UNITED STATES ENVIRONMENTAL PROTECTION AGENCY WASHINGTON D. C., 20460 OFFICE OF PREVENTION, PESTICIDES AND TOXIC SUBSTANCES PC Codes: 012501, 012502, 013802, 013803, 013806 DP Barcode: D309097 Date: March 29, 2006 DRINKING WATER ASSESSMENT SUBJECT: Response to Registrant Phase I Error Only Comments: Drinking Water Assessment for Organic Arsenical Herbicides for the Reregistration Eligibility Decision ( RED)

TO: Diana Locke, Risk Assessor Reregistration Branch II Health Effects Division ( 7509C)

Lance Wormell, Chemical Review Manager Reregistration Branch II Special Review and Reregistration Division ( 7508C)

FROM: Keara Moore, Chemist Environmental Risk Branch III Environmental Fate and Effects Division ( 7507C)

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) 2 Attached, please find the revised Drinking Water Assessment for organic arsenical herbicides. Revisions have been made to the previous Drinking Water Assessment ( DP Barcode 309098; 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. Clarification has also been added in discussion of the application rates and half lives used in modeling and in discussion of the sorption properties of arsenicals. Uncertainty in estimates of surface water exposure resulting from use on turf has been emphasized by adding further discussion of the issue in the executive summary of the document. 3 EXECUTIVE SUMMARY This memorandum presents the results of the Environmental Fate and Effects Division's ( EFED) drinking water assessment, conducted to support the human health aggregate risk assessment for the reregistration of organic arsenicals. Five individual herbicides are included in this assessment: cacodylic acid ( DMA; dimethylarsinic acid), sodium cacodylate ( DMA Na), monosodium methanearsonate ( MSMA), disodium methanearsonate ( DSMA), and calcium acid methanearsonate ( CAMA). Past assessments have handled cacodylic acid and sodium cacodylate separately from the methylarsonate salts ( MSMA, DSMA, CAMA). The current assessment considers all of these pesticides together as the " organic arsenicals." These pesticides are all alike in that they contain arsenic in a methylated form. They have similar chemical structures and environmental fate properties. All have the potential to metabolize to the more toxic inorganic arsenic. Because arsenic is elemental and does not degrade, application of any organic arsenical pesticide may contribute to total arsenic loading, whether as parent compound or inorganic arsenic, in surface water, groundwater, soil, or plants, depending on environmental conditions. In light of these considerations, the environmental fates of these pesticides are discussed as a group. In addition to estimating concentrations resulting from application of individual compounds, situations where multiple organic arsenicals may be applied to the same field are considered, and the uncertainty resulting from the possibility of multiple uses in a single watershed is discussed.

The estimated drinking water concentrations ( EDWC) are based on application rates from the set of master labels provided by the Methanearsonic Acid Research Task Force ( MAATF), dated November 14, 2005. These encompass all uses except for application of DMA to turf and non crop areas, which will be addressed in a future label. It is the assumption that any labels for formulated products which exceed these maximum application rates will be revised to comply with the master labels. Nearly 3.5 million pounds of these pesticides are applied annually, with approximately 70% of the total use as an herbicide or a desiccant on cotton and another 27% as an herbicide on turf. These use data do not include residential uses, which are expected to be a small percentage of the total use. Because of the predominance of the cotton and turf uses, EDWCs are based on those uses.

EFED estimated drinking water concentrations for exposure to surface water are presented in Table 1. These values were modeled using the Pesticide Root Zone Model ( PRZM 3.12) and the Exposure Analysis Modeling System, ( EXAMS 2.98.04) with the pe4v01 graphical interface and represent the two most vulnerable available scenarios. The species of concern in this assessment   the methylarsonate salts, DMA, and inorganic arsenic   all have distinct toxicities; exposure to each needs to be considered individually. Individual EDWCs are therefore provided for each, including DMA and inorganic arsenic as potential metabolites as well as the parent compounds. The total arsenic value incorporates all arsenical species that may be present as a result of pesticide application, including inorganic arsenic. The total arsenic EDWC can be compared to regulatory levels, all of which are defined by total arsenic. For drinking water, the 4 Table 1. EDWCs ( ppb) from maximum labeled rates for major uses of arsenicals.

Acute Chronic Cancer TURF MMA1 250.5 127.5 74.6 DMA 102.3 46.5 28.1 Total As2 135.2 68.8 40.3 COTTON MMA1 37.4 11.0 5.3 DMA 23.6 7.4 4.3 Total As2 20.9 7.2 3.9 1 Monomethyl arsonic acid ( MMA) is the acid equivalent form of the methylarsonate salts. 2 Total arsenic is the sum of arsenic ( as ppb As) that may be present from all applied and metabolite species. It also represents the maximum EDWC of inorganic arsenic.

Maximum Contaminant Level ( MCL) for total arsenic is 10 ppb and is based on concerns for long term exposure. As a conservative assumption, the entire estimated total arsenic EDWC may be present as inorganic arsenic. No EDWCs are provided for exposure to groundwater. Based on the environmental fate properties of organic arsenicals, leaching to groundwater is not expected to contribute significantly to the already existing burden of arsenic in groundwater except in highly vulnerable situations.

Organic arsenicals are stable to hydrolysis and photolysis but in many conditions, they 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 environmental conditions. Although it may convert to different forms, however, the arsenic in these pesticides does not disappear; arsenic from pesticides is not lost but redistributed and transformed throughout the environment. Organic arsenicals and their metabolites are strongly sorbing and are expected to be relatively immobile in soil in most conditions.

Arsenic occurs in the environment naturally in variable concentrations. A USGS statistical analysis of 50,000 groundwater samples from 30,000 locations found that nearly half of the groundwater samples had total arsenic concentrations < 1 ppb while about 10% exceeded 10 ppb. Areas of high arsenic groundwater are scattered throughout the country but are more likely to be found in the Intermountain West and Pacific Coast regions. A less thorough consideration of a USGS dataset including 40,000 samples from 4500 lake and stream locations found that in surface water also, half of the samples had total arsenic concentrations   1.1 ppb and 10% of the samples exceeded 10 ppb. The limited data available for speciated arsenic suggest that in both surface and groundwater, natural arsenic is present primarily in the inorganic form.

Potential exposure to arsenic in drinking water from pesticide application, separate from natural background levels, was assessed through evaluation of targeting monitoring data from surface and groundwater in pesticide use areas as well as through the surface water 5 modeling discussed above. A summary of groundwater monitoring in areas in North Dakota, South Dakota, Wisconsin, and Minnesota where high arsenic concentrations regionally coincided with agricultural use found that in these areas, the groundwater concentrations were largely unaffected by use of arsenical pesticides. In Florida golf courses, however, groundwater detections of total arsenic as high as 123 ppb, significantly higher than local background, have been attributed to use of MSMA. Targeted surface water monitoring has been conducted in cotton use areas as well as in golf course ponds in Florida. In cotton growing regions of Mississippi, surface water concentrations up to 5.5 ppb total arsenic were measured. This appears to be higher than natural background arsenic levels, but the results are inconclusive. In limited sampling of Florida golf course ponds, the highest total arsenic level found was 24 ppb. For all arsenic monitoring data, there is some uncertainty in determining the source of elevated levels.

Uncertainties, limitations, and assumptions in the drinking water assessment include the following:

  Environmental fate data for DSMA and CAMA are primarily based on studies for MSMA because all of these methylarsonate salts dissociate to monomethyl arsonic acid ( MMA) in aqueous solution. Environmental fate studies for both MMA and DMA are included in a single discussion based on the assumption that general fate processes are similar, even if specific rates and sorption coefficients are not identical.

  For fate discussions and modeling, environmental fate data for all organic arsenicals are based on open literature studies as well as registrant submitted data. The majority of metabolism data are from non GLP studies that were conducted for less than one year. Transformation of applied organic arsenicals to volatile alkylarsines is assumed to be a minor dissipation route.

  Modeling to estimate drinking water concentrations is based on application of pesticide to an entire watershed. The results are then modified using a percent cropped area factor ( PCA). Because of the variety of sites where use on turf is possible, no PCA has been developed and the estimated concentration is based on the unmodified, entire watershed result. This is likely an overestimation of exposure but the extent of overestimation is undefined. Comparing the modeled result to the very limited monitoring data which are available, the highest concentration observed is approximately one half of the chronic concentration estimated by modeling. For the cotton use, the cotton PCA was used in estimating exposure despite the fact that arsenicals may be applied to other crops in the same watershed. The cotton exposure estimate may therefore be an underestimate, but limited monitoring data suggest that the results are reasonable.

  Several additional assumptions are made in modeling exposure to arsenic species other than the parent compound. The estimate of metabolism to DMA is based on the assumption that 35% is the maximum amount of MMA that may be present as 6 DMA at any one time. Because significant transformation from DMA to MMA has not been confirmed in field or laboratory studies, in modeling it is assumed that no transformation occurs. . Maximum potential concentrations of inorganic arsenic are represented by the total arsenic EDWCs, calculated as a molar conversion of the EDWCs of parent compounds. This is a general estimate of the amount of inorganic arsenic that may reach surface water rather than a direct calculation based on specific physical processes.

