1

 UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

 WASHINGTON, D.C. 20460

OFFICE OF

PREVENTION, PESTICIDES

AND TOXIC SUBSTANCES

March 05, 2002

PC Code:109001

DP Barcode D280320

MEMORANDUM

Subject: EFED Risk Assessment for the Reregistration Eligibility Decision of Oxadiazon

To: Margaret Rice, Branch Chief

Veronique LaCapra, Chemical Review Manager

Reregistration Branch II

Special Review and Reregistration Division (7508C)

From: Miachel Rexrode, Ph.D., Aquatic Biologist

José Luis Meléndez, Chemist

Rodolfo Pisigan, Jr., Ph.D., Chemist

Environmental Risk Branch V

Environmental Fate and Effects Division (7507C)

Through: Mah Shamim, Ph.D., Chief

Jean Holmes, Risk Assessment Process Leader

Environmental Risk Branch V

Environmental Fate and Effects Division (7507C)

This memo provides a summary of the EFED Environmental Risk Assessment for the Oxadiazon

Reregistration Eligibility Document (RED). Oxadiazon is registered for use on terrestrial non-food crop

sites, including golf courses, landscape (turf and ornamentals), nurseries, and roadside areas. Based on

laboratory and field data, oxadiazon is a persistent, lipophilic compound that has a low mobility in most

soils, and may be susceptible to aqueous photolysis. Oxadiazon is also a light-dependent peroxidizing

herbicide (LDPH) that has the potential for the induction of phototoxicity. 

Our risk assessment shows that oxadiazon use has the potential for chronic exposure to aquatic

organisms that could result in reproductive effects to estuarine/marine fish and aquatic invertebrates. 

Other aquatic issues of concern include oxidiazon’s ability to bind and accumulate in the sediment thus

resulting in possible toxic exposure to aquatic organisms that live in or near the benthos. Since oxadiazon

exposure to light has the potential for free radical generation, phototoxicity may be another issue of toxic

concern for aquatic organisms.
2

Acute exposure of oxadiazon (emulsifiable concentrate and granular) to birds and mammals should

not present short term toxic risk. However, the potential for chronic risk to mammals and birds may result

in reproductive effects.

EFED also has a concern that acute exposure of oxadiazon to aquatic and terrestrial systems may

result in the potential for risk to endangered species that include mammals, birds, fish and aquatic

invertebrates. Since this compound is a herbicide, there can be an assumption of potential risk to nontarget 

plants (terrestrial, semi-aquatic and aquatic). The studies that have been submitted show that this

compound is potentially a toxic risk to aquatic vascular and nonvascular plants. However, this possible

risk to nontarget terrestrial plants cannot be fully assessed at this time due to the lack of acceptable data.

Outstanding Data Requirements

Environmental Fate:

The environmental fate data base is largely complete. EFED will not require additional studies at this

time.

Ecological Effects:

72-4 (a) Early-Life Stage Estuarine Fish 

72-4 (b) Life Cycle Estuarine Invertebrate

123-1 Seedling Emergence and Vegetative Vigor- using a liquid TEP to represent both granular

and liquid formulations (note in the case that liquid formulations are not supported for

reregistration, only seedling emergence testing would be required; vegetative vigor testing

is not required for granular formulations) 

123-2 Aquatic plant testing (TierII) is required because oxadiazon has outdoor nonresidential

terrestrial uses that may move off-site by runoff and/or aerial spray drift. The following

species should be tested at Tier II: Kirchneria subcapitata (Selenastrum capricornutum),

Skeletonema costatum, and Anabaena flos-aquae.

70-1 Acute and Chronic Sediment Toxicity Testing - Oxadiazon shows a high KOC, combined

with a high persistence exhibited in the aerobic soil metabolism, and the anaerobic aquatic

metabolism (>10 days). These fate properties indicate that there may be risk to benthicdwelling aquatic invertebrates, however the potential for risk cannot be assessed until data

have been submitted. The Chronic Sediment Toxicity Testing data requirement is triggered,

with Chironomus tentans and the Acute Chronic Sediment Toxicity Testing data

requirement is triggered, with both Hyalella azteca, and Chironomus tentans.

70-1 Phototoxicity studies on fathead minnow. A subchronic exposure duration would be

adequate for proof of concept. Behavioral observations should be conducted in addition to
3

mortality, growth, and morphology. All studies should be conducted under defined light

conditions.

Endangered Species

The Agency has developed the Endangered Species Protection Program to identify pesticides

whose use may cause adverse impacts on endangered and threatened species, and to implement mitigation

measures that address these impacts. The Endangered Species Act requires federal agencies to ensure that

their actions are not likely to jeopardize listed species or adversely modify designated critical habitat. To

analyze the potential of registered pesticide uses to affect any particular species, EPA puts basic toxicity

and exposure data developed for REDs into context for individual listed species and their locations by

evaluating important ecological parameters, pesticide use information, the geographic relationship

between specific pesticide uses and species locations, and biological requirements and behavioral aspects

of the particular species. This analysis will take into consideration any regulatory changes recommended

in this RED that are being implemented at this time. A determination that there is a likelihood of potential

impact to a listed species may result in limitations on use of the pesticide, other measures to mitigate any

potential impact, or consultations with the Fish and Wildlife Service and/or the National Marine Fisheries

Service as necessary. 

The Endangered Species Protection Program as described in a Federal Register notice (54 FR

27984-28008, July 3, 1989) is currently being implemented on an interim basis. As part of the interim

program, the Agency has developed County Specific Pamphlets that articulate many of the specific

measures outlined in the Biological Opinions issued to date. The Pamphlets are available for voluntary

use by pesticide applicators on EPA’s website at www.epa.gov/espp. A final Endangered Species

Protection Program, which may be altered from the interim program, is scheduled to be proposed for

public comment in the Federal Register before the end of 2001.

Endocrine Disruption

EPA is required under the FFDCA, as amended by FQPA, to develop a screening program to

determine whether certain substances (including all pesticide active and other ingredients) “may have an

effect in humans that is similar to an effect produced by a naturally-occurring estrogen, or other such

endocrine effects as the Administrator may designate.” Following the recommendations of its Endocrine

Disruptor Screening and Testing Advisory Committee (EDSTAC), EPA determined that there was

scientific basis for including, as part of the program, the androgen and thyroid hormone systems, in

addition to the estrogen hormone system. EPA also adopted EDSTAC’s recommendation that the Program

include evaluations of potential effects in wildlife. For pesticide chemicals, EPA will use FIFRA and, to

the extent that effects in wildlife may help determine whether a substance may have an effect in humans,

FFDCA authority to require the wildlife evaluations. As the science develops and resources allow, 

screening of additional hormone systems may be added to the Endocrine Disruptor Screening Program

(EDSP). 
4

When the appropriate screening and or testing protocols being considered under the Agency’s

Endocrine Disruptor Screening Program have been developed, oxadiazon, may be subjected to additional

screening and or testing to better characterize effects related to endocrine disruption. Issues that have

raised this concern include fish reproduction effects (larval and embryo survival, egg hatchability) and

invertebrate reproduction effects (reduced neonate production) also suggest endocrine disruption.

Uncertainties

Environmental Fate and Exposure:

There is some uncertainty in using the FIRST and GENEEC2 models respectively for drinking

water and aquatic assessment. These two models are typically used for Tier I screening purposes for

pesticides applied to soils. In turf environments, the fate characteristics and transport behavior of

oxadiazon may be different than those in soils. Whether the difference is significant is not known at the

present. Hence, it is difficult to estimate the magnitude of uncertainty. The turf scenario for

PRZM/EMAMS Tier II modeling is not available at this time so turf EECs can not be further refined..

Ecological Effects:

Since oxadiazon is a herbicide, there is a potential for risk to nontarget plants. Lack of adequate

data represents an uncertainty with regards to the risk, which may be further clarified through the

submission of data.

In the absence of data on chronic effects of oxadiazon to estuarine aquatic organisms, chronic

testing results from freshwater fish and invertebrates species were extrapolated, representing an

uncertainty which may be satisfied through the submission of appropriate data.

The high persistence and lipophilicity of this chemical and its likelihood to accumulate in the

sediment suggest that there may be risk to benthic and epibenthic aquatic life (fish and aquatic

invertebrates). However the potential for risk cannot be further refined until additional data (sediment

toxicity tests) have been submitted.

Enhanced toxicity of oxadiazon to aquatic organisms after light exposure is an uncertainty. The

inhibition of protoporphyrinogen oxidase, the rapid accumulation of protoporphyrin IX with the resulting

generation of singlet oxygen (free radicals)and eventual cell membrane destruction suggest that exposure

to this compound may increase toxicity to aquatic organisms

Label Recommendations:

EFED recommends that the labels for all oxadiazon products carry the following statements:
5

Environmental Hazards

i. Manufacturing Use Product:

This pesticide is toxic to fish and aquatic invertebrates. Do not discharge effluent containing this

product into lakes, streams, ponds, estuaries, oceans, or other waters unless in accordance with the

requirements of a National Pollutant Discharge Elimination System (NPDES) permit and the permitting

authority has been notified in writing prior to discharge. For guidance, contact your State Water Board or

Regional Office of the EPA. 

ii. End-Use Product:

This pesticide is toxic to fish and aquatic invertebrates. 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 washwaters or rinsate. Do not apply when weather conditions favor

drift from treated areas. Runoff and drift from treated areas may be hazardous to aquatic organisms in

neighboring areas. Do not allow this product to drift.
6

SUPPLEMENT

Possible Risk Mitigation Measures:

To reduce risk to plants and aquatic organisms, possible risk mitigation measures may include, but are not

limited to:

C The addition of a well maintained buffer zone can also mitigate the risk. It is known that buffer

zones can decrease the amount of spray drift reaching bodies of water.

C The current label suggests that to improve the efficacy, prior to application, the turf should be

mowed and after application it should be irrigated if rain is not expected shortly. Making this

suggestion compulsory would assure that most of the chemical reaches the soil surface, where it is

less prone to runoff.
7

N

N

O

Cl

Cl

CH O O

C

CH3

CH3

CH3

H C3

H C3

Environmental Fate and Ecological Risk Assessment

for the Reregistration Eligibility Decision

Oxadiazon; Ronstar®

2-tert-Butyl-4-(2,4-dichloro-5-isopropoxyphenyl)-delta2-1,3,4-oxadiazoline-5-one and 3-[2,4-

Dichloro-5-(1-methylethoxy)phenyl]-5-(1,1-dimethyl-ethyl)-1,3,4-oxadiazol-2(3H)-one

Shaughnessy Number: 109001

CAS Number: 19666-30-9

Prepared by:

Miachel Rexrode, Ph. D.

José L. Meléndez

Environmental Risk Branch V

Reviewed by: Environmental Fate and Effects Division

Rodolfo Pisigan Jr., Ph.D.

Mah T. Shamim, Ph.D.
8

TABLE OF CONTENTS

CHAPTER 1: ENVIRONMENTAL RISK CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

CHAPTER 2: INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

CHAPTER 3: INTEGRATED ENVIRONMENTAL RISK CHARACTERIZATION . . . . . . . . . . . . . 15

CHAPTER 4: ENVIRONMENTAL FATE AND TRANSPORT ASSESSMENT . . . . . . . . . . . . . . . . 19

CHAPTER 5: DRINKING WATER ASSESSMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

CHAPTER 6: AQUATIC EXPOSURE AND RISK ASSESSMENT . . . . . . . . . . . . . . . . . . . . . . . . . . 21

CHAPTER 7: TERRESTRIAL EXPOSURE AND RISK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

CONCLUSIONS......................................................................................................................................... 32

APPENDIX A: REFERENCES CITED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

APPENDIX B: FATE SUMMARIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

APPENDIX C: ECOLOGICAL TOXICITY DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

APPENDIX D: FATE TERRESTRIAL MODEL RUNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

APPENDIX E: DRINKING WATER CONCENTRATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

APPENDIX F: EXPOSURE AND RISK CHARACTERIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

APPENDIX G: ENVIRONMENTAL FATE AND ECOLOGICAL EFFECTS DATA 

 REQUIREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

APPENDIX H: QUALITATIVE USE ASSESSMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

APPENDIX I: GENEEC 2.0 INPUT PARAMETERS, RESULTS, AND OUTPUTS . . . . . . . . . . . . . . 75

APPENDIX J: REQUEST for PHOTOTOXICITY STUDY PROTOCOL for LIGHT-DEPENDENT 

 PEROXIDIZING

HERBICIDES...........................................................................................80

ATTACHMENT I........................................................................................................................................83
9

CHAPTER 1: ENVIRONMENTAL RISK CONCLUSIONS

a. Registered Uses

Oxadiazon is a selective pre-emergent and early post-emergent herbicide used to control grassy

and broadleaf weeds in turf and ornamentals. Application rates range from 2 to 4 lbs ai/A, with usually

only one application made per season. Most of the products are in granular form. The herbicide’s

primary use is on golf courses, turf farms and ornamental plantings. The registrant, Aventis, is supporting

a maximum yearly use of eight pounds of active ingredient per acre. Aerial applications are not being

supported (SRRD communication).

. 

Table 1. Oxadiazon Registered Use on Turf (Golf Courses), Nursery and Roadsides.

Usage Maximum

Application Rate

 (lbs ai/A)

Number of

Applications

Minimum

Application Interval

(days)

Maximum

Application Rate per

Season 

(lbs ai/A)

Turf

 (Ground Spray)

4.0 2 182 8.0 

4.0 1 NA 4.0

3.0 1 NA 3.0

2.0 1 NA 2.0

Turf

(Ground Spray)

Split Application

1.0 8 42 8.0

1.3 6 56 8.0

Turf

(Granular) 4.0 2 182 8.0

4.0 1 NA 4.0

3.0 1 NA 3.0

2.0 1 NA 2.0

b. Major Risk Concerns

Our assessment shows that oxadiazon exposure in the aquatic environment can present significant

chronic risk to freshwater fish and aquatic invertebrates. The chronic Level of Concern (LOC) was

exceeded by up to 132-fold for fish and 37-fold for aquatic invertebrates. Although this Tier I risk

assessment suggests that acute exposure of oxadiazon to aquatic systems should result in relatively lower

short term risk to non endangered fish and aquatic invertebrates there is uncertainty regarding possible

phototoxicity. Since oxadiazon is an LDPH compound, enhanced toxicity through exposure to high levels

of solar radiation is a possible concern that could impact aquatic organisms that inhabit small, shallow

water bodies. Oxadiazon is also a lipophilic compound that has the capacity to strongly bind to

particulates and organic carbon. This binding can result in accumulation in the sediment raising concerns

for toxic risk to benthic and epibenthic aquatic organisms (aquatic insects, amphipods, crustaceans,

mollusks, bivalves, etc). Since sediment act as a reservoir for lipophilic persistent compounds, sediment
10

bound oxadiazon presents a high risk potential for aquatic life because of direct contact with various

organisms through respiration, ingestion, dermal contact, or indirectly through alterations of the food

chain. The herbicidal properties of this compound also strongly suggest that there is a potential for toxic

risk to aquatic plants (monocots and dicots) which may result in an indirect impact on aquatic systems

through habitat alteration. Endangered species concerns have also been triggered for fish, aquatic

invertebrates and plants. 

Terrestrial exposure of this compound to mammals and birds can result in potential chronic risk

while the acute risk to terrestrial organisms (birds, mammals, and honey bees) from the registered use of

oxadiazon appears low. However, information on the herbicidal mode of action of oxadiazon strongly

suggests that there is a potential for acute risk to nontarget aquatic and terrestrial plants. The limited plant

data for oxadiazon shows that this compound can present a toxic risk to nontarget plants (EC25 values

reported in the seedling emergence/vegetative vigor tests were as low as about a tenth of a pound per

acre). Although there does not appear to be an acute risk to endangered birds and mammals there may be

chronic concerns as reflected in the two-fold LOC exceedences for non endangered terrestrial animals.

Therefore, our assessment suggests that endangered terrestrial species (birds, mammals, and terrestrial

plants) may be at risk.

c. Oxadiazon Incident History

There are no confirmed incidents associated with the use of oxadiazon in the Environmental Fate

and Effects Division EIIS Database. However, this data base is compiled through voluntary submissions

that may only capture a small fraction of actual incidents.

d. Likelihood of Water Contamination

The potential impact to water quality from the use of oxadiazon on turf is essentially due to the

parent (as opposed to possible degradates). Oxadiazon appears to be very persistent under most

environmental conditions making the chemical available for surface runoff. Moreover, the remaining

important factor which affects the impact of oxadiazon on water quality is its mobility. A soil column

leaching study, and supplemental batch equilibrium studies indicated that oxadiazon has low mobility in

the various soils tested. Ordinarily this would mean that the chemical would remain soil bound and would

be transported to a water body on eroded soil. Turf scenarios, however, offer different challenges than

other conventional crops. The turf itself offers a vegetative interception layer (including thatch) that

prevents rapid deposition of the oxadiazon on the surface of the soil. Both liquid and granular

formulations labels of oxadiazon specify that the chemical’s effectiveness is improved if it is wetted in

after application. Furthermore, both labels recommend mowing the grass prior to application. Oxadiazon

is expected to bind to soil particles, but turf scenarios offer vegetation interception.

The models used for the determination of the water exposures were FIRST, GENEEC 2.0 for

surface waters, and SCIGROW for ground waters. The models are screening models designed to provide

upper-bound estimates of the concentrations that might be found due to the use of oxadiazon. For

drinking water worst case scenario (4 lb a.i./A applied at 6-months interval) was used. Further

refinements of our computer models were not possible at this time. The EFED is currently developing a
11

turf scenario, which is expected to be ready in the near future. Surface water monitoring data for

oxadiazon is very limited and cannot be used to represent possible concentrations of oxadiazon in surface

waters. The chemical is not included in the NAWQA monitoring studies. The STORET database

contained only two samples taken from the same location within an interval of only four days. The

estimated recommended acute, and long term drinking water concentrations are detailed in Chapter 6.

Oxadiazon has a high affinity to soils and sediments KOC.2357, combined with the high

persistence exhibited in the aerobic soil metabolism (>>1year), as well as the anaerobic aquatic

metabolism (.1 year studies). It appeared that oxadiazon would be a persistent chemical in sediment

environments. 

Although oxadiazon exhibits high affinity to soils and a relatively high bioconcentration factor

(Kow = 63100; BCF’s of 368X, 2239X, and 1111X for muscle, viscera, and whole fish, respectively), the

rate of depuration was relatively rapid (half-life of about one day). 

e. Recommended Drinking Water Concentrations for HED

As per HED’s request, the drinking water assessment for oxadiazon is as follows: the peak

untreated surface water concentration is 246 ppb, and the annual average untreated water concentration is

100 ppb . These values represent upper-bound estimates of the concentrations of oxadiazon that might be

found in surface water due to the use of oxadiazon on turf at the maximum application rate of 8.0 lb

a.i./A/season. The recommended oxadiazon ground water concentration is 0.6 ppb.

Recently EFED validated a new turf scenario that can be used to model golf courses. 

Additionally, new PCA’s have been developed for golf courses (tees, greens and fairways). EFED will

model Tier II drinking water concentrations accordingly. These calculations will be performed during the

revisions of the 60-days comments period.

f. Monitoring and Modeling

As shown in Table 3, the groundwater concentration estimated from SCIGROW is 0.6 ppb which

is about two orders of magnitude lower than those of surface water. This concentration may be used for

both acute and chronic values. The low concentration is consistent with both laboratory and field studies

that indicate the low mobility of oxadiazon, and subsequently, its reduced potential to reach

groundwater.
12

CHAPTER 2: INTRODUCTION 

a. Mode of Action

Oxadiazon is a selective pre-emergent and early post-emergent herbicide used to control grassy

weeds (e.g., crabgrass and goosegrass) and broadleaf weeds. The primary mode of action of oxadiazon is

inhibition of protoporphyrinogen oxidase (Protex), a critical enzyme in the biosynthesis of chlorophyll

and heme (Matringe et al., 1989). Consistent with protoporphyrinogen oxidase- inhibiting herbicides,

tissue exposed in darkness accumulate protoporphyrin IX, which can lead to a photodynamic loss of cell

membrane integrity (free radical development) upon exposure to light.

b. Use Characterization

The formulation types include granular (39 products; predominant formulation), wettable powder

(2 products), soluble concentrate (1 product) and emulsifiable concentrate (1 product). Aventis is the sole

technical registrant. Oxadiazon is registered for use on terrestrial non-food crop sites, including golf

courses; landscape (turf and ornamentals); nursery; and roadside.

