Monitoring of Microplastics in Dutch Marine Sediments 2025

Following earlier projects comparing different methods for microplastic analysis, Rijkswaterstaat (RWS) has commissioned NIVA the Dutch marine sediment monitoring program for microplastics in 2025, 2026 and 2027.

An optimized method, incorporating a three-stage chemical digestion and density separation, was applied to samples from seven locations, producing cleaner filters for more reliable µFTIR analysis.

The integration of the new mpAI software significantly improved particle detection, identifying on average 2.5 times more microplastics than the previously used siMPle software.

Polyethylene (PE) and polypropylene (PP) were the prevalent polymers, with higher concentrations and polymer diversity in fine-grained, organic-rich coastal sediments, while offshore sandy sites showed lower levels, correlating with reduced organic carbon. The method validation demonstrated high recovery efficiency (>96%) for particles >100 µm.

Limit of Detection (LOD) was calculated using a Poisson model, which is more appropriate for count data like microplastics. Some sandy sites had particle counts near or below the LOD, suggesting larger sample volumes are needed. Despite higher absolute concentrations due to methodological improvements, spatial patterns matched previous studies.

Main recommendations include increasing sample volumes in sandy samples, increasing number of procedural blanks, refining MicroparticlesAI software, and performing recovery test with smaller microplastic particles.