Green roofs provide many ecosystem services in urban areas, such as thermal insulation of buildings, reduction of stormwater runoff, mitigation of heat island effect and noise pollution, aesthetic improvement. A less investigated aspect is the contribution to the reduction of urban runoff pollution by absorbing pollutants deriving from stormwater. An experimental research in two areas, North and South Milan (Brugherio and San Giuliano Milanese), was carried out to test the potential of this technology in a heavily impervious metropolitan area. A total of 24 different green roof arrangements (each with a 0.38 m2 surface area and 2 replicates, for a total of 25.5 m2) were equipped by combining the following characteristics: substrate type (3 types, i.e. 2 different soil types with about 5% organic matter, and type 2 enriched with organic matter up to 10%), substrate depth (8 cm vs. 12 cm vs. 15 cm), vegetation type (Sedum spp. vs. grassfield vs. no vegetation), slope (2% vs. 10%), fertilization (no fertilization vs. fertilization). The performance in retaining stormwater, analyzed between May 2018 and September 2019, resulted in 73% runoff reduction. Macro- and micro-element, nutrient and PAHs concentrations and loads in green roof runoff were compared with those of stormwater in three precipitation events. In November 2019 and April 2019 events, experimental green roofs retained 88% of N-NH4 deriving from stormwater, as well as 10%-35% of Zn, Ni, Pb, Se, Co e Cd. Vegetation and substrate type resulted as key factors determining the capability to retain part of precipitation and pollutants. In the winter event (February 2019), retention capability was strongly reduced, probably due to inactive vegetation and frozen substrate, except for PAHs (60% retention) probably due to the absorption on the organic fraction of the substrate. However, green roof runoff was characterized by a strong enrichment in suspended solids (30±40 mg/L), DOC (13.3-55.3 mg/L), nutrients such as TP and TN, cations as K e Mg, macro- e micro-element such as Al, Fe, Mn, Ba, Sr e Cr, leached from substrates, fertilizers or drainage systems. If correctly designed and managed, these systems may be an effective adapting strategy to climate change, achieving a strong capability to reduce stormwater annual runoff in impervious areas. However, materials and arrangements need to be correctly chosen to reduce significantly pollutant loads.
Green roofs in Milan metropolitan area: runoff water quality and quantity
Laura Marziali;Lucia Valsecchi;Diego Copetti;Alfredo Schiavon;Franco Salerno;Gianni Tartari;Benedetta Barozzi;Alice Bellazzi;
2020
Abstract
Green roofs provide many ecosystem services in urban areas, such as thermal insulation of buildings, reduction of stormwater runoff, mitigation of heat island effect and noise pollution, aesthetic improvement. A less investigated aspect is the contribution to the reduction of urban runoff pollution by absorbing pollutants deriving from stormwater. An experimental research in two areas, North and South Milan (Brugherio and San Giuliano Milanese), was carried out to test the potential of this technology in a heavily impervious metropolitan area. A total of 24 different green roof arrangements (each with a 0.38 m2 surface area and 2 replicates, for a total of 25.5 m2) were equipped by combining the following characteristics: substrate type (3 types, i.e. 2 different soil types with about 5% organic matter, and type 2 enriched with organic matter up to 10%), substrate depth (8 cm vs. 12 cm vs. 15 cm), vegetation type (Sedum spp. vs. grassfield vs. no vegetation), slope (2% vs. 10%), fertilization (no fertilization vs. fertilization). The performance in retaining stormwater, analyzed between May 2018 and September 2019, resulted in 73% runoff reduction. Macro- and micro-element, nutrient and PAHs concentrations and loads in green roof runoff were compared with those of stormwater in three precipitation events. In November 2019 and April 2019 events, experimental green roofs retained 88% of N-NH4 deriving from stormwater, as well as 10%-35% of Zn, Ni, Pb, Se, Co e Cd. Vegetation and substrate type resulted as key factors determining the capability to retain part of precipitation and pollutants. In the winter event (February 2019), retention capability was strongly reduced, probably due to inactive vegetation and frozen substrate, except for PAHs (60% retention) probably due to the absorption on the organic fraction of the substrate. However, green roof runoff was characterized by a strong enrichment in suspended solids (30±40 mg/L), DOC (13.3-55.3 mg/L), nutrients such as TP and TN, cations as K e Mg, macro- e micro-element such as Al, Fe, Mn, Ba, Sr e Cr, leached from substrates, fertilizers or drainage systems. If correctly designed and managed, these systems may be an effective adapting strategy to climate change, achieving a strong capability to reduce stormwater annual runoff in impervious areas. However, materials and arrangements need to be correctly chosen to reduce significantly pollutant loads.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
