Irrigation and tillage effects on atrazine and metabolite leaching from a sandy soil

The degree to which management practices such as irrigation scheduling and tillage affect the amount of pesticide lost to groundwater is not well understood. The rate and amount of atrazine movement through a sandy soil were measured at a site along the Lower Wisconsin River Valley In Iowa County, Wisconsin, during the 1991 and 1992 growing seasons. Three irrigation water management regimes were compared: no-irrigation, irrigation according to water balance, and surplus irrigation. Tillage by moldboard plowing and no-till were also compared. Daily drainage was predicted with a soil-plant-atmosphere model and average atrazine [6-chloro-N-ethyl-N'-(1-methyethyl)-1,3,5-triazine-2,4-diamine] and metabolite (1992 only) concentrations in soil-water were interpolated to obtain daily concentrations. Daily solute concentrations and drainage were used to calculate solute fluxes. Modeled cumulative seasonal drainage below the root zone was 12 to 40% (47-278 mm) of total water input for all treatments in the 2-yr period. Treatment-averaged atrazine concentration in soil-water ranged from 1 to 4 mu g L(-1) throughout much of the growing season. Seasonal atrazine flux density below the root zone ranged from 0.05% (0.04 mg m(-2)) of applied rate for nonirrigated plots to 0.6% (0.52 mg m(-2)) for surplus irrigated conditions. Deethylatrazine [6-chloro-N'-(1-methylethyl)-1,5-triazine-2,4-diamine] was the primary form of atrazine residue that leached from the root zone of all treatments, followed by atrazine and deisopropylatrazine [6-chloro-N-ethyl-1,3,5-triazine-2,4-diamine]. Attenuation of DEA movement under no-tillage appeared to be greater than under plow tillage.