TRANSPORT OF IMAZETHAPYR IN UNDISTURBED SOIL COLUMNS

The disappearance of imazethapyr [(+/-)-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazole-2-yl]-5-ethyl-3-pyridinecarboxylic acid] from soil solution was investigated to evaluate the transport of imazethapyr in undisturbed soil columns. Soil treated with imazethapyr was incubated for 0.15, 1, 2, 4, 8, and 16 d, at which times soil solution was obtained by vacuum displacement. Bromide and imazethapyr wert then applied to the surface of undisturbed soil columns followed by application of deionized water at approximately 3 cm d-1. The removal of imazethapyr from solution (i.e., sorption) exhibited biphasic kinetics and was well described (R2 = 0.99) by two simultaneous first-order reactions. The initial reaction was essentially instantaneous and the secondary reaction was time dependent. The classical convective-dispersive ( CD) equation was used to describe solute transport through undisturbed soil columns. Bromide and imazethapyr breakthrough curves (BTCs) were displaced to the left of one pore volume and showed considerable tailing, with imazethapyr BTCs retarded in relation to Br-. This was indicative of preferential solute mobility in the soil. Imazethapyr BTCs were similar to model simulations using transport parameters determined from Br BTCs, kinetic parameters from the imazethapyr solution concentration decay curves, and retardation factors calculated from the distribution coefficient at the 1-d incubation time. Model simulations using retardation factors from later times predicted increasingly delayed imazethapyr breakthrough with lower peak concentrations than measured. Preferential flow paths apparently reduced attenuation of imazethapyr as predicted from equilibrium adsorption measurements.