Environmental Rate of
Pesticides - 85
 
 

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Project Leader and Principal UC Investigators

D. G. Crosby, Department of Environmental Toxicology, University of California, Davis

 

Objectives

Purposes of this project are to develop information that will permit use of rice pesticides without creating environmental problems outside the immediate area of use and to meet the regulatory requirements for pesticide use.

Specific objectives in 1985 were 1) to identify the environmental factors that govern movement and chemical fate of pesticides, 2) to estimate the relative importance of these factors to practical pesticide use, and 3) to apply the results toward meeting regulatory requirements and to improve management of rice pesticides.

Catalysts to Dissipate Pesticide Residues

Under natural field conditions, rice pesticides degrade to other substances and lose their effectiveness. The objective of field management is to retain pesticide effectiveness long enough to kill the weeds (with herbicides) and then have them degrade rapidly to substances that are biologically inactive and do not create flavors or odors in drainage waters. Thiobencarb (Bolero), a particularly effective herbicide, is of special interest because one of its breakdown products, thiobencarb sulfoxide, is suspected of contributing to the off-flavor of Sacramento drinking water.

In laboratory and field studies done in 1984, solid zinc oxide (ZnO) and titanium dioxide (TiO2) induced rapid degradation of Ordram 10G. The studies were expanded in 1985 to determine if these catalysts would reduce or eliminate herbicide residues in drainage water.

Amy Pettigrove, graduate .student working with Dr. Donald Crosby, experiments with ZnO effects on pesticide residues in field plots.

ZnO and TiO2 were irradiated with pesticides in water for several hours in a sunlight simulator, and the nontoxic breakdown products were identified. The half-lifes (when half the chemical has broken down) of molinate, thiobencarb, methyl parathion and carbofuran catalyzed by ZnO were 4, 11, 20, and 16 minutes, respectively.

These results indicate the broad applicability of this method of residue destruction. ZnO was not effective below pH 7, but its catlytic effect improved with increasing pH up to pH 10. Rice field water becomes more alkaline toward mid-day, making this the optimum time for ZnO application. Unlike ZnO, TiO2 used as the catalyst was not affected by pH or sunlight and was too persistent to be practical.

Bolero (thiobencarb) Dissipation Studies

The dissipation rates of Bolero were determined in a field study when Zn0 was applied at 2.7, 5.3, and 10.7 pounds per acre. The thiobencarb residue dropped 93 percent within 2 hours after Zn0 application at 10.7 pounds per acre, with proportionate decreases at the lower application rates.

Herbicide levels in the water of plots not treated with zinc were still increasing three days after application, as the Bolero granules and soil continued to release the herbicide. The herbicide release was still not complete at the end of the experiment nine days after herbicide application. This explains some of the Bolero persistence problems.

The Bolero degradation product, thiobencarb sulfoxide, chat was targeted as the cause of the off-flavor in drinking water was dissipated quickly after zinc application but slowly increased again as fresh thiobencarb came off the Bolero granules. However, the levels were far below those in Bolero treated plots not receiving zinc treatments.

Molinate (Ordram) Dissipation Studies

The effects of water depth and holding period on residues of Ordram were examined as a follow-up to experiments done in 1984. The conclusions:

  • Water depth determines the initial Ordram residues, with lower residues in deeper water, but after eight days, residues for all depths were about the same.
  • Water management after the holding period has little influence on either the Ordram dissipation rate or discharge level.
  • Dissipation rates (half-lives) alone may not accurately predict residue levels at the time of water release.
  • Random water sampling can lead to erroneous estimates of average Ordram levels because of short-term fluctuations in residues.
  • Preflood applications of Ordram results in residues chat are generally less than two percent of those from postflood applications. Pre-flood application of Ordram warrants more attention.

Apparently, some unknown substance in field water at flooding possesses the same analytical properties as Ordram. Identification of the substance is in progress, and since it was present in all preflood application samples, further study of its sources and occurrence is important so that residues will not appear too high.

In another dissipation study, Ordram 10G was applied at 5 pounds per acre when rice was at the 3-leaf stage. After three days, zinc was applied as zinc oxide or zinc sulfate at 6 pounds per acre on zinc pretreated plots and 9 pounds per acre on plots not pre-treated.

Results of the experiment were somewhat impaired because rapid water loss from seepage through the soil caused rapid loss of the Ordram. Although the Ordram concentrations were low when ZnO applications were made, the half-life of the Ordram was reduced from 22 to 2.1 hours. The rice was not harmed by zinc treatments.

Environmental Degradation of Londax, Whip and Basagran

  • Londax appears to be quite persistent, with a half-life of more than 45 days in field water and sunlight.
  • Whip decomposed rapidly in both the photoreactor and in field plots. The half-life was 2.6 hours due to photolysis and chemical hydrolysis.
  • Basagran showed intermediate persistence with a half-life of 2.9-days in field plots. More than 20 complex breakdown products were detected.

 

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