Environmental Fate of
Rice Pesticides-94



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Project Leader and Principal UC Investigators

Donald G. Crosby, Professor, Dept. of Environmental Toxicology, UC Davis

J. Ito, staff research associate, Dept. of Environmental Toxicology, UC Davis

C. Johnson, student assistant, Dept. of Environmental Toxicology, UC Davis

S.A. Mabury, post-graduate research chemist, Dept. of Environmental Toxicology, UC Davis

E. Simon, student assistant, Dept. of Environmental Toxicology, UC Davis

Enviro1.jpg (181241 bytes)Knowledge of what happens to rice pesticides in soil and water is critical to decisions made about their management and regulation. Environmental toxicologists working on this area of research are continually searching for improved methods of analyzing the fate of pesticides in the environment; gauging the relative importance of those factors to the practical use of rice pesticides; and applying their research results toward meeting regulatory requirements and improved management practices.

Analytical Techniques

Researchers continued their work on developing new analytical techniques that can be used to measure the persistence of pesticides in field water. In previous work they had determined that hydroxyl radicals, which occur naturally by the action of sunlight, help pesticides to break down in field water. Last year's work focused on the influence carbonate radicals have on levels of hydroxyl radicals. Their findings suggest that high levels of carbonate - or "hard water" - may help pesticides break down at a faster rate.

Enviro2.jpg (94756 bytes)Bluestone

Growers apply relatively high amounts of copper sulfate (bluestone) to control algae and tadpole shrimp. Toxicologists wanted to analyze where the copper goes after it is applied. Their examination of rice soils in Colusa, Glenn and Sutter counties showed that very little copper remains in the water column - often not enough for effective action. Almost all the copper resides in the sediment within about 15 minutes after application. Almost none appeared to have left the field in tailwater.

Analysis of soil samples from 32 rice fields in Colusa, Glenn and Sutter counties showed that total copper ranged from 3.9 parts per million (ppm) to 301 ppm, with an average of 50 ppm. Researchers estimate that each 10-pound-per-acre application of bluestone theoretically increases soil copper levels by 3 ppm.

In an effort to determine how increasing copper levels might affect rice plant growth, researchers also conducted several greenhouse experiments and field tests. Phytotoxicity appeared when copper levels rose to 150 ppm and became increasingly severe to lethal at 700 ppm. Surprisingly, rice yields were reduced by only 10 percent at 250 ppm and 20 percent at 500 ppm, suggesting that most rice soils are a long way from harmful copper levels.

Herbicide Toxicity

Continuing research on Bolero confirms concerns that a breakdown product called DCT is causing phytotoxicity to rice under certain conditions and that the problem could spread. DCT is formed under flooded conditions and degrades when soil is exposed to air, suggesting that extended flooding may not be advisable for some fields. The experiment is being repeated to confirm results.

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