  Because of the multiple potential sources of arsenic, both natural and anthropogenic, it can be difficult to determine the source of arsenic found in monitoring studies. Sampling in studies of background arsenic is typically not uniformly distributed and not designed to limit consideration to only uncontaminated regions. In targeted studies, detailed land use histories and pesticide application information are not always available and conclusions are based primarily on general trends. 7 TABLE OF CONTENTS PESTICIDE USE AND APPLICATION..................................................................... 9 MSMA and DSMA .................................................................................................... 9 CAMA..................................................................................................................... 11 Cacodylic Acid ....................................................................................................... 11 Co occurrence ........................................................................................................ 11 ENVIRONMENTAL FATE AND TRANSPORT CHARACTERIZATION........... 12 GROUNDWATER EXPOSURE................................................................................ 15 Groundwater Monitoring........................................................................................ 15 Background Arsenic ( Non targeted Monitoring) ..................................................... 15 Pesticide Impacts ( Targeted Monitoring)................................................................ 18 SURFACE WATER EXPOSURE.............................................................................. 21 Surface Water Monitoring ...................................................................................... 21 Background Arsenic ( Non targeted Monitoring) ..................................................... 21 Pesticide Impacts ( Targeted Monitoring)................................................................ 23 Surface Water Modeling.......................................................................................... 24 Input Parameters: Metabolism Rates ..................................................................... 26 Input Parameters: Soil Mobility............................................................................. 29 Calculation of Metabolites...................................................................................... 30 Percent Cropped Area ............................................................................................ 31 Characterization..................................................................................................... 31 SOIL ACCUMULATION........................................................................................... 35 UNCERTAINTY......................................................................................................... 36 8 Monitoring ............................................................................................................... 36 Surface Water Modeling.......................................................................................... 36 REFERENCES............................................................................................................ 39 Appendix A: PRZM/ EXAMS Input Files ................................................................ 42 Appendix B: PRZM/ EXAMS Output Files.............................................................. 62 9 PESTICIDE USE AND APPLICATION Organic arsenicals are nonselective contact herbicides used to control grasses and broadleaf weeds in a variety of agricultural and non agricultural applications. This assessment is based on master labels provided by the registrants on 11/ 14/ 05 which have not yet been finalized but which are assumed to invalidate all previous labels. Variance from these rates could change the conclusions of this assessment. Table 2 provides a summary of maximum application rates allowed by these master labels.

MSMA and DSMA Monosodium methanearsonate ( MSMA) and disodium methanearsonate ( DSMA) are used in a wide variety of applications. Although applied as different parent compounds, MSMA and DSMA end up as the same chemical in the environment, monomethyl arsonic acid ( MMA). For this reason, labels for all uses specify that the maximum number of applications applies to " either DSMA or MSMA or their combination per crop, per year." Rates for both are reported here in acid equivalents ( ae) to represent the amount applied as MMA. The main agricultural application for both is as a pre/ postemergent herbicide on cotton applied prior to the first bloom. The major use area for both MSMA and DSMA is in the southeastern US, including Georgia, Arkansas, Mississippi, Alabama, Louisiana, and North and South Carolina. USGS pesticide maps, based on data from 1995 1998, show use of MSMA, but not DSMA, in Texas, New Mexico, Arizona, and California as well ( USGS, 2003).

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 2.

BEAD estimates that 100% of agricultural use of DSMA is on cotton at approximately 200,000 lbs/ yr, based on use data from 1998 to 2003 ( BEAD, 2005b). Limited information on non agricultural uses of DSMA is available but it is expected to be a small percentage of total use. Use of MSMA is much higher, for both agricultural and non agriculture uses, and so more data are available to quantify non agricultural use. On cotton, use of MSMA is estimated at 2 million lb/ yr, two thirds of the total MSMA usage of 3 million lbs/ yr. The remaining one third of use is primarily on turf, estimated at 930,000 lb/ yr and including sod farms, golf courses, lawn care operators, and other turf uses. This estimate does not include individual residential use. Golf courses account for approximately half of the MSMA applied on turf. Interestingly, lawn care operators treated almost as many acres as golf courses but used around a quarter of the pounds. Estimates of non agricultural use of MSMA are based on proprietary data from 2000. Other uses found for MSMA, in order of decreasing percentage, include pasture, citrus, pistachios, walnuts, watermelon, apples, and pecans. Combined, these uses contributed less than 3% of total annual use ( BEAD, 2005a & b). 10 Table 2. Application Rates for Organic Arsenicals, based on 11/ 14 Master Labels Use Maximum App. Rate ( lb ai/ A) Max. No. Apps. Application Interval Application Method DSMA1 1.7 1 n/ a g round or aerial Cotton 1.7 2 1   3 weeks ground ( directed)

Turf 2 2.5 4 14 days g round spray Orchards, Citrus, Vineyards 3 3.7 3 not specified g round ( directed)

Non crop 4 3.9 4 10   14 days ground spray Grass for seed 5 3.3 1 n/ a n ot specified MSMA1 1.7 1 n/ a g round or aerial Cotton 1.7 2 not specified g round ( directed)

Turf  Sod Farms6  Golf Course6 3.4 2.2 4 10   14 days ground spray Orchards, Citrus, Vineyards 3 3.7 3 10   14 days ground spray Non crop 4 3.4 4 10   14 days ground spray Grass for seed 5 5.3 1 n/ a n ot specified CAMA1 3.6 4 not specified ground spray Turf 2.2   4.4 2 not specified ground spray Cacodylic Acid / Sodium Cacodylate Cotton 1.2 1 n/ a g round or aerial Citrus 3 5.0 3 not specified g round spray Non crop 4 8.1 3 not specified g round spray Turf  general  lawn renovation 5 lb/ 100gal 8.4  21 not specified not specified not specified 5 days ground spray ground spray 1 Reported as lbs acid equivalent/ A. 2 Prohibited on golf course greens. 3 Non bearing only  not to be used within one year of harvest. Restricted to spot treatments in Florida. 4 Non crop = " drainage ditchbanks, rights of way, storage yards and similar areas". 5 Pacific Northwest only. 6 Sod farm rate allowed on sod farms and established Bermuda and Zoysiagrass. Golf course rate allowed on athletic fields, golf courses, and parks. 5 Pacific Northwest only. 11 CAMA Calcium acid methanearsonate ( CAMA) is a minor use contact herbicide applied to control crabgrass in residential areas. It is only sold at the retail level and not to commercial or industrial applicators. As with DSMA and MSMA, in aqueous solution CAMA dissociates to MMA and so rates are reported as acid equivalents. The maximum labeled application rate for CAMA is 4 applications at 3.6 lb ae/ A. Limited usage data are available for residential uses, but the total usage of CAMA is estimated to account for less than 1% of total national usage of the methanearsonate salts.

Cacodylic Acid Cacodylic acid ( DMA), sometimes formulated with its sodium salt, sodium cacodylate ( DMA Na), is used primarily as a defoliant on cotton. Current labels for irrigated and dryland cotton allow application of up to 1.2 lb ai/ A annually by either aerial or ground spray. Treatment on cotton is made when 50% or more of the bolls have opened ( 7 to 10 days prior to the anticipated picking date). The most recent USGS pesticide use maps, based on data from 1995 1998, indicate that use of DMA on cotton is limited to California and Arizona ( USGS, 2003). Based on use data from 1998 2003, BEAD estimates average annual use of DMA on cotton of 80,000 lbs/ yr and for DMA Na, use is estimated at 100,000 lbs/ yr. The total use of cacodylate on cotton, then, is 180,000 lb/ yr. The current BEAD analysis also estimates that 20,000 lb/ yr of DMA is used on corn. EFED is not aware of any labeled uses on corn; it is not a use supported under the proposed master label ( BEAD, 2005b).

A small proportion of DMA/ DMA Na use is on non crop sites, including utilities, residential outdoor, and weed treatment around the bases of citrus trees. Application rates for DMA/ DMA Na to non crop sites are mostly higher than for cotton; for many products, however, directions simply specify mixing a certain amount of product in a specified number of gallons of water and applying the spray solution to the point of runoff. The labels imply, however, that in these uses DMA is expected to be applied to a very small area, with most labeled rates described in terms of how much product to apply to areas from 40 to 1000 square feet. Application for non crop sites is by ground only. Because DMA/ DMA Na produces a top kill only, repeat applications are needed for season long weed control. Some labels limit the number of applications to three per year but many simply state that repeat applications can be made as needed. The registrants are preparing a master label for these uses also; that label was not complete at the time of the current assessment and so has not been considered here. No national use data are available for non crop applications. BEAD's agricultural usage analysis found no use on citrus ( BEAD, 2005b).

Co occurrence On cotton, it is possible that DSMA or MSMA, as early season herbicides, and DMA, as a late season desiccant, could be applied to the same field. BEAD estimates that this cooccurrence is likely to affect less than 1% of the cotton crop. It is also possible that DMA, MSMA, DSMA, and CAMA could be applied to multiple crops within the same watershed. 12 ENVIRONMENTAL FATE AND TRANSPORT CHARACTERIZATION Note: What follows is a summary of the environmental fate and transport characteristics of organic arsenicals. A detailed discussion of these complex processes is included as an appendix in the Ecological Risk Assessment document ( DP Barcode D309100). That discussion includes references to all registrant conducted and open literature reports considered.

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. Physicochemical properties of these compounds are listed in Tables 3a and 3b.

Table 3a. Physicochemical Properties for MSMA and DSMA.

DSMA MSMA Molecular Structure As O O O C H 3 Na + Na + As OH O O C H 3 Na +

Empirical Formula CH3AsNa2O3 CH4AsNaO3 Molecular Weight 183.92 161.94 CAS No. 144 21 8 2163 80 6 PC Code 013802 013803 Melting Point ( º C) > 300 116 121 Density ( g/ mL) 1.04 1.65 Vapor Pressure ( mm Hg) 1 x 10 7 7.5 x 10 7 log Kow < 1 < 1 Solubility: Water ( mg/ L) 3.4 x 105 104 Methanol 2.6 x 105 16 Hexanol 25 0.005 pKa1,2 ( approx.) 4.0, 9.0 4.0, 9.0 13 Table 3b. Physicochemical Properties for Cacodylic Acid and Sodium Cacodylate.

