That turns out to be a tricky question. USGS scientists plotted the microplastics data for the river water samples collected at various locations. Then they compared those data to land-use types found at those same sampling locations to determine if there seemed to be any patterns in the data.
Hover over the data on the graph below for more information.
Though the relationship is modest, it appears that urban watersheds tend to be associated with rivers that have high concentrations of microplastics in the water, which is especially true for plastic fragments, films, and foams. However, scientists were surprised to find no apparent pattern between land use and the most commonly observed microplastic type - fibers.