Plastic pollution in the ocean is a global environmental issue, with buoyant debris accumulating at the surface and posing long-term ecological threats. Although sediments are the ultimate sink for plastics, a mismatch between observed surface concentrations and estimated inputs implies the understanding of vertical sedimentation mechanisms and rates are inaccurate. Here, we present a coupled fragmentation-sedimentation model that quantitatively predicts the vertical transport and long-term fate of buoyant plastic debris and microplastics (MPs, less than 5 mm). Using a representative 10 mm polyethylene (PE) particle, we show that fragmentation into small MPs is essential for their incorporation into marine snow aggregates (MSAs) and subsequent settling. Even after 100 yr, ca. 10% of the initial plastic mass still remains at the surface providing a continual source of small MPs to ocean surface waters. This study provides the first mechanistic framework linking large plastic degradation to size selective sedimentation, demonstrating that plastic pollution will persist at our ocean surfaces for over a century even if inputs cease. Our findings highlight the need for mitigation strategies beyond input reduction and ocean clean-up, addressing the long-term removal of existing ocean plastics.This article is part of the Theo Murphy meeting issue 'Sedimentology of plastics: state of the art and future directions'.

Many studies have reported the paradoxical observation of high concentrations of low-density microplastics (plastic particles < 5 mm) in deep-sea sediments despite their buoyancy. The incorporation of buoyant microplastics into marine snow has been observed to enhance microplastic settling. Previous studies on the vertical movement of buoyant microplastics have been unable to theoretically account for these ocean observations and no study has comprehensively elucidated microplastic transport pathways in the ocean from the surface to seafloor. Here, we establish a one-dimensional theoretical model, that embraces key elements of the flocculation process, to explain how marine snow acts as a vector to transport buoyant microplastics to deep water and the ocean bottom. Microplastics reach the ocean floor through multiple cycles of aggregation, settling, and disaggregation between marine snow and microplastics. Each settling cycle results in a net settling of 200-400 m. We demonstrate that microplastics with different sizes show distinct vertical settling behaviors and only microplastics less than 100 mu m in diameter can reach the ocean bottom. This theoretical model refines our ability to predict and understand the global and long-term fate, transport, and inventory of microplastics in the ocean interior, the influence of microplastics on the biological carbon pump and the efficacy of plastic management policies.

The ocean is considered a sink for plastic waste, with buoyant plastics remaining at the surface for up to 50 years. Removal of plastic waste from the ocean by manpower is not feasible, but natural-based removal strategies are promising. Among these, the biological pump stands out as a crucial process responsible for transferring materials and nutrients from the surface to the deep ocean. In this context, plastic particles can be incorporated into and transported with marine snow aggregates, providing a potential mechanism for removing plastic waste from the upper ocean (human food chain) to the deep ocean. However, the longevity of plastic debris at the ocean surface remains poorly understood. To address this gap, we developed a degradation-aggregation model to predict the longevity of different types, sizes and shapes of buoyant plastics at the ocean surface. Our results show that the longevity of plastic debris is primarily determined by the time it takes for plastic debris to degrade sufficiently and become small enough (< 100 µm) to be incorporated into marine snow aggregates. Larger plastics take decades to degrade into small microplastics that can be trapped in marine snow, prolonging their presence in surface waters. Conversely, once microplastics are small enough to be incorporated into marine snow, they can reach the seafloor sediment within two years. Interestingly, our model highlights that vertical settling of microplastics occurs through multiple incorporation and settling via vector transport of marine snow, which differs from previous studies reporting oscillations of microplastics in the ocean subsurface. This study explains the mechanisms of plastic debris removal by the biological pump and the longevity of plastic debris at the ocean surface. In addition, this theoretical model can be extended to different aquatic ecosystems to predict the fate and longevity of plastic debris in different environments.

Low lying, flood prone, coastal areas have historically been identified as ideal locations to dispose of landfill waste due to their low land values. It is estimated that there are in excess of 10,000 such landfills in Europe alone, many of which are now threatened with erosion as sea level rise driven by anthropogenic climate change renders flood defences ineffective. Many of these historic coastal landfills do not have records of the quantity or composition of the waste stored within them and in many of these locations waste is already being eroded into the coastal zone. The potential consequences of such waste release are wide ranging, impacting human health, coastal communities and marine ecosystems. Geomorphometric techniques can be used to quantify volumes of released waste and landfill erosion rates, which when combined with geochemical investigation of landfill waste can form a critical component of hazard assessment, landscape management and remediation efforts. Here, we report the preliminary results and future plans of a new monitoring programme, for one such historic coastal landfill in the South East of England. This work integrates terrestrial and aerial laser scanning, aerial photography and fieldwork to constrain the volume of waste contained within this site, and estimates the rate of erosion of landfill material into the marine environment.

