Runoff water quality from intensive and extensive vegetated roofs

Transcription

1 ecological engineering 35 (2009) available at journal homepage: Runoff water quality from intensive and extensive vegetated roofs Justyna Czemiel Berndtsson a,, Lars Bengtsson a, Kenji Jinno b a Department of Water Resources Engineering, Lund University, Box 118, SE Lund, Sweden b Institute of Environmental Systems, Kyushu University, Hakozaki, Higashi-ku, Fukuoka-shi , Japan article info abstract Article history: Received 5 July 2007 Received in revised form 18 September 2008 Accepted 28 September 2008 Keywords: Green roof Heavy metals Nutrients Runoff quality Urban Vegetated roof Vegetated roofs are becoming a trend in urban design, among others as a tool for city greening, mitigating urban heat island effect, and lowering urban storm runoff. Additionally, pollutant removal within vegetated roofs is often expected; however, it is commonly not a design feature. This study investigated influence on runoff water quality from two fullscale vegetated roofs (an intensive from Japan and an extensive from Sweden). Results show that both extensive and intensive vegetated roofs are a sink of nitrate nitrogen and ammonium nitrogen with similar performance. The intensive vegetated roof is also a sink of total nitrogen in contrast to the extensive roof. Phosphorus release is observed from the extensive vegetated roof but not from the intensive vegetated roof; release of dissolved organic carbon and potassium is observed from both roofs. The vegetated roofs, if not retaining the metal pollutants, were generally not a significant source. The increase of average ph during rainwater passage through the intensive vegetated roof indicated rapid neutralization of the acid depositions Elsevier B.V. All rights reserved. 1. Introduction Vegetated roofs, also referred to as green-, living-, and ecoroofs are roofs covered with soil and vegetation. Vegetated roofs can be constructed as intensive, simple intensive, and extensive (FLL, 2002). Intensive vegetated roofs can be designed as gardens with deep soil layers supporting bigger plants such as trees and bushes. They require maintenance (e.g. weeding, fertilizing, and watering). Simple intensive vegetated roofs consist of lawns and ground covering plantations and also would require gardening maintenance. Extensive vegetated roofs would typically support vegetation of type succulents, herbs, grasses, and mosses and are often planned as maintenance-free. Typically a thin substrate is used on extensive vegetated roofs. Vegetated roofs can provide many general environmental and associated aesthetic and health benefits (English Nature, 2003) and can play an important role in increasing the green space in densely populated urban areas. In Japan, vegetated roofs are constructed to provide amenity space for building users. The evaporative cooling effect is also considered important (Onmura et al., 2001). In Sweden, an important reason for establishment of vegetated roofs is also aesthetics. In some areas reducing urban storm runoff is a priority (Bengtsson et al., 2005). At present, in Sweden, extensive vegetated roofs gain most attention. Interest in vegetated roofs is increasing and more and more such installations are being established. However, the performance of vegetated roofs in achieving desired benefits has not been sufficiently investigated. Some aspects of vegetated Corresponding author. Tel.: ; fax: address: justyna.czemiel berndtsson@tvrl.lth.se (J.C. Berndtsson) /$ see front matter 2008 Elsevier B.V. All rights reserved. doi: /j.ecoleng

2 370 e c o l o g i c a l e n g i n e e r i n g 3 5 ( ) Fig. 1 Location of the vegetated roofs study sites. roofs, for example, their influence on energy flows through the roof, have received more attention than others, as, for example, roofs influence on storm water quality. Runoff water quality from vegetated roofs is an important environmental aspect in particular if vegetated roofs are combined with open storm water systems. Although sufficient evidence is lacking, it is often assumed that vegetated roofs would contribute to runoff water quality improvement as compared with hard roofs runoff (English Nature, 2003). A reason behind this assumption is a fact that the total annual runoff from a vegetated roof due to evapotranspiration is usually less than runoff volume from a hard surface (Getter et al., 2007; Bengtsson et al., 2005; Villarreal and Bengtsson, 2005). Thus, assuming the same concentrations of pollutants from a vegetated roof and a hard roof, annual pollutant load from a vegetated roof would be less. However, concentrations of pollutants in runoff from vegetated roofs are not necessarily the same as in runoff from hard roofs. For example, release of fertilizers and/or soil particles can potentially cause runoff water contamination by nutrients and/or suspended solids (Czemiel Berndtsson et al., 2006; Emilsson et al., 2006; Moran et al., 2005). Pollutant removal within vegetated roofs is often an expected by-product of vegetated roofs. As long as water quality issues remain outside the design phase there is