Linking Urban Sewage and Rainfall Patterns to Shifting Water Chemistry

In a recent study, researchers at Hebei University of Science and Technology have identified nitrogen pollution, water temperature, and pH as the primary drivers behind seasonal changes in dissolved organic matter (DOM) in Shijiazhuang's urban rivers. The findings for this study, which were published in the journal Water Cycle, illuminate a potential pathway for cities to navigate and manage water quality during these extreme weather events.¹

What is dissolved organic matter?

Dissolved organic matter refers to organic mixtures that are characterized by their composition and structure.² DOMs are normally found in commonly found in rivers, lakes, streams, wetlands, groundwater, oceans, and soils, where it plays a key role in nutrient cycling, carbon transport, and aquatic ecosystem health.^2–4

What was the question the researchers were trying to answer in their study?

In their study, the researchers examined how DOM changes across seasons and locations in a warm-temperature temperate monsoon climate zone. The city investigated in the study was Shijiazhuang, which roughly 11 million people call home.¹

Using fluorescence spectroscopy combined with machine learning (ML), the research team analyzed water samples from upstream and downstream sections of the city's rivers following rainfall events. The downstream sites, which receive discharge from sewage treatment plants and industrial sources, showed significantly higher total nitrogen and chemical oxygen demand than upstream sites, keeping the rivers in what researchers classified as a "mesotrophic," or moderately nutrient-enriched, state throughout the study period.¹

What were the components of DOM?

After conducting their analysis, the researchers determined that there were three components of DOM. These three components are two humic-like substances typically associated with soil and plant decay, and one protein-like substance generally linked to microbial or algal activity and, in urban settings, wastewater.¹ The researchers also found that the DOM component found in the water differed between the upstream and downstream waters. To highlight this point, the researchers stated that the protein-like component dominated the upstream waters, accounting for roughly 30% and 87% of DOM depending on the season.¹ Downstream waters, by contrast, were dominated by humic-like substances, which made up as much as 91% of the total in some samples.¹

So why did this distinction occur? According to the researchers, it all comes down to how water moves through and collects urban runoff.

What did the researchers observe about the nitrogen concentration?

Applying ML to the spectral data, researchers pinpointed nitrogen concentration, pH, and water temperature as the environmental variables most strongly associated with shifts in DOM composition.¹ Total nitrogen showed the greatest overall influence on how DOM varied across seasons and locations. The upstream river network also displayed more complex interactions between DOM and environmental factors than downstream sections, which the authors attribute to the combined effects of natural processes and human activity in less-developed catchment areas.¹

Meanwhile, path analysis further showed that the factors shaping DOM properties were not uniform across time and place. In upstream waters during summer, the source of the organic matter itself had the largest effect on its properties.¹ In nearly every other combination of season and location examined, dissolved iron had the greatest influence instead, which is a distinction the researchers say underscores how monsoon rainfall and urban infrastructure interact in ways that are not constant throughout the year.¹

What are the main takeaways from this study, particularly for water managers?

There are several takeaways from this study, particularly when it comes to controlling nitrogen inputs. Tying control of nitrogen inputs with the amount of wastewater discharge would help improve ecological resilience in urban rivers. The authors argue that making improvements in sewage pipe networks would also help improve water management.¹

References

1. Shi, K.; Zhang, J.; Zhao, Y.; Zhou, S. Seasonal Effects of Rainfall on Characteristics and Environment Response of Dissolved Organic Matter (DOM) in Urban Rivers: Insights from Spectroscopy and Machine Learning. Water Cycle 2026, 7, 57–67. DOI: 10.1016/j.watcyc.2025.06.005
2. Chasse, J. Spectroscopy and GPC to Evaluate Dissolved Organic Matter. Spectroscopy Online, 2026. https://www.spectroscopyonline.com/view/spectroscopy-and-gpc-to-evaluate-dissolved-organic-matter (accessed July 8, 2026).
3. Wegley Kelly, L.; Nelson, C .E.; Petras, D.; Koester, I.; Quinlan, Z. A.; Arts, M. G. I.; Nothias, L. F.; Comstock, J.; White, B. M.; Hopmans, E. C.; van Duyl, F. C.; Carlson, C. A.; Aluwihare, L. I.; Dorrestein, P. C.; Haas, A. F. Distinguishing the Molecular Diversity, Nutrient Content, and Energetic Potential of Exometabolomes Produced by Macroalgae and Reef-Building Corals. Proc. Natl. Acad. Sci. USA 2022, 119 (5), e2110283119. DOI: 10.1073/pnas.2110283119
4. Chen, M.; Xu, J.; Tang, R.; Yuan, S.; Min, Y.; Xu, Q,; Shi, P. Roles of Microplastic-Derived Dissolved Organic Matter on the Photodegradation of Organic Micropollutants. J. Hazard. Mater. 2022, 440, 129784. DOI: 10.1016/j.jhazmat.2022.129784