Measuring Plastic Pollution In Water: Methods And Solutions

Plastic pollution is a pressing global issue that poses a significant threat to the environment and human health. With the increasing production of disposable plastic products, the world is struggling to manage the growing plastic waste problem. Plastic pollution is pervasive, found not only in oceans, rivers, and lakes but also on land, affecting wildlife, ecosystems, and communities worldwide. To address this challenge, it is crucial to measure and understand the extent and impact of plastic pollution in water bodies. This involves identifying the sources, types, quantities, and potential solutions for mitigating plastic pollution. Developing accurate measurement methods is essential for tackling this complex issue and protecting the planet for future generations.

Using optical satellite data to find plastic patches

Optical satellite data has been used to find plastic patches in coastal waters, with promising results. The Sentinel-2 satellites of the European Space Agency (ESA) have been used to collect optical data for this purpose. Sentinel-2A and 2B Earth Observation satellites were launched in 2015 and 2017, respectively, and they have improved resolution, which is crucial for detecting floating macroplastics.

The detection of marine plastic litter with satellites is still in the research and development stage, and operational satellite monitoring for plastic litter is not yet possible. However, studies have shown that optical satellite data can be used to detect and distinguish plastic patches from naturally occurring materials such as seaweed and sea foam. This is possible because plastic has distinct reflectance properties in the near-infrared range compared to other materials.

To identify and distinguish plastic patches, spectral signatures are derived from the mean spectra of deployed plastic targets. These signatures are then compared to the signatures of other materials such as seaweed, seawater, timber, and non-plant debris. Sentinel-2 satellites can detect materials on the ocean surface using a Floating Debris Index (FDI) developed for the Sentinel-2 Multi-Spectral Instrument (MSI).

The use of satellite data to detect plastic patches is a valuable tool for monitoring and addressing plastic pollution in the marine environment. However, it is important to note that individual pieces of plastic are still very difficult to detect by satellite. Instead, satellites are more likely to detect larger patches of plastic that have aggregated under the influence of ocean currents, winds, and vertical mixing.

In situ measurements with instruments like manta trawls

Manta trawls are particularly useful for collecting samples from the ocean surface, such as plastic pieces and plankton. They are easy to use and can skim a large volume of samples, making them a popular choice for in situ measurements. However, one of the drawbacks of manta trawls is their cut-off size, which can affect the representativeness of the samples. For example, the 330 μm cut-off size of the manta trawl may not capture the majority of particles smaller than 330 μm in a real-world scenario.

To address this limitation, in situ pump filtration systems have been developed as a complementary technique to manta trawling. These filtration systems can sample lower sizes in a cascade filtration setup, which manta trawls cannot. The two methods sample at slightly different depths, with the manta trawl sampling the full surface microlayer, making it more efficient at capturing microplastics that float on the water's surface, such as expanded cellular plastics.

It is important to note that the lack of standardized sampling protocols can hinder effective comparisons between studies. To improve the quality control of monitoring plans, there is a critical need for standardization and harmonization of the methodology for microplastic sampling. This includes considerations such as mesh size, trawling duration, speed, distance, and net opening dimensions, which can impact the efficiency and comparability of studies.

Overall, manta trawls are a valuable tool for in situ measurements of plastic pollution in water, but they should be used in conjunction with other methods to ensure comprehensive and accurate data collection.

Visual surveys for large plastic items

Visual surveys are an important method for detecting large plastic items in water. These surveys involve trained observers on vessels who visually scan the ocean surface for large plastic debris. The observers typically look out to a distance of 20 meters, noting and recording any large debris items they spot. This method was employed by van Sebille et al. in 2015, and the collected observations were later compared with oceanic models.

Visual surveys are a type of in situ observation, which refers to on-site plastic samples or measurements collected at various locations over a given period. In situ measurements play a crucial role in validating and calibrating plastic particle tracking models or satellite measurements. By conducting visual surveys and collecting in situ data, scientists can gain valuable insights into the abundance and distribution of large plastic items in water.

One challenge with visual surveys is the potential for limited coverage. As the observers can only scan a specific distance from the vessel, there may be large plastic items outside of this range that go undetected. Additionally, visual surveys are typically suited for detecting larger plastic items, while smaller microplastics or nanoplastics may be more challenging to identify through visual means alone.

To overcome these limitations, visual surveys are often combined with other measurement techniques. For example, the manta trawl is a net system resembling a manta ray that can be towed behind a scientific research vessel. It uses a thin mesh net to collect samples from the surface of the ocean, including plastic pieces and associated plankton. However, the mesh size of these nets, typically around 0.3 mm, restricts the size of the debris that can be captured.

