Vertical distribution and post-depositional translocation of microplastics in a Rhine floodplain soil

Abstract Microplastics (MPs) are ubiquitous in the environment, but their vertical movement in undisturbed soils is poorly understood. This study investigates MP distribution and transport in one 110 cm soil profile from a Rhine River floodplain, Germany. Nine soil samples were analyzed for MP content, grain size distribution, bulk density, porosity, and organic carbon content. MPs (10 $$\upmu$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>μ</mml:mi> </mml:math> m to 5 mm) were characterized by size, shape, and polymer type using ATR-FTIR and $$\upmu$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>μ</mml:mi> </mml:math> FTIR. Biological activity was assessed through earthworm species identification and abundance, and sediment deposition was dated using optically stimulated luminescence (OSL). To our knowledge, this is one of the first studies to integrate detailed field observations with dating results to draw conclusions about the vertical displacement of microplastics. MPs were detected at all depths, with 81.3%–96.6% being 10–150 $$\upmu$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>μ</mml:mi> </mml:math> m in size, predominantly fragments and spheres. Concentrations peaked at 790,497 particles $$kg^{-1}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>k</mml:mi> <mml:msup> <mml:mi>g</mml:mi> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> dry soil between 38 and 45 cm, where root- and earthworm-formed macropores facilitated transport. Below this depth, reduced porosity limited movement, favoring smaller MPs. OSL dating indicated sedimentation before the 1950s/60s below 20 cm, suggesting vertical transport of MPs. Therefore, we would advise to be careful using small MPs as stratigraphic markers for the Anthropocene. Although analysing just one soil profile, this study highlights the role of biological activity and soil structure in MP translocation and emphasizes the need to account for small MPs in floodplain studies to avoid underestimating their environmental presence. Graphical Abstract