How many samples do we need to be representative? Grid sampling in Danish waters for assessing the distribution of microplastics and tire wear particles in seabed sediments

This study investigates the short-distance variability in microplastics (MPs) and tire wear particles (TWPs) concentrations in coastal sediment environments, aiming to refine sampling strategies for accurate environmental assessments. Grid sampling was conducted at two Danish sites, Strandby (∼0.57 km, 16 sampling points with distance ranging from 215 to 1070 m) and Odense (∼0.95 km, 13 sampling points, with distances ranging from 215 to 1577 m), followed by MP and TWP extraction and quantification. The results revealed significant variation in MP and TWP concentrations within and between sites, with Odense showing much higher contamination levels than Strandby likely due to the proximity to pollution sources and differences in depositional environments. No TWPs were detected at Strandby, likely due to its distance to road surfaces, as TWPs are made from the friction between tires and road surfaces. Monte Carlo simulations indicated that with 10 sampling points, the mean of the samples has a 50 % probability of being within 94-119 % of the true mean, defined here as the overall mean obtained from this study. The findings underscore the importance of collecting multiple samples to accurately represent MP pollution in the sediment compartment and provide recommendations for future monitoring efforts and sampling strategies.