|Environmental Fate of
Rice Pesticides - 93
Project Leader and Principal UC Investigators
Donald G. Crosby, professor, Department of Environmental Toxicology, UC Davis
J.M. McFarland, staff research associate
S.A. Mabury, graduate research assistant
J.R. Gever, graduate research assistant
J. Ito, staff research associate
L. Rucoba, undergraduate assistant
|Knowledge of what happens to pesticides used in rice farming
is critical to decisions affecting their management and regulation. Environmental
toxicologists working on this area of research are continually searching for improved
methods of analyzing the factors that affect 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
In previous work toxicologists established a direct relationship between the rate at which pesticides break down in field water and their reactivity toward hydroxyl radicals, -which are created naturally by the action of sunlight on field water. Their next step was to analyze field water for hydroxyl radicals and also for some of its principal sources.
Using a variety of sophisticated laboratory techniques, the scientists were able to determine the relative importance of different substances as sources of hydroxyl radicals. While dissolved iron, copper and hydrogen peroxide all contribute to the natural capacity of rice field water to degrade pesticides, nitrate (largely from fertilizer) is generally the most important source of hydroxyl radicals. Another significant but previously unrecognized source of the degradative agent is the carbonate radical generated from hydroxyl and dissolved carbonate minerals.
Researchers continued their studies of BoleroŽ (thiobencarb) and concerns that this herbicide may be to blame for severe stunting observed in parts of the Central Valley. Last year laboratory analysis showed no likely connection between the compound and typical California rice field soils. In 1993, however, further analysis has led toxicologists to believe that a breakdown byproduct of thiobencarb may be to blame.
The findings "strongly implicate" a toxic byproduct of thiobenearb as the cause of injury. Scientists warn that the factors necessary to create further injury exist in the Central Valley. They also suggest that there may be a connection to straw incorporation and continuous flooding.
Copper sulfate or "bluestone" is used heavily by rice growers for control of algae and tadpole shrimp. In 1991 more than 1.5 million pounds of bluestone were applied to 97,000 acres of rice - roughly 15 pounds per acre, Toxicologists wanted to analyze where all this copper goes, how much leaves the field in tailwater, how much is added to the soil, how much of it drifts off-site, and how fast it dissipates from water.
Although small proportions of aerially applied copper were found in adjacent air, soil and water, 88 percent of the application was found to have precipitated from the water in solid form within six hours and was thus largely unavailable to control algae and other aquatic pests. Total copper in 11 Sutter County soils ranged from 15 to 54 ppm and bioavailable copper from 4 to 10 ppm. Fifty ppm is roughly equivalent to 160 pounds copper per acre. The researchers now are measuring the effects of these and higher levels of copper on rice plants.
Previously toxicologists had analyzed the nature of an ultra-thin "microlayer" of organic liquid found on the surface of rice field water. They determined that this film is comprised primarily of fats from decaying plants and animals.
Researchers last year developed a "revolutionary" new technique for collecting microlayer samples with silicone-treated glass plates. The data they collected showed convincingly that certain "hydrophobic" pesticides have an affinity for this microlayer. Concentrations of carbofuran (FuradanŽ) reached about 10,000 ppm in the film, while thiobencarb (BoleroŽ) concentrations were even higher. These pesticides dissipated more slowly from the microlayer than from subsurface water, although photodegradation and transfer into water eventually reduced pesticide concentrations.
Novel Sampling Method
Toxicologists also developed a novel method for detecting and measuring fluorine- containing pesticides, such as diflubenzuron (DimilinŽ), and also their breakdown products in soil and water. Nuclear magnetic resonance spectrometry - the scanning technique widely used in hospitals - fits the bill. It requires no extraction or sample cleanup and recognizes only the fluorinated compounds of interest.
In addition to use in further studies of diflubenzuron fate, the technique also holds promise for investigation and monitoring of other fluorinated compounds which now represent 12 percent of all new pesticides.