Preferential movement of pesticides and tracers in agricultural soils

Carbofuran, alachlor, atrazine, bromide (a tracer), and nitrate are applied to two tile-drained plots of com, one of which is conventionally tilled and another that is no-till. Soil water samples are collected from the plots with suction lysimeters and ground-water samples are collected from wells and tile drains. The no-till plot first showed low concentrations of atrazine and alachlor ranging from 0.1 to 0.4 μg/L below the root zone one month after application, but the conventionally tilled plot showed no pesticides below the root zone until late fall, when atrazine was detected at 0.4 μg/L. Dye studies indicated that, in the no-till plots, some of the pesticide bypasses the root zone through surface-connected macropores, whereas in the conventionally tilled plot, most of the pesticide is adsorbed within the root zone as a result of greater contact with the soil. Concentrations of bromide (which is not adsorbed) in samples collected from wells and suction lysimeters do not differ between the two plots. Nitrate is found only in that part of the profile that remains unsaturated throughout the experiment.