Seasonal changes of runoff water quality from an extensive vegetated roof

Transcription

1 11 th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 28 Seasonal changes of runoff water quality from an extensive vegetated roof J. Czemiel Berndtsson Department of Water Resources Engineering, University of Lund, Box 118, 221 Lund, Sweden ABSTRACT Seasonal changes of runoff water quality from an extensive vegetated roof located in southern Sweden are studied. The vegetated roof consists of a prefabricated sedum-moss vegetation layer grown in a 3 cm thick soil substrate. Four precipitation events are studied for water quality during two autumn and two spring seasons. Bulk samples of rainwater and of runoff water from the vegetated roof are analyzed for potassium (K), nitrate nitrogen (NO3-N), total nitrogen (Tot-N), phosphate phosphorus (PO4-P), total phosphorus (Tot-P), and dissolved organic carbon (DOC). After water passage through the vegetated roof a substantial increase of concentration is observed for PO4-P, Tot-P, K and DOC while concentration of NO3-N substantially decreases. Tot-N concentration is only somewhat decreasing. Loads of contaminants from the vegetated roof and from rainwater per unit area are also calculated. The results show that the loads of K, PO4-P and Tot-P from the vegetated roof are decreasing with the roof age. The loads of DOC in runoff water are higher in spring seasons than in autumn seasons and also decrease with the roof age. The ability of the vegetated roof to adsorb NO3-N is decreasing with time. KEYWORDS Green roof; nutrients; runoff quality; urban INTRODUCTION In many countries interest in vegetated roofs is increasing. Vegetated roofs are becoming a trendy city greening tool often happily used by architects. Indeed, vegetated roofs have a tremendous potential of providing attractive green space in downtown areas where the green space on the ground is limited. Vegetated roofs can be equally appreciated in scarcely populated areas where they are often used to emphasize the connection between the building and the surrounding nature. But vegetated roofs are much more than an architects dream: introduced to an urban landscape they interact with and influence other city infrastructure as well as the local environment in the city. In qualitative terms vegetated roofs may provide, among others, reduction and attenuation of stormwater runoff, influence of stormwater quality, absorption of dusts and air pollutants, increased humidity and reduction of urban heat island effect, provision of wildlife habitat, and protecting roof fabric from uv-radiation and mechanical damage, reducing diurnal/seasonal temperature variation, improving thermal isolation. Still, the quantitative evidence on different aspects of vegetated roofs influence on other building elements, city infrastructure, and environment is often insufficient. A lot of information about vegetated roofs is often based on speculations rather than replicable scientific studies. Concerning runoff water quality it is common to expect that vegetated roofs Czemiel Berndtsson 1

2 11 th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 28 would contribute to water quality improvements (English Nature, 23). However, recent water quality studies do not seem to support that statement revealing nutrients leakage from newly established vegetated roofs (Monterusso et al., 24; Moran et al., 25; Czemiel Berndtsson et al., 26; Emilsson et al., 26). Attention need to be brought to the fact that the results from different vegetated roofs may not be comparable due to the large performance dependence on factors such as local climate, type of vegetated roofs and construction materials, type of plantation, management and age of a vegetated roof. This paper reports results of a water quality study repeated on one full scale installation of vegetated roof from southern Sweden during two autumn and two spring seasons over four year period. Results provide a ground for discussion of runoff water quality changes between autumn and spring season as well as changes linked to age of a vegetated roof. STUDY SITE AND METHODS Studied vegetated roof is located in southern Sweden, in the city of Malmö residential area Augustenborg. Malmö City has 27, inhabitants. Average annual precipitation in the region is 6 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 21 when the area was undergoing renovation. One of the objectives with the renovation was to minimise flooding problems from combined sewers 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. Vegetated roofs are a part of the open stormwater system contributing to runoff attenuation and reduction. The 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. This type of vegetated roofs with a thin soil substrate is often referred to as extensive roof. The soil substrate is commercially available and made of crushed lava, natural calcareous soil, clay, and shredded peat; the organic content is about 5%. A bitumen membrane reinforced with a polyester layer is situated beneath the vegetated roof. Details on vegetated roof material and construction can be found in Czemiel Berndtsson et al. (26). Hydrological function of the roof is described in Bengtsson et al. (25). When the vegetated roofs were constructed it was decided that they should also serve for scientific research. On a part of the vegetated roof a number of plots were designed for research on roof s hydrological function. They allow collection of runoff from each plot separately. Vegetation mats are identical on all the plots but the drainage layers differ. The studied vegetated roof plot from which data is presented here 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 was fertilized during spring 21 and spring 22 and there was no fertilization since 23. The view of the study site is shown in Fig. 1. Four to five precipitation events were studied for water quality during Autumn 23, Spring 25, Autumn 26, and Spring 27. During the studied events a bulk sample of rainwater and a bulk sample of runoff water from the vegetated roof plot were collected in HDPE containers. The container for collection of precipitation was with an open water-catching surface of.1 m 2. The container for runoff collection was 3 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 (25) 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. 2 Runoff water quality from an extensive vegetated roof

