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Nine-month evaluation of runoff quality and quantity from an experiential green roof in Missouri, USA
G Model ECOENG 3033 No. of Pages 7
Ecological Engineering xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Nine-month evaluation of runoff quality and quantity from an experiential green roof in Missouri, USA Grace E. Harper, Matt A. Limmer, W. Eric Showalter, Joel G. Burken * Department of Civil, Architectural and Environmental Engineering, Missouri University of Science and Technology, 1401 N Pine Street, Rolla, MO 65409, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 November 2013 Received in revised form 10 June 2014 Accepted 12 June 2014 Available online xxx
A better understanding of green roof stormwater performance is needed to assess and predict benefits of proposed green roof projects. A nine-month green roof pilot study was conducted in mid-Missouri to evaluate runoff quantity and quality under field conditions for two different media, both tested under planted and unplanted conditions. Water quantity results showed a 40% reduction in runoff from the unplanted growing media and a 60% reduction in runoff from the planted growing media. A water balance model was developed that incorporated water storage in the media and evapotranspiration (ET) from the media based upon local weather conditions using the Penmen–Monteith ET method. Water quality monitoring showed a first-order decline of excess nutrients in the first few months of green roof operation. Total phosphorus >30 mg-P/L and total nitrogen concentrations >60 mg-N/L were observed in green roof runoff initially, with concentrations decreasing over nine months to 5 mg-P/L and 10 mg-N/ L, respectively. In addition, elevated total organic carbon concentrations were observed, with concentrations of 500 mg/L initially, decreasing after a few weeks to below 50 mg/L. Media type and age were the largest influences on carbon and nutrient loading from the green roof media tested. ã 2014 Elsevier B.V. All rights reserved.
Keywords: Green roof Evapotranspiration Climate model Nitrate Phosphorous Nutrients
1. Introduction Urbanization has drastically impacted the water cycle. As buildings and paved surfaces are added to the landscape, pervious soil surface area and rainfall infiltration decrease, causing stormwater to rapidly transfer from impervious hardscapes to grey infrastructure, such as curb and gutters and underground sewers. Such rapid conveyance results in erosion of surface water channels and downstream flooding while reducing evapo-transpiration (ET) that would normally occur from vegetated soils. In cities with combined stormwater and sanitary sewers, overflows result in the release of sanitary wastewater and associated pathogens into waterways, which is regulated by the USEPA’s Clean Water Act. Several US cities are under consent decrees from the USEPA for their combined sewer overflows. St. Louis, MO, USA is committing to expending $4.7 billion on upgrades to prevent combined sewer overflows (WEF, 2011). To address the effects of urbanization on stormwater quantity and quality, green infrastructure approaches have been developed and implemented using natural treatment systems (Tzoulas et al., 2007). By strategically integrating vegetation, easily draining soils
* Corresponding author. Tel.: +1 573 341 6547. E-mail address: [email protected] (J.G. Burken).
and natural storage into the urban landscape, rainfall can be treated, stored and evapo-transpired to avoid excess stormwater runoff from urban impervious surfaces. Green roofs are an example of green infrastructure implemented at a primary source of stormwater collection in the built environment, building rooftops, designed to store rainfall on-site and release much of the water through evapotranspiration (Czemiel Berndtsson 2010). Through such green infrastructure, the quantity of water that must be conveyed through conventional storm sewers is reduced, preventing costly traditional grey infrastructure upgrades to handle increased flows and mitigate excessive downstream flows. Green roofs are categorized in two major classes: intensive and extensive. Intensive green roofs feature deeper soil profiles to support a diverse set of vegetation (Czemiel Berndtsson 2010). Extensive roofs, the focus of this study, are often installed primarily for their stormwater benefit and are characterized by growing media depths less than 15 cm (Kosareo and Ries 2007). Structural reinforcement needs are often low as extensive green roofs do not result in large live loads or people using the space. In addition, extensive roofs have fewer vegetation options and are commonly planted with succulents such as sedums due to their ability to thrive in harsh rooftop conditions (Blanusa et al., 2013). Green roof media available for commercial use most often has proprietary compositions, but most media contain similar components. Growing media is generally lighter than topsoil and chosen based
http://dx.doi.org/10.1016/j.ecoleng.2014.06.004 0925-8574/ ã 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: Harper, G.E., et al., Nine-month evaluation of runoff quality and quantity from an experiential green roof in Missouri, USA. Ecol. Eng. (2014), http://dx.doi.org/10.1016/j.ecoleng.2014.06.004
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upon its ability to drain and support plant growth. Additional fertilizers are also added to the mix to sustain vegetative growth. Because green roofs are a living system of several components, understanding the dynamics of the system is necessary to predict the environmental impacts at specific sites. Several studies have reported varying impacts of green roofs on water quantity, as impacts are dependent upon roof slope, growing media, plant type, antecedent rainfall, green roof age, and weather conditions (Carpenter and Kaluvakolanu 2011; Carter and Rasmussen 2006; Getter et al., 2007; Hilten et al., 2008; Metselaar 2012; Nagase and Dunnett 2012; Rezaei et al., 2005). Due to the desire to predict green roof performance, models to account for the effects of many of these variables have been proposed. However, the resulting models are often experiment- and site-specific and are not translatable to other installations. Some models consider soil characteristics for varying media depths and/or roof slopes, such as those estimating curve numbers, but neglect antecedent rainfall as well as evapotranspiration losses (Carter and Rasmussen 2006). Other models account for hydrologic processes while greatly simplifying ET (Hilten et al., 2008; Kasmin et al., 2010; She and Pang 2010). The effect of ET on green roof performance is dependent both on climate and plant type (Nagase and Dunnett 2012), fundamental factors which must be understood to ensure a broadly predictive model. Capturing the effects of these variables is not trivial, as demonstrated by a multiple linear regression approach intending to provide a transparent predictive model, although prediction quality was poor, even on the roof for which the model was parameterized (Stovin et al., 2012). A widely applicable predictive green roof runoff model has yet to come to fruition. Since their industrial use as stormwater management tools in Germany beginning in the 1970’s, much research has been done to quantify water quality impacts of green roofs. Several studies have evaluated metals content in runoff, generally finding limited impact of green roofs (Alsup et al., 2010; Mickovski et al., 2013; Ye and Liu, 2013). Studies of nutrients and organics in runoff have shown mixed findings, which has left the unanswered question of how green roofs affect surface water eutrophication. Nutrient loads from green roofs generally decrease over time, but loadings vary with media type, media age, amounts of irrigation, roof slope, and media depth, as well as vegetation type (Czemiel Berndtsson et al., 2006, 2009; Gregoire and Clausen 2011; Monterusso et al., 2005; Teemusk and Mander 2007; Toland et al., 2012; Vijayaraghavan et al., 2012). Similarly, suspended solids studies have shown a first flush effect in potted studies, dependent on media type and planted conditions (Morgan et al., 2011). The objective of this study was to assess the runoff quantity and quality from a green roof in mid-Missouri. In particular, two commercially available media was tested to quantify the resulting nutrient loads to assess potential nutrient loading to water bodies. The runoff quantity, media properties and meteorological data were combined to parameterize a parsimonious runoff model. Such a model can provide estimates of green roof water quantity performance from easily attainable variables in a variety of climates.
