Environmental Fate of
Pesticides-87

 
 

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Project Leader and Principal UC Investigators

Donald R. Crosby, Dept. of Environmental Toxicology, UC Davis

 

Isolation and analysis of breakdown products caused by the photodegradation of bentazon (Basagran) resulted in the discovery of three compounds, identified by their chemical structure, that have not been reported previously in research literature. Two of them were further degraded in sunlight, but one was stable.

The half-life of bentazon was on the order of three to six days in both the laboratory and the field, and sunlight (ultraviolet) energy was essential for its dissipation. However, one of the photodegradation products persisted for at least a week; the toxic implications and final fate remain unknown.

Bentazon was rapidly photo-oxidized in the presence of zinc oxide in the laboratory, but attempts to use this technique to reduce field residues were inconclusive.

Londax Degradation Discrepancies Explained

Previous studies on the environmental persistence of Londax indicated that the chemical degraded slowly (half-life greater than 30 days) in water under sunlight or indoor ultraviolet radiation. However, manufacturer's data indicated a half-life of four hours.

Repeating the experiment using equipment and procedures similar to those used by the manufacturer traced rapid photodegradation of Londax to the container, light source, and the presence of acetone in the water used to dissolve the Londax. Sunlight caused no degradation of Londax in the absence of acetone, supporting the conclusions from earlier studies that the herbicide is environmentally unreactive.

A commercial field application of Londax (one ounce per acre) dissipated from an initial high concentration of 154 parts per billion to half that amount in 13 hours. However, irregularities in the dissipation curve made it impossible to determine a "true" half-life for the chemical. The concentration fell to 22 ppb in six days and was undetectable within 12 days. Londax concentrations in the sediment peaked at 175 ppb 67 hours after application and then slowly declined to 85 ppb 59 days after application.

Quinclorac Shows Lab/Field Differences

Laboratory methods, sensitive to concentrations as low as one part per billion, were developed to detect residues of quinclorac (BAS 514 H) in soil and water.

Subsequent measurements showed that quinclorac was stable in distilled water but that it slowly (10 percent in 30 days) degraded in unsterilized field water. Degradation rate was about the same in both dark and light, suggesting primarily a slow breakdown by microorganisms. The principal photodegradation product was identified as 3,7-dichloroquinoline and it occurred along with three unidentified minor products.

Under field conditions, quinclorac dissipated to undetectable levels during a 31-day period. Rapid dissipation (75 to 80 percent) during the first day was contrary to the slower rate indicated in the laboratory tests. Soil residue levels also were low. Dispersion into surrounding water may account for the low residue levels found.

Volatized Chemicals Little Threat to Clean Water

A test to see if reabsorption of volatized molinate (Ordram) and thiobencarb (Bolero) could be a source of residues in Sacramneto River water produced no detectable levels of the chemicals in distilled water monitored for several days during the peak herbicide application period. These results suggest that reabsorption from the atmosphere is not a significant source of herbicide residues in the Sacramento River.

A comparison of thiobencarb dissipation rates in recirculating water and unrecirculated water produced no significant differences.

Bleach Away Excess Spray

A combination of hypochlorite bleach (Clorox) and sunlight effectively degraded MCPA spray waste.

Zinc Oxide Treatments Produce Mixed Results

Bentazon (Basagran) was rapidly photo-oxidized in the presence of zinc oxide in the laboratory, but attempts to use this technique to reduce field residues were inconclusive.

Aerial applications of zinc oxide appeared to immediately reduce bentazon residues by 20 percent in treated waters, but this effect was overcome within two days. Bentazon levels then declined with a halflife on the order of five days. The normal half-life for bentazon dissipation under field conditions is three to five days.
 

Environmental fate of Pesticides Important to Rice Culture

"The title tells you as much as anything," says Donald R. Crosby, project leader and environmental toxicology professor for more than 25 years at UC Davis. "Where do pesticides go and what happens to them?"

UC Davis environmental toxicologist Donald G. Crosby tracks the path of rice herbicides through the environment.

"Our first objective is to make the rice growers, the University and the public aware of the persistence of rice pesticides in the environment," Crosby explains. "Our second objective is to try and examine the environmental forces that act on rice herbicides and do some predicting, so they can make some management decisions."

This year Crosby's group made major advances in studying the effects of sunlight on pesticides in water. "We found that rice pesticides do break down quite readily in water under sunlight," Crosby said. "The sunlight creates oxidants that react with the pesticide and break it down."

In one experiment, zinc oxide proved "enormously" successful at accelerating the breakdown of Basagran (bentazon). "There's no doubt in my mind that this works. We could kill the residues within a couple of hours," Crosby said.

Such a tool would help eliminate residues from improper or accidental applications, but Crosby says it is no panacea. Due to natural environmental variables - water levels, temperature and acidity consistent results would be impossible to guarantee. Furthermore, too much zinc in the environment can prove toxic, although many rice growers with poor soils must add this nutrient.

Another significant finding concerned the "hot potato" of rice pesticides - Ordram and Bolero. There had been speculation that somehow the herbicides were volatilizing into the atmosphere and reappearing in downstream river water, but Crosby's group proved that was not the case.

 

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