GROUNDWATER RESIDUES OF ATRAZINE AND ALACHLOR UNDER WATER-TABLE MANAGEMENT-PRACTICES

This study was conducted to investigate the effects of various water-t able management (WTM) practices on the concentrations of two surface-a pplied herbicides, atrazine and alachlor in a shallow groundwater syst em. Groundwater samples were collected by installing piezometers and s uction tubes at Iowa State University's research centers near Ames and Ankeny during three corn-growing seasons, 1989-1991. At the Ames site , experiments were conducted by maintaining constant water-table depth s (WTD) of 0.3, 0.6, and 0.9 m in nine field-type lysimeters, and grou ndwater samples were collected from various depths during the corn-gro wing seasons. At the Ankeny site, a dual-pipe subirrigation system was installed on a 0.85 ha field, and variable water-table depths were ma intained. Analysis of water samples collected in 1989, 1990, and 1991 clearly indicates that atrazine and alachlor concentrations in groundw ater could be substantially reduced by maintaining shallow WTD during the growing season. It was also observed that atrazine concentrations were higher than those of alachlor. Alachlor was not detected in many samples; however atrazine was detected in all samples, with high conce ntrations at the Ames site at the 0.9 m WTD, and at the Ankeny site at deeper WTD. Pesticide concentrations in groundwater decreased with so il depth and time. Results of this study suggest a positive influence of WTM practices in reducing pesticide concentrations in groundwater.