Chlorine Dioxide

Environmental Hazard and Risk Assessment

Case 4023

Genevieve Angle

Office of Pesticide Programs

Antimicrobials Division

U.S. Environmental Protection Agency

  SEQ CHAPTER \h \r 1 1200 Pennsylvania Avenue, NW

Washington, DC 20460

July 13, 2006

Chlorine Dioxide and Sodium Chlorite: Environmental Hazard and Risk
Assessment

Introduction

The toxicity endpoints (e.g. LC50, NOEC) included in this section are
based on the results of sodium chlorite and chlorine dioxide toxicity
studies that were submitted to EPA for review. Chlorine dioxide and
sodium chlorite are used as antimicrobial pesticides at numerous use
sites.  Sodium chlorite is used as a precursor in the generation of
chlorine dioxide.  The antimicrobial registered uses of chlorine
dioxide/sodium chlorite fall into several major categories including use
in the treatment of human drinking water systems; in industrial process
and water systems; as a materials preservative; and as a general
disinfectant in medical, residential, agricultural, commercial and
industrial settings. The once-through cooling tower use of chlorine
dioxide/sodium chlorite has been selected for risk assessment because
out of all the uses of these chemicals, it is the one expected to have
the most potential for environmental exposure.  The environmental risk
assessment will be conducted using sodium chlorite endpoints because
under environmental conditions, chlorine dioxide converts mostly into
chlorite ions.  Currently, a quantitative evaluation of the likelihood
of the ecological exposures resulting from the industrial uses of
chlorine dioxide and sodium chlorite overlapping spatially and
temporally with the exposures resulting from the agricultural uses of
chlorine dioxide and sodium chlorite is not possible.  Therefore, the
ecological risks are not aggregated in this assessment.

Data Gaps

Several data requirements, necessary to support the once-through cooling
tower use of chlorine dioxide/sodium chlorite, are not fulfilled:

72-4/850.1300		Fish early life-stage testing-freshwater			TGAI

72-4b/850.1400	Invertebrate life-cycle testing - freshwater			TGAI

123-1/850.4225	Seedling emergence dose-response in rice			TEP

123-1/850.4250	Vegetative vigor dose-response in rice			TEP

123-2/850.4400	Aquatic vascular plant dose-response toxicity- Lemna sp.
TGAI or TEP

123-2/850.5400	Acute algal dose-response toxicity - 4 species		TGAI or
TEP

Ecological Hazard Assessment:

I. Toxicity to Terrestrial Animals

A. Birds, Acute and Subacute

An acute oral toxicity study using the technical grade of the active
ingredient (TGAI) is required to establish the toxicity of chlorine
dioxide/sodium chlorite to birds.  The preferred test species is either
mallard duck (a waterfowl) or bobwhite quail (an upland game bird).  The
results of several toxicity studies are provided in the following table.

Table 1.  Acute Oral Toxicity of Chlorine Dioxide and Sodium Chlorite to
Birds

Substance/%

Active Ingredient

(AI)	

Organism	

Endpoints/Results

(95% conf. interval)

mg/kg	

Reference	

Study Classification



Sodium Chlorite/80%	

Northern bobwhite

(Colinus virginianus)	

LD50 = 382 (300-520)

NOAEL = 175	

Robaidek, 1985 

ACC # 259373	

acceptable



Sodium Chlorite/80%	

Northern bobwhite

(Colinus virginianus)	

LD50 = 390 (310-490)

NOAEL = N.R.	

Robaidek and Johnson, 1985

ACC # 257341	

acceptable



Sodium Chlorite/80%	

Northern bobwhite

(Colinus virginianus)	

LD50 = 395 (272-573)

NOAEL = N.R.	

Fletcher, 1984

ACC # 253378	

acceptable



Sodium Chlorite/83%	

Northern bobwhite

(Colinus virginianus)	

LD50 = 660 (540-810)

	

Fletcher, 1973

MRID # 31610	

acceptable



Sodium Chlorite/80%	

Northern bobwhite

(Colinus virginianus)	

LD50 = 467 (372-585)

	

Beavers, 1984

ACC # 254177	

acceptable



Sodium Chlorite/80%	

Mallard Duck (Anas platyrhynchos)	

LD50 > 31.25

	

Beavers, 1984

ACC # 254176	

supplemental



Sodium Chlorite/25%	

Northern bobwhite

(Colinus virginianus)

	

LD50 = 797(420-2594)

NOAEL= 125	

MBA Laboratories, 1984

ACC# 252854	

acceptable

Note: LD50=Median Lethal Dose; NOAEL=No Observed Adverse Effect Level;
N.R. = Not Reported

These results indicate that chlorine dioxide/sodium chlorite range from
slightly to highly toxic to birds on an acute oral basis.  The guideline
requirement (71-1/OPPTS 850.2100) is fulfilled.

Two subacute dietary studies using the technical grade of the active
ingredient are required to establish the toxicity of a pesticide to
birds.  The preferred test species are mallard duck (a waterfowl) and
bobwhite quail (an upland gamebird).  Results of avian subacute dietary
tests are tabulated below.

Table 2.  Avian Subacute Dietary Toxicity of Chlorine Dioxide and Sodium
Chlorite



Substance/% AI	

Organism	

LC50

(ppm)

(95 % c.i.)	

NOAEC

(ppm)	

Reference	

Study Classification



Sodium Chlorite/80%	

Mallard Duck (Anas platyrhynchos)	

>5000	

5000	

Johnson, 1984 

MRID # 94068008	

acceptable



Sodium Chlorite/80%	

Northern bobwhite (Colinus virginianus)	

> 5000	

N.R.	

Fletcher, 1984

ACC # 253378	

acceptable



Sodium Chlorite/80%	

Mallard Duck (Anas platyrhynchos)	

> 5000	

N.R.	

Fletcher, 1984

ACC # 253378	

acceptable



Sodium Chlorite/80%	

Northern bobwhite (Colinus virginianus)	

> 5000	

N.R.	

Johnson, 1984

MRID # 94068005	

acceptable



Sodium Chlorite/80%	

Mallard Duck (Anas platyrhynchos)	

> 5620	

N.R.	

Beavers, 1984

ACC # 254178	

acceptable



Sodium Chlorite/80%	

Northern bobwhite

(Colinus virginianus)	

> 5620	

N.R.	

Beavers, 1984

ACC # 254179	

acceptable



Sodium Chlorite/80%	

Northern bobwhite

(Colinus virginianus)	

>10,000	

N.R.	

Fink, 1977

MRID # 130649	

acceptable



Sodium Chlorite/80%	

Mallard Duck (Anas platyrhynchos)	

>10,000	

N.R.	

Fink, 1977

MRID # 130650	

acceptable



Sodium Chlorite/25%	

Mallard Duck (Anas platyrhynchos)	

18686 (8186-109184)	

N.R.	

MBA Laboratories, 1983

ACC # 252854	

acceptable



Sodium Chlorite/25%	

Northern bobwhite (Colinus virginianus)	

2031(1226-3903)	

417	

MBA Laboratories, 1984

ACC #252854	

acceptable

Note: LC50=Median Lethal Concentration; NOAEC=No Observed Adverse Effect
Concentration; N.R. = Not Reported

These results indicate that chlorine dioxide/sodium chlorite range from
slightly toxic to practically non-toxic to birds on a subacute dietary
basis.  The guideline requirement (71-2/OPPTS 850.2200) is fulfilled.