Cacodylic Acid ( DMA) Sodium Cacodylate ( DMA Na)

Molecular Structure O || CH3  As  OH | CH3 O || CH3  As  O Na+ | CH3 Empirical Formula C2H7AsO2 C2H6AsNaO2 Molecular Weight 138.0 160.0 CAS No. 75 60 5 124 65 2 PC Code 012501 012502 Melting Point ( º C) 192 194 77   79.5 Density ( g/ mL) 1.10 1.10 Vapor Pressure ( mm Hg) Non volatile No data Kow < 0.028 No data Solubility: Water ( mg/ L) ~ 1 to 3 x 106 No data Methanol 3.63 x 105 No data Hexanol 1.02 x 10 1 No data pKa 6.2 6.2 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 14 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 transformation. 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 degradation 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. 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. Remobilization of sorbed arsenic with changing environmental conditions is also possible. 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 Kd for MMA is 11.4 mL/ g. For 20 tested soils, the range of Kds spans two orders of magnitude 15 ( 0.5 to 95 mL/ g, mean 37 mL/ g). For DMA, the lowest non sand Kd from 16 soils is 8.2 mL/ g ( range 8.2 to 33 mL/ g, mean 18 mL/ g).

GROUNDWATER EXPOSURE Arsenic compounds sorb strongly to soil and are relatively immobile in most environments, as discussed in the Environmental Fate Summary. Significant leaching of organic arsenicals applied according to the labels is therefore unlikely in most conditions. This conclusion is supported by several field studies which have not detected arsenic from pesticide application below the top layers of soil. Natural background levels of arsenic in groundwater are extremely variable and can reach levels greater than 100 ppb. National monitoring has found no relationship between agricultural use of arsenicals and widespread high arsenic concentrations, but recent investigations in Florida have detected elevated arsenic levels in groundwater below golf courses. In most situations, organic arsenical pesticides should not contribute significantly to the already existing burden of arsenic in groundwater from all sources, natural and anthropogenic. In certain vulnerable circumstances in areas with low background arsenic, application of organic arsenicals may lead to an increase in groundwater total arsenic.

Modeling of estimated groundwater concentrations was not conducted. SCI GROW, the Agency's current screening level groundwater model, is based on data from groundwater studies for a number of organic pesticides. The environmental behavior of arsenicals is quite different from that of typical organic pesticides. A primary difference is that arsenical sorption tends to correlate with soil mineral properties rather than the amount of organic matter. For this reason, SCI GROW is not an appropriate tool for estimating the groundwater concentrations that may result from application of organic arsenical pesticides.

Groundwater Monitoring Background Arsenic ( Non targeted Monitoring) Data on arsenic in groundwater are abundant, but the majority of sampling is not targeted specifically to determine the impacts of organic arsenicals applied as herbicides. Additionally, most monitoring is conducted for total arsenic, consistent with the regulatory limits, rather than for speciated forms of arsenic. These non targeted, unspeciated results are most useful as a measure of background concentrations.

The most extensive consideration of groundwater arsenic has been done by the USGS. A USGS map of total arsenic distribution in ground waters of the U. S. is shown below in Figure 1 ( Ryker, 2001). In one study, USGS authors retrieved water quality data from the National Water Information System ( which includes National Water Quality Assessment [ NAWQA] data) and other sources totaling about 50,000 samples from about 30,000 locations ( Welch, 2000). A subset of about 20,000 analyses from the years 1973 1997 which met certain criteria were selected for statistical testing. Only a single analysis for a particular well or spring was used to avoid bias towards frequently sampled 16 Figure 1. Arsenic in Groundwater in the United States. Equal area map representing arsenic concentrations found in at least 25% of ground water samples within a moving 50km radius, based on USGS NAWQA data. ( Ryker, 2001). 17 sources. Sample sites were not uniformly distributed across the country and were not specifically targeted for areas where arsenical pesticides were applied.

Based on this statistical analysis, nearly half of the groundwater samples were found to have total arsenic concentrations < 1 ppb while about 10% exceeded 10 ppb ( Welch, 2000). The authors conclude that natural arsenic concentrations exceeding 10 ppb are more widespread than previously recognized, although they still find that " ground water in the U. S. typically contains low to very low arsenic concentrations, particularly in [ the East]." Some areas of high arsenic concentrations were associated with point source pollution but most appeared to be natural. High arsenic concentrations were found throughout the study, but in general, the Appalachian Highlands and the Atlantic Plain had very low levels with somewhat greater concentrations in the Rocky Mountain System and Interior Plains and the highest concentrations found most frequently in the Intermontane Plateaus and the Pacific Mountain System. The survey authors report that even in areas that tend to have naturally high arsenic concentrations there exists some ground water with low to very low arsenic concentrations. Steep lateral and vertical concentration gradients exist, showing inherent local spatial variability. This article provides a thorough review of the environmental geochemistry of arsenic and arsenic occurrence in relation to natural sources.

Speciated monitoring data are limited. The Welch report cites groundwater sampling in northwestern Nevada in an area where organic arsenical herbicides are not applied ( 2000). In 30 samples tested for MMA and DMA, all results were < 0.3 ppb. An additional USGS study analyzed 6 groundwater samples from Idaho, 10 from Illinois, and 8 from Nevada ( total of 24 sites) for arsenite [ As( III)], arsenate [ As( V)], MMA and DMA ( Garbarino and Burkhardt, 1998). Again, none of the sites were associated with the application of arsenical pesticides. With a detection limit of approximately 0.2 ppb, there were no measurable concentrations of the methylated compounds in ground water. In contrast, arsenite or arsenate concentrations in these groundwater samples ranged from less than the method detection limit to a concentration as high as approximately 900 ppb. Their data show that either arsenite or arsenate or both were detected in all samples, typically at 10 50 ppb.

The USGS water resources database was found to include 798 groundwater samples with speciated measurement of organic arsenicals, collected between 2002 and 2004. ( The NWISWeb database is available online at http:// waterdata. usgs. gov/ nwis. Information about data quality procedures and about the studies represented in the database can be found at this site.) More than half of the samples were from Kansas ( 56%) with nearly a third from Idaho ( 27%). The remaining samples were from Maryland, Nevada, New Jersey, Oklahoma, and West Virginia. No information is available about the land uses in the areas sampled; the sites are not evenly distributed and are not targeted to pesticide use. For both DMA and MMA, the median result was less than the detection limit, which were generally between 0.1 and 1.2 ppb. All values are reported as arsenic. The averages were 0.23 and 0.15 ppb and the 90th percentile values were 0.5 and 0.3 ppb for DMA and MMA, respectively. The maximum concentrations for DMA and MMA, 14.9 and 34.8 ppb, respectively, were both from the same site in Idaho. On the same date, 18 total arsenic at this site was measured at 858 ppb. Generally, no data were available about total arsenic at these sites.

Pesticide Impacts ( Targeted Monitoring) The USGS report on background arsenic in groundwater includes a discussion of monitoring done in study areas where " high arsenic concentrations regionally coincide with agricultural use" ( Welch, 2000). Based on studies of agricultural use of inorganic arsenical pesticides in North Dakota, South Dakota, Wisconsin, and Minnesota, the authors find that " although some contribution of arsenic from historic uses is possible, [ these studies] all conclude that ground water is largely unaffected by use of arsenical pesticides." In the several sites where groundwater was determined to be affected by anthropogenic arsenic sources, the detections were associated with disposal or other types of point source pollution.

The EPA Pesticides in Groundwater Database includes monitoring data for two sites, both of which show high concentrations of total arsenic in a high percentage of water wells ( USEPA, 1992). In localized areas of Texas, concentrations of 10 680 ppb were found in 91 of 247 wells with limits of detection ( LODs) of 10 25 ppb. The Agency independently concluded that these detections were associated with use of cacodylic acid, but not from non point source, labeled pesticidal application ( Aurelius, 1988). Rather, the high concentrations were most likely caused by cacodylic acid treated cotton gin waste which was spread in the vicinity of poorly cased wells, and by somewhat higher natural concentrations of arsenic. In the State of Washington, where total arsenic was found at 1.6 13.3 ppb in 15 of 20 wells ( LOD 0.2 ppb), no connection was found with organic arsenical pesticides ( Erickson, 1990). In this area, natural conditions including historical volcanic activity; strongly alkaline, sandy soils; hydrology favorable to the migration of soluble constituents; and thermal waters may have contributed to the elevated concentrations, as well as possible inputs from former heavy use of calcium or lead arsenates in orchards ( sometimes hundreds of lbs/ A annually) or wastes from mining operations.

All of the targeted monitoring studies reviewed below were conducted in Florida, an area of concern because of its very sandy soils and shallow water tables. Although arsenicals are strongly sorbing, lab studies have shown that leaching in typical Florida soils may be important. Golf courses, major users of MMA, may be particularly susceptible to leaching because they typically have well drained soils and are heavily irrigated.

Given the variability of natural arsenic, it is useful to have information about regional background levels to compare with targeted results. The USGS water resources database included 322 arsenic groundwater samples from Florida collected at 235 sites between 1990 and 2004. This sampling found an average arsenic concentration of 1.1 ppb with a 90th percentile value of 2.5 ppb. ( All arsenic concentrations in this discussion of monitoring data are unspeciated, reported as total arsenic, unless otherwise specified.) This sampling is not uniformly distributed and not directed to specifically capture only control, non use areas. When available, background data specific to targeted monitoring locations are included in this discussion. 19 One Florida monitoring study, while not targeted to specific uses, found higher arsenic in groundwater in an urban area than a control region in the same aquifer ( German, 1996). In the control area there were no detections of arsenic in 10 samples while around the Orlando urban area, arsenic was detected in 6 of 23 samples with a maximum of 13 ppb. This demonstrates that anthropogenic inputs of arsenic may reach groundwater but provides no information about the specific source. In urban areas, possible inputs include but are not limited to application of organic arsenicals to golf courses and residential lawns.

