HERBICIDE AND TRACER MOVEMENT TO FIELD DRAINAGE TILES UNDER SIMULATED RAINFALL CONDITIONS

The extent of herbicide and tracer leaching to field drainage tiles may help to predict chemical movement to deeper groundwater systems. Field experiments were conducted in 1988 and 1989 to measure herbicide and tracer movement to tile lines during and immediately after a simulated 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-(methoxymethyl) acetamide), cyanazine (2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methylpropanenitrile) and Rhodamine WT dye were applied to 4.5-m2 plots directly over field drainage tiles. In 1989, alachlor, cyanazine, Rhodamine WT, and pendimethalin (N-(1-ethylpropyl)-3,4 dimethyl-2,6 dinitrobenzenamine) were used. Chloride or bromide tracers were also soil applied. The plots were irrigated 24 h after chemical application with a rainfall simulator. In 1988, 53 mm of rainfall was applied, whereas 81 mm of rain was applied in 1989. Tile effluent was intensively sampled for 8 h after irrigation. In 1988, herbicide and Rhodamine dye concentration peaks ranged from 1 to 38 mug l-1. In 1989, concentrations were higher than in 1988, with alachlor and cyanazine concentration peaks exceeding 500 mug l-1 from one tile line. In contrast, pendimethalin was found in only one sample, barely above the detection limit. Rhodamine WT 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 containing Rhodamine WT, alachlor and cyanazine were also detected. In both years, herbicide and dye concentrations peaked within 130 min after the start of rainfall. The rapid solute movement to the 1.2 m tile depth suggests that preferential flow is an important mechanism affecting chemical transport through structured soils.