Five years ago, an article in WIREs Water provided the first comprehensive analysis of historic (legacy) landfill sites vulnerable to coastal flooding and erosion at a national scale (England). This update expands upon that article by considering the potential impacts of climate change upon inland historic landfills. Globally, there are hundreds of thousands of landfills that predate modern environmental regulations, and where waste is not isolated from the surrounding environment, but climate change impacts on the pollution risk from historic landfills in freshwater environments has received little attention. Where climate change causes an increase in the frequency and magnitude of fluvial flood events, this will increase leachate generation and the probability of landfill erosion and solid waste release. Where there is increased drought the landfill capping materials may crack, opening up new pollutant pathways, and increasing the risk of solid waste release. Changes to groundwater movement resulting from climate change may open new leachate pathways, and in England alone, thousands of historic landfills are in (groundwater) Source Protection Zones where modern regulations to protect drinking water supplies would not permit their construction. This increased contaminant release from historic landfills in freshwater environments may impact surface and/or groundwater quality and ecological health, increase costs for drinking water monitoring/treatment, or make some abstraction sources unviable. This is especially of concern where receptors are subject to multiple pressures and may cause tipping points to be reached. Further research is warranted into contaminant behavior, receptor vulnerability, historic landfill risk prioritization, and mitigation/remediation methods.This article is categorized under:Engineering Water > Engineering WaterScience of Water > Water QualityScience of Water > Water and Environmental ChangeWater and Life > Stresses and Pressures on Ecosystems

Microplastics (MPs) are an important component of suspended particulate matter in aquatic environments with two main transport modes, that is, as individual entities or in flocs. Despite its importance to MP pollution management, understanding and predicting MP flocculation remains a challenge. In this Article, we combined a meta-analysis of published data (>2,000 measurements) with new experimental data (>4,000 measurements) to investigate which size fraction of MPs can be incorporated into and transported by flocs in the aquatic environment. The size relationship between MPs and flocs can be used to predict the flocculation of MPs in various aquatic environments, and we have proposed a mathematical model to show that small MPs (<162 m) are predominantly transported as flocs, regardless of the physicochemical characteristics of the MPs or water body. This provides valuable information to predict the transport modes of MPs, presenting a critical insight for multiple environmental settings and future pollution control strategies.

&lt;p&gt;Impacts of climate change &amp;#8211; sea level rise, more frequent storms and coastal flooding will exacerbate coastal erosion, resulting in the erosion of coastal historic landfills. These historic landfills are particularly vulnerable to such erosion as they typically have no lining or leachate management, limited information of the proportion and/or types of waste mixtures they contain and inaccurate records of their location and waste volumes. There are over 1200 coastal historic landfills in England alone, and over 10,000 such sites are at risk of release both solid waste and soluble contaminants across Europe. The contaminated matrix and solid wastes make landfills a major sink and source of microplastics and heavy metal, posing a threat to the coastal and marine environment.&lt;/p&gt; &lt;p&gt;We investigated heavy metal and microplastic pollution on the beach and foreshore in three coastal historic eroding landfills, East Tilbury (n = 32 samples), Lynemouth (n = 33 samples), Northam Burrows Tilbury (n = 33 samples), UK. Samples were collected every 50 meters along the shoreline, with 100g of surface soil from the landfill edge, and 1kg of beach and intertidal sediment collected from each transect. The metal concertation was measured with handheld X-ray Fluorescence (XRF). Microplastics were density separated with a zinc chloride solution (1.5 g cm&lt;sup&gt;&amp;#8722;3&lt;/sup&gt;), after the samples were dried and digested with hydrogen peroxide. The extracted microplastics were recorded under stereomicroscope at 50&amp;#215; magnification with a digital camera, and characterized with Fourier-transformed infrared (ATR-FTIR) spectroscopy.&lt;/p&gt; &lt;p&gt;This study is one of the first few to investigate the impacts of eroding historic landfill. Our preliminary findings suggest that eroding landfill are releasing significant amounts of microplastics and heavy metal pollution. These findings will be crucial to assess the impacts of eroding landfills, identify solutions and raise public attention to this environmental problem.&lt;/p&gt;