a large potential for vegetated roofs to be contra-productive towards improvement of urban runoff quality. Roesner (1999) expresses the opinion that most structural storm water BMPs (Best Management Practices) in the world are improperly designed hydrologically and hydraulically. Until flow management and water quality management are jointly considered in BMP design, we will continue to be disappointed with their performance. Once again this proves to be valid when applied to vegetated roofs, in which case flow management is recognized but water quality issues have not been considered until recently. In order to refine the design of vegetated roofs for runoff water quality improvement, further research is required. In this study the influence of extensive (from Sweden) and intensive (from Japan) vegetated roofs on runoff water quality is investigated. Location of the study sites is shown in Fig. 1. These vegetated roofs are to be seen as example installations; they are different in design (including soil composition, thickness of soil layer, and type of vegetation) and are placed in different urban environments (among others with regard to city size, climate and precipitation). The goal of the study is to investigate if studied example vegetated roofs behave as sink or source of chemical substances in runoff. Sink, as used in this paper, is a situation when concentrations of studied chemical compounds and elements are larger in input water (precipitation) than in output water (roof runoff); sink means that a part of studied substance is retained in a vegetated roof soil and/or vegetation. A vegetated roof is a source of a chemical substance when concentrations in runoff water are larger than in input water. The questions asked in this paper are: what is the current role of vegetated roofs in urban drainage with regard to runoff water quality and does the runoff water quality differ significantly from different vegetated roofs? To answer these questions, runoff water quality from two different vegetated roofs is investigated and runoff water quality from vegetated roofs is compared with example runoff quality from urban surfaces as found in literature. 2. Study sites and methods 2.1. Intensive vegetated roof at ACROS Fukuoka, Japan Fukuoka City, 1.4 million inhabitants, is situated in south-west Japan on the Kyushu Island. The average annual precipitation in Fukuoka region is 1600 mm, early summer rainy season is

3 ecological engineering 35 (2009) Fig. 2 Study site at the intensive vegetated roof at ACROS Fukuoka, Japan. in June-July. Fukuoka ACROS is a commercial public building located in downtown Fukuoka. The building and the vegetated terraces on its south side were constructed in An important reason behind the establishment of vegetated terraces at the ACROS building was achieving aesthetical benefits: the terraces face a small park and give a natural extension of green area into the built environment. Terraces are accessible for the public during the day time with the exception of rainy days; they are used as a public garden and provide a place for relaxation from the city stress. The terraces are constructed with artificial inorganic lightweight soil made of perlite naturally occurring siliceous rock containing mainly silicon dioxide, aluminium-, potassium-, and sodium-oxides. The commercially available type of soil used is Aquasoil (AquaSoil Ikegami Inc., 2002). It has a large water retention ability and high drainage capacity when saturated. Saturated hydraulic conductivity is 5148 mm/h (AquaSoil Ikegami Inc., 2002). The underlying drainage layer is made of plastic. More than 70 different plant species have been planted; however, new ones have appeared in the area due to seeds spread by wind and birds. The plantations are dominated by leave trees and bushes. (A. Hara, ACROS Fukuoka Building Management, personal communication, 2005). The microclimate around the ACROS building and the cooling effect of the vegetation has been studied (Hagishima et al., 2003). The influence of the vegetated terraces at the ACROS building on storm water runoff quality has not previously been studied. A terrace between the floor 12th and 13th was used in this study. The choice was dictated by the accessibility of the site and the possibility to sample runoff directly and exclusively from each section of vegetated roofs. The study sections are 0.8 m wide, 2.7 m long, and flat, with a soil layer thickness of 0.4 m. They are further referred to as J1 and J2 (J indicates study site from Japan). Both sections are constructed with the same soil material and vegetation and there is no design difference between these sections. The view of the study site at the intensive vegetated roof at ACROS Fukuoka, Japan, is shown in Fig. 2. Five precipitation events between November 2005 and April 2006 were sampled from J1 and four from J2. Notice that snow is uncommon and all studied events were rain events. During the studied events a precipitation sample and runoff from two sections of the vegetated roof (J1 and J2) were collected in HDPE (high-density polyethylene) containers. The container for