ATRAZINE DISSIPATION IN UNSATURATED SOIL 
UNDER DRY AND FLOOD CONDITIONS 

William R. Roy, Ivan G. Krapac, S.F. Joseph Chou, and F. William Simmons

Illinois State Geological Survey and University of Illinois at Urbana-Champaign
 

Abstract

The chemical fate and movement of pesticides may be influenced by transient storage in unsaturated soils during periods of light rainfall. The objective of this study was to conduct a field-scale experiment to determine the relative importance of transient storage of atrazine in unsaturated soil, and its subsequent release into potential aquifers during flooding conditions. 

The field-scale experiment was conducted under a rain exclusion shelter. The study soil was a Flanagan silt loam (fine smectitic mesic Aquic Argiudoll) developed in loess over Wisconsinan glacial till. The thickness of the unsaturated zone varied from 125 to 193 cm. Water was applied to simulate rain. One plot served as a control; it received a total of 28 cm of rain during a 95-day period. The treatment plot first received 5 cm of rain during a 14-day period, then a drought was imposed for 44 days, followed by a 37-day period when heavy rains (72 cm) were applied once or twice a week. Atrazine was applied in an amount that was four times greater than that usually applied to fields (6.7 kg ha^-1) without incorporation. 

Water and soil samples were collected to characterize the fate of atrazine under the two moisture regimes. The dry period imposed on the treatment plot did not appear to result in any obvious storage of the chemicals, whereas the wet period resulted in greater leaching of atrazine, although the resulting concentrations were still less than the Maximum Contaminant Level of 3  mg L^-1. Atrazine, de-ethylatrazine, and deisopropylatrazine were detected only in the near-surface (0 to 15 cm) soil samples. Most of the applied herbicide dissipated before the onset of the dry period in the treatment plot. 

The results of this field study demonstrated that atrazine was not sufficiently persistent to be stored and then released in significantly large concentrations to the saturated zone, even when applied in relatively large amounts. The dissipation half-life of atrazine was about 44 days. This study, in addition to others, suggests that atrazine may be less persistent in surface soil than has been generally reported.