Table 1 Green roof media agronomic properties. Agronomic Property
Arkalyte New
pH Phosphorus (mg/kg) Potassium (mg/kg) Calcium (mg/ kg) Magnesium (mg/kg) Organic Matter (%) CEC (meq)
GAF Aged 9 months
7.4 60
New cubic foot sacks
7.7 46
7.6 219
7.8 212
Aged 1 month, supersack 7.9 82
121
49
1065
215
137
3405
1930
1815
1794
2151
208
101
334
286
348
12.7
9.0
6.4
7.9
7.3
19.1
10.6
14.6
11.9
14.0
from GAF and had not been used on any previous projects. The media was characterized by the MU Agronomic Soil Testing Services (Columbia, MO) after sieving (#10 sieve) (Table 1). The Bray I phosphorous (P) for Arkalyte and GAF media were 60 mg-P/kg and 219 mg-P/kg, respectively, showing very high initial P concentrations in the GAF media before testing. The P concentration in soil for agriculture recommended by the MU Agronomic lab is 60 mg-P/kg. The green roof media was re-tested after 9 months of exposure during the pilot study; compositional changes minimal in most cases (reported error 10%). The media were also tested for particle size distribution, water holding capacity and bulk density (Table 2). With the assistance of Jost Greenhouses (St. Louis, MO) succulents were selected based upon their survivability in the Missouri Ozarks region. A regional mix of 18 different species (Sedum acre, S. oreganum, S. aizoon, S. pulchellum, S. album, S. reflexum, S. ellacombianum, S. sexangulare, S. floriferum, S. seiboldii, S. hispanicum, S. spurium, S. stoloniferum, S. rupestre, S. kamtschaticum, S. telephium, S. hybridum ‘Czar’s Gold’, and Phedimus takesimensis) were chosen and planted on a 5 5 grid in 60.8 cm 60.8 cm 10.2 cm Green Roof Blocks, hereafter referred to as trays (Green Roof Blocks, Lake Saint Louis, MO). For the Arkalyte trays, a root barrier was placed between the tray and media to aid in media retention. For the GAF trays, the GAF DuraGroTM system was placed in the bottom of the tray. This system provides a thin drainage layer and a root barrier. The trays and plants were grown in a greenhouse for three months to allow the plants to become established. During establishment, minimal water was provided to the trays to prevent any runoff from the trays. Due to the drought conditions during the summer of 2012, Table 2 Green roof media physical properties. Media property
GAF
Arkalyte
3/8” #4 #8 #10 #16 #30 #40 #50 #100 #200
98.3% 78.7% 51.4% 45.9% 33.0% 21.6% 17.4% 13.9% 9.3% 4.5%
95.2% 62.1% 36.9% 33.2% 26.0% 19.6% 15.5% 10.1% 4.9% 2.3%
Bulk density (g/L)
635
654
Water holding capacity (L/L)
0.35
0.26
Particle size analysis: percent passing sieve
2. Materials and methods 2.1. Experimental setup The two media tested were an Arkalyte mix and GAF’s GardenscapesTM green roof media (GAF, Wayne NJ). The Arkalyte mix was stored outside for one year, being excess from a previous green roof research project (Kelly Luckett, personal communication, 2011). The GAF Gardenscapes media was delivered directly
Aged 9 months
Please cite this article in press as: Harper, G.E., et al., Nine-month evaluation of runoff quality and quantity from an experiential green roof in Missouri, USA. Ecol. Eng. (2014), http://dx.doi.org/10.1016/j.ecoleng.2014.06.004
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Fig. 1. Pilot scale testing of media and sedums, with schematic of test system on left and photo of Green Roof Block as placed on right.
the trays were not placed on the roof until late August 2012. Once the trays were moved to the roof, plants relied solely on rainwater. Data were collected through May 2013. For seasonal comparisons, water quality results in fall (September through November), winter (December through February), and spring (March through May) were compared. To simulate field conditions in a controlled study, these trays were tested atop the Butler-Carlton Civil Engineering Hall in Rolla, MO and runoff was sampled after each rain event within 24 h after drainage from the trays had stopped. This set-up allowed for testing of different media and planted/non-planted conditions on a small scale with a controlled runoff collection system for accurate measurement of runoff as well as easy sampling for chemical analysis. The experimental setup included two factors, each with two levels: media type (Arkalyte and GAF) and plants (planted and unplanted) (Fig. 1). Each treatment combination was performed in triplicate, with an empty tray used as a control. The experimental design allowed two-way analysis of variance (ANOVA). For analysis of seasonality, data were split into fall (October 31–December 20), winter (December 21–March 20) and spring (March 21–June 21) datasets. All statistical analyses were performed using SAS 9.1 (SAS Institute, Cary, NC). 2.2. Water balance Using runoff volumetric data, meteorological data, and media properties a daily water balance was developed to predict stormwater runoff as a function of meteorology and green roof design.