collection of precipitation has an open watercatching surface of 0.17 m 2. Capacity of the HDPE containers for runoff collection was 20 l each and they were connected with a roof drainage discharge through plastic pipes. Plastic pipes discharged at the bottom of each container. It has been tested for the studied sections of the vegetated roof that it takes about 30 mm rain to generate first runoff for preceding dry period of 7 days. Thus, containers are suitable for collection of all runoff from up to about 40 mm rain-depth events. For rain events of greater depths containers would overflow. Before each sampling event, containers were cleaned and rinsed three times with distillated water. From each collecting container a bulk (1 dm 3 ) water sample was taken for each rain-runoff event included in this study. Runoff samples were collected by lowering HDPE sampling bottles in each container. A precipitation sample was collected by pouring water from the collecting container into a sampling bottle. On one sampling occasion in December 2006, runoff sample from roof J2 was not collected; consequently four precipitation events have been sampled from roof J2. All water samples were refrigerated until they were analyzed. Storing of samples was no longer than 1 week. Samples collected during the first precipitation event (November 2005) were analyzed for dissolved metals (cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), potassium

4 372 ecological engineering 35 (2009) Table 1 Japanese and Swedish methods for chemical analyses of water and detection limits applied (mg/l). Component Analyses method, Japan Detection limit, Japan Analyses method, Sweden Detection limit, Sweden ph JIS K Instrument manuals a Tot-N JIS K Instrument manuals b NO 3 N JIS K ISO NH 4 N JIS K ISO Tot-P JIS K Instrument manuals c PO 4 P JIS K ISO DOC JIS K Instrument manuals b 0.1 Ca JIS K Cd JIS K Instrument manuals c Cr JIS K Instrument manuals c Cu JIS K Instrument manuals c Fe JIS K Instrument manuals c K JIS K Instrument manuals c Mn JIS K Instrument manuals c Pb JIS K Instrument manuals c Zn JIS K Instrument manuals c a ph instrument model Ionanalyzer/901 from Orion Research, Boston, MA, USA. b TOC-analysator model TOC-VCPH med total nitrogen measuring unit TNM-1 from Shimatzu, Kyoto, Japan. c Optical ICP AES technique using a PerkinElmer OPTIMA 3000 DV instrument; Norwalk, CT, USA. (K), manganese (Mn), lead (Pb), and zinc (Zn)), nitrate nitrogen (NO 3 N), ammonium nitrogen (NH 4 N), total nitrogen (Tot- N), phosphate phosphorus (PO 4 P), total phosphorus (Tot-P), and dissolved organic carbon (DOC)), and ph was measured. Samples collected during subsequent precipitation events were also analyzed for calcium (Ca) and all above listed metals and nutrients except Cd, Cr, Cu, and Mn. It is because Cd, Cr, Cu and Mn remained below detection limit in all samples collected during first studied event (first collected precipitation and runoff samples from vegetated roof sections J1 and J2). It indicated the vegetated roof is not a substantial source of these metals. All analyses were performed according to Japanese standard methods listed in Table Extensive vegetated roofs in Augustenborg Malmö, Sweden Malmö City, 270,000 inhabitants, is situated in southern Sweden, and Augustenborg is a residential area in the city. Average annual precipitation in the region is 600 mm, climate is cool temperate with occasional snow in winter. The vegetated roofs were established on a number of buildings in Augustenborg since the year 2001 when the area was undergoing renovation. One of the objectives with the renovation was to minimize flooding problems which have been a frequent nuisance in the neighbourhood. Storm water has been disconnected from combined sewers and an open storm water system has been constructed. Some of the vegetated roofs are accessible for Fig. 3 Study site at the extensive vegetated roof at Augustenborg, Malmö, Sweden.

5 ecological engineering 35 (2009) Fig. 4 Range and average values for nutrients in extensive S (Sweden) and intensive J1 and J2 (Japan) vegetated roofs runoff and in rain from study sites in Sweden (S) and Japan (J). Four samples have been analyzed for sites rain S, roof S, and rain J2; five samples have been analyzed for sites rain J and Roof J1. public during scheduled hours in summer months and have an educational purpose. The extensive vegetated roofs consist of a prefabricated sedum-moss vegetation layer grown in a 3-cm thick soil substrate, a geotextile filter layer, and usually some kind of a drainage layer. The soil substrate is commercially available and made of crushed lava, natural calcareous soil, clay, and shredded peat. Organic content in the soil is 5%. A bitumen membrane reinforced with a polyester layer is situated beneath the vegetated roof. Details on vegetated roof material can be found in Emilsson and Rolf (2005). Hydrological function of the roof is described in Bengtsson et al. (2005). The studied section of the extensive vegetated roof is 1.25 m wide and 4 m long, sloping 2.6%. It is with an underlying 2 cm thick drainage layer made of shingle (coarse