**Ammonium Thiosulfate (ATS)**

(NH~4~)~2~S~2~O~3~

CAS Registry Number
[7783-18-8](http://www.sigmaaldrich.com/catalog/search/SearchResultsPage?Query=7783-18-8&Scope=CASSearch&btnSearch.x=1)
(Solution)

CAS Registry Number 7664-41-7 (Anhydrous)

PC Code: 080103

> Thiosulfate Ammonium Counter Cation

![](media/image1.jpeg){width="1.1159722222222221in"
height="0.8965277777777778in"}
![](media/image2.jpeg){width="0.7284722222222222in"
height="0.8965277777777778in"}

**End-Use Product:** Oxalis/Spurge X

**EPA Reg.No.** 9499-1

**Formulation:** Concentrate (55.5% ATS)

**Uses:** Herbicide to control spurge and oxalis in dichondra lawns

**Prepared By:** Colleen Flaherty, M.S., Biologist

James A. Hetrick, Ph.D., Senior Scientist

Silvia Carlota Termes, Ph.D., Chemist

Reviewed by: Stephanie Syslo, M.S., Environmental Scientist

Ecological Risk Branch III

Environmental Fate and Effects Division

Approved by: Karen Whitby, Ph.D.

Ecological Risk Branch III

Environmental Fate and Effects Division

**Date:** 13 September 2007

**Table of Contents**

[**I.** **Executive Summary** 3](#__RefHeading___Toc177444902)

[**A.** **Nature of the Chemical Stressor**
3](#__RefHeading___Toc177444903)

[**B.** **Potential Risks to Non-target Organisms**
3](#__RefHeading___Toc177444904)

[**C.** **Environmental Fate and Exposure**
3](#__RefHeading___Toc177444905)

[**D.** **Ecological Effects** 4](#__RefHeading___Toc177444906)

[**E.** **Uncertainties and Data Gaps** 4](#__RefHeading___Toc177444907)

[**II.** **Problem Formulation** 5](#__RefHeading___Toc177444908)

[**A.** **Nature of Regulatory Action** 5](#__RefHeading___Toc177444909)

[**B.** **Stressor Source and Distribution**
5](#__RefHeading___Toc177444910)

[**1.** **Nature of the Chemical Stressor**
5](#__RefHeading___Toc177444911)

[**2.** **Identification of the Chemical Stressor; Physical and Chemical
Properties** 5](#__RefHeading___Toc177444912)

[**3.** **Overview of Pesticide Usage** 8](#__RefHeading___Toc177444913)

[**C.** **Receptors** 8](#__RefHeading___Toc177444914)

[**1.** **Aquatic and Terrestrial Effects**
8](#__RefHeading___Toc177444915)

[**D.** **Analysis Plan** 9](#__RefHeading___Toc177444916)

[**III.** **Analysis** 10](#__RefHeading___Toc177444917)

[**A.** **Use Characterization** 10](#__RefHeading___Toc177444918)

[**B.** **Exposure Characterization** 10](#__RefHeading___Toc177444919)

[**1.** **Environmental Fate and Transport**
10](#__RefHeading___Toc177444920)

[**2.** **Aquatic Exposure Assessment**
13](#__RefHeading___Toc177444921)

[**C.** **Effects Characterization** 16](#__RefHeading___Toc177444922)

[**IV.** **Risk Characterization** 17](#__RefHeading___Toc177444923)

[**A.** **Aquatic Risk** 17](#__RefHeading___Toc177444924)

[**B.** **Terrestrial Risk** 19](#__RefHeading___Toc177444925)

[**C.** **Federally Threatened and Endangered (Listed) Species
Concerns** 19](#__RefHeading___Toc177444926)

[**D.** **Discussion of Uncertainties, Limitations, and Data Gaps**
19](#__RefHeading___Toc177444927)

[**1. Exposure For All Taxa** 19](#__RefHeading___Toc177444928)

[**a. Maximum Use Scenario** 19](#__RefHeading___Toc177444929)

[**b. Additive and/or Synergistic Effects**
19](#__RefHeading___Toc177444930)

[**2.** **Exposure For Aquatic Species**
20](#__RefHeading___Toc177444931)

[**3.** **Data Gaps** 20](#__RefHeading___Toc177444932)

[**V.** C**ited Literature** 21](#__RefHeading___Toc177444933)

