Co-occurrence and Ecological Risk of Microplastics and Polycyclic Aromatic Hydrocarbons in a Coastal Small Watershed: A Case Study of the Houxi River, Xiamen, China

Abstract Microplastics (MPs) and polycyclic aromatic hydrocarbons (PAHs) are two prominent classes of environmental contaminants that frequently co-exist in aquatic sediments, yet their co-occurrence patterns in coastal small watersheds remain poorly understood. This study provides the first integrated assessment of MPs and PAHs in the Houxi River watershed, a typical coastal small watershed in southeastern China, by simultaneously sampling water, surface sediments, and riparian soils across 18 sites spanning the upstream reservoir area (Zone A), midstream rural-urban transition zone (Zone B), and downstream urbanized area (Zone C). MPs were detected at a 100% detection rate across all three media. The mean abundance of MPs in water, sediments, and soil was 3.65 +/- 0.51 n/L, 354.56 +/- 18.22 n/kg, and 1509.55 +/- 69.90 n/kg, respectively. MPs were predominantly small-sized (< 0.5 mm, > 70%), fragment-shaped (> 50%), and composed of polyethylene (PE, > 50%). The total concentrations of 16 US EPA priority PAHs (Σ-16 PAHs) in surface sediments ranged from 115.02 to 807.27 ng/g (mean: 371.33 ng/g), with 4-ring and 5-ring PAHs being the dominant components (each > 25%), indicating primarily pyrogenic sources. PAH diagnostic ratios further confirmed mixed combustion origins dominated by biomass and fossil fuel burning. Both MPs and PAHs exhibited a pronounced upstream-to-downstream increasing gradient, strongly linked to the intensity of anthropogenic activities along the urbanization continuum. Spearman correlation analysis revealed a significant positive correlation between sediment MP abundance and Σ-16 PAHs concentrations (r_s = 0.91, p < 0.001), suggesting shared anthropogenic sources and potential synergistic accumulation mechanisms. Risk assessment using the polymer-specific hazard index (H) and pollution load index (PLI) indicated that MPs posed a low-risk (Level I) but were approaching Level II thresholds. Sediment quality guidelines (SQGs) assessment showed that PAH ecological risks were predominantly between the Rare Effect Level (REL) and Threshold Effect Level (TEL), with localized exceedances to the Occasional Effect Level (OEL) at downstream sites, particularly for benzo[a]pyrene (BaP, 44.4% of sites reaching OEL). The co-occurrence of MPs and PAHs in sediments raises concerns about combined ecological risks, as the dominant polymer types (PE and PP) have demonstrated high sorption affinities for PAHs in laboratory studies. These findings provide critical baseline data for understanding the coupled behavior of emerging and legacy contaminants in coastal small watersheds and inform integrated pollution management strategies for land-sea coordinated environmental governance.