Human-associated modulation of urban DOM heterogeneity: transformation toward aquatic carbon sink-like and carbon...

Dissolved organic matter (DOM) plays a vital role in the safety and stability of drinking water, yet the molecular characteristics of DOM across urban aquatic systems remain insufficiently elucidated. In this study, advanced spectroscopic techniques and Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) were employed to characterize DOM in municipal tap water (TW) and corresponding source water (SW) from Shenzhen, China. Comparative analyses revealed significant reductions in humic-like fluorescence, molecular weight, aromaticity, unsaturation, and N/S-containing molecules from SW to TW, primarily attributable to water treatment and distribution processes. Notably, while DOM in SW samples exhibited high molecular similarity across regions, TW samples displayed marked regional heterogeneity within the city. Specifically, TW from the eastern region contained higher concentrations of aliphatic compounds, peptides, and P-containing molecules, while TW in the middle and western regions possessed a greater abundance of highly unsaturated compounds. These findings enabled the further identification of two distinct human-associated transformation pathways: the eastern region favored the formation of carbon source-like compounds (e.g., lipids and peptides), whereas the western and middle regions promoted the accumulation of carbon sink-like substances (e.g., highly unsaturated and carboxyl-rich alicyclic molecules). This divergence indicates region-specific carbon cycling dynamics within the urban water system, highlighting a potential trade-off between risks of secondary pollution and toxic compound enrichment. Molecular-level insights generated by this study are essential for understanding DOM fate and carbon cycling within urban water systems, ultimately informing strategies to ensure the reliable and safe provision of drinking water.