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**UNITED STATES ENVIRONMENTAL PROTECTION AGENCY**

WASHINGTON D.C., 20460

OFFICE OF

PREVENTION, PESTICIDES AND

TOXIC SUBSTANCES

Number range CHAPTER**PC Codes:** 012501, 012502, 013802, 013803, 013806

**DP Barcode:** D327974

**Date**: March 29, 2006

**ADDENDUM TO RED CHAPTER**

**SUBJECT**: Response to Registrant Phase I Error Only Comments:

Addendum to EFED RED Chapters for Organic Arsenicals Accounting for
Updated Label Rates and Potential for Long Term Buildup in Soil

> \[*Cacodylic Acid and its Sodium Salt (D210451, D212449, D255226) and
> Sodium and Calcium Salts of Methanearsonic Acid (MSMA / DSMA / CAMA;
> D277233)*\]

**TO:** Lance Wormell, Chemical Review Manager

Reregistration Branch II

Special Review and Reregistration Division (7508C)

**FROM**: Keara Moore, Chemist

Number range CHAPTEREnvironmental Risk Branch III

Environmental Fate and Effects Division (7507C)

Thuy Nguyen, Senior Environmental Scientist

Number range CHAPTEREnvironmental Risk Branch III

Environmental Fate and Effects Division (7507C)

Number range CHAPTER**THROUGH:** Mark Corbin, Senior Environmental
Scientist

Environmental Risk Branch III

Environmental Fate and Effects Division (7505C)

**APPROVED** Daniel Rieder, Branch Chief

**BY:** Environmental Risk Branch III

> Environmental Fate and Effects Division (7507C)

Attached, please find the revised addendum to the EFED RED Chapters for
Organic Arsenicals Accounting for Updated Label Rates and Potential for
Long Term Buildup in Soil. Revisions have been made to the previous EFED
RED Addendum (DP Barcode 309100; February 3, 2006) to correct any
mathematical/ typographical errors and provide clarification in areas
identified by the registrants in their Phase 1 response. Additional
comments were received regarding EPA's methodology and environmental
fate results; these comments will be addressed after the 60-day Phase 3
public comment period scheduled to begin in April 2006.

The revisions made in this document do not alter any EFED conclusions
from the previous assessment. Where appropriate, references to
"degradation" have been replaced with the more suitable term
"metabolism." Language has been added to clarify the discussion of the
potential for transformation of organic arsenicals to inorganic arsenic
and for transformation of dimethylated arsenical species to
monomethylated species. A more thorough discussion of terrestrial field
dissipation data is also included, presented in the soil accumulation
section in the environmental fate appendix.

**I. EXECUTIVE SUMMARY**

This addendum updates the ecological risk assessments for the organic
arsenicals, which include cacodylic acid (DMA; dimethylarsinic acid),
sodium cacodylate (DMA-Na), monosodium methanearsonate (MSMA), disodium
methanearsonate (DSMA), and calcium acid methanearsonate (CAMA).
Originally, these pesticides were assessed as two separate groups: the
methylarsonate salts (MSMA, DSMA, CAMA; DP Barcode D277233) and the
cacodylates (DMA and DMA-Na; DP Barcodes D210451, D212449, D255226).
Although risk estimates are still calculated based on the original
groupings, these two groups are being included in a single document
because the discussion of their fate characteristics has been combined.
They all have similar chemical structures and similar environmental fate
profiles, and they all contribute to total arsenic environmental
loading, an important issue in the current assessment. The updated
drinking water assessment (DP Barcode D309098) also considers these
compounds as a group.

For MSMA and DSMA, risk estimates are updated based on new master labels
that have lowered application rates for most uses. The new master label
for cacodylic acid (dimethylarsinic acid; DMA) had not been completed at
the time of this assessment and so has not been included here. No new
toxicity data have been considered and so calculation of risk is all
based on toxicity conclusions from the original assessment. With the
lower application rates, most risk quotients (RQs) have dropped as well,
but the overall risk conclusions have not changed. There are no
exceedances of levels of concern (LOC) for aquatic species, including
fish, invertebrates, and plants. All categories of terrestrial exposure
lead to exceedances of LOCs, including small mammals, avian species, and
upland and semi-aquatic plants. Because of lower applications rates, in
many cases the terrestrial exceedances are not as great as in the
original assessment and there are fewer exceedances in the highest risk
category. The endangered species portion of this assessment has not been
updated. Number range CHAPTERThe original assessment found that
endangered species risk quotients exceed the Agency's LOC for birds,
mammals and terrestrial plants from exposure to DSMA and MSMA. There are
still exceedances in each of these categories. In the original
assessment, however, one use led to exceedances for freshwater fish and
the new rates lead to RQs below the LOC.

The discussion of the environmental fate and transport of organic
arsenicals has been expanded. Greater attention is paid to the
possibility of transformation of these compounds to the more toxic
inorganic arsenic. Additionally, the issue of long term accumulation of
total arsenic in soil after repeated applications of organic arsenicals
has been considered. Field studies, monitoring data, and modeling all
indicate that accumulation of arsenical in soil is likely. Based on PRZM
modeling of soil concentrations, maximum rates of MSMA, DSMA, and DMA on
turf are expected to exceed the ecological soil screening levels set by
the EPA's Office of Solid Waste and Emergency Response (OSWER). Modeling
suggests that applications of these products to cotton will not lead to
soil level exceedances.

**II. PESTICIDE USE CHARACTERIZATION**

This update is based on master labels for MSMA and DSMA. These labels,
provided by the registrants on 11/14/05, have not yet been finalized but
are assumed to invalidate all previous labels. Variance from these rates
could change the conclusions of this assessment.

The main agricultural application for both MSMA and DSMA is as a
pre/post-emergent herbicide on cotton applied prior to the first bloom.
The only other agricultural uses of MSMA and DSMA supported by master
labels are in non-bearing orchards, citrus, and vineyards.
Non-agricultural uses of MSMA and DSMA include use on turf and non-crop
uses. The turf use is both residential and commercial, including
residential lawns, sod farms, golf courses, parks, and other areas. The
non-crop use includes drainage ditch banks, rights-of-way, storage yards
and similar areas.

The maximum application rates for all of these uses are included in
Table 1. Although applied as different parent compounds, MSMA and DSMA
end up as the same chemical in the environment, monomethyl arsonic acid
(MMA). Rates for both are reported here in acid equivalents (ae) to
represent the amount applied as MMA. Labels for all uses specify that
the maximum number of applications applies to "either DSMA or MSMA or
their combination per crop, per year." Exposure was therefore estimated
based on the maximum MMA application rate for each use for *either* MSMA
or DSMA. The maximum rate for each use, marked in bold on Table 1, was
considered in exposure estimates. For the non-crop use, the master label
single application rate is lower than that allowed by previous labels,
but the number of applications has increased so the annual application
rate has not changed. For all other uses, the master label annual
application rate is lower than the rates considered in the previous
assessment.

**III. EXPOSURE CHARACTERIZATION**

A.  **Environmental Fate and Transport Summary**

The environmental fate section has been updated to discuss the organic
arsenicals (MSMA, DSMA, CAMA, DMA) as a group rather than in separate
documents. It has been expanded to include consideration of the
potential for long term accumulation of total arsenic in soil and of
metabolism of organic arsenicals to inorganic arsenic. The general
conclusions about the fate and transport of organic arsenicals have not
changed, but re-evaluation of the available data has led to revision of
several modeling inputs. A detailed discussion of environmental fate
properties is provided in Appendix A and summarized here. References to
all of the registrant studies and open literature reports taken into
account in this fate characterization are included in the Appendix.

MSMA, DSMA, and CAMA are salts of the dibasic weak acid MMA. In aqueous
solution, they dissociate to MMA and the associated companion cations.
DMA is a weak acid with two methyl groups attached to the central
arsenic atom, rather than one as in MMA. These pesticides are all
non-volatile solids that are highly soluble in water.

**Table 1. Application Rates for MSMA and DSMA, based on 11/14 Master
Labels.**

+--------------+-----------+-------------+-----------+---+--------------+
| **Number     | **Maximum | **Max.**    | **Appl    | * |              |
| range        | App.      |             | ication** | * |              |
| CHAPTER**    | Rate**    | **No.**     |           | A |              |
|              |           |             | **I       | p |              |
| **Use**      | **(lb     | **Apps.**   | nterval** | p |              |
|              | ae/A)**   |             |           | l |              |
|              |           |             |           | i |              |
|              |           |             |           | c |              |
|              |           |             |           | a |              |
|              |           |             |           | t |              |
|              |           |             |           | i |              |
|              |           |             |           | o |              |
|              |           |             |           | n |              |
|              |           |             |           | * |              |
|              |           |             |           | * |              |
|              |           |             |           |   |              |
|              |           |             |           | * |              |
|              |           |             |           | * |              |
|              |           |             |           | M |              |
|              |           |             |           | e |              |
|              |           |             |           | t |              |
|              |           |             |           | h |              |
|              |           |             |           | o |              |
|              |           |             |           | d |              |
|              |           |             |           | * |              |
|              |           |             |           | * |              |
+==============+===========+=============+===========+===+==============+
| **DSMA**     |           |             |           |   |              |
+--------------+-----------+-------------+-----------+---+--------------+
| Cotton       | 1.7       | 1           | n/a       |   | ground or    |
|              |           |             |           |   | aerial       |
+--------------+-----------+-------------+-----------+---+--------------+
|              | **1.7**   | **2**       | 1 -- 3    |   | ground       |
|              |           |             | weeks     |   | (directed)   |
+--------------+-----------+-------------+-----------+---+--------------+
| Turf         | 2.5       | 4           | 14 days   |   | ground spray |
+--------------+-----------+-------------+-----------+---+--------------+
| Orchards,    | **3.7**   | **3**       | not       |   | ground       |
| Citrus,      |           |             | specified |   | (directed)   |
| Vineyards    |           |             |           |   |              |
| ^1^          |           |             |           |   |              |
+--------------+-----------+-------------+-----------+---+--------------+
| Non-crop ^2^ | **3.9**   | **4**       | 10 -- 14  |   | ground spray |
|              |           |             | days      |   |              |
+--------------+-----------+-------------+-----------+---+--------------+
| Grass for    | 3.3       | 1           | n/a       |   | not          |
| seed ^3^     |           |             |           |   | specified    |
+--------------+-----------+-------------+-----------+---+--------------+
| **MSMA**     |           |             |           |   |              |
+--------------+-----------+-------------+-----------+---+--------------+
| Cotton       | 1.7       | 1           | n/a       |   | ground or    |
|              |           |             |           |   | aerial       |
+--------------+-----------+-------------+-----------+---+--------------+
|              | **1.7**   | **2**       | not       |   | ground       |
|              |           |             | specified |   | (directed)   |
+--------------+-----------+-------------+-----------+---+--------------+
| Turf         | **3.4**   | **4**       | 10 -- 14  |   | ground spray |
|              |           |             | days      |   |              |
| \- Sod       | **2.2**   |             |           |   |              |
| Farms^4^     |           |             |           |   |              |
|              |           |             |           |   |              |
| \- Golf      |           |             |           |   |              |
| Course^4^    |           |             |           |   |              |
+--------------+-----------+-------------+-----------+---+--------------+
| Orchards,    | 3.7       | 3           | 10 -- 14  |   | ground spray |
| Citrus,      |           |             | days      |   |              |
| Vineyards    |           |             |           |   |              |
| ^1^          |           |             |           |   |              |
+--------------+-----------+-------------+-----------+---+--------------+
| Non-crop ^2^ | **3.9**   | **4**       | 10 -- 14  |   | ground spray |
|              |           |             | days      |   |              |
+--------------+-----------+-------------+-----------+---+--------------+
| Grass for    | 5.3       | 1           | n/a       |   | not          |
| seed ^3^     |           |             |           |   | specified    |
+--------------+-----------+-------------+-----------+---+--------------+

^1^ Non-bearing only - not to be used within one year of harvest.
Restricted to spot treatments in Florida.

^2^ Non-crop = "drainage ditchbanks, rights-of-way, storage yards and
similar areas".

^3^ Pacific Northwest only.

^4^ Sod farm rate allowed on sod farms and established Bermuda and
Zoysiagrass. Golf course rate allowed on athletic fields, golf courses,
and parks.

Environmental fate laboratory studies show that organic arsenicals are
stable under all tested abiotic conditions; they do not degrade by
hydrolysis or by aquatic or soil photolysis. Arsenicals can be subject
to microbial metabolism in soil under aerobic and anaerobic conditions.
The occurrence, rate, and products of this metabolism are variable,
dependent on environmental conditions. Persistence of applied parent
compounds can range from days to years, depending on soil properties and
ambient conditions such as soil moisture, temperature, chemical
concentration, bacterial population, and amount of organic matter.
Regardless of the form it takes, however, the total amount of arsenic
present does not change; these arsenicals and their transformation
products, in combination with arsenic from the natural background and
from other anthropogenic sources, maintain the total, immutable arsenic
load. Arsenic from pesticides is not lost but redistributed and
transformed throughout the environment (plants, animals, air, soil,
sediment, water) into other arsenic containing substances.

Metabolism rates do not appear to depend linearly on arsenical
concentration; the kinetics are therefore not necessarily first-order
and so "half-life" may not be an appropriate constant for all
concentrations. Despite the uncertainty, first-order half-lives have
been calculated for modeling purposes and as a convenient measure to
compare laboratory results. The estimated half-lives, used in EFED's
current models, may underestimate the faster initial rate of metabolism
but adequately portray the overall transformation and so are assumed to
be protective for chronic exposure, a major concern for arsenicals. The
modeled aerobic soil half-life for MMA, based on two studies with
similar results, is 240 days. No anaerobic soil half-life was determined
for MMA. For DMA, the Agency derived aerobic soil half-life is 173 ± 115
days with a standard upper 90% confidence limit on the mean of 240 days.
The anaerobic soil half-life for DMA was calculated to be 128 ± 38 days
with a standard upper 90% confidence limit on the mean of 168 days.

The effects of environmental factors on the rate of arsenical metabolism
are complex and poorly defined, with different studies leading to
conflicting results. An increase in temperature leads to increased
metabolism. The observed influences of soil organic matter or applied
arsenical concentrations are contradictory. The effect of aerobic versus
anaerobic conditions on metabolism rates is also ambiguous.

Potential metabolites of applied arsenicals include volatile
alkylarsines and inorganic arsenic (as arsenate or arsenite) along with
carbon dioxide. Additionally, DMA may be present as a metabolite of MMA
as well as applied directly. As with the rate, the metabolism pathway is
sensitive to environmental conditions in indeterminate ways with the
major metabolites occurring in widely variable proportions.
Transformation to volatile alkylarsines, the only metabolism route that
would directly reduce soil arsenic loading, has been shown to be
possible in certain circumstances but is generally not expected to be a
major route of dissipation. A maximum of 35% of applied MMA is expected
to be present as DMA at any one time. Theoretically, there is some
possibility for MMA to metabolize to DMA, but significant transformation
has not been observed in current acceptable field or laboratory studies.
Observed metabolism of MMA and DMA to inorganic arsenic has ranged from
undetected after several years to more than 80% transformation in
several months. Generally, arsenate \[As(V)\] is expected to be the
dominant species of inorganic arsenic, but in reducing conditions,
arsenite \[As(III)\] may be more stable.

