From land to sea: Hydrological source tracking of microplastics in coastal sediments

Land-based sources are the predominant contributors to marine microplastic pollution; however, identifying specific inland origins and their correspondence transport paths remain challenging at large-scale. In Denmark, terrestrial-marine connectivity is largely mediated by a complex network of streams, that further complicates hydrological connections between diverse inland discharges and the final coastal receptor. This study presents a GIS-based hydrological source tracking approach to identify the relevant inland discharges and further delineating their transportation paths for microplastics (≥10 μm) sampled from coastal sediments. After analyzed with Focal Plane Array-based Fourier Transform Infrared (FPA-μFTIR) imaging, samples from 13 Danish coastal monitoring stations were tested for souce tracking operations. The tracking tool first screened the stream network across Denmark to identify both point and non-point inland sources-stormwater runoff, wastewater effluent, and combined sewer overflow (CSO)-contributing microplastics to the selected coastal zones. Microplastics were detected at all stations, with variations in abundance, polymer composition, particle shape, and size. Significant positive correlations (p < 0.05) were observed between discharge volume and the composition of certain microplastics: wastewater effluent positively correlates with fiber and polyvinylchloride (PVC), while stormwater-associated discharges (including CSOs) correlated with polypropylene (PP). However, total microplastic concentration was not significantly linked to the discharges. Nevertheless, the GIS-based hydrological tool demonstrated an early-screening tool to identify inland source of marine microplastics. The results underscored the significant role of inland discharges in transporting distinct microplastics to coastal environment, offering valuable insights for authorities to further implement more targeted hydrodynamic modelling, source-specific mitigation strategies, as well as optimised station deployment strategies in future.