Improving estimates of microplastic pollution in freshwater

Microplastic (MP) pollution is a global problem. Despite this, there is no standard method for determining the MP concentration in rivers, which are the main points of entry of MP into the oceans. This leads to random sampling and inaccurate assessments. Now, researchers from Japan have proposed a method that determines the appropriate number of samples for accurate determination of MP concentration in freshwater. The method could significantly reduce the time and resources required for MP surveys.

The extent of plastic pollution in the form of microplastics (MP) remains largely hidden: plastic particles with a diameter of less than 5 mm. Because plastics are slow to degrade, they break down into tiny particles that eventually contaminate entire ecosystems. In the years since their discovery in the early 1970’s, MPs have become a ubiquitous and global problem. MPs are on land, in the air, in the water, and in the food we eat, especially seafood. This is because freshwater sources such as rivers often carry MPs into the oceans, where they accumulate.

However, despite its prevalence, there is currently no standard method for measuring and quantifying MP concentration in rivers. Plankton nets, originally designed to collect plankton samples, are often used to capture MPs in rivers. To prevent these nets from clogging and to ensure a large sample size, multiple samples are collected from fixed locations along the river and the MP concentration is calculated as the average of all sample results. However, most studies do not account for uncertainties and sampling errors, leading to erroneous assessments of MP concentrations, particularly in relation to the sample amounts required for accurate MP assessments.

Now in a recent study Published in environmental pollutionhave dr Mamoru Tanaka and Professor Yasuo Nihei from Tokyo University of Science together with Associate Professor Tomoya Kataoka from Ehime University in Japan improved the MP concentration estimate by accounting for the variability between estimates from different samples. The variance can help estimate the appropriate number of samples needed for an accurate representation of MP contamination. “For an on-site sampling of microplastics, we proposed a method to determine the appropriate number of iterations in each contamination situation.” says dr Tanaka.

In addition, the variance can provide information about how MPs are distributed in the body of water. For example, if they are evenly distributed in the flow, the between-sample variances would be small. On the other hand, a high variance would indicate an uneven clump distribution.

To assess the variances in MP concentration between samples, the scientists borrowed another method originally intended for zooplankton. “It turns out that the numerical concentration ranges of microplastics in rivers overlap with those of zooplankton,” explains Dr. Tanaka on the similarity in both sampling method and concentration estimates between MPs and zooplankton. According to this method, the between-sample variance is proportional to the average or mean of the concentration estimates.

For the MP concentrations, the team collected 10 samples in plankton nets at two locations along the Ohori River and the Tone-unga (Unga) Canal in Chiba, Japan — two bodies of water that flow through urban areas and contain high concentrations of plastic waste . They identified a total of 1333 MP particles at the sampling points. The average MP concentrations measured in the Ohori at 5.23 particles/m3 and the Unga at 15.22 particles/m3 were higher than the reported average of MPs in Japanese rivers (4.3 particles/m3). In addition, the calculated means and variances at both sites agreed using a simple linear regression. “The variance steadily increased with increasing mean numerical concentrations,” points to dr. to Tanaka. The regression analysis further suggested that the MPs in the rivers do not interact with each other, leading to random particle distributions.

Most importantly, the team found that when MP concentrations are high, two replicate samples are sufficient to accurately measure MP concentrations. “We found that the mean of two replicates retained a reasonable accuracy of less than 30% for conditions with high concentrations greater than 3 particles/m3,” says dr Tanaka.

The problem of MPs has been recognized in recent years and various countries, including Japan, have enacted legislation to ensure better surveillance and control of MPs in the environment. With this in mind, this study could help improve sampling methodology and reduce the time and resources required for MP assessment surveys.

Relation: Tanaka M, Kataoka T, Nihei Y. Variance and precision of microplastic sampling in urban rivers. Vicinity. pollution. 2022;310:119811. doi: 10.1016/j.envpol.2022.119811

This article was republished from the following materials. Note: The material may have been edited for length and content. For more information, please refer to the given source.

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