Herbicide and tracer movement in soil
Field movement of several herbicides, rhodamine WT, chloride, and bromide was measured by sampling field drainage tiles 1.2 m deep. Under natural rainfall conditions, alachlor (2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide) and metribuzin (4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one) were detected in tile effluent samples 12 to 18 hours after a 30.7 mm rainstorm. Rainfall occurring 1 to 2 days after chemical application was primarily responsible for the elevated concentrations found. In a second experiment, a rainfall simulator was used to apply 50 to 80 mm of water to 8 plots, 24 hours after chemical application. Field drainage tiles were intensively sampled for 8 hours following the simulated rainstorm. Although tile flow rate and backfill material significantly affected solute breakthrough curves, alachlor, cyanazine (2- ( (4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl) amino) -2-methylpropanenitrile), rhodamine WT, chloride, and bromide were detected in tile effluent. The rapid breakthrough of both adsorbed and non-adsorbed solutes indicated that preferential flow is an important mechanism for the movement of surface-applied chemicals in a structured soil. Solute concentrations peaked within 130 minutes after the start of a simulated rainfall, but then decreased rapidly as matrix flow increased;Laboratory experiments were conducted to study the effect of a large, continuous macropore on chloride and herbicide movement. Packed soil columns were modified by removing a 6 mm diameter core from the center. The importance of a single macropore for water and solute movement was governed largely by the hydraulic conductivity of the surrounding soil matrix. Although higher initial breakthrough concentrations of chloride was measured in columns with a continuous macropore, total chloride recovered was less. In contrast, herbicides were only detected in drainage from soil columns containing a continuous macropore. In soil columns without a continuous macropore, no herbicides were found above the 0.4 ppb detection limit.