0.1-0.2) 0.61 75 2.4 Loamy sand

NC coast (0.2-0.4) 0.19 55 20 Sandy clay loam

WI sand (0-0.1) 0.46 95 2.3 Sand

WI sand (0.1-0.2) 0.46 95 2.3 Sand

WI sand (0.2-0.4) 0.14 96 2.5 Sand

As an alternative to the rights-of-ways use, runs with a lower value in the application rate were 

conducted for the pasture/rangeland use, at 2 lb a.i./A. This is the application rate recommended 

for sandy soils for pasture/rangeland. All other parameters were the same than the ones 

previously described. Results are summarized in Table 19. As expected, since the application 

rate for this use is 1⁄4 the one modeled previously for rights-of-way, the EDWCs were 

approximately 4-fold lower.

Table 19. Hexazinone Groundwater Estimated Drinking Water Concentrations (EDWCs) 

Resulting from the Use of the Chemical on Pasture/Rangeland, for the TTRs (Excluding 

the UERs), Applied at 2 lb a.i./A

Crop/Scenario

Highest Daily 

Value

(μg/L)

Post Breakthrough 

Average

(μg/L)

Average Simulation 

Breakthrough Time

(days)

Rangeland/Tier I Delmarva 616 480 1752

Rangeland/Tier I FL citrus 551 460 1294

Rangeland/Tier I FL potatoes 270 238 1800

Rangeland/Tier I GA coast 236 191 2166

Rangeland/Tier I NC coast 773 552 2485

Rangeland/Tier I WI sand 632 564 3141

Bolded values represent the highest groundwater EDWCs.

Monitoring Data

There is available monitoring data for this chemical. Hexazinone in groundwater was previously 

considered by the California Department of Pesticide Regulation (CDPR), as shown in the 

following site: http://www.cdpr.ca.gov/docs/emon/grndwtr/hexazinone.htm (accessed 

01/12/2015). Their findings were as follows: “The Director concurs with the subcommittee that 

based on the detected concentrations of hexazinone (ranging from 0.05 to 0.27 μg/L) and the 

health-protective drinking water level of 170 μg/L established by the Office of Environmental 

35
Health Hazard Assessment (OEHHA), hexazinone cannot be found to have polluted ground 

water.”

Hexazinone and a number of its degradates were found to be monitored in the USGS National 

Water Quality Assessment Data Warehouse (NAWQA), when verified by constituent finder (at

http://cida.usgs.gov/nawqa_public/apex/f?p=136:1:0, accessed 01/12/2015). Table 20

summarizes the findings for hexazinone. Across the U.S. (excluding territories), a total of 3872 

samples were analyzed for hexazinone, of which 1265 (33%) were positive detections. Of the 

detections, 157 (4.1%) were positive groundwater detections. Additionally, certain degradates of 

hexazinone were tested for in the NAWQA database.

Table 20. Summary of Hexazinone Monitoring Data from the NAWQA Database

State Number of Detections Number of Samples Number of Groundwater Detections

Alabama 39 189 0

Alaska 0 2 0

Arizona 13 103 1

Arkansas 4 19 0

California 427 >511 20

Connecticut 39 123 0

Delaware 0 33 0

District of 

Columbia 0 0 0

Florida 99 337 49

Georgia 80 270 20

Hawaii 4 15 4

Idaho 9 148 4

Illinois 3 188 0

Indiana 4 133 0

Iowa 7 220 0

Kansas 0 31 0

Kentucky 1 4 0

Louisiana 61 274 0

Maine 0 37 0

Maryland 3 128 0

Massachusetts 2 86 2

Michigan 0 113 0

Minnesota 4 172 3

Mississippi 28 97 0

Missouri 1 43 0

Montana 0 43 0

Nebraska 2 105 0

Nevada 2 55 0

New 

Hampshire 0 35 0

New Jersey 36 264 0

New Mexico 3 157 3

New York 7 131 4

North 

Carolina 25 126 0

North Dakota 0 0 0

Ohio 8 78 1

Oklahoma 0 12 0

36
State Number of Detections Number of Samples Number of Groundwater Detections

Oregon 181 252 0

Pennsylvania 0 40 0

Rhode Island 1 5 1

South 

Carolina 42 211 3

South Dakota 0 0 0

Tennessee 6 113 6

Texas 43 414 13

Utah 4 124 3

Vermont 0 3 0

Virginia 36 110 0

Washington 41 275 10

West Virginia 0 5 0

Wisconsin 0 123 0

Wyoming 0 6 0

Total 1265 3872 157

It is noted that sampling and testing for hexazinone in the NAWQA database does not represent 

necessarily source (reservoir) or finished (treated) drinking waters. Despite the relatively high 

level of detections, EFED believes that modeling reflects concentrations in surface and ground 

waters, which are realistically conservative for each site represented. The NAWQA monitoring 

program analyzed for this drinking water assessment does not specifically target hexazinone. 

Consequently, detections cannot be directly associated with a particular use pattern or site, nor 

are the detections expected to represent the potential peak hexazinone exposure. This result is 

attributed to the fact that water monitoring sampling programs do not specifically target 

hexazinone use areas, may not represent high hexazinone use areas, and may not correspond with 

applications of hexazinone; therefore, peak concentrations of hexazinone may not be detected.

Sampling frequencies are not designed to capture peak concentrations. Model generated times 

series data suggest that monitoring programs need to sample daily in order to capture peak 

concentrations of hexazinone in surface water. There are also limited sampling data available for 

the U.S. In order to use monitoring data as an exposure endpoint for a chemical with an acute

exposure concern, sampling needs to be intensive (likely daily) in order to capture the peak 

exposure concentrations.

Because currently available monitoring data likely underestimate hexazinone concentrations, 

monitoring results are not an appropriate prediction of the potential exposure resulting from 

hexazinone use. For these reasons, EFED does not recommend that hexazinone water 

monitoring data be used by HED in the human health risk assessment.

Drinking Water Treatment

There is no readily available data on impacts of water treatment on the dissipation and/or the 

formation of transformation products that may occur. Hexazinone is persistent under hydrolysis 

and aqueous photolysis condition. Further, its half-lives under aquatic metabolism condition also 

indicate high persistence. Its mean KOC of ~58 mL/gOC shows that the chemical is mobile (FAO 

2000). It appears unlikely that primary treatment may reduce the levels of hexazinone due to its 

low tendency to bind.