Several other Florida monitoring studies focus on groundwater that may be impacted by golf course uses. The 2 studies reviewed here look at groundwater at 14 golf courses. The first study, conducted by the USGS in conjunction with the Florida Department of Environmental Protection ( FDEP), was designed to determine if pesticides used in turfgrass management were migrating to shallow groundwater ( Swancar, 1996). Most of the 9 golf courses tested were in central Florida with one to the south in Broward County and one in the panhandle, in Pensacola. Samples were taken quarterly for a year at 39 wells, most at tees and greens with 3 wells in pesticide mixing and loading areas. Arsenic was detected in 6 wells on 4 golf courses. On greens, 4 wells at 3 golf courses had detections, with concentrations ranging from 11   120 ppb. The average arsenic concentrations from the 4 samples taken at each well were 21, 28, 30, and 64 ppb. Arsenic was detected in wells from 1 tee site and 1 mixing/ loading area, with average concentrations of 3 and 32 ppb, respectively ( Swancar, 1996).

This is not a controlled study and detailed pesticide application histories are not available. The study shows that in general, pesticides applied to Florida golf courses may reach groundwater, finding that of the more than 40 pesticides and metabolites tested for, 13 were detected in golf course groundwater. Specific to arsenic, recent pesticide application data coupled to concentration trends, along with known historical uses, point to MSMA application as a likely source of groundwater arsenic at these golf courses. At Ventura Golf Course, where arsenic was detected at 1 green well at an average of 28 ppb, MSMA had been applied 10 days prior to the 2nd sampling event, when arsenic concentrations were found to have increased from 11 ppb to 48 ppb. Concentrations steadily decreased at the following 2 sampling events, reaching 22 ppb by the last sample, taken approximately 6 months later. This concentration trend provides evidence that MSMA application resulted in elevated groundwater arsenic levels. Historically, at least 12 years prior to monitoring, part of this site was a dairy farm. MSMA was applied monthly at the Highland Golf Course, where arsenic was detected at 1 green well at levels as high as 120 ppb with a mean of 64 ppb. Prior to development as a golf course, this was open land or pasture with citrus groves nearby. Bonaventura Golf Course, in south Florida, had the most arsenic detections, in 2 green wells and 1 mixing/ loading area. The first application of MSMA during the sampling period was between the 2nd and 3rd sampling events in areas not specific to the greens. There is no particular trend in arsenic levels at these wells. Before the golf course was established, this land was part of Everglades National Park ( Swancar, 1996). 20 The results of this study, along with documented violations of soil cleanup target levels for arsenic, led the Miami Dade County Department of Environmental Resources Management ( DERM) to conduct monitoring at 5 county golf courses ( 2002). At each golf course, three well clusters were installed including one shallow ( 10 15 ft) and one deep ( 21 28 ft) well. All golf courses had one well cluster in a pesticide mixing and loading area with the other wells evenly divided between tees, greens, and fairways, and wells were sampled quarterly for one year. In shallow wells, 76% of all samples exceeded 10 ppb as arsenic with 32% exceeding 50 ppb as arsenic. Groundwater arsenic concentrations at greens were not found to be significantly higher than those at tees and fairways, although there were higher concentrations at mixing and loading areas. In greens, the median concentration was 13.6 ppb with a maximum of 55 ppb. On fairways, the median was 17.8 ppb and the maximum was 56 ppb. In tee areas, the median was 81.8 ppb and a maximum of 123 ppb. In mixing/ loading areas, concentrations reached as high as 815 ppb with a median of 31.8 ppb ( DERM, 2002). Concentrations were much lower in deeper wells, with a maximum of 44 ppb. Arsenic was only detected in deeper groundwater at 3 of the 5 golf courses, with 18% of the samples having concentrations greater than 10 ppb. The highest concentrations were found in mixing/ loading areas. Only tees, with a maximum of 11 ppb and a median of 3 ppb, exceeded the detection limit in more than 50% of the samples ( DERM, 2002).

To establish background levels, three pre existing shallow monitoring wells in nearby residential areas were monitored concurrently. Based on 10 total samples, the maximum concentration was 5 ppb and the median was less than the detection limit. Additionally, historical data was gathered from wells at similar depths in the Ambient Water Quality Monitoring network. Based on 22 total samples, the maximum was 13.9 ppb and the median was 2.0 ppb ( DERM, 2002). These data indicate that the golf course arsenic levels are higher than background levels.

Other potential arsenic sources in these areas are discussed, including historical land uses and arsenic from fertilizers. All golf courses in the study were established prior to 1975 with some more than 50 years old. One course was built on undeveloped land, one on farmland, and a portion of one was used as a municipal dump 25 years prior to sampling. No historical land use information is provided for the other two courses. The highest shallow groundwater concentrations in mixing/ loading areas and fairways are from the previously undeveloped site while the landfill site had more detections of arsenic in deep groundwater than most other golf courses. These past uses are potential arsenic sources, but at those courses, the available data are not sufficient to rule out modern or historical contributions to arsenic contamination. Fertilizer samples were collected from the participating golf courses and analyzed for arsenic. Arsenic concentrations in fertilizer ranged from below detection limit to 15 mg/ kg, with a mean of 3.27 mg/ kg ( DERM, 2002).

A third report, by the Florida Department of Environmental Protection, describes investigations done at golf course sites where levels of arsenic have been found to exceed local regulatory limits ( FDEP, 2002). In some cases, FDEP was able to rule out pesticide applications as a primary source of arsenic contamination, but in at least 3 sites, pesticide 21 application was linked to contamination. This report focuses on soil arsenic levels, but groundwater arsenic is measured as well. At two golf courses, although groundwater levels reach 472 ppb at one and 1300 ppb at the other, concentrations are only reported for maintenance areas, so it cannot be determined whether there are impacts from nonpoint sources. At the Palm Beach Lakes Golf Course, two sets of shallow groundwater sampling were conducted. The first, focusing on former green areas, found arsenic levels exceeding 10 ppb in all 20 samples. The second set, in a different parcel of the course, consisted only of playing areas and found exceedances of the 10 ppb MCL in 22 of 27 samples.

These are not controlled studies. Nevertheless, considered as a group, they show a trend of groundwater arsenic at Florida golf courses well above local background levels and in many cases exceeding the federal MCL. Although evidence of the source of arsenic is generally not conclusive, pesticide application is a probable contributor to these levels, and in some cases, appears to be the likely source. The registrants and the State of Florida are currently negotiating a plan for a prospective groundwater study designed to investigate potential leaching of organic arsenicals in this vulnerable setting. This study is expected to lead to more conclusive evidence of whether or not applied arsenicals may reach groundwater.

SURFACE WATER EXPOSURE Arsenical pesticides and their metabolites may be transported to surface waters and sediments through runoff water, eroding soils, or drift during application. Both monitoring data and modeling results, discussed below, suggest that these routes of exposure are likely to lead to local elevations above background arsenic levels in surface water bodies. This is true for both the turf and the cotton uses, although the higher application rates for turf and potentially more widespread application lead to higher estimated concentrations.

Surface Water Monitoring Background Arsenic ( Non targeted Monitoring) As with groundwater, unspeciated surface water arsenic data not targeted to pesticide use areas are abundant. Unlike groundwater, no major statistical effort has been undertaken to interpret these surface water data. A less complete consideration of these data can still provide useful information about typical arsenic concentrations. Unless otherwise specified, all USGS data discussed in this section is from the publicly available NWISWeb database. The database, as well as information about data quality procedures and about the studies represented, can be accessed at http:// waterdata. usgs. gov/ nwis. The USGS dataset includes nearly 40,000 lake and stream samples collected nationally between 1990 and 2004 at around 4500 locations. Sample sites were not uniformly distributed across the country and these data have not been corrected for bias resulting from frequently sampled sources. The data also include sampling from point sources such as mine waste but specific land use information has not been considered. Half of the samples had total arsenic concentrations   1.1 ppb and 10% of the samples exceeded 22 the MCL of 10 ppb. With the exception of a mine in Pennsylvania, the very high arsenic concentrations, greater than 100 ppb, are all in the western U. S. with New Mexico and North Dakota as the furthest east samples. For concentrations between 50 and 100 ppb, there are only 2 sites in the eastern region not obviously associated with a point source. Perhaps coincidentally, both are in heavy cotton producing areas, one in Texas and one in Mississippi. The USGS groundwater report included a brief consideration of surface water arsenic, finding that the most common natural cause of elevated arsenic in surface water is discharge of geothermal water ( Welch, 2002).

The BASINS and STORET databases maintained by the US EPA also provide a large body of data ( many tens of thousands of entries) on total arsenic concentrations in surface waters of the U. S. As with the USGS surface water data, the wide ranging concentrations in space and time and the multiplicity of possible major sources of arsenic make any attempt to associate these with pesticide use impractical. Nevertheless, even casual inspection of the data from the databases shows that it is not unusual for total arsenic concentrations in raw surface water in many different sites to be several parts per billion. Spatial variability is the rule. The data for Georgia and Texas are discussed here as illustrative of this variability and are not meant to imply a special situation. In Georgia, as presented in BASINS, upper 85th percentile concentrations of total arsenic at different sites ranged from less than 1 ppb to 338 ppb. More typical high values fall in the range of 10 to 50 ppb. The most probable concentration appears to fall in the range of < 1 to 2 or 3 ppb. The STORET database for Georgia and Texas show similar results, with Texas seeming to average several parts per billion more. Some of the highest values reported in STORET for Georgia and Texas ( 79,000 entries) approach 1000 ppb and can often be associated with anthropogenic point source pollution. As previously mentioned, natural sources ( especially in places in the western U. S.) also produce such high concentrations.

