Occurrence and Spatial Distribution of Microplastics in River Shore Sediments of the Rhine-Main Area in Germany

TLDR

Plastic debris, especially durable synthetic polymers, are widespread pollutants that accumulate in aquatic sediments worldwide, yet freshwater sediment contamination has received less attention than marine systems. This study aimed to analyze shore sediments from the Rhine and Main rivers in the Rhine‑Main area of Germany. Samples were collected from shore sediments of the Rhine, a major European river, and the industrially influenced Main, across sites with varying population densities and near nature reserves. All samples contained microplastics up to 1 g kg⁻¹ (≈4000 particles kg⁻¹), predominantly polyethylene, polypropylene, and polystyrene (>75 %), with evidence of short‑distance transport, underscoring rivers as vectors of microplastic pollution to the ocean.

Abstract

Plastic debris is one of the most significant organic pollutants in the aquatic environment. Because of properties such as buoyancy and extreme durability, synthetic polymers are present in rivers, lakes, and oceans and accumulate in sediments all over the world. However, freshwater sediments have attracted less attention than the investigation of sediments in marine ecosystems. For this reason, river shore sediments of the rivers Rhine and Main in the Rhine-Main area in Germany were analyzed. The sample locations comprised shore sediment of a large European river (Rhine) and a river characterized by industrial influence (Main) in areas with varying population sizes as well as sites in proximity to nature reserves. All sediments analyzed contained microplastic particles (<5 mm) with mass fractions of up to 1 g kg⁻¹ or 4000 particles kg⁻¹. Analysis of the plastics by infrared spectroscopy showed a large abundance of polyethylene, polypropylene, and polystyrene, which covered more than 75% of all polymer types identified in the sediment. Short distance transport of plastic particles from the tributary to the main stream could be confirmed by the identification of pellets, which were separated from shore sediment samples of both rivers. This systematic study shows the emerging pollution of inland river sediments with microplastics and, as a consequence thereof, underlines the importance of rivers as vectors of transport of microplastics into the ocean.