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PT05 Atmospheric Transport and Fate
(PT079) Contribution of atmospheric deposition to pesticide loads in surface water runoff.
Majewski, M1, Zamora, C1, Foreman, W2, Kratzer, C1, 1 US Geological Survey, Sacramento, California, USA2 US Geological Survey, Denver, Colorado, USA
ABSTRACT- A four-year study was conducted in which wet and dry atmospheric deposition samples were collected at six sites in the central San Joaquin Valley, California. Wet deposition samples consisted of individual rain events. Dry or bulk deposition samples were collected for periods ranging from three weeks to four months. Each sample was analyzed for 41 currently used pesticides and 23 transformation products, including the oxygen analogs of nine organophosphorus insecticides. Seven compounds were detected in 75% of the rainfall samples, and 9 were detected in 50% of the dry deposition samples. The herbicides dacthal, metolachlor, pendimethalin, and simazine, and the insecticides chlorpyrifos and diazinon were the most frequently detected pesticides in both rainfall and dry deposition. The oxygen analog concentrations (maximum/median) of chlorpyrifos (0.063/0.013 g/L) and diazinon (0.299/0.025 g/L) in rainfall were at times equivalent to or greater than the parent. A comparison of the depositional amounts (micrograms per square meter) showed that the magnitude of dry deposition was about 40% higher than wet deposition for the 11 most frequently detected pesticides over a three year period. During one storm event in a small urban watershed, 68% of the diazinon in the dissolved phase runoff water (mean of 0.76 g/L) could be attributed to coming from the rainfall (mean of 0.52 g/L). The more water soluble pesticides carbaryl, metolachlor, napropamide, and simazine also showed this trend. Chlorpyrifos, dacthal, pendimethalin, and trifluralin - compounds with water solubilities less than 1.25 x 10-3 mole/m3 and log Koc values of greater than 2.2 - had higher concentrations in rainfall than in the filtered runoff water and are presumed to have partitioned onto suspended sediments and soil organic matter on the ground.
Key words: runoff, atmospheric deposition, pesticides
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