Memorandum Amended 8/ 26/ 00 from 01/ 13/ 00 from 10/ 07/ 99 from 8/ 26/ 99 To: Christina Scheltema, Chemical Review Manager Special Review & Reregistration Division 7508W From: EFED Disulfoton Team Henry Craven, Biologist John Jordan, Microbiologist James Wolf, Soil Scientist Mary Frankenberry, Statistician Thru: Arnet Jones, Chief Environmental Risk Branch III Environmental Fate & Effects Division 7507C Subject: Reregistration Eligibility Document for Disulfoton ( D237134)

Attached to this memorandum is the revised EFED RED chapter for disulfoton. EFED has reviewed the public comments and has modified the chapter in response to the comments. This transmittal memo summarizes EFED   s findings and recommendations for potential mitigation, monitoring and labeling.

The risk assessment was performed by evaluating use information listed in both the BEAD LUIS report for disulfoton as well as information supplied by Bayer Corporation, the major registrant for disulfoton products, and current labels ( EPA Reg. No. 3125 172; 3125 307) .

Background Disulfoton is an organophosphate insecticide/ acaricide used on a variety of terrestrial food crops, terrestrial feed crops, and terrestrial nonfood crops. Disulfoton is formulated as 15% granules, 8% emulsifiable systemic, 95% cotton seed treatment, systemic granules ( 1, 2, 5, 10% ) , and 68% concentrate for formulating garden products. Directions regarding application intervals, number of applications and total application per year or crop cycle are not always specified by the label.

1 Environmental Fate Summary Parent disulfoton has low to intermediate potential mobility ( Kocs 386 888) and is neither persistent ( average ( half life) T1/ 2 is 4.8 days) nor volatile. Disulfoton photo degrades within 2.4 days on soil and in water under natural sunlight the T1/ 2 is 4 days. Disulfoton is essentially stable to hydrolysis at 20 E C at pH 5, 7, and 9, but hydrolyzes much more rapidly at 40 E C. Soil applied disulfoton will be degraded rapidly oxidized by chemical reaction and microbial metabolism to its corresponding D. sulfoxide and D. sulfone. Aerobic soil metabolism data indicated that the sulfoxide ( T1/ 2 > 17days) and sulfone ( T1/ 2 > 120 days) degradates of disulfoton are more persistent and mobile then parent disulfoton. In a recently submitted leaching study, nine additional metabolism products were identified, at least three may have human toxicity issues. Field dissipation information also indicates that the degradates may persist longer in the environment, D. sulfoxide has a half life of 8 to 10 weeks and D. sulfone remained fairly stable over a 294 day period. There is insufficient environmental fate information on the degradates to fully characterize their fate and transport. The half life for total disulfoton residues was greater than 170 days. Open literature suggests that D. sulfoxide can be reduced to back to disulfoton. Information is not available to assess the significance of the reduction of D. sulfoxide. Aerobic and anaerobic aquatic metabolism studies which could provide valid model inputs for the degradates disulfoton sulfone and disulfoton sulfoxide have not been submitted. Although the registrant provided the Agency with additional information concerning the fate of disulfoton residues in water under controlled artificial conditions ( MRID 43568501 and LaCorte et al. , 1995) , this information is limited and should not be used for model inputs. Specifically, these studies provide information concerning the combined effects of hydrolysis, photolysis, and metabolism, with photodegradation contributing significantly to the dissipation.

Water Resources Summary The Water Resources Assessment considered the potential of disulfoton and its degradates, D. sulfoxide and D. sulfone, to contaminate ground water, surface water, and drinking water from labeled uses. The assessment included a TIER II ( PRZM/ EXAMS) analysis which estimates environmental concentrations ( EECs) in surface water for disulfoton parent and for total disulfoton residues, TDR ( sum of disulfoton, sulfoxide, and sulfone) , applied at the maximum label rate and number of applications to barley, cotton, potatoes, spring wheat, and tobacco. The OPP standard farm pond was used for ecological exposure assessment and the Index Reservoir and Percent Crop Area ( PCA) were factored into the drinking water assessment. These crops represent major uses and generally reflect the highest use rates and total amounts. The potential for disulfoton parent residues ( and TDR) to contaminate ground water was assessed using the EFED ground water concentration screening model ( SCI GROW) and monitoring data available in EFED   s Pesticides in Ground Water Data Base ( PGWDB) , EPA' s STORET data base, and in the USGS National Water Quality Assessment Program ( NAWQA) . Surface water monitoring data sources available in the USGS NAWQA program and the EPA   s STORET data base were also considered.

Disulfoton is likely to be found in runoff water and sediment from treated and cultivated fields.

2 The fate of disulfoton and its degradates once in surface water and sediments, and the likely concentrations therein, cannot be modeled with a high degree of certainty since data are not available for the aerobic and anaerobic aquatic degradation rates. Estimates of disulfoton concentrations in ground water did not consider the anaerobic soil metabolism, as studies have been submitted by the registrant, but have not reviewed by EFED. The anaerobic soil metabolism rate for disulfoton appears to be slower than the aerobic soil metabolism rate. For this assessment, the aerobic aquatic metabolism rate, required by EXAMS, was estimated by using EFED' s recommended guidance to estimate an aerobic aquatic metabolism rate from aerobic soil metabolism rates ( e. g. , multiply the soil aerobic metabolism rate used in PRZM by 0.5 ( doubles the half life) ) . In lieu of actual data on persistence of disulfoton in an aquatic environment, the assumed aquatic metabolism rate for EXAMS will reduce the estimated concentrations, but not the uncertainty. Considering the relatively rapid rate of microbial degradation in the soil and aquatic photolysis in surface water, parent disulfoton may degrade fairly rapidly, whereas the degradates are more persistent and may not degrade as rapidly in water. As noted above the registrant has submitted additional information suggesting a fairly rapid degradation of disulfoton and D. sulfoxide and D. sulfone in natural water under artificial conditions.

Sorption data ( reflection of mobility, e. g. , Kds) are also not available for the sulfoxide and sulfone degradates ( and other degradates) , were considered to be equal to the parent in the modeling. Typically, however, the D. sulfoxide and D. sulfone degradates are more mobile than the parent. The peak concentrations of parent disulfoton appear capable of being quite high, especially when high, foliar application rates are used and coincide with a rainfall event. Limited monitoring confirms this ( VA, CO) . A large degree of latitude available in the disulfoton labels also allows for wide variation in possible application rates, total amounts of disulfoton applied, application methods, and intervals between applications. Lower application rates would result in lower estimated concentrations ( EECs) . Additionally, considerable uncertainty exists because the percent crop area or PCA value was not known, thus, the default value was applied.

The low concentrations typically reported in available ground water and surface water monitoring data of parent disulfoton tends to confirm fairly rapid degradation and low mobility, but do not preclude potentially high peak values ( few reported high values) . Although no assessment can be made for degradates due to lack of data, limited data suggests that the degradates are more persistent than disulfoton, suggesting their presence in water for a longer period of time than the parent Surface Water Modeling:

In the Tier II PRZM/ EXAMS assessment, the overall estimate of the multiple year mean concentrations of disulfoton in a farm pond over multiple years simulated ranged from 0.21 µ g/ L for two applications at the maximum rate ( 1.00 lb ai/ A) to barley in Virginia to 1.14 µ g/ L for potatoes in Maine with three applications at the maximum application rate ( 1.00 lb ai/ A) . Maximum, or peak, estimated concentrations of 26.75 µ g/ L occurred for one 4.00 lb. ai/ ac application of disulfoton to tobacco. For the other scenarios, the maximum concentrations ranged from 7.14 to 18.46 µ g/ L.

3 The estimated drinking water concentrations using the Index Reservoir ( IR) and PCA ( PCA) concepts for the same scenarios were evaluated. The long term mean of the parent disulfoton concentration in the Index Reservoir and by PCA ranged from 0.23 to 1.31 µ g/ L for cotton and tobacco, respectively. The 1 in 10 year estimated annual mean concentration ranged from 0.43 to 2.77 µ g/ L for cotton and tobacco, respectively. The peak 1 in 10 year estimated drinking water concentration for parent disulfoton ranged from 7.13 to 44.20 µ g/ L.

The Tier II modeling results from PRZM/ EXAMS fall within the range of concentrations for surface water reported in the STORET database ( 0.0 to 100 µ g/ L, 96 percent of 8137 samples were reported as less than 16 µ g/ L) , a Virginia monitoring study ( 0.37 to 6.11 µ g/ L) and NAWQA ( 0.010 to 0.060 µ g/ L) . But because some of the data in STORET have a high degree of uncertainty because many samples were only listed as   actual value is known to less than given value   , the maximum concentration of samples was not always known ( see Appendix III) . The modeled concentration estimates are generally greater than those seen in the monitoring data. The modeling results therefore cannot be confirmed by the monitoring data.

Because the degradates of disulfoton ( including oxygen analogs) : sulfoxide and sulfone are also toxic, the EECs of the total disulfoton residue ( TDR) in a farm pond was also considered. The overall estimated of the multiple year mean concentrations of TDR in a farm pond over multiple years simulated ranged from 3.89 µ g/ L for two applications at the maximum rate ( 1.00 lb ai/ A) to barley in Virginia to 9.32 µ g/ L for tobacco in Georgia with one application at the maximum application rate ( 4.00 lb ai/ A) . Maximum, or peak, estimated TDR concentrations of 58.47 µ g/ L occurred for one 4.00 lb. ai/ ac application of disulfoton to tobacco. For the other scenarios, the maximum TDR concentrations ranged from 15.32 to 52.93 µ g/ L. There are no monitoring data to evaluate these concentration estimates from PRZM/ EXAMS modeling.

Total disulfoton residues using the IR and PC concepts were also considered for drinking water. The long term mean of the total disulfoton residues ( TDR) in the Index Reservoir and by PCA ranged from 2.55 to 10.42 µ g/ L for cotton and potatoes, respectively. The 1 in 10 year estimated annual mean TDR concentrations in the IR ranged from 5.10 to 16.72 µ g/ L for cotton and potatoes, respectively. The peak 1 in 10 year estimated TDR concentrations in the IR ranged from 20.83 to 104.92 µ g/ L. There are no monitoring data to evaluate these concentration estimates from PRZM/ EXAMS modeling.

Uncertainty surrounds these estimates because the sites selected for modeling represent sites though to be representative of vulnerable sites. Additionally, the IR was generic ( to each scenario) and data to fully understand of the fate of disulfoton and disulfoton residues is available. Evidence suggests that the concentrations will not be as high as suggest by the modeled estimates. The PCA values have been estimated by OPP for spring wheat ( 0.56) and cotton ( 0.20) . The default for value for all agricultural land of 0.87 was used for the barley, potatoes, and tobacco scenarios. Better estimates of the PCA for these crops would reduce the uncertainty associated with the estimated drinking water concentrations.

4 Ground Water Modeling:

The maximum disulfoton concentration predicted in ground water by the SCI GROW model ( using the maximum rate 4 lb. a. i. / ac and 2 applications  potatoes) was 0.05 µ g/ L. The maximum total disulfoton residue concentration predicted in ground water by the SCI GROW model for the same scenario is 3.19 µ g/ L. The SCI GROW model represents a " vulnerable site" , but not necessarily the most vulnerable, treated ( here) with the maximum rate and number of disulfoton applications, while assuming conservative environmental properties ( 90 percent upper confidence bound on the mean aerobic soil half life and an average Koc value) . Monitoring data has reported a few disulfoton concentrations higher than those estimated by SCI GROW.

Disulfoton Monitoring Data:

Based upon the fate properties of disulfoton parent, which is not very persistent, or mobile you would not expect to observe disulfoton in ground water. The Pesticides in Ground Water Data Base ( USEPA, 1992) summarizes the results of a number of ground water monitoring studies conducted which included disulfoton ( and rarely the disulfoton degradates D. sulfone and D. sulfoxide) . Monitoring, with no detections ( limits of detections ranged from 0.01 to 6.0 µ g/ L) , has occurred in the following states ( number of wells) : AL ( 10) , CA ( 974) , GA ( 76) , HI ( 5) , IN ( 161) , ME ( 71) , MS ( 120) , MN ( 754) , OK ( 1) , OR ( 70) , and TX ( 188) . The range of detection limits, especially the high ones ( e. g. , 6 µ g/ L) reduce the certainty of these data. Disulfoton residues were detected in ground water in Virginia and Wisconsin. In Virginia, 6 of the 12 wells ( 8 monitoring wells) sampled monthly from June 1986 through December 1990 had disulfoton detections ranging from 0.04 to 2.87 µ g/ L. In Wisconsin, 14 of 26 wells ( municipal, community, and home wells) sampled, during May and June 1982, had disulfoton residues ranging from 4.0 to 100.0 µ g/ L, with a mean of concentration of 38.4 µ g/ L. Although the Wisconsin study has received some criticism, particularly over QA/ QC issues, EFED believes that this study needs to be considered in the risk assessment. The Wisconsin study was conducted in the Central Sand Plain of Wisconsin which is extremely vulnerable to ground water contamination. Detections of other pesticides in this area have often tended to be orders of magnitude greater than those seen other areas. One hundred twenty wells were analyzed in MS for disulfoton degradates sulfone and sulfoxide and 188 wells were analyzed in TX for sulfone. Limits of detection were 3.80 and 1.90 µ g/ L for the sulfone and sulfoxide degrade, respectively, in MS. There were no degradates reported in these samples. In a more recent ground water monitoring study conducted in North Carolina, there were no detections of disulfoton, disulfoton sulfoxide, and disulfoton sulfone. Efforts were made in the study to place the wells in vulnerable areas where the pesticide use was known, so that the pesticide analyzed for would reflect the use history around the well. Limitations of the study include that sites were sampled only twice and the limits of detections were high ( e. g. , > 1.0 µ g/ L) for some of disulfoton analytes ( NCIWG, 1997; DP Barcode 267486) .

Surface water samples were also collected ( same Virginia study as noted above) in study to evaluate the effectiveness of Best Management Practices ( BMP) in a Virginia watershed. Approximately half of the watershed is in agriculture and the other half is forested. Parent 5 disulfoton was detected in several surface water samples with concentrations ranging from 0.037 to 6.11 µ g/ L. These levels are within the same order of magnitude of the estimated environmental concentrations ( EECs) obtained from the PRZM/ EXAMS models for parent disulfoton which range from 0.21 to 1.14 µ g/ L for annual mean daily concentrations and 7.14 to 26.75 µ g/ L for peak daily values.