&lt;p&gt;Easily transportable microplastics (plastic particles &lt; 5 mm) have become an increasingly important component of suspended particulate matter (SPM) in the aquatic environment, and their fate is significantly influenced by aggregation and flocculation. Aggregation modifies particle properties (e.g., size) controlling the hydrodynamics of SPM in the aquatic environment. Hence, understanding and quantifying aggregation is key to predicting the behaviour of both SPM and associated microplastics. However, quantifying the aggregation degree of microplastics with complex parameters in various water environments is very difficult.&lt;/p&gt;&lt;p&gt;Here, an extensive range of microplastics including 8 polymer types, 3 shapes, different weathering conditions and different sizes (10-300 &amp;#181;m for fragments and microbeads, and 10-1500 &amp;#181;m in length for microfibers, respectively), were used to explore the aggregation dynamics of microplastics. Over 4000 measurements of incorporated microplastics were collected, and we found microplastic size (MinFeret diameter of fragments, diameter of microfibers) is the key parameter to determining the aggregation behavior. Our results simplified the aggregation of microplastics with a wide range of properties in various water ecosystems into two parameters, the size of microplastics and the size of aggregates. A boundary curve for microplastics was fitted based on size relationships between microplastics and aggregates to divide microplastics into aggregable and un-aggregable groups. This study can aid better understanding the fate of microplastics in various aquatic environments at multiple scales.&lt;/p&gt;

Globally there are significant numbers of historic landfills, and in England alone there are over 1200 in low-lying coastal areas. Approximately one-third of these historic coastal landfills are near designated ecological sites, and without intervention, 10% are expected to start eroding within 40 years. Indeed, some sites are already eroding and releasing waste, and erosion is likely to become more common with the anticipated effects of climate change. Mitigating the pollution risk from all historic coastal landfills under threat of erosion would be prohibitively expensive; consequently, it is necessary to understand which sites pose the greatest pollution risk to prioritise management resources. This paper proposes a new risk screening assessment that can support coastal managers in identifying which historic coastal landfills pose the greatest pollution risk at a national scale for minimal cost using existing datasets. The proposed method determines an overall risk index for each site by considering the risk of pollution from eroding historic coastal landfills in two stages: the first stage assesses the risk of waste being released (waste release index), and the second assesses the risk to various receptors (pollution index). The highest risk sites can then be prioritised for further investigation or remediation.

Clay-rich flocculated suspended sediments are an important constituent of estuarine and coastal systems globally. They are responsible for the host, movement and deposition of a variety of pollutants, contaminants and sediment itself. Accurate modelling of the movement of these sediments is crucial for a number of industries including fisheries, aquaculture, shipping and waste management. This requires an accurate and reliable measurements of the physical properties of flocs and their behaviour. Porosity is a key element in floc structures, and this research provides updated 3D quantified porosity and pore space morphological data in relation to influences on floc settling behaviour. We report the questionable relationship between floc size and settling velocity, and explore alternative influences such as floc composition, porosity and pore morphology. These outcomes suggest that a shift in focus from floc size to a combination of factors is necessitated to understand the complex movement behaviour of flocculated suspended sediments.

Flocculated suspended sediments (flocs) are found in a variety of environments globally, and their transport and behavior bear substantial importance to several industries including fisheries, aquaculture, and shipping. Additionally, the modelling of their behavior is important for estuarine and coastal flood prediction and defence, and the process of flocculation occurs in other unrelated industries such as paper and chemical production. Floc porosity is conventionally assessed using inferential indirect or proxy data approaches. These methods underestimate floc porosity % by c. 30% and cannot measure the micro-scale complexity of these pore spaces and networks, rendering inputs to models sub-optimal. This study introduces a novel 3D porosity and pore space quantification protocol, that produces directly quantified porosity % and pore space data.•3D floc data from micro-CT scanning is segmented volumetrically•This segmented volume is quantified to extract porosity and several pore space parameters from the floc structure

