Stormwater pits are part of urban drainage and conveyance structures, but they are also small biogeochemical zones where organic materials from streets, trees, soils, rooftops and urban debris are leached, transformed and transported during rainfall events. In these systems, dissolved organic matter (DOM) dynamics are influenced by both the type of organic material entering the pit and the hydrological conditions that determine retention and flushing.
This study used thirty-three concrete stormwater pits to investigate how evergreen and deciduous street-swept detritus interact with contrasting hydrological regimes to influence the quantity and composition of DOM in stormwater. Evergreen material was dominated by Eucalyptus leaf litter, while deciduous material included common urban tree genera such as Platanus, Gleditsia, Liquidambar, Quercus and Zelkova. These materials were exposed to low-flow and high-flow regimes, and DOM responses were assessed using dissolved organic carbon, UV-visible absorbance, fluorescence indices and parallel factor (PARAFAC) modelling.
Deciduous detritus produced a much stronger early carbon pulse, releasing approximately 11.4-13.3 mg C g-¹ detritus within the first week, compared with 3.0-3.7 mg C g-¹ from evergreen detritus. Carbon release declined after the second week, indicating rapid depletion of readily leachable compounds. Low-flow conditions supported DOM accumulation and more humified signatures, whereas high-flow events flushed more labile material from the pits. Fluorescence and PARAFAC results showed a gradual shift from humic-like and fulvic-like DOM toward a greater contribution of protein-like, microbially influenced material over time.
Overall, vegetation type was the most important driver of DOM concentration, with deciduous detritus releasing substantially greater amounts of carbon than evergreen detritus. Hydrological regime played a secondary but important role by regulating the retention, transformation and export of DOM. These findings highlight the need to consider urban vegetation composition, litter management and stormwater flow conditions when managing carbon enrichment and water quality in urban catchments.