University of California Water Resources Center

The three largest coastal rivers discharging to Monterey Bay, the Salinas, Pajaro, and San Lorenzo Rivers potentially serve as a route of transport of pyrethroids and other pesticides into the shelf waters of Monterey Bay and deeper areas within Monterey Canyon. Suspended sediment samples from the lower reaches of the rivers were collected over several rain events in the winters of 2007/2008 and 2008/2009, and analyzed for pyrethroids and one organophosphate pesticide. Bed sediments from Elkhorn Slough, Monterey Bay, and Monterey Canyon were also analyzed for these same substances. Nearly all suspended sediments contained measurable pyrethroids, with the pyrethroids bifenthrin and permethrin being the most commonly detected. The differences in pyrethroid composition between the rivers, some with predominantly an agricultural watershed and some with substantial urban influence, were relatively minor reflecting the broad uses for many of the insecticides within this class. While it is clear that all three coastal rivers are introducing substantial amounts of pyrethroids into coastal waters, there is sufficient degradation and/or dilution with uncontaminated material such that bed sediments in Monterey Bay and Monterey Canyon contained no quantifiable pyrethroids. Toxicity to sensitive invertebrates due to pyrethroids is likely in the bed sediments of the lower reaches of the Salinas River, and potentially the other rivers, but pyrethroid-related toxicity is not likely in Elkhorn Slough or adjacent coastal waters.