 		B.  Birds, Chronic

Avian reproduction studies using the technical grade of the active
ingredient are required for a pesticide when any of the following
conditions are met: (1) birds may be subject to repeated or continuous
exposure to the pesticide, especially preceding or during the breeding
season, (2) the pesticide is stable in the environment to the extent
that potentially toxic amounts may persist in animal feed, (3) the
pesticide is stored or accumulated in plant or animal tissues, and/or
(4) information derived from mammalian reproduction studies indicates
reproduction in terrestrial vertebrates may be adversely affected by the
anticipated use of the product.  The currently registered uses of
chlorine dioxide/sodium chlorite do not require avian reproduction
testing.

II.   Toxicity to Freshwater Aquatic Animals

A.  Freshwater Fish, Acute

Two fish toxicity studies using the TGAI are required to establish the
toxicity of a pesticide to freshwater fish.  The preferred test species
are rainbow trout (a cold water fish) and bluegill sunfish (a warm water
fish).  The results of studies submitted for sodium chlorite are
provided in the following table.

Table 3.  Acute Toxicity of Chlorine Dioxide and Sodium Chlorite to
Freshwater Fish

Substance/%

Active Ingredient

(AI)	

Organism	

Endpoints/Results

(ppm)

(95% conf. interval)	

Reference	

Study Classification



Sodium Chlorite/80%	

Rainbow trout (Oncorhynchus mykiss)	

LC50 = 360 (216-600)

NOAEC = 216	

Barrows, 1984

MRID # 94068007	

acceptable



Sodium Chlorite/80%	

Bluegill

(Lepomis macrochirus)	

LC50 = 244 (196-304)

NOAEC = 108

	

Larkin, 1984

ACC # 254181	

acceptable



Sodium Chlorite/80%	

Rainbow trout (Oncorhynchus mykiss)	

LC50 = 360 (216-600)

NOAEC = 216	

Larkin, 1984

ACC # 254180	

acceptable



Sodium Chlorite/80.25%	

Bluegill

(Lepomis macrochirus)	

LC50 = 265 (231-309)

NOAEC = 130	

EG&G Bionomics, 1978

ACC # 69809	

supplemental



Sodium Chlorite/79%	

Bluegill

(Lepomis macrochirus)	

LC50 = 208 (165-262)

NOAEC = N.R.	

Sleight III, 1971

MRID # 131351	

supplemental



Sodium Chlorite/79%	

Rainbow trout (Oncorhynchus mykiss)	

LC50 = 50.6 (38.8-65.8)

NOAEC = N.R.	

Sleight III, 1971

MRID # 131351	

supplemental



Sodium Chlorite/80%	

Rainbow trout (Oncorhynchus mykiss)	

LC50 >100

NOAEC = N.R.	

McMillen, 1985  

ACC # 253743	

supplemental



Sodium Chlorite/80%	

Bluegill (Lepomis macrochirus)	

LC50 >100

NOAEC = N.R.	

McMillen, 1985  

ACC # 253743	

supplemental



Sodium Chlorite/25%	

Rainbow trout (Oncorhynchus mykiss)	

LC50 = 203 (175-236)

NOAEC = 100	

MBA Laboratories, 1984  

ACC # 252854	

acceptable



Sodium Chlorite/25%	

Bluegill (Lepomis macrochirus)	

LC50 = 222 (207-237)

NOAEC = 186	

MBA Laboratories, 1983  

ACC # 252854	

supplemental



Sodium Chlorite/81.5%	

Bluegill (Lepomis macrochirus)	

LC50 = 310 (270-350)

NOAEC = 220	

Sousa, 1981  

ACC # 245697	

acceptable



Sodium Chlorite/80.25%	

Rainbow trout (Oncorhynchus mykiss)	

LC50 = 290 (250-340)

NOAEC = <70

	

EG&G Bionomics, 1979

ACC # 69810	

acceptable



Sodium Chlorite/80%	

Rainbow trout (Oncorhynchus mykiss)	

LC50 = 340 (220-600)

NOAEC = 130	

Sousa and Surprenant, 1984

ACC # 253379	

acceptable



Sodium Chlorite/80%	

Bluegill (Lepomis macrochirus)	

LC50 = 420 (220-600)

NOAEC = 220	

Sousa and Surprenant, 1984

MRID # 94068006	

acceptable

Note: LC50=Median Lethal Concentration; NOAEC=No Observed Adverse Effect
Concentration; N.R. =Not Reported

These results indicate that chlorine dioxide/sodium chlorite range from
slightly toxic to practically non-toxic to freshwater fish on an acute
basis.  The guideline requirement (72-1/OPPTS 850.1075) is fulfilled.

B.  Freshwater Fish, Chronic

A freshwater fish early life-stage test using the technical grade of the
active ingredient is required for a pesticide when it may be applied
directly to water or if the end-use product is expected to be
transported to water from the intended use site, and any of the
following conditions are met: (1) the pesticide is intended for use such
that its presence in water is likely to be continuous or recurrent
regardless of toxicity, (2) any aquatic acute LC50 or EC50 is less than
1 mg/l, (3) the EEC (estimated environmental concentration) in water is
equal to or greater than 0.01 of any acute LC50 or EC50 value, or (4)
the actual or estimated environmental concentration in water resulting
from use is less than 0.01 of any acute LC50 or EC50 value and any one
of the following conditions exist: studies of other organisms indicate
the reproductive physiology of fish may be affected, physicochemical
properties indicate cumulative effects, or the pesticide is persistent
in water (e.g., half-life greater than 4 days).  The preferred test
species is rainbow trout, but other species may be used.  Freshwater
fish early life-stage testing is required for chlorine dioxide and
sodium chlorite in order to support once-through cooling tower uses. No
data under this topic have been submitted; therefore Guideline
72-4/OPPTS 850.1300 is not fulfilled.

C.  Freshwater Invertebrates, Acute

A freshwater aquatic invertebrate toxicity test using the TGAI is
required to establish the toxicity of chlorine dioxide and sodium
chlorite to freshwater aquatic invertebrates.  The preferred test
species is Daphnia magna.  Results of this test are tabulated below.

Table 4.  Acute Toxicity of Chlorine Dioxide and Sodium Chlorite to
Freshwater Invertebrates

Substance/%

Active Ingredient

(AI)	

Organism	

Endpoints/Results

(ppm)

(95% conf. interval)	

Reference	

Study Classification



Sodium Chlorite/80%	

Daphnia magna	

EC50 = 0.027 (0.021-0.031)

NOAEC = 0.003	

Barrows, 1984  

MRID # 146162	

acceptable



Sodium Chlorite/80%	

Daphnia magna	

EC50 = 0.39 (0.32-0.54)

NOAEC =N.R.	

Hoberg and Surprenant, 1984

MRID # 141149	

acceptable



Sodium Chlorite/79%	

Daphnia magna	

EC50 = 0.29

(0.25-0.33)

NOAEC = 0.10	

Vilkas, 1976

MRID # 131350	

acceptable



Sodium Chlorite/80%	

Daphnia magna	

EC50 = 0.08

(0.06-0.10)

NOAEC = 0.06	

Larkin, 1984

ACC # 254182	

acceptable



Sodium Chlorite/80%	

Daphnia magna	

EC50 = 0.146 

 (0.12 - 0.18)

NOAEC = 0.06	

Nackord, 1984  

MRID # 94068009	

acceptable



Sodium Chlorite/25%	

Daphnia magna	

EC50 = 1.4 (1.0-1.9 ) 

NOAEC = 0.4	

MBA Laboratories, 1984  

ACC # 252854	

supplemental

Note: EC50=Median Effective Concentration; NOAEC=No Observed Adverse
Effect Concentration; N.R. =Not Reported

These studies indicate that chlorine dioxide and sodium chlorite range
from very highly toxic for technical grade sodium chlorite a.i. to
moderately toxic for the formulated product to aquatic invertebrates on
an acute basis.  The guideline requirement (72-2/OPPTS 850.1010) is
fulfilled.

