Water quality assessment of a long-term established extensive green roof under tropical conditions

Green roofs comprise vegetated systems that aim to offset the effects of urbanisation towards more sustainable environmental conditions. While these vegetated systems are a key component of sustainable drainage systems, reducing stormwater runoff volume, their role as a source or sink of pollutants remains debated, particularly in tropical climates where long-term data for mature systems are critically scarce. This study aims to assess the water quality of stormwater runoff from a mature extensive green roof (established for 7 years) in Rio de Janeiro, Brazil, under tropical climate conditions. Two identical (1.8 m²) green roof and metallic roof prototypes were monitored over an 11-month period, encompassing 11 rainfall events. Samples of rainwater (RW) and stormwater runoff from the metallic roof (MR) and green roof (GR) were analysed for physicochemical and microbiological parameters, such as colour, pH, turbidity, total dissolved solids (TDS), total organic carbon (TOC), total nitrogen (TN), total coliforms (TC) and Escherichia coli (EC). Results indicate that green roof runoff exhibited significantly higher concentrations of colour, TDS, TOC, TN and microbiological contaminants than rainwater and metallic roof stormwater runoff. Given that these findings highlight the role of green roofs in influencing physicochemical and microbiological parameters in rainwater, the overall environmental impact must be weighed against the significant reduction in runoff volume they provide. The mitigation of stormwater runoff by green roofs can offset their role as a source of pollutants in rainwater. However, it must be emphasised that integrated load-based assessments and careful substrate management are needed in tropical urban settings.