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(P447) Changes in Organophosphate Pesticide Concentrations, 1992-2002, San Joaquin River, California.
Kuivila, Kathryn*,1, Orlando, James1, 1 U.S. Geological Survey, Sacramento, CA, USA
ABSTRACT- Two organophosphate pesticides, diazinon and methidathion, are applied to orchards in the Central Valley as dormant sprays in the winter, then transported offsite primarily by rainfall. Pulses of these pesticides have been detected in the San Joaquin River at concentrations that are toxic to aquatic invertebrates. Concentrations of diazinon and methidathion are compared from two time periods, 1992-1994 and 2000-2002, at the San Joaquin River near Vernalis. Water samples were collected daily during January through March each year and analyzed for dissolved pesticides by gas-chromatography/mass spectrometry. Maximum concentrations for each year, from 1992 to 1994, ranged from 290 to 1100 ng/L for diazinon and from 210 to 800 ng/L for methidathion. The observed variation in maximum concentration from year to year can be explained by the amount and timing of pesticide application relative to rainfall. In all three years, the peak concentrations were high enough to cause acute toxicity to Ceriodaphnia dubia in 7-day bioassays. In contrast, maximum concentrations of diazinon and methidathion during January through March 2000-2002 were nearly an order of magnitude lower. Most of the observed decrease in concentration can be attributed to a decrease in pesticide application amounts. Approximately one-sixth the amount of diazinon and one-third the amount of methidathion were applied as dormant sprays in the San Joaquin River watershed in 2000 as compared with 1992. The concentrations of diazinon and methidathion measured during 2000-2002 are below the values shown to cause mortality to C. dubia. Although the decreased use of organophosphate pesticides has resulted in lower concentrations and thus, a decrease in related toxicity, there has been a corresponding increase in the use of pyrethroid pesticides of which the biological effects are still uncertain.
Key words: organophosphate pesticides, toxicity, central valley
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