Some of the variability in metabolism processes is associated with
variability in sorption, because microbial transformation is only likely
to occur while compounds remain dissolved in pore water. Mobility of
arsenicals is typically very low to intermediate and appears to be
independent of organic matter content. Instead, sorption is higher in
soils with higher percentage of clay or with more iron or aluminum
content. One study found by direct comparison that all arsenicals were
more strongly sorbed than phosphate in the increasing order: phosphate
**\<** DMA **\<** arsenate \~ MMA. The lowest non-sand K~d~ for MMA is
11.4 mL/g. For 20 tested soils, the range of K~d~s spans two orders of
magnitude (0.5 to 95 mL/g, mean 37 mL/g). For DMA, the lowest non-sand
K~d~ from 16 soils is 8.2 mL/g (range 8.2 to 33 mL/g, mean 18 mL/g).

[Surface Water Exposure Conclusions]{.underline}. Arsenical pesticides
and their metabolites may be transported to surface waters and sediments
through runoff water, eroding soils, or drift during application. These
routes of exposure are likely to lead to local, temporal elevations
above background arsenic levels in surface water bodies. Tier I surface
water modeling for MSMA and DSMA estimated surface water concentrations
as high as 360 ppb, as MMA. Limited targeted monitoring has found
elevated total arsenic levels in surface water bodies in MMA use areas.
In cotton growing areas in Mississippi, surface water concentrations of
MMA up to 5 ppb were detected. In golf courses in Florida, 6 of 10 ponds
tested had elevated arsenic levels with a maximum of 34 ppb, as total
arsenic. These monitoring results are discussed in more detail in the
organic arsenical Drinking Water Assessment (DP Barcode 309098).

[Soil Accumulation Conclusions]{.underline}. The relative immobility of
arsenicals along with arsenic's elemental nature make buildup in soil
after repeated applications an important consideration. Controlled field
studies, monitoring targeted to pesticide use areas, and soil modeling
results all indicate that soil buildup is a likely result of long term
organic arsenical application. Arsenic accumulation is likely to be
limited to the top layers of soil, with studies suggesting that it is
unlikely to occur at depths greater than 30 cm. These conclusions are
discussed in more detail in the terrestrial exposure section.

B.  **Aquatic Exposure**

Number range CHAPTERFor surface water contamination, the estimated
environmental concentrations (EEC) were calculated using GENEEC2
([GEN]{.underline}eric [E]{.underline}stimated
[E]{.underline}nvironmental [C]{.underline}oncentration). This Tier-I
model uses the soil/water partition coefficient and degradation kinetic
data to estimate runoff from a ten hectare field into a one hectare by
two meter deep \"standard\" pond. This model is designed as a coarse
screen and estimates conservative pesticide concentrations in surface
water from a few basic chemical parameters and pesticide label use and
application information. This model is used to screen chemicals in order
to determine which ones potentially pose sufficient risk to warrant
higher level modeling. Additional information on this model can be found
at:
[[http://www.epa.gov/oppefed1/models/water/index.htm]{.underline}](http://www.epa.gov/oppefed1/models/water/index.htm).

The input parameters used to estimate the surface water EECs for aquatic
exposure assessment were selected from the environmental fate data
submitted by the registrant and from the open literature, and in
accordance with USEPA-OPP EFED water model parameter selection
guidelines, *Guidance for Selecting Input Parameters in Modeling the
Environmental Fate and Transport of Pesticides*, Version II, February
28, 2002. The maximum rate for each use, either from MSMA or DSMA, was
modeled. Both are present in aqueous solution as monomethyl arsonic acid
(MMA) and are therefore modeled in acid equivalents (ae) to represent
the amount applied as MMA. With the exception of the sorption
coefficient, which was reduced from 13 mL/g to 11.2 mL/g, all modeling
inputs are the same as in the original RED (DP Barcode 277233). The
general input values used in the model runs are included in Appendix B,
along with the model outputs. The results are presented in Table 2.

**Table 2. EECs** for MMA in the aquatic environment.

+-------------+------------------+-------------+----------------------+
| **Crop**    | **Application    | **Number of | **Peak EEC**         |
|             | rate ^2^**       | App         |                      |
| *           |                  | lications** | **(ppb as MMA)**     |
| *(Chemical) | **(lbs ae/A)**   |             |                      |
| ^1^**       |                  |             |                      |
+-------------+------------------+-------------+----------------------+
| Cotton      | 1.74             | 2           | 84                   |
|             |                  |             |                      |
| (DSMA)      |                  |             |                      |
+-------------+------------------+-------------+----------------------+
| Non-crop    | 3.86             | 4           | 360                  |
|             |                  |             |                      |
| (MSMA)      |                  |             |                      |
+-------------+------------------+-------------+----------------------+
| Orchard     | 3.70             | 3           | 260                  |
|             |                  |             |                      |
| (DSMA)      |                  |             |                      |
+-------------+------------------+-------------+----------------------+
| Turf; max   | 2.23             | 4           | 310                  |
|             |                  |             |                      |
| (MSMA)      |                  |             |                      |
+-------------+------------------+-------------+----------------------+
| Turf; golf  | 3.35             | 4           | 210                  |
|             |                  |             |                      |
| (MSMA)      |                  |             |                      |
+-------------+------------------+-------------+----------------------+

^1^ The pesticide with the highest application rate for each use was
modeled as the acid equivalent MMA.

^2^ All application rates are based on ground application. The cotton
use is labeled for aerial application as well but a higher rate is
allowed for ground application.

C.  **Terrestrial Exposure**

> 1\. *Number range CHAPTERAnimals*

Number range CHAPTERTerrestrial wildlife are exposed to pesticides
through the plant or animal material that they consume as food. The
Kenaga nomogram as modified by Fletcher, a series of tables that relate
food item residues to pesticide application rate based on a database of
actual measured pesticide residue values on plants, are used to estimate
exposure to terrestrial organisms. The TREX model (version 1.22) was
used to calculate EECs, presented in Table 3, for MMA from application
of MSMA and DMSA. The equations and assumptions used by TREX are
discussed in Appendix C.

**Table 3. EECs (ppm MMA) on terrestrial food items resulting from
direct applications of DSMA and MSMA**.

+-----------+----------+--------+----------+---+-----------+-----------+
| **Crop**  | **Appl.  | **No.  | **P      |   |           |           |
|           | rate**   | of     | redicted |   |           |           |
| **(       |          | A      | Maximum  |   |           |           |
| Chemical) | **(lbs   | ppl.** | EEC on   |   |           |           |
| ^1^**     | ae/A)**  |        | Food     |   |           |           |
|           |          |        | Items**  |   |           |           |
+-----------+----------+--------+----------+---+-----------+-----------+
|           |          |        | *Short   | * |           | *Seeds*   |
|           |          |        | grasses* | B |           |           |
|           |          |        |          | r |           |           |
|           |          |        |          | o |           |           |
|           |          |        |          | a |           |           |
|           |          |        |          | d |           |           |
|           |          |        |          | l |           |           |
|           |          |        |          | e |           |           |
|           |          |        |          | a |           |           |
|           |          |        |          | f |           |           |
|           |          |        |          | p |           |           |
|           |          |        |          | l |           |           |
|           |          |        |          | a |           |           |
|           |          |        |          | n |           |           |
|           |          |        |          | t |           |           |
|           |          |        |          | s |           |           |
|           |          |        |          | & |           |           |
|           |          |        |          | i |           |           |
|           |          |        |          | n |           |           |
|           |          |        |          | s |           |           |
|           |          |        |          | e |           |           |
|           |          |        |          | c |           |           |
|           |          |        |          | t |           |           |
|           |          |        |          | s |           |           |
|           |          |        |          | * |           |           |
+-----------+----------+--------+----------+---+-----------+-----------+
| Cotton    | 1.74     | 2      | 734      |   | 413       | 46        |
|           |          |        |          |   |           |           |
| (DSMA)    |          |        |          |   |           |           |
+-----------+----------+--------+----------+---+-----------+-----------+
| Non-crop  | 3.86     | 4      | 2557     |   | 1438      | 160       |
|           |          |        |          |   |           |           |
| (MSMA)    |          |        |          |   |           |           |
+-----------+----------+--------+----------+---+-----------+-----------+
| Orchard   | 3.70     | 3      | 2071     |   | 1165      | 129       |
|           |          |        |          |   |           |           |
| (DSMA)    |          |        |          |   |           |           |
+-----------+----------+--------+----------+---+-----------+-----------+
| Turf; max | 3.35     | 4      | 2225     |   | 1251      | 139       |
|           |          |        |          |   |           |           |
| (MSMA)    |          |        |          |   |           |           |
+-----------+----------+--------+----------+---+-----------+-----------+
| Turf;     | 2.23     | 4      | 1482     |   | 833       | 93        |
| golf      |          |        |          |   |           |           |
|           |          |        |          |   |           |           |
| (MSMA)    |          |        |          |   |           |           |
+-----------+----------+--------+----------+---+-----------+-----------+

^1^ The pesticide with the highest application rate for each use was
modeled as the acid equivalent MMA.

> 2\. Number range CHAPTER*Plants*

Exposure to upland and wetland plants is estimated using the TerrPlant
(v1.0) screening model and the EECs are presented in Table 4. TerrPlant
estimates potential exposure using default assumptions for runoff and
spray drift. It should be emphasized that TerrPlant is only used for
estimating environmental concentrations based on a single application.
Most uses of MSMA and DSMA allow for multiple applications. TerrPlant
model details are included in Appendix D.

**Table 4. EECs (lb MMA/A) in off-site terrestrial environments
resulting from drift and runoff from single applications of DSMA and
MSMA.**

+----------+--------+---------+--------+--------+----------+--------+
| **Crop** | *      | **R     |        | **D    | **Total  |        |
|          | *Appl. | unoff** |        | rift** | Load**   |        |
| **(C     | rat    | (lb     |        |        |          |        |
| hemical) | e^2^** | ae/A)   |        | **     |          |        |
| ^1^**    |        |         |        | only** |          |        |
|          | (lbs   |         |        |        |          |        |
|          | ae/A)  |         |        | (lb    |          |        |
|          |        |         |        | ae/A)  |          |        |
+----------+--------+---------+--------+--------+----------+--------+
|          |        | *Upl    | *Semi  |        | *Up      | *Semi  |
|          |        | and^3^* | -Aquat |        | land^3^* | -Aquat |
|          |        |         | ic^4^* |        |          | ic^4^* |
+----------+--------+---------+--------+--------+----------+--------+
| Cotton   | 1.74   | 0.087   | 0.87   | 0.017  | 0.104    | 0.887  |
|          |        |         |        |        |          |        |
| (DSMA)   |        |         |        |        |          |        |
+----------+--------+---------+--------+--------+----------+--------+
| Non-crop | 3.86   | 0.193   | 1.93   | 0.039  | 0.232    | 1.969  |
|          |        |         |        |        |          |        |
| (MSMA)   |        |         |        |        |          |        |
+----------+--------+---------+--------+--------+----------+--------+
| Orchard  | 3.70   | 0.185   | 1.85   | 0.037  | 0.222    | 1.887  |
|          |        |         |        |        |          |        |
| (DSMA)   |        |         |        |        |          |        |
+----------+--------+---------+--------+--------+----------+--------+
| Turf;    | 2.23   | 0.167   | 1.67   | 0.033  | 0.201    | 1.709  |
| golf     |        |         |        |        |          |        |
|          |        |         |        |        |          |        |
| (MSMA)   |        |         |        |        |          |        |
+----------+--------+---------+--------+--------+----------+--------+
| Turf;    | 3.35   | 0.111   | 1.11   | 0.022  | 0.134    | 1.137  |
| max      |        |         |        |        |          |        |
|          |        |         |        |        |          |        |
| (MSMA)   |        |         |        |        |          |        |
+----------+--------+---------+--------+--------+----------+--------+

^1^ The pesticide with the highest application rate for each use was
modeled, both as the acid equivalent MMA.

^2^ All application rates are based on ground application. The cotton
use is labeled for aerial application as well but a higher rate is
allowed for ground application.

^3^ Sheet runoff + drift.

^4^ Channelized runoff + drift.

> 3\. *Number range CHAPTERSoil Accumulation*

Long term accumulation of pesticides in soil is not generally considered
in ecological risk assessments. Organic arsenicals are unique, however,
due to the elemental nature of arsenic. Arsenic does not degrade; along
with arsenicals' relative immobility in soil, this makes long term
impacts an important consideration. Field studies, monitoring of soil in
use areas, and modeling all suggest that it is likely that applied
arsenicals will build up in soil over time. Arsenic accumulation is
likely to be limited to the top layers of soil, with studies suggesting
that it is unlikely to occur at depths greater than 30 cm. A more
detailed discussion of the potential for arsenic to accumulate in soil
is included in the Environmental Fate and Transport section in Appendix
A.

Registrant terrestrial field dissipation studies have measured the
impact of one season of pesticide applications on soil concentrations.
Two studies at rates similar to the maximum labeled cotton and turf
rates found that a single year of application is unlikely to lead to
significantly elevated soil arsenic levels. Since most of the applied
arsenic remained in the top 6 inches of soil, however, repeated
application may lead to significant accumulation. Higher application
rates of DMA, possible in some of the non-crop uses, led to elevated
soil arsenic levels after a single year of application. Few studies have
evaluated the longer term impact of repeated application of organic
arsenicals. Those that have been conducted have conflicting results,
with some reporting no arsenic buildup despite very high application
rates and others finding substantial buildup at rates similar to current
labels. Most reports do not contain adequate explanations to account for
observed loss of arsenic. Monitoring of soil in areas where arsenicals
are known to have been applied, including golf courses and roadside
areas, has found significant increases of arsenic levels relative to
background concentrations. In Miami, an area with low background arsenic
levels, arsenic was the most common contaminant found in documented soil
violations at golf courses.

Number range CHAPTERSoil accumulation values are generated by PRZM as
part of the process of modeling runoff concentrations. PRZM was run with
a modified version of the pe4 v01 shell program to estimate arsenic
accumulation in the top 10 cm of soil. Results are presented in Table 5.
Maximum application rates for MMA and DMA on turf and cotton were used
and the modeling assumed median sorption (K~d~ = 30 for MMA and K~d~ =
20 for DMA). The annual application rates for DMA on non-crop areas and
ornamentals are nearly 50% higher than the turf rate, and so would be
expected to lead to higher soil concentrations. Because of limitations
in the available data and modeling capabilities, soil concentrations
were modeled as total arsenic, rather than speciated forms. Application
rates were therefore calculated to represent applied arsenic and
infinite half-lives were used to capture all forms of arsenic that may
be present. Routes of dissipation accounted for in the modeling include
runoff, leaching, and soil erosion; transformation to volatile species
and plant uptake are not accounted for. Of available scenarios, PA turf
and NC cotton scenarios led to the highest soil concentrations. Because
soil concentrations are based on local application rather than input
from an entire watershed, use of a percent cropped area (PCA) factor was
unnecessary.

**Table 5. Modeled soil concentrations for total arsenic resulting from
long term application of MMA or DMA.**

+------------+-----------+-------------+-----------+-----------------+
| **Parent   | **Use**   | **App.      | **No. of  | **Chronic EEC   |
| Compound** |           | rate**      | Appli     | ^1^**           |
|            |           |             | cations** |                 |
|            |           | (lbs As/A)  |           | (ppm, total     |
|            |           |             |           | arsenic)        |
+------------+-----------+-------------+-----------+-----------------+
| **MSMA**   | Cotton    | 0.95        | 2         | 13              |
+------------+-----------+-------------+-----------+-----------------+
|            | Turf      | 1.80        | 4         | 45              |
+------------+-----------+-------------+-----------+-----------------+
| **DMA**    | Cotton    | 0.65        | 1         | 2               |
+------------+-----------+-------------+-----------+-----------------+
|            | Turf ^2^  | 4.18        | 4         | 77              |
|            |           |             |           |                 |
|            | (lawn     |             |           |                 |
|            | edging)   |             |           |                 |
+------------+-----------+-------------+-----------+-----------------+

^1^ "Chronic" concentrations are the upper 90^th^ percent confidence
limit on the annual average, or the 1-in-10 year peak annual
concentration.

