Characterizing export of land-based microplastics to the estuary - Part II: Sensitivity analysis of an integrated geospatial microplastic transport modeling assessment of tire and road wear particles.

Integrated models addressing microplastic (MP) generation, terrestrial distribution, and freshwater transport are useful tools characterizing the export of MP to marine waters. In Part I of this study, a baseline watershed-scale MP mass balance model was developed for tire and road wear particles (TRWP) in the Seine watershed. In Part II, uncertainty and sensitivity analysis (SA) methods were used to identify the parameters that determine the transport of these particles to the estuary. Local differential, local range and global first-order variance-based SA identified similar key parameters. The global SA (1000 Monte Carlo simulations) indicated that most of the variance in TRWP exported to the estuary can be apportioned to TRWP diameter (76%), TRWP density (5.6%), the fraction of TRWP directed to combined sewers with treatment (3.9%), and the fraction of TRWP distributed to runoff (versus roadside soil; 2.2%). The export fraction was relatively insensitive to heteroaggregation processes and the rainfall intensity threshold for road surface washoff. The fraction of TRWP exported to estuary in the probabilistic assessment was centered on the baseline estimate of 2%. This fraction ranged from 1.4 to 4.9% (central tendency defined as 25th to 75th percentile) and 0.97% to 13% (plausible upper bound defined as 10th to 90th percentiles). This study emphasizes the importance of in situ characterization of TRWP diameter and density, and confirms the baseline mass balance presented in Part I, which indicated an appreciable potential for capture of TRWP in freshwater sediment.

Characterizing export of land-based microplastics to the estuary - Part I: Application of integrated geospatial microplastic transport models to assess tire and road wear particles in the Seine watershed.

Human and ecological exposure to micro- and nanoplastic materials (abbreviated as MP, < 5 mm) occurs in both aquatic and terrestrial environments. Recent reviews prioritize the need for assessments linking spatially distributed MP releases with terrestrial and freshwater transport processes, thereby providing a better understanding of the factors affecting MP distribution to the sea. Tire and road wear particles (TRWP) have an estimated generation rate of 1 kg tread inhabitant-1 year-1 in Europe, but the fate of this MP source in watersheds has not been systematically assessed. An integrated temporally and geospatially resolved watershed-scale MP modeling methodology was applied to TRWP fate and transport in the Seine (France) watershed. The mass balance considers TRWP generation and terrestrial transport to soil, air, and roadways, as well as freshwater transport processes including particle heteroaggregation, degradation and sedimentation within subcatchments. The per capita TRWP mass release estimate in the Seine watershed was 1.8 kg inhabitant-1 yr-1. The model estimates indicated that 18% of this release was transported to freshwater and 2% was exported to the estuary, which demonstrated the potential for appreciable capture, degradation, and retention of TRWP prior to export. The modeled pseudo-steady state sediment concentrations were consistent with measurements from the Seine watershed supporting the plausibility of the predicted trapping efficiency of approximately 90%. The approach supported the efficient completion of local and global sensitivity analyses presented in Part II of this study, and can be adapted to the assessment of other MPs.