Impact of the superblock model on key components of the urban water cycle: Trace metals and dissolved organic matter dynamics across the built environment

Climate change, growing urban pollution, and increasing water scarcity are forcing cities to adopt strategies that enhance resilience to both climatic and anthropogenic pressures. The Barcelona Superblock model is a novel urban planning strategy that highly restricts vehicle traffic, converts streets into pedestrian-priority corridors, and promotes green spaces. Within this framework, green infrastructures, also called sustainable urban drainage systems (SUDS), are implemented as local, site-specific measures to capture (i.e., flood control), treat, and infiltrate treated stormwater (i.e., aquifer recharge). To evaluate the Superblock model impact on urban water quality, we conducted seven sampling campaigns across three Barcelona districts, targeting rainfall, stormwater "first-flush" from roads and pedestrianized streets, as well as SUDS influent and effluent within and in the vicinity to Superblocks, with a focus on dissolved trace metals and dissolved organic matter (DOM). Results showed that Superblocks reduced pollutant loads and mitigated ecotoxicological risks. Trace metal and DOM concentrations followed the trend: Rain < SUDS effluent < Pedestrian Street runoff < Road runoff, highlighting traffic-related impacts and SUDS treatment capacity (23-70% in best case scenario). Risk assessment indicated episodic ecotoxicological risk in stormwater, especially in road runoff due to elevated concentrations of Cu and Zn, while SUDS consistently remained below risk thresholds. SUDS also transform DOM into more stable, humic-like forms. Trace metals and DOM emerged as biogeochemical proxies for stormwater quality, enabling more effective and sustainable urban water management. These findings support the integration of Superblock-like strategies into urban planning to control and reduce contaminant urban discharges.