Comment:

Response: Predicted exposure estimates from PRZM/EXAMS cannot be directly compared with the monitoring data because there are no monitoring data that specifically targeted phosmet use (application period and/or site s). NAWQA is a non-targeted monitoring study that includes analysis for a wide range of pesticides. Genera lly, targeted monitoring data are collected based on a sampling program designed to capture, both spatially and temporally, the maximum use of a particular pesticide. Typically, sampling fr equencies employed in non-targeted monitoring

studies are insufficient to documen t peak exposure values. The l ack of targeted data coupled with the fact that these data are not temporally or spatially corre lated with pesticide application times and/or areas limit the utility of these data in estimating e xposure concentrations for risk assessment purposes. Therefore model-generate d values are used for estimating acute and chronic exposure values, and the non-targeted monitoring da ta (such as NAWQA monitoring data) are used for qualitative ch aracterization. Further, the dr inking water assessment may be revised with the submission of an acceptable hydrolysis study (OPPTS 835.2120). At the writing of these responses, EFED has not yet receivedan acceptable hydrolysis study. The estimates assumed conservative values for input paramete rs based on core guideline studies which have

not been fulfilled.

Page 2 of 4Comment:

Response: EFED recognizes the qualitative use of the data presented by Ka mel et al. These data were incorporated into the phosmet dr inking water exposure assessment to highlight the uncertainty surrounding the formation of the potentially mo re toxic oxon degradat e in drinking water sources. Further, NAWQA data reported six det ections of phosmet oxon. These detections were

not concurrent with detections of phosmet parent; however given th e generally short half-lives of

oxons in natural waters and the limitations of monitoring data (see response to Comment 3.1), monitoring detections are evidence of the poten tial for acute environmental exposure to oxons. Therefore, the data presented by Kamel et al. and the NAWQA monitoring data show that the phosmet oxon degradate may form in the en vironment and under oxidative chlorination treatment processes. Further, data show that oxon degradates of many organophosphate pesticides are stable in the presence of chlorine and do not degrade until th e chlorine leaves the system. Thus oxons can persist wh ile they are in the distribution system. The data presented in Kamel et al., 2008 indicate that this may not be the case for phosmet oxon; however, these data

were used to show the formation of the oxon de gradate under oxidative ch lorination system, not

Page 3 of 4to quantitatively estimate the persistence of phosmet oxon in the di stribution system. Additional information is needed to better quantify the e xposure of the oxon under environmental conditions (e.g., additional targeted studies of finished and raw water). W ithout the additional information, the formation and persistence of phosmet oxon and the exposure of this potentially more toxic degradate remains an uncertainty and is thus qualitatively discussed and cannot be dismissed

from the assessment.

Comment:

Response: EFED believes that a discussion of the validity of the use of Kenaga nomograph values, as modified by Fletcher et al. must recognize that the values are based on a robust set of actual field residue data. Hoerger and Kenaga (1972) state that the upper limit valu es from the nomograph, represent the 95 th percentile of residue values from actua l field measurements. The Fletcher et
al. (1994) modifications to the Kenaga nomograp h are also based on measured field residues from 249 published research pa pers, including information on 118 species of plants, 121 pesticides, and 17 chemical classes. These modifications represent the 95 th percentile of the expanded data set.

EFED will consider chemical- and use-specific residue data, provided the data are sufficiently robust to account for intra- and inter-site variability as well as account for temporally variable

environmental conditions. Gowan should cite the MRIDs it believes should be reviewed by EFED for the upcoming risk assessment. These data will be consider ed in the course of conducting the risk assessment. For example, for the phosmet risk assessment, the available foliar dissipation half-life of four days, the uppe r-bound on the mean of the available data points, will be used instead of EFED’s default value of 35 days.

Page 4 of 4