Disulfoton residues have been detected in surface water at a low frequency in the USGS NAWQA study. The percentage of detections with disulfoton concentrations > 0.01 µ g/ L for all samples, agricultural streams, urban streams were 0.27% , 0.20, and 0.61% , respectively. The corresponding maximum concentrations were 0.060, 0.035, and 0.037 µ g/ L. Disulfoton has not been detected in ground water in the NAWQA study. Although pesticide usage data is collected for the different NAWQA study units, the studies are not targeted, specifically for disulfoton.

Limitations for the monitoring studies include the use of different limits of detection between studies, lack of information concerning disulfoton use around sampling sites, and lack of data concerning the hydro geology of the study sites.

About 50 percent of the well samples reported in STORET had low levels ( < 1 µ g/ L) of disulfoton residues. However, there were indications of some high concentrations ( the other 50% were reported as < 250 µ g/ L) , which may be a reflection of how the data were reported as the disulfoton concentrations in the monitoring were not always known. This is because the detection limit was extremely high or not specified, and/ or the limit of quantification was not stated or extremely high. Disulfoton concentrations were simply given as less than a value. Therefore, considerable uncertainty exists with respect to the STORET monitoring data. The spatial and temporal relationship between disulfoton use, rainfall/ runoff events and the location and time of sampling frequently cannot be adequately determined.

Toxicity Summary The available acute toxicity data on the TGAI indicate that disulfoton is: highly to very highly toxic to birds on an acute oral basis ( LD50 = 3.2 to 39 mg/ kg) ; moderately to highly toxic to birds on a dietary basis ( LC50 = 333 to 622 ppm) ; very highly toxic to mammals on an acute oral basis ( LD50 = 1.9 to 15 mg/ kg) ; moderately toxic to bees ( LD50 = 4.1 µ g/ bee) ; very highly toxic to moderately toxic to freshwater fish ( LC50 = 39 to 7,200 ppb) ; very highly toxic to freshwater invertebrates ( LC50 = 3.9 to 52 ppb) ; highly toxic to marine/ estuarine fish ( LC50 = 520 ppb) and very highly toxic to marine/ estuarine invertebrates ( LC50 or EC50 = 15 to 900 ppb) . Acute toxicity for the sulfone degradate indicate that it is highly toxic to birds on an acute oral basis ( LD50 = 18 mg/ kg) , moderately toxic to birds on a dietary basis ( LC50 = 558 to 622 ppm) , highly toxic to mammals on an acute oral basis ( LD50 = 11.24 mg/ kg) , highly toxic to bees ( LD50 = 0.96 F g/ bee) , highly to moderately toxic to freshwater fish ( LC50 = 112 to > 9,200 ppb) , very highly toxic to freshwater invertebrates ( LC50 = 35.2 ppb) , and moderately toxic to marine/ estuarine fish ( LC50 = 1,060 ppb) . The sulfoxide metabolite is very highly toxic to birds on an acute oral basis ( LD50 = 9.2 mg/ kg) ; moderately to highly toxic to birds on a dietary basis ( LC50 = 456 to 823 ppm) ; moderately toxic 6 to bees ( LD50 = 1.11 µ g/ bee) ; highly to slightly toxic to freshwater fish ( LC50 = 188 to 60,300 ppb) ; very highly toxic to freshwater invertebrates ( LC50 = 64 ppb) ; and slightly toxic to marine/ estuarine fish ( LC50 = 11,300 ppb) .

Chronic toxicity studies on disulfoton established the following NOAEC values: 37 ppm for birds, 0.8 ppm for small mammals, 220 ppb for freshwater fish ( 4.6 ppb for bluegill sunfish, using the factor of chronic to acute values for the rainbow trout) , 0.037 ppb for freshwater invertebrates, 16.2 ppb for marine/ estuarine fish early life stage, 0.96 ppb for marine/ estuarine fish for life cycle, and 2.35 ppb for marine/ estuarine invertebrates. There are chronic invertebrate studies on the 2 major degradates  sulfone ( NOAEC 0.14 ppb) and sulfoxide ( NOAEC 1.53 ppb) .

Risk Assessment Summary Risk Characterization A. Characterization of the Fate and Transport of Disulfoton I. Water Exposure ( a) Surface Water Disulfoton is likely to be found in runoff water and sediment from treated and cultivated fields. However, the fate of disulfoton and its degradates once in surface water and sediments, and the likely concentrations therein, cannot be modeled with a high degree of certainty since data are not available for the aerobic and anaerobic aquatic degradation rates. Surface water concentrations of disulfoton and total disulfoton residues were estimated by using PRZM3 and EXAMS models using several different scenarios ( barley, cotton, potato, tobacco, and spring wheat) . The large degree of latitude available in the disulfoton labels also allows for a wide range of possible application rates, total amounts, application methods, and intervals between applications. Considering the relatively rapid rate of microbial degradation in the soil ( < 20 day aerobic soil metabolism half life) and direct aquatic photolysis, disulfoton parent may degrade fairly rapidly in surface water. However, peak concentrations of disulfoton in the farm pond appear capable of being quite high, with 1 year in 10 peak surface water concentrations of 7.14 to 26.75 F g/ L and 90 day concentrations of 1.73 to 6.87 µ g/ L for the parent compound. The mean EECs of the annual means of disulfoton ranged from 0.21 to 1.14 µ g/ L. Although there is a lack of some environmental fate data for the degradates, the assessment suggests that the degradates will reach higher concentrations than the parent because they are more persistent and probably more mobile. The estimated peak concentrations for the total disulfoton residues in the farm pond ranged from 15.43 to 58.48 µ g/ L, 90 day average ranged from 12.20 to 35.30 µ g/ L, and the mean of the annual means ranged from 3.89 to 9.32 µ g/ L. Water samples collected at the site of a fish kill in Colorado contained D. sulfoxide at levels of 29.5 48.7 µ g/ L, and D. sulfone at 0.0199 0.214 µ g/ L. The aerobic soil metabolism studies show that the maximum sulfoxide residues are about 58 percent of total radioactive material, thus, the sulfoxide concentrations suggest that parent 7 disulfoton concentrations could range from 50.8 to 83.9 µ g/ L. The ratio of the disulfoton sulfoxide concentration to the average maximum disulfoton concentration was higher ( 74% ) in the microcosm study ( MRID # 4356501) than in the soil residues ( 58% ) .

The estimated drinking water concentrations ( EDWC) for parent disulfoton and total disulfoton residues were also determined using the IR and PCA concepts. The peak concentrations of disulfoton in IR appear capable of being quite high, with 1 year in 10 peak surface water concentrations of 7.13 to 44.20 F g/ L and annual mean concentrations of 0.43 to 2.77 µ g/ L for the parent compound. The mean EECs of the annual means of disulfoton ranged from 0.23 to 1.31 µ g/ L. Although there is a lack of some environmental fate data for the degradates, the assessment suggests that the degradates will reach higher concentrations than the parent because they are more persistent and probably more mobile. The estimated 1 in 10 year peak concentrations for the total disulfoton residues in the IR ranged from 20.83 to 104.92 µ g/ L and annual mean ranged from 5.10 to 16.25 µ g/ L, and the mean of the annual means ranged from 2.55 to 10.42 µ g/ L. These values will be highly effected by the value selected for PCA. The PCA values have been estimated by OPP for spring wheat ( 0.56) and cotton ( 0.20) . The default for value for all agricultural land of 0.87 was used for the barley, potatoes, and tobacco scenarios. Better estimates of the PCA for these crops would reduce the uncertainty associated with the estimated drinking water concentrations.

Surface water samples were collected in a study to evaluate the effectiveness of Best Management Practices ( BMP) in a Virginia watershed. Approximately half of the watershed is in agriculture and the other half is forested. The detections of parent disulfoton in surface water samples ranged from 0.037 to 6.11 µ g/ L and fell within an order of magnitude with the estimated environmental concentrations ( EECs) obtained from the PRZM/ EXAMS models.

Surface water monitoring by the USGS in the NAWQA ( USGS, 1998) project found relatively few detections of disulfoton in surface water with a maximum concentration of 0.060 µ g/ L. As noted above disulfoton degradates were reported in surface water, when a rainfall event occurred following application to wheat, where fish kills occurred; pesticide residue concentrations ranged from 29.5 to 48.7 µ g/ L for D. sulfoxide and 0.02 to 0.214 µ g/ L ( Incident Report No. I001167 001) .

A search of the EPA   s STORET ( 10/ 16/ 97) data base resulted in the identification of disulfoton residues at a number of locations. Often the values ranged from 0.01 to 100.0 F g/ L with most of the values reported as   actual value is less than this value.   Thus, , when a value of 100.00 µ g/ L is reported, it is not known how much less than 100.0 F g/ A the actual value is known to be less. Thus there is considerable uncertainty surrounding some of the data in STORET.

( b) Ground Water The SCI GROW ( Screening Concentration in Ground Water) screening model developed in EFED was used to estimate disulfoton concentrations in ground water ( Barrett, 1997) . SCI  GROW represents a " vulnerable site" , but not necessarily the most vulnerable conditions, treated 8 ( here) with the maximum rate and number of disulfoton applications, while assuming conservative environmental properties ( 90 percent upper confidence bound on the mean aerobic soil half life of 6.12 days and an average Koc value of 551 mL/ g) . The maximum disulfoton concentration predicted in ground water by the SCI GROW model ( using the maximum rate 4 lb. a. i. / ac and 2 applications  potatoes) was 0.05 µ g/ L. The maximum total disulfoton residue concentration predicted in ground water by the SCI GROW model for the same scenario is 3.19 µ g/ L ( except 90 percent upper bound on mean half life of total residues is 259.6 days) .

Ground water monitoring data generally confirms fairly rapid degradation and low mobility, because of the relatively low levels and frequency of detections of parent disulfoton in ground water. There were no ground water detections of parent disulfoton in the USGS NAWQA ( USGS, 1998) with a limit of detections of 0.01 or 0.05 µ g/ L, depending upon method. . Most of the studies recorded in the PGWDB ( USEPA, 1992) also reported no disulfoton detections. Disulfoton residues ranging from 0.04 to 100.00 µ g/ L were reported for studies conducted in Virginia ( 0.04 to 2.87 µ g/ L) and Wisconsin ( 4.00 to 100.00 µ g/ L) . Of specific interest are areas where the concentrations of parent disulfoton reported in the studies ( VA and WI) exceeded the estimate of 0.05 µ g/ L obtained from EFED' s SCI GROW ( ground water screening model) model. It should be noted that the Wisconsin data received some criticism which influences the certainty of these detections, no such criticisms or limitations exist for the Virginia study.

The major issues, concerning the Wisconsin study ( Central Sands) were that the study may not have followed QA/ QC on sampling and the failure of follow up sampling to detect disulfoton residues in ground water as suggested by Holden ( 1986) , have been considered by EFED in the ground water quality assessment. The Central Sands of Wisconsin are known to be highly vulnerable to ground water contamination. There are regions within the United States that have conditions that are highly vulnerable to ground water contamination and regularly have pesticides detected in ground water which far exceeds values seen elsewhere. Several of these areas are well documented, e. g. , Long Island, Suffolk County, NY and Central Sands in WI. Although, some questions have been levied against the disulfoton detections in Wisconsin, the occurrence of disulfoton at the levels reported cannot be ruled out.

There were no detections of disulfoton, disulfoton sulfoxide, and disulfoton in the ground water monitoring study conducted in North Carolina. Efforts were made to place the wells in vulnerable areas where the pesticide use was known, so that the pesticide analyzed for would reflect the use history around the well. Seven Christmas tree, one wheat, and two tobacco growing areas were sampled for disulfoton. Limitations of the study include that sites were sampled only twice and the limits of detections were high ( e. g. , > 1.0 µ g/ L) for some of disulfoton analytes. Uncertainties associated with the study include whether two samples from eight wells are adequate to represent the ground water concentrations of disulfoton residues, did DRASTIC correctly identify a site' s vulnerability, and were the wells placed down gradient of the use areas.

The SCI GROW model represents a " vulnerable site" , but not necessarily the most vulnerable.

9 Several things should be considered. First, the Virginia and Wisconsin monitoring studies were probably conducted in areas vulnerable to ground water contamination. The level of certainty with respect to vulnerability is probably greater for Wisconsin ( relatively less uncertainty) than for Virginia ( relatively more uncertainty ) . The occurrence of preferential flow and transport processes has been also noted in Wisconsin ( and is also possible in Virginia) and may ( speculation) have contributed to the " high" concentrations ( especially in WI) when the initial sampling occurred, but not necessarily in the follow up sampling) . The knowledge concerning the disulfoton use in areas in association with the wells is not well known ( high uncertainty) . Some notable limitations of modeling and monitoring are presented elsewhere in this document ( c) Drinking Water The Agency recommends that the 1 out of 10 year peak values be used the acute surface drinking water level for parent disulfoton, and for chronic levels use either the 90 day and annual average. The maximum values are: 44.20, 2.77, and 1.31 µ g/ L or the peak, 90 day mean, and long term mean, respectively. For the total disulfoton residues the peak, 90 day mean, and long term mean are 104.92, 53.47, and 10.42 µ g/ L. The EDWCs for both parent disulfoton and TDR exceed the DWLOC values estimated by the Agency. The EDWCs values for the parent disulfoton have less uncertainty than the total residue, because there is more certainty surrounding the " estimated" aerobic aquatic metabolism half life for the estimated aerobic aquatic half life for the total disulfoton residues. It is recommended that the Virginia data be considered in the " quantitative" drinking water assessment for ground water exposure. The Wisconsin data should be noted and addressed more qualitatively. Highly vulnerable areas, such as the Central Sand Plain, do not represent the entire use area and can probably be better mitigated or managed a local or state level. Specifically, it is recommended that the 2.87 µ g/ L be used for acute and chronic exposure from ground water. Based upon the fate properties of disulfoton, the sulfoxide and sulfone degradates ( more persistent and probably more mobile) have a greater probability of being found in ground water. It is likely that a ground water study ( ies) may be required to better assess the potential exposure from the degradates ( and also parent) .

B. Characterization of risk to nontarget species from Disulfoton Birds: Acute risk to birds is predicted especially for use patterns involving the 15 G formulation. All modeled application rates and methods for the 15 G formulation exceed the acute risk level of concern for birds, regardless of size. Robins were reported to have been killed following the application of a disulfoton granular product to a tree nursery. Carcasses were found during terrestrial field testing of disulfoton on potatoes, confirming the presumption of acute risk to birds. Since disulfoton is a systemic pesticide, the granular formulations can result in exposure through food items due to uptake by the plant tissues in addition to direct exposure to any unincorporated granules.