37
EXPOSURE CHARACTERIZATION 

Tables 21 and 22 provide a summary of the maximum EDWCs obtained from each of the 

models used in this DWA.

Table 21. Maximum Estimated Drinking Water Concentrations for the Use of Hexazinone, 

for the TTRs (excluding the UERs)

DRINKING 

WATER SOURCE 

(MODEL USED)

SCENARIO (USE) (rate modeled per year) MAXIMUM ESTIMATED DRINKING WATER 

CONCENTRATION (EDWC) ( ppb)

Groundwater 

(PRZM-GW)

NC coast1 (rights-of-way) (8 lb a.i.) Acute 3090

WI sand1 (rights-of-way) (8 lb a.i.) Chronic 2260

Surface water

(SWCC)

LA sugarcane with HI meteorological file2

(sugarcane in HI) (3.6 lb a.i.)

Acute 502

Chronic 206

Cancer/Chronic 77.3

1 Scenario was used as a surrogate and run with the maximum application rate for rights-of-ways.

2 Represents applications of hexazinone on sugarcane in Hawaii.

Table 22. Maximum Estimated Drinking Water Concentrations for the Use of Hexazinone, 

for the TTRs + UERs

DRINKING 

WATER SOURCE 

(MODEL USED) 

SCENARIO (USE) (rate modeled per year) MAXIMUM ESTIMATED DRINKING WATER 

CONCENTRATION (EDWC) ( ppb)

Groundwater 

(PRZM-GW) NC coast1 (rights-of-way) (8 lb a.i.) Acute 3710

Chronic 2600

Surface water

(SWCC)

LA sugarcane with HI meteorological file2

(sugarcane in HI) (3.6 lb a.i.)

Acute 502

Chronic 207

Cancer/Chronic 77.6

1 Scenario was used as a surrogate and run with the maximum application rate for rights-of-ways.

2 Represents applications of hexazinone on sugarcane in Hawaii.

Results presented in Table 23 represent EDWCs for refined groundwater estimates for the use of 

hexazinone on pastures/rangeland, assuming that the soils are sands or sandy loams.

Table 23. Refined Estimated Groundwater Drinking Water Concentrations for the Use of 

Hexazinone, for the TTRs

DRINKING 

WATER SOURCE 

(MODEL USED)

SCENARIO (USE) (rate modeled per year) MAXIMUM ESTIMATED DRINKING WATER 

CONCENTRATION (EDWC) ( ppb)

Groundwater 

(PRZM-GW)

NC coast1 (pastures/rangeland) (2 lb a.i.) Acute 773

WI sand1 (pastures/rangeland) (2 lb a.i.) Chronic 564

1 Scenario was used as a surrogate and run with the prescribed application rate for sandy loam and loamy sand soils 

on pasture/rangeland.

Input Parameters

It appears that the main route of dissipation of hexazinone is aerobic soil metabolism (even 

though it occurs slowly). The aquatic metabolism is very slow (both aerobic and anaerobic). 

There are four aerobic soil half-lives available. Three of them were obtained at 200C; 

38
temperature adjustment was required for PRZM-GW and the SWCC, resulting in shorter halflives.

As indicated earlier, the environmental fate database for hexazinone is essentially complete. It 

was noted that for the aquatic metabolism studies, the study duration was short compared to the 

half-lives obtained. The aerobic soil metabolism study (MRID 48898001) was conducted for a 

period of only 150 days (in a study where three soils were tested), with half-lives of 54-143 days 

(note that these half-lives do not include the total residues or the unextracted residues, they only 

include the parent compound). The aerobic aquatic metabolism study (MRID 49109901) was 

conducted for a period of only 100 days, compared to half-lives of 528-719 days (for the parent). 

Meanwhile, the anaerobic aquatic metabolism study (MRID 49109902) was conducted for 101 

days, compared to a half-life of 482-961 days (for the parent). In this assessment two aerobic 

and two anaerobic aquatic systems were available. Further four aerobic soils were available. 

There is also an anaerobic soil metabolism study (four soils, MRID 48898002), but the results of 

this study will not affect this DWA since they are not used in hexazinone drinking water models. 

The fact that multiple soils and sediments were available tends to reduce the uncertainty; 

however, the short duration of the studies brings uncertainty in the half-life estimates.

Application Rates

In this DWA, a number of uses were assessed; rights-of-ways (for groundwater) and sugarcane 

(for surface water) were found to yield the highest EDWCs. The range of application rates on 

rights-of-ways is ~2 to 8 lb a.i./A (depending on the weeds controlled and the soil texture), and 

on HI sugarcane it is ~0.45 to 3.6 lb a.i./A (depending on the soil texture). In both cases the 

maximum application rate was assumed to be applied in a single application per year. It is 

uncertain the degree to which this is true under actual use conditions.

Additional refinements were conducted for the pastures/rangeland use, and using the application 

rate for coarse textured soils since the six PRZM scenarios in PRZM-GW have coarse textured 

soils. This modeling was performed only to obtain groundwater EDWCs since according to 

HED there was no acute concern with results obtained from the rights-of-way and other 

modeling runs.

In this assessment, applications were assumed to occur at once and every year for 30 consecutive 

years. Due to possible pest resistance (as indicated in the labels), consecutive applications for 

such an extended period of time appear to be unlikely.

Percent Crop Area

It is noted that for the model that yielded the highest EDWCs (PRZM-GW for groundwater), a 

PCA is not applicable. A percent cropped area (PCA) adjustment factor should not be applied to 

PRZM-GW model output data to generate EDWCs. The conceptual model for groundwater 

exposure, as implemented using PRZM-GW, represents pesticide concentrations in a vulnerable 

groundwater supply that is located directly beneath a single, rural agricultural field and assumes 

the entire field (zone of influence) is treated. The zone of influence does not represent an entire 

watershed.

39
For surface waters, the PCA for turf-all agriculture was used (1.00) and for the non-agricultural 

uses, no PCA adjustment was used since no values have been established for such uses (in other 

words, a PCA of 1.00 was assumed).

Residues of Concern

As indicated earlier, the MARC committee had decided that for hexazinone the residues of 

concern were comprised by the parent compound and all its identified metabolites. Citation is as 

follows: “For water, the Committee concluded that for risk assessment purposes, hexazinone, 

G3170, and all metabolites with conjoined cyclohexyl and triazine rings should be included.” 

(Source: Memorandum from Sherrie Kinard, DP Barcode D282111, April 25, 2002).

