Temporal Dynamics of PPD-Class Antioxidants and Transformation Products in a Small Roadway-Runoff-Impacted Watershed

The occurrence and transport dynamics of 6PPD-quinone (N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine-quinone; 6PPDQ) and other PPD-derived contaminants (N,N′-substituted p-phenylenediamines; PPDs) remain poorly characterized in receiving waters. To define contaminant-hydrograph dynamics, we conducted time-resolved sampling during 17 storm events over three years in Miller Creek, a small roadway-impacted watershed in the Puget Sound region (Washington, USA). We quantified seven PPD antioxidants, five quinone transformation products, and five other 6PPD transformation products (TPs). 6PPDQ concentrations ranged from 50 value (41 ng/L) for 0–9 h (median: 5 h) and exceeded the U.S. EPA Acute Aquatic Life Screening values (11 ng/L) for 8–19 h (median: 13 h) during storms. Peak 6PPDQ concentrations showed no significant seasonal variation, suggesting year-round risks to coho salmon across multiple life stages. Beyond 6PPDQ, other frequently detected compounds included 6PPD (