Spatial distribution of atrazine residues in soil and shallow groundwater: effect of tillage and rainfall timing

A field study began in 1986 to determine the effects of no-till (NT) and conventional-till (CT) management practices on the movement and leaching characteristics of several commonly used pesticides to shallow groundwater during corn (Zea mays L.) production. Specifically, the effect of tillage and rainfall timing on the spatial distribution of atrazine in the 0-30 cm soil depth and in shallow groundwater (less than 1 m depth) was evaluated in 1987, 1988, and 1989. These 3 years were selected because nearly the same amount of rain fell, but at different times and intensities, within the period between atrazine application and the first sampling. The average atrazine residues in the top 10 cm of soil of the two CT plots were much higher than the two NT plots in all years, regardless of the rainfall timing. This difference was about 23% in 1987 (first rain occurred 3 days after application), 56% in 1988 (12 h after application), and 63% in 1989 (6 days after application). Atrazine residue levels of 663 and 424 microgram 1 -1 were found in the samples collected, from the wells above the clay layer, 3 days after the first rain event in 1988, in the two NT plots. However, the concentrations decreased to 133 microgram l-1 and 105 microgram l-1 and 6 microgram l-1 and 5 microgram l-1 after 14 and 40 days, respectively. These relatively high residue levels in 1988 presumably resulted because the first rain event which contributed 48 mm of rain began 12 h after application and lasted about 2 days. The average residue levels of atrazine in the wells below the clay layer in the NT plots were higher than the CT plots in all 3 years. The semi-variograms for atrazine residues in the 0-30 cm soil depth showed that the residue values were spatially related for the separation distance of less than 16 m. For the well water samples, this distance was estimated to be about 30 m.