Land use-based characterization and source apportionment of microplastics in urban storm runoffs in a tropical region

Urban stormwater runoff has been suggested as one important land-based pathway of microplastics (MPs) entering the oceans, in which the abundance and characteristics of MPs may be influenced by urban land use types. However, little information has been reported regarding this, especially in the tropical monsoon region. This study first reports the MPs in urban stormwater runoffs in a tropical monsoon region that were collected from four typical urban land use types, including industrial, highways, commercial, and residential areas. The average MP particle count and mass concentration were measured as 4.7 ± 3.5 particles/L and 3.8 ± 2.9 mg/L, respectively. MP abundances showed clear urban land use gradients following the order of industrial > transportation > commercial > residential area. In terms of the seasonal variation in MP abundances, a slightly increasing particle count in the dry season was noted for the residential site. Source apportionment of MPs in stormwater runoffs was demonstrated based on the land use type, particle morphology, and chemical compositions. With the simple apportionment approach, approximately 85% of the MP sources were able to be identified in the industrial, transportation, and residential sites. However, the commercial site showed high variability in terms of the morphology and polymer type of MPs. Furthermore, significantly positive correlations between MP abundance and runoff turbidity, TSS, COD, and rainfall intensity were identified, while, no significant correlation was found between MP characteristics and selected water quality/meteorological parameters.