SWCC

In general, the linked SWCC model produces EDWCs that are expected to be exceeded once 

within a ten-year period. The Pesticide Root Zone Model is a process or “simulation” model that 

calculates what happens to a pesticide in an agricultural field on a day-to-day basis. It considers 

factors such as rainfall and plant transpiration of water, as well as how and when the pesticide is 

applied. It has two major components: hydrology and chemical transport. Water movement is 

simulated by the use of generalized soil parameters, including field capacity, wilting point, and 

saturation water content. The chemical transport component can simulate pesticide application 

on the soil or on the plant foliage. Dissolved, adsorbed, and vapor-phase concentrations in the 

soil are estimated by simultaneously considering the processes of pesticide uptake by plants, 

surface runoff, erosion, decay, volatilization, foliar wash-off, advection, dispersion, and 

retardation.

Uncertainties associated with each of these individual components add to the overall uncertainty 

of the modeled concentrations. Additionally, model inputs from the environmental fate 

degradation studies are chosen to represent the upper confidence bound on the mean values that 

are not expected to be exceeded in the environment approximately 90 percent of the time. 

Mobility input values are chosen to be representative of conditions in the environment. The 

natural variation in soils adds to the uncertainty of modeled values. Factors such as application 

date, crop emergence date, and canopy cover can also affect estimated concentrations, adding to 

the uncertainty of modeled values. Factors within the ambient environment such as soil 

temperatures, sunlight intensity, antecedent soil moisture, and surface water temperatures can 

cause actual aquatic concentrations to differ for the modeled values.

PRZM-GW

PRZM-GW has been determined to be an effective tool for establishing upper bound pesticide 

concentrations in groundwater for national as well as site-specific assessments. As such, it could 

be used as a screening model for Tier I drinking water assessments. Since hydrolysis is 

generally the only transformation pathway considered at soil depths greater than one meter, it 

can have a large impact on the PRZM-GW EDWCs. Hexazinone is stable to hydrolysis in a pH 

range of 5 to 9. For Tier I assessments, all six scenarios should be run for national scale 

pesticide assessments. However, if the use is restricted by geography, the user should identify 

the best representative surrogate scenario(s). Since the use on rights-of-ways can occur 

nationwide, in a first tier try, all the scenarios were modeled.

40
In cases where the “throughputs” value for a model run are less than one, modeling is repeated 

with the appropriate extended weather file. An extended weather file contains the same weather 

as the standard 30-year weather file and allows the user to run the simulation for up to 100 years 

in order to observe breakthrough. This was not needed for hexazinone.

Even though six scenarios modeled are not strictly rights-of-way scenarios, they were regarded 

as if they were rights-of-way, in the first tier screening-level exposure assessment. Additionally, 

the maximum application rate was used, despite the fact that a wide range of application rates is 

prescribed in the labels, including that for coarse textured soils, the rate is lower than the 

maximum. Note that all the PRZM scenarios in PRZM-GW have coarse textured top soils.

Furthermore, available evidence suggests that the application rate on rights-of-way are usually 

around 10% the value indicated in the labels. As an additional refinement, EFED conducted 

modeling of the pastures/rangeland use, with the application rate prescribed in the labels for 

coarse textured soils (2 lb a.i./A).

It appears that both the rights-of-way and pasture/rangeland uses are less likely to be in the 

vicinity of groundwater wells. Agricultural uses like sugarcane, blueberries or alfalfa, all of 

which have lower application rates than rights-of-way and pastures/rangeland, appear more 

likely to be present in the proximity of drinking water wells. Furthermore, three of the six 

standard scenarios (FL citrus, WI sand and Delmarva) represent rare, but realistic for the specific 

sites, vulnerability conditions that are unlikely to be associated with many of the hexazinone use 

sites.

The results of the above stated uncertainties suggest the highly conservative nature of the 

groundwater EDWCs noted in this DWA.

SCI-GROW

Pesticide concentrations estimated by SCI-GROW represent conservative or high-end exposure 

values because the model is based on groundwater monitoring studies which were conducted by 

applying pesticides at maximum allowed rates and frequency to vulnerable sites (i.e., shallow 

aquifers, sandy, permeable soils, and substantial rainfall and/or irrigation to maximize leaching). 

In most cases, a large majority of the use areas will have ground water that is less vulnerable to 

contamination than the areas used to derive the SCI-GROW estimate. The SCI-GROW estimate 

is based on environmental fate properties of the pesticide, the application rate, and the aerobic 

soil metabolism of the chemical. Experience shows that if a pesticide is substantially subject to 

abiotic hydrolysis (i.e., not stable over the 30-day period used in most hydrolysis studies), this 

generally mitigates its long-term groundwater contamination potential. If the hydrolysis data 

demonstrate that the pesticide in question is subject to hydrolysis, then this should be noted 

because it means that the SCI-GROW value may be an overestimate even for highly vulnerable 

groundwater. For hexazinone, this is not the case. Hexazinone is stable to hydrolysis at pH 7. 

The SCI-GROW concentration has usefulness as a screening or upper-bound value because the 

regression is based solely upon scenarios in which the vulnerability of groundwater 

contamination was much higher than the vast majority of agricultural use scenarios.

All Models

41
This assessment is national in character and EDWCs should be regarded as upper bound 

estimates. National PCAs were used in SWCC, while PCAs are not applicable for the PRZMGW or SCI-GROW. For the non-agricultural uses of hexazinone, no PCA adjustment factor was 

applied (assumed to be 1.00).

A more definitive assessment could be performed, for example, using regional PCAs in the 

SWCC should they be applicable, or performing additional refinements with PRZM-GW 

(groundwater). For PRZM-GW a number of refinement strategies have been identified and they 

can be used in definitive assessments. Additionally, typical application rates may be modeled for 

characterization purposes.

REFERENCES 

Food and Agriculture Organization of the United Nations 2000. 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. Accessed at the following URL 08/15/2013:

http://www.fao.org/DOCREP/003/X2570E/X2570E00.htm.

U.S. Environmental Protection Agency (USEPA) 2002. Hexazinone. The Outcome of the HED 

Metabolism Assessment Review Committee for Water. OCSPP/OPP/Health Effects 

Division. DP Barcode: D282111. Dated 04/25/2002.

USEPA 2006. Tier I Drinking Water Assessment Guidance and Guidance for Developing a Tier I 

Drinking Water Exposure Assessment. OCSPP/OPP/Environmental Fate and Effects 

Division. Dated 01/27/2006.

