Predicting the fate of plaSTic On beaches by theiR 3D-distribution and weAtherinG procEsses

Informations projet

STORAGE

N° de convention de subvention: 101061749

DOI

10.3030/101061749

Projet clôturé

Date de signature de la CE 15 Juin 2022

Date de début 18 Août 2022

Date de fin 17 Août 2025

Financé au titre de

• Marie Skłodowska-Curie Actions (MSCA)

Coût total

Aucune donnée

Contribution de l’UE

€ 276 681,60

Coordonné par

INSTITUT FRANCAIS DE RECHERCHE POUR L'EXPLOITATION DE LA MER
France

Periodic Reporting for period 2 - STORAGE (Predicting the fate of plaSTic On beaches by theiR 3D-distribution and weAtherinG procEsses)

Période du rapport: 2024-08-18 au 2025-08-17

Résumé du contexte et des objectifs généraux du projet

Context
Plastic pollution is one of the most significant environmental challenges worldwide. Since the onset of synthetic polymer production in the 1950s, global models of plastic pollution estimate that approximately two-thirds of the plastic mass released into the ocean has either stranded or settled around the world’s shorelines. Furthermore, plastic debris transported by ocean currents and wind can travel vast distances across the world’s oceans far from its source, eventually converging in one of the five subtropical gyres, where it persists as “legacy plastic.” The accumulation of floating plastic debris in these gyres is commonly referred to as “Garbage Patches.” Shorelines near these gyres, such as those of Hawaiʻi, are particularly vulnerable to accumulating large amounts of plastic debris.

Even with major reductions in plastic inputs, surface ocean plastic debris is projected to continue increasing for decades. Exacerbating this trend is the ongoing degradation of legacy plastics, which leads to further fragmentation, abrasion, and breakdown into smaller fragments. These fragments generate secondary microplastics—plastic particles of 5 mm or smaller formed through the physical and chemical weathering of larger plastics. The continuous influx of these microplastic fragments increases the exposure risk of plastic particles to marine organisms, ultimately jeopardizing the ecological integrity, cultural practices, well-being, and livelihoods of communities reliant on healthy ocean and coastal ecosystems. With the world currently navigating the final negotiations of the Global Plastics Treaty, it is increasingly important to both stop the flow of plastic into the ocean and address legacy plastic pollution, particularly in accumulation hotspots. Understanding where, how, and when legacy plastic fragments can inform targeted interventions such as beach and open-ocean cleanup efforts, offering a last opportunity to remove plastics before they degrade into secondary microplastics.

The STORAGE project aimed to improve understanding of the fate of ocean plastic pollution, with a particular focus on its transformation and transport within coastal environments. By investigating the beaches of Hawaiʻ STORAGE sought to better understand how ocean plastic debris behaves once it reaches shore, including how it becomes buried, degraded, and fragmented over time. The project’s overall objective was to generate knowledge that can inform strategies to reduce microplastic formation and strengthen global understanding of plastic transport pathways. By clarifying where and how plastics accumulate, transform, and persist, STORAGE aimed to contribute data that can contribute to the development of more effective mitigation and prevention measures that protect marine ecosystems and coastal communities from the long-term impacts of plastic pollution.

Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent

The STORAGE project was structured around three key objectives, each addressing a knowledge gap in the understanding of ocean plastic pollution fate in coastal environments. Over the course of the project, field work and data analyses was completed, revealing new insights into the vertical distribution, degradation, and transport mechanisms of plastics in high-accumulation coastal zones.

- Objective 1: Depth distribution of plastic in the sandcolum of beaches

Activities:
Field sampling was conducted on the island of Oʻahu, focusing on three windward beaches: Kahuku (James Campbell), Kokololio (Lāʻie), and Waimānalo. During quarterly surveys between November 2022 and February 2024, plastic debris (>0.5 mm) was collected across ten depth intervals down to 1 m using 60 × 60 cm quadrats. The study was also expanded to Hawaiʻi Island to investigate the depth distribution of plastic debris on a rocky shoreline, allowing for a broader understanding of how different coastal environments influence plastic burial and retention.

Outcomes:
This study provided the first survey of buried plastic distribution along the sand columns of Oʻahu’s beaches. Results revealed that 91% of all recovered plastics were buried below the surface, extending down to 1 m depth. The buried fraction—largely invisible to surface surveys—highlights the complexity of plastic pollution distribution on beaches and indicates that surface-only assessments may capture only a small portion of the total load. The majority of particles were small, hard fragments primarily composed of polyethylene (PE) and polypropylene (PP). This substantial yet often unseen plastic reservoir points to a more severe pollution issue on Hawaiʻi’s beaches than previously recognized. Additionally, a potential positive correlation between plastic particle length and sand grain size was observed which could suggest that the physical characteristics of beaches may influence plastic retention and burial behavior.

