Environmental Fate of Pesticides-87 |
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Project Leader and Principal UC Investigators Donald R. Crosby, Dept. of Environmental Toxicology, UC Davis |
Isolation and analysis of breakdown products caused by the photodegradation
of bentazon (Basagran) resulted in the discovery of three compounds,
identified by their chemical structure, that have not been reported
previously in research literature. Two of them were further degraded in
sunlight, but one was stable. The half-life of bentazon was on the order of three to six days in both the laboratory and the field, and sunlight (ultraviolet) energy was essential for its dissipation. However, one of the photodegradation products persisted for at least a week; the toxic implications and final fate remain unknown. Bentazon was rapidly photo-oxidized in the presence of zinc oxide in the laboratory, but attempts to use this technique to reduce field residues were inconclusive. Londax Degradation Discrepancies ExplainedPrevious studies on the environmental persistence of Londax indicated that the chemical degraded slowly (half-life greater than 30 days) in water under sunlight or indoor ultraviolet radiation. However, manufacturer's data indicated a half-life of four hours. Repeating the experiment using equipment and procedures similar to those used by the manufacturer traced rapid photodegradation of Londax to the container, light source, and the presence of acetone in the water used to dissolve the Londax. Sunlight caused no degradation of Londax in the absence of acetone, supporting the conclusions from earlier studies that the herbicide is environmentally unreactive. A commercial field application of Londax (one ounce per acre) dissipated from an initial high concentration of 154 parts per billion to half that amount in 13 hours. However, irregularities in the dissipation curve made it impossible to determine a "true" half-life for the chemical. The concentration fell to 22 ppb in six days and was undetectable within 12 days. Londax concentrations in the sediment peaked at 175 ppb 67 hours after application and then slowly declined to 85 ppb 59 days after application. Quinclorac Shows Lab/Field DifferencesLaboratory methods, sensitive to concentrations as low as one part per billion, were developed to detect residues of quinclorac (BAS 514 H) in soil and water. Subsequent measurements showed that quinclorac was stable in distilled water but that it slowly (10 percent in 30 days) degraded in unsterilized field water. Degradation rate was about the same in both dark and light, suggesting primarily a slow breakdown by microorganisms. The principal photodegradation product was identified as 3,7-dichloroquinoline and it occurred along with three unidentified minor products. Under field conditions, quinclorac dissipated to undetectable levels during a 31-day period. Rapid dissipation (75 to 80 percent) during the first day was contrary to the slower rate indicated in the laboratory tests. Soil residue levels also were low. Dispersion into surrounding water may account for the low residue levels found. Volatized Chemicals Little Threat to Clean WaterA test to see if reabsorption of volatized molinate (Ordram) and thiobencarb (Bolero) could be a source of residues in Sacramneto River water produced no detectable levels of the chemicals in distilled water monitored for several days during the peak herbicide application period. These results suggest that reabsorption from the atmosphere is not a significant source of herbicide residues in the Sacramento River. A comparison of thiobencarb dissipation rates in recirculating water and unrecirculated water produced no significant differences. Bleach Away Excess SprayA combination of hypochlorite bleach (Clorox) and sunlight effectively degraded MCPA spray waste. Zinc Oxide Treatments Produce Mixed ResultsBentazon (Basagran) was rapidly photo-oxidized in the presence of zinc oxide in the laboratory, but attempts to use this technique to reduce field residues were inconclusive.
Aerial applications of zinc oxide appeared to immediately reduce bentazon
residues by 20 percent in treated waters, but this effect was overcome
within two days. Bentazon levels then declined with a halflife on the order
of five days. The normal half-life for bentazon dissipation under field
conditions is three to five days.
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