An annual estimate of oxadiazon’s total usage is 249,000 pounds of active ingredient on 52,000

acres. Most of the use is on golf courses, which accounts for about 65% of all use. Application rates

range from two to four pounds active ingredient per acre. According to SRRD, Aventis is supporting a

maximum application of 4.0 lb ai/A per six month period, equivalent to 8.0 lbs ai/year and aerial

applications are not being supported. Since efficacy (pre emergent control)is based on oxadiazon reaching

and remaining in the soil, product labels may specify to mow, if necessary, before application, and to

irrigate, if rain is not expected shortly. Oxadiazon may also be used for early post-emergent control, but

this is to a much lesser extent. (usage information was obtained from BEAD’s Qualitative Use

Assessment, Appendix K).

Oxadiazon is classified as an oxadiazole herbicide. The chemical name is: 5-tert-Butyl-3-(2,4-

dichloro-5-isopropoxyphenyl)-1,3,4-oxadiazol-2(3H)-one. Other chemical names are: (IUPAC). 2-tertButyl-4-(2,4-dichloro-5-isopropoxyphenyl)-delta2

-1,3,4-oxadiazoline-5-one and 3-[2,4-Dichloro-5-(1-

methylethoxy)phenyl]-5-(1,1-dimethyl-ethyl)-1,3,4-oxadiazol-2(3H)-one. Trade names include Ronstar,

RP-17623, and G 315.

c. Approach to Risk Assessment

In order to conduct an ecological risk assessments on this compound, EFED used dosage rate

information obtained from SRRD and BEAD. The evaluation of the potential risk to aquatic and terrestrial

organisms from the use of oxadiazon, was assessed through the calculation of risk quotients (RQs) that

were derived from the ratio of estimated environmental concentrations (EECs) to ecotoxicity values (see

Appendix F). EECs were based on the maximum and typical application rate of oxadiazon to turf. These

RQs are then compared to the Levels of Concern (LOC) (Appendix F) criteria used by EFED for

determining potential risk to nontarget organisms and the subsequent need for possible regulatory action. 
1

EFED examined two DER’s that provide data (2-7 day half-life) on the decay of transferable residues of 

oxadiazon from turf surfaces to a cotton cloth, EFED chose not to use these studies because one study was

conducted on a granular formulation and the other study presented a quantitative concern.

13

Terrestrial exposure was evaluated using EECs generated from ELL-FATE spreadsheet-based

model that calculates the decay of a chemical applied to foliar surfaces for single and multiple

applications. The model assumes initial concentrations on plant surfaces based on Kenaga predicted

maximum residues as modified by Fletcher et al. (1994) and assumes 1st order dissipation. Kenaga

estimates and an explanation of the model with sample output are presented in Appendix F. In the

absence of foliar dissipation half-life data for oxadiazon a 35-day half-life was used. The selection of this

half-life was based on the upper limit of pesticide, foliar dissipation half-lives provided in the half-life

listing of Willis and McDowell, 1987. EFED uses this value as a default equivalent when the foliar

dissipation for a particular pesticide is unknown or in question1

. The terrestrial and aquatic risk

assessment was also based on the three maximum application rates of 4.0, 3.0, and 2.0 lbs ai/A at 2

applications each and a 4.0 lbs ai/A for granular, at 2 applications. Additional exposure scenarios for split

application (1.0 and 1.3 lbs ai/A, at 6 and 8 week intervals, respectively) were conducted for terrestrial

exposure.Aquatic exposure was evaluated using EECs generated from the Tier I GENEEC2 model. 
14

Aquatic and terrestrial risk assessments were conducted by using worst case ecotoxicity endpoints (i.e.,

LD50 and LC50 values, NOAEC values). The toxicity endpoints chosen for use in the ecological risk

assessment are summarized below.

Table 2. Selection of Toxicological Endpoints Used to Determine Risk Quotients (RQs)

Type Of Toxicity Organism Species Toxicological Endpoint

Oral Acute

 Bird

Mallard 1040 mg/kg

Dietary Bobwhite/Mallard >5000 ppm

Chronic Bobwhite 500 ppm 1

Oral Acute

 Mammal

Rat >5000 mg/kg

Chronic Rat 200 ppm 2

 Acute

 Freshwater Fish

Rainbow trout/Bluegill 0.88 ppm

Chronic Rainbow trout 0.88 ppb 3

Acute

 Freshwater

 Invertebrates

Daphnid 2.18 ppm

Chronic Daphnid 0.03 ppm 

Acute

 Estuarine Fish

Sheepshead Minnow 1.5 ppm

Chronic Sheepshead Minnow 0.0015 ppm 4

Acute

 Estuarine Invertebrates

Mysid 0.27 ppm

Chronic Mysid 0.0037ppm4

Acute Aquatic Plants

 (vascular)

 (Nonvascular)

duckweed 

marine diatom

EC50 =41 ppb;

NOAEC = <8 ppb

EC50 = 5.2 ppb

1

 No effects on any reproductive parameter or viability of of F1 offspring at the highest dose tested, 1000 ppm; however due to

 excessive mortality (33%) of adult female birds in that dose level, a NOAEC for chronic effects was set at 500 ppm. 2

 LOAEL of >38 mg/kg/ day for inactive mammary tissue and fetal/pup death observed in the one year range-finding test of a rat

reproduction study. NOAEC >200 ppm. 

3

 Rainbow trout was more sensitive than the fathead minnow (fathead minnow NOAEC= 33 ppb). 4 Extrapolation from acute/chronic ratio.
15

CHAPTER 3: INTEGRATED ENVIRONMENTAL RISK CHARACTERIZATION

Oxadiazon is a persistent, lipophilic compound that has low mobility in most soils (not expected to

move to ground water), and may be susceptible to aqueous photolysis. Oxadiazon is also a light-dependent

peroxidizing herbicide (LDPH) that has the potential for the induction of phototoxicity (exposure to light

results in the development of free radicals that can destroy cell membranes). Our risk assessment shows

that chronic exposure of this compound to aquatic organisms (estuarine/marine fish and aquatic

invertebrates) can result in significant reproductive effects (EFED’s runoff and drift exposure scenarios).

Aquatic risk is further compounded by oxadiazon’s ability to bind and accumulate in the sediment. As a

contrast, terrestrial concerns for this compound are mixed. The potential for chronic risk to mammals

appears very high and could result in significant reproductive effects. However, chronic risk to birds

appeared to be a relatively lower concern although values still exceed EFED’s level of concern (LOC).

Our analysis also noted that acute exposure of oxadiazon (emulsifiable concentrate and granular) to birds

and mammals should not present significant short term toxic risk. EFED also has a concern that exposure

of oxadiazon to aquatic and terrestrial systems may result in a potential risk to endangered species that can

include mammals, birds, fish and aquatic invertebrates. Since this compound is a herbicide, there is the

potential for impact to nontarget plants (terrestrial, semi-aquatic and aquatic). However, this possible risk

to nontarget plants cannot be fully assessed at this time due to the lack of acceptable data.

The focus of this risk assessment is based on toxicity and exposure values (risk quotients or RQs

as the ratio of exposure/toxicity), the disposition (fate) of oxadiazon in the environment, and its mode of

action as a phototoxic compound. In order to evaluate the potential for risk to non target organisms, our

assessment is divided into aquatic and terrestrial exposure scenarios. The aquatic component was

evaluated through GENEEC2 pond scenario while terrestrial impact was assessed through the ELL-FATE

model. Since oxadiazon is primarly used as a herbicide on turf, especially golf courses, EFED has

evaluated the proximity of these areas to estuarine/marine environments, and the ecological significance

of application timing. 

Oxadiazon is a stable and persistent compound. However, direct aqueous photolysis half-life of

about 3 days suggests that in clear and shallow surface water bodies where sunlight penetration can be

significant, photolytic degradation of oxadiazon is possible. However, this photolytic effect may also

substantially diminish in turbid and deeper water bodies. Soil photolysis and hydrolysis under acidic and

basic conditions do not appear to be an important dissipation mechanism. Microbial metabolism in soil

and aquatic environments under either aerobic and anaerobic condition is not expected to cause any

significant transformation of oxadiazon. Studies on equilibrium sorption and aged/unaged oxadiazon

indicate that the pesticide has low environmental mobility (Kd's ranged from 8.17 to 22.83; Koc’s ranged

from 1409 to 3268). Thus, oxadiazon can be transported on erodible soil particles via runoff events to

nearby surface water bodies. Leaching from surficial soils to groundwater is expected to be low or

negligible, unless the soil is very porous or has some cracks that favor preferential flow. Oxadiazon

exhibited slow dissipation in two field terrestrial studies conducted in California and North Carolina.

Our review has found that golf courses can represent about 2,300,000 acres in the USA. About half

of this acreage is located in counties that are considered coastal and close to estuarine/marine

environments and tributaries. Because of the proximity to these aquatic habitats to golf courses, EFED 
16

has a concern for any persistent compound that has the potential for runoff and toxicity to aquatic systems

that include estuaries. Many of these aquatic areas have significant fisheries that can account for over 65%

of the commercial catches for the USA. (e.g. Chesapeake Bay, Long Island Sound, The Gulf of Mexico,

San Diego Bay, San Francisco Bay, Puget Sound, etc.). Impact to this resource could effect not only the

ecological value but the livelihood of fishing communities and markets at a local and national level. 

Since oxadiazon is stable to hydrolysis and persistent in the environment, the results from our Tier

I (GENEEC2) pond scenario model suggest that chronic exposure of oxadiazon can result in significant

long term risk to freshwater and estuarine/marine fish and aquatic invertebrates. Our screening level

assessment shows that the RQ values that were generated exceeded the LOC by significant amounts of 4 -

132 fold (application rates of 2.0 - 4.0 lbs ai/A EC and granular formulation). The issue of chronic toxicity

is compounded by the lipophilic nature of oxadiazon. Since this stable compound can be absorbed to

particulate and organic carbon, oxadiazon residues can accumulate in sediments and increase the potential

for chronic risk to benthic and epibenthic organisms (aquatic organisms that live in or on the sediment).

Acting as a repository for lipophilic compounds, sediments can impact aquatic organisms through

respiration, ingestion, dermal contact, and/or indirect impact through alterations of the food chain. This

can present a significant risk to aquatic organisms because about 80% of all aquatic life in estuaries is in

contact with the benthos. Therefore, in order to better understand this potential risk, EFED is requiring

appropriate sediment toxicity testing (acute and chronic) on this compound. Another issue of concern is

the uncertainty regarding the degree of phototoxicity of this compound to aquatic organisms. Since

oxadiazon is a light-dependent peroxidizing herbicide (LDPH), enhanced toxicity through exposure to

high levels of solar radiation may increase toxic risk to aquatic organisms that inhabit small, shallow

water bodies (toxicity is increased through the production of free radicals which actively destroy cell

membranes). This can be very critical to several species of aquatic organisms (fish, crabs, etc) whose early

life stages are dependent upon these relatively shallow areas for their development. The herbicidal

properties of oxadiazon also suggest the potential for acute toxicity to aquatic plants and the possibility of

aquatic habitats alterations. This can potentiate an indirect effect to aquatic populations through a decrease

in plant cover. In addition to toxic risk to non target aquatic organisms, oxadiazon may also impact

endangered species (fish and invertebrates).

The potential for birds and mammals to be exposed to pesticides through a turf use has been

documented (e.g. chlorpyrifos, lindane). The application of oxadiazon in the spring as noted from the

label, can coincide with several avian and mammalian reproductive cycles, as well as spring migrations

(avian). In order to evaluate the potential for risk to terrestrial organisms, EFED has conducted a Tier I 

assessment by using the ELL-FATE model. In order to evaluate possible toxic risk to terrestrial

organisms, three application rates (4.0, 3.0, and 2.0 lbs ai/A, at 2 applications/6 months) and two split

applications (1.0 lbs ai/A applied 4 times/6 month and 1.3 lbs ai/A applied 3 times/6 month) were run. Our

objective was to find not only the highest rate that may cause toxic risk, but the rate that might result in

lower risk. Our assessment noted that acute risk to birds and mammals was minimal and should not

present any short term toxic concern to these organisms. However, all application scenarios showed that

chronic exposure could result in significant risk to mammalian herbivores and insectivores (15g, 35g, and

1000g) with RQ exceedences of 1.5 - 9.9 fold. In contrast to mammalian chronic risk, our assessment also

noted that chronic exposure to birds could result in relatively lower RQ values that showed exceedence of

about 1 to 2 fold the LOC. This could be interpreted as potentially low toxic risk (chronic) to birds that
17

feed on plants and grass (e.g. ducks, geese). A reduction in chronic risk to birds was noted with the split

application scenarios (RQ = < 1), but chronic risk to mammals was still very high even with this scenario.

Exposure from the granular formulation was evaluated because birds may be exposed to granular

pesticides through ingestion when foraging for food or grit. RQ values were calculated for three weight

classes of birds (1000g waterfowl, 180g upland game bird, and 20g songbird). All scenarios for the

granular resulted in no acute risk to birds (EFED does not conduct a chronic assessment from granular

exposure). 

The potential for chronic risk (high for mammals but relatively low for birds) that has been noted

for terrestrial organisms suggests that oxadiazon may present a risk to both avian and mammalian

endangered species (RQ > 0.1), even though the acute LOC values were not exceeded. Although, risk to

terrestrial plants could not be conducted at this time (lack of data), oxadiaxon’s herbicidal mode of action

suggests that there is a potential for risk to nontarget terrestrial plants, as well as endangered plants. Since

oxadiazon is practically non-toxic to the honey bee, minimal risk to these organisms is anticipated. 

The Agency is currently engaged in a Proactive Conservation Review with FWS and the National

Marine Fisheries Service under section 7(a)(1) of the Endangered Species Act. The objective of this

review is to clarify and develop consistent processes for endangered species risk assessments and

consultations. Subsequent to the completion of this process, the Agency will reassess the potential effects

of oxadiazon use to federally listed threatened and endangered species. At that time the Agency will also

consider any regulatory changes recommended in the RED that are being implemented. Until such time

as this analysis is completed, the overall environmental effects mitigation strategy articulated in this

document and any County Specific Pamphlets which address oxadiazon, will serve as interim protection

measures to reduce the likelihood that endangered and threatened species may be exposed to oxadiazon at

levels of concern. The endangered species LOCs for liquid and granular formulations of oxadiazon are

exceeded for chronic risks to birds and mammals and acute/chronic risk to freshwater and estuarine fish

and invertebrates and aquatic vascular plants. Although the terrestrial plant data are outstanding, it is

assumed that endangered terrestrial plants are at risk since oxadiazon is an herbicide. Although the

endangered species LOC for estuarine invertebrates has been exceeded, there are no listed species in this

group.

EPA is required under the FFDCA, as amended by FQPA, to develop a screening program to

determine whether certain substances (including all pesticide active and other ingredients) “may have an

effect in humans that is similar to an effect produced by a naturally-occurring estrogen, or other such

endocrine effects as the Administrator may designate.” Following the recommendations of its Endocrine

Disruptor Screening and Testing Advisory Committee (EDSTAC), EPA determined that there was

scientific basis for including, as part of the program, the androgen- and thyroid hormone systems, in

addition to the estrogen hormone system. EPA also adopted EDSTAC’s recommendation that the Program

include evaluations of potential effects in wildlife. For pesticide chemicals, EPA will use FIFRA and, to

the extent that effects in wildlife may help determine whether a substance may have an effect in humans,

FFDCA authority to require the wildlife evaluations. As the science develops and resources allow,

screening of additional hormone systems may be added to the Endocrine Disruptor Screening Program

(EDSP).
18

When the appropriate screening and or testing protocols being considered under the Agency’s

Endocrine Disruptor Screening Program have been developed, oxadiazon, may be subjected to additional

screening and or testing to better characterize effects related to endocrine disruption. Issues that have

raised this concern include fish reproduction effects (larval and embryo survival, egg hatchability) and

invertebrate reproduction effects (reduced neonate production). 
19

CHAPTER 4: ENVIRONMENTAL FATE AND TRANSPORT ASSESSMENT

Basic Physicochemical Parameters

The important properties of oxadiazon are summarized below. Oxadiazon is a high-molecularweight compound with fairly low solubility in water and high solubility in organic solvents. It has a low

vapor pressure and Henry’s Law Constant (<< 1 x 10-3 atm-m-3/mol) suggesting that volatilization from

soil and surface water environments is not important. Its high Kow value tends to indicate that

bioconcentration in aquatic organisms such as fish is possible. Nevertheless, the high bioconcentration

factors observed in studies using bluegill sunfish can be offset by fast depuration rate.

Molecular formula: C15H18Cl2N2O3.

 Molecular weight: 345.2.

 Physical state: Colorless crystals.

 Vapor pressure (20°C):1.00x10-6 mm Hg

 Henry’s Constant: 4.51x10-7 Atm•m3/mol

 Solubility (20°C): 1 ppm water (25°C)

600 g/L acetone, acetophenone, anisole

1 kg/L benzene, chloroform, toluene

100 g/L ethanol, methanol

 Kow: 63,100

 log10 Kow: 4.8

Fate and Transport Processes - Summary

Based on fate studies reviewed, oxadiazon would be stable and persistent under typical natural

environment. However, direct aqueous photolysis half-life of about 3 days (summer sunlight conditions in

Florida) suggests that in clear and shallow surface water bodies where sunlight penetration can be

significant, photolytic degradation of oxadiazon is possible. The photolytic effect though may

substantially diminish in turbid and deeper water bodies. Soil photolysis and hydrolysis under acidic and

basic conditions do not appear to be an important dissipation mechanism. Microbial metabolism in soil

and aquatic environments under either aerobic and anaerobic condition is not expected to cause any

significant transformation of oxadiazon. A number of degradates have been reported from the different

chemical and biological fate studies. The nomenclature of these degradates are summarized in Appendix 3

(move nomenclauture on page to 20 to this Appendix 3).

Studies on equilibrium sorption and aged/unaged oxadiazon indicate that the pesticide has low

environmental mobility (Kd's ranged from 8.17 to 22.83; Koc’s ranged from 1409 to 3268). Thus,

oxadiazon can be transported as sorbed species to erodible soil particles via surface runoff to nearby

surface water bodies. Leaching from surficial soils to groundwater is expected to be low or negligible,

unless the soil is very porous or has some cracks that favor preferential flow. Oxadiazon exhibited slow

dissipation in two field terrestrial studies conducted in California and North Carolina. Details of all the

fate and transport studies are discussed in Appendix B.
20

CHAPTER 5: DRINKING WATER ASSESSMENT

a. Estimated Environmental Concentrations and Drinking Water Concentration Estimates

TierI screening models, FIRST and SCIGROW, were used to determine estimated environmental

concentrations (EECs) of oxadiazon in surface water and groundwater associated with the ground spray

application of 4.0 lbs a.i./A (applied two times a year) in turf. FIRST estimates surface water

concentrations resulting from runoff of applied pesticides from a treated area to an adjacent index water

reservoir in which the percent or fraction of cropped area (0.87) is taken into account. SCIGROW predicts

groundwater concentrations after leaching of pesticides from the surficial soils and/or subsurface horizons

to the aquifer. The screening concentrations derived from the two models are used in the evaluation of

human exposure to contaminated drinking water. Details about the two models, including the input

parameters and computer output printouts for turf scenario, are presented in Appendix E (Drinking Water

Memo).

Surface Water

The results of FIRST modeling for the acute and chronic surface water EECs are summarized in

the table below. The acute (246 ppb) and chronic (100 ppb) values represent the peak and annual average

concentrations predicted by the model. These values generally represent upper bound estimates. The

values are relatively higher than the two similar oxadiazon detections (0.05 ug/L) in Larue, KY reported

in the 1997 surface water monitoring data of the STORET system. Therefore, based on the data available,

EFED conservatively recommends to use the model-predicted values for surface derived drinking water

concentrations.

