6PPD-quinone concentrations in the Sacramento-San Joaquin Delta, California, 2018-2024

The Sacramento-San Joaquin Delta is a crucial river delta that supplies a significant portion of California's freshwater for agricultural and residential use and serves as critical habitat for numerous state and federally listed species of concern. In several areas, urban stormwater drains discharge directly into the Delta. 6PPD-quinone (6PPD-Q, 2-anilo-5-[(4-methylpentan-2- yl)amino]cyclohexa-2,5-diene-1,4-dione), an ozonation byproduct of the tire antiozonant 6PPD (N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine), has been shown to enter surface water through these pathways and can cause various toxicological effects, including acute urban mortality syndrome in coho salmon (Oncorhynchus kisutch (Walbaum, 1792)) ) at low concentrations (LC50 = 41 nanograms per liter [ng/L] for juveniles and 95 ng/L for adults). In this study, we quantified 6PPD-Q in 64 archived Delta water extracts collected between 2018 and 2024, with detected levels reaching up to 20.64 ng/L. All detections were observed in samples collected during or immediately following precipitation events, consistent with the general understanding of 6PPD-Q transport. Reported concentrations are to be considered as the minimum amount present due to unknowns surrounding long storage times of archived extracts. Additionally, silicone bands were identified as potential passive samplers for semi-quantitative 6PPD-Q research. This work provides environmentally relevant concentration data that complements ongoing toxicological investigations of 6PPD-Q in Delta organisms and highlights the need for a more comprehensive survey of 6PPD-Q inputs into the Delta.