2.3. Runoff quality analysis Water samples taken after each storm event were tested for total nitrogen (TN), total phosphorous (TP), total organic carbon (TOC), and total suspended solids (TSS). TSS were measured by standard method 2540 D (APHA, 2012). TP was measured using a Hach DR/2400 Spectrophotometer following EPA method 365.2 for freshwater samples (USEPA, 1983). Briefly, samples were digested in acid and heat to allow for hydrolysis of inorganic forms; organic phosphorous was converted to orthophosphate through heating and reaction with persulfate. Once cooled, the sample was mixed with ascorbic acid and reacted with molybdate to produce a phosphate/molybdate complex. TOC and TN were tested using a Shimadzu TOC-L analyzer. TOC was tested using a 680 C combustion catalytic oxidation method for TOC. TN was tested using a 720 C catalytic thermal decomposition/chemiluminescence method. 3. Results 3.1. Runoff quantity Each tray exhibited a reduction in runoff when compared to the empty control tray and calculated runoff from an impervious surface (Fig. 2 and Table 3). The substantial reduction in green roof runoff can be attributed to both the plants and the growing media. Sixty percent reduction in runoff from the planted GAF media as
W i ¼ W i1 þ ðP AÞ ðK c ET0 Þ A where Wi is the water content of the media on day i (L3), Kc is the crop coefficient (dimensionless), P is precipitation (L), A is surface area of green roof (L2),and ET0 is the predicted reference ET (L). When the calculated water content exceeded the field capacity of the media, the difference was taken as the runoff. The model was calculated on a daily time step in MATLAB. A nonlinear least squares approach was used to estimate the crop coefficient using the measured runoff data. Estimates of ET0 followed the Penman– Monteith procedure described by the Food and Agriculture Organization of the United Nations (Allen et al., 1998). Most weather data were taken from the Vichy, MO National Weather Service station, approximately 18 km from the study location, with the exception of solar radiation, vapor pressure deficit and rainfall. Solar radiation and vapor pressure deficit were obtained from the agricultural weather station in Cook Station, MO, approximately 32 km from the study location. Rainfall was taken from the Missouri S&T weather station, part of the Global Historical Climatology Network, located approximately 400 m from the study location.
Fig. 2. Cumulative runoff from pilot scale tests performed. Errors bars denote range of 3 replicate trays.
Please cite this article in press as: Harper, G.E., et al., Nine-month evaluation of runoff quality and quantity from an experiential green roof in Missouri, USA. Ecol. Eng. (2014), http://dx.doi.org/10.1016/j.ecoleng.2014.06.004
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Table 3 Total runoff during the study for sampled storm.
3.3. Runoff quality
Condition
Total runoff during study (mm)
Total rainfall Planted Arkalyte Unplanted Arkalyte Planted GAF Unplanted GAF
1059 532 33 617 43 463 23 497 33
Values are mean one standard deviation, n = 3.
compared to the empty tray control was the highest cumulative reduction over the nine-month study. The seasonal impact of plant ET was evident as the reduction in runoff was dependent on the season. Plants were a statistically significant factor in fall and spring, but not in winter (Table 4). Storm size also influenced the runoff reduction with larger storms in spring resulting in a smaller relative runoff reduction on a percent basis and with wet seasons also showing a reduced overall impact of media and planting on total flow. When the plants were dormant over winter, less variation between the planted and unplanted trays was observed, as would be expected under low ET conditions. The plants had a >20% additional reduction (for example, 14 mm compared to 21 mm of runoff during a 29 mm storm) in stormwater runoff in the fall, despite relatively recent establishment. Throughout the duration of the fall, planted GAF showed an additional reduction of runoff of 20 mm and planted Arkalyte reduced runoff by an additional 54 mm when compared to the unplanted trays of the same media. Greater impacts of plants are expected as they mature and increase in coverage and develop deeper, broader root structure. Media type significantly impacted storm water runoff (Table 4). The differences between the two unplanted media trays were constant throughout the experiment with a 20% decrease in runoff for GAF media relative to Arkalyte and >100 mm in additional reduction in runoff from unplanted GAF than unplanted Arkalyte. As the media each had different compositions and field capacity (GAF: 0.35 L/L, Arkalyte: 0.26 L/L), water retention capacity was expected to vary accordingly. 3.2. Water balance The water balance model utilized local meteorological data, measured runoff, and media properties to estimate crop coefficients. The crop coefficients for planted trays showed a slight increase over unplanted treatments, resulting in planted crop coefficients 0.8. The estimated 95% confidence intervals range of 0.45–1.1 for all planted trays (n = 6). Unplanted tray crop coefficients generally showed greater variability than planted trays, perhaps reflecting the inability of Penman-Monteith to accurately describe direct soil-based evaporation. The water balance model performed adequately when calculating runoff volumes (Fig. 3). Note that the asterisks represent dates when measurement buckets overflowed, resulting in an undercounting of true runoff. The most substantial overflow event in March showed a step-divergence between the measured and calculated runoff values. However, the fit before and after this event were quite good considering the modelling assumptions made.
TN concentrations were expected to demonstrate a “first flush” of TN and then reduce quickly over time. However, the dissolved TN did not show a rapid wash out of highly soluble TN in the “first flush”. Instead, a first-order decrease in TN runoff concentrations was observed throughout the duration of this study (Fig. 4). When considering the TN mass produced from each tested condition, the largest total nitrogen releases occurred in the winter months, likely due to freeze-thaw, and GAF media had consistently higher loadings of TN than Arkalyte. Both plants and media influenced cumulative TN mass per area (p-values < 0.05). The cumulative TN runoff per area shows a large discrepancy between planted and unplanted Arkalyte (Fig. 5). Planted GAF trays had less TN discharge than unplanted trays. In addition, a 15 increase in TN runoff from planted GAF media, as compared to the control, shows potentially large TN loading from green roofs immediately after installation. TP concentrations varied greatly through the test period, and TP discharged from GAF media was consistently higher than Arkalyte, showing media composition had a large effect on TP (Fig. 6). The planted trays did show lower TP concentrations throughout the testing, likely due to plant roots stabilizing the media and preventing erosion. GAF had much higher TP concentrations in runoff, which was expected given the high P concentrations measured in the media (Table 1). When considering impacts to water bodies downstream, concentrations are much less important than TP load. Despite accounting for the decrease in runoff quantity with a green roof, an elevated load of TP was observed when compared to the control (Fig. 7). Media type significantly influenced TP loading (p-values < 0.001) throughout the year. In addition, plant and media interaction was significant (p-value 0.0011). TOC concentrations, similar to the nutrient concentrations, were elevated for both media when compared to the control. Media type affected TOC concentrations, with unplanted GAF runoff containing on average 208 mg/L TOC as compared to unplanted Arkalyte, which contained 32 mg/L TOC on average. Media type also affected TOC mass loading in cumulative runoff, with Arkalyte loading significantly lower when compared to GAF loading (p < 0.01). GAF TOC concentrations did not drop below 100 mg/L for several months. The presence of plants had minimal impact on TOC, with average concentrations of 36 mg/L and 187 mg/L for Arkalyte and GAF, respectively.
Table 4 2-way ANOVA p-values of plant and media factors on total runoff volume by season. Factor
Fall
Winter
Spring
Combined
Plants Media Planted media
0.0031** 0.0828 0.8387
0.1181 0.0003** 0.8611
0.0445* 0.0779 0.2648
0.016* 0.0013** 0.224
* **
Significant at a = 0.05. Significant at a = 0.01.
Fig. 3. Cumulative runoff from pilot scale tests performed in Green Roof BlocksTM compared to the runoff predicted from a water-balance model with planted GAF.
