Effects of Water on the Fate of Herbicides in Irrigated Soils

The wide use of soil-applied herbicides necessitates an understanding of their fate in soil if we are to use them in the most beneficial manner. All herbicides have some degree of water solubility and the field performance of many are dependent on ample, timely availability of soil moisture. It is important that different aspects of soil-herbicide-water interactions be thoroughly investigated. An excess use of water for irrigation purposes can result in the downward movement of herbicides in the soil profile. Subsoils are less adsorptive and a decreased degradation potential exists when herbicides move below the top 15 cm. In sandy soils, or other areas where extensive irrigation is planned it is prudent to attempt to choose a herbicide with high adsorptive capacity and low water solubility. Leaching of metribuzin, metolachlor, and fluometuron was an important dissipation process for each of the chemicals over the winter months when degradation was slow. Pendimethalin dissipation was greater in alternatively flooded and dried soil than with soil water content at 1/3 bar tension or with a continuous flood. In a laboratory degradation study, over 59% of the oxadiazon persisted after 20 weeks. In a greenhouse study its biological activity was reduced and its persistence increased when applied below the soil surface

Minimizing the Potential for Groundwater Contamination from Agricultural Point Sources

An activated charcoal filtration unit was designed to remove pesticides from leftover pesticide solutions and rinsates generated under farm-like conditions. The system, fabricated for less than $1400 using readily available components, effectively removed the pesticides atrazine, benomyl, carbaryl, fluometuron, metolachlor, and trifluralin from wastewater generated on the University of Arkansas Agronomy Farm located in Fayetteville, AR. A total of 2253 L of wastewater were treated using the system. Of these 1768 L were generated from washing out the spray tank (rinsates) while 485 L stemmed from leftover pesticide solutions that were mixed, but not applied. Typical initial pesticide concentrations in the wastewater were on the order of 500 to 1000 parts per million (ppm). The final pesticide concentrations remaining after charcoal filtration were generally less than 10 ppm. Approximately 1514 L of wastewater was treated with 23 kg of charcoal before the charcoal was replaced. This resulted in an estimated pesticide loading rate on the charcoal of 0.05 to 0.10 kg pesticide active ingredient per kg activated charcoal. Incubation of alachlor-treated charcoal with a mixed culture of microorganisms resulted in approximately a 30% loss of alachlor after 21 d. These results suggest that on-site degradation of spent charcoal may be a feasible alternative to incineration, however more research is needed to fully determine its potential. A reduced adsorption of methylene blue dye with increasing amounts of trifluralin sorbed to charcoal occurred. Activated charcoal treated with 222 mg/g trifluralin sorbed only 19% of the amount sorbed by the control with no trifluralin present. These results suggest that methylene blue or other dyes might be used to indicate the remaining adsorptive capacity of a charcoal used for removing pesticides from wastewater

Residual Fluometuron Levels in Three Arkansas Soils under Continuous Cotton (Gossypium hirsutum) Production

Evidence has shown that fluometuron {N,N- dimethyl-N\u27- [3-(trifluoromethyl)phenyll urea} persists be- yond the end of the growing season when used in continuous cotton (Gossypium birsutum L.) production. Samples were taken from three soils following cotton production in 1980, 1981, and 1982. All three soils had been in production under the same herbicide use regime, fluometuron preemergence followed by fluometuron plus MSMA (monosodium methane- arsonate), since either 1976 or 1977. The fluometuron remaining in each soil was quantified using a greenhouse bioassay and a chemical extraction technique followed by high-performance liquid chromatography determinations. The fluometuron concentrations determined by bioassay and chemical extraction methods had partial correlation coefficients of 0.62, 0.91, and 0.72 for a Sharkey silty clay, a Dundee silt loam, and a Loring silt loam, respectively. Predictive equations were determined for each soil to relate chemical extraction findings to plant response. Bioassay analysis indicated nearly 2 ppmw of fluometuron in the Sharkey silty clay in October 1980, with 1 ppmw in the Dundee silt loam, and approximately 0.27 ppmw in the Loring silt loam with annual application rates of 4.0, 2.9, and 3.5 kg/ha, respectively. Fluometuron concentrations as determined by chemical analysis were 0.83, 0.34, and 0.14 ppmw, respectively. Fluometuron concentrations declined over the winter in all three soils. Samples taken in March of 1981, 1982, and 1983 showed little difference in carryover levels in the Sharkey silty clay but more yearly variation in the other two soils. Fluometuron was found in all three soils to depths of 60 cm, but more than 55% of the fluometuron was found in the upper 15 cm of each soil. A controlled laboratory study conducted with the three soils showed that both cold and dry conditions reduced fluometuron dissipation rates. In the laboratory under conditions favorable for dissipation, fluometuron had a half- life of 26 days in the Dundee silt loam, 43 days in the Loring silt loam, and 73 days in the Sharkey silty clay. In the field, dissipation was very rapid in the Loring silt loam compared to the Dundee silt loam and the Sharkey silty cla