Neuer Zeitschriftenbeitrag von Nieß et al., 2026
Green roofs are widely promoted as sustainable urban infrastructure for mitigating climate change impacts, yet their role in stormwater quality remains controversial. This study identifies the primary sources of contaminant leaching from all green roof construction materials by testing material-by-material under standardized laboratory conditions. A total of 19 green roof construction materials, including substrates, fleeces, retention boxes, and root barrier layers, were evaluated in accordance with European and German standards. Results show that green waste compost is the dominant source of pollutants with maximum concentrations for Cu (170 µg/L), and Zn (404 µg/L). Expanded clay released Ni (77.3 µg/L), while crushed bricks contributed to high concentrations of V (99.5 µg/L) and nitrate (1175 mg/L). Leaching from substrates declines over time, often falling below threshold values at high liquid-to-solid ratios. Bitumen sheets with chemical root barriers released Mecoprop, with cumulative release reaching 26.4 mg/m². Mecoprop leaching is characterized by initial wash-off, followed by a diffusion-driven release. A rough estimate based on average daily rainfall suggests that leaching might take several years. However, this estimate does not account for factors such as vegetation uptake, atmospheric deposition, intermittent rainfall, drying-wetting cycles, ageing, preferential flow, and evapotranspiration. Nevertheless, the findings demonstrate that green roof construction materials, particularly substrates and root barriers, can be a major source of pollution in green roof runoff. The results underscore the importance of source control through material selection: replacing green waste compost with low-contaminant alternatives and using non-chemical or encapsulated root barriers. Without such measures, green roof runoff might pose long-term risks to aquatic ecosystems. Sustainable green roof design must prioritize material quality and contaminant screening to ensure true environmental benefit.