Please cite this article in press as: Harper, G.E., et al., Nine-month evaluation of runoff quality and quantity from an experiential green roof in Missouri, USA. Ecol. Eng. (2014), http://dx.doi.org/10.1016/j.ecoleng.2014.06.004
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Fig. 4. TN concentrations for each storm event, with error bars representing the range of 3 replicate trays.
To assess water quality impacts due to particles in the runoff, TSS were measured for each rain event. TSS remained relatively unchanged over time and at median concentrations <10 mg/L. TSS total mass was also calculated (Fig. 8). TSS data from unplanted and planted media were log transformed and assessed via ANOVA, revealing both media and planted conditions were significant (p < 0.05). The TSS of the Arkalyte media was very similar to the control TSS, whereas GAF TSS was approximately three times larger. 4. Discussion 4.1. Runoff quantity Green roof runoff quantitiy reductions are understandably variable in the literature. A recent review noted average retention values range of 45–78%, with a range of 5–100% (Czemiel Berndtsson 2010). Such variance results from meteorological factors such as antecedant rainfall, rainfall size and intensity, relative humidity and time of year. Green roof properties provide an additional source of variance, with media water holding capacity, vegetation, roof slope and roof age affecting runoff reductions. While the measured data fall comfortably within the literature range, care should be taken in extrapolating the performance of any green roof to a proposed green
roof with one or more differences in meteorological or engineered properties. 4.2. Water balance The water balance model estimated crop coefficients of 0.8 using the pilot green roof data from late summer to spring. Values of crop coefficients from other green roof plants are similar, such as those of Rezaei et al. (2005) who found crop coefficients of CAM plants on extensive green roofs of 0.74 in winter and 1.97 in fall/ spring. Similarly, Starry (2013) found crop coefficients ranging from 0.27 to 0.79 for three sedum species. In the model, several simplifying assumptions were made to generate a parsimonious model. First, the entire soil moisture compartment was considered available for ET; however, some of the moisture will not be accessible by roots (Kasmin et al., 2010). In addition, heat lost by the tray was neglected in the Penman– Monteith energy balance. Depending on the configuration of the roof, heat may be transferred to the below living space. Additionally, succulents have the ability to perform CAM photosynthesis, which is unlikely to be well described by the Penman–Monteith equation. Violations of these assumptions likely lead to some of the observed error in fitting the model. Despite these assumptions, the model performed well on the particular roof and meteorological conditions for which it was calibrated, although validation on roofs in different locations needs to be performed. Differing media types, media thicknesses and climatology are needed to robustly validate the model. The benefit of the water balance model is the ability to predict runoff volumes for specific media of a design-specific media depth and in any selected climate, as the Penman–Monteith prediction is based on the local climate data and a crop coefficient. A reasonable range of crop coefficients can be used to simulate runoff using historical or typical meteorological conditions (e.g., Typical Meteorological Year data) for a site considering a green roof application in any climate and utilizing different media and depth. Such a modeling approach to forecast impacts is more powerful and valuable than experiment-specific percent reductions in runoff typically reported. 4.3. Runoff quality
Fig. 5. Cumulative TN per area over the course of the study. Error bars represent the range of 3 replicate trays.
TN values measured in this study did not approach the literature range of 0.2–6.9 mg-N/L for several months, particularly for GAF (Bliss et al., 2009; Czemiel Berndtsson et al., 2009; Hathaway et al., 2008; Moran et al., 2005; Van Seters et al., 2009). This sustained loading is hypothesized to originate from nitrogen moving from non-available forms to a more labile form through slow breakdown of the media and subsequent flushing.
Please cite this article in press as: Harper, G.E., et al., Nine-month evaluation of runoff quality and quantity from an experiential green roof in Missouri, USA. Ecol. Eng. (2014), http://dx.doi.org/10.1016/j.ecoleng.2014.06.004
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Fig. 6. Median TP concentrations over time. Error bars represent the range of replicate trays.
TP concentrations in runoff from extensive green roofs are quite variable in the literature (Czemiel Berndtsson 2010). Teemusk and Mander (2007) reported concentrations ranging from 0.012 mg-P/ L to 0.09 mg-P/L for a 1–2 year old green roof. Others have reported higher concentrations of 0.23–0.45 mg-P/L (Van Seters et al., 2009), 0.6–1.5 mg-P/L (Moran et al., 2005), 0.31 mg-P/L (Czemiel Berndtsson et al., 2009), 0.6–1.4 mg/L (Hathaway et al., 2008) and 2–3 mg-P/L (Bliss et al., 2009). An initial flush of TP from a green roof planted with Sedum was 10.3 mg-P/L (Beck et al., 2011). The concentrations of TP observed from the Arkalyte media fit within the reported range. However, the TP measured in runoff from GAF media were much higher, likely due to excess P reported by the agronomic tests (Table 1). Plants reduced the amount of TP discharged in both media. Discharge of P most often occurs in natural systems from adsorption to soil grains that are then eroded away into a water body (Wendt and Alberts, 1984), suggesting the roots acted as a media stabilizer. Plant uptake of phosphate could also be reducing labile phosphate dissolving into the water. However, the differences remain throughout the winter, when the plants were dormant, supporting hypothesis of roots acting as a media stabilizer. Also of note are the effects of freeze-thaw, particularly
evident for GAF during the winter months, as TP concentrations in runoff notably increased. Initial TOC concentrations bracketed those of Beck et al. (2011); decreasing over time to less than 50 mg/L for all media, however no immediate substantial decrease after the “first flush” was observed. GAF contained higher TOC concentrations, despite containing less organic matter than Arkalyte (Table 1), although GAF contained more fines (Table 2). With TOC concentrations measured as high as 500 mg/L, GAF organic matter was more easily leached than that in the Arkalyte media. Previous literature has reported an initial flush of TSS of 120 mg/L for Arkalyte, decreasing to 10 mg/L after 15 watering events (Morgan et al., 2013). The observation of a first flush is likely dependent on rainfall intensity and the rooting barrier. TSS concentrations are expected to drop over time with TSS values reported less than 2.5 mg/L for a green roof 1–2 years old (Van Seters et al., 2009). TSS concentrations from wastewater treatment plants into water bodies are often set at limits of 20 mg/L (Tchobanoglous et al., 2003). TSS values generally remained below this standard, which shows the effectiveness of the rooting barriers designed to support and retain the growth media. The differences in TSS between the two media is likely due to the age of the media. Arkalyte’s one-year aging likely reduced the number of fines observed in the media (Table 2). The TSS increased for the
Fig. 7. Cumulative TP mass per area of green roof. Error bars denote the range of three replicate trays.
