Use of GIS and PRZM-GW to Predict Groundwater Concentrations for a Modified Acetochlor Soil Restriction On Corn, Cotton, and Soybeans.

The United States Environmental Protection Agency in collaboration with Canada’s Pest Management Regulatory Agency has recently developed a new Tier II modeling approach for predicting pesticide transport to groundwater.  This draft approach uses a modified version of the Pesticide Root Zone Model (PRZM) to predict long-term (e.g., > 30 years) groundwater concentrations at a desired depth.  In this paper we demonstrate the next generation groundwater model, referred to as PRZM-GW, and how Geographic Information Systems (GIS) can be used to help predict the conservative leaching potential of a pesticide.  

Groundwater concentrations were predicted for acetochlor and its two primary soil degradation products, acetochlor ethanesulfonic acid (Ac-ESA) and acetochlor oxanilic acid (Ac-OXA) following application to corn, cotton, and soybeans grown on coarse textured soils with low organic matter, soils that were previously restricted from use. Model parameterization relied heavily on a detailed GIS analysis of soils throughout the southeastern US and field dissipation data collected from numerous field leaching studies. Well setbacks, which accounted for abiotic degradation over a range of setback distances and groundwater flow rates, were used to evaluate the effectiveness of restricting acetochlor use on coarse textured soils near down-gradient wells.

Results of batch simulations for various setback distances, soil, crop, and physicochemical properties demonstrated that conservative upper bound groundwater concentrations were effectively reduced via well setbacks. For a typical groundwater velocity (0.066 ft/day) and a 150 foot well setback, maximum concentrations were reduced by over 95%.  Assuming a conservative (i.e., fast) groundwater velocity of 0.49 ft/day and a short well setback distance of 50 feet, maximum concentrations still decreased by nearly 20%.  Based on the results of field studies, Tier II modeling, and USEPA evaluation of risks, the label was revised such that acetochlor is restricted from use only on coarse textured soils that are within 150 feet from a well and where the groundwater is less than 30 feet below the ground surface.