Field evaluation of the movement of agricultural chemical contaminants in alluvial soils with a shallow water table

The movement of agricultural chemical contaminants in alluvial soils with a shallow water table in a warm, humid environment was evaluated in this study. Bromide, a non-adsorbent tracer, was used to assess the pathways of water and solute in the soil profile. The movement and fate of three commonly used herbicides: Trifluralin, Metolachlor, and Metribuzin were determined. The data gathered from the herbicide study were used to compare with the results of simulation to validate the pesticide component of the GLEAMS model.;The study was conducted at the Ben Hur Research Farm of the Louisiana Agricultural Experiment Station. Five relatively flat plots (;Bromide concentrations were determined with an ion chromatograph from samples collected from two monitoring tubes installed at 1 m and 2 m depths in each plot. The average Br concentrations were significantly higher in the 1 m than in the 2 m depth in both drained and nondrained plots indicating that agricultural chemicals do not migrate deep into the aquifer but stay near the surface of the water table. Soils with a shallow water table are probably more vulnerable to contamination than soils with a deep water table. Higher Br concentrations at 2 m depth were observed in the drained plots suggesting a possibility of a higher pollution potential risks in areas with subsurface drainage.;Results of herbicide study showed the direct relationship of surface runoff occurrence with the time of herbicide application. The highest concentrations were contained in the first surface runoff after the application. Herbicide loss depends on runoff volume which is a function of precipitation among other variables. Estimated half life of Trifluralin in soil was 46 days, Metolachlor, 27 days, and Metribuzin, 16 days.;GLEAMS simulation results showed underprediction of runoff loss for Metolachlor and Metribuzin while Trifluralin runoff loss was overpredicted by the model.