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Domagalski, J.L. and Dubrovsky, N.M.
Sorenson, S.K., ed., Proceedings abstracts of the American Water
Resources Association's symposium on the National Water Quality Assessment
(NAWQA) Program--November 7-9, 1004, Illinois: U.S. Geological Survey Open-File
Report 94-397, p. 8.
The organophosphate pesticide, diazinon, is applied as a spray to dormant
almond and stone-fruit orchards in the San Joaquin Valley, California,
during late December through January. A storm on the evening of February
7 and morning of February 8, 1993, with more than an inch and one-half
of rain, produced runoff in the San Joaquin Valley and adjacent Coast Ranges.
Two distinct pulses of pesticide inputs to the San Joaquin River resulted
from contrasts between the soil texture and hydrology of the eastern and
western valley. The fine soil texture and small size of the western tributary
basins resulted in rapid runoff. Diazinon concentrations at Orestimba Creek
peaked (3.8 micrograms per liter) within hours of the end of rainfall and
then decreased because of a combination of dilution with pesticide-free
runoff from the nearby Coast Ranges and decreasing concentration into the
agricultural runoff. Data for the Merced River, a large eastern tributary,
are sparse but suggest that peak concentrations occurred at least a day
after those at Orestimba Creek. This may be attributable to well-drained
soils and poorly integrated surface-water drainage networks that dominate
the eastern valley, and the larger size of the Merced River basin.
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