Ecological risks of diazinon from agricultural use in the Sacramento-San Joaquin River Basins, California

A probabilistic risk assessment was conducted to evaluate the likelihood an d ecological significance of potential toxic effects of diazinon in the Sac ramento-San Joaquin system. Diazinon, an organophosphorus insecticide, is u sed in the Sacramento-San Joaquin River Basin as a dormant spray on almonds and other tree crops, as well as for other agricultural and urban applicat ions. Diazinon and other pesticides have been detected in the Sacramento an d San Joaquin Rivers and their tributaries. Diazinon exposure was character ized based on monitoring programs conducted in 1991-94. Diazinon effects we re characterized using laboratory toxicity data for 63 species, supplemente d by results from field mesocosm and microcosm studies. The assessment addr essed the possibility that reductions in invertebrate populations could lea d to impacts on species of fish that feed on those invertebrates. The risk assessment concluded that fish in these rivers are not at risk from the dir ect effects of diazinon in the water. Invertebrates are at greater risk, es pecially in agriculturally dominated streams and drainage channels during J anuary and February. Cladocerans-including Daphnia magna and Ceriodaphnia d ubia, two common bioassay species-are especially sensitive to diazinon and other organophosphates and are likely to be subject to acute toxic effects in some locations at some times. Any ecological damage that may occur, howe ver, is brief and limited to cladocerans. None of the fish species of conce rn depend on cladocerans as critical components of their diet. Invertebrate s that are not affected by observed concentrations of diazinon (copepods, m ysids, amphipods, rotifers, and insects) are preferred foods for fish in th e Sacramento-San Joaquin system.