3 11 th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 28 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 from collecting containers in 1 ml HDPE bottles. Samples were collected by pouring water from collecting containers into sampling bottles. All the samples were refrigerated until analysis. Samples were analysed for potassium (K), nitrate nitrogen (NO3-N), total nitrogen (Tot-N), phosphate phosphorus (PO4-P), total phosphorus (Tot-P), and dissolved organic carbon (DOC). Figure 1. View of the study site at the extensive vegetated roof in Augustenborg, Malmö Sweden. Picture was taken in June 27. Samples were analysed for potassium with the optical ICP AES technique using a Perkin- Elmer OPTIMA 3 DV instrument; analyses were performed following the instrument manuals; detection limit (DL) was 5μg/l. Samples collected during autumn 23 were analysed for NO3-N, Tot-N, PO4-P, and Tot-P using a FIA 5 instrument from FOSS- Tecator; nitrate nitrogen was analysed according to ISO 13395, total nitrogen according to ISO 1195, phosphate phosphorus according to ISO , and total phosphorus was analysed according to the CAS 535 (FOSS, Customer Application Summary Note). Samples collected during spring 25, autumn 26 and spring 27 were analysed for NO3-N and PO4-P according to methods listed above (DL=1 μg/l); for Tot-P (DL=8 μg/l) with the optical ICP AES technique using a Perkin-Elmer OPTIMA 3 DV instrument; for Tot-N (DL 5 μg/l) and DOC (DL,1 mg/l) using TOC analyzer model TOC-VCPH with Tot-N measuring unit TNM-1 from Shimatzu. Czemiel Berndtsson 3

4 11 th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 28 RESULTS AND DISCUSSION Results from concentration measurements of studied substances in roof runoff water and rainwater during four different seasons (Autumn 23, Spring 25, Autumn 26, and Spring 27) are presented in Fig. 2. Average values within the range registered are shown. Measured concentrations of studied compounds in rainwater (input) and roof runoff (output) water are compared. After water passage through the vegetated roof an increase of concentration is observed for PO4-P, Tot-P, K, and DOC. PO4-P increases about 1 times in spring seasons and more than 2 times in autumns; Tot-P increases less, about 5 to 8 times in spring and more than 1 times in autumns. K increases about 1 times in spring season, 2 times in autumn 26, but much more in the first studied autumn 23. Concentrations of PO4-P, Tot-P, and K in water after passing through vegetated roof increase more in autumn seasons than in spring seasons. DOC concentrations in roof runoff water are 15 times higher than in rainwater during the first studied autumn and 4 times during the second; they are 19 times higher in the first studied spring, and 11 times during the second spring season. A substantial decrease of NO3-N concentrations is observed after water passage through the vegetated roof. The decrease is about 2 times in autumn seasons and less in springs; however, Tot-N decreases little accept autumn 26 when a decrease of about 4 times is observed. 4 Runoff water quality from an extensive vegetated roof

