some of the most vulnerable settings in the corn and sorghum belt for atrazine. Since 2010, this conclusion has been reinforced by the tracking of individual grower applications in AEEMP watersheds with the highest exposures, collected to evaluate conditions and practices driving high exposures. By tracking individual applications and agronomic practices coupled with daily monitoring, there is high confidence that AEEMP monitoring reflects actual use in these settings. This information, along with the fact that these watersheds yield some of the highest exposures across all monitored sites, indicates that these data provide a direct measure of atrazine exposure appropriate for risk assessment purposes. A subset (29) of the AEEMP sites, which had daily or near daily monitoring data from 2011 to 2014, was selected for an alternatives analysis. The subset of 29 sites draws from the original 40 sites that continually had high levels of atrazine in water, as well as 25 additional sites identified as vulnerable based on a revised set of watershed criteria described in the 2009 SAP (http://www.regulations.gov/#!docketBrowser;rpp=25;po=0;dct=SR;D=EPA-HQ-OPP-2009-0104). These 25 new sites are selected based on refined vulnerability characteristics that further maximized the potential for high atrazine concentrations. A summary of the monitoring results and the properties of the watersheds and water bodies are provided in Table A 3-1.12. The Maumee River dataset is a collection of samples collected by the Heidelberg University National Center for Water Quality Research from 1983 to 1999. Years 1987 and 1994 have been selected, as m3/s. these are the years with annual average flow rates closest to the target flow rate for Bin 4 of 100 µg/L Maximum daily average atrazine concentrations ranged from 2 to 39 from 1983 to 1999, with a µg/L maximum average daily concentration of 10 and 4 for 1987 and 1994, respectively. While the sampling intervals varied from 2 to 30 days, sampling became more frequent (daily) from May to August. Data provided by the registrant and derived from sales data, dealership interviews, registrant surveys, and farmer interviews indicate that applications occurred in sampled areas and were slightly lower than label maximum rates (i.e., generally around 1.5-2 lbs/acre/year instead of 2.5 lbs/acre/year). Atrazine applications to corn are most often pre-emergence (mid-April through mid-May in the major corn- growing areas). Post-emergence applications are most likely to occur up through the end of June, until corn reaches 12" in height. There is also some variability in timing based on geographical location. As a result, monitoring data were collected from approximately March to August. Applications to corn are modeled using a range of application dates from March 1 to August 15. Fate parameters used in aquatic modeling are provided in Table A 3-1.13. Monitored concentrations confirm that applications occurred during the predicted window, with little to no atrazine initially and increasing concentrations thereafter. Modeling is conducted using watershed and water body sizes for the various HUC2 regions. Watershed areas representative of the monitoring datasets (HUC2 regions 5, 6, 7, 8, 10, 11, and 12) range from m2 m2 5.05E+07 to 3.41E+08 for Bin 3 and 4.61E+09 to 2.16E+11 for Bin 4.

Modeling results for the refinements are presented in Table A 3-1.14 for Bins 3 and 4 (Maumee HUC 4). The monitoring results presented in Table A 3-1.12 have not been adjusted to account for a sampling interval that may have missed the peak. In most cases, the sampling was done on a daily or near daily basis, so an adjustment for these values would not be needed. Also, the modeled application rates are slightly higher than those reported by the registrant for the monitoring time periods. Therefore, modeling results may be lower than those reported in Table A 3-1.14, if the application rates are reduced.