Sources and transformations of dissolved organic matter in urban impervious surface sediments: Insights from optical signatures and environmental drivers

Despite the critical role of dissolved organic matter (DOM) in urban aquatic environments, knowledge about DOM characteristics in urban impervious surface sediments—a major source of non-point pollution—remains limited. This study addresses this gap by investigating DOM spatiotemporal distribution, sources, and transformation mechanisms through an innovative integration of optical analysis with multidimensional environmental factors. Surface sediment samples (n = 171) were collected across different functional zones and impervious surfaces in Xi'an, China. Three major fluorescent components were identified: UV humic-like C1 predominantly from soil/vegetation, visible-light humic-like C2 mainly from atmospheric deposition, and protein-like C3 primarily from microbial/human activities. Redundancy analysis revealed that meteorological factors, land use, and air quality collectively explained 35.78 % of DOM variability, and structural equation modeling (SEM) further quantified their direct/indirect effects on DOM sources and transformation. Significant seasonal and spatial heterogeneity was observed: dry periods facilitated atmospherically deposited DOM accumulation, while rainfall preferentially washed off soluble low-molecular-weight fractions. Built-up area proportion negatively impacted C1, confirming its natural source origin. Atmospheric pollutants strongly influenced C2 composition and aromaticity, while temperature enhanced microbial processes, promoting C3 formation. These findings provide a scientific foundation for developing targeted non-point source pollution control strategies and deepen our mechanistic understanding of urban DOM cycling in rapidly urbanizing regions.