Environmental Fate of Rice Pesticides - 88 |
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Project Leader and Principal UC Investigators Donald Crosby, professor, Department of Environmental Toxicology, UC Davis |
Continuing research to measure the degradation
rate of rice herbicides and their breakdown products resulted in these
findings during 1988:
For the second year, dissipation of thiobencarb (Bolero®) was compared in a field where water was recirculated after six days and one where it remained static. During the first 12 days, dissipation was slower in the recirculated water; within 30 days the herbicide concentrations were the same (20 parts per billion) in both fields; and within 60 days no thiobencarb could be detected in the water in either field. In the soil, levels of thiobencarb remained relatively constant for almost two months in the static field (980 ppb at 54 days); in the recycle field they even seemed to increase somewhat. In previous research, Londax® appeared much more persistent in the laboratory than under field conditions. However, two facts became apparent: (1) concentrations of the herbicide used in the laboratory were much higher than in the field, and (2) degradation of Londax® is due largely to small amounts of environmental oxidants in the water. Perhaps, the researchers theorized, the natural oxidants were being depleted too rapidly under the unrealistic laboratory conditions. In a trial at a lower concentration of 100 ppb-twice the maximum found naturally but still far below previous laboratory trials-Londax® had a half-life of about three days.
Other laboratory and field tests of the environmental properties of rice pesticides showed that:
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