Table 3. Estimated Tier I concentrations of oxadiazon in drinking water

 Chemical

 

 Surface Water (ug/L) Groundwater (ug/L)

 Acute Chronic Acute and Chronic

 Oxadiazon 246 100 0.6

Groundwater

As shown in Table 3, the groundwater concentration estimated from SCIGROW is 0.6 ppb which

is about two orders of magnitude lower than those of surface water. This concentration may be used for

both acute and chronic values. The low concentration is consistent with both laboratory and field studies

that indicate the low mobility of oxadiazon, and subsequently, its reduced potential to reach groundwater.
21

CHAPTER 6: AQUATIC EXPOSURE AND RISK ASSESSMENT

a. Aquatic (Acute/Chronic Hazard Summary)

Oxadiazon is considered to be moderately toxic on an acute basis to freshwater fish ( LC50 = 0.88-

1.2 ppm) and estuarine/marine fish ( LC50 = 1.5 ppm). However, chronic NOAEC/LOAEC were

determined for freshwater fish at 0.88/1.7 ppb with egg hatchability as the endpoint effected. Oxadiazon

has the potential for high acute toxicity to estuarine/marine invertebrates (EC50 = 0.27 - 0.7 ppb) but

appears to be moderately toxic to freshwater invertebrates (LC50 = 2.18 - 2.4 ppm). Chronic toxicity to

freshwater invertebrates shows reproductive effects (mean time to first brood, # young/adult/reproductive

day, survival, growth) with a NOAEC/LOAEC = 30.0/35.0 ppb. The limited data on plant toxicity shows

that oxadiazon is toxic to non-vascular aquatic plants (marine diatom EC50 = 5.2 ppb) and vascular aquatic

plants (duckweed EC50 = 41 ppb).

b. Risk to Aquatic Organisms (Acute/Chronic)

Tables 4 and 5 provide acute and chronic RQ values for oxadiazon exposure to freshwater and

estuarine/marine species relative to turf use patterns (application rates for EC at 2.0 - 4.0 lbs ai/A and 4.0

lbs ai/A for granular). Our Tier I (GENEEC) risk assessment suggests that chronic exposure of this

compound can result in significant chronic risk to freshwater and estuarine/marine fish (RQ = 39.3

- 131.8) and aquatic invertebrates (RQ = 3.9 - 36.7). Although our assessment further suggests that

oxadiazon acute exposure may result in low acute risk to fish (RQ = 0.1 - 0.2) and invertebrates (RQ

= 0.3 - 0.5), there is uncertainty regarding the potential for enhanced risk that may occur through

phototoxicity. Since oxadiazon is a light-dependent peroxidizing herbicide (LDPH), enhanced toxicity

through exposure to high levels of solar radiation is a possible concern regarding aquatic organisms that

inhabit small, shallow water bodies. Endangered species concerns are also suggested with RQ = 0.1.

Aquatic plant acute high risk levels of concern are exceeded (Table 6 and 7) for both vascular and

nonvascular plants. The exceedences range 1 - 4 fold for vascular plants and 8.5 - 33 fold for nonvascular plants. The acute plant high risk level of concern is exceeded for vascular plants with an

exceedence range of 5.5 -22 fold. Currently, EFED does not perform assessments for chronic risk to

aquatic plants.
22

Table 4. Acute and chronic RQ’s for evaluating toxic risk of oxadiazon exposure to fish (freshwater

and estuarine/marine). RQ’s are based on the bluegill (Lepomis macrochirus) LC50 = 0.88 ppm,

rainbow trout (Oncorhynchus mykiss) NOAEC = 0.00088 ppm and sheepshead minnow (Cyprinodon

variegatus) LC50 = 1.5 ppm., NOAEC = 0.0015 ppm1

. EEC values are generated from GENEEC and

reflect three of the highest proposed EC application rates, and the maximum granular use rate (4.0,

3.0, and 2.0 lbs ai/A, 2 applications each; 4.0 lbs ai/A, 2 applications, respectively) for turf use.

Crop App.

Rate (lbs

ai/A; # App.)

Organism LC50

(ppm)

NOAEC

(ppm)

EEC 

Peak

(ppm)

EEC 

60-Day Ave.

(ppm)

Acute 

RQ

(EEC/LC50)

Chronic 

RQ 

(EEC/NOAEC)

Turf (EC)

4.0 (2)

Freshwater 0.88 0.00088 0.143 0.116 0.22 131.83

Estuarine/

Marine 

1.5 0.00151 0.143 0.116 0.12 77.33

Turf (EC)

3 (2)

Freshwater 0.88 0.00088 0.130 0.122 0.12 139.03

Estuarine/

Marine 

1.5 0.00151 0.130 0.122 0.12 81.33

Turf (EC)

2 (2)

Freshwater 0.88 0.00088 0.088 0.083 0.12 94.33

Estuarine/

Marine 

1.5 0.00151 0.088 0.083 0.0 55.33

Turf

(Granular)

4.0 (2)

Freshwater 0.88 0.00088 0.122 0.099 0.12 112.53

Estuarine/

Marine 

1.5 0.00151 0.122 0.099 0.12 66.03

1

 Extrapolated chronic value using acute/chronic freshwater toxicity ratio

2

 Acute restrictive use (> 0.1), acute species 3

 Chronic concern (> 1.0)
23

Table 5. Acute and chronic risk RQ’s for evaluating toxic risk of oxadiazon exposure to aquatic

invertebrates (freshwater and estuarine / marine). RQ’s are based on Daphnia (Daphnia magna)

EC50 = 2.18 ppm, NOAEC = 0.03 ppm and the Mysid shrimp (Americamysis bahia) EC50 = 0.27 ppm,

NOAEC = 0.0037 ppm1. EEC values are generated from GENEEC and reflect three of the highest

proposed EC application rates, and the maximum granular use rate (4.0, 3.0, and 2.0 lbs ai/A, 2

applications each; 4.0 lbs ai/A, 2 applications, respectively) for turf use.

Crop App.

Rate (lbs

ai/A) # App.

(days)

Organism EC50

(ppm)

NOAEC

(ppm)

EEC Peak

(ppm)

EEC

 21-Day Ave.

(ppm)

Acute

RQ

(EEC/

LC50)

Chronic 

RQ 

(EEC/NOAEC)

Turf (EC)

4.0 (2)

Freshwater 2.18 0.03 0.143 0.136 0.12 4.51

Estuarine/

Marine

0.27 0.0037 0.143 0.136 0.52 36.71

Turf (EC)

3.0 (2)

Freshwater 2.18 0.03 0.130 0.127 0.52 4.21

Estuarine/

Marine

0.27 0.0037 0.130 0.127 0.52 34.33

Turf (EC)

2.0 (2)

Freshwater 2.18 0.03 0.088 0.086 0.0 2.93

Estuarine/

Marine

0.27 0.0037 0.088 0.086 0.32 23.23

Turf

(Granular)

4.0 (2)

Freshwater 2.18 0.03 0.122 0.116 0.0 3.91

Estuarine/

Marine

0.27 0.0037 0.122 0.116 0.42 31.31

1

 Extrapolated chronic value using acute/chronic freshwater toxicity ratio

2

 Acute restrictive use (> 0.1) 3

 Chronic concern (> 1.0)
24

Exposure and Risk to Aquatic Plants

Exposure to nontarget aquatic plants may occur through runoff or spray drift from adjacent treated

sites. An aquatic plant risk assessment for acute high risk is usually made for aquatic vascular plants from

the surrogate duckweed Lemna gibba. Non-vascular acute high aquatic plant risk assessments are

performed using either algae or a diatom, whichever is the most sensitive species. Runoff and drift

exposure are computed from GENEEC2 and the risk quotient is determined by dividing the pesticide's

initial or peak concentration in water by the plant EC50 value. Acute risk quotients for vascular and nonvascular plants are tabulated in Table 6. 

Table 6. Acute Risk Quotients for Aquatic Plants based upon a duckweed (Lemna gibba) EC50 of 

41 ppb and a nonvascular plant (marine diatom) EC50 of 5.2 ppb.

Turf/ Rate of Application 

in lbs ai/A (Number of Applications). 

Species EC50

 (ppm)

EEC

(ppm)

Non-target plant

RQ (EEC/EC50)

4 (1) duckweed 0.041 0.173 4.2

4 (1) “ 0.041 0.089 2.2

3 (1) “ 0.041 0.067 1.6

2 (1) “ 0.041 0.044 1.1

4 (2) diatom 0.0052 0.173 33.3

4 (1) “ 0.0052 0.089 17.1

3 (1) “ 0.0052 0.067 12.9

2 (1) “ 0.0052 0.044 8.5

The acute high risk levels of concern for aquatic plants are exceeded for both vascular and

nonvascular plants. The exceedences range 1 - 4 fold for vascular plants and 8.5 - 33 fold for nonvascular plants. Currently, EFED does not perform assessments for chronic risk to aquatic plants.
25

Table 7. Species Risk Quotients on turf for aquatic plants based upon a duckweed (Lemna gibba)

NOAEC of <8 ppb.

Rate of Application 

in lbs ai/A (Number of Applications). 

Species EC50

(ppm)

EEC

(ppm)

Non-target plant

RQ (EEC/EC50)

4 (2) duckweed 0.008 0.173 21.6

4 (1) 0.008 0.089 11.1

3 (1) 0.008 0.067 8.4

2 (1) 0.008 0.044 5.5

An analysis of the results indicate that the plant acute high risk level of concern is exceeded for

vascular plants with exceedences ranging 5.5 -22 fold. 
26

CHAPTER 7: TERRESTRIAL EXPOSURE AND RISK

a. Terrestrial Hazard Summary

The available toxicity data are listed in Appendix D. Oxadiazon appears to be practically non-toxic

to avian species on an subacute basis (Northern Bobwhite quail and mallard duck LC50 > 5,000 ppm) and

slightly to practically non-toxic to birds on an acute basis (Bobwhite quail LD50 >2,150 mg/kg; mallard

LD50 = 1,040 mg/kg). Chronic testing showed no reproductive effects at 500 ppm.. At greater than 1,000

ppm mortality was noted for adult females (Bobwhite quail).

Mammalian toxicity data suggest that this compound is practically non-toxic to small mammals on

an acute basis (rat LD50 > 5,000 mg/kg). Reproductive effects were noted at > 200 ppm that resulted in

inactive mammary tissue and fetal/neonatal death. Acute toxicity studies to honey bees show that

oxadiazon was practically non-toxic (LD50 > 25 ug/bee).

b. Risk to Avian Species (Acute/Chronic)

Table 8 provides avian acute and chronic RQs from exposure to multiple applications of oxadiazon

EC to turf for the maximum three application rates (4.0, 3.0 and 2.0 lbs ai/A) and two split applications

(1.0 lb ai/A, 4 times/6 months; 1.3 lbs ai/A, 3 times/6 months). The maximum three applications have the

potential for chronic exposure to birds that feed on plants and grass (e.g. ducks, geese) and may result in

toxic risk to these birds (RQ = 1.0 - 2.0). The split application appears to lower this chronic exposure and

risk (RQ = < 1). Exposure from the granular formulation was evaluated (Appendix F) because birds may

be exposed to granular pesticides through ingestion when foraging for food or grit. RQ values were

calculated for three weight classes of birds (1000g waterfowl, 180g upland game bird, and 20g songbird).

All scenarios for the granular resulted in no acute risk to birds (RQ < 1.5 - 2.0). However, the potential

chronic concern noted for non endangered birds suggest that oxadiazon may present a risk to endangered

species (RQ > 0.1)

The estimated environmental concentration (EEC) values used for foliar terrestrial exposure are

derived from the Kenega nomograph, as modified by Fletcher et al. (1994), based on a large set of actual

field residue data. The upper limit values from the nomograph represent the 95th percentile of residue

values from actual field measurements (Hoerger and Kenega, 1972). The Fletcher et al., (1994)

modifications to the Kenega nomograph are based on measured field residues from 249 publications,

including information on 118 species of plants, 121 pesticides, and 17 chemical classes. These

modifications represent the 95th percentile of the expanded data set. Risk quotients are based on the most

sensitive LC50 and NOAEC for birds. EFED also used the ELL- FATE model for multiple applications,

incorporating the appropriate dissipation half-life to generate EECs. Single application EECs reflect day

zero maximum Fletcher residue values (lbs ai/A x 240; 110; 135; 15 ppm). 

Current EFED policy assumes that pesticide dissipation from foliar surfaces is primarily due to

degradation or dissipation by one or more processes including, photolysis, hydrolysis, microbial

degradation and volatilization. If adequate foliar dissipation data are not available then a half-life not to
27

exceed 35 days will be used in the EEC calculations.

Table 8. Avian acute and chronic risk quotients (RQ’s) as generated through ELL-FATE for

broadcast ground spray applications for oxadiazon. RQ’s are based on mallard duck LC50 > 5,000

ppm and NOAEC = 500 ppm. The EEC reflects the turf use with the three highest use rate (4.0, 3.0

and 2.0 lbs ai/A, 2 applications) and two split applications (1.0 lb ai/A, 4 times/6 months; 1.3 lbs

ai/A, 3 times/6 months).

Site Application Rate

lbs ai/A

 (# appl)

Food Item Maximum EECs

(ppm)

Acute RQ

(EEC/LC50)

Chronic RQ

(Max.

EEC/NOAEC)

Turf (EC)

4.0 (2)

Short grass

Tall grass

Broadleaf

plants/insects

Seeds

984.1

451.1

553.6

61.5

< 0.2

< 0.1

< 0.1

< 0.0

2.0

1.0

0.1

0.1

Turf (EC)

3.0 (2)

Short grass

Tall grass

Broadleaf

plants/insects

Seeds

739.6

339.0

416.0

46.2

<0.1

 0.0

<0.1

 0.0

1.5

1.0

1.0

0.1

Turf (EC)

2.0 (2)

Short grass

Tall grass

Broadleaf

plants/insects

Seeds

493.1

226.0

277.3

30.8

<0.1

0.0

0.0

0.0

1.0

0.4

0.5

0.1

Turf (EC)

1.0 (split 4

applications/ 6

months)

Short grass

Tall grass

Broadleaf

plants/insects

Seeds

424.4

194.5

238.7

26.5

<0.1

0.0

0.0

0.0

1.0

0.4

0.5

0.1

Turf (EC)

1.3 (split 3

applications/ 6

months)

Short grass

Tall grass

Broadleaf

plants/insects

Seeds

257.0

117.8

144.6

16.1

<0.1

0.0

0.0

0.0

1.0

0.4

0.5

0.1
28

c. Risk to Mammalians (Acute, Chronic)

Our assessment (Table 9 and 10) suggests that the proposed use rates (4.0, 3.0 and 2.0 lbs ai/A), as

well as the split use rates (1.0 and 1.3 lbs ai/A) should not result in acute risk to mammals (RQ < 0.2).

However, these application scenarios can result in significant chronic exposure and risk to mammalian

herbivores and insectivores (15g, 35g, and 1000g) with RQ values ranging from 1.5 - 9.9. This chronic

risk to non endangered mammalian species also suggests the potential for impact to endangered species.

Estimating the potential for adverse effects to wild mammals is based upon EFED’s draft 1995

SOP of mammalian risk assessments and methods used by Hoerger and Kenaga (1972) as modified by

Fletcher et al., (1994). The concentration of oxadiazon in the diet is expected to be acutely toxic to 50% of

the test organisms is determined by dividing the LD50 value (usually the rat LD50 ) by the per cent body

weight consumed. A risk quotient is then determined by dividing the EEC by the acute toxicity value. 

 RQ = EEC (ppm) 

 LD50 (mg/kg)/ % Body weight consumed

RQ values are calculated for four different kinds of food (short grass, tall grass, forage/insects, and

seeds) that are expected to be consumed by mammalian herbivores, insectivores, and granivores. The per

cent body weight consumed for herbivores and insectivores corresponding to the three weight categories

(15, 35, and 1000 g) is assumed to be 95%, 66%, and 15%, respectively. Granivores are expected to have

a different per cent body weight consumption for the same weight categories (21%, 15%, and 3%,

respectively). Chronic toxicity values were based on the NOAEC from a rat reproductive study. In order

to evaluate chronic concerns, a maximum EEC was generated through the ELL-FATE model that takes

into consideration pesticide half-life, application rate, number of applications, and intervals between

applications (first order kinetics model). In order to evaluate possible toxic risk to terrestrial organisms,

three application rates (4.0, 3.0, and 2.0 lbs ai/A, at 2 applications/6 months) and two split applications

(1.0 lbs ai/A applied 4 times/6 month and 1.3 lbs ai/A applied 3 times/6 month) were run. Our objective

was to find not only the highest rate that may cause toxic risk, but the lowest rate that might result in 

lower risk.
29

Table 9. Mammalian acute risk quotients as generated through ELL-FATE for ground application

of oxadiazon (EC). RQ’s are based on rat (Rattus norvegicus) LD50 > 5,000 mg/kg,. The EEC reflects

the three highest use rate (4.0, 3.0 and 2.0 lbs ai/A, 2 applications) and two split applications (1.0 lb

ai/A, 4 times/6 months; 1.3 lbs ai/A, 3 times/6 months).

Crop

Application Rate lbs

ai/A ( # of

applications)

Body Wt.

(g)

% Body Wt.

Consumed

Acute

RQ

Short

Grass

Acute

RQ

Forage and

Small Insects

Acute

RQ

Large Insects

Acute

RQ

Seeds

Turf (EC)

4.0 (2)

15

35

1000

95/21

66/15

15/3

<0.2

<0.1

 0.0

<0.1

<0.1

 0.0

<0.1

<0.1

 0.0

0.0

0.0

0.0

Turf (EC)

3.0 (2)

15

35

1000

95/21

66/15

15/3

<0.1

<0.1

 0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

Turf (EC)

2.0 (2)

15

35

1000

95/21

66/15

15/3

<0.1

<0.1

 0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

Turf (EC)

1.0 (split 4 

applications/

 6 months)

15

35

1000

95/21

66/15

15/3

<0.1

<0.1

 0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

Turf (EC)

1.3 (split 3 

applications/

 6 months)

15

35

1000

95/21

66/15

15/3

<0.1

<0.1

 0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

1

 Acute species concerns (> 0.1) 2 Acute restricted use (> 0.2)
30

Table 10. Mammalian chronic risk quotients as generated through ELL-FATE for ground

application of oxadiazon are based on rat (Rattus norvegicus) NOAEC = 100 ppm. The EEC reflects

the three highest use rate (4.0, 3.0 and 2.0 lbs ai/A, 2 applications) and two split applications (1.0 lb

ai/A, 4 times/6 months; 1.3 lbs ai/A, 3 times/6 months).

Crop

Application Rate lbs ai/A

 ( # of applications)

Food Items Max.

EEC

 (ppm)

Chronic RQ

(Max. EEC/NOAEC)

Turf (EC)

4.0 (2)

Short Grass

Tall Grass

Broadleaf plant/ Insects

Seeds

986.1

452.0

554.7

61.6

9.91

4.51

5.51

1.01

Turf (EC)

3.0 (2)

Short Grass

Tall Grass

Broadleaf plant/ Insects

Seeds

739.6

339.0

416.0

46.2

7.41

3.41

4.21

0.5

Turf (EC)

2.0 (2)

Short Grass

Tall Grass

Broadleaf plant/ Insects

Seeds

493.1

226.0

227.3

30.8

4.91

2.31

2.81

0.3

Turf (EC)

1.0 (split 4 

applications/

 6 months)

Short Grass

Tall Grass

Broadleaf plant/ Insects

Seeds

424.4

194.5

238.7

26.5

3.21

1.51

1.81

0.2

Turf (EC)

1.3 (split 3 

applications/

 6 months)

Short Grass

Tall Grass

Broadleaf plant/ Insects

Seeds

257.0

117.8

144.6

16.1

4.71

2.21

2.71

0.3

1 Chronic risk (LOC > 1)

d. Risk to Non-target Insects

EFED does not do risk assessments on insects. However, it appears that oxadiazon exposure to

honeybees should present low risk.

e. Risk to Terrestrial Plants

The risk assessment of oxadiazon to terrestrial plants and aquatic plants (vascular and

nonvascular) cannot be completed because of an inadequate data base. It should be noted that the

assessment for nonvascular plants provided here is incomplete in that the assessment is based on a

supplemental study and additional nonvascular plant species testing is being recommended.
31

f. Endocrine Disruption

EPA is required under the FFDCA, as amended by FQPA, to develop a screening program to

determine whether certain substances (including all pesticide active and other ingredients) “may have an

effect in humans that is similar to an effect produced by a naturally-occurring estrogen, or other such

endocrine effects as the Administrator may designate.” Following the recommendations of its Endocrine

Disruptor Screening and Testing Advisory Committee (EDSTAC), EPA determined that there was

scientific basis for including, as part of the program, the androgen- and thyroid hormone systems, in

addition to the estrogen hormone system. EPA also adopted EDSTAC’s recommendation that the Program

include evaluations of potential effects in wildlife. For pesticide chemicals, EPA will use FIFRA and, to

the extent that effects in wildlife may help determine whether a substance may have an effect in humans,

FFDCA authority to require the wildlife evaluations. As the science develops and resources allow,

screening of additional hormone systems may be added to the Endocrine Disruptor Screening Program

(EDSP).

