Herbicide and tracer movement in soil columns containing an artificial macropore

Preferential flow has been shown to be an important mechanism affecting water and solute movement in some soils. The movement of agricultural chemicals to groundwater is of special concern. Soil columns containing an artificial macropore were used to study alachlor [2-chloro-N-(2,6- diethylphenyl)-N-(methoxymethyl)acetamide], cyanazine [2-{[4-chloro-6- (ethylamino)-1,3,5-triazin-2-yl]amino}-2-methylpropanenitrile], pendimethalin [N-(1-ethylpropyl)-3,4 dimethyl-2,6 dinitrobenzenamine], and chloride movement. Packed columns were modified by removing a 6-mm diam. core from the center. Herbicides and chloride were applied to the soil surface and columns were irrigated with 63 mm of a 0.0075 M calcium sulfate solution. Initial chloride breakthrough occurred much sooner in columns with a continuous macropore than in columns with either a partial or no macropore. Total chloride loss in column drainage, however, was less in columns containing a continuous macropore than in those without a continuous macropore. In contrast, alachlor, cyanazine, and pendimethalin were only detected in drainage from columns with a continuous macropore. Because herbicides were not detected in column drainage unless a continuous macropore was present, leaching studies using packed soil columns may significantly underestimate the extent of herbicide movement through a structured soil. Pesticide leaching experiments using soil columns with an artificial macropore may provide estimates that are more representative of field behavior and could be used to supplement current pesticide mobility studies required by the USEPA to support product registration.