D.  Freshwater Invertebrates, Chronic

A freshwater aquatic invertebrate life-cycle test using the technical
grade of the active ingredient is required for a pesticide if the
end-use product may be applied directly to water or expected to be
transported to water from the intended use site, and any of the
following conditions are met: (1) the pesticide is intended for use such
that its presence in water is likely to be continuous or recurrent
regardless of toxicity, (2) any aquatic acute LC50 or EC50 is less than
1 mg/l, (3) the EEC in water is equal to or greater than 0.01 of any
acute EC50 or LC50 value, or (4) the actual or estimated environmental
concentration in water resulting from use is less than 0.01 of any
aquatic acute EC50 or LC50 value and any of the following conditions
exist: studies of other organisms indicate the reproductive physiology
of invertebrates may be affected, physicochemical properties indicate
cumulative effects, or the pesticide is persistent in water (e.g.,
half-life greater than 4 days).  The preferred test species is Daphnia
magna. Freshwater fish early life-stage testing is required for chlorine
dioxide/sodium chlorite in order to support once-through cooling tower
uses.  No data under this topic have been submitted; therefore Guideline
72-4b/OPPTS 850.1400 is not fulfilled.  

E.  Freshwater Field Studies

Freshwater field testing is required for pesticides in cases where there
are risks of concern from labeled use patterns.  This testing was not
required for chlorine dioxide and sodium chlorite and is reserved at the
present time.  

III.   Toxicity to Estuarine and Marine Animals

A.  Estuarine and Marine Fish, Acute

Acute toxicity testing with estuarine/marine fish using the technical
grade of the active ingredient is required for a chemical when the
end-use product is intended for direct application to the
marine/estuarine environment or the active ingredient is expected to
reach this environment because of its use in coastal counties.  The
preferred test species is sheepshead minnow.  Results of this test are
tabulated below.

Table 5.  Acute Toxicity of Chlorine Dioxide/Sodium Chlorite to
Estuarine/Marine Fish

Substance/% Active Ingredient (AI)	

Organism	

96 hr LC50 

mg/L (ppm) 

(95% conf. interval)	

NOAEC mg/L (ppm)	

Reference	

Study Classification



Sodium Chlorite/79%	

Sheepshead minnow

(Cyprinodon variegatus)	

75 (62.6-89.8)

	

13.9	

Yurk and Overman, 1994

MRID # 43259401	

acceptable

Note: LC50=Median Lethal Concentration; NOAEC=No Observed Adverse Effect
Concentration

The results indicate that chlorine dioxide/sodium chlorite are slightly
toxic to estuarine/marine fish on an acute basis.  The guideline
requirement (72-3a/OPPTS 850.1025) is fulfilled.

B.  Estuarine and Marine Fish, Chronic

Estuarine/marine fish early life-stage testing is not required for the
currently registered uses of chlorine dioxide and sodium chlorite. 
Since freshwater and estuarine/marine fish are comparably sensitive to
sodium chlorite on an acute basis, the freshwater early life-stage
endpoints will suffice for assessing risk to estuarine/marine fish
species.  However, as noted above, the data requirement for freshwater
fish early life-stage testing (72-4a/850.1300) has not been fulfilled.

C.  Estuarine and Marine Invertebrates, Acute

Acute toxicity testing with estuarine/marine invertebrates using the
technical grade of the active ingredient is required for a pesticide
when the end-use product is intended for direct application to the
marine/estuarine environment or the active ingredient is expected to
reach this environment because of its use in coastal counties.  The
preferred test species are the mysid shrimp and eastern oyster.  Results
of these tests are tabulated below.

Table 6:  Acute Toxicity of Chlorine Dioxide and Sodium Chlorite to
Estuarine/Marine Invertebrates

Substance/

% Active Ingredient (AI)	

Organism	

96-hour

EC50 mg/L (ppm)

(95% conf. interval)	

Reference	

Study Classification



Sodium Chlorite/79%	

Eastern oyster 

(Crassostrea virginica)	

21.4 (14.3-27.1)

NOAEC = 14.3	

Yurk and Overman, 1994

MRID # 43259403	

acceptable



Sodium Chlorite/79%	

Mysid

(Mysidopsis bahia)	

0.576 (0.44-0.75)

NOAEC= N.R.	

Yurk and Overman, 1994

MRID # 43259402	

acceptable

Note: EC50=Median Effective Concentration; NOAEC=No Observed Adverse
Effect Concentration

The results indicate that chlorine dioxide/sodium chlorite range from
highly toxic to slightly toxic to estuarine/marine invertebrates on an
acute basis.  The guideline requirements (72-3b and 72-3c/OPPTS 850.1035
and 850.1045) are fulfilled. 							

D.  Estuarine and Marine Invertebrate, Chronic

An estuarine/marine invertebrate life-cycle toxicity test is required
for a pesticide if the end-use product may be applied directly to
estuarine or marine waters, or is expected to be transported to these
waters from the intended use site, and any of the following conditions
are met: (1) the pesticide is intended for use such that its presence in
water is likely to be continuous or recurrent regardless of toxicity,
(2) any aquatic acute LC50 or EC50 is less than 1 mg/l, (3) the EEC in
water is equal to or greater than 0.01 of any acute EC50 or LC50 value,
or (4) the actual or estimated environmental concentration in water
resulting from use is less than 0.01 of any aquatic acute EC50 or LC50
value and any of the following conditions exist: studies of other
organisms indicate the reproductive physiology of invertebrates may be
affected, physicochemical properties indicate cumulative effects, or the
pesticide is persistent in water (e.g., half-life greater than 4 days). 
Due to the high acute toxicity of sodium chlorite to estuarine/marine
invertebrates, life-cycle testing is necessary; however, freshwater
invertebrates tend to be more sensitive than estuarine/marine
invertebrates to sodium chlorite on an acute basis, and, therefore,
freshwater life-cycle endpoints will suffice for assessing risk to
estuarine/marine invertebrate species. As noted above, freshwater
invertebrate life-cycle testing (72-4b/850.1400) has not been submitted.

E.  Estuarine and Marine Field Studies

No estuarine or marine field study testing is currently required for
chlorine dioxide and sodium chlorite. 

IV.   Toxicity to Plants

A. Terrestrial/Semi-aquatic

Currently, terrestrial plant testing is not required for pesticides
other than herbicides except on a case-by-case basis (e.g., labeling
bears phytotoxicity warnings, incidents of plant damage have been
reported, or literature indicating phytotoxicity is available). The
once-through cooling tower use of chlorine dioxide/sodium chlorite
requires such testing, due to the algaecidal nature of the chemical and
the likelihood of exposure to semi-aquatic plants along surface waters
receiving industrial facility outfall from the cooling system.  Results
of these tests are tabulated below.