Foliar applications of liquid formulations present the greatest risk to herbivorous birds. Based on 10 the results of field studies, the residue levels on sampled invertebrates are well below those predicted by EFED' s models, consequently insectivores did not appear to be at risk. However, there is field evidence suggesting that some species are extremely sensitive to disulfoton such that even low concentrations caused mortality. The Swainson   s hawk kill appears to be the result of consuming grasshoppers. The hawks crop contents were analyzed and contained residues around 8 ppm. Finally, live blue jays collected 6 to 7 hrs after a pecan orchard was sprayed at 0.72 lbs ai/ A had brain cholinesterase inhibition from 32 to 72% ( White et al. 1990) . Although it is unknown whether these birds would eventually die, Ludke et al. 1975 suggest that inhibition > 50% in carcasses is evidence that death was caused by some chemical agent. Furthermore, it should be recognized that these birds were not only feeding on contaminated food, but also were impacted by dermal and inhalation exposure.

Ground applications of liquid formulations to soil, even at 4.0 lb ai/ A would not be expected to cause mortality to birds. Field studies have demonstrated that residue concentration within food items   vegetation, invertebrates and seeds   in or on the edge of fields are well below those used in screening level assessments and empirically derived from aerial applications. However, in light of the points made in the previous paragraph, some mortality is possible given the possible multiple routes of exposure and hypersensitivity of some species.

Chronic risk to herbivorous birds are predicted from exposure to disulfoton when assuming birds are exposed to peak residues for a short period of time or average Fletcher maximum residues for longer periods. Based on reduced hatchling weight, the NOAEC is 37; both for bobwhite quail and mallard duck. Foliar applications and aerially applied soil sprays are estimated to result in 30 day average residues ( based on maximum Fletcher values) on vegetation exceeding the avian chronic level of concern for application rates equal or greater than a single application of 1 lb ai/ A. A residue monitoring study for Di syston 8E in potatoes showed the peak residues on vegetation was 105 ppm after the initial application and 152 ppm following a second application 6 to 10 days later. In the same study, the means of the 3 applications for vegetation in and adjacent to fields were 41 and 14 ppm respectively. The upper bound 95% mean for the vegetation adjacent to the fields was 71 ppm. Therefore even empirically derived residues suggest that the chronic LOC is exceeded on foliage, but not invertebrates for a short time following aerial applications. It is anticipated that since the sulfone and sulfoxide degradates of disulfoton were similar in acute toxicity to parent disulfoton they would have similar chronic NOAECs. These degradates extend the time that total disulfoton residues are available for consumption. Since many of the applications of disulfoton occur in the spring, overlapping the breeding season for most bird species, there is the potential for significant reproductive impacts.

Mammals: Acute risk to mammals is expected for use patterns involving the 15 G formulation. All modeled application rates and methods exceed the acute risk level of concern for mammals, regardless of the mammals   size. . Small mammal carcasses were found during terrestrial field testing of disulfoton on potatoes, confirming the presumption of acute risk to mammals. Since disulfoton is a systemic pesticide, the granular formulations can result in exposure through food items due to uptake by the plant tissues in addition to direct exposure to any unincorporated granules.

11 Applications of the liquid formulations especially by air can result in mammals being exposed to multiple routes of exposure   dermal, inhalation, drinking contaminated water as well as ingestion of contaminated food items. The persistent sulfone and sulfoxide degradates are also toxic to mammals, thereby increasing the potential risk from the application of disulfoton. The registrant has suggested that mammals as well as birds can consume an equivalent of 2 to 3 LD50' s as part of their diet and not be adversely effected. Although this may be true for a population of laboratory test animals, individuals will vary in their sensitivity and can die as a result of inability to avoid predation, secure prey or thermoregulate. Numerous pen studies were conducted with cottontail and jack rabbits exposed to single applications ranging from 1 to 25 lbs ai/ A. While no mortality occurred to cottontails, at the 2 lb ai/ A rate and above jackrabbits suffered 100% mortality. Secondary poisoning did not occur when the jackrabbit carcasses were fed to a number of avian and mammalian carnivores. The apparent difference between the pen study results and the acute mortality predicted in the risk assessment screen is largely due to the possibility that the calculated 1 day LC50s ( ranging from 2 to 12.7 ppm) discounts the rapid metabolism of disulfoton. However, using the demeton LC50 of 320 ppm with its wide ranging confidence interval ( 0 to infinity) also adds uncertainty to the question of disulfoton   s acute risk to mammals.

Chronic risk to mammals is predicted. As was previously discussed in the above acute and chronic sections for birds, there are several reasons why small mammals are likely to be at even greater risk, not the least of which is the extremely low NOAEC of 0.8 ppm. All modeled and empirically derived residues for all sites exceed the chronic risk level of concern for mammals. Finally, the persistence of the sulfone and sulfoxide degradates, which are also toxic to mammals, increases the likelihood of chronic risk to mammals.

Non target Insects: Disulfoton and its sulfoxide and sulfone degradates are moderately to highly toxic to bees, however a residual study with honey bees indicated no toxicity for applications up to 1 lb ai/ A.

Freshwater Fish: Most of the modeled use patterns did not exceed the acute risk levels of concern for freshwater fish. Only the two soil applications at 4.0 lb ai \ A of the liquid formulation exceeded acute risk. All other scenarios exceeded the restricted use and endangered species levels of concern. There is, however, a large amount of variation in freshwater fish species   sensitivity to disulfoton, , as evidenced in the toxicity data table. The microcosm study included bluegill sunfish. Following the last application of 30 ppb, 10% of the fish died. Several kills of freshwater fish have occurred from applications of disulfoton to different crops  both as registered uses as well as from misuse.

Chronic risk to freshwater fish may occur from uses where single application rates are equal to 4 lb ai/ a and from 3 applications of 1 lb ai/ A. . The single freshwater fish species ( rainbow trout) , for which chronic toxicity data was available, demonstrates significantly less sensitivity to disulfoton than several other species ( bluegill sunfish, bass, guppy) . Therefore, an estimated chronic NOEC value was calculated using the chronic to acute ratio for the rainbow trout, as described earlier. Based on the estimated chronic NOAEC for bluegill, chronic effects would 12 occur from the present uses on tobacco, foliar treatments of potatoes and repeated soil treatments of cotton. Christmas tree plantations were not modeled, however the high application rate ( possibly 47 lbs ai/ A) and sloped land may be a potentially risky site.

Freshwater Invertebrates: All modeled crop scenarios exceeded the acute risk level of concern, but the highest risk quotients were less than 10. Again, the risk is further increased due to the toxicity and persistence of the degradates of disulfoton. Microcosm study results indicated that there was recovery of most phyla examined at 3 ppb and long term impacts for most phyla at 30 ppb. Therefore 10 ppb is probably a concentration where short term effects will occur, but recovery can be anticipated.

Chronic risk to freshwater invertebrates is predicted to from the use of disulfoton. All of the modeled crop scenarios greatly exceeded the level of concern, sometimes by a factor of several hundred. Invertebrate life cycle testing with disulfoton shows that it impacts reproductive parameters ( number of young produced by adults) in addition to survival and growth. The 21 day average EECs for the modeled sites ranged from 4.3 to 17.9 ppb. For the most part these EECs are within the range where recovery was occurring in the microcosm. However there is uncertainty as to how much more reliable the microcosm may be as a predictor of safety.

Estuarine and Marine Fish: Although acute and restricted risk levels of concern were not exceeded for estuarine and marine fish, the endangered species level of concern was exceeded for several of the modeled crop scenarios ( cotton, potatoes and wheat) . As was note among the freshwater fish, there can be substantial species differences in sensitivity to disulfoton. Therefore, it is possible that the single marine/ estuarine fish species tested ( Sheepshead minnow) does not fully represent the true range of sensitivity found in a marine or estuarine ecosystem, and this assessment may therefore underestimate the true risk to marine/ estuarine fish. There is also some uncertainty in using the PRZM/ EXAMS EECs derived for ponds to predict exposure to marine/ estuarine organisms. The scenarios modeled are based on hydrologic data for freshwater habitats. The exposure in a marine or estuarine habitat may be higher or lower than that predicted for a freshwater habitat, resulting in higher or lower risk to marine/ estuarine organisms.

Chronic risk to estuarine and marine fish is predicted from the use of disulfoton. Both early life stage and full life cycle testing demonstrated a variety of effects at low levels of disulfoton. Risk quotients based on the early life stage toxicity endpoint exceeded the level of concern for cotton, potatoes and tobacco. The highest risk quotients were based on numerous life cycle toxicity endpoints   fecundity, hatching success and growth; consequently the chronic level of concern was exceeded for all modeled scenarios. Estuarine fish spawning in the upper reaches of tributaries of bays would be a greatest risk. However the likelihood of this risk is uncertain for several reasons: 1) the required time the adults must be exposed to disulfoton in order for their reproductive systems to be effected and 2) the residency time of disulfoton residues in tidal or flowing water. Even if adults are effected after an exposure of only a week, disulfoton may be moved out of an area within several days.

Estuarine and Marine Invertebrates: Three of the five modeled scenarios ( cotton, potatoes, and tobacco) resulted in exceedences of the estuarine/ marine invertebrate acute risk level of concern. All the remaining uses exceeded the restricted use level of concern. Similar uncertainty 13 exists as to the validity of the exposure scenario for invertebrates as was just described for estuarine fish.

Chronic risk to marine/ estuarine invertebrates is predicted. All of the modeled crop scenarios exceeded the chronic level of concern. The much shorter life cycle of invertebrates as compared to fish, increases the likelihood that only a brief exposure ( a few day or even hours) of adults to disulfoton concentrations around the NOAEC is sufficient to negatively impact reproduction. The degree to which the freshwater microcosm is a predictor of safety for the estuarine invertebrates in highly uncertain. Only the mysid shrimp has been tested and it was acutely and chronically less sensitive than freshwater Daphnia. Therefore, on the basis of this limited data, the chronic impact to estuarine invertebrates not only appears to be lower than for freshwater invertebrates, but is likely to be low.

Nontarget Plants: Currently, terrestrial and aquatic plant testing is not required for pesticides other than herbicides except on a case by case basis. Nontarget plant testing was not required for disulfoton, so the risk to plants could not be assessed at this time. There are phytotoxicity statements on the label, however, as well as some incident reports of possible plant damage from the use of disulfoton, so there is the potential for risk to nontarget plants.

Summary of Risk Assessment of North Carolina 24c for use in Christmas Tree Farms Christmas tree farms and the adjacent areas   forests and/ or pasture   provide excellent habitat for a great variety of wild life. The use of granular disulfoton suggests that there is acute risk to small birds and mammals. The North Carolina Christmas Tree community has submitted numerous testimonials emphasizing the ever increasing numbers and diversity of wild life . This includes game animals such as turkey rearing young amidst the Christmas trees, song birds, rodents and foxes. Although this information is intended to suggest there is little or no negative impact from not only disulfoton, but other pesticides or cultural practices as well, the Agency would prefer to receive documented surveys or research before making a final determination.

There were no detections of disulfoton or its metabolites in the ground water monitoring study conducted in North Carolina by the North Carolina Departments of Agriculture and Environment, Health, and Natural Resources. Seven Christmas tree, one wheat, and two tobacco growing areas were sampled for disulfoton residues. Limitations of the study include that sites were sampled only twice and the limits of detections were high ( e. g. , > 1.0 µ g/ L) for some of disulfoton analytes. Uncertainties associated with the study include whether two samples from eight wells are adequate to represent the ground water concentrations of disulfoton residues, did DRASTIC correctly identify a site' s vulnerability, and were the wells placed down gradient of the use areas.

The use of Disulfoton 15 G in Christmas tree farms at this time cannot be modeled for potential surface water contamination. EFED assumes the estimated concentration for the North Carolina 24 ( c) use pattern   2.75 lbs ai/ A unincorporated   may be similar to the values for the single 4.0 lb ai/ A incorporated application of granular disulfoton to tobacco. Based on this assumption there is acute risk to aquatic invertebrates and chronic risk to freshwater fish and aquatic invertebrates.

14 The North Carolina Christmas tree industry submitted two surveys of streams in the Westerns region. The surveys followed a protocol for looking at macro invertebrates to assess the impact of agricultural practices associated with Christmas tree farming. In summary, the two surveys suggests that when conservation measures associated with Christmas tree farming in the Western counties of North Carolina are implemented, there may be only slight, short term impact to aquatic macro invertebrates from disulfoton use. Aquatic macro invertebrates appear to have the capacity to recover from any impact that could be caused by disulfoton use on Christmas trees in Western North Carolina.

C. Mitigation The use of disulfoton at single application rates of 1.0 lb ai/ A and greater, and multiple application rates of 0.5 lb ai/ A and greater, poses an acute risk to birds, mammals, fish, and aquatic invertebrates, as well as to nontarget insects. EFED believes that amending label rates to the lowest efficacious rate as a maximum, as well as restricting the number of applications per year and lengthening the application interval, would reduce acute risk to terrestrial and aquatic organisms. Requiring in furrow applications wherever feasible, and eliminating banded applications of granular disulfoton with narrow row spacing, would also reduce the risk to nontarget organisms, especially birds and mammals. Eliminating aerial applications of disulfoton and imposing buffer strips around aquatic habitats would reduce the risk to aquatic organisms. Risk to bees and other nontarget insects could be lowered by not applying disulfoton when the insects are likely to be visiting the area.

Qualitative comparative ecological risk assessment between present and proposed disulfoton uses.

Bayer has proposed the following changes to some use patterns assessed by the Agency that would reduce the ecological risk from Di syston 8E:

* cancel aerial applications to cotton and wheat. * cancel foliar applications to cotton.

The table reflects additional changes proposed by Bayer.