**Appendix A**. Sulfur chemical species (other than thiosulfate) of
environmental significance

**Appendix B.** Aquatic Exposure Model Estimates (GENEEC Version 2.0)

I.  []{#__RefHeading___Toc177444902 .anchor}**Executive Summary**

```{=html}
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```
A.  []{#__RefHeading___Toc177444903 .anchor}**Nature of the Chemical
    Stressor**

Ammonium thiosulfate (CAS No.
[7783-18-8](http://www.sigmaaldrich.com/catalog/search/SearchResultsPage?Query=7783-18-8&Scope=CASSearch&btnSearch.x=1)),
an inorganic compound, is the chemical stressor considered in this
assessment. Ammonium thiosulfate dissociates completely in water. The
dissociation products are the thiosulfate anion and its ammonium counter
cation. This is the first step in the behavior of thiosulfate in the
environment. Both thiosulfate and ammonium are considered in this risk
assessment.

Thiosulfate is a metastable, moderately reducing oxyanion of sulfur.
Chemical reactions (redox reactions) of thiosulfate generate chemical
species of sulfur that differ in their oxidation state. The ammonium
counter cation in ammonium thiosulfate is a source of "ammoniacal
nitrogen." Its major chemical reaction in the environment is oxidation
to nitrate nitrogen, which is known as nitrification. Ammonium also
exists in equilibrium with ammonia gas.

Ammonium thiosulfate is registered to kill oxalis and spurge in
dichondra lawns. The only end-use product is "Oxalis/Spurge X,
Concentrate Foliar Spray" (USEPA Reg. No. 9499-1; Approval date of last
label 9/30/2003). This product is a concentrate foliar spray containing
55.0% ammonium thiosulfate. Ammonium thiosulfate is also used as a
fertilizer.

B.  []{#__RefHeading___Toc177444904 .anchor}**Potential Risks to
    Non-target Organisms**

This ecological risk assessment considers the limited use of ammonium
thiosulfate as an herbicide on dichondra lawns in residential/homeowner
settings in California. This assessment is qualitative in nature; that
is, the risk quotient approach is not utilized. The nature of ammonium
thiosulfate and its predicted behavior in the environment make it
difficult to provide meaningful quantitative exposure estimates for
aquatic and terrestrial systems. Since ammonium thiosulfate is being
used as an herbicide, there is a possibility that non-target plants may
be at risk; however, given that it is only used in residential/homeowner
settings, the potential for adverse effects to non-target plants seems
highly unlikely. Based on this qualitative assessment for the pesticidal
use of ammonium thiosulfate, risk to non-target aquatic and terrestrial
organisms (including Federally-listed species) is presumed to be
negligible.

C.  []{#__RefHeading___Toc177444905 .anchor}**Environmental Fate and
    Exposure**

The thiosulfate anion is only stable in neutral or alkaline media. It is
unstable in acid media. Thiosulfate acid cannot be generated by
acidification in aqueous solutions of its salts because it decomposes to
sulfuric acid and a mixture of elemental sulfur, hydrogen sulfide,
polysulfide, and sulfur dioxide. In aqueous media, thiosulfate
irreversibly disproportionates to sulfide and sulfate.

The chemistry of sulfur in soils (and in water) is complex because of
its many oxidation states of sulfur, which not only involves oxysulfur
species, but also species containing only sulfur (e.g., elemental
sulfur, sulfide). The predominance of a given species is dependent on pH
and redox potential of the media, as well as by the type of bacteria
present in soils. The terminal, thermodynamically-favored reaction
product of thiosulfate is sulfate, except in highly reduced soils. As an
anion, sulfate is expected to be mobile in soils. Sulfate is ubiquitous
and is present in most natural waters, where it is considered a
permanent solute. Sources of sulfate in natural water are dissolution of
naturally occurring sulfate salts, runoff of oxidized sulfide ores, acid
rain deposition, industrial waste, and fertilizers. No aquatic exposure
modeling was performed for sulfate, and risk was discussed
qualitatively.

For the ammonium counter cation, its major chemical reaction in the
environment is oxidation to nitrate. Ammonium is expected to be in
equilibrium with ammonia gas in the soil air voids. The volatilization
of ammonia is dependent on temperature and pH of the media and increase
with increasing temperature and pH. As a cation, ammonium can adsorb to
soil colloid or organic matter through electrostatic cation exchange.
For ammonia, a screening-level aquatic exposure assessment using the
GENEEC model (Version 2.0) was conducted to estimate surface water
concentrations of ammonium from use of ammonium thiosulfate as a
pesticide.

D.  []{#__RefHeading___Toc177444906 .anchor}**Ecological Effects**

There are no acceptable registrant-submitted ecotoxicity data available
for fish, aquatic invertebrates, aquatic plants, birds, terrestrial
invertebrates, or terrestrial plants for consideration in this risk
assessment for ammonium thiosulfate. The aquatic assessment relies on
the USEPA ambient water quality criteria for ammonia (1999 Update). The
terrestrial assessment is qualitative in nature, but it does consider
mammalian toxicity data provided by the Health Effects Division.
Ammonium thiosulfate is practically non-toxic to mammals on an acute
oral basis.

E.  []{#__RefHeading___Toc177444907 .anchor}**Uncertainties and Data
    Gaps**

No acceptable environmental fate or ecotoxicity guideline studies are
available for ammonium thiosulfate. However, given the nature of the
chemical and its predicted behavior in the environment, this was not an
impediment to the risk assessment. At this time, the EFED is not
requesting any environmental fate or ecotoxicity studies to be submitted
for ammonium thiosulfate.

[]{#__RefHeading___Toc177444908 .anchor}**II. Problem Formulation**

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

[]{#__RefHeading___Toc177444909 .anchor}**A. Nature of Regulatory
Action**

This ecological risk assessment is in support of the Reregistration
Eligibility Decision (RED) for ammonium thiosulfate use as an herbicide.
There is one end-use product considered in this assessment,
Oxalis/Spurge (55.0% active ingredient ammonium thiosulfate), which is
used to kill oxalis and spurge in dichondra lawns in California.

[]{#__RefHeading___Toc177444910 .anchor}**B. Stressor Source and
Distribution**

[]{#__RefHeading___Toc177444911 .anchor}**1. Nature of the Chemical
Stressor**

Ammonium thiosulfate (CAS No.
[7783-18-8](http://www.sigmaaldrich.com/catalog/search/SearchResultsPage?Query=7783-18-8&Scope=CASSearch&btnSearch.x=1))
is the chemical stressor considered in this assessment. Ammonium
thiosulfate is an inorganic compound. Both ammonium and thiosulfate are
considered in the assessment.

Ammonium thiosulfate is not listed as a High Production Volume (HPV)
chemical or in the Toxic Release Inventory (TRI), but it is listed in
the Toxic Substance Control Act (TSCA) (8b) Inventory of Chemical
Substances (Substance of Commerce).

[]{#__RefHeading___Toc177444912 .anchor}**2. Identification of the
Chemical Stressor; Physical and Chemical Properties**

The first step in the environmental fate of ammonium thiosulfate will be
dissociation into ammonium and thiosulfate. Thiosulfate is one of the
oxyanions of sulfur. The chemical reactions of thiosulfate in the
environment are mostly controlled by the redox potential of the media,
as well as pH. The mode of action of ammonium thiosulfate as an
herbicide to control oxalis and spurge is presumably via fertilizer burn
(salt injury).

Identification and information on ammonium thiosulfate/thiosulfate anion
is summarized in Table II-1. Figure II.1 presents the ammonium counter
cation and its equilibrium with ammonia. Sulfur compounds have an
extensive and often ambiguous nomenclature, and the chemical names for
sulfur chemical species relevant to thiosulfate are summarized in
**Appendix A**.

**Table II.1 Chemical Identification and General Information on Ammonium
Thiosulfate (ATS) and the Thiosulfate anion**

+---------------+------------------------------------------------------+
| **Type of     | **Information**                                      |
| Information** |                                                      |
+---------------+------------------------------------------------------+
| Common        | Ammonium Thiosulfate; ATS                            |
| Name(s)       |                                                      |
+---------------+------------------------------------------------------+
| Chemical      | Ammonium Thiosulfate (sometimes the S~2~O~3~^2−^     |
| Name-         | anion is referred as thiosulfites or hyposulfites)   |
| Synonyms      |                                                      |
|               | Thiosulfuric acid, diammonium salt                   |
|               |                                                      |
|               | Diammonium Thiosulfate                               |
|               |                                                      |
