To Lich river faces long-term microplastic accumulation risk

Research findings from the Institute of Geological Sciences and Mineral Resources reveal that microplastics are widespread along the entire length of the To Lich River, with particularly high concentrations in areas receiving domestic wastewater and urban stormwater runoff.

The findings stem from a project titled "Modeling Microplastic Accumulation Processes in the Bottom Sediments of the To Lich River," led by Dr. Pham Dieu Linh, a researcher in the Department of Hydrogeology, Engineering Geology, and Environmental Geology. The project was carried out under the Plastic Innovation Program (PIP), funded by the United Nations Development Programme.

According to the research team, microplastics are no longer an unfamiliar form of pollution in urban environments, but this marks the first time that microplastic accumulation and dispersal processes in the To Lich River have been surveyed and modeled with reasonable comprehensiveness using hydrodynamic simulation tools.

To assess the current state of contamination, the team collected 28 surface water samples and 28 sediment samples from 14 locations along the To Lich River during both the rainy and dry seasons. Results showed microplastics appearing in multiple forms, including fibers, fragments, films, and pellets, with fibers representing the dominant type by a clear margin.

At numerous discharge points, microplastic concentrations in the water ranged from 3,500 to 18,000 particles or fibers per cubic meter. Areas with the highest densities were typically located near outlet pipes receiving mixed wastewater from residential zones and urban combined sewer overflow systems.

Beyond the water column, microplastics accumulate at far higher densities in the river's bottom sediment layer. Sediment samples simultaneously recorded microplastic fibers, fragmented plastic debris, and dark-colored polymer materials, distinctive markers of an urban environment heavily impacted by domestic wastewater and traffic-related runoff.

According to Dr. Pham Dieu Linh, the most concerning characteristic of microplastics lies in their extraordinarily long environmental persistence. "Unlike many pollutants that dissolve in water, microplastics can sink to the riverbed and accumulate in sediment layers for years. When heavy rain, dredging, or strong flow fluctuations occur, they can be resuspended and dispersed back into the water column," she said.

The research team also documented that prior to the full commissioning of the Yen Xa wastewater collection system, more than 200 direct discharge outlets lined both banks of the river, channeling untreated domestic wastewater and stormwater directly into the watercourse. Each day, the river received approximately 150,000 cubic meters of urban wastewater.

Dr. Pham Dieu Linh concluded that given the river's slow flow velocity, high organic content, and prolonged sedimentation processes, the To Lich River has effectively become a microplastic "sink" for the urban area of Hanoi.

Monitoring microplastics through long-term data

A particularly notable methodological contribution of the study is its application of the MIKE 21 HD and MIKE 21 PT models to simulate microplastic dispersal processes along the To Lich River.

According to the model outputs, during the rainy season, significantly elevated flow volumes substantially increase the diffusion capacity for microplastics compared to the dry season. Rainy season flow rates can reach approximately 6.2 cubic meters per second, compared to just around 1.1 cubic meters per second during the dry season.

By applying hydrodynamic and microplastic transport models, the research team has been able to make an initial identification of river sections with elevated microplastic accumulation risk, particularly segments receiving domestic wastewater and major urban discharge outlets.

Dr. Pham Dieu Linh described this as an important scientific foundation enabling management authorities to identify microplastic pollution hotspots; prioritize monitoring and discharge source control; support the planning of urban drainage and wastewater treatment systems; and assess environmental risks during inner-city river and lake rehabilitation and dredging operations.

The research results also showed that the majority of samples fell into two primary material categories: Polyester and Polyurethane. Among these, PET plastic appeared with high frequency across many analyzed samples.

Dr. Pham Dieu Linh assessed that this points to the highly diverse origins of microplastic generation in the urban environment, not limited to household waste but also linked to traffic, construction, industrial materials, and riverside service activities.

Experts have warned that the risks posed by microplastics extend beyond their small particle size to include their capacity to adsorb heavy metals and persistent organic compounds. Once entering the aquatic environment, they can function as vectors transporting toxic substances through aquatic ecosystems.

According to Dr. Pham Dieu Linh, alongside completing and efficiently operating centralized wastewater treatment facilities such as the Yen Xa system, Hanoi must simultaneously implement a range of measures to reduce microplastic emissions at source. These include reducing single-use plastic consumption; strengthening plastic waste sorting and collection; controlling synthetic microfibers generated from washing and cleaning activities; limiting plastic waste leakage from the urban drainage system; and intensifying monitoring of microplastics in surface water, sediments, and dredged sludge.

Dr. Linh also recommended that Hanoi move quickly to establish a long-term database on microplastics across its inner-city river systems, enabling trend tracking of accumulation over time and assessment of the effectiveness of plastic pollution control measures.

The research conducted by the Institute of Geological Sciences and Mineral Resources does not stop at detecting microplastics beneath the bed of the To Lich River. It opens a scientific foundation for managing urban pollution in the years ahead, one that Hanoi's environmental authorities will need to act on with urgency if the city's waterways are to be meaningfully restored.