Publication Summary

EPA was expected to finalize a rule this year directing states to develop pesticide management plans for four restricted-use pesticides: atrazine, simizine, metolachlor, and alachlor. Under this rule, EPA-approved plans would be required to continue use of these pesticides in the state. The Washington State Department of Agriculture is the lead agency. The U.S. Geological Survey and the Washington State Department of Ecology tested the feasibility of using EPA′s Pesticide Root Zone Model (PRZM2) model and a Geographic Information System (GIS) to screen for susceptibility on a regional scale. Ecology ran the model and mapped the results. The USGS has completed a review and evaluation comparing model results to observations and data obtained during the National Water Quality Assessment (NAWQA) study, which will be published in a separate report.

EPA was expected to finalize a rule this year directing states to develop pesticide management plans for four restricted-use pesticides: atrazine, simizine, metolachlor, and alachlor. Under this rule, EPA-approved plans would be required to continue use of these pesticides in the state. The Washington State Department of Agriculture is the lead agency. The U.S. Geological Survey and the Washington State Department of Ecology tested the feasibility of using EPA′s Pesticide Root Zone Model (PRZM2) model and a Geographic Information System (GIS) to screen for susceptibility on a regional scale. Ecology ran the model and mapped the results. The USGS has completed a review and evaluation comparing model results to observations and data obtained during the National Water Quality Assessment (NAWQA) study, which will be published in a separate report.

Ecology produced susceptibility maps based on PRZM2 model results for atrazine, using NRCS county-level soil data in the Columbia Basin Irrigation Project Area, Franklin, and Grant Counties. Other inputs include meteorological, crop, and pesticide data obtained from the EPA Center for Exposure Assessment Modeling. Programming assistance was provided by the U.S.G.S.

PRZM2 is a one-dimensional model that tracks the mass balance of pesticide and recharge in the soil column over time. PRZM2 is coupled with a vadose zone model, VADOFT, which can extend the modeling to a specified depth. If you know the depth to water, soils characteristics, and vadose zone characteristics below the soil profile, you can model the fate of the pesticide at the top of the water table.

Because the soils data is of much better resolution than vadose zone information, the strategy was to model the susceptibility of the soils first, group the results, and run scenarios for the vadose zone. Two trial VADOFT runs, one in the Quincy Basin and one in the Pasco Basin, were completed. Both trials demonstrate how the model results at succeeding depths can be compared to a critical concentration, such as the maximum contaminant level (the maximum concentration allowed in drinking water under the Safe Drinking Water Act). With further modeling, the depth to water at which an impact is probable for a range of soil conditions could be explored.

Surficial geology and well logs were used to guide the inputs to the vadose zone model. A much-generalized well depth map for the area, based mostly on electronic well records, was generated to identify areas with the potential for shallow groundwater resources at risk. Ultimately, the PRZM2 modeling results could be compared to the hydrogeologic setting in terms of the surficial geology and the depth to water map. This is important for identifying the hydrogeologic settings that are most vulnerable to transport of pesticides to groundwater.

It is very important to keep limitations, assumptions, uncertainty, and the appropriate use of the model and the model results in mind. The results do not in any sense predict atrazine occurrence in ground water. They point to patterns and conditions that are likely to result in relatively higher vulnerability to atrazine transport through the surface soils.

Landscape scale patterns in soil characteristics become apparent when the model results are mapped. These patterns are related to the soil depositional environment, and also relate to how well atrazine would travel down through different soil profiles, given enough rainfall or irrigation. The model results presented in Appendices A through D and the maps provide a convenient way of comparing which soils and soil/recharge combinations are more sensitive.

The model used together with GIS allows us to track and process large amounts of information about the environment that relate to pesticide fate in the environment. It also gives us flexibility so that we can change the inputs as we come to have better estimates and information.

This project is meant to demonstrate the utility of using the PRZM model and GIS as a tool to aid in planning.

NOTE: The draft document title was: State Pesticide Management Plan Technical Support -- Aquifer Vulnerability Project