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Microplastic retention during a flood event by floodplain vegetation and their infiltration into the Rhine floodplain soil
Summary
Scientists found that when rivers flood, tiny plastic particles get trapped by plants and then sink deep into floodplain soil, with the highest concentrations found in the top 15-18 inches of soil. Thicker, more complex vegetation captures more of these microplastics from floodwater, but the plastics don't stay on the surface—smaller pieces move deeper underground through soil cracks and animal burrows. This matters because floodplains near rivers may be accumulating microplastics that could potentially affect soil health and crops grown in these areas.
Floodplains are recognized as significant sinks for sediments and pollutants transported by rivers, particularly during flood events which mobilize and redistribute contaminants like microplastics (MPs). This study investigates the role of floodplain vegetation and rough surfaces in the retention of microplastics and fine sediments during overbank flow, and the subsequent fate of these particles within the soil profile.The research combined column experiments and field observations to quantify particle deposition. We found that vegetation characteristics are a primary driver for the retention of suspended particles. Specifically, plant biomass and structural diversity were positively correlated with the amount of sediment deposited on the vegetation surface. This suggests that denser and more structurally complex vegetation enhances the capture of both sediments and microplastics from the water column. To differentiate the effects of biological surfaces from purely physical ones, deposition on vegetation was compared with deposition on metal sheets of varying surface areas. The deposition results potentially reveal the impact of surface areas and roughness on microplastic retention.Following the initial retention process, microplastics can infiltrate into the floodplain soil. The analysis of flood simulated soil column experiments confirm a heterogenous and rapid microplastic breakthrough. Additionally, under field conditions soil profiles confirms that floodplains act as major sinks, with the highest concentrations of MPs found in the upper 38-45 cm of soil depth. However, MPs are not permanently sequestered at the surface. The vertical distribution of microplastics is influenced by particle characteristics such as size and shape and soil properties. Smaller, spherical particles tend to infiltrate deeper into the soil compared to larger fragments and fibers. This downward translocation can be facilitated by processes such as preferential flow through soil structure and biopores.In conclusion, floodplain vegetation plays a critical role in intercepting microplastics during floods, initiating their transfer from the aquatic to the terrestrial environment. The retention efficiency is closely linked to vegetation biomass and structure. Our findings highlight that vegetation structure is one factor for MP sedimentation from flooding, while soil structure and biopores control infiltration and vertical transport of MP into the soil matrix. Subsequent infiltration and distribution in the soil profile are governed by a complex interplay between MP particle traits, soil texture, and biological activity. These findings highlight the importance of floodings, vegetation cover, soil structure in the transport of microplastics at the interface of aquatic and terrestrial ecosystems.