^2^ Application rate based on master label from 12/12/05.

**IV. RISK CHARACTERIZATON**

> **A. Risk Estimation**

Number range CHAPTERA risk quotient (RQ)-based approach is used in this
assessment, comparing the ratio of exposure concentrations to effects
endpoints with predetermined levels of concern (LOCs). Although risk is
often defined as the likelihood and magnitude of adverse ecological
effects, the risk quotient-based approach does not provide a
quantitative estimate of likelihood and/or magnitude of an adverse
effect. The primary change in these RQs, compared to those in the 2001
RED (DP Barcode D277233), is that they are calculated using EECs based
on application rates from updated master labels. No new toxicity data
have been considered and most toxicity endpoints used in calculations
have not changed. For terrestrial avians and small mammals, the LD50s
that were used in calculating acute RQs was adjusted for the purity of
the test material. This calculation is reported in the original RED, but
for those earlier calculations, the unadjusted LD50s were used. Further
discussion of the toxicity can be found in the 2001 RED.

> 1\. *Non-target Aquatic Number range CHAPTERAnimals*

Acute RQs for aquatic organisms, based on the aquatic EECs listed in
Table 2, are shown below in Table 6. All calculated RQs for fish and
aquatic invertebrates are \< 0.05 and below the level of concern (LOC).
(This table updates Table 10 from the 2001 RED).

**Table 6. Acute risk quotients for MMA exposure to aquatic organisms.**

+---+-----------+-----------+---------+----------+---+----------+----------+
| * |           | **Peak    | **Fre   |          |   | **       |          |
| * |           | EEC**     | shwater |          |   | Estuarin |          |
| C |           |           | RQ      |          |   | e/marine |          |
| r |           | **(ppb)** | ^1^**   |          |   | RQ ^1^** |          |
| o |           |           |         |          |   |          |          |
| p |           |           |         |          |   |          |          |
| * |           |           |         |          |   |          |          |
| * |           |           |         |          |   |          |          |
|   |           |           |         |          |   |          |          |
| * |           |           |         |          |   |          |          |
| * |           |           |         |          |   |          |          |
| ( |           |           |         |          |   |          |          |
| C |           |           |         |          |   |          |          |
| h |           |           |         |          |   |          |          |
| e |           |           |         |          |   |          |          |
| m |           |           |         |          |   |          |          |
| i |           |           |         |          |   |          |          |
| c |           |           |         |          |   |          |          |
| a |           |           |         |          |   |          |          |
| l |           |           |         |          |   |          |          |
| ) |           |           |         |          |   |          |          |
| * |           |           |         |          |   |          |          |
| * |           |           |         |          |   |          |          |
+===+===========+===========+=========+==========+===+==========+==========+
|   |           |           | *F      | *Inv     |   | *        | *Inv     |
|   |           |           | ish^2^* | ert.^3^* |   | Fish^4^* | ert.^5^* |
+---+-----------+-----------+---------+----------+---+----------+----------+
|   | Cotton    | 84        | \<0.05  | \<0.05   |   | \*       | \*       |
|   |           |           |         |          |   |          |          |
|   | (*DSMA*)  |           |         |          |   |          |          |
+---+-----------+-----------+---------+----------+---+----------+----------+
|   | Non-crop  | 360       | \<0.05  | \<0.05   |   | \<0.05   | \<0.05   |
|   |           |           |         |          |   |          |          |
|   | (*MSMA*)  |           |         |          |   |          |          |
+---+-----------+-----------+---------+----------+---+----------+----------+
|   | Orchard   | 260       | \<0.05  | \<0.05   |   | \*       | \*       |
|   |           |           |         |          |   |          |          |
|   | (*DSMA*)  |           |         |          |   |          |          |
+---+-----------+-----------+---------+----------+---+----------+----------+
|   | Turf; max | 310       | \<0.05  | \<0.05   |   | \<0.05   | \<0.05   |
|   |           |           |         |          |   |          |          |
|   | (*MSMA*)  |           |         |          |   |          |          |
+---+-----------+-----------+---------+----------+---+----------+----------+
|   | Turf;     | 210       | \<0.05  | \<0.05   |   | \<0.05   | \<0.05   |
|   | golf      |           |         |          |   |          |          |
|   |           |           |         |          |   |          |          |
|   | (*MSMA*)  |           |         |          |   |          |          |
+---+-----------+-----------+---------+----------+---+----------+----------+

^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^ LC50 = 323 ppm for MSMA

^5^ EC50 = 160 ppm for MSMA

\* No toxicity data for DSMA and marine/estuarine organisms were
presented, so no RQs were calculated.

2.  *Non-target Aquatic Number range CHAPTERPlants*

Risk quotients for aquatic plants, based on toxicity values from the
original RED and EECs determined using GENEEC2, are summarized below in
Table 7. (This table updates Table 12 from the 2001 RED). No risks to
non-endangered or endangered aquatic plants exceed the level of concern
of 1.

**Table 7. Risk quotients for MMA exposure to aquatic plants.**

+---+-----------+-----------+--------------+--------------+-------------+
| * |           | **Peak    | **RQ ^1^**   |              |             |
| * |           | EEC**     |              |              |             |
| C |           |           |              |              |             |
| r |           | **(ppb)** |              |              |             |
| o |           |           |              |              |             |
| p |           |           |              |              |             |
| * |           |           |              |              |             |
| * |           |           |              |              |             |
|   |           |           |              |              |             |
| * |           |           |              |              |             |
| * |           |           |              |              |             |
| ( |           |           |              |              |             |
| C |           |           |              |              |             |
| h |           |           |              |              |             |
| e |           |           |              |              |             |
| m |           |           |              |              |             |
| i |           |           |              |              |             |
| c |           |           |              |              |             |
| a |           |           |              |              |             |
| l |           |           |              |              |             |
| ) |           |           |              |              |             |
| * |           |           |              |              |             |
| * |           |           |              |              |             |
+===+===========+===========+==============+==============+=============+
|   |           |           | *Non         | *Non         | *           |
|   |           |           | -endangered* | -endangered* | Endangered* |
|   |           |           |              |              |             |
|   |           |           | *Vascular    | *Non-        | *Vascular   |
|   |           |           | spp.^2^*     | vascular^3^* | spp.^2^*    |
+---+-----------+-----------+--------------+--------------+-------------+
|   | Cotton    | 84        | \<0.1        | \<0.1        | \<0.1       |
|   |           |           |              |              |             |
|   | (DSMA)    |           |              |              |             |
+---+-----------+-----------+--------------+--------------+-------------+
|   | Non-crop  | 360       | \<0.1        | 0.13         | \<0.1       |
|   |           |           |              |              |             |
|   | (MSMA)    |           |              |              |             |
+---+-----------+-----------+--------------+--------------+-------------+
|   | Orchard   | 260       | \<0.1        | 0.17         | \<0.1       |
|   |           |           |              |              |             |
|   | (DSMA)    |           |              |              |             |
+---+-----------+-----------+--------------+--------------+-------------+
|   | Turf; max | 310       | \<0.1        | 0.11         | \<0.1       |
|   |           |           |              |              |             |
|   | (MSMA)    |           |              |              |             |
+---+-----------+-----------+--------------+--------------+-------------+
|   | Turf;     | 210       | \<0.1        | \<0.1        | \<0.1       |
|   | golf      |           |              |              |             |
|   |           |           |              |              |             |
|   | (MSMA)    |           |              |              |             |
+---+-----------+-----------+--------------+--------------+-------------+

^1^ RQ = EEC / EC50 (non-endangered species) or NOEC (endangered
species)

^2^ vascular (duckweed) EC50\'s and NOECs: 72.7 ppm & 20.5 ppm (DSMA),
53 ppm & 29 ppm (MSMA)

^3^ non-vascular (algae or diatom) EC50\'s: 1.5 ppm (DSMA), 2.8 ppm
(MSMA)

*3. Non-target Terrestrial Animals*

Acute and chronic risk quotients for terrestrial small mammals are
summarized below in Table 8. These RQs are calculated based on toxicity
values from the original RED, adjusted for the purity of the test
material, and EECs determined using TREX. (This table updates Tables 3
and 17 from the 2001 RED). These RQs are applicable to mammals with body
weights of 35 g that consume green vegetation or insects equivalent to
66% of their body weight (herbivores and insectivores) or seeds
equivalent to 15% of their body weight (granivores). The purpose of this
addendum is only to update RQs from the 2001 RED, so RQs have not been
calculated for mammals with body weights of 10 g or 1000 g, as would be
done for a current assessment. Most of these acute RQs, excepting those
for granivores, exceed the endangered species LOC of 0.1 while some also
exceed the restricted use and high risk LOCs of 0.2 and 0.5. All but 2
of the chronic RQs exceed the chronic risk LOC of 1.

**Table 8. Risk quotients for small mammals (35 g) from exposure to
MMA.**

+----------+---------+---------+---------+---------+---------+---------+
| **Crop** | **Acute |         |         | **      |         |         |
|          | RQs**   |         |         | Chronic |         |         |
| **(Che   | ^1^     |         |         | RQs**   |         |         |
| mical)** |         |         |         | ^2^     |         |         |
+----------+---------+---------+---------+---------+---------+---------+
|          | *Her    | *Insec  | *Gra    | *Her    | *Insec  | *Gra    |
|          | bivore* | tivore* | nivore* | bivore* | tivore* | nivore* |
+----------+---------+---------+---------+---------+---------+---------+
| Cotton   | 0.17\*  | 0.10\*  | \<0.1   | 7.3^‡^  | 4.1^‡^  | 0.46    |
|          |         |         |         |         |         |         |
| (DSMA)   |         |         |         |         |         |         |
+----------+---------+---------+---------+---------+---------+---------+
| Non-crop | 6.0     | 3.3     | \<0.1   | 26^‡^   | 14^‡^   | 1.6^‡^  |
|          | 4\*\*\* | 9\*\*\* |         |         |         |         |
| (MSMA)   |         |         |         |         |         |         |
+----------+---------+---------+---------+---------+---------+---------+
| Orchard  | 0       | 0       | \<0.1   | 21^‡^   | 12^‡^   | 1.3^‡^  |
|          | .48\*\* | .27\*\* |         |         |         |         |
| (DSMA)   |         |         |         |         |         |         |
+----------+---------+---------+---------+---------+---------+---------+
| Turf;    | 5.2     | 2.9     | \<0.1   | 22^‡^   | 13^‡^   | 1.4^‡^  |
| max      | 5\*\*\* | 5\*\*\* |         |         |         |         |
|          |         |         |         |         |         |         |
| (MSMA)   |         |         |         |         |         |         |
+----------+---------+---------+---------+---------+---------+---------+
| Turf;    | 3.5     | 1.9     | \<0.1   | 15^‡^   | 8.3^‡^  | 0.93    |
| golf     | 0\*\*\* | 7\*\*\* |         |         |         |         |
|          |         |         |         |         |         |         |
| (MSMA)   |         |         |         |         |         |         |
+----------+---------+---------+---------+---------+---------+---------+

^1^ Acute RQ = EEC / LD50, corrected for body weight; LD50s = 1599 mg/kg
(rat) for DSMA, 157 mg/kg (rat) for MSMA, adjusted for purity of test
material.

^2^ Chronic RQ = EEC/NOEC, corrected for body weight; NOEC = 100 ppm
(rat) for MMA

\*\*\* exceeds LOCs for high risk (0.5), restricted use (0.2), and
endangered species (0.1)

\*\* exceeds the LOCs for restricted use and endangered species

\* exceeds the LOC for endangered species

^‡^ exceeds the chronic risk LOC (1)

Acute RQs for avian species are presented below in Table 9. These values
are based on toxicity values from the original RED, adjusted for the
purity of the test material, and EECs determined using TREX. (This table
updates Table 4 from the 2001 RED). As with terrestrial mammals, most
RQs, with the exception of granivores, exceed the restricted use and
endangered species LOCs of 0.2 and 0.1 while some also exceed the high
risk LOC of 0.5.

**Table 9. Risk quotients for avians from exposure to MMA.**

+-----------------+-----------------+-----------------+-----------------+
| **Crop**        | **Acute RQ      |                 |                 |
|                 | ^1^**           |                 |                 |
| **(Chemical)**  |                 |                 |                 |
+=================+=================+=================+=================+
|                 | *Herbivores*    | *Insectivores*  | *Granivores*    |
+-----------------+-----------------+-----------------+-----------------+
| Cotton          | 0.16\*          | \<0.1           | \<0.1           |
|                 |                 |                 |                 |
| (DSMA)          |                 |                 |                 |
+-----------------+-----------------+-----------------+-----------------+
| Non-crop        | 1.53\*\*\*      | 0.86\*\*\*      | 0.1\*           |
|                 |                 |                 |                 |
| (MSMA)          |                 |                 |                 |
+-----------------+-----------------+-----------------+-----------------+
| Orchard         | 0.54\*\*\*      | 0.30\*\*        | \<0.1           |
|                 |                 |                 |                 |
| (DSMA)          |                 |                 |                 |
+-----------------+-----------------+-----------------+-----------------+
| Turf; max       | 1.33\*\*\*      | 0.75\*\*\*      | \<0.1           |
|                 |                 |                 |                 |
| (MSMA)          |                 |                 |                 |
+-----------------+-----------------+-----------------+-----------------+
| Turf; golf      | 0.89\*\*\*      | 0.50\*\*\*      | \<0.1           |
|                 |                 |                 |                 |
| (MSMA)          |                 |                 |                 |
+-----------------+-----------------+-----------------+-----------------+

^1^ RQ = EEC /LC50; LC50s = 4695 mg/kg (DSMA) and 1667 mg/kg (MSMA),
both for northern bobwhite, adjusted for purity of the test material.

\*\*\* exceeds LOCs for high risk (0.5), restricted use (0.2), and
endangered species (0.1)

\*\* exceeds the LOCs for restricted use and endangered species

\* exceeds the LOC for endangered species

> 4\. *Non-target Terrestrial and Semi-Aquatic Plants*

Risk quotients for terrestrial and semi-aquatic plants exposed to drift
and/or runoff are summarized below in Table 10. These are based on
toxicity values from the 2001 RED and EECs determined using TerrPlant.
Most RQs for endangered and non-endangered plants, both upland and
semi-aquatic, exceed the LOC of 1 for exposure from runoff and drift.
None of the drift only RQs exceed the LOC.

