A watershed-scale, citizen science approach to quantifying microplastic concentration in a mixed land-use river

Microplastic (particles < 5 mm) pollution dynamics are well documented in oceans and increasingly studied in freshwater. We used a watershed-scale approach to examine spatial and temporal patterns in microplastic concentrations in the Gallatin River watershed (Montana, USA). At 72 sites, trained volunteers collected ∼1-L grab samples at 4 seasons per year over 2 years (n = 714 samples). Microplastics were found in 57% of the samples (mean = 1.2 particles L). The majority of particles were fibers (80%), 0.1-1.5 mm long. Chemical identification determined 93% of particles measured by μFT-IR were synthetic or semi-synthetic materials. Microplastic concentration differed significantly among dates, but showed no longitudinal pattern or relationship to land-use among subwatersheds. At two sites with gaging stations, microplastic was negatively related to discharge when compared across dates. This suggests stormwater is not a source of microplastic in this watershed, but instead dilutes microplastic inputs from other sources. We conclude that microplastic sources are diverse, and measurements of microplastic deposition, resuspension, and transport may be needed to clarify the role of land-use patterns on microplastic pollution. This large scale, citizen science based approach provides a model for future analysis which can further expand microplastic collection at the watershed scale.