Assessment of the Fate of Two Herbicides in a Wyoming Rangeland Soil: Column Studies

Extensive use of dicamba (2-methoxy-3,6-dichlorobenzoic acid) and picloram (4-amino-3,5,6-trichloropicolinic acid) in arid Wyoming, along with large volumes of irrigation water used in some areas, has created a concern for the potential contamination of surface and groundwaters by these herbicides. Persistence and mobility of dicamba and picloram were investigated in a Wyoming rangeland soil using batch adsorption and soil column studies. The objectives of this study were to characterize soil chemical and physical properties that affect herbicide transport, examine herbicide sorption, model herbicide movement, and estimate degradation rate constants. Essentially no sorption of dicamba was detected; however, picloram sorption was greatest in the highest organic C content horizon. Both saturated (5.90, 2.96, and 0.82 kg ha⁻¹ dicamba and 1.85, 0.97, and 0.47 kg ha⁻¹ picloram) and unsaturated (2.76 and 1.00 kg ha⁻¹ for dicamba and picloram, respectively) column experiments were conducted. The herbicides and Br tracer (34, 38, 69, and 137 mg L⁻¹) were displaced through the soil columns using distilled water that was added in daily increments (60 mL d⁻¹). Degradation rate constants were calculated using both a simple recovery fraction technique and by matching LEACHP-generated breakthrough curves to experimental data. For the two columns receiving intermediate application rates, anaerobic picloram dissipation was more rapid (t₁/₂ = 19 d) than for aerobic conditions (t₁/₂ = 87 d). The rate of dissipation of dicamha was approximately the same under aerobic and anaerobic conditions (t₁/₂ = 15 and 17 d in the saturated and unsaturated columns, respectively). Piclorum and dicamba dissipation was more rapid at the lowest application rates, with t₁/₂ of 13 and 10 d. At the highest application rates, t₁/₂ of 23 and 17 d were measured for picloram and dicamba, respectively. Both herbicides were found to be highly mobile, with the mobility of picloram increasing at higher pore-water velocities.