Natural sediment flocs are fragile and highly heterogeneous aggregates of biogenic and minerogenic material typically with high porosity and low density. In aquatic environments dominated by fine, cohesive or mixed sediments they can dominate suspended sediment flux. Consequently, monitoring and modelling the behaviour, transport and distribution of flocs is very important for many aquatic industries, maintenance of waterways and conservation and management of aquatic waterbodies. Mathematical models that predict the behaviour of flocs rely on the accurate assessments of the size, shape, density, porosity and fractal dimension of flocs. These inherently 3-dimensional (3D) characteristics are typically derived from 2-dimensional (2D) data, largely due to the challenges associated with sampling, capturing, imaging and quantifying these fragile aggregates. We have developed new volumetric microscopy techniques which can quantify 3D internal and external structures and characteristics of sediment flocs. Here, these techniques were applied to quantify the 3D size (volume), shape and fractal dimension of natural and artificial sediment flocs and compare them to standard 2D approaches. Our study demonstrates that 2D approaches are under-estimating shape complexity and over-estimating the size and mass settling flux of flocs by up to two orders of magnitude, and the discrepancy between 2D and 3D is most marked for natural, organic rich macroflocs. Our study has significant implications for estimations of sediment flux at local to global scales within in aquatic environments. These new data and approaches offer the potential to improve the current parameterisation of sediment transport models and to improve the accuracy of current field-monitoring techniques.

Flocculation is a key process for controlling the fate and transport of suspended particulate matter (SPM) in water environments and has received considerable attention in the field of water science (e.g., oceanography, limnology, and hydrology), remaining an active area of research. The research on flocculation has been conducted to elucidate the SPM dynamics and to diagnose various environmental issues. The flocculation, sedimentation, and transportation of SPM are closely linked to the compositional and structural properties of flocs. In fact, flocs are highly heterogeneous in terms of composition. However, the lack of comprehensive research on floc composition and structure has led to misconceptions regarding the temporal and spatial dynamics of SPM. This review summarizes the current understanding of the heterogeneous composition of flocs (e.g., minerals, organic matter, metals, microplastic, engineered nanoparticles) and its effect on their structure and on their fate and transport within aquatic environments. Furthermore, the effects of human activities (e.g., pollutant discharge, construction) on floc composition are discussed.

Salt marshes provide diverse ecosystem services including coastal protection, habitat provision and carbon sequestration. The loss of salt marshes is a global scale phenomenon, of great socio-economic concern due to the substantial benefits that they provide. However, the causes of spatial variability in marsh loss rates are inadequately understood for the purposes of predicting future ecosystem distributions and functions under global environmental change. This study investigated the relationship between the presence of different saltmarsh plants and the mechanical properties of the underlying substrate that relate to its vulnerability to erosion. Relationships between three halophytes (Puccinellia spp., Spartina spp. and Salicornia spp.) and sediment stability were assessed and compared to unvegetated substrates using in-situ and laboratory tests of substrate geotechnical properties and sediment characteristics. Sampling was conducted at two UK sites with contrasting sedimentology, one sand-dominated and one clay-rich. Sediment samples, collected simultaneously with measurements of shear strength, were analysed for moisture content, particle size and organic, carbonate and mineral compositions. These data were then used to explore the contribution of plant type, alongside the sedimentological parameters, to measured shear strength.Shear strength of the sediment varied between and, to a lesser extent, within sites, with the four cover types having a similar effect on shear strength within sites relative to each other. Sediments covered by Puccinellia spp exhibit the highest shear strength, while bare sediments exhibit the lowest. The effect of vegetation type on shear strength was greater in the coarser sediments of Warton Sands. Surface cover type made a significant contribution to exploratory statistical models developed for the prediction of sediment shear strength. The findings support existing recognition that vegetation can enhance sediment shear strengths but extend the insight reveal differences in this effect that show generality between sedimentological settings. Further, the combination of methods provides insight into the fundamental mechanics by which various measures of sediment stability may be affected by different surface cover types. Cohesion appears to be a more appropriate descriptor sediment erodibility than shear strength or friction angle and is most greatly enhanced by the presence of a fine, fibrous root system such as that of Puccinellia. A more detailed understanding of the multi-scale mechanisms which plants confer strength to substrates is needed to better anticipate their impact on sediment erodibility, and therefore salt marsh vulnerability.

