UNITED STATES ENVIRONMENTAL PROTECTION AGENCY WASHINGTON D. C., 20460 OFFICE OF PREVENTION, PESTICIDES AND TOXIC SUBSTANCES MEMORANDUM May 18, 2006 SUBJECT: Qualitative Assessment of Dichlorvos ( DDVP) in Drinking Water and Volatilization from Use of Trichlorfon Turf DP Barcode: 327193 PC Codes: 057901 ( Trichlorfon) 084001 ( Dichlorvos)

TO: Dayton Eckerson, Chemical Review Manager Special Review Branch Special Review and Reregistration Division Sue Hummel, Chemist Reregistration Branch 4 Health Effects Division FROM: R. David Jones, Ph. D., Senior Agronomist Environmental Risk Branch 4 THROUGH: Dana Spatz, Risk Assessment Process Leader Environmental Risk Branch 2 Elizabeth Behl, Chief Environmental Risk Branch 4 Environmental Fate and Effects Division The purpose of this document is to assess the nature of exposure to dichlorvos ( PC Code 084001), also known as dichlorvos, formed from the application of trichlorfon ( PC Code 057901). The reregistration of trichlorfon was completed in January 1997 ( OPP, 1997) and tolerance reassessment was completed in September 2001 ( OPPTS, 2001). In those documents, only trichlorfon parent was considered in the risk assessment, and it was decided that assessment and management of risk of dichlorvos resulting from trichlorfon application would be handled in the reregistration process for dichlorvos.

This memorandum describes qualitatively the nature of dichlorvos formation from trichlorfon and the implications for evaluating the resultant dichlorvos exposure through drinking water and inhalation. The fate and transport properties of both chemicals are briefly reviewed, as well as the use patterns for trichlorfon. This is followed by a discussion of drinking water. There are a number of assumptions in estimates of dichlorvos formation from trichlorfon that add uncertainty to the estimated drinking water concentrations reported in the existing trichlorfon assessment:

  Conversion of trichlorfon to dichlorvos: Data reviewed indicates that trichlorfon mostly degrades to dichlorvos, which subsequently degrades to other compounds and that the rate of formation and degradation are pH dependant. Where trichlorfon is used and pH is neutral or low, the assumption used in modeling that 100% of trichlorfon is converted to dichlorvos is a conservative one. This is particularly the case east of the Mississippi River. In these areas, the maximum fraction of dichlorvos formed from trichlorfon will be less than the approximately 25% seen in the pH 7 hydrolysis study. In areas with higher pH the conversion efficiency could run as high as the 52% seen in the pH 9 hydrolysis study. Higher pHs are found in arid and semi arid areas in the western United States where pedogenic lime accumulates, and soils formed from carbonatic parent materials, e. g. limestone. The practical lower pH limit for turf is 6, as lime is generally applied to maintain turf at this pH. On the upper end, pH s higher than 8.5 are seldom seen in the environment where plant stands can be maintained. Most commonly, soil pH for turf is between 6 and 7. Based on this information, the modeled estimates of DDVP derived from trichlorforn use on turf would be estimated to decrease from the concentrations estimated in the risk assessment. The peak 1 in 10 year Drinking Water Estimated Concentration would be in the range of 122 µ g L  1 in areas where the pH is 9 or greater. If the trichlorfon turf use area were limited to east of Mississippi where pH is 7 or less ( i. e. other than karst areas and where soils with higher pH occur), the peak 1 in 10 year Drinking Water Estimated Concentration would be in the range of 59 µ g L  1 .

  Use of adjustment factors and model scenario: Drinking water estimates from turf uses were adjusted in previous modeling by the default PCA for agricultural crops ( 0.87); this factor is not generally used for turf. Golf course adjustment factors are currently available to take into account percentages of areas treated in golf courses. However, golf course adjustment factors do not apply to other turf uses ( e. g. urban uses, sod farms) and trichlorfon use is dominated by use on residential lawns. Loading to surface water from application in Florida does provide estimated drinking water concentrations that are greater than those that would occur in most of the country; these values do not reflect exposure in other parts of the country where less intense rainfall occurs.

  Label requirement to   wet in  trichlorfon after application: Provided irrigation is not applied in amounts that cause runoff to occur, this practice will significantly reduce off site movement due to runoff from storm events. However, without significant additional analysis, the amount of reduction cannot be quantified.

