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:

A VIEW FROM ABOVE-An aerial shot, location uncertain, shows well the symmetry in a typical rice field. Researchers are determining what fate holds a variety of herbicides in these fields.

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.

"...degradation of Londax® is due largely to small amounts of environmental oxidants in the water."

Other laboratory and field tests of the environmental properties of rice pesticides showed that:

  • Quinclorac (Facet®) appeared relatively persistent under rice field conditions.
  • Bentazon (Basagran®) produces several major and relatively persistent photodegradation products. Its rate of degradation is independent of pH, and the laboratory half-life was 22 hours, comparable to the three days of outdoor sunlight measured last year.
  • Unused MCPA spray can be successfully degraded with a combination of sunlight and common bleach. Creation of small amounts of breakdown products (chlorinated solvents and chloramine) might be avoided by using a transparent plastic cover to force complete oxidation of the compound.

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