Diazinon concentrations in the Sacramento and San Joaquin Rivers and San Francisco Bay, California, February 1993

The distribution and possible biological effects of a dormant spray pesticide, diazinon, were examined by measuring pesticide concentrations and estimating toxicity using bioassays at a series of sites in the Sacramento-San Joaquin Delta and San Francisco Bay. Pulses of diazinon were observed in early February 1993 in the Sacramento and San Joaquin Rivers after heavy rains, with elevated concentrations measured for a few days to weeks at a time. The pulse of diazinon in the Sacramento River was followed from Sacramento through Suisun Bay, the eastward embayment of San Francisco Bay. In the central delta, well-defined pulses of diazinon were not observed at the Old and Middle River sites; instead, the concentrations steadily increased throughout February. Ceriodaphnia dubia mortality was 100% in water samples collected for 12 consecutive days (February 8-19) from the San Joaquin River at Vernalis. The bioassay mortality corresponded with the peak diazinon concentrations. Conversely, no toxicity was observed in water collected before or after peaks of diazinon concentration. Other pesticides present also could contribute to the toxicity.