Speciated data on arsenic in surface waters are sparse. A search of the USGS database found only 26 samples, not enough to overcome the limitations of the dataset with regard to site distribution and unknown land uses. One open literature study tested 10 natural waters in the vicinity of Tampa, Florida ( Braman and Foreback, 1973). There were six surface fresh water bodies ( two rivers, two ponds, two lakes) and three saline waters ( two bays, one tidal flat) included in the study. One groundwater site was also included, a well in a remote camping area. Analysis was for the four arsenic species: DMA, MMA, arsenite, and arsenate. All concentrations reported below are as arsenic equivalents. Concentrations of DMA in the natural waters ranged up to 1.0 ppb. At one site, the Hillsborough River, concentrations were below a remarkably low detection limit of 0.02 ppb, while the median value for DMA at the other nine sites was approximately 0.3 ppb As. Concentrations of arsenate, detected at all ten sites, were similar to cacodylic acid. Arsenite was detected at only six of the ten sites, but had the highest concentration ( 2.7 ppb) of all species. MMA, detected in eight of the ten sites, was generally present at lower concentrations, the highest being 0.22 ppb As. However, in a broader sense, considering the few samples, surface water concentrations of all four were similar. Total arsenic concentrations ranged from approximately 0.3 to 3.6 ppb. The extent to which these concentrations represent the natural background in the Tampa area or are influenced 23 by introduction of artificial sources is unknown. However, the study authors considered the sampled sites as " natural" waters.

A California study measured speciated arsenic in 14 water bodies ( 10 lakes, estuaries, or reservoirs, and 4 rivers or creeks) in order to compare concentrations of methylated arsenical forms to inorganic forms ( Anderson, 1991). The authors indicate that the sampling represents drainage basins both affected and unaffected by agricultural runoff but do not distinguish between the two. All sites had measurable arsenic, although in 8 of the 14 sites, total arsenic was less than 2 ppb and in 1 site, Mono Lake, inorganic arsenic was extremely high ( 17 ppm), likely due to its unique hydrogeology. At Mono Lake, analysis for organic arsenic was impossible and so this site will not be considered further in this discussion. The highest measured concentrations of MMA and DMA were equivalent to 0.6 ppb and 2.5 ppb as arsenic, respectively. These were found in the Salton Sea, where the total arsenic concentration was 12.8 ppb. The Salton Sea receives a substantial amount of agricultural runoff, including from cotton growing areas, but also has geothermal activity, a possible source of natural arsenic. The stream sites had total arsenic concentrations ranging from 0.7 to 7.4 ppb but had no detectable concentrations (< 0.1 nM) of MMA or DMA. Of the 9 other sites, 7 had measurable concentrations of MMA and all 9 had measurable concentrations of DMA. In these sites, the average concentration of MMA, as arsenic, was 0.15 ppb with a standard deviation of 0.2 ppb. The average concentration of DMA was higher, at 0.62 ppb with a standard deviation of 0.8 ppb. Except for one site, which had 57% of total arsenic present in methylated form, inorganic arsenic was found in much higher concentrations than organic, which ranged from 1% to 39% of total arsenic.

Pesticide Impacts ( Targeted Monitoring) There are several monitoring studies available targeted to measure the impact of organic arsenical herbicides on surface water. One study collected surface water samples at one or two week intervals from early March through the middle of September of 1997 from two drainage basins in high cotton producing areas of Mississippi ( Bednar, 2002). Sampling dates and times varied between sites and were not correlated. A total of 24 samples were analyzed from each the Yazoo and the Bogue Philia drainages. Analyses were for MMA, DMA, arsenite, and arsenate. Maximum concentrations, as arsenic, of MMA at Yazoo and Bogue Philia were approximately 2 ppb and 5 ppb, respectively, and were relatively short lived. The highest concentrations were seen in June and July, at the end of the period when MSMA is most likely to be applied. Concentrations of DMA were at or below the detection limit of 0.2 ug/ L, except for one sample which registered approximately 0.6 ug/ L. Arsenate and arsenite reached maximum concentrations similar to those from MMA; both approximately 2 ppb at Yazoo and approximately 3.5 ppb ( arsenite) and 5.5 ppb ( arsenate) at Bogue Philia. The higher concentrations of inorganic arsenic were measured more frequently than those for MMA. In all samples, one species tended to dominate; the comparable concentrations of arsenite and arsenate is also noteworthy. At the Bogue Philia site, inorganic arsenic concentrations increased after the MMA maximum was measured, possibly indicating transformation of the organic arsenical or a secondary source of arsenic ( Bednar, 2002; in previous REDs, the same data were cited from Garbarino, 1998). 24 Detailed histories of MSMA application in this area are not available and the location of water bodies relative to application sites was not published. Without baseline data for arsenic concentrations in areas without arsenical application, which are not provided in this study, it cannot be concluded whether the fluctuations seen here are part of the natural arsenic cycle or influenced by pesticide application. For comparison, the Agency retrieved surface water arsenic concentrations from the USGS database, which included 65 samples in Mississippi. The samples are not uniformly distributed and information about land use patterns is not available. The average total arsenic concentration found was 3.4 ppb with a 90th percentile value of 4 ppb. All samples with arsenic concentrations higher than 4 ppb were from a single water body where levels reached as high as 72 ppb. Presumably this water body is impacted by a pollution source and therefore not representative of background levels. If these data are not included, the average of the 60 remaining samples is 1.1 ppb with a 90th percentile of 2 ppb. While still not conclusive, this suggests that at least the higher arsenic concentrations found in the Bednar report, above 5 ppb, are higher than would be expected from background arsenic alone.

Limited monitoring has been done in surface water bodies on golf courses as well. Golf course ponds are relatively small water bodies and are typically completely surrounded by potential application areas, and therefore runoff sources. Consequently, these ponds are likely to represent the upper end of possible surface water concentrations. Limited monitoring, however, cannot be expected to capture peak concentrations. One USGS study, also discussed in the groundwater monitoring section, sampled ponds at 6 golf courses, all in central Florida, for multiple pesticide residues ( Swancar, 1996). Ponds were sampled 4 or 5 times over the course of one year. Arsenic was below the detection limit ( 1 ppb) at 2 of the golf courses and at a third, was detected in only 1 out of 5 samples, measured at the MCL of 10 ppb. At the other 3 golf course ponds, concentrations ranged from 5 to 24 ppb. Of the 13 samples from these 3 ponds, the mean total arsenic concentration was 11.6 ppb. It is interesting to note that detectable levels of arsenic were not found in any of the groundwater samples collected at these 3 locations.

Surface Water Modeling Modeling of the application of arsenical pesticides was performed to predict acute and chronic concentrations that may reach drinking water. Although there are extensive monitoring data available for arsenic, some of it even targeted to heavy use areas, modeling remains a valuable tool to supplement the limitations of monitoring. Most monitoring measures total arsenic and does not provide information on speciation. Monitoring data do not generally allow for determination of the source of contamination. This is particularly important for arsenic, which has multiple potential sources, both from natural background and from various anthropogenic activities. Even for those monitoring studies targeted to heavy use areas, specific information about application rates and land use history are not always available. Additionally, even targeted studies cannot be expected to capture short lived peak concentrations. In light of these factors, and considering the variability of arsenic's environmental behavior with variable environmental conditions, modeling is useful for providing high end estimates of 25 speciated arsenic concentrations resulting from pesticide applications under the most vulnerable conditions.

To determine estimated drinking water concentrations ( EDWCs) in surface water, the Pesticide Root Zone Model ( PRZM 3.12; 5/ 7/ 98), which simulates transport off the agricultural field, is run in tandem with the Exposure Analysis Modeling System, ( EXAMS 2.98.04; 6/ 13/ 97), which simulates the fate of chemicals in the water body. These are operated using the pe4v01 shell program ( 8/ 13/ 03). Additional information about these models can be found at the EPA's water modeling website, http:// www. epa. gov/ oppefed1/ models/ water/. The simulated watershed is based on an Index Reservoir ( IR) scenario, and a percent cropped area ( PCA) adjustment factor is used to adjust for the area within the watershed that is planted to the modeled crop ( OPP, 2000). Models are run for 30 years and the reported EDWCs represent the values that are expected once every ten years, based on the 30 years of daily values generated during the simulation.

The crop scenarios used in PRZM/ EXAMS represent sites that are highly vulnerable to runoff. In this assessment, the Mississippi cotton and Florida turf scenarios are modeled to represent the major uses of arsenicals. Other cotton and turf scenarios were run to confirm that these are the most vulnerable of available scenarios. The Mississippi cotton scenario is located in Yazoo County, the number one county in a state that ranks fourth in cotton production ( EFED, 2003). The Florida turf scenario is located in Osceola County. Golf courses and other areas of turf cultivation in Florida use a significant amount of arsenical pesticides each year ( Swancar, 1995; Ma, 2002). Minor crop scenarios, including those for orchards and citrus, were also modeled to provide characterization.