15 Table 1 Comparison of present and proposed changes in 4 use patterns of Di syston 8E Present Use Proposed Use Rate / Number of Applications / Interval / Incorp. Depth/ method 1 Rate/ Number of Applications / Interval/ Incorp. Depth/ method 1 lb. ai/ A / # app. / days / inches lb. ai/ A / # app. / days / inches cotton 1.0/ 3/ 21/ 0/ gs cotton 1.0/ 1/  / 0/ gs potatoes 4.0/ 2/ 14/ 2.5/ gs potatoes 3.0/ 1/  / 2.5/ gs potatoes 1.0/ 3/ 14/ 0/ af potatoes 0.5/ 3/ 14 / 0/ af wheat 0.75/ 2/ 30/ 0/ gs wheat 0.75/ 1/  / 0/ gs 1 Method of application: f = foliar and s = soil; gs = ground spray, af = aerial spray foliar Risk to Birds and Mammals Canceling aerial application to wheat and cotton reduces significantly the potential for exposing edge of field food items and vegetation. Canceling foliar applications to cotton reduces the opportunity for exposure, by reducing the food items that are directly sprayed. As the discussion below explains, field monitoring indicates that ground spray to soil reduces substantially the residues on food items from those residues predicted from the nomograph.

Potato aerial foliar at 0.5 lb ai/ acre Biological field testing ( MRID 41359101) suggests that significant acute risk to mammals from foliar sprays is unlikely at a single application of 1 lb ai/ acre or lower. Reducing the potato rate from 1 lb ai/ acre 3 times, to 0.5 lb ai/ acre 3 times, substantially lowers the acute risk to mammals.

Wheat, potato and cotton ground spray to soil Field residue monitoring ( MRID 41118901) indicates that residues on food items following ground applications to soil are significantly lower than would be expected from direct application to vegetation. Peak residues following the first of two treatments at 3 lb ai/ acre ( in furrow) ranged from 0.9 ppm ( invertebrates and edge of field vegetation) , to 26 ppm ( potato foliage) . The second treatment at 3 lb ai/ acre side dressing ( 6 7 weeks later) resulted in peak residues of 1.8 ( invertebrates) , 44 ppm potato foliage, and 54 ppm ( edge of field vegetation) . The residues from these applications are not only lower than those estimated using the nomograph, but also lower than the field residues resulting from foliar applications. In the foliar residue monitoring study ( 3 aerial applications at 1.0 lb ai/ acre) the peaks were: invertebrates ( 16 ppm) and vegetation ( 154 ppm) . The proposed changes would greatly reduce exposure terrestrial species.

16 Table 2 Comparison of potential acute and chronic risk resulting from proposed changes in 4 use patterns of Di  syston 8E for birds and mammals Present Use Birds Mammals Proposed Use Birds Mammals Rate / Number of Apps / Interval / Incorp. Depth/ method 1 ac ch ac ch Rate / Number of Apps / Interval / Incorp. Depth/ method 1 ac ch ac ch lb. ai/ A / # app. / days / inches lb. ai/ A / # app. / days / inches cotton 1.0/ 3/ 14/ 0/ gs E Y R Y cotton 1.0/ 1/  / 0/ gs no Y E Y potatoes 4.0 / 2/ 14/ 2.5/ gs R Y A Y potatoes 3.0/ 1/  / 2.5/ gs E Y R Y potatoes 1.0/ 3/ 14/ 0/ af R Y A Y potatoes 0.5/ 3/ 14 / 0/ af R Y R Y wheat 0.75/ 2/ 30/ 0/ gs E Y R Y wheat 0.75/ 1/  / 0/ gs no Y E Y 1 Method of application: f = foliar and s = soil; g = ground and a = aerial Acute = ac; Chronic = ch Acute risk LOC is exceeded= A; Restricted use LOC is exceeded= R; Endangered Species LOC is exceeded= E; No acute LOC is exceeded= no; LOC for chronic risk is exceeded= Y; LOC for chronic risk is not exceeded= N.

Risk to fish and aquatic invertebrates The following table summarizes the results of modeling the proposed new uses. The EECs were reduced from the present registered use patterns:

17 Table 3 Tier II Upper Tenth Percentile EECs for Disulfoton Parent based on proposed new maximum label rates and management scenarios for cotton, potatoes, and spring wheat in farm pond. Estimated using PRZM3/ EXAMS.

Crop Disulfoton Application Concentration ( µ g/ L)

( 1 in 10 annual yearly maximum value) Mean of Annual Means ( µ g/ L)

Rate / Number of Apps / Interval / Incorp.

Depth/ method 1 lb. ai/ A / # / days / inches Peak 96 Hour Avg. 21 Day Avg. 60 Day Avg. 90 Day Avg. Annual Avg.

Cotton 1. 00/ 1/  / 0/ gs 10.31 9.38 6.83 3.54 2.42 0.62 0.23 Potatoes 3.00/ 1/  / 2.5/ gs 2.42 2.18 1.67 0.84 0.57 0.15 0.12 Potatoes 0.5/ 1/  / 0/ af 7.51 6.62 5.20 3.45 2.42 0.62 0.57 Spr. Wheat 0.75/ 1/  / 0/ gs 1.02 0.91 0.67 0.41 0.28 0.08 0.05 1 Method of application: f = foliar and s = soil; g = ground and a = aerial 18 The following tables reflect a qualitative comparative risk assessment for aquatic and estuarine organisms.

Table 4 Comparison of potential acute and chronic risk resulting from proposed changes in 4 use patterns of Di  syston 8E for freshwater fish and invertebrates Present Use Fish Invertebrates Proposed Use Fish Invertebrates Rate / Number of Apps / Interval / Incorp. Depth/ method 1 ac ch ac ch Rate / Number of Apps / Interval / Incorp. Depth/ method 1 ac ch ac ch lb. ai/ A / # / days / inches lb. ai/ A / # / days / inches cotton 1.0/ 3/ 14/ 0/ gs R Y A Y cotton 1.0/ 1/  / 0/ gs R N A Y potatoes 4.0/ 2/ 14/ 2.5/ gs R Y A Y potatoes 3.0/ 1/  / 2.5/ gs E N A Y potatoes 1.0/ 3/ 14/ 0/ af R Y A Y potatoes 0.5/ 3/ 14 / 0/ af R N A Y wheat 0.75/ 2/ 30/ 0/ gs R N A Y wheat 0.75/ 1/  / 0/ gs no N R Y 1 Method of application: f = foliar and s = soil; g = ground and a = aerial Acute = ac; Chronic = ch Acute risk LOC is exceeded= A; Restricted use LOC is exceeded= R; Endangered Species LOC is exceeded= E; No acute LOC is exceeded= no; LOC for chronic risk is exceeded= Y; LOC for chronic risk is not exceeded= N.

Table 5 Comparison of potential acute and chronic risk resulting from proposed changes in 4 use patterns of Di  syston 8E for estuarine fish and invertebrates Present Use Fish Invertebrates Proposed Use Fish Invertebrates Rate / Number of Apps / Interval / Incorp. Depth/ method 1 ac ch ac ch Rate / Number of Apps / Interval / Incorp. Depth/ method 1 ac ch ac ch lb. ai/ A / # / days / inches lb. ai/ A / # / days / inches cotton 1.0/ 3/ 14/ 0/ gs no Y A Y cotton 1.0/ 1/  / 0/ gs no Y A Y potatoes 4.0/ 2/ 14/ 2.5/ gs no Y R Y potatoes 3.0/ 1/  / 2.5/ gs no N R N potatoes 1.0/ 3/ 14/ 0/ af no Y A Y potatoes 0.5/ 3/ 14 / 0/ af no Y A Y wheat 0.75/ 2/ 30/ 0/ gs no Y A Y wheat 0.75/ 1/  / 0/ gs no N E N 1 Method of application: f = foliar and s = soil; g = ground and a = aerial Acute = ac; Chronic = ch Acute risk LOC is exceeded= A; Restricted use LOC is exceeded= R; Endangered Species LOC is exceeded= E; No acute LOC is exceeded= no; LOC for chronic risk is exceeded= Y; LOC for chronic risk is not exceeded= N.

19 Summary EFED supports the proposed use modifications, and concurs that generally they reduce risk to nontarget organisms to varying degrees. Although there remains the concern for hypersensitive birds and mammals, the acute risk to most birds and mammals is reduced substantially. The greatest risk reduction to fish and aquatic invertebrate are soil applications to potatoes and wheat. There appears to be little changes in acute risk to aquatic organisms from the proposed modifications to cotton and potatoes ( aerial application) . Chronic risk to terrestrial and aquatic organisms are likely to be reduced; but with less certainty, because the duration of exposure required to produce adverse chronic effects in the field are not available.

Data Gaps:

The following environmental fate requirements are not satisfied for disulfoton, D. sulfoxide, and D. sulfone: 162 3: Anaerobic Aquatic Metabolism 162 4: Aerobic Aquatic Metabolism 163 1: Mobility  Leaching and adsorption/ desorption for D. sulfone and D. sulfoxide.

Additionally, there is limited environmental fate data available for the sulfone and sulfoxide degradates. Data on the fate of these degradates in soil and water would allow additional characterization of the risks they present to nontarget organisms.

The following ecological effects data requirements are not satisfied for disulfoton: 122 1: Tier I Terrestrial Plant Testing 122 2: Tier I Aquatic Plant Testing ( 123 1 and 123 2, Tier II testing, are reserved pending the results of Tier I testing) . 71 3 Wild mammal testing subacute dietary ( LC50) .

The value added for the wild mammal test is high. This study could resolve the issue between the calculated 1 day LC50 ( ranging from 2 12 ppm) derived from the acute rat acute oral of 1.9 mg/ kg and the demeton LC50 study ( 320 ppm) with 95% C. I. ( 0 to infinity) . The risk assessment for mammals would be refined with greater certainty.

Manufacturing Use Products   This pesticide is extremely toxic to birds, mammals, fish and aquatic invertebrates. Do not discharge effluent containing this product into lakes, streams, ponds, estuaries, oceans, or public waters unless this product is specifically identified and addressed in an NPDES permit. do not discharge effluent containing this product to sewer systems without previously notifying the sewage treatment plant authority. For guidance, contact your State Water Board or Regional Office of the EPA.   20 End use Products Non granular products:   This pesticide is extremely toxic to birds, mammals, fish and aquatic invertebrates. Do not apply directly to water, or to areas where surface water is present or to intertidal areas below the mean high water mark. Drift and runoff may be hazardous to aquatic organisms in neighboring areas. Do not contaminate water when disposing of equipment washwater or rinsate.   Granular products: This pesticide is extremely toxic to birds, mammals, fish and aquatic invertebrates. Collect granules spilled during loading or application. . Do not apply directly to water, or to areas where surface water is present or to intertidal areas below the mean high water mark. Runoff may be hazardous to aquatic organisms in neighboring areas. Do not contaminate water when disposing of equipment washwater or rinsate.   Disulfoton Bee Mitigation  Suggested Precautionary Label Language for non granular products:   This pesticide is toxic to bees. Application should be timed to coincide with periods of minimum bee activity, usually between late evening and early morning.   Surface Water Advisory   This product may contaminate water through drift of spray in wind. This product has a high potential for runoff for several months. Poorly draining soils and soils with shallow watertables are more prone to produce runoff that contains this product.   Labels for farmers should add the following to the previous statement:

  A level, well maintained vegetative buffer strip between areas to which this product is applied and surface water features such as ponds, streams, and springs will reduce the potential for contamination of water from rainfall runoff. Runoff of this product will be reduced by avoiding applications when rainfall is forecasted to occur within 48 hours.   Labels for home owners should add the following to the previous statement:

  Avoid applying this product to ditches, swales, and drainage ways. Runoff of this product will be reduced by avoiding applications when rainfall is forecasted to occur within 48 hours.

Ground Water Advisory Note to CRM: Disulfoton residue detections in ground water range from 0.04 to 100 ppb; detections are up to 300 times the Health Advisory ( 0.3 ppb) . There is a high potential for degradates to contaminate ground water. Because disulfoton degradates are persistent, apparently mobile, and parent disulfoton has been found in ground water, a ground water label advisory is required. The following label language is appropriate:

21 " Disulfoton is known to leach through soil into ground water under certain conditions as a result of label use. Use of this chemical in areas where soils are permeable, particularly where the water table is shallow, may result in ground water contamination. "

Spray Drift Since disulfoton can be applied aerially, current cautionary labeling for the spray drift of aerially applied pesticides must be used.

22 Table of Contents Amended 8/ 26/ 00 from 01/ 13/ 00 from 10/ 07/ 99 from 8/ 26/ 99 1. Use Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2. Exposure Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 A. Chemical profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 B. Environmental Fate Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 C. Terrestrial Exposure Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 D. Water Resources Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 I. Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 ii. Application rates used in modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 iii. Modeling scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 iv. Modeling procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 v. Modeling results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 vi. Monitoring data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 vii. Limits of this analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 3. Ecological Effects Hazard Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 A. Toxicity to Terrestrial Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 I. Birds, Acute and Subacute . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 ii. Birds, Chronic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 iii. Mammals, Acute and Chronic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 iv. Insects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 v. Terrestrial Field Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 B. Toxicity to Freshwater Aquatic Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 I. Freshwater Fish, Acute . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 ii. Freshwater Fish, Chronic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 iii. Freshwater Invertebrates, Acute . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 iv. Freshwater Invertebrates, Chronic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 v. Freshwater Field Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 C. Toxicity to Estuarine and Marine Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 I. Estuarine and Marine Fish, Acute . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 ii. Estuarine and Marine Fish, Chronic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 iii. Estuarine and Marine Invertebrates, Acute . . . . . . . . . . . . . . . . . . . . . . . . 44 iv. Estuarine and Marine Invertebrates, Chronic . . . . . . . . . . . . . . . . . . . . . . . 45 v. Estuarine and Marine Field Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 D. Toxicity to Plants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 I. Terrestrial Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 ii. Aquatic Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 4. Ecological Risk Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 A. Risk to Nontarget Terrestrial Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 I. Birds and Mammals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 ii. Insects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 B. Risk to Nontarget Aquatic Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 .