- Objective 2: Assessing Polymer Transformation Upon Weathering

Activities:
Analytical methods including Fourier Transform Infrared (FT-IR) spectroscopy, Gel Permeation Chromatography (GPC) were used to assess the degradation state of the sampled plastics in the sand-column. Natural weathering experiments were carried out at the Centre for Marine Debris Research in Waimānalo, Hawaiʻi. Plastic films (PP and PE) were weathered for up to three months, with continuous monitoring of temperature, humidity, and solar radiation (UVA/UVB). FT-IR spectroscopy, GPC and mechanical testing were used to follow molecular and structural degradation. A fragmentation simulator for swash zone experiments was used to reproduce the mechanical stresses driving onshore fragmentation of un-weathered and weathered plastic samples.

Outcomes:
The analysis of sampled plastics from the sand-column revealed advanced polymer degradation, with low molecular weights (as low as 14 kg/mol for PE and 7 kg/mol for PP). To our knowledge such low molecular weight of plastic debris values in the environment has not been reported. Approximately 92% of analyzed particles exhibited brittleness, making them prone to fragmentation into secondary microplastics under minimal stress. The strong link between brittleness, presence of oxidation, and chain scission confirms that high probability of plastic fragmentation on beaches. The fragmentation simulator experiments validated the role of physical abrasion and wave impact on embrittled weathered plastic samples. Together, these findings advance scientific understanding of polymer weathering processes leading to brittleness and fragmentation under coastal conditions.

- Objective 3: Understanding Fragmentation and the Capture & Release of Plastics on Beaches

Activities:
Data collected through Objectives 1 and 2 - including the distribution of plastics in sand columns and the assessment of polymer weathering and fragmentation - were used to investigate how plastics are captured, retained, and released on Hawaiian beaches. To better understand these processes, we collaborated with researchers specializing in beach dynamics on Hawaiian shores. Field surveys were complemented with GPS mapping, camera monitoring, and satellite data, capturing beach accretion, erosion, and sediment movement of the three beaches where the sand-column surveys were conducted. Environmental parameters such as waves, tides, and wind patterns are also considered to link beach physical dynamics with plastic behavior.

Outcomes:
The integration of plastic distribution data, polymer degradation analyses, and beach dynamics has begun to reveal the drivers of plastic retention and fragmentation on sandy and rocky beaches. Initial analyses show that beach morphology, sediment movement, and seasonal hydrodynamics can potentially influence where plastics accumulate, where they fragment into secondary microplastics, and where they are subsequently released back into the ocean. These findings could inform strategies for targeted cleanup and mitigation, by identifying beaches and zones most at risk of generating secondary microplastics.

Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent)

The STORAGE project has provided new insights into the fate of marine plastic debris (MPD) on Hawaiian beaches, highlighting aspects of plastic pollution that are often overlooked.

Key Findings:

Subsurface Plastic Distribution: Surveys revealed that a large portion of plastics—over 90% on Oʻahu’s sandy beaches—are buried below the surface, extending down to 1 m. This suggests that surface surveys alone may only capture a fraction of total plastic loads, emphasizing the need to consider buried plastics in monitoring and management strategies.

Plastic Fragmentation and Degradation: Most plastics collected were small, brittle fragments of polyethylene (PE) and polypropylene (PP), showing advanced degradation and low molecular weight. These characteristics indicate that plastics can rapidly fragment into secondary microplastics once on beaches, particularly under environmental stressors.

Influence of Beach Dynamics: By combining plastic distribution and degradation data with observations of beach morphology and dynamics, initial patterns are emerging regarding where plastics accumulate, where fragmentation is accelerated, and how plastics are released back into the environment.

Potential Implications:

Guiding Mitigation: Understanding where and under which conditions plastics are likely to fragment or persist could inform more targeted beach cleanup efforts and interventions aimed at reducing secondary microplastic formation.

Informing Research and Modeling: The findings suggest that including buried plastics and their fragmentation behavior in models could improve predictions of plastic transport and accumulation in coastal systems.

Supporting Policy Development: Recognizing the role of beach dynamics on plastic fragmentation may help policymakers and coastal managers better identify priority areas for action.

Key Needs for Further Uptake:

Extended Monitoring: Additional studies across seasons and different beach types could refine understanding of plastic burial, fragmentation, and transport.

Integration into Models: Linking detailed field data on buried plastics and polymer degradation with predictive coastal plastic transport models could enhance decision-making.

Demonstration Projects: Testing how these insights can improve cleanup efficiency or prevent secondary microplastic formation may help translate findings into practice.

Broader Collaboration: Comparative studies in other coastal regions could help generalize findings and support international strategies to manage plastic pollution.

Standardization: Developing shared methods for monitoring buried plastics and assessing degradation would help ensure results are comparable and usable in broader contexts.

Overall, STORAGE’s findings suggest that coastal plastic pollution is complex, with subsurface plastics, fragmentation, and local beach dynamics playing important roles. These insights could guide further research, management strategies, and policy approaches aimed at mitigating the impact of marine plastics.

STORAGE project findings reveal the hidden accumulation of plastic pollution on beaches