  Maximum label rate for trichlorfon: Modeling of dichlorvos concentrations from trichlorfon application were based on molecular weight and the labeled maximum application rate of trichlorfon on turf. The maximum application rate of trichlorfon ( 8.2 lb. a. i./ A) is significantly higher than that of dichlorvos ( 0.2 lb. a. i./ A), with more frequent and more closely spaced applications allowed.

  Effects of volatilization: The amount of dichlorvos available for volatilization will be somewhat mitigated by the irrigation (  wetting in ) required on the trichlorfon labels, but the extent of the reduction cannot be quantified without significant further analysis. The maximum amount of dichlorvos available for volatilization from trichlorfon application varies with pH, and is of most concern in places with soil pH above 7.

Fate and Transport Properties of Trichlorfon The fate and transport properties of trichlorfon are discussed in the RED for trichlorfon ( OPP, 1997). Trichlorfon degradation appears to be strongly influenced by pH. In a hydrolysis study conducted at at 25 ° C, the trichlorfon degradation half life was 104 days at pH 5; 34 hours at pH 7; and 31 minutes at pH 9. The maximum measured dichlorvos formed from trichlorfon also varied with pH, with a maximum percentage converted of 2.1% at pH 5; 25% at pH 7; and 52% at pH 9. The formation of dichlorvos from trichlorfon is not a   hydrolysis reaction  per se, but a dehydrochlorination. This reaction occurs abiotically in aqueous solution and the reaction products are found in the   hydrolysis  study. The other degradates found in the hydrolysis study are des methyldichlorvos, and dichloroacetaldehyde, resulting from hydrolysis of dichlorvos directly ( Edwards, 2000), thus, they are secondary degradates formed from dichlorvos. The maximum amount of dichlorvos measured in the hydrolysis study is controlled by the relative formation and degradation rates of dichlorvos rather than a competitive degradation pathway for trichlorfon. However, because the original data for the hydrolysis study is not available, this supposition cannot be confirmed at present. Aqueous photolysis is very slow, with an estimated half life greater than 2000 days ( after correction for degradation in the dark control). Soil photolysis is also slow, with a corrected half life of 54 days or more, although this result is highly uncertain. Given the background variability in the soil photolysis study it is not certain that soil photolysis is even occurring, as all identified degradates were found in other degradation studies.

There are three metabolism studies for trichlorfon: an aerobic soil metabolism study; an anaerobic soil metabolism study; and a pond water study. These studies provide information on aerobic degradation in the water column, independent of the bed sediment. Trichlorfon degraded with a 6.4 day half life in the aerobic soil metabolism study; no DDVP was detected in the study. However, the experiment was designed so that any dichlorvos that formed and volatilized would be converted to CO 2 for measurement. The other degradates found were 2, 2, 2 trichloroacetic acid, 2, 2 dichloracetic acid and ( 1 hydroxy 2,2 dichlorovinyl) phosphate ( HDCP). The results of this study would be consistent with the hydrolysis results if the study was conducted at low pH, but the pH of this study was apparently not reported. Trichlorfon degraded with a 1.8 day half life under anaerobic conditions in soil. There are indications in the Data Evaluation Record that this study was carried out at pHs between 7 and 9, but the specific pH is not reported. Degradates were glyoxilic acid, dichloroacetic acid, dichlorvos, HDCP and 2,2 dichloroethanol. However, DDVP was never present at more than 1% of the applied trichlorfon concentration. The pond water study was conducted at two pH s   5 and 8.5. At pH 5, no degradation was observed over the 24 duration of the study. At pH 8.5, there was rapid loss of trichlorfon with no detectable amounts after 8 h. Dichlorvos was formed, and the concentration peaked at 56% at 8 h after study initiation. It is not clear whether additional degradates were monitored or identified.

Trichlorfon is highly mobile in soil, with estimated K d   s ranging from 0.4 to 0.6 L kg  1 . Trichlorfon does not appear to be volatile. A soil volatility study found less than 1% of the applied trichlorfon in organic traps. However, the study was conducted at pH 4.3, so little dichlorvos would be expected to be formed.

