Masters Theses

Date of Award

8-1989

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant, Soil and Environmental Sciences

Major Professor

John G. Graveel

Committee Members

G. Neil Rhodes, John T. Ammons

Abstract

The soil fate and transport of imazethapyr [(±)-2-[4,5- dihydro-4-methyl-4- (1-methylethyl) -5-oxo-1H-imidazole-2-yl] - 5-ethyl-3-pyridinecarboxylic acid] was evaluated in soil solution and undisturbed column leaching studies. Soil solution studies provided information as to the diffusion controlled fate of imazethapyr and column leaching indicated the relative magnitude of diffuse versus channelized flow to the vertical transport of imazethapyr through the soil profile.

Laboratory experiments were conducted to determine the effects of application rate, wheat (Triticum aestivum L.) straw addition, and incubation time on the concentration of imazethapyr in soil solution. Field-moist loam (clayey, kaolinitic, thermic Typic Hapludult) Ap horizon was amended with finely ground wheat straw at rates of 0 and 2 g kg-1 and imazethapyr was applied at rates of 0, 31, 62, and 124 μg kg-1. Soil solution was obtained by vacuum displacement at times of 0, 1, 2, 4, 8, and 16 days after application and analyzed to determine liquid-phase imazethapyr concentration. The concentration of imazethapyr in solution decreased exponentially as a function of time, and tended to be higher in solution from the straw-amended samples. Variation in Kd as a function of equilibration time may have been a consequence of diffusion-limited movement of imazethapyr into micropores. For days 0 to 8, the Kd tended to be lower for straw-amended versus unamended soil, perhaps as a consequence of competitive exclusion of imazethapyr by straw for sorption sites in the soil solid phase.

A tractor-mounted core sampler was constructed to obtain large undisturbed soil columns for a leaching study. An 89 000 N hydraulic cylinder was used to push a steel sampling cylinder containing schedule 40 PVC pipe into the soil. The hydraulic cylinder was then retracted to pull the pipe and enclosed soil from the ground, with the soil separating evenly at the bottom of the column. Columns 30 cm in diameter were taken to a depth of 68 cm, with compaction observed to be less than three percent.

Undisturbed soil columns were mounted vertically in the laboratory and deionized water applied at a rate of approximately 90 mL h-1. After two weeks of wetting, a tension of -0.5 kPa was applied to the bottom of one column, while another column was subjected to a tension of -2 kPa. A 25 mL pulse of 100 mmol L-1 Br-1 and 0.518 mmol L-1 imazethapyr was applied to the surface of each column. Leachate was collected as a function of time and analyzed to determine Br-1 and imazethapyr concentrations. Breakthrough curves were asymmetrical and characterized by rapid appearance of both solutes in the column effluent, with maximum concentrations occurring well ahead of one pore volume of the column. Breakthrough curves were analyzed with the one-dimensional convection-dispersion equation to determine transport parameters and predict the elution of the solutes. The first order decay constant for imazethapyr in soil solution was useful in predicting the breakthrough of imazethapyr after determining transport parameters from the Br-1 data, but distribution coefficients predicted by the model were lower than those determined from soil solution.

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