Fig. 8. Cumulative TSS in green roof runoff during the study. Error bars denote the range of three trays.
Please cite this article in press as: Harper, G.E., et al., Nine-month evaluation of runoff quality and quantity from an experiential green roof in Missouri, USA. Ecol. Eng. (2014), http://dx.doi.org/10.1016/j.ecoleng.2014.06.004
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unplanted condition supporting the hypothesis that plants reduce erosion during storm runoff and therefore reduce TP loading. Large variation in the empty control tray resulted from large storm events, especially with high winds, when some media from adjacent trays was observed to have blown into the control trays contributing to control tray TSS. Similar effects would be observed during large events on a full-scale, conventional roof, as nearby debris and leaves would be carried toward the roof drain. 5. Conclusions Green roofs have been championed as an effective resource for urban water management by reducing the stormwater loads reaching grey infrastructure and surface waters. Runoff reduction of over 60% for storms below 5 cm was shown and further support the ability of green roofs to reduce urban stormwater. However, the concentrations of TN and TP leaching out of new green roof media are a concern for water quality downstream, as excess nutrient loads have the potential to increase eutrophication risk for lakes and rivers. A substantial nutrient load was observed in runoff emanating from media tested, likely due to excess nutrients in the manufactured media. In addition, the organics dissolved in the green roof runoff can add to the total biological oxygen demand in the water. Such effects will not be observed on a watershed scale from any single roof, but with city policies encouraging green roofs to be incorporated into urban structures, all effects, both positive and negative, from this implementation must be considered. Altering the amount of organic matter, type of organic matter, and/or fertilizers used could all lead to a “greener” green roof. Alternatively, in areas where nutrient loads are particularly problematic, an alternative media without leachable nutrients but with sufficient water holding capacity may be able to provide much of the same engineering benefits without the risk of lowquality runoff. Acknowledgements The authors thank the USGS Water Center, Columbia Missouri for funding (Project Number: 2011MO122B). We also thank Kelly Luckett of Green Roof Blocks and Helene Hardy-Pierce of GAF for their personal insight and coordinating donation of materials for the study and full scale green roof. We also appreciate input on plant selection and cultivation by Vic Jost of Jost Greenhouses. The authors also greatly appreciate assistance with data collection and analysis by Lea Ahrens, Elise Kittrell, Honglan Shi and Amanda Holmes of Missouri S&T. References Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop Evapotranspiration– Guidelines for Computing Crop Water Requirements. Food and Agriculture Organization of the United Nations, Rome. Alsup, S., Ebbs, S., Retzlaff, W., 2010. The exchangeability and leachability of metals from select green roof growth substrates. Urban Ecosyst. 13, 91–111. American Public Health Association, 2012. Standard Methods for the Examination of Water and Wastewater, twenty-second ed. APHA. Beck, D.A., Johnson, G.R., Spolek, G.A., 2011. Amending greenroof soil with biochar to affect runoff water quantity and quality. Environ. Pollut. 159, 2111–2118. Blanusa, T., Monteiro, Vaz, Fantozzi, M.M., Vysini, F., Li, E., Cameron, Y., R.W.F, 2013. Alternatives to Sedum on green roofs: can broad leaf perennial plants offer better ‘cooling service'? Build. Environ. 59, 99–106.
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Bliss, D.J., Neufeld, R.D., Ries, R.J., 2009. Storm water runoff mitigation using a green roof. Environ. Eng. Sci. 26, 407–417. Carpenter, D.D., Kaluvakolanu, P., 2011. Effect of roof surface type on storm-water runoff from full-scale roofs in a temperate climate. J. Irrigation Drain Eng. 137, 161–169. Carter, T.L., Rasmussen, T.C., 2006. Hydrologic behavior of vegetated roofs. J. Am. Water Resour. Assoc. 42, 1261–1274. Czemiel Berndtsson, J.C., Emilsson, T., Bengtsson, L., 2006. The influence of extensive vegetated roofs on runoff water quality. Sci. Tot. Environ. 355, 48–63. Czemiel Berndtsson, J.C., Bengtsson, L., Jinno, K., 2009. Runoff water quality from intensive and extensive vegetated roofs. Ecol. Eng. 35, 369–380. Czemiel Berndtsson, J.C., 2010. Green roof performance towards management of runoff water quantity and quality: a review. Ecol. Eng. 36, 351–360. Getter, K.L., Rowe, D.B., Andresen, J.A., 2007. Quantifying the effect of slope on extensive green roof stormwater retention. Ecol. Eng. 31, 225–231. Gregoire, B.G., Clausen, J.C., 2011. Effect of a modular extensive green roof on stormwater runoff and water quality. Ecol. Eng. 37, 963–969. Hathaway, A.M., Hunt, W.F., Jennings, G.D., 2008. A field study of green roof hydrologic and water quality performance. Transactions ASABE 51, 37–44. Hilten, R.N., Lawrence, T.M., Tollner, E.W., 2008. Modeling stormwater runoff from green roofs with HYDRUS-1D. J. Hydrol. 358, 288–293. Kasmin, H., Stovin, V.R., Hathway, E.A., 2010. Towards a generic rainfall-runoff model for green roofs. Water Sci. Technol. 62, 898–905. Kosareo, L., Ries, R., 2007. Comparative environmental life cycle assessment of green roofs. Build. Environ. 42, 2606–2613. Metselaar, K., 2012. Water retention and evapotranspiration of green roofs and possible natural vegetation types. Resour. Conserv. Recycl. 64, 49–55. Mickovski, S.B., Buss, K., McKenzie, B.M., Sökmener, B., 2013. Laboratory study on the potential use of recycled inert construction waste material in the substrate mix for extensive green roofs. Ecol. Eng. 61, 706–714. Monterusso, M.A., Bradley Rowe, D., Rugh, C.L., 2005. Establishment and persistence of Sedum spp. and native taxa for green roof applications. HortScience 40, 391–396. Moran, A., Hunt, B., Smith, J., 2005. Hydrological and water quality performance from green roofs in Goldsboro and Raleigh, North Carolina. Green Roofs for Healthy Cities Conference, May 2005, Washington, DC. Morgan, S., Alyaseri, I., Retzlaff, W., 2011. Suspended solids in and turbidity of runoff from green roofs. Int. J. Phytorem. 