Flowing Fibers: Subsurface Sampling Is Key to Understanding Natural and Plastic Textile Fiber Pollution in Rivers

There is a pressing need to understand the pathways of textile fibers as anthropogenic pollutants in the environment. Current efforts to understand textile fiber pollution in waterways have relied on surface-sampling methodologies without consideration for environmental heterogeneity. Moreover, how nonplastic textile fibers behave in the environment is not known. Here, for the first time, we experimentally quantify the role that fiber type (cotton, wool, polyester, and acrylic) and riverbed roughness (flat, fine gravel, and coarse gravel) have on the vertical distribution of transported fibers using an experimental, recirculating flume. Analysis of the vertical profile distributions of 18,793 cotton, wool, polyester, and acrylic fibers indicated that bed substrate significantly altered fiber transport pathways, which was consistent across all tested fiber types. Our findings indicate that surface-only sampling will substantially under-record fiber fluxes, but such biases did not differ between any tested fiber types. Our findings provide key insights into fiber/bed interactions and transport pathways and imply that current monitoring methodologies significantly underestimate lotic (and potentially lentic) populations of fibers. We argue that it is crucial to sample for all fiber types, throughout the water column in all riverbed types, to understand fully the scale of riverine textile fiber pollution.