When the appropriate screening and or testing protocols being considered under the Agency’s

Endocrine Disruptor Screening Program have been developed, oxadiazon, may be subjected to additional

screening and or testing to better characterize effects related to endocrine disruption. Issues that have

raised this concern include the findings from fish reproduction effects (larval and embryo survival, egg

hatchability) and invertebrate reproduction effects (reduced neonate production) also suggest endocrine

disruption. 
32

CONCLUSIONS

The Tier I GENEEC calculated RQ values for the use of oxadiazon on turf suggests that

chronic exposure of this compound has the potential for toxic risk to freshwater and estuarine/marine fish

(RQ = 39.3 - 131.8) and aquatic invertebrates (RQ = 3.9 - 36.7). The chronic Level of Concern (LOC) was

exceeded by up to 132-fold for fish and 37-fold for aquatic invertebrates. Although our initial risk

assessment suggests that acute exposure of oxadiazon to aquatic systems should result in relatively lower

short term risk to non endangered fish and aquatic invertebrates (RQ = 0.1 - 0.5) there is uncertainty

regarding possible risk enhancement through phototoxicity. Since oxadiazon is a light-dependent

peroxidizing herbicide (LDPH), enhanced toxicity through exposure to high levels of solar radiation is a

possible concern regarding aquatic organisms that inhabit small, shallow water bodies. Oxadiazon is also

a lipophilic, persistent compound that can be absorbed to particulate and sediment. This combination of

chemical/physical attributes and the relatively high toxicity profile to fish and invertebrates suggest

concern for accumulation in the sediments. Since sediments can act as a repository for lipophilic

compounds, there can be direct impact to aquatic organisms through respiration, ingestion, dermal contact,

and/or indirect impact through alterations of the food chain. The herbicidal properties of this compound

also suggest toxicity to aquatic plants and the resulting alteration of habitats.

Our terrestrial risk assessment for the oxadiazon EC use on turf was conducted by using the ELLFATE model. An evaluation of EECs generated for each of the three application rates (4.0, 3.0, and 2.0 lbs

ai/A) and split applications (1.0 and 1.3 lbs ai/A) showed that oxadiazon chronic exposure to mammals

(RQ = 1.5 - 9.9) has the potential for toxic risk. Chronic risk to mammalians can be upto 10 fold greater

than the LOC with the potential to impact herbivores, granivores and insectivores. Relative to mammalian

effects, chronic risk to avian species (RQ = 1.0 - 2.0) appears lower but still exceeds EFEDs LOC (RQ

=1). This exposure may result in impact to herbivorous birds which feed on grass, broadleaf plants, etc.

Although acute exposure of this compound should not present a toxic risk to non endangered avian or

mammalian species (RQ < 0.1), the potential for chronic risk suggests a possible endangered avian species

concern. Exposure from the granular formulation was evaluated (Appendix F) because birds may be

exposed to granular pesticides through ingestion when foraging for food or grit. RQ values were

calculated for three weight classes of birds (1000g waterfowl, 180g upland game bird, and 20g songbird).

The maximum use rate scenarios for the granular resulted in acute risk to small songbirds (RQ = 1.5 -

2.0). 

Since oxadiazon is practically non-toxic to the honey bee, minimal risk to these organisms is

anticipated. However, since oxadiazon is a herbicide, risk to non-nontarget aquatic and terrestrial plants

can be anticipated. RQ’s generated for Tier I testing of aquatic plants (vascular RQ = 1.1 - 4.2 and

nonvascular RQ = 8.5 - 33.3) show the potential for toxic risk to aquatic plants. Although there does not

appear to be an acute risk to endangered birds and mammals there may be chronic concerns as reflected in

the two-fold LOC exceedences for non endangered terrestrial animals. Therefore, our assessment suggests

that endangered terrestrial species (birds, mammals, and terrestrial plants) may be at risk.

Aquatic studies that showed fish reproduction effects (larval and embryo survival, egg

hatchability) and invertebrate reproduction effects (reduced neonate production) suggest that oxadiazon

may be subject to additional screening or testing to better characterize effects related to possible endocrine
33

disruption. 
34

APPENDIX A: REFERENCES CITED

Supplemental and Core Ecotoxicity Studies Cited

MRID 111806 Posner, S.; McGee, G.; Freeman, L. (1971) Acute Toxicity (LD50) in Mallard Ducks:

[RP-17623 Technical Assay 99.1]: Experimental Reference No. A-408. (Unpublished study received

Aug 23, 1972 under 359-658; prepared by Biometric Testing, Inc., submitted by Rhone-Poulenc, Inc.,

Monmouth Junction, NJ; CDL:003179-B)

MRID 111807 Posner, S.; McGee, G.; Freeman, L. (1971) Acute Toxicity (LD50) in Bobwhite Quail:

[RP-17623 Technical Assay 99.1]: Experimental Reference No. A-408. (Unpublished study received Aug

23, 1972 under 359-658; prepared by Biometric Testing, Inc., submitted by Rhone-Poulenc, Inc.,

Monmouth Junction, NJ; CDL:003179-C)

MRID 112622 Posner, S.; McGee, G.; Freeman, L. (1971) Acute Toxicity (LD50) in Bobwhite Quail:

Experimental Reference No. A-408. (Unpublished study received Oct 14, 1972 under 2F1269; prepared

by Biometric Testing, Inc., submitted by Rhodia, Inc., New Brunswick, NJ;

CDL:091824-D)

MRID 41610101 Pedersen, C. (1990) Oxadiazon Technical: 21-Day Acute Oral LD50 Study in Bobwhite

Quail: Lab Project Number: BLAL/NO/89 QD 139. Unpublished study prepared by Bio-Life Associates,

Ltd. 35 p.

MRID 41610102 Pedersen, C. (1990) Oxadiazon Technical: 8-Day Acute Dietary LC50 Study in

Bobwhite Quail: Lab Project Number: BLAL/NO/89 QC 141.Unpublished study prepared by Bio-Life

Associates, Ltd. 82 p.

MRID 41610103 Pedersen, C. (1990) Oxadiazon Technical: 8-Day Acute Dietary LC50 Study in Mallard

Ducklings: Lab Project Number: BLAL/NO/89 DC 137. Unpublished study prepared by Bio-Life

Associates, Ltd. 80 p.

MRID 41610105 Giddings, J. (1990) Oxadiazon Technical-Toxicity to the Marine Diatom Skeletonema

costatum: Lab Project Number: 90-7-3384: 10566- 1089-6137-450. Unpublished study prepared by

Springborn Laboratories, Inc. 55 p.

MRID 41610106 Giddings, J. (1990) Oxadiazinon Technical-Toxicity to the Freshwater Diatom Navicula

pelliculosa: Lab Project Number:90-8-3423; 10566-1089-6137-440. Unpublished study prepared by

Springborn Laboratories, Inc. 52 p.

MRID 41610108 Giddings, J. (1990) Oxadiazon Technical-Toxicity to the Freshwater Green Alga

Selenastrum capricornutum: Amended Report: Lab Project Number: 90-8-3422; 10566.1089.6137.437. 

Unpublished study prepared by Springborn Laboratories, Inc. 52 p.

MRID 41610107 Giddings, J. (1990) Oxadiazon Technical-Toxicity to the Duckweed Lemma gibba G3:
35

Final Report: Lab Project Number: 90-7-3389; 10566.1089.6137.410. Unpublished study prepared by

Springborn Laboratories, Inc. 48 p.

MRID 41784301 Blakemore, G.; Burgess, D. (1991) Chronic Toxicity of Oxadiazon Technical to

Daphnia magna under Flow-thru Conditions: Final Reort: Lab Project Number: 38369. Unpublished

study prepared by Analytical Bio-Chemistry Labs., Inc. 349 p.

MRID 41898001 Hoberg, J. (1991) Oxadiazon Technical--Determination of Effects on Seed Germination,

Seedling Emergence and Vegetative Vigor of Ten Plant Species: Final Report: Lab Project Number:

90-11-3547: 10566-0790-6165-610. Unpublished study prepared by Springborn Laboratories, Inc. 164 p.

MRID 41993201 Fletcher, D.; Pedersen, C. (1991) Oxadiazon Technical: Toxicity and Reproduction

Study in Mallard Ducks: Lab Project Number: 89 DR 35. Unpublished study prepared by Bio-Life

Associates, Ltd.138 p.

MRID 41993202 Fletcher, D.; Pedersen, C. (1991) Oxadiazon Technical: Toxicity and Reproduction

Study in Bobwhite Quail: Lab Project Number: 89 QR 39. Unpublished study prepared by Bio-Life

Associates, Ltd.145 p.

MRID 42330401 Sword, M.; Northup, R. (1992) Acute Flow-Through Toxicity of Oxadiazon to Rainbow

Trout (Oncorhynchus mykiss): Lab Project Number: 39729. Unpublished study prepared by ABC

Laboratories, Inc. 211 p.

MRID 42331801 Blasberg, J.; Bowman, J. (1992) Acute Toxicity of Oxadiazon to Daphnia magna under

Flow-through Conditions: Amended Final Report: Lab Project Number: 39730. Unpublished study

prepared by ABC Labs, Inc. 254 p.

MRID 42350601 Sword, M.; Northup, R. (1992) Acute Flow-through Toxicity of Oxadiazon to Bluegill

(Lepomis macrochirus): Final Report: Lab Project Number: 39728. Unpublished study prepared by ABC

Labs., Inc. 194 p.

MRID 42468301 Beevers, M. (1992) Acute Contact Toxicity of Oxadiazon Technical to Honey Bees

(Apis mellifera L.): Lab Project Number: CAR 160-92. Unpublished study prepared by California

Agricultural Research, Inc. 14 p. 42570301 Dionne, E. (1992) Oxadiazon Technical-- Acute Toxicity to

Eastern Oyster (Crassostrea virginica) under Flow-through Conditions: Final Report: Lab Project Number:

92-7-4329: 10566.0392.6238.504. Unpublished study prepared by Springborn Labs, Inc. 63 p.

MRID 42615801 Machado, M. (1992) Oxadiazon Technical--Acute Toxicity to Sheepshead Minnow

(Cyprinodon variegatus) under Flow-through Conditions: Final Report: Lab Project Number: 92-8-4383 

 10566.0392.6237.505. Unpublished study prepared by Springborn Labs, Inc. 66 p.

MRID 42615802 Machado, M. (1992) Oxadiazon Technical--Acute Toxicity to Mysid Shrimp

(Mysidopsis bahia) under Flow-through Conditions:Final Report: Lab Project Number: 92-7-4348:

10566.0392.6236.515. Unpublished study prepared by Springborn Labs, Inc. 65 p.
36

MRID 42659001 Mihaich, E. (1993) Response to EPA Review of Oxadiazon Anabaena flos-aquae Study

(MRID 41610104) and Selenastrum caprocornutum (sic) Study (MRID 41610108): Lab Project No.

NS/EMM-93-03. Unpublished study prepared by Rhone-Poulenc Ag Co. and Springborn Labs., Inc. 10 p.

MRID 42921601 Rhodes, J. (1993) Early Life-Stage Toxicity of Oxadiazon Technical to the Fathead

Minnow (Pimephales promelas) Under Flow-Through Conditions: Lab Project Number: 40024.

Unpublished study prepared by ABC Labs. Inc., Environmental Toxicology. 432 p.

Literature Citation, Ecotoxicity Study

Guardigli, A, et. al., “Residue Uptake and Depletion Measurements of Dietary Oxadiazon in Mammalian

and Avian Species.” Archives of Environmental Contamination and Toxicology Vol. 4, 145-154 (1976)

Supplemental and Core Environmental Fate Studies Cited

(MRID# 42226701; DP Barcode D192825) Armstrong, K., B.D. Cameron, S.A. Chapleo, B.E. Hall, and

A.Haswell. 1991. Oxadiazon: Bioaccumulation test in bluegill sunfish. IRI Project No. 381195; Report

No. 8385. Unpublished study performed by Inversk Research International, Tranent, Scotland, and

submitted by Rhône-Poulenc Ag Company, Research Triangle Park, NC

Manley, J.D., I.A.J. Hardy, and E.A. Savage. 1992. Herbicides: Oxadiazon spectroscopic investigation

of metabolites from a [14C]-oxadiazon bioaccumulation test in bluegill sunfish. IRI Project No. 381195. 

Unpublished study performed by Rhône-Poulenc Agriculture Limited, Ongar, United Kingdom, and

submitted by Rhône-Poulenc Ag Company, Research Triangle Park, NC (No Study ID)

(MRID# 41898201; DP Barcode D165559) Das, Y.T. 1991. Photodegradation of [Phenyl(U)-14C]

Oxadiazon on Soil under Artificial Sunlight. Unpublished study performed by Innovative Scientific

Services, Inc. Piscataway, N.J., and sponsored and submitted by Rhône-Poulenc Ag Company, Research

Triangle Park, NC

(MRID# 41897201; DP Barcode D192825) Corgier, M.M.C., and A.P. Plewa. 1991. 14Coxadiazon photodegradation in aqueous solution. Study No. 90-29. Filing Reference

AG/CRLD/AN/9115609. Unpublished study performed by Rhône-Poulenc Secteur Agro, Lyon, France,

and submitted by Rhône-Poulenc Ag Company, Research Triangle Park, NC 

(MRID# 41863603; DP Barcode D165559) Corgier, M.M.C., and Robin, J.M. 1991. 14COxadiazon Hydrolysis at 25°C. Unpublished study performed by Rhône-Poulenc, Lyon, France, and

submitted by Rhône-Poulenc, Research Triangle Park, NC

(MRID# 41898202, DP Barcode D165559) Dykes, J. 1991. Soil Adsorption/Desorption with 14COxadiazon. An unpublished study performed by Analytical Bio-Chemistry Laboratories, Inc., Columbia,

MO, nad submitted by Rhône Poulenc Ag Company, Research Triangle Park,NC
37

(MRID# 41767401; DP Barcode D192825) Norris, F.A. 1991. A terrestrial field soil dissipation study

with oxadiazon. Study No. EC/P-89-0014. File No. 40642. Unpublished study performed and submitted

by Rhône-Poulenc Ag Company, Research Triangle Park, NC 

(MRID# 41889501; DP Barcode D165559) Priestley, D.B., Lowden, P., and Savage, E.A. 1991.

Oxadiazon-14C: Leaching Study with Four Soils. Unpublished study performed by Rhône-Poulenc

Agriculture Limited, Essex, England, and submitted by Rhône-Poulenc Ag Company, Research Triangle

Park, NC 

(MRID# 42772801; DP Barcode D192825) Waring, A.R. 1993a. [14C]Oxadiazon: Aerobic soil 

metabolism. HUK Study No. 68/111; Report No. 7218. Unpublished study performed by Hazleton UK,

Harrogate, North Yorkshire, England, and submitted by Rhône-Poulenc Agriculture Company, Research

Triangle Park, NC 

(MRID# 42773802; DP Barcode D192825) Waring, A.R. 1993b. [14C]Oxadiazon: Anaerobic

aquatic metabolism. HUK Study No. 68/112; Report No. 7214. Unpublished study performed by

Hazleton UK, Harrogate, North Yorkshire, England, and submitted by Rhône-Poulenc Agriculture

Company, Research Triangle Park, NC

Supplemental HED Study, for Environmental Fate Use

MRID# 44995501 Howell, C., and Wedekind, W. 1999. “Oxadiazon: Determination of Transferable

Turf Residues on Turf Treated with CHIPCO®G.” An unpublishded study sponsored by Rhône Poulenc

Corporation, NC, and performed by ABC Laboratories, Inc., MO, Test Guideline 875.2100, ABC Study

Number 44951.

MRID# 44995502 Howell, C. 1999. “Oxadiazon: Determination of Transferable Turf Residues on Turf

Treated with CHIPCO®RONSTAR®50WSP.” An unpublishded study sponsored by Rhône Poulenc Ag

Company, NC, and performed by ABC Laboratories, Inc., MO, Test Guideline 875.2100, ABC Study

Number 44952.
38

APPENDIX B: FATE SUMMARIES

161-1 Hydrolysis (MRID# 41863603; Core)

[

14C]-Oxadiazon (phenyl ring-labeled), at 0.48 mg/L, was stable in pH 4, 5, and 7 sterile aqueous buffered

solutions incubated in the dark at 25°C for 31 days.

At pH 9, oxadiazon was hydrolyzed with a calculated half-life of 38 days. Oxadiazon averaged 93.64% of

the applied at 5 days, and 49.98% at 31 days. The main degradate found was

 ! 1-trimethyl acetyl-2-(2,4-dichloro-5-isopropoxyphenyl) hydrazine (RP26123), which increased to

45% of the applied at 31 days.

All other metabolites were present at <10% of the applied.

161-2 Photodegradation in Water (MRID# 41897201; Core)

[

14C]oxadiazon (phenyl ring-labeled), at 0.5 mg/L, photodegraded with a half-life of 21.2 hours (or the

equivalent of 2.75 days of summer sunlight in Florida) in pH 5 buffered solutions that were continuously

irradiated with a xenon arc lamp at 25 + 1°C for up to 42 hours. Oxadiazon declined from an average of

98.68% of the recovered immediately posttreatment, to 42.46% at 26 hours. In the dark controls, no

degradation was observed for 42 hours.

The degradates identified were RP36939 and RP37084, present at up to 4.8% and 11.5% of the applied

radioactivity, respectively. Up to 20 degradates were isolated, present at <8% of the applied radioactivity. 

The registrant did not provide the chemical names for RP36939 and RP37084. The later one was a

maximum of 11.5% of the applied at 42 hours (last test interval), when the level of oxadiazon had

decreased to <28% of the applied. It is not likely that RP37084 would be formed in significantly higher

quantities.

161-3 Photolysis on Soil (MRID# 41898201; Core)

[

14C]oxadiazon (phenyl ring-labeled), at 9.4-11.3 ppm, degraded slowly with a calculated half-life of 165

days on a sandy loam soil irradiated with xenon arc lamp intermitently at 25°C. There was no significant

breakdown of the parent under non-irradiated conditions. In the irradiated samples, oxadiazon averaged

90.2% of the applied at day 0 posttreatment, and averaged 86.6% at day 30. In the dark samples,

oxadiazon averaged 90.2% of the applied at day 0, and 90.8% at day 30. The following minor degradates

were observed in small quantities.

 ! 2-tertiobutyl-4-(2,4-dichloro-5-hydroxyphenyl)-5-oxo-1,3,4-oxadiazolin (RP25496), and

 ! 3-(2,4-dichloro-5-methoxyphenyl)-5-tertiobutyl-1,3,4->4-oxadiazolin-2-one (RP17272).
39

161-4 Photodegradation in Air (Waived)

163-2 Laboratory Volatility (Waived)

163-3 Field Volatility (Waived)

All three data requirements were waived, based on the relatively low vapor pressure (1.0x10-6 mm Hg)

and Henry’s Law Constant (calc. 4.51x10-7 AtmA m3/mol) of oxadiazon. EFED believes that this value is

relatively low and that volatility of oxadiazon may not be an important route of dissipation for oxadiazon. 

This conclusion is further confirmed by the results of the Aerobic Soil Metabolism study (MRID#

42772801), which shows only a small fraction of the applied was volatilized after 1 year (see below).

162-1 Aerobic Soil Metabolism (MRID# 42772801; Core)

[

14C]-Oxadiazon (uniformly ring labeled) degraded slowly in sandy loam soil that was incubated

aerobically in the dark at about 25°C and approximately 75% of soil water capacity at 0.33 bar for 1 year. 

The registrant-calculated initial half-life was well beyond the experimental time frame (t1⁄2 = 841 days). 

Oxadiazon comprised 98.87-92.33% of the applied immediately posttreatment and decreased slowly to

72.11-76.47% of the applied after 365 days. Unextracted [14C] residues, and volatilized [14C] residues

comprised 4.82% and 2.95% of the applied at 365 days, respectively.

Five degradates were identified:

 ! 3-(2,4-dichloro-5-methoxyphenyl)-5-tertiobutyl-1,3,4->4-oxadiazolin-2-one (RP17272)

 ! 2-(carboxy-2-propyl)-4-(2,4-dichloro-5-hydroxyphenyl)-5-oxo-1,3,4-oxadiazolin (RP26471)

 ! 2-tertiobutyl-4-(2,4-dichloro-5-hydroxyphenyl)-5-oxo-1,3,4-oxadiazolin (RP25496)

 ! 2-(2-carboxy-2-propyl)-4-(2,4-dichloro-5-isopropoxyphenyl)-5-oxo-1,3,4-oxadiazolin (RP26449)

and

 ! 1-trimethylacetyl-2-(2,4-dichloro-5-isopropoxyphenyl)hydrazine (RP26123).

These degradates were present at concentrations #1.51% of the applied throughout the study. Three other

areas of radioactivity were isolated, but not identified, at #1% of the applied.

