Fate of pesticides in tropical soils of Brazil under field conditions

The potential of pesticides for nonpoint ground water pollution depends on their dissipation and leaching behavior in soils. We investigated the fate of 10 pesticides in two tropical soils of contrasting texture in the Brazilian Cerrado region near Cuiabá during an 80-d period, employing topsoil dissipation studies, soil core analyses, and lysimeter experiments. Dissipation of pesticides was rapid, with field half-lives ranging from 0.8 to 20 d in Ustox and 0.6 to 11.8 d in Psamments soils. Soil core analyses showed progressive leaching of polar pesticides in Psamments, whereas in Ustox pesticides were rapidly transported to 40 cm soil depth regardless of their sorption properties, suggesting that leaching was caused by preferential flow. In lysimeter experiments (35 cm soil depth), cumulative leaching was generally low, with < or = 0.02% and < or = 0.19% of the applied amounts leached in Ustox and Psamments, respectively. In both soils, all pesticides but the pyrethroids were detected in percolate at 35 cm soil depth within the first 6 d after application. Cumulative efflux and mean concentrations of pesticides in percolate were dosely correlated with their Groundwater Ubiquity Score (GUS). The presence of alachlor (2-chloro-2', 6'-diethyl-N-methoxymethylacetanilide), atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine), metolachlor 2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide], simazine 2-chloro-4,6-bis(ethylamino)-1,3,5-triazine], and trifluralin (2,6-dinitro-N,N-dipropyl-4-trifluoromethylaniline) throughout the soil profile and in percolate of wick lysimeters at 95 cm soil depth indicated that a nonpoint pollution of ground water resources in tropical Brazil cannot be ruled out for these substances.