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i ~ {, ~ ~ UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

~ I ~ wASHINGTON D.C., 20460 ""~ (j."~ '1£. PRO~~ OFFICE OF CHEMICAL SAFETY 

MEMORANDUM 

Subject: 

To: 

·Through: 

From: 

AND POLLUTION PREVENTION 

PC Code: 013803 

DP Barcode: 406606 

Date: March 12, 2013 

Registration Review: Preliminary Problem Formulation for 

Monosodium Acid Methanearsonate (MSMA) 

Garland Waleko, Chemical Review Manager 

Kevin Costello, Chief 

Risk Management and Implementation Branch II 

Pesticide Re-evaluation Division (7508P) 

James Hetrick, Ph.D., Senior Science Adviso~ a. 

Melissa Panger, Ph.D., Senior Scientist 

Environmental Risk Branch III 

Environmental Fate and Effects Division (7507P) 

The Environmental Fate and Effects Division (EFED) has completed the problem 

formulation in support of the Registration Review of the herbicide monosodiurri acid 

methanearsonate [MSMA (PC code: 013803)] (attached). This document is intended to 

provide an overview of what is currently known regarding the environmental fate and 

ecological effects associated with MSMA and its transformation products. Uncertainties 

regarding attributes of the parent compound and its transformation products are also 

outlined. It describes the preliminary ecological risk hypothesis and the processes that 

will be used during the completion of ecological risk assessment and drinking water 

exposure assessment in support of registration review. 

Page 1 of 58 

' 
Page 2 of 58 

Contents

1. Introduction .............................................................................................................. 4

2. Conclusions from Previous Risk Assessments ........................................................ 6

2.1. Ecological Risk Assessments .......................................................................... 6

2.2. Drinking Water Exposure Assessments ........................................................... 7

2.3. Clean Water Act Programs ............................................................................... 8

3. Environmental Fate and Transport ............................................................................. 9

4. Receptors................................................................................................................ 11

4.1. Effects to Aquatic Organisms ......................................................................... 12

4.2. Effects to Terrestrial Organisms ..................................................................... 16

4.3. Ecological Incidents ....................................................................................... 17

5. Conceptual Model .................................................................................................. 19

6. Analysis Plan ........................................................................................................... 19

6.1. Stressors of Concern ....................................................................................... 19

6.2. Measures of Exposure ................................................................................... 20

6.3. Measures of Effect .......................................................................................... 20

6.4. Endangered Species Assessments .................................................................. 21

6.5. Endocrine Disruptor Screening Program ........................................................ 21

7. Preliminary Identification of Data Gaps ................................................................ 21

7.1. Environmental Fate ............................................................................................ 21

7.2. Ecological Effects .............................................................................................. 22

8. References ................................................................................................................ 28

APPENDIX A: Currently Registered MSMA Products. .............................................. 29

APPENDIX B: Currently Registered MSMA USES. ................................................... 31

APPENDIX C: BEAD Chemical Profile for MSMA: ................................................. 41

APPENDIX D. Chemical Equilibria Model for As. ..................................................... 50

APPENDIX E. Results of the Screening Imbibation Program (SIP v. 1.0). ................ 57

 
Page 3 of 58 

Definition of Terms 

Background arsenic: “Background” arsenic is used to describe arsenic in the environment 

that is present as a result of natural geological processes and/or as a result of anything 

other than organic arsenical herbicide use. 

Organic arsenic: The “organic” form of arsenic includes the pesticides cacodylic acid, 

MSMA, DSMA, and CAMA. These arsenic compounds can also be found naturally in 

the environment. 

Inorganic arsenic: Found naturally in the environment, “inorganic” arsenic species are the 

elemental forms of arsenic (As) in the environment. It can exist in the oxidation states of 

As(III) and As(V). 

Total arsenic: “Total” arsenic is used to describe all the arsenic present in a sample 

regardless of its form (i.e., organic arsenic + inorganic arsenic) and source (i.e., 

background + pesticidal); EPA and state/Federal agencies measure and/or establish 

regulatory limits in soil and water for total arsenic. 

Transformation: The process of arsenic changing forms (i.e., organic to inorganic or vice 

versa). 

Speciating: Quantifying the concentration of inorganic and organic arsenic in a soil or 

water sample (as opposed to only total arsenic); “speciated” data provide a breakdown of 

organic and inorganic arsenic whereas “unspeciated” data provide only total arsenic. 

Oxidation State: The oxidation state of an atom in a molecule gives the number of 

valence electrons it has gained or lost. The oxidation states of As are -3,+3, and +5. 

Methylation and demethylation: The chemical process that transforms metalloids such as 

arsenic by the addition (methylation) or removal (demethylation) of methyl groups (CH3) 

to the molecule. 

Monomethyl methanearsonate (MMA): MMA refers to organic arsenic compounds with 

a single methyl group; in these assessments, the term “MMA” is used collectively to refer 

to MSMA, DSMA, and CAMA (salts of MMA that readily dissociate to MMA in water). 

It can form with As in the oxidation states of As(III) as MMA(III) or As(V) as MMA(V). 

Dimethyl methanearsonate (DMA): DMA refers to organic arsenic compounds with two 

methyl groups such as cacodylic acid and its salt. It can form with As in the oxidation 

states of As(III) as DMA(III) or As(V) as DMA(V). 

Trimethyl methanearsonate (TMA): TMA refers to organic arsenic compounds with three 

methyl groups. It can form with As in the oxidation states of As(III) as TMA(III) or 

As(V) as TMA(V). 
Page 4 of 58 

1. Introduction

Monosodium acid methanearsonate, or MSMA (an organic arsenical), is a broadspectrum herbicide used to control grasses and broadleaf weeds in cotton and turf. 

MSMA’s herbicidal mode of action, along with other organic arsenicals, is unknown. 

However, it is suspected that the organic arsenicals, including MSMA, destroy cell 

membranes in plants, leading to the herbicidal activity. 

In 2006 a Reregistration Eligibility Decision (RED) was completed. The RED 

considered two groups of organic arsenicals; the methylarsonate salts [MSMA, DSMA 

(PC code: 013802), CAMA (PC code: 013806)] and the cacodylates [DMA (PC code: 

012501) and DMA-Na (PC code: 012502)]. The 2006 RED determined that none of the 

registered uses of organic arsenicals (i.e., CAMA, cacodylic acid, DSMA, and MSMA) 

were eligible for reregistration. 

In January and February 2009, EPA reached an agreement in principle with the technical 

registrants of all existing organic arsenical pesticide products to eliminate, phase out, 

and/or modify certain uses. The result of this agreement was the EPA issuance of a 

revised RED in April 2009 and a cancellation order in September 2009 that terminated 

the last remaining uses of DSMA, CAMA, and DMA in the United States. The order 

also terminated certain MSMA product registrations and amended others (decreased rates 

and/or limited applications). For MSMA, only the cotton use was determined to be 

eligible for reregistration, based on high potential benefits. The remaining uses of 

MSMA on turf (i.e., golf courses, sod farms, and highway rights-of-way) were to be 

phased out with these turf uses, and use of MSMA is prohibited after December 31, 2013. 

The 2009 agreement in principle also provided for a scientific peer review related to the 

carcinogenic mode of action (MOA) of inorganic arsenic (iAs) prior to the final phase out 

of the remaining MSMA uses. OPP was initially considering use of a joint review by the 

EPA Science Advisory Board (SAB) and the FIFRA Science Advisory Panel (SAP). 

However, since that time, the Agency has announced review of iAs by the National 

Academy of Sciences (NAS) to be used in lieu of an SAB/SAP review. Therefore, EPA 

will postpone any decision regarding the phase-out of the use of MSMA on golf courses, 

sod farms, and highway rights-of-way until such time that the NAS review is complete 

and the Agency has analyzed and responded to the NAS report. 

The risk assessment conducted for MSMA will be based on uses registered at the time of 

the assessment. Since the turf uses (i.e., golf courses, sod farms, and highway rights-ofway) have not yet been phased out, they are considered here along with the cotton use. 

MSMA is currently formulated in 10 active end-use products as a granular, flowable 

concentrate, ready-to-use, and emulsifiable concentrate. Eight of the products contain a 

single active ingredient (a.i.) (i.e., MSMA), while two of the products contain multiple 

a.i.’s (see Appendix A). Applications to cotton are via aerial or ground equipment, while 

applications to turf are limited to ground applications. Appendix B is a detailed list of 

the use patterns and maximum application rates for the current uses of MSMA (they are 

summarized below in Table 1). 
Page 5 of 58 

TABLE 1. Currently Registered Uses of MSMA and Maximum Labeled Application Rates. 

USE APP. 

METHOD 

SINGLE 

MAX. 

RATE 

MAX NO. 

APPS 

PER 

SEASON 

MIN. 

APP. 

INTERVAL 

MAX SEASONAL APP 

RATE 

COMMENTS 

Cotton Ground 

Aerial 

2 lb 

a.i/acre 2 7-day 4 lb a.i./acre 

- Do not apply this product 

in FL, except for in the 

following counties: 

Calhoun, Columbia, 

Escambia, Gadsden, 

Hamilton, Holmes, 

Jackson, Jefferson, 

Okaloosa, Santa Rosa, 

Suwannee, Walton, and 

Washington 

- 2nd application is allowed 

only for a salvage 

operation 

- Some (but not all) labels 

have the following 

restrictions: 50 ft buffer 

from water; cannot be 

applied post-bloom; in FL, 

only apply as a band 

treatment 

Golf Course 

(newly 

constructed) 

Ground 4 lb 

a.i./acre 1 N/A1

4.0 lb a.i./acre 

(does not 

include 

additional 

spot 

treatments) 

- Do not apply in FL 

Golf Course 

(NOT newly 

constructed; 

spot treatment 

only) 

Ground 4 lb 

a.i./acre NS NS2

NS (not to 

exceed 25% 

of total golf 

course 

acreage/yr) 

- Do not apply in FL 

- 100 ft2

 max per spot 

Sod Farms 

(broadcast) Ground 4 lb a.i./acre 2 NS 

8 lb a.i./acre 

(based on 2 x 

max single 

rate) 

- Do not apply in FL 

- 25 ft buffer from 

permanent water bodies or 

aquatic habitat 

Highway 

Rights-of-Way Ground NS3

 2/yr3 NS NS3

- Do not apply in FL 

- 100 ft buffer from 

permanent water bodies or 

aquatic habitat 

1

 N/A – Not applicable 

2

 NS – Not specified on the label 

3

 The information is specified on four labels (19713-42; 19713-528; 19713-550; and 42519-3): the highest 

application rate is 5 lb a.i./acre (single app.); 2 app./yr; 10 lb a.i./acre app. rate (based on 2 apps./yr); 90-

day app. interval 

Private market usage data from 2000 – 2010 show that approximately one million pounds 

of active ingredient (lb a.i.), on average, were used per year for agricultural uses (97% of 

the agricultural use was on cotton). The average number of acres treated was 850,000 
Page 6 of 58 

(see Appendix C for details). Most of the usage is in the Southeastern United States. 

For the non-agricultural uses, the available usage data show that from 2002 – 2006, there 

was an average of just over one million pounds of MSMA applied per year (including 

many of the uses that have been phased out since the usage data were collected). 

2. Conclusions from Previous Risk Assessments

EFED evaluated the most recent ecological risk assessments for [MSMA (PC code: 

013803)] and other arsenicals (organic and inorganic) in association with any updated 

toxicity, exposure, and usage information to determine if further updates are needed to 

support registration review, and if so, if sufficient data are available. The most recent 

risk assessment for MSMA is a 2006 assessment that was an addendum to the ecological 

risk assessment conducted for the Reregistration Eligibility Decision (RED) (D309100) 

(see ‘Introduction’). The RED was originally signed in 2006 and an amended RED was 

signed in 2009. The 2006 RED determined that none of the organic arsenicals (i.e., 

CAMA, cacodylic acid, DSMA, and MSMA) were eligible for reregistration. In 2009, 

the registrants voluntarily cancelled all uses of CAMA, cacodylic acid and DSMA. At 

that time, most of the uses of MSMA were either voluntarily cancelled or were scheduled 

for a voluntary phase out. Cotton was the only MSMA use that was determined to be 

eligible for reregistration in 2009, based on high potential benefits. 

Based upon a reevaluation and reconsideration of the available data, a more refined 

approach than the one used for the MSMA RED document is proposed for the risk 

assessment to be conducted for registration review. Because MSMA can speciate to a 

variety of different organic and inorganic arsenicals in the environment, the MSMA 

assessment will consider the available information on the fate properties and toxicity data 

for a variety of arsenicals (see below for details). 

2.1. Ecological Risk Assessments 

The EFED’s risk assessment for the organic arsenicals (including MSMA) RED was 

completed in 2006 (USEPA, 2006). The RED document for the organic arsenicals serves 

as the starting point for this problem formulation document (USEPA, 2006). The results 

of the ecological assessment conducted for the RED are briefly summarized below. 

TABLE 2. Summary of Risk Concerns Identified for Organic Arsenicals (Including MSMA) in 

Previous Assessments

Birds Mammals Terrestrial 

Plants 

Terrestrial

Inverts Fish Aquatic Inverts 

Aquatic 

Plants 

Groundwater 

Contamination Persistence Degradates of Concern 

√ √ √ √ -- -- -- -- √ √1

1-MSMA degrades to form inorganic arsenic (iAs). Because As is an element, it does not further degrade in the 

environment. However, it does form numerous inorganic and organic species in the environment. For the purposes of 

this assessment, only the inorganic and methylated forms of arsenic are considered in this problem formulation. 

For aquatic organisms (fish, invertebrates, and plants), none of the levels of concern 

(LOC) were exceeded, indicating a low potential for risk from the use of MSMA. For 

terrestrial organisms (birds, mammals, and plants), exposures led to LOC exceedances, 
Page 7 of 58 

indicating a potential for risk to these taxa. Risks to terrestrial invertebrates were not 

assessed in the RED. 

The aquatic exposure assessment for MSMA was conducted in the RED. The daily peak 

estimated environmental concentration (EEC) for MSMA ranged from 26 μg/L from use 

on kenaf at an annual application rate of 2 lb a.i./A, to 605 μg/L from use on orchards at 

an annual application rate of 46.5 lb a.i./A. The daily peak EEC for MSMA was 78 μg/L 

from use on cotton at an annual application rate of 6 lb a.i./A. The daily peak EEC for 

MSMA was 164 μg/L from use on turf at an annual application rate of 12.6 lb a.i./A. 

Surface water monitoring data for total As as well as speciated As (including arsenate, 

arsenite, MMA, DMA) were reported in the RED. The monitoring data for total As 

indicated that concentrations are typically less than 1 μg/L to several parts per billion. 

However, there are exceptions in the monitoring data depending on local geology and 

anthropogenic activities. The maximum total As concentrations in surface water 

approach 1000 μg/L (1 mg/L). Speciated monitoring programs for As were conducted in 

Florida and the Lower Mississippi River Basin (Braman and Foreback and Garbarino and 

Burkhurdt, 1998). The maximum concentration of various As species was 1.8 μg/L for 

cacodylic acid, 5 μg/L for methylarsonic acid, 3 μg/L for arsenite, and 5.5 μg/L for 

arsenate. 

2.2. Drinking Water Exposure Assessments 

A drinking water assessment was conducted to support the RED for MSMA (USEPA, 

2002). The drinking water assessment was conducted using PRZM/EXAMS modeling 

using the index reservoir. MSMA applications to cotton were modeled based on 2 aerial 

applications at 3.6 lb a.i./A with an application interval of 14 days. EDWCs for MSMA 

are shown in Table 3. 

Table 3. Tier II PRZM/EXAMS Estimated Drinking Water Concentrations of 

MSMA.

 Use Pattern Arsenic Species 

1 in 10 year Concentration 

Peak Annual Average 

μg/L 

Cotton MSMA 83 21 

Citrus MSMA 512 136 

A more recent drinking water exposure assessment for organic arsenical herbicides 

(DSMA, CAMA, MSMA, DMA) was conducted for MSMA in 2006 (USEPA, 2006). 

This exposure assessment was designed to provide estimated drinking water 

concentrations for methylarsonate salts (MSMA, CAMA, and DSMA), cacodylates 

(DMA) and total arsenic. PRZM/EXAMS modeling was conducted using the index 

reservoir (Table 4). MSMA applications to turf and cotton were modeled at 2 

applications at 1.74 lb acid equivalent (ae)/A and at 4 applications at 3.35 lb ae/A, 

respectively. The modeling accounted for MSMA transformation to cacodylates (DMA) 

in soil. The extent of DMA formation is based on a maximum of 35% formation of 

DMA from MSMA as reported in an aerobic soil metabolism study (MRID 44767601). 
Page 8 of 58 

Ground water modeling was not conducted in this assessment. Surface water and ground 

water monitoring data also were considered in this assessment. 

Table 4. Tier II PRZM/EXAMS Estimated Drinking Water Concentrations for 

Various As Species from MSMA Application.

 Use Pattern Arsenic Species 

1 in 10 year Concentration 30 year Annual 

Peak Annual Average Average 

μg/L 

Turf 

MMA 250.5 127.5 74.6 

DMA 102.3 46.5 28.1 

Total As 135.2 68.8 40.3 

Cotton 

MMA 37.4 11.0 5.3 

DMA 23.6 7.4 4.3 

Total As 20.9 7.2 3.9 

2.4. Clean Water Act Programs

An Aquatic Life Ambient Water Quality Criteria document has not been published 

specifically for MSMA; however, one has been published for arsenic (USEPA, 1995). 

The criteria were derived from data for arsenic (III), but it is applied for total arsenic. 

The freshwater and saltwater criteria are expressed in terms of the dissolved As in the 

water column. The Aquatic Life Ambient Water Quality Criteria for arsenic are listed in 

the table below (Table 5). 

 
Page 9 of 58 

Table 5. EPA Aquatic Life Ambient Water Quality Criteria for Arsenic (μg/L).

Freshwater Saltwater 

Criteria Maximum 

Concentration (acute) 

Criterion Continuous 

Concentration (chronic) 

Criteria Maximum 

Concentration (acute) 

Criterion Continuous 

Concentration (chronic) 

340 150 69 36 

Arsenic can enter water bodies via natural and/or anthropogenic sources. Total 

Maximum Daily Load (TMDL) criteria have been developed for arsenic in several states 

for several water bodies found in the U.S. The TMDLs do not distinguish the source of 

arsenic in water, so it is not possible to identify water bodies that are impaired by arsenic 

as a result of pesticidal use using the available data. Based on EPA’s WATERS Expert 

Query Tool (http://iaspub.epa.gov/pls/waters/f?p=ASKWATERS:EXPERT:0:::::), 

arsenic (total, from both natural and anthropogenic sources) has been identified as a cause 

of water body impairment 2118 times under section 303(d) of the Clean Water Act. 

3. Environmental Fate and Transport

3.1 Fate Characterization 

Unlike other pesticides that degrade over time, MSMA contains the element arsenic 

which does not degrade. Arsenic can, however, transform (i.e., change forms) or be 

redistributed through runoff, leaching, erosion, volatilization, or plant uptake. The extent 

and speed of transformation and redistribution of the organic arsenical herbicides in soil 

is highly variable and depends mostly on localized environmental conditions. Thus, 

persistence of applied organic arsenical herbicides can range from days to years, 

depending on soil properties and ambient conditions such as soil moisture, temperature, 

chemical concentration, bacterial population, and amount of organic matter. To fully 

describe the environmental fate of MSMA, consideration must be given to both the rate 

of transformation (demethylation/methylation) as well as chemical equilibria of inorganic 

and organic As species in terrestrial and aquatic environments. 

The aerobic and anaerobic metabolism studies in soil and water are useful in describing 

the transformation rate of MSMA into other forms of arsenic [e.g., As(V), As(III), TMA 

(III), DMA(III), MMA(III), TMA(V), DMA(V), MMA(V), etc.]. MSMA was essentially 

stable under all test systems, except for biotic degradation studies (Table 6). MSMA has 

a half-life of 240 days (Gao and Burau, 1997; MRID 44767601) in aerobic soil. In 

anaerobic environments, MSMA has a half-life of 776 days (MRID 44767602). It is 

stable to both hydrolysis and photolysis (MRID 42363001; MRID 41903902). No aerobic 

aquatic metabolism data are available for MSMA. Although these studies indicate that 

MSMA is persistent in aquatic and terrestrial environments, MSMA and its 

transformation products, in combination with As from natural background and other 

anthropogenic sources, represent a total arsenic load, which can be redistributed into 

different forms of As in the environment. Redistribution of arsenic can be influenced by 

redox potential, pH, microbial activity, and presence of counter ions (Al3+, Fe3+, etc). 

 
Page 10 of 58 

Table 6: Summary of Available Data to Assess the Environmental Fate of MSMA. 

Guideline MRID Summary Data 

Acceptability 

Hydrolysis 42363001 Stable @ pH 5,7, 9 Acceptable 

Photolysis in water 41903902 Stable in irradiated pH 7 buffer Acceptable 

Photolysis on soil 41903901 Stable on irradiated sandy loam Acceptable 

Aerobic Soil Metabolism 41886901 Arsenic acid major soil metabolite; 

Unable to reproduce formation/ 

decline of parent and degradation 

products; study did not differentiate 

background from introduced As. 

Upgradable 

44767601 t1/2=240 days; DMA was major 

degradate(31% @ 2-12 months) 

Acceptable 

Anaerobic Aquatic Metabolism 41996501 Poor material balance Not acceptable 

44767602 t1/2=776 days Acceptable 

Leaching Adsorption/Desorption 41651906 Lowest non-sand Kd 11.4 ml/g. Not acceptable 

 

Terrestrial Field Dissipation 42526001 

42616201 

CA study-t1/2=55 days; cacodylic 

acid identified 

AR study- t1/2=11 days; cacodylic 

acid identified 

Upgradable 

Accumulation in Fish 42432501 Mean BCF <1X in all fish tissue Acceptable 

The biogeochemical cycle of As is dependent on the microbial mediated 

methylation/demethylation, precipitation into As minerals, and sorption onto soil 

colloids. Inorganic As is prone to undergo methylation in anoxic (low redox conditions) 

environments. This methylation process can form various methylated forms of As 

including TMA (III), DMA(III), MMA(III), TMA(V), DMA(V), MMA(V), etc. The 

predominance and distribution of these species is dependent on pH and redox potential. 

The inorganic As can exist in soil pore water or surface water as soluble hydrolysis 

species of As(III) and As(V). As(III) is the predominate inorganic As species in anoxic 

and suboxic environments. As(V) is the predominate As species in oxic environments. 

Hydrolysis species of As(V) and As(III) are formed through dissociation of arsenic acid 

(H3AsO4) or arsenous acid (H3AsO3), respectively. These soluble species can be sorbed 

on soil colloids or mineral surfaces. Additionally, the soluble species can precipitate to 

form As minerals. 

3.2 Speciation and Transformation Products

Potential metabolites of applied organic arsenicals include volatile alkylarsines and 

inorganic arsenic (as arsenate or arsenite) along with carbon dioxide. Additionally, DMA 

may be present as a metabolite of MMA. As with the rate, the metabolism pathway is 

sensitive to environmental conditions with the major metabolites occurring in variable 

proportions under different environmental conditions. Transformation to volatile 

alkylarsines, the only metabolism route that would directly reduce soil arsenic loading, 

has been shown to be possible in certain circumstances but is generally not expected to be 

a major route of dissipation. A maximum of 35% of applied MMA is expected to be 

present as DMA at any one time. Theoretically, there is some possibility for MMA to 

metabolize to DMA, but significant transformation has not been observed in available 

field or laboratory studies. Observed metabolism rates of MMA and DMA to inorganic 
Page 11 of 58 

arsenic greatly varies, ranging from undetected after several years to more than 80% 

transformation in several months. Generally, arsenate [As(V)] is expected to be the 

dominant species of inorganic arsenic, but in reducing conditions, arsenite [As(III)] may 

be more stable. 

As stated in previous risk assessments for MSMA, the biogeochemical cycle for As is 

complex because it has multiple oxidation states (+5 and +3) which can form numerous 

inorganic As complexes and minerals, as well as organoarsenic compounds (for example, 

methylarsonic acid, methylarsinic acid, arsenocholine, arsenobetaine, roxarsone). Figure 

1 shows a diagram of selected As forms in the biogeochemical cycle. Although these As 

forms (methylated compounds, minerals, and soluble species) do not represent the 

complete universe of As compounds, they do represent important As species for 

quantifying the potential amount of inorganic As from methylated As compounds. 

Speciation was not quantitatively considered in the 2006 Reregistration Eligibility 

Assessment (RED) for MSMA (013803) and disodium methylarsonic acid [DSMA 

(013802)], it will be considered in this updated assessment. 

Chemical equilibria modeling, including soluble inorganic and methylated species of As, 

suggest that methylated forms of arsenic are expected to be thermodynamically stable As 

species under anoxic (pe+pH > 4) environmental conditions (Appendix D). The 

inorganic As(III) species are expected to be the predominate As species in suboxic 

(pe+pH = 4 to 8) environmental conditions. The inorganic As(V) species are expected to 

be the predominate As species in suboxic to oxic (pe+pH > 8) environmental conditions. 

These data suggest that MSMA is expected to be transformed to inorganic As species in 

most surface waters and soil conditions. This analysis, however, does not provide an 

estimate on the rate of transformation of MSMA to inorganic As. 

4. Receptors

The inorganic forms of arsenic are generally considered more toxic (as direct toxins) than 

the organic forms. For aquatic species, toxicity data for the organic (MSMA) and 

inorganic As species are expected in a dynamic equilibria with microbes, minerals, water, 

air in aquatic and/or soil conditions (see discussion above) (Lindsay, 1979). For 

terrestrial species, however, only the toxicity data for the MSMA will be 

considered because the dynamic equilibria of As is not expected due to the limited 

quantity of minerals and microbial populations on the surface of terrestrial food items 

(e.g., foliage). The most sensitive endpoint from each taxon is provided here in the 

tables. No toxicity data are currently available for methylated arsenicals with a +3 

valence. Tables 11 through 15 provide a summary of the aquatic taxonomic groups and 

most sensitive surrogate species tested to characterize the potential acute and chronic 

ecological effects of MSMA, methylated arsenicals (+5), methylated arsenicals (+3), 

inorganic arsenicals (+5), and inorganic arsenicals (+3). Tables 16 through 18 provide a 

summary of the terrestrial taxonomic groups and most sensitive surrogate species tested 

to characterize the potential acute and chronic ecological effects of MSMA, methylated 

arsenicals (+5), and methylated arsenicals (+3). 
Page 12 of 58 

4.1. Effects to Aquatic Organisms 

Table 11. Summary of the Most Sensitive Endpoints from Aquatic Toxicity Studies 

for MSMA. 

STUDY TAXA FORM. ENDPOINT MRID 

STUDY 

CLASSIFICATION/ 

COMMENTS 

Acute freshwater 

fish1

Rainbow trout 

(Oncorhynchus 

mykiss) 

51% a.i. LC50 > 85 mg a.i./L 41747301 

Acceptable; NOAEC = 85 

mg a.i./L (no effects were 

noted in the study) 

Chronic 

freshwater fish No data available 

Acute estuarine/ 

marine fish 

Sheepshead 

minnow 

(Cyprinodon 

variegates) 

99.4% a.i. 

(MAA 

Methanearsoni

c acid) 

LC50 = 323 mg a.i./L 42414102 

Acceptable; NOAEC = 170 

mg a.i./L (based on 

mortality) 

Chronic 

estuarine/ marine 

fish 

No data available 

Acute freshwater 

invertebrate 

Freshwater 

invertebrate 

(Daphnia 

pulex) 

51% a.i. EC50 = 77 mg a.i./L 41940605 

Acceptable; no observations 

of sublethal effects were 

made during the study; 

NOAEC = 12.3 mg a.i./kg 

(based on mortality); slope 

= 2.7. 

Chronic 

freshwater 

invertebrate 

No data available 

Acute estuarine/ 

marine 

invertebrate 

Estuarine/ 

Marine 

invertebrate 

(Americamysis 

bahia - 

formerly 

Mysidopsis 

bahia) 

99.4% a.i. 

(MAA 

Methanearsoni

c acid) 

LC50 = 173 mg a.i./L 42414101 

Supplemental (the test 

organisms were 24-48-hr 

old instead of <24-hr) 

(adequate for RQ 

calculation); NOAEC = 93.5 

mg a.i./L (based on 

mortality) 

Acute estuarine/ 

marine 

invertebrate 

Estuarine/ 

Marine 

invertebrate – 

oyster 

(Crassostrea 

virginica) 

50.4% a.i. EC50 = 160 mg a.i./L 

(shell deposition) 42464801 

Acceptable; NOAEC = 49 

mg a.i./L (based on shell 

deposition); no mortality 

occurred during the test 

Chronic 

estuarine/ marine 

invertebrate 

No data available 

Vascular aquatic 

plant 

Vascular 

aquatic plant 

(Lemna gibba) 

51% a.i. 

EC50 = 104 mg a.i./L 

(reduced number of 

fronds) 

NOAEC = 56.9 mg 

a.i./L (reduced 

number of fronds) 

41748202 

Acceptable; provided meanmeasured values so assume 

concentrations were 

corrected for % a.i. 

Non-vascular 

aquatic plant 

Non-vascular 

aquatic plant 51% a.i. EC50 = 5.63 mg a.i./L (reduced cell 41748201 Acceptable; did not use the lowest two nominal 
Page 13 of 58 

STUDY TAXA FORM. ENDPOINT MRID 

STUDY 

CLASSIFICATION/ 

COMMENTS 

(Selenastrum 

capricornutum) 

number) 

NOAEC < 0.3 mg 

a.i./L (reduced cell 

number) 

concentrations because no 

detectable concentration 

could be found; provided 

mean-measured values so 

assume concentrations were 

corrected for % a.i. 

Bioconcentration 

factor study 

BCF study 

(bluegill 

sunfish) 

14C residues 

BCF < 1x in edible 

(fillet), nonedible 

(viscera and carcass), 

and whole fish 

42432501 

Acceptable; fish exposed for 

28 days to 0.49 mg a.i./L; 

max concentrations in each 

tissue was measured on Day 

7 1 Freshwater fish may be surrogates for aquatic-phase amphibians. 

Table 12. Summary of the Most Sensitive Endpoints from Aquatic Toxicity Studies 

for Methylated Arsenicals (+5 Valence) (Including MSMA). 

TAXA ARSENICAL (PC 

CODE) FORM. ENDPOINT MRID 

Acute 

Freshwater Fish 

Cacodylate acid (5) 

(012501 and 012502) 100% (acid) LC50 = 17 mg a.i./L (bluegill) 40098001 

Acute Estuarine/ 

Marine Fish 

Cacodylate acid 

Equivalence (5) (a.e.) 

(012501 and 012502) 

29.4% (a.e.) LC50 = 291 mg a.i./L 

(sheepshead minnow) 42433301 

Fish 

Reproduction No data available 

Acute 

Freshwater 

invertebrate 

Cacodylate acid 

Equivalence (5) (a.e.) 

(012501 and 012502) 

29.4% (a.e.) EC50 = 18.1 mg a.e./L 

(daphnid) 41747901 

Acute Estuarine/ 

Marine 

invertebrate 

Cacodylate acid 

Equivalence (5) (a.e.) 

(012501 and 012502) 

29.4% (a.e.) LC50 = 9.7 mg a.e./L 

(mysid) 42433302 

Acute Estuarine/ 

Marine 

Invertebrate 

(oyster) 

MSMA (5) (013803) 50.4% a.i. EC50 = 160 mg a.i./L (shell 

deposition) 42464801 

Aquatic 

Invertebrate 

Reproduction 

No data available 

Vascular aquatic 

plant (Lemna 

gibba) 

Cacodylate acid 

Equivalence (5) (a.e.) 

(012501 and 012502) 

29.4% (a.e.) EC50 = 30.9 mg/L 

NOAEC = 2.4 mg a.i./L 41791105 

Non-vascular 

aquatic plant DSMA (5) (013802) 81.35% a.i. 

EC50 = 1.5 mg a.i./L (bluegreen algae) 

NOAEC = 0.9 mg a.i./L 

(blue-green algae) 

43184502 

BCF study (fish) MSMA (5) (013803) 14C residues 

BCF < 1x in edible (fillet), 

nonedible (viscera and 

carcass), and whole fish 

(bluegill) 

42432501 

 
Page 14 of 58 

Table 13. Summary of the Most Sensitive Endpoints from Aquatic Toxicity Studies 

for Methylated Arsenicals (+3 Valence). 

TAXA ARSENICAL (PC 

CODE) FORM. ENDPOINT MRID 

All No data available 

Table 14. Summary of the Most Sensitive Endpoints from Aquatic Toxicity Studies 

for Inorganic Arsenicals (+5 Valence). 

TAXA ARSENICAL(3 or 

5)/PC CODE FORM. ENDPOINT MRID* 

Acute 

Freshwater Fish 

Arsenic Acid (5)/ 

006801 75% a.i. LC50 = 50 mg a.i./L (bluegill) 

41950601 (Machado, 

1991) 

Acute Estuarine/ 

Marine Fish 

Arsenic Acid (5)/ 

006801 76.1% LC50 = 2 mg a.i./L (mysid) 41620004 (LeLievre, 1990) 

Fish 

Reproduction 

Arsenic Acid (5)/ 

006801 75.8% a.i. LC50 = 50 mg a.i./L 41802201 (Machado, 1991) 

Acute 

Freshwater 

invertebrate 

Arsenic Acid (5)/ 

006801 76.1% EC50 = 15 mg a.i./L (daphnid) 

41620001 (LeLievre, 

1990) 

Chronic 

Freshwater 

Invertebrate 

Chromated 

Arsenicals (5)/ 

128977 

ACZA leachate 

(from treated 

wood) [copper, 

zinc, and arsenic 

(<0.95 arsenic)] 

NOAEC < 0.95 μg a.i./L 

(LOAEC <0.95 μg a.i./L 

based on survival, length 

and dry weight) (Daphnia) 

44797501 (Putt, 1997) 

(supplemental) 

Acute Estuarine/ 

Marine 

invertebrate 

Arsenic Acid (5)/ 

006801 76.1% LC50 = 2 mg a.i./L (mysid) 41620002 (LeLievre, 1990) 

Acute Estuarine/ 

Marine 

Invertebrate 

(oyster) 

No data available 

Estuarine/ 

Marine 

Invertebrate 

Reproduction 

Chromated 

Arsenicals (5)/ 

128977 

** From OPPIN Bibliography 44797502 (Putt, 1997) 

Vascular aquatic 

plant (Lemna 

gibba) 

Arsenic Acid (5)/ 

006801 75.8% a.i. 

EC50 >9.8 mg a.i./L 

[19% (statistically 

significant) reduction in 

frond number at limit dose] 

42290901 (Hoberg, 1992) 

[Acceptable] 

Non-vascular 

aquatic plant 

Arsenic Acid (5)/ 

006801 75.8% a.i. EC50 = 0.0092 mg a.i./L (Skeletonema costatum) 

42278801 (supplemental, a 

NOAEC could be 

established) 

BCF study (fish) Arsenic Acid (5)/ 

006801 ** From OPPIN Bibliography 40435801 (Envirosphere, 1987) 

 
Page 15 of 58 

Table 15. Summary of the Most Sensitive Endpoints from Aquatic Toxicity Studies for Inorganic 

Arsenicals (+3 Valence). 

TAXA ARSENICAL(3 or 

5)/PC CODE FORM. ENDPOINT MRID* 

Acute 

Freshwater Fish 

Arsenic Trioxide (3)/ 

007001 99.31% a.i. LC50 = 25.6 mg a.i./L (trout) 44423602 

Acute Estuarine/ 

Marine Fish No data available 

Acute 

Freshwater 

invertebrate 

Sodium Arsenite (3)/ 

013603 NaAs02 (99%) EC50 = 4.89 mg a.i./L (daphnid) 00114493 

Chronic 

Freshwater 

Invertebrate 

Sodium Arsenite 

(3)/013603 NaAs02 (99%) 

NOAEC = 0.37 mg a.i./L 

(daphnid) 

LOAEC = 0.75 mg a.i./L 

(based on reduced 

production of young and 

length of surviving adult) 

02025917 

Acute Estuarine/ 

Marine 

invertebrate 

No data available 

Acute Estuarine/ 

Marine 

Invertebrate 

(oyster) 

No data available 

Estuarine/Marine 

Invertebrate 

Reproduction 

No data available 

Vascular aquatic 

plant (Lemna 

gibba) 

No data available 

Non-vascular 

aquatic plant No data available 

BCF study (fish) No data available 
Page 16 of 58 

4.2. Effects to Terrestrial Organisms 

Table 16. Summary of the Most Sensitive Endpoints from Terrestrial Toxicity Studies for MSMA. 

TAXA FORM. ENDPOINT MRID STUDY 

CLASSIFICATION 

Acute avian oral 

Bobwhite quail 

(Colinus 

virginianus) 

Arsonate 

liquid blend 

(51% a.i.) 

LD50 = 425.2 mg 

a.i./kg-bw 41610002 

Acceptable; NOAEL < 149 

mg a.i./kg-bw (effects were 

noted in all of the test 

concentrations – signs of 

toxicity and reduced body 

weight/food consumption) 

Sub-acute avian 

dietary 

Bobwhite quail 

(Colinus 

virginianus) 

Arsonate 

liquid blend 

(51% a.i.) 

LC50 = 1,667 mg 

a.i./kg-diet 41610003 

Acceptable; NOAEC < 287 

mg a.i./kg-diet (effects were 

noted in all test 

concentrations – reduced 

body weight) 

Chronic avian No data available 

Acute oral rat Rat (Rattus 

norvegicus) 34.35% a.i. LD50 = 157 mg 

a.i./kg-bw 41610003 Acceptable 

Chronic rat No data available 

Acute honey bee Honey bee 

(Apis mellifera) 

Arsonate 

liquid blend 

(51% a.i.) 

LD50 = 35 μg a.i./bee 

(contact) 41610007 Acceptable; endpoint corrected for purity 

Terrestrial plant 

Seedling 

emergence – 

Tier II 

51% a.i. 

EC25 = 0.116 lb 

a.i./acre (cabbage; 

reduced dry weight) 

NOAEC 0.018 lb 

a.i./acre (cabbage; 

plant height) 

41705501 

42572501 

Supplemental (a NOAEC 

could not be determined for 

cabbage) ; Acceptable in 

conjunction with MRID 

42572501 

Terrestrial plant Vegetative 

vigor – Tier II 51% a.i. 

EC25 = 0.418 lb 

a.i./acre (oat; reduced 

dry weight) 

NOAEC = 0.14 (oat, 

dry weight) 

41705502 Acceptable 

1 Birds represent surrogates for terrestrial-phase amphibians and reptiles. 

 
Page 17 of 58 

Table 17. Summary of the Most Sensitive Endpoints from Terrestrial Toxicity Studies for 

Methylated Arsenicals (+5 Valence) (Including MSMA). 

TAXA ARSENICAL (PC 

CODE) FORM. ENDPOINT MRID 

Acute Avian 

(oral) MSMA (5) (013803) Arsonate liquid blend (51% a.i.) 

LD50 = 425.2 mg a.i./kgbw 41610002 

Sub-acute Avian 

(dietary) MSMA (5) (013803) Arsonate liquid blend (51% a.i.) 

LC50 = 1,667 mg a.i./kgdiet (quail) 41610003 

Avian 

Reproduction 

No data available 

Acute Mammal MSMA (5)/ 013803 34.35% a.i. LD50 = 157 mg a.i./kg-bw 

(rat) 00105171 

Mammal 

Reproduction 

Methanearsonic Acid 

(5) (128876) 99.4% a.i. NOAEL = 100 mg a.i./kgdiet 43178301 

Honey bee (Apis 

mellifera) MSMA (5) (013803) Arsonate liquid blend (51% a.i.) 

LD50 = 35 μg a.i./bee 

(contact) ; endpoint 

corrected for purity 

41610007 

Seedling 

emergence – Tier 

II 

MSMA (5) (013803) 51% a.i. 

EC25 = 0.116 lb a.i./acre 

(cabbage; reduced dry 

weight 

NOAEC = 0.018 lb 

a.i./acre (cabbage; plant 

height) 

42572501 

Vegetative vigor 

– Tier II DSMA (5) (013802) 82.7% a.i. 

EC25 = 0.354 lb a.i./acre 

(corn; reduced dry weight 

NOAEC < 0.30 lb a.e./acre 

(corn; reduced dry weight) 

41905604 

TABLE 18. Summary of the Most Sensitive Endpoints from Terrestrial Toxicity Studies for 

Methylated Arsenicals (+3 Valence). 

TAXA ARSENICAL (PC 

CODE) FORM. ENDPOINT MRID 

All No data available 

4.3. Ecological Incidents 

For this problem formulation, the Environmental Incident Information System (EIIS), the 

Incident Database System (IDS) (for aggregate incidents), and the American Bird 

Conservancy’s Avian Incident Management System (AIMS) were searched on October 9, 

2012, for ecological incident reports associated with MSMA. Additionally, because of 

the way that MSMA can speciate to other organic and inorganic arsenicals, the EIIS and 

IDS databases were searched for incident data on all of the organic and inorganic 

arsenicals to help provide insight on the toxicity of these chemicals to non-target 

organisms under natural conditions. Because of the time investment required to search 

for incidents associated with each of the organic and inorganic arsenicals, only those 

arsenicals associated with the most ecological incidents were search for in AIMS (i.e., 

MSMA, CAMA, DSMA, and cacodylic acid). The results are summarized below. 
Page 18 of 58 

MSMA: 

EIIS returned nine ecological incidents (eight involving terrestrial plants and one aquatic 

incident) for MSMA (PC Code: 013803). The plant incidents were associated with plant 

damage from direct treatment (five incidents) or drift to nearby plants (three incidents). 

The aquatic incident involved mortality of several fish (a variety of species) from runoff 

into a canal from the registered use of MSMA on a golf course in Louisiana. All of these 

incidents were listed as ‘possible’ or ‘probable’ for their certainty indices and occurred 

between 1972 and 2001. Three of the incidents were from registered uses, three were 

from accidental misuses, and the legality of use was undetermined in the remaining three 

incidents. Three incidents were associated with use on rights-of-way, one was from a 

golf course use (i.e., the aquatic incident), two were from residential lawn use, one was 

from a peanut use, and two were from use on cotton. 

The IDS database returned a total of 187 aggregated incidents (all plant incidents) for 

MSMA (PC Code: 013803). All of the aggregated incidents for MSMA occurred 

between 1998 and 2006. The AIMS database returned one incident for MSMA involving 

birds and rabbits. However, the certainty index was listed as ‘unlikely’ for this incident. 

Other Organic Arsenicals: 

EIIS returned 13 ecological incidents (12 involving terrestrial plants and one bird 

incident) for organic arsenicals, excluding MSMA. The bird incident was associated with 

a single bird mortality and a registered homeowner use of cacodylic acid (PC Code 

012501) in 1995. The certainty index for this incident is classified as ‘possible’. Of the 

12 plant incidents, 11 were associated with CAMA (PC Code 013806) and two were 

associated with DSMA (PC Code 013802). All of these incidents were listed as 

‘possible’ or ‘probable’ for their certainty indices. All of the CAMA incidents occurred 

in 1999 and were from registered uses on residential lawns. There were no dates 

associated with the DSMA incidents and the legality of use for these incidents was 

undetermined. One of the DSMA incidents was from a rice use; the use for the other 

DSMA incident was not reported. 

The IDS database returned a total of 592 aggregated incidents (all plant incidents) for 

CAMA (PC Code: 013806). All of the aggregated incidents for CAMA occurred 

between 1995 and 2011. There were no incident reports for CAMA, DSMA, or cacodylic 

acid in the AIMS database. 

Inorganic Arsenicals: 

EIIS returned 8 ecological incidents (seven terrestrial and one aquatic) for inorganic 

arsenicals. The aquatic incident was associated with fish mortality (several species) from 

the use of sodium arsenate (PC Code 013505) on cotton in 1972. The certainty index for 

this incident is classified as ‘possible’ and the legality of use was undetermined. Two of 

the incidents involved arsenic acid (PC Code 006801) and the death of deer [one in 1987 
Page 19 of 58 

(forest use) and one in 1995(use not reported)]. One of the incidents involved the 

registered use of arsenic oxide (PC Code 006802) around a residential building that was 

associated with the death of two squirrels in 2006. The certainty index for this incident is 

listed as ‘probable’. There are four incidents associated with lead arsenate hydroxide (PC 

Code 013502). Two of these incidents are classified as ‘unlikely’; the other two are 

reported as ‘highly probable’. Of the latter two, one involved the death of an eagle in 

1989 (use site not reported) and the other involved the death of four peregrine falcons in 

1993 (building use). In both of these incidents, the legality of use was undetermined. 

There were no aggregate incidents reported for inorganic arsenicals in the IDS database. 

5. Conceptual Model

The conceptual model for MSMA indicates that spray drift, runoff, leaching, and 

volatilization are potential exposure pathways of MSMA and its As transformation 

products to aquatic and terrestrial environments. Because As in MSMA is an element, it 

will not degrade and, therefore, will be transformed to various organic and inorganic As 

species depending on site-specific environmental conditions. The transformation of 

MSMA into various soluble As inorganic and As organic compounds is expected to 

influence the exposure pathways. For example, the inorganic As from MSMA will be 

reduced under anaerobic environments to form arsine AsH3(g). Arsine is toxic gas formed 

from application of methylated arsenic pesticides in soil (Gao and Burau, 1997). Factors 

controlling the predominance of As species include pH, redox potential, microbial 

community, and mineralogy. 

6. Analysis Plan

6.1. Stressors of Concern 

The stressors of concern are parent MSMA as well as all the soluble As transformation 

products from MSMA in soil and aquatic environments. For aquatic organisms, stressors 

of concern are for both methylated and inorganic arsenicals, since MSMA can speciate to 

these forms in aquatic conditions. For terrestrial animals, the stressor of concern is 

limited to MSMA and other methylated arsenic compounds since speciation to other 

arsenicals is less likely in the terrestrial environment (than in aquatic environments). 

Because the methylated (III) and inorganic As (III) are expected to form in conditions 

under low redox potential (i.e., under sub-oxic and anoxic conditions), the greatest 

concern for the As(III) species are for sediment-dwelling organisms. Arsenic minerals 

are not considered as a stressor of concern because they simply serve to buffer the soluble 

As concentration. Soluble As species considered in this assessment are discussed in 

Section 3. Environmental Fate and Transport Assessment. The stressors of concern are 

expected to be controlled by the natural As biogeochemical cycle. Factors controlling the 

predominance of stressors of concern include pH, redox potential, microbial community, 

and mineralogy. 
Page 20 of 58 

6.2. Measures of Exposure for Ecological Effects and Drinking Water 

EFED will use standard available models to evaluate potential exposures to aquatic and 

terrestrial organisms as described at http://www.epa.gov/pesticides/science 

/models_db.htm. The aquatic and drinking water exposure assessment will be conducted 

using a tiered approach. PRZM/EXAMS modeling will be conducted to estimate MSMA 

concentrations in the standard pond and index reservoir. The total mass of MSMA in the 

standard pond and index reservoir is assumed to be completely transformed into 

inorganic As. This assumption is used to conservatively address toxicity concerns for 

MSMA and inorganic As. Site-specific water quality conditions will not be considered in 

this exposure assessment. If additional refinement is needed in the exposure assessments, 

a chemical equilibria model using site specific water quality data will be used to speciate 

As in the standard pond and index reservoir water into the inorganic As (III and V) and 

methylated arsenic compounds (MMA,DMA,TMA). Please see Appendix D for details 

on the chemical equilibria model. Summation of molar concentrations of As species into 

various As forms [including inorganic As(V) species; inorganic As(III) species; DMA, 

MMA, TMA(V); MMA, DMA, TMA(III)]. If additional refinement is needed then 

concentrations of individual inorganic and organic As species will considered. 

For terrestrial animal exposures, it will be assumed that dietary exposures are to residues 

of MSMA (and other methylated arsenic compounds), since speciation to other arsenicals 

is less likely (and less understood) in the terrestrial environment [e.g., on potential food 

items (e.g., foliage)] than in water. Although MSMA has the potential to speciate into 

volatile forms (see Tables 8 and 9), the volatile arsenical species would only be a 

concern under highly reduced conditions in the terrestrial environment. Therefore, for 

birds and mammals, exposure via inhalation will not be assessed for MSMA. 

To determine the potential for drinking water to be an exposure concern for birds and 

mammals the Screening Imbibation Program (SIP v. 1.0) was run. For the SIP run, the 

highest solubility limit for calcium arsenate (the stable mineral phase arsenical formed 

under most sub-oxic and oxic conditions) and corresponding toxicity data for inorganic 

arsenicals were used (see Appendix E). Based on this analysis, drinking water is a 

potential route of concern for birds (acute exposure) and mammals (acute and chronic 

exposures). There are currently no reproduction data available for inorganic arsenics and 

birds, therefore, the potential for drinking water to be a potential route of concern for 

birds from chronic exposure could not be modeled. However, the potential risk to birds 

from drinking water (chronic exposure) is assumed since the risk cannot currently be 

precluded and the results of the acute run (SIP) for birds and acute and chronic 

mammalian SIP runs indicate that drinking water is a potential route of concern. 

6.3. Measures of Effect 

Toxicity data presented in Section 4 of this problem formulation will be used to calculate 

risk quotients. Any additional information submitted by the registrant or found in the 

open literature prior to conduct of the risk assessment will also be considered. The open 

literature studies are identified using EPA’s ECOTOXicology database (ECOTOX) 
Page 21 of 58 

(USEPA, 2007c), which employs a literature search engine for locating chemical toxicity 

data for aquatic life, terrestrial plants, and wildlife. The evaluation of both sources of 

data can also provide insight into the direct and indirect effects of pesticides on biotic 

communities from loss of species that are sensitive to the chemicals and from changes in 

structure and functional characteristics of the affected communities. 

6.4. Endangered Species Assessments 

Consistent with the Agency’s responsibility under the Endangered Species Act (ESA), 

the Agency will evaluate risks to federally listed threatened and/or endangered (listed) 

species from registered uses of pesticides in registration review. These assessments will 

be conducted in accordance with standard EPA procedures and good scientific judgment. 

6.5. Endocrine Disruptor Screening Program 

MSMA (or any arsenical) was not included in the first group of 67 chemicals issued an 

order to conduct Tier 1 EDSP testing. For additional information the EDSP program, 

visit http://www.epa.gov/endo/. 

7. Preliminary Identification of Data Gaps 

7.1. Environmental Fate 

Table 19 identifies environmental fate studies by MRID that offer data for each guideline 

requirement, as well as study classifications and whether or not further data are needed in 

order to support risk assessment. 

Table 19. Submitted Environmental Fate Data for MSMA. 

OCSPP 

Guideline Data Requirement Submitted Studies (MRID)

Study 

Classifications

Are data 

needed to 

conduct risk 

assessment?

Current Additional Data Need 

835.2120 Hydrolysis 42363001 Acceptable No 

835.2240 Photodegradation in 

Water 

41903902 Acceptable No 

835.2410 Photodegradation on 

Soil 

41903901 Acceptable No 

835.4100 Aerobic Soil 41886901 Upgradable No 

44767601 Acceptable 

Gao and Burau, 

1997 

Supplemental 

 151476 Supplemental 

 151475 Supplemental 

835.4200 Anaerobic Soil Not submitted NA No Anaerobic aquatic metabolism data 

(MRID 44767602) provide sufficient 

data on MSMA degradation under 

anaerobic conditions 

835.4300 Aerobic Aquatic 43314801 Not classified Study is in review 
Page 22 of 58 

OCSPP 

Guideline Data Requirement Submitted Studies (MRID)

Study 

Classifications

Are data 

needed to 

conduct risk 

assessment?

Current Additional Data Need 

835.4400 Anaerobic Aquatic 44767602 Acceptable No 

835.1230 

835.1240 

Leaching and 

adsorption/desorption 

41651906 

152662 Supplemental 

835.1410 Volatility-Laboratory No 

835.6100 Terrestrial Field 

Dissipation 

42526001 

42616201 

Supplemental No 

835.6200 Aquatic Field 

Dissipation 

Not submitted No 

165-4 

(850.1730) 

Fish Bioaccumulation 42432501 Acceptable No 

7.2. Ecological Effects 

Tables 20 through 23 identify aquatic ecological effects studies by MRID that offer data 

for each guideline requirement, as well as study classifications and whether or not further 

data are needed for methylated arsenicals(V) (including MSMA), methylated arsenicals 

(III), inorganic arsenicals(V), and inorganic arsenicals (III). Tables 24 identifies 

terrestrial ecological effects studies that offer data for each guideline requirement and 

whether or not further data are needed for methylated arsenicals(V) (including MSMA) 

and methylated arsenicals(III). 

There are data gaps for the methylated arsenicals (both V and III valences). For aquatic 

species, there are currently no chronic data available for any animal for the methylated 

arsenicals(V). There are currently no toxicity data available for any methylated arsenical 

(III). Because methylated arsenicals (III) are most likely to form under low redox 

potential conditions, exposure to benthic organisms is of greatest toxicity concern. 

Therefore, EFED recommends requesting several toxicity studies for methylated 

arsenicals and aquatic organisms (see the tables below). 
Page 23 of 58 

For the inorganic arsenicals(V), there are a few data gaps; however, currently available 

data can be used to adequately address the gaps (see Table 22 for details). For inorganic 

arsenicals(III), again, because these are most likely to form in sediments (under low 

redox potential conditions), benthic organisms are of greatest toxicity concern. 

Therefore, EFED recommends requesting toxicity studies for inorganic arsenicals (+3) 

and sediment-dwelling organisms (see Table 23). 

 

For terrestrial organisms, data gaps include an acute oral study with passerines and avian 

reproduction studies for methylated arsenicals (V) (see Table 24). Therefore, EFED 

recommends requesting these data. The inorganic arsenicals and the methylated 

arsenicals (III) will not be considered in the terrestrial assessment for MSMA, therefore, 

toxicity data for terrestrial organisms and these arsenicals are not needed at this time. 

Table 20. Summary of Submitted Aquatic Ecological Effects Data for Methylated 

Arsenics (+5) (Including MSMA). 

OCSPP 

Guideline 

Data 

Requirement 

Submitted 

Studies 

(MRID) 

Study 

Classifications 

Are data 

needed to 

conduct 

risk 

assessment? 

Comment 

 850.1010 

Freshwater 

invertebrate 

acute toxicity 

41747901 Acceptable No -- 

850.1025 

850.1035 

850.1045 

850.1055 

Saltwater 

invertebrate 

acute toxicity 

42433302 Decision 

deferred* No 

This study will be reviewed 

prior to conducting the risk 

42464801 Acceptable assessment 

 850.1075 

Freshwater 

fish acute 

toxicity 

40098001 Acceptable No -- 

 850.1075 Saltwater fish 

acute toxicity 42433301 Acceptable No -- 

 850.1300 

Freshwater 

invertebrate 

life cycle 

(None) (N/A) Yes

Currently, no data are available; 

therefore, a guideline compliant 

freshwater invertebrate life 

cycle test is needed. 

 850.1350 

Saltwater 

invertebrates 

life cycle 

(None) (N/A) Yes 

Currently, no data are available; 

therefore, a A guideline 

compliant saltwater invertebrate 

life cycle test is needed. 

 850.1400 

Freshwater 

fish 

early-life 

stage 

(None) (N/A) No 

A guideline compliant 

freshwater fish early life-stage 

test would not be needed if a 

fish life-cycle test was available.
Page 24 of 58 

OCSPP 

Guideline 

Data 

Requirement 

Submitted 

Studies 

(MRID) 

Study 

Classifications 

Are data 

needed to 

conduct 

risk 

assessment? 

Comment 

 850.1400 

Saltwater fish 

early-life 

stage 

(None) (N/A) No – see 

comment 

A guideline compliant saltwater 

fish early life-stage test would 

not be needed if a fish life-cycle 

test was available. 

 850.1500 Fish life 

cycle (None) (N/A) Yes

Currently, no data are available; 

therefore, a guideline compliant 

freshwater fish life-cycle test is 

needed. 

850.4400 

Aquatic plant 

growth: 

Tier II 

vascular 

plants 

41791105 Acceptable No -- 

850.5400 

Aquatic Plant 

Growth: 

Tier II algae 

43184502 Acceptable No -- 

Table 21. Summary of Submitted Sediment Effects Data for Methylated Arsenics 

(+3). 

OCSPP 

Guideline 

Data 

Requirement 

Submitted 

Studies 

(MRID) 

Study 

Classifications 

Are data 

needed to 

conduct 

risk 

assessment? 

Comment 

850.1735 

Whole 

sediment: 

acute 

freshwater 

invertebrates 

(None) (N/A) Yes 

Methylated arsenicals are 

expected to form under low 

redox potential conditions, 

exposure to benthic organisms is 

of greatest toxicity concern. A 

guideline compliant whole 

sediment acute freshwater 

invertebrates test is needed. 

Nonguideline1

Whole 

sediment: 

chronic 

freshwater 

invertebrates 

(None) (N/A) Yes 

Methylated arsenicals are 

expected to form under low 

redox potential conditions; 

exposure to benthic organisms is 

of greatest toxicity concern. A 

whole sediment chronic 

freshwater invertebrates test is 

needed. 1 Although there is no guideline for the chronic sediment toxicity study, the following methods by the 

USEPA’s Office of Research and Development may be consulted: ORD study method EPA 600/R-099-

064: chronic freshwater sediment testing flowthrough (Hyalella azteca); ORD study method EPA 600/R099-064: chronic freshwater sediment testing flowthrough (Chironomus dilutus or C. tentans). 
Page 25 of 58 

Table 22. Summary of Submitted Aquatic Ecological Effects Data for Inorganic 

Arsenicals (+5). 

OCSPP 

Guideline 

Data 

Requirement 

Submitted 

Studies 

(MRID) 

Study 

Classifications 

Are data 

needed to 

conduct 

risk 

assessment? 

Comment 

 850.1010 

Freshwater 

invertebrate 

acute toxicity 

41620001 Acceptable No -- 

850.1025 

850.1035 

850.1045 

850.1055 

Saltwater 

invertebrate 

acute toxicity 

41620002 Acceptable No -- 

 850.1075 

Freshwater 

fish acute 

toxicity 

41950601 Acceptable No -- 

 850.1075 Saltwater fish 

acute toxicity 41620004 Acceptable No -- 

 850.1300 

Freshwater 

invertebrate 

life cycle 

44797501 Acceptable No -- 

 850.1350 

Saltwater 

invertebrates 

life cycle 

(None) (N/A) No 

The chronic saltwater 

invertebrate endpoint will be 

based on an acute-to-chronic 

ratio using data from freshwater 

invertebrates. 

 850.1400 

Freshwater 

fish 

early-life 

stage 

41802201 Acceptable No -- 

 850.1400 

Saltwater fish 

early-life 

stage 

41802201 Acceptable No -- 

 850.1500 Fish life 

cycle (None) (N/A) No 

The chronic freshwater fish 

endpoint will be based on the 

results from the available 

freshwater fish early life-stage 

study. 

850.4400 

Aquatic plant 

growth: 

Tier II 

vascular 

plants 

42290901 Acceptable No -- 

850.5400 

Aquatic Plant 

Growth: 

Tier II algae 

42278801 

Supplemental 

(a NOAEC 

could not be 

established) 

No -- 
Page 26 of 58 

Table 23. Summary of Submitted Sediment Ecological Effects Data for Inorganic 

Arsenicals (+3). 

OCSPP 

Guideline 

Data 

Requirement 

Submitted 

Studies 

(MRID) 

Study 

Classifications 

Are data 

needed to 

conduct 

risk 

assessment? 

Comment 

850.1735 

Whole 

sediment: 

acute 

freshwater 

invertebrates 

(None) (N/A) Yes 

Arsenite is expected to form 

under low redox potential 

conditions; exposure to benthic 

organisms is of greatest toxicity 

concern. A guideline compliant 

whole sediment acute freshwater 

invertebrates test is needed. 

Nonguideline1

Whole 

sediment: 

chronic 

freshwater 

invertebrates 

(None) (N/A) Yes 

Arsenite is expected to form 

under low redox potential 

conditions; exposure to benthic 

organisms is of greatest toxicity 

concern. A whole sediment 

chronic freshwater invertebrates 

test is needed. 1 Although there is no guideline for the chronic sediment toxicity study, the following methods by the 

USEPA’s Office of Research and Development may be consulted: ORD study method EPA 600/R-099-

064: chronic freshwater sediment testing flowthrough (Hyalella azteca); ORD study method EPA 600/R099-064: chronic freshwater sediment testing flowthrough (Chironomus dilutus or C. tentans). 

Table 24. Summary of Submitted Terrestrial Ecological Effects Data for 

Methylated Arsenics (+5) (Including MSMA). 

OCSPP 

Guideline 

Data 

Requirement 

Submitted 

Studies 

(MRID) 

Study 

Classifications 

Are data 

needed for 

risk 

assessment? 

Comment 

 850.2100 Avian oral 

toxicity 

41610002 Acceptable Yes Acute oral toxicity data are 

necessary for a passerine 

species. 

 850.2200 Avian dietary 

toxicity 

41610003 Acceptable No -- 

 850.2300 Avian 

reproduction 

(None) (N/A) Yes No data are available to assess 

potential chronic toxicity to 

avian species. A guideline 

compliant avian reproduction 

test is needed. 

Nonguideline Avian 

inhalation 

(None) (N/A) No Although MSMA has the 

potential to speciate into 

volatile forms, this would only 

be a concern under highly 

reduced conditions. Therefore, 

for birds, exposure via 

inhalation will not be assessed 

for MSMA and avian inhalation 

data are not needed. 
Page 27 of 58 

OCSPP 

Guideline 

Data 

Requirement 

Submitted 

Studies 

(MRID) 

Study 

Classifications 

Are data 

needed for 

risk 

assessment? 

Comment 

 850.3020 Honey bee 

acute contact 

toxicity 

00018842 Acceptable No -- 

850.4100 Terrestrial 

plant toxicity: 

Tier II 

seedling 

emergence 

42572501 Supplemental No -- 

850.4150 Terrestrial 

plant toxicity: 

Tier II 

vegetative 

vigor 

41905604 Supplemental No -- 
Page 28 of 58 

8. References 

Dombrowski, Paul. M., Wei Long, Kevin J. Farley, John D. Mahony, Joseph F. Capitani, 

and Dominic M. Di Toro. 2005. Thermodynamic analysis of arsenic methylation. 

Environ. Sci. Technol. 39:2169-2176. 

Gao, S. and R. G. Burau. 1997. Environmental factors affecting rates of arsine evolution 

from and mineralization of arsenicals in soil. Environ. Qual. 26:753-763. 

Lindsay, W.L. 1979. Chemical Equilibria in Soils. Wiley and Sons, Inc. United States. 

Sadiq, Muhammad, Tahir H. Zaidi and Attique A. Mian. 1995. Environmental behavior 

of arsenic in soils: Theoretical. Water, Air, and Soil Pollution 20:369-377. 

Sadiq, Muhammad. 1997. Arsenic chemistry in soils: An overview of thermodynamic 

predictions and field observations. Water, Air, and Soil Pollution 93:117-136. 

 
Page 29 of 58 

APPENDIX A: Currently Registered MSMA Products.

Table 1. Currently Registered MSMA Products.

LABEL EPA 

REG. NO. 

ACTIVE 

INGREDIENT(S) 

(A.I.) 

% A.I. USE(S) 

QUADMEC® TURF 

HERBICIDE 2217-709 

MSMA 

18% a.i. 

[total Arsenic (as 

elemental) all in water 

soluble form 8.33%] 

Golf courses; Sod farms 

Dimethylamine salt 

of 2,4-

dichlorophenoxyacet

ic acid 

5.83% 

Dimethylamine salt 

of (+)-(R)-2-(2-

methyl-4-

chlorophenoxy)prop

ionic acid 

2.93% 

Dimethylamine salt 

of dicamba: 3,6-

dichloro-o-anisic 

acid 

1.46% 

DrexarTM 530 

Herbicide 19713-40 MSMA 

 35.78%) 

[total arsenic (as 

elemental), all in watersoluble form is 16.55%) 

Cotton; Highway rights-ofway; Golf courses; Sod 

farms 

MSMA 6.6 MSMA 

Liquid 19713-41 MSMA 

(51.3%) 

[total Arsenic as 

elemental) all in water 

soluble form is 23.61%] 

Cotton; Highway rights-ofway; Golf courses; Sod 

farms 

MSMA 6 Plus 

MSMA Liquid Plus 

Surfactant 

19713-42 MSMA 

47.6% 

 [total Arsenic (as 

elemental), all in watersoluble form, is 22%] 

Cotton; Highway rights-ofway; Golf courses; Sod 

farms 

MSMA 6.6 Liquid 19713-269 MSMA 

51.0% a.i. 

[total Arsenic (as 

elemental) all in water 

soluble form is 23.61%] 

Cotton; Highway rights-ofway; Golf courses; Sod 

farms 

Diumate Herbicide 19713-528 MSMA 

34.6% a.i. 

[Total Arsenic (as 

elemental) all in water 

soluble form is 16.00% 

Cotton; Highway rights-ofway 

Diuron 5.18% a.i.i 

MSMA 660 

Herbicide 19713-531 MSMA 

51.3% a.i. 

[Total Arsenic (as 

elemental) all in water 

soluble form is 23.61%) 

Cotton; Highway rights-ofway; Golf courses; Sod 

farms 

MSMA 120 

Arsonate Liquid for 

Postemergence 

Weed Control 

19713-550 MSMA 

52.8% a.i. 

[Total Arsenic (as 

elemental), all in watersoluble form, is 23.7%] 

Cotton; Highway rights-ofway; Golf courses; Sod 

farms 
Page 30 of 58 

LABEL EPA 

REG. NO. 

ACTIVE 

INGREDIENT(S) 

(A.I.) 

% A.I. USE(S) 

TARGET® 6.6 42519-1 MSMA 

51.0% a.i. 

[Total Arsenic (as 

elemental) all in water 

soluble form 23.61%] 

Cotton; Highway rights-ofway; Golf courses; Sod 

farms 

TARGET® 6 Plus 42519-3 MSMA 

48.3% a.i. 

[Total arsenic (as 

elemental) all in water 

soluble form 22.3%] 

Cotton; Highway rights-ofway; Golf courses; Sod 

farms 
Page 31 of 58 

APPENDIX B: Currently Registered MSMA USES.Table 1. CURRENTLY REGISTERED MSMA USES:

LABEL/EPA 

REG. NO. 

A.I.(S) APP 

METHOD 

USE(S) SINGLE 

MAX. 

APPL. 

RATE 

MAX. NO. 

APP. 

/SEASON 

MIN. APP. 

INTERVAL 

MAX. 

SEASONAL 

APP.RATE 

SPECIFIC 

COMMENT

QUADMEC®

TURF HERBICIDE/ 2217-709 

Monosodium acid methanearsonate (18% a.i.) [total Arsenic (as elemental) all in water soluble form 8.33%] 

Ground Newly 

constructed golf courses (broadcast) 

3 lb a.i./acre 1 Not 

applicable (N/A) 

3 lb a.i./acre (does not include additional spot treatments) 

None 

Dimethylamine salt of 2,4-dichlorophenoxyacetic acid (5.83%) 

Other golf courses (spot treatments only) 

1.8 lb a.i./acre (3 fl oz of product/ 1,000 ft2) 

NS NS NS 100 ft2 max per s

not to exceed 25total golf course acreage/yr 

Dimethylamine salt of (+)-(R)-2-(2-methyl-4-chlorophenoxy)propionic acid (2.93%) 

Sod farms (broadcast) 

3 lb a.i./acre 2 30-day 6 lb a.i./acre 

(total for product) 

25 ft buffer from permanent water aquatic habitat 

Dimethylamine salt of dicamba: 3,6-dichloro-oanisic acid (1.46%) 

Sod farms (spot treatments) 

1.8 lb a.i./acre (3 fl oz of product/ 1,000 ft2) 

NS NS 6 lb a.i./acre 

(total for product) 

100 ft2 max per s

DrexarTM 530 Herbicide/ 19713-40 

Monosodium acid methanearsonate 

Ground Aerial 

Cotton 2 lb a.i./acre 2 (the 

second application 

7-day 4 lb a.i./acre - Postemergence 

not apply preplan- In FL, applicati
Page 32 of 58 

LABEL/EPA 

REG. NO. 

A.I.(S) APP 

METHOD 

USE(S) SINGLE 

MAX. 

APPL. 

RATE 

MAX. NO. 

APP. 

/SEASON 

MIN. APP. 

INTERVAL 

MAX. 

SEASONAL 

APP.RATE 

SPECIFIC 

COMMENT

(35.78%) [Total arsenic (as elemental), all in water- soluble form is 16.55%) 

is allowed for a salvage operation) 

should be confinband treatments - 50 ft buffer fropermanent water bodies or aquatic habitat - Do not apply pbloom 

Ground Highway 

rights-ofway 

NS 2/yr 

(broadcast) 

NS NS - 100 ft buffer fr

permanent water bodies or aquatic habitat - Use is prohibiteafter Dec. 31, 20

Ground Golf courses 

(newly constructed) (broadcast) 

2.75 lb a.i./acre (2 fl oz of product to 1,000 ft2) 

1 N/A 2.75 lb 

a.i./acre (does not include additional spot treatments) 

- Use is prohibiteafter Dec. 31, 20

Ground Other golf 

courses (spot treatments only) 

2.75 lb a.i./acre (2 fl oz of product to 1,000 ft2) 

NS NS NS - 100 ft2 per spot 

- Not to exceed 2of the total golf course acreage p- Use is prohibiteafter Dec. 31, 20

Ground Sod farms 

(broadcast) 

2.75 lb a.i./acre (2 fl oz of product to 1,000 ft2) 

2 NS 5.5 lb 

a.i./acre (based on 2 x the max single rate) 

- 25 ft buffer fropermanent water bodies or aquatic habitat - Use is prohibiteafter Dec. 31, 20
Page 33 of 58 

LABEL/EPA 

REG. NO. 

A.I.(S) APP 

METHOD 

USE(S) SINGLE 

MAX. 

APPL. 

RATE 

MAX. NO. 

APP. 

/SEASON 

MIN. APP. 

INTERVAL 

MAX. 

SEASONAL 

APP.RATE 

SPECIFIC 

COMMENT

MSMA 6.6 MSMA Liquid/ 19713-41 

Monosodium acid methanearsonate (51.3%) [total Arsenic as elemental) all in water soluble form is 23.61%] 

Ground Aerial 

Cotton 2 lb a.i./acre 2 (the 

second application is allowed for a salvage operation) 

7-day 4 lb a.i./acre - Postemergence 

not apply preplan- In FL, applicatishould be confinband treatments - 50 ft buffer fropermanent water bodies or aquatic habitat - Do not apply affirst bloom 

Ground Highway 

rights-ofway 

NS 2/yr 

(broadcast) 

NS NS - 100 ft buffer fr

permanent water bodies or aquatic habitat - Use is prohibiteafter Dec. 31, 20

Ground Golf courses 

(newly constructed) (broadcast) 

4 lb a.i./acre (1.8 fl oz of product/ 1,000 ft2) 

1 N/A 4 lb a.i./acre - Use is prohibite

after Dec. 31, 20

Ground Other golf 

courses (spot treatments only) 

4 lb a.i./acre (1.8 fl oz of product/ 1,000 ft2) 

NS NS NS - 100 ft2 per spot 

- Not to exceed 2of the total golf course acreage p- Use is prohibiteafter Dec. 31, 20

Ground Sod farms 

(broadcast) 

4 lb a.i./acre (1.8 fl oz of product/ 1,000 ft2) 

2 NS 8 lb a.i./acre 

(based on 2 x max single rate) 

- 25 ft buffer fropermanent water bodies or aquatic habitat - Use is prohibiteafter Dec. 31, 20
Page 34 of 58 

LABEL/EPA 

REG. NO. 

A.I.(S) APP 

METHOD 

USE(S) SINGLE 

MAX. 

APPL. 

RATE 

MAX. NO. 

APP. 

/SEASON 

MIN. APP. 

INTERVAL 

MAX. 

SEASONAL 

APP.RATE 

SPECIFIC 

COMMENT

MSMA 6 Plus MSMA Liquid Plus Surfactant/ 19713-42 

Monosodium acid methanearsonate (47.6%) [total Arsenic (as elemental), all in water-soluble form, is 22%] 

Aerial Ground 

Cotton 2 lb a.i./acre 2 (the 

second application is allowed for a salvage operation) 

7-day 4 lb a.i./acre - Postplant (befo

cotton emergencand postemergen- Applications in should be confinband treatments - Do not apply affirst bloom 

Ground Highway 

rights-ofway 

4.5 lb a.i./acre (broadcast) [4.1 lb a.i./acre for ‘small areas’] 

2/yr (broadcast) 

NS 9.0 lb 

a.i./acre (broadcast) (based on 2 x max single rate) 

- 100 ft buffer frpermanent water bodies or aquatic habitat - Use is prohibiteafter Dec. 31, 20

Ground Golf courses 

(newly constructed) (broadcast) 

4.0 lb a.i./acre (2 fl oz of product/ 1,000 ft2) 

1 N/A 4.0 lb 

a.i./acre (does not include additional spot treatments) 

- Use is prohibiteafter Dec. 31, 20

Ground Other golf 

courses (spot treatments only) 

4.0 lb a.i./acre (2 fl oz of product/ 1,000 ft2) 

NS NS NS - 100 ft2 per spot 

- Not to exceed 2of the total golf course acreage p- Use is prohibiteafter Dec. 31, 20

Ground Sod farms 

(broadcast) 

4.0 lb a.i./acre (2 fl oz of product/ 1,000 ft2) 

2 NS 8.0 lb 

a.i./acre (broadcast) (based on 2 x max single rate) 

- 25 ft buffer fropermanent water bodies or aquatic habitat - Use is prohibiteafter Dec. 31, 20
Page 35 of 58 

LABEL/EPA 

REG. NO. 

A.I.(S) APP 

METHOD 

USE(S) SINGLE 

MAX. 

APPL. 

RATE 

MAX. NO. 

APP. 

/SEASON 

MIN. APP. 

INTERVAL 

MAX. 

SEASONAL 

APP.RATE 

SPECIFIC 

COMMENT

MSMA 6.6 Liquid/ 19713-269 

51.0% a.i. [Total Arsenic (as elemental) all in water soluble form is 23.61%) 

Aerial Ground 

Cotton 2 lb a.i./acre 2 (the 

second application is allowed for a salvage operation) 

7-day 4.0 lb 

a.i./acre 

- Postplant and postemergence - Application in should be confinband treatments - 50 ft buffer fropermanent water bodies and aquathabitats 

Ground Golf courses 

(newly constructed) (broadcast) 

4.0 lb a.i./acre (1.8 fl oz of product/ 1,000 ft2) 

1 N/A 4.0 lb 

a.i./acre (does not include additional spot treatments) 

- Use is prohibiteafter Dec. 31, 20

Ground Other golf 

courses (spot treatments only) 

4.0 lb a.i./acre (1.8 fl oz of product/ 1,000 ft2) 

NS NS NS - 100 ft2 per spot 

- Not to exceed 2of the total golf course acreage p- Use is prohibiteafter Dec. 31, 20

Ground Sod farms 

(broadcast) 

4.0 lb a.i./acre (1.8 fl oz of product/ 1,000 ft2) 

2 NS 8.0 lb 

a.i./acre (broadcast) (based on 2 x max single rate) 

- 25 ft buffer fropermanent water bodies or aquatic habitat - Use is prohibiteafter Dec. 31, 20

Ground Highway 

rights-ofway 

NS 2/yr 

(broadcast) 

NS NS - 100 ft buffer fr

permanent water bodies and aquathabitats - Use is prohibiteafter Dec. 31, 20
Page 36 of 58 

LABEL/EPA 

REG. NO. 

A.I.(S) APP 

METHOD 

USE(S) SINGLE 

MAX. 

APPL. 

RATE 

MAX. NO. 

APP. 

/SEASON 

MIN. APP. 

INTERVAL 

MAX. 

SEASONAL 

APP.RATE 

SPECIFIC 

COMMENT

Diumate Herbicide/ 19713-528 

Monosodium acid methanearsonate (34.6% a.i.) [Total Arsenic (as elemental) all in water soluble form is 16.00% 

Aerial Ground 

Cotton 2 lb a.i./acre 2/yr (the 

second application is allowed for a salvage operation) 

21-day 4.0 lb 

a.i./acre 

- Do not apply affirst bloom - Applications in should be limited band treatments - 50 ft buffer fropermanent water bodies and aquathabitats 

Diuron (5.18% a.i.i) 

Ground Highway 

rights-ofway 

5 lb a.i./acre (1.25 gallon of product/ acre) 

2/yr 90-day 10 lb a.i/acre 

(based on 2 x max. single rate) 

- 100 ft buffer frpermanent water bodies and aquathabitats - Use is prohibiteafter Dec. 31, 20

MSMA 660 Herbicide/ 

Monosodium acid 

Aerial Ground 

Cotton 2 lb a.i./acre 2/yr (the 

second 

7-day 4.0 lb 

a.i./acre 

- Postemergence applications 
Page 37 of 58 

LABEL/EPA 

REG. NO. 

A.I.(S) APP 

METHOD 

USE(S) SINGLE 

MAX. 

APPL. 

RATE 

MAX. NO. 

APP. 

/SEASON 

MIN. APP. 

INTERVAL 

MAX. 

SEASONAL 

APP.RATE 

SPECIFIC 

COMMENT

19713-531 methanearsonate 

(51.3% a.i.) [Total Arsenic (as elemental) all in water soluble form is 23.61%) 

application is allowed for a salvage operation) 

- Applications in confined to band treatments - 50 ft buffer fropermanent water bodies and aquathabitats 

Ground Golf courses 

(newly constructed) (broadcast) 

4.0 lb a.i./acre (1.8 fl oz of product/ 1,000 ft2) 

1 N/A 4.0 lb 

a.i./acre (does not include additional spot treatments) 

- Use is prohibiteafter Dec. 31, 20

Ground Other golf 

courses (spot treatments only) 

4.0 lb a.i./acre (1.8 fl oz of product/ 1,000 ft2) 

NS NS NS - 100 ft2 per spot 

- Not to exceed 2of the total golf course acreage p- Use is prohibiteafter Dec. 31, 20

Ground Sod farms 

(broadcast) 

4.0 lb a.i./acre (1.8 fl oz of product/ 1,000 ft2) 

2 NS 8.0 lb 

a.i./acre (broadcast) (based on 2 x max single rate) 

- 25 ft buffer fropermanent water bodies or aquatic habitat - Use is prohibiteafter Dec. 31, 20

Ground Highway 

rights-ofway 

NS 2/yr 

(broadcast) 

NS NS - 100 ft buffer fr

permanent water bodies and aquathabitats - Use is prohibiteafter Dec. 31, 20
Page 38 of 58 

LABEL/EPA 

REG. NO. 

A.I.(S) APP 

METHOD 

USE(S) SINGLE 

MAX. 

APPL. 

RATE 

MAX. NO. 

APP. 

/SEASON 

MIN. APP. 

INTERVAL 

MAX. 

SEASONAL 

APP.RATE 

SPECIFIC 

COMMENT

MSMA 120 Arsonate Liquid for Postemergence Weed Control/ 19713-550 

Monosodium acid methanearsonate (52.8% a.i.) [Total Arsenic (as elemental), all in watersoluble form, is 23.7%] 

Aerial Ground 

Cotton 2 lb a.i./acre 2/yr (the 

second application is allowed for a salvage operation) 

7-day 4.0 lb 

a.i./acre 

- Postemergence applications - Applications in confined to band treatments - 50 ft buffer fropermanent water bodies and aquathabitats 

Ground Golf courses 

(newly constructed) (broadcast) 

4.0 lb a.i./acre (1.8 fl oz of product/ 1,000 ft2) 

1 N/A 4.0 lb 

a.i./acre (does not include additional spot treatments) 

- Use is prohibiteafter Dec. 31, 20

Ground Other golf 

courses (spot treatments only) 

4.0 lb a.i./acre (1.8 fl oz of product/ 1,000 ft2) 

NS NS NS - 100 ft2 per spot 

- Not to exceed 2of the total golf course acreage p- Use is prohibiteafter Dec. 31, 20

Ground Sod farms 

(broadcast) 

4.0 lb a.i./acre (1.8 fl oz of product/ 1,000 ft2) 

2 NS 8.0 lb 

a.i./acre (broadcast) (based on 2 x max single rate) 

- 25 ft buffer fropermanent water bodies or aquatic habitat - Use is prohibiteafter Dec. 31, 20

Ground Highway 

rights-ofway 

4.5 lb a.i./acre [4.0 lb a.i./acre (1.8 fl oz of product/ 1,000 ft2) for 

2/yr (broadcast) 

NS 9.0 lb a.i/acre 

(based on 2 x max. single rate) (does not include applications 

- 100 ft buffer frpermanent water bodies and aquathabitats - Use is prohibiteafter Dec. 31, 20
Page 39 of 58 

LABEL/EPA 

REG. NO. 

A.I.(S) APP 

METHOD 

USE(S) SINGLE 

MAX. 

APPL. 

RATE 

MAX. NO. 

APP. 

/SEASON 

MIN. APP. 

INTERVAL 

MAX. 

SEASONAL 

APP.RATE 

SPECIFIC 

COMMENT

‘smaller areas’] 

to ‘smaller areas’) 

TARGET®

6.6/ 42519-1 

Monosodium acid methanearsonate (51.0% a.i.) [Total Arsenic (as elemental) all in water soluble form 23.61%] 

Aerial Ground 

Cotton 2 lb a.i./acre 2/yr (the 

second application is allowed for a salvage operation) 

7-day 4.0 lb 

a.i./acre 

- Postemergence - 50 ft buffer fropermanent water bodies and aquathabitats 

Ground Golf courses 

(newly constructed) (broadcast) 

2.24 lb a.i./acre (1 fl oz of product/ 1,000 ft2) 

1 N/A 2.24 lb 

a.i./acre (does not include additional spot treatments) 

- Use is prohibiteafter Dec. 31, 20

Ground Other golf 

courses (spot treatments only) 

2.24 lb a.i./acre (1 fl oz of product/ 1,000 ft2) 

NS NS NS - 100 ft2 per spot 

- Not to exceed 2of the total golf course acreage p- Use is prohibiteafter Dec. 31, 20

Ground Sod farms 

(broadcast) 

2.24 lb a.i./acre (1 fl oz of product/ 1,000 ft2) 

2 10-day 4.5 lb 

a.i./acre (broadcast) (based on 2 x max single rate) 

- 25 ft buffer fropermanent water bodies or aquatic habitat - Use is prohibiteafter Dec. 31, 20

Ground Highway 

rights-ofway 

NS 2/yr 

(broadcast) 

NS NS - 100 ft buffer fr

permanent water bodies and aquathabitats - Use is prohibiteafter Dec. 31, 20
Page 40 of 58 

LABEL/EPA 

REG. NO. 

A.I.(S) APP 

METHOD 

USE(S) SINGLE 

MAX. 

APPL. 

RATE 

MAX. NO. 

APP. 

/SEASON 

MIN. APP. 

INTERVAL 

MAX. 

SEASONAL 

APP.RATE 

SPECIFIC 

COMMENT

TARGET® 6 

Plus/ 42519-3 

Monosodium acid methanearsonate (48.3% a.i.) [Total arsenic (as elemental) all in water soluble form 22.3%] 

Aerial Ground 

2 lb a.i./acre 2/yr (the 

second application is allowed for a salvage operation) 

7-day 4.0 lb 

a.i./acre 

- Postemergence only - 50 ft buffer from permanent water bodies and aquatic habitats 

- Postemergence 

Ground Golf courses 

(newly constructed) (broadcast) 

4 lb a.i./acre (2 fl oz of product/ 1,000 ft2) 

1 N/A 4 lb a.i./acre 

(does not include additional spot treatments) 

- Use is prohibiteafter Dec. 31, 20

Ground Other golf 

courses (spot treatments only) 

4 lb a.i./acre (2 fl oz of product/ 1,000 ft2) 

NS NS NS - 100 ft2 per spot 

- Not to exceed 2of the total golf course acreage p- Use is prohibiteafter Dec. 31, 20

Ground Sod farms 

(broadcast) 

4lb a.i./acre (1 fl oz of product/ 1,000 ft2) 

2 NS 8 lb a.i./acre 

(broadcast) (based on 2 x max single rate) 

- 25 ft buffer fropermanent water bodies or aquatic habitat - Use is prohibiteafter Dec. 31, 20

Ground Highway 

rights-ofway 

4.5 lb a.i./acre 

2/yr (broadcast) 

10 9 lb a.i./acre 

(broadcast) (based on 2 x max single rate) 

- 100 ft buffer frpermanent water bodies and aquathabitats - Use is prohibiteafter Dec. 31, 20
Page 41 of 58 

APPENDIX C: BEAD Chemical Profile for MSMA:
Page 42 of 58 
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Page 50 of 58 

Appendix D. Chemical Equilibria of As 

The speciation of As can be roughly divided into three main forms: soluble methylated 

species, soluble inorganic species, and minerals. Within each As form, the valence state 

of As can be As5+ and As3+ depending on redox conditions. The As forms are discussed 

in the context of chemical equilibria in the As biogeochemical cycle. Chemical equilibria 

analysis is based on thermodynamic data from Sadiq et al., 1982, Sadiq, 199X and 

Dombrowski, et al., 2005. 

As Minerals 

Arsenic minerals (As) can form as oxides (e.g., As2O3,As2O5), sulfides (AsS, As2S2, and 

As2S3), and with various metals minerals(AlAsO4, Ca3(As04)2, Cd(AsO4)2, Cu(AsO4)2, 

FeAsO4, Mn(AsO4)2, Ni(AsO4)2, Pb3(AsO4)2, and Zn(AsO4)2 (Sadiq et al., 1982). The 

solubility of each mineral is dependent on numerous factors including redox condition, 

counter ion chemistry, and pH. The least soluble (i.e., most stable) arsenate minerals 

were Ca3(As04)2 in alkaline and neutral environments and Mn(AsO4)2 in acid 

environments (Figure 2). For the other metal arsenate (V) minerals and oxides, the 

mineral solubility from highest to lowest is As2O5 > Cd(AsO4)2> Pb3(AsO4)2> Cu(AsO4)2>

AlAsO4> FeAsO4> Zn(AsO4)2> Ni(AsO4)2.

pH

456789

Log (HAsO42-)

-10

-8

-6

-4

-2

0

2

 Ca3(AsO4)2-Calcite 

AlAsO4-amorp.- Al(OH)3 

 AlAsO4-gibbsite 

FeAsO4-ferrihydrite 

 FeAsO4-geothite 

 Zn3(AsO4)2-Soil Zn 

Cu3(AsO4)2-Soil Cu 

Figure 2: Arsenate Mineral Stability Diagram in Oxic Conditions (pe+pH=16) 

 
Page 51 of 58 

Table 7: Chemical Equilibria Equations for As Minerals as a Function of pH.

Mineral Equilibria Equilibrium Equation 

Ca3(AsO4)

2

↔CaCO3 Log(HAsO4

2-)=15.565-1.5pCO2+2pH 

Ca3(AsO4)2↔Soil Ca Log (HAsO4

2- )= 2.795-pH 

AlAsO4↔amorp Al(OH)3 Log(HAsO4

2

)= -14.26+2pH 

AlAsO4↔ð-Al(OH)3 Log(HAsO4

2

)= -13.1+2pH 

FeAsO4↔amorp Fe(OH)3 Log(HAsO4

2

)= -12.13+2pH 

FeAsO4↔ Fe(OH)3 Log(HAsO4

2

)= -8.57+2pH 

Fe3(AsO4)2↔FeCO3 Log(HAsO4

2

)=12.96-2pH+3/2 pCO2

Fe3(AsO4)2↔Fe2SiO4 Log(HAsO42-)=-23.82+log(H4SiO4)+2pH 

Zn3(AsO4)2↔Soil Zn Log(HAsO4

2

)= -12.8+2pH 

Cu3(AsO4)2↔Soil Cu Log(HAsO4

2

)= -11.68+2pH 

An important aspect of As chemistry is the effect of redox in controlling As mineral 

stability. Arsenite (III) sulfide minerals are predicted to be the most stable arsenic 

mineral at suboxic to anoxic redox potentials (pe+pH≤7) (Figure 3). At higher redox 

potentials (pe+pH>7),Ca3(As04)2 is predicted to be the most stable arsenate (V) mineral. 

pe+pH

0 5 10 15 20

log (HAsO42-) -20

-15

-10

-5

0

 Arsinolite (As203) 

 Claudenite(As2O3) 

 Realger (AsS) 

 Orpiment (As2S2) 

 As2S3

 Ca3(As04)2

Figure 3: Arsenate Mineral Stability Diagram as a Function of Redox Conditions (pe+pH) 

Table 8. Chemical Equilibria Equations for As Minerals as a Function of Redox Potential. 

Mineral Equilibria Equilibrium Equation 

As203↔ HAsO4

2

 Log(HAsO4

2-)= -29.325+2pH+ 2(pe+pH) 

AsH3(g)↔ HAsO4

2- Log(HAsO42-)= -28.92+2pH+ 2(pe+pH) 

AsS↔HAsO4

2- + SO42- Log(HAsO42-)= -83.13-log(SO42-)+4pH+11(pe+pH) 

As2S2↔ HAsO4

2- + SO42- Log(HAsO42-)= -90.21-log(SO42-)+0.5pH+11(pe+pH) 

As2S3↔ HAsO4

2- + SO42- Log(HAsO42-)= -109.57-1.5 log(SO42-)+ 4pH + 15 (pe+pH) 

Cu3(AsO4)↔CuFe204 +Fe3O4 Log(HAsO4

2-)= -26.1+2pH+(pe+pH) 
Page 52 of 58 

Cu3(AsO4)↔CuFe204 +Fe3(OH)8 Log(HAsO4

2-)= -18.05+2pH+(pe+pH) 

Mn(AsO4)2↔ß-MnO2 Log(HAsO4

2-)= -58.58+2pH+3(pe+pH) 

Mn(AsO4)2↔ ð-MnO1.9 Log(HAsO4

2-)= -23.73+ 0.9(pe+pH) 

Mn(AsO4)2↔ ð-MnO1.8 Log(HAsO4

2-)= -22.24+ 0.8(pe+pH) 

FeAsO4↔ Amorp Fe(OH)3 Log(HAsO4

2-)= -12.9+2pH 

FeAsO4↔ Geothite ά-FeOOH Log(HAsO4

2-)= -9.43+2pH 

Fe3(AsO4)2↔ Amorp Fe(OH)3 Log(HAsO4

2-)= -33.74+2pH+1.5(pe+pH) 

Fe3(AsO4)2↔ Geothite ά-FeOOH Log(HAsO4

2-)= -28.53+2pH+1.5(pe+pH) 

Soluble Inorganic As Species 

The soluble As species is a mixture of arsenic acid (H3AsO4) ions, arsenious acid 

(As(OH)3) ions, and halides of As (Sadiq, et al., 1982). In oxic environments 

(pe+pH>8), the predominate arsenic acid species are H2AsO4

-

 and HAsO4

2- in 

environmental pH range of 5 to 8. Under suboxic and anoxic conditions (pe+pH<8), the 

predominant inorganic arsenious acid species are HAsO2

o

 and H3AsO3

o

. Arsine gas 

(AsH3) is only a predominate species in low redox (pe+pH<0.5) and acidic environments. 

Table 9. Chemical Equilibria Equations for Inorganic As Species. 

 Equilibria Equilibrium Equation 

 AsBr3(g)↔HAsO4

2- Log(AsBr3(g)) =14.32 + log(HAsO42-)+3log(Br-)-5pH-2(pe+pH) 

 AsCl3(g)↔HAsO4

2- Log(AsCl3(g)) =15.24 + log(HAsO42-)+3log(Cl-)-5pH-2(pe+pH) 

 AsF3(g)↔HAsO4

2- Log(AsF3(g)) =50.44 + log(HAsO42-)+3log(F-)-5pH-2(pe+pH) 

AsH3(g)↔ HAsO4

2

 Log(AsH3)=28.92+log(HAsO4

2-)-2pH-8(pe+pH) 

AsO2

-

↔ HAsO4

2- Log(AsO2-

)=19.32+log(HAsO4

2-)-pH-2(pe+pH) 

AsO3

3-↔ HAsO42- Log(AsO33-)=6.13+log(HAsO42-)-pH-(pe+pH) 

HAsO3

2-↔ HAsO42- Log(HAsO33-)=2.27+log(HAsO42-)-2(pe+pH) 

H2AsO3

-

↔ HAsO4

2- Log(H2AsO3-

)=19.40+log(HAsO4

2-)-pH-2(pe+pH) 

H3AsO3↔ HAsO4

2- Log(H3AsO3)=-28.63+log(HAsO42-)-2pH-2(pe+pH) 

AsO4

3-↔ HAsO4

2- Log(AsO43- )= 11.50+log(HAsO42-)+pH 

HAsO2↔ HAsO4

2- Log(HAsO2)=28.60-2(pe+pH)-2pH+log(HAsO42-) 

H3AsO4↔ HAsO4

2- Log(H3AsO4)=-9.28+log(HAsO42-)-2pH 

H2AsO4

-

↔ HAsO4

2- Log(H2AsO4-

)=-6.94+ log(HAsO4

2-)-pH 

Soluble Methylated As Species 

The methylation of As has been described using the Challenger pathway (Bentley and 

Chasteen, 2002) (Figure 4; Table 10). This pathway is a sequential series of two electron 

reductions of arsenate (V) to arsenite (III) which are linked to a methylation-oxidation 

reaction forming a sequential methylarsenate (V) species. The reductants in the pathway 

are thiols, glutathione (GSH), and lipoic acid. The methyl donor in the pathway is Sadenosylmethionine (SAM). The methylation process of arsenic has been identified in 

mammals, fungi, and bacteria. 
Page 53 of 58 

Figure 4: Proposed Challenger pathway for As Methylation. 

Domrowski et al., 2005 used chemical equilibria to predict the distribution of methylated 

As species in human urine. Equilibrium constants (log Ko) for methylated arsenic species 

were estimated from the standard free energy change of the products and reactants in 

their standard states (25o

C). Methane concentrations (CH4) were assumed to be 

controlled by equilibria with CO2. Chemical equilibria reactions for methylated As 

species in Table 10 correspond to chemical reaction numbers in Figure 5. 

Table 10. Chemical Equilibria Equations for Methylated As Species.

Reaction Equilbria Chemical Reaction 

1+2 As5+↔MMA(V) Log[MMA(V)]=53.44+log(HAsO4

2-)-pCO2-2pH-10(pe+pH) 

3 MMA(V) ↔MMA(III) Log[MMA(III)]=64.86+log(HAsO4

2-)-pCO2-2pH-12(pe+pH) 

4 MMA(III) ↔DMA(V) Log[DMA(V)]=106.35+log(HAsO4

2-)-2pCO2-pH-20(pe+pH) 

5 DMA(V) ↔DMA(III) Log[DMA(III)]=115.90+log(HAsO4

2-)-2pCO2-pH-22(pe+pH) 

6 DMA(III) ↔TMA(V) Log(TMA(V)]=122.19+log(HAsO4

2-)-3pCO2-32(pe+pH) 

7 TMA(V) ↔TMA(III) Log(TMA(V)]=133.29+log(HAsO4

2-)-34(pe+pH) 

Using the chemical equilibria equations in Tables 9 and 10, a mole fraction diagram was 

developed to show their predominance of As species as a function of redox potential 

(pe+pH) at a specific pH (Figure 5). 
Page 54 of 58 

pe+pH

0 5 10 15 20

log (mole fraction)

-20

-15

-10

-5

0

 MMA(V) 

 MMA(III) 

DMA(V) 

 DMA(III) 

 TMA(V) 

TMA(III) 

 AsBr3(g) 

 AsBr3(g) 

 AsF3(g) 

 AsH3 

 AsO2- 

HAsO32- 

 H2AsO3- 

H3AsO3 

 AsO43- 

HAsO2 

 H3AsO4 

 H2AsO4- 

 HAsO42- 

Figure 5: Mole Fraction Diagram (pH=6) for Soluble Methylated As and Inorganic As Species 

APPENDIX E. Chemical Equilibria Model for As Speciation.

A chemical equilibria model was developed to predict the concentration of individual As 

species in surface water. Chemical equilibrium constants were derived from Sadiq, 1983 

and Domrowski et al., 2005. The chemical equilibria model is based on estimating the 

concentration of individual As species from a total soluble As concentration (expressed 

as HAsO4

 2-) and general water quality data for surface water. The total soluble As 

concentration is equal to the summation of molar concentrations of individual As species 

(Equation 1).

Equation 1: Distribution of Individual As Species 

 

[Total As]= [HAsO4

2-]+ [H2AsO4-

]+ [AsO4

3-]+[H3AsO3]+[H2AsO3-

]+[HAsO3

2-]+ 

[AsO3

3-]+ [HAsO2]+[AsO2-

]+ [AsH3(g)]+ [AsF3(g)]+ [AsCl3(g)]+ AsBr3(g)] 

+[MMA(III)]+[DMA(III)]+[TMA(III)] +[MMA(V)]+[DMA(V)]+[TMA(V)] 

The concentrations of the individual As species in Equation 1 can be expressed in terms 

of chemical activities (or effective concentration) using Equation 2. 

Equation 2: Chemical Activity Equations 

[Concentration]*δ (activity coefficient)= (chemical activity) 

or 

[Concentration] = (chemical activity)/ δ (activity coefficient) 

The concentration of each As species can be calculated from chemical activities using 
Page 55 of 58 

Equation 3. 

Equation 3: Chemical Concentrations in Terms of Chemical Activities 

[Total As]= (HAsO4

2-)/δHAsO42- + (H2AsO4-

)/δH2AsO4- + (AsO4

3- )/δAsO43- + (H3AsO3)/ 

δH3AsO3 + (H2AsO3

- )/δ H2AsO3- + (HAsO32-) /δ HAsO32- + (AsO33-)/δAsO33- + (HAsO2)/δHAsO2

+ (AsO2

- )/δAsO2- + (AsH3(g))/δAsH3 + (AsF(g))/δAsF3 + (AsCl3(g))/δAsCl3(g) + 

(AsBr3(g))/δAsBr3(g) +(MMA(III))/δMMA(III) + (DMA(III))/δ DMA(III)+ ( TMA(III))/δTMA(III) + 

(MMA(V))/δMMA(V) + (DMA(V))/δDMA(V)+ (TMA(V))/δTMA(V)

The activity for each species is calculated using the equilibrium equations shown in 

Tables 1. The ionic strength is calculated using the concentrations of Na+

, Mg2+, Ca2+ , 

K+, SO4

2-, and Cl- from surface water quality data. Charge balance was maintained by 

ensuring the molar concentration*valence for anions and cations are equal. Any 

difference in charge balance was adjusted by additional concentration of a monovalent 

ion (eg., NO3

-

). The ionic strength was calculated using Equation 4. 

Equation 4: Ionic Strength 

μ=1/2∑CiZi

2

where: Ci= molar concentration of ion 

 Zi=valance of ion 

 μ=ion strength 

 

The Davies Equation was used to estimate the activity coefficient (δ) for each As species 

(Equation 5). 

Equation 5: Davies Activity Coefficient (δi) 

Log δi=-AZi2

(μ1/2/1+μ 1/2 – 0.3μ) 

where: δi= activity coefficient 

 A= constant at 25o

C 

 Zi=valance of ion 

 μ=ion strength (calculated in Equation 4) 

Tables 1: Chemical Equilibria Equations for Inorganic As Species 

 Equilibria Equilibrium Equation 

 AsBr3(g)↔HAsO4

2- Log(AsBr3(g)) =14.32 + log(HAsO42-)+3log(Br-)-5pH-2(pe+pH) 

 AsCl3(g)↔HAsO4

2- Log(AsCl3(g)) =15.24 + log(HAsO42-)+3log(Cl-)-5pH-2(pe+pH) 

 AsF3(g)↔HAsO4

2- Log(AsF3(g)) =50.44 + log(HAsO42-)+3log(F-)-5pH-2(pe+pH) 

AsH3(g)↔ HAsO4

2

 Log(AsH3)=28.92+log(HAsO4

2-)-2pH-8(pe+pH) 

AsO2

-

↔ HAsO4

2- Log(AsO2-

)=19.32+log(HAsO4

2-)-pH-2(pe+pH) 
Page 56 of 58 

AsO3

3-↔ HAsO42- Log(AsO33-)=6.13+log(HAsO42-)-pH-(pe+pH) 

HAsO3

2-↔ HAsO42- Log(HAsO33-)=2.27+log(HAsO42-)-2(pe+pH) 

H2AsO3

-

↔ HAsO4

2- Log(H2AsO3-

)=19.40+log(HAsO4

2-)-pH-2(pe+pH) 

H3AsO3↔ HAsO4

2- Log(H3AsO3)=-28.63+log(HAsO42-)-2pH-2(pe+pH) 

AsO4

3-↔ HAsO4

2- Log(AsO43- )= 11.50+log(HAsO42-)+pH 

HAsO2↔ HAsO4

2- Log(HAsO2)=28.60-2(pe+pH)-2pH+log(HAsO42-) 

H3AsO4↔ HAsO4

2- Log(H3AsO4)=-9.28+log(HAsO42-)-2pH 

H2AsO4

-

↔ HAsO4

2- Log(H2AsO4-

)=-6.94+ log(HAsO4

2-)-pH 

As5+↔MMA(V) Log[MMA(V)]=53.44+log(HAsO4

2-)-pCO2-2pH-10(pe+pH) 

MMA(V) ↔MMA(III) Log[MMA(III)]=64.86+log(HAsO4

2-)-pCO2-2pH-12(pe+pH) 

MMA(III) ↔DMA(V) Log[DMA(V)]=106.35+log(HAsO4

2-)-2pCO2-pH-20(pe+pH) 

DMA(V) ↔DMA(III) Log[DMA(III)]=115.90+log(HAsO4

2-)-2pCO2-pH-22(pe+pH) 

DMA(III) ↔TMA(V) Log(TMA(V)]=122.19+log(HAsO4

2-)-3pCO2-32(pe+pH) 

TMA(V) ↔TMA(III) Log(TMA(V)]=133.29+log(HAsO4

2-)-34(pe+pH) 

The concentration of each As species was estimated using the following equation. 

Equation 6: As Species Concentration Estimation 

[As Species]= [Total As Concentration in Water moles/liter]*MF(As species)/δarsenic species 

 
Page 57 of 58 

APPENDIX E. Results of the Screening Imbibation Program (SIP v. 1.0).

The highest solubility limit for calcium arsenate (the stable mineral phase arsenical 

formed under most sub-oxic and oxic conditions) (between pH 5 and 9) was used since 

the solubility limit is pH-dependent (solubility = 467.3 mg As+5/L). 

For the mammalian and avian toxicity endpoints, the most sensitive acute and chronic 

endpoints from all of the inorganic arsenicals were used. For mammals, an acute 

endpoint for arsenic acid (LD50 = 52 mg a.i./kg-bw, rat; MRID 0026356) and a chronic 

endpoint for arsenic acid (NOAEL = 32 mg a.i./kg-bw, mouse; from the chromated 

arsenicals RED) were used for the modeling. For birds, an acute endpoint for arsenic 

acid (LD50 = 28.9 mg a.i./kg-bw, bobwhite quail; MRID 40409013) was used. A chronic 

endpoint for birds and inorganic arsenicals is not currently available. 

Table 1. Inputs 

Parameter Value 

Chemical name Arsenicals (Calcium Arsenate) 

Solubility (in water at 25oC; mg/L) 467.3 

 

Mammalian LD50 (mg/kg-bw) 52 

Mammalian test species laboratory rat 

 

Mammalian NOAEL (mg/kg-bw) 32 

Mammalian test species other 

Body weight (g) of "other" mammalian species 20 

 

Avian LD50 (mg/kg-bw) 28.9 

Avian test species northern bobwhite quail 

Mineau scaling factor 1.15 

 

Table 2. Mammalian Results 

Parameter Acute Chronic 

Upper bound exposure (mg/kg-bw) 80.3756 80.3756 

Adjusted toxicity value (mg/kg-bw) 39.9963 12.0339 

Ratio of exposure to toxicity 2.0096 6.6791 

Conclusion* 

Exposure through drinking 

water alone is a potential 

concern for mammals 

Exposure through drinking 

water alone is a potential 

concern for mammals 

 

 
Page 58 of 58 

Table 3. Avian Results 

Parameter Acute Chronic 

Upper bound exposure (mg/kg-bw) 378.5130 378.5130 

Adjusted toxicity value (mg/kg-bw) 20.8204 0.0000 

Ratio of exposure to acute toxicity 18.1799 0.0000 

Conclusion* 

Exposure through drinking 

water alone is a potential 

concern for birds 

Due to insufficient data, risk 

cannot be precluded 

 

*Conclusion is for drinking water exposure alone. This does not combine all routes of exposure. Therefore, when 

aggregated with other routes (i.e., diet, inhalation, dermal), pesticide exposure through drinking water may 

contribute to a total exposure that has potential for effects to non-target animals.