Environmental Fate of Pesticides-80



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Project Leader and Principal UC Investigators

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


The continued use of pesticides is essential to the economic health of California rice production. We have striven to use pesticides responsibly with due concern for the quality of the environment. Thus, research has been sponsored annually to improve efficiency and safety of all types of pesticides. The research has three main objectives: 1) to identify and investigate environmental factors governing movement and chemical transformations of rice pesticides, 2) to estimate the relative importance of each factor to the practical use of specific rice pesticides, 3) to apply research results toward registration and improved management of rice pesticides.

The Environmental Fate of Duter

The reaction of TPTH (Duter) with hydrogen sulfide in rice field mud remains a possible explanation for the apparent dissipation of the fungicide.

What happens to MCPA? In sunlight, MCPA (dimethylamine salt) is rapidly degraded, both as spray and concentrate, to produce primarily 4-chlorocresol. While the MCPA salt remains in the form of a sticky particulate once spray water has evaporated, the chlorocresol and related photoproducts evaporate and account for the characteristic odor associated with MCPA application. Atmospheric ozone, which reacts rapidly with MCPA at higher concentrations, probably is not an important factor at field levels. The particles become increasingly acidic due to further MCPA breakdown, and any remaining dissolved chlorocresol also decomposes. The life of air-borne MCPA is estimated to be about one week.

The application efficiency of MCPA-amine spray may be as low as 50 percent, but most of the rest remains unaccounted for. Both MCPA and chlorocresol are detectable in air near treated fields during and after spraying. As expected, the ratio of chlorocresol to MCPA increases with both time and distance from the spray site as photodegradation proceeds and the chlorocresol evaporates. ,

Since 1977 UC water scientists, under an EPA grant, and in cooperation with local irrigation and drainage districts and the S.C.S., have been studying the sediment and turbidity problem in the Colusa Basin Drain. This key waterway drains over one million acres o f which several hundred thousand are devoted to rice. Researchers are shown sampling Colusa Drain water for laboratory analysis.

The continuous turbidity o f the drain is not caused by rice spill water. Studies by UC researchers, partially funded by the Rice Research Board, which began in 1972 have shown that the quality of rice spill water is very little different from that of the water entering at the upper end o f the field. Salinity and nitrogen content may be increased slightly, but turbidity is decreased because rice fields serve as settling basins.


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