Recent deposition and mobility of microplastics in proximal floodplain sediments

With the exponential expansion of global plastic production since the 1950s, plastics were able to enter the environment and freshwater systems now for 70 years. Meanwhile it is known that microplastics (MP) are transported through river systems as part of the global plastic cycle and can be deposited in river or floodplain sediments. However, it remains unclear to what extent and since when MP deposition takes place in alluvial sediments and whether MP remains stable within sediments? To address this question, we combined the analysis of MP (>500 µm) based on density separation, visual fluorescence identification, and ATR-FTIR analysis, with the analysis of sediment stratigraphy, textures, and 210Pb-137Cs dating. Within the framework of a recent case study, we performed those analyses for two sediment cores, located in proximal floodplains within the upper and lower reaches of the Lahn River (Germany). We found that MP concentrations range from 0–21.12 p kg-1, with a significant enrichment in upper (0–50 cm) sediments. During radionuclide analyses we found peaks in depths between 30–50 cm, indicating that MP enriched legacy sediments were deposited after the 1960s. Our results therefore prove an accumulation of MP through youngest flood dynamics. The additional detection of MP below the recent sedimentary deposits, with average concentrations of 0.69 p kg-1 in deeper sections (50–200 cm) may refer to in-situ vertical transfer and a mobility of MP particles. We can therefore conclude that MP entered freshwater systems with the onset of increased use and production, and has since been deposited intermittently in legacy sediments of proximal floodplains.  In addition to our findings, we would like to discuss the opportunities and limitations of our combined method approach for further MP research within the aquatic-terrestrial interface.