|               | Ammonium thiosulfate solution (60% or less)          |
|               |                                                      |
|               | As per US EPA Substance Registry System              |
+---------------+------------------------------------------------------+
| CAS Registry  | [7783-18-8](htt                                      |
| Number        | p://www.sigmaaldrich.com/catalog/search/SearchResult |
|               | sPage?Query=7783-18-8&Scope=CASSearch&btnSearch.x=1) |
| EPA PC Code   | (Solution)                                           |
| No.           |                                                      |
|               | 7664-41-7 (Anhydrous)                                |
|               |                                                      |
|               | 080103                                               |
+---------------+------------------------------------------------------+
| Empirical     | (NH~4~)~2~S~2~O~3~                                   |
| Formula       |                                                      |
+---------------+------------------------------------------------------+
| Molecular     | 148.21 (Anhydrous)                                   |
| Weight        |                                                      |
+---------------+------------------------------------------------------+
| Molecular     | ![](media/image3.png){width="1.5625in"               |
| Structure of  | height="1.2715277777777778in"}                       |
| the anion     |                                                      |
|               | Note the S-S bond (disulfide linkage)                |
|               |                                                      |
|               | ![](media/image4.png){width="2.0819444444444444in"   |
|               | height="1.2083333333333333in"}                       |
|               |                                                      |
|               | Note: The purple symbol stands for Na^+^ or K+       |
|               | cations. See below for ammonium                      |
|               |                                                      |
|               | [Crystal Structure]{.underline}:                     |
|               |                                                      |
|               | S. T. TENG, H. FUESS AND J. W. BATS. 1979. *Acta     |
|               | Cryst.* B35, 1682-1684                               |
|               |                                                      |
|               | The structure consists of                            |
|               |                                                      |
|               | NH ~4~ and S~2~O~3~ tetrahedra interconnected by     |
|               | hydrogen bonds. All but two H atoms were located by  |
|               | different methods. One of the NH~4~ groups is        |
|               | possibly disordered at room temperature.             |
+---------------+------------------------------------------------------+
| Oxidation     | The two different S atoms in thiosulfate have charge |
| state of      | densities corresponding to V and -I oxidation states |
| sulfur in     |                                                      |
| thiosulfate   | \[NH4+\].\[S-\]S(=O)(=O)\[O-\].\[NH4+\]              |
|               |                                                      |
| Smiles        |                                                      |
| Notation      |                                                      |
+---------------+------------------------------------------------------+

![](media/image5.png){width="2.7083333333333335in"
height="0.9388888888888889in"}

![](media/image6.jpeg){width="1.5715277777777779in"
height="1.5625in"}Ammonium ion ![](media/image7.jpeg){width="1.34375in"
height="1.1756944444444444in"}

**Figure II.1** Equilibrium Between Ammonium Cation and Ammonia

NH~3~(g) + H^+^↔ \[NH~4~^+^\]; Log K^o^=11.04, where K^o^ is the
equilibrium constant expressed in terms of activity.

The ammonium ion has a tetrahedral structure. The formal oxidation state
of nitrogen in ammonia and ammonium cation is --III. Ammonia-based
fertilizers serve as a source of nitrogen.

**Physical and Chemical Properties**

The physical and chemical properties of ammonium thiosulfate are shown
in **Table II.2**.

**Table II.2 Physical and Chemical Properties of Ammonium Thiosulfate**

+------------------------------------+---------------------------------+
| **Property**                       | **Value**                       |
+------------------------------------+---------------------------------+
| Melting Point                      | Decomposes at 160 C             |
|                                    |                                 |
| Boiling Point                      |                                 |
+------------------------------------+---------------------------------+
| Vapor Pressure                     | Reported as 0 mmHg at 25C       |
|                                    |                                 |
| Henry's Law Constant               | Not applicable                  |
+------------------------------------+---------------------------------+
| Solubility in Water                | 103.3 g/100 ml water @ 100 deg. |
|                                    | C.                              |
| (Thiosulfate is not stable under   |                                 |
| acidic pHs)                        | Complete dissociation           |
+------------------------------------+---------------------------------+
| Solubility in Non-aqueous Solvents | Easily soluble in cold water,   |
|                                    | hot water.                      |
|                                    |                                 |
|                                    | Slightly soluble in acetone.    |
|                                    |                                 |
|                                    | Insoluble in diethyl ether.     |
|                                    |                                 |
|                                    | Insoluble in alcohol            |
|                                    |                                 |
|                                    | Unstable in acid pH             |
+------------------------------------+---------------------------------+

**Behavior of Ammonium Thiosulfate, Thiosulfate and Ammonium in the
Environment**

Ammonium thiosulfate dissociates completely in water. The dissociation
products are the thiosulfate anion and the ammonium cation. This is the
first step in the behavior of thiosulfate in the environment.

Thiosulfate is a metastable, moderately reducing oxyanion of sulfur.
Chemical reactions (redox reactions) of thiosulfate generate chemical
species of sulfur that differ in their oxidation state.

The main types of chemical reactions of thiosulfate relevant in the
environment are:

1.  Disproportionation (chemical and microbially mediated)

2.  Chemical and microbially mediated redox reactions

3.  Complexation with metal ions

4.  Bimolecular nucleophilic substitution reactions (SN2 mechanisms);
    dihalogenation

The ammonium counter cation in ammonium thiosulfate is a source of
"ammoniacal nitrogen". Its major chemical reaction in the environment is
oxidation to nitrate nitrogen, that is N(-III)→ N(V), which is known as
nitrification.

[]{#__RefHeading___Toc177444913 .anchor}**3. Overview of Pesticide
Usage**

Ammonium thiosulfate (ATS) is registered to kill oxalis and spurge in
dichondra lawns. The only end-use product is "Oxalis/Spurge X,
Concentrate Foliar Spray" (USEPA Reg. No. 9499-1; Approval date of last
label 9/30/2003). This product is a concentrate foliar spray containing
55.0% ATS.

The major use of ammonium thiosulfate is not as a pesticide, but rather
as a fertilizer (soil nutrient and amendment). In 2005, over 700 million
pounds (350,580 short tons) of ammonium thiosulfate was used as a
fertilizer (USDA/NASS). Compared to its use as a pesticide, less than
6500 pounds of ammonium thiosulfate is used annually.

In minerals processing, ATS is increasingly used as a leaching agent for
gold and silver extraction from auriferous ores. It is increasingly used
as an alternative for conventional cyanide leaching.

[]{#__RefHeading___Toc177444914 .anchor}**C. Receptors**

[]{#__RefHeading___Toc177444915 .anchor}**1. Aquatic and Terrestrial
Effects**

The receptor is the biological entity that is exposed to the stressor
(EPA, 1998). In general, the receptors considered in ecological risk
assessments for pesticides are aquatic organisms, such as fish
(surrogate for aquatic-phase amphibians), invertebrates, and plants
(non-vascular and vascular), and terrestrial organisms, such as birds
(surrogate for reptiles and terrestrial-phase amphibians), mammals,
invertebrates, and plants.

[]{#__RefHeading___Toc177444916 .anchor}**D. Analysis Plan**

*Available Data*

No environmental fate guideline studies (Subdivision N) are available
for ammonium thiosulfate; thus, the environmental fate and exposure
assessment will be based on open literature data. EPISuite Version 3.2,
