Field-scale pesticide transport and degradation in three Missouri soils

Public attention of groundwater contamination has increased interest in solute transport through soils to groundwater. A series of experiments were conducted to study the transport and degradation parameters of selected pesticides under field conditions in three Missouri soils. Soil samples were removed at selected dates after the initiation of each experiment for chemical analysis. Other samples were removed for quantifying specific characteristics of the soils involved in the transport of pesticides. Transport parameters were determined using the method of moments and a non-linear estimation routine. A method was developed to analyze soil samples for bromide at low concentrations using epithermal neutron activation analysis (ENAA). This method provided quantitative results of bromide at low concentrations which is necessary for tracing the pathways of pesticide leaching via soil cracks. This method does not require extraction, and measures bromide in any chemical or physical state. Transport parameters were determined from the chemical concentrations and comparisons were made among the treatments. The pore water velocity, atrazine degradation rate, and atrazine retardation factor were spatially autocorrelated to a distance of 15-20 meters in an alluvial soil of the Missouri River floodplain. The dispersion coefficients for bromide and atrazine were not found to be autocorrelated. The pore water velocity was highly correlated with the sand content of the surface horizon. Non-equilibrium transport of aldicarb and carbofuran was found in the Menfro silt loam soil in the initially dry plots as compared to the initially wet plots. This preferential flow was attributed to macropores present due to low moisture content in the dry plots. However, the difference between dry and wet plots was not significant due to a high degree of variability. Initial irrigation water applied immediately compared to a 24 hour delay did not have a significant influence on the transport of the pesticides. Preferential flow of atrazine and alachlor and a bromide tracer was found in the initially dry Mexico silt loam. The deeper movement of herbicides in the initially dry plots was attributed to the presence of cracks resulting from low soil water content. Knowledge of this information can be used in scheduling application of chemicals. The retardation factor estimated from soil properties was significantly higher than that estimated from the relative velocity of bromide to that of the pesticides. Degradation rates of the pesticides were higher in the initially wet plots compared to the initially dry plots.