gravel). It is further referred to as roof S (S indicates study site in Sweden). The extensive vegetated roof was fertilized during spring 2001 and spring 2002 and there was no fertilization since The type of fertilizers and the application rate can be found in Czemiel Berndtsson et al. (2006). The view of the study site at the extensive vegetated roof at Augustenborg, Malmö, Sweden, is shown in Fig. 3. Four precipitation events in the period of May June 2005 were sampled. During the studied events a bulk sample of rainwater and a bulk sample of runoff water from a section of vegetated roof were collected in HDPE containers. The container for collection of precipitation was with an open water-catching surface of 0.1 m 2. The container for runoff collection was 30 l large; it was connected with a roof drainage discharge through a plastic pipe. Plastic pipe discharged at the bottom of each container. Bengtsson et al. (2005) showed that it takes about 9 mm rain to generate first runoff from the vegetated roof which was in an unsaturated condition at the beginning of a rainfall. Thus the container is suitable for collection of all runoff from about 17 mm rain-depth events. For rain events of greater depths the container would overflow. Before each sampling event, containers were rinsed three times with distillated water. For each studied precipitation event a bulk precipitation sample and a bulk runoff water samples were taken in two 100 ml HDPE bottles each; half

6 374 ecological engineering 35 (2009) Table2 Maximum, minimum and average concentrations of Cu and Mn in rain and extensive vegetated roof runoff, Sweden and concentrations of Pb in rain and intensive vegetated roof runoff (J1 and J2) in Japan. Samples Cu (mg/l) Mn (mg/l) Pb (mg/l) Rain S a Roof S a Rain S a Roof S a Rain J b Roof J1 b Roof J2 b Max Min Average a Four bulk rainwater samples and four bulk runoff water samples were analyzed. b Five bulk rainwater samples and five bulk runoff water samples were analyzed. of each sample was acidified (0.1 M HNO 3 ) and later used for analyses of heavy metals. Samples were collected by pouring water from collecting containers into sampling bottles. All the samples were refrigerated until analysis; storing time was no longer than 1 week. Samples were analyzed for dissolved metals (Cd, Cr, Cu, Fe, K, Mn, Pb, and Zn), and nutrients (NO 3 N, NH 4 N, Tot-N, PO 4 P, Tot-P, and DOC) and ph was measured. Analyses were performed according to methods (in Sweden) listed in Table Results and discussion In this study the combined (source sink) influence of an extensive and an intensive vegetated roofs on runoff water quality was studied. It is difficult to make quantitative source apportionment of pollutants but qualitatively runoff pollutants would originate from soil and roof material, additives (fertilizer), vegetation, and atmospheric deposition. Average values Fig. 5 Range and average values for metals and ph in extensive S (Sweden) and intensive J1 and J2 (Japan) vegetated roofs runoff and in rain from study sites in Sweden (S) and Japan (J). Four samples have been analyzed for sites rain S, roof S, and rain J2; five samples have been analyzed for sites rain J and Roof J1.

7 ecological engineering 35 (2009) within the range registered in the study are presented in Fig. 4 (nutrients) and Fig. 5 (Fe, Pb, Zn, K, Ca and ph). Ca has been analyzed exclusively in samples taken from the sections of the intensive vegetated roof (J1 and J2). Cd, Cu, Cr, and Mn in rain samples and runoff samples from the intensive vegetated roof (J1 and J2) were below detection limits of analyses performed in Japan (Table 1). Cd, Cr and Pb in rain samples and runoff samples from the extensive vegetated roof (S) were also below detection limits of analyses performed in Sweden (Table 1). The measured maximum, minimum, and average concentrations of Cu and Mn in rain and runoff samples collected in Sweden are presented in Table Rainwater quality Bulk rainwater samples were collected at study sites on the selected events to compare pollutants concentrations in atmospheric wet deposition and runoff from vegetated roofs. The average concentrations of Tot-N, NO 3 N, NH 4 N, Tot-P, PO 4 P, K and DOC in bulk rainwater samples collected within this study in Japan and in Sweden are very similar, although the range of measured concentrations is larger for rainwater samples from Japan (Figs. 4 and 5). Average concentrations and the maximum measured values of Zn, Pb, and Fe are higher in rainwater samples collected in Japan. Average ph of rainwater samples collected in Japan is lower than in samples from Sweden Nitrogen The results show that both extensive (S) and intensive (J1, J2) vegetated roofs are a sink of NO 3 N and NH 4 N. The intensive vegetated roof is also a sink of Tot-N in contrast to the extensive roof in which the reduction of average concentration of Tot-N was small. Moran et al. (2005) reported substantial release of Tot-N from two studied extensive vegetated roofs in North Carolina, USA. Tot-N concentrations in runoff varied between 6.9 and 0.8 mg/l with average about 3.6 mg/l (Moran et al., 2005). The