A field lysimeter experiment was conducted during the summers of 1993 and 1994, at the River Laboratory, Utah State University, Logan, Utah. The intent was to study the fate and disposition of metolachlor and 2,4-D in a one dimensional soil system. Metolachlor was applied to corn and 2,4-D was applied to turf. The mobility, persistence, and degradation of these herbicides were investigated in the field lysimeters using four irrigation treatment levels on corn and two irrigation levels on turf. The four irrigation levels applied to corn were 0, 30, 60, and 120 percent of potential evapotranspiration. The two irrigation levels applied to turf were 75 and 120 percent of potential evapotranspiration. The experimental design is a randomized block design with each irrigation treatment replicated three times. Soil moisture content was measured in all lysimeters at five depths twice weekly before and after irrigation to monitor soil moisture fluctuation. A neutron probe was used to measure soil moisture content. Potential evapotranspiration was determined from a weather station at the site. Irrigation amount was then computed based on actual plant evapotranspiration demand. Pesticide mobility and persistence was determined by collecting soil samples from four depths once every week after the second irrigation. Samples were collected from all lysimeters to determine the effect of irrigation level on pesticide mobility. Results indicate that under field conditions irrigation levels for both metolachlor and 2,4-D have no effect on pesticide mobility and concentration in the soillysimeters. Neither the amount of water applied nor the irrigation system used significantly affected pesticide leaching in the lysimeters especially if the amount of applied water was less than the crop potential evapotranspiration. Irrigation levels also seem to have little effect on pesticide half-life. Metolachlor half-life ranged from 12 to 18 days in 1993, and from 15 to 25 days in 1994. The half-life of 2,4-D was about 18 days in 1993 and 16 days in 1994. The metolachlor adsorption coefficient was found to range between 0.4 and 0.57 in the top 0-30 em and between 0.21 and 0.45 in the 30-60 em interval. The degradation coefficient for 2,4-D could not be determined in the laboratory. The reason for failing to determine the adsorption of 2, 4-D is not known. It might be due to the high volatility of the compound. A pesticide simulation model using chemical and soil parameters determined in this field experiment was applied. It indicated that proper herbicide and irrigation management would allow enough time for the pesticide to degrade in the soil and prevent its leaching to the water table. The rate at which a herbicide reaches the water table depends also on the depth to the water table and the presence of preferential flow paths in the soil. Proper consideration of these factors is essential to prevent groundwater contamination
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