All species of concern in this assessment ( MMA, DMA, and iAs) have distinct toxicities; exposure to each needs to be considered individually. Estimated concentrations of each of these compounds are therefore provided for each use. Compounds resulting from metabolism are considered as well as directly applied pesticides. Applied MMA may metabolize to DMA. On cotton, MMA and DMA can be applied to the same field. Reported cotton MMA EDWCs, therefore, result from direct application while cotton DMA EDWCs result both from direct application and from transformation of MMA. On turf, all EDWCs are the result of MMA application. Both MMA and DMA may also metabolize to inorganic arsenic. It is impossible to determine the exact concentration of each species that will be present at any one time and each species will reach its highest level at different times; the reported EDWCs for parent and metabolite compounds represent maximum potential concentrations for each species and are not additive to a total possible concentration. When the concentration of one species is at its highest, the other species' concentration will be lower. An EDWC for " total arsenic" has been reported to indicate the maximum amount of arsenic that may be in surface water as a sum of all species present. It is this EDWC that would be compared to regulatory levels, which are set as total arsenic. Since it is possible that all arsenic would be present in the form of inorganic arsenic, this EDWC also represents the maximum potential concentration of inorganic arsenic. EDWCs resulting from the maximum labeled application rates for cotton and turf are presented in Table 4. The input parameters used 26 Table 4. EDWCs ( ppb) from maximum labeled rates for major uses of arsenicals.

Acute Chronic Cancer TURF1 MMA 250.5 127.5 74.6 DMA 102.3 46.5 28.1 Total As3 135.2 68.8 40.3 COTTON2 MMA 37.4 11.0 5.3 DMA 23.6 7.4 4.3 Total As3 20.9 7.2 3.9 1 MSMA applied 4 times at 3.35 lbs ae/ A. 2 DSMA applied 2 times at 1.74 lbs ae/ A & DMA applied 1 time at 1.2 lb/ A. 3 Total arsenic is the sum, reported as ppb As, of arsenic that may be present from all applied and metabolite species. It also represents the maximum EDWC of inorganic arsenic.

are presented in Tables 5 and 6 and more detailed discussion of parameter selection, methods of determining metabolites, and uncertainty is included below. The PRZM/ EXAMS input files are provided in Appendix A and the output files in Appendix B.

Input Parameters: Metabolism Rates The input parameters used in PRZM/ EXAMS modeling in this assessment ( Tables 5 and 6) are based primarily on the values determined for the most recent REDs ( DP Barcodes D210451, D212449, D255226 and DP Barcode D277223). The validity of each parameter has been considered individually and, while most have been confirmed, several have been adjusted to take into account new data or different interpretations of older data. In part, this process has been a response to concerns expressed by the registrant regarding the parameterization of the original modeling. In particular, the registrant argues that thesoil metabolism half life inputs used do not account for the biphasic nature of arsenical metabolism ( MAATF, 2005). It is true that arsenical metabolism appears to be biphasic, occurring more rapidly prior to sorption. Current EFED modeling requires first order linear rate constants. Even if biphasic rates could be incorporated, because of the variability of sorption processes there are insufficient data to allow for determination of two separate rates. Because chronic exposure is a major concern for arsenicals, it is assumed to be protective to use a long term half life that is less accurate in representing the faster initial rate of metabolism but that better portrays the overall transformation.

For MSMA and DSMA, the modeled half lives are unchanged from those used in the 2001 RED ( DP Barcode D277223). The aerobic soil and aquatic metabolism half lives are based on one registrant study ( MRID 44767601; Acceptable) and on a non GLP open literature study reviewed in the previous RED ( Gao and Burau, 1997). The registrant submitted a review of additional literature which includes several studies that quantify shorter half lives than those used in the initial RED ( MAATF, 2005). Even if those values are included in the calculation, the 90% confidence half life only changes from 240 days to 220 days, which has a minimal effect on the outcome and does not justify the 27 Table 5. PRZM/ EXAMS Input Parameters for DSMA/ MSMA on Cotton and Turf Model Parameter Value Comments Source Single Application Rate (# of applications) Cotton: 1.95 kg ae/ ha ( 2 apps) Turf: 3.75 kg ae/ ha ( 4 apps) Master Label Application Date ( Intervals) Cotton: 5/ 7 ( 7 days)

Turf: 3/ 14 ( 10 days) Cotton: USDA NASS Planting & Harvest Dates ( 12/ 97) 1 Turf 2 Application Method Cotton & Turf: Foliar; ground applied3 Leads to default values of 6.4% Spray drift 99% App. efficiency Incorporation Depth Cotton & Turf: 0 cm Aerobic Soil Metabolism Half Life 240 days 90% upper confidence bound, based on 2 values. Gao & Burau, 1997 MRID 44767601 Anaerobic Soil Metabolism Half Life  No reported data, not used as input parameter Aerobic Aquatic Metabolism Half life 480 days No reported data, use ½ of aerobic soil rate Input Parameter Guidance4 Anaerobic Aquatic Metabolism Half life 2300 days 3 times single reported value MRID 44767602 Aqueous Photolysis Half life Stable MRID 41903902 Hydrolysis Half life Stable Stable at all pH MRID 42363001 Kd 11.4 ml/ g 5 Lowest non sand Kd from 19 soils. Wauchope, 1975 MRID 41651906 Molecular Weight 139 g/ mol 5 as MMA Water Solubility 1 x 106 mg/ L 2001 RED ( D277223)

Vapor Pressure 1 x 10 7 torr 2001 RED ( D277223)

Percent Cropped Area ( PCA) Cotton: 20% Turf: 100% Drinking Water Screening Level Assessment Guidance, Part B 6 1 http:// usda. mannlib. cornell. edu/ reports/ nassr/ field/ planting/ uph97. html 2 A spring application date was chosen to represent the period when crabgrass and goosegrass are most prevalent. MSMA can be applied at any time of year. 3 Cotton is labeled for aerial application but ground application leads to more conservative EDWCs. 4 http:// www. epa. gov/ oppefed1/ models/ water/ input_ guidance2_ 28_ 02. htm 5 Different from input for 2001 RED ( DP Barcode D277223). 6 http:// www. epa. gov/ oppfead1/ trac/ science/ reservoir. pdf 28 Table 6. PRZM/ EXAMS Input Parameters for Cacodylic Acid on Cotton Model Parameter Value Comments Source Single Application Rate (# of applications) 1.34 kg/ ha ( 1 application) Master Label Application Date 10/ 10 USDA NASS Planting and Harvest Dates ( 12/ 97) 1 Application Method Foliar; aerial applied Leads to default values of 16% Spray drift 95% App. efficiency Incorporation Depth 0 cm Aerobic Soil Metabolism Half Life 240 days2 90% upper confidence bound, based on 6 values. Woolson & Kearney, 1973 Woolson, 1982 Gao & Burau, 1997 MRID 44767601 Anaerobic Soil Metabolism Half Life 168 days 90% upper confidence bound, based on 3 values. Woolson & Kearney, 1973 Aerobic Aquatic Metabolism Half life 480 days2 No reported data, use ½ of aerobic soil rate Input Parameter Guidance3 Anaerobic Aquatic Metabolism Half life 336 days No reported data, use ½ of anaerobic soil rate Input Parameter Guidance3 Aqueous Photolysis Half life Stable MRID 41662601 Hydrolysis Half life Stable Stable at all pH MRID 42059201 Kd 8.2 ml/ g Lowest non sand Kd from 16 soils. Reviewer calculated. Wauchope, 1975 Molecular Weight 138 g/ mol Water Solubility 1 x 106 mg/ L 2000 RED ( D210451, D212449, 255226)

Vapor Pressure 0 2000 RED ( D210451, D212449, 255226)

Percent Cropped Area ( PCA) 20% Drinking Water Screening Level Assessment Guidance, Part B 4 1 http:// usda. mannlib. cornell. edu/ reports/ nassr/ field/ planting/ uph97. html 2 Different from input for 2000 RED ( DP Barcodes D210451, D212449, 255226). 3 http:// www. epa. gov/ oppefed1/ models/ water/ input_ guidance2_ 28_ 02. htm 4 http:// www. epa. gov/ oppfead1/ trac/ science/ reservoir. pdf 29 inclusion of unreviewed non GLP studies from an incomplete literature review. The anaerobic aquatic metabolism half life is based on the value from one registrant study ( MRID 44767602; Acceptable) which is multiplied by three to account for the uncertainty when only a single value is available, as directed by the input guidance ( EFED, 2002). This leads to a half life that is significantly longer than the aerobic values. Although this is not entirely supported by the literature, which indicates that anaerobic metabolism may be faster than aerobic metabolism ( Akkari, 1986; Gao and Burau, 1997), adjusting this value makes no difference in the results and so no change was made.

For DMA, the soil and aquatic metabolism half lives are all based on non GLP data. Although these values have uncertainty, there are no acceptable GLP studies of DMA available. The aerobic soil metabolism half life is based on the same 3 literature studies considered in the 2000 RED ( DP Barcodes D210451, D212449, D255226) as well as data on DMA as a metabolite from a GLP study of MSMA ( MRID 44767601). For one study, the half lives used were recalculated based on data from the complete 32 week study rather than data from the first 24 weeks, as was originally used ( Woolson & Kearney, 1973). Because of decreasing transformation with time, this leads to a longer half life. A half life for DMA was derived from the MSMA study by performing non linear regression on its formation and decline using StatMost software ( StatMost, 1994). The half life was calculated using the simplifying assumption that metabolism occurred as two individual first order processes: MMA transforming to DMA followed by transformation to inorganic arsenic. Although there is some uncertainty in this approach, this is the only GLP study with data for DMA metabolism and the only study available that was carried out for a full year so it still provides valuable information. The aerobic aquatic metabolism half life is based on the aerobic soil value, as per EFED guidance ( EFED, 2002) and so increased as well. The anaerobic aquatic metabolism half life was calculated based on anaerobic soil metabolism data from one literature study ( Woolson & Kearney, 1973). Another study was discussed in the 2000 RED ( Woolson, 1982), but because it was carried out in atypical conditions it was not included in calculations.