rainfall concentrations were below 1 mg/l except of one event when concentration of 2.1 mg/l was registered. These show both larger variation and larger release of Tot-N than what was found in this study (Fig. 4). Rainfall concentrations of Tot-N reported by Moran et al. (2005) were smaller as compared to this study (Fig. 4). The results of this study show that sink of NH 4 N is not accompanied by increase of NO 3 N which would indicate nitrification. It is possible that the missing inorganic nitrogen was transformed to organic nitrogen within the vegetated roofs. For roof S it is observed that Tot-N in runoff is similar to that of precipitation which suggests release of organic nitrogen from the roof (to account for retained inorganic nitrogen). This released organic nitrogen might originate from moss material which has a high capacity to fast adsorb nitrogen from precipitation and release it in organic form while decomposed. Intensive vegetated roofs support larger plants. It is possible that inorganic nitrogen taken up by plants changes to organically bound nitrogen and remains within the vegetation. This would explain the observed sink of Tot-N on intensive vegetated roof. There are no published studies of processes involved in nitrogen transformations within vegetated roofs. However, the processes within vegetated roofs might be comparable with processes occurring during water passage through gravel roofs and urban runoff infiltration. For example, Mason et al. (1999) studied runoff quality from gravel roofs. They suggest that the differences in NH 4 N and NO 3 N concentrations indicated nitrification occurring on gravel covered roofs with longer residence time. The sum of NO 3 N and NH 4 N in gravel roof runoff was about 50% of the concentrations monitored in atmospheric deposition. Mason et al. (1999) suggest that the larger part of this missing inorganic nitrogen was retained as organically bound nitrogen which regularly accounted for 20 50% of total nitrogen in runoff. This is similar to the result of our study of the vegetated roofs. Mason et al. (1999) also studied roof runoff infiltration through soil. In contrast to our results they showed that NO 3 N behaved conservatively during infiltration, whereas concentration of NH 4 N decreased, as suggested by Mason et al. (1999), probably as a consequence of nitrification. Dietz and Clausen (2005) investigated the performance of two rain-garden facilities designed for retention (first 2.54 cm) and treatment of urban runoff in Connecticut, USA. Rain gardens, also termed bioretention areas, are shallow depressions in the landscape that are planted with trees and shrubs, covered with a bark mulch layer or ground cover; treatment is expected to take place due to adsorption, decomposition, ion exchange and volatilization (Dietz and Clausen, 2005). However, poor treatment of NO 3 N, TKN, and organic-n was observed in the study by Dietz and Clausen (2005). The only pollutants significantly lower in the effluent than in the influent were NH 3 N in both gardens and Tot-N in one garden. They concluded that rain gardens had little impact on pollutant concentration in percolate. This result is different from our findings for vegetated roofs which showed reduction of NO 3 N, NH 4 N and Tot-N (the last for J1 and J2) Phosphorus Release of phosphorus was observed from the extensive roof S, most of it in the form of PO 4 P. In contrast, the intensive vegetated roof showed no release of phosphorus (Fig. 4). The probable source of phosphorus to runoff water from the extensive vegetated roof S is fertilizer used in previous years and phosphorus available from the roof soil. Recall that the latest recorded fertilization took place 2 years before the sampling. Also, Moran et al. (2005) showed substantial release of Tot-P from two studied vegetated roofs in North Carolina, USA. Tot-P concentrations as found by Moran et al. (2005) varied between 0.6 and 1.5 mg/l with an average about 1 mg/l, which is three times more than found in our study for the roof S. The rainfall concentrations registered by Moran et al. (2005) were about 0.05 mg/l comparing to 0.02 mg/l (Sweden) and 0.01 mg/l (Japan) found in our study. According to Moranetal. (2005), phosphorus in runoff originated from the soil nutrients which contained compost material. Mason et al. (1999) in their study of roof runoff infiltration through soil showed that PO 4 P behaved conservatively during infiltration. Dietz and Clausen (2005) in their investigation of the performance of two rain-garden facilities (Connecticut, USA) found out that Tot-P concentrations increased after passage of the rain gardens and that the increase was larger at the beginning