USEPA 2007a. Tier II Drinking Water Exposure Assessment Guidance. OCSPP/OPP/

Environmental Fate and Effects Division. Dated 05/28/2007.

USEPA 2007b. Characterization of Drinking Water Exposure for Dietary Risk Assessment. 

OCSPP/OPP/Environmental Fate and Effects Division. Dated 12/13/2007.

USEPA 2009. Guidance for Selecting Input Parameters in Modeling the Environmental Fate and 

Transport of Pesticides Version 2.1. OCSPP/OPP/Environmental Fate and Effects 

Division. Dated 10/22/2009.

USEPA 2010a. Development and Use of the Index Reservoir in Drinking Water Exposure 

Assessments. OCSPP/OPP/Environmental Fate and Effects Division. Dated 04/15/2010 

and approved 09/14/2010.

USEPA 2010b. Guidance for Making Temperature Adjustments to Metabolism Inputs to 

EXAMS and PE5 and WQTT Advisory Note Number 9: Temperature Adjustments for 

Aquatic Metabolism Inputs to EXAMS and PE5. OCSPP/OPP/Environmental Fate and 

Effects Division. Dated 09/21/2010 and approved 10/18/2010.

42
USEPA 2011. Finalization of Guidance on Incorporation of Water Treatment Effects on 

Pesticide Removal and Transformations in Drinking Water Exposure Assessments and 

The Incorporation of Water Treatment Effects on Pesticide Removal and Transformations 

in Food Quality Protection Act (FQPA) Drinking Water Assessments. OCSPP/OPP/

Environmental Fate and Effects Division. Dated 12/05/2011.

USEPA 2012a. Guidance for Selecting Input Parameters for Modeling Pesticide Concentrations 

in Groundwater Using the Pesticide Root Zone Model. OCSPP/OPP/Environmental Fate 

and Effects Division. Dated 10/15/2012.

USEPA 2012b. Guidance for Using PRZM-GW in Drinking Water Exposure Assessments. 

OCSPP/OPP/Environmental Fate and Effects Division. Dated 10/15/2012.

USEPA 2012c. Guidance for Evaluating and Calculating Degradation Kinetics in Environmental 

Media, Health Canada, U.S. Environmental Protection Agency, OCSPP/OPP/

Environmental Fate and Effects Division. Dated December 21, 2012.

USEPA 2012d. Standard Operating Procedure for Using the NAFTA Guidance to Calculate 

Representative Half-life Values and Characterize Pesticide Degradation. OCSPP/OPP/

Environmental Fate and Effects Division. Dated November 30, 2012 and approved 

December 17, 2012.

USEPA 2014a. Guidance on Development and Use of Community Water System Drinking 

Water Intake Percent Cropped Area Adjustment Factors for use in Drinking Water 

Exposure Assessments: 2014 Update. OCSPP/OPP/ Environmental Fate and Effects 

Division. Dated September 9, 2014 and approved September 12, 2014.

USEPA 2014b. Guidance for Addressing Unextracted Residues in Laboratory Studies. 

OCSPP/OPP/ Environmental Fate and Effects Division. Approved September 12, 2014.

43
APPENDIX A

Example Model Input and Output Files

SWCC

Run for LA Sugarcane, for the Total Toxic Residues, Excluding the Unextracted Residues

Summary of Water Modeling of Hexazinone and the USEPA Standard 

Reservoir

Estimated Environmental Concentrations for Hexazinone are presented in Table 1 for the 

USEPA standard reservoir with the LAsugarcaneSTD field scenario. A graphical presentation of 

the year-to-year peaks is presented in Figure 1. These values were generated with the Surface 

Water Concentration Calculator (SWCC Version 1.106). Critical input values for the model are 

summarized in Tables 2 and 3.

This model estimates that about 3.6% of Hexazinone applied to the field eventually reaches the 

water body. The main mechanism of transport from the field to the water body is by runoff ( 

88% of the total transport), followed by spray drift (11.5%) and erosion (0.53%).

In the water body, pesticide dissipates with an effective water column half-life of 114.0 days. 

(This value does not include dissipation by transport to the benthic region; it includes only 

processes that result in removal of pesticide from the complete system.) The main source of 

dissipation in the water column is washout (effective average half-life = 124.4 days) followed by 

metabolism (1363.8 days) and volatilization (1.085964E+08 days).

In the benthic region, pesticide dissipation is negligible (1833.0 days). The main source of 

dissipation in the benthic region is metabolism (effective average half-life = 1833 days). Most of 

the pesticide in the benthic region (86%) is sorbed to sediment rather than in the pore water.

Table 1. Estimated Environmental Concentrations (ppb) for Hexazinone.

Peak (1-in-10 yr) 502.

4-day Avg (1-in-10 yr) 495.

21-day Avg (1-in-10 yr) 469.

60-day Avg (1-in-10 yr) 422.

365-day Avg (1-in-10 yr) 206.

Entire Simulation Mean 77.3

44
Table 2. Summary of Model Inputs for Hexazinone.

Scenario LAsugarcaneSTD

Cropped Area Fraction 1.0

Koc (ml/g) 57.5

Water Half-Life (days) @ 25 °C 1343

Benthic Half-Life (days) @ 25 °C 1805

Photolysis Half-Life (days) @ 40 

°Lat

0

Hydrolysis Half-Life (days) 0

Soil Half-Life (days) @ 25 °C 512.2

Foliar Half-Life (days) 0

Molecular Wt 252.32

Vapor Pressure (torr) 1.9E-7

Solubility (mg/l) 29800

Table 3. Application Schedule for Hexazinone.

