Pyrethroid-driven toxicity and its relationships with urban land use: Findings from 15 years of statewide monitoring in California watersheds.

The Stream Pollution Trends Monitoring Program (SPoT) is one of several statewide programs under California's Surface Water Ambient Monitoring Program (SWAMP). SPoT has been generating sediment toxicity and pollutant data since 2008 from up to 100 diverse watersheds. Analysis has included toxicity testing with the amphipod Hyalella azteca and, more recently, the midge Chironomus dilutus, as well as the measurement of metals, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), legacy pesticides, current use pesticides, and emerging contaminants such as fipronil, polybrominated diphenyl ethers (PBDEs) and per- and poly-fluoroalkyl substances (PFAS) in watershed sediments. This extensive data set has been analyzed in the context of watershed land use to understand the nature and spatiotemporal trends of identified aquatic pollutants. The most significant relationships were observed among amphipod toxicity, pyrethroid insecticides and urban land use. Using a toxic unit approach, we demonstrate increased potential for pyrethroid-driven toxicity statewide over the last 15 years. Significant relationships between standard toxicity test results and pyrethroid toxic units were identified. Out of eight measured pyrethroid pesticides, bifenthrin accounted for about 50 % of the calculated toxic units, and its concentrations significantly increased at nearly 25 % of SPoT monitoring sites. Decreased amphipod survival was significantly associated with urban land use, as were elevated concentrations of pyrethroids. Based on amphipod survival data, 72 % of toxic samples had more than one pyrethroid toxic unit indicating pyrethroid-driven toxicity. Diagnostic testing at an environmentally relevant temperature provided further evidence for pyrethroid contributions to observed toxicity.