Another complementary technique involves the use of ocean colour to detect water-leaving radiance. By analysing the radiance of the water, researchers can identify large patches of plastic on the sea surface. Advances in imagery resolutions further enable more precise monitoring of smaller plastic plots. These combined efforts help address the challenges of measuring plastic pollution in water and provide valuable data for understanding the scope and impact of plastic pollution on ecosystems and human health.

Modelling to understand plastic fate and impact

Fate and Transport Models

Fate and transport models are essential tools for understanding the movement and ultimate fate of plastic particles in water systems. These models can account for various processes such as advection, dispersion, diffusion, settling, dissolution, and biochemical degradation by light and temperature. For example, the National Integrated Catchment-based Eco-hydrology (NICE) model and its extension, NICE-BGC (BioGeochemical Cycle), are process-based eco-hydrology models that have been applied to study plastic debris in rivers and estuaries. These models help simulate the impact of estuaries on plastic budgets and the seasonal variability caused by the settling and resuspension of plastic particles.

Spatio-Temporal Variability

Modelling plastic pollution in water involves understanding its spatial and temporal distribution. Models can simulate the size distribution of plastics in water and riverbed sediments, revealing that coarser particles tend to be more prevalent in riverbeds. Flood events significantly impact plastic mobilisation and inter-annual variability, with higher plastic transport during flood periods. Modelling studies have also focused on the spatial and seasonal distribution of floating plastics in tropical coastal waters, considering factors like wind and hydrodynamic conditions.

Multi-Pollutant and Multi-Particle Approaches

Plastic pollution in water is often accompanied by other pollutants, and a multi-pollutant modelling approach can help better understand and manage water quality issues. Additionally, a multi-particle approach is necessary to study the complex interactions between biota and various types of plastic materials, including microplastics and nanoplastics, to assess their ecological and health risks accurately.

Numerical Frameworks

Numerical frameworks, such as CaMPSim-3D, have been developed to predict the fate and transport of microplastics in different aquatic settings. This framework combines a Lagrangian, three-dimensional (3D) particle-tracking model (PTM) with an Eulerian-based hydrodynamic modelling system (TELEMAC) to provide accurate simulations and facilitate informed decision-making for targeted prevention and clean-up activities.

Process-Based Mass-Balance Models

Addressing the complex physical and chemical properties of plastic pollution can be challenging. Process-based mass-balance models have been proposed as a solution to evaluate environmental exposure to plastic pollution by synthesising knowledge about its measurable intrinsic properties, including composition, size, shape, and associated chemicals.

Developing methods to identify and quantify plastic particles

Plastic pollution is a pressing environmental issue, with plastic waste pervading marine environments and endangering wildlife and ecosystems. Developing methods to identify and quantify plastic particles is crucial for understanding the scope and impact of plastic pollution and devising effective solutions.

One approach to identifying and quantifying plastic particles is in situ plastic measurements, which involve collecting on-site samples or measurements at various locations over a given period. For instance, the manta trawl, a net system resembling a manta ray, is used to collect samples from the ocean surface. However, the mesh size of such nets limits the size of debris that can be collected. In situ measurements help validate and calibrate plastic particle tracking models and satellite measurements.

Optical satellite data provided by Sentinel-2 satellites has also been used to detect floating aggregations of seaweed, sea foam, and macroplastics. By exploiting the spectral shape, including reflectance properties, researchers can distinguish macroplastics from other materials. This method has successfully identified plastic with high reflectance in the near infrared compared to other materials.

To address the challenge of identifying microscopic nanoplastics, researchers have modified a powerful imaging technique called stimulated Raman scattering (SRS) microscopy. SRS microscopy uses two laser beams focused on samples to stimulate certain molecules to emit unique detectable light signals. This technique does not depend on labeling specific molecules, making it useful for detecting micro- and nanoplastics at the single-particle level.

Additionally, laboratory analyses, such as FTIR Spectroscopy, py-GC/MS, and Raman Spectroscopy, are employed to detect and quantify microplastic concentrations in water samples. These methods can determine the number of microplastic particles by type and size range and quantify specific types of microplastics in mg/l.

The development of accurate methods for identifying and quantifying plastic particles is essential for understanding plastic pollution's sources, transport, fate, and impacts on different environmental compartments. This knowledge will inform targeted solutions, such as source reduction and recycling initiatives, to mitigate plastic pollution's effects on ecosystems and human health.

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