13, 179–193. Morgan, S., Celik, S., Retzlaff, W., 2013. Green roof storm-water runoff quantity and quality. J. Environ. Eng. 139, 471–478. Nagase, A., Dunnett, N., 2012. Amount of water runoff from different vegetation types on extensive green roofs: Effects of plant species, diversity and plant structure. Landsc. Urban Plan. 104, 356–363. Rezaei, F., Jarrett, A.R., Berghage, R.D., Beattie, D.J., 2005. Evapotranspiration rates from extensive green roof plant species. Paper number 052150, 2005, ASAE Annual Meeting . She, N., Pang, J., 2010. Physically Based Green Roof Model. J. Hydrol. Eng. 15, 458–464. Starry, O., 2013. The Comparative Effects of Three Sedum Species on Green Roof Stormwater Retention, Ph.D Thesis. University of Maryland. Stovin, V., Vesuviano, G., Kasmin, H., 2012. The hydrological performance of a green roof test bed under UK climatic conditions. J. Hydrol. 414–415, 148–161. Tchobanoglous, G., Burton, F.L., Stensel, H.D., 2003. Wastewater Engineering: Treatment and Reuse, fourth ed. McGraw-Hill Companies, New York. Teemusk, A., Mander, U., 2007. Rainwater runoff quantity and quality performance from a greenroof: The effects of short-term events. Ecol. Eng. 30, 271–277. Toland, D.C., Haggard, B.E., Boyer, M.E., 2012. Evaluation of nutrient concentrations in runoff water from green roofs, conventional roofs, and urban streams. Transactions ASABE 55, 99–106. Tzoulas, K., Korpela, K., Venn, S., Yli-Pelkonen, V., Ka zmierczak, A., Niemela, J., James, P., 2007. Promoting ecosystem and human health in urban areas using Green Infrastructure: a literature review. Landsc. Urban Plann. 81, 167–178. United States Environmetal Protection Agency (USEPA), 1983. Methods For Chemical Analysis Of Water And Wastes. Method 365.2, second ed. U.S. Environmental Protection Agency, Washington, DC. Van Seters, T., Rocha, L., Smith, D., MacMillan, G., 2009. Evaluation of Green Roofs for Runoff Retention, Runoff quality, and Leachability. Water Qual. Res. J. Can. 44, 33–47. Vijayaraghavan, K., Joshi, U.M., Balasubramanian, R.A., 2012. Field study to evaluate runoff quality from green roofs. Water Res. 46, 1337–1345. Water Environment Federation (WEF), 2011. Stormwater Report. http://stormwater. wef.org/2011/07/st-louis-sewer-district-to-spend-4-7-billion-to-eliminatecsos-and-ssos/. (accessed April 2013). Wendt, R.C., Alberts, E.E., 1984. Estimating labile and dissolved inorganic phosphate concentrations in surface runoff. J. Environ. Qual. 13, 613–618. Ye, J., Liu, C., 2013. Heavy metals in plants and substrate from simulated extensive green roofs. Ecol. Eng. 55, 29–34.
Please cite this article in press as: Harper, G.E., et al., Nine-month evaluation of runoff quality and quantity from an experiential green roof in Missouri, USA. Ecol. Eng. (2014), http://dx.doi.org/10.1016/j.ecoleng.2014.06.004
Ecological Engineering xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Nine-month evaluation of runoff quality and quantity from an experiential green roof in Missouri, USA Grace E. Harper, Matt A. Limmer, W. Eric Showalter, Joel G. Burken * Department of Civil, Architectural and Environmental Engineering, Missouri University of Science and Technology, 1401 N Pine Street, Rolla, MO 65409, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 November 2013 Received in revised form 10 June 2014 Accepted 12 June 2014 Available online xxx
A better understanding of green roof stormwater performance is needed to assess and predict benefits of proposed green roof projects. A nine-month green roof pilot study was conducted in mid-Missouri to evaluate runoff quantity and quality under field conditions for two different media, both tested under planted and unplanted conditions. Water quantity results showed a 40% reduction in runoff from the unplanted growing media and a 60% reduction in runoff from the planted growing media. A water balance model was developed that incorporated water storage in the media and evapotranspiration (ET) from the media based upon local weather conditions using the Penmen–Monteith ET method. Water quality monitoring showed a first-order decline of excess nutrients in the first few months of green roof operation. Total phosphorus >30 mg-P/L and total nitrogen concentrations >60 mg-N/L were observed in green roof runoff initially, with concentrations decreasing over nine months to 5 mg-P/L and 10 mg-N/ L, respectively. In addition, elevated total organic carbon concentrations were observed, with concentrations of 500 mg/L initially, decreasing after a few weeks to below 50 mg/L. Media type and age were the largest influences on carbon and nutrient loading from the green roof media tested. ã 2014 Elsevier B.V. All rights reserved.
Keywords: Green roof Evapotranspiration Climate model Nitrate Phosphorous Nutrients
1. Introduction Urbanization has drastically impacted the water cycle. As buildings and paved surfaces are added to the landscape, pervious soil surface area and rainfall infiltration decrease, causing stormwater to rapidly transfer from impervious hardscapes to grey infrastructure, such as curb and gutters and underground sewers. Such rapid conveyance results in erosion of surface water channels and downstream flooding while reducing evapo-transpiration (ET) that would normally occur from vegetated soils. In cities with combined stormwater and sanitary sewers, overflows result in the release of sanitary wastewater and associated pathogens into waterways, which is regulated by the USEPA’s Clean Water Act. Several US cities are under consent decrees from the USEPA for their combined sewer overflows. St. Louis, MO, USA is committing to expending $4.7 billion on upgrades to prevent combined sewer overflows (WEF, 2011). To address the effects of urbanization on stormwater quantity and quality, green infrastructure approaches have been developed and implemented using natural treatment systems (Tzoulas et al., 2007). By strategically integrating vegetation, easily draining soils
* Corresponding author. Tel.: +1 573 341 6547. E-mail address: [email protected] (J.G. Burken).