Table 7.  Toxicity of Chlorine Dioxide/Sodium Chlorite to
Terrestrial/Semi-aquatic Plants

Substance/% Active Ingredient (AI)	

Organism	

EC25

(ppm)	

Reference	

Study Classification



Sodium Chlorite/80%	

Monocots & Dicots (10 Species)	

>3.5

	

Backus et al., 1990

MRID # 41843101	

acceptable



Sodium Chlorite/80%	

Monocots & Dicots (9 Species)	

>3.5	

Backus et al., 1990

MRID # 41843102	

acceptable



Sodium Chlorite/80%	

Buckwheat (Polygonum convolvulus)	

<3.5	

Backus et al., 1990

MRID # 41843102	

acceptable

Note: EC25=Effective Concentration (25th Percentile)

The results indicate that chlorine dioxide/sodium chlorite are
moderately toxic to terrestrial plants.  The guideline requirement
(122-1/OPPTS 850.4100 and 850.4150) is fulfilled.  However, since the
maximum label rate for many of the chlorine dioxide/sodium chlorite
once-through cooling labels was not used in these tests, it is necessary
to conduct Tier II testing with rice.  Guideline 123-1/OPPTS 850.4225
and 850.4250, Seedling Emergence and Vegetative Vigor testing with Rice
(Oryza sativa) is not fulfilled. 



B.  Aquatic

The once-through cooling tower use of chlorine dioxide/sodium chlorite
requires that 5 aquatic plant tests be conducted due to the algaecidal
nature of the chemical and the likelihood of exposure to aquatic plants
in surface waters receiving industrial facility outfall from the cooling
system.  Guideline 123-2/OPPTS 850.4400 and 850.5400, Vascular Aquatic
Plant and Algal dose-response toxicity testing (5 species), is not
fulfilled because only one study (1 species) under this topic has been
submitted and 5 are required.  The results of this test are tabulated
below.  The following aquatic plant studies are still required:
blue-green cyanobacteria (Anabaena flos-aquae), freshwater diatom
(Navicula pelliculosa), marine diatom (Skeletonema costatum) and
floating macrophyte (Lemna gibba).

Table 8.  Toxicity of Chlorine Dioxide/Sodium Chlorite to Aquatic Plants

Substance/% Active Ingredient (AI)	

Organism	

96 hr EC50 (ppm) (95% conf. interval)	

Reference	

Study Classification



Sodium Chlorite/80%	

Green algae (Selenastrum capricornutum)	

1.32 (1.18-1.47)

NOAEC = <0.62

	

Ward and Boeri, 1991

MRID # 41880403	

supplemental

Note: EC50=Median Effective Concentration; NOAEC=No Observed Adverse
Effect Concentration

The results indicate that chlorine dioxide/sodium chlorite are
moderately toxic to aquatic plants.  

Risk Assessment and Characterization:

Risk assessment integrates the results of the exposure and ecotoxicity
data to evaluate the likelihood of adverse ecological effects. One
method of integrating the results of exposure and ecotoxicity data is
called the quotient method.  For this method, risk quotients (RQs) are
calculated by dividing exposure estimates by ecotoxicity values, both
acute and chronic:  

       

           RQ = EXPOSURE/TOXICITY 

 

RQs are then compared to levels of concern (LOCs).  These LOCs are
criteria used by OPP to indicate potential risk to nontarget organisms
and the need to consider 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 - the 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 this may be mitigated through restricted use
classification; (3) acute endangered species - the potential for acute
risk to endangered 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, AD does not
perform assessments for chronic risk to plants, acute or chronic risks
to nontarget insects, or chronic risk from granular/bait formulations to
mammalian or avian species.

The ecotoxicity test values (i.e., measurement endpoints) used in the
acute and chronic risk quotients are derived from the results of
required studies.  Examples of ecotoxicity values derived from the
results of 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). The
NOAEC value is used as the ecotoxicity test value in assessing chronic
effects. 

Risk presumptions, along with the corresponding RQs and LOCs are
tabulated below.

Risk Presumptions for Terrestrial Animals



Risk Presumption	

RQ	

LOC



Birds and Wild Mammals



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 Endangered Species	

EEC/LC50 or LD50/sqft or LD50/day 	

0.1



Chronic Risk	

EEC/NOEC	

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 Animals	 



Risk Presumption	

RQ 	

LOC



Acute High Risk	

EEC1/LC50 or EC50	

0.5



Acute Restricted Use	

EEC/LC50 or EC50	

0.1



Acute Endangered Species	

EEC/LC50 or EC50	

0.05



Chronic Risk	

EEC/MATC2 or NOEC	

1



 1  EEC = (ppm or ppb) in water

 2  MATC = maximum allowable toxicant concentration

Risk Presumptions for Plants	

	





Risk Presumption	

RQ	

LOC



Terrestrial and Semi-Aquatic Plants 



Acute High Risk	

EEC/EC25	

1



Acute Endangered Species	

EEC/EC05 or NOEC	

1



Aquatic Plants



Acute High Risk	

EEC1/EC50	

1



Acute Endangered Species	

EEC/EC05 or NOEC 	

1



 1  EEC = (ppb/ppm) in water 

1.  Environmental Fate Assessment Summary

This section is a summary of the Environmental Fate Risk Assessment.

Chlorine dioxide and sodium chlorite are assessed together because
chlorine dioxide is produced by a reaction of sodium chlorite (and
sometime sodium chlorate) and hypochlorite/acid. In addition, chlorite
is a breakdown product of chlorine dioxide.  

Chlorine dioxide has a short half life and in the presence of sunlight
and will break down into chloride and chlorate ions (between pH 4 and
7).  At pH lower than 4, its breakdown products are chlorite and
chlorate. Chlorite is the dominant breakdown product.  Ultimately,
oxygen is formed.  Sodium chlorite dissolves in water, breaking down
into chloride and chlorate ions under similar conditions as chlorine
dioxide. Chemical degradation of sodium chlorite commonly occurs in
water as well as in the presence of suspended soil particles containing
ions, like Fe(II), Mn(II),  I-, and S-2, through redox reactions. The
final breakdown products are chloride and oxygen.  These same end
products are obtained when sodium chlorite is heated.

Chlorate and chlorite ions tend to only undergo biodegradation only
under anaerobic conditions.  Biodegradation of chlorate and chlorite
have been observed in anoxic groundwater, sediments and some soils. The
end products are the same as stated above: chloride and oxygen.  No
adsorption/desorption constants (Kds) have been measured or reported in
published literature for either chlorite or chlorate. These ions are
likely to be mobile and may travel from surface to groundwater easily. 
The estimated log Kow of chlorine dioxide is -3.22 and for sodium
chlorite is -7.17.  It is not expected that either would bioaccumulate
in aquatic organisms.  

2.  Environmental Exposure Assessment

The Probabilistic Distribution Model version 4 (PDM4) was used to
estimate exposure from once-through cooling tower uses.  The details of
this model can be found in the   SEQ CHAPTER \h \r 1 Chlorine Dioxide
and Sodium Chlorite Environmental Modeling Chapter.  This
screening-level model was used to provide the percentage of days per
year various concentrations are exceeded for several different flow,
application, and dosing scenarios.

Three different flow regimes were considered: high flow [power plants
with average stream flow rates of 1000±50 million gallons per day
(MGD)]; medium flow (power plants with average stream flow rates of
500±50  MGD); and low flow (power plants with average stream flow rates
of 100±10 MGD).  These plants were taken from the NPDES database of
plant codes; 12 plants were used in each the low and medium flow
category, and 6 plants were used in the high flow category (see
Environmental Modeling Chapter, page 5, for a listing of these plants). 
The 7Q10 flow rate (a flow rate that a stream is expected to be below
for seven consecutive days once every ten years) for each of the
selected plants was used to develop a mean 7Q10 flow rate for each
category.  This mean 7Q10 flow rate was used as the input parameter for
flow in the PDM4 model.

