Hyporheic exchange processes increase retention of microplastics in rivers

, Drummond Jennifer D., Nel Holly A., Packman Aaron I., Krause Stefan.

Microplastics are abundantly found in streambed sediments, including both small and low-density particles of neutral and positive buoyancy. Hyporheic exchange, the two-way movement between the overlying water and the streambed sediments, is driven by both turbulence in the near bed region and pressure variations at the streambed surface that force water and suspended particles (e.g., microplastics, fine sediments, microbes) into and out of the sediment porewater. In fact, hyporheic exchange is occurring to some extent in most river systems, which leads to the accumulation of even very small particles with very low settling rates within the hyporheic region. However, the relative importance of hyporheic exchange compared to other mechanisms that lead to streambed retention of microplastics has not yet been assessed. Here we evaluate the effects of hyporheic exchange on microplastics by calculating and comparing the rates of microplastic delivery to streambed sediments by hyporheic exchange and gravitational settling for combinations of particle size and density most commonly found in streams. In a field stream study, we found that 23% of all microplastic combinations have a higher hyporheic exchange rate than settling rate. This fraction was as high as 42% for microplastics composed of low-density polymers, such as polyethylene. We then expand these findings to consider a wide range of hydrodynamic conditions in rivers and demonstrate that hyporheic exchange is important for the transport and fate of particles smaller than 100 µm diameter, irrespective of polymer type. Hyporheic exchange can substantially increase the retention of microplastics in rivers and facilitate modification of these particles prior to reaching the oceans (e.g., through the combination of increased retention and riverbed processes such as fragmentation, biofouling, and aggregation). Models that do not include hyporheic exchange are therefore likely to substantially underestimate the deposition, retention, and long-term accumulation of microplastics in streambed sediments.

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