U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Distribution and Possible Biological Effects of Diazinon in the San Joaquin River and the Sacramento-San Joaquin Delta, California, February 1993

Abstract

The distribution and possible biological effects of a dormant spray pesticide, diazinon, were examined by measuring pesticide concentrations and estimating toxicity by means of bioassays at a series of sites in the Sacramento-San Joaquin Delta. A double peak of diazinon concentration was observed in early February 1993 at Vernalis on the San Joaquin River after heavy rains; similar pulses were detected at Stockton 2 days later. Two geographically separate sources of diazinon--orchards along the western side of the San Joaquin River near Patterson and orchards along the Merced River--could account for the observed pattern of diazinon concentrations in early February. Distributions of diazinon at central delta sites on Old and Middle Rivers were not pulselike; instead, the concentrations steadily increased during the sampling period. Seven-day bioassays indicated that San Joaquin River water at Vernalis was acutely toxic to Ceriodaphnia dubia (water flea) for the 12 consecutive days (February 8-19) with the highest diazinon concentrations. Examination of 96-hours LC₅₀ values (lethal concentration that kills 50 percent of test organisms in 96 hours) indicates that measured diazinon concentrations could not account for all the observed toxicity. Other pesticides present could contribute to the toxicity.