Lead halide perovskite solar cells (PSCs) have emerged as a highly promising next‐generation photovoltaic (PV) technology that combines high device performance with ease of processing and low cost. However, the potential leaching of lead is recognized as a major environmental concern for their large‐scale commercialization, especially for application areas with significant overlap with human life. Herein, a quantitative kinetic analysis of the Pb leaching behavior of five types of benchmark PSCs, namely, MAPbI 3 , FA 0.95 MA 0.05 Pb(I 0.95 Br 0.05 ) 3 , Cs 0.05 (FA 0.85 MA 0.15 ) 0.95 Pb(I 0.85 Br 0.15 ) 3 , CsPbI 3 , and CsPbI 2 Br, under laboratory rainfall conditions is reported. Strikingly, over 60% of the Pb contained in the unencapsulated perovskite devices is leached within the first 120 s under rainfall exposure, suggesting that very rapid leaching of Pb can occur when indoor and outdoor PV devices are subject to physical damage or failed encapsulation. The initial Pb leaching rate is found to be strongly dependent on the types of PSCs, pointing to a potential route toward Pb leaching reduction through further optimization of their materials design. The findings offer kinetic insights into the Pb leaching behavior of PSCs upon aqueous exposure, highlighting the urgency to develop robust mitigation methods to avoid a potentially catastrophic impact on the environment for their large‐scale deployment.

Purpose Flocculated cohesive suspended sediments (flocs) play an important role in all aquatic environments, facilitating the transport and deposition of sediment and associated contaminants with consequences for aquatic health, material fluxes, and morphological evolution. Accurate modelling of the transport and behaviour of these sediments is critical for a variety of activities including fisheries, aquaculture, shipping, and waste and pollution management and this requires accurate measurement of the physical properties of flocs including porosity. Methods Despite the importance of understanding floc porosity, measurement approaches are indirect or inferential. Here, using μCT, a novel processing and analysis protocol, we directly quantify porosity in natural sediment flocs. For the first time, the complexity of floc pore spaces is observed in 3-dimensions, enabling the identification and quantification of important pore space and pore network characteristics, namely 3D pore diameter, volume, shape, tortuosity, and connectivity. Results We report on the complexity of floc pore space and differentiate effective and isolated pore space enabling new understanding of the hydraulic functioning of floc porosity. We demonstrate that current methodological approaches are overestimating floc porosity by c. 30%. Conclusion These new data have implications for our understanding of the controls on floc dynamics and the function of floc porosity and can improve the parameterisation of current cohesive sediment transport models.

Microplastics (MPs) are becoming an important component of suspended particulate matter (SPM), especially in estuaries which are hotspots of MPs pollution. Most SPM in estuarine water are removed via flocculation and further deposited. Therefore, we hypothesise that there is an efficient removal process for MPs by flocculation, which are expected to decrease the overall load of MPs in the marine environment. Here we systematically studied and quantified the influence of MPs properties (size, shape, density, polymer type and weathering condition) on flocculation behavior with suspended sediment in estuary conditions. We chose over 20 types of MPs with different properties for 8 size ranges from (10-300 µm for fragments and microbeads, and 10-1000 µm in length for microfibers, respectively). The MPs with different properties and suspended sediment were flocculated in artificial seawater, and the MPs in the system were observed using fluorescence microscopy to distinguish the incorporated MPs and suspended MPs. The incorporation rate (IR) is the ratio of incorporated MPs to total MPs, which is the parameter to evaluate the interaction between MPs and flocs. The IR decreased with increasing size for fragments and microbeads, and also decreased as the diameter of microfibers rose. The IR for fragments smaller 20 µm is extremely high, from 94.8% to 100%, but gradually decreased with increasing size. For fragments larger than 200 µm, the IR of Polyethylene (PE), Polypropylene (PP) and Polystyrene (PS) are lower than 20%, while higher than 50% for Polyethylene terephthalate (PET) and Polyvinyl chloride (PVC). The IR of PE microbeads is significantly lower than those of fragments. When the diameter of microfibers are smaller than 20 µm, the IRs are always higher than 90%, and the length has no effect on IR. While the IR of microfibers decreased with the length increasing when the diameter of microfibers is larger than 30 µm. According to extensive comparisons between 20 types of MPs, we found that the IR is normally higher in the small size, elongated, angular, high density, weathered, chemically active MPs, while lower in the large size, spherical, low density, pristine and chemically inactive MPs. The size plays the most important role, followed by shape. There is an evidence that MPs are likely to be removed from the water column and hence estuary sediments are important sinks for MPs. This process reduces overall load to marine environments, but this is selective and depends on the characteristics of MPs. This study offers a reference to predict the preferential removal behavior of MPs in the estuary.