Experimental field evidence for transport of microplastic tracers over large distances in an alluvial aquifer

Highlights: • Microplastic tracer particles (MPs) can be transported in an alluvial aquifer. • MP peak concentrations and mean flow velocities can exceed those of solutes. • Transport parameters vary with flow distance. • Rainfall might remobilize MPs. • Large flow distances as investigations scales (up to ~100 m) are important. The transport of microplastic tracer particles in comparison to the solute conservative tracer uranine was experimentally investigated in a shallow alluvial aquifer over distances from 3.1 to 200 m by means of a natural-gradient tracer test. The microplastic particles (MPs) with diameters of 1, 2 and 5 µm were artificially injected into an observation well to simulate microplastic transport; water samples were taken at eleven observation wells further downgradient over a time span of 171 days. In total, 44 individual breakthrough curves of microplastics and uranine were obtained at all sampling sites, allowing a detailed analysis of the size-dependency of microplastics transport in porous media at field scale. Results clearly show that (i) microplastics of 1–5 µm can be transported in significant amounts in sand-and-gravel aquifers; (ii) peak concentrations of microplastics can exceed those of conservative solutes, in particular for longer flow distances; (iii) microplastic peak velocities are in a similar range or exceed those of conservative solutes; (iv) retardation and filtration processes did not efficiently attenuate microplastics in groundwater at the study site. To our best of knowledge, this is the first experimental field evidence for microplastics transport over large distances in an alluvial aquifer.