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The effects of large roughness elements on the in-stream transport and retention of polystyrene microplastics
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Laboratory flume experiments showed that large roughness elements like boulders in riverbeds significantly alter how polystyrene microplastics are transported and retained, with two distinct flow regimes determining whether particles travel quickly or get trapped. Understanding these river dynamics is essential for predicting where microplastics accumulate in freshwater systems and designing realistic environmental risk models.
The mechanisms controlling transport and retention of microplastics (MPs) in riverine systems are not understood well. We investigated the impact of large roughness elements (LREs) on in-stream transport and retention of the ubiquitous polystyrene-microplastics (PS-MPs). Scaled experiments were conducted with and without LREs under various shear Reynolds numbers (Re*) in an ecohydraulics flume. Our results, for the first time, demonstrated a clear dependence of the MPs' velocity on Re* in LREs-dominated channel. Two distinct regimes and thresholds were identified: lower Re* (≤ 15,000) regime corresponding to higher velocities of MPs ([Formula: see text]> 0.45), and higher Re* (> 15,000) to lower [Formula: see text]< 0.45). The presence and higher density of LREs increased Re*, decreased [Formula: see text], and enhanced the PS-MPs capture. The LREs-generated turbulence kinetic energy (TKE) was found to be a good predictor of PS-MPs transport and retention rates, indicating the effectiveness of LREs in retaining PS-MPs in streams and rivers.
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