Submitted field dissipation studies were not acceptable because of analyte recovery problems. A lysimeter study was submitted for consideration as a field dissipation study, but was found to be unacceptable for that purpose. The pH in the lysimeter study was 5.5, which is lower than soil pHs normally found in soils where turf is grown ( turf is usually limed to pH s between 6 and 7). Furthermore, a pH of 5.5 would not be conducive to the formation of large amounts of dichlorvos from trichlorfon, based on laboratory degradation studies.

Fate and Transport Properties of Dichlorvos The fate and transport properties of dichlorvos are described more completely in the EFED RED chapter for dichlorvos ( Edwards, 2000). Dichlorvos appears to dissipate through volatilization, abiotic degradation, and perhaps also by aerobic metabolic pathways. Dichlorvos is prone to volatilization, with a vapor pressure of 1.2 x 10  2 torr. Degradation half life by hydrolysis at pH 5 is 11 days and decreases to 124 hours and 21.1 hours at pH 7 and 9 respectively. Degradates formed by hydrolysis are DCA, DAA, desmethyl dichlorvos, and glyoxilic acid. There was no discernable aqueous photolysis, and the soil photolysis half life was estimated at 10.6 days. As with trichlorfon, the uncertainties around the dichlorvos soil photolysis half life are very high. The half life of dichlorvos in an aerobic soil metabolism study was 10 hours, with DAA, DCA and 2, 2 dichloroethanol detected as degradates. In an anaerobic soil metabolism study conducted at a pH of 6.8, dichlorvos degraded with a half life of 6.3 days. Degradates in this study were dichloroacetic acid, dichloroacetaldehyde and dichloroethanol. Dichlorvos is mobile through soil with a K oc of 36.9 L kg OC  1 but is not likely to be persistent enough to occur in groundwater. With two exceptions, 2, 2, 2 trichloroacetic acid and methyl ( 2, 2, 2 trichloro hydroxyethyl) phosphonate, all degradates found in the trichlorfon degradation studies were found in the dichlorvos studies. Both of the trichloro degradates were found only in small amounts, less than 2% of the applied trichlorfon. This strongly suggests that trichlorfon mostly degrades to dichlorvos, which subsequently degrades to other compounds. The increasing maximum dichlorvos fractions found in the trichlorfon hydrolysis study would be consistent with the changing ratios of the degradation rates with pH. At pH 5, trichlorfon is forming dichlorvos at a rate 1/ 10 as fast of that rate at which dichlorvos is degrading, so little dichlorvos is found from trichlorfon at this pH. In contrast, at pH 9, dichlorvos is being formed from trichlorfon about 10 times faster than it is degrading, so dichlorvos accumulates before it can be degraded.

Trichlorfon Use Based on usage data from 1994 to 1999, about 1 million pounds of trichlorfon are used per year. Most of this is used by lawn care operators ( 74%), though a significant fraction is used on golf courses ( 18%). The remainder of the use is in minor uses such as landscaping, institutional turf, greenhouses, nurseries, and livestock. For comparison, the estimated use of dichlorvos on turf and turf type sites is less than 6000 lb/ year.

The trichlorfon label allows a maximum single application rate on turf of 8.2 lb a. i./ acre, with a maximum of 3 applications per year at a 7 day interval ( ground spray). In contrast, the dichlorvos label allows a maximum of 4 applications per year at 0.2 lb a. i./ acre with a 30 day interval.

Drinking water characterization A quantitative drinking water exposure assessment at Tier 2 was conducted by Ibrahim Abel Saheb ( 2003) for dichlorvos. This assessment included dichlorvos from the degradation of naled and trichlorfon. The EECs for the dichlorvos derived from the turf use of trichlorfon are 235 µ g L  1 for the peak 1 in ten year DWEC; 6.24 µ g L  1 for the 1 in ten year annual mean; and 3.25 µ g L  1 for the overall mean. These values reflect the use of the default PCA of 0.87, a conversion to a dichlorvos based on molecular weight, and the assumption of 100% conversion of trichlorfon to dichlorvos. The simulation was conducted using the Florida turf scenario.

The PCA of 0.87 reflects the maximum fraction of agricultural land found in any HUC 8 basin in the United States. Given that the default PCA reflects agricultural land, it is not particularly apt for turf use, which is dominantly an urban, mostly non agricultural use. Standard practice is to use a PCA of 1.0 for the turf use because of this fact, which is likely an overestimate for turf uses. In cases where the only turf use is on golf courses, there are factors that can be applied when the use is restricted to tees and greens, or tees, greens, and fairways ( OPP, 2005). However, these factors are not applicable to urban or other turf uses of trichlorfon, which is dominated by residential applications. While estimates of the fraction of turf in drinking watersheds are not available, it is known that the amount of urban land in some drinking water watersheds can be significant and that pesticides, particularly insecticides that frequently used on lawns are found with high frequency in surface water downstream from urban areas.