Input Parameters: Soil Mobility For MMA, the Kd value used in the 2000 RED, 13 mL/ g, was not referenced and so could not be confirmed. Data from a registrant study ( MRID 41651906; Supplemental) were considered along with data reported by Wauchope ( 1975) leading to a lowest non sand Kd value of 11.4 mL/ g. For DMA, the original Kd value of 8.2 mL/ g was not changed. It is the lowest non sand value calculated from data reported by Wauchope ( 1975).

Use of the lowest non sand Kd provides a conservative estimate of potential runoff in agricultural soils that do not promote sorption. The effect of variable Kd values on modeling results was examined. For MMA, the median of the 19 Kd values considered was 28 and the average was 37 mL/ g ( Wauchope, 1975; MRID 41651906). For DMA, 16 values were considered with a median of 15 and an average of 18 mL/ g ( Wauchope, 1975). To estimate exposure from typical soils, a Kd of 20 was used for modeling DMA and 30 for MMA. This led to decreased peak EDWCs for cotton and had less of an impact on chronic values. Little change was seen in turf peak or chronic EDWCs, while 30 the cancer EDWCs actually increased slightly. See the characterization discussion for more detail.

Calculation of Metabolites MMA metabolism to DMA was simulated by modeling DMA applied at 35% of the MMA rate with DMA input parameters. For cotton, where DMA and MMA can both be applied to the same field at different times in the same season, the time series of concentrations resulting from DMA applied directly was added to that resulting from DMA as a metabolite of MMA. The upper 90% confidence limit was determined for peak and average annual values from this 30 year time series.

This simulation is based on the assumption that a maximum of 35% of applied MMA may be present as DMA at any one time. This is based on one registrant aerobic soil study submitted in 2001 ( MRID 44767601; Acceptable). After 1 year, 32% of the applied MSMA was present as DMA. At the end of this study, the amount of DMA present was still rising, so it is possible that after 1 year, a greater percentage of the applied amount is present as DMA. The 35% estimate is supported by calculations of the formation and decline of DMA using the same half lives as included in the PRZM/ EXAMS modeling.

There is uncertainty associated with this assumption, which is based on the results of a lab study on the metabolism of MMA and on calculations using the modeled half lives of MMA and DMA. In both of these approaches, the DMA present is being metabolized as it is formed and so, without metabolism, would be present in higher amounts. The estimate of 35%, therefore, already takes into account some amount of transformation. The model then simulates metabolism as well, possibly leading to an underestimation of the maximum amount of DMA present at any time. Despite this uncertainty, 35% is considered sufficiently conservative because even incorporating some transformation, it is still significantly higher than the amount of DMA seen in any other field or lab study reviewed, many of which found no DMA at all. Additional uncertainty results from modeling the DMA metabolite as if it were applied at the same time as the MMA parent. Metabolism to DMA is not a rapid transformation and so in fact, when present, the metabolite will appear at a later date dependent on the environmental conditions.

Total arsenic was estimated as a direct molar conversion of the EDWC predicted for the applied, parent organic arsenical( s). ( The reported EDWCs for parent and metabolite compounds represent maximum potential concentrations for each species and are not additive to a total possible concentration. When the concentration of one species is at its highest, the other species' concentration will be lower.) The total arsenic value is also used as the inorganic arsenic EDWC. For maximum inorganic arsenic, a direct molar conversion of the parent compound's aquatic concentration does not necessarily represent the actual physical process   inorganic arsenic is likely formed through metabolism in the soil, rather than after reaching surface water. This calculation still provides a conservative estimate of the amount of inorganic arsenic that may reach surface water. The estimate is supported by a targeted monitoring study in cotton growing areas which found that typically, one arsenic species at a time was dominant. Immediately following 31 the period of typical application, MMA was the dominant species and several weeks to months later it the primary form of arsenic was inorganic, present at approximately the same total arsenic concentration as the earlier MMA ( Bednar, 2002).

Percent Cropped Area For cotton, a 20% PCA is applied to the modeling results, according to division policy which estimates that 20% is the maximum area of any watershed planted in cotton ( OPP, 2000). It is possible that arsenicals are applied to other crops within the same watershed, which could lead to higher amounts of pesticide reaching surface water. Use as an herbicide on turf is the only other application of arsenicals extensive enough to potentially increase watershed scale concentrations. Monitoring data suggest, however, that the 20% PCA still leads to protective estimates of exposure. A study targeted to estimate the impacts of arsenical use on cotton sampled surface water weekly from May to September in heavy cotton growing areas in Yazoo County, Mississippi ( site of the Mississippi cotton scenario), and in Arkansas ( Bednar, 2002). The highest concentration of total arsenic found in any of those samples ( 6 ppb as arsenic = 11 ppb as MMA) is similar to the modeled chronic EDWC for MMA on cotton as parent compound only and so is less than the chronic total arsenic concentrations, which would include metabolites as well. Weekly targeted monitoring of this type can be expected to capture chronic concentrations but not acute concentrations. These monitoring values, in the range of modeled chronic concentrations and less than modeled acute concentrations, suggest that the EDWCs are reasonable.

No appropriate PCA has been determined for application of pesticides to turf. In order to be protective, at this time division policy is to not apply any PCA for turf. The modeled EDWCs for turf are therefore based on the assumption that the entire modeled watershed has been treated with pesticide, which may lead to overestimation of potential exposure. Limited surface water monitoring data targeted to golf course use are available. Two studies measured arsenic in golf course ponds. In one study, 6 ponds were sampled with arsenic concentrations ranging from below detection limits to 24 ppb ( as total arsenic; Swancar, 1996). In the other study, arsenic concentrations at four ponds ranged from below detection limits to 30 ppb with a median of 19.75 ppb ( as total arsenic). These values were reported to be an order of magnitude higher than other surface water concentrations in the same area ( DERM, 2002). This monitoring involves relatively small surface water bodies that are completely surrounded by turf that may be treated with arsenicals, so they represent high potential exposure relative to the index reservoir. Nevertheless, they demonstrate that a significant amount of arsenic may reach surface water from turf applications of arsenicals and that modeled turf EDWCs are on the same scale as concentrations that have been measured in monitoring. The chronic turf total arsenic EDWC is 68.8 ppb, approximately twice that found in monitoring. These data are discussed in more detail in the monitoring section.

Characterization The reported EDWCs are based on maximum labeled application rates under the most vulnerable circumstances. A variety of management practices, environmental conditions, and application rates are possible and can lead to different concentrations. This section 32 characterizes how exposure to arsenicals may be affected by less vulnerable soils, typical application rates, application to minor crops, or practices such as irrigation or spot treatment.

Sorption of arsenical pesticides varies with soil characteristics. Sorption is expected to be higher in soils with a higher percentage of clay or with more iron or aluminum content ( Matera, 2001). Surface water concentrations were estimated using higher Kd values to characterize possible exposure from application to less vulnerable soils. Considering the parent compound only, for application of MMA or DMA to cotton, the typical Kd led to peak concentrations approximately 30% less than those from more vulnerable soils. Kd had less of an effect on chronic exposure from cotton application with a 15% decrease for MMA and a 3% decrease for DMA. There was little change for cancer EDWCs. For MMA applied to turf, a higher Kd led to less than a 10% decrease for peak and chronic exposures and actually led to a slight increase in estimated cancer concentrations.

EDWCs are modeled using the maximum labeled application rates. For turf, this maximum rate is allowed for sod farms and other well established Bermudagrass and Zoysiagrass, but use on golf courses, parks, and athletic fields is limited by the label to a lower application rate. For MSMA, the " golf course" rate is 2.6 lb/ A, compared to the maximum rate of 3.9 lb/ A. Additionally, the new master label specifies that arsenicals cannot be applied to golf course greens. While greens generally make up less than 3% of golf course area ( EFED, 2005), they are typically the areas treated most heavily with pesticides and are also designed to promote drainage, so limiting application to greens may lead to lower concentrations. A 2002 report by the Florida Center for Solid and Hazardous Waste Management at the University of Florida estimated that typical use of MSMA on golf courses was 3 annual applications at 3 lbs/ A. Based on responses from 15% of Florida golf courses, the report estimated that 80% of golf courses apply MSMA Table 7. Surface water concentrations ( ppb) from applications of MSMA to turf at less than the maximum labeled rate. Acute Chronic Cancer Golf course rate1 MMA 166.7 85.2 49.7 DMA 68.0 33.7 18.9 Total As 89.9 46.0 26.8 " Typical" use rate2 MMA 180.6 80.5 45.2 DMA 64.6 27.6 14.8 Total As 97.4 43.4 24.4 Single app.; max rate3 MMA 77.7 34.8 18.5 DMA 33.2 13.4 7.0 Total As 41.9 18.9 10.0 1 On golf courses, parks, and athletic fields, maximum rate is 4 applications at 2.49 lbs ae/ A . 2 " Typical use" on golf courses = 3 applications MSMA at 2.57lbs ae/ A. ( Ma, 2002) 3 One application of MSMA at 3.35 lbs ae/ A. 33 at this rate or lower ( Ma, 2002). These application rates were modeled with PRZM/ EXAMS and the estimated concentrations are presented in Table 7. It should be emphasized that modeling at the maximum labeled rate is the most protective and these concentrations are provided for characterization only.

The most conservative estimate of use of arsenicals on cotton assumes that DSMA or MSMA is applied as an early season herbicide followed by DMA as a late season defoliant on the same field. This combined use is allowed by the labels and is known to be used in some areas. It is more common that either one or the other product is used individually ( BEAD, 2005a) and so these uses were modeled as well for characterization. Table 8 includes estimated concentrations for maximum labeled applications to cotton of only one type of arsenical, either MMA as an herbicide or DMA as a defoliant. These values are based on the same input parameters described in Table 5 and Table 6 including the 20% PCA adjustment.