Date (Mon/Day) Type Amount (kg/ha) Eff. Drift

01/10 Foliar 4.03 0.95 0.135

45
Figure 1. Yearly Peak Concentrations

SCI-GROW

Total Toxic Residues, Excluding the Unextracted Radioactivity

 SCIGROW

 VERSION 2.3

 ENVIRONMENTAL FATE AND EFFECTS DIVISION

 OFFICE OF PESTICIDE PROGRAMS

 U.S. ENVIRONMENTAL PROTECTION AGENCY

 SCREENING MODEL

 FOR AQUATIC PESTICIDE EXPOSURE

SciGrow version 2.3

chemical:Hexazinone

time is 12/11/2014 12: 2:45

------------------------------------------------------------------------

 Application Number of Total Use Koc Soil Aerobic

 rate (lb/acre) applications (lb/acre/yr) (ml/g) metabolism (days)

------------------------------------------------------------------------

 8.000 1.0 8.000 5.75E+01 475.0

------------------------------------------------------------------------

groundwater screening cond (ppb) = 1.80E+02 

************************************************************************

46
Total Toxic Residues, Including the Unextracted Radioactivity

 SCIGROW

 VERSION 2.3

 ENVIRONMENTAL FATE AND EFFECTS DIVISION

 OFFICE OF PESTICIDE PROGRAMS

 U.S. ENVIRONMENTAL PROTECTION AGENCY

 SCREENING MODEL

 FOR AQUATIC PESTICIDE EXPOSURE

SciGrow version 2.3

chemical:Hexazinone

time is 12/ 4/2014 8:23:10

------------------------------------------------------------------------

 Application Number of Total Use Koc Soil Aerobic

 rate (lb/acre) applications (lb/acre/yr) (ml/g) metabolism (days)

------------------------------------------------------------------------

 8.000 1.0 8.000 5.75E+01 1051.0

------------------------------------------------------------------------

groundwater screening cond (ppb) = 5.27E+02 

************************************************************************

PRZM-GW

Run of the Total Toxic Residues, Excluding the Unextracted Residues, NC Coast

Groundwater Analysis for Hexazinone and the NC Eastern Coastal 

Plain - Norfolk Loamy Sand - Cotton Scenario

Estimated groundwater concentrations and breakthrough times for Hexazinone are presented 

in Table 1 for the NC Eastern Coastal Plain - Norfolk loamy sand - Cotton groundwater scenario. 

A graphical presentation of the daily concentrations in the aquifer is presented in Figure 1. 

These values were generated with the PRZM-GW (Version 1.07). Critical input values for the 

model are summarized in Tables 2 and 3.

Table 1. Groundwater Results for Hexazinone and the NC Eastern Coastal Plain - Norfolk 

loamy sand - Cotton Scenario.

Peak Concentration (ppb) 3.09E+03

Post-Breakthrough Mean 

Concentration (ppb)

2.21E+03

Entire Simulation Mean 

Concentration (ppb)

1.68E+03

Average Breakthrough 

Time (days)

2484.538

Throughputs 4.411282

47
Table 2. Chemical Properties for Groundwater Modeling of Hexazinone.

Koc (ml/g) 57.5

Surface Soil Half Life (days) 512.2

Hydrolysis Half Life (days) 0

Diffusion Coefficint Air (cm2/day) 0.0

Henry's Constant 0.0

Enthalpy (kcal/mol) 0.0

Table 3. Pesticide application scheme used for Hexazinone. This application scheme was 

applied every year of the simulation.

Application Date

(Month/Day)

Application Method Application Rate

(kg/ha)

03/15 Above canopy application 8.97

Figure 1. Aquifer Breakthrough Curve for Hexazinone and the NC Eastern Coastal Plain -

Norfolk Loamy Sand - Cotton Scenario

48
APPENDIX B

Hexazinone and Its Environmental Transformation Products. A

Code Name/ 

Synonym Chemical Name Chemical Structure Study Type MRID Maximum %AR (day)

Final %AR 

(study 

length)

PARENT

Hexaxinone (DPX-A3674) IUPAC: 3-Cyclohexyl-6-

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

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

CAS: 3-Cyclohexyl-6-

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

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

CAS No.: 51235-04-2

Formula: C12H20N4O2

MW: 252.3 g/mol

SMILES: 

O=C(N=C(N(C1=O)C)N(C)C)N1C

(CCCC2)C2

O

N

C H 3

N

C H 3

C H 3

O

N

N

Hydrolysis 41587301 pH 5 99.5 (0 d) 99.6 (30 d)

Hydrolysis 41587301 pH 7 99.3 (0 d) 99.4 (30 d)

Hydrolysis 41587301 pH 9 99.6 (0 d) 99.5 (30 d)

Aq Photol 41300801 Irradiated 92.8 (0 d) 86.6 (30 d)B

Aq Photol 41300801 Dark (pH 7) 94.3 (0 d) 93.6 (30 d)

Soil Photol 41300802 Sandy loam 88.1 (0 d) 67.5 (30 d)C

Soil Photol 41300802 SL dark ctrl 88.9 (0 d) 86.7 (30 d)

835.4100

Aerobic 

soil 

metabolism

41807401/ 

42635001

Non-sterile 

sandy loam 92.6 (0 d) 34.3 (365 d)

Sterile 91.5 (0 d) 82.3 (365 d)

48898001

Clay loam 101.2% (0 d) 15.5% (150 d)

Sandy loam 103.1% (0 d) 52.8% (150 d)

Loam 100.0% (0 d) 38.6% (150 d)

835.4200

Anaerobic 

soil 

metabolism

48898002

Sandy clay 

loam 100.7% (0 d) 60.7% (180 d)

Clay loam 101.2% (0 d) 36.1% (180 d)

Sandy loam 100.7% (0 d) 61.4% (180 d)

Loam 100.4% (0 d) 69.6% (180 d)

835.4300

Aerobic 

aquatic 

metabolism

41811801

Non-sterile 

sandy loam 99.7 (3) 80.3 (56)

Sterile 99.9 (7) 86.8 (56)

49109901

Wabasha 

water:sandy 

loam

101.3% (0 d) 87.4% (100 d)

Tift 

water:sandy 

loam

100.5% (0 d) 91.6% (100 d)

835.4400 41807402

Non-sterile 

loam 96.7 (7 d) 39.7 (365 d)

Sterile 96.0 (0 d) 81.3 (365 d)

49
Code Name/ 

Synonym Chemical Name Chemical Structure Study Type MRID Maximum %AR (day)

Final %AR 

(study 

length)

Anaerobic 

aquatic 

metabolism 49109902

Wabasha 

water:sandy 

loam

101.6% (0 d) 86.5% (101 d)

Tift 

water:sandy 

loam

100.5% (0 d) 92.1% (101 d)

MAJOR (>10%) TRANSFORMATION PRODUCTS

IN-G3170; Met. G IUPAC: 1-Methyl-6-

(methylamino)-1,3,5-triazine-2,4-

dione

Formula: C5H8N4O2

MW: 156.14 g/mol

SMILES: 

[H]n1c(=O)nc(n(c1=O)C)N([H])C H

N

O N

C H 3

N

C H 3

H

N

O

835.4100

Aerobic 

soil 

metabolism

48898001

Clay loam 42.2% (150 d) 42.2% (150 d)

