Pesticide displacement along preferential flow pathways in a Brazilian Oxisol

Previous studies conducted in tropical soils showed that pesticides of different polarity reached subsoil lysimeters within the same time intervals. This cannot be explained by pesticide transport under matrix flow conditions. The objective of this study was to elucidate pesticide transport in an Oxisol under preferential flow conditions. In a field experiment near Cuiabá, Brazil, alachlor, atrazine, chlorpyrifos, λ-cyhalothrin, deltamethrin, endosulfan-α, metolachlor, monocrotophos, simazine, and trifluralin were applied onto a Typic Haplustox. Afterwards, 40 mm day−1 of tracer solution (containing 5 g l−1 of the dye Brilliant Blue FCF and 0.015 M KBr) were applied in duplicate experiments over a period of 3 days, using either a tension infiltrometer (3.3 cm tension) or manual irrigation with a watering can. Soil monoliths were laid open, and the soil layers of 0–5, 5–10, 10–20, 20–30, and 30–40 cm were quantitatively removed. The soil of each depth interval was separated into a “blue”, a “nonblue”, and, if necessary, into a “nonseparable” fraction. Pesticide concentrations in 10–30 cm soil depth were 2.0–3.5 times higher at the dye front than in the entirely blue fraction. In the Oxisol under study, transport along preferential flow pathways contributed a major part to total pesticide displacement. This relative contribution was two to five times higher for the nonpolar than for the polar pesticides. The measured pesticide displacement reached deeper soil layers than that simulated for these compounds with the leaching model PEARL. Can irrigation, causing an occasional ponding, enhanced leaching of the corn herbicides (simazine, atrazine, alachlor, metolachlor) from the top 5 cm by a factor of 2.8–4.2, compared with strictly unsaturated infiltrometer irrigation. For the other compounds, differences in leaching behaviour between plots exceeded those between the different infiltration regimes.