24 September 2001

RE-REGISTRATION ELIGIBILITY DOCUMENT 

FOR

SODIUM AND CALCIUM SALTS OF METHANEARSONIC ACID

(MSMA/DSMA/CAMA)
TABLE OF CONTENTS

Page

List of Tables 4

Ecological Risk Conclusions 5

1. Terrestrial Risk Conclusions 5

2. Aquatic Risk Conclusions 5

Environmental Fate and Drinking Water Conclusions 6

1. Environmental Fate Conclusions 6

2. Drinking Water Conclusions 7

Introduction 9

1. Usage 9

2. Background Environmental Concentrations 9

3. Arsenic Environmental Chemistry 10

4. General Toxicology 11

Risk Characterization 12

1. Terrestrial Risk 12

a. mammals and birds 12

b. terrestrial plants 14

c. summary of terrestrial risk 15

2. Aquatic Risk 15

a. summary of aquatic risk 16

3. Endangered Species 16

Environmental Fate Assessment: Perspective, Basis, and Summary 21

1. Perspective and Basis 21

2. Summary 24

3. Chemical Structures and Physicochemical Properties 25

4. Transformation 27

5. Metabolites 28

6. Bioconcentration 29

7. Transport/Mobility 29

Drinking Water Assessment 30

1. Groundwater Exposure Conclusions 31

2. Surface Water Exposure 32

3. Index Drinking Water Reservoir Simulation Results 35

4. Surface Water Exposure Conclusions 36
Aquatic Exposure and Risk Assessment 37

1. Exposure Assessment 37

2. Hazard Assessment 38

a. freshwater animals 38

b. marine/estuarine animals 38

c. plants 38

3. Risk Assessment 39

a. animals 39

b. plants 39

Terrestrial Exposure and Risk Assessment 39

1. Exposure Assessment 39

a. food items 39

b. off-site plant 39

2. Hazard Assessment 40

a. birds 40

b. mammals 40

c. insects 41

d. non-target plants (Tier II) 41

3. Risk Assessment 41

a. birds 41

b. mammals 42

c. non-endangered plants 42

d. endangered species 42

Appendices

Appendix 1. Toxicity, EEC, and risk quotient tables 44

Appendix 2. Definitions and explanations of terms and acronyms 62

Appendix 3. Formulas for off-site terrestrial plant exposure 64

Appendix 4. Estimating residues on terrestrial food items 65

Appendix 5. Cacodylic acid drinking water assessment 66

Appendix 6. Index reservoir drinking water concentrations 71

Appendix 7. GENEEC model/output tables for aquatic ecological assessment 76

Appendix 8. Data requirements tables 77

Appendix 9. References 83

Appendix 10. Arsenic concentrations in US ground waters, USGS map 86
LIST OF TABLES

Table # Page #

1 Acute toxicities to mammals 45

2 EECs on terrestrial food items 46

3 Risk quotients for mammals 47

4 Risk quotients for avian species 48

5 Acute toxicities to avian species 49

6 EECs at off-site terrestrial environments 50

7 Risk quotients for non-endangered terrestrial plants 51

8 Risk quotients for endangered terrestrial plants 52

9 Acute toxicities to freshwater aquatic fish and invertebrates 53

10 Risk quotients for aquatic fish and invertebrates 54

11 Acute toxicities to estuarine/marine aquatic fish and invertebrates 55

12 Risk quotients for non-endangered aquatic plants 56

13 EECs in aquatic environment 57

14 Acute toxicities to aquatic plants 58

15 Acute toxicities to honey bees 59

16 Acute toxicities to non-target terrestrial plants (Tier II) 60

17 Chronic risk quotients for small mammals 61
5

ENVIRONMENTAL RISK CONCLUSIONS

Disodium methanearsenate (DSMA), monosodium methanearsenate (MSMA), and calcium

methanearsenate (CAMA) are used as contact herbicides on a wide variety of predominantly

agricultural, but also non-agricultural sites. Approximately four times as much MSMA (4,333,000 lbs

a.i. weighted avg) is applied annually in the United States as compared to DSMA (1,077,000 lbs a.i.

weighted avg) with the predominant usage for both being on cotton (QUA case no. 2395, PC code

nos. 13803 and 13802). Very little information was available relating to CAMA usage or application

rates other than it is used for residential crabgrass control (QUA case no. 2395, PC code no. 13806).

Terrestrial Risk Conclusions

Acute risk exists within the terrestrial environment near the site of application, for a short period after

application, due to the high use rates for these chemicals at certain sites. Toxicity classifications of the

chemicals to birds and mammals range from moderately toxic to practically non-toxic, indicating that the

inherent toxicity of these chemicals to animals is not great. However, the use rates are high enough to

lead to exceedences of acute risk levels of concern (LOC). For example, MSMA is applied to

orchards up to three times, each at a rate as high as 15.5 lbs a.i./A, leading to estimated environmental

concentrations (EECs) on grasses that exceed the acute risk LOC by 12 times for mammalian

herbivores. Use on cotton may result in EECs on grasses that exceed the acute risk by as much as 1.8

times for mammalian herbivores. Therefore, the potential risk of these chemicals in the terrestrial

environment is related mostly to their high use rates.

Chronic risk for these chemicals in the terrestrial environment is uncertain. Modeling predicts the

residues to be present on food items for up to several months, but the persistence of the food items may

not be long enough to present residues to organisms over that period of time. Chronic reproductive

studies with methylarsenate, a component of all three herbicides, indicate that they are chronically toxic

to mammals. As such, they may also be chronically toxic to avian species. Unfortunately, no chronic

avian toxicity studies were submitted. Their submission is required.

Aquatic Risk Conclusions

There does not appear to be considerable acute risk to aquatic organisms. Acute toxicity studies

revealed that the three herbicides are considered to be slightly toxic to practically non-toxic to aquatic

organisms. Combining this with the modeled aquatic EECs revealed that no acute risk LOCs were

exceeded.

Chronic risk is uncertain. These herbicides are heavily applied in the environment meaning that their

continuous low level release into the aquatic environment is possible, especially near sites of repeated

applications. As such aquatic organisms would be chronically exposed to these chemicals. 

Unfortunately, no chronic aquatic toxicity studies were submitted. Their submission is required.
6

ENVIRONMENTAL FATE AND DRINKING WATER CONCLUSIONS

Environmental Fate Conclusions

As explained in special context in the Environmental Fate Assessment Section in the main body of this

document, formal data requirements for MSMA/DSMA/CAMA are satisfied at this time, except for 1)

submission and EPA laboratory validation of analytical chemical methods for detection and

measurement in water and soil and 2) field spray-drift and droplet-size spectral data for these

herbicides.

Results of the environmental fate assessment were used integrally to estimate exposures for the

preceding ecological risk characterizations and conclusions. The environmental fate of arsenic

compounds is complex, and entwined as part of a global biogeochemical cycle. Rather than

attempt out-of-context and oversimplified conclusions here, the reader is directed to the

Environmental Fate Assessment Section proper, which begins with a perspective and basis,

followed by a “boxed” abstract of the assessment which then follows. However, the potential for

long-term accumulation of arsenical residues at sites receiving repeated applications over many years

bears repeating here.

These arsenicals, in combination with their transformation products, including the pesticide cacodylic

acid as well as other natural arsenic containing species with known toxicities, maintain the total,

immutable arsenic environmental load. That is, arsenic from the pesticides is not lost, but redistributed

and transformed (recycled) throughout the environment (plants, animals, air, soil, sediment, water) into

other arsenic containing substances. It is also crucial not to confuse transformation with detoxification. 

Currently, the Agency has a limit of 43 parts per million (ppm) of total (unspeciated) arsenic in soil as

the concentration beyond which remediation is necessary (for example, at Superfund sites impacted by

various sources of arsenic and currently on The American University campus in Washington, DC,

where World War I chemical warfare agents containing the poisonous gas lewisite (ClCHCHAsCl2)

were tested and disposed).

As an example of potential accumulation, let us take MSMA at a constant, annual application rate of

one pound per acre (all actual MSMA rates are higher). Without redistribution of arsenic from soil

which originally may have contained only a few parts per million of natural arsenic at the site of

application, the regulatory total arsenic limit would be reached in approximately 190 years. At the

maximum cotton use rate, the limit would be reached in around 30 years; at the maximum orchard use

rate, in only 3 years. However, since we know that global redistribution, transformation, and recycling

of arsenic in all possible environmental compartments occurs in the guise of different arsenical

compounds such as those given in the environmental fate assessment, different maximum concentrations

would be reached at indefinite future times. This would depend on local quasi-equilibrium conditions

for arsenic chemistry and entwined environmental factors (biogeochemistry). Thus, although there is no

doubt that local arsenic concentrations will be elevated, the degree of long-term or chronic exposure
7

and potential effects are largely unknown from the standpoint of our assessment. This uncertainty

suggests the need for systematic, long-term field monitoring for potential buildup of total arsenic

residues.

Drinking Water Conclusions for Humans

MSMA, DSMA, CAMA, and cacodylic acid are four of several arsenic containing pesticides which

the Agency is considering for re-registration eligibility. Ultimately, in another action, the Office of

Pesticide Programs (OPP) will assess the potential overall (aggregate, cumulative) drinking water

exposure to these compounds and common transformation products. The Agency’s Office of Water

(OW) is scheduled to issue a final regulation on the larger issue of “arsenic” in drinking water during the

year 2001.

We conclude from monitoring and modeling that surface drinking water concentrations originating from

use of these salts of methanearsonic acid will impose an additive, localized burden on background

concentrations of these and other chemical species of arsenic, including cacodylic acid. Ground water

should not be impacted incrementally.

Based on our surface water assessment, application of arsenical pesticides will result in a superposition

upon existing local, “non-pesticidal” (anthropogenic and natural) background concentrations of arsenic

species in surface drinking water. In some locations, these incremental amounts caused by pesticidal

application would be relatively small compared to some of the existing concentrations. In other

locations where the background is low, the superposition could be relatively large. Provided

repeated, long-term pesticidal applications at the higher labeled rates do not occur, it is most

likely that local biogeochemistry of arsenic (redistribution, transformation, recycling) would moderate or

prevent significant alteration in local, long-term arsenic equilibria. Therefore, provided local

biogeochemistry is not overwhelmed by excessive application of arsenical pesticides (or arsenic

compounds from other human activities), local, long-term average concentrations of available arsenic

should remain relatively unaffected.

Whatever the sources of arsenic, natural or otherwise, if there are toxicological concerns near the

monitored, modeled, and/or regulatory limit concentrations, then, clearly, this invites broader scale,

systematic environmental sampling and analysis for speciated arsenic, not just total arsenic, in order to

determine statistically the associations, ranges of exposure, and possible effects. Adequate analytical

methods for speciating arsenic are now readily available.

The effects of water treatment on raw water concentrations are known, and should be considered for

“public water systems” (as defined in 40 CFR 141.2). However, it also should be acknowledged that

many users of surface drinking water do not drink from regulated public water systems. According to

the USEPA Office of Drinking Water, which is currently engaged in new regulation for total arsenic in

drinking water in public water systems, many larger water treatment facilities use sedimentation-
8

flocculation procedures that are capable of lowering total arsenic to 1 ppb (part per billion) or less. 

However, in smaller public water systems, the method is less effective, and can lower concentrations to

only approximately 20-25 ppb. At the present time, public water systems only assure protection

against excursions of total arsenic beyond the regulatory 50 ppb MCL (maximum contaminant level)

(10 ppb if pending regulation is enacted). Furthermore, within the regulatory limit for total arsenic in

drinking water are inorganic forms that are most often in higher concentrations and generally accepted

to be far more toxic than the organic.

Determining the kind (chemical speciation) and degree of acceptable arsenic exposure in drinking water

involves many regulatory considerations (for example, cost/benefit) representing complex issues for

EPA Program Offices. However, the Office of Drinking Water, after extensive project funding,

protracted deliberations with expert panels, practical considerations, and public/stakeholder input, has

set the regulatory limit for public water systems (which are not inclusive of all drinking water sources) at

50 ppb (10 ppb if current proposed regulation is enacted) for total arsenic (which includes organic and

inorganic forms), even if that limit should happen to exceed the Office of Pesticide’s level of concern for

arsenical pesticides or their by-products.
9

INTRODUCTION

While some arsenicals are present naturally within the environment, continued heavy use of the arsenical

herbicides could present a risk in use areas. Indeed, both inorganic and organic arsenicals have been

shown to have deleterious toxic consequences within an organism, among which are those leading to

cancer. Therefore, while the arsenicals may be naturally present, they may occur at concentrations

which can lead to harm or death.

Usage 

Monosodium methanearsenate (MSMA), disodium methanearsenate (DSMA), and calcium

methanearsenate (CAMA) are contact herbicides used in a wide variety of agricultural and nonagricultural applications. The major application is cotton with over 93% of total usage. As a result, the

southeastern US is the major area of use including Georgia, Arkansas, Mississippi, Louisiana, North

Carolina, and Texas. Other uses for arsenicals include sugarcane, sod farms, corn, wheat, pasture and

others too numerous to list. Orchards have the highest use rate, with as many as three applications

annually, each at 15.5 lb a.i./acre (annual total of 46.5 lb/acre). All totaled, these other uses account

for approximately 5% of the total national usage.

With reference to cotton, MSMA and DSMA are generally applied to a field multiple times during a

single growing season. MSMA is a pre/post-emergent herbicide applied before or sometime after

cotton emergence. Prior to harvest, DSMA may be applied along with another arsenical not addressed

in this risk assessment, cacodylic acid, to defoliate cotton. While applied as different product names

initially, MSMA and DSMA end up as the same chemical in the environment, monomethylarsenate

(MMA), also known as methanearsonic acid (MAA). Therefore, while the arsenical herbicides are

different formulations and are applied separately, in essence, when they are applied in the field, MAA is

applied each time leading to potential arsenical accumulation in the field.

CAMA is a minor use contact herbicide applied to control crabgrass in residential areas. Its total usage

accounts for less than 1% of total arsenical usage nationally.

Background Environmental Concentrations

Arsenic presence is widespread within the environment. The primary natural source of arsenic is from

bedrock from which it is weathered, but it is also emitted from industrial processes and various

agricultural practices. Total arsenic concentrations in air range from 0.01 :g/m3 to as high as

0.75 :g/m3 in smelter emissions. Surface and ground water total arsenic concentrations are generally

quite low. Surface waters of a southeastern, cotton producing state can reach several parts per billion

(ppb) arsenic according to EPA's BASINS and STORET databases, but are typically less than 3 ppb. 

High arsenic concentrations ranged between 10 and 50 ppb. For comparison, arsenic concentrations

in raw water samples from Washington, DC area rivers were not above the lower detection limit of 2
10

ppb. However, water concentrations of arsenic can be quite high. For example, 86 of 2125 sampled

water wells in Wisconsin had arsenic concentrations in excess of 50 ppb (Burke and Stoll 1999). The

source of that arsenic was determined to be a sandstone aquifer over which the wells were drilled. 

Arsenic concentrations in ground water of the lower Madison river valley range from 16 to 176 ppb

(Nimick 1998). The source of that arsenic was believed to be from the Madison river, which has

arsenic concentrations up to 100 ppb. Soils and sediments will contain greater arsenic concentrations,

typically in the range of 1 to 50 parts per million (ppm), but possibly greater than that in areas near

bedrock outcrops and mine spoilings. More substantive discussion on the prevalence of arsenic in

water and other environment compartments is reserved for the environmental fate and drinking water

sections of this document.

Arsenic can also be found in various food items. Arsenic concentrations of 1.6 to 2.4 :g/g (ppm) were

found in cans of tuna (Higham and Tomkins 1993) while samples of fruits, vegetables, grains, fast

foods, and dairy products had an average arsenic concentration of 2 :g/g dry weight (Nielson et al.

1991). Even alcoholic beverages had arsenic concentrations ranging from 1.5 to 12.4 ppb (HercePagliai et al. 1999). Due to the widespread occurrence of arsenic, determination of arsenical risk

through simple presence alone is difficult.

Arsenic Environmental Chemistry

Discerning arsenic risk in the environment would be furthered by understanding arsenic chemistry. 

Microbiota mediated adsorption/desorption processes take place between arsenic and both aluminum

and iron oxide fractions in soils and sediments (Menzer and Nelson 1986). These processes control

the fate of arsenic in the environment. Once sorbed to the clay fractions of soils, they may be

transported to water bodies and deposited into sediments, the ultimate sink for arsenic. The statement

of the sediment being the ultimate sink for arsenic is corroborated by Woolson (1975) who stated that

the residence time of arsenic in air is around 9 days, but is millions of years in sediments.

The chemistry of arsenic is very complex, and is the reason that detection of the arsenical herbicides

would be difficult following their dissolution in water. Like many metals, once exposed to the

environment, arsenic is oxidized to various forms, generally to trivalent arsenite (AsO3

3-) and

pentavalent arsenate (AsO4

3-). These two species, commonly encountered in environmental soils,

waters, and sediments, may be further transformed into methylated arsenic chemicals such as monomethylarsenate (MMA or MAA), di-methylarsenate (DMA also known as cacodylate), trimethylarsine oxide and the reduced, highly volatile mono-methylarsine, di-methylarsine, and trimethylarsine (Bright et al. 1996). These methylated arsenic species are all generated through biological

processes. Once the methylated arsenic species are in the environment, they may just as easily be demethylated back to their inorganic forms (arsenite and arsenate) by soil bacteria (Gao and Burau

1997). Many mammalian species also successively methylate the oxidized, inorganic arsenic species to

MMA and DMA (Hughes et al. 2000, Vahter 2000, Moore et al. 1997). MMA and DMA are both

more readily excreted from the body relative to the inorganic species (Hughes et al. 2000). 
11

Interestingly, the ability to methylate arsenic varies among species, and even among different age

groups, genders, and communities within a species (Vahter 2000). This variability, resulting from

various factors including the ability to produce enzymes responsible for arsenic methylation (methyltransferases) and nutritional status, can alter individual susceptibility to the arsenic species. The

preponderant commentary on arsenic environmental chemistry is given in the environmental fate section

of this document.

It is evident that arsenic can be found in the environment in many different forms, due to natural

processes. Addition of the methylated arsenic species (i.e., DSMA, MSMA, CAMA) to the

environment could lead to the temporary accumulation of inorganic arsenic species in the environment,

and at the same time be difficult to discern from naturally occurring arsenic. Considering this, how

could arsenic present a risk in the environment? Paracelsus once said, "All substances are poisons;

there is none which is not a poison. The right dose differentiates a poison from a remedy." Excessively

high concentrations of a chemical in an environment where such occurrence is abnormal could

present a risk to that environment and its inhabitants.

General Toxicology

Despite the obvious widespread presence of arsenic and its multiple forms in the environment,

considerable concern over its toxicity exists as incidents of cancer have been associated with arsenic

exposure (Goering et al. 1999). The inorganic forms of arsenic are generally considered more potent

direct toxins than the organic forms. Trivalent arsenite has a high affinity for protein sulfhydryl groups,

but appears to be selective for closely spaced dithiols, which are common in DNA-binding proteins,

transcription factors, and enzymes involved in the repair of damaged DNA (Vahter 2000). Pentavalent

arsenate can replace phosphate and uncouple cellular respiration, thereby impairing cellular function

(Vahter 2000). Also, one study found the relative, direct genotoxic potencies of arsenite and arsenate,

when compared to MMA and DMA, differ by a few orders of magnitude (Moore et al. 1997). Active

concentrations of those chemicals were 1-2 ug/L, 10-14 ug/L, 2,500-5,000 ug/L, and 10,000 ug/L,

respectively. This indicates that the methylated arsenicals have a much lower affinities for the same

substrates and are less potent direct toxins at equivalent concentrations. This is why some state the

metabolic process leading to methylation of arsenic could be considered a detoxification process

(Vahter 2000). In addition, methylation leads to quicker elimination of arsenic from the body (Hughes

et al. 2000) leading it away from the site of action. Therefore, metabolic transformation of arsenic

may mediate some of its risk.

On the other hand, it appears that the organic arsenic species are also toxic in that they are inducers of

oxidative stress (Schlenk et al. 1997, Yamanaka and Okada 1994, Kreppel et al. 1993). Oxidative

stress is the condition, within cells, in which highly reactive chemicals become involved in

oxidative/reductive reactions with cellular components, resulting in cellular component degradation. 

This degradation leads to mutagenicity (Bartsch and Nair 2000) and is a precursor to carcinogenicity

(Bartsch and Nair 2000, Bostwick et al. 2000). However, unlike the direct action of the inorganic
12

arsenic species mentioned earlier, it is an indirect mode of action intiated by methylated arsenic species. 

According to one author, the methylated species are slightly more potent inducers of oxidative stress

than are the inorganic species. For example, channel catfish (Ictalurus punctatus) exposed to 1ppm

of MSMA experienced twice the oxidative stress compared to catfish exposed to the same

concentration of sodium arsenite (Schlenk et al. 1997). Another author states that the inorganic arsenic

species is a more potent inducer of oxidative stress (Kreppel et al. 1993). Regardless of the relative

potencies, while methylation can reduce the direct toxicity of arsenic, it can enhance the indirect toxicity

yielding species that lead to oxidative stress. Furthermore, it demonstrates the potential toxicity of the

methylated arsenic. In fact, di-methylarsinic acid (DMAA also known as cacodylic acid) orally

administered to mice induced lung specific DNA damage attributed to reactive chemical species in lung

tissues (Yamanaka and Okada 1994). Fortunately, methylation also appears to hasten elimination as

the half-life for the methylated arsenic species in rats is less than 24h (Hughes et al. 2000). However,

risk of the arsenicals would be increased with continued chronic exposure to levels exceeding

normal background levels.

RISK CHARACTERIZATION

Terrestrial

Mammals and Birds

What is known about potential risk from MSMA, DSMA, and CAMA is the hazard of the various

formulations of each chemical to various laboratory organisms. The toxicity classifications range from

moderately toxic to practically non-toxic (Tables 1 and 5). Of course, there is always the uncertainty of

extrapolating chemical toxicity from laboratory animals to other species in the field. But there is also the

uncertainty of trying to predict the complex environmental exposure scenarios. Even given this

uncertainty, it might still be assumed that these chemicals are not considerably hazardous to terrestrial

organisms.

However, application rates of the three herbicides are great enough that acute risk LOCs are exceeded

for both chemicals, provided adequate exposure occurs. Estimated environmental concentrations

(EECs) on grasses following MSMA use in orchards exceed the mammalian herbivore acute LOC by

12 times. In fact, ten of the eleven arsenical uses presented in table 3 equal or exceed the mammalian

acute risk LOC for herbivores, while seven of eleven uses exceed the acute avian LOC for herbivores

(Table 4). 

The environmental fate of these chemicals is variable and not fully understood. These chemicals can

enter into very stable chemical associations with clays, and as such, would likely be biologically

unavailable. On the other hand, if the chemicals are bioavailable, both animals and microorganisms can

transform them, altering the toxicity of the original chemical. In addition, the central structure of these

herbicides is part of a complex suite of naturally occurring arsenic-based chemicals ubiquitous within the
13

environment, making exposure differentiation between herbicidal and naturally occurring arsenic-based

chemicals difficult. Even though the identity of the actual toxic moiety in the environment is uncertain,

heavy use of these herbicides does present an increased exposure risk of the arsenic-based chemicals

to organisms near the application site.

Given that exposure does take place, how long will residue levels on potential food items remain high

enough to present a risk to animals that enter the field to feed? Application of DSMA on cotton does

not result in acute risk LOC exceedence for any longer than a few days (Fig 1). On the other hand,

MSMA residues on grasses, following application to cotton, would remain high enough to exceed the

same mammalian acute risk LOC for up to 77 days (Fig. 2). The highest use, MSMA in orchards,

presents the longest period of potentially hazardous exposure with residues on grasses exceeding the

mammalian herbivore acute risk LOC for nearly 140 days (Fig 4). Therefore, there is the potential for

extended exposure of organisms to arsenicals.

Multiple applications of the same or a different arsenical would increase the time frame for animal

exposure to potentially hazardous arsenical residue levels on food items. For example, MSMA is used

as a pre/post-emergent herbicide on cotton, applied either before cotton emergence or at some point

post emergence. Several months later, DSMA and/or cacodylic acid (another arsenical) is applied

shortly before harvest to defoliate cotton. Under this scenario, applications of different arsenicals at the

same site can occur during one growing season, a situation depicted in figure 3. MSMA residues on

grasses and insects would potentially remain above the acute risk LOC for 80 and 40 days,

respectively, from the first day of application in the spring. Several months later, the same site is

exposed to DSMA. DSMA residues on grasses and insects result in decidedly lower RQs, but there is

a period of a few days during which an acute risk LOC would be exceeded for mammalian herbivores. 

This scenario indicates potential chronic exposure and toxicity to organisms due to repeated

applications.

In reality, persistence on certain food items is not likely to be great. Considering that these chemicals

are herbicides, grasses are likely to die, or at least the edible portion will be desiccated, to later be

replaced by new, unexposed leaves. What is unknown is how long the exposed edible portion will be

available for consumption. On the other hand, some plants have been shown to be resistant to MSMA

toxicity increasing the potential exposure period. Feeding behavior could also influence exposure. If an

organism prefers to consume living plant material, then risk is limited by the survival period of the plant. 

If the animal will consume dead plant material, then risk is not limited by plant survival and will be

extended beyond the risk presented by living plants alone. Therefore, while herbicide residues on

grasses may potentially be available for extended periods of time, the actual risk due to availability of

exposed grasses is unknown.

Risk to insectivorous organisms will be related to insect survival and mobility. Considering that insects

are mobile, they may be at the application site during application and move off site after application

presenting exposure to organisms off-site. But judging from the low acute toxicity of MSMA and
14

DSMA to honey bees, the insects may not immediately die from exposure to the chemicals and will be

able to move off site presenting exposure to organisms off-site. On the other hand, arsenicals have

been used as insecticides, indicating these chemicals have insecticidal activity. Even if insects do die

shortly after exposure, their carcasses would be available for opportunistic foraging by insectivores. It

is uncertain if an insectivore will consume a dead, motionless insect, or if some motion cue is necessary

to trigger a feeding response in the animal. These aspects of insectivore exposure are unknown.

Mammalian reproductive studies have been submitted for methanearsonate (MRID 431783-01). 

These tests with laboratory rats found that exposure to methanearsonate resulted in reduced fertility and

reduced litter survival. Other chronic studies (MRID 416690-01) found that, following oral gavage,

methanearsonate led to affects on body weight gain and food consumption as well as leading to

development of parathyroid adenomas. This latter endpoint supports information presented in the

introduction describing the relationship between arsenicals, oxidative stress, and carcinogenicity. 

Terrestrial Plants

Low risk from DSMA exposure is presumed for non-endangered plants inhabiting upland areas

adjacent to application sites (Table 6). Offsite loading of DSMA due to drift, and due to drift plus

sheet run-off, presented exposure that would be less than the acute risk LOC (Table 7).

Greater risk is presumed for non-endangered semi-aquatic plants, which inhabit lowland areas

surrounding the application site, exposed to DSMA. Channelized runoff and drift result in predicted

exposures that result in exceedences up to three times the acute risk LOC.

Due to its apparently greater inherent toxicity, MSMA use presents a potentially greater risk to nonendangered terrestrial plants than does DSMA despite the similarity of exposure levels for MSMA and

DSMA (Table 6). Predicted MSMA exposure resulted in acute risk LOC exceedences up to 8 times

the LOC for upland terrestrial plants exposed to drift and sheet run-off adjacent to the application sites,

and up to 75 times the LOC for semi-aquatic plants exposed to drift and channelized run-off (Table 7). 

These exceedences indicate a high level of risk for terrestrial and semi-aquatic plants.

However, there is uncertainty in the prediction of terrestrial and semi-aquatic exposure levels. The

assumptions for the exposure scenarios, presented in appendix 3, are subject to debate. For instance,

5% of the applied chemical is assumed to be in the channelized run-off for highly soluble chemicals

(solubility >100ppm). Since arsenicals are highly soluble, this would be the percentage of the applied

arsenicals assumed to be in runoff. However, given that these arsenicals can strongly bind to soils, and

that these chemicals are applied to fields, it is uncertain what proportion of the arsenicals in the run-off

are actually dissolved in water, hence biologically available, and what proportion is sorbed to soil

particles suspended in water, and thereby less biologically available. The actual percentage of the

applied chemicals in run-off could be significantly higher than 5% due to erosion of soils in humid areas

that receive considerable rainfall, or less than 5% due to the lack of erosion in arid regions.
15

Summary of Terrestrial Risk

MSMA appears to be inherently more toxic than DSMA as evidenced by lower acute toxicity values

(greater toxicity) for a number of organisms (Tables 1, 5, and 16). This apparent toxicity difference is a

significant area of uncertainty since the two chemicals are essentially the same. Both chemicals are

sodium salts, one of methanearsonate (DSMA) and the other of protonated methanearsonate

(MSMA). This area needs further investigation, especially since it leaves unanswered the question of

what is the toxic moiety in the field.

The overall risk of these herbicides in the terrestrial environment is uncertain. They are not highly toxic

to mammals and birds as evidenced by the toxicity values, but the use rates yield EECs on food items

that exceed acute risk LOCs for the majority of uses. Therefore, while short term risk is evident, long

term risk is uncertain. Registrant submitted studies indicate these compounds are relatively long-lived

(see fate assessment below), but peer reviewed literature indicates considerable variability. It is

possible that they are more persistent in relatively sterile soils once adsorbed to the clay component, but

in environmental soils, they may be degraded by soil microbiota. In addition, they may be eroded from

the application site while adsorbed to soils, and deposited into sediments. From there, they may be

further transformed by sediment microbiota, or, possibly absorbed by the roots of semi-aquatic plants. 

Considerable uncertainty exists for predicting long-term risk. Given the uncertainty of environmental

persistence and potentially long-term exposure, chronic toxicity in the environment is a possibility.

How long these chemicals may remain on potential food items is also unknown, as well as the time

frame during which the food item would effectively present the chemicals to potential consumers.

Avian chronic toxicity studies would be essential to fully understand the potential of risk to avian

species, especially in light of the acute toxicity of arsenicals to birds and the demonstrated reproductive

and chronic toxicity of methanearsenate to mammals.

Aquatic

Arsenical use appears to present minimal acute risk to organisms within the aquatic environment. The

toxicity classifications for the tested arsenical formulations range from slightly toxic to practically nontoxic (Tables 9, 11, 14). Adjustment of some of the formulations for active ingredient purity changed

the classification to moderately toxic, but there is uncertainty if such an adjustment would accurately

reflect the true hazard of 100% active ingredient. Comparing the low hazard of these chemicals to the

aquatic EECs for these chemicals (Table 13) resulted in the exceedence of no acute high risk LOCs for

any aquatic plant or animal (Tables 10 and 12).

Potential effects on benthic organisms is uncertain. Eroded or suspended soil settles to the bottom of a

water body and presents arsenical exposure to benthic organisms. While sorption of chemicals reduces

biological availability in water, ingestion of contaminated sediments and microbiological activity can
16

enhance exposure.

No data regarding the potential acute toxicity of DSMA to estuarine/marine organisms was presented. 

These studies are necessary to evaluate the risk DSMA presents within the marine/estuarine

environment. Also no chronic studies were submitted for any of the arsenical herbicides. While there

appears to be low acute toxicity to many of the tested species, chronic toxicity may still be a concern at

low exposure concentrations, especially in water bodies that repeatedly receive run-off from sites of

arsenical application. 

Summary of Aquatic Risk

Overall risk of arsenicals within the aquatic environment is uncertain. Data gaps remain. Acute toxicity

along with EECs indicate low risk. Available data are insufficient to assess chronic risk to aquatic

organisms. Like for terrestrial animals, the toxicity is not considerable, but the exposure level may

present significant risk. In addition, with the heavy use rates and possible episodic (rain events)

introduction of these chemicals into water bodies, chronic risk is a possibility.

Data gaps regarding acute toxicity of DSMA in estuarine/marine environments, and chronic

toxicity of these arsenicals need to be filled.

Endangered Species

The endangered species risk quotients exceed the Agency’s LOC for birds, mammals and terrestrial

plants from exposure to DSMA and MSMA. The risk quotients also exceed the LOC for freshwater

fish exposed to MSMA. 
17

Risk Quotient Decrease due to DSMA Residue Dissipation

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0 28 56 84 112 140 168

Avian Herbivores Avian Insectivores

Mammalian Herbivores Mammalian Insectivores

Figure 1. Risk quotient (EEC/LC50) decrease due to DSMA dissipation (35 day half-life) from food items (grasses and insects) following two

applications (3.6 lbs a.i./A) on cotton, 14 days apart. The levels of concern(LOC) are 0.5 for acute risk, 0.2 for restricted use, and 0.1 endangered

species.
18

Risk Quotient Decrease due to MSMA Dissipation

0.00

0.50

1.00

1.50

2.00

0 28 56 84 112 140 168

Avian Herbivores Avian Insectivores

Mammalian Herbivores Mammalian Insectivores

Figure 2. Risk quotient (EEC/LC50) decrease due to MSMA dissipation (35 day half-life) from food items (grasses and insects) following two

applications (3 lbs a.i./A) on cotton, 14 days apart. The levels of concern(LOC) are 0.5 for acute risk, 0.2 for restricted use, and 0.1 endangered

species. Note the greater degree of LOC exceedences for MSMA use on cotton compared to DSMA use on cotton.
19

Risk Quotient Decrease due to MSMA and DSMA Dissipation

0.00

0.50

1.00

1.50

2.00

0 28 56 84 112 140 168 196 224 252 280

Avian Herbivores Avian Insectivores

Mammalian Herbivores Mammalian Insectivores

Figure 3. Risk quotient (EEC/LC50) decrease due to dissipation of MSMA and DSMA residues from potential food items (grasses and seeds)

following application of first MSMA and then DSMA. This figure represents application of MSMA in cotton fields at the time of cotton emergence

followed by DSMA application five months later to desicate the crop prior to cotton harvest. The levels of concern(LOC) are 0.5 for acute risk, 0.2

for restricted use, and 0.1 endangered species. Note the extended period of potentially hazardous exposure to organisms resulting from multiple

arsenical applications on cotton fields.
20

Risk Quotient Decrease due to MSMA Dissipation in Orchards

0

2

4

6

8

10

12

14

0 28 56 84 112 140 168

Avian Herbivores Avian Insectivores

Mammalian Herbivores Mammalian Insectivores

Figure 4. Risk quotient (EEC/LC50) decrease due to MSMA residue dissipation from potential food items (grasses and insects) in orchards

following three applications (15.5 lbs/A), 14 days apart. The levels of concern(LOC) are 0.5 for acute risk, 0.2 for restricted use, and 0.1

endangered species. Note the considerable LOC exceedences for this use, and the length of time over which the LOCs are exceeded.
21

ENVIRONMENTAL FATE ASSESSMENT: PERSPECTIVE, BASIS, AND SUMMARY

Perspective and Basis

This fate assessment for the salts of methanearsonic acid, is intimately and inextricably linked with that

previously completed for the Reregistration Eligibility Document (RED) for their close arsenical ally,

cacodylic acid (and its sodium salt). Chemically speaking, all these arsenical species are better treated

in a single document, but logistical and proprietary constraints precluded this. It was intended that the

cacodylic acid RED be the more inclusive and comprehensive on the environmental chemistry and

transport of this class of compounds, and most of that material is not repeated here. However, some of

the general fate and most of the drinking water assessment conclusions expressed in the main text

below are taken directly from that document, but, in effect, with a simple change of subject noun(s). 

The previous cacodylic acid RED (including an expanded environmental fate assessment in its

Appendix 6) is indispensable for arsenical chemical perspective, additional details,

interpretation, and references to eclectic sources which may not be included here. However, for

ready reference and reader convenience, we have extracted the drinking water assessment and citations

from the more technical Appendix 6 of the Cacodylic acid RED, and copied it here as Appendix 5.

Previous OPP/EFED reviews (for example, Edelstein, 1994) have correctly pointed out deficiencies

and seeming inconsistencies in registrant generated data. In the context of this RED and the cacodylic

acid RED, which attempt to show the many facets of arsenic chemistry through sources in addition to

registrant generated data, the nature of the discrepancies is more understandable. In a formal sense, the

Agency could “require” more research data and/or literature review from the registrant. However,

based upon the available information and the manner in which we have used the information to conduct

the present risk assessment, in a realistic regulatory sense, there would be little practical value added by

additional submissions. While we have considered in detail the specific influences of

MSMA/DSMA/CAMA in our drinking water assessment, the larger, general EPA issue of arsenic

compounds from all sources in drinking water (which we touch on below) is reserved for another

forum. Therefore, on this basis, we consider formal data requirements for MSMA/DSMA/CAMA

satisfied at this time, except for submission and EPA laboratory validation of analytical

chemical methods for detection and measurement in water and soil and for field spray-drift and

droplet-size spectral data for these herbicides.

To establish proper context and some perspective, in addition to that given in the previous introductory

section of this document, it is worth copying here nearly verbatim (beginning in the next paragraph in

italics below) an excerpt from the cacodylic acid RED. The principal non-verbatim exceptions are the

additions of “MSMA”, “DSMA”, and “CAMA” or their collective use; the addition of supplemental

chemical information on the element arsenic; changes for agreement of subject and predicate or similar

editorial changes; and updating the calendar. The fourth of the italicized paragraphs below emphasizes

the fact that the conditions of standard FIFRA environmental fate study requirements, which are suited

more for neutral organic substances, are generally unsuited for compounds such as these organo-
22

metalloids with multiple oxidation states and a biogeochemical history. The fourth paragraph also

indicates the eclectic nature of the fate assessment. With the stated minor alterations (by the same,

present author), here is the excerpt from the cacodylic acid RED:

MSMA, DSMA, CAMA, and cacodylic acid are four of several arsenic containing pesticides

which the Agency is, or soon will be, considering for reregistration eligibility. Ultimately, in

another action, the Agency will assess the potential overall (aggregate, cumulative) drinking

water exposure to these compounds and common transformation products. The EPA Office of

Water (OW) is scheduled to issue a final regulation on the larger issue of “arsenic” in drinking

water during the year 2001.

Because of historical toxicological interest in arsenic compounds (the name arsenic has been

practically synonymous with "poison" for thousands of years), there is a substantial body of

published and unpublished scientific study on their chemistry and environmental fate. Modern

literature clearly shows that the biogeochemical interactions of these compounds are complex,

and still the subject of active research. However, it is not within the scope of this document to

comprehensively review this fascinating area. This has been done to a large extent in the past

by the EPA in 1988, the National Academy of Sciences (NAS, 1977), the U.S. Department of the

Interior’s Fish and Wildlife Service (FWS, 1988), other agencies, individual scientists (e.g., ACS,

1975 Symposium), and very recently by the National Research Council (NRC, 1999; sponsored

by EPA) and by the U.S. Geological Survey (USGS, see below). Although the focus here is on

MSMA, DSMA, CAMA, and cacodylic acid, their place in the larger scheme should be

understood at the outset.

Essentially all the arsenic since the earth's formation is still present and virtually immutable

(barring nuclear transmutation), with new amounts added daily as cosmic dust. Arsenic is

everywhere in the form of many different organic and inorganic compounds. The uncombined

element arsenic is a steel grey, very brittle, crystalline, semi-metallic (metalloid) solid., which

tarnishes in air. Because of its reactivity, arsenic is found only occasionally as the free element

in nature, but is usually found chemically combined in a number of minerals, in plants and

animals, and in products of decay or metabolism. Soils, for example, commonly contain from 1-

50 parts per million (ppm) of total arsenic in various combined forms, averaging around 5 ppm. 

Sediments average somewhat higher. Many of the compounds of arsenic, including our subject

chemicals, are synthetically manufactured (ultimately from some primary mineral source) and

occur naturally and ubiquitously as well. All are subject to the same fate and transport

processes, and are entwined inseparably as part of the global arsenic biogeochemical cycle. 

These compounds introduced locally through agriculture cannot be distinguished molecularly

from that which is present in the natural background. Only unnaturally high localized

concentrations would serve as a telltale. Likewise, other chemical forms of arsenic (organic or

inorganic/mineral) resulting from the transformation of arsenic-containing pesticides or other

arsenic sources which are either naturally present or introduced by human activities (e.g., fossil
23

or wood fuels; agricultural burning; waste incineration; spreading of sewage sludge;

pharmaceuticals; poultry and swine manure; mining; smelting; congener contaminant in

phosphate detergents and fertilizers; industrial production of semiconductors and glass) cannot,

in general, be distinguished as originating exclusively from pesticidal application. There is

certainly much more arsenic in all environmental compartments (air, soil, sediments, water,

plants, animals) than could ever be attributed to pesticidal application of MSMA, DSMA, CAMA,

cacodylic acid, and other arsenicals.

Because of the established complexity, the environmental fate of the subject arsenicals and other

arsenicals cannot be adequately captured by standard FIFRA, 1988, Guideline studies, which are

designed primarily for aspects of xenobiotic organic compounds subjected to a very limited set

of experimental conditions. Therefore, in addition to the registrant’s FIFRA studies performed

under Good Laboratory Practices (GLP), the Agency relies substantially on the open scientific

literature, including very recent publications (or some in press), for nuggets of information which

directly impact the Office of Pesticide Programs (OPP) environmental fate and effects risk

assessment. Even so, evidence is often highly variable or seemingly conflicting, indicating that

processes, speciation, and environmental interactions (including effects of moisture, pH, and

oxidation-reduction conditions) among the various organic and inorganic species of arsenic are

in constant biogeochemical flux and still incompletely understood.

Recent outbreaks of serious arsenic poisoning originating from drinking water wells in India and

Bangladesh, as well as those long-established in Taiwan, have attracted the worldwide attention

of newspapers and public health officials. Although these events are not associated with use of

MSMA, DSMA, CAMA, or cacodylic acid, they have had the collateral effect of stimulating

renewed scientific interest in the environmental chemistry and fate of arsenicals (Nickson, et al.,

1998; Masud, 2000; numerous others). In addition, as will be discussed under the drinking water

exposure assessment section of this document, recently completed projects by the USGS and the

pending regulation by the EPA Office of Water also attest to the interest in arsenicals in our

environment. A better understanding of the many facets of the environmental chemistry and

potential risks of environmental arsenic is evolving. (End of excerpt from cacodylic acid RED)
24

Environmental Fate Summary for MSMA/DSMA/CAMA
25

ABSTRACT. This environmental fate assessment is based on registrant submissions and published technical

literature. The previous cacodylic acid RED serves as the primary, more comprehensive document on the nature

of the environmental chemistry and transport of arsenical pesticides.

The environmental fates of MSMA, DSMA, and CAMA, sodium salts of methanearsonic acid (MAA or MMA)

are, for our purposes, virtually indistinguishable. Transformation is by aerobic and anaerobic microbial

metabolism. Aerobic soil metabolism rates depend critically on ambient conditions such as soil moisture, pH,

oxidation-reduction potential, and their complex interrelationships. Thus, the concept of simple first-order (“halflife”) degradation is simplistic. Direct abiotic transformation processes are ineffectual. Aerobic soil “half-lives”

may range from a few weeks under favorable conditions to several years under dry conditions not conducive to

agriculture. Registrant submitted field studies did not adequately account for dissipation. The Agency has

selected eight months (240 days) as an effective aerobic soil metabolism “half-life”. The aerobic aquatic

metabolism half-life is similar. The anaerobic (flooded) soil metabolism half-life appears to be measured in years. 

With the selected aerobic soil metabolism “half-life” of eight months, the amount of parent material remaining one

year after an initial application would be approximately 35% of applied. The asymptotic first-order kinetics limit

for build-up parent after several years of successive applications is approximately 50% of an annually applied

amount. However, it is crucial not to confuse transformation with detoxification. Total arsenic, although

redistributed, is immutable. Regulatory limits of total arsenic in soil may be reached in a few years in some cases. 

Metabolites of MSMA (see below) include the pesticide cacodylic acid as well as other arsenic species with

known toxicities.

Major metabolites occur inconstantly and in variable proportions for complex, unclear reasons. Principal

identified metabolites are cacodylic acid (which could reach up to approximately 30-35% chemical equivalent

units of parent material in the environment), arsenate, carbon dioxide, and the volatile compounds dimethylarsine

and trimethylarsine. Arsenite was either not detected or not speciated, but would be subject to formation under 

conditions of its natural occurrence. In the biosphere, soils/sediments ultimately sequester the more

thermodynamically stable mineral forms of arsenic; for example, sulfides.

Volatilization of parent materials is not a significant route of dispersal. However, volatile arsenicals are part of the

global arsenic cycle.

Sorption to numerous diverse soils varied tremendously, but indicates intermediate to low mobility. Simple

sorption coefficients (Kd) in over 20 soils ranged from 2.5 to 110 mL/g with with a median of around 30 mL/g. 

That there is no correlation with soil organic carbon is demonstrated by pseudo soil organic carbon sorption

coefficients (Koc) widely ranging from approximately 400 to 50,000 mL/g of organic carbon. Sorption is correlated

better with clay and iron and aluminum oxide content. If pursued, there would most likely be a strong correlation

with pH, reflecting acid/base chemical speciation. Field studies provide some evidence supporting little

likelihood of leaching.

Bioconcentration is not indicated for parent compounds. Although many plant and animal species have

selectively high bioconcentration factors for arsenic, these are for other complex organic forms of arsenic which

are reported as practically nontoxic.

Drinking Water for Humans: We conclude from monitoring and modeling that surface drinking water

concentrations originating from use of these salts of methanearsonic acid will impose an additive, localized

burden on natural background concentrations of these and other chemical species of arsenic, including cacodylic

acid. Ground water should not be impacted incrementally. Any drinking water levels of concern should be

coordinated with the pending regulatory plan of the EPA Office of Water for arsenic.
26

As O

O

O

H3C

Na

+

Na

+ As OH

O

O

H3C

Na

+

MSMA, DSMA, and CAMA are salts of the dibasic weak acid methanearsonic acid (MAA) (also

variously abbreviated in this document and in the published literature as MMA or MMAA for

monomethylarsonic acid). Chemical structures and some of the physicochemical properties for MSMA

and DSMA are given below. For comparison, the structure of cacodylic acid (sometimes abbreviated

herein as CA or DMA for dimethylarsonic acid) and its sodium salt, which have two methyl groups

instead one attached to the central arsenic atom, are also given.

Structures and Physicochemical Properties for MSMA, DSMA, and Cacodylic Acid

 DSMA

Empirical Formula: CH3AsNa2O3

Molecular Weight: 183.92

CAS Registry No.: 144-21-8

PC Code: 013802

 MSMA

Empirical Formula: CH4AsNaO3

Molecular Weight: 161.94

CAS Registry No.: 2163-80-6

PC Code: 013803

M.P.: >300 C M.P.: 116-121 C

Density: 1.04 g/mL Density: 1.65 g/mL

V.P.: 1 x 10-7 mm Hg V.P.: 7.5 x 10-7 mm Hg

log Pow < 1 log Pow < 1

Water Sol. 34 g/100mL

Methanol Sol. 26 g/100mL

Hexane Sol. 0.0025 g/100 mL

Water Sol. 104 g/100mL

Methanol Sol. 16 g/100mL

Hexane Sol. 0.005 g/100 mL

pKa1, 2 4.0, 9.0 (approx.) pKa1, 2 4.0, 9.0 (approx.)
27

 Cacodylic Acid Sodium Cacodylate

 O O

 || ||

CH3–As–OH CH3–As–O- Na+

 | |

 CH3 CH3

EPA PC Code: 012501

Chem. Abs. No.: 75-60-5

Other Names: dimethyl arsenic acid, hydroxydimethylarsine oxide

Empirical Formula: C2H7AsO2

Molecular Weight: 138.0 daltons

pKa: 6.2

Once dispersed and diluted to trace concentrations by water and physical disturbance in environmental

media (parts per million and below), the sodium ions become part of the background morass of natural

metal cations (positive ions), while the methanearsonate anions (singly and doubly charged) tend to

associate with hydrogen (hydronium) ions through the process of simple ionic hydrolysis. The process

is analogous to the familiar hydrolysis of the basic carbonate ion to bicarbonate ion and carbonic acid

(carbon dioxide in water). The three arsenic species are symbolized here as H2MMA, HMMA1-, and

MMA2-. The degree of association depends on pH. Reported titration studies for MSMA and DSMA

yield base strengths for the anions corresponding to pKa values for the acid MMA of approximately 4

and 9. Assuming these pKa values, at pH 5, 7, and 9, for example, the ratios of acid to respective

anions H2MMA / HMMA1- / MMA2- are approximately:

pH 5 pH 7 pH 9

1 000 / 10 000 / 1 1 / 1 000 / 10 1 / 100 000 / 100 000

Hence, we are discussing the pH dependent equilibria and environmental fate of acid, anions, and

possibly other unknown soil/sediment/solution associated complexes or precipitates. Thus, for our

purposes, and based on the available data, the environmental fates of MSMA, DSMA, and CAMA are

virtually indistinguishable. Therefore, fate data for MSMA serve as surrogate data for DSMA/CAMA

and conversely. Although imprecise, we will continue to simply refer to the environmental collection of

chemical species (acid, anions, other complexes) as if they were the pure, active starting material

MSMA (equivalently DSMA/CAMA). Perhaps a more appropriate designation would be MMA,

which connotes the acid nature of the underlying chemical structure, or HMMA1- since it tends to

dominate in the range of typical agricultural pHs. The important point is that in the environment

chemical speciation is in dynamic flux, depending on variations in pH and intimate contact (affected by

transport, mixing, diffusion, etc.), in accordance with principles of chemical equilibria and kinetics.
28

Transformation Is By Microbes

Environmental fate laboratory studies showed that MSMA (equivalently DSMA/CAMA) was

essentially stable under all tested conditions, except microbial metabolism. That is, transformation did

not occur appreciably under the influence of abiotic (sterile) chemical or photochemical processes in

soil or water. Aerobic and anaerobic microbial processes were slow. Field studies which the registrant

submitted did not adequately account for dissipation. The aerobic soil "half-life” which we have

selected for modeling purposes is approximately eight months (240 days). This value derives from data

from a one-year laboratory study (MRID 44767601) which the registrant submitted for one soil

(approximately 35% of the dose remained after one year). This value is essentially the same as that

which the reviewer estimated from two data points reported in a published seventy-day study of several

arsenicals (Gao and Burau, 1997) after rough normalizations for soil moisture (suction) and

concentration effects based on comparison with more complete results for cacodylic acid. However,

based on these and other published results (confer cacodylic acid RED), the concept of simple firstorder kinetics (which uniquely gives rise to the “half-life” concept) is overly simplistic, with

transformation rates punctuated dramatically by varying ambient conditions such as soil moisture, redox

potential, pH, and temperature. We proffer that sensitivity to local, transitory conditions may be so

exquisite that under suitable conditions degradation rates could suddenly be much faster or much

slower. An aerobic aquatic metabolism study conducted for only 30 days yielded a rough extrapolated

half-life similar to the eight months selected for aerobic soil. The anaerobic "half-life" appears to be

measured in years (a recent study submission indicated an extrapolated “half-life” of roughly two years

in an anaerobic aquatic metabolism study lasting one year).

With the selected aerobic soil metabolism half-life of eight months, the amount of parent MSMA

material remaining in soil (without offsite transport) one year after an initial application would be

approximately 35% of applied. The asymptotic first-order kinetics limit for build-up in soil (without

offsite transport) of parent after several years of successive applications would be approximately 50%

of an annually applied amount.

It is crucial not to confuse transformation with detoxification. Metabolites of MSMA, to be discussed

below, include the pesticide cacodylic acid as well as other arsenic species with known toxicities. In

combination, all of these maintain the total, immutable arsenic load. Currently, the Agency has a limit of

43 parts per million of total arsenic in soil as the concentration beyond which remediation is necessary

(for example, at Superfund sites impacted by various sources of arsenic and currently on The American

University campus in Washington, DC, where World War I chemical warfare agents containing the

poisonous gas lewisite (ClCHCHAsCl2) were tested and disposed). Thus, without redistribution of

arsenic from soil which originally may have contained only a few parts per million of natural arsenic at

the site of application, a constant annual rate of application of one pound per acre of MSMA (all actual

rates are higher), the regulatory arsenic limit would be reached in approximately 190 years. At the

maximum cotton use rate, the limit would be reached in around 30 years; at the maximum orchard use

rate, in only 3 years. However, since we know that global redistribution, transformation, and recycling
29

of arsenic in all possible environmental compartments (plants, animals, air, soil, sediment, water) occurs

in the guise of compounds such as those given in the metabolites section below, different maximum

concentrations would be reached at indefinite future times, depending on local quasi-equilibrium

conditions for arsenic chemistry and entwined environmental factors. Thus, although there is no doubt

local arsenic concentrations will be elevated, the degree of long-term or chronic exposure and potential

effects are largely unknown from the standpoint of this assessment.

Furthermore, it must be remembered that total arsenic is only a crude measure of potential toxicity. 

Ideally and logically, toxicity should be based on the specific kinds and proportions of arsenic moieties. 

Whenever arsenic concentrations are elevated above local background, to speciate is common sense.

Metabolites

The major metabolites identified in published sources and registrant submissions are inconstant--

sometimes detected and sometimes not. They also occur in widely variable proportions. Reasons for

this are unclear, but are most likely associated with exquisite sensitivity to the ambient conditions

mentioned above, and as discussed in more detail in the cacodylic acid RED. Listed below with

chemical formulas and names are the principal metabolites variously identified: cacodylic acid, arsenate,

carbon dioxide, and volatile alkylarsines. Arsenite was not detected under aerobic soil conditions, or

was otherwise not speciated, but would be subject to formation under conditions of its natural, local

occurrence. According to the published literature, in biogeochemical time frames, sediment is the sink

that sequesters the thermodynamically more stable, less soluble mineral forms of arsenic; for example,

sulfides. In seawater, concentrations of total arsenic are typically in the range of 1 to 2 ug/L (ppb).

Arsenic Species Commonly Found in Environmental Samples and Cited Above

 “Organic” Species “Inorganic” Species

(CH3)2AsO(OH) cacodylic acid AsO4

3- arsenate

(CH3)2AsH dimethylarsine AsO3

3- arsenite

(CH3)3As trimethylarsine As2S3, As4,S4 sulfides

With the selected "half-life" for MSMA (equivalently DSMA/CAMA) of approximately 240 days and

that for cacodylic acid of approximately 100 days (value selected for the cacodylic acid RED), then the

maximum amount of cacodylic acid would be reached approximately 220 days after pesticidal

application (assuming first-order kinetics), and would be chemically equivalent to approximately 23% of

the initial amount of parent. At the same time (220 days after application), parent compound would be

at approximately 53% of original amounts. These values could be considered "typical", but widely

variable, in accordance with widely variable environmental behavior. Depending on whether

conservative focus is on parent material or cacodylic acid, legitimate adjustments to "half-lives" could be
30

made to alter relative proportions at various time intervals. However, based on the available data, it is

not likely that cacodylic acid would ever reach much more than 30-35% of chemical equivalent units of

parent compound in the environment.

Bioconcentration

Since MSMA did not bioconcentrate significantly in bluegill sunfish and both MSMA and DSMA have

low octanol/water partitioning ratios (log Pow < 1), there is little potential for bioconcentration. 

Although the scientific literature (for example, ACS, 1975; NAS, 1977) shows that many plant and

animal species (shrimp, for example) have selectively high bioconcentration factors for arsenic, these

are for other complex organic forms of arsenic (such as organobetaines) which are reported as

practically nontoxic. The published literature also shows the absence of biomagnification of arsenic.

Transport/Mobility

In Air. Based on physical properties tabulated above, volatilization of parent materials would not

expected to be a significant route of dispersal. Consistent with this expectation, volatilization of parent

was not reported in any lab study. However, as mentioned above, volatile arsines produced by

metabolism are part of the global arsenic transformation and transport cycle.

In Soil. Sorption to numerous diverse soils varied tremendously, but indicates intermediate to low

mobility. In a study with five soils, simple sorption coefficients (Kd) ranged from 2.5 to 110 mL/g with

a median of 40 mL/g. That there is no correlation with soil organic carbon is demonstrated by pseudo

soil organic carbon sorption coefficients (Koc, in units of mL/g of organic carbon) ranging from a low of

approximately 400, for a heavy clay with over 9% organic carbon, to 50,000 for a sandy clay with

0.22 organic carbon. A sand soil with less than 0.1% organic carbon had a pseudo Koc of 4,200. In

a published study (Wauchope, 1975) with 16 Mississippi River flood plain soils at an experimentally

fixed (artificial) pH of 5.6, Kds ranged from a reviewer calculated 17 to 95 mL/g, with a median value

of 28 units. Again, there was poor correlation with organic carbon. No doubt, if pursued, there would

be a correlation with natural pH which would reflect the chemical speciation discussed above and in

more detail in the cacodylic acid RED. Thus, at higher pHs more typical of agricultural soils, the Kd

values for the 16 soils would be expected to be significantly lower. In the 16 soils, sorption at fixed pH

was better correlated with clay and iron and aluminum oxide content. By direct comparison with

phosphate, the three arsenic chemical species cacodylate, arsenate, and MSMA (methylarsonate )

were more strongly sorbed than phosphate in the increasing order: 

phosphate < cacodylate < arsenate ~ methylarsonate.

In this respect, the organic arsenicals (cacodylic acid and methylarsonate) behaved like the inorganic

arsenate and phosphate.
31

The drinking water section of this document, which follows below, expands upon the subject of

transport and monitoring for ground and surface water exposure.

DRINKING WATER ASSESSMENT (The Cacodylic Acid RED document, especially its

Appendix 6 Environmental Fate Assessment, is the primary source of information for this

drinking water assessment. Its Appendix 6 has substantial additional details and cited references

not included below. For convenience and ready reference, the drinking water portion of that

Appendix 6 is included here as Appendix 5.)

The drinking water exposure assessment was based primarily on 1) limited monitoring for MAA

(analytical form of MSMA/DSMA/CAMA) and other speciated forms of arsenic, including the

cacodylic acid metabolite and the inorganic arsenite and arsenate; 2) extensive monitoring for total

(unspeciated) arsenic; and 3) submitted environmental fate studies and associated simulation modeling. 

Although monitoring data for individual chemical forms of arsenic were sparse, the available measured

data were revealing. Surface water conclusions from these sources were compared with simulation

model predictions.

Monitoring sources included USGS (mostly ground water, but with some surface water), BASINS,

STORET (surface water only), and published literature (surface and groundwater). Limited, but

valuable, surface and groundwater data exist for speciated arsenic [MAA, cacodylic acid (abbreviated

as CA or DMA), arsenite, and arsenate]. In most cases, however, monitoring was for total arsenic

(unspeciated) in areas not targeted for MSMA, DSMA, or CAMA use. Thus, most monitoring data

are useful for assessing total exposure to arsenic, but are generally not useful for estimating exposure to

MSMA, DSMA, CAMA, or CA as such. Furthermore, in most cases, monitoring was in areas not

specifically targeted for arsenical pesticide use, and very often had high total arsenic reporting limits

(presumably because of established regulatory limits). What the data do clearly show, however, is the

frequent presence of significant concentrations of the organic MAA and CA and generally higher

concentrations of inorganic arsenic (arsenite and arsenate).

The primary monitoring sources are identified briefly as follows:

1) The United States Geological Survey (USGS) has recently completed an extensive survey report

entitled “Arsenic in Ground Water of the United States: Occurrence and Geochemistry” (Welch,

1999). Water analyses were almost entirely for undifferentiated or total arsenic. Although the survey

targeted ground water, it also included some useful information for surface water evaluation.

2) A separate, limited investigation by USGS research scientists (Garbarino and Burkhardt, 1998),

provided data on the arsenic species CA, MAA (equivalent to MSMA/DSMA/CAMA), arsenite, and

arsenate in ground water not associated with arsenical pesticide application and in surface waters in

areas which were associated with the application of MSMA.
32

3) The EPA Pesticides in Groundwater Database (EPA, 1992) provided monitoring data for localized

areas in Texas and Washington state.

4) The BASINS and STORET databases maintained by the US EPA provided a large body of data

on total arsenic concentrations in surface waters of the United States.

5) Chemical speciation data published in 1973 (Braman, 1973) on six fresh surface water bodies, one

well, and three saline waters around Tampa, Florida, revealed local background concentrations for CA,

MAA, arsenite, and arsenate.

6) Chemical speciation data published in 1991 (Anderson, 1991) on a total of 14 California water

bodies for concentrations of CA, MAA, and inorganic arsenic.

Conclusions drawn from all sources follow below. As stated above, Appendix 5, taken from the

cacodylic acid RED, has additional supporting details and references.

Groundwater Exposure Conclusions

Incremental exposure to MSMA/DSMA/CAMA or the CA metabolite from groundwater due to

agricultural application is not expected to be significant. Based on soil sorption characteristics

discussed in the Transport/Mobility Section above and supplemental field evidence, significant leaching

of MSMA/DSMA/CAMA , CA or other metabolites to ground water from normal agricultural (diffuse

or non-point source) use is not expected. Although monitoring for MSMA (MAA) and CA indicated a

minor presence in ground water, this cannot be reasonably attributed to labeled agricultural use. 

Therefore, no modeling for ground water estimated concentrations was conducted. (Additionally, the

currently available Agency SCI-GROW groundwater regression model is not well-suited for these

compounds because their sorption does not correlate with soil organic matter as previously discussed.)

This conclusion should not be interpreted to mean that the U.S. population is not exposed to arsenicals

(organic and inorganic) in drinking water from groundwater sources, which it certainly is, only that the

labeled use of MSMA, DSMA, or CAMA should not contribute significantly to the already

existing burden of arsenic from groundwater sources. In fact, a USGS map of total arsenic

distribution in ground waters of the U.S., given in Appendix 10 and discussed in Appendix 5, shows

surprisingly high concentrations of total arsenic at many sites. The survey authors concluded that while

slightly less than half the groundwater samples had total arsenic concentrations < 1 ug/L, about 10%

exceeded 10 ug/L. They further concluded that “natural” arsenic concentrations exceeding 10 ug/L are

more widespread and common than previously recognized. Some areas of high arsenic concentrations

(defined as > 50 ug/L) were associated with point source pollution (for example, Superfund Sites

where about 30% of approximately 1200 sites list arsenic as a “contaminant of concern”), but most

appear to be natural areas.
33

In a limited, but provocative, separate study by USGS research scientists (Garbarino and Burkhardt,

1998), 6 groundwater samples from Idaho, 10 from Illinois, and 8 from Nevada (total of 24 sites) were

analyzed for arsenite, arsenate, monomethylarsonate (MMA, equivalent to MSMA/DSMA/CAMA),

and cacodylic acid. Again, none of the sites were associated with the application of arsenical

pesticides. With a new method detection limit of approximately 0.2 ug/L, there were no measurable

concentrations of the methylated compounds in ground water. In contrast, arsenite or arsenate

concentrations in these groundwater samples ranged from less than the method detection limit of

approximately 0.2 ug/L to a concentration as high as approximately 900 ug/L. Their data show that

either arsenite or arsenate or both were detected in all samples, typically at 10-50 ug/L. Arsenite and

arsenate concentrations were unexpectedly comparable. In a personal communication, one of the study

authors expressed surprise at this latter comparability, since arsenate is generally assumed to be the

dominant of the two species.

Therefore, as will be stated again for emphasis in the surface water conclusions below, if total

arsenic concentrations at any location should exceed established levels of concern for any

known, combined chemical form of arsenic, then it would be prudent to speciate to determine

true individual identities and probably source(s), natural or otherwise.

Surface Water Exposure

In contrast with ground water, significant incremental exposure to MSMA/DSMA and the metabolite

CA from surface water from labeled use is expected. MSMA/DSMA use is focused primarily on

cotton and citrus, and such use could result in substantial areas of potential exposure.

In accordance with the fate characteristics given in the environmental fate section above, eroding soil

and runoff water would transport MSMA/DSMA and metabolites (cacodylic acid, for example) to

surface waters. The extensive USGS report cited above on arsenic in ground water (which had a

modicum of surface water information), as well as other sources (e.g., Hem, 1985), indicate that, in

general, total arsenic concentration in the majority of U.S. rivers and streams is less than 1 ug/L (ppb). 

However, spatial and temporal variability are the rule, and many notable exceptions of higher arsenic

concentrations occur. Surface water bodies such as lakes and reservoirs appear to have higher

concentrations more frequently than streams. Although in raw (untreated) surface waters in many areas

there are excursions far beyond the Office of Water’s current total arsenic Maximum Contaminant

Level (MCL) in drinking water of 50 ppb (a reduction to 10 ppb is pending approval), it would be

difficult to associate these findings in most of these areas with current arsenical pesticide use. Highest

concentrations are most likely to occur in geothermal areas and in water draining mineralized areas.

The BASINS and STORET databases maintained by the US EPA have a large body of data (many

tens of thousands or more of entries) on total arsenic concentrations in surface waters of the U.S.. 

However, because of the wide ranging concentrations (see below) in space and time, and the

multiplicity of possible major sources of arsenic, any attempt to associate these with arsenical pesticide
34

use would likely be a fruitless exercise, and would certainly require a major resource effort akin to that

by the USGS for groundwater. Nevertheless, even casual inspection of the data from the databases

shows that is not unusual for total arsenic concentrations in raw surface water in many different sites to

be several parts per billion. (Reporting limits in many cases appear to be 1 ppb, but in many other

cases are in excess of 10 ppb and up to approximately 50 ppb. The 10 ppb and 50 ppb limits may be

associated with the human health limits currently set for total arsenic set by the World Health

Organization and the US EPA, respectively.)

Spatial variability is the rule. The following data for Georgia and Texas are illustrative of this variability,

and are not meant to imply a special situation. For example, in Georgia, as presented in BASINS,

upper 85th percentile concentrations of total arsenic at different sites ranged from less than 1 ppb to

338 ppb. More typical high values would fall in the range of 10 to 50 ppb. The most probable

concentration appears to fall in the range of <1 to 2 or 3 ppb. The STORET database for Georgia and

Texas show similar results, with Texas seeming to average several parts per billion more. Some of the

highest values reported in STORET for Georgia and Texas (79,000 entries) approach 1000 ppb and

can often be associated with past point-source pollution by humans. Such pollution is becoming less

serious through environmental conscientiousness and governmental regulation. However, as previously

mentioned, natural sources (especially in places in the western U.S.) also produce such high

concentrations.

As stated above, speciated arsenic data are sparse, but informative in clearly showing the presence of

significant concentrations organic arsenic and generally higher concentrations of inorganic arsenic. In

1997, very limited USGS surface water monitoring data in arsenical pesticide use areas targeted four

arsenic species: MAA (MSMA/DSMA/CAMA equivalent), cacodylic acid (CA), arsenite, and

arsenate. USGS research scientists collected surface water samples from streams at one or two week

intervals from early March through the middle of September of 1997 from each of three sites

associated with different basins or sub-basins in an agricultural region potentially impacted by

application of MSMA (application is normally from May through July). A total of 24 samples were

analyzed from each site. Unfortunately, there were no baseline data for areas not associated with

application. Analyses showed maximum concentrations of MAA at the three sites were approximately

2, 3, and 5 :g/L (ppb), and were relatively short-lived. (The multiplicative factor for converting MAA

to MSMA is 1.2; thus 5 ppb of MAA converts to 6 ppb of MSMA.) Although it was apparently not in

use, concentrations of CA, a metabolite of MAA, were at or below the detection limit of 0.2 :g/L,

except for one sample which registered approximately 0.6 :g/L. For comparison, maximum measured

arsenite and arsenate concentrations were approximately 3 ppb and 5.5 ppb, respectively. The extent

of MSMA application and distance of the streams from application sites were not published, and there

were no baseline data for comparable areas not associated with MSMA application..

Sampling for the Florida study, published in 1973, was for 10 natural waters in the vicinity of Tampa. 

There were six surface fresh water bodies (two rivers, two ponds, two lakes), three saline waters (two

bays, one tidal flat), and one water well at a remote camping area. Tampa tap water was also
35

analyzed. Presumably, all water sites were sampled once with two or more replicates analyzed;

however, the study authors did not report specifics on sampling dates and the number of samples

analyzed. Concentrations of CA in the natural waters ranged up to 1.0 ppb. At one site, the

Hillsborough River, concentrations were below the detection limit of 0.02 ppb, while the median value

for CA at the other nine sites was approximately 0.3 ppb. The well water concentration was 0.2 ppb. 

MAA, detected in eight of the ten sites, was generally present at lower concentrations, the highest being

0.22 ppb. Total arsenic concentrations ranged from approximately 0.3 to 3.6 ppb. The extent to

which these concentrations represented the natural background (baseline) in the Tampa area or were

influenced by introduction of artificial sources is unknown. However, the study authors considered the

sampled sites as “natural” waters. Tampa tap water contained only traces of arsenate at the time.

Sampling for the cited California study published in 1991 was brought to our attention by the Office of

Water’s proposed rule for the regulation of arsenic (22 June 2000). MAA and CA concentrations in a

total of 14 California water bodies were measured (total of ten lakes, estuaries, or reservoirs, and a

total of four rivers or creeks). Highest measured concentrations of MAA and CA were equivalent to

1.2 ppb and 4.6 ppb, respectively (excluding one anomalous site that was so high in inorganic arsenic

as to swamp the analysis for the organic). There were no detectable concentrations (<0.1 nM) of

MAA or CA in the four streams, but eight of the ten other sites had measurable concentrations of MAA

and all ten had measurable concentrations of CA. Inorganic arsenic was present at all sites. Except for

the Davis Creek Reservoir, inorganic arsenic was in much higher concentrations than the organic. At

five sites, total arsenic concentrations were far above the current MCL of 50 ppb. The extent to which

these measurements may be affected by agriculture or the use of arsenical pesticides is unclear.

It is informative to compare the various actual measured results presented above with environmental

simulation model predictions, and then draw appropriate conclusions for surface water exposure. For

surface drinking water, the Agency has recently adapted the standard PRZM/EXAMS simulation

model to estimate runoff to an Index Drinking Water Reservoir (IR), and also made an allowance for

the portion of surrounding land which may be cropped. The resulting acute and chronic drinking water

concentrations for MSMA/DSMA for the IR scenario for cotton and citrus were based on 1) current

labeled application rates, and 2) field trial data application rates. Field trial data, from which the dietary

(food) exposure assessment was based, were collected at application rates lower than the label rates. 

The Agency is recommending that the labels be amended to reflect the lower rates (or that field trial

data be conducted at the current label rates). Details of the modeling inputs and assumptions used to

calculate index reservoir concentrations (IRC) given below can be found in Appendix 6.
36

Index Drinking Water Reservoir Simulation Results for MSMA and DSMA Use

Cotton: application rates and intervals

Labeled rate MSMA: 2 aerial applications, each at 3.0 lb a.i./acre, 14 days apart

Field trial rate MSMA: 2 aerial applications, each at 2.0 lb a.i./acre, 14 days apart

Labeled rate DSMA: 2 aerial applications, each at 3.6 lb a.i./acre, 14 days apart

Field trial rate DSMA: 2 aerial applications, each at 2.0 lb a.i./acre, 14 days apart

Cotton: Drinking Water Index Reservoir Concentrations (IRC)

Labeled / Field Trial Labeled / Field Trial

Acute IRC* Chronic IRC*

MSMA 83 / 56 :g/L (ppb) 21 / 14 :g/L

DSMA 100 / 56 :g/L 25 / 14 :g/L

*Acute = 1-in-10 year peak concentration, Chronic = 1-in-10 year average annual mean

Citrus application rates and intervals

Labeled rate MSMA: 3 ground applications each, at 4.7 lb a.i./acre, 14 days apart

Field trial rate MSMA: 3 ground applications, each at 4.0 lb a.i./acre, 14 days apart

Labeled rate DSMA: 3 ground applications, each at 6.3 lb a.i./acre, 14 days apart

Field trial rate DSMA: 3 ground applications, each at 4.9 lb a.i./acre, 14 days apart

Citrus: Drinking Water Index Reservoir Concentrations (IRC)

Labeled / Field Trial Labeled / Field Trial

Acute IRC Chronic IRC

MSMA 512 / 436 :g/L (ppb)136 / 116 :g/L

DSMA 687 / 534 :g/L 182 / 141 :g/L

Modeled concentrations are roughly and, at least superficially, akin to the higher total arsenic

concentrations reported in the BASINS and STORET databases. Modeled results for MSMA and

DSMA can be converted to arsenic equivalent concentrations by multiplying the tabulated results by a

factor of approximately one-half (0.46 and 0.41, respectively) and inversely, but this factor is really
37

immaterial when considering variabilities and uncertainties. However, as stated above, any attempt to

correlate the monitoring data for total arsenic with pesticidal use is a major resource effort, and requires

establishing natural background concentrations in use areas. Even if there were correlation, there would

still be niggling doubt of association without knowledge of arsenic speciation.

The USGS sampling results for individually speciated arsenic components (approx. 5-6 ug/L max. for

MSMA/DSMA) are seemingly lower than those from modeling for cotton by a rough factor of ten. 

(We do not have similar data for citrus use areas for comparison.) However, the USGS surface water

results were from flowing water bodies in non-targeted, general use areas, limited to a few months in

1997, and limited to so few samples over the time period as to have little statistical weight. The

speciated data from the Tampa, Florida, area were from “natural” waters and even sparser than that of

the USGS. Likewise, the representativeness of the California measurements which yield organic

arsenic results similar to those of the USGS is uncertain. In contrast, the index reservoir scenario yields

values intended to represent reservoirs in the upper 90% of susceptibility to runoff/erosion when viewed

over many years, and represents a rather quiescent water body in an area where use would be most

extensive.

Agency regulations currently assure that community water systems will provide public drinking water

which contains total arsenic at no more than the maximum contaminant level (MCL) of 50 :g/L (ppb). 

At the time of this writing, a new rule for a proposed MCL of 10 ppb is pending final approval. 

Consequently, community water systems are protected against all the modeled surface water

concentrations in excess of these limits.

Surface Water Exposure Conclusions

Runoff/erosion of applied surface deposits to surface waters and sediments is expected to cause

significant local, temporal perturbations above background concentrations for MSMA/DSMA and

CA. The higher measured total arsenic concentrations agree qualitatively with the Index Reservoir

model for uses on cotton. However, this agreement is probably coincidental because the measured

values are entangled with undetermined, highly variable natural background concentrations for which

there has been no attempt to separate use and non-use areas. The total arsenic data are therefore

inconclusive about arsenic origin. Speciated data are too sparse, not suitably targeted, and also

generally open-ended about origin. However, the speciated data do clearly and consistently show the

presence of MAA and CA, as well as comparable or larger amounts of arsenite and arsenate,

regardless of source. Also, independent of source, the limited available speciated data may

underestimate potential concentrations of pesticide-related arsenicals in drinking water reservoirs in

major use areas.

Therefore, whatever the source of arsenic, if there are toxicological concerns near the monitored,

modeled, and/or regulatory limit concentrations, then, clearly, this invites broader scale, systematic

environmental sampling and analysis for speciated arsenic, not just total arsenic, in order to determine
38

statistically the associations, ranges of exposure, and possible effects. Adequate analytical methods for

speciating arsenic are now readily available.

The effects of water treatment on raw water concentrations are known, and should be considered for

“public water systems” (as defined in 40 CFR 141.2). However, it also should be acknowledged that

many users of surface drinking water do not drink from regulated public water systems. According to

the USEPA Office of Drinking Water (personal communication from Charles Abernathy), which is

currently engaged in new regulation for total arsenic in drinking water in public water systems, many

larger water treatment facilities use sedimentation-flocculation procedures that are capable of lowering

total arsenic to 1 ppb or less. However, in smaller public water systems, the method is less effective,

and can lower concentrations to only approximately 20-25 ppb. At the present time, public water

systems only assure protection against excursions of total arsenic beyond the 50 ppb MCL (10 ppb if

pending regulation is enacted). Furthermore, within the regulatory limit for total arsenic in drinking

water are inorganic forms that are most often in higher concentrations and generally accepted to be far

more toxic than the organic.

Based on our surface water assessment, it is certain that application of arsenical pesticides will result in

a superposition upon existing local background concentrations of arsenic species in surface drinking

water. In some locations, these incremental amounts caused by pesticidal application would be

relatively small compared to some of the existing or “non-pesticidal” background concentrations. In

other locations where the background is low, the superposition could be relatively large. Provided

repeated, long-term pesticidal applications at the higher labeled rates do not occur, it is most

likely that local biogeochemistry of arsenic (redistribution, transformation, recycling) would moderate or

prevent significant alteration in local, long-term arsenic equilibria (for possible long-term accumulations,

see above Environmental Fate Subsection, Transformation Is By Microbes, third paragraph). 

Therefore, provided local biogeochemistry is not overwhelmed by excessive application of arsenical

pesticides, local, long-term average concentrations of available arsenic should remain relatively

unaffected.

Determining the kind (chemical speciation) and degree of acceptable arsenic exposure in drinking water

involves many regulatory considerations (for example, cost/benefit) representing complex issues for

EPA Program Offices. However, the Office of Drinking Water, after extensive project funding,

protracted deliberations with expert panels, practical considerations, and public/stakeholder input, has

set the regulatory limit for public water systems at 50 ppb (10 ppb if current proposed regulation is

enacted) for total arsenic (which includes organic and the more toxic inorganic forms), even if that limit

should happen to exceed the Office of Pesticide’s level of concern for arsenical pesticides or their byproducts.
39

AQUATIC EXPOSURE AND RISK ASSESSMENT

Exposure Assessment

Estimated environmental concentrations (EECs) for MSMA and DSMA are reported in Table 13. The

values were calculated using GENEEC with constant input parameters for interval between applications

(5 days), Koc (1121 ml/g), aerobic soil half-life (240 days), aerobic aquatic half-life (480 days), and

solubility in water (104,000 mg/L). The variable input parameters were application rate (conservatively

used maximum rate), number of applications, and application method (aerial for cotton, ground for noncrop, turf, and orchard). Depending on application rates and the number of applications, the values for

DSMA ranged from 98 ppb near cotton applications to 270 ppb near turf applications. MSMA values

ranged from 26 ppb near Kenaf applications to 605 ppb near orchard applications.

Hazard Asessment

Freshwater Animals

Toxicity data for freshwater organisms are presented in Table 9. DSMA could be considered

practically non-toxic to the tested fish species as the LC50s were greater than 112 ppm. MSMA 

toxicity range from slightly to moderately toxic to the tested fish species with the majority of LC50s

being between 10 and 100 ppm. The toxicity categories ranged from slightly toxic to practically nontoxic to the tested freshwater invertebrates. The LC50s of MSMA and DSMA to the water flea

(Daphnia magna) were 77.5 and 156 ppm, respectively.

The guidelines (72-1, 72-2) are fulfilled for DSMA and MSMA. Data are not required for CAMA.

Marine/Estuarine Animals

MSMA toxicity to sheepshead minnows was classified as practically non-toxic while the categories for

MSMA toxicity to marine/estuarine invertebrates ranges from moderately toxic to practically non-toxic

(Table 11). The LC50 of MSMA to the sheepshead minnow was considerably higher (323ppm) than

the range of LC50s reported for the tested marine/estuarine invertebrates (>1 to 173ppm).

The guidelines (72-3a,b,c) are fulfilled for MSMA but not for DSMA since no data were reported for

DSMA. Data were not required for CAMA.

Aquatic Plants

The EC50s of DSMA to different plant species range from 1.5 to 96.2ppm (Table 14). The EC50s of

MSMA to green algae and duckweed were 5.6ppm and 104.1ppm, respectively. Although a complete
40

complement of aquatic plant studies were not submitted for MSMA, EFED previously determined that

testing either DSMA or MSMA would suffice for both active ingredients. Testing was not required for

CAMA.

The guideline (123-2) is fulfilled for all three active ingredients.

Risk Assessment

Aquatic Animals

Based on the acute toxicity information provided to EFED, and the model generated EEC values, the

overall risk of MSMA toxicity to aquatic organisms would be considered low at any use site. The risk

quotients (EEC/effects endpoint concentration) were all below 0.5, the acute risk presumption RQ for

aquatic organisms (Table 11).

No risk is presumed for aquatic fish and invertebrates exposed to DSMA at the predicted

environmental concentrations (Table 9). Risk quotients remained <0.01 for all applications for all

marine fish and invertebrates, and for freshwater invertebrates. No toxicity data were presented for

freshwater fish so no RQs could be calculated.

Aquatic Plants

The acute LOC for non-endangered and endangered aquatic plants (RQ = 1) was not exceeded for

either MSMA or DSMA (Table 12) allowing for a presumption of low risk to aquatic plants exposed

to the predicted MSMA and DSMA concentrations.

TERRESTRIAL EXPOSURE AND RISK ASSESSMENT

Exposure Assessment

Food items

The EECs on potential bird and mammal food items are based on Hoerger and Kenaga (1972) as

modified by Fletcher et al. (1994), which presumes a 1 lb ai/acre pesticide application yielding the

following maximum EECs: 240 ppm for short grass; 135 ppm for broadleaved plants and insects; and

15 ppm for seeds, pods, and fruits. Using those values, the EECs for DSMA and MSMA ranged from

108 to 3078 ppm and 30 to 6277 ppm, respectively, depending on application rates, number of

applications, and food item (Table 2). Typically, fruits and seeds had lower predicted residue levels as

compared to short grasses and broad-leaved plants. The exposure levels for insects are predicted to

be similar to that for broad-leaved plants because the surface area of a leaf is assumed to approximate

that of an insect.
41

Off-site exposure for plants

Due to sheet runoff and spray drift, the EECs of upland plant exposure range from 0.12 to 3.36 lb ai/A

for MSMA and 0.216 to 0.403 lb ai/A for DSMA (Table 6). For semi-aquatic plants, the EECs range

from 1.12 to 8.68 lb ai/A for MSMA and 1.836 to 3.876lb ai/A for DSMA. EECs for upland (dry)

and lowland (semi-aquatic) off-site areas are for a single application. 

Hazard Assessment

Birds

Based on acute oral (LD50) and dietary tests (LC50) with northern bobwhite quail (Colinus

virginianus) and mallard ducks (Anas platyrhynchos), DSMA, MSMA, CAMA (all 82% a.i.) were

classified as being moderately to practically non-toxic (Table 5).

For DSMA, the LD50 to bobwhite was 627 mg/kg (moderately toxic) while the LC50s for bobwhite

and mallards were 4695 ppm (slightly toxic) and >5620 ppm (practically non-toxic), respectively. The

disparity between the LD50s and LC50s is likely due to absorptive efficiency differences between the

mode of exposure for oral and dietary tests, i.e. absorption efficience is likely to be greater for an oral

test as compared to a dietary test. Therefore, in comparison to an oral dose, a greater dose will be

necessary in a dietary test in order to get the same amount of active ingredient to the site of toxic action.

The LD50 of MSMA (51% a.i.) to bobwhite was 834 mg/kg (slightly toxic), while the LC50s were

3269 ppm (slightly toxic) and 5620 ppm (practically non-toxic) for the bobwhite and mallard,

respectively. When the MSMA LC50 values were adjusted to toxicity for 100% a.i., the values for the

bobwhite and mallard decreased to 425 ppm (moderately toxic) and 1667 ppm (slightly toxic),

respectively.

CAMA was classified as practically non-toxic to both bobwhite quail and mallard ducks. However,

the percent active ingredient was 10% indicating significant uncertainty about CAMA’s toxicity to avian

species.

Avian reproduction studies (71-4) are required but have not been submitted for any of these chemicals. 

Guidelines 71-1 and 71-2 are fulfilled for DSMA and MSMA but not for CAMA for which the

required studies have not been submitted. Guideline 71-4 is not fulfilled for any of these active

ingredients.
42

Mammals

The acute toxicity classifications of MSMA, DSMA, and CAMA to laboratory rats range from

moderately toxic to practically non-toxic (Table 1).

The LD50s for MSMA (34.35% a.i.), DSMA (82.7% a.i.), and CAMA (10.3% a.i.) are reported to

be 458 (moderately toxic), 1934 (slightly toxic), and >5000 mg/kg (practically non-toxic), respectively. 

Extrapolating the toxicity to 100% a.i. yields LD50s of 157 mg/kg (moderately toxic), 1599mg/kg

(slightly toxic), and 500mg/kg (moderately toxic. 

A no-observeable-adverse-effect-level (NOAEL) of 100ppm was determined for laboratory rats

exposed to methanearsonic acid (anion of MSMA, DSMA, and CAMA) during a two generation,

reproductive toxicity tests (Table 1).

Insects

MSMA, DSMA, and CAMA are practically non-toxic to honey bees (Table 15). The LD50s for each

were 68 (51% a.i.), >20.7 (82.7% a.i.), and >25 ug/bee (10.1% a.i.), respectively, for MSMA,

DSMA, and CAMA. Extrapolating the toxicity of MSMA to 100% a.i. would approximately double

the toxicity (35 ug/bee) of MSMA to honey bees. 

Because all the active ingredients are practically nontoxic to the honey bee, precautionary labeling is not

needed. The guideline (141-1) is fulfilled.

Non-target Terrestrial Plants (Tier II)

DSMA (82.7% a.i.) and MSMA (51% a.i.) toxicity to non-target agricultural plants are reported in

Table 16. Very little information was presented for the EC25 on seedling emergence. The most

sensitive species to MSMA, with respect to the EC25, was cabbage (0.116 lb a.i./A). The remainder

of the EC25 values for both chemicals ranged from 1.07 to 1.44 lb a.i./A. The majority of the seedling

emergence toxicity values were the NOECs. These ranged from <0.3 to 4.86 lb a.i./A for DSMA, and

0.018 to 2.24 lb a.i./A for MSMA. More toxicity information was presented for vegetative vigor. The

EC25s ranged from 0.354 to 2.43 lb a.i./A for DSMA and from 0.418 to >2.24 lb a.i./A for MSMA. 

The NOECs ranged from <0.30 to 4.86 lb a.i./A for DSMA and from 0.14 to 2.24 lb a.i./A for

MSMA. Only the most sensitive endpoint is reported for each species.
43

Risk Assessment

Birds

The RQ values for acute lethality to avian species is divided into three categories: herbivores,

insectivores, and granivores (Table 4). For DSMA, acute risk (RQ = 0.5) to herbivorous birds is

presumed for all applications except on cotton, for which the restricted use LOC (RQ = 0.2 is

exceeded. The RQs range from 0.38 to 1.2, the highest RQ from turf applications. The acute risk

LOC is also exceeded for insectivorous birds for cotton and non-crop applications of DSMA. None

of the DSMA applications exceeded the LOCs for granivores. 

For MSMA, the acute risk LOCs are exceeded for herbivorous species non-crop, orchard, and turf

applications, for insectivorous species near non-crop and orchard applications, and not at all for

granivorous species. The restricted use LOC was exceeded for herbivorous species near cotton and

seed grass applications, for insectivorous species near cotton, seed grass, and turf applications, and not

at all for granivorous species.

CAMA, while the tested formulation would be classified as practically nontoxic to birds, it must be

understood that the formulation was only 10% a.i. The acute LOC was exceed for herbivorous

species, while the restricted use LOC was exceeded for insectivorous species. No levels of concern

were exceed for granivorous species.

Avian reproductive risk cannot be assessed until toxicity data are submitted. A potential for

reproductive risk exists from avian exposure to DSMA and MSMA, because applications may occur in

at use sites during the avian reproductive season.

Mammals

The acute risk LOC (RQ > 0.5) is exceed for small herbivorous mammals exposed to DSMA in all

uses (Table 3). The RQs ranged from 0.5 to 1.45 for DSMA. The RQs were greatest for DSMA use

on turf grass. The acute risk LOC was also exceed for insectivorous mammals for all uses except on

cotton, a use for which the restricted use LOC (RQ > 0.2) was exceeded.

All MSMA uses except Kenaf resulted in acute risk exceedences for both herbivorous and

insectivorous species. The only exceedence for granivorous species was for endangered species near

orchard applications.

While CAMA was classified as practically non-toxic to mammals, the percent active ingredient was

10%. Considerable uncertainty exists about CAMA’s inherent toxicity. The acute toxicity LOC was

exceeded for herbivorous species, while the restricted use LOC was exceeded for insectivorous

species. No LOC was exceeded for granivorous species.
44

The reproductive risk LOC (RQ = 1) was exceeded for all MSMA, DSMA, and CAMA uses except

for granivores exposed to DSMA near cotton fields, and to MSMA near cotton fields, seed grass, and

kenaf applications (Table 17).

Non-endangered plants

Risk is presumed for non-endangered plant species inhabiting upland and especially lowland areas that

are exposed to MSMA runoff and drift (i.e., total loading) from all use sites as the LOC (RQ > 1) is

exceeded for all uses (Table 7). Also at risk are non-endangered plants inhabiting lowland areas

receiving runoff and drift of DSMA from all use sites. Minimal risk from DSMA is presumed for

upland plants and areas receiving only drift exposure.

Endangered Species

LOCs for endangered species are exceeded for birds (MSMA, DSMA), mammals (MSMA, DSMA),

freshwater fish (MSMA), and terrestrial plants (MSMA, DSMA).

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

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

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

that their actions are not likely to jeopardize listed species or adversely modify designated critical

habitat. To analyze the potential of registered pesticide uses to affect any particular species, EPA puts

basic toxicity and exposure data developed for REDs into context for individual listed species and their

locations by evaluating important ecological parameters, pesticide use information, the geographic

relationship between specific pesticides uses and species locations, and biological requirements and

behavioral aspects of the particular species. This analysis will include consideration of the regulatory

changes recommended in this RED. A determination that there is a likelihood of potential impact to a

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

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

as necessary.

At present, the program is being implemented on an interim basis as described in a Federal Register

notice (54 FR 27984-28008, July 3, 1989). A final program, which may be altered from the interim

program, will be proposed in a Federal Register notice scheduled for publication in autumn of 2001.
45

APPENDIX 1

Toxicity, EEC, and Risk Quotient Tables
46

Table 1. Toxicity of DSMA, MSMA, and CAMA to laboratory rats.

Chemical/

Test species % ai

Acute

LD50

(mg/kg)

Reproductive

NOAEL

(ppm)

Toxicity

Classification

 MRID 

 No.

DSMA

 Lab. rat 82.7 1934*/15991 NA slightly toxic 418920-04

MSMA

 Lab. rat 34.35 458*/ 1571 NA moderately toxic 00105171

CAMA

 Lab. rat 10.3 >5000/5001 NA # 010991

Methanearsonic acid

 Lab. rat 99.4 NA 100 431783-01

1

 adjusted for the purity of the test material

* Acute values utilized for risk quotient calculation

# No toxicity classification was designated since actual toxicity is unknown.
47

Table 2. Estimated environmental concentrations (EECs) (ppm) of DSMA, MSMA, and CAMA on

terrestrial food items resulting from direct application of active ingredient.

Chemical/Site

Max. Appl.

Rates

(lb ai/A)

No.

Appl.

Predicted Maximum EEC

on Food items1

Short grasses Broadleaf plants and

Insects

Seeds

DSMA

cotton 3.6 2 1519 854 95

non-crop 5.7 3 3190 1795 199

orchard 6.3 3 3526 1983 220

turf 7.6 3 4253 2393 266

MSMA

cotton 3 2 1265 712 79

non-crop 6 3 3358 1889 210

orchard 15.5 3 8676 4880 542

Kenaf 2 1 480 270 30

seed grass 6 1 1440 810 90

turf 4.2 3 2351 1322 147

CAMA

turf 5 3 2799 1574 175

* See appendix four for assumptions and formulae used to calculate residue levels on food items.
48

Table 3. Risk quotients for small mammal exposure to DSMA and MSMA.

Chemical/Site Appl. Rate

(lb ai/A)

EEC (ppm) Acute RQ1,2

Grasses Insects Seeds Herbivores3 Insectivores3 Granivores4 

DSMA

Cotton 3.6 1519 854 95 0.52 *** 0.29 ** <0.1

Noncrop 5.7 3190 1795 199 1.09 *** 0.61 *** <0.1

Orchard 6.3 3526 1983 220 1.20 *** 0.68 *** <0.1

Turf 7.6 4253 2393 266 1.45 *** 0.82 *** <0.1

MSMA

Cotton 3 1265 712 79 1.82 *** 1.02 *** <0.1

Noncrop 6 3358 1889 210 4.84 *** 2.72 *** <0.1

Orchard 15.5 8676 4880 542 12.50 *** 7.03 *** 0.18 *

Kenaf 2 480 270 30 0.69 *** 0.39 ** <0.1

Seed grass 6 1440 810 90 2.07 *** 1.17 *** <0.1

Turf 4.2 2351 1322 147 3.39 *** 1.90 *** <0.1

CAMA

turf 5 2799 1574 175 0.37 *** 0.21 ** <0.1

1

 RQ = EEC / [LD50 / food eaten expressed as % of bw]

2

 LD50s = 1934 mg/kg for DSMA, 458 mg/kg for MSMA, and 5000 mg/kg for CAMA 

3

 for a 35 g mammal that consumes an amount of green vegetation or insects equivalent to 66% of its body weight 

4

 for a 35 g mammal that consumes an amount of seeds equivalent to 15% of its body weight

*** exceeds the LOCs for high risk (RQ >0.5), restricted use (RQ >0.2), and endangered species (RQ >0.1)

 ** exceeds the LOCs for restricted use and endangered species

 * exceeds the LOC for endangered species
49

Table 4. Risk quotients for avian species exposure to DSMA and MSMA.

Chemical/Site

Appl.

Rate

(lb ai/A)

EEC (ppm) Acute RQ1,2

Grasses Insects Seeds Herbivores Insectivores Granivores

DSMA

Cotton 3.6 1519 854 95 0.32 ** 0.18 * <0.1

Noncrop 5.7 3190 1795 199 0.68 *** 0.38 ** <0.1

Orchard 6.3 3526 1983 220 0.75 *** 0.42 ** <0.1

Turf 7.6 4253 2393 266 0.90 *** 0.51 *** <0.1

MSMA

CAMA

Cotton 3 1265 712 79 0.39 ** 0.22 ** <0.1

Noncrop 6 3358 1889 210 1.03 *** 0.58 *** <0.1

Orchard 15.5 8676 4880 542 2.65 *** 1.49 *** 0.17 *

Kenaf 2 480 270 30 0.15 * <0.1 <.0.1

Seed Grass 6 1440 810 90 0.44 ** 0.25 ** <0.1

Turf 4.2 2351 1322 147 0.72 *** 0.40 ** <0.1

turf 5 2799 1574 175 0.50 *** 0.28 ** <0.1

1

 RQ = EEC/LC50 

2

 LC50s = 4695 (northern bobwhite) for DSMA, 3269 ppm (northern bobwhite) for MSMA, and 5620 for CAMA

*** exceeds the LOCs for high risk (RQ >0.5), restricted use (RQ >0.2), and endangered species (RQ >0.1)

 ** exceeds the LOCs for restricted use and endangered species

 * exceeds the LOC for endangered species
50

Table 5. Acute toxicity of DSMA and MSMA to northern bobwhite (Colinus virginianus) and mallard ducks (Anas platyrhynchos).

Chemical/

Test species

Test % ai Toxicity

Value*

Toxicity

Classification MRID No.

Study

Classification

DSMA

 Northern bobwhite Acute oral 82 627/5141slightly toxic 418920-01 core

 Northern bobwhite Dietary 82 4695 **/38501slightly toxic 418920-03 core

 Mallard Dietary 82 >5620/46061 practically non-toxic 418920-02 core

 Northern bobwhite Reproduction no data outstanding

 Mallard Reproduction no data outstanding

MSMA

 Northern bobwhite Acute oral 51 834/4251 moderately toxic 416100-02 core

 Northern bobwhite Dietary 51 3269 **/16671slightly toxic 416100-03 core

 Mallard Dietary 51 >5620/29121 practically non-toxic 416100-04 core

 Northern bobwhite Reproduction no data outstanding

 Mallard Reproduction no data outstanding

CAMA

 Northern bobwhite Acute oral 10.1 >2250/>2271 # 433164-03 outstanding

 Northern bobwhite Dietary 10.1 >5620 **/>5641 # 433164-02 outstanding

 Mallard Dietary 10.1 >5620/>5641 # 433164-01 outstanding

 Northern bobwhite Reproduction no data outstanding

 Mallard Reproduction no data outstanding

1

 adjusted value based on the purity of the test material

* toxicity values for acute oral and dietary tests are in mg/kg and ppm, respectively

** Acute values utilized for risk quotient calculations
51

# No toxicity classification was designated since actual toxicity is unknown. 
52

Table 6. Estimated environmental concentrations* (EECs) (lb ai/A) in terrestrial environments off-site from the application site resulting from drift and

runoff from different application rates.

Chemical/Site Appl. Rate

(lb ai/A)

Runoff (lb ai/A)

Drift

(lb ai/A)

Total Load (lb ai/A) 

Sheet Channelized Upland1 Semi-aquatic2

DSMA

cotton 3.6 0.18 1.8 0.036 0.216 1.836

non-crop areas 5.7 0.285 2.85 0.057 0.342 2.907

orchard 6.3 0.315 3.15 0.063 0.378 3.213

turf 7.6 0.38 3.8 0.076 0.403 3.876

MSMA

cotton 3 0.15 1.5 0.03 0.18 1.53

noncrop areas 6 0.3 3 0.06 0.36 3.36

orchard 15.5 0.775 7.75 0.155 0.93 8.68

Kenaf 2 0.100 1.000 0.020 0.120 1.120

seed grass 6 0.300 3.000 0.060 0.360 3.060

turf 4.200 0.210 2.100 0.042 0.240 2.142

CAMA

turf 5 0.25 2.5 0.05 0.3 2.55

1

 sheet runoff plus drift

2

 channelized runoff plus drift

*

 See appendix three for formulae and assumptions utilized for calculating off-site terrestrial exposure levels.
53

Table 7. Risk quotients for non-endangered terrestrial plants from a single application.

Chemical/Site

Appl. Rate

(lb ai/A) 

Appl.

Method

Total Loading (lb ai/A) Drift

(lb ai/A)

RQ3,4

Upland1Lowland2 Upland Lowland Drift

DSMA

 Cotton

 3.6

ground 0.216 1.836 0.036 <1 <1 - 2 * <1

aerial 0.315 1.530 0.18 <1 <1 - 1 * <1

 Noncrop 2.3 - 5.7 ground 0.138 - 0.342 1.173 - 2.907 0.023 - 0.057 <1 <1 - 2 * <1

 Orchard 4.0 - 6.3 ground 0.204 - 0.378 2.040 - 3.213 0.040 - 0.063 <1 2 - 3 * <1

 Turf 2.3 - 7.6 ground 0.138 - 0.403 1.173 - 3.876 0.023 - 0.076 <1 <1 - 3 * <1

MSMA

 Cotton 1 - 3

ground 0.060 - 0.180 0.510 - 1.530 0.010 - 0.030 <1 - 2 * 4 - 13 * <1

aerial 0.088 - 0.263 0.430 - 1.280 0.050 - 0.150 <1 - 2 * 4 - 11 * <1

 Noncrop 2.1 - 6 ground 0.126 - 0.360 1.070 - 3.360 0.021 - 0.060 1 - 3 * 9 - 29 * <1

 Orchard 2 - 15.5 ground 0.120 - 0.930 1.120 - 8.680 0.020 - 0.155 1 - 8 * 10 - 75 * <1

 Kenaf 2

ground 0.120 1.120 0.020 1 * 10 * <1

aerial 0.175 0.850 0.100 2 * 7 * <1

 Seed grass 6

ground 0.360 3.060 0.060 3 * 26 * <1

aerial 0.525 2.550 0.300 5 * 22 * <1

 Turf 3 - 4.2 ground 0.180 - 0.240 1.530 - 2.142 0.030 - 0.042 2 * 13 - 19 * <1

1

 sheet runoff and drift

2

 channelized runoff and drift 

3

 RQ = lb ai/A / EC25 (seedling emergence for total loading and vegetative vigor for drift)

4

 Seedling emergence EC25s = 1.25 and 0.116 lb ai/A for DSMA and MSMA, respectively;

 vegetative vigor EC25s = 0.354 and 0.418 lb ai/A for DSMA and MSMA, respectively

* exceeds the LOC (RQ >1) for nontarget plants
54

Table 8. Risk quotients for endangered terrestrial plants from a single application.

Chemical/Site Appl. Rate

(lb ai/A) 

Appl.

Method

Total Loading (lb ai/A) Drift

(lb ai/A)

RQ3,4

Upland1Lowland2 Upland Lowland Drift5

DSMA

 Coton 1.1 - 3.6

ground 0.066 - 0.216 0.561 - 1.836 0.011 - 0.036 <1 2 - 6 * <1

aerial 0.096 - 0.315 0.465 - 1.530 0.055 - 0.180 <1 - 1 * 2 - 5 * <1

 Noncrop 2.3 - 5.7 ground 0.138 - 0.342 1.173 - 2.907 0.023 - 0.057 <1 - 1 * 4 - 10 * <1

 Orchard 4.0 - 6.3 ground 0.204 - 0.378 2.040 - 3.213 0.040 - 0.063 <1 - 1 * 7 - 11 * <1

 Turf 2.3 - 7.6 ground 0.138 - 0.403 1.173 - 3.876 0.023 - 0.076 <1 - 1 * 4 - 13 * <1

MSMA

 Cotton 1 - 3

ground 0.060 - 0.180 0.510 - 1.530 0.010 - 0.030 3 - 10 * 28 - 85 * <1

aerial 0.088 - 0.263 0.430 - 1.280 0.050 - 0.150 5 - 15 * 24 - 71 * <1 - 1 *

 Noncrop 2.1 - 6 ground 0.126 - 0.360 1.070 - 3.360 0.021 - 0.060 7 - 20 * 59 - 187 * <1

 Orchard 2 - 15.5 ground 0.120 - 0.930 1.120 - 8.680 0.020 - 0.155 7 - 52 * 62 - 482 * <1 - 1 *

 Kenaf 2

ground 0.120 1.120 0.020 7 * 62 * <1

aerial 0.175 0.850 0.100 10 * 47 * <1

 Seed grass 6

ground 0.360 3.060 0.060 20 * 170 * <1

aerial 0.525 2.550 0.300 29 * 142 * 2 *

 Turf 3 - 4.2 ground 0.180 - 0.240 1.530 - 2.142 0.030 - 0.042 10 - 13 * 85 - 119 * <1

1

 sheet runoff and drift

2

 channelized runoff and drift

3

 RQ = lb ai/A / NOEC (seedling emergence for total loading and vegetative vigor for drift)

4

 Seedling emergence NOECs = 0.30 and 0.018 lb ai/A for DSMA and MSMA, respectively;

 vegetative vigor NOECs = <0.30 and 0.14 lb ai/A for DSMA and MSMA, respectively

* exceeds the LOC (RQ >1) for nontarget plants

5

 used NOEC of 0.3 for DSMA affect on vegetative vigor
55

Table 9. Acute toxicity of DSMA and MSMA to various freshwater fish species and an aquatic invertebrate.

Chemical/

 Test Species % ai

Test

Conditions

LC50

 (ppm)

Toxicity

Classification MRID No.

Study

Classification

DSMA

Bluegill 82.7 flow-through >112 * practically non-toxic 419056-01 core

Rainbow trout 82.7 flow-through >114 practically non-toxic 419056-02 core

Water flea 82.7 static 153 * practically non-toxic 419406-04 core

MSMA

Bluegill 51 flow-through >93/<47.41# 417480-01 core

Bluegill 51.2 static >100/<511# 400980-01 core

Bluegill 34.8 static 12 */ 4.11slightly - moderately toxic 400980-01 core

Bluegill 37.7 static 49.2 / 18.51slightly toxic 400980-01 core

Rainbow trout 51 flow-through >167/<851# 417473-01 core

Rainbow trout 51.2 static 78 / 401slightly toxic 400980-01 core

Fathead minnow 34.8 static 13.3 / 4.61slightly - moderately toxic 400980-01 supplemental

Channel catfish 37.7 static 26.8 / 10.11slightly toxic 400980-01 supplemental

Goldfish 34.8 static 31.1 / 10.81slightly toxic 400980-01 supplemental

Water flea static 77.5 */401slightly toxic 419406-05 core

Scud 34.8 static >100<511# 400980-01 supplemental

1

 adjusted LC50 or EC50 is based on the purity of the test material

* Acute toxicity values used for risk quotient calculation

# No toxicity classification was designated since actual toxicity is unknown.
56

Table 10. Risk quotients for DSMA and MSMA exposure to aquatic organisms.

Chemical/

Site

Appl.

Rate

(lb ai/A)

No.

Appl.

Peak

EEC

(ppb)

Acute RQ1 - Freshwater Acute RQ1 - Estuarine/marine

Fish2Invert.3 Fish5Invert.4

DSMA

Cotton 3.6 2 98 <0.05 <0.05 * *

Non-crop 5.7 3 202 <0.05 <0.05 * *

Orchard 6.3 3 270 <0.05 <0.05 * *

Turf 7.6 3 224 <0.05 <0.05 * *

MSMA

Cotton 3 2 78 <0.05 <0.05 <0.05 <0.05

Non-crop 6 3 234 <0.05 <0.05 <0.05 <0.05

Orchard 15.5 3 605 0.05 * <0.05 <0.05 <0.05

Kenaf 2 1 26 <0.05 <0.05 <0.05 <0.05

Seed grass 6 1 78 <0.05 <0.05 <0.05 <0.05

Turf 4.2 3 164 <0.05 <0.05 <0.05 <0.05

1

 RQ = EEC/LC50 (fish) or EC50 (invertebrates)

2

 LC50s = 112 ppm for DSMA and 12 ppm for MSMA 

3

 EC50s = 153 ppm for DSMA and 77.5 ppm for MSMA

4

 EC50 = 160 ppm for MSMA

5

 LC50 = 323 ppm for MSMA

* No toxicity data for DSMA and marine/estuarine organisms were presented, so no RQs were calculated.
57

Table 11. Acute toxicity of DSMA and MSMA to various estuarine and saltwater fish and invertebrate species.

Chemical/

 Test Species % ai

Test 

Conditions

LC50/EC50

(ppm)

Toxicity

Classification MRID No.

Study

Classification

DSMA

Sheepshead minnow no data outstanding

Mysid shrimp no data outstanding

Oyster no data outstanding

MSMA

Sheepshead minnow 99.4 static 323* practically non-toxic 424141-02 core

Mysid shrimp 99.4 static 173 practically non-toxic 424141-01 supplemental

Eastern oyster 50.4 flow-through 160* / 801practically non-toxic / 

moderately toxic

424648-01 core

Eastern oyster 63 flow-through >1/>0.631# 400980-01 supplemental

Pink shrimp 63 flow-through >1/>0.631# 400980-01 supplemental

1

 adjusted for the purity of the test material

* Acute toxicity values utilized for risk quotient calculation

# No toxicity classification was designated since actual toxicity is unknown
58

Table 12. Risk quotients for non-endangered aquatic plant exposure to DSMA and MSMA.

Chemical/

 Site

Appl. Rate

(lb ai/A)

No.

Appl.

Peak EEC

(ppb)

 RQ1,2

non-endangered 

vascular spp.

non-endangered 

non-vascular spp.

endangered 

vascular spp.

DSMA

Cotton 1.1 - 3.6 2 32 - 98 <0.1 <0.1 <0.1

Noncrop 2.3 - 5.7 3 67 - 202 <0.1 <0.1 0.13

Orchard 4 - 6.3 3 90 - 270 <0.1 <0.1 0.18

Turf 2.3 - 7.6 3 74 - 223 <0.1 <0.1 0.15

MSMA

Cotton 1 - 3 2 26 - 78 <0.1 <0.1 <0.1 

Noncrop 2.1 - 6 3 82 - 156 <0.1 <0.1 <0.1 

Orchard 2 - 15.5 3 78 - 605 <0.1 0.2 <0.1 

Kenaf 2 1 26 <0.1 <0.1 <0.1 

Seed grass 6 1 78 <0.1 <0.1 <0.1 

1

 RQ = EEC / EC50 (non-endangered species) or NOEC (endangered species)

2

 vascular (duckweed) EC50's and NOECs are 72.7 ppm and 20.5 ppm for DSMA and 53 and 29 ppm for MSMA; non-vascular

(algae or diatom) EC50's are 1.5 ppm for DSMA and 2.8 ppm for MSMA
59

Table 13. Estimated environmental concentrations (EECs) for MSMA and DSMA in the aquatic

environment.

Application rate

(lbs a.i./A)

Number of

Applications Peak EEC (ppb)

DSMA

Cotton 3.6 2 98

Non-crop 5.7 3 202

turf 7.6 3 270

orchard 6.3 3 223

MSMA

cotton

3 2 78

noncrop areas

3 3 234

orchard

15.5 3 605

Kenaf

6 1 26

seed grass 

6 1 78

turf 4.2 3 164
60

Table 14. Acute toxicity of DSMA and MSMA to aquatic algal, diatom, and plant species.

Chemical/

Test Species % ai

EC50

(ppm)

NOEC

(ppm) MRID No.

Study 

Classification

DSMA

Duckweed 82.7 72.7 20.5 419406-03 core

Green algae 82.7 96.2 <67 417911-01 core

Marine diatom 81.35 38.8 <26.6 431845-03 core

Marine diatom 81.35 211 32.9 431845-01 core

Blue-green algae 81.35 1.5 0.9 431845-02 core

MSMA

Duckweed 51 104.1 / 531* 56.8 / 291* 417482-02 core

Green algae 51 5.6 / 2.81* <0.3 417482-01 core

* Acute toxicity values utilized for risk quotient calculation

1

 adjusted value based on the purity of the test material
61

Table 15. Acute toxicity of MSMA, DSMA, and CAMA to honey bees.

Chemical % ai

LD50

(μg/bee)

Toxicity

Classification MRID No.

Study 

Classification

DSMA

 Honey bee 82.7 >20.7 practically non-toxic 419354-01 core

MSMA

 Honey bee 51 68

351

practically non-toxic 416100-07 core

CAMA

 Honey bee 10.1 >25 practically non-toxic 430511-01 core

1

 adjusted value based on the purity of the test material 
62

Table 16. Acute toxicity of DSMA, MSMA, and CAMA to non-target algricultural plants (Tier II).

Chemical/

Test Species

Seedling Emergence Vegetative Vigor

Endpoint 1 EC25

(lb ai/A)

NOEC

(lb ai/A)

Endpoint 1 EC25

(lb ai/A)

NOEC

(lb ai/A)

DSMA 1,2 

 monocots:

 Corn none n/a 4.86 dry weight 0.354* <0.30

 Oat phytotoxicity <0.30 phytotoxicity n/a 0.61

 Ryegrass none n/a 4.86 none n/a 4.86

 Onion none n/a 4.86 none n/a 4.86

 dicots:

 Soybean phytotoxicity n/a 1.22 none n/a 4.86

 Carrot none n/a 4.86 phytotoxicity n/a 2.43

 Tomato none n/a 4.86 dry weight 0.666 0.30

 Cabbage dry weight 1.25* 1.22 dry weight 0.419 <0.30*

 Cucumber none n/a 4.86 dry weight 2.43 0.61

 Lettuce dry weight 1.44 0.30* dry weight 0.705 0.30

MSMA 4,5

 monocots:

 Corn none n/a 2.24 none n/a 2.24

 Oat none n/a 1.12 dry weight 0.418* 0.14*

 Ryegrass none n/a 2.24 none n/a 2.24

 Onion none n/a 2.24 plant height >2.24 0.56

 dicots:

 Soybean none n/a 2.24 dry weight 0.712 0.56

 Carrot none n/a 2.24 none n/a 2.24

 Tomato none n/a 2.24 plant height 1.104 0.28

 Cabbage dry weight 0.116* 0.0184* plant height 0.516 0.286

 Cucumber none n/a 2.24 dry weight >2.24 0.56

1

 only the most sensitive endpoint has been tabulated for each species

2

 the test material was 82.7% ai DSMA

3

 MRID Nos 419056-03 for seedling emergence (supplemental) and MRID No. 419056-04 for vegetative vigor (supplemental)
63

4

 the test material was 51% ai MSMA

5

 MRID Nos 417055-01 and 425725-01 for seedling emergence (core) and 417055-02 for vegetative vigor (core)

6

 based on phytotoxicity

* Acute toxicity values utilized for risk quotient calculations
64

Table 17. Chronic risk quotients for small mammals from exposure to methanearsonic acid (MMA)*.

Chemical/Site

Max. Appl.

Rates

(lb ai/A)

No.

Appl.

Predicted Maximum EEC

on Food items1

Reproductive Risk Quotients1 **

Short

grasses

Broadleaf plants

and Insects

Seeds Herbivores Insectivores Granivores

DSMA

cotton 3.6 2 1519 854 95 15.19 8.54 0.95

non-crop 5.7 3 3190 1795 199 31.90 17.95 1.99

orchard 6.3 3 3526 1983 220 35.26 19.83 2.20

turf 7.6 3 4253 2393 266 42.53 23.93 2.66

MSMA

cotton 3 2 1265 712 79 12.65 7.12 0.79

non-crop 6 3 3358 1889 210 33.58 18.89 2.1

orchard 15.5 3 8676 4880 542 86.76 48.80 5.42

Kenaf 2 1 480 270 30 4.80 2.70 0.3

seed grass 6 1 1440 810 90 14.40 8.10 0.9

turf 4.2 3 2351 1322 147 23.51 13.22 1.47

CAMA

turf 5 3 2799 1574 175 27.99 15.74 1.75

* Methanearsonic acid is the anion of MSMA, DSMA, and CAMA, and is generated upon dissolution of MSMA, DSMA, and CAMA in water.

** The level of concern for reproductive risk is 1.

1

 Reproductive risk RQ = EEC/NOEC (100ppm)
65

APPENDIX 2

DEFINITIONS AND EXPLANATIONS OF TERMS AND ACRONYMS

RISK QUOTIENTS AND LEVELS OF CONCERN

Risk quotients (RQs) are compared to levels of concern (LOCs) to assess the potential for adverse ecological effects

to nontarget organisms. An RQ is an index comparing estimated environmental concentrations (EECs) with the most

sensitive ecotoxicity values, where

 

 RQ = EEC / TOXICITY 

Risk presumptions are made by comparing acute and chronic RQs to LOCs for birds, mammals, plants, and aquatic

organisms. Exceedance of an LOC indicates the potential for serious risk to nontarget organisms and the need for

the Agency to consider regulatory action. LOCs address the following risk presumption categories: 

acute high risk: regulatory action may be warranted to eliminate or reduce risk; 

acute restricted use: risk may be mitigated by restricted use classification; 

acute endangered species: regulatory action may be warranted to protect endangered species; and 

reproductive and chronic risk: regulatory action may be warranted to eliminate or reduce reproductive

or chronic risk. 

The ecotoxicity values for acute effects are: 

LC50: birds and fish

LD50: mammals 

EC50: aquatic plants and aquatic invertebrates 

EC25: terrestrial plants

NOAEL: endangered terrestrial and aquatic plants

The NOAEL is the ecotoxicity value for reproductive and chronic effects for birds, mammals, fish, and aquatic

invertebrates. 

Risk presumptions are tabulated below.
66

Risk Presumption RQ LOC

Birds and Mammals

Acute High Risk EEC1/LC50 or LD50/sqft2 or LD50/day30.5

Acute Restricted Use EEC/LC50 or LD50/sqft or LD50/day (or LD50 < 50 mg/kg) 0.2

Acute Endangered Species EEC/LC50 or LD50/sqft or LD50/day 0.1

Reproductive Risk EEC/NOAEL 1

Aquatic Animals

Acute High Risk EEC4/LC50 or EC50 0.5

Acute Restricted Use EEC/LC50 or EC50 0.1

Acute Endangered Species EEC/LC50 or EC50 0.05

Chronic Risk EEC/NOAEL 1

Plants

Acute Risk EEC5/EC25 (terrestrial) or EEC4/ EC50 (aquatic) 1

Endangered Species EEC/EC05 or NOAEL 1

 

1

 ppm ai on avian and mammalian food items (grass, insects, seeds) 

 

2

 mg ai per ft2 ÷ [LD50 * animal wt (kg)] 

 

3

 mg ai consumed per day ÷ [LD50 * animal wt (kg)]

 

4

 aquatic concentration of ai 

 

5

 lb ai/A 
67

APPENDIX 3

FORMULAS AND ASSUMPTIONS FOR OFF-SITE TERRESTRIAL PLANT EXPOSURE

EECs for Terrestrial Plants Inhabiting Upland Areas Adjacent to Treatment Sites 

 Ground application:

 Runoff = maximum application rate (lb ai/A) x runoff value (1%, 2%, or 5%)

 Drift = maximum application rate (lb ai/A) x 1% (drift) 

 Total Loading = runoff (lb ai/A) + drift (lb ai/A) 

 Aerial application:

 Runoff = max. appl. rate (lb ai/A) x 75% (application efficiency) x runoff value

 Drift = max. appl. rate (lb ai/A) x 5% (drift)

 Total Loading = runoff (lb ai/A) + drift (lb ai/A)

EECs for Terrestrial Plants Inhabiting Lowland Areas

 Ground application:

 Runoff = max. appl. rate (lb ai/A) x runoff value x 10 acres

 Drift = max. appl. rate x 1%

 Total Loading = runoff (lb ai/A) + drift (lb ai/A) 

 Aerial application:

 Runoff = max. appl. rate (lb ai/acre) x 75% x runoff value x 10 acres

 Drift = max. appl. rate (lb ai/A) x 5% 

 Total Loading = runoff (lb ai/A) + drift (lb ai/A)

Terrestrial plants inhabiting dry and semi-aquatic areas may be exposed to pesticides from runoff, spray drift or

volatilization. Semi-aquatic areas are those low-lying wet areas that may be dry at certain times of the year. EFED's

runoff scenario is: (1) based on a pesticide's water solubility and the amount of pesticide present on the soil surface

and its top one inch, (2) characterized as "sheet runoff" (one treated acre to an adjacent acre) for dry areas, (3)

characterized as "channelized runoff" (10 treated acres to a distant low-lying acre) for semi-aquatic areas, and (4)

based on % runoff values of 0.01, 0.02, and 0.05 for water solubility of <10 ppm, 10-100 ppm, and >100 ppm,

respectively. 

Spray drift exposure from ground application is assumed to be 1% of the application rate. Spray drift from aerial,

airblast, forced-air, and chemigation applications is assumed to be 5% of the application rate. 

EECs are calculated for the following application methods: (1) unincorporated ground applications, (2) incorporated

ground application, and (3) aerial, airblast, forced-air, and chemigation applications. Formulas for calculating EECs

for dry areas adjacent to treatment sites and EECs for semi-aquatic areas are in an addendum.
68

APPENDIX 4

ESTIMATING RESIDUES ON TERRESTRIAL FOOD ITEMS

The Agency estimates the residues on terrestrial food items from multiple applications and over time by taking into

account:

- application rate for a single application, 

- number of applications,

- application interval, and

- dissipation rate

The model of Hoerger and Kenega (1972), as modified by Fletcher et al. (1994) was used to estimate pesticide

concentrations on selected avian or mammalian food items. This model predicts the maximum concentrations that

may occur immediately following a direct application at 1 lb ai/A. For 1 lb ai/A applications, concentrations on short

grass, broadleaf plants, and fruits are predicted to be 240 and 85 ppm, 135 and 45 ppm, and 15 and 7 ppm for maximum

and mean residues, respectively. The predicted maximum concentration for broadleaf plants and fruits are used to

represent maximum concentrations that may occur on small and large insects, respectively. Linear extrapolation is

then used to estimate maximum terrestrial EEC’s for single applications at other application rates.

The Agency uses this as a screening level exposure assessment tool to determine peak terrestrial EECs resulting

from multiple applications. After application, residues on food items are predicted to decline according to a firstorder exponential model. If the maximum initial concentration is C0

 and the half-life for the exponential dissipation of

the active ingredient is t1/2, the remaining concentration at time t is given by the following formula:

 or Ct = C0(1/2)t/t1/2 Ct C e (the latter form shows the direct use of the half-life concept).

t

t =

−

0

2

1/2

ln

The general formula for the peak EEC (Cpeak) following multiple applications is:

Cpeak C e or in simpler, directly calculable, single equation form,

i

n I n i

t =

=

−

−

∑ 0

1

2

1 2

( )ln

/

Cpeak = C0[1-(1/2)nI/t1/2]

 [1- (1/2)I/t1/2]

where C0 is the maximum initial concentration after one application, I is the time interval between applications, n is

the number of applications, and t1/2 is the half-life of the active ingredient.
1

Typical or conventional laboratory analysis does not differentiate among the various inorganic species

and organic species of arsenic. Analysis for total arsenic, of course, sets an upper bound for all arsenic-containing

species from all sources. Any contribution specifically from cacodylic acid would be masked.

69

APPENDIX 5*

[*This appendix is a verbatim copy of the expanded drinking water assessment which was a major portion of

Appendix 6 of the Cacodylic Acid RED. It has coequal information on cacodylic acid and MDMA/DSMA, as well as

other arsenical species. References are listed at the end.]

DRINKING WATER EXPOSURE ASSESSMENT

Background and Basis.

As stated in the introduction, cacodylic acid is one of several arsenical pesticides which the Agency is or will soon

be considering for reregistration. Ultimately, in another action, the Agency will assess the potential overall

(aggregate, cumulative) drinking water exposure to these compounds and common transformation products. The

EPA Office of Water (OW) is scheduled to issue a proposed and final regulation on the larger issue of “arsenic” in

drinking water by the years 2000 and 2001, respectively.

This drinking water assessment is based primarily on 1) fate studies reviewed in the Environmental Fate Assessment

Section of this document, 2) limited monitoring for cacodylic acid and other speciated forms of arsenic in a mixed

assortment from pesticidal use or non-use areas, and 3) extensive monitoring for total1 arsenic.

 Although monitoring for individual chemical forms of arsenic is sparse, the available measured data are revealing. 

Surface water conclusions from these sources are compared with simulation model predictions. The primary

monitoring sources are identified briefly as follows:

1) The United States Geological Survey (USGS) has recently completed an extensive survey report entitled “Arsenic

in Ground Water of the United States: Occurrence and Geochemistry” (Welch, 1999), which is soon to be published

in the journal Groundwater. Alan H. Welch, a principal investigator, has provided the Agency with an advance

version of the report and, through telephone discussions, provided valuable insights. Water analyses were almost

entirely for undifferentiated or total arsenic, as footnoted. Although the survey was dedicated to ground water, it

also includes some useful information for surface water. In addition to survey information and statistical analysis, it

provides a comprehensive bibliography and discusses the many of the facets of the biogeochemistry of arsenic.

2) A separate, limited investigation by USGS research scientists (Garbarino and Burkhardt, 1998), provided data on

the arsenic species cacodylic acid, monomethylarsonate, arsenite, and arsenate in ground water not associated with

arsenical pesticide application and in surface waters in areas which are associated with the application of MSMA, a

compound closely related to cacodylic acid.

3) The EPA Pesticides in Groundwater Database (EPA, 1992) provided groundwater monitoring data for a localized

area in Texas and another in the State of Washington.

4) The BASINS and STORET databases maintained by the US EPA provided a large body of data (many tens of

thousands or more of entries) on total arsenic concentrations in surface waters of the U.S.

5) Chemical speciation data published in 1973 on six fresh surface water bodies, one well, and three saline waters

around Tampa, Florida, revealed background concentrations for cacodylic acid, monomethylarsonate, arsenite, and

arsenate.
70

Groundwater Exposure

The USGS (Welch, 1999) retrieved water-quality data for about 50,000 samples from about 30,000 sample locations

from the USGS’s National Water Information System (NWIS, which includes National Water Quality Assessment

(NAWQA) data), and additional data from many other sources. A subset of about 20,000 analyses from the years

1973-1997 which met certain criteria were selected (censored) for statistical testing. Only a single analysis for a

particular well or spring was used to avoid bias towards frequently sampled sources. Sample sites were not

uniformly distributed across the country and were not specifically targeted for areas where arsenical pesticides were

applied. The survey authors conclude that while slightly less than half the groundwater samples had total arsenic

concentrations < 1 ug/L, about 10% exceeded 10 ug/L. They further conclude that “natural” arsenic concentrations

exceeding 10 ug/L are more widespread and common than previously recognized. Some areas of high arsenic

concentrations

(defined as > 50 ug/L) were associated with point source pollution (for example, Superfund Sites where about 30% of

approximately 1200 sites list arsenic as a “contaminant of concern”), but most appear to be natural areas. A USGS

map of total arsenic distribution in ground waters of the U.S. is given in Appendix 7.

Based on a few regional studies of agricultural use of inorganic arsenical pesticides (not the “organic” cacodylic

acid), the survey authors conclude that groundwater arsenic concentrations do not correlate well with use, and that

groundwater is largely unaffected by their use in those areas (North Dakota, South Dakota, Wisconsin, Minnesota). 

Almost all instances of significant detections of the inorganics are associated with disposal or other types of point

source pollution.

The survey authors report that even in areas that tend to have naturally high arsenic concentrations there exists

some ground water with low to very low arsenic concentrations. Steep lateral and vertical concentration gradients

exist, showing inherent local spatial variability. The USGS scientists state that a statistically based evaluation of

these observations requires work on, among other things, development of lithochemical geologic/aquifer

relationships. Similar reports of high spatial variability over small, localized areas come from scientists investigating

numerous recent newsworthy instances of serious arsenic poisoning deriving from drinking water wells in India

(West Bengal) and Bangladesh (Nickson, et al., 1998; Masud, 2000; numerous others). 

The USGS reports few analyses of groundwater samples specifically for methylated arsenic compounds such as

cacodylic acid. However, about 30 groundwater samples taken in northwestern Nevada where these compounds

were not used were analyzed for background monomethylarsonate and cacodylic acid (also identified as

dimethylarsinate). The data suggested to the authors that groundwater in areas where arsenical pesticides are not

applied contains < 1ug/L of methylated arsenic (all analyses were < 0.3 ug/L).

In a separate study by USGS research scientists (Garbarino and Burkhardt, 1998), 6 groundwater samples from Idaho,

10 from Illinois, and 8 from Nevada (total of 24 sites) were analyzed for arsenite, arsenate, monomethylarsonate, and

cacodylic acid. Again, none of the sites were associated with the application of arsenical pesticides. With a

detection limit of approximately 0.2 ug/L, there were no measurable concentrations of the methylated compounds in

ground water. In contrast, arsenite or arsenate concentrations in these groundwater samples ranged from less than

the method detection limit of approximately 0.2 ug/L to a concentration as high as approximately 900 ug/L. Their data

show that either arsenite or arsenate or both were detected in all samples, typically at 10-50 ug/L. Arsenite and

arsenate concentrations were comparable. In a personal communication, one of the study authors expressed

surprise at this latter comparability, since arsenate is generally assumed to dominate.

As part of a study of 10 natural waters (one ground water, nine surface waters) in the vicinity of Tampa, Florida

(Braman and Foreback,1973), one water well at a remote camping area was sampled. Tampa tap water was also

analyzed. Details of this study are given below in the Surface Water Exposure Section. The well water concentration

for cacodylic acid was 0.4 ug/L (ppb); for methylarsonic acid, 0.2 ug/L; and for arsenate (as arsenic), 0.3 ug/L. 

Concentration of arsenite (as arsenic) was less than the detection limit of 0.02 ug/L (20 parts per trillion). Tampa tap
71

water “contained only traces of arsenate”.

Reported monitoring data in the EPA Pesticides in Groundwater Database show relatively high concentrations of

total arsenic in a high percentage of water wells in localized areas of Texas [concentrations of 10-680 ppb in 91 of

247 wells with limits of detection (LODs) for most samples of 25 ppb; fewer with LODs of 10 ppb]; and in the State of

Washington (1.6-13.3 ppb in 15 of 20 wells with LOD of 0.2 ppb). The Texas data (paragraph 1 below) were

associated with use of cacodylic acid; the Washington data (paragraph 2 below) were not.

1) From the Texas report (Aurelius, 1988), the Agency independently concludes that these detections are not

reasonably attributable to labeled pesticidal application (non-point source) of cacodylic acid or other

currently used arsenicals. Rather, the high concentrations were most likely caused by cacodylic acid-treated

cotton gin waste which was spread in the vicinity of poorly cased wells, and by somewhat higher natural

concentrations of arsenic.

2) In Washington (Erickson, 1990), natural conditions including historical volcanic activity; strongly alkaline,

sandy soils; hydrology favorable to the migration of soluble constituents; former heavy use of calcium or lead

arsenates (sometimes hundreds of pounds/acre annually) in orchards; thermal waters; or wastes from mining

operations may have contributed to the elevated concentrations.

Groundwater Conclusion. In view of 1) the extensive USGS survey report on arsenic (principally total arsenic);

2) the limited, monitoring data for speciated arsenicals, including cacodylic acid; 3) the relative immobility of

cacodylic acid indicated by laboratory sorption and comparison with phosphate; and 4) the insignificant

leaching of cacodylic acid observed in a supplemental field study, it is reasonable for the Agency to conclude that

highly variable background concentrations of arsenic species in groundwater (including arsenite and arsenate,

the mineral allies/metabolites of cacodylic acid) should be largely unaffected by the labeled, agricultural (nonpoint source) use of cacodylic acid on cotton.

This conclusion should not be interpreted to mean that humans are not exposed to arsenicals in their drinking

water from groundwater sources. It is patently clear from all sources that humans are exposed to significant

concentrations of chemical forms of arsenic (including cacodylic acid) in their drinking water from groundwater

sources. However, labeled use of cacodylic acid should not contribute significantly to the already existing burden

of arsenic in groundwater from all sources, natural and anthropogenic. Furthermore, the prevalence of so much

natural total arsenic, including arsenite and arsenate, suggests that any concerns be directed toward other

arsenic species. It is therefore desirable that whenever future groundwater samples are analyzed for total arsenic

(as has usually been the case), that arsenic be speciated whenever total arsenic concentrations justify possible

toxicity concerns.

Surface Water Exposure

As discussed under “mobility” in the environmental fate section above, eroding soil and runoff water would

transport cacodylic acid and metabolites to surface waters. The extensive USGS report cited above on arsenic in

ground water (which had a modicum of surface water information), as well as other sources (e.g., Hem, 1985), indicate

that total arsenic concentration in the majority of U.S. rivers and streams is less than 1 ug/L. However, as for ground

water, many notable exceptions of higher arsenic concentrations occur; the highest most likely to occur in

geothermal areas and in water draining mineralized areas.

The BASINS and STORET databases maintained by the US EPA have a large body of data (many tens of thousands

or more of entries) on total arsenic concentrations in surface waters of the U.S.. However, because of the wide

ranging concentrations (see below) in space and time, and the multiplicity of possible major sources of arsenic, any

attempt to associate these with cacodylic acid use would likely be a fruitless exercise, and would certainly require a

major resource effort akin to that by the USGS for groundwater. Nevertheless, even casual inspection of the data
72

from the databases shows that is not unusual for total arsenic concentrations in raw surface water in many different

sites to be several parts per billion. (Reporting limits in many cases appear to be 1 ppb, but in many other cases are

in excess of 10 ppb and up to approximately 50 ppb. The 10 and 50 ppb limits may be associated with the human

health limits currently set for total arsenic set by the World Health Organization and the US EPA, respectively.)

Spatial variability is the rule. The data for Georgia and Texas given below are illustrative of this variability and are

not meant to imply a special situation. For example, in Georgia, as presented in BASINS, upper 85th percentile

concentrations of total arsenic at different sites ranged from less than 1 ppb to 338 ppb. More typical high values

would fall in the range of 10 to 50 ppb. The most probable concentration appears to fall in the range of <1 to 2 or

3 ppb. The STORET database for Georgia and Texas show similar results, with Texas seeming to average several

parts per billion more. Some of the highest values reported in STORET for Georgia and Texas (79,000 entries)

approach 1000 ppb and can often be associated with past point source pollution by humans. Such pollution is

becoming less serious through environmental conscientiousness and governmental regulation. However, as

previously mentioned, natural sources (especially in places in the western U.S.) also produce such high

concentrations. It is noteworthy that most major drinking water facilities which use surface water often use water

treatment techniques (e.g., flocculation with compounds of aluminum or iron) which are generally effective in

reducing total arsenic concentrations in the observed ranges to less than 1 ppb. As was established above, if

desired, it is possible to measure and speciate lower concentrations.

As previously stated, data specifically on methylated forms of arsenic in surface waters are sparse. One precocious

study (partially described in the groundwater section above) for 10 natural waters in the vicinity of Tampa, Florida,

published in 1973 (Braman and Foreback) revealed background concentrations of cacodylic acid to range up to 1.8

ug/L. There were six surface fresh water bodies (two rivers, two ponds, two lakes), three saline waters (two bays,

one tidal flat), and one water well at a remote camping area. Tampa tap water was also analyzed. Analysis was for

the four arsenic species: cacodylic acid (dimethylarsenic acid), methylarsonic acid (methyl arsonate, MMA or

MSMA), arsenite, and arsenate. Presumably, all water sites were sampled once with two or more replicates analyzed;

however, the study authors did not report specifics on sampling dates and the number of samples analyzed or

replicated. All concentrations reported below are as arsenic equivalents (to get cacodylic acid concentrations, for

example, multiply all values given below for cacodylic acid by the relative chemical weights of approximately 138/75

or 1.8). [In addition to the water samples, human urine (15 ppb cacodylic acid, N=8; 3 ppb methylarsonic acid, N=8; 2

ppb arsenite, N=4; and 4 ppb arsenate, N=4), bird eggshells, seashells, and a limestone rock, were analyzed.]

Concentrations of cacodylic acid (reported as arsenic) in the natural waters ranged up to 1.0 ppb (1.0 x 1.8 = 1.8 ppb

cacodylic acid). At one site, the Hillsborough River, concentrations were below a remarkably low detection limit of

0.02 ppb, while the median value for cacodylic acid (as arsenic) at the other nine sites was approximately 0.3 ppb

(0.5 ppb cacodylic acid). The well water concentration, as given previously, was 0.2 ppb (0.4 cacodylic acid). 

Concentrations of arsenate, detected at all ten sites, were similar to cacodylic acid. Arsenite was detected at only six

of the ten sites, but had the highest concentration (2.7 ppb) of all species. Methylarsonic acid, detected in eight of

the ten sites, was generally present at lower concentrations, the highest being 0.22 ppb. However, in a broader

sense, considering the few samples, surface water concentrations of all four were similar. Total arsenic

concentrations ranged from approximately 0.3 to 3.6 ppb. The extent to which these concentrations represent the

natural background in the Tampa area or are influenced by introduction of artificial sources is unknown. However,

the study authors considered the sampled sites as “natural” waters. As previously given, Tampa tap water

“contained only traces of arsenate”.

In another arsenic speciation study, USGS research scientists (Garbarino and Burkhurdt, 1998) collected surface

water samples at one or two week intervals from early March through the middle of September of 1997 from each of

three sites associated with different basins or sub-basins in an agricultural region potentially impacted by

application of MSMA (application is normally from May through July). MSMA can be used as a surrogate for

cacodylic acid because it is chemically closely related to cacodylic acid; produces cacodylic acid as a metabolite; is a

minor metabolite of cacodylic acid; and has a similar use and environmental fate profile (cacodylic acid is slightly
73

more mobile). Sampling dates and times varied between sites and were not correlated. A total of 24 samples were

analyzed from each site. Analyses were for MSMA (as the acid MMA), cacodylic acid (DMA), arsenite, and

arsenate. Maximum concentrations of MMA at the three sites were approximately 2, 3, and 5 ug/L, and were

relatively short-lived. As a surrogate and with adjustment for application rates, similar concentrations would be

expected for cacodylic acid. Concentrations of cacodylic acid (a possible metabolite of MSMA), were at or below

the detection limit of 0.2 ug/L, except for one sample which registered approximately 0.6 ug/L. Maximum arsenite

concentrations were approximately 2, 3, and 3 ug/L. Maximum arsenate concentrations were approximately 2, 5.5, and

5.5 ug/L. The comparable concentrations of arsenite and arsenate is again noteworthy. The inorganic species

generally increased later in the summer, possibly indicating alkylarsenic degradation (oxidation) or a secondary

source of inorganic arsenic. However, until similar baseline data for surface water sources where no arsenicals are

applied are compared, no conclusions can be reached about whether the fluctuations are part of the natural arsenic

cycle or influenced by pesticide application.

It is informative to compare the preceding surface water results with environmental simulation model predictions for

potential increases in surface water concentrations. The Agency has recently adapted the standard PRZM/EXAMS

model for runoff to an Index Drinking Water Reservoir (IR), and also makes an allowance for an estimated maximum

of surrounding land which may be cropped. Model input values specific to cacodylic acid are given in an

attachment. Resulting acute and chronic drinking water concentrations for the IR scenario for cotton grown in

Mississippi with one annual application of cacodylic acid at a rate of 0.60 lb/acre and with a multiplicative factor for

percent cropped area (PCA) of 0.20 (20% cropped) are:

Acute (one-in-ten year peak concentration x PCA): 29 ug/L x 0.20 ~ 6 ug/L (ppb) 

Chronic (one-in-ten year peak concentration x PCA): 7.0 ug/L x 0.20 ~ 1 ug/L (ppb)

Conclusion for Surface Drinking Water. Whether coincidence or not, and whatever the source, the modeled values

for cacodylic acid for surface drinking water are similar to those measured and discussed above for both use and

non-use areas, and would, therefore, generally represent an additional, localized burden above background. If there

are toxicological concerns at or near the monitored and modeled concentrations, then, clearly, this observation

invites broader scale, systematic environmental sampling and analysis for cacodylic acid and other speciated

arsenicals in order to determine statistically the associations, ranges of exposure, and possible effects. As is

obvious from the Florida and USGS work, adequate analytical methods for arsenical chemical speciation are

available.

While all of the pesticidal arsenicals certainly can contribute to total arsenic in surface water in use areas, their

additive effects would be generally comparable to the lower limits of most observed concentrations of total arsenic

(including arsenite and arsenate) or a small superposition on larger collective contributions at the upper limits of

concentrations from other sources, natural or anthropogenic. However, considering the speciated measurements by

the USGS researchers, those by Braman and Foreback, and the similarity of model predictions, if there is concern for

concentrations of cacodylic acid per se, then its pesticidal application clearly appears to represent an additional,

localized exposure burden of magnitude comparable to background concentrations.
74

APPENDIX 6

MSMA/DSMA

INDEX RESERVOIR DRINKING WATER CONCENTRATIONS

 PRZM/EXAMS MODEL INPUTS AND OUTPUTS FOR

DEFOLIANT/HERBICIDAL USE WITH COTTON AND CITRUS

Note: Modeled uses (cotton and citrus) for MSMA and DSMA are the same. Except for differences in application

rates, any differences in model inputs for MSMA and DSMA, the mono- and disodium salts methanearsonic acid

(MMA), are virtually inconsequential to model output. Thus, only the simulations for DSMA on cotton (aerial spray)

and citrus (ground spray) are given below. MSMA and DSMA concentrations for other rates, as given in the Drinking

Water Section of the main text from which this appendix is referenced, are proportional to the application rates.

PRZM (Version 3.12) INPUTS 

Application Information, including Rates and Intervals (also given in table below):

Cotton: Aerial spray applied to mature cotton for defoliation of cotton plant prior to harvest. Two applications

of DSMA, each at 4.0 kg/ha (3.6 lb/acre) with a 14-day interval (seasonal max. of 8.0 kg/ha). No

incorporation.

Citrus: Ground spray applied for weed control around citrus. Three applications of DSMA, each at 7.1 kg/ha

(6.3 lb/acre) applied at 14-day intervals (seasonal max. of 21 kg/ha). No incorporation.

Variable

(units)

Variable 

Description

Input Value Source of

Info/Reference

DWRATE(1)1

(day-1)

Dissolved phase

pesticide decay

rate constant in

surface horizon

DWRATE(1) = DSRATE(1)

k = ln2/t1/2 = ln2/242

k = 2.9 x 10-3/day

Aerobic soil

metabolism study (GLN

162-1)

2 soils (t /2 = 240 and

241days), upper 90%

C.L. = 242 days

DSRATE(1)1

(day-1)

Adsorbed phase

pesticide decay

rate constant in

surface horizon

DWRATE(2)

(day-1)

DWRATE(3)

(day-1)

Dissolved phase

decay rate const.

in 1st, and 2nd

subsurface

horizon

DWRATE(2) = DSRATE(2)

k = ln2/t1/2 = ln2/2300

k = 3.0 x 10-4/day

DWRATE(3) = DSRATE(3)

Same as above:

k = 3.0 x 10-4/day 

Anaerobic soil/

anaerobic aquatic

metabolism study

(GLN 162-2/3)

One anaerobic aquatic

study, t1/2 = 2.1 years

3 x 2.1 = 6.3 years =

2300 days

DSRATE(2)

(day-1)

DSRATE(3)

(day-1)

Adsorbed phase

decay rate const.

in 1st and 2nd

subsurface

horizon
75

KD(1)

KD(2)

KD(3)

(cm3 gm-1 or

mL g-1 or

L kg-1)

Pesticide

partition or

distribution

coefficients for

each horizon

 Kd = 13 mL/g (loamy sand) (use for all horizons)

Mobility -

Adsorption/Desorption 

study (GLN 163-1)

No OC correlation.

Lowest non-sand Kd of

21 U.S. soils =13 mL/g

DEPI 

(cm)

Incorporation

depth

Actual or pesticide label

0 cm (no incorporation)

Product label

TAPP 

(kg ha-1)

Application rate Maximum label rate

DSMA on Cotton: Aerial spray. Two

applications of 4.0 kg/ha with a 14-day interval

(seasonal max. of 8.0 kg/ha).

DSMA on Citrus: Ground spray. Three

applications, each at 7.1 kg/ha with a 14-day

interval (seasonal max. of 21 kg/ha).

Product label

APPEFF

(decimal)

Application

efficiency

0.95 for aerial spray and ground spray Current default

DRFT Spray drift

fraction

0.16 for aerial spray; 0.014 for ground spray. Current defaults 
76

MSMA/DSMA USES ON COTTON AND CITRUS

Input Parameters for EXAMS (Version 2.97.5)

Variable (units) Variable 

Description

Input Value Source of

Info/Reference

HENRY 

(atm-m3-mole-1) 

Henry's law

constant

MSMA: 1.5 x 10-13 atm m3 mole -1

DSMA: 7.1 x 10-14 atm m3 mole-1

Calculated from

V.P.and

solubility 

KBACW1

(cfu/mL)-1 hour-1

Bacterial biolysis

in water column

Aerobic soil t1/2 x 2 = 242days x 2 x 24hr/day

t1/2 = 1.16 x 104 hr

k = ln2/1.16 x 104/hr = 6.0 x 10-5/hr

Aerobic aquatic

metabolism

(GLN 162-4)

half-life

( units change)

KBACS1

(cfu/mL)-1 hour-1

Bacterial biolysis

in benthic

sediment

Anaerobic aquatic study

One value, t1/2 = 2.1 years

 2.1 x 3 = 6.3 years = 2300 days x 24hr/da = 5.5 x 104 hr

k = ln2/t1/2 = 1.3 x 10-5/hr

 

Anaerobic

aquatic

metabolism

(GLN 162-3)

(units change)

Anaerobic soil

(GLN 162-2)

KDP

(hour-1)

Direct

photolysis

Stable (0.0) Photolysis in

water (161-2)

KBH

(mole-1 hour-1)

KNH 

(hour-1)

KAH 

(mole-1 hour-1)

Base hydrolysis

Neutral

hydrolysis

Acid hydrolysis 

0.0

0.0

0.0

Hydrolysis

study

(GLN 161-1)

Stable

KOC

(mL g-1 O.C.)

Partition 

coefficient for

organic carbon

Lowest non-sand Kd 

Kd = 13 mL/g

Mobility -

Ads/Des study

(GLN 163-1)

No OC corr.

Lowest nonsand Kds for

21 soils.

MWT

(g mole-1)

Molecular

weight

MSMA: 162

DSMA: 184

Chemical

formula

SOL

(mg L-1)

Aqueous

solubility

MSMA: 1.0 x 106 mg/L

DSMA: 3.4 x 105 mg/L

Product Chem.

QUANT Direct

photolysis

quantum yield 

0 NA

VAPR (torr) Vapor pressure MSMA: 7.5 x 10-7 Torr

DSMA: 1 x 10-7 Torr

Product Chem.
77

Output for DSMA use on cotton - Yazoo County, Mississippi

Major Land Resource Area 134: Southern Mississippi Valley Silty Uplands

Loring Silt Loam (Hydraulic Soil Group: C) 

 2 aerial applications at 3.6 lb a.i./ac 

 MS cotton - IR w/PCA for drinking water (PPB)

 YEAR PEAK 96 HOUR 21 DAY 60 DAY 90 DAY YEARLY

 ---- ---- ------- ------ ------ ------ ------

 1948 362.000 352.000 338.000 299.000 224.000 67.800

 1949 281.000 274.000 250.000 224.000 201.000 122.000

 1950 156.000 152.000 136.000 115.000 109.000 61.800

 1951 269.000 262.000 254.000 239.000 191.000 74.590

 1952 218.000 213.000 196.000 162.000 143.000 86.110

 1953 143.000 140.000 129.000 111.000 95.890 46.070

 1954 177.000 173.000 165.000 144.000 134.000 65.230

 1955 228.000 222.000 205.000 165.000 146.000 87.050

 1956 135.000 132.000 128.000 108.000 95.190 61.370

 1957 552.000 537.000 503.000 416.000 341.000 107.000

 1958 332.000 323.000 294.000 252.000 217.000 116.000

 1959 180.000 176.000 160.000 133.000 125.000 71.060

 1960 459.000 446.000 426.000 358.000 316.000 114.000

 1961 247.000 242.000 229.000 157.000 125.000 82.730

 1962 200.000 194.000 177.000 158.000 138.000 68.220

 1963 245.000 238.000 218.000 153.000 116.000 44.210

 1964 207.000 202.000 180.000 150.000 146.000 112.000

 1965 280.000 272.000 243.000 192.000 164.000 83.390

 1966 218.000 212.000 129.000 117.000 104.000 50.500

 1967 204.000 199.000 178.000 157.000 143.000 100.000

 1968 164.000 159.000 142.000 125.000 117.000 53.420

 1969 396.000 379.000 303.000 262.000 205.000 79.310

 1970 385.000 375.000 335.000 275.000 244.000 126.000

 1971 129.000 126.000 112.000 78.020 69.510 38.640

 1972 539.000 525.000 489.000 424.000 337.000 115.000

 1973 315.000 308.000 278.000 228.000 196.000 127.000

 1974 153.000 148.000 135.000 108.000 91.900 54.250

 1975 155.000 150.000 141.000 136.000 110.000 46.200

 1976 166.000 161.000 151.000 123.000 117.000 76.100

 1977 162.000 158.000 152.000 130.000 129.000 57.220

 1978 568.000 552.000 498.000 418.000 387.000 142.000

 1979 484.000 471.000 425.000 348.000 299.000 144.000

 1980 197.000 192.000 175.000 150.000 147.000 65.280

 1981 374.000 364.000 331.000 273.000 221.000 80.820

 1982 283.000 278.000 254.000 221.000 205.000 117.000

 1983 146.000 142.000 127.000 101.000 85.690 41.250

 upper

 10th 500.500 487.200 444.900 375.400 322.300 126.300

 percentile

 MEAN OF ANNUAL VALUES = 82.906

 STANDARD DEVIATION OF ANNUAL VALUES = 30.285

 UPPER 90% CONFIDENCE LIMIT ON MEAN = 90.380

PCA for cotton = 0.20
78

Acute: 1-in-10-year peak concentration x PCA = 500.5 x 0.20 = 100 ug/L

Chronic: 1-in-10-year average annual mean x PCA = 126.3 x 0.20 = 25 ug/L
79

 Output for DSMA use on citrus - Osceola County, Florida

Major Land Resource Area 156A: Florida Everglades and Associated Area

Adamsville Sand (Hydraulic Soil Group: C) 

 3 ground applications at 6.3 lb a.i./ac 

 FL citrus - IR w/ PCA for drinking water (PPB)

 YEAR PEAK 96 HOUR 21 DAY 60 DAY 90 DAY YEARLY

 ---- ---- ------- ------ ------ ------ ------

1948 622 602 550 488 429 165

1949 360 347 308 293 266 109

1950 473 460 414 350 328 140

1951 334 326 292 242 242 116

1952 571 553 494 428 401 191

1953 586 568 543 519 478 203

1954 443 429 406 354 324 144

1955 310 300 269 232 221 107

1956 565 553 513 430 368 138

1957 844 817 747 627 556 223

1958 389 376 338 296 266 116

1959 554 538 494 441 387 160

1960 766 741 705 578 489 186

1961 328 318 279 253 231 106

1962 532 515 473 421 368 142

1963 654 633 575 460 384 150

1964 1094 1059 928 766 682 243

1965 506 490 446 385 354 172

1966 363 352 320 266 239 109

1967 576 565 513 435 410 168

1968 695 672 599 511 474 179

1969 454 440 406 346 309 134

1970 414 401 359 279 233 95

1971 396 389 358 285 260 111

1972 438 425 382 315 298 120

1973 623 608 550 462 396 150

1974 562 544 487 402 342 135

1975 278 270 251 219 212 97

1976 667 646 585 490 421 158

1977 429 417 367 298 266 118

1978 156 152 139 119 118 62

1979 419 405 361 337 331 137

1980 593 579 552 471 419 171

1981 1155 1120 1015 901 774 279

1982 274 266 248 202 194 105

1983 501 485 445 374 323 124

 upper

 10th 789 764 718 592 508 208

 percentile

 MEAN OF ANNUAL VALUES = 146

 STANDARD DEVIATION OF ANNUAL VALUES = 44

 UPPER 90% CONFIDENCE LIMIT ON MEAN = 157

PCA for citrus = 0.87 (default)

Acute: 1-in-10-year peak concentration x PCA = 789 x 0.87 = 687 ug/L
80

Chronic: 1-in-10-year average annual mean x PCA = 208 x 0.87 = 182 ug/L
81

APPENDIX 7

GENEEC MODEL INPUT/OUTPUT TABLES FOR AQUATIC ECOLOGICAL ASSESSMENT

EXAMPLE: DSMA USE ON COTTON

DSMA INPUT

Two applications, each at 3.6 lb/acre, 14 days apart

-------------------------------------------------------------------------------------------------------------------------------------------

 RATE (#/AC) APPLICATIONS SOIL SOLUBILITY % SPRAY INCORP

 ONE(MULT) NO.-INTERVAL Koc (Kd) (PPM) DRIFT DEPTH(IN)

-------------------------------------------------------------------------------------------------------------------------------------------

 3.6 (7.06) 2 14 1120 (13) 3 x 105 5.0 0

 FIELD AND STANDARD POND HALFLIFE VALUES (DAYS) 

 ----------------------------------------------------------------------------------------------------------------------------------------

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

 (FIELD) RAIN/RUNOFF (POND) (POND-EFF) (POND) (POND) 

 ----------------------------------------------------------------------------------------------------------------------------------------

 240 0 N/A .00- .00 ***** 480

DSMA OUTPUT: GENERIC EECs (IN PPB)

---------------------------------------------------------------------------

PEAK AVERAGE 4 AVERAGE 21 AVERAGE 56 

GEEC DAY GEEC DAY GEEC DAY GEEC 

---------------------------------------------------------------------------

 97 95 81 64

________________________________________________
82

APPENDIX 8

Data Requirements for: DSMA Chemical Number: 013802

Data Requirement

Use

Pattern1

Does EPA Have

Data To Satisfy

This Requirement?

(Yes, No, or

Partially)

Bibliographic Citation

MRID #

Must Additional

Data Be Submitted

Under FIFRA

3(c)(2)(B)?

§158.490 WILDLIFE AND AQUATIC ORGANISMS

71-1(a) Acute Avian Oral, Quail/Duck Yes 418920-01

71-2(a) Acute Avian Diet, Quail Yes 418920-03

71-2(b) Acute Avian Diet, Duck Yes 418920-02

71-3 Wild Mammal Toxicity No

71-4(a) Avian Reproduction, Quail No Yes

71-4(b) Avian Reproduction, Duck No Yes

71-5(a) Simulated Terrestrial Field Study No

71-5(b) Actual Terrestrial Field Study 

 

No

72-1(a) Acute Fish Toxicity, Bluegill Yes 419056-01

72-1(b) Acute Fish Toxicity, Bluegill (TEP) No

72-1(c) Acute Fish Toxicity,

RainbowTrout 

Yes 419056-02

72-1(d) Acute Fish Toxicity, Rainbow

Trout (TEP)

No

72-2(a) Acute Aquatic, Invertebrate Yes 419406-04

72-2(b) Acute Aquatic, Invertebrate (TEP) No

72-3(a) Acute Est/Mar Toxicity, Fish No Yes

72-3(b) Acute Est/Mar Toxicity, Mollusk No Yes

72-3(c) Acute Est/Mar Toxicity, Shrimp No Yes

72-3(d) Acute Est/Mar Toxicity, Fish

(TEP)

No

72-3(e) Acute Est/Mar Toxicity, Mollusk

(TEP) 

No
Data Requirement

Use

Pattern1

Does EPA Have

Data To Satisfy

This Requirement?

(Yes, No, or

Partially)

Bibliographic Citation

MRID #

Must Additional

Data Be Submitted

Under FIFRA

3(c)(2)(B)?

83

72-3(f) Acute Est/Mar Toxicity, Shrimp 

(TEP) 

No

72-4(a) Early Life Stage, Fish No Yes

72-4(b) Life Cycle, Aquatic Invertebrate No Yes

72-5 Life Cycle, Fish No Yes

72-6 Aquatic Organism Accumulation No

72-7(1) Simulated Aquatic Field Study No

72-7(b) Actual Aquatic Field Study No

§158.540 PLANT PROTECTION

122-1(a) Seedling Emergence (Tier I) No

122-1(b) Vegetative Vigor (Tier I) No

122-2 Aquatic Plant Growth (Tier I) No

123-1(a) Seedling Emergence (Tier II) Yes 417055-01, 425725-

01

123-1(b) Vegetative Vigor (Tier II) Yes 417055-02

123-2 Aquatic Plant Growth (Tier II) Yes 419406-03, 417911-

01, 431845-03,

431845-02

124-1 Terrestrial Field Study (Tier III) No

124-2 Aquatic Field Study (Tier III) No

§158.490 NONTARGET INSECT TESTING

141-1 Honey Bee Acute Contact Yes 419354-01

141-2 Honey Bee Residue on Foliage No

141-5 Field Test for Pollinators No

§158.290 ENVIRONMENTAL FATE

Degradation Studies-Lab:

 161-1 Hydrolysis

 161-2 Photodegradation In Water

 161-3 Photodegradation On Soil

Metabolism Studies-Lab:
Data Requirement

Use

Pattern1

Does EPA Have

Data To Satisfy

This Requirement?

(Yes, No, or

Partially)

Bibliographic Citation

MRID #

Must Additional

Data Be Submitted

Under FIFRA

3(c)(2)(B)?

84

 162-1 Aerobic Soil

 162-3 Anaerobic Aquatic

 162-4 Aerobic Aquatic

Mobility Studies:

 163-1 Leaching- Adsorption/Desorp.

 163-2 Volatility (Lab)

Dissipation Studies-Field:

 164-1 Soil

 164-2 Aquatic (Sediment)

 164-3 Forestry 

 164-5 Soil, Long-term

Accumulation Studies:

 165-4 In Fish

Ground Water Monitoring Studies:

 166-1 Small-Scale Prospective

§158.440 SPRAY DRIFT

 201-1 Droplet Size Spectrum

 202-1 Drift Field Evaluation

FOOTNOTES:

1. 1=Terrestrial Food; 2=Terrestrial Feed; 3=Terrestrial Non-Food; 4=Aquatic Food; 5=Aquatic Non-Food(Outdoor); 6=Aquatic NonFood (Industrial); 7=Aquatic Non-Food (Residential); 8=Greenhouse Food; 9=Greenhouse Non-Food; 10= Forestry; 11=Residential

Outdoor; 12=Indoor Food; 13=Indoor Non-Food; 14=Indoor Medicinal; 15=Indoor Residential.
85

Data Requirements for: MSMA Chemical Number: 013803 

Data Requirement

Use

Pattern1

Does EPA Have

Data To Satisfy

This

Requirement?

(Yes, No, or

Partially)

Bibliographic

Citation

MRID #

Must Additional Data

Be Submitted Under

FIFRA 3(c)(2)(B)?

§158.490 WILDLIFE AND AQUATIC ORGANISMS

71-1(a) Acute Avian Oral, Quail/Duck Yes 416100-02

71-2(a) Acute Avian Diet, Quail Yes 416100-03

71-2(b) Acute Avian Diet, Duck Yes 416100-04

71-3 Wild Mammal Toxicity No

71-4(a) Avian Reproduction, Quail No Yes

71-4(b) Avian Reproduction, Duck No Yes

71-5(a) Simulated Terrestrial Field Study No

71-5(b) Actual Terrestrial Field Study No

72-1(a) Acute Fish Toxicity, Bluegill Yes 400980-01,

417480-01

72-1(b) Acute Fish Toxicity, Bluegill (TEP)

72-1(c) Acute Fish Toxicity, RainbowTrout 

 

Yes 417473-01

72-1(d) Acute Fish Toxicity, Rainbow Trout

(TEP)

No

72-2(a) Acute Aquatic, Invertebrate Yes 419406-05

72-2(b) Acute Aquatic, Invertebrate (TEP)

72-3(a) Acute Est/Mar Toxicity, Fish Yes 424141-02

72-3(b) Acute Est/Mar Toxicity, Mollusk Yes 424648-01,

400980-01

72-3(c) Acute Est/Mar Toxicity, Shrimp Yes 424141-01,

400980-01

72-3(d) Acute Est/Mar Toxicity, Fish (TEP)

72-3(e) Acute Est/Mar Toxicity, Mollusk (TEP) 

72-3(f) Acute Est/Mar Toxicity, Shrimp (TEP) 

72-4(a) Early Life Stage, Fish No Yes
Data Requirement

Use

Pattern1

Does EPA Have

Data To Satisfy

This

Requirement?

(Yes, No, or

Partially)

Bibliographic

Citation

MRID #

Must Additional Data

Be Submitted Under

FIFRA 3(c)(2)(B)?

86

72-4(b) Life Cycle, Aquatic Invertebrate No Yes

72-5 Life Cycle, Fish No Yes

72-6 Aquatic Organism Accumulation No

72-7(1) Simulated Aquatic Field Study No

72-7(b) Actual Aquatic Field Study No

§158.540 PLANT PROTECTION

122-1(a) Seedling Emergence (Tier I) No

122-1(b) Vegetative Vigor (Tier I) No

122-2 Aquatic Plant Growth (Tier I) No

123-1(a) Seedling Emergence (Tier II) Partial 419056-03

123-1(b) Vegetative Vigor (Tier II) Partial 419056-04

123-2 Aquatic Plant Growth (Tier II) Yes 417482-02,

417482-01

124-1 Terrestrial Field Study (Tier III) No

124-2 Aquatic Field Study (Tier III) No

§158.490 NONTARGET INSECT TESTING

141-1 Honey Bee Acute Contact Yes 416100-07

141-2 Honey Bee Residue on Foliage No

141-5 Field Test for Pollinators No

§158.290 ENVIRONMENTAL FATE

Degradation Studies-Lab:

 161-1 Hydrolysis

 161-2 Photodegradation In Water

 161-3 Photodegradation On Soil

Metabolism Studies-Lab:

 162-1 Aerobic Soil

 162-3 Anaerobic Aquatic

 162-4 Aerobic Aquatic
Data Requirement

Use

Pattern1

Does EPA Have

Data To Satisfy

This

Requirement?

(Yes, No, or

Partially)

Bibliographic

Citation

MRID #

Must Additional Data

Be Submitted Under

FIFRA 3(c)(2)(B)?

87

Mobility Studies:

 163-1 Leaching- Adsorption/Desorp.

 163-2 Volatility (Lab)

Dissipation Studies-Field:

 164-1 Soil

 164-2 Aquatic (Sediment)

 164-3 Forestry 

 164-5 Soil, Long-term

Accumulation Studies:

 165-4 In Fish

Ground Water Monitoring Studies:

 166-1 Small-Scale Prospective

§158.440 SPRAY DRIFT

 201-1 Droplet Size Spectrum

 202-1 Drift Field Evaluation

FOOTNOTES:

1. 1=Terrestrial Food; 2=Terrestrial Feed; 3=Terrestrial Non-Food; 4=Aquatic Food; 5=Aquatic Non-Food(Outdoor); 6=Aquatic NonFood (Industrial); 7=Aquatic Non-Food (Residential); 8=Greenhouse Food; 9=Greenhouse Non-Food; 10= Forestry; 11=Residential

Outdoor; 12=Indoor Food; 13=Indoor Non-Food; 14=Indoor Medicinal; 15=Indoor Residential.
88

APPENDIX 9

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91

APPENDIX 10

ARSENIC IN GROUND WATER; USGS DATA

Printed March 2, 2000

http://co.water.usgs.gov/trace/pubs/usgs_as_county.jpeg