Urban rivers bear distinct chemical fingerprints of landscape and human activity

A collaborative team of researchers from the University of Science and Technology, Beijing, and the Chinese Research Academy of Environmental Sciences has provided an unprecedented molecular-level view into the water quality of urban rivers. The investigation focused on dissolved organic matter (DOM), a complex mixture of carbon-based compounds that influences aquatic ecosystems and drinking water safety. By analyzing the intricate chemical makeup of DOM, scientists can trace its origins, whether from natural soil and plant decay or from human-caused pollution. This new work offers a powerful diagnostic approach for understanding the health of waterways in densely populated areas.

The investigation centered on two vital Beijing waterways with differing roles and surrounding environments: the Yongding River (YDH) and the Beiyun River (BYH). The YDH, known as Beijing's "mother river," primarily serves water supply functions and flows through mountainous, forested terrain. In contrast, the BYH courses through the city’s urban sub-center, receiving significant amounts of domestic sewage and agricultural runoff. This intentional comparison allowed the scientific team to isolate how distinct landscapes and anthropogenic pressures imprint unique chemical signatures on the rivers’ dissolved organic matter pools.

A Tale of Two Rivers: Molecular Clues in Water

To achieve a comprehensive characterization, the team employed a powerful combination of analytical techniques. They first used optical spectroscopy to gain a broad overview of the DOM's properties, such as its aromaticity and molecular weight. Following this, they utilized a highly sensitive technique called Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). This advanced method provided ultra-high-resolution data, allowing for the assignment of exact molecular formulas to thousands of individual compounds within the DOM, effectively creating a detailed chemical fingerprint for each water sample.

The results painted two very different pictures. The Yongding River, influenced by its forested catchment, contained DOM dominated by terrestrially-derived humic substances. These compounds, originating from soil and decaying plant material, were characterized by higher aromaticity, larger molecular weights, and a more refractory nature, meaning they are less easily broken down. This profile is consistent with a river system whose chemistry is governed more by natural, allochthonous inputs from the surrounding landscape.

Fingerprinting Pollution Sources

Conversely, the chemical signature of the Beiyun River clearly reflected its urban setting. Its DOM was composed mainly of microbial and sewage-derived proteins, which are indicative of wastewater discharges and the biological activity they stimulate. These molecules were less aromatic, had lower molecular weights, and were more labile or easily degradable. The analysis also identified a significant presence of anthropogenic surfactants, compounds found in household detergents. The detection of these specific molecules provides unequivocal evidence of the impact of urban wastewater on the river's composition.

A concerning trend was observed in both rivers: the concentration and complexity of DOM increased from upstream to downstream. This downstream accumulation of more refractory and aromatic compounds suggests that as water flows through the landscape, more easily degradable organic matter is consumed by microbes, leaving behind a persistent signature of pollution. Dr. Shasha Liu, the corresponding author from the University of Science and Technology, Beijing, explains, "Our findings demonstrate that urbanization broadly modifies the molecular structure of DOM to be more aliphatic and saturated. By combining optical and mass spectrometry techniques, we can not only identify the dominant sources of organic matter but also track its transformation along the river. This provides a much-needed scientific basis for developing more precise and effective water management strategies in complex urban ecosystems."

The Path Forward for Water Management

The researchers note that biogeochemical processes, such as microbial degradation and sedimentation, can alter DOM molecules in the water, which can present challenges for precise source identification. Future investigations could build upon this work by examining these processes across different seasons and hydrological conditions. Such efforts would help develop a more dynamic understanding of how carbon and contaminants cycle through urban river networks.

Ultimately, this work provides deep molecular insight into how human activities and natural landscapes interact to define the chemical composition of urban rivers. The detailed characterization offers a powerful tool for water resource managers to monitor pollution, identify its sources with greater accuracy, and protect the ecological health and safety of critical freshwater resources in rapidly growing metropolitan regions worldwide.

Corresponding Author: Shasha Liu

Original Source: https://doi.org/10.1007/s44246-024-00151-y

Contributions: Yi Ren analyzed the data and wrote the manuscript. Shasha Liu designed the experiment and modified the manuscript. Leizhen Liu, Chengyu Suo and Rui Fu assisted in completing the experiment. Yuxin Zhang and Yue Qiu revised the manuscript. Fengchang Wu provided constructive guidance. All authors read and approved to the final manuscript.