The organophosphate insecticide diazinon is used for many agricultural and urban applications. The main agricultural application of diazinon in the San Joaquin River Basin occurs during the winter to control wood-boring insects in dormant almond orchards. This application period coincides with the rainy season.
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Almond orchard spraying (photograph by Dave Kim, California Department of Pesticide Regulation).
San Joaquin River Basin showing boundaries of Orestimba Creek, Bear Creek, Merced River, Tuolumne River, and Stanislaus River Basins (almond orchards shown in green).
Diazinon concentrations during winter storm runoff in Orestimba Creek, and in the Merced, Tuolumne, and San Joaquin Rivers frequently exceeded 0.35 µg/L, a concentration shown to be acutely toxic to water fleas (Kuivila and Foe, 1995; Domagalski and others, 1997; Kratzer, 1997). Although this level is acutely toxic to water fleas, the effect on other organisms is largely unknown. Concentrations in the Stanislaus River never exceeded 0.35 µg/L. On the basis of daily samples from the San Joaquin River during 1991-94, diazinon concentrations only exceeded 0.35 µg/L during January and February storm runoff (MacCoy and others, 1995).
Diazinon concentrations at the San Joaquin River near Vernalis exceeded concentrations toxic to aquatic life (0.35 microgram per liter) only during January and February because of storm runoff.
The main factors involved in the transport of diazinon in the San Joaquin River are the timing of diazinon applications and the occurrence of sizable storms during January and February. During 1991-93, 74 percent of diazinon transport in the San Joaquin River occurred during January and February. In 1994, about half of the diazinon application in agricultural areas of the San Joaquin River Basin occurred during two dry periods preceding sampled storms during January and February. The overall amount of diazinon transported in the San Joaquin River during these storms was only about 0.05 percent of the amount applied during the preceding dry periods.
About half of the total agricultural application of diazinon in 1994 occurred during two dry periods preceding storms.
A dye-tracer study was done during the February 1994 storm to estimate traveltimes in the San Joaquin River system (Kratzer and Biagtan, 1997). On the basis of storm sampling during 1993-94 and estimated traveltimes, ephemeral west-side creeks probably were the main diazinon source early during the storms, whereas the Tuolumne and Merced Rivers and east-side drainages directly to the San Joaquin River were the main sources later (Domagalski and others, 1997; Kratzer, 1997).
Most diazinon in the San Joaquin River comes from west-side creeks, the Tuolumne and Merced Rivers, and direct drainage from the east side.
The occurrence, concentrations, and transport of dissolved pesticides in storm runoff were compared in the Tuolumne River Basin for two land uses: agricultural areas and the Modesto urban area. Both storms followed the main application period of pesticides on dormant almond orchards. Six pesticides were detected in runoff from agricultural areas, and 15 pesticides were detected in runoff from urban areas. Chlorpyrifos, diazinon, DCPA, metolachlor, and simazine were detected in almost every sample. Median concentrations were higher in runoff from urban areas for all pesticides except napropamide and simazine. The lower occurrence and concentrations in agricultural runoff was partly attributed to dilution by nonstorm base flow in the Tuolumne River and by storm runoff from nonagricultural land (primarily native vegetation) (Kratzer, in press).
Sampling a storm drain during February 1995 storm (photograph by Charles R. Kratzer, U.S. Geological Survey).
Transport of chlorpyrifos, diazinon, metolachlor, napropamide, and simazine was greater from agricultural areas than from urban areas. Transport of DCPA was about the same from agricultural and urban areas. The main source of transport for the other pesticides could not be determined.
In most cases, the occurrence and relative concentrations of pesticides in storm runoff from agricultural and urban areas were related to pesticide applications. Some pesticides detected frequently, and in relatively high concentrations, in the storm drains did not relate to reported use. However, unlike agricultural use, reporting of pesticide use in urban areas is incomplete and only includes use by licensed pest control operators.
More pesticides were detected, and generally in higher concentrations, in storm runoff from urban areas compared with storm runoff from agricultural areas.