Dissolved organic matter pathways and transformation across the urban environment

Today, traditional water sources are starting to struggle to meet current urban demands. Alternative sources such as rainwater, urban stormwater runoff, or aquifers must be considered. Stormwater could represent a very interesting option to augment local water supplies. However, it is widely recognized that stormwater is a large contributor to diffuse pollution. In this context, DOM can be used as a biogeochemical cycle tracer and a proxy for stormwater quality, enabling an effective and sustainable urban water management. However, few studies have addressed the quality at small-scale catchment. To address this, we conducted six sampling campaigns in the city of Barcelona including different water matrices: rainwater, urban runoff (pedestrian and vehicular streets), and influents/effluents from Sustainable Urban Drainage Systems (SUDS). SUDs are urban green infrastructures primarily designed to prevent city flooding by enhancing permeability and facilitating aquifer recharge. Dissolved organic matter (DOM) quality was evaluated by spectroscopic techniques, i.e., measuring specific ultraviolet absorbance (SUVA254), along with fluorescence excitation-emission matrices (FEEM). Results highlighted that concentration of dissolved organic carbon (DOC) followed: rain < SUDS < Pedestrian Street < Conventional Street, providing an initial estimation of contamination levels. The SUVA254 index followed the sequence: rain < Pedestrian Street < Conventional Street < SUDs, indicating the enrichment in aromaticity of Dissolved Organic Matter (DOM) after percolating through the SUDS. Furthermore, calculated fluorescence indices (HIX, BIX, FI, α/β) aided in identifying the origin and maturity of organic matter in the different matrices. Our findings suggest that rainwater was comprised mostly of fresh, microbially derived DOM, while SUDS contained more matured DOM, primarily from terrestrial origin. In contrast, streets contained a different DOM composition, predominantly freshly and microbially derived. Additionally, FEEM together with the MCR-ALS chemometric method allowed us to identify up to six components (i.e., soluble microbial-like, fulvic-like, protein-like tryptophan, protein-like tyrosine, terrestrial humic-anthropic-like, marine and terrestrial-like). Our observations showed that the quantity of DOM decreased through its passage in the SUDS (street to SUD), indicating that these systems can change the quality and quantity of urban organic matter, potentially impacting the water quality of aquifers.