C. Risk to Nontarget Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 5. Endangered Species Consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 6. Ecological Incident Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 7. Risk Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 A. Characterization of Fate and Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 I. Water Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 a. Surface Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 b. Ground Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 B. Characterization or Risk to Nontarget Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 C. Mitigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 8. References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 9. Appendices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 I. Use of Disulfoton by Crop and State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 II. Chemical Structure of Disulfoton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 III. Assessment of STORET Monitoring Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 IV. Environmental Fate and Chemistry Study Identification . . . . . . . . . . . . . . . . . . . . . . . . 92 V. Environmental Fate Data Requirements Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 VI. Ecological Effects Data Requirements Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 VII. Percent Crop Area Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 VIII. PRZM and EXAMS values for Index Reservoir . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 99 IX. PRZM and EXAMS Input and EXAMS Output files Table . . . . . . . . . . . . . . . . . . . . 100 1 1. Use Characterization for Disulfoton Disulfoton is a systemic organophosphate insecticide, acaracide ( miticide) registered for use to control aphids, thrips, mealybugs, other sucking insects, and spider mites on a variety of terrestrial food crops ( coffee, peppers, broccoli, brussels sprouts, cabbage, cauliflower, lettuce, spinach, asparagus, pecan, radish, and raspberries) , terrestrial food and feed crops ( tomato, barley, corn, oats, triticale, wheat, cotton, peanut, peas, sorghum, soybeans, potatoes, beans, and lentils) , terrestrial feed crops ( bermudagrass, and alfalfa) , and terrestrial nonfood crops ( Christmas tree plantations, ornamentals, and non bearing fruit) . The total use of disulfoton for 1997 was approximately 1.7 million lbs ai. Cotton has the greatest use of disulfoton ( 420,000 840,000 lb ai/ yr) , accounting for 61% of the disulfoton market. Wheat has the next largest percentage of the market, at 16% ( 180,000 354,000 lb ai/ yr) . The largest use state is California ( 16% of the market, 272,000 lb ai/ yr) , followed by Louisiana ( 11% of the market, 187,000 lb ai/ yr) . Rankings of disulfoton usage by crop and by state are provided in Appendix I.

Disulfoton is formulated as 15% granules, 8% emulsifiable systemic, 95% cotton seed treatment, systemic granules ( 1, 2, 5, 10% ) , and 68% concentrate for formulating garden products. Applications are generally soil applied: in furrow, broadcast, or row treatment followed by 2 3 inch soil incorporation. It can also be applied as a foliar treatment and in irrigation water. Cotton seeds can also be directly treated and planted. Disulfoton can be applied in multiple applications ( up to three) at intervals from 7 to 21 days depending upon the crop. Application rates typically range from about 0.5 to 4.00 lb ai/ A. A Section24( c) Registration for North Carolina Christmas trees allows up to 4.5 lb ai/ A and for the same use, the Federal Section 3 Registration allows for greater than 57 lb ai/ A.

2. Exposure Characterization A. Chemical Profile 1. Common name: disulfoton 2. Chemical name: O, O'  diethyl S [ 2 ethylthio) ethyl ] phosphorothioate 3. Trade Names: Di Syston 4. Physical/ Chemical properties: Molecular formula: C8H1802PS3 Molecular weight: 274.39 Physical state: colorless liquid Specific gravity. 1.144 a 20 E C. Henry' s Law Constant: 2.60E 6 Atm. M3/ Mol ( measured) Boiling point: 62 E C at 0.01 mmHg Vapor pressure: ( 20 E C) = 1.8 X 10  4 mmHg Solubility: in water at 20 E C= 25 ppm; miscible in n hexane, dichloromethane, 2 propanol, toluene 2 B. Environmental Fate Assessment I. Environmental Fate and Chemistry Data The environmental fate and chemistry data base for disulfoton is incomplete for the parent compound. Less fate data are available for the degradation products. The major routes of dissipation are chemical reaction and microbial degradation in aerobic soil and aqueous photolysis and soil photolysis. Volatilization from soil and water is not expected to be significant. Data are unavailable for aerobic and anaerobic aquatic environments. The anaerobic soil metabolism studies have been submitted to the Agency, and will be reviewed by EFED. Disulfoton is essentially stable to hydrolysis at 20 E C at the three pH values tested but is influenced by temperature as hydrolysis is fairly rapid at 40 E C. The overall results of these mechanisms of dissipation appear to indicate that disulfoton has low persistence. Limited data suggests that the degradates are much more persistent. Disulfoton also appears to be more persistent under anaerobic soil conditions than aerobic soil conditions. The adsorption/ desorption studies indicate that disulfoton is slightly to somewhat mobile depending upon the soil. Aged leaching studies suggested that D. sulfoxide and D. sulfone degradates did not leach which is inconsistent with the field data, terrestrial field dissipation studies showed that both degradates leached. Sulfoxide and sulfone degradates of other organophosphate pesticides tend to be more mobile than the parent compound. The individual studies are summarized below.

Hydrolysis ( 161 1)

The primary hydrolysis products were the disulfoton oxygen analog ( POS) at pH 4, a mixture of des ethyl disulfoton metabolites of which the major one is des ethyl POSO2 at pH 7 and a product obtained at pH 9 which converted to 2 2 ( ethylsulfonyl) ethane sulfonic acid upon treatment with potassium permanganate. The reported hydrolysis half lives are 1174 days, 323 days, and 231 days in sterile aqueous buffered solutions at pH   s 4, 7, and 9, respectively, for a 30 day study. Consequently, disulfoton is essentially stable to abiotic degradation at 20 E C. At 40 E C, the half lives were 30, 23.2, and 22.7 days at pH 4, 7, and 9, respectively. The hydrolysis guideline requirement ( 161 1) is fulfilled ( MRID 00143405) .

Photodegradation in water ( 161 2)

Disulfoton had a T1/ 2 of 93 hours. The half life for aqueous photolysis ( corrected for the dark control) is 93 hours in a pH 5 buffered solution exposed to natural sunlight ( Latitude 38.05 N; Longitude 84.30 W. ; October 5 15. 1987; average temperature 19.4 + 2.08 C ) . For the purpose of modeling ( in the water body) , the rate of disulfoton photolysis in water was considered. Disulfoton sulfoxide was the major degradation product. Control ( dark) samples degraded with a half life of > 300 hours. Both reactions followed zero order kinetics ( independent of concentration) . The guideline requirement for photo degradation in water ( 161 2) is fulfilled ( MRID 40471102) .

Photodegradation on soil ( 161 3) 3 The half life of disulfoton was 2.4 days on sandy loam soil plates exposed to natural sunlight. The primary photoproduct was disulfoton sulfoxide in irradiated and dark samples. Less than 10% disulfoton oxygen analog sulfoxide and disulfoton sulfone were detected in the light exposed samples after two days of irradiation. MRID 40789701 was rejected on 8/ 23/ 89 since the proportion of metabolites formed was not presented in the study report. The registrant provided this information in a letter dated 2/ 11/ 92. The photo degradation on soil data requirement ( 161 3) is fulfilled ( MRID 40471103) .

Aerobic soil metabolism ( 162 1)

Literature suggests that disulfoton is transformed in soil via microbial metabolism and chemical oxidation ( Howard et al. , 1991) . Primary transformation products are D. sulfoxide and D. sulfone. Five oxidative metabolites, that persisted for more than 12 weeks ( 84 days) , have been identified in a paddy soil ( Howard et al. , 1991) . Data generally suggests that in soil disulfoton will initially decline rapidly in soil, but this decline slows with time. Reported " half lives" of disulfoton tend to be generally less than 5 days. In soil, the metabolites, D. sulfoxide and D. sulfone, appear to be more persistent > 17 days and > 150 days, respectively ( MRID# 4437391) .

The registrant has submitted a several studies to assess the aerobic metabolism rate in soil ( MRID # s 43800100, 40042201; 41585101) . The aerobic soil half life was calculated by the registrant to be 15.6 days, however, the reaction did not follow first order kinetics ( MRID 43800101) . It was recalculated ( see next paragraph) . Less than 20% of the amount applied remained 7 days after treatment; < 3% remained 60 days after treatment. The major degradates are the sulfoxide ( 58.7% ) at 7 days, and sulfone ( 72% ) at 90 days. At the end of the study ( 367 days) , the sulfone was present at 35% of the applied amount, and the sulfoxide at 2% of the applied amount. Except for the sulfone and sulfoxide degradates, residues were not detectable at 367 days. The aerobic soil metabolism guideline requirement ( 162 1) is fulfilled ( MRID 43800101) .

As noted above there is an issue as to whether the decline of disulfoton in soil follows first order kinetics in this study ( MRID 43800101) . The information reported in MRID 43800101 suggests non first order kinetics and a half life less than the " calculated" 15.6 days as indicated. The 15.6 day half life was calculated by the registrant and only represents a portion of the data ( days 0 through 90, days 122, 241, and 367 were not included) . The slope ( decay rate constant, k) of the transformed ( natural log or ln) ( ln C( t) = ln Co  kt, where Co is the initial concentration, C is concentration, and t is time) was significant with p= 0.0001 and a r 2 of 0.888. From a statistical standpoint, a first order model using transformed data provides a reasonable estimate of the decline rate. However, the time that the initial pesticide concentration reaches half the initial concentration ( e. g. , half life) is less than the 15.6 days suggested by the analysis of the transformed data. The decay rate of disulfoton appears to follow the pseudo first order type kinetics over the entire study duration better than when nonlinear regression is applied to untransformed data ( C= Coe  kt ) where Co is the initial concentration, C is concentration, t is time, and k is the decay rate constant. The parameter k was estimated by non linear regression of C versus time. The half life ( when C/ Co = 0.5) was estimated to be 2.57 days ( r 2 = 0.93) . The linear regression of the ln transformed dated tended to over estimate disulfoton residues with time whereas the non linear regression of the non transformed data under estimated the disulfoton residues with time. Approximately, 10 percent of applied radio labeled disulfoton ( Di 4 Syston) was reported to be in the sulfoxide state at time zero ( day 0 < then 6 hours) which suggests rapid oxidation to the corresponding sulfoxide metabolite.

Two additional aerobic soil metabolism studies ( MRID# s 40042201; 41585101) submitted by the registrant, determined to be supplemental studies by EFED, also provided additional information which was considered. These studies had estimated aerobic half lives of 2.4 and 1.9 days, respectively. A half life of 1.9 days ( MRID 41585101) was estimated using the ln transformed disulfoton percentages from only the first three days ( 0, 0.25, 1, and 3 days) of the experiment, the remaining days 7, 14, 30, 90, 189, 270 are not considered. The decline of parent disulfoton in these studies also appeared not to follow first order kinetics, but pseudo first order kinetics.

The registrant indicated in a response ( 3/ 8/ 99 To: P. Poli, From: J. S. Thornton) that the half lives for the studies submitted as MRID # 43800101 and 41585101 were 5.5 and 4.1 days, respectively. Because these half lives are longer ( more conservative) than those estimated by EFED ( see above) , these values were used in the modeling for the water assessment.

The metabolites ( D. sulfoxide and D. sulfone) tended to be more persistent with T1/ 2 of ~ 17 days and ~ 150 days, respectively ( MRID# 4437391) . The registrant indicates, non guideline study ( modeling exercise) that the DT50 for disulfoton, sulfoxide, and sulfone is 5.5, 17, and 150 days, respectively ( MRID 4437391) . The equations used to estimate these values were not specified, thus, the DT50s ( rate constants) could not be confirmed.

Anaerobic soil metabolism ( 162 2)

Several anaerobic soil metabolism studies have been submitted to the EPA ( MRID# s 43512201, 43042503. The studies indicate that disulfoton is more persistent under anaerobic soil conditions compared to aerobic soil conditions. EFED will conduct a detailed review of these studies.

Anaerobic aquatic metabolism ( 162 3)

This study ( MRID 43042503) cannot be used to fulfill data requirement 162 3. Material balances were too low, declining from 106% immediately post treatment to 78.7% at 202 days. Only 65% of the intended application was available at the start of the study. The study cannot be upgraded; a new anaerobic aquatic study or an anaerobic soil metabolism study must be submitted for disulfoton.

Aerobic aquatic metabolism ( 162 4)

No data on aerobic aquatic metabolism of disulfoton or its metabolites have been submitted. This information must be submitted by the registrant.

Mobility  Leaching and Adsorption/ Desorption. ( 163 1)

Adsorption/ desorption studies of disulfoton indicated that it is slightly mobile to somewhat mobile depending on the soil. Adsorption/ desorption coefficients of various soil types are 5 tabulated below.

Table 1. Kd and Koc Adsorption/ Desorption Values for Disulfoton for four soils Soil Texture Silt Loam Sand Clay Loam Sandy Loam Kd 6.85 4.67 4.47 9.66 Koc ( ads. ) 449 888 386 483 Koc ( des. ) 629 1340 547 791 The average organic carbon normalized Freundlich Kads was estimated to be 551.5 mL/ g soil carbon from the data summarized in the above Table 1. The Koc ( ads. ) model generally appears to be appropriate as Kads increase with organic carbon content and the 1/ n term in the Freundlich equation were close to 1 ( so Kads ~ Kd) .

In a second report, # 66792, parent Freundlich K values ( 7.06 to 14.29) indicate that disulfoton is adsorbed to a moderate degree which also reflects low mobility in soils. The average Di Syston Rf value was 0.22 on six soils which also indicates low mobility of the parent disulfoton. The correlation coefficients describing the degree of data conformity to the Freundlich equation ranged from 90.3 to 99.9% . The 1/ n values for the three soils were 1.002, 0.980, and 0.975. Calculated Kocs were 641, 752, and 839. The mobility leaching and adsorption/ desorption guideline requirement ( 163 1) is fulfilled ( MRID # 443731 03 and 00145469) . These data were also recorded in Bayer' s 11/ 30/ 93 letter to SRRD, MRID  430425 00 pages 3 and 4. )

Adsorption/ desorption data are needed for D. sulfoxide and D. sulfone.

Mobility  Leaching of Aged Di Syston ( 163 1)

This 1986 study ( Acc. # 00145470) was not conducted in accordance with acceptable guidelines, and the 1986 results were not consistent with current data using guideline studies. Recent data indicate that the degradates will leach to lower depth, but the 1986 study indicated no leaching of sulfoxide and sulfone degradates. A new column leaching study is not required, because other existing data fulfill the requirement.

Laboratory Volatility ( 163 2)

Disulfoton volatilized at maximum of 0.026 and 0.096 µ g/ Cm 2/ hr from sand soil adjusted to 25% and 75% of field capacity at 0.33 bar respectively, incubated in dark for 21 days at 25 E C with an air flow of approximately 300 mL/ minute. Maximum volatilization occurred within 24 hours following treatment. The vapor pressure of disulfoton was reported to be 7.2 X 10  5 mBar at 20 E C and 1.3 X 10  5 mBar at 25 E C. Freundlich Kads for the sand soil was determined to be 0.172. The guideline requirement for laboratory volatility ( 163 2) has been fulfilled ( MRID 42585802) 6 Field Volatility ( 163 2)

Maximum concentration observed in air at 1 foot above ground was 22.2 ng/ L. Disulfoton concentrations, after 6 hours, at the 5 foot level were not detectable. Bayer, Inc. submitted additional data, e. g. , ads. / des. Kds, and cloud covering on the days of the experiment. The guideline requirement for field volatility ( 163 2) has been fulfilled ( MRID 40471105) .