**Table 10. Risk quotients for terrestrial and semi-aquatic plants from
a single application of DSMA or MSMA.**

+----------+---------+---------+---------+---------+---------+---------+
| **Crop** | **      |         |         | **End   |         |         |
|          | Non-End |         |         | angered |         |         |
| **(Che   | angered |         |         | RQs**   |         |         |
| mical)** | RQs**   |         |         | ^2^     |         |         |
|          | ^1^     |         |         |         |         |         |
+----------+---------+---------+---------+---------+---------+---------+
|          | *       | *Se     | *Drift  | *       | *Se     | *Drift  |
|          | **Uplan | mi-Aqua | Only*   | **Uplan | mi-Aqua | Only*   |
|          | d***^3^ | tic*^3^ |         | d***^3^ | tic*^3^ |         |
+----------+---------+---------+---------+---------+---------+---------+
| Cotton   | \<1     | \<1     | \<1     | \<1     | 3\*     | \<1     |
|          |         |         |         |         |         |         |
| (DSMA)   |         |         |         |         |         |         |
+----------+---------+---------+---------+---------+---------+---------+
| Non-crop | 2\*     | 17\*    | \<1     | 13\*    | 109\*   | \<1     |
|          |         |         |         |         |         |         |
| (MSMA)   |         |         |         |         |         |         |
+----------+---------+---------+---------+---------+---------+---------+
| Orchard  | \<1     | 1.5\*   | \<1     | \<1     | 6\*     | \<1     |
|          |         |         |         |         |         |         |
| (DSMA)   |         |         |         |         |         |         |
+----------+---------+---------+---------+---------+---------+---------+
| Turf;    | 1.7\*   | 15\*    | \<1     | 11\*    | 95\*    | \<1     |
| golf     |         |         |         |         |         |         |
|          |         |         |         |         |         |         |
| (MSMA)   |         |         |         |         |         |         |
+----------+---------+---------+---------+---------+---------+---------+
| Turf;    | 1.1\*   | 9.8\*   | \<1     | 7.4\*   | 63\*    | \<1     |
| max      |         |         |         |         |         |         |
|          |         |         |         |         |         |         |
| (MSMA)   |         |         |         |         |         |         |
+----------+---------+---------+---------+---------+---------+---------+

^1^ RQ = EEC / EC25. For total loading use seedling emergence EC25 (1.25
and 0.116 lb ai/A for DSMA and MSMA, respectively). For drift use
vegetative vigor EC25 (0.354 and 0.418 lb ai/A for DSMA and MSMA,
respectively)

^2^ RQ = EEC / NOEC. For total loading use seedling emergence NOEC (0.30
and 0.018 lb ai/A for DSMA and MSMA, respectively). For drift use
vegetative vigor NOEC (\<0.30 and 0.14 lb ai/A for DSMA and MSMA,
respectively)

^3^ Upland EEC based on sheet runoff + drift; Semi-Aquatic EEC based on
channelized runoff + drift.

\* exceeds the LOC (RQ [\>]{.underline}1) for nontarget plants

5.  *Soil Accumulation*

Because it is not typical for EFED to assess environmental risk
resulting from accumulation of pesticides in soil, there are no
established methods for calculating RQs based on this type of exposure.
The EPA's Office of Solid Waste and Emergency Response (OSWER) has
established procedures for dealing with the issue of contaminated soils.
Rather than calculating RQs resulting from accumulation of arsenic in
soil, EFED has compared the estimated soil concentration to ecological
soil screening levels (SSL) for arsenic set by OSWER in March, 2005
(OSWER, 1995). SSLs are defined as "concentrations of contaminants in
soil that are protective of ecological receptors that commonly come into
contact with soil or ingest biota that live in or on soil." More
information about how these levels were set can be obtained at
<http://www.epa.gov/ecotox/ecossl/pdf/eco-ssl_arsenic.pdf>.

Table 11 presents the SSLs set to protect plants, birds, and mammals.
Insufficient data were available to set an SSL for soil invertebrates.
Arsenic SSLs are based on concentrations of *total arsenic*. The soil
concentrations estimated using PRZM, also as total arsenic, are included
for comparison. Application of either MMA or DMA at the maximum rate for
cotton does not lead to exceedance of any SSLs, but the maximum rates of
MMA or DMA applied to turf exceed SSLs for all groups.

**Table 11. Soil EECs for MSMA and DMA compared to OSWER arsenic SSLs.**

+-------------+------------+--------------+--------------+------------+
| **Parent    | **Use**    | **Chronic    | **OSWER      |            |
| Compound**  |            | EEC**        | Eco-SSLs**   |            |
|             |            |              |              |            |
|             |            | (ppm, total  | (mg/kg dry   |            |
|             |            | arsenic)     | weight)      |            |
+-------------+------------+--------------+--------------+------------+
| **MSMA**    | Cotton     | 13           | Plants       | 18         |
+-------------+------------+--------------+--------------+------------+
|             | Turf       | 45           | Avian        | 43         |
|             |            |              | Wildlife     |            |
+-------------+------------+--------------+--------------+------------+
| **DMA**     | Cotton     | 2            | Mammalian    | 46         |
|             |            |              |              |            |
|             |            |              | Wildlife     |            |
+-------------+------------+--------------+--------------+------------+
|             | Turf       | 77           | Soil         | NA         |
|             |            |              | I            |            |
|             | (lawn      |              | nvertebrates |            |
|             | edging)    |              |              |            |
+-------------+------------+--------------+--------------+------------+

> **B. Risk Description**
>
> *1. Risks to Aquatic Organisms*

Use of MSMA and DSMA present minimal risk to aquatic species, including
fish, invertebrates, and plants. In the original assessment, the one
exception found to this conclusion was risk to freshwater fish from the
very high application rate of MSMA to orchards. The lower application
rates on the current master label have reduced this risk quotient below
the LOC.

> *2. Risks to Terrestrial Organisms*

With the lower application rates, most terrestrial risk quotients (RQs)
have dropped but the overall risk conclusions have not changed.
Application of organic arsenicals presents risk to all categories of
terrestrial organisms, including small mammals, avian species, and
upland and semi-aquatic plants. Because of lower applications rates, in
many cases the exceedances are not as great as in the original
assessment. Additionally, there are fewer exceedances in the highest
risk category.

> *3. Federally Threatened and Endangered (Listed) Species Concerns*

The endangered species portion of this assessment has not been updated.
Number range CHAPTERThe original assessment found that application of
DSMA and MSMA present risk to endangered species for birds, mammals and
terrestrial plants. There are still exceedances in each of these
categories. The original assessment found risk to endangered freshwater
fish from the high application rate for MSMA on orchards. Limiting this
application rate lowered the risk for freshwater fish below the LOC.

The Agency's preliminary risk assessment for endangered species
indicates that organic arsenic applied as a pesticide results in a
determination of \"no effect\" to listed aquatic animal and plant
species on an acute or chronic basis. RQs exceed endangered species LOCs
for terrestrial plants, birds, and mammals. These findings are based
solely on EPA's screening level assessment and do not constitute "may
affect" findings under the Endangered Species Act.

> *4. Uncertainties*

The uncertainties of this risk assessment were discussed in the original
assessments. Further discussion of the uncertainty regarding the
environmental fate and transport properties of organic arsenicals has
been included in Appendix A, the Environmental Fate and Transport
section. Uncertainties in the modeling input parameters are discussed in
the organic arsenicals Drinking Water Assessment (DP Barcode D309098).

One additional source of uncertainty is relevant to this addendum. All
exposure estimates for MSMA and DSMA in this document are based on
application rates as MMA, the acid equivalent of the methanearsonate
salts. For determining terrestrial risk, in some cases it is unclear
which form of the parent compound was used as the active ingredient used
to determine toxicity. Some of these endpoints may have been based on
concentrations as MSMA rather than as MMA. If the toxicity is based on
arsenic as MSMA, the units of the toxicity estimate would have to be
converted MMA to be appropriately compared to the exposure as MMA. This
would lead to a lower toxicity value and therefore a higher risk
quotient and a higher estimate of risk. It is possible, then, that risk
was underestimated in some of the terrestrial categories. Even if true,
though, this would not lead to significant changes in the risk
conclusions.

**APPENDIX A:**

**ENVIRONMENTAL FATE OF ORGANIC ARSENICALS**

**Physicochemical Properties**

MSMA, DSMA, and CAMA are salts of the dibasic weak acid
monomethylarsonic acid (MMA; also variously abbreviated in other
documents and the published literature as MMAA or MAA, for
methanearsonic acid). In aqueous solution these compounds dissociate
into MMA and the associated ions, either sodium or calcium. Cacodylic
acid (DMA, for dimethylarsonic acid) is a weak acid with two methyl
groups attached to the central arsenic atom, rather than one as in MMA.
In some formulations, DMA is mixed with its sodium salt (DMA-Na), which
in aqueous solution is also dissociated to DMA and a sodium ion.
Chemical structures and some physicochemical properties for MSMA, DSMA,
DMA, and DMA-Na are presented in Tables A1a and A1b below. This group of
compounds is referred to as "organic arsenicals" throughout this
document. Most environmental fate studies have been conducted on MSMA or
DMA as representatives of the monomethylated and dimethylated species,
respectively.

**Table A1a. Physicochemical Properties for MSMA and DSMA**

+-----------------------+-----------------------+---------------------+
|                       | **DSMA**              | **MSMA**            |
+-----------------------+-----------------------+---------------------+
| Molecular Structure   | ![](medi              | ![](media/im        |
|                       | a/image2.wmf){width=" | age3.wmf){width="0. |
|                       | 1.2236111111111112in" | 9965277777777778in" |
|                       | height="0.76875in"}   | height="0.76875in"} |
+-----------------------+-----------------------+---------------------+
| Empirical Formula     | CH~3~AsNa~2~O~3~      | CH~4~AsNaO~3~       |
+-----------------------+-----------------------+---------------------+
| Molecular Weight      | > 183.92              | > 161.94            |
+-----------------------+-----------------------+---------------------+
| CAS No.               | > 144-21-8            | > 2163-80-6         |
+-----------------------+-----------------------+---------------------+
| PC Code               | > 013802              | > 013803            |
+-----------------------+-----------------------+---------------------+
| Melting Point (*ºC*)  | > \>300               | > 116-121           |
+-----------------------+-----------------------+---------------------+
| Density (*g/mL*)      | > 1.04                | > 1.65              |
+-----------------------+-----------------------+---------------------+
| Vapor Pressure (*mm   | > 1 x 10^-7^          | > 7.5 x 10^-7^      |
| Hg*)                  |                       |                     |
+-----------------------+-----------------------+---------------------+
| log K~ow~             | > \< 1                | > \< 1              |
+-----------------------+-----------------------+---------------------+
| Solubility: Water     | > 3.4 x 10^5^         | > 104               |
| (*mg/L*)              |                       |                     |
+-----------------------+-----------------------+---------------------+
| Methanol              | > 2.6 x 10^5^         | > 16                |
+-----------------------+-----------------------+---------------------+
| Hexanol               | > 25                  | > 0.005             |
+-----------------------+-----------------------+---------------------+
| pK~a1,2~ (approx.)    | > 4.0, 9.0            | > 4.0, 9.0          |
+-----------------------+-----------------------+---------------------+

**Table A1b. Physicochemical Properties for Cacodylic Acid**

+-----------------------+-----------------------+---------------------+
|                       | **Cacodylic Acid**    | **Sodium            |
|                       |                       | Cacodylate**        |
|                       | (DMA)                 | (DMA-Na)            |
+-----------------------+-----------------------+---------------------+
| Molecular Structure   | Number range CHAPTER  | O                   |
|                       | O                     |                     |
|                       |                       | \|\|                |
|                       | \|\|                  |                     |
|                       |                       | CH~3~--As--O^-^     |
|                       | CH~3~--As--OH         | Na^+^               |
|                       |                       |                     |
|                       | \|                    | \|                  |
|                       |                       |                     |
|                       | CH~3~                 | CH~3~Number range   |
|                       |                       | CHAPTER             |
+-----------------------+-----------------------+---------------------+
| Empirical Formula     | Number range          | C~2~H~6~AsNaO~2~    |
|                       | CHAPTERC~2~H~7~AsO~2~ |                     |
+-----------------------+-----------------------+---------------------+
| Molecular Weight      | > 138.0               | > 160.0             |
+-----------------------+-----------------------+---------------------+
| CAS No.               | > Number range        | > 124-65-2          |
|                       | > CHAPTER75-60-5      |                     |
+-----------------------+-----------------------+---------------------+
| PC Code               | > 012501              | > 012502            |
+-----------------------+-----------------------+---------------------+
| Melting Point (*ºC*)  | > Number range        | > 77 -- 79.5        |
|                       | > CHAPTER192-194      |                     |
+-----------------------+-----------------------+---------------------+
| Density (*g/mL*)      | > 1.10                | > 1.10              |
+-----------------------+-----------------------+---------------------+
| Vapor Pressure (*mm   | > Non-volatile        | > No data           |
| Hg*)                  |                       |                     |
+-----------------------+-----------------------+---------------------+
| K~ow~                 | > \<0.028             | > No data           |
+-----------------------+-----------------------+---------------------+
| Solubility: Water     | > Number range        | > No data           |
| (*mg/L*)              | > CHAPTER\~1 to 3 x   |                     |
|                       | > 10^6^               |                     |
+-----------------------+-----------------------+---------------------+
| Methanol              | > Number range        | > No data           |
|                       | > CHAPTER3.63 x 10^5^ |                     |
+-----------------------+-----------------------+---------------------+
| Hexanol               | > Number range        | > No data           |
|                       | > CHAPTER1.02 x       |                     |
|                       | > 10^-1^              |                     |
+-----------------------+-----------------------+---------------------+
| pK~a~                 | > 6.2                 | > 6.2               |
+-----------------------+-----------------------+---------------------+

As weak acids, the form in which MMA and DMA are present is dependent on
pH, which affects the degree of association with hydrogen ions and
therefore the charge of the species. For MMA, the three possible species
are symbolized H~2~MMA, HMMA^1-^, and MMA^2-^. Reported titration
studies for MSMA and DSMA yield base strengths for the anions
corresponding to pK~a~ values for the acid MMA of 4 and 9. Under
agriculturally relevant conditions, then, HMMA^1-^ would dominate at pH
5 to 7 as approximately 90% to 99% of the total, while at pH 9, it would
be approximately equal to the amount present as MMA^2-^. DMA can be
present as neutral cacodylic acid or as the cacodylate anion and has a
pK~a~ of 6.2. At a pH of 5, then, neutral cacodylic acid dominates at
approximately 94% of the total, while at pHs of 7 to 9, the anionic
species is dominant, going from 87% of the total to more than 99%. In
the environment, chemical speciation is in dynamic flux, depending on
variations in pH and affected by transport, mixing, diffusion, etc., in
accordance with principles of chemical equilibria and kinetics.

**Degradation and Metabolism (Rate and Environmental Conditions)**

Environmental fate laboratory studies show that organic arsenicals are
stable under all tested abiotic conditions. Registrant submitted studies
of DMA and MMA found both compounds to be stable to hydrolysis at all
pHs (MRIDs 42059201 and 42363001). DMA and MMA were also found to be
stable to photolysis in both aquatic and soil environments (DMA: MRIDs
41662601 & 41662602; MMA: MRIDs 41903902 & 41903901).

Organic arsenicals can be subject to microbial metabolism in soil under
aerobic or anaerobic conditions. The occurrence, rate, and products of
this metabolism are variable, dependent on environmental conditions. The
observed persistence of organic arsenicals in aerobic soil has ranged
from weeks to years, depending on soil properties and ambient conditions
such as soil moisture, temperature, chemical concentration, and amount
of organic matter. The extreme of this range is seen in several
registrant submitted studies of microbial metabolism which observed no
transformation at all (MRIDs 42616001, 42572601, 43036101). These
studies were determined to be scientifically sound but were found to be
not fully acceptable because they provided no explanation for this
result in light of well established evidence of organic arsenical
metabolism. Although these studies demonstrate that there are conditions
in which metabolism does not occur, possibly due to non-viable soils,
this assessment is based on the understanding that transformation of
organic arsenicals is an important, although variable, process.

