Significance of Hyporheic Exchange for Predicting Microplastic Fate in Rivers

Microplastics are abundantly found in streambed sediments, including both small and low-density particles of neutral and positive buoyancy. Although the flow of water into streambed sediments (hyporheic exchange) has previously been shown to increase the rate of delivery of fine particles to the streambed, the influence of hyporheic exchange on microplastic fate in aquatic environments has not yet been assessed in detail. Here we evaluate the effects of hyporheic exchange on microplastics by calculating and comparing the rates of delivery of microplastics 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 hyporheic exchange rate that is higher than their 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 that are <100 μm in diameter, irrespective of polymer type. 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.