and natural storage into the urban landscape, rainfall can be treated, stored and evapo-transpired to avoid excess stormwater runoff from urban impervious surfaces. Green roofs are an example of green infrastructure implemented at a primary source of stormwater collection in the built environment, building rooftops, designed to store rainfall on-site and release much of the water through evapotranspiration (Czemiel Berndtsson 2010). Through such green infrastructure, the quantity of water that must be conveyed through conventional storm sewers is reduced, preventing costly traditional grey infrastructure upgrades to handle increased flows and mitigate excessive downstream flows. Green roofs are categorized in two major classes: intensive and extensive. Intensive green roofs feature deeper soil profiles to support a diverse set of vegetation (Czemiel Berndtsson 2010). Extensive roofs, the focus of this study, are often installed primarily for their stormwater benefit and are characterized by growing media depths less than 15 cm (Kosareo and Ries 2007). Structural reinforcement needs are often low as extensive green roofs do not result in large live loads or people using the space. In addition, extensive roofs have fewer vegetation options and are commonly planted with succulents such as sedums due to their ability to thrive in harsh rooftop conditions (Blanusa et al., 2013). Green roof media available for commercial use most often has proprietary compositions, but most media contain similar components. Growing media is generally lighter than topsoil and chosen based
http://dx.doi.org/10.1016/j.ecoleng.2014.06.004 0925-8574/ ã 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: Harper, G.E., et al., Nine-month evaluation of runoff quality and quantity from an experiential green roof in Missouri, USA. Ecol. Eng. (2014), http://dx.doi.org/10.1016/j.ecoleng.2014.06.004
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upon its ability to drain and support plant growth. Additional fertilizers are also added to the mix to sustain vegetative growth. Because green roofs are a living system of several components, understanding the dynamics of the system is necessary to predict the environmental impacts at specific sites. Several studies have reported varying impacts of green roofs on water quantity, as impacts are dependent upon roof slope, growing media, plant type, antecedent rainfall, green roof age, and weather conditions (Carpenter and Kaluvakolanu 2011; Carter and Rasmussen 2006; Getter et al., 2007; Hilten et al., 2008; Metselaar 2012; Nagase and Dunnett 2012; Rezaei et al., 2005). Due to the desire to predict green roof performance, models to account for the effects of many of these variables have been proposed. However, the resulting models are often experiment- and site-specific and are not translatable to other installations. Some models consider soil characteristics for varying media depths and/or roof slopes, such as those estimating curve numbers, but neglect antecedent rainfall as well as evapotranspiration losses (Carter and Rasmussen 2006). Other models account for hydrologic processes while greatly simplifying ET (Hilten et al., 2008; Kasmin et al., 2010; She and Pang 2010). The effect of ET on green roof performance is dependent both on climate and plant type (Nagase and Dunnett 2012), fundamental factors which must be understood to ensure a broadly predictive model. Capturing the effects of these variables is not trivial, as demonstrated by a multiple linear regression approach intending to provide a transparent predictive model, although prediction quality was poor, even on the roof for which the model was parameterized (Stovin et al., 2012). A widely applicable predictive green roof runoff model has yet to come to fruition. Since their industrial use as stormwater management tools in Germany beginning in the 1970’s, much research has been done to quantify water quality impacts of green roofs. Several studies have evaluated metals content in runoff, generally finding limited impact of green roofs (Alsup et al., 2010; Mickovski et al., 2013; Ye and Liu, 2013). Studies of nutrients and organics in runoff have shown mixed findings, which has left the unanswered question of how green roofs affect surface water eutrophication. Nutrient loads from green roofs generally decrease over time, but loadings vary with media type, media age, amounts of irrigation, roof slope, and media depth, as well as vegetation type (Czemiel Berndtsson et al., 2006, 2009; Gregoire and Clausen 2011; Monterusso et al., 2005; Teemusk and Mander 2007; Toland et al., 2012; Vijayaraghavan et al., 2012). Similarly, suspended solids studies have shown a first flush effect in potted studies, dependent on media type and planted conditions (Morgan et al., 2011). The objective of this study was to assess the runoff quantity and quality from a green roof in mid-Missouri. In particular, two commercially available media was tested to quantify the resulting nutrient loads to assess potential nutrient loading to water bodies. The runoff quantity, media properties and meteorological data were combined to parameterize a parsimonious runoff model. Such a model can provide estimates of green roof water quantity performance from easily attainable variables in a variety of climates.
Table 1 Green roof media agronomic properties. Agronomic Property
Arkalyte New
pH Phosphorus (mg/kg) Potassium (mg/kg) Calcium (mg/ kg) Magnesium (mg/kg) Organic Matter (%) CEC (meq)
GAF Aged 9 months
7.4 60
New cubic foot sacks
7.7 46
7.6 219
7.8 212
Aged 1 month, supersack 7.9 82
121
49
1065
215
137
3405
1930
1815
1794
2151
208
101
334
286
348
12.7
9.0
6.4
7.9
7.3
19.1
10.6
14.6
11.9
14.0
from GAF and had not been used on any previous projects. The media was characterized by the MU Agronomic Soil Testing Services (Columbia, MO) after sieving (#10 sieve) (Table 1). The Bray I phosphorous (P) for Arkalyte and GAF media were 60 mg-P/kg and 219 mg-P/kg, respectively, showing very high initial P concentrations in the GAF media before testing. The P concentration in soil for agriculture recommended by the MU Agronomic lab is 60 mg-P/kg. The green roof media was re-tested after 9 months of exposure during the pilot study; compositional changes minimal in most cases (reported error 10%). The media were also tested for particle size distribution, water holding capacity and bulk density (Table 2). With the assistance of Jost Greenhouses (St. Louis, MO) succulents were selected based upon their survivability in the Missouri Ozarks region. A regional mix of 18 different species (Sedum acre, S. oreganum, S. aizoon, S. pulchellum, S. album, S. reflexum, S. ellacombianum, S. sexangulare, S. floriferum, S. seiboldii, S. hispanicum, S. spurium, S. stoloniferum, S. rupestre, S. kamtschaticum, S. telephium, S. hybridum ‘Czar’s Gold’, and Phedimus takesimensis) were chosen and planted on a 5 5 grid in 60.8 cm 60.8 cm 10.2 cm Green Roof Blocks, hereafter referred to as trays (Green Roof Blocks, Lake Saint Louis, MO). For the Arkalyte trays, a root barrier was placed between the tray and media to aid in media retention. For the GAF trays, the GAF DuraGroTM system was placed in the bottom of the tray. This system provides a thin drainage layer and a root barrier. The trays and plants were grown in a greenhouse for three months to allow the plants to become established. During establishment, minimal water was provided to the trays to prevent any runoff from the trays. Due to the drought conditions during the summer of 2012, Table 2 Green roof media physical properties. Media property
GAF
Arkalyte
3/8” #4 #8 #10 #16 #30 #40 #50 #100 #200
98.3% 78.7% 51.4% 45.9% 33.0% 21.6% 17.4% 13.9% 9.3% 4.5%
95.2% 62.1% 36.9% 33.2% 26.0% 19.6% 15.5% 10.1% 4.9% 2.3%
Bulk density (g/L)
635
654
Water holding capacity (L/L)
0.35
0.26
Particle size analysis: percent passing sieve
2. Materials and methods 2.1. Experimental setup The two media tested were an Arkalyte mix and GAF’s GardenscapesTM green roof media (GAF, Wayne NJ). The Arkalyte mix was stored outside for one year, being excess from a previous green roof research project (Kelly Luckett, personal communication, 2011). The GAF Gardenscapes media was delivered directly
Aged 9 months
Please cite this article in press as: Harper, G.E., et al., Nine-month evaluation of runoff quality and quantity from an experiential green roof in Missouri, USA. Ecol. Eng. (2014), http://dx.doi.org/10.1016/j.ecoleng.2014.06.004
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Fig. 1. Pilot scale testing of media and sedums, with schematic of test system on left and photo of Green Roof Block as placed on right.
