Isokinetic pump sampling – first application results and contribution of smallest microplastic fractions to riverine transport

Building on the methodological development carried out within the “Alplast” project, a newly developed isokinetic pump was successfully applied in combination with established net-based sampling methodology at several riverine measurement sites. This integrated approach allows for a comprehensive assessment of microplastic transport across a wide range of particle sizes, including the finest fractions that cannot be captured by net-based methods.

The combined application of net sampling and isokinetic pump sampling has proven to be robust and operational under varying field conditions form small streams to large rivers. While net sampling continues to effectively target coarser microplastic particles and enables the filtration of large water volumes, the isokinetic pump has delivered reliable results for the smallest size fractions. First experiences with laboratory analysis and data evaluation indicate that these fine particles represent a significant proportion of the total microplastic mass, hence they contribute substantially to overall microplastic transport, also due to their high mobility and widespread spatial distribution within the flow. The isokinetic sampling principle ensured that flow conditions at the intake were representative of the ambient river velocity, thereby minimizing sampling bias and enabling direct weighting of transport across the cross-section. This proved especially advantageous for capturing the variability in microplastic concentrations while keeping the number of required samples manageable.

Overall, the results confirm that the combination of net-based sampling and pump sampling with the isokinetic pump significantly enhances the representativeness of microplastic transport assessments in rivers. The methodology provides a sound basis for future studies aiming to quantify the role of fine microplastic fractions and contribute to standardized monitoring approaches.