Adding to the complexity is that metabolism rates do not appear to
depend linearly on organic arsenical concentration; in some studies,
transformation decreased with increasing concentration while in others,
concentration had no effect on metabolism. Hence, the kinetics are not
necessarily first-order, and "half-life" is therefore not necessarily an
appropriate constant for all concentrations. Keeping this uncertainty in
mind, first-order half-lives have been calculated as required by EFED's
current models and also as a convenient measure to compare results from
laboratory studies. The Agency derived values for a DMA half-life from
four studies representing a total of six soils. Together, these yielded
an effective average aerobic soil half-life for DMA of
173 [+]{.underline} 115 days with a standard upper 90% confidence limit
on the mean of 240 days (Woolson and Kearney, 1973; Woolson, 1982; Gao
and Burau, 1997; and MRID 44767601). The average half-life of DMA in
three anaerobic (flooded) soils was 128 [+]{.underline} 38 days with a
standard upper 90% confidence limit on the mean of 168 days (Woolson and
Kearney, 1973). For MMA, two studies in two soils both resulted in
aerobic soil half-lives of approximately 240 days (Gao and Burau, 1997;
MRID 44767601). No anaerobic soil half-life was determined for MSMA. The
studies on which these estimates are based are discussed in more detail
below and the calculations are also described in the surface water
modeling section of the Drinking Water Assessment (DP Barcode D309098).

Some of the variability in metabolism processes is associated with
variability in sorption processes. Soil microbial metabolism of organic
arsenicals only occurs while the compounds remain dissolved in pore
water (NRC, 2003). As the organic arsenicals sorb to soil, they become
less accessible to microbes and therefore less likely to be metabolized
(Woolson and Kearney, 1973). Sorption variability, discussed in more
detail in the mobility section (p. 28), is largely controlled by soil
properties including the clay content, the iron and aluminum content,
and the soil pH (Wauchope, 1975; Matera and La Hecho, 2001). Laboratory
studies have shown that in some situations, significant sorption of
arsenic compounds may occur within hours of application, while in
others, a large portion of applied arsenic remains in water-soluble
forms for days or months after application (Onken and Adriano, 1997;
Sarkar *et al.*, 2005). Remobilization of sorbed arsenic with changing
environmental conditions is also possible (Matera and La Hecho, 2001).

The effects of other environmental factors on the rate of organic
arsenical metabolism are complex and poorly defined with different
studies leading to conflicting results. An increase in temperature has
been shown to lead to increased metabolism (Akkari *et al.*, 1986; Gao
and Burau, 1997) but results on the impact of soil organic matter or
applied organic arsenical concentrations are contradictory (Dickens and
Hiltbold, 1967; Von Endt *et al.,* 1968; Woolson and Kearney, 1973;
Woolson, 1982; Akkari *et al.*, 1986; Gao and Burau, 1997).

The influence of aerobic versus anaerobic conditions on metabolism rates
is also ambiguous. Based on a comparison of the results of studies of
MMA applied to an aerobic soil and to an anaerobic sediment/aquatic
system, the registrants conclude that aerobic metabolism is more
significant than anaerobic metabolism (MAATF, 2005). After one year of
incubation, 35% of the parent MMA remained in aerobic soil (MRID
44767601), while in the anaerobic system, only 3% of applied MMA had
been metabolized (MRID 44767602). Both studies ended with a significant
amount of unidentified arsenic compounds bound to the soil (10-25%). Two
studies by Woolson also concluded that greater metabolism occurred in
aerobic than in anaerobic conditions. In one study, an average of 24% of
^14^C from applied DMA remained after 24 weeks incubation of 100 ppm DMA
in three aerobic soils while 39% remained in flooded soils (Woolson,
1973). In the other study, the amounts of applied DMA remaining after 60
days were similar at soil moisture levels between 77% and 125% of field
capacity but in flooded soils (230% field capacity), more parent
compound remained, indicating slower metabolism in anaerobic conditions
(Woolson, 1982).

Two more recent studies led to the opposite result, finding that
metabolism of organic arsenicals is more significant in anaerobic
conditions. In 3 soils incubated at 30°C for 120 days, Akkari found
half-lives in flooded conditions (150% field capacity) of approximately
1 month while in low-moisture conditions, (20% field capacity), the
half-lives were 2 to 5 months. One of these soils was also tested at 75%
field capacity and found to have a half-life between the flooded and
low-moisture soils. In this study, the reported half-life also includes
"deactivation by processes such as salt formation, irreversible
absorption, and ion-exchange" (Akkari *et al*, 1986). Gao and Burau
(1997) incubated one soil at a range of soil moistures for 70 days and
found significantly higher mineralization in flooded conditions (\>350 g
H~2~0/kg soil). In soil with moisture levels ≤350 g H~2~0/kg soil, 88%
to 97% of applied DMA was recovered as parent compound, while for
flooded soils, recovered parent compound was as low as 13%, indicating a
significant increase in metabolism in anaerobic conditions.

Of the studies reviewed for this document, the most complete, systematic
investigation of the metabolism of organic arsenicals under variable
conditions is by Gao and Burau (1997). In particular, this study is
valuable because the products are speciated, including trapping of
volatiles, with very good recovery. The mass balance is greater than
90%, even for inorganic arsenic, so, unlike some studies reviewed here,
these half-lives include only metabolism and not other routes of
dissipation such as sorption. Gao and Burau were motivated by noticing
differences in published results on transformation processes (rates and
products) and by a concern for the potential for arsenic to accumulate
in soils. The extent to which an applied organic arsenical and its
arsenic containing byproducts accumulate locally depends on both
mineralization and volatilization. The 1997 Gao and Burau study serves
to systematically complement, and, to a great extent, unify existing
data. The study is designed to measure the influence of four
factors\--concentration, soil moisture, temperature, and soil amendment
with organic carbon (cellulose)\--on the rates and routes of
transformation of organic arsenicals in soil. Gao and Burau primarily
tested DMA in the form of sodium cacodylate, although production of
volatile arsines was measured for MMA, sodium arsenate (As^V^), and
sodium arsenite (As^III^), and one treatment compared the metabolism
rates of DMA and MMA under the same conditions. The study was limited to
one California soil, a Sacramento silty clay (noncalcareous).

Gao and Burau's results show the great influence of soil environmental
conditions on transformation rates. After 70 days of incubation,
depending on conditions, the amount of parent cacodylate remaining
ranged from a high of about 97% to a low of about 13%[^1]. Under all
conditions, production of volatile arsines was less than 0.5% of the
applied arsenic. Arsenite was not a detected product under these
conditions and both DMA and MMA metabolized without detection of
methylation or demethylation. The amount of parent compound remaining at
the end of the study depended strongly on all tested factors. Unless
noted otherwise, the following tests were all carried out with DMA at
100 mg As/kg soil and soil moisture content of 350 g water/kg soil
(approximately 1/3-bar suction \[-0.03 Mpa\]).

1)  [Soil Moisture.]{.underline} As mentioned above, metabolism of DMA
was measured at five soil moisture contents of 50, 250, 350, 450, and
550 g water/kg soil \[from approximately 7% (-23.7 MPa suction) to 81%
(-0.0005 MPa) of saturation\], at 22 ^o^C. Mineralization to arsenate
increased with soil moisture from 2.7% at the lowest soil moisture to
86.6% at the highest, showing the tremendous influence of soil water.

2) [Temperature.]{.underline} At 5 and 25 ^o^C, mineralization was
approximately 5.2 and 23.8%, respectively. This result is roughly as
would be predicted from the Arrhenius relationship which yields rate
doubling for every 10 ^o^C increase in temperature. \[The 23.8%
mineralization result can be compared with the result of 12.1%
mineralization, from the soil moisture series, as a measure of
variability under essentially the same conditions, except for an
uncorrected 3 ^o^C difference in temperature.\]

3) [Concentration.]{.underline} At increasing cacodylate concentrations
equivalent to arsenic concentrations of 10, 30, and 100 mg As/kg soil at
25 ^o^C, decreasing percentages of cacodylate mineralized were
approximately 82, 31 and 24%, respectively, indicating that the rate
process is not first-order in concentration. One possible reason for the
slowing of metabolism at increasing concentrations could be toxicity to
soil microorganisms, however, the two upper concentrations correspond to
application rates much higher than those currently labeled.

4\) [Organic Matter.]{.underline} Increasing cellulose additions of 0.0
(unamended), 0.2, 1.0, and 5.0 g per 100 g of soil (original soil
organic carbon content of 1.8%) with sodium cacodylate added at the
arsenic equivalent of 10 mg As/kg soil at 25^0^C decreased
mineralization from around 77% down to around 49%. (As a measure of
variability in this experiment under the same conditions, the first
value of 77% can be compared directly with the 82% result from the
concentration experiment above.) Some reports show the same trend of
decreasing mineralization with increasing concentration of organic
matter (e.g., Woolson, 1982), but others report the opposite trend
(e.g., Dickens and Hiltbold, 1967). Types of added organic matter were
generally different, and could perhaps account for the differences.
However, in view of the sensitivity of metabolism to the other cited
factors, lack of extremely careful control of experimental conditions
could also be a major factor for the difference.

5\) [Arsenical Species.]{.underline} Metabolism rates of MMA and DMA
were compared by applying both at 100 mg As/kg soil under the same
environmental conditions. After 70 days at 22^0^C, more than twice as
much DMA as MMA had metabolized. Parent DMA remained at only 26.7% of
the applied, while 57.1% of the applied MMA remained.

Using the Gao and Burau data to estimate an "effective half-life" for
DMA (realizing that the process is apparently not first-order with
concentration), the Agency calculated half-lives for each trial, based
on the initial and final concentrations of parent compound. These were
interpolated as a function of soil moisture at the tested arsenic
equivalent concentration of 100 mg/kg soil to the standard 75% of
1/3-bar soil moisture content using the soil moisture retention function
(the logarithm of the absolute value of soil tension vs. soil water
concentration) which the authors gave for the Sacramento silty clay
soil. The interpolation gives a moisture concentration of approximately
27%, or 266 g water per kilogram of dry soil, and a corresponding DMA
half-life of 642 days (1.8 years). In tandem, the Agency then made a
simple, proportionate adjustment of the rate constant corresponding to
this half-life at the 100 mg/kg concentration to the more agriculturally
relevant 10 mg/kg arsenic equivalent (18 mg/kg cacodylic acid). This
range is relatively low when compared to most study concentrations which
were typically 10 to 180 ppm in cacodylic acid. This calculation leads
to a normalized DMA half-life (at 75% of 1/3-bar soil moisture and an
arsenic equivalent concentration of 10 mg/kg) of 102 days. The more
limited data for MMA was converted by comparison to the DMA
normalization, resulting in a normalized half-life of 241 days.

Additional reports contributing to the discussion of organic arsenical
soil metabolism include two published laboratory studies authored by
Woolson and others[^2]. In the first, ^14^C-cacodylic acid was applied
at three concentrations (1, 10 and 100 ppm DMA; arsenic equivalent 0.5,
5, and 54 ppm) to each of three soils of varying iron and aluminum
content (Woolson and Kearney, 1973). Soil moistures were brought to 75%
of field capacity and the temperature was 25 ^0^C. For several reasons
this study does not meet current guideline standards but was not
specifically cited as deficient in the 1986 Agency review and provides
valuable information. The authors found that "rate of application had no
appreciable effect on disappearance" and so reported the amount of ^14^C
remaining in each soil after 32 weeks as an average from the three
application rates. Based on these final values, the Agency calculated
assumed first-order aerobic half-lives of 325, 245, and 106 days, from a
loamy sand, a silty clay loam, and a clay loam, respectively. These are
considerably longer than the half-lives reported in the 2001 RED (117,
89, and 48 days; DP Barcodes D210451, D212449, D255226), which were
calculated based on the ^14^C remaining at 24 weeks rather than 32
weeks. Recalculation based on the complete length of the study led to
these longer half-lives which are more representative of the long term
behavior of applied organic arsenicals and closer to the guideline
standard of a study length of one year. Less data were reported for
anaerobic (flooded) soil metabolism and so half-lives were calculated
based on data from 24 weeks for cacodylic acid applied at 100 ppm.
Apparent anaerobic half-lives, as reported in the 2000 RED, were 86,
138, and 159 days. Due to the different dataset used, these half-lives
are not directly comparable to the calculated aerobic half-lives. They
would be expected to follow the same trend, with the cacodylic acid
metabolizing more slowly over time. Again, these estimated results are
dependent on professional judgment and interpretation. The nature of
transformation products revealed in this and other studies is integrated
below in the "metabolites" subsection.

Woolson *et al* (1982) applied cacodylate at an initial arsenic
equivalent concentration of 10 ppm to a Mattapeake silt loam soil
treated under several different conditions of soil moisture (77-230% of
field capacity) with unamended soil and with a variety soil amendments.
After 60 days of aerobic incubation at 25 ^0^C, the residual cacodylic
acid was reported to average approximately 15-30%. Results were similar
under all conditions. Based on these published data, the estimated
average aerobic soil half-life (assumed first-order) for all unamended
treatments was approximately 20 days. The estimated half-life from
amended soil, with a higher organic matter content, was approximately
31 days.

Again, none of these published studies meet current Guideline and GLP
criteria. Half-life estimates for all were based on single time
intervals, rather than complete time series. The Woolson studies
generally had poor recoveries with the loss accounted for through
inferred metabolism. These studies appeared formally in peer-reviewed
scientific publications, however, as is normal for most scientific work
outside of the regulatory process. They have been cited in the
literature numerous times and they were submitted by the registrant in
the past. These studies provide a body of evidence on the environmental
fate of organic arsenicals that cannot be dismissed.

One GLP aerobic soil metabolism study is available for MSMA and has been
rated acceptable according to guideline standards (MRID 44767601). In
this study, ^14^C-labeled MSMA was applied to a sandy loam soil at 6.1
ppm (2.7 ppm as arsenic) and incubated at 25°C and 75% of field moisture
capacity for 1 year. Based on measurements taken throughout the course
of the study, the estimated first-order half-life for MMA was 240 days.
This half-life is calculated from the percent of the applied dose
remaining as the parent compound. As much as 11% of the applied dose was
bound to soil and unextractable, so it is possible that this half-life
takes into account some amount of sorption as well as transformation.
The metabolism did not occur at a constant rate; 50% of the parent
compound had dissipated by 60 days, but after this time, metabolism
slowed to the extent that approximately 35% of the applied dose remained
after one year. DMA was formed as a metabolite in this study and data on
its formation and decline led to a half-life of 242 days, as discussed
in the surface water modeling section.

**Metabolites**

Potential metabolites of applied DMA and MMA include volatile
alkylarsines and inorganic arsenic (as arsenate or arsenite) along with
carbon dioxide. Cacodylic acid may be present as a metabolite of MMA as
well as an applied parent compound. The formulas and names of the
principal species are listed below in Table A2. 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
likely associated with the sensitivity to the ambient conditions,
mentioned above, although some variability is likely due to difficulty
in analysis as well. Regardless of the form it takes, however, the total
amount of arsenic present does not change; these organic arsenicals and
their transformation products, in combination with arsenic from the
natural background and from other anthropogenic sources, maintain the
total, immutable arsenic environmental load. Arsenic from pesticides is
not lost but redistributed and transformed throughout the environment
(plants, animals, air, soil, sediment, water) into other arsenic
containing substances.