the trays were not placed on the roof until late August 2012. Once the trays were moved to the roof, plants relied solely on rainwater. Data were collected through May 2013. For seasonal comparisons, water quality results in fall (September through November), winter (December through February), and spring (March through May) were compared. To simulate field conditions in a controlled study, these trays were tested atop the Butler-Carlton Civil Engineering Hall in Rolla, MO and runoff was sampled after each rain event within 24 h after drainage from the trays had stopped. This set-up allowed for testing of different media and planted/non-planted conditions on a small scale with a controlled runoff collection system for accurate measurement of runoff as well as easy sampling for chemical analysis. The experimental setup included two factors, each with two levels: media type (Arkalyte and GAF) and plants (planted and unplanted) (Fig. 1). Each treatment combination was performed in triplicate, with an empty tray used as a control. The experimental design allowed two-way analysis of variance (ANOVA). For analysis of seasonality, data were split into fall (October 31–December 20), winter (December 21–March 20) and spring (March 21–June 21) datasets. All statistical analyses were performed using SAS 9.1 (SAS Institute, Cary, NC). 2.2. Water balance Using runoff volumetric data, meteorological data, and media properties a daily water balance was developed to predict stormwater runoff as a function of meteorology and green roof design.
2.3. Runoff quality analysis Water samples taken after each storm event were tested for total nitrogen (TN), total phosphorous (TP), total organic carbon (TOC), and total suspended solids (TSS). TSS were measured by standard method 2540 D (APHA, 2012). TP was measured using a Hach DR/2400 Spectrophotometer following EPA method 365.2 for freshwater samples (USEPA, 1983). Briefly, samples were digested in acid and heat to allow for hydrolysis of inorganic forms; organic phosphorous was converted to orthophosphate through heating and reaction with persulfate. Once cooled, the sample was mixed with ascorbic acid and reacted with molybdate to produce a phosphate/molybdate complex. TOC and TN were tested using a Shimadzu TOC-L analyzer. TOC was tested using a 680 C combustion catalytic oxidation method for TOC. TN was tested using a 720 C catalytic thermal decomposition/chemiluminescence method. 3. Results 3.1. Runoff quantity Each tray exhibited a reduction in runoff when compared to the empty control tray and calculated runoff from an impervious surface (Fig. 2 and Table 3). The substantial reduction in green roof runoff can be attributed to both the plants and the growing media. Sixty percent reduction in runoff from the planted GAF media as
W i ¼ W i1 þ ðP AÞ ðK c ET0 Þ A where Wi is the water content of the media on day i (L3), Kc is the crop coefficient (dimensionless), P is precipitation (L), A is surface area of green roof (L2),and ET0 is the predicted reference ET (L). When the calculated water content exceeded the field capacity of the media, the difference was taken as the runoff. The model was calculated on a daily time step in MATLAB. A nonlinear least squares approach was used to estimate the crop coefficient using the measured runoff data. Estimates of ET0 followed the Penman– Monteith procedure described by the Food and Agriculture Organization of the United Nations (Allen et al., 1998). Most weather data were taken from the Vichy, MO National Weather Service station, approximately 18 km from the study location, with the exception of solar radiation, vapor pressure deficit and rainfall. Solar radiation and vapor pressure deficit were obtained from the agricultural weather station in Cook Station, MO, approximately 32 km from the study location. Rainfall was taken from the Missouri S&T weather station, part of the Global Historical Climatology Network, located approximately 400 m from the study location.
Fig. 2. Cumulative runoff from pilot scale tests performed. Errors bars denote range of 3 replicate trays.
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Table 3 Total runoff during the study for sampled storm.
3.3. Runoff quality
Condition
Total runoff during study (mm)
Total rainfall Planted Arkalyte Unplanted Arkalyte Planted GAF Unplanted GAF
1059 532 33 617 43 463 23 497 33
Values are mean one standard deviation, n = 3.
compared to the empty tray control was the highest cumulative reduction over the nine-month study. The seasonal impact of plant ET was evident as the reduction in runoff was dependent on the season. Plants were a statistically significant factor in fall and spring, but not in winter (Table 4). Storm size also influenced the runoff reduction with larger storms in spring resulting in a smaller relative runoff reduction on a percent basis and with wet seasons also showing a reduced overall impact of media and planting on total flow. When the plants were dormant over winter, less variation between the planted and unplanted trays was observed, as would be expected under low ET conditions. The plants had a >20% additional reduction (for example, 14 mm compared to 21 mm of runoff during a 29 mm storm) in stormwater runoff in the fall, despite relatively recent establishment. Throughout the duration of the fall, planted GAF showed an additional reduction of runoff of 20 mm and planted Arkalyte reduced runoff by an additional 54 mm when compared to the unplanted trays of the same media. Greater impacts of plants are expected as they mature and increase in coverage and develop deeper, broader root structure. Media type significantly impacted storm water runoff (Table 4). The differences between the two unplanted media trays were constant throughout the experiment with a 20% decrease in runoff for GAF media relative to Arkalyte and >100 mm in additional reduction in runoff from unplanted GAF than unplanted Arkalyte. As the media each had different compositions and field capacity (GAF: 0.35 L/L, Arkalyte: 0.26 L/L), water retention capacity was expected to vary accordingly. 3.2. Water balance The water balance model utilized local meteorological data, measured runoff, and media properties to estimate crop coefficients. The crop coefficients for planted trays showed a slight increase over unplanted treatments, resulting in planted crop coefficients 0.8. The estimated 95% confidence intervals range of 0.45–1.1 for all planted trays (n = 6). Unplanted tray crop coefficients generally showed greater variability than planted trays, perhaps reflecting the inability of Penman-Monteith to accurately describe direct soil-based evaporation. The water balance model performed adequately when calculating runoff volumes (Fig. 3). Note that the asterisks represent dates when measurement buckets overflowed, resulting in an undercounting of true runoff. The most substantial overflow event in March showed a step-divergence between the measured and calculated runoff values. However, the fit before and after this event were quite good considering the modelling assumptions made.
TN concentrations were expected to demonstrate a “first flush” of TN and then reduce quickly over time. However, the dissolved TN did not show a rapid wash out of highly soