UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

OFFICE OF 

CHEMICAL SAFETY AND 

POLLUTION PREVENTION 

July 11 , 2013 

PC Code: 013803 

Barcode: D412157 

MEMORANDUM 

SUBJECT: 

TO: 

THROUGH: 

FROM: 

Response to Public Comments on the Monosodium Methanearsonate (MSMA) 

Problem Formulation for Ecological Risk and Drinking Water Exposure 

Assessment. 

Garland Waleko, Chemical Review Manager 

Risk Management and Implementation Branch II 

Pesticide Re-evaluation Division (7508P) 

Dana Spatz, Branch Chie~ -- - _ <:::::-~ i \\\ \ \a 

Environmental Risk ranc~~ Environmental Fate and Effects Division (7507P) 

James A. Hetrick, Ph.D., Senior Science Adviso~ C. 

Melissa Panger, Ph.D., Senior Biologist , 

Environmental Risk Branch 3 --

Environmental Fate and Effects Division (7507P) 

This memorandum provides EFED's responses to Organic Arsenical Products Task Force 

(OAPTF) comments on the preliminary problem formulation for the ecological risk and drinking 

water exposure assessment to be conducted for MSMA. OAPTF comments and the respective 

EFED responses are shown below. 

ENVIRONMENTAL FATE AND EXPOSURE MODEUNG 

OAPTF Comment 1: 

OPP should utilize more realistic modeling to estimate transformation of MSMA to inorganic 

arsenic As(i) and transport from treated fields to surface water. 
2

EFED Response:

The exposure assessment section in the problem formulation describes the unique and complex 

processes involved in controlling the environment fate and transport of MSMA and its 

degradation products in soil and aquatic environments. To predict the fate of MSMA and its 

degradation products, we plan to conduct exposure modeling that considers the rate of 

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

inorganic and organic As species in terrestrial and aquatic environments. 

OAPTF Comment 2:

The OAPTF believes that PRZM/EXAMS modeling is not an appropriate Tier 2 modeling 

approach because of conservative assumptions regarding extent of demethylation of MSMA into 

As(i), MSMA application rate, percent of crop treated with MSMA, and frequency of MSMA 

applications. 

EFED Response:

EFED plans to account for the rate of demethylation of MSMA into As(i) in the PRZM/EXAMS 

modeling. Additionally, the modeling will be conducted to represent the maximum MSMA 

application rate and application frequency as stipulated on the current label. Although the percent 

crop treated is not currently a factor considered in the drinking water or ecological exposure 

assessment modeling, the appropriate percent crop area(s) will be used in the drinking exposure 

assessment.

OAPTF Comment 3:

The OAPTF believes index reservoir modeling is only screening level assessment. Therefore, a 

more refined modeling approach is needed. 

EFED Response:

The index reservoir scenario serves as a representative scenario for representing a vulnerable 

community water system (FIFRA SAP, 1998). Refined modeling approaches can be employed 

to account for the rate of transformation of MSMA (demethylation /methylation) as well as 

chemical equilibria of inorganic and organic As species in terrestrial and aquatic environments.

OAPTF Comment 4:

The OAPTF believes a Tier 3 model for MSMA exposure should consider soil kinetics and 

transformation. OAPTF believes the PRZM/EXAMS modeling should be modified to account 

for soil kinetics and transformation of MSMA.
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EFED Response:

See response to comment 1.

OAPTF Comment 5:

The OAPTF believes that incorporation of a speciation model for As would improve the 

exposure modeling.

EFED Response:

Although the exposure modeling for MSMA is a difficult and complex exercise, the complexity 

of As speciation is an important consideration in the exposure assessment. Chemical speciation 

modeling can provide a bounding level assessment on the thermodynamic stability of various As 

species. This type of modeling, however, does not account for the rate of transformations among 

the various As species. In order to consider chemical kinetics, demethylation rates of MSMA 

(DMA) to As(i) are intended to be used in PRZM/EXAMS modeling. Appropriate fate 

properties for DMA and As(i) will be considered in both PRZM and EXAMS modeling to 

provide an estimated concentration of DMA and As(i) in soil and aquatic environments. 

Chemical speciation modeling of As in the water column of the standard pond and index 

reservoir will be conducted to provide bounding estimates on the variety of inorganic and 

methylated As species and their concentration. 

OAPTF Comment 6:

The OAPTF believes MSMA transformation into As(i) in soil needs to be considered in the 

modeling. This modeling should be compared to experimental data to allow for calibration of 

the modeling. 

EFED Response:

The plan is to use demethylation rates of MSMA (DMA) in PRZM/EXAMS modeling. 

Appropriate fate properties for DMA and As(i) will be considered in both PRZM and EXAMS 

modeling to provide an estimated concentration of DMA and As(i) in soil and aquatic 

environments. Chemical speciation modeling of As in the water column of the standard pond and 

index reservoir is planned to provide a bounding estimate of the concentrations and variety of 

inorganic and methylated As species.

OAPTF Comment 7:

The OAPTF believes the exposure modeling should represent actual use conditions in 

watersheds with MSMA usage. The modeling should consider risk mitigation measures for 

currently registered MSMA uses (cotton, golf courses, sod farms, and highway rights-of-way). 
4

EFED Response:

The exposure modeling is conducted to represent current labels. The mitigation measures on the 

current label will be considered, when possible, in the PRZM/EXAMS modeling. 

ECOLOGIAL EFFECTS 

OAPTF Comment 8: 

“OPP has proposed to require aquatic invertebrate life cycle testing with methylated pentavalent

arsenic for freshwater (Guideline 850.1300) and for saltwater (Guideline 850.1350).

 In

evaluating life cycle testing needs for inorganic pentavalent arsenic, EFED determined that

a chronic endpoint for saltwater invertebrates can be “based on an acute-to-chronic ratio

using data from freshwater invertebrates.” OAPTF notes that there is an existing acute toxicity

study with mysid shrimp (a saltwater invertebrate) that indicates that MSMA is practically

non-toxic with an LC50/EC50 of 173 mg/L.

 Just as EFED determined that life cycle testing

with saltwater aquatic invertebrates is not needed for inorganic pentavalent arsenic, it should

also be practicable to utilize an acute-to-chronic ratio derived from the freshwater

aquatic invertebrate life cycle testing proposed by EPA and the available acute data to determine 

chronic toxicity to saltwater invertebrates.” (p. 21 – 22; footnotes have been omitted)

EFED Response:

Based on the available information, there is no indication that estuarine/marine animals are more 

sensitive to inorganic pentavalent arsenic than freshwater animals. Therefore, unless data 

become available that indicate otherwise, EFED agrees that an acute-to-chronic ratio derived

from an acceptable freshwater aquatic invertebrate life cycle test could be used with the available

acute data to determine chronic toxicity to saltwater invertebrates (using an acute-to-chronic 

ratio). Therefore, EFED could concur with a waiver of the requirement for an estuarine/marine 

invertebrate life cycle test with methylated pentavalent arsenic if an acceptable freshwater 

invertebrate life cycle test with methylated pentavalent arsenic is submitted.

OAPTF Comment 9: 

In lieu of a freshwater fish life cycle test with methylated pentavalent arsenic, OAPTF suggests 

using the acute and chronic data for freshwater fish and sodium arsenite [As(III)] to derive a 

chronic endpoint for MSMA using an acute-to chronic ratio. This is based on the following: 1) 

EPA used the AS(III) chronic fish data to derive the ambient water quality criteria for arsenic; 

and 2) the acute toxicity of As(III) to fish appears similar to that of MSMA. OAPTF state that it 

is not necessary to require either a life-cycle study or an early life-stage study with freshwater 

fish and methylated pentavalent arsenic for these reasons.

EFED Response:

EPA’s national recommended water quality criteria provide guidance for states and tribes to use 

in adopting water quality standards for the protection of aquatic life and human health and, for 
5

aquatic life, are not taxon specific. EPA’s ecological risk assessments conducted for pesticide 

registrations are taxon specific. Therefore, data adequate for use in setting national water quality 

standards may not be fully adequate for assessing taxon-specific risks associated with pesticide 

use. Additionally, there are currently no acceptable chronic data available for methylated 

arsenicals (including both III and V valences) for any aquatic species. Therefore, the relative 

toxicities among the arsenicals (organic and inorganic arsenicals, valences III and V) to fish on a 

chronic basis are unknown at this time. Therefore, EFED recommends requesting chronic 

toxicity data for freshwater fish and methylated pentavalent arsenic (i.e., an early life stage 

and/or a life cycle study).

OAPTF Comment 10:

OAPTF states that whole sediment acute tests for freshwater invertebrates should not be required 

for methylated trivalent arsenic or for inorganic trivalent arsenic because, “...EFED has not cited 

any data that demonstrate conversion of MSMA to trivalent arsenic compounds in sediment”. (p. 

23). They continue, “(i)t appears that EFED has based its recommendation that the two forms of 

trivalent arsenic be tested in whole sediment solely on chemical equilibria modeling” (p. 23 – 24) 

and the analysis has not been provided. OAPTF states that it is unreasonable to require these 

data without a detailed explanation for the conclusion that MSMA will transform to trivalent 

arsenic in sediment. They also argue that it will be costly and redundant to conduct the tests with 

more than one compound.

EFED Response:

Based primarily on the chemical equilibria modeling described in the MSMA Problem 

Formulation (USEPA, 2013), both methylated trivalent arsenic and inorganic trivalent arsenic

are expected to form under low redox potential conditions. Therefore, unless data to show 

otherwise become available, EFED has a concern for benthic organisms and their potential 

exposure to trivalent arsenic from the use of MSMA. Additionally, there are currently no data 

available to indicate the relative toxicities of methylated trivalent arsenic and inorganic trivalent 

arsenic to benthic organisms. Therefore, EFED recommends requesting whole sediment tests for 

acute exposure to both methylated trivalent arsenic and inorganic trivalent arsenic.

OAPTF Comment 11:

“OPP has also proposed to require testing whole sediment for chronic effects in freshwater 

invertebrates using technical grade active ingredients for methylated trivalent arsenic and for 

inorganic trivalent arsenic. These would both be non-guideline studies. EPA’s ecological 

effects testing requirements for pesticides state that chronic sediment toxicity tests will only be 

required if: ‘The estimated environmental concentration (EEC) in sediment is > 0.1 of the acute 

LC50/EC50 values....’ Thus, any proposal by EPA to require chronic sediment toxicity testing 

for trivalent arsenic is premature, and imposition of such a requirement should be deferred until 

completion of any acute sediment toxicity testing.

In addition, OAPTF notes that existing methods for chronic sediment toxicity tests with 

freshwater invertebrates are based on the testing of contaminated sediments and do not 
6

appropriately address method modifications needed for testing with pesticides or pesticide 

degradates. Contract laboratories are encountering a number of difficulties in trying to 

implement chronic sediment testing. Accordingly, MSMA registrants should not be required to 

conduct this type of test until OPP has issued further guidance on the methodology for this test.” 

(p. 23 – 24; footnotes have been omitted)

EFED Response:

As stated above, EFED has a concern for benthic organisms and their potential exposure to 

trivalent arsenic from the use of MSMA (this concern is for both acute and chronic exposure). 

EFED recommends requesting these data if ‘(t)he estimated environmental concentration (EEC) 

in sediment is > 0.1 of the acute LC50/EC50 values....’. EFED defers to PRD on whether or not 

to delay the studies until the completion of the acute sediment toxicity testing. 

As stated in the MSMA Problem Formulation, “(a)lthough there is no guideline for the chronic 

sediment toxicity study, the following methods by the USEPA’s Office of Research and 

Development may be consulted: ORD study method EPA 600/R-099-064: chronic freshwater 

sediment testing flowthrough (Hyalella azteca); ORD study method EPA 600/R-099-064: 

chronic freshwater sediment testing flowthrough (Chironomus dilutus or C. tentans).” (USEPA, 

2013, p. 26). EFED recommends that the registrant submit a protocol for review before 

conducting the studies. EFED is not aware of any ‘difficulties’ that contract laboratories are 

encountering when conducting chronic sediment testing with freshwater organisms. 

REFERENCES:

USEPA, 2013. Registration Review: Preliminary Problem Formulation for Monosodium Acid 

Mathanearsonate (MSMA). Environmental Fate and Effects Division, Office of Pesticide 

Programs, United States Environmental Protection Agency. Washington, DC. March 12, 2013.

USEPA. 1998. FIFRA Scientific Review Panel: Proposed Methods for Basin-scale Estimation 

of Pesticide Concentrations in Flowing Water and Reservoirs for Tolerance Reassessment.