- • Heidelberg College monitoring data from the Maumee River in Ohio, discussed above, present 6th 7th daily samples from a to order stream. These data are used to qualitatively represent Bin 4 estimates.

The key concept to consider when making these comparisons is the relative magnitudes of exposures from the monitoring data and how Bin 3 and 4 EECs might be expected to change relative to the modeled Bin 2 estimates. For example,

- • Edge of field monitoring from Wauchope shows that in the most vulnerable case (i.e. headwater stream adjacent to treated field) that atrazine typically occurs between 1 and 5 ppm.
- • Conversely, the AEEMP data, which represents an integration of multiple applications across a mid-sized watershed, shows a spread of exposures over time for atrazine, typically between 200 and 400 ppb, representing a 5x decrease in peak exposures.
- • Finally, the Maumee monitoring data typically ranges between 1 and 10 ppb (though up to 60 ppb was detected in years with less frequent sampling) representing an order of magnitude decrease in peak exposure from the edge of field setting to a larger flowing river.

An additional line-of-evidence for a decrease in pesticide concentration with increasing water body size and flow is to consider the relative impact of dilution that would occur to an instantaneous pesticide release into the different water bodies from the same adjacent field. Assuming the same pesticide mass loading from the field, the level of dilution, based on the volume of the water bodies, would be 40 times greater for Bin 3 as compared to Bin 2 [1 m x 8 m / (0.1 m x 2 m) = 40] and would be 10 times greater for Bin 4 as compared to bin 3 [2 m x 40 m / (1 m x 8 m) = 10]. As the flow rates for these water bodies also m3/s m3/s m3/s increases by orders of magnitude (0.001 for Bin 2 to 1 for Bin 3 to 100 for Bin 4), an increase due to advection-dispersion would also be expected, reducing the pesticide concentrations in the water bodies further. While this approach does not account for additional pesticide loadings from other sources upstream of the field, it does provide the relative reduction in loading potential one might expect at the edge of the field. The monitoring data sets, when viewed together and in sequence from the headwater to the major river, show a clear pattern that matches the concept of downstream dispersion one would expect as a pesticide moves from Bin 2 to Bin 4 successively (Figure A 3-1.6). Coupled with the relative increase in dilution potential, based on the increase in volume and flowrate, as one moves from Bin 2 to Bin 4, these lines-of-evidence can be used to characterize the expected EECs in the various flowing bins. As such, an approach is being used where Bin 2 EECs are generated using the PRZM5/VVWM. Bin 3 EECs are characterized as being conservatively 5 and 10 times lower than the Bin 2 EECs, and the Bin 4 EECs are characterized as being conservatively 5 and 10 times lower than the Bin 3 EECs. Bin 2 EECs will also be provided for species that inhabit Bins 3 and 4 as a metric for exposure that could occur near shorelines where complete mixing has not yet occurred. It should be noted that, as a result of using this scaling technique, there is uncertainty around the Bin 3 and 4 EECs.

### 3.1.4.5. Modifications Evaluation, Pilot Chemicals, Final BEs

A number of recommendations received during the public comment period which are discussed below, were incorporated into EFED’s methodology for estimating PRZM5/VVWM model generated Bin 3 and 4 EECs.

Specifically, daily (24-hour) mean concentrations have been adopted in place of the initial (time zero) concentrations that had been previously used as acute EECs in the draft BEs. From an exposure