The only metabolism pathway that could directly reduce soil arsenic
loading is transformation to volatile alkylarsines. Microbial species
capable of metabolizing dissolved organic arsenical compounds to gaseous
arsines have been identified (Cox and Alexander, 1973; Turpeinen, 2001).
Dimethylarsine and trimethylarsine are the most likely volatile products
of soil metabolism (Cox and Alexander, 1973; Woolson, 1977). Some early
studies of arsenic fate in soil concluded that volatilization was a
major route of dissipation. Woolson (1973) attributed losses of applied
arsenic of up to 60% to

**Table A2. Arsenic species commonly found in environmental samples.**

+--------------------+--------------+----------------+----------------+
| "Organic" Species  |              | "Inorganic"    |                |
|                    |              | Species        |                |
+--------------------+--------------+----------------+----------------+
| dimethylarsine     | (            | > arsenate     | > AsO~4~^3-^   |
|                    | CH~3~)~2~AsH | > \[As(V)\]    |                |
+--------------------+--------------+----------------+----------------+
| trimethylarsine    | (CH~3~)~3~As | > arsenite     | > AsO~3~^3-^   |
|                    |              | > \[As(III)\]  |                |
+--------------------+--------------+----------------+----------------+
| cacodylic acid     | (CH~3        |                |                |
|                    | ~)~2~AsO(OH) |                |                |
| > (dimethylarsenic |              |                |                |
| > acid,            |              |                |                |
| > hyd              |              |                |                |
| roxydimethylarsine |              |                |                |
| > oxide)           |              |                |                |
+--------------------+--------------+----------------+----------------+

volatilization. This conclusion was inferred indirectly from mass
balance calculations and was not confirmed analytically. In a later
study designed to investigate the possibility of volatilization, Woolson
found that less than 0.5% of applied arsenic was trapped as volatile
arsines from unamended soils, and only 1.4% was found in amended soils
(1982). Several other studies found volatilization of up to 15% of
applied arsenic from amended soils, but no more than 2.2% volatilization
was found in unamended soils (Woolson, 1977; Akins and Lewis, 1976;
Cheng and Focht, 1979). More recent studies with improved analytical
techniques have achieved mass balance without volatilization (Onken and
Adriano, 1997; MRID 44767601) and other experiments designed
specifically to consider volatilization have not found alkylarsines
produced at greater than 0.5% of applied arsenic (Gao and Burau, 1997;
Turpeinen *et al*, 2001). Metabolism to volatile alkylarsines is
possible under certain conditions but is generally not likely to be a
major route of dissipation. The possibility of volatilization was
therefore not included in calculations on the fate of applied organic
arsenical pesticides.

Other routes of metabolism include methylation and demethylation. DMA
has two methyl groups attached to a central arsenic, MMA has one, and
inorganic arsenic has none. Theoretically, any of the methyl groups on
DMA or MMA are subject to removal, while one methyl group could be added
to convert inorganic arsenic to MMA which could be further methylated to
DMA. In fact, not all of these transformations are likely to occur.
There is some uncertainty associated with all speciation of soil arsenic
due to limitations in extraction and analytical techniques. Organic
arsenical species may be transformed to some extent during the process
of extraction and measurement. This is not an issue when results are
reported as total arsenic, as done in many studies, but those results
are less useful in a discussion of metabolites.

Application of MMA may lead to both DMA and iAs as end products. A
registrant aerobic soil metabolism study found that after 1 year, 35% of
MMA applied at 6.1 ppm remained present as the parent compound, 32% was
present as DMA, and 19% had evolved as carbon dioxide, indicating
demethylation to inorganic arsenic (MRID 44767601; Acceptable).
Conversely, Gao and Burau (1997) detected no DMA resulting from
application of MMA despite significant metabolism to inorganic arsenic.
In the few other laboratory studies reviewed for this document that
speciated end products, variable DMA resulting from application of MMA
was found up to a maximum of 9% (MRID 43314801; MRID 44767602; Von Endt
*et al.*, 1968). Field studies found some DMA after application of MMA,
but typically in very small amounts (MRID 42526001; MRID 42616201).
Transformation to inorganic arsenic is discussed further below.

For metabolism of DMA, there are fewer studies available with speciated
results. A registrant submitted aerobic soil metabolism study detected
MMA in soil samples following application of DMA, but both the study
author and the EPA reviewer concluded that the MMA detected was an
artifact of the analytical methodology (MRID 42616001; Unacceptable).
Gao and Burau (1997) found that after 70 days, up to 82% of applied DMA
had metabolized, but only inorganic arsenic, and not MMA, was detected.
Given the relative stability of MMA, it is unlikely that MMA metabolized
too quickly to accumulate, so this result suggests that DMA metabolizes
directly to inorganic arsenic by loss of both methyl groups, rather than
stepwise going through a monomethylated intermediate. As discussed
above, other experiments have shown that Gao and Burau's results with
respect to MMA are not true in all situations. Their results for DMA are
more conclusive than those for MMA, however, because DMA's metabolism
was investigated under at least 15 sets of varying conditions, while MMA
was only studied in one. In another laboratory study, Woolson *et al*.
(1982) applied DMA at 10 ppm (as arsenic) in aerobic and anaerobic
soils, either unamended or amended with sewage sludge, dairy manure, or
hay. In unamended soils, the highest reported detection of MMA was at
0.8 ppm as arsenic. In amended soils in anaerobic conditions, detection
of MMA went as high as 3.6 ppm as arsenic. A registrant submitted
terrestrial field dissipation study looked for MMA in soil after two
applications of DMA at 24 lb/A (MRID 43485301; Supplemental). A small
amount of MMA (1.65 ppm as MSMA, equivalent to 0.76 ppm as arsenic) was
detected immediately after the second application and was undetected by
the end of the study. Based on the lack of significant transformation
seen in these studies, MMA was not treated as a metabolite of DMA in
modeling potential exposure to both species, although there is some
uncertainty in this assumption.

Inorganic forms of arsenic are generally more toxic than the organic
forms, so it is very important to assess the possibility of
mineralization of organic arsenical pesticides to arsenate \[As(V)\] and
arsenite \[As(III)\]. Results of studies investigating conversion to
inorganic arsenic are extremely variable, ranging from no mineralization
in one year to approximately 80% mineralization in 2 months. The only
GLP study carried out for a full year, as required by FIFRA guidelines,
found that 19% of MMA applied at 6.1 ppm mineralized to inorganic
arsenic. Mineralization, as measured by production of carbon dioxide,
was most rapid in the first three months and then slowed, with very
little, if any, occurring in the period from 9 to 12 months (MRID
44767601; Acceptable). These results are consistent with the findings of
several shorter term published studies which determined that metabolism
to inorganic arsenic is most likely to occur soon after application
(Abdelghani *et al.*, 1977; Von Endt *et al.*, 1968; Dickens and
Hiltbold, 1967). All of those studies, which lasted several weeks to
several months, ended with inorganic arsenic at less than 19% of the
applied amount, suggesting that as a reasonable upper bound.

Two other studies, however, present very different conclusions. One
study applied DMA at the arsenic equivalent of 10 ppm and found that in
aerobic conditions, 52% was demethylated after 60 days, while in
anaerobic conditions, 27% mineralization occurred. Mineralization was
significantly lower in soils amended with sewage sludge, manure, or hay
(Woolson, 1982). In another study, DMA was applied at 10 to 100 ppm in a
variety of conditions and incubated for 70 days. The least amount of
mineralization, all less than 7% of the applied, occurred in low
moisture soils and at low temperatures. In moister soils at 22-25°C,
demethylation ranged from approximately 12% to approximately 86% (Gao
and Burau, 1997). In the one treatment where MMA was applied,
approximately 43% demethylation occurred, relative to 73% for DMA in the
same conditions (100 ppm as arsenic, 22°C, soil moisture approximately
at field capacity). Terrestrial field dissipation studies also suggest
that conversion to inorganic arsenic can occur, with total soil arsenic
levels remaining elevated over control values even after applied parent
compounds are no longer detectable (MRIDs 43485301, 42526001). These
results are discussed in more detail in the soil buildup section (p.
30).

In the one available aerobic soil laboratory study where there was
analysis for arsenite \[As(III)\], a possible mineral transformation
product, it was not detected (Gao and Burau, 1997). Consistent with this
result, and as part of the same study, applied arsenite was converted to
arsenate. It is generally accepted and consistent thermodynamically that
arsenate rather than arsenite is the prevalent form in aerobic soils.
There were no reported tests for arsenite production from cacodylic acid
under anaerobic or flooded conditions. However, comparable
concentrations of arsenate and arsenite can thermodynamically coexist
under certain environmental conditions, including those found in
groundwater and surface water and such concentrations have been
measured, as discussed in the monitoring section of the Drinking Water
Assessment (DP Barcode D309098).

**Mobility**

[In Air.]{.underline} 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.]{.underline}

Sorption to numerous diverse soils varies tremendously, but indicates
intermediate to low mobility. Laboratory studies have shown that in some
situations, significant sorption of arsenic compounds may occur within
hours of application, while in others, a large portion of applied
arsenic remains in water-soluble forms for days or months after
application (Onken and Adriano, 1997; Sarkar *et al.*, 2005).
Remobilization of sorbed arsenic with changing environmental conditions
is also possible (Matera and La Hecho, 2001).

To fulfill data requirements, the registrant submitted a published
non-FIFRA, non-GLP study by Wauchope (1975) for Agency review (part of
27 June 1986 review package, EPA Accession No. 260061) as well as one
FIFRA GLP study (MRID 41651906; Supplemental). Additional information
about the pH dependency of sorption of arsenic was obtained from an open
literature report by Smith (1999).

Wauchope measured the simple batch equilibrium adsorption of 16
Mississippi River alluvial flood plain soils, none of which were in the
"sand" textural class, two of which had a "clay" texture, and 14 of
which had a "loam" texture. The main study objectives were to correlate
sorption with soil properties and to make direct experimental comparison
of the relative sorptions of phosphate (as H~2~PO~4~^1-^ , a relatively
immobile soil chemical), cacodylate/cacodylic acid, arsenate/arsenic
acid, and methylarsonate/methylarsonic acid (at an adjusted pH of 5.6
for all soils and chemicals). Phosphate, a large magnitude agricultural,
industrial, and naturally occurring mineral with established relative
immobility, is a well-suited benchmark for comparing the suite of
organic arsenicals. Phosphorous and arsenic are also adjacent periodic
table group Vb.

Wauchope equilibrated arsenical slurries at a 1:20 soil:solution ratio
in systems experimentally fixed at a pH of 5.6. Wauchope did not
explicitly calculate sorption coefficients, so the Agency calculated
simple soil sorption coefficients (K~d~) based on the results at the
low-end initial concentrations, 3.2 x 10^-4^ M for cacodylate and 2.5 x
10^-4^ M for arsenate and MMA. The K~d~s for cacodylate ranged from 8.2
to 33 mL/g with a median of 16 mL/g and for MMA, the range was from 17
to 95 mL/g with a median of 28 mL/g. The results were relatively
independent of organic matter content. (Pseudo organic carbon sorption
coefficients (K~oc~) range from around 700 to 7000 mL/g oc, but do not
correlate with the K~d~ values given above.) Wauchope found that
sorption was best correlated with clay and iron and aluminum oxide
content. In this respect, the two organic arsenicals behaved like the
inorganic arsenate and phosphate. By direct comparison, all arsenicals
were more strongly sorbed than phosphate in the increasing order:
phosphate **\<** DMA **\<** arsenate [\~]{.underline} MMA. Although
arsenicals sorb more strongly than phosphate, phosphate is still
expected to strongly compete for binding sites (Matera and Le Hecho,
2001).

Additional K~d~ values for MMA were obtained from a registrant
adsorption/desorption study following standard FIFRA guidelines (MRID
41651906). MMA was equilibrated with soil at a 1:10 ratio with initial
concentrations ranging from 1 to 18 ppm. Four soils were tested with
average K~d~s of 0.5 mL/g (sand), 11.4 mL/g (silty loam), 18.7 mL/g
(silty clay), and 39.4 mL/g (sandy loam). Again, there was no
correlation between organic matter content and sorption. Based on these
two studies, the lowest non-sand K~d~ values for DMA and MMA were 8.2
mL/g and 11.4 mL/g, respectively.

pH could have a major influence on sorption because of the anionic
nature of the tested chemicals. Wauchope experimentally fixed the pH at
5.6 for the 16 soil/water systems (natural soil pHs ranged from 4.8 to
7.6) so correlation with pH cannot be determined from this study. At a
pH of 5.6, DMA would be expected to be present primarily in the
protonated, uncharged form. At neutral to alkaline pHs, however, the
negatively charged cacodylate ion dominates. Generally, anionic
(negatively charged) species tend to be less strongly sorbed by soil
surfaces which tend to maintain a negative (repelling) charge; the
surface charge also tends to increase (become more negative) at higher
pHs. Thus, at more nearly neutral or alkaline pHs, sorption coefficients
could be considerably lower than those given above, and mobility
correspondingly higher. A recent publication on arsenate and arsenite
sorption in Australian soils (Smith *et al.*, 1999) provides some
insight on the potential degree of importance of pH on sorption of acid
and anion couples. Although somewhat tenuous, Wauchope's four species
might be considered a homologous series with the congeners phosphorus or
arsenic at the central core. Therefore, Smith's arsenate and arsenite
data would serve as a relational link to the possible effect of pH on
the mobility of cacodylic acid/cacodylate.

In four soils selected to vary widely in chemistry and mineralogy, Smith
showed that in the experimentally adjusted pH range of 2.0 to 8.5
(adjusted with dilute nitric acid or sodium hydroxide) and ionic
strength range of 0.003 to 0.3 mol/L (adjusted with sodium nitrate)
there are complicated pH and ionic strength dependencies. However, the
Agency observes from the data in the more environmentally relevant range
of pHs from 5 to 8.5 and ionic strength of 0.003 molar, that arsenate
decreased in sorption with increasing pH by a maximum factor of only
approximately two. This decrease is not dramatic compared with the much
larger variability in simple, standard sorption K~d~s which Wauchope
measured above and which Smith measured for a total of 10 soils (the
four for the detailed pH and ionic strength dependencies plus six others
for a rudimentary subset for K~d~ measurement and soil correlations).
Smith's simple K~d~s for arsenate in the 10 soils were distributed in
the range from 1.7 to 62 mL/g.

Although Smith did not report any calculated sorption coefficients for
arsenite, his comments and plotted data of sorbed amounts show arsenite
to be moderately less sorbed than arsenate, consistent with its lower
negative charge. A more recent study comparing sorption of arsenate and
arsenite confirms this results, finding that arsenite is more available
to dissolution in water than arsenate (Onken and Adriano, 1997) However,
in contrast with the *decreasing* sorption of arsenate in the four soils
in the pH range of 5 to 8.5 and ionic strength of 0.003 M, Smith found
sorption of arsenite in two soils was fairly constant from approximately
pH 2 to pH 5, but *increased* significantly from approximately pH 5 to
the maximum reported pH 7 for arsenite. Sorption increased by a maximum
factor of approximately five in the most sensitive of the two soils.
Effect of ionic strength on sorption of arsenite was small and complex
in the tested ranges of pH and ionic strength. Overall, the Agency
concludes from Smith's surrogate data in the more environmentally
relevant range of pHs from 5 to 8.5 and ionic strength of 0.003 molar,
that sorption of cacodylate/cacodylic acid should not decrease (increase
mobility) dramatically with pH when compared to the much larger
variability in soil sorption found with different soils.