162-3 Anaerobic Aquatic Metabolism (MRID# 42773802: Supplemental)

[

14C] Oxadiazon (uniformly ring labeled) degraded slowly with an observed half-life of approximately 1

year in anaerobic (flooded plus nitrogen atmosphere) sandy loam soil that was incubated in the dark at

about 25°C for 1 year; oxadiazon comprised 91.7-91.8% of the applied immediately posttreatment and

decreased to 47.3-47.9% of the applied at 366 days. At 366 days, unextracted [14C] residues were 2.47%

of the applied and [14C] volatiles totaled 0.02% of the applied.

Five degradates were identified:

 ! 2-(carboxy-2-propyl)-4-(2,4-dichloro-5-hydroxyphenyl)-5-oxo-1,3,4-oxadiazolin (RP26471)

 ! 2-tertiobutyl-4-(2,4-dichloro-5-hydroxyphenyl)-5-oxo-1,3,4-oxadiazolin (RP25496)

 ! 2-(2-carboxy-2-propyl)-4-(2,4-dichloro-5-isopropoxyphenyl)-5-oxo-1,3,4-oxadiazolin (RP26449)

 ! 2,4-dichloroisopropoxybenzene (RP36227) and
40

 ! 1-trimethylacetyl-2-(2,4-dichloro-5-isopropoxyphenyl)hydrazine (RP26123).

All the degradates were present at concentrations #3.76% of the applied througout the experiments. In

addition, one "cluster" of [14C] residues was isolated from the floodwater at a maximum of 18.2% of the

applied, and one was isolated from the soil at a maximum of 20.8% of the applied at 181 day. These [14C]

residues were not further characterized.

163-1 Mobility in Soil (MRID# 41889601; Core)

[

14C]-Oxadiazon (phenyl ring labeled), at 4 ppm showed a low mobility in soil leaching columns

containing sand, loam, and two sandy loam soils. The material was either freshly applied (parent

pesticide), or applied after 30 days of aerobic incubation.

A similar profile was observed in the aged and unaged soil columns. The majority of the radioactivity

($80.5%) remained in the upper 0-6 inches of the 36 inches long columns, indicating a low mobility for

parent oxadiazon in these soil. The amount of radiaoctivity recovered from the leachate was small

(#0.2%). Solvent extracts were shown to contain only parent compound. The total recoveries of

radioactivity were 92.6-102.4%.

163-1 Mobility - Leaching and Adsorption/Desorption (MRID# 41898202; Supplemental)

Based on batch equilibrium studies, at nominal concentrations of 0.1-0.3 ppm and 25°C, [14C]Oxadiazon

demonstrated slight mobility in sand and sandy loam, and low mobility in a clay and silt loam. The Kd

and Koc constants obtained from the Freundlich isotherms were as follows:

Table 1. Mobility of Oxadiazon adsorption. desorption

Soil type %OC Kd Koc Kd Koc

silt loam 1.2 16.91 1409 21.35 1779

clay 1.2 22.83 1903 51.72 4310

sandy loam 0.4 11.39 2848 41.00 10250

sand 0.25 8.17 3268 10.34 `4136

The soil treatment included grinding in a grist mill. It is not known what is the particle size distribution. 

The particle size of the soil is a determinant of its adsortivity. A cursory revision of the DER for this

study indicates that 1/n values were not reported for any soil type.
41

164-1 Terrestrial Field Dissipation (MRID# 41767401; Core)

Oxadiazon slowly dissipated from two field plots located in San Juan Bautista, California (sandy loam

soil) and Clayton, North Carolina (loamy sand) and planted with Junipers and Azaleas, respectively. The

plots were treated with oxadiazon at 4.48 kg ai/ha (4.06 lb ai/A). The registrant-calculated initial half-life

of oxadiazon in the California site was 65 days, and 40 days in the North Carolina site. The dissipation

rate was near zero during the winter season in both locations. Oxadiazon appears to persist over time. In

the 0- to 15-cm soil depth of the California site, oxadiazon was 1.80-3.60 ppm immediately posttreatment

and decreased to 0.08-0.18 ppm at 12-16 months. In the North Carolina site oxadiazon was 1.08-2.05

ppm immediately posttreatment and decreased to 0.02-0.43 ppm at 12-16 months. In the 15- to 30-cm soil

depth of both plots, oxadiazon was #0.12 ppm through 2 months and #0.01 ppm thereafter.

Generally, oxadiazon residues were detected only in the upper 30 cm of the soil, with occasional small

detections in the 15- to 30-cm soil depth. The degradates

 ! 3-(2,4-Dichloro-5-methoxyphenyl)-5-tertiobutyl-1,3,4->4-oxadiazolin-2-one (RP-17272) and 

 ! 2-(2-Carboxy-2-propyl)-4-(2,4-dichloro-5-isopropoxyphenyl)-5-oxo-1,3,4-oxadiazolin (RP-26449)

were each present at <0.02 ppm in the 0- to 15-cm soil depth, and were not detected in the deeper soil

layers. Total irrigation plus rainfall during the study period was 14.03 cm in the California site, and 20.03

cm in the North Carolina site.

165-4 Bioaccumulation in Fish (MRID# 42226701; Core)

Oxadiazon residues accumulated in bluegill sunfish continuously exposed to 8.8 ug/L of oxadiazon, with

average bioconcentration factors of 368x for the edible tissues (muscle), 2239x for the nonedible tissues

(viscera), and 1111x for the whole fish. Maximum mean [14C]residue concentrations were 4.26 ug/g for

the edible tissues, 26.83 ug/g for the nonedible tissues, and 11.94 ug/g for whole fish. Steady state

concentrations were observed within 3 days of exposure. Depuration was rapid, with an observed half-life

of about 1 day; by day 3 of the depuration period, 83% of the accumulated [14C]residues had been

eliminated from whole fish, and by day 14, >97% were eliminated.

Parent [14C]oxadiazon was detected only once in the fish inedible tissues on day 14; three degradates were

identified in the fish samples:

 ! 5-(1-hydroxymethyl-1-methylethyl)-3-(2,4-dichloro-5-isopropoxyphenyl)-1,3,4-oxadiazol-2(3H)-

one (M8), which was 15.8-24.6% of the [14C] residues extracted from the edible fish;

 ! An ether glucuronide conjugate of M8 (chemical name not provided), which was 25.8-31.6% of

the [14C] residues extracted from the edible fish; and

 ! 5-(1-hydroxymethyl-1-methylethyl)-3-(2,4-dichloro-5-hydroxyphenyl)-1,3,4-oxadiazol-2(3H)-one

(M10), which was 8.5-13.4% of the [14C] residues extracted from the edible fish.
42

875.2100 Transferable Oxadiazon Residues on Turf Treated with the Product in the Granular

Formulation (MRID# 44995501; supplemental)

EFED had available a review by Versar, Inc., a contractor for HED studies. This study and the review

were prepared as a requirement of the Health Effects Division. Only some highlights of the study are

mentioned here for information only, since EFED investigated the usefulness of the study for modeling

purposes.

This test was performed with the granular formulation of oxadiazon. Samples were taken with cotton

cloth sheets, which were exposed to a large turf area. These samples were taken at various test intervals,

starting from prior to application, from three geographical locations. It appears that the registrant did not

intend to compare the actual amount of oxadiazon residues present on and in turf, compared to the amount

present in the cotton cloth. The registrant may have developed that information, but it was not available

to EFED at this time.

The reported LOQ was 25 :g/sample, while the LOD was not provided. Several results were less than the

LOQ.

The authors intended also to report the degradates RP25496+RP17272 (together), and RP26449. These

degradates, however, were not detected in any sampling interval through the study.

Half-life of the transferable residues may be around 2-7 days for the granular formulation. By no means

that would definitely mean that 7 days would be representative of the half-life of oxadiazon in the turf

because there is no evidence that the methodology used was in some way quantitatively measuring the

total levels of oxadiazon in the foliage. For the purpose of running EFED models, this study can only be

regarded at best as supplemental.

875.2100 Transferable Oxadiazon Residues on Turf Treated with the Product in the Liquid

Formulation (MRID# 44995502; supplemental)

EFED had available a review by Versar, Inc., a contractor for HED studies. This study and the review

were prepared as a requirement of the Health Effects Division. Only some highlights of the study are

mentioned here for information only, since EFED investigated the usefulness of the study for modeling

purposes.

The product was formulated in Wettable Soluble Packets (of powder containing oxadiazon at about 2% of

a.i.). The product was applied at 3 lb a.i./A. Samples were taken in triplicate with cotton cloth sheets,

which were exposed to a large turf area. These samples were taken at various test intervals, starting from

prior to application, from two geographical locations. Sampling occurred between March 26 and April 8,

1999. It appears that the registrant did not intend to compare the actual amount of oxadiazon residues

present on turf and in turf, compared to the amount present in the cotton cloth. The registrant may have

developed that information, but it was not available to EFED at this time.

The reported LOQ was 25 :g/sample, while the LOD was not provided. Several results were less than the
43

LOQ. In addition, the authors intended also to report the degradates RP25496+RP17272 (together), and

RP26449. It appears, however, that these degradates were not detected in any sampling interval through

the study.

In the California site, the data was very variable, and it was judged unreliable by the authors of the study. 

The review does not offer the author’s detailed rationale with respect to this invalidation. On the other

hand, a study conducted in Georgia, yielded most samples at > LOQ, although the range of the actual

results was several orders of magnitude larger than range of the fortified samples. Based on the results of

the 27 samples available, the half-life was about 2.3 days, with a correlation coefficient (R2) of 0.7. By no

means that would definitely mean that 2 days would be representative of the half-life of oxadiazon in the

turf because there is no evidence that the methodology used was in some way quantitatively measuring the

total levels of oxadiazon in the foliage. Furthermore, out of two studies conducted with the liquid

formulation, only one produced results with acceptable concentrations above the LOQ. EFED believes

that for the purpose of running EFED models, this study can only be regarded at best as informative, and,

for the safety of the public, the default value of 35 days should be used.
44

APPENDIX C: ECOLOGICAL TOXICITY DATA

Toxicity testing reported in this section is not representative of the wide diversity of terrestrial and

aquatic organisms in the United States. Two surrogate bird species, the bobwhite quail and the mallard

duck, are used for the 680 plus species of birds found in this country. For mammals, acute studies are

usually limited to the Norway rat or the house mouse. Reptiles are not tested, as these are assumed to be

subject to similar toxicological effects as birds. Of approximately 100,000 species of insects, spiders, and

other terrestrial arthropods, toxicity tests are usually required only for the honey bee. Only two surrogate

fish species (rainbow trout and bluegill sunfish) are used to represent the over 2,000 species of freshwater

fish found in this country. Amphibians are not tested, as these are assumed to be subject to similar

toxicological effects as fish. One crustacean, the water flea, is used to represent all freshwater

invertebrates. Estuarine/marine animal acute toxicity testing is usually limited to a crustacean, a mollusk,

and a fish. Testing on aquatic plants is limited to one species of vascular plant (duckweed) and four

species of algae and diatoms. 

Toxicity to Terrestrial Organisms

Birds, Acute, Subacute and Chronic

An acute oral toxicity study using the technical grade of the active ingredient (TGAI) is required to

establish the toxicity of oxadiazon to birds. The avian oral LD50 is an acute, single-dose laboratory study

designed to estimate the quantity of toxicant required to cause 50% mortality in a test population of birds. 

The preferred test species is either the Mallard Duck, a waterfowl, or Bobwhite quail, an upland gamebird. 

The TGAI is administered by oral intubation to adult birds, and the results are expressed as LD50

milligrams (mg) active ingredient (a.i.) per kilogram (kg). Toxicity category descriptions are as follows:

 If the LD50 is less than 10 mg a.i./kg, then the test substance is very highly toxic.

If the LD50 is 10-to-50 mg a.i./kg, then the test substance is highly toxic.

If the LD50 is 51-to-500 mg a.i./kg, then the test substance is moderately toxic.

If the LD50 is 501-to-2,000 mg a.i./kg, then the test substance is slightly toxic.

If the LD50 is greater than 2,000 mg a.i./kg, then the test substance is practically nontoxic.
45

Study results are in the table below. 

Table C.1. Avian Acute Oral Toxicity 

Species % ai LD50

(mg/kg) 

Toxicity

Category

MRID/

Lab/Year Classification

Northern Bobwhite

(Colinus virginianus)

99.1 6000 practically

nontoxic

111807 (also under

112622)

Biometric Testing, Inc.

1971 

Supplemental1

Mallard

(Anas platyrhynchos)

99.1 1040 slightly toxic 111806

Biometric Testing, Inc.

1971 

Supplemental1

Northern bobwhite

(Colinus virginianus)

97.49 >2150

(no bird

mortality)

practically

nontoxic

41610101

Bio-Life Associates, Ltd 

1990

Core

1

 studies are scientifically sound; although deemed satisfactory for registration of oxadiazon in the early 1970's, EFED required a new

study in 1991under Phase IV Reregistration. 

Based on results of the above studies, oxadiazon may be categorized slightly to practically

nontoxic to birds on an acute oral basis. The guideline 71-1(a ) is fulfilled (MRID 41610101). 

Two dietary studies using the TGAI are required to establish the toxicity of oxadiazon to birds. 

These avian dietary LC50 tests, using the Mallard Duck and Bobwhite Quail, are acute, eight-day dietary

laboratory studies designed to estimate the quantities of toxicant required to cause 50% mortality in the

two respective test populations of birds. The TGAI is administered by mixture to juvenile birds' diets for

five days followed by three days of "clean" diet, and the results are expressed as LC50 parts per million

(ppm) active ingredient (a.i.) in the diet. Toxicity category descriptions are as follows: 

If the LC50 is less than 50 ppm a.i., then the test substance is very highly toxic.

If the LC50 is 50-to-500 ppm a.i., then the test substance is highly toxic.

If the LC50 is 501-to-1,000 ppm a.i., then the test substance is moderately toxic.

If the LC50 is 1001-to-5,000 ppm a.i., then the test substance is slightly toxic.

If the LC50 is greater than 5,000 ppm a.i., then the test substance is practically nontoxic.

Study results are tabulated below. 

Table C.2. Avian Subacute Dietary Toxicity

Species % ai LC50 (ppm) Toxicity

Category

MRID/Lab/

Year

Study

Classification

Bobwhite Quail (Colinus

virginianus)

97.49 >5,000

(no bird mortality)

practically

nontoxic

41610102

Bio-Life Associates, Ltd

1990

Core

Mallard Duck (Anas

platyrhynchos)

97.49 >5000

(no bird mortality)

practically

nontoxic

41610103

Bio-Life Associates, Ltd

1990

Core
46

Based on results of the above studies, oxadiazon may be characterized practically nontoxic to birds

on a subacute basis. The guideline 71-2(a) for bobwhite (MRID 41610102) and 71-2(b) for mallard duck

(MRID 41610103) are fulfilled. 

Avian reproduction tests are designed to estimate the quantity of toxicant required to adversely

affect the reproductive capabilities of a test population of birds. The TGAI is administered by mixture to

breeding birds' diets throughout their breeding cycle. Test birds are approaching their first breeding

season and, generally, are 18-to-23 weeks old. The onset of the exposure period is at least 10 weeks prior

to egg laying. Exposure period during egg laying is generally 10 weeks with a withdrawal period of three

additional weeks if reduced egg laying is noted. Results are expressed as No Observed Adverse Effect

Concentration (NOAEC) and various observable effect levels, such as the Lowest Observable Adverse

Effect Concentration (LOAEC), quantified in units of parts per million of active ingredient (ppm) in the

diet. Study results are tabulated below . 

Table C.3. Avian Chronic Toxicity 

Species/ Study

Duration % ai NOAEC/

LOAEC (ppm) LOAEC Endpoints MRID/Lab/

Year Classification

Mallard Duck

(Anas platyrhynchos)

20 weeks

97.49 >1000 (highest dose

tested)/ LOAEC not

determined

not determined 41993201

Bio-Life Associates,

Ltd 1991

Supplemental1

Northern Bobwhite

(Colinus virginianus)

21 weeks

97.49 500/1000 mortality among adult

females

41993202

 Bio-Life Associates,

Ltd 1991

Core

1

 study was classified supplemental because a NOAEC was not established.

Based on the results of the Bobwhite reproduction study, the ingestion of oxadiazon at levels up to

1,000 ppm, the highest dose concentration tested, had no effect on any reproductive parameter or viability

of F1 the offspring (reproductive NOAEC >1000 ppm). However, mortality among females at that level

was quite high (33%). The study authors stated that due to the inconsistency and lack of dose-related

pathology observations in birds found dead or sacrificed at study termination, the pathology observations

were attrbuted to factors other than the test substance. EFED, in the absence of information on the cause

of the deaths, considered the mortality attributable to treatment. The chronic NOAEC was set at 500

ppm. The guideline 71-4(a) is fulfilled (MRID 41993202). 

The avian reproduction study using mallard resulted in a NOAEC greater than 1000 ppm, the

highest dose tested. This study was classified supplemental because a NOAEC was not established. 

Although the study is classified supplemental, it does not have to be repeated because (1) the Bobwhite

was more sensitive in testing, and (2) the highest dose tested is greater than the highest estimated

environmental concentration for the highest application rate (turf; 4 lb ai/A; maximum Fletcher value 240

x 4 = 960 ppm). The guidelines 71-4(a) for the Bobwhite (MRID 41993202) and 71-4(b) for the mallard

(MRID 41993201) are considered fulfilled.
47

Mammals, Acute and Chronic

Wild mammal testing is required on a case-by-case basis, depending on the results of lower tier

laboratory mammalian studies, intended use pattern and pertinent environmental fate characteristics. In

most cases, rat or mouse toxicity values obtained from the Agency's Health Effects Division (HED)

substitute for wild mammal testing. The acute toxicity values below were taken from HED’s Tox OneLiners. Chronic toxicity information was obtained from the Health Effects Division Hazard Identification

Assessment Review Committee (HIARC report HED DOC. NO. 014469; February 8, 2001).

Table C.4. Mammalian Acute Toxicity 

Species % ai

Test

Type LD50 (mg/kg)

Toxicity

Category) MRID 

laboratory rat 

(Rattus norvegicus)

97.5 oral - single

dose

>5,000

(combined sexes)

practically

nontoxic

41866501

Table C.5. Mammalian Chronic Toxicity 

Species % ai Test

Type 

NOAEC/LOAEC

(ppm) Affected Endpoints MRID

laboratory rat 

(Rattus

norvegicus)

96.6 Three generation

reproduction

study

(feeding)

100/200 Fewer and lighter pups at birth

With a rat LD50 >5,000 mg /kg, oxadiazon may be characterized practically nontoxic to

mammals on an acute oral basis. The rat reproduction study showed a NOAEC/LOAEC of 100/200 ppm.

Chronic effects included fewer and lighter pups at birth.

Insect Acute Contact

A honey bee acute contact study using the TGAI is required to support outdoor uses. The acute

contact LD50, using the honey bee, Apis mellifera, is an acute contact, single-dose laboratory study

designed to estimate the quantity of toxicant required to cause 50% mortality in a test population of bees. 

The TGAI is administered by one of two methods: whole body exposure to technical pesticide in a

nontoxic dust diluent; or, topical exposure to technical pesticide via micro-applicator. The median lethal

dose (LD50) is expressed in micrograms of active ingredient per bee (ug a.i./bee). Toxicity category

descriptions are as follows:

If the LD50 is less than 2 :g a.i./bee, then the test substance is highly toxic.

If the LD50 is 2 to less than 11 :g a.i./bee, then the test substance is moderately toxic.

If the LD50 is 11 :g a.i./bee or greater, then the test substance is practically nontoxic

Study results are tabulated below.
48

Table C.6. Nontarget Insect Acute Contact Toxicity 

Species % ai LD50

(μg/bee)

Toxicity

Category

MRID/Lab/

Year

Study

Classification

Honey bee

(Apis mellifera)

95.9 > 25 practically 

nontoxic

42468301

California

Agricultural Research

Inc. 1992

Core

The LD50 for oxadiazon is greater than 25 ug per bee, characterizing oxadiazon practically

nontoxic to bees. The guideline (141-1) is fulfilled (MRID 42468301).

Insect Residual Contact

Honey bee toxicity of residues on foliage study is required on an end-use product for any pesticide

intended for outdoor application when the proposed use pattern indicates that honey bees may be exposed

to the pesticide and when the formulation contains one or more active ingredients having an acute contact

honey bee LD50 which falls in the moderately toxic or highly toxic range. Since oxadiazon is practically

nontoxic to honey bees a honey bee toxicity of residues on foliage (Guideline 141-2) is not required.

Terrestrial Plant Testing

The data were deemed inadequate for determining the EC25/NOAEC values of the most sensitive

species (Reference: D166982; 1995 memo to SRRD requesting repeat of all ten species due to very poor

study with numerous deficiencies and guideline deviations). To date, the studies have not been submitted

to EFED.