Two pesticide application scenarios, continuous feed and intermittent
feed, were used in the modeling, based on label instructions.  For
continuous feed use, the label rates ranged from 0.10 ppm to 2.0 ppm
chlorine dioxide/sodium chlorite in the water.  For intermittent use,
the label rates ranged from 0.10 ppm to 25 ppm chlorine dioxide/sodium
chlorite in the water.  A single label contained the rate of 800 ppm
chlorine dioxide/sodium chlorite and did not specify whether this was
for continuous or intermittent use.  It is believed that the single
label with the 800 ppm dose rate will be either cancelled or amended by
the registrant to delete this dose. The concentrations (the
concentrations of concern, or COC) used in the model were endpoints from
aquatic organism toxicity studies with sodium chlorite.

Exceedance values for average and worst-case situations were modeled. 
The average values were calculated by averaging all of the values for a
given flow category.  The worst-case values were calculated by averaging
the highest (peak) values for a given flow category.

Additional details on the PDM4 model and the specific inputs,
assumptions, and output can be found in the Environmental Modeling
Chapter.

Risk to Aquatic Organisms from Once-Through Cooling Use of Chlorine
Dioxide/Sodium Chlorite

Since the modeling for chlorine dioxide/sodium chlorite provides results
as percent days per year a particular concentration is exceeded, Risk
Quotients were not used in the usual way to provide numeric estimates of
risk.  Instead, the endpoints from various toxicity studies were
adjusted to determine the numeric Level of Concern (LOC) for each taxa
for both acute and chronic effects.  The adjustment factor is the same
as the one used with the RQ method, e.g. 0.5 * LC50 or EC50 for acute
effects, and 0.05 * LC50 or EC50 for acute endangered species risks. 
The chronic LOC needs no adjustment.  The modeling results provided the
percentage of days concentrations were exceeded for a range encompassing
the numeric LOCs.  When a specific LOC was not listed in the modeling
output tables, the percent days exceeded for the LOC was interpolated
from the closest numbers above and below the specific LOC.  The
percentage of days was then multiplied by 365 to provide the number of
days per year the LOC is exceeded.

Values for LOC percent days exceeded were interpolated where necessary,
using Tables 5 through 10 and 23 through 28 of the Environmental
Modeling Chapter.  If the number of days an LOC is exceeded is greater
than the number of days used to determine the endpoint in toxicity
testing, risk is assumed.  This assumes that the number of days exceeded
are consecutive, a conservative but not impossible assumption.

In the following tables, “risk” indicates that the LOC was exceeded
for more than 2 days per year for freshwater invertebrates and for more
than 4 days per year for all other organisms.  Where there is risk, the
number of days the LOC was exceeded is indicated in bold type.

Tables 9A-L: Number of Days per Year Levels of Concern are Exceeded by
the Once-Through Cooling Use of Chlorine Dioxide/Sodium Chlorite

A.  Continuous feed, 800 ppm, high flow scenario

	

	

Levels of Concern (LOC)



Taxa	

Endpoint value from study (ppm)	

Acute Nontarget (0.5*LC50) [average # of days exceeded]

{worst case # of days exceeded}	

Acute Endangered Species (0.05*LC50) [average # of days exceeded]

{worst case # of days exceeded}



FW fish	

96hLC50 = 50.6	

25.3 ppm

[262.44 days]

{363.54 days}	

2.53 ppm 

[340.74 days]

{365 days}



FW invert.	

48hEC50 = 0.027	

0.0135 ppm

[365 days]

{365 days}	

0.001 ppm

[365 days]

{365 days}



ME fish	

96hLC50 = 75	

37.5 ppm

[246.74 days]

{359.16 days}	

3.75 ppm

[325.86 days]

{365 days}



ME mollusk	

96hEC50 = 21.4	

10.7 ppm

[286.45 days]

{365 days}	

1.07 ppm

[358.54 days]

{365 days}



ME invert.	

96hLC50 = 0.576	

0.288 ppm

[365 days]

{365 days}	

0.029 ppm

[365 days]

{365 days}



Aquatic plant	

96hEC50 = 1.32	

1.32 ppm (LC50)

[355.49 days]

{365 days}	

0.62 ppm (NOEC)

[363.70 days]

{365 days}



The average scenario of a continuous feed of 800 ppm in a high flow
facility results in acute risk to endangered and non-endangered
freshwater fish, endangered and non-endangered freshwater invertebrates,
endangered and non-endangered marine/estuarine fish, endangered and
non-endangered marine/estuarine invertebrates and endangered and
non-endangered aquatic plants.  The worst-case scenario of a continuous
feed of 800 ppm in a high flow facility results in acute risk to
endangered and non-endangered freshwater fish, endangered and
non-endangered freshwater invertebrates, endangered and non-endangered
marine/estuarine fish, endangered and non-endangered marine/estuarine
invertebrates and endangered and non-endangered aquatic plants. 

 

B. Continuous feed, 800 ppm, low flow scenario



	

	

Levels of Concern (LOC)



Taxa	

Endpoint value from study (ppm)	

Acute Nontarget (0.5*LC50) [average # of days exceeded]

{worst case # of days exceeded}	

Acute Endangered Species (0.05*LC50) [average # of days exceeded]

{worst case # of days exceeded}



FW fish	

96hLC50 = 50.6	

25.3 ppm

[170.82 days]

{363.54 days}	

2.53 ppm 

[296.74 days]

{365 days}



FW invert.	

48hEC50 = 0.027	

0.0135 ppm

[365 days]

{365 days}	

0.001 ppm

[365 days]

{365 days}



ME fish	

96hLC50 = 75	

37.5 ppm

[147.46 days]

{359.16 days}	

3.75 ppm

[273.16 days]

{365 days}



ME mollusk	

96hEC50 = 21.4	

10.7 ppm

[216.34 days]

{365 days}	

1.07 ppm

[324.96 days]

{365 days}



ME invert.	

96hLC50 = 0.576	

0.288 ppm

[355.88 days]

{365 days}	

0.029 ppm

[365 days]

{365 days}



Aquatic plant	

96hEC50 = 1.32	

1.32 ppm (LC50)

[320.12 days]

{365 days}	

0.62 ppm (NOEC)

[335.35 days]

{365 days}



The average scenario of a continuous feed of 800 ppm in a low flow
facility results in acute risk to endangered and non-endangered
freshwater fish, endangered and non-endangered freshwater invertebrates,
endangered and non-endangered marine/estuarine fish, endangered and
non-endangered marine/estuarine invertebrates and endangered and
non-endangered aquatic plants.  The worst-case scenario of a continuous
feed of 800 ppm in a low flow facility results in acute risk to
endangered and non-endangered freshwater fish, endangered and
non-endangered freshwater invertebrates, endangered and non-endangered
marine/estuarine fish, endangered and non-endangered marine/estuarine
invertebrates and endangered and non-endangered aquatic plants.  

C. Intermittent feed, 800 ppm, high flow scenario

	

	

Levels of Concern (LOC)



Taxa	

Endpoint value from study (ppm)	

Acute Nontarget (0.5*LC50) [average # of days exceeded]

{worst case # of days exceeded}	

Acute Endangered Species (0.05*LC50) [average # of days exceeded]

{worst case # of days exceeded}



FW fish	

96hLC50 = 50.6	

25.3 ppm

[1.11 days]

{3.46 days}	

2.53 ppm 

[139.49 days]

{223 days}



FW invert.	