**Soil Buildup/Field Studies**

The relative immobility of arsenicals along with arsenic's elemental
nature make buildup in soil after repeated applications an important
consideration. Arsenic does not break down; it can only be redistributed
through runoff, leaching, erosion, volatilization, or plant uptake. Even
at the lowest non-sand K~d~s of 8.2 and 11.4 mL/g, sorption of
arsenicals would be significant, and the range of possible K~d~s extends
to much higher values, especially if the possibility of transformation
to more strongly sorbing inorganic arsenic is considered. As a result,
significant leaching is unlikely in most conditions and the potential
for runoff is likely to decrease over time. Volatilization is also
likely only in specific circumstances, leaving soil erosion and plant
uptake as the sole routes of dissipation of organic arsenicals applied
to soil.

Several terrestrial field dissipation studies have been conducted by
registrants to explore the environmental fate of applied organic
arsenicals. They are discussed here with the qualification that,
although most have been determined to be scientifically sound, unless
otherwise indicated here, they have not been recognized as fulfilling
the data requirement because they do not provide an adequate theoretical
explanation for observed loss of arsenic.

In the most recent study, rated supplemental, DMA was applied twice at
24 lb a.i./A to bare ground plots of sandy loam soil (MRIDs 42843101 and
43485301). The annual application rate used in this study, 48 lb a.i./A,
is considerably higher than that allowed on cotton (1.2 lb a.i./A/yr;
12/12/05 Master Label) but it is similar to the labeled rate on non-crop
areas and ornamentals (44 lb a.i./A/yr; 12/12/06 Master Label). DMA in
the top 6 inches of soil reached a peak concentration of 32.8 ppm (as
DMA) 10 days after the second application and was non-detectable by 243
days. MSMA reached a peak concentration of 1.65 ppm immediately
following the second application.

Total arsenic peaked at 20.37 ppm (as As), also 10 days after the second
application. Following the peak, total arsenic decreased more slowly
than DMA with little dissipation in the final 6 months, reaching 10.68
ppm on day 366. These values can be compared to the average arsenic
background of 2.00 ± 0.24 ppm found in the top 6 inches of the control
plot. Some of the dissipation in the top soil layer can be accounted for
by increasing arsenic in the next soil layer. At the 6 to 12 inch depth,
total arsenic increased steadily from 1.95 ppm prior to DMA application
to 4.47 ppm (as As) on day 366. Beginning on day 182, slightly elevated
arsenic levels were observed in the 12 to 18 inch soil layer as well but
a statistical evaluation found these results to be inconclusive. This
shows a significant persistence of applied arsenic, with the study
author concluding that "by \[day 93\] the organic arsenicals...were
largely mineralized, leaving elemental arsenic in the surface soil."

Two other registrant studies monitored the terrestrial field dissipation
of MSMA. In one, MSMA was applied twice to cotton in a silty loam soil
at 2 lb a.i./A (MRID 42616201). In the other, turf application was
simulated by applying MSMA three times at 4.95 lb a.i./A to bare ground
sandy loam (MRIDs 42526001 and 4322801). In both, residual MSMA
decreased to undetectable levels, by 2 months in the cotton field and by
day 365 in the bare ground plot. Both studies showed some transformation
to DMA, but at very low levels, peaking at less than 0.5 ppm in a short
time frame after application. In the cotton study, soil arsenic levels
were "variable with no discernable pattern of increase or decline." In
the bare ground study, total arsenic levels in the top 6 inches of soil
were consistently higher in treated plots than in control plots and
higher than initial values. Total soil arsenic peaked three times,
corresponding to the three applications. In the study period following
the third peak, days 41 to 365, the average total arsenic in the treated
plots was 12.16 ± 0.77 ppm compared to an average in the control plot
over the same period of 9.79 ± 1.76 ppm. It is difficult to quantify the
impacts on total soil arsenic, in large part due to the variability in
measurements in the control plot, but these results suggest that arsenic
levels in the treated plot were elevated over background levels by at
least 2 ppm.

These studies show that except at high application rates, a single year
of application is unlikely to lead to quantifiable buildup of soil
arsenic, but they show that a large fraction of applied arsenic remains
in the top layers of soil, opening the possibility that soil buildup is
a long term concern. As a worst case scenario, all applied arsenic would
remain in the top soil layer with no dissipation. In the DMA field
dissipation study (MRIDs 42843101 and 43485301), the worst case scenario
would lead to arsenic levels in the top 6 inches of soil approximately
12.9 ppm^3^ higher than background. As discussed above, by the end of
the study, total arsenic in the top layer of soil is elevated at least 8
ppm over background levels, meaning that nearly two-thirds of the
applied arsenic remained in the top layer of soil at the end of a year,
and additional arsenic was found from 6 to 12 inches.

At the application rates used in the studies of MSMA on cotton and turf,
the worst case scenario would have led to soil arsenic levels of
approximately 0.9 ppm and 3.3 ppm above background, respectively[^3].
The first value is low enough to be within the natural range of
variation in measurements of background arsenic levels, so it is
impossible to identify arsenic resulting from pesticide application. For
the bare ground plot, the elevated soil level, at least 2 ppm greater
than the control, is a significant portion of the worst case level of
3.3 ppm, showing that a substantial amount of the applied arsenic
remained in the top 6 inches of soil. After repeated applications for
multiple years, then, soil arsenic levels could be expected to increase,
possibly to levels of concern.

Because long term impacts are of concern, it is important to look at
studies conducted over a longer period of time. One older registrant
study applied MSMA and DMA to cropped fields at annual rates of up to 6
lb ai/A and 7.5 lb ai/A, respectively (MRID 117165). After 6 years soil
arsenic levels in the top 6 inches of soil, although somewhat uncertain
because of large variation in residue levels, were as high as 14 ppm and
15.5 ppm, respectively, compared to an average background level of 11
ppm. This shows some buildup, although less than worst case calculations
would predict. The study authors hypothesized that the arsenic loss was
due to volatilization. The authors concluded, however, that "after 6
annual applications all rates of MSMA and DMA resulted in poorly defined
but significant buildup of arsenic" in the 0-6 inch layer.

An incomplete review of the open literature found several long term
field dissipation studies with some reporting no arsenic buildup despite
very high application rates and others finding substantial buildup at
rates similar to current labels. Robinson (1975) applied MSMA for 5
years at annual rates varying from 4.4 to 288 kg MSMA/ha and observed an
increase in total arsenic only for application rates greater than 36 kg
ai/ha. Hiltbold et al (1974) applied MSMA to plots in three different
cotton fields at variable rates. After 6 years at an application rate
comparable to the maximum labeled MSMA rate[^4], the amounts of soil
arsenic in the top 15 cm, corrected for background levels, were 13.0,
13.2, and 8.0 ppm. These levels represent 57%, 53%, and 32% of the
applied pesticide, respectively. In the second soil, an additional 15%
of the applied arsenic was detected in the 15-30 cm soil layer, while in
the other soils no residual arsenic was detected below 15 cm. The
authors suggest that the unrecovered arsenic could be accounted for
through gaseous losses. In another field study, Woolson and Isensee
(1981) monitored soil arsenic levels in soybean fields in which DMA,
MSMA, and sodium arsenite were applied annually. The data are presented
only as line plots, but show that in most cases, after 6 years, at least
half of the applied arsenic was recovered in the top 15 cm of soil. For
MSMA applied at 11.2 kg ai/ha (compare to maximum labeled MSMA rate on
turf of 17.5 kg ai/ha/yr), the line plot shows recovered soil arsenic of
approximately 8 ppm. DMA applied at 11.2 kg ai/ha (a higher application
as total arsenic than the same rate for MSMA) led to soil levels of
approximately 10 ppm arsenic after 6 years.

Additional information about the potential for arsenic soil buildup
comes from monitoring studies in areas where organic arsenicals are
known to be used. One of these studies measured soil arsenic
concentrations at 5 Florida golf courses that use MSMA. 9 samples from
greens, tees, and fairways found arsenic levels ranging from
non-detectable to 50 ppm. The median value was 13 ppm and the 90^th^
percentile approximately 30 ppm. The estimated mean background arsenic
level in this area is 1.2 ppm (DERM, 2002). Another study measured
arsenic levels in roadside areas where the Louisiana Department of
Transportation applied arsenicals. Out of 559 samples, 42% had
concentrations between 21 and 50 ppm and 22% had concentrations greater
than 50 ppm. Background arsenic in this area was found to be between 4
and 14 ppm (LDOTD, 1984). Detailed histories of pesticide applications
are not available in these studies and other potential sources of
arsenic, including historical land uses, are not accounted for. These
results, then, are inconclusive as to the source of the arsenic buildup,
but they add to the weight of evidence from controlled field studies and
modeling.

Number range CHAPTERSoil accumulation values are generated by PRZM as
part of the process of modeling runoff concentrations. PRZM was run with
a modified version of the pe4 v01 shell program to estimate soil
accumulation. Except for exceptions described here, the modeling inputs
were the same as those included for surface water modeling (DP Barcode
D309098). Maximum application rates for MMA and DMA on turf and cotton
were used and the modeling assumed median sorption (K~d~ = 30 for MMA
and K~d~ = 20 for DMA). Because of limitations in the available data and
modeling capabilities, soil concentrations were modeled as total
arsenic, rather than speciated forms. Application rates were therefore
calculated to represent applied arsenic and infinite half-lives were
used to capture all forms of arsenic that may be present. A wide range
of sorption is possible; lower sorption, used to provide protective
estimates for surface water modeling (K~d~ = 11.4 for MMA and K~d~ = 8.2
for DMA), would lead to lower soil concentrations, while greater
sorption leading to higher concentrations is also possible. The
possibility of transformation to volatile species was not included in
modeling, and modeling does not account for the possibility of plant
uptake of soil arsenic. The results presented are for the scenarios that
led to the highest soil concentrations (PA turf and NC cotton). Because
soil concentrations are based on local application rather than input
from an entire watershed, use of a percent cropped area (PCA) factor was
unnecessary.

In the top 10 cm of soil, modeling predicts that arsenic will accumulate
with very little dissipation for several years and then level off. Over
the long term, the buildup of total arsenic from MMA application is
predicted to reach chronic concentrations of approximately 13 ppm and 45
ppm on cotton and turf, respectively. For DMA on turf, the highest
application rate leads to a modeled chronic soil concentration of
approximately 77 ppm, assuming annual application at that rate. For DMA
on cotton, the chronic soil concentration is 2 ppm. ("Chronic"
concentrations are the upper 90^th^ percent confidence limit on the
annual average, or the 1-in-10 year peak annual concentration.)
Considering another 10 cm, the modeled concentrations reach similar
levels, but after longer periods time. If deeper soils are included, the
overall concentrations would be lower. Most studies suggest that
concentrations are highest in the surface layers and buildup is
typically limited to the top 30 cm.

The same issue of arsenic buildup, supported for soil by field studies,
monitoring, and modeling, is relevant to concentrations of arsenic in
sediment as well. Arsenic that reaches surface water is likely to end up
in sediments. A registrant study in an aerobic aquatic/sediment system
found that after 30 days, 25% of applied DMA was found in the sediment
(MRID 43036101). For MMA, 39% of the applied amount ended up in sediment
after 30 days, most of it out of the water within the first week (MRID
43314801; Acceptable). In an anaerobic system, after 1 year 61% to 95%
of applied MMA was found in the sediment (MRID 44767602; Acceptable).
For DMA, approximately 95% of the applied amount was found in the
sediment after one year, most of it reaching there within the first
month (MRID 42572601). A literature review of arsenic fate reports that
in Lake Michigan, arsenic concentrations in water are generally much
less than in the sediments, and refers to oceanic sediments as "the
ultimate sink for arsenic" (Woolson, 1977).

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1977. Demethylation of MSMA by soil microorganisms. In *Trace Substances
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Missouri, Columbia, pp. 419-426.

Akins, M.B.; Lewis, R.J. Chemical distribution and gaseous evolution of
arsenic-74 added to soils as DSMA-^74^As. *Soil Sci. Soc. Am. J.*,
**1976**, *40*, 655-658.

Akkari, K.H.; Frans, R.E., Lavy, T.L. Factors affecting degradation of
MSMA in soil. *Weed Sci.* **1986**, *34*, 781-787.

Bednar, A.J; Garbarino, J.R.; Ranville, J.F.; Wildeman, T.R. Presence of
organoarsenicals used in cotton production in agricultural water and
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Cheng, C.N.; Focht, D.D. Production of arsine and methylarsines in soil
and in culture. *Applied Environ. Microbiol.* **1979**, *38*, 494-498.

Cox, D.P.; Alexander, M. Production of trimethylarsine gas for various
arsenic compounds by three sewage fungi. *Bull. Environ. Contam.
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DERM, 2002. *Environmental Quality Monitoring at Five Municipal Golf
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of Environmental Resources Management: Florida.

<http://www.miamidade.gov/derm/land/library/golf_course.pdf>

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Gao, S.; Burau, R.G. Environmental factors affecting rates of arsine
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Hiltbold, A.E.; Hajek, B.F.; Buchanan, G.A. Distribution of arsenic in
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Kuhlmeier, P.D. Partitioning of arsenic species in fine-grained soils.
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Enviromed Laboratories, Inc. Baton Rouge, LA. June, 1984. Rpt. No.
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MAATF, 2005. *The Environmental Fate of Monosodium Methane Arsonate
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Matera, V; Le Hecho, I. 2001. Arsenic behavior in contaminated soils:
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H.M.; Sparks, D.M.), CRC Press, Boca Raton, pp. 207-235.

National Research Council (NRC), 2003. *Bioavailability of Contaminants
in Soils and Sediments*. National Academies Press, Washington, D.C.

Onken, B.M.; Adriano, D.C. Arsenic availability in soil with time under
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*61*, 746-752.

Robinson, E.L. Arsenic in soil with five annual applications of MSMA.
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Smith, E., Naidu, R., Alston, A.M. Chemistry of arsenic in soils: I.
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Swancar, A. *Water Quality, Pesticide Occurrence, and Effects of
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<http://fl.water.usgs.gov/PDF_files/wri95_4250_swancar.pdf>

Tu, C.; Ma, L.Q.; Zhang, W.; Cai, Y.; Harris, W.G. Arsenic species and
leachability in the fronds of the hyperaccumulator Chinese brake
(*Pteris vittara L.*). *Environmental Pollution.* **2003**, *124*,
223-230.

Turpeinen, R.; Pantsar-Kallio, M.; Kairesalo, T. Influence of microbes
on the mobilization, toxicity, and biomethylation of arsenic in soil.
*Sci. Total Environ.* **1999**, *236*, 173-180.

Turpeinen, R.; Pantsar-Kallio, M.; Kairesalo, T. Role of microbes in
controlling the speciation of arsenic and production of arsines in
contaminated soils. *Sci. Total Environ.* **2001**, *285*, 133-145.

Von Endt, D.W.; Kearney, P.C.; Kaufman, D.D. Degradation of MSMA by soil
microorganisms. *Ag. Food Chem.* **1968**, *16(1)*, 17-20.

Wauchope, R.D. Fixation of arsenical herbicides, phosphate, and arsenate
in alluvial soils. *J. Environ. Qual*. **1975**, *4(3)*, 355-358. (EPA
Accession No.  260782 and associated Nos. 259582 and 260061.)

Woolson, E.A.; Kearney, P.C. Persistence and reactions of 14C-cacodylic
acid in soils. *Environ. Sci. Technol.*, **1973**, *7*, 47-50. (EPA
Accession Nos. 259582.)