Aquatic Organism Toxicity

Toxicity to Freshwater Organisms

Freshwater Fish, Acute

Two freshwater fish toxicity studies using the TGAI are required to establish the toxicity of

oxadiazon to fish. The preferred test species are rainbow trout (a coldwater fish) and bluegill sunfish (a

warmwater fish). Toxicity category descriptions are as follows:

If the LC50 is less than 0.1 ppm a.i., then the test substance is very highly toxic.

If the LC50 is 0.1-to-1.0 ppm a.i., then the test substance is highly toxic.

If the LC50 is greater than 1 and up through 10 ppm a.i., then the test substance is moderately toxic.

If the LC50 is greater than 10 and up through 100 ppm a.i., then the test substance is slightly toxic.

If the LC50 is greater than 100 ppm a.i., then the test substance is practically nontoxic.

Study results are tabulated below.
49

Table C.7. Freshwater Fish 96-hr Acute Toxicity

Species/

Flow-through or

Static

% ai LC50 (ppm) Toxicity

Category

MRID /Lab/

Year

Study Classification

Bluegill sunfish 

(Lepomis macrochirus)

/static

97.4 0.88 (nominal) highly toxic

McCann / 1977

Supplemental1

Bluegill sunfish 

(Lepomis macrochirus /

flow-through

95.9 1.2 (measured) moderately 

toxic

42350601

ABC Labs., Inc.

1992

Core

Rainbow Trout

(Oncorhynchus 

mykiss)/static

97.4 1.05 (nominal) moderately 

toxic McCann /1977

Supplemental1

Rainbow Trout

(Oncorhynchus 

mykiss) / flow-through

95.9 1.2 (measured) moderately

toxic

42330401

ABC Labs, Inc.

1992

Core

1

 EFED considers McCann studies as scientifically sound and useful for risk assessment purposes, even though studies do not follow current

protocols and raw data are not available for verification of results. 

Based on the above studies, oxadiazon may be characterized moderately to highly toxic to

freshwater warmwater fish on an acute basis. The guideline 72-1(a) for bluegill is fulfilled (MRID

42350601 and McCann study). Oxadiazon may be characterized as moderately toxic to freshwater

coldwater fish on an acute basis. The guideline 72-1(c) for rainbow trout is fulfilled (MRID 42350401

and McCann study). 

Freshwater Fish, Chronic

A freshwater fish early life-stage test using the TGAI is required for oxadiazon because the enduse product may be transported to water from the intended use site, and an acute aquatic toxicity value is

less than 1 ppm. Acceptable freshwater test species are rainbow trout, brook trout, coho salmon, chinook,

bluegill, brown trout, lake trout, northern pike, fathead minnow, white sucker and channel catfish. The

fish early life-stage is a laboratory test designed to estimate the quantity of toxicant required to adversely

effect the reproductive capabilities of a test population of fish. The TGAI is administered into water

containing the test species, providing exposure throughout a critical life-stage, and the results are

expressed as a No Observed Adverse Effect Concentration (NOAEC) and LOAEC (Lowest Observed

Adverse Effect Concentration). Testing results are summarized below.
50

Table C.8. Freshwater Fish Chronic Toxicity

Species/Static or

Flow-through

Study Duration

% ai

NOAEC/LOAEC 

(ppb)/

(measured/nominal)

Endpoints

Affected

MRID/Lab/

Year

Study

Classification

Rainbow trout

(Oncorhynchus

mykiss)

>98% 

Radiopurity

0.88/1.7 

(measured)

egg hatch

ability

41811601

Analytical Biochemistry Labs, Inc.

1991

Supplemental1

Fathead minnow 

(Pimephales

promelas) / flowthrough/ 48 days

>98.5

Radiopurity

33 /84 

(measured) 

growth 

(length of fry)

42921601

Analytical Biochemistry Labs, Inc.

1993

Core

1 individual growth data were not included in report and were requested so statistics could not be verified.

The rainbow trout was found to be more sensitive than the fathead minnow in fish early life stage

testing. The study results of the rainbow trout study could not be verified, because individual growth data

were not included in the report. Although this information was requested (Reference: D165510; 1/13/97

asking for raw data, and also informing the registrant that the study was acceptable for use in a risk

assessment), the information has not been submitted. The guideline 72-4(a) for early life-stage fish testing

is fulfilled.

Freshwater Invertebrates, Acute

A freshwater aquatic invertebrate toxicity test using the TGAI is required to establish the toxicity

of oxadiazon to aquatic invertebrates. The preferred test organism is Daphnia magna, but early instar

amphipods, stoneflies, mayflies, or midges may also be used Study results are tabulated below.

Table C.9. Freshwater Invertebrate Acute Toxicity (48-hour)

Species/Static or Flowthrough % aiEC50 (ppm)/

(nominal/measured)

Toxicity

Category

MRID/Lab/

Year

Study

Classification

Daphnid

(Daphnia magna)/

static 

97.4 2.18 (nominal) moderately 

toxic McCann /1977

Supplemental1

Daphnid

(Daphnia magna) /

flow-through

95.9 >2.4 (measured) moderately toxic 42331801

Analytical biochemistry Labs., Inc.

1992

Core

1

 EFED considers McCann studies as scientifically sound and useful for risk assessment purposes.

Based on results of the above studies, oxadiazon may be categorized moderately toxic to

freshwater aquatic invertebrates on an acute basis. The guideline 72-2(a) is fulfilled (MRID 42331801

and McCann study).
51

Freshwater Invertebrate, Chronic

A freshwater aquatic invertebrate life-cycle test using the TGAI is required because the end-use

product is expected to be transported to water from the intended use site, and an aquatic acute LC50 is less

than 1.0 ppm. The preferred test species is Daphnia magna.

Table C.10. Freshwater Invertebrate Chronic 

Species/Static or Flowthrough/ Duration% ai NOAEC/LOAEC

 ppb

(nominal/measured)

Endpoints Affected MRID/Lab/

Year

Study

Classification

Daphnid

(Daphnia magna)/

flow-through/ 21-day

97.49 30 / 55

(measured)

survival; adult mean

length; mean time in

days to first brood and

young/adult/

reproduction day

41784301

Analytical Biochemistry Labs., 

1991 

Core

Based on the results of a 21-day daphnid chronic test survival with effects on adult growth, time in

days to first brood and number of young/adult/reproduction day at a LOAEL of 55 ppb , the NOAEC is 30

ppb. The guideline 72-4(b) for invertebrate life-cycle testing is fulfilled (MRID 41784301).

Toxicity to Estuarine and Marine Organisms

Estuarine and Marine Fish, Acute

Acute toxicity testing with estuarine and marine fish using the TGAI is required for oxadiazon

because the end-use product may reach the marine/estuarine environment. The preferred test organism is

the sheepshead minnow. Study results are tabulated below. 

Table C.11. Estuarine/Marine Fish Acute Toxicity

Species/static or

flowthrough % a.i.

LC50) ppm/

(measured/nominal)

Toxicity

Category

MRID/Lab/

Year Classification

Sheepshead minnow/

(Cyprinodon

variegatus)/ flowthrough

95.9 1.5 (measured) moderately toxic 42615801

Springborn Labs,

Inc.

1992

Core

Based on results of the above study, oxadiazon may be categorized moderately toxic to estuarine fish on

an acute basis. The guideline 72-3(a) is fulfilled (MRID 42615801).

Estuarine and Marine Fish, Chronic

No data are available.
52

Estuarine and Marine Invertebrates, Acute

Acute toxicity testing with estuarine/marine invertebrates using the TGAI is required for

oxadiazon because the end-use product may reach the marine/estuarine environment. The preferred test

species are mysid shrimp and eastern oyster. Study results are tabulated below. 

Table C.12 . Estuarine/Marine Invertebrate Acute Toxicity

Species/Static or 

Flow-through % a.i.

96-hour

EC50 (ppm)/

(measured/nominal)

Toxicity

Category

MRID/Lab/

Year

Study

Classification

Eastern oyster 

(Crassostrea virginica)/flowthrough (shell deposition)

95.9 0.7 (measured) highly toxic 42570301

Springborn 1992

Supplemental1

Mysid 

(Americamysis bahia)/flowthrough 

95.9 0.27 (measured) highly toxic 42615802

Springborn 1992

Core

1

 classified supplemental because average growth in controls was less than 2 mm.

Based on the results of the above studies, oxadiazon is considered to be highly toxic to estuarine

invertebrates on an acute basis.

Although the oyster study is classified supplemental, the study does not need to be repeated, since

the mysid was the more sensitive of the two species, and will be used for risk assessment purposes

(Reference: D182582; 3/16/95). The guideline 72- 3(b) for the oyster is considered fulfilled (MRID.

42570301). The guideline 72-3(c) for the mysid is fulfilled (MRID 42615802). 

Estuarine and Marine Invertebrate, Chronic

No data are available. The guideline 72-4(b) for the estuarine/marine invertebrate life cycle is not

fulfilled. 

Aquatic Plants

Tier I or Tier II aquatic plant growth testing using the TEP is required for fungicides. The

recommendation is for five species: freshwater green alga (Selenastrum capricornutum), duckweed

(Lemna gibba), marine diatom (Skeletonema costatum), blue-green algae (Anabaena flos-aquae), and a

freshwater diatom. Results of testing with the TGAI are below.
53

Table C.13. Nontarget Aquatic Plant Toxicity (Tier II)

Species/duration % A. I.

EC50/NOAEC

(ppb)

MRID No.

Author/year Classification1

duckweed

(Lemna gibba)/

14 day

97.49 41 / <8 

(measured)

41610107

Springborn Laboratories Inc.

1990

Supplemental1

freshwater green

algae

(Selenastrum

capricornutum)

/120 hrs.

97.49 8 /5.6

(measured)

41610108

Springborn Laboratories Inc.

1990

Core

marine diatom

(Skeletonema

costatum)/120

hrs.

97.49 5.2 / 1.4

(measured)

41610105

Springborn Laboratories Inc.

1990

Core

freshwaterr

diatom

(Navicula

pelliculosa)/120

hrs.

97.49 126 / 27

(measured)

41610106

Springborn Laboratories Inc.

1990

Core

1 the study was classified supplemental primarily because the exposure concentrations used in the test were too high to establish a 

NOAEC. 

2

 study deemed invalid because EC50 not established; registrant was asked to repeat study (Reference: D188355; 4/7/95).

With an EC50 of 5.2 ppb, the marine diatom appears to be the most sensitive non-vascular aquatic plant

species tested.

Guideline 123-2 (Tier II) is fulfilled for four of the five species required (MRIDs 41610195

through 41610108). Although the duckweed study was classified supplemental, it does not have to be

repeated since adequate information was provided for risk assessment purposes (Reference: D182582;

3/16/95). A data gap exists for freshwater blue-green algae (Anabaena flos-aquae).
54

APPENDIX D: TERRESTRIAL MODEL RUNS

ELL-Fate Version 1.2 July 19, 2001

Developed by Laurence Libelo. February, 1999

This spreadsheet based model calculates the decay of a chemical applied to foliar surfaces 

for single or multiple applications. It uses the same principle as the batch code models 

FATE and TERREEC for calculating terrestrial estimates exposure (TEEC) concentrations on

plant surfaces following application.

A first order decay assumption is used to determine the concentration at each day after initial

application based on the concentration resulting from the initial and additional applications. The

decay is calculated by from the first order rate equation:

CT = Cie-kT

or in integrated form:

ln (CT/Ci) = kT

Where 

CT = concentration at time T = day zero.

Ci = concentration, in parts per million (PPM) present initially (on day zero) on the surfaces.

Ci is calculated based on Kenaga and Fletcher by multiplying the Ci is calculated based on the Kanaga

nomogram (Hoerger and Kenaga, (1972) as modified Fletcher (1994). For maximum concentration the

application rate, in pounds active ingredient per acre, is multiplied by 240 for Short Grass, 110 for Tall

Grass, and 135 for Broad leafed plants/insects and 15 for Seeds. Additional applications are converted

from pounds active ingredient per acre to PPM on the plant surface and the additional mass added to the

mass of the chemical still present on the surfaces on the day of application.

k = degradation rate constant determined from studies of hydrolysis, photolysis, microbial

degradation etc. Since degradation rate is generally reported in terms of half-life the rate constant is

calculated from the input half-life (k = ln 2/T1/2) instead of being input directly. Choosing which

processes controls the degradation rate and which half-life to use in terrestrial exposure calculations is

open for debate and should be done by a qualified scientist.

T = time, in days, since the start of the simulation. The initial application is on day 0. The

simulation is hardwired to run for 365 days.

The program calculates concentration on each type of surface on a daily interval for one year. 
55

The maximum concentration during the year and the average concentration during the first 56

days are calculated.

The inputs used to calculate the amount of the chemical present are in highlighted in yellow on

the spread sheet. Outputs are in blue. The inputs required are:

Application Rate: The maximum label application rate (in pounds ai/acre)

Half-life: The degradation half-life for the dominate process(in days)

Frequency of Application: The interval between repeated applications, from the label (in days)

Maximum # Application per year: From the label

The calculated concentrations are used to calculate Avian and Mammalian RQ values. The maximum

calculated concentration is divided by user input values of Chronic No Observable Adverse Effects Level

and acute LC50 to give RQs for each plant type.

The rat LC 50 is calculated by dividing the mammalian LD 50 by 0.05 (to correct for actual food

consumption)

For 15g, 35g and 1000 g mammals the RQ values are calculated by dividing the maximum concentration

for each surface by the LD 50 or NOAEL corrected for consumption (0.95, 0.66 and .15 body wt. for

herbivores and insectivores and 0.21, 0.15 and 0.3 body wt. for granivore)

The number of days that the input value of Chronic No Observable Adverse Effects Level and

acute LC50 are exceeded in the first 56 days is calculated by comparing the input value to the

calculated concentration.

A graph of concentration on each plant surface vs time is plotted and a "level of concern" line

can be added at a user specified level.

The maximum single application which can be applied and not exceed the toxicity input values if

calculated by dividing the input value by the Kenaga maximum concentration for Short Grass (240).
56

Chemical Name: Oxadiazon Use turf

 Formulation EC

Inputs Application Rate 4 lbs a.i./acre

Half-life 35 days 

Frequency of Application 182 days

Maximum # Apps./Year 2

Outputs Maximum 56 day Average

Concentration Concentration 

(PPM) (PPM)

Short Grass 986.12 585.83

Tall Grass 451.97 268.51 # days 

Broadleaf plants/Insects 554.69 329.53 Exceeded

Seeds 61.63 36.61 on short grass

(in first 56)

Avian Acute LC 50 (ppm) 5000 0

Chronic NOAEC (ppm) 500 33 Max Single Application

which does NOT exceed

Acute RQ Chronic RQ Avian Acute 20.833

(Max. res. mult. apps.) Avian Chronic 2.083 (lb a.i.)

Short Grass 0.20 1.97

Tall Grass 0.09 0.90 # days Mammalian Acute 138.89

Broadleaf plants/Insects 0.11 1.11 Exceeded Mammalian Chronic 0.42

Seeds 0.01 0.12 on short grass

(in first 56)

Mammalian Acute LD 50 (mg/kg) 5000 0 Rat Calculated LC 50 (ppm) 100000

Chronic NOAEL (mg/kg) 100 56

15 g mammal 35 g mammal 1000 g mammal

Rat Acute Rat Chronic

Acute RQ Acute RQ Acute RQ Dietary Dietary

(mult. apps) (mult. apps) (mult. apps) RQ RQ

Short Grass 0.19 0.13 0.03 0.01 9.86

Broadleaf plants/ insects 0.09 0.06 0.01 0.00 4.52

Large Insects 0.11 0.07 0.02 0.01 5.55

Seeds (granivore) 0.00 0.00 0.00 0.00 0.62
57

Chemical Name: Oxadiazon Use turf

 Formulation EC

Inputs

Application Rate 3 lbs a.i./acre

Half-life 35 days 

Application Interval 182 days

Maximum # Apps./Year 2

Outputs Maximum 56 Day Average

Concentration Concentration 

(PPM) (PPM)

Short Grass 739.59 439.37

Tall Grass 338.98 201.38 # days 

Broadleaf plants/Insects 416.02 247.15 Exceeded

Seeds 46.22 27.46 on short grass

(in first 56)

Avian Acute LC50 (ppm) 5000 0

Chronic NOAEC (ppm) 500 19 Max Single Application

which does NOT exceed

Acute RQ Chronic RQ Avain Acute 20.833

(Max. res. mult. apps.) Avian Chronic 2.083 (lb a.i.)

Short Grass 0.15 1.48

Tall Grass 0.07 0.68 # days Mammalian Acute 138.89

Broadleaf plants/Insects 0.08 0.83 Exceeded Mammalian Chronic 2.78

Seeds 0.01 0.09 on short grass

(in first 56)

Mammalian Acute LD50 (mg/kg) 5000 0 Rat Calculated LC 50 (ppm) 100000

Chronic NOAEL (mg/kg) 100 56

15 g mammal 35 g mammal 1000 g mammal

Chronic RQ Chronic RQ Chronic RQ Rat Acute Rat Chronic

Acute RQ (Max. res. ) Acute RQ (Max. res. ) Acute RQ (Max. res. ) Dietary Dietary

(mult. apps) mult. apps.) (mult. apps) mult. apps.) (mult. apps) mult. apps.) RQ RQ

Short Grass 0.14 7.03 0.10 4.88 0.02 1.11 0.01 7.40

Tall Grass 0.06 3.22 0.04 2.24 0.01 0.51 0.00 3.39

Broadleaf plants/Insects 0.08 3.95 0.05 2.75 0.01 0.62 0.00 4.16

Seeds 0.01 0.44 0.01 0.31 0.00 0.07 0.00 0.46
58

Chemical Name: Oxadiazon Use turf

 Formulation EC

Inputs Application Rate 2 lbs a.i./acre

Half-life 35 days 

Application Interval 182 days

Maximum # Apps./Year 2

Outputs Maximum 56 Day Average

Concentration Concentration 

(PPM) (PPM)

Short Grass 493.06 292.92

Tall Grass 225.99 134.25 # days 

Broadleaf plants/Insects 277.35 164.77 Exceeded

Seeds 30.82 18.31 on short grass

(in first 56)

Avian Acute LC 50 (ppm) 5000 0

Chronic NOAEC (ppm) 500 0 Max Single Application

which does NOT exceed

Acute RQ Chronic RQ Avain Acute 20.833

(Max. res. mult. apps.) Avian Chronic 2.083 (lb a.i.)

Short Grass 0.10 0.99

Tall Grass 0.05 0.45 # days Mammalian Acute 138.89

Broadleaf plants/Insects 0.06 0.55 Exceeded Mammalian Chronic 2.78

Seeds 0.01 0.06 on short grass

(in first 56)

Mammalian Acute LD 50 (mg/kg) 5000 0 Rat Calculated LC 50 (ppm) 100000

Chronic NOAEL (mg/kg) 100 56

15 g mammal 35 g mammal 1000 g mammal

Chronic RQ Chronic RQ Chronic RQ Rat Acute Rat Chronic

Acute RQ (Max. res. ) Acute RQ (Max. res. ) Acute RQ (Max. res. ) Dietary Dietary

(mult. apps) mult. apps.) (mult. apps) mult. apps.) (mult. apps) mult. apps.) RQ RQ

Short Grass 0.09 4.68 0.07 3.25 0.01 0.74 0.00 4.93

Tall Grass 0.04 2.15 0.03 1.49 0.01 0.34 0.00 2.26

Broadleaf plants/Insects 0.05 2.63 0.04 1.83 0.01 0.42 0.00 2.77

Seeds 0.01 0.29 0.00 0.20 0.00 0.05 0.00 0.31

units = weeks not days
59

Chemical Name: Oxadiazon Use turf

 Formulation EC

Inputs Application Rate 1.33 lbs a.i./acre

Half-life 5 days 

Application Interval 8 days

Maximum # Apps./Year 6

Outputs Maximum 56 Day Average

Concentration Concentration 

(PPM) (PPM)

Short Grass 475.72 257.05

Tall Grass 218.04 117.82 # days 

Broadleaf plants/Insects 267.59 144.59 Exceeded

Seeds 29.73 16.07 on short grass

(in first 56)

Avian Acute LC 50 (ppm) 5000 0

Chronic NOAEC (ppm) 500 0 Max Single Application

which does NOT exceed

Acute RQ Chronic RQ Avain Acute 20.833

(Max. res. mult. apps.) Avian Chronic 2.083 (lb a.i.)

