development for use in these assessments. The methods explored may be an oversimplification of hydrologic process in the environment, but afford a means to estimate conservative pesticide concentrations in vulnerable water bodies. Recommendations for improving upon modeling these waterbodies from stakeholders, the scientific community, and the public are welcome and encouraged.

##### 3.1.4.2. Modifications to Modeling Approach

Existing aquatic exposure modeling tools (PRZM5/VVWM) were used to evaluate aquatic EECs in the BEs, rather than models still under development such as the SAM. The SAM uses high resolution spatial data capturing the diversity of soil, land cover, weather, hydrology, and crop conditions/management practices and key fate and transport processes to estimate daily pesticide exposure concentrations in U.S. streams, rivers, lakes, and reservoirs. The SAM is based on a new conceptual approach for exposure assessment, and will address many of the limitations identified in the modeling of medium and high-flowing water bodies (Bins 3 and 4). Some of the improvements developed as part of the SAM are examined further below. It should be noted that the SAM conceptual approach and its methods have not be reviewed by the FWS and NMFS, so the methods discussed below are considered preliminary investigations by the USEPA into methods that could improve Bin 3 and 4 EECs. In the initial alpha version of SAM, like PRZM5/VVWM, pesticide loadings and runoff are assumed to enter receiving water bodies instantaneously. Water bodies are assumed to be completely mixed, with steady flow throughout the day. First-order washout occurs, and a mean daily concentration is calculated based on the pesticide loading, water body volume, and flow rate. Subsequent versions of SAM, however, account for longitudinal dispersion (concentration peak spreading and flattening) and the temporal re-distribution of pesticide concentration pulses at the pour point or receiving water body, times.15 based on in-stream travel In the ESA modeling approach, flowing water bodies (i.e., Bins 2, 3 and 4) should similarly account for these processes. As a contaminant moves downstream by advection and dispersion, the magnitude and distribution of its concentration changes. As seen in Figure A 3-1.6, a contaminant pulse tends to dampen and elongate as it moves from a headwater to successively larger water bodies. This can be represented by a pesticide load moving from a headwater stream (Strahler 1st order) such as Bin 2, to a higher order stream (3rd or 4th order) such as Bin 3, to a larger main stem river such as Bin 4. Smaller waterbodies, equivalent to Bin 2, are clustered around the headwater fringes of a watershed, while Bin 3 represents the habitat in the mid-reaches of the watershed and Bin 4 at the pour point. Regardless of how the model is parameterized and the magnitude of the predicted exposures, the relationship between the exposures from Bin 2 to Bin 3 to Bin 4 must reflect what is expected in the environment based on watershed hydrology and contaminant transport. Initial modeling for the three pilot chemicals has demonstrated that, contrary to known contaminant transport processes, estimates have increased in magnitude and retained a sharp-pulsed shape. As a result, the following sections discuss alternatives explored to simulate these relationships.