5 11 th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 28 2,5,5 NO3-N (mg/l) 2 1,5 1,5 Tot-P (mg/l),4,3,2,1 A3rain A3roof S5rain S5roof A6rain A6roof S7rain S7roof A3rain A3roof S5rain S5roof A6rain A6roof S7rain S7roof Tot-N (mg/l) K (mg/l) A3rain A3roof S5rain S5roof A6rain A6roof S7rain S7roof A3rain A3roof S5rain S5roof A6rain A6roof S7rain S7roof PO4-P (mg/l),4,3,2,1 A3rain A3roof S5rain S5roof A6rain A6roof S7rain S7roof Figure 2. Results of concentration measurements of studied substances in rainwater (marked on diagrams as rain) and vegetated roof runoff water (marked on diagrams as roof) during autumn 23 (A3), spring 25 (S5), autumn 26 (A6), and spring 27 (S7); average values within the range registered are shown. DOC (mg/l) A3rain A3roof S5rain S5roof A6rain A6roof S7rain S7roof Loads of contaminants from vegetated roofs and from rainwater per unit area are also calculated. This is done using the average precipitation data for the study region during period (SCB, 27) and the measured average concentrations of studied compounds during each of the study seasons. The results of loads calculations are presented in Fig. 3. They show that the loads of K, Tot-P and PO4-P from the vegetated roof are decreasing with the roof age. It can be caused by the decrease of influence from fertilizers used during vegetation establishment. The loads of DOC in runoff water are higher in spring seasons than in autumn seasons. If loads of DOC in roof runoff are compared between the same seasons (autumn 23 and autumn 26, spring 25 and spring 27) a decrease of load is observed with the roof age. The ability of the vegetated roof to adsorb NO3-N and Tot-N is decreasing with a roof age. It can be due to the lower ability of established sedum vegetation to use nitrogen compounds. Czemiel Berndtsson 5

6 11 th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 28 Load NO3-N (mg/m2) Load Tot-N (mg/m2) Load PO4-P (mg/m2) Autumn3 Spring5 Autumn6 Spring7 Autumn3 Spring5 Autumn6 Spring7 Autumn3 Spring5 Autumn6 Spring7 Load DOC (mg/m2) Load K (mg/m2) Load Tot-P (mg/m2) Autumn3 Spring5 Autumn6 Spring Autumn3 Spring5 Autumn6 Spring Autumn3 Spring5 Autumn6 Spring7 Figure 3. Calculated loads of contaminants (mg/m 2 ) from rainwater (light blue staple on diagrams) and from the vegetated roof (dark purple staple on diagrams) during studied seasons; March, April, and May are considered as spring months and September, October, and November are autumn months. CONCLUSION After water passage through the studied extensive vegetated roof a substantial increase of concentration is observed for PO4-P, Tot-P, K, and DOC during all studied seasons. Concentrations of PO4-P, Tot-P, and K increase more in autumn seasons than in spring seasons. NO3-N concentrations decrease substantially in runoff as compared to rainwater and largest decrease is observed in autumn seasons; however, Tot-N concentrations in runoff water decrease little. It is also found that the loads of K, Tot-P, PO4-P and DOC from the vegetated roof are decreasing with the roof age. At the same time the ability of the vegetated roof to adsorb nitrogen compounds is decreasing with a roof age. 6 Runoff water quality from an extensive vegetated roof

7 11 th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 28 REFERENCES Bengtsson, L., Grahn, L., and Olsson, J. (25), Hydrological function of a thin extensive green roof in southern Sweden, Nordic Hydrology, 36(3), Czemiel Berndtsson, J., Emilsson, T., and Bengtsson, L. (26), The influence of extensive vegetated roofs on runoff water quality, Science of the Total Environment, 355, Emilsson, T.U., Czemiel Berndtsson, J., Mattson, J.E., and Rolf, K. (26), Nutrient runoff from extensive vegetated roofs after fertilization with conventional and controlled release fertilizer, Ecological Engineering, 29, English Nature (23), Green roofs: Their existing status and potential for conserving biodiversity in urban areas, English Nature Research Reports, Report no 498, English Nature, Northminster House, Peterborough, UK. Monterusso, M.A., Rowe, D.B., Rugh, C.L., and Russell, D.K. (24), Runoff water quantity and quality from green roof systems, Acta Hort. 639, Moran, A., Hunt, B., and Smith, J. (25), Hydrological and water quality performance from green roofs in Goldsboro and Raleigh, North Carolina, In proceedings of the Green Roofs for Healthy Cities Conference, Washington DC, May 25. SCB (27), Statistisk årsbok för Sverige 27, Miljö och väder, Statistiska centralbyrån (in Swedish). Czemiel Berndtsson 7