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 <MDL to 160 ng/L, with storm peaks of 33–160 ng/L (median: 98 ng/L); these concentrations exceeded the lowest published LC50 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 (<MDL–130 ng/L) and its TP 1,3-DMBA (30–4800 ng/L). Storm loads were 0.078–2.7 g/storm for 6PPDQ and 10–77 g/storm for all PPD-related contaminants. "Middle flush" and transport-limited dynamics were most observed for PPD-related contaminants, while "last flush" events were also observed for low-intensity drizzles and short duration storms. Collectively, multiple PPD-derived contaminants demonstrated substantial and sustained mass transport throughout storm events of all seasons.