8 376 ecological engineering 35 (2009) Table 3 Nutrients concentrations in urban storm water runoff. Collection site Parameter DOC (mg/l) Tot-N (mg/l) NH 4 N (mg/l) NO 3 N (mg/l) Tot-P (mg/l) Street a Mean 2.2 Site 1, median Site 2, median Site 3, median Tile roof c Mean for 0 2 mm rain Polyester roof c Mean for 0 2 mm rain Gravel roof c Mean for rainfall Urban d Mean Roof runoff e Mean After infiltration e Site 1, mean After infiltration e Site 2, mean Site 1, mean Site 2, mean Site 3, mean Site 4, mean Residential driveways g Asphalt Mean Permeable pavers Mean Crushed stone Mean Event 1, mean Event 2, mean Event 3, mean Site 1, mean Site 2, mean Site 3, mean a Barraud et al. (1999), February November, 1996, urban street runoff at Valence, France. b Barrett et al. (1998), sampling of highway runoff during April 1994 May 1995 at 3 locations on the MoPac Expressway, Austin, TX, USA. c Zobrist et al. (2000), measurements in Switzerland, gravel roof partly covered with natural vegetation. d Taebi and Droste (2004), urban stormwater runoff at Siosepol catchments, Iran, 10 rainfall events, e Dietz and Clausen (2005), since 2002 during 56-weeks, roof runoff and effluent after infiltration through constructed rain gardens sampled at Haddam, CT, USA. f Goonetilleke et al. (2005), urban residential catchments Highland Park, Alextown, Gumbeel, and Birdlife at Gold Coast, Queensland State, Australia. g Gilbert and Clausen (2006) urban runoff monitoring during 1 year ( ) in residential neighborhood Waterford, CT, USA. h Uchimura et al. (1997), urban runoff monitoring, three events during May 1995, Tokyo, Japan. i Lee and Bang (2000), urban runoff from separate storm sewers in Korea in three intensively developed residential and commercial watersheds investigated during , mean wet weather concentrations. of the study. It can be concluded that phosphorus release from vegetated roofs can be linked to use of fertilizer and the composition of soil material Dissolved organic carbon Substantial release of DOC was observed from the extensive roof S; the average concentration in runoff water was about 20 times more than that in precipitation. In the case of the intensive vegetated roof, the average concentrations in runoff exceeded that of precipitation twice and four times, respectively. The source of DOC is organic material from the roof soil and the carbon originated from vegetation decomposition. The difference in performance of studied extensive and intensive vegetated roofs can be explained by soil composition: the first with 5% organic content and the second being primarily mineral. The results of a study by Mason et al. (1999) of roof runoff infiltration through soil showed that a major part of DOC behaved conservatively during infiltration. Adsorption of DOC during infiltration through soil seems unlikely. In case there is excess carbon available in the soil/topsoil, some degree of DOC release may be typical of vegetated roofs Potassium and calcium Studied vegetated roofs (S, J1, and J2) showed substantial release of K (Fig. 5); average concentrations of K in runoff water from vegetated roofs was about seven times more than that of rainwater collected at corresponding sites. Ca was investigated exclusively for the study site in Japan. The results show

9 ecological engineering 35 (2009) Table 4 Heavy metals concentrations in urban storm water runoff. Collection site Parameter Cd (mg/l) Cr (mg/l) Cu (mg/l) Mn (mg/l) Pb (mg/l) Zn (mg/l) Street a Mean < Site 1, median Site 2, median Site 3, median Tile roof c Mean 0 2 mm rain Polyester roof c Mean 0 2 mm rain Gravel roof c Mean for rainfall Urban area d Mean Residential driveways e Asphalt Mean Permeable pavers Mean Crushed stone Mean Site 1, mean Site 2, mean Highway g Bridge g Street h Median roofs h Median Event 1 EMC Event 2 EMC Event 3 EMC Event 4 EMC Event 5 EMC Motorway j Median Site 1 median Site 2 median Site 3 median Event 1 EMC Event 2 EMC Event 3 EMC Event 4 EMC Highway m Mean Urban area n Site 1 mean ND ND ND Site 2 mean ND ND ND Road o Mean Roof o Mean ND not detectable, EMC event mean concentrations. a Barraud et al. (1999), February November, 1996, urban street at Valence, France. b Barrett et al. (1998), sampling of highway runoff during April 1994 May 1995 at 3 locations on the MoPac Expressway, Austin, TX, USA. c Zobrist et al. (2000), measurements in Switzerland, gravel roof partly covered with natural vegetation. d Taebi and Droste (2004), urban stormwater runoff at Siosepol catchments, Iran, 10 rainfall events, e Gilbert and Clausen (2006), urban runoff monitoring during 1 year ( ) in residential neighborhood Waterford, CT, USA. f Hares and Ward (1999), measurements at London Orbital, stations Leatherhead and Oxted, UK. g Yousef et al. (1984), measurements at locations in Central Florida, USA. h Gromaire-Mertz et al. (1999), measurements in central Paris, 16 rain events i Turer et al. (2001), samples collected during April October 1995 from I-75 Cincinnati, OH, USA. j Legret and Pagotto (1999), samples collected during 49 rain events at 275 m motorway, Nantes, France. k Wu et al. (1998), 10 storm events, city of Charlotte, NC, USA. l Shinya et al. (2000), samples collected during 4 rain events during August November 1997 from urban highway in Osaka, Japan. m Barbosa and Hvitved-Jacobsen (1999), 10 rain events, IP 4 mountain road in the north-east Portugal. n Chui (1997), 9 and 6 storm events at two sites respectively, Stamford Canal watershed, Singapore. o Gnecco et al. (2005), urban runoff sampled during 12 rainfall events, , Genoa, Italy.

