Tracking 6PPD-Quinone Dynamics in a Coho Salmon-Bearing Stream Following Rain Reveals Elevated Concentrations for Multihour Periods During High Flow

Urban runoff mortality syndrome in coho salmon (Oncorhynchus kisutch) is attributed primarily to N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine-quinone (6PPD-Q), principally from car tires, that is delivered to streams by surface runoff. However, time-resolved stream 6PPD-Q concentration data remain scarce. We measured rainfall, discharge, and 6PPD-Q concentrations in an urban salmon-bearing stream over four rain events to capture the antecedent dry weather (baseflow) and the increased streamflow (stormflow). Portable autosamplers proved to be effective for continual water sampling. The resulting time series revealed 6PPD-Q flushing during stormflow, likely influenced by surface runoff from roads upstream. Following the initial rise in stream stage (+5% depth), mean 24 h time-averaged 6PPD-Q concentration was 45.0 ng/L, 25-fold higher than baseflow, and the mean 24 h load was 3.73 g, 220-fold higher than the preceding 24-h period. Measured 6PPD-Q concentrations, which peaked at 237.0 ng/L, exceeded the LC₅₀ reported for juvenile coho salmon for periods ranging from 3.5 to 18.7 h during rain events, but never during the preceding dry period. Accordingly, coho salmon habitat can be subjected to repeated pulses of 6PPD-Q over the course of the wet season during elevated streamflow periods, which may expose spawning adults, juveniles, and hatching fry to toxic concentrations for considerable periods.