48hEC50 = 0.027	

0.0135 ppm

[356.19 days]

{365 days}	

0.001 ppm

[365 days]

{365 days}



ME fish	

96hLC50 = 75	

37.5 ppm

[0.35 days]

{1.25 days}	

3.75 ppm

[89.53 days]

{147.50 days}



ME mollusk	

96hEC50 = 21.4	

10.7 ppm

[9.34 days]

{21.77 days}	

1.07 ppm

[199.29 days]

{313.35 days}



ME invert.	

96hLC50 = 0.576	

0.288 ppm

[259.52 days]

{362.81 days}	

0.029 ppm

[338.65 days]

{365 days}



Aquatic plant	

96hEC50 = 1.32	

1.32 ppm (LC50)

[189.05 days]

{297.88 days}	

0.62 ppm (NOEC)

[220.75 days]

{341.31 days}



The average scenario of an intermittent feed of 800 ppm in a high flow
facility results in acute risk to endangered freshwater fish, endangered
and non-endangered freshwater invertebrates, endangered marine/estuarine
fish, endangered and non-endangered marine/estuarine invertebrates and
endangered and non-endangered aquatic plants.  The worst-case scenario
of an intermittent feed of 800 ppm in a high flow facility results in
acute risk to endangered freshwater fish, endangered and non-endangered
freshwater invertebrates, endangered marine/estuarine fish, endangered
and non-endangered marine/estuarine invertebrates and endangered and
non-endangered aquatic plants.  	

D. Intermittent feed, 800 ppm, low flow scenario

	

	

Levels of Concern (LOC)



Taxa	

Endpoint value from study (ppm)	

Acute Nontarget (0.5*LC50) [average # of days exceeded]

{worst case # of days exceeded}	

Acute Endangered Species (0.05*LC50) [average # of days exceeded]

{worst case # of days exceeded}



FW fish	

96hLC50 = 50.6	

25.3 ppm

[1.73 days]

{18.54 days}	

2.53 ppm 

[73.88 days]

{230.80 days}



FW invert.	

48hEC50 = 0.027	

0.0135 ppm

[320.76 days]

{365 days}	

0.001 ppm

[364.64 days]

{365 days}



ME fish	

96hLC50 = 75	

37.5 ppm

[0.89 days]

{10.11 days}	

3.75 ppm

[47.09 days]

{168.96 days}



ME mollusk	

96hEC50 = 21.4	

10.7 ppm

[6.54 days]

{53.54 days}	

1.07 ppm

[105.97 days]

{304.80 days}



ME invert.	

96hLC50 = 0.576	

0.288 ppm

[165.71 days]

{359.16 days}	

0.029 ppm

[293.61 days]

{365 days}



Aquatic plant	

96hEC50 = 1.32	

1.32 ppm (LC50)

[100.48 days]

{292.13 days}	

0.62 ppm (NOEC)

[120.43 days]

{329.19 days}



The average scenario of an intermittent feed of 800 ppm in a low flow
facility results in acute risk to endangered freshwater fish, endangered
and non-endangered freshwater invertebrates, endangered marine/estuarine
fish, endangered and non-endangered marine/estuarine invertebrates and
endangered and non-endangered aquatic plants.  The worst-case scenario
of an intermittent feed of 800 ppm in a low flow facility results in
acute risk to endangered and non-endangered freshwater fish, endangered
and non-endangered freshwater invertebrates, endangered and
non-endangered marine/estuarine fish, endangered and non-endangered
marine/estuarine invertebrates and endangered and non-endangered aquatic
plants.  	

E.  Intermittent feed, 25 ppm, high flow scenario

	

	

Levels of Concern (LOC)



Taxa	

Endpoint value from study (ppm)	

Acute Nontarget (0.5*LC50) [average # of days exceeded]

{worst case # of days exceeded}	

Acute Endangered Species (0.05*LC50) [average # of days exceeded]

{worst case # of days exceeded}



FW fish	

96hLC50 = 50.6	

25.3 ppm

[9.78x10-7 days]

{5.40x10-6 days}	

2.53 ppm 

[1.03 days]

{3.05 days}



FW invert.	

48hEC50 = 0.027	

0.0135 ppm

[249.07 days]

{343.39 days}	

0.001 ppm

[329.96 days]

{365 days}



ME fish	

96hLC50 = 75	

37.5 ppm

[1.24x10-7 days]

{7.01x10-7 days}	

3.75 ppm

[0.53 days]

{1.58 days}



ME mollusk	

96hEC50 = 21.4	

10.7 ppm

[7.49x10-5 days]

{3.88x10-4 days}	

1.07 ppm

[1.63 days]

{4.80 days}



ME invert.	

96hLC50 = 0.576	

0.288 ppm

[12.15 days]

{27.23 days}	

0.029 ppm

[204.15 days]

{293.61 days}



Aquatic plant	

96hEC50 = 1.32	

1.32 ppm (LC50)

[1.52 days]

{4.50 days}	

0.62 ppm (NOEC)

[2.91 days]

{7.65 days}



The average scenario of an intermittent feed of 25 ppm in a high flow
facility results in acute risk to endangered and non-endangered
freshwater invertebrates and endangered and non-endangered
marine/estuarine invertebrates.  The worst-case scenario of an
intermittent feed of 25 ppm in a high flow facility results in acute
risk to endangered and non-endangered freshwater invertebrates,
endangered and non-endangered marine/estuarine invertebrates and
endangered and non-endangered aquatic plants.  

F. Intermittent feed, 25 ppm, low flow scenario

	

	

Levels of Concern (LOC)



Taxa	

Endpoint value from study (ppm)	

Acute Nontarget (0.5*LC50) [average # of days exceeded]

{worst case # of days exceeded}	

Acute Endangered Species (0.05*LC50) [average # of days exceeded]

{worst case # of days exceeded}



FW fish	

96hLC50 = 50.6	

25.3 ppm

[4.42x10-5 days]

{5.29x10-4 days}	

2.53 ppm 

[1.34 days]

{13.82 days}



FW invert.	

48hEC50 = 0.027	

0.0135 ppm

[163.81 days]

{341.49 days}	

0.001 ppm

[267.55 days]

{365 days}



ME fish	

96hLC50 = 75	

37.5 ppm

[5.11x10-6 days]

{6.13x10-5 days}	

3.75 ppm

[0.71 days]

{7.33 days}



ME mollusk	

96hEC50 = 21.4	

10.7 ppm

[2.55x10-3 days]

{0.03 days}	

1.07 ppm

[2.09 days]

{21.58 days}



ME invert.	

96hLC50 = 0.576	

0.288 ppm

[8.03 days]

{61.32 days}	

0.029 ppm

[125.34 days]

{292.38 days}



Aquatic plant	

96hEC50 = 1.32	

1.32 ppm (LC50)

[1.96 days]

{20.25 days}	

0.62 ppm (NOEC)

[2.92 days]

{27.80 days}



The average scenario of an intermittent feed of 25 ppm in a low flow
facility results in acute risk to endangered and non-endangered
freshwater invertebrates and endangered and non-endangered
marine/estuarine invertebrates.  The worst-case scenario of an
intermittent feed of 25 ppm in a low flow facility results in acute risk
to endangered freshwater fish, endangered and non-endangered freshwater
invertebrates, endangered marine/estuarine fish, endangered and
non-endangered marine/estuarine invertebrates and endangered and
non-endangered aquatic plants.  

