Environmental Fate of Rice Pesticides-91



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Project Leader and Principal UC Investigators

D.G. Crosby, Environmental Toxicology Dept., UC-Davis

J.M. McFarland, Staff Research Associate

S.A. Mabury, Graduate Research Assisistant

J.R. Gever, Graduate Research Assistant

C. Schrenk, Postgraduate Research Chemist.



Scientists conduct experiments exploring the environmental fate of pesticides important to rice culture to meet three general objectives:
  • To identify, investigate and model environmental factors governing the movement and chemical fate of rice pesticides.
  • To estimate the relative importance of such factors to the practical use of specific rice pesticides.
  • To apply research results toward meeting regulatory requirements and improved management of rice pesticides.

The following narrative summarizes the most important findings from the scientists' 1991 studies.

Scientists are exploring how a substance that occurs naturally infield water might be used to predict how long pesticides will persist in the environment.

A very reactive substance known as a hydroxyl radical, which is created naturally in field water by sunlight, plays an important role in the degradation of rice pesticides. These hydroxyl radicals react very rapidly with other chemicals and might be used to help predict the persistence of rice pesticides in the environment. Experiments showed that the technique worked well with carbaryl, carbofuran, MCPA and hexazinone but not with quinclorac.

Researchers know that the generation of hydroxyl radicals in field water - and, therefore, pesticide dissipation rate - is dependent upon the intensity of ultraviolet radiation from sunlight. Consequently, the researchers sought to develop a simple and inexpensive method by which sunlight UV could be measured by the rate of a photochemical reaction.

They found, unexpectedly, that the efficiency of light absorption - known as "quantum yield" - for a photolysis technique they developed is strongly affected by temperature. This finding is significant because the standard method of measuring quantum yield holds that temperature is not a factor. This will undoubtedly affect a great deal of already published literature.

Last year's research into diflubenzuron (Dimilin®) the experimental chemical proposed for control of rice water weevil - showed that sunlight was the principal factor in its dissipation under both laboratory and field conditions. Consequently, 1991 experiments were aimed at detection and measurement of the chemical's photolysis products.

One byproduct, p-chlorophenylurea (CPU); was more persistent than the parent compound. Although its halflife was only 20 hours, the potency of diflubenzuron against aquatic invertebrates is so high that treated water may have to be held as much as seven days before being safe to release.

Additionally, the researchers noted that rice field water was covered by a very thin film of organic matter - a "surface microlayer" - composed mostly of fats from decaying plants and animals. Within this microlayer, the scientists observed a much greater concentration of diflubenzuron than in the adjacent water column, a factor that may affect both pesticide fate and residue analysis.

Researchers working on this project also attempted to ascertain the reasons for the rapid dissipation of quinclorac (Facet®) from field water during the first 24 hours after application. This is a major barrier to its registration as an herbicide for commercial rice culture.

The scientists concluded the short half-life (16 hours) was not due to volatilization, photolysis or soil adsorption but that it migrates rapidly into subsurface water.

These researchers also began studying two new experimental herbicides from a Japanese company.

They found that the first chemical, KI-2023, has a short half life and may move readily into subsurface water. The second chemical, KI-6127, dissipated partly by volatilization and partly by photolysis within 25 hours. The scientists note they had insufficient samples to determine the mobility of KI-6127 through soil but that its other properties looked promising.


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