Sandy loam 16.4% (150 d) 16.4% (150 d)

Loam 26.8% (150 d) 26.8% (150 d)

835.4200

Anaerobic 

soil 

metabolism

48898002

Sandy clay 

loam 4.8% (46 d) 4.7% (180 d)

Clay loam 25.7% (122 d) 20.4% (180 d)

Sandy loam 8.9% (180 d) 8.9% (180 d)

Loam 11.1% (122 d) 10.4% (180 d)

835.4300

Aerobic 

aquatic 

metabolism

49109901

Wabasha 

water:sandy 

loam

0.2% (15 d) ND (100 d)

Tift 

water:sandy 

loam

0.2% (15 d) ND (100 d)

835.4400

Anaerobic 

aquatic 

metabolism

49109902

Wabasha 

water:sandy 

loam

1.3% (70 d) ND (101 d)

IN-JS472; Met. 1 IUPAC: 6-(Dimethylamino)-1-

methyl-3-(4-oxocyclohexyl)-1,3,5-

triazine-2,4-dione

Formula: C12H18N4O3

MW: 266.3 g/mol

SMILES: 

Cn1c(nc(=O)n(c1=O)C2CCC(=O)C

C2)N(C)C

835.4100

Aerobic 

soil 

metabolism

41807401/ 

42635001

Non-sterile 

sandy loam 13.9 (180 d) 10.8 (365 d)

Sterile 4.0 (120 d) ND (365 d)

48898001

Clay loam 9.4% (10 d) 4.7% (150 d)

Sandy loam 8.3% (60 d) 7.5% (150 d)

Loam 13.3% (150 d) 13.3% (150 d)

835.4200

Anaerobic 

soil 

metabolism

48898002

Sandy clay 

loam 6.6% (31 d) 5.5% (180 d)

Clay loam Aerobic

14.8%

(14 d) ND (180 d)

50
Code Name/ 

Synonym Chemical Name Chemical Structure Study Type MRID Maximum %AR (day)

Final %AR 

(study 

length)

O

N N

N

C H 3

C H 3

N

C H 3

O

O

Anaerobic 14.7%

(33 d)

Sandy loam 8.5% (33 d) 5.0% (180 d)

Loam 8.1% (33 d) 6.7% (180 d)

835.4300

Aerobic 

aquatic 

metabolism

41811801

Non-sterile 

sandy loam 6.5 (28 d) 3.7 (56 d)

Sterile 2.0 (56 d) 2.0 (56 d)

49109901

Wabasha 

water:sandy 

loam

5.8% (100 d) 5.8% (100 d)

Tift 

water:sandy 

loam

4.4% (100 d) 4.4% (100 d)

835.4200

Anaerobic 

soil 

metabolism

49109902

Wabasha 

water:sandy 

loam

6.5% (101 d) 6.5% (101 d)

IN-B2838; Met. D IUPAC: 1-Cyclohexyl-3-methyl1,3,5-triazinane-2,4,6-trione

Formula: C10H15N3O3

MW: 225.2 g/mol

SMILES: 

Cn1c(=O)[nH]c(=O)n(c1=O)C2CC

CCC2

N

O N

C H 3

O

N

O

835.4100

Aerobic 

soil 

metabolism

41807401/ 

42635001

Non-sterile 

sandy loam 5.1 (365 d) 5.1 (365 d)

Sterile 4.1 (365 d) 4.1 (365 d)

48898001

Clay loam 1.1% (150 d) 1.1% (150 d)

Sandy loam 4.2% (150 d) 4.2% (150 d)

Loam 1.3% (150 d) 1.3% (150 d)

835.4200

Anaerobic 

soil 

metabolism

48898002

Sandy clay 

loam 32.0% (180 d) 32.0% (180 d)

Clay loam 2.2% (180 d) 2.2% (180 d)

Sandy loam 2.9% (180 d) 2.9% (180 d)

Loam 2.9% (150 d) 2.8% (180 d)

835.4300

Aerobic 

aquatic 

metabolism

41811801

Non-sterile 

sandy loam 1.3 (56 d) 1.3 (56 d)

Sterile 0.1 (56 d) 0.1 (56 d)

49109901

Wabasha 

water:sandy 

loam

0.8% (15, 70 d) 0.4% (100 d)

51
Code Name/ 

Synonym Chemical Name Chemical Structure Study Type MRID Maximum %AR (day)

Final %AR 

(study 

length)

Tift 

water:sandy 

loam

1.8% (100 d) 1.8% (100 d)

835.4400

Anaerobic 

aquatic 

metabolism

41807402

Non-sterile 

loam 24.2 (365 d) 24.2 (365 d)

Sterile 12.5 (90 d) 1.9 (365 d)

49109902

Wabasha 

water:sandy 

loam

0.4% (70 d) 0.3% (101 d)

Tift 

water:sandy 

loam

1.6% (70, 101 d) 1.6% (101 d)

IN-G3453; Met. A-1 IUPAC: 6-(Dimethylamino)-3-(2-

hydroxycyclohexyl)-1-methyl-1,3,5-

triazine-2,4-dione

Formula: C12H20N4O3

MW: 268.3 g/mol

SMILES: 

Cn1c(nc(=O)n(c1=O)C2CCCCC2O

)N(C)C O H

O

N

N

C H 3

N

C H 3

C H 3

N

O

835.4100

Aerobic 

soil 

metabolism

41807401/ 

42635001

Non-sterile 

sandy loam 19.5 (365 d) 19.5 (365 d)

Sterile 2.0 (120 d) ND (365 d)

835.4200

Anaerobic 

soil 

metabolism

48898002

Clay loam 9.5% (180 d) 9.5% (180 d)

Sandy loam 4.0% (150 d) 2.6% (180 d)

Loam 0.7% (122 d) 0.6% (180 d)

835.4300

Aerobic 

aquatic 

metabolism

41811801

Non-sterile 

sandy loam 5.1 (14 d) 5.0 (56 d)

Sterile 6.8 (56 d) 6.8 (56 d)

IN-A3928; Met. B IUPAC: 3-Cyclohexyl-1-methyl-6-

(methylamino)-1,3,5-triazine-2,4-

dione

Formula: C11H18N4O2

MW: 238.28 g/mol

SMILES: 