Loading to surface water from application in Florida, as simulated using the Florida turf scenario does provide DWECs that are greater than those that would occur in most of the country and are thus used for screening level assessment. However, these values do not necessarily reflect exposure in other parts of the country where less intense rainfall occurs.

As noted above, the conversion efficiency of trichlorfon to dichlorvos is pH dependent, ranging from 2% at pH 5, to 52% at pH 9. The practical lower pH limit for turf is 6, as lime is generally applied to maintain turf at this pH. On the upper end, pH s higher than 8.5 are seldom seen in the environment where plant stands can be maintained. Commonly, soil pH is between 6 and 7. Higher pHs are found in arid and semi arid areas in the western United States where pedogenic lime accumulates, and soils formed from carbonatic parent materials, e. g. limestone. Based on this information, the modeled estimates of DDVP derived from trichlorforn use on turf would be estimated to decrease from the concentrations estimated in the risk assessment. The peak 1 in 10 year Drinking Water Estimated Concentration from the drinking water assessment would be in the range of 122 µ g L  1 in areas where the pH is 9 or greater ( based on 52% formed). If the trichlorfon turf use area were limited to east of Mississippi where pH is 7 or less ( i. e. other than karst areas and where soils with higher pH occur), the peak 1 in 10 year Drinking Water Estimated Concentration would be in the range of 59 µ g L  1 ( 25% formed).

It is a label requirement that trichlorfon be   wetted in  after application. This likely is necessary so that it is carried into the soil to the target organisms, and also to decrease loss from volatilization. Provided irrigation is not applied in amounts that cause runoff to occur, this practice will significantly reduce off site movement due runoff from storm events. However, with significant additional analysis, the amount of reduction cannot be quantified.

Volatilization As noted above, it appears that regardless of pH, trichlorfon mostly forms dichlorvos in the degradation process. However, the relative rate of formation and degradation of dichlorvos at different pH s controls the maximum amount of dichlorvos that accumulates from a particular application of trichlorfon. As noted above, most soils nationally are below pH 7. This is particularly the case east of the Mississippi River. In these areas, the maximum fraction of dichlorvos formed trichlorfon will be less than the approximately 25% as measured in the pH 7 hydrolysis study. In areas with higher pH, which will usually be arid or semiarid, or in soils with a carbonatic parent material ( these are often karst areas), the conversion efficiency could run as high as the 52% seen at pH 9. The amount of dichlorvos available for volatilization will be somewhat mitigated by the irrigation required on the trichlorfon labels, but the extent of the reduction cannot be quantified without significant further analysis.

References D288834. Abdel Saheb, Ibrahim. 2003. Revised Drinking Water Assessment for DDVP ( PC Code 084001), from Naled ( PC Code34401), and from Trichlorfon ( PC Code 057901). Internal EPA Memorandum to Eric Olsen and Susan Hummel, dated March 16, 2003.

Edwards, Joanne, 2000, Phase 1 Comments for Dichlorvos. Internal EPA Memorandum to Pam Noyes, http:// www. epa. gov/ oppsrrd1/ op/ ddvp/ efedrisk. pdf Office of Pesticide Programs. 2005. Golf Course Adjustment Factors for Modifying Estimated Drinking Water Concentrations and Estimated Environmental Concentrations Generated by Tier I ( FIRST) and Tier II ( PRZM/ EXAMS) Models. http:// www. epa. gov/ oppefed1/ models/ water/ golf_ course_ adjustment_ factors. htm Office of Pesticide Program. 1997. Reregistration Eligibility Decision: Trichlorfon. EPA 738 R 96 017. http:// www. epa. gov/ oppsrrd1/ REDs/ 0104. pdf Office of Prevention, Pesticides, and Toxic Substances. 2001. Report on FQPA Tolerance Reassessment Progress and Interim Risk Management Decision ( TRED): Trichlorfon. EPA 738 R 01 009. http:// www. epa. gov/ oppsrrd1/ REDs/ trichlorfon_ tred. pdf cc actions