Short Grass 0.10 0.95

Tall Grass 0.04 0.44 # days Mammalian Acute 138.89

Broadleaf plants/Insects 0.05 0.54 Exceeded Mammalian Chronic 2.78

Seeds 0.01 0.06 on short grass

(in first 56)

Mammalian Acute LD 50 (mg/kg) 5000 0 Rat Calculated LC 50 (ppm) 100000

Chronic NOAEL (mg/kg) 100 52

15 g mammal 35 g mammal 1000 g mammal

Chronic RQ Chronic RQ Chronic RQ Rat Acute Rat Chronic

Acute RQ (Max. res. ) Acute RQ (Max. res. ) Acute RQ (Max. res. ) Dietary Dietary

(mult. apps) mult. apps.) (mult. apps) mult. apps.) (mult. apps) mult. apps.) RQ RQ

Short Grass 0.09 4.52 0.06 3.14 0.01 0.71 0.00 4.76

Tall Grass 0.04 2.07 0.03 1.44 0.01 0.33 0.00 2.18

Broadleaf plants/Insects 0.05 2.54 0.04 1.77 0.01 0.40 0.00 2.68

Seeds 0.01 0.28 0.00 0.20 0.00 0.04 0.00 0.30

units = weeks not days
60

Chemical Name: Oxadiazon Use turf

 Formulation EC

Inputs Application Rate 1 lbs a.i./acre

Half-life 5 days 

Application Interval 8 days

Maximum # Apps./Year 8

Outputs Maximum 56 Day Average

Concentration Concentration 

(PPM) (PPM)

Short Grass 358.09 66.16

Tall Grass 164.13 30.32 # days 

Broadleaf plants/Insects 201.43 37.21 Exceeded

Seeds 19.95 4.13 on short grass

(in first 56)

Avian Acute LC 50 (ppm) 5000 0

Chronic NOAEC (ppm) 500 0 Max Single Application

which does NOT exceed

Acute RQ Chronic RQ Avain Acute 20.833

(Max. res. mult. apps.) Avian Chronic 2.083 (lb a.i.)

Short Grass 0.06 0.64

Tall Grass 0.03 0.29 # days Mammalian Acute 138.89

Broadleaf plants/Insects 0.04 0.36 Exceeded Mammalian Chronic 2.78

Seeds 0.00 0.04 on short grass

(in first 56)

Rat Calculated LC 100000 50 Acute LD 5000 0 (ppm) 50 Mammalian (mg/kg)

Chronic NOAEL (mg/kg) 100 16

15 g mammal 35 g mammal 1000 g mammal

Chronic RQ Chronic RQ Chronic RQ Rat Acute Rat Chronic

Acute RQ (Max. res. ) Acute RQ (Max. res. ) Acute RQ (Max. res. ) Dietary Dietary

(mult. apps) mult. apps.) (mult. apps) mult. apps.) (mult. apps) mult. apps.) RQ RQ

Short Grass 0.06 3.03 0.04 2.11 0.01 0.48 0.00 3.19

Tall Grass 0.03 1.39 0.02 0.97 0.00 0.22 0.00 1.46

Broadleaf plants/Insects 0.03 1.71 0.02 1.18 0.01 0.27 0.00 1.80

Seeds 0.00 0.19 0.00 0.13 0.00 0.03 0.00 0.20
61

APPENDIX E: DRINKING WATER CONCENTRATIONS

The Tier I Estimated Environmental Concentrations were calculated using the computer models

FIRST (surface waters) and SCIGROW (ground waters). A copy from the electronic document generated

by EFED appears next.

Drinking Water Memo:

U. S. ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, DC 20460

. OFFICE OF 

PREVENTION, PESTICIDES

AND TOXIC SUBSTANCES 

May 8, 2001

DPBarcode: D273599

PC Code 109001

MEMORANDUM

SUBJECT: Tier I Estimated Environmental Concentrations of Oxadiazon

FROM: José Luis Meléndez, Chemist

Environmental Risk Branch V

Environmental Fate and Effects Division (7507C)

THROUGH: Mah T. Shamim, Ph.D., Chief

Environmental Risk Branch V

Environmental Fate and Effects Division (7507C)

TO: Margaret Rice, Acting Branch Chief

Veronique LaCapra, CRM

and Tom Myers, Team Leader

Special Review and Reregistration Division (7508C)

This memo presents the Tier I Estimated Environmental Concentrations (EECs) for oxadiazon, calculated

using FIRST (surface water) and SCIGROW (ground water) for use in the human health risk assessment. For

surface water, the acute (peak) value is 246 ppb and the annual average value is 100 ppb. The groundwater

screening concentration is 0.6 ppb. These values generally represent upper-bound estimates of the

concentrations that might be found in surface water and groundwater due to the use of oxadiazon on turf,

which is the major use of the chemical.

Background Information on FIRST:

FIRST is a new screening model designed to estimate the pesticide concentrations found in water for use in
62

drinking water assessments. It provides high-end values on the concentrations that might be found in a small

drinking water reservoir due to the use of pesticide. Like GENEEC, the model previously used for Tier I

screening level, FIRST is a single-event model (one run-off event), but can account for spray drift from

multiple applications. FIRST takes into consideration the so called Index Drinking Water Reservoir by

representing a larger field and pond than the standard GENEEC scenario. The FIRST scenario includes a 427

acres field immediately adjacent to a 13 acres reservoir, 9 feet deep, with continuous flow (two turnovers per

year). The pond receives a spray drift event from each application plus one runoff event. The runoff event

moves a maximum of 8% of the applied pesticide into the pond. This amount can be reduced due to

degradation on field and the effect of binding to soil. Spray drift is equal to 6.4% of the applied concentration

from the ground spray application and 16% for aerial applications.

FIRST also makes adjustments for the percent crop area. While FIRST assumes that the entire watershed

would not be treated, the use of a PCA is still a screen because it represents the highest percentage of crop

cover of any large watershed in the US, and it assumes that the entire crop is being treated. Various other

conservative assumptions of FIRST include the use of a small drinking water reservoir surrounded by a

runoff-prone watershed, the use of the maximum use rate, no buffer zone, and a single large rainfall

Background Information on SCIGROW:

SCIGROW provides a groundwater screening exposure value to be used in determining the potential risk to

human health from drinking water contaminated with the pesticide. Since the SCIGROW concentrations are

likely to be approached in only a very small percentage of drinking water sources, i.e., highly vulnerable

aquifers, it is not appropriate to use SCIGROW for national or regional exposure estimates.

SCIGROW estimates likely groundwater concentrations if the pesticide is used at the maximum allowable

rate in areas where groundwater is exceptionally vulnerable to contamination. In most cases, a large majority

of the use area will have groundwater that is less vulnerable to contamination than the areas used to derive

the SCIGROW estimate.

Modeling Inputs and Results:

Tables 1 and 2 summarize the input values used in the model runs for FIRST 1.0 and SCIGROW,

respectively. The lowest non-sand KD was used in FIRST 1.0. The median KOC value was used in

SCIGROW. The available aerobic soil metabolism half-life for oxadiazon was extremely high. For FIRST,

stability was assumed, while the extrapolated value of 841 days was used in SCIGROW. The modeling

results associated with maximum allowable rate per year (4 lb ai/A applied twice at 6 months interval) are

presented in Table 3. Attached to this memo are copies of the original printouts generated from FIRST and

SCIGROW runs.

cc: Nancy McCarroll (HED)
63

Table 1. Environmental Fate and Other Input Parameters for the Estimation of Oxadiazon using FIRST

Parameter Value Source

Water Solubility (25°C) 1 ppm One-Liner

Hydrolysis Half-Life (pH 7) stable MRID 41863603

Aerobic Soil Metabolism Half-Life (from 6 values) essentially stable MRID 42772801

Aerobic Aquatic Metabolism Half-life not available N/A

Aqueous Photolysis Half-Life 2.75 days MRID 41897201

Soil/Water Partition Coefficient (Lowest non-sand Kd) 16.9 MRID 41898202

Pesticide is Wetted-In Yes Labels

PCA (turf) 0.87 Default

Depth of Incorporation (Broadcast) 0.0 inch Labels

Table 2. Environmental Fate Input Parameters for the Estimation of Oxadiazon using SCIGROW.

Parameter Value Source

Organic Carbon Partition Coefficient (median KOC) 2376 MRID 41898202

Aerobic Soil Metabolism Half-Life (median) 841 days MRID 42772801

Table 3. Modeling Results for Use of Oxadiazon on (Turf) Golf Courses

Parameter Value Source

Application Method Ground Spray Labels

Application Rate 4.0 lb a.i./A Registrant

Applications Permitted per Year 2 Registrant***

Application Interval (days) 182 Registrant

FIRST 1.0 Peak Untreated Water Concentration 246 ppb N/A

FIRST 1.0 Annual Average Untreated Water Concentration 100 ppb N/A

SCIGROW Ground Water Concentration 0.6 ppb N/A

***The Registrant supports multiple applications, at lower application rates.
64

 RESULTS OBTAINED USING FIRST

 RUN No. 1 FOR OXADIAZON ON Turf (Golf * INPUT VALUES * 

 --------------------------------------------------------------------

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE %CROPPED INCORP

 ONE(MULT) INTERVAL Kd (PPM ) (%DRIFT) AREA (IN)

 --------------------------------------------------------------------

 4.000( 8.000) 2 182 16.9 1.0 GROUND( 6.4) 87.0 .0

 FIELD AND RESERVOIR HALFLIFE VALUES (DAYS) 

 --------------------------------------------------------------------

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

 (FIELD) RAIN/RUNOFF (RESERVOIR) (RES.-EFF) (RESER.) (RESER.) 

 --------------------------------------------------------------------

 .00 0 N/A 2.75- 341.00 .00 341.00

 UNTREATED WATER CONC (MICROGRAMS/LITER (PPB)) Ver 1.0 MAY 1, 2001

 --------------------------------------------------------------------

 PEAK DAY (ACUTE) ANNUAL AVERAGE (CHRONIC) 

 CONCENTRATION CONCENTRATION 

 --------------------------------------------------------------------

 246.388 100.013

 RESULTS OBTAINED USING SCIGROW

 RUN No. 1 FOR OXADIAZON INPUT VALUES

 --------------------------------------------------------------------

 APPL (#/AC) APPL. URATE SOIL SOIL AEROBIC

 RATE NO. (#/AC/YR) KOC METABOLISM (DAYS)

 --------------------------------------------------------------------

 4.000 2 8.000 2376.0 841.0

 GROUND-WATER SCREENING CONCENTRATIONS IN PPB

 --------------------------------------------------------

 .592986

 --------------------------------------------------------

 A= 836.000 B= 2381.000 C= 2.922 D= 3.377 RILP= 1.821

 F= -1.130 G= .074 URATE= 8.000 GWSC= .592986
65

APPENDIX F: EXPOSURE AND RISK CHARACTERIZATION

Risk characterization integrates the results of the exposure and ecotoxicity data to evaluate the likelihood of

adverse ecological effects. The means of this integration is called the quotient method. Risk quotients (RQs)

are calculated by dividing exposure estimates by acute and chronic ecotoxicity values: RQ =

exposure/toxicity. To evaluate the potential risk to aquatic and terrestrial organisms from the use of

Oxadiazon, risk quotients (RQs) are calculated from the ratio of estimated environmental concentrations

(EECs) to ecotoxicity values. 

For risk assessments, EFED used dosage rate information obtained from SRRD and BEAD. Since most of

the use is on golf courses, turf was chosen to represent all sites. 

Terrestrial and aquatic risk assessments were based on:

- 4 lb ai/A liquid and granular product at 1 and 2 applications per year with a six month reapplication

interval

- 2.4 lb ai/A liquid product at 2 application per year with a six month reapplication interval

(NOTE: aquatic risk assessments did not include granular formulations)

RQs are then compared to levels of concern (LOC) criteria used by OPP for determining potential risk to

nontarget organisms and the subsequent need for possible regulatory action. The criteria indicate that a

pesticide used as directed has the potential to cause adverse effects on nontarget organisms. LOCs currently

address the following risk presumption categories: (1) acute high -- potential for acute risk is high; regulatory

action may be warranted in addition to restricted use classification, (2) acute restricted use -- the potential

for acute risk is high, but it may be mitigated through restricted use classification, (3) acute species - the

potential for acute risk to species is high, and regulatory action may be warranted, and (4) chronic risk - the

potential for chronic risk is high, and regulatory action may be warranted. Currently, EFED does not perform

assessments for chronic risk to plants, acute or chronic risks to nontarget insects, or chronic risk from

granular/bait formulations to birds or mammals.

The ecotoxicity test values (measurement endpoints) used in the acute and chronic risk quotients are derived

from required studies. Examples of ecotoxicity values derived from short-term laboratory studies that assess

acute effects are: (1) LC50 (fish and birds), (2) LD50 (birds and mammals), (3) EC50 (aquatic plants and aquatic

invertebrates) and (4) EC25 (terrestrial plants). Examples of toxicity test effect levels derived from long-term

laboratory studies that assess chronic effects are: (1) LOAEC (birds, fish, and aquatic invertebrates) and (2)

NOAEC (birds, fish and aquatic invertebrates). Generally, the most sensitive species tested are used. The

NOAEC is used as the ecotoxicity test value in assessing chronic effects to birds, mammals, fish, and aquatic

invertebrates. 

Risk presumptions and the corresponding RQs and LOCs, are tabulated in Table 1.
66

Table 1. Risk presumptions for terrestrial organisms

Risk Presumption RQ LOC

Birds

Acute High Risk EEC1

/LC50 or LD50/sqft2 or LD50/day3 0.5

Acute Restricted Use EEC/LC50 or LD50/sqft or LD50/day (or LD50 < 50

mg/kg)

0.2

Acute Species EEC/LC50 or LD50/sqft or LD50/day 0.1

Chronic Risk EEC/NOAEC 1

Wild Mammals

Acute High Risk EEC/LC50 or LD50/sqft or LD50/day 0.5

Acute Restricted Use EEC/LC50 or LD50/sqft or LD50/day (or LD50 < 50

mg/kg)

0.2

Acute Species EEC/LC50 or LD50/sqft or LD50/day 0.1

Chronic Risk EEC/NOAEC 1

1

 abbreviation for Estimated Environmental Concentration (ppm) on avian/mammalian food items 

2

 mg/ft2 3 mg of toxicant consumed/day

 LD50 * wt. of bird LD50 * wt. of bird 

Risk presumptions for aquatic organisms

Risk Presumption RQ LOC

Acute High Risk EEC1

/LC50 or EC50 0.5

Acute Restricted Use EEC/LC50 or EC50 0.1

Acute Species EEC/LC50 or EC50 0.05

Chronic Risk EEC/NOAEC 1

1

 EEC = (ppm or ppb) in water
67

Risk presumptions for plants

Risk Presumption RQ LOC

Plant Inhabiting Terrestrial and Semi-Aquatic Areas 

Acute High Risk EEC1

/EC25 1

Acute Species EEC/EC05 or NOAEC 1

Aquatic Plants

Acute High Risk EEC2

/EC50 1

Acute Species EEC/EC05 or NOAEC 1

1

 EEC = lbs a.i./A 

2

 EEC = (ppb or ppm) in water 
68

Table 2. Selection of Toxicological Endpoints Used to Determine Risk Quotients (RQs)

Type Of Toxicity Organism Species Toxicological Endpoint

Oral Acute

 Bird

Mallard 1040 mg/kg

Dietary Bobwhite/Mallard >5000 ppm

Chronic Bobwhite 500 ppm 1

Oral Acute

 Mammal

Rat >5000 mg/kg

Chronic Rat 100 ppm 2

 Acute

 Freshwater Fish

Rainbow trout/Bluegill 0.88 ppm

Chronic Rainbow trout 0.88 ppb 3

Acute

 Freshwater

 Invertebrates

Daphnid 2.18 ppm

Chronic Daphnid 0.03 ppm 

Acute

 Estuarine Fish

Sheepshead Minnow 1.5 ppm

Chronic Sheepshead Minnow 0.0015 ppm4

Acute

 Estuarine

Invertebrates

Mysid 0.27 ppm

Chronic Mysid 0.0037 ppm4

Acute Aquatic Plants

 (vascular)

 Aquatic Plants

 (Nonvascular)

duckweed 

marine diatom

EC50 =41 ppb;

NOAEC = <8 ppb

EC50 = 5.2 ppb

1

 No effects on any reproductive parameter or viability of of F1 offspring at the highest dose tested, 1000 ppm; however due to

 excessive mortality (33%) of adult female birds in that dose level, a NOAEC for chronic effects was set at 500 ppm. 2

 Based on LOAEL of >38 mg/kg/ day for inactive mammary tissue and fetal/pup death observed in the one year range-finding 

test of a rat reproduction study. NOAEC > 100 ppm.

3

 Rainbow trout was more sensitive than the fathead minnow (fathead minnow NOAEC= 33 ppb). 

 4 Extrapolation from acute/chronic ratio.
69

Table 6. Environmental Fate Input Parameters for GENEEC 2.0.

Chemical Oxadiazon

PC Code 109001

Water Solubility (25°C) 1 ppm

Hydrolysis Half-Life (pH 7) stable

Aerobic Soil Metabolism Half-Life stable

Aerobic Aquatic Metabolism Half-life not available

Photolysis Half-Life 2.75 days

Soil/Water Equilibrium Partition Coefficient (Kd) 16.91

Depth of Incorporation (Broadcast) 0.0 in.

Wetted-In Yes

Table 7. Modeling Results for Use on Turf

Ground spray1 granular

Application Rate 2.0 3.0 4.0 4.0 4.0

Application Frequency 1 1 1 2 2

Application Interval (days) N/A N/A 182 182 182

GENEEC 2.0

Peak EEC 44 67 89 173 150

21-Day EEC 43 65 87 170 147

60-Day EEC 42 63 84 163 142

1. Low boom ground sprayer with fine spray quality (EFED defaults), no buffer (no spray zone).
70

APPENDIX G: ENVIRONMENTAL FATE AND ECOLOGICAL EFFECTS DATA

REQUIREMENTS

Table of Data Requirements of Ecological Effects for Oxadiazon

Guideline # Data Requirement

Is Data

Requirement

Satisfied? MRID #’s

Study

Classification

71-1 850.2100 Avian Oral LD50 yes 41610101 core

71-2 850.2200 Avian Dietary LC50 yes 41610102

41610103

core

core

71-4 850.2300 Avian Reproduction yes 41993201

41993202

supplemental

core

72-1 850.1075 Freshwater Fish LC50 yes 42350601

42330401

core

core

72-2 850.1010 Freshwater

Invertebrate Acute

LC50

yes 41784301 core

72-3(a) 850.1075 Estuarine/Marine

Fish LC50

yes 42615801 core

72-3(b) 850.1025 Estuarine/Marine

Mollusk EC50

yes 42570301 core

72-3(c) 850.1035

850.1045

Estuarine/Marine

Shrimp EC50

yes 42615802 core

72-4(a) 850.1400 Estuarine Fish Early

Life-Stage

no

72-4(b) 850.1300

850.1350

Estuarine/ Marine

Invertebrate LifeCycle

no

72-5 850.1500 Freshwater Fish Full

Life-Cycle

na - -

122-

1(a)

850.4100 Seed Germ./Seedling

Emergence

no - -

122-

1(b)

850.4150 Vegetative Vigor no - -
71

122-2 850.4400 Aquatic Plant

Growth

partially - -

123-

1(a)

850.4225 Seed Germ./Seedling

Emergence

no

123-

1(b)

850.4250 Vegetative Vigor no

123-2 850.4400 Aquatic Plant

Growth

partially 5 41610105

41610106

41610106

41610108

core

141-1 850.3020 Honey Bee Acute

Contact LD50

yes 4268301 core

141-2 850.3030 Honey Bee Residue

on Foliage

not required

70-1 Acute and Chronic

Sediment Toxicity

Testing

no6

70-1 Aquatic

Phototoxicity Studies 

no

1

 Although the mallard study was supplemental since a NOAEC was not established, the study does not have to be repeated; the bobwhite was

more sensitive and was used for risk assessment purposes.

2

 Early-life stage fish testing with an estuarine species is required. The raw data for the rainbow trout study MRID 41811601 must be

submitted. This information was requested in 1997 under D165510. 

4

 The rates used should be low enough to elicit an NOAEC or allow for accurate estimation of the EC05 for all measured parameters. The

measured endpoints should include: shoot length, root length and/or height, and a phytotoxic rating of the visible effects. Testing must be

conducted with a liquid typical end-use product, rather than technical product, due to the insolubility of the material and since historically,

plant species have been found to be more sensitive to the end-use product, than technical. Concentrations must be measured, and results must

be based on measured concentrations. The nominal concentrations used in statistical analyses most likely did not represent actual exposure. 

This information was requested in 1995 under D166982.

5

 A freshwater blue-green algae (Anabaena flos-aquae) is required; the study submitted (MRID 41610104) was invalid.