[H]N(C)c1nc(=O)n(c(=O)n1C)C2C

CCCC2

Soil Photol 41300802 Sandy loam 10.1 (30 d)C 10.1 (30 d)C

SL dark ctrl 1.4 (30 d) 1.4 (30 d)

835.4100

Aerobic 

soil 

metabolism

41807401/ 

42635001

Non-sterile 

sandy loam 2.4 (60 d) 2.3 (365 d)

Sterile 2.3 (120 d) 1.8 (365 d)

48898001

Clay loam 3.7% (122 d) 3.2% (150 d)

Sandy loam 5.3% (150 d) 5.3% (150 d)

Loam 3.5% (150 d) 3.5% (150 d)

835.4200

Anaerobic 

soil 

metabolism

48898002

Sandy clay 

loam 1.1% (33, 180 d) 1.1% (180 d)

Clay loam 2.6% (180 d) 2.6% (180 d)

Sandy loam 6.2% (180 d) 6.2% (180 d)

52
Code Name/ 

Synonym Chemical Name Chemical Structure Study Type MRID Maximum %AR (day)

Final %AR 

(study 

length)

N

O N

C H 3

N

H

C H 3

N

O

Loam 3.2% (46 d) 2.8% (180 d)

835.4300

Aerobic 

aquatic 

metabolism

41811801

Non-sterile 

sandy loam 5.3 (28 d) 0.7 (56 d)

Sterile 6.3 (56 d) 6.3 (56 d)

49109901

Wabasha 

water:sandy 

loam

2.2% (100 d) 2.2% (100 d)

Tift 

water:sandy 

loam

0.2% (70 d) ND (100 d)

835.4400

Anaerobic 

aquatic 

metabolism

41807402

Non-sterile 

loam 3.5 (365 d) 3.5 (365 d)

Sterile 7.1 (270 d) 5.3 (365 d)

49109902

Wabasha 

water:sandy 

loam

1.6% (70 d) ND (101 d)

Tift 

water:sandy 

loam

0.2% (70 d) ND (101 d)

IN-JT677; Met. 2 IUPAC: 6-(Dimethylamino)-1-

methyl-3-(2-oxocyclohexyl)-1,3,5-

triazine-2,4-dione

Formula: C12H18N4O3

MW: 266.3 g/mol

SMILES: 

Cn1c(nc(=O)n(c1=O)C2CCCCC2=

O)N(C)C

O

N

O N

C H 3

N

C H 3

C H 3

N

O

835.4100

Aerobic 

soil 

metabolism

48898001

Clay loam 1.4% (122 d) ND (150 d)

Sandy loam 0.8% (122 d) ND (150 d)

Loam 1.1% (122 d) ND (150 d)

835.4200

Anaerobic 

soil 

metabolism

48898002

Clay loam 1.9% (150 d) 1.8% (180 d)

Sandy loam 1.2% (150 d) ND (180 d)

Loam 1.0% (180 d) 1.0% (180 d)

835.4300

Aerobic 

aquatic 

metabolism

49109901

Wabasha 

water:sandy 

loam

1.3% (15 d) ND (100 d)

835.4400

Anaerobic 

aquatic 

metabolism

41807402

Non-sterile 

loam 27.5 (365 d) 27.5 (365 d)

Sterile 2.7 (90 d) ND (365 d)

49109902

Wabasha 

water:sandy 

loam

1.3% (70 d) ND (101 d)

IUPAC: Carbon dioxide Soil Photol 41300802 Sandy loam 0.23 (30 d)C 0.23 (30 d)C

53
Code Name/ 

Synonym Chemical Name Chemical Structure Study Type MRID Maximum %AR (day)

Final %AR 

(study 

length)

Carbon dioxide

Formula: CO2

MW: 44 g/mol

SMILES: C(=O)=O

O C O

SL dark ctrl 0.02 (30 d) 0.02 (30 d)

835.4100

Aerobic 

soil 

metabolism

41807401/ 

42635001

Non-sterile 

sandy loam 2.0 (365 d) 2.0 (365 d)

Sterile <0.1 (365 d) <0.1 (365 d)

48898001

Clay loam 9.7% (150 d) 9.7% (150 d)

Sandy loam 2.8% (150 d) 2.8% (150 d)

Loam 2.8% (150 d) 2.8% (150 d)

835.4200

Anaerobic 

soil 

metabolism

48898002

Sandy clay 

loam 0.5% (180 d) 0.5% (180 d)

Clay loam 1.3% (90-180 d) 1.3% (180 d)

Sandy loam 0.4% (180 d) 0.4% (180 d)

Loam 0.7% (180 d) 0.7% (180 d)

835.4300

Aerobic 

aquatic 

metabolism

41811801

Non-sterile 

sandy loam 0.5 (56 d) 0.5 (56 d)

Sterile 0.1 (56 d) 0.1 (56 d)

49109901

Wabasha 

water:sandy 

loam

0.1% (100 d) 0.1% (100 d)

Tift 

water:sandy 

loam

0.1% (100 d) 0.1% (100 d)

Unextracted residues

NA NA

835.4100

Aerobic 

soil 

metabolism

48898001

Clay loam 25.6% (150 d) 25.6% (150 d)

Sandy loam 14.3% (150 d) 14.3% (150 d)

Loam 14.3% (150 d) 14.3% (150 d)

835.4200

Anaerobic 

soil 

metabolism

48898002

Sandy clay 

loam 11.0% (33 d) 5.1% (180 d)

Clay loam 28.2% (31 d) 12.6% (180 d)

Sandy loam 10.6% (180 d) 10.6% (180 d)

MINOR (<10%) TRANSFORMATION PRODUCTS

IN-T3937; Met. A IUPAC: 6-(Dimethylamino)-3-(4-

hydroxycyclohexyl)-1-methyl-1,3,5-

triazine-2,4-dione

Formula: C12H20N4O3

MW: 268.3 g/mol

835.4100

Aerobic 

soil 

metabolism

48898001

Clay loam 0.5% (150 d) 0.5% (150 d)

Sandy loam 0.5% (150 d) 0.5% (150 d)

Loam 2.6% (150 d) 2.6% (150 d)

835.4200 48898002 Sandy clay 

loam 1.0% (60 d) ND (180 d)

54
Code Name/ 

Synonym Chemical Name Chemical Structure Study Type MRID Maximum %AR (day)

Final %AR 

(study 

length)

SMILES: 

Cn1c(nc(=O)n(c1=O)C2CCC(CC2)

O)N(C)C

O

N N

N

C H 3

C H 3

N

C H 3

O

H O Anaerobic 

soil 

metabolism

Clay loam 5.8% (180 d) 5.8% (180 d)

Sandy loam 3.4% (180 d) 3.4% (180 d)

Loam 2.5% (90 d) 1.3% (180 d)

A AR means “applied radioactivity”. MW means “molecular weight”. ND means “not detected”. NA means “not applicable”.

