#### 3.1.4. Issues Modeling Medium- and High-Flowing Waterbodies

##### 3.1.4.1. Overview of Issues

Preliminary PRZM5/VVWM modeling of ESA aquatic Bins 3 (medium flow) and 4 (high flow) resulted in peak concentrations greater than several parts per million for all of the HUC2 regions and scenarios µg/L, (e.g., for chlorpyrifos 44,000-121,000,000 30 to 86,000 times higher than the solubility limit of µg/L). chlorpyrifos, 1,400 Several assumptions and model limitations have been identified as contributing to these overestimates:

- • The large ratio of the watershed drainage area to the water body capacity.
- • The entire watershed is treated with pesticide on the same application days (e.g., when a runoff event occurs, the pesticide loading is from the entire watershed).
- • Flow contributions from groundwater or sources upstream are not accounted for, and only runoff influences daily flows.
- • Bins 3 and 4 are much larger water bodies than the PRZM5/VVWM Standard Pond and Index Reservoir.
- • The instantaneous peak concentrations may not adequately reflect the pesticide concentrations in medium and high flowing water bodies (residence times << 1 day) within large watersheds. Though instantaneous peak concentrations have been used for smaller watersheds with water body residence times of > 1 day (e.g., Standard Pond, Index Reservoir), larger watersheds may have runoff and pesticide loadings reaching the water body at varying times based on their upstream origin.

Several modifications are examined to address these issues and limitations in the ESA aquatic modeling data14 approach for Bins 3 and 4. Atrazine monitoring are also used in a case study to evaluate the recommended refinements, given the fit-for-purpose daily sampling. The atrazine monitoring data do not account for specifically where and when applications occur, as outlined in the NAS report, so some uncertainty may exist around whether the measured concentrations capture the maximum potential concentration that could occur along the length of the receiving stream rather than at the measured location. However, as discussed in Section 3.1.4.4, the monitoring data do reflect the integration of processes that are occurring within that watershed (i.e., multiple applications, spread out over weeks, over a variety of landscapes), which is important when considering the estimation of aquatic concentrations in Bins 3 and 4. Additionally, there is a high degree of confidence in the data set due to daily sampling at a temporal and spatial scale consistent with the highest use of atrazine on corn. The PRZM5/VVWM models are edge-of-field models, designed to assess pesticide and water runoff from treated fields into water bodies much smaller than those being assessed for the Bin 3 and 4 modeling. For the pilot draft BEs, EPA and the Services agreed to use existing tools for aquatic modeling as opposed to developing new models. For this reason, a watershed/basin-scale model is not incorporated for the pilot draft BEs. The techniques discussed below for estimating exposure in Bins 3 and 4 are being considered as interim approaches for improving pesticide concentration estimates in these flowing water bodies. EPA is currently developing a basin scale model, Spatial Aquatic Model (SAM), which may provide better estimates for Bins 3 and 4. However, this model is not far enough along in its