Microplastic beaching dependence on sediment grain size

Microplastic (MP) beaching is the buildup of microplastics (MPs) in sand and along the coastline. It is a growing environmental issue in which beaches act as traps for MPs, impacting marine life and potentially human health. This paper supports the hypothesis that microplastics (MPs) accumulation on sandy beaches is directly influenced by both sediment granulometry and proximity to anthropogenic sources. Two sampling campaigns were carried out at Pletera Beach, a nationally protected site in northeastern Spain on the western Mediterranean coast. This study focuses primarily on fibre and fragment abundances, as these were the dominant MP particles, accounting 95.88 and 84.40% of total MPs in the two sampling campaigns, respectively. The study further discusses how the surface distribution of MPs on beaches results from the capacity of microfibers and microfragments to infiltrate beach sediments. MP retention was higher in fine-grained sand sizes, suggesting that fine-sand beaches may be more prone to elevated surface MP concentrations due to a lower infiltration, whereas beaches characterized by coarser bed grains tend to present lower MP abundances at the surface due to the higher infiltration capacity. For the medium-grained sands, microfiber abundances were 7.6 fold higher than microfragments. Other grain-size fractions could not be considered in this study. Greater microfiber abundances are most likely caused by their entanglement within the interstitial spaces between sediment grains, indicating that differences in MP shape will result in differential depositional patterns. Higher MP abundances were found at the north of the Pletera Beach, and these were linked to both anthropogenic tourism and by the nearby port activities. Likewise, the stations closest to the Ter River had the lowest Pollution Load Indexes (PLI), which might be associated to the flushing and transport by the river while the stations closest to the northern urban area had the highest values. This study also reveals the high level of pollution at Pletera Beach, with localized MP abundances exceeding 6,000 MPs/kg, corresponding to Level 1 on the Higher Pollution Load Index.