10 378 ecological engineering 35 (2009) release of Ca from the vegetated roof sections J1 and J2 with average runoff concentrations being ten times larger than concentrations in rainwater; however, variations of concentration in runoff water are large. Mason et al. (1999) evaluated the behavior of roof runoff during infiltration through specially designed infiltration pit suggested that concentrations of K and Ca were regulated by dissolution of soil material. In the same study, runoff from a gravel roof proved to be a significant source of K and Ca. This is similar to our finding for studied vegetated roofs Metals and ph The results show that for the roof S the average concentration of Zn in runoff water was larger than that in rainwater and concentration of Fe did not change (Fig. 5). Release of Cu and some minor release of Mn were observed from roof S. Concentrations of Cd, Cr and Pb were below detection limits. For the roof sections J1 and J2 average concentrations of Fe, Pb, and Zn decreased in roof runoff as compared to rainwater (Fig. 5; Table 2) indicating the potential of the intensive vegetated roof functioning as a sink for these metals. The results of studies by Mason et al. (1999) showed that artificial infiltration of roof runoff could lead to accumulation of heavy metals in the top soil levels (especially Pb and Zn). Cu tends to be only partly retained by the soil and a significant fraction transports down with the soil water through the unsaturated zone and remained mobile. These results, with regard to Pb and Zn, are similar to our findings for vegetated roofs (the later for J1 and J2) in which sink of these metals was observed. The roof S showed to be a source of Zn and Cu. The study by Zobrist et al. (2000) indicated similar result for Zn but different for Pb. They found out that Zn occurred in the labile form in the roof runoff, meaning that it will react on the infiltration path and will be eliminated from water. The largest part of Cu and Pb were present in the reactive form meaning that they will only be retained gradually, i.e. on the longer infiltration path. The increase of average ph during rainwater passage through the roof sections J1 and J2 from about 5 in precipitation to 7.5 in the roof runoff indicated rapid neutralization of the acid depositions. This is an environmental benefit in case roof runoff is directly discharged to natural water recipients. For the extensive vegetated roof no substantial change of ph has been observed; ph in both rainwater and roof runoff was about 6.0 (Fig. 5) Comparison of vegetated roof runoff and urban runoff quality To estimate the role of vegetated roofs in urban drainage with respect to water quality, the runoff water quality from vegetated roofs as measured in this study is compared with example urban runoff quality as published in literature and presented in Table 3 (nutrients) and 4 (metals). Average concentration of Tot-N in runoff from the extensive roof (Fig. 4) compares well with results obtained for urban and street runoff by Barraud et al. (1999), Goonetilleke et al. (2005), and Uchimura et al. (1997); it is less than findings by Taebi and Droste (2004) and exceeds values obtained by Dietz and Clausen (2005). Runoff from the intensive vegetated roof showed concentrations of Tot-N much below typical values of urban runoff. A research study presented concentration of NH 4 N for three roofs in Switzerland (Zobrist et al., 2000) in which gravel roof showed similar values as obtained for the intensive vegetated roof. In contrast, tile and polyester roofs showed higher concentrations of NH 4 N. For NO 3 -N, runoff from vegetated roofs showed lower concentrations than those reported for urban runoff by Barrett et al. (1998), Dietz and Clausen (2005), Gilbert and Clausen (2006), and Lee and Bang (2000). Concentrations of Tot-P in runoff from the intensive vegetated roof (J1 and J2) were much lower than values reported for urban runoff (Table 3). Tot-P in the extensive roof runoff is similar to average values for urban runoff as reported by Barrett et al. (1998) (site 1), Taebi and Droste (2004), Goonetilleke et al. (2005) (sites 1 and 2), and Gilbert and Clausen (2006) (asphalt driveway). DOC concentrations found in this study for the extensive vegetated roof exceed values reported for urban runoff by Zobrist et al. (2000). Our findings for the intensive vegetated roof show DOC concentrations in a similar range as in Zobrist et al. (2000). Concentrations of Zn in runoff from vegetated roofs studied (Fig. 5) are either similar to or less than results obtained for urban runoff in studies of urban runoff as reported in literature and presented in Table 4. Pb, Cd, Cr and Mn concentrations in runoff from studied vegetated roofs were very small, typically below values reported for urban runoff. Only mean concentration of Cu obtained in runoff from the extensive roof somewhat exceeds values reported by Yousef et al. (1984), Gromaire-Mertz et al. (1999), Turer et al. (2001) (for