HERBICIDE AND TRACER MOVEMENT TO FIELD DRAINAGE TILES UNDER SIMULATEDRAINFALL CONDITIONS

The extent of herbicide and tracer leaching to field drainage tiles ma y help to predict chemical movement to deeper groundwater systems. Fie ld experiments were conducted in 1988 and 1989 to measure herbicide an d tracer movement to tile lines during and immediately after a simulat ed heavy rainstorm. The eight tile lines monitored were 1.2 m deep and 3.4 m long. In 1988, alachlor (2-chloro-N-(2,6-diethylphenyl)-N-(meth oxymethyl) acetamide), cyanazine -1,3,5-triazin-2-yl]amino]-2-methylpr opanenitrile) and Rhodamine WT dye were applied to 4.5-m2 plots direct ly over field drainage tiles. In 1989, alachlor, cyanazine, Rhodamine WT, and pendimethalin (N-(1-ethylpropyl)-3,4 dimethyl-2,6 dinitrobenze namine) were used. Chloride or bromide tracers were also soil applied. The plots were irrigated 24 h after chemical application with a rainf all simulator. In 1988, 53 mm of rainfall was applied, whereas 81 mm o f rain was applied in 1989. Tile effluent was intensively sampled for 8 h after irrigation. In 1988, herbicide and Rhodamine dye concentrati on peaks ranged from 1 to 38 mug l-1. In 1989, concentrations were hig her than in 1988, with alachlor and cyanazine concentration peaks exce eding 500 mug l-1 from one tile line. In contrast, pendimethalin was f ound in only one sample, barely above the detection limit. Rhodamine W T was found to be useful as a tracer to study the movement of alachlor and cyanazine in the soil profile. In all tile effluent samples conta ining Rhodamine WT, alachlor and cyanazine were also detected. In both years, herbicide and dye concentrations peaked within 130 min after t he start of rainfall. The rapid solute movement to the 1.2 m tile dept h suggests that preferential flow is an important mechanism affecting chemical transport through structured soils.