DEGRADATION AND TRANSPORT OF DICAMBA IN A CLAY SOIL

Dicamba (3,6-dichloro-2-methoxybenzoic acid) has been identified as one of five pesticides present in Montana groundwaters. We determined the effects of degradation and time of water application on the transport of dicamba in a Lohmiller clay soil (fine, montmorillonitic, mesic Ustic Torrifluvent). Carbon 14-labeled dicamba was surface applied (0.35 kg ha-1) to disturbed soil columns (5.0 cm diam; 29 cm length) previously conditioned with 3 mM CaCl2. The columns were allowed to incubate (23.5-degrees-C) in triplicate for 0, 14, 21, 28, and 42 d. Following incubation, the columns were attached to a vacuum chamber containing a fraction collector and leached with 3 mM CaCl2 under unsaturated conditions. Dicamba breakthrough curves were determined for each incubation period. The percentage of dicamba recovered in the column effluent decreased from 85% (of total applied) after no incubation to 9.5% after 42 d of incubation. The decline of dicamba in the effluent coincided with an accumulation of dichlorosalicyclic acid at the soil surface. Dicamba half-lives determined under batch conditions were 23.5 d at 28-degrees-C, 38 d at 20-degrees-C, and 151 d at 12-degrees-C, and were all higher than the half-life estimated from the decrease in column effluent concentrations over time (13.5 d). The simulation model, LEACHM, was used to predict transport of dicamba after the different incubation periods. LEACHM adequately estimated the mass of dicamba leached, but underestimated the maximum dicamba concentrations observed in the effluent. Both simulated and observed results indicated that the transport of dicamba can be greatly reduced if sufficient degradation of dicamba is allowed to occur before irrigation or precipitation.