Terrestrial Field Dissipation ( 164 1)

Disulfoton applied at 8 lbs. / ac dissipated with a T1/ 2 of 2 to 4 days from the upper 6 inches of sand/ sandy loam and loamy sand/ sandy loam plots in California. Parent disulfoton was detected only in the upper 6 inches of soil, the sulfoxide and sulfone degradates were detected to a depth of 18 inches. The guideline requirement for terrestrial field dissipation ( 164 1) has been fulfilled ( MRID 43042502) .

Fish Bioaccumulation ( 165 4)

From 60.8 to 85.9 ppb 14 C residues in edible fish and 38.1 to 39.9 ppb in the inedible fish tissues were not characterized. After 14 days depuration, fillet contained 21% of the applied residues, viscera 18.1% , and whole fish 22% . Bioconcentration factors were 460X for whole fish, 700X for viscera, and 460X for fillet. Bayer submitted data, at the Agency   s request, which indicated that there was no mortality and no growth during the study. The bioaccumulation guideline ( 165 4) has been partially fulfilled ( MRID 43042501, 43060101, 40471106, and 40471107) . No further bioaccumulation testing is required for parent disulfoton, however, bioaccumulation information, or at least Kow determination, for the sulfone and sulfoxide degradates would be helpful for risk assessment purposes.

Foliar Dissipation ( Non Guideline Study  Supporting Information)

The foliar dissipation rate of 3.3 days is based on field monitoring data ( MRID # 41201801) . Disulfoton was aerially applied to potatoes 3 times at 1 lb ai/ acre in Michigan. Potato foliage was collected from five different treated fields with six sampling stations in each field. Samples were collected the day before and the day after each of the three treatments, and then on day 7 and 14 after the third ( final) treatment. The foliar dissipation rate estimates are based on the samples collected after the third treatment. The following table shows the average residue levels on potato foliage on days 1, 7 and 14 from the five fields, across all 6 sample stations and the average for all fields.

EFED determined that the 90 th percentile upper bound foliar dissipation half life for disulfoton of 3.3 days is used for both terrestrial exposure assessment, and in PRZM EXAMS when foliar dissipation is applicable 7 Table 2. Residue data and the calculated foliar dissipation half life based on measured residues of disulfoton on potato foliage after the third application. Residues in g/ g ( ppm) .

Time field 1* field 2* field 3* field 4* field 5* average of all fields day 1513634404040 day 7 4.8 4.7 8.5 5.3 5.9 5.8 day 14 1 0.9 1.9 1.6 4.2 1.9 half life ( days) 2.3 2.4 3.1 2.8 4 2.98 upper 90% CL 3.3 * average across all stations C. Terrestrial Exposure Assessment For pesticides applied as a nongranular product ( e. g. , liquid, dust) , the estimated environmental concentrations ( EECs) on food items following product application are compared to LC50 values to assess risk. The predicted 0 day maximum and mean residues of a pesticide that may be expected to occur on selected avian or mammalian food items immediately following a direct single application at 1 lb ai/ A are tabulated below.

Table 3. Estimated Environmental Concentrations on Avian and Mammalian Food Items ( ppm) Following a Single Application at 1 lb ai/ A)

Food Items EEC ( ppm) Predicted Maximum Residue 1 EEC ( ppm) Predicted Mean Residue 1 Short grass 240 85 Tall grass 110 36 Broadleaf/ forage plants, and small insects 135 45 Fruits, pods, seeds, and large insects 15 7 1 Predicted maximum and mean residues are for a 1 lb ai/ A application rate and are based on Hoerger and Kenaga ( 1972) as modified by Fletcher et al. ( 1994) .

Predicted residues ( EECs) resulting from multiple applications are calculated in various ways. For this assessment, maximum disulfoton EECs were calculated using Hoerger and Kenaga ( 1972) as modified by Fletcher et al. ( 1994) . These EECs served as inputs into the FATE 8 program. The FATE program is a first order dissipation model, i. e. , the pesticide is applied repeatedly, but degrades over time from the first application to some assigned time there after. In the case of disulfoton the time period was 30 days. A foliar degradation half life of 3.3 days was selected based on a field monitoring study ( MRID # 41201801) . EEC values for a variety of crops and application rates/ methods are provided in the risk quotient tables in Section 4,   Ecological Risk Assessment.   D. Water Resources Assessment i. Summary and Conclusions This section presents the assessment of the potential of disulfoton ( and degradates) to contaminate surface water and ground water from label uses. The assessment includes a Tier II estimates of environmental concentrations ( EECs) of disulfoton and total disulfoton residues ( TDR sum of disulfoton, D. sulfoxide, and D. sulfone) in surface water and SCI GROW estimates of ground water concentrations, and the available monitoring data which primarily addresses only parent disulfoton. Tier I was not included because EECs levels of concern are generally exceeded for organophosphate insecticides, thus, necessitating a more refined evaluation. The ecological exposure assessment used the standard farm pond scenarios and the drinking water assessment utilized the Index Reservoir and Percent Crop Area concepts.

The Tier II modeling of disulfoton residue concentrations in surface water used the PRZM3 and EXAMS models as applied to barley, cotton, potatoes, tobacco, and spring wheat, using maximum label application rates and several application methods. Surface water monitoring data collected by the USGS as part of the National Water Quality Assessment ( NAWQA) ( Gilliom, 1995; USGS, 1997) program, USEPA' s STORET, and any State study that measured disulfoton in surface water were also considered. The potential for disulfoton residues in ground water is assessed using the EFED ground water concentration screening model ( SCI GROW) and the monitoring data available in EFED   s Pesticides in Ground Water Data Base ( PGWDB) ( USEPA, 1992) , USGS NAWQA study ( USGS, 1997) , and STORET ( search date 10/ 16/ 97) . The purpose of this analysis is to provide an estimate of environmental concentrations of disulfoton ( and degradates) in surface water bodies and ground water for use in the human health and ecological risk assessment as part of the registration process. The environmental fate data base is not complete for disulfoton. Limited data indicates that the degradates are much more persistent and mobile than parent disulfoton. Organophosphate degradates are often as toxic as the parent compound and are considered in this assessment as total disulfoton residues ( TDR) . However, as noted, since data are lacking there is considerable uncertainty in these estimates.

Surface and ground water monitoring data available in STORET were evaluated in detail, but were generally not considered due to limitations associated with high detection limits and difficulty in interpreting the data. Detailed discussion of the STORET findings is presented in the Appendix III .

The Tier II EEC assessment uses a single site, or multiple single sites, which represents a high end exposure scenario from pesticide use on a particular crop or non crop use site for ecological exposure assessments. The EECs for disulfoton were generated for multiple crop scenarios using PRZM3.12 ( Carsel, 1997; 5/ 7/ 98) which simulates the erosion and run off from an 9 agricultural field and EXAMS 2.97.5 ( Burns, 1997; 6/ 13/ 97) which simulates the fate in a surface water body. PRZM3 and EXAMS estimates for a single site, over multiple years, EECs for a 1 ha surface area, 2 m deep farm pond draining an adjacent 10 ha barley, cotton, potato, tobacco, or spring wheat field. Each scenario, or site, was simulated for 20 to 40 ( depending on data availability) years. EFED estimated 1 in 10 year maximum peak, 4 day average, 21 day average, 60 day average, 90 day, annual average concentrations, and the mean of the annual averages. Disulfoton ( Di Syston) formulations were based upon registered uses on the specific crops. The application rates ( maximum on label ; EPA Reg. No. 3125 172, 3125 307) , numbers, and intervals are listed in Tables 7a. and 7b. and Tables 8a. and 8b. and environmental fate inputs are listed in Table 6. PRZM simulations were run using maximum application rates, maximum number of yearly applications, and the shortest recommended application interval. Spray drift is determined by method of pesticide application ( and assumed to be 5% for aerial spray; 1% for ground spray, 0% for granular or soil incorporated applications) per EFED guidance for the pond scenarios ( USEPA, 1999) .

The PRZM/ EXAMS EECs are generated for high exposure agricultural scenarios and represent one in ten year EECs in a stagnant pond with no outlet that receives pesticide loading from an adjacent 100% cropped, 100% treated field for parent disulfoton and total disulfoton residues. As such, the computer generated EECs represents conservative screening levels for ponds, lakes, and flowing water and should only be used for screening purposes. The EECs have been calculated so that in any given year, there is about a 10% probability that the maximum average concentration of that duration in that year will equal or exceed the EEC at the site. Tier II upper tenth percentile EECs for disulfoton and total disulfoton residues are presented in Tables 7a. and 7b. and 8a. and 8b. for the pond and the index reservoir with PCA adjustment, respectively.

The sites selected are currently used by EFED ( standard scenarios) to represent a reasonable   at risk   soil for the region or regions being considered. . The scenarios selected represent high end exposure sites. The sites are selected so that they generate exposures larger than for most sites ( about 90 percent) used for growing the selected crops. An   at risk   soil is one that has a high potential for run off and soil erosion. Thus, these scenarios are intended to produce conservative estimates of potential disulfoton concentrations in surface water. The crop, MLRA, state, site, and soil conditions for each scenario are given in Tables 4 and 5.

The SCI GROW ( Screening Concentration in Ground Water) screening model developed in EFED ( Barrett, 1997) was used to estimate potential ground water concentrations for disulfoton parent and total disulfoton residues under   generic   hydrologically vulnerable conditions. . SCI GROW provides a screening concentration, an estimate of likely ground water concentrations if the pesticide is used at the maximum allowed label rate in areas with ground water exceptionally vulnerable to contamination. In most cases, a majority of the use area will have ground water that is less vulnerable to contamination than the areas used to derive the SCI GROW estimate.

ii. Application Rates Used in Modeling The application rates ( Tables 7a and b, 8a and 8b) selected for use in the modeling scenarios 10 were based upon information submitted by the registrant, analysis conducted by BEAD, and the disulfoton ( Di Syston) labels. Four factors went into selecting the application rate: 1) the range of ounces or pounds a. i. ; 2) the area or length of row per acre ( which is influenced by row spacing) ; 3) the number of applications; and 4) the application interval. The maximum rate ( ounces or pounds a. i. per crop simulated) and the shortest application intervals were selected. The shorter the distance between the crop rows the greater the application rates on an area basis.

iii. Modeling Scenarios Surface Water: The disulfoton scenarios ( Tables 4 and 5) are representative of high run off sites for barley in the Southern Piedmont of Virginia ( MLRA 136) , cotton in the Southern Mississippi Valley Silty Uplands of Mississippi ( MLRA 134) , potatoes in the New England and Eastern New York Upland of Maine ( MLRA 144A) , tobacco in Southern Coastal Plain of Georgia ( MLRA 133A) , and spring wheat in the Rolling Till Prairie of South Dakota ( MLRA 102A) . The scenarios chosen are professional best judgement sites expected to produce run off greater than would be expected at 90% of the sites where the appropriate crop is grown. Soils property data ( Table 5) and planting date information were obtained from the PRZM Input Collator ( PIC) data bases ( Bird et al, 1992) . The Percent Crop Area ( PCA) values used for the five scenarios for estimated drinking water concentrations are also given in Table 4. 11 Table 4. Crop, location, soil and hydrologic group for each modeling scenario.

Crop MLRA 1 State Soil Series Soil Texture Hydrologic Group Period ( Years) PCA 2 Barley 136 VA Gaston sandy clay loam C270. 87 Cotton 131 3 MS Loring silt loam C 20 0. 20 Potatoes 144A ME Paxton sandy loam C 36 0. 87 Tobacco 133A GA Emporia loamy sand C 36 0. 87 Spr. Wheat 102A SD Peever clay loam C 40 0. 56 1 MLRA is major land resource area ( USDA, 1981) . 2 PCA is the Percent Crop Area. 3 Meteorological file met131. met is used in the EFED standard cotton scenario, since the weather station is closer to the simulated site.

Table 5. Selected soil properties used modeling.

Soil Series ( MLRA) Depth ( in) Bulk Density ( g/ cm 3 ) Organic Carbon ( % ) Field Capacity ( cm 3 / cm 3 ) Wilting Point ( cm 3 / cm 3 )

Gaston ( 136) 16 1.6 1.740 0.246 0.126 84 1.6 0.174 0.321 0.201 50 1.6 0.116 0.222 0.122 Loring ( 131) 10 1.6 1.160 0.294 0.094 10 1.6 1.160 0.294 0.094 105 1.8 0.174 0.147 0.087 Paxton ( 144A) 20 1.6 2.90 0.166 0.66 46 1.8 0.174 0.118 0.38 34 1.8 0.116 0.085 0.035 Emporia ( 133A) 38 1.4 1.16 0.104 0.054 62 1.6 0.174 0.225 0.125 50 1.6 0.116 0.135 0.056 Peever ( 102A) 18 1.35 1.740 0.392 0.202 82 1.60 0.116 0.257 0.177 50 1.60 0.058 0.256 0.176 Ground Water: The SCI GROW ( Screening Concentration in Ground Water) screening model developed in EFED ( Barrett, 1997) was used to estimate potential ground water concentrations for disulfoton parent and total disulfoton residues under   generic   hydrologically vulnerable 12 conditions, but necessarily the most vulnerable conditions. The SCI GROW model is based on scaled ground water concentrations from ground water monitoring studies, environmental fate properties ( aerobic soil half lives and organic carbon partitioning coefficients Koc' s) and application rates.

iv. Modeling Procedure Environmental fate parameters used in PRZM3 and EXAMS runs are summarized in Table 6. The standard EFED pond ( mspond) was used. The PRZM3 simulations were run for a period of 36 years on potatoes, and tobacco, beginning on January 1, 1948 and ending on December 31, 1983. Barley was run for 27 years ( 1956 1983) and spring wheat was run for 40 years ( 1944 1983) . Cotton was run for 20 years of data ( January 1, 1964 December 31, 1983) . Scenario information is summarized in Tables 4 and 5. The EXAMS loading ( P2E C1) files, a PRZM3 output, were pre processed using the EXAMSBAT post processor. EXAMS was run for the 20 40 years using Mode 3 ( defines environmental and chemical pulse time steps) . For each year simulated, the annual maximum peak, 96 hour, 21 day, 60 day, 90 day values, and annual means in addition to the mean of annual means were extracted from the EXAMS output file REPORT. XMS with the TABLE20 post processor. The 10 year return EECs ( or 10% yearly Exceedence EECs) listed in Tables 7a. , 7b. , 8a. and 8b. were calculated by linear interpolation between the third and fourth largest values by the program TABLE20. Table 6. Disulfoton fate properties and values used in ( PRZM3/ EXAMS) modeling.

