Isokinectic pump sampling – a methodoligy addressing the small size ranges of microplastic in rivers

Due to their persistent presence in the environment and the largely unknown long-term impacts on biota, plastic waste has increasingly become the focus of scientific research in recent years. Particularly in the field of microplastic monitoring in rivers, there remains a significant need for research. As methodologies are not yet standardized, and often not representative and comparable, efficient sampling poses a major challenge.Within the framework of the "Alplast" project, intensive efforts were made to develop a methodology capable of accurately capturing microplastic transport in rivers. Various carrier systems tailored to different river sizes were developed for net sampling, which targets the coarser fractions of microplastics (> 250 µm). In addition, a novel isokinetic pump was designed to analyze finer microplastic fractions from 50 to 250 µmGiven the spatial and temporal variability of plastic transport, multipoint sampling under varying hydrological conditions is strongly recommended. Larger microplastic particles occur less frequently, making it essential to sample large volumes of water to representatively capture this size range. Using net sampling, up to 1,000 m³ of water can be filtered at a single sampling point. However, finer particles, which cannot be captured by the limited mesh size of nets due to clogging especially under turbid boundary conditions, must be analyzed using pump samples. The number of sampling points across the river profile is often limited related to the high costs of analysis.The newly developed isokinetic pump addresses this gap by measuring flow velocity at the intake area of the sampling device. A control unit regulates the pump speed so that the pumped water volume matches the natural flow in the sampling cross-section. This isokinetic sampling approach offers two major advantages. First, the plastic flow is not disproportionately altered during sampling, and second, a direct weighting of the spatial distribution in flow and concentration is automatically accounted for. This significantly reduces the number of required samples, which is particularly beneficial given the high costs associated with sample analysis.The new methodology was successfully combined with net sampling and applied at multiple measurement sites. Results demonstrate that this approach enables efficient and representative monitoring of microplastic transport in rivers.