Environmental Fate of Pesticides-81



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Project Leader and Principal UC Investigators

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


The use of pesticides to control weeds and other rice pests is essential for rice production in California. Funding has been provided annually from the Rice Research Board not only to help support research on the effective use of pesticides but also to provide for their safe use, with special attention to maintaining environmental quality.

"MCPA environmental fate still is an important pesticide issue in California."

The main objectives are: 1) to identify and investigate the environmental factors that govern the movement and chemical transformations of rice pesticides, 2) to estimate the relative importance of these factors to the practical use of specific rice pesticides, and 3) to apply research results toward registration and improved management of rice pesticides.

MCPA environmental fate still is an important pesticide issue in California. Thirty days after application, chlorocresol remains the principal "terminal residue" of MCPA/DMA salt in the environment, except for parent herbicides. The atmospheric detection of this substance as methylether after MCPA application now is complicated by discovery that this ether is itself a major volatile and stable MCPA degradation product, although attempts to isolate it from water or the atmosphere were unsuccessful in the field.

MCPA degradation in water and bulk spray was accurately shown by measurements on single isolated spray droplets - a new technique - although MCPA also was found to be broken down primarily in particles or on surfaces rather than in liquid droplets. Lightenergized oxidation was the principal pathway for degradation. Despite some persistence in surface water, MCPA was not detected by measurements accurate down to 10 ppb in any of five 160-foot wells in treated rice areas: our successful analytical method for environmental tin compounds showed that while trimethyltin hydroxide was methylated in active sewage sludge or rice-field mud, triphenyltin hydroxide (Duter) appeared unreactive. If so, this removes a major potential for field dissipation of Duter but also reduces the possibility of bioconcentration.


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