G. Continuous feed, 2.0 ppm, high flow scenario

	

	

Levels of Concern (LOC)



Taxa	

Endpoint value from study (ppm)	

Acute Nontarget (0.5*LC50) [average # of days exceeded]

{worst case # of days exceeded}	

Acute Endangered Species (0.05*LC50) [average # of days exceeded]

{worst case # of days exceeded}



FW fish	

96hLC50 = 50.6	

25.3 ppm

[9.89x10-3 days]

{0.04 days}	

2.53 ppm 

[47.99 days]

{77.19 days}



FW invert.	

48hEC50 = 0.027	

0.0135 ppm

[324.83 days]

{364.86 days}	

0.001 ppm

[364.64 days]

{365 days}



ME fish	

96hLC50 = 75	

37.5 ppm

[2.03x10-3 days]

{9.75x10-3 days}	

3.75 ppm

[25.71 days]

{42.54 days}



ME mollusk	

96hEC50 = 21.4	

10.7 ppm

[0.23 days]

{0.83 days}	

1.07 ppm

[74.64 days]

{118.66 days}



ME invert.	

96hLC50 = 0.576	

0.288 ppm

[165.35 days]

{275.21 days}	

0.029 ppm

[303.90 days]

{364.52 days}



Aquatic plant	

96hEC50 = 1.32	

1.32 ppm (LC50)

[70.08 days]

{111.56 days}	

0.62 ppm (NOEC)

[91.07 days]

{145.89 days}



The average scenario of a continuous feed of 2.0 ppm in a high flow
facility results in acute risk to endangered freshwater fish, endangered
and non-endangered freshwater invertebrates, endangered marine/estuarine
fish, endangered and non-endangered marine/estuarine invertebrates and
endangered and non-endangered aquatic plants.  The worst-case scenario
of a continuous feed of 2.0 ppm in a high flow facility results in acute
risk to endangered freshwater fish, endangered and non-endangered
freshwater invertebrates, endangered marine/estuarine fish, endangered
and non-endangered marine/estuarine invertebrates and endangered and
non-endangered aquatic plants.  

H. Continuous feed, 2.0 ppm, low flow scenario

	

	

Levels of Concern (LOC)



Taxa	

Endpoint value from study (ppm)	

Acute Nontarget (0.5*LC50) [average # of days exceeded]

{worst case # of days exceeded}	

Acute Endangered Species (0.05*LC50) [average # of days exceeded]

{worst case # of days exceeded}



FW fish	

96hLC50 = 50.6	

25.3 ppm

[0.10 days]

{1.19 days}	

2.53 ppm 

[22.60 days]

{98.70 days}



FW invert.	

48hEC50 = 0.027	

0.0135 ppm

[259.22 days]

{364.57 days}	

0.001 ppm

[348.58 days]

{365 days}



ME fish	

96hLC50 = 75	

37.5 ppm

[0.03 days]

{0.41 days}	

3.75 ppm

[12.82 days]

{62.60 days}



ME mollusk	

96hEC50 = 21.4	

10.7 ppm

[0.68 days]

{7.74 days}	

1.07 ppm

[34.31 days]

{141.90 days}



ME invert.	

96hLC50 = 0.576	

0.288 ppm

[78.84 days]

{264.26 days}	

0.029 ppm

[229.80 days]

{363.55 days}



Aquatic plant	

96hEC50 = 1.32	

1.32 ppm (LC50)

[32.30 days]

{134.50 days}	

0.62 ppm (NOEC)

[42.03 days]

{165.88 days}



The average scenario of a continuous feed of 2.0 ppm in a low flow
facility results in acute risk to endangered freshwater fish, endangered
and non-endangered freshwater invertebrates, endangered marine/estuarine
fish, endangered and non-endangered marine/estuarine invertebrates and
endangered and non-endangered aquatic plants.  The worst-case scenario
of a continuous feed of 2.0 ppm in a low flow facility results in acute
risk to endangered freshwater fish, endangered and non-endangered
freshwater invertebrates, endangered marine/estuarine fish, endangered
and non-endangered marine/estuarine invertebrates and endangered and
non-endangered aquatic plants.  

I. Continuous feed, 0.10 ppm, high flow scenario

	

	

Levels of Concern (LOC)



Taxa	

Endpoint value from study (ppm)	

Acute Nontarget (0.5*LC50) [average # of days exceeded]

{worst case # of days exceeded}	

Acute Endangered Species (0.05*LC50) [average # of days exceeded]

{worst case # of days exceeded}



FW fish	

96hLC50 = 50.6	

25.3 ppm

[negligible]

{negligible}	

2.53 ppm 

[0.06 days]

{0.24 days}



FW invert.	

48hEC50 = 0.027	

0.0135 ppm

[200.47 days]

{310.07 days}	

0.001 ppm

[294.19 days]

{365 days}



ME fish	

96hLC50 = 75	

37.5 ppm

[negligible]

{negligible}	

3.75 ppm

[0.03 days]

{0.12 days}



ME mollusk	

96hEC50 = 21.4	

10.7 ppm

[6.55x10-7 days]

{3.96x10-6 days}	

1.07 ppm

[0.09 days]

{0.37 days}



ME invert.	

96hLC50 = 0.576	

0.288 ppm

[1.28 days]

{3.94 days}	

0.029 ppm

[143.21 days]

{225.21 days}



Aquatic plant	

96hEC50 = 1.32	

1.32 ppm (LC50)

[0.09 days]

{0.35 days}	

0.62 ppm (NOEC)

[0.23 days]

{0.79 days}



The average scenario of a continuous feed of 0.10 ppm in a high flow
facility results in acute risk to endangered and non-endangered
freshwater invertebrates and endangered marine/estuarine invertebrates. 
The worst-case scenario of a continuous feed of 0.10 ppm in a high flow
facility results in acute risk to endangered and non-endangered
freshwater invertebrates and endangered marine/estuarine invertebrates. 


J. Continuous feed, 0.10 ppm, low flow scenario

	

	

Levels of Concern (LOC)



Taxa	

Endpoint value from study (ppm)	

Acute Nontarget (0.5*LC50) [average # of days exceeded]

{worst case # of days exceeded}	

Acute Endangered Species (0.05*LC50) [average # of days exceeded]

{worst case # of days exceeded}



FW fish	

96hLC50 = 50.6	

25.3 ppm

[1.53x10-7 days]

{1.84x10-6 days}	

2.53 ppm 

[0.25 days]

[2.92 days]



FW invert.	

48hEC50 = 0.027	

0.0135 ppm

[109.28 days]

{304.56 days}	

0.001 ppm

[228.86 days]

{365 days}



ME fish	

96hLC50 = 75	

37.5 ppm

[1.10x10-8 days]

{1.20x10-7 days}	

3.75 ppm

[0.13 days]

{1.52 days}



ME mollusk	

96hEC50 = 21.4	

10.7 ppm

[3.20x10-5 days]

{3.85x10-4 days}	

1.07 ppm

[0.39 days]

{4.60 days}



ME invert.	

