Multi-scale analysis of 6PPD-quinone: distribution, environmental risk, and urbanization influence in the Delaware River Basin

Tire wear particles (TWPs) represent a pervasive and chemically complex class of urban contaminants. Among these, 6PPD-quinone (6PPDQ) an oxidative transformation product of the anti-degradant i.e., N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD), have emerged as contaminant of critical concern due to their acute toxicity to aquatic species, particularly salmonids. While significant research has focused on Pacific Northwest, European, and East Asian water systems, environmental data from northeastern U.S. freshwater ecosystems remain limited. This dissertation addresses this gap through a multiscale investigation of spatiotemporal distribution, ecological risk, urbanization impact and leaching dynamics of 6PPDQ across two major urban watersheds: the Schuylkill River and the Delaware River Basin both vital for drinking water supply and ecological integrity. Field sampling was conducted across 16 sites in the Schuylkill River and 23 in the Delaware River Basin over multiple seasons (2024-25’). Using EPA Draft Method 1634 and LC-MS/MS, the analysis revealed seasonal accumulation of 6PPDQ, with elevated concentrations during summer and autumn, particularly after storm events. Multiple sites exceeded the EPA’s freshwater screening value (11 ng/L), with urbanized tributaries showing the highest concentrations. Chapter 4 expands this assessment through ecological risk evaluation using both threshold-based risk quotients and species-specific toxic unit (TU) approaches, revealing significant risks to sensitive trout populations. Further analysis identified urbanization indicators like traffic volume, population density, and proximity to tire-related industries as key predictors of 6PPDQ distribution. Statistical and spatial regression modeling demonstrated strong positive correlations between these drivers and contaminant levels, especially in tributaries. Chapter 5 presents laboratory leaching experiments with new and used tire particles, simulating environmentally relevant conditions to explore the leaching dynamics of 6PPD and 6PPDQ. Spectroscopic characterization (NMR) confirmed surface oxidation and persistence of 6PPDQ in aqueous phases. Overall, these findings provide the first regionally focused, mechanistically informed assessment of 6PPDQ in northeastern U.S. rivers. By linking occurrence, toxicity, and urban drivers, this dissertation informs regulatory science and urban watershed management. The research underscores the importance of stormwater controls, targeted monitoring during hydrologic events, and innovation in tire material design to reduce environmental and ecological risks.