Time-resolved co-transport dynamics of microplastics and antibiotic resistance genes during stormwater first flush

Urban stormwater runoff is increasingly recognized as a key yet insufficiently characterized pathway for the dissemination of microplastics (MPs) and antibiotic resistance genes (ARGs), but their time-resolved co-occurrence during individual rainfall events remains poorly understood. Water quality indicators (SS, TN, TP, TOC), MPs, tire and road wear particles (TRWPs), and ARG abundances were monitored from runoff initiation to recession during a 90-min rainfall event. First, MPs and TRWPs showed an early-event first-flush pulse: MP counts declined from ∼3100 particles/L at 0 min to 480 particles/L at 10 min, whereas TRWPs increased from 3313.78 μg/L at 0 min to 3931.46 μg/L at 10 min and then fell to 62.96 μg/L at 90 min. Detected MPs were yielding an event mean concentration of 415.78 particles/L (total load: 1.3 × 10⁸ particles). Second, ARG abundances ranged from 10⁵ to 10⁹ copies/L, and sul1 showed the highest event load (1.1 × 10¹⁴ copies). In addition, Pearson correlation and redundancy analyses revealed strong associations among MPs, TOC/TN, and several ARGs (sul1, qnrS, floR; r > 0.7), whereas TRWPs showed weaker links with ARG profiles. These results further indicate that a subset of ARGs co-varied with particulate organic matter and MPs during early runoff based on correlation analyses, while other genes (e.g., ermB) follow distinct pathways. Finally, the study provides field-based evidence of co-occurrence patterns between MPs and selected ARGs and suggests that routinely monitored indicators may enable proxy-based screening of micro-pollutant dynamics. Although derived from a single event, the dataset underscores the importance of first-flush control and multi-event validation for improved assessment of ARG dissemination via stormwater.