96hLC50 = 0.576	

0.288 ppm

[1.88 days]

{19.97 days}	

0.029 ppm

[77.42 days]

{234.40 days}



Aquatic plant	

96hEC50 = 1.32	

1.32 ppm (LC50)

[0.37 days]

{4.31 days}	

0.62 ppm (NOEC)

[0.59 days]

{6.67 days}



The average scenario of a continuous feed of 0.10 ppm in a low flow
facility results in acute risk to endangered and non-endangered
freshwater invertebrates and endangered marine/estuarine invertebrates. 
The worst-case scenario of a continuous feed of 0.10 ppm in a low flow
facility results in acute risk to endangered and non-endangered
freshwater invertebrates, endangered and non-endangered marine/estuarine
invertebrates and endangered and non-endangered aquatic plants.  

K. Intermittent feed, 0.10 ppm, high flow scenario

	

	

Levels of Concern (LOC)



Taxa	

Endpoint value from study (ppm)	

Acute Nontarget (0.5*LC50) [average # of days exceeded]

{worst case # of days exceeded}	

Acute Endangered Species (0.05*LC50) [average # of days exceeded]

{worst case # of days exceeded}



FW fish	

96hLC50 = 50.6	

25.3 ppm

[negligible]

{negligible}	

2.53 ppm 

[negligible]

{negligible}



FW invert.	

48hEC50 = 0.027	

0.0135 ppm

[0.09 days]

{0.34 days}	

0.001 ppm

[16.06 days]

{34.31 days}



ME fish	

96hLC50 = 75	

37.5 ppm

[negligible]

{negligible}	

3.75 ppm

[negligible]

{negligible}



ME mollusk	

96hEC50 = 21.4	

10.7 ppm

[negligible]

{negligible}	

1.07 ppm

[negligible]

{negligible}



ME invert.	

96hLC50 = 0.576	

0.288 ppm

[negligible]

{1.83x10-8 days}	

0.029 ppm

[0.05 days]

{0.22 days}



Aquatic plant	

96hEC50 = 1.32	

1.32 ppm (LC50)

[negligible]

{negligible}	

0.62 ppm (NOEC)

[negligible]

{negligible}



The average scenario of an intermittent feed of 0.10 ppm in a high flow
facility results in acute risk to endangered freshwater invertebrates. 
The worst-case scenario of an intermittent feed of 0.10 ppm in a high
flow facility results in acute risk to endangered freshwater
invertebrates.

L. Intermittent feed, 0.10 ppm, low flow scenario

	

	

Levels of Concern (LOC)



Taxa	

Endpoint value from study (ppm)	

Acute Nontarget (0.5*LC50) [average # of days exceeded]

{worst case # of days exceeded}	

Acute Endangered Species (0.05*LC50) [average # of days exceeded]

{worst case # of days exceeded}



FW fish	

96hLC50 = 50.6	

25.3 ppm

[0 days]

{0 days}	

2.53 ppm 

[negligible]

{negligible}



FW invert.	

48hEC50 = 0.027	

0.0135 ppm

[0.37 days]

{4.26 days}	

0.001 ppm

[9.82 days]

{70.08 days}



ME fish	

96hLC50 = 75	

37.5 ppm

[0 days]

{0 days}	

3.75 ppm

[negligible]

{negligible}



ME mollusk	

96hEC50 = 21.4	

10.7 ppm

[0 days]

{0 days}	

1.07 ppm

[negligible]

{negligible}



ME invert.	

96hLC50 = 0.576	

0.288 ppm

[8.40x10-8 days]

{1.01x10-6 days}	

0.029 ppm

[0.24 days]

{2.74 days}



Aquatic plant	

96hEC50 = 1.32	

1.32 ppm (LC50)

[negligible]

{negligible}	

0.62 ppm (NOEC)

[negligible]

{negligible}



The average scenario of an intermittent feed of 0.10 ppm in a low flow
facility results in acute risk to endangered freshwater invertebrates. 
The worst-case scenario of an intermittent feed of 0.10 ppm in a low
flow facility results in acute risk to endangered and non-endangered
freshwater invertebrates.

3.  Environmental Risk Assessment

a.  Terrestrial Organisms

No model is available to estimate exposure and risk to birds and mammals
from discharge of once-through cooling system effluents into surface
waters.  The rapid degradation of the chemicals, coupled with the
generally low toxicity of chlorine dioxide and sodium chlorite to birds
and mammals, make risk to these organisms unlikely.  The very limited
data available to assess the phytotoxicity of chlorine dioxide/sodium
chlorite make it difficult to determine the risk to
terrestrial/semi-aquatic plants.

b.  Aquatic Organisms

Acute risk to aquatic organisms may occur from the use of chlorine
dioxide/sodium chlorite in once-through cooling towers based on the
screening level assessment conducted.  At the highest doses, the model
shows risk to freshwater and marine/estuarine fish and invertebrates and
aquatic plants, and at the lowest doses the model shows risk only to
freshwater invertebrates.  See Table 9 A-L for details. 

Chronic risk to aquatic organisms cannot be assessed at this time due to
the lack of chronic toxicity endpoints for fish and aquatic
invertebrates.  When the required aquatic chronic toxicity testing
described above is submitted, chronic risk to these organisms will be
assessed.

4.  Listed Species

Section 7 of the Endangered Species Act, 16 U.S.C. Section 1536(a)(2),
requires all federal agencies to consult with the National Marine
Fisheries Service (NMFS) for marine and anadromous listed species, or
the United States Fish and Wildlife Services (FWS) for listed wildlife
and freshwater organisms, if they are proposing an “action” that may
affect listed species or their designated habitat.  Each federal agency
is required under the Act to insure that any action they authorize,
fund, or carry out is not likely to jeopardize the continued existence
of a listed species or result in the destruction or adverse modification
of designated critical habitat.  To jeopardize the continued existence
of a listed species means “to engage in an action that reasonably
would be expected, directly or indirectly, to reduce appreciably the
likelihood of both the survival and recovery of a listed species in the
wild by reducing the reproduction, numbers, or distribution of the
species.” 50 C.F.R. § 402.02.

To facilitate compliance with the requirements of the Endangered Species
Act subsection (a)(2) the Environmental Protection Agency, Office of
Pesticide Programs has established procedures to evaluate whether a
proposed registration action may directly or indirectly reduce
appreciably the likelihood of both the survival and recovery of a listed
species in the wild by reducing the reproduction, numbers, or
distribution of any listed species (U.S. EPA, 2004).  After the
Agency’s screening-level risk assessment is performed, if any of the
Agency’s Listed Species LOC Criteria are exceeded for either direct or
indirect effects, a determination is made to identify if any listed or
candidate species may co-occur in the area of the proposed pesticide
use.  If determined that listed or candidate species may be present in
the proposed use areas, further biological assessment is undertaken. 
The extent to which listed species may be at risk then determines the
need for the development of a more comprehensive consultation package as
required by the Endangered Species Act.

Acute risks to listed birds and mammals are not anticipated from the use
of chlorine dioxide and sodium chlorite products due to low exposure and
low toxicity.  The screening level model used in this assessment
indicates that there may be acute risks to listed aquatic organisms from
the once through cooling tower use of chlorine dioxide/sodium chlorite. 
Further, potential indirect effects on any species dependent upon a
species that experiences effects from use of chlorine dioxide/sodium
chlorite cannot be precluded based on the screening level ecological
risk assessment.  These findings are based solely on EPA’s screening
level assessment and do not constitute “may effect” findings under
the Endangered Species Act.

Chronic risks to listed aquatic organisms cannot be assessed at this
time; this risk will be assessed when required chronic toxicity data are
submitted to and evaluated by the Agency.  



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TGAI = technical grade active ingredient

TEP = typical end-use product

 EC50=Median Effective Concentration

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