Illinois State Geological Survey

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 and subsequent release of agrichemicals from the vadose zone into potential aquifers. The field-scale experiment was conducted under a rain exclusion shelter. Atrazine and chlorpyrifos were applied at application-rate equivalents to two experimental plots beneath the shelter. Water was applied to simulate rain. One plot received normal (average Illinois) rainfall amounts for a three-month period. The other plot first received normal rain for eight days, then a drought was imposed for 14 days, followed by a 70-day period of heavy rain. Water and soil samples were collected to characterize the concentration and distribution of the atrazine and chlorpyrifos. Atrazine was detected in the water samples, whereas chlorpyrifos was not detected in the majority of the samples. The dry period imposed on the one plot did not appear to result in any obvious storage of the chemicals, whereas the wet period resulted in greater leaching of atrazine to lower depths, although the resulting concentrations were still less than the maximum contaminant level (MCL) of 3 mg/L. Both chemicals were detected in soil samples collected from a 20- to 30-cm depth, but it appeared that both chemicals were degraded and/or transported out of the study area before the field experiment was concluded. The results of mass-transport modeling also suggested that the solution concentration of both pesticides in the vadose zone would be relatively small during both the dry and wet periods. It appeared that the one-time application of atrazine and chlorpyrifos at the label rates did not result in a sufficient mass to be stored and flushed in significant concentrations to the saturated zone. The impact of larger concentrations of chemicals such as those associated with spills or other nonroutine applications remains unknown.