Microplastic Pollution in Benthic Midstream Sediments of the Rhine River

Rivers are major transport vectors for microplastics (MP) toward the sea. However, there is evidence that MP can temporarily or permanently be inhibited from migrating downstream by retention in sediments or ingestion by organisms. MP concentrations, compositions, and fate within the different compartments of the fluvial environment are poorly understood. Here, benthic, midstream sediments of two undammed, open-flowing stretches were investigated in the Rhine River, one of the world’s busiest inland waterways. Twenty-five samples were collected at ten sites via riverbed access through a diving bell or dredging. We performed the first comprehensive analysis of riverbed sediment aliquots that avoids visual selection bias using state-of-the art automated micro-Fourier-transform infrared spectroscopy (μFTIR) imaging. MP numbers ranged between 0.26 ± 0.01 and 11.07 ± 0.6 × 103 MP kg–1 while MP particles 1 g cm–3) dominated (85 ± 18%), which contrasts the large proportions of low-density polymers previously reported in near-surface compartments of the Rhine.