a tool commonly used in the ecological risk assessment process when
environmental fate data are unavailable, cannot be used to estimate the
environmental fate and transport of ammonium thiosulfate because it is
not an appropriate model for inorganic chemicals.

There are no acceptable registrant-submitted ecotoxicity data available
for fish, aquatic invertebrates, aquatic plants, birds, terrestrial
invertebrates, or terrestrial plants for consideration in this risk
assessment for ammonium thiosulfate. The aquatic assessment will rely on
the U.S.E.P.A. ambient water quality criteria for ammonia (1999
Update).[^1] The terrestrial assessment will rely on the mammalian
toxicity data provided by the Health Effects Division.

*Aquatic Risk*

For sulfate, the most likely sulfur species in water, the Agency has
established a Secondary Maximum Contaminant Level (SMCL) of 250 mg/L. It
is highly unlikely that the aquatic exposures of sulfate would approach
that level as a result of its limited use as an herbicide to control
spurge and oxalis in dichondra lawns. Therefore, no aquatic exposure
modeling will be performed for sulfate.

For ammonium, aquatic exposure estimates will be generated using the
Tier I GENEEC model (GENeric Estimated Exposure Concentrations, Version
2.0). Assumptions will be made to select the environmental fate input
parameters. Further, since the label does not specify an application
rate in terms of pounds of ATS per acre, assumptions must be made to
estimate the application rates in terms of pounds per acre. Given that
the product is applied in residential/homeowner settings, assumptions
will be made to estimate the percent of treated area, which will be used
to correct the model-generated aquatic exposure estimates. It is
recognized that these assumptions will result in highly conservative
aquatic exposure estimates.

Given the multiple assumptions and the complexity of the sulfur and
nitrogen systems (multiple chemical species in different oxidation
states), the aquatic exposure assessment carries a very high degree of
uncertainty. Despite this uncertainty, the model estimates will be
compared to the U.S.E.P.A. ambient water quality criteria for ammonia to
determine if there is a potential risk to aquatic animals as a result of
the pesticidal use of ammonium thiosulfate.

*Terrestrial Risk*

Given the expected behavior of ammonium thiosulfate in the environment,
it is inappropriate to use the Tier 1 T-REX model (Version 1.3.1) to
estimate terrestrial dietary exposures. The potential risk of ammonium
thiosulfate (when used as a pesticide) to terrestrial organisms will be
discussed qualitatively.

[]{#__RefHeading___Toc177444917 .anchor}**III. Analysis**

[]{#__RefHeading___Toc177444918 .anchor}**A. Use Characterization**

Ammonium thiosulfate (CAS No.
[7783-18-8](http://www.sigmaaldrich.com/catalog/search/SearchResultsPage?Query=7783-18-8&Scope=CASSearch&btnSearch.x=1))
is the chemical stressor considered in this assessment. The end-use
product, Oxalis/Spurge X (55.0% active ingredient ammonium thiosulfate),
is used to kill oxalis and spurge in dichondra lawns in California.

Dichondra (*Dichondra repens;* kidney weed dichondra) is a low growing,
creeping perennial that spreads by underground runners. The leaves are
kidney-shaped and dark green in color. The overall appearance is that of
a flat cover. It has a neat rich velvety appearance throughout the year
and needs no mowing.

According to the product label, the dichondra lawn is to be watered the
day before applying the ATS product, but the foliage must be dry at the
time of application. The range of temperature at which Oxalis/Spurge X
is to be applied is between 72 and 80° F (22 to 26.7° C). Above 80° F.,
it could cause severe, temporary damage to the dichondra foliage. The
product can be used on young dichondra.

The label also indicates that the product can be used in bladegrass
lawns. Bladegrass can tolerate temperatures up to 95° F (35° C), but it
is not to be used until a minimum of three mowings. The product should
not be applied immediately after mowing. Weeds should be killed before
they go to seed.

The product is claimed to work best in sunlight.

The label recommends mixing 4 oz/gallon of water per 100 feet of area to
be treated. It is recommended the addition of a "good spreader-sticker"
and use of pressure type tank sprayer because hose-end garden sprayers
and water cans are ineffective. Leaves must be thoroughly saturated.
According to the label, application can be repeated every 10 days or two
weeks, but the number of applications per year (maximum total
application rate) is not specified in the label.

[]{#__RefHeading___Toc177444919 .anchor}**B. Exposure Characterization**

[]{#__RefHeading___Toc177444920 .anchor}**1. Environmental Fate and
Transport**

The first step in the environmental fate of ammonium thiosulfate,
(NH~4~)~2~S~2~O~3~, will be dissociation \[i.e., (NH~4~)~2~S~2~O~3~ ↔ 2
NH~4~^+^ + S~2~O~3~ ^2-^ \]. Because (NH~4~)~2~S~2~O~3~ is a salt,
dissociation will be controlled by the water solubility of
(NH~4~)~2~S~2~O~3~ (i.e., 103.3 g/100 mls of H~2~O @ 100^o^C or
1,033,000 mg/L; 1,033 g/L). Concentrations of (NH~4~)~2~S~2~O~3~ below
the water solubility will cause complete dissociation to 2NH~4~^+^ and
S~2~O~3~^2-^. As a first approximation, (NH~4~)~2~S~2~O~3~
concentrations in soil solution based on the maximum application rate
and different depths of soil incorporation are not expected to exceed
the water solubility of (NH~4~)~2~S~2~O~3~ (Table II.2). Therefore,
(NH~4~)~2~S~2~O~3~ is not expected to exist as the (NH~4~)~2~S~2~O~3~
salt (i.e., as a discrete, non-dissociated entity) when applied because
it will completely dissociate to 2 NH~4~^+^ and S~2~O~3~^2--^ ions.

The thiosulfate anion is only stable in neutral or alkaline media. It is
unstable in acid media. Thiosulfate acid cannot be generated by
acidification in aqueous solutions of its salts because it decomposes to
sulfuric acid and a mixture of elemental sulfur, hydrogen sulfide,
polysulfide, and sulfur dioxide. In aqueous media, thiosulfate
irreversibly disproportionates to sulfide and sulfate[^2]. The rate of
disproportionation is pH-dependent, with the rate of disproportionation
being directly proportional to the hydrogen ion concentration (i.e, with
decreasing pH). Sulfite is a transient intermediate in this reaction
Another reaction intermediate of thiosulfate is tetrathionate
(S~4~O~6~^2-^), which undergoes further reactions to sulfate, the
thermodynamically stable sulfur species.

Chemical and microbial reactions are involved in the transformation of
thiosulfate (and of other sulfur species) in the environment. Both
sulfide and thiosulfate are the most abundant reduced species of sulfur
in the environment and both are converted to sulfate in the oxidative
half of the sulfur cycle.

Redox and disproportionation reactions of thiosulfate (chemical;
bacterial) are involved in the sulfur cycle in the environment. For
example, disproportionation has been identified as an important pathway
in bacterial energy metabolism in anoxic marine sediments. In the
environment, sulfur (elemental sulfur, S°) is cycled between

S(VI)-Sulfate and S(-II)-sulfide soluble and insoluble sulfur species,
such as thiosulfate. Because thiosulfate, sulfite, and sulfate are
chemical species relevant to the natural sulfur cycling, it would be
difficult to distinguish between these reactions from those arising from
the addition of thiosulfate as an herbicide. In any case, added
thiosulfate would be incorporated into the natural sulfur cycle, where
the thermodynamically-stable end product is sulfate.

Among the types of enzymes involved in the oxidation-reduction of
thiosulfate and other sulfur species are rhodanese (thiosulfate
cleaving) and sulfite oxidase (plus electron transfer). Several
*Thiobacillus* species are known to be involved in the formation or
transformation of thiosulfate[^3]. The enzymatic reaction products are
dependent on the conditions of the media (e.g., presence of absence of
oxygen). An important *Thiobacillus* species involved in the oxidation
of thiosulfate is *Thiobacillus thiooxidans*.

Besides enzymatic reactions, thiosulfate can coordinate with several
metals (i.e., thiosulfate is a ligand; thiosulfate complexes). These
metal complexes serve as catalysts to the oxidation of thiosulfate
(e.g., Co(III)-thiosulfate complexes). In leaching of gold from
auriferous ores using ammonium thiosulfate, Cu(II)-amine complexes can
catalyze the undesirable conversion of thiosulfate (oxidation) to
polythionates instead of the desirable of thiosulfate with gold and
prevent the desirable complexation of thiosulfate with gold. The
photoreduction of thiosulfate in semiconductor dispersions (TiO~2~)
reduce the efficiency of thiosulfate conversion to sulfide and
sulfite/sulfate. However, the extent of complexation and surface
reactions of thiosulfate in the environment is not well documented.

In summary, the chemistry of sulfur in soils (and in water) is complex
because of its many oxidation states, which not only involve oxysulfur
