Plastic pollution has become a major global environmental concern as modern societies rely increasingly on plastic products. Much of this plastic waste eventually reaches the ocean, with rivers acting as the main transport routes from urban, agricultural, and other landscapes, thereby affecting the lives of marine organisms. Over time, larger plastic items break down into smaller pieces known as microplastics (less than 5 millimeters) and mesoplastics (between 5 and 25 millimeters). These particles can spread through diverse ecosystems and are now found in many living organisms, including humans. To develop effective countermeasures, it is essential to accurately estimate how much plastic the rivers carry to the ocean.
Many previous studies have examined microplastic and mesoplastic (MMP) concentrations in river water during normal, low-flow conditions. However, they have largely overlooked what happens during floods. Similar to suspended sediment (SS), large amounts of plastic are thought to be transported to the ocean during high-flow conditions, but how exactly MMP concentrations vary during flooding remains unclear. Moreover, few studies have examined the combined effects of urbanization and high flows.
To address this gap, Assistant Professor Mamoru Tanaka and Professor Yasuo Nihei from the Department of Civil Engineering at Tokyo University of Science, Japan, conducted a comprehensive field study to examine how high flows influence MMP concentration. "Previous studies have shown that MMP concentration increases significantly in flooded rivers, likely due to runoff from urban roads that carry MMP into rivers through sewer pipes and other channels," explains Dr. Tanaka. "In this study, for the first time in the world, we clarified this transport by directly collecting river water samples during flood events from four Japanese rivers at six times, capturing how plastic transport changes as water levels rise and fall." Their findings were made available online on December 23, 2025, and published in Volume 291 of Water Research on March 01, 2026.
The team conducted field campaigns in four Japanese rivers with catchments that included urban, agricultural, and forest areas with relatively high population densities. The field campaign covered six rainfall events with total precipitation ranging from 8.8 to 117.9 millimeters. During each event, surface water samples were collected hourly for 12–15 hours, covering both the rising and falling stages of river flow. Alongside MMP concentration, they also conducted turbidity measurements.
The results showed that MMP concentrations during high-flow conditions increased by one to four orders of magnitude compared to low-flow conditions. Using the observational data, the researchers identified a general trend between the load (L) and discharge (Q), also known as the L-Q relationship. This relationship is commonly used to estimate the amount of microplastics and mesoplastics discharged from the river mouth. "Very few studies have quantified the L–Q relationships for plastics," notes Dr. Tanaka.
This relationship can be applied to calculate the total amount of MMP discharged from the river mouth in a year under a wide range of conditions. While the researchers identified distinct trends for each river, they did not find clear correlations between river-specific L–Q relationships and catchment characteristics.
Importantly, the analysis of the L-Q relationship showed that plastic transport is highly concentrated in short periods of high flow. In one river, 90% of the annual mesoplastic load was transported in just 43 days within a year. This means that overlooking high-flow conditions could lead to a serious underestimation of how much plastic rivers deliver to the ocean. The annual microplastic load showed a similar but less prominent pattern. Furthermore, turbidity or SS measurements were significantly correlated with MMPs, suggesting that routine SS monitoring can potentially be used as a proxy for estimating MMP concentration.
"These findings offer valuable insights for understanding riverine transport of plastic debris during floods. Anyone can easily estimate their plastic waste volume, thereby visualizing the burden on rivers numerically. These findings will play a major role in educating people about the environment," remarks Prof. Nihei. "Using the identified L-Q relationship, the annual plastic discharge can be better visualized, supporting better monitoring and more effective policy decisions."
Overall, this work highlights the crucial role of floods in transporting plastic pollution and underscores the need to include high-flow events when assessing plastic emissions.