Parameter Value Source Molecular Weight 274.39 MRID 150088 Water Solubility 15 mg/ l @ 20 MRID 150088 Henry   s Law Coefficient 2. 60 atm m3/ mol EFED One liner 05/ 21/ 97 Partition Coefficient ( Koc) 551.5 ( mean of 4 ) MRID 43042500 Vapor Pressure 1.8E 04 mmHg EFED One liner 05/ 21/ 97 Hydrolysis Half lives @ pH 4 pH 7 pH 9 1174 days 323   231   MRID 143405 Hydrolysis Rate Constants ( needed for EXAMS derived from Hydrolysis halflives Kah = ( negative) Knh = 8.88E 05 Kbh = 3.58 Aerobic Soil Half life ( Disulfoton) 6.12 days ( 0.113/ d) Upper 90% confidence bound on the mean of " half lives" for the two aerobic soils tested in the laboratory. MRIDs 40042201, 41585101, 43800101 Aerobic Soil Half life 1 ( Total Disulfoton Residues) 259.63 days ( 2.67E 03/ d) Upper 90% confidence bound on the mean of half lives for the two aerobic soils tested in the laboratory. MRIDs 40042201, 41585101, 43800101 Water Photolysis 3.87 days ( pH = 5) ( 0.179/ d) MRID 40471102 Aerobic Aquatic Half life ( Disulfoton) ( Kbaws, Kbacs) 12.2 days ( 0.05682/ day) Estimated per EFED guidance Aerobic Aquatic Half life ( Total Disulfoton Residues) ( Kbaws, Kbacs) 259.63 days ( 2.67E 03/ d) Did not multiple half life by 2 per EFED guidance to account for uncertainty. Half lives greater than a year would show residue accumulation.

Foliar Dissipation Rate 3.3 days ( 0.21/ d) MRID 41201801 1 Half lives for total residues were determined from the total residues at each sampling interval. Total disulfoton residues did follow first order kinetic decay ( The slope ( decay rate constant, k) of the transformed ( natural log or ln) ( ln C( t) = ln Co  kt, where Co is the initial concentration, C is concentration, and t is time) ) .

v. Modeling Results 13 a. Surface water In the Tier II assessment, the mean of the annual mean concentrations of disulfoton ( Table 7a) in a farm pond over multiple years simulated ranged from 0.21 µ g/ L for a two applications ( @ 0.83 lb ai/ a) to barley in Virginia to 1.14 µ g/ L for potatoes in Maine with the three applications at the maximum application rate ( @ 1.00 lb ai/ ac) . The one in ten year maximum, or peak, estimated concentrations of 26.75 µ g/ L occurred for one 4.0 lb. ai/ ac applications of disulfoton to tobacco in Georgia. For the other scenarios or recommended application rates, the maximum concentrations ranged from 7.14 to 18.46 µ g/ L. Because of limited data, the modeling results, therefore, cannot be confirmed by the monitoring data.

Because the degradates of disulfoton ( including oxygen analogs) : sulfoxide and sulfone are also toxic, the EECs of the total disulfoton residue ( TDR) in a farm pond was also considered ( Table 7b) . The overall estimated of the multiple year mean concentrations of TDR in a farm pond over multiple years simulated ranged from 3.89 µ g/ L for two applications at the maximum rate ( 1.00 lb ai/ A) to barley in Virginia to 9.32 µ g/ L for tobacco in Georgia with one application at the maximum application rate ( 4.00 lb ai/ A) . Maximum, or peak, estimated TDR concentrations of 58.47 µ g/ L occurred for one 4.00 lb. ai/ ac application of disulfoton to tobacco. For the other scenarios, the maximum TDR concentrations ranged from 15.32 to 52.93 µ g/ L. There are no monitoring data to evaluate these concentration estimates from PRZM/ EXAMS modeling. Water samples collected, following a fish kill incident in Colorado, contained disulfoton sulfoxide at levels of 29.5 48.7 µ g/ L, and disulfoton sulfone at 0.0199 0.214 µ g/ L. The source of the disulfoton was Di Syston E. C. applied to wheat which was followed by heavy rain fall. ( Incident Report No. I001167 001) .

The PRZM/ EXAMS estimated disulfoton residue concentrations in surface water appear to be strongly related to the application rate, number of applications, application interval, and method of application and timing to application to rainfall events.

14 Mean of Annual Means ( µ g/ L) 0. 50 0.21 0.48 0. 33 1.14 0.66 0.42 0. 66 Table 7a. Tier II Upper Tenth Percentile EECs for Disulfoton Parent Used on barley, cotton,

potatoes, tobacco, and spring wheat for several application ( Label maximum) rates and management scenarios estimated using PRZM3/ EXAMS in standard farm pond.

Concentration ( µ g/ L)

( 1 in 10 annual yearly maximum value) Annual Avg. 0. 79 0.49 0.92 0. 44 1.23 1.72 1.15 0. 73 90 Day Avg. 2. . 82 1.73 3.44 1. 80 4.89 6.87 4.64 2. 76 60 Day Avg. 3. 79 2.37 4.91 2. 59 6.89 9.94 6.74 3. 81 21 Day Avg. 5. 96 4.36 8.05 4. 51 10.40 17.89 12.54 5. 47 96 Hour Avg. 7. 93 6. 32 12.96 6. 40 13.24 24.33 16.79 7. 95 Peak 9. 20 7. 14 14.79 7. 14 15.02 26.75 18.46 8. 90 Disulfoton Application Rate / Number of Apps / Interval / Incorp.

Depth / method 1 lb. ai/ A / # / days / inches 1.0 / 2/ 21/ 0/ f 0.83/ 2/ 21/ 0/ s ( granular)

1. 0 / 3/ 21/ 0/ s 4.0 / 2/ 14/ 2.5/ s 1.0/ 3/ 14/ 0/ f 4.0/ 1/ 0/ 2.5/ s ( granular)

4.0/ 1/ 0/ 2.5/ s 0.75/ 2/ 30/ 0/ f Crop Barley Barley Cotton Potatoes Potatoes Tobacco Tobacco Spr. Wheat 1 Method of application: f = foliar and s = soil 15 Mean of Annual Means ( µ g/ L) 4.94 3.89 9.13 4.48 8.37 9.32 7.16 4.73 Table 7b. Tier II Upper Tenth Percentile EECs for Total Disulfoton Residues Used on barley,

cotton, potatoes, tobacco, and spring wheat for several application ( Label maximum) rates and management scenarios estimated using PRZM3/ EXAMS in standard farm pond.

Concentration ( µ g/ L)

( 1 in 10 annual yearly maximum value) Annual Avg. 7.51 6.60 15.61 6.02 9.75 15.23 13.36 5.65 90 Day Avg. 16.48 14.46 32.41 10.97 19.33 35.30 31.94 11.29 60 Day Avg. 17.35 15.02 34.37 12.20 20.88 39.57 35.68 12.56 21 Day Avg. 19.27 17.44 39.27 13.51 23.92 49.54 44.76 14.91 96 Hour Avg. 20.99 19.34 43.50 14.94 26.59 56.35 51.03 16.36 Peak 21.77 19.95 44.78 15.43 27.36 58.47 52.93 16.92 Disulfoton Application Rate / Numberof Apps / Interval / Incorp.

Depth / method 1 lb. ai/ A / # / days / inches 1.0 / 2/ 21/ 0/ f 0.83/ 2/ 21/ 0/ s ( granular)

1. 0 / 3/ 21/ 0/ s 4.0 / 2/ 14/ 2.5/ s 1.0/ 3/ 14/ 0/ f 4.0/ 1/ 0/ 2.5/ s ( granular)

4.0/ 1/ 0/ 2.5/ s 0.75/ 2/ 30/ 0/ f Crop Barley Barley Cotton Potatoes Potatoes Tobacco Tobacco Spr. Wheat 1 Method of application: f = foliar and s = soil 16 Surface Water Drinking Water Assessment with Index Reservoir and Percent Crop Area The estimated drinking water concentrations ( EDWCs) were evaluated using the methodology outlined in EPA OPP draft Guidance for Use of the Index Reservoir and Percent Crop Area Factor in Drinking Water Exposure Assessments ( USEPA, 2000) . This generally results in the modification of the scenarios developed for farm ponds to scenarios for the index reservoirs.

The purpose the Index Reservoir ( IR) scenario and the Percent Crop Area ( PCA) for use in estimating the exposure in drinking water derived from vulnerable surface water supplies. Since the passage of the Food Quality Protection Act ( FQPA) in 1997, the Agency has been using the standard farm pond as an interim scenario for drinking water exposure and has been assuming that 100% of this small watershed is planted in a single crop. The Agency is now implementing the index reservoir to represent a watershed prone to generating high pesticide concentrations that is capable of supporting a drinking water facility in conjunction with the percent cropped area ( PCA) which accounts for the fact that a watershed large enough to support a drinking water facility will not usually be planted completely to a single crop. These two steps are intended to improve the quality and accuracy of the drinking water exposure for pesticides obtained by models.

Percent Crop Area ( PCA) : PCA is a generic watershed based adjustment factor that will be applied to pesticide concentrations estimated for the surface water component of the drinking water exposure assessment using PRZM/ EXAMS with the index reservoir scenario. The output generated by the linked PRZM/ EXAMS models is multiplied by the maximum percent of crop area ( PCA) in any watershed ( expressed as a decimal) generated for the crop or crops of interest. Currently, OPP has PCA adjustments for four major crops   corn, , cotton, soybeans, and wheat. Two are appropriate for disulfoton, cotton and wheat.

The concept of a factor to adjust the concentrations reported from modeling to account for land use was first proposed in a presentation to the SAP in December 1997 ( Jones and Abel, 1997) . This guidance results from a May 1999 presentation to the FIFRA Scientific Advisory Panel ( SAP) , Proposed Methods For Determining Watershed derived Percent Crop Areas And Considerations For Applying Crop Area Adjustments to Surface Water Screening Models , and the response and recommendations from the panel. A more thorough discussion of this method and comparisons of monitoring and modeling results for selected pesticide/ crop/ site combinations is located at: http: / / www. epa. gov/ scipoly/ sap/ 1999/ may/ pca_ sap. pdf.

The Agency will continue to develop PCAs for other major crops in the same manner as was described in the May 1999 SAP presentation. However, the Agency expects that it will use smaller watersheds for these calculations in the near future. For minor use crops, the SAP found that the use of PCAs produced less than satisfactory results and advised OPP to further investigate possible sources of error. Thus, for the near term, OPP is not be using PCAs in a crop specific manner for both major crops that do not yet have PCAs and minor use crops. Instead it will use a default PCA that reflects the total agricultural land in an 8 digit Hydrologic Unit Code ( HUC) . The PCA values used in this assessment are listed in Appendix VII.

17 The OPP guidance document provides information on when and how to apply the PCA to model estimates, describes the methods used to derive the PCA, discusses some of the assumptions and limitations with the process, and spells out the next steps in expanding the PCA implementation beyond the initial crops. Instructions for using the index reservoir and PCA are provided below. Discussion on some of the assumptions and limitations for both the PCA and Index Reservoir are included in the Reporting section. One should note that there is an entry for   All Agricultural Land   in Appendix VII. . This is a default value to use for crops for which no specific PCA is available. It represents the largest amount of land in agricultural production in any 8 digit hydrologic unit code ( HUC) watershed in the continental United States.

The unadjusted EDWC ( PRZM/ EXAMS output) is multiplied by the appropriate PCA for that crop to obtain the final estimated drinking water concentration ( EDWC) . Note that if Tier 2 modeling is done for an area other than the standard scenario, the PCA would still be applied, since it represents the maximum percent crop area for that particular crop. ( As regional modeling efforts are expanded, regional PCAs could be developed in the future. ) As an example, for a pesticide used only on cotton, the PRZM/ EXAMS estimated environmental concentrations would be multiplied by 0.20. This factor would be applied to the standard PRZM/ EXAMS scenario for cotton or any non standard cotton scenario until such time as regional PCAs are developed.

When multiple crops occur in the watershed, the co occurrence of these crops needs to be considered. The PCA approach assumes that the adjustment factor represents the maximum potential percentage of a watershed that could be planted to a crop. If, for example, a pesticide is only used on cotton, then the assumption that no more than 20% of the watershed ( at the current HUC scale used) would be planted to the crop is likely to hold true.

The Index Reservoir ( IR) : IR is intended as a drop in replacement for the standard pond for use in drinking water exposure assessment. It is used in a manner similar to the standard pond, except that flow rates have been modified to reflect local weather conditions. The PRZM and EXAMS input files for the standard pond and index reservoir are in Appendix IX. This guidance results from a July, 1998 presentation to the FIFRA Science Advisory Panel. The materials for that presentation are at: http: / / www. epa. gov/ scipoly/ sap/ 1998/ index. htm Barley, cotton, potatoes, tobacco, and spring were considered because they represent significant uses, maximum application rates, and are grown in vulnerable regions of the United States. For the PRZM, the input files for each IR scenario are essentially the same as its farm pond scenario. Three parameters in the PRZM input file require modification, AFIELD, HL, and DRFT. These changes are shown in Appendix VIII.

The estimated drinking water concentrations using the Index Reservoir ( IR) and PCA ( PCA) concepts for the same scenarios used for ecological exposure assessments were evaluated ( Tables 8a and 8b) . The long term mean of the parent disulfoton concentration in the Index Reservoir and by PCA ranged from 0.23 to 1.31 µ g/ L for cotton and tobacco, respectively. The 1 in 10 year estimated annual mean concentration ranged from 0.43 to 2.77 µ g/ L for cotton and tobacco, respectively. The peak 1 in 10 year estimated drinking water concentration for parent 18 disulfoton ranged from 7.13 to 44.20 µ g/ L.

The Tier II modeling results from PRZM/ EXAMS fall within the range of concentrations for surface water reported in the STORET database ( 0.0 to 100 µ g/ L, 96 percent of 8137 samples were reported as less than 16 µ g/ L) , a Virginia monitoring study ( 0.37 to 6.11 µ g/ L) and NAWQA ( 0.010 to 0.060 µ g/ L) . But because some of the data in STORET have a high degree of uncertainty because many samples were only listed as   actual value is known to less than given value   , the maximum concentration of samples was not always known ( see Appendix III) . The modeled concentration estimates are generally greater than those seen in the monitoring data. The modeling results therefore cannot be confirmed by the monitoring data.