three events), Legret and Pagotto (1999), Wu et al. (1998), Shinya et al. (2000) and Gnecco et al. (2005), which represent roof and street runoff. In summary, the concentrations of nitrogen and phosphorus compounds as well as heavy metals studied in runoff from vegetated roofs are in similar range or below the corresponding concentrations in urban runoff. This fact can be interpreted as in favor of vegetated roofs which, in contrast to urban impermeable surfaces, provide also a benefit of runoff volume reduction. Vegetated roofs may not provide the benefit of rainwater treatment (with exception to nitrate nitrogen) but neither would other urban impermeable surfaces. The attention shall be directed towards the influence of materials and substances used in vegetated roofs construction and maintenance and their potential release of chemicals into urban water. The materials and substances which would, because of their concentration or toxicity, have detrimental effect on runoff quality (e.g. fertilizers, contaminated soil, toxic chemicals that may be used in gardening) shall be avoided. 4. Conclusion Both extensive (S) and intensive (J1, J2) vegetated roofs behave as sinks of NO 3 N and NH 4 N. The intensive vegetated roof is also a sink of Tot-N in contrast to extensive roof in which the reduction of average concentration of Tot-N was small. Release of phosphorus was observed from the studied extensive vegetated roof, most of it in the form of PO 4 P. In contrast,

11 ecological engineering 35 (2009) the intensive vegetated roof showed no release of phosphorus. The probable source of phosphorus to runoff from the extensive vegetated roof is fertilizer and soil. The intensive vegetated roof supports larger plants while the extensive one is planted with sedum which has low nutrient requirement. Thus more nitrogen is potentially used and retained by vegetation on the intensive vegetated roof being the reason for observed sink of total nitrogen and no release of Tot-P. Substantial release of DOC was observed from the extensive vegetated roof, and smaller release was observed from the intensive vegetated roof. The source of DOC is organic material from the roof soil and the carbon originating from decomposition of vegetation. Some release of DOC might be typical of vegetated roofs. Studied vegetated roofs (S, J1, and J2) showed substantial release of K and the intensive roof also showed greater release of Ca which is likely caused by dissolution of soil material. Average concentrations of Fe, Pb, and Zn decreased in runoff from intensive vegetated roof as compared to rainwater indicating the potential of vegetated roof functioning as a sink for these metals. The average concentration in extensive vegetated roof runoff increased for Zn and was unchanged for Fe as compared to concentrations in rainwater. Pb has not been detected in extensive roof runoff water. Release of Cu and some minor release of Mn were observed from the extensive roof. For vegetated roofs, not being designed for metal treatment, it is likely that metals are transported through the vegetated roof although temporary transient retention can take place. The results of the study indicated that studied vegetated roofs, if not retaining the metal pollutants, were generally not a significant source of metals. The increase of average ph during rainwater passage through the intensive vegetated roof indicated rapid neutralization of the acid depositions. Concentrations of studied chemical elements and compounds in extensive and intensive vegetated roofs runoff were similar or in the lower range of example values reported in literature for urban runoff. Our recommendation is that vegetated roofs should not be seen as a tool for improving runoff water quality by reducing concentration of pollutants found in precipitation. The construction of vegetated roofs may be justified by a broad range of other benefits including contribution to runoff volume reduction. However, it remains important to assure that these installations would not have detrimental effect on urban runoff quality. All the construction elements of vegetated roofs (materials and substances) shall be tested for their influence on passing water quality before the full scale installations are put in place. Acknowledgments Mr. Akiyoshi Hara and Mr. Masafumi Sugi from ACROS Fukuoka Building Management kindly provided information about the green terraces at ACROS and supported research activities on the terraces. Research in Sweden was financially supported by Åke and Greta Lissheds Stifftelse. Research in Japan was possible due to a grant from the Japanese Society for the Promotion of Science. All contributions are gratefully acknowledged. references AquaSoil Ikegami Inc., Properties of AquaSoil. Product information (in Japanese). Ikegami Inc., Japan. Barbosa, A.E., Hvitved-Jacobsen, T., Highway runoff and potential for removal of heavy metals in an infiltration pond in Portugal. Sci. 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