B Equivalent to 69.0 days of natural sunlight.

C Equivalent to 84.0 days of natural sunlight.

55
APPENDIX C. Excerpts from the Atrazine Endangered Species Assessment

EFED conducted in 2006 an analysis of the percentage of land that is covered by rights-of-ways 

in the Chesapeake Bay area. The analysis can be found in Section 3.2.1.2 of the atrazine 

endangered species assessment located at: 

http://www.epa.gov/oppfead1/endanger/litstatus/effects/atrazine/2007/determination-cheasp.pdf

(accessed 03/05/2015) (excerpts are reproduced below).

Right-of-way - Surrogate scenario for all rights of way uses including roads, fence line, power 

lines, and railroads.

Unlike most of EFED existing scenarios, the scenario is conceptually different in that it 

represents a linear surface that drains into an adjacent water body (drainage ditch). However, for 

this exercise, EFED assumes that while conceptually different, the scenario is for practicality 

purposes developed in a similar manner as a standard scenario that assumes a 10 hectare field 

draining into a 1 hectare static pond. For purposes of this assessment, EFED will assume 

equivalency between the concept of the adjacent drainage ditch (assumed to be perennial) and 

EFED’s standard pond. This conceptual model also assumes that the receiving drainage ditch is 

adjacent and connected to the standard pond. Development of this scenario should focus on 

curve numbers identified in Table 2-2a from the USDA Publication Urban Hydrology for Small 

Watersheds, Technical Release 55 (TR-55) dated June 1986 for impervious areas - paved open 

ditches (including right-of-way). EFED geospatial analysis indicates that the Barton Springs

Segment in Hays and Travis counties (see figure 1 in Attachment B) is dominated by Hydrologic 

Group C and D soil types and emphasis should be placed on identifying a representative soil 

series which will yield high end runoff and sediment loads. This scenario will need research into 

the appropriateness of this cover type and the percentage of impervious surface typically present 

in rights-of-ways where pesticides would be applied. 

Conceptually, this scenario assumes that the entire 10 hectare treated area will drain directly into 

either an adjacent waterbody or into a drainage ditch/channel which in turn flows directly into a 

receiving waterbody. 

For the rights-of-way scenario, it was assumed that rights-of-way consist of 50% impervious and 

50% pervious cover. In addition, it was assumed that no single watershed will be completely 

covered by a rights-of-way use. This assumption seems reasonable given that rights-of-way 

(roads, rail and utility lines) are typically long linear features that traverse a watershed. For the 

screening level assessment, it was assumed that no more than 10% of the watershed is covered in 

rights-of-way. 

An analysis was completed for the Chesapeake Bay endangered species assessment for atrazine 

to assess the amount of rights-of-way likely to be present in the action area. In this analysis, 

spatial data specific to the Chesapeake Bay watershed compiled by the Chesapeake Bay Program 

(CBP) for roads and railways and obtained on July 12, 2006 that may be found at;

(http://www.chesapeakebay.net/data/data_desc.cfm?DB=CBP_GIS)

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and internal Agency data for pipelines was obtained (spatial data for utility easements was 

unavailable). The road, rail, and pipeline land cover data were added to a GIS map of the action 

area (Figure 3-2) and a comparison of the density of the total network of potential use sites was 

made. Each land cover feature in the GIS map is presented as a line with no width associated. 

EFED applied a buffer using the Arc Toolbox within Arc Map in order to account for the 

potential width of the each linear feature. This assignment of area to each feature was done in 

order to compare the total area of each feature type (e.g. railways) with the total area of the 

action area.

For each feature, an assumption was made about the typical width of the feature (e.g. width of 

the road surface plus shoulders) plus the right of way area adjacent to the feature that could 

potentially be treated. In each case, a conservative assumption for the width of the feature zone 

plus the potentially treated area surrounding each was made. These assumed widths were based 

on professional judgment but skewed the total feature estimate to the largest feature in the class. 

For example, it was assumed that a national highway would yield that largest width which was 

then applied to all primary and secondary highways within the action area. This approach is also 

assumed to be conservative because it is unlikely that all features within the available data will 

be treated with atrazine because many of the areas are likely to be maintained using mechanical 

methods (e.g. mowing) or not treated at all. Using the CBP data different road types were able to 

be distinguished using the USGS’ classification scheme for digital line graph (DLG) data. In 

these data set distinguishing between highways (primary and secondary roads), state and county 

highways, state and local streets, unimproved trails, as well as minor features such as 

interchanges and traffic circles was possible. Different buffer widths were applied to each 

category of roadway depending upon the general use of the feature. For example, it is generally 

assumed that primary and secondary roads are wider than state/county roads, which in turn are 

generally wider than local streets. Based on this approach the following assumptions were made 

for the width of each feature.

• Primary/Secondary Roads – 200 feet

• Class 3 Roads (State/County) – 100 feet

• Class 4 Roads (Streets in built up areas) – 50 feet

• Unimproved Trails – 25 feet

• Rail – 200 feet

• Pipeline – 100 feet

• Utility Line – 200 feet

Given these assumptions the percentage of rights-of-way land cover types plus associated buffers 

for roads, railways, and pipelines within the action area for the Chesapeake Bay is 0.2% of the 

total area for rail, 3.6% for all primary and secondary roads (interstates, national and state 

highways), and 0.5% for pipelines. Including all classes of roads in the data set yields a road 

density of 9.5%. This is believed to be an over estimation because it includes a high number of 

roads in urban and suburban areas unlikely to be treated with pesticides for right of way control. 

Locally, it appears that higher percentages occur near more urbanized areas, however, it was 

assumed that less right of way application of pesticides occur in urbanized areas. Additional 

roads may be present in the action area not captured by the available spatial data and the analysis 

above does not include utilities for which no spatial data are available. Therefore, the10% 

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assumption of rights-of-way in the action area used in this assessment is an over-estimation but 

given the uncertainties is reasonable while still being conservative and protective.

Figure 3-2 Density of Road, Railways, and Pipelines as Surrogate for Right of Way Density 

in Chesapeake Bay Watershed (Action Area)

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