Because the degradates of disulfoton ( including oxygen analogs) : sulfoxide and sulfone are also toxic, the EECs of the total disulfoton residue ( TDR) in the index reservoirs was also considered. The long term mean of the total disulfoton residues ( TDR) in the Index Reservoir and by PCA ranged from 2.55 to 10.42 µ g/ L for cotton and potatoes, respectively. The 1 in 10 year estimated annual mean TDR concentrations in the IR ranged from 5.10 to 16.72 µ g/ L for cotton and potatoes, respectively. The peak 1 in 10 year estimated TDR concentrations in the IR ranged from 20.83 to 104.92 µ g/ L. There are no monitoring data to evaluate these concentration estimates from PRZM/ EXAMS modeling.

Uncertainty surrounds these estimates because the sites selected for modeling represent sites thought to be representative of vulnerable sites. Additionally, the IR was generic ( to each scenario) and data to fully understand of the fate of disulfoton and disulfoton residues is not available. Evidence suggests that the concentrations will not be as high as suggest by the modeled estimates. The PCA values have been estimated by OPP for spring wheat ( 0.56) and cotton ( 0.20) . The default for value for all agricultural land of 0.87 was used for the barley, potatoes, and tobacco scenarios. Better estimates of the PCA for these crops would reduce the uncertainty associated with the estimated drinking water concentrations.

19 Table 8a. Tier II Upper Tenth Percentile EECs for Disulfoton Parent Used on barley, cotton, potatoes,

tobacco, and spring wheat for several application ( Label maximum) rates and management scenarios estimated using PRZM3/ EXAMS in Index Reservoir with Percent Crop Area. Mean of Annual Means ( µ g/ L) 0.95 0.51 0.23 1.05 0.94 1.31 0.86 0. 38 Concentration ( µ g/ L)

( 1 in 10 annual yearly maximum value) Annual Avg. 1.61 1.22 0.43 1.30 1.09 2.77 2.38 0. 48 90 Day Avg. 6.09 4.16 1.61 5.22 4.38 11.14 9.62 1. 79 60 Day Avg. 7.69 5.59 2.32 7.53 6.19 16.23 14.09 2. 41 21 Day Avg. 11.67 9.82 3.86 12.73 9.59 30.14 26.56 3. 88 96 Hour Avg. 14.18 13.57 6.24 17.17 11.77 40.39 35.24 5. 76 Peak 15.51 14.88 7.13 18.83 13.09 44.20 38.57 6. 32 Disulfoton Application Rate / Number of Apps / Interval / Incorp.

Depth / method 1 lb. ai/ A / # / days / inches 1.0 / 2/ 21/ 0/ f 0.83/ 2/ 21/ 0/ s ( granular)

1. 0 / 3/ 21/ 0/ s 4.0 / 2/ 14/ 2.5/ s 1.0/ 3/ 14/ 0/ f 4.0/ 1/ 0/ 2.5/ s ( granular)

4.0/ 1/ 0/ 2.5/ s 0.75/ 2/ 30/ 0/ f Crop 2 Barley Barley Cotton Potatoes Potatoes Tobacco Tobacco Spr. Wheat 1 Method of application: f = foliar and s = soil 2 PCA Barley, Potatoes, Tobacco = 0.87 ( default value for all ag. land) ; cotton = 0.20, Spring wheat = 0.56 20 Table 8b. Tier II Upper Tenth Percentile EECs for Total Disulfoton Residues Used on barley, cotton,

potatoes, tobacco, and spring wheat for several application ( Label maximum) rates and management scenarios estimated using PRZM3/ EXAMS with Index Reservoir and Percent Crop Area. Mean of Annual Means ( µ g/ L) 4.21 5.42 2.55 10.42 9.49 8.70 8.01 3.68 Concentration ( µ g/ L)

( 1 in 10 annual yearly maximum value) Annual Avg. 7.62 10.01 5.10 13.44 16.72 16.25 15.99 4.88 90 Day Avg. 18.04 26.30 12.82 26.91 25.85 53.36 5347.00 11.03 60 Day Avg. 22.33 27.99 14.10 30.06 27.87 66.65 63.97 12.24 21 Day Avg. 29.47 32.50 17.91 32.41 30.21 85.43 85.04 13.84 96 Hour Avg. 33.30 37.64 20.22 35.64 33.56 100.31 99.44 15.09 Peak 34.53 39.05 20.83 36.57 34.37 104.92 103.79 15.48 Disulfoton Application Rate / Numberof Apps / Interval / Incorp.

Depth / method 1 lb. ai/ A / # / days / inches 1.0 / 2/ 21/ 0/ f 0.83/ 2/ 21/ 0/ s ( granular)

1. 0 / 3/ 21/ 0/ s 4.0 / 2/ 14/ 2.5/ s 1.0/ 3/ 14/ 0/ f 4.0/ 1/ 0/ 2.5/ s ( granular)

4.0/ 1/ 0/ 2.5/ s 0.75/ 2/ 30/ 0/ f Crop 2 Barley Barley Cotton Potatoes Potatoes Tobacco Tobacco Spr. Wheat 1 Method of application: f = foliar and s = soil 2 PCA values for Barley, Potatoes, Tobacco = 0.87 ( default value) ; cotton = 0.20, Spring wheat = 0.56 21 b. Ground water For this assessment, the maximum rate and number of disulfoton applications were used, while assuming conservative environmental properties ( 90 percent upper confidence bound on the mean aerobic soil half life of 6.12 days and an average Koc value of 551 mL/ g) . The maximum disulfoton concentration predicted in ground water by the SCI GROW model ( using the maximum rate 4 lb. a. i. / ac and 2 applications  potatoes) was 0.05 µ g/ L. The maximum total disulfoton residue concentration predicted in ground water by the SCI GROW model for the same scenario is 3.19 µ g/ L ( except 90 percent upper bound on mean half life of total residues is 259.6 days) .

It should be noted that all the detections of disulfoton residues in ground water in Wisconsin ( range 4.0 to 100.0 µ g/ L) and some detections in Virginia ( range 0.04  2.87 µ g/ L) exceeded the concentrations predicted by SCI GROW ( 0.05 µ g/ L) . Although SCI GROW, which is thought to be conservative ( e. g. , a vulnerable site) , is based on a regression relationship between monitoring data ( detected concentrations) and pesticide fate chemistry at vulnerable sites, SCI  GROW does not account for preferential flow, point source contamination, pesticide spills, misuses, or pesticide storage sites. Many unknowns, data limitations, such as on site variability, are also present in the prospective ground water monitoring studies which were not included when developing SCI GROW. The difference between monitoring and modeling is discussed further in the next section.

vi. Disulfoton Monitoring Data Ground Water: Monitoring Studies With No Disulfoton Residues Detections in Ground Water: The Pesticides in Ground Water Data Base ( USEPA, 1992) summarizes the results of a number of ground water monitoring studies conducted which included disulfoton ( and rarely the disulfoton degradates D. sulfone and D. sulfoxide) . Monitoring, with no detections ( limits of detections ranged from 0.01 to 6.0 µ g/ L) , has occurred in the following states ( number of wells) : AL ( 10) , CA ( 974) , GA ( 76) , HI ( 5) , IN ( 161) , ME ( 71) , MS ( 120) , MN ( 754) , OK ( 1) , OR ( 70) , and TX ( 188) . The range of detection limits, especially the high ones ( e. g. , 6 µ g/ L) reduce the certainty of these data.

One hundred twenty wells were analyzed in MS for disulfoton degradates sulfone and sulfoxide and 188 wells were analyzed in TX for sulfone. Limits of detection were 3.80 and 1.90 µ g/ L for the sulfone and sulfoxide degrade, respectively, in MS. There were no degradates reported in these samples.

North Carolina: The North Carolina Departments of Agriculture ( NCDA) and Environment, Health, and Natural Resources ( DEHNR) conducted a cooperative study under the direction of the North Carolina Pesticide Board ( NCIWP, 1997) . The purpose of the statewide study was to determine if the labeled uses of pesticide products were impacting the ground water resources in North Carolina. The study was conducted in two phases. In phase one, 55 wells in the DEHNR Ground Water 22 Section' s ambient monitoring network representing the major drinking water aquifers of the state were sampled at least twice and analyzed for selected pesticides. In phase two, 97 cooperator monitoring wells were installed and subsequently sampled at least twice in 36 counties across the North Carolina. Sites for the cooperator monitoring wells were chosen based on an evaluation of the vulnerability of ground water to risk of contamination from the use of pesticides.

Monitoring wells were located adjacent to and down gradient from areas where pesticides were reported to have been applied ( within 300 feet) during the previous five years. Wells were constructed so that the shallowest ground water could be collected for analysis. The objective of these criteria was to use a scientific method for determining monitoring well locations so that the results could be used as an early indication of the potential for problems associated with pesticides leaching to ground water. Disulfoton residues were monitored for in five North Carolina counties, Alleghany, Ash, Beaufort, Madison, and Robeson. Seven wells were located in Christmas Tree growing areas, one in wheat growing county, and two in tobacco areas. The study authors make the following statement, " Results cannot be interpreted as representing the quality of ground water near pesticide use areas statewide because the study methods targeted areas of highly vulnerable ground water" .

There were no detections of disulfoton, disulfoton sulfoxide, and disulfoton in the ground water monitoring study conducted in North Carolina. Efforts were made to place the wells in vulnerable areas where the pesticide use was known, so that the pesticide analyzed for would reflect the use history around the well. Limitations of the study include that sites were sampled only twice and the limits of detections were high ( e. g. , > 1.0 µ g/ L) for some of disulfoton analytes. Uncertainties associated with the study include whether two samples from eight wells are adequate to represent the ground water concentrations of disulfoton residues, did DRASTIC correctly identify a site' s vulnerability, and were the wells placed down gradient of the use areas.

The study used tools and information available at the time of the study to identify vulnerable locations for well placement. This included statewide agricultural data from the N. C. Agricultural Statistics which were used to identify crop growing areas, the USEPA DRASTIC method ( Aller et al. , 1987) was used to locate the most vulnerable locations in the target crop growing areas, and local county agents of the USDA Natural Resources Conservation Service ( NRCS) helped identify cooperators farmers for placement of wells. The Pesticide Study staff and county agents also met with the cooperators to obtain pesticide use information. Other studies have shown that DRASTIC is not as good a method to identify vulnerable areas as hoped. The study appeared to QA/ QC practices.

Monitoring Studies With Disulfoton Detections in Ground Water: Two of the studies cited in the PGWDB ( USEPA, 1992) report the detection of disulfoton residues in ground water. The disulfoton detections in ground water in occurred studies conducted by Virginia Polytechnic Institute and State University ( VPI& SU, Mosaghimi, 1989) in Virginia where disulfoton concentrations ranged from 0.04 to 2.87 µ g/ L and in a Wisconsin Department of Natural Resources study in Wisconsin ( WDNR, after Barton, 1982) where concentrations ranged from 4.00 to 100.00 µ g/ L. Of specific are the disulfoton concentrations of parent disulfoton reported in these studies ( VA and WI) that exceeded the estimate of 0.05 µ g/ L obtained from EFED' s 23 SCI GROW ( ground water screening model) model.

Virginia: A monitoring study was conducted to evaluate the effectiveness of Best Management Practices ( BMP) in a 3616 acre watershed in the Nomini Creek Watershed, Westmoreland County, Virginia. Approximately half of the watershed is in agriculture and the other half is forested. The major focus of this study was surface water quality rather than ground water quality. However, in addition to the surface water monitoring, twelve wells were analyzed for pesticides, including disulfoton.

Samples were taken in 1985 and 1986 from four household wells in the Nomini Creek Watershed ( NCW) . Water samples from these wells were analyzed for 24 pesticides. Detectable levels of ( not specified) pesticides were found in all four wells at concentrations below the respective MCL. One of these four household wells consistently had higher pesticide levels than the other wells. The study authors suggested that this household well was not " sufficiently protected and was contaminated by surface runoff from adjacent land" .

Based upon these results of the four household wells sampled, eight pairs of ground water monitoring wells ( 39 to 54 feet deep) were installed at eight sites in the NCW and sampled approximately monthly from June 1986 through December 1990. Information concerning farming practices in the watershed was obtained from farmer interviews and questionnaires. Disulfoton residues ( 0.04, 0.10, 0.10, 0.13, 0.16, and 2.87 µ g/ L) were detected in wells at five of the eight monitoring sites during the period 11/ 86 to 12/ 90. The average detection was 0.57 µ g/ L ( standard deviation = 1.13 µ g/ L) . Since the study authors present no information or discussion questioning the pesticide detections which occurred in the monitoring wells ( notably site GN3, the well with 2.87 µ g/ L) , the disulfoton detections found in the monitoring wells should be included in this assessment.

Table 9. Summary of Disulfoton Detections in ground water from the eight ground water monitoring wells in Nomini Creek Watershed ( Virginia) , during 1986 and 1987.

Sampling Date Well Site Number Concentration ( g/ L)

11/ 5/ 86 GN3 2.87 11/ 5/ 86 GN6 0.04 3/ 13/ 87 GN4 0.10 8/ 20/ 87 GN1 0.13 8/ 20/ 87 GN2 0.16 8/ 20/ 87 GN3 0.10 24 The study was conducted under a Quality Assurance/ Quality Control Plan. Pesticides were determined using GLC methods with an EC Ni63 detector. The study reportedly ran until 1995 ( data available only goes through 1990) .

Wisconsin: Barton, 1982. In May and June 1982, the Wisconsin Department of Natural Resources ( WDNR) sent twenty nine water samples from wells in the Central Sands area of Wisconsin to the EPA' s Office of Pesticide Programs for pesticide residue analysis. Samples were taken from one municipal well, two or three community wells, and twenty five home wells; all of which were sources of drinking water. Of the 29 samples, 15 samples were reported as no detects whereas 14 samples were reported disulfoton detections. Disulfoton detections ranged from 4.00 to 100.00 µ g/ L, with a mean ( samples with detections) of 38.43 µ g/ L and standard deviation of 31.56 µ g/ L. No detection limit was specified for disulfoton, although detections as low as 1 µ g/ L are reported for other pesticide residues ( aldicarb, and aldicarb sulfone, dinoseb, sencor, linuron, carbofuran, and Lasso/ Bravo) .