Table 8. Surface water concentrations ( ppb) from applications of individual arsenicals to cotton. Acute Chronic Cancer Max rate, DMA only1 MMA

DMA 18.3 2.9 2.0 Total As 9.9 1.6 1.1 Max rate, MMA only2 MMA 37.4 11.0 5.3 DMA 17.8 5.1 2.3 Total As 20.2 5.9 2.9 Single app., MMA only3 MMA 18.4 5.4 2.6 DMA 8.7 2.5 1.2 Total As 9.9 2.9 1.4 1 Based on 1 application of DMA as a defoliant at 1.2 lb/ A. 2 Based on 2 applications of DSMA as an herbicide at 1.74 lbs ae/ A. 3 Based on 1 application of DSMA as an herbicide at 1.74 lbs ae/ A.

Labels for DSMA and MSMA also allow application to orchards, citrus, and vineyards and the DMA label includes a use on citrus, although this is prohibited in Florida. All available scenarios were modeled to characterize concentrations resulting from application to these minor crops. The Oregon grass seed scenario was modeled as well to account for the labeled application of DSMA and MSMA to grass seed in the Pacific Northwest. Concentrations resulting from application of arsenicals to the most vulnerable of these scenarios are presented in Table 9. Although application rates for some of these crops are higher than for use on turf or cotton, turf and cotton represent at least 97% of the total national use of arsenicals and so those uses are expected to have the greatest impact ( BEAD, 2005a & b). 34 The values in Table 9 are screening estimates that do not account for the labeled restriction that arsenicals can only be applied to non bearing fields of orchards, citrus, and vineyards and cannot be applied within one year of harvest. As a worst case scenario, then, arsenicals would only be applied to these crops every second or third year so a refined estimate would limit the years of application, leading to much lower estimated concentrations. Additionally, these estimated concentrations use the default PCA adjustment of 87%. Given the low estimated usage of arsenicals on these crops, the actual usage area within a watershed is likely notably lower than that. These modeled values therefore overestimate the likely concentrations of arsenicals on orchards, citrus, and vineyards. The EDWCs from application to turf are considered protective relative to these uses.

Table 9. Screening estimate of surface water concentrations ( ppb) of parent compound from application of MMA or DMA to minor crops.

Acute Chronic Cancer MMA1 CA almond 193.0 128.2 80.0 GA pecan 330.6 81.4 44.5 MI cherry 191.4 143.2 102.2 OR grass seed 145.9 102.5 76.4 DMA2 CA citrus 116.3 93.7 76.8 1 Orchard, citrus, and vineyard based on 3 applications of DSMA at 3.7 lb ae/ A. Grass seed based on 1 application of MSMA at 5.3 lb ae/ A. 2 Based on 2 applications of DMA at 4.96 lbs/ A.

The reported EDWCs do not account for the potential impact of irrigation because it is not calculated effectively by the model versions currently available. Irrigation tends to be very important in turf cultivation; golf courses often irrigate at rates of over 100,000 gallons per day ( Swancar, 1996) with some estimates of average irrigation as high as 350,000 gpd ( Ma, 2002). A provisional version of PRZM has been designed to account for the effects of irrigation, although this irrigated model is currently approved only to provide characterization of modeling results and not to quantify EDWCs. This model, run using a version of the Florida turf scenario edited to include irrigation, estimated that surface water concentrations of MMA from irrigated turf can be expected to be 8 to 33% lower than those from non irrigated fields. Using the inputs listed in Table 5, the irrigated model estimates MMA surface water concentrations of 230 ppb ( acute), 102 ppb ( chronic), and 50 ppb ( cancer). Especially on well drained soils like those in Florida golf courses, irrigation can decrease runoff by increasing the potential for leaching to groundwater. 35 Turf EDWCs are also expected to be lower if arsenicals are applied as spot treatments rather than as broadcast sprays, which appears to be a common practice. There is a possibility that this will be included as a restriction on the labels for DSMA and MSMA. Spot treatments are applied to less area than broadcast sprays and so lead to lower exposure. If application rate or area is limited on the label by a quantifiable amount, the estimated concentrations will be reduced by a proportional amount. For example, if treatment of arsenicals on all turf uses is limited to 50% of a field, the EDWCs will go down to one half of the original value.

SOIL ACCUMULATION Accumulation of persistent pesticides in soil is not typically addressed in drinking water assessments. Because of the possible implications for human health, a summary of the issue is presented here. A more detailed discussion of potential buildup of arsenic in soil resulting from pesticide application can be found in the Ecological Risk Assessment ( DP Barcode D309100). The relative immobility of arsenicals along with arsenic's elemental nature make buildup in soil after repeated applications 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.

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 but, since most of the applied arsenic remained in the top 6 inches of soil, repeated application may lead to significant accumulation. Higher application rates, 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, although 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.

Soil modeling based on median sorption and maximum application rates suggest that over the long term, the buildup of total arsenic in the top 10 cm of soil for MMA would level off at chronic concentrations of approximately 13 ppm and 45 ppm on cotton and turf, respectively. The higher application rates allowed for DMA in non crop and some turf uses would be expected to lead to higher soil concentrations, assuming annual application at those rates. Routes of dissipation accounted for in the modeling include runoff, 36 leaching, and soil erosion. The field and monitoring studies support these modeling results as a reasonable representation of potential accumulation.

UNCERTAINTY Monitoring For arsenic, monitoring data serve two separate functions   to establish natural background arsenic levels in surface and groundwater and to provide information about potential impacts from pesticide applications. The uncertainty associated with the monitoring data is different for these different roles.

For most of the data used in discussing natural background arsenic levels, data collection sites are not uniformly distributed and have not been specifically selected to exclude anthropogenic arsenic sources   some sites even represent known point sources of arsenic. The percentage of sites impacted by these sources, however, is quite small relative to the large national USGS datasets, each with more than 30,000 samples. For groundwater, the uncertainty was further reduced by using statistical criteria to limit the data incorporated and therefore minimizing potential bias. This was not done for the surface water dataset which therefore may include some bias towards more frequently sampled sites. The data on individual arsenic species, rather than total arsenic, are not nationally representative, for surface or groundwater. There is uncertainty in extrapolating the results from these sites to a wider assumption regarding background levels.

There is greater uncertainty in the targeted monitoring data. Specific sources of uncertainty are included in the discussion of each individual study. In general, even targeted sampling is likely to underestimate both acute exposures and the frequency of occurrence due to the limited number of samples and the extended time between sampling events. For arsenic, additional uncertainty comes from the multiple possible arsenic sources. In most cases, detailed pesticide application and land use history information are not available. Natural background arsenic levels are not well defined in all areas.

Surface Water Modeling Modeling relies on estimated fate parameters and assumed agricultural practices to predict concentrations of pesticides to which humans may be exposed. There is uncertainty in all fate inputs used in this assessment. The values used as half lives include uncertainty both due to the limited available data and due to the complexity of arsenic's environmental fate. The majority of data are from non GLP studies that were conducted for less than one year, ranging in length from 60 days to 32 weeks. Additionally, in most cases complete time series were not available, so rates were calculated based on initial and final concentrations only. One set of values included in calculations was normalized by the reviewer to standard conditions, based on up to six replications performed under a variety of conditions ( Gao, 1997). All of these factors 37 could have an impact on the calculated half lives. Modeling with infinite half lives provides an upper boundary of the extent to which a modification in half life inputs could change estimated results. Using infinite half lives, acute EDWCs are increased by up to 17% while chronic and cancer EDWCs are increased by approximately one third.

As discussed above, there is also uncertainty associated with the method of estimating concentrations of the major metabolites, DMA and inorganic arsenic. The estimate of metabolism to DMA is based on the assumption that 35% is the maximum amount of MAA that may be present as DMA at any one time. Because this value is based on data that already includes some transformation, it may lead to some underestimation of DMA concentrations. The estimation of concentrations of inorganic arsenic is also uncertain. Maximum potential concentrations of inorganic arsenic are represented by the total arsenic EDWCs, calculated as a molar conversion of the EDWCs of parent compounds. This is a general estimate of the amount of inorganic arsenic that may reach surface water rather than a direct calculation based on specific physical processes. Transformation of applied organic arsenicals to inorganic arsenic is a long term process that occurs primarily through microbial activity in the soil, not a rapid conversion upon reaching water. Because of the nature of these transformation processes and because of inorganic arsenic's tendency to bind more strongly to soil than the organic arsenicals do, the EDWC of the parent compounds represents a reasonable upper bound on the EDWCs of inorganic arsenic. This assumption is supported by empirical evidence from a targeted monitoring study which found that the maximum concentrations of inorganic arsenic found in surface water in areas of heavy application was similar to the total arsenic resulting from parent compounds ( Bednar, 2002).

PRZM/ EXAMS requires information on agricultural practices as inputs, such as specific application dates and rates to be applied. In reality, application dates and rates applied across the United States and even within a single watershed will vary depending on geography, pest pressure, climatic factors, and changes in agricultural cropping patterns. EFED attempts to capture some of this variability by modeling as many representative scenarios as possible and by using meteorological data which covers a time span sufficient to capture climatic variations which are likely to occur. However, the model is limited in its ability to capture all of the natural variation which occurs for any pesticide application and this adds uncertainty to the drinking water assessment. This is of particular importance for the turf scenario, because the application of pesticides to turf is not limited to a specific planting season. It can be applied most of the year, especially in areas like that modeled, where winter dormancy is not expected. In this case, a spring application date was chosen because the primary use of arsenicals on turf is to control crabgrass and goosegrass, pests which emerge in the spring months. Application of arsenicals to turf at other times of year is possible and could lead to higher exposure, especially if applied during periods of heavier rainfall which would increase runoff and leaching. For use on cotton, the application dates within the typical use season that led to the most conservative EDWCs were chosen.

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