Woolson, E.A. Fate of arsenicals in different environmental substrates.
*Environ. Health Perspectives*. **1977**, *19*, 73-81.

Woolson, E.A.; Isensee, A.R. Soil residue accumulation from three
applied arsenic sources. *Weed Sci. Soc. Am. J.* **1981**, *29*, 17-21.

Woolson, E. A.; Aharonson, N.; Iadevaia, R. Application of
high-performance liquid chromatography - flameless atomic absorption
method to the study of alkyl arsenical herbicide metabolism in soil. *J.
Agric. Food Chem.*, **1982**, *30*, 580-584. (EPA Accession Nos. 259582,
260061, 260782.)

**APPENDIX B: GENEEC2 INPUTS**

**Table B1. Number range CHAPTER GENEEC2 Input Parameters for DSMA and
MSMA**

+----------------+---------------+-----------------+------------------+
| **Property**   | **Value**     | **Comments**    | **Source /       |
|                |               |                 | MRID**           |
+----------------+---------------+-----------------+------------------+
| Solubility     | 1 x 10^6^     |                 | 2000 RED         |
| (water)        | mg/L at 20 C |                 | (D210451,        |
|                | -             |                 | D212449, 255226) |
+----------------+---------------+-----------------+------------------+
| Hydrolysis     | 0 day         | stable at all   | 42059201         |
| t~1/2~         |               | pHs             |                  |
+----------------+---------------+-----------------+------------------+
| Aquatic        | 0 day         | stable          | 41662601         |
| Photolysis     |               |                 |                  |
| t~1/2~         |               |                 |                  |
+----------------+---------------+-----------------+------------------+
| Aerobic Soil   | 240 days      | 90^th^          | Woolson &        |
| Metabolism     |               | percentile,     | Kearney, 1973;   |
| t~1/2~         |               | based on 3      | Woolson 1982;    |
|                |               | values          |                  |
|                |               |                 | Gao & Burau,     |
|                |               |                 | 1997;            |
|                |               |                 |                  |
|                |               |                 | 44767601         |
+----------------+---------------+-----------------+------------------+
| Aerobic        | 480 days      | 2x240 days. No  |                  |
| Aquatic        |               | data available, |                  |
| Metabolism     |               | 2 x aerobic     |                  |
| t~1/2~         |               | soil t~1/2~     |                  |
|                |               | since           |                  |
|                |               | hydrolysis      |                  |
|                |               | assumed stable  |                  |
+----------------+---------------+-----------------+------------------+
| K~d~           | 11.4 mL/g     | lowest non      | Wauchope, 1975   |
|                |               | sand, from 20   |                  |
|                |               | soils           | 41651906         |
+----------------+---------------+-----------------+------------------+
| Application    | cotton: 1.74  |                 |                  |
| Rate (MMA - lb | / 2 (ground)  |                 |                  |
| ae/A) /        |               |                 |                  |
|                | orchard: 3.70 |                 |                  |
| No. of Appl    | / 3           |                 |                  |
| per year       |               |                 |                  |
|                | non-crop:     |                 |                  |
|                | 3.86 / 4      |                 |                  |
|                |               |                 |                  |
|                | turf (max):   |                 |                  |
|                | 3.35 / 4      |                 |                  |
|                |               |                 |                  |
|                | turf (golf):  |                 |                  |
|                | 2.23 / 4      |                 |                  |
+----------------+---------------+-----------------+------------------+

Number range CHAPTER**APPENDIX C. T-REX Model (T-REX Version 1.22, July
11, 2004) -- Model Overview and Results**

**1. Introduction**

This spreadsheet based model calculates the decay of a chemical applied
to foliar surfaces for single or multiple applications. It uses the same
principle as the batch code models FATE and TERREEC for calculating
terrestrial estimates exposure (TEEC) concentrations on plant surfaces
following application. A first order decay assumption is used to
determine the concentration at each day after initial application based
on the concentration resulting from the initial and additional
applications. The decay is calculated by from the first order rate
equation:

C~T~ = C~i~e^-kT^

or in log form:

ln (C~T~/C~i~) = kT

Where:

**C~T~ =** concentration at time T = day zero.

**C~i~ =** concentration, in parts per million (PPM) present initially
(on day zero) on the surfaces. Ci is calculated based on Kenaga and
Fletcher by multiplying the Ci based on the Kenega nomogram (Hoerger and
Kenaga, (1972) as modified Fletcher (1994). For maximum concentration
the application rate, in pounds active ingredient per acre, is
multiplied by 240 for Short Grass, 110 for Tall Grass, and 135 for Broad
leafed plants/small insects and 15 for fruits/pods/lg insects.
Additional applications are converted from pounds active ingredient per
acre to PPM on the plant surface and the additional mass added to the
mass of the chemical still present on the surfaces on the day of
application.

**k =** If the foliar dissipation data submitted to EFED are found
scientifically valid and statistically robust for a specific pesticide,
the 90% upper confidence limit of the mean half-lives should be used.
When scientifically valid, statistically robust data are not available
TETT recommends the using a default half-life value of 35 days. The use
of the 35 day half-life is based on the highest reported value (36.9
days) reported by Willis and McDowell (Pesticide persistence on foliage,
Environ. Contam.Toxicol, 100:23-73, 1987).

**T =** time, in days, since the start of the simulation. The initial
application is on day 0. The simulation is designed to run for 365 days.

The program calculates concentration on each type of surface on a daily
interval for one year. The maximum concentration during the year are
calculated for both maximum and mean residues. The inputs used to
calculate the amount of the chemical present are in highlighted in light
blue on the spread sheet. Outputs are in yellow. The inputs required
are:

**Application Rate:** The maximum label application rate (in pounds
ai/acre)

**Half-life**: The degradation half-life for the dominant process(in
days)

**Frequency of Application:** The labeled interval, in days, between
repeated applications

**Maximum \# Application per year:** From the label

The calculated concentrations are used to calculate Avian and Mammalian
RQ values. The maximum calculated concentration is divided by user input
values for acute and chronic endpoints to give RQs for each type of
plant surface.

**2. Avian Species**

For calculating dose-based RQs in birds, the maximum and mean Kenaga
residue values are adjusted for avian class and food consumption based
on the following scaling factor (USEPA, 1993):

FI (g/d) = 0.648 (g bw)\^0.651

For the 3 avian weight classes considered (20, 100 and 1000 g), this
results in % body weight consumption of:

Weight(g) FI wet FI % bw consumed

20 4.555599463 22.77799731 114

100 12.98897874 64.94489369 65

1000 58.15338588 290.7669294 29

**A. Dose-Based Acute RQs**

Dose-based acute RQs are then calculated using the formula:

RQ = adjusted EEC/LD~50~ or NOAEL

where the adjusted EEC is considered to be the daily dose weighted for %
body weight consumed of a given food source.

**B. Dietary-Based RQs**

For dietary-based RQs, two values are given for each food group. First,
the consumption-weighted RQ for each weight class (20, 100, and 1000g
birds) is displayed and calculated using the equation:

RQ = EEC/((LC~50~ or NOAEC)/(%bw consumed))

In the second method, no adjustment is made for consumption differences
among the weight classes. This RQ is calculated:

RQ = EEC/LC~50~ or NOAEC

**3. Mammalian Species**

**A. Dose-Based RQs**

For calculating dose-based RQs in mammals, the maximum and mean Kenaga
values are adjusted for mammalian class and food consumption (0.95, 0.66
and 0.15 body weight for herbivores and insectivores and 0.21, 0.15, and
0.03 body wt. for granivores). Dose-based acute and chronic RQs are then
calculated by dividing the adjusted EECs (daily dose) by the LD~50~ or
NOAEL.

**B. Dietary-Based RQs**

Dietary-based RQs are calculated using the equation:

RQ = EEC/((LC~50~ or NOAEC)/(% bw consumed))

**4. Graph**

A graph of concentration on each plant surface vs time is plotted and a
concentration of concern line can be added at a user specified level.
The concentration of concern (*e.g.*, avian LC~50~, mammalian NOAEL)
label should be entered in the cell underneath the value. The graph
automatically plots a line at this concentration and the label is
extracted from that cell. The graph is plotted for the first 100 days
post-application. Graphs displaying acute and chronic LOCs for both
birds and mammals are displayed in the \"Graph\" worksheet. These graphs
may be useful as a visual aid to communicate risk in your assessment and
can be copy/pasted into your document. To help with scaling issues on
the y axis, you may want to delete one of the endpoints.

**5. New Version Notes**

A new look is used in this update in an effort to decrease confusion and
increase transparency in the risk assessment process. This version of
T-REX (v1.12) incorporates the ability to calculate EECs and RQs for
maximum and mean residues. Mean residues are calculated exactly as the
maximum residues are, except the corresponding Kenaga values are: 85 for
Short Grass, 36 for Tall Grass, and 45 for Broad leafed plants/small
insects and 7 for fruits/pods/lg insects. Version 1.22 provides
additional improvements.

**6. References**

Fletcher, J.S., J.E. Nellesson and T. G. Pfleeger. 1994. Literature
review and evaluation of the EPA food-chain (Kenaga) nomogram, an
instrument for estimating pesticide residues on plants. Environ. Tox.
and Chem. 13(9):1383-1391.

Hoerger, F. and E.E. Kenaga. 1972. Pesticide residues on plants:
correlation of representative dada as a basis for estimation of their
magnitude in the environment. IN: F. Coulston and F. Corte, eds.,
Environmental Quality and Safety: Chemistry, Toxicology and Technology.
Vol 1. Georg Theime Publishers, Stuttgart, Germany. pp. 9-28.

USEPA. 1993. Wildlife Exposure Factors Handbook. Volume I of II.
EPA/600/R-93/187a. Office of Research and Development, Washington, D. C.
20460. Willis and McDowell. 1987.Pesticide persistence on foliage.
Environ. Contam. Toxicol. 100:23-73.

**Appendix D: TerrPlant 1.0 Details**

Number range CHAPTERExposure to Terrestrial Plants including Wetlands
(August 8, 2001; version 1.0)

Terrestrial plants inhabiting dry and semi-aquatic (wetland) areas may
be exposed to pesticides from runoff and/or spray drift. Semi-aquatic
areas are low-lying wet areas that may dry up at times throughout the
year.

EFED\'s runoff scenario is (1) based on a pesticide\'s water solubility
and the amount ot 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 \"channel runoff\"
(10 acres to a distant low-lying acre) for semi-aquatic or wetland
areas, and (4) based on percent runoff values of 0.01, 0.02, and 0.05
for water solubilities of \<10, 10-100, and \>100 ppm, respectively.

EFED\'s Spray Drift scenario is assumed as (1) 1% for ground
application, and (2) 5% for aerial, airblast, forced air, and spray
chemigation applications. The spray drift ratio used here is in
agreement with the policy procedures at the time the worksheet was
designed.

Currently, 1) this worksheet is designed to derive the plant exposure
concentrations from a single, maximum application rate only. 2) For
pesticide applications with incorporation of depth of less than 1 inch,
the total loading EECs derived for the incorporation method will be same
as the unincorporated method.

To calculate RQ values for Non-Endangered Terrestrial Plants:

Terrestrial Plants Inhabiting Areas Adjacent to Treatment Site:

Emergence RQ = Total Loading to Adjacent Area or EEC/Seedling Emergence
EC25

Drift RQ = Drift EEC/Vegetative Vigor EC25

Terrestrial Plants Inhabiting Semi-aquatic Areas Adjacent to Treatment
Site:

Emergence RQ = Total Loading to Semi-aquatic Area or EEC/Seedling
Emergence EC25 Drift RQ = Drift EEC/Vegetative Vigor EC25

To calculate RQ values for Endangered Terrestrial Plants:

Endangered Terrestrial Plants Inhabiting Areas Adjacent to Treatment
Site:

Emergence RQ = Total Loading to Adjacent Area or EEC/Seedling Emergence
EC05

Drift RQ = Drift EEC/Vegetative Vigor EC05 or NOAEC

Endangered Terrestrial Plants Inhabiting Semiaquatic Areas Near
Treatment Site:

Emergence RQ = Total Loading to Semiaquatic Area or EEC/Seedling
Emergence EC05

Drift RQ = Drift EEC/Vegetative Vigor EC05 or NOAEC

Formulas used to calculate EEC values (8/08/01; version 1.0)

To calculate EECs for terrestrial plants inhabiting in areas adjacent to
treatment sites

Un-incorporated Ground Application (Non-granular):

Sheet Runoff = Application Rate (lb ai/A) x Runoff Value

Drift = Application Rate (lb ai/A) x 0.01

Total Loading = EEC = Sheet Runoff + Drift

Incorporated Ground Application with Drift (Non-granular):

Sheet Runoff = \[Application Rate (lb ai/A)/Incorporation Depth (inch)\]
x Runoff Value

Drift = Application Rate (lb ai/A) x 0.01

Total Loading = EEC = Sheet Runoff + Drift

Un-incorporated Ground Application (Granular):

Sheet Runoff = EEC = Application Rate (lb ai/A) x Runoff Value

Incorporated Ground Application without Drift (Granular):

Sheet Runoff = EEC = \[Application Rate (lb ai/A)/Incorportion Depth
(inch)\]

x Runoff Value

Aerial/Airblast/Spray Chemigation Applications:

Sheet Runoff = Application Rate (lb ai/A) x Runoff Value x Application
Efficiency of 0.6

Drift = Application Rate (lb ai/A) x 0.05

Total Loading = EEC = Sheet Runoff + Drift

Runoff Value = 0.01, 0.02, or 0.05 when the solubility of the chemical
is \<10 ppm, 10-100 ppm, or\>100 ppm, respectively

Incorporation Depth: Use the minimum incorporation depth reported on the
label.

[^1]: Gao and Burau report some of their results as "percent
    mineralized", calculated as 100% minus the measured percent of
    parent compound remaining. The authors explain this as an assumption
    for the sake of consistency. The reported percent mineralized, then,
    includes the \<0.5% volatilized as well as any unrecovered arsenic.
    The actual concentrations of inorganic arsenic resulting from
    degradation were measured directly with 91% recovery, so 9% is the
    most unrecovered arsenic which may be accounted for in the reported
    demethylation. In this discussion, in order to avoid this
    uncertainty, the percentage of parent compound remaining was
    determined based on the reported percent mineralized.

[^2]: These studies were submitted under Accession No. 259582 and
    associated Accession Nos. 260061 and 260782. The study by Woolson
    and Kearney (1973) was specifically evaluated at the time but
    Woolson et al, 1982 was not. Both studies were discussed in the 1986
    Agency review and in the 2000 RED for cacodylic acid.

[^3]: Calculations of soil arsenic levels for worst case scenarios
    assumed that all applied arsenic remained in the top 15-18 cm of
    soil. The applied rate was converted to mg As/ha and then divided by
    the mass of one hectare of soil to a depth of 15-18 cm, estimated to
    be approximately 2.25 million kg.

[^4]: Hiltbold indicates MSMA was applied at rates of 10, 20, and 40
    kg/ha for 6 years. The total applied, as arsenic, over the course of
    the study amounted to 27, 55, and 110 kg/ha, indicating average
    annual application rates, as arsenic, of 4.5, 9, and 18 kg/ha/yr.
    The current MSMA label allows a maximum application rate (on turf)
    of 3.9 lb ae/A, 4 times a year. This is equivalent to an annual rate
    as arsenic of 8.5 kg/ha/yr, comparable to the experimental average
    annual rate of 9 kg/ha/yr leading to a total of 55 kg/ha.