Traffic-Driven Spatial and Seasonal Variability of Rainwater Chemistry: Insights from Multivariate Analysis in Bucharest, Romania

Urban rainwater is an increasingly important alternative water resource, but its suitability for harvesting and reuse depends on a water quality that varies strongly with local emissions and season—a variability not previously characterised for Bucharest, the Romanian capital. This study addresses that gap by evaluating the physicochemical quality of directly collected rainwater at six contrasting sites (four traffic-influenced, two urban-green) in District 1, Bucharest, over 37 rainfall events sampled between January and November 2025. Samples were analysed for pH, electrical conductivity, turbidity, true colour, total hardness, chloride, inorganic nitrogen species (NO3−–N, NO2−–N, NH4+–N) and trace metals (Zn, Cd, Pb), and interpreted using descriptive statistics, Pearson’s correlation and Principal Component Analysis (PCA). Traffic-influenced sites were consistently more contaminated than urban-green sites, with mean concentrations higher by 47% for electrical conductivity (EC), 64% for cadmium and 67% for zinc. PCA resolved two factors: traffic-related particulate pollution (Cd, Pb, Zn) and secondary atmospheric nitrogen processing. Cadmium reached the World Health Organisation (WHO) indicative guideline, and lead exceeded the European Union (EU) parametric value at traffic sites, while ammonium exceeded 0.50 mg/L in most traffic-site events. These findings show that harvested rainwater quality is strongly site-dependent and that high-traffic locations require targeted treatment before any potable reuse.