Environmental Fate of
Rice Pesticides-95



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Project Leader and Principal UC Investigators

Donald G. Crosby, Professor Emeritus, Department of Environmental Toxicology, UC Davis

Charles Johnson, Undergraduate Research Assistant, Department of Environmental Toxicology, UC Davis

Gregory Jorgensen, Graduate Research Assistant, Department of Environmental Toxicology, UC Davis

Scott Maybury, Postgraduate Researcher, Department of Environmental Toxicology, UC Davis

Knowledge of what happens to the chemicals used in the production of California rice is crucial to decisions affecting their use and regulation. Toward that end toxicologists working on this project search for improved methods of analyzing pesticides in the environment; estimate the relative importance of these factors to the practical use of rice pesticides; and apply research results toward meeting regulatory requirements and improved management of rice pesticides.

Analytical Techniques

Building on previous research, toxicologists confirmed their suspicions that sunlight-generated carbonate radicals are the principal agents responsible for breaking down pesticides in field waters. Carbonate radicals are formed by the reaction of naturally occurring hydroxyl radicals and natural bicarbonate. In other words, hard water helps to break down pesticides at a faster rate. This has important implications for testing procedures, since the Environmental Protection Agency and pesticide manufacturers seldom carry out their tests under conditions where these radicals exist.

Copper Studies

Last year's report concluded that copper sulfate (blue- stone) applied to control algae and tadpole shrimp quickly precipitates out of field water. However, the continued concern of state regulatory agencies over the possibility of toxic levels of copper in released tailwater prompted further investigation.

Toxicologists applied copper sulfate in several field experiments. Results were similar to those from the previous year: combined particulate and dissolved copper never exceeded 400 parts per billion, below the federal water quality standard for drinking water (1 part per million). However, this amount exceeds the federal-state water quality standards for the protec- tion of aquatic animals. Researchers suggest that some type of simple filtration of tailwater or a holding period might sufficiently reduce the levels leaving the fields.

How might continued copper applications affect the productivity of rice fields? Soil levels of copper above 100 ppm cause stunting of rice plants and subsequently reduce grain yields. Current copper levels in rice soils average about 50 ppm. Although at least 20 years would be required for copper levels to reach serious and permanent yield reduction at present bluestone application rates, new rice varieties and alternative crops could be affected in the future. Toxicologists suggest a reduction in copper applications and a search for a more satisfactory algicide.

Herbicide Toxicity

Soils from fields treated with the herbicide thiobencarb (Bolero) can cause stunting and toxicity in rice due to the formation of a breakdown product called DCT. The substance is formed by the action of anaerobic soil bacteria on thiobencarb. DCT can be removed or prevented by drying soil in air or by application of antibiotics such as streptomycin. This may have important implications for growers who flood their fields in winter.

Fipronil Fate

Enviro.jpg (186003 bytes)Fipronil is a new insecticide under study for control of rice water weevil. Toxicologists developed an analytical method to detect and measure Fipronil in ricefield soil and water at levels down to less than one part per billion. With this new technique researchers determined that the new compound dissipates rapidly in field water. Residual levels dropped to 4-6 ppb soon after a standard application and declined to 0.5 ppb after 400 hours.

Photodegradation is the principal route of Fipronil loss. Toxicologists observed the formation of a more persistent desulfurized product, which reached maximum concentrations of 2 ppb. Further study may be warranted. Otherwise, toxicologists report that 'the properties of Fipronil look very promising.

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