Relation of landscape position and irrigation to concentrations of alachlor, atrazine, and selected degradates in regolith in northeastern Nebraska

Concentrations of alachlor, its ethanesulfonic acid degradate, atrazine and its degradates, deethylatrazine and deisopropylatrazine, in the upper regolith and associated shallow aquifers were determined in relation to landscape position (floodplains, terraces, and uplands) and irrigation (nonirrigated and irrigated corn cropland) in 1992. Irrigated and nonirrigated sites were located on each landscape position. Samples were collected from three depths. Canonical discriminant and multivariate analyses were used to interpret data. Herbicides and their degradation products tended to be present in soils with high percent organic matter, low pH, and low sand content. Atrazine was present more frequently on the floodplain at all depths than the other compounds. Atrazine (maximum 17.5 µg/kg) and ethanesulfonic acid (maximum 10 µg/kg) were associated with landscape position, but not with irrigation. Alachlor (maximum 24 µg/kg), deethylatrazine (maximum 1.5 µg/kg), and deisopropylatrazine (maximum 3.5 µg/kg) were not significantly associated with either landscape position or irrigation. Ground-water analytical results suggested that concentrations of these herbicides and degradates in ground water did not differ among landscape position or between irrigated and nonirrigated corn cropland.