species, but also species containing only sulfur. The predominance of a
given species is dependent on pH and redox potential of the media, as
well as by the type of bacteria present in soils (see the "Ammonium
Thiosulfate Behavior in Soils" section). The terminal, thermodynamically
favored reaction product of thiosulfate is sulfate, except in highly
reduced soils. Sulfate would form insoluble compounds by precipitation
with cations such as Ba^2+^ and Ca^2+^ if the concentrations of sulfate
and cation exceed the solubility product. Other sulfur species formed
from thiosulfate, such as sulfide and polysulfide(s)[^4], may be
transient and unstable products under most environmental conditions.

In addition, oxidation of thiosulfate to sulfate is involved in the
degradation of volatile, halogenated hydrocarbons fumigants by
dehalogenation through a bimolecular nucleophilic substitution (SN2)
mechanism. Thiosulfate dehalogenation reactions have been reported for
methyl bromide; methyl iodide, and chloroacetanilide herbicides, for
example:

CH~3~Br + S~2~O~3~ ^2-^ → CH~3~S~2~O~3~ + Br^-^

Although the chemical reactions of thiosulfate are independent of the
nature of the counter cation, the use of ATS as a sulfur and nitrogen
fertilizer/soil amendment appears to present several advantages. Both
thiosulfate and its intermediate tetrathionate have been found to
inhibit nitrification (oxidation reaction) of ammonia and hydrolysis of
urea, thus increasing nitrogen efficiency.

For the ammonium counter cation, its major chemical reaction in the
environment is oxidation to nitrate. Ammonium is expected to be in
equilibrium with ammonia gas in the soil air voids. The volatilization
of ammonia is dependent on temperature and pH of the media and increases
with increasing temperature and pH. As a cation, ammonium can adsorb to
soil colloid or organic matter through electrostatic cation exchange.

[]{#__RefHeading___Toc177444921 .anchor}**2. Aquatic Exposure
Assessment**

For sulfate, the most likely sulfur species in water, the Agency has
established a Secondary Maximum Contaminant Level (non-enforceable),
SMCL of 250 mg/L for drinking water. Ambient surface water
concentrations of sulfate range from 0.1 to 12,000 mg/L (USGS NWIS,
March 21, 2005). The maximum sulfate concentration was detected at sites
in Montana, Wyoming, and Arizona. These sites were generally associated
with mining activities. It is very unlikely the concentration of sulfate
from ammonium thiosulfate would approach the maximum concentration
sulfate in surface water or the SMCL as a result of its limited use as
an herbicide to control spurge and oxalis in dichondra lawns. No aquatic
exposure modeling will be performed for sulfate, and risk will be
discussed qualitatively.

For ammonia, a screening-level aquatic exposure assessment using the
GENEEC model (Version 2.0) was conducted to estimate surface water
concentrations of ammonium from use of ammonium thiosulfate as a
pesticide. Ammonium concentrations in surface water were used to
estimate ammonia concentrations using chemical equilibria. Additionally,
the Henry's constant was used to estimate the partitioning of ammonia
between water and air. It is important to note that the GENEEC model is
not designed to address environmental behavior of inorganic compounds.
Therefore, the Tier I modeling required conservative assumptions for
input parameters (**Table III.1**). This exposure assessment is
conservative because it assumes 100% of ammonium from ammonium
thiosulfate is available for runoff. Competing environmental fate
processes for ammonium fixation (sorption on sediment and soil) and
transformation (nitrification, denitrification, etc.) are not considered
in the assessment.

The Tier 1 aquatic exposure modeling scenario assumes a 10-hectare field
is 100% treated with ammonium thiosulfate at 74.31 lbs/A. Because the
registered pesticidal use of ammonium thiosulfate is in
residential/homeowner settings, it is highly conservative to assume 100%
of the area is treated. According to the U.S. Census data, a typical
house (residence) area is 1000 sq-foot (0.023 acre) located within a
0.25 acre plot. It is then assumed that the area in the plot not
occupied by the house is a dichondra lawn and that all of the lawn is
treated with ATS at the estimated application rate. With these
assumptions, the percent treated area is 91%. This percent treated area
correction is used as a refinement in the exposure assessment.

**Table III.1. Environmental fate, physical/chemical, and use
information input parameters selected GENEEC model to estimate ammonia
concentrations in surface water resulting from ammonium thiosulfate
application to dichondra lawns.**

  ------------------ ----------- --------------- ----------------------------
  **Input            **Value**   **Source**      **Comments**
  parameter**                                    

  Hydrolysis         0           Assumed         pH-dependence of redox
  half-life (days)                               reactions are not taken into
                                                 account

  Photolysis in      0           Assumed         Photooxidation reactions not
  Water half-life                                taken into account
  (days)                                         

  Aerobic Soil       0           Assumed         Microbial nitrification not
  Metabolism                                     considered
  half-life (days)                               

  Anaerobic Aquatic  0           Assumed         Microbial nitrification not
  Metabolism                                     considered
  half-life (days)                               

  Solubility in      1030        Open literature This solubility is based on
  Water, mg/L                                    the solubility of ATS, which
                                                 is mostly reported as "very
                                                 soluble"; "totally soluble".
                                                 The value used to run GENEEC
                                                 is the reported solubility
                                                 of ATS at 100°C. The
                                                 solubility would be lower at
                                                 25°C.

  K~oc~              0           Assumed         Ammonium is known to sorb
                                                 onto clay surfaces. For
                                                 screening level purposes, a
                                                 K~oc~ of zero was assumed

  Method of          Ground,     Assumed from    Spot applications are
  Application        Broadcast   the available   probably more typical for
                                 label           this product

  Frequency of       4 times per Assumed         The label does not specify
  Application        year                        an upper limit of number of
                                                 applications per year

  Interval between   10          Shorter         N/A
  applications                   interval        
  (days)                         specified in    
                                 the label       

  Application Rate,  18.09       Estimated       The application rate for
  lb Ammonium/A                                  ammonium was based on an
                                                 estimate of the application
                                                 rate of ATS in terms of lb
                                                 ATS/A
  ------------------ ----------- --------------- ----------------------------

The GENEEC estimated environmental concentrations (EECs) for the
ammonium cation are summarized in **Table III.2.** It includes
uncorrected concentrations corrected for percent treated area. The
concentrations appear to remain stable as a result of the environmental
fate assumptions used for model input parameters. The GENEEC model
output is provided in **Appendix B**.

**Table III.2. Aquatic EECs (in mg/L) for Ammonium Cation in surface
water (Source: Ammonium Thiosulfate; Uncorrected and corrected for
percent of treated area, PTA,** 0.91)

  ------------- ---------- --------------- --------------- --------------- ---------------
                **Peak**   **Maximum 4     **Maximum 21    **Maximum 60    **Maximum 90
                           day-average**   day-average**   day-average**   day-average**

  Uncorrected   4.29       4.29            4.29            4.29            4.29

  PTA Corrected 3.90       3.90            3.90            3.90            3.90
  ------------- ---------- --------------- --------------- --------------- ---------------

The relative concentration of the ammonium and ammonia gas depend on the
pH of the media according to the equilibrium expression for the
reaction,

NH~3~(g) + H^+^↔ \[NH~4~^+^\];

K^o^ = \[NH~4~^+^\]/ \[NH~3~(g)\] \[H^+^\],

where K^o^ is the equilibrium constant expressed in terms of activity

Assuming the activity coefficient (γ) to be 1 (i.e., ionic strength not
