Pesticide Concentrations observed in groundwater after normal uses are highly variable and particularly dependent on the conditions soon after application. The mass of a given pesticide leaching through the soil profile can vary significantly over small to large scales in relation to soil chemistry, soil structure, topography, and the nature and activity of microorganisms. The environmental fate data submitted for registration are focused on the behavior of the pesticide in the top few inches of the soil profile or the A horizon. The implications for this are that highly standardized and simplified treatment is used of the chemical behavior as it travels through the vadose zone and groundwater. We explored the sensitivity of the PRZM-GW (now a part of the Pesticide Water Calculator) model to changes in chemical inputs with an emphasis in the impact of assumptions regarding persistence through the entire profile. Modeling was compared to laboratory and field studies of chemical subsurface behavior (especially persistence). Enhanced environmental fate datasets from subsurface fate (hydrolysis and biotic degradation) studies can assist in providing model inputs that more closely match field behavior.