6

 The high KOC of oxadiazon, combined with the high persistance exhibited in the aerobic soil metabolism, as well as the anaerobic aquatic

metabolism (>>10 days) trigger the requirement of a Chronic Sediment Toxicity Testing with both Hyalella azteca and Chironomus tentans.
2

 Waived due to the relatively low vapor pressure for oxadiazon (1.00x10-6 mm Hg).

3

Satisfied by submission of an Anaerobic Aquatic Metabolism study.

72

Table of Data Requirements of Environmental Effects for Oxadiazon

Guideline # Data Requirement

Is Data

Requirement

Satisfied?

MRID #’s Study

Classificatio

n

161-1 835.2120 Hydrolysis Yes 41863603 acceptable

161-2 835.2240 Photodegradation in

Water

Yes 41897201 acceptable

161-3 835.2410 Photodegradation on

Soil

Yes 41898201 acceptable

161-4 835.2370 Photodegradation in

Air

waived2 N/A N/A

162-1 835.4100 Aerobic Soil

Metabolism

Yes 42772801 acceptable

162-2 835.4200 Anaerobic Soil

Metabolism

Yes3 NA N/A

162-3 835.4400 Anaerobic Aquatic

Metabolism

Yes 42773802 supplemental

162-4 835.4300 Aerobic Aquatic

Metabolism

not required N/A N/A

163-1 835.1240

835.1230

LeachingAdsorption/DesorptionYes 44555608

,

41898202

acceptable,

supplemental

163-2 835.1410 Laboratory Volatility waived1 N/A N/A

163-3 835.8100 Field Volatility waived1 N/A N/A

164-1 835.6100 Terrestrial Field

Dissipation

Yes 41767401 acceptable

164-2 835.6200 Aquatic Field

Dissipation

not required N/A N/A

164-3 835.6300 Forestry Dissipation not required N/A N/A
73

164-4 835.6400 Combination Products

and Tank Mixes

Dissipation

not required N/A N/A

165-4 850.1730 Accumulation in Fish Yes 42226701 acceptable

165-5 850.1950 Accumulation- aquatic

non-target

not required N/A N/A

166-1 835.7100 Ground Water- small

prospective

not required N/A N/A

201-1 840.1100 Droplet Size Spectrum reserved N/A N/A

202-1 840.1200 Drift Field Evaluation reserved N/A N/A
74

APPENDIX H: QUALITATIVE USE ASSESSMENT

Case No.: 2485 PC Code:109001

Date: 01-10-01 Analyst: Stephen Smearman

Oxadiazon is a selective preemergence and early post emergence herbicide used primarily to control

annual grasses and broadleaf weeds. The tradename for Oxadiazon in the US is Ronstar (formerly

Chipco Ronstar) and formulations are available as emulsifiable concentrates, granules, flowable and

wetable powders.

Based upon the available EPA data and other pesticide usage survey information for Oxadiazon on all

sites for the years 1989 through 1999, an annual estimate of Oxadiazon’s total usage on all sites averaged

249,000 pounds of active ingredient (a.i.) on an average of 52,000 acres treated over the last 10 years. 

Most of the acreage is treated with up to 2.4 pounds of a.i. per acre owing most of the usage applies to

golf courses which based on reported usage has higher application rates than other uses. Oxadiazon’s

largest markets in terms of total pounds of active ingredient is allocated to golf courses (65%). The

remaining usage is primarily for horticultural/nursery uses and on processed tomatoes (22%). Other

reported uses include potatoes and barley which accounts for an respective 1.5% and less than 1% of the

total pounds a.i. annually. However, there are no tolerances nor labeled uses for these site and therefore

should not be considered during risk assessment. However, there is international reported use of

Oxadiazon on rice in China and on cotton in Mexico and Sudan.

Additional estimates of total acres grown and total acres treated for the non-crop sites of road right of

ways (ROW), landscape maintenance, horticultural/nursery and park uses are not currently available

although there is estimates of pounds of a.i.applied. The following table illustrates the usage of

Oxadiazon. 
75

USAGE OF OXADIAZON

Site Acres 

Grown

(000)

Acres Treated

(000)

% of Crop

Treated

LB AI Applied

(000)

Average Application

Rate

Wtd

 Avg

Est

Max

Wtd

 Avg

Est

Max

Wtd

 Avg

Est

Max

lb ai/ 

acre/yr

#appl

/ yr

lb ai/ 

A/appl

*Potatoes 1,373 2 4 0% 0% 4 8 2.0 1.0 2.0 FL

*Barley 7,326 0 1 0% 0% 0 1 1.0 1.0 1.0 CA

Lots/Farmsteads/etc 56,000 1 3 0% 0% 1 4 1.3 1.0 1.3 CA

Golf Courses 1,618 49 98 3% 6% 160 235 2.4 1.0 2.4

Landscape Mainten - - - - - 12 24 3.0 - -

Rights of Way - - - - - 5 10 - - -

Parks - - - - - 11 22 - - -

Horticultural

Nurseries

- - - - - 56 112 4.0 - -

Total 51.915 106 249.03 416

COLUMN HEADINGS

Wtd Avg = Weighted average--the most recent years and more reliable data are weighted more heavily.

Est Max = Estimated maximum, which is estimated from available data.

Average application rates are calculated from the weighted averages.

NOTES ON TABLE DATA

Usage data primarily covers 1988 - 1998. Calculations of the above numbers may not appear to agree because they are displayed as rounded

to the nearest 1000 for acres treated or lb. a.i. (Therefore 0 = < 500)

to the nearest whole percentage point for % of crop treated. (Therefore 0% = < 0.5%)

* = Available EPA sources indicate that usage is observed for potatoes and barley in the reported data for this site. However, there are no tolerances or labeled

uses for these site. Reason for reported usage is undetermined and therefore usage for these sites should not be used for risk assessment.

- = missing information or lack of confidence in the data to determine an accurate estimate of usage. However, these sites were included in the table because of

indicated usage.

SOURCES: EPA data, 1988-98; USDA, NASS, 1999
76

APPENDIX I. GENEEC 2.0 INPUT PARAMETERS, RESULTS, AND OUTPUTS

Environmental Fate Input Parameters for GENEEC 2.0.

Chemical Oxadiazon

PC Code 109001

Water Solubility (25°C) 1 ppm

Hydrolysis Half-Life (pH 7) stable

Aerobic Soil Metabolism Half-Life stable

Aerobic Aquatic Metabolism Half-life not available

Photolysis Half-Life 2.75 days

Soil/Water Equilibrium Partition Coefficient (Kd) 16.91

Depth of Incorporation (Broadcast) 0.0 in.

Wetted-In Yes

Table 2. Modeling Results for Use on Turf

Ground spray1 granular

Application Rate 2.0 3.0 4.0 4.0 4.0

Application Frequency 1 1 1 2 2

Application Interval (days) N/A N/A 182 182 182

GENEEC 2.0

Peak EEC 44 67 89 173 150

21-Day EEC 43 65 87 170 147

60-Day EEC 42 63 84 163 142

1. Low boom ground sprayer with fine spray quality (EFED defaults), no buffer (no spray zone).
77

 RUN No. 1 FOR Oxadiazon ON Turf * INPUT VALUES * 

 --------------------------------------------------------------------

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP

 ONE(MULT) INTERVAL Kd (PPM ) (%DRIFT) (FT) (IN)

 --------------------------------------------------------------------

 4.000( 8.000) 2 182 16.9 1.0 GRHIFI( 6.6) .0 .0

 FIELD AND STANDARD POND HALFLIFE VALUES (DAYS) 

 --------------------------------------------------------------------

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

 (FIELD) RAIN/RUNOFF (POND) (POND-EFF) (POND) (POND) 

 --------------------------------------------------------------------

 .00 0 N/A 2.75- 341.00 .00 341.00

 GENERIC EECs (IN MICROGRAMS/LITER (PPB)) 

 --------------------------------------------------------------------

 PEAK MAX 4 DAY MAX 21 DAY MAX 60 DAY MAX 90 DAY

 GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC

 --------------------------------------------------------------------

 173.14 172.44 169.50 163.00 158.23

 RUN No. 2 FOR Oxadiazon ON Turf * INPUT VALUES * 

 --------------------------------------------------------------------

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP

 ONE(MULT) INTERVAL Kd (PPM ) (%DRIFT) (FT) (IN)

 --------------------------------------------------------------------

 2.000( 4.000) 2 42 16.9 1.0 GRHIFI( 6.6) .0 .0

 FIELD AND STANDARD POND HALFLIFE VALUES (DAYS) 

 --------------------------------------------------------------------

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

 (FIELD) RAIN/RUNOFF (POND) (POND-EFF) (POND) (POND) 

 --------------------------------------------------------------------

 .00 0 N/A 2.75- 341.00 .00 341.00

 GENERIC EECs (IN MICROGRAMS/LITER (PPB)) 

 --------------------------------------------------------------------

 PEAK MAX 4 DAY MAX 21 DAY MAX 60 DAY MAX 90 DAY

 GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC

 --------------------------------------------------------------------

 88.20 87.84 86.34 83.03 80.60
78

 RUN No. 3 FOR Oxadiazon ON Turf * INPUT VALUES * 

 --------------------------------------------------------------------

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP

 ONE(MULT) INTERVAL Kd (PPM ) (%DRIFT) (FT) (IN)

 --------------------------------------------------------------------

 3.000( 6.000) 2 182 16.9 1.0 GRHIFI( 6.6) .0 .0

 FIELD AND STANDARD POND HALFLIFE VALUES (DAYS) 

 --------------------------------------------------------------------

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

 (FIELD) RAIN/RUNOFF (POND) (POND-EFF) (POND) (POND) 

 --------------------------------------------------------------------

 .00 0 N/A 2.75- 341.00 .00 341.00

 GENERIC EECs (IN MICROGRAMS/LITER (PPB)) 

 --------------------------------------------------------------------

 PEAK MAX 4 DAY MAX 21 DAY MAX 60 DAY MAX 90 DAY

 GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC

 --------------------------------------------------------------------

 129.85 129.33 127.12 122.25 118.67

 RUN No. 4 FOR Oxadiazon ON Turf * INPUT VALUES * 

 --------------------------------------------------------------------

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP

 ONE(MULT) INTERVAL Kd (PPM ) (%DRIFT) (FT) (IN)

 --------------------------------------------------------------------

 4.000( 8.000) 2 182 16.9 1.0 GRANUL( .0) .0 .0

 FIELD AND STANDARD POND HALFLIFE VALUES (DAYS) 

 --------------------------------------------------------------------

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

 (FIELD) RAIN/RUNOFF (POND) (POND-EFF) (POND) (POND) 

 --------------------------------------------------------------------

 .00 0 N/A 2.75- 341.00 .00 341.00

 GENERIC EECs (IN MICROGRAMS/LITER (PPB)) 

 --------------------------------------------------------------------

 PEAK MAX 4 DAY MAX 21 DAY MAX 60 DAY MAX 90 DAY

 GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC

 --------------------------------------------------------------------

 150.41 149.80 147.25 141.60 137.46
79

 RUN No. 1 FOR Oxadiazon ON Turf * INPUT VALUES * 

 --------------------------------------------------------------------

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP

 ONE(MULT) INTERVAL Kd (PPM ) (%DRIFT) (FT) (IN)

 --------------------------------------------------------------------

 2.000( 2.000) 1 1 16.9 1.0 GRHIFI( 6.6) .0 .0

 FIELD AND STANDARD POND HALFLIFE VALUES (DAYS) 

 --------------------------------------------------------------------

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

 (FIELD) RAIN/RUNOFF (POND) (POND-EFF) (POND) (POND) 

 --------------------------------------------------------------------

 .00 0 N/A 2.75- 341.00 .00 341.00

 GENERIC EECs (IN MICROGRAMS/LITER (PPB)) 

 --------------------------------------------------------------------

 PEAK MAX 4 DAY MAX 21 DAY MAX 60 DAY MAX 90 DAY

 GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC

 --------------------------------------------------------------------

 44.39 44.21 43.46 41.79 40.57

 RUN No. 1 FOR Oxadiazon ON Turf * INPUT VALUES * 

 --------------------------------------------------------------------

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP

 ONE(MULT) INTERVAL Kd (PPM ) (%DRIFT) (FT) (IN)

 --------------------------------------------------------------------

 3.000( 3.000) 1 1 16.9 1.0 GRHIFI( 6.6) .0 .0

 FIELD AND STANDARD POND HALFLIFE VALUES (DAYS) 

 --------------------------------------------------------------------

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

 (FIELD) RAIN/RUNOFF (POND) (POND-EFF) (POND) (POND) 

 --------------------------------------------------------------------

 .00 0 N/A 2.75- 341.00 .00 341.00

 GENERIC EECs (IN MICROGRAMS/LITER (PPB)) 

 --------------------------------------------------------------------

 PEAK MAX 4 DAY MAX 21 DAY MAX 60 DAY MAX 90 DAY

 GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC

 --------------------------------------------------------------------

 66.59 66.32 65.19 62.69 60.85
80

 RUN No. 1 FOR Oxadiazon ON Turf * INPUT VALUES * 

 --------------------------------------------------------------------

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP

 ONE(MULT) INTERVAL Kd (PPM ) (%DRIFT) (FT) (IN)

 --------------------------------------------------------------------

 4.000( 4.000) 1 1 16.9 1.0 GRHIFI( 6.6) .0 .0

 FIELD AND STANDARD POND HALFLIFE VALUES (DAYS) 

 --------------------------------------------------------------------

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

 (FIELD) RAIN/RUNOFF (POND) (POND-EFF) (POND) (POND) 

 --------------------------------------------------------------------

 .00 0 N/A 2.75- 341.00 .00 341.00

 GENERIC EECs (IN MICROGRAMS/LITER (PPB)) 

 --------------------------------------------------------------------

 PEAK MAX 4 DAY MAX 21 DAY MAX 60 DAY MAX 90 DAY

 GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC

 --------------------------------------------------------------------

 88.79 88.43 86.92 83.59 81.14
4

Matringe, M., J.-M. Camadro, P. Labbe, and R. Scalla. 1989. Protoporphyrinogen oxidase as a molecular

target for diphenyl ether herbicides. Biochem. J. 260: 231-235.

5

Birchfield, N.B., and J.E. Casida. 1997. Protoporphyrinogen oxidase of mouse and maize: Target site selectivity and thiol effe

peroxidizing herbicide action. Pesticide Biochemistry and Physiology 57, 36-43.

6

Halling, B.P., D.A. Yuhas, V.F. Fingar, and J.W. Winkleman. 1994. "Protoporphyrinogen oxidase

inhibitors for tumor therapy" in Porphyric Pesticides: Chemistry, Toxicology, and Pharmaceutical Applications,

(S.O. Duke and C.A. Rebeiz, Eds.) pp. 280-290, American Chemical Society Symposium Series 559, Am. Chem.

Soc., Washington, D.C., 1994. 

7

Birchfield, N.B. Protoporphyrinogen Oxidase as a Herbicide Target: Characterization of the

[

3

H]Desmethylflumipropyn Binding Site. Dissertation. University of California, Berkeley. 1996. 

81

APPENDIX J. PHOTOXICITY STUDY PROTOCOL for LIGHT-DEPENDENT

PEROXIDIZING HERBICIDES

The light-dependent peroxidizing herbicides (LDPHs) are a growing class of weed control

chemicals (see partial listing attached). They act in plants by inhibiting the enzyme protoporphyrinogen

oxidase (protox), which is the last common enzyme in the heme and chlorophyll biosynthetic pathways.4

Protox exists in both plants and animals and the enzyme from both sources has been shown to be highly

sensitive to many LDPHs.5

 

LDPH protox inhibition in plants results in a rapid accumulation of protoporphyrin IX, a phototoxic

heme and chlorophyll precursor. In the presence of light, protoporphyrin IX is a powerful generator of

singlet oxygen which in plants causes lipid membrane peroxidation leading to a rapid loss of turgidity

and foliar burns. LDPH exposure in mammals has been shown to result in excretion of porphyrins in

urine (porphynuria) and feces, increased liver weight, elevated blood porphyrin levels, developmental

abnormalities, and cancer. Humans with a hereditary protox disorder (variegate porphyria) which

results in lowered protox activity exhibit many symptoms similar to LDPH exposure in addition to

photosensitivity. However, photosensitivity is not a commonly reported symptom of LDPH exposure in

animals. 

An LDPH-induced occurrence of phototoxicity in rats6

 and increased cytotoxicity to human skin cells

grown in culture in the presence of light and an LDPH7

 have been reported but many other LDPH

toxicity studies make no mention of phototoxicity in animals. The scarcity of phototoxicity data in

animals could result from physiological or biochemical distinctions from plants. For instance, animals

exposed to LDPHs may not normally accumulate protoporphyrin IX in their epidermis. However,

phototoxicity may not be reported in many LDPH toxicity tests because of relatively low light

conditions in laboratories and/or protection afforded by the animals' fur or feathers. Animals without fur

or feathers existing in sunny environments would be expected to be at highest risk for potential

phototoxic effects. 

The Aquatic Biology Tech Team (ABTT) recommends that phototoxicity studies be conducted on
8

American Society for Testing and Materials. 1994. Standard guide for conducting the frog embryo

teratogenesis assay-Xenopus. E 1439-91. In Annual Book of ASTM Standards, Vol 11.5, pp. 825-835. Philadelphia,

PA.

82

herbicides with this mode of action to determine if animals exposed to LDPHs and intense light (similar

to sunlight) show increased toxicity relative to controls exposed to LDPHs and low intensity light. The

results of these studies will help to determine if animals that are exposed to sunlight in LDPH use areas

are at higher risk than guideline toxicity studies suggest. 

The ABTT is requesting that a LDPH phototoxicity protocol be submitted for review and agreement by

EFED and the registrant prior to study initiation. Protocols for standard toxicity tests have also been

published.8

 In nature, fish and other aquatic organisms are expected to be exposed to LDPHs through

run-off and spray drift. Aquatic organisms inhabiting small, shallow water bodies, exposed to high

levels of solar radiation would be expected to be at greatest risk for potential phototoxic effects. 

Therefore, the ABTT is requesting a small fish species be used in a phototoxicity assay to assess the

potential of light to increase LDPH toxicity. 

The ABTT requests that the study adequately address the following issues and suggests the paper,

"Photoenhanced Toxicity of a Carbamate Insecticide to Early Life Stage Anuran Amphibians",5

 and

other studies in the peer-reviewed scientific literature serve as sources of additional guidance:

Species 

The fathead minnow may be an appropriate test species because of existing toxicity protocols which

may be adapted for this study. 

Exposure duration

A subchronic exposure duration would be adequate for proof of principle. A single exposure may not

allow adequate time for porphyrin accumulation, however, a life-cycle is not necessary to identify a

phototoxic effect. 

Dosing

A range finding study should be conducted under defined low light conditions to identify an LC50 value

and lower dose levels expected to be similar to controls. Doses used in the phototoxicity study should

not be expected to result in significant mortality in low light controls. Dissolved concentrations of the

test chemical should be confirmed by an appropriate analytical method. 

Endpoints

Behavioral observations should be made in addition to measurements of mortality, growth, weight,

morphology, and appearance. Ideally, measurements of protoporphyrin and heme concentrations in the

blood and protox activity in the liver of each test organisms should be made. 

Light sources

Artificial light may be preferred to natural light that will vary in different regions and seasons as well as

with weather. If artificial light is used, the light should resemble full, natural sunlight as closely as
83

possible, particularly around 400 nm. The most important wavelength for porphyrin induced

phototoxicity in ~400 nm. No matter what the light source, the duration and intensity of UV and visible

light should be reported at all wavelengths (200-800 nm). At this point EFED does not have a specific

recommendation for an artificial light source.

Dark, light, and positive controls

As this study is intended to identify potential effects of light on LDPH toxicity, an appropriate study

protocol should include a dark, or low light, control group. Another group not exposed to chemicals but

exposed to full light should be included (a full light control). In addition to the dark and light controls, a

positive control group using protoporphyrin IX may be useful.

Exposure chambers and light filters

Light intensity should be measured inside test chambers if glass or any other material is placed between

the light source and the test animals. Any filters should be cured under the study light for 72-hours prior

to study initiation to ensure consistent transmittance.
84

ATTACHMENT 1.

The following list of herbicides are believed to act by inhibiting protoporphyringen oxidase in the heme

and chlorophyll biosynthetic pathway.

acifluorfen 

azafenidin 

carfentrazone-ethyl 

flumiclorac-pentyl

flumioxazin

fluthiacet-methyl

fomesafen

lactofen 

oxadiargyl

oxadiazon

oxyfluorfen 

sulfentrazone 

thidiazimin