considered), the equilibrium constant is then expressed in terms of
molar concentration. That is, K^o^ becomes K and Log K^o^=11.04 becomes
Log K = 11.04 (Lindsay, 1979):

Log\[NH~3~(g)\]**= -**11.04 + log\[NH~4~^+^\]+ pH

This equation is used to estimate the relative concentrations of
ammonium and ammonia gas in the receiving water body as controlled by
the pH of the water. This equation indicates that theoretically, the
concentration of dissolved ammonia gas increases with increasing pH.

**Table III.3. GENEEC Environmental Exposure Concentration (EECs) of
Ammonia in Surface Water (C**orrected for percent of treated area, 0.91)

+--------------------+------------------+----------+---------+--------+
| **GENEEC EECS**    | **Peak Ammonium  | **Peak   |         |        |
|                    | EEC (mg/L)**     | Ammonia  |         |        |
|                    |                  | EEC^1^** |         |        |
|                    |                  |          |         |        |
|                    |                  | **(mg    |         |        |
|                    |                  | N/L)**   |         |        |
+--------------------+------------------+----------+---------+--------+
|                    |                  | **pH 5** | **pH    | **pH   |
|                    |                  |          | 7**     | 9**    |
+--------------------+------------------+----------+---------+--------+
| PCT Corrected      | 3.90             | 2.77E-6  | 2.79E-4 | 0.027  |
+--------------------+------------------+----------+---------+--------+

^1^Estimated using following equation: log(NH~3~(g)**)=-**11.04 + log
(NH~4~^+^)+ pH

These are overall, upper bound concentrations resulting solely from the
use of ammonium thiosulafate on dichondra lawns. These concentrations do
not differentiate between solvated and ammonia gas. Ammonia has a
Henry's Law Constant of ammonia at 25° C is 3.5 x 10^-6^ atm-m^3^/mole
(1.41 x 10^-4^, unitless). Volatilization of ammonia is dependent on
temperature and pH of the media and increase with increasing temperature
and pH. In addition, these exposure estimates do not take into account
assimilation (uptake by organisms), binding to humic acids and clays in
soils, nitrification in soil/water body, atmospheric oxidation reactions
of volatilized ammonia, or the contribution from ammonia fertilizers.

> **3. Terrestrial Exposure Assessment**

Based on basic physical-chemical principles, ammonium thiosulfate will
dissociate to form ammonium (NH~4~^+^) and thiosulfate (S~2~O~3~^2-^)
ions when used as an herbicide application in a residential setting to
control spurge and oxalis in dichondra lawns. Dissociation in water is
the first step involved in the environmental fate of ammonium
thiosulfate.

Given the environmental fate of ammonium thiosulfate in terrestrial
systems (see Section III.B.1), it is not appropriate to model
terrestrial dietary residues using the T-REX model. The potential
terrestrial risks associated with using ammonium thiosulfate as a
pesticide will be discussed qualitatively.

[]{#__RefHeading___Toc177444922 .anchor}**C. Effects Characterization**

For ammonium thiosulfate, there are no acceptable registrant-submitted
ecotoxicity data available for any receptors other than mammals. That
is, there are no acceptable data for the following taxa: fish (surrogate
for aquatic-phase amphibians), aquatic invertebrates, aquatic plants,
birds (surrogate for reptiles and terrestrial-phase amphibians),
terrestrial invertebrates, and terrestrial plants. A search of the
publicly-accessible ECOTOX website (<http://www.epa.gov/ecotox>)
revealed one aquatic toxicity study that is unacceptable for use in this
risk assessment[^5], and there were no terrestrial toxicity data for
ammonium thiosulfate.

As described in the Exposure Characterization section above, the aquatic
risk assessment for ammonium thiosulfate will focus on ammonia. Ammonia
is known to exhibit aquatic toxicity and has been studied extensively in
the open literature. The available evidence indicates that the toxicity
of ammonia can depend on ionic composition, pH, and temperature. The
Agency (Office of Water) has developed ambient water quality criteria
for ammonia,[^6] which will be used in this risk assessment for ammonium
thiosulfate use as a pesticide.

The terrestrial risk assessment will be qualitative for ammonium
thiosulfate due to its environmental fate properties and predicted
behavior in the environment. For the sake of being complete, **Table
III.4** summarizes the available toxicity data for mammals (provided by
the Health Effects Division). Ammonium thiosulfate is practically
non-toxic to mammals on an acute oral basis, and the acute dermal and
inhalation toxicity studies revealed no mortalities.

Table III.4. Toxicity of ammonium thiosulfate to mammals.

+--------+-------------+-----------------------------+-------+-------+
| **Test | **Test      | **Results**                 | **Tox | **M   |
| Type** | Organism**  |                             | icity | RID** |
|        |             |                             | Categ |       |
|        |             |                             | ory** |       |
+--------+-------------+-----------------------------+-------+-------+
| Acute  | Rat         | 5-day LD50 (both sexes) =   | P     | 416   |
| oral   |             | 3824 mg/kg                  | racti | 47405 |
|        | (Spra       |                             | cally |       |
|        | gue-Dawley) | 5-day LD50 (males) = 4054   | non-  |       |
|        |             | mg/kg                       | toxic |       |
|        |             |                             |       |       |
|        |             | 5-day LD50 (females) = 3500 |       |       |
|        |             | mg/kg                       |       |       |
+--------+-------------+-----------------------------+-------+-------+
| Acute  | Rabbit      | 3-day LD50 \> 2000 mg/kg bw | N/A   | 416   |
| dermal |             |                             |       | 47406 |
|        |             | (Male and female rabbits    |       |       |
|        |             | were treated dermally with  |       |       |
|        |             | 2000 mg/kg bw ammonium      |       |       |
|        |             | thiosulfate. All rabbits    |       |       |
|        |             | survived the treatment. No  |       |       |
|        |             | abnormal clinical signs or  |       |       |
|        |             | gross findings were noted.  |       |       |
|        |             | Erythema was noted in 3     |       |       |
|        |             | males and in all 5 females; |       |       |
|        |             | however, there were no skin |       |       |
|        |             | reactions after day 3.)     |       |       |
+--------+-------------+-----------------------------+-------+-------+
| Acute  | Rat         | Male and female             | N/A   | 416   |
| inha   |             | Sprague-Dawley rats were    |       | 47407 |
| lation | (Spra       | exposed to ammonium         |       |       |
|        | gue-Dawley) | thiosulfate aerosol (1.79   |       |       |
|        |             | mg/L) for four hours. Signs |       |       |
|        |             | included polypnea and       |       |       |
|        |             | dyspnea, languid behavior,  |       |       |
|        |             | hunched appearance,         |       |       |
|        |             | prostration and tremors,    |       |       |
|        |             | rhinorrhea,                 |       |       |
|        |             | chromodacryorrhea, and      |       |       |
|        |             | salivation. There were no   |       |       |
|        |             | effects on body weights and |       |       |
|        |             | all animals survived the    |       |       |
|        |             | treatment.                  |       |       |
+--------+-------------+-----------------------------+-------+-------+

[]{#__RefHeading___Toc177444923 .anchor}**IV. Risk Characterization**

[]{#__RefHeading___Toc177444924 .anchor}**A. Aquatic Risk**

As described in the Exposure Characterization (Section III.B), ammonium
thiosulfate will quickly dissociate in the environment into ammonium and
thiosulfate, both of which are considered in this aquatic risk
assessment. Thiosulfate would be incorporated into the natural sulfur
cycle, where the thermodynamically-stable end product is sulfate. Since
the Agency has established an SMCL of 250 mg/L for sulfate in drinking
water, and it is highly unlikely that sulfate levels would approach this
level (as a result of this ammonium thiosulfate use), no aquatic
exposure modeling was performed for sulfate. The risk of sulfate to
aquatic organisms via the pesticidal use of ammonium thiosulfate is
presumed to be negligible.

For ammonium, aquatic exposures were estimated using the Tier 1 GENEEC
model and adjusted for the percent of the area treated. Ammonium EECs
were then adjusted using equilibrium equations to estimate ammonia
concentrations at three pH levels (5, 7, and 9). Based on the Agency's
national criterion for ammonia in fresh water[^7], aquatic life should
be protected if both of the following conditions are satisfied for the
temperature, T, and pH of the waterbody:

1.  The one-hour average concentration of total ammonia nitrogen (in mg
    N/L) does not exceed, more than once every three years on the
    average, the CMC (acute criterion) calculated using the following
    equations:

Where salmonid fish are present:

**CMC = (0.275 / (1 + 10 ^7.204\ --\ pH^)) + (39.0 / (1 + 10
^pH\ -\ 7.204^))**

> Or, where salmonid fish are not present:

**CMC = (0.411 / (1 + 10 ^7.204\ --\ pH^)) + (58.4 / (1 + 10
^pH\ -\ 7.204^))**

2.  The thirty-day average concentration of total ammonia nitrogen (in
    mg N/L) does not exceed, more than once every three years on the
    average, the CCC (chronic criterion) calculated using the following
    equations.

> When fish early life stages are present:
>
> **CCC = \[(0.0577 / (1 + 10 ^7.688\ --\ pH^)) + (2.487 / (1 + 10
> ^pH\ --7.688^))\]**
>
> **\* MIN(2.85, 1.45\*10^0.028^ \* ^(25-T)^ )**

When fish early life stages are absent:

> **CCC = \[(0.0577 / (1 + 10 ^7.688\ --\ pH^)) + (2.487 / (1 + 10
> ^pH\ --7.688^))\]**
>
> **\* 1.45\*10^0.028^ \* ^(25-MAX\ (T,7))\ ^**

Using these equations, these criteria were calculated assuming a
temperature of 25^o^C at pH 5, 7, and 9. As shown in **Table IV.1**, the
highly conservative GENEEC model estimated aquatic EECs for ammonia are
all below the national water criteria. It is important to keep in mind
that the GENEEC modeled EECs are upper bound concentrations and do not
take into account uptake by organisms, binding to humic acids and clays
in soils, nitrification in soil/water body, or atmospheric oxidation
reactions of volatilized ammonia. This analysis suggests that the
pesticidal use of ammonium thiosulfate poses negligible risk to aquatic
organisms.

**Table IV.1. Comparison of GENEEC modeled aquatic EECs versus national
water criteria for ammonia.**

+-------------------------+----------------+--------+--------+--------+
| **GENEEC EECS**         | **Peak         | **Peak |        |        |
|                         | Ammonium       | A      |        |        |
|                         | C              | mmonia |        |        |
|                         | oncentration** | Concen |        |        |
|                         |                | tratio |        |        |
|                         | **(mg/L)**     | n^1^** |        |        |
|                         |                |        |        |        |
|                         |                | **(mg  |        |        |
|                         |                | N/L)** |        |        |
+-------------------------+----------------+--------+--------+--------+
|                         |                | **pH   | **pH   | **pH   |
|                         |                | 5**    | 7**    | 9**    |
+-------------------------+----------------+--------+--------+--------+
| PTA Corrected           | 3.90           | 2      | 2      | 0.027  |
|                         |                | .77E-6 | .79E-4 |        |
+-------------------------+----------------+--------+--------+--------+
|                         |                |        |        |        |
+-------------------------+----------------+--------+--------+--------+
| National Water Acute    |                | 38.759 | 24.103 | 0.885  |
| Criterion (Salmonid)^2^ |                |        |        |        |
+-------------------------+----------------+--------+--------+--------+
| National Water Acute    |                | 58.039 | 36.093 | 1.324  |
| Criterion               |                |        |        |        |
| (Non-Salmonid)^2^       |                |        |        |        |
+-------------------------+----------------+--------+--------+--------+
| National Water Chronic  |                | 3.599  | 3.000  | 0.233  |
| Criterion^2^            |                |        |        |        |
|                         |                |        |        |        |
| (Fish early life        |                |        |        |        |
| stage-present)@ 25^o^C  |                |        |        |        |
+-------------------------+----------------+--------+--------+--------+
| National Water Chronic  |                | 3.599  | 3.000  | 0.233  |
| Criterion^2^            |                |        |        |        |
|                         |                |        |        |        |
| (Fish early life        |                |        |        |        |
| stage-absent)@ 25^o^C   |                |        |        |        |
+-------------------------+----------------+--------+--------+--------+

^1^ Ammonia concentration estimated using following equation:
log(NH~3~(g))**=-**11.04 + log (NH~4~^+^)+ pH

[]{#__RefHeading___Toc177444925 .anchor}**B. Terrestrial Risk**

Terrestrial exposures of ammonium and thiosulfate were not
quantitatively estimated in this assessment. As described in the
Exposure Characterization (Section III.B), the environmental fate of
thiosulfate will be dependent on soil redox potential. In suboxic and
oxic environments (where oxygen, nitrogen and iron act as electron
acceptors), thiosulfate will be oxidized to form sulfate. In anoxic
environment, thiosulfate will be reduced to form sulfide. The
environmental fate of ammonium is predominantly dependent on
nitrification and sorption on soil colloids and organic matter. Ammonium
may also form ammonia (NH~3~(g)) in very alkaline and sodic soil
environments (pH 9-10).

Sulfur and nitrogen are essential nutrients for plants and animals.
Available mammalian data indicate that ammonium thiosulfate is
practically non-toxic on an acute oral basis. There are no toxicity data
available for birds, terrestrial invertebrates, or terrestrial plants.
Since ammonium thiosulfate is being used as an herbicide, there is a
possibility that non-target plants may be at risk. However, given that
it is only used in residential/homeowner settings in California on
dichondra lawns, the potential for adverse effects to non-target plants
(including Federally-listed species) seems highly unlikely.

Given the nature of the chemical and its expected behavior in the
environment, and because sulfur and nitrogen are essential nutrients for
plants and animals, risk to terrestrial organisms is not expected as a
result of ammonium thiosulfate use as an herbicide on dichondra lawns.

[]{#__RefHeading___Toc177444926 .anchor}**C. Federally Threatened and
Endangered (Listed) Species Concerns**

This risk assessment suggests that Federally-listed threatened and
endangered aquatic and terrestrial species are not at risk as a result
of the limited use of ammonium thiosulfate as an herbicide on dichondra
lawns.

[]{#__RefHeading___Toc177444927 .anchor}**D. Discussion of
Uncertainties, Limitations, and Data Gaps**

[]{#__RefHeading___Toc177444928 .anchor}**1. Exposure For All Taxa**

[]{#__RefHeading___Toc177444929 .anchor}**a. Maximum Use Scenario**

The screening-level risk assessment focuses on qualitatively
characterizing potential ecological risks resulting from a maximum use
scenario, which is determined from labeled statements of maximum
ammonium thiosulfate application rate and number of applications with
the shortest time interval between applications. The frequency at which
actual uses approach this maximum use scenario may be dependent on
herbicide resistance, timing of applications, cultural practices, and
market forces.

[]{#__RefHeading___Toc177444930 .anchor}**b. Additive and/or Synergistic
Effects**

It was assumed that aquatic and terrestrial organisms were exposed only
to ammonium thiosulfate use as an herbicide. Ecological risks associated
with exposure to a mixture of ammonium thiosulfate and other pesticides,
adjuvants, heavy metals, industrial chemicals, pharmaceuticals, etc.
were not considered in this risk assessment.

[]{#__RefHeading___Toc177444931 .anchor}**2. Exposure For Aquatic
Species**

The Tier I (screening) simulation model GENEEC was used to estimate the
exposure concentrations (EECs) of thiosulfate and the ammonium cation.
This model is not suitable for inorganic chemicals, particularly those
that undergo extensive, pH-dependent redox reactions. Assumptions had to
be made for the selection of environmental fate parameters (hydrolysis,
photolysis, biotransformation). It was assumed that thiosulfate and
ammonium were stable (i.e, half-lives = 0). Therefore, these EECs
represent an upper bound concentration of thiosulfate and ammonium
cation that do not consider any redox reaction products in soil and
water.

The available product label does not provide clear information on
application rates and frequency of application. Given that the product
is only used in residential settings, an application rate had to be
estimated from U.S. Census 2000 data for a typical residential plot and
assumed treated area, which then was used to estimate the percent
treated area (PTA). Since GENEEC estimates assume that all of the area
gets treated, the PTA served to correct the aquatic EECs.

[]{#__RefHeading___Toc177444932 .anchor}**3. Data Gaps**

There were no acceptable registrant-submitted environmental fate or
ecotoxicity data available for ammonium thiosulfate. However, given the
nature of the chemical and its predicted behavior in the environment,
this was not an impediment to the risk assessment. At this time, the
EFED is not requesting any environmental fate or ecotoxicity studies to
be submitted for ammonium thiosulfate.

[]{#__RefHeading___Toc177444933 .anchor}**V.** C**ited Literature**

Bamford, V.A., Bruno, S., Rasmussen, T., Appia-Ayme, C., Cheesman, M.R.,
Berks, B.C., and Hemmings, A.M. 2002. Structural basis for the oxidation
of thiosulfate by a sulfur cycle enzyme. The EMBO (European Molecular
Biology Organization) Journal, Vol.21(21), pp.5599-5610.

Boparal, H.K., Shea, P.J., Comfort, S.D., and Snow, D.D. 2006.
Eviron.Sci.Technol., Dechlorinating chloroacetanilide herbicides by
dithionite-treated aquifer sediment and surface soil. Vol 10(9),
pp.3043-3049.

Borgello, E., Desilvestro, J., Grätzel, and Pelizzetti. 1998.
Phoreduction of Thiosulfate in Semiconductor Dispersions. Helvetica
Chimica Acta, Vol.66(6), pp 1827-1834.

Brown, T., Fischman, A., Spiccia, L., and McPhail, D.C. 2003.
Hydrometallurgy 2003-Fifth International Conference in Honor of
Professor Ian Ritchie-Volume 1: Leaching and Solution Purification,
Edited by

Cotton, F.A. and Wilkinson, G. Advanced Inorganic Chemistry, Fifth
Edition, John Wiley and Sons, New York, 1988.

Gan, J., Yates, S.R., Ole-Becker, J. and Wang, D. 1998. Surface
Amendment of Fertilizer Ammonium Thiosulfate to Reduce Methyl Bromide
Emission from Soil. Environ. Sci. Technol. Vol 32, pp 2438-2441.

Gan, J., Becker, J.O.., Ernst, F.F., Hutchinson, C., Knuteson, J.A., and
Yates, S.R. 2000. Surface application of ammonium thiosulfate fertilizer
to reduce volatilization of 1,3-dichloropropene from soil Pest
Mnanagement Science, Vol. 56(3) pp.264-270.

Jeffery, R., Masau, Y, Oh, J.K., and Suzuki, I. 2001. Mechanism of
oxidation of inorganic sulfur compounds by thiosulfate-grown
*Thiobacillus thiooxidans*. Can. J. Microbiol., Vol 47, pp 348-358.

Johnson, M.D. and Balahura, R.J. 1988. Kinetics and Mechanism of the
Oxidation of Coordinated Thiosulfate by Peroxymonosulfate. Inorg.Chem.,
Vol.27, pp 3104-3107.

Lens, P.N.L and Kuenen, J.G. 2001. The biological surface cycle: novel
opportunities for environmental biotechnology. Water Science and
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Lindsay, W.L. Chemical Equilibria in Soils, John Wiley and Sons, New
York, 1979.

Pryor, W.A. 1960. The Kinetics of Disproportionation of Sodium
Thiosulfate to Sodium Sulfide and Sulfate. J.Am.Chem.Soc. Vol. 82,
pp.4794-4798.

Schippers, A.L. and Wolfang, S. 1999. Bacterial Leaching of Metal
Sulfides Proceeds by Two Indirect Mechanisms via Thiosulfate or via
Polysulfides and Sulfur. Applied and Environmental Microbiology, Vol 65
(1), pp. 319- 321.

Suzuki, I. 1999. Oxidation of inorganic compounds: Chemical and
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**Appendix A. Sulfur chemical species (other than thiosulfate) of
environmental significance**

+-----------+----------+----------------------+-----------------------+
| **Name of | **O      | **Chemical Name of   | **Compounds**         |
| Sulfur    | xidation | the Acid Form**      |                       |
| Species** | State of |                      |                       |
|           | Sulfur** |                      |                       |
+-----------+----------+----------------------+-----------------------+
| Sulfide   | -II      | Hydrogen sulfide,    | Metal Sulfides ,      |
|           |          | H~2~S                | mostly minerals such  |
| S^2-^     |          |                      | as:                   |
|           |          | Hydrosulfide, HS-    |                       |
|           |          |                      | Galena (PbS)          |
|           |          |                      |                       |
|           |          |                      | Sphalerite (ZnS)      |
+-----------+----------+----------------------+-----------------------+
| Po        | -II      | Acid form is not     | In aqueous solution   |
| lysulfide |          | stable               | only                  |
|           |          |                      |                       |
| S~n~^2-^  |          |                      | S~3~^2-^ and S~4~^2-^ |
|           |          |                      | are stable.           |
+-----------+----------+----------------------+-----------------------+
| Sulfur    | 0        | No acid form         | Not applicable        |
| (         |          |                      |                       |
| elemental |          | The                  |                       |
| sulfur)   |          | thermodynamically    |                       |
|           |          | stable form is       |                       |
|           |          | cyclooctasulfur      |                       |
|           |          | (cyclo-S~8~);        |                       |
|           |          | orthorhombic         |                       |
+-----------+----------+----------------------+-----------------------+
| Sulfur    | IV       | Not applicable       | Not applicable        |
| Dioxide,  |          |                      |                       |
| SO~2~     |          |                      | It can be generated   |
|           |          |                      | from burning of       |
|           |          |                      | S-fuels It is of      |
|           |          |                      | environmental concern |
|           |          |                      | because it forms      |
|           |          |                      | sulfuric acid         |
+-----------+----------+----------------------+-----------------------+
| Hydr      | IV       | Acid form not stable | As salts              |
| osulfite, |          |                      |                       |
| sulfite,  |          |                      |                       |
| HSO~3~^-^ |          |                      |                       |
|           |          |                      |                       |
| Sulfite,  |          |                      |                       |
| SO~3~^-^  |          |                      |                       |
+-----------+----------+----------------------+-----------------------+
| Sulfate,  | VI       | Sulfuric acid,       | Sulfate salts (sodium |
| SO~4~^2-^ |          | H~2~SO~4~            | potassium)            |
|           |          |                      |                       |
|           |          |                      | Naturally occurring   |
|           |          |                      | sulfate minerals      |
|           |          |                      | (e.g., barite)        |
+-----------+----------+----------------------+-----------------------+

Sulfur oxyanions containing 3 or more sulfurs:

Polythionates (S~n~)O~6~^2-^, of which tetrathionate is an example,
S~4~O~6~ ^2-^

**Appendix B.** Aquatic Exposure Model Estimates (GENEEC Version 2.0)

RUN No. 1 FOR Ammonium Cation ON Dichondra \* INPUT VALUES \*

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP

ONE(MULT) INTERVAL Koc (PPM ) (%DRIFT) (FT) (IN)

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

18.090( 72.360) 4 10 .0 1030.0 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 2 N/A .00- .00 .00 .00

GENERIC EECs (IN MILLIGRAMS/LITER (PPM)) Version 2.0 Aug 1, 2001

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

PEAK MAX 4 DAY MAX 21 DAY MAX 60 DAY MAX 90 DAY

GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

4.29 4.29 4.29 4.29 4.29

[^1]: http://www.epa.gov/waterscience/criteria/ammonia/99update.pdf

[^2]: Disproportionation reactions apply to reactions in which an
    element in one of the reaction products is in a lower oxidation
    state and the other is of a higher oxidation than the oxidation
    state of the parent species. For example, 2U(V)→ U(IV) + U(VI) in
    water.

[^3]: ^?^ Another important *Thiobacillus* species is *Thiobacillus
    ferroxidans*. It not only oxidizes sulfide to sulfate, but above pH
    3 it also oxidizes Fe(II) to Fe(III), mostly from pyrite and/or
    marcasite. As the pH increase, Fe(III) generates a series of
    hydrolytic reactions hydroxyoxides, with the concomitant formation
    of hydronium ions (H~3~O^+^). These reactions are responsible for
    acid mine drainage, which has negative impact on the environment.
    *Thiobacillus thiooxidans* oxidizes sulfur only.

[^4]: S^2-^ may polymerize to polysulfides (S~n~^2-^). In aqueous media,
    the polysulfide with the highest number of sulfurs is S~4~^2-^.
    Polysulfide can also oxidize to oxyanions of sulfur (oxic condition)
    or breakdown polysulfides with a lower number of sulfurs, including
    S^2-^.

[^5]: Ecotox. Ref. No. 14566. Fathead minnow acute toxicity study.
    Substance purity and mortality data were not reported.

[^6]: For more information visit:
    http://www.epa.gov/waterscience/criteria/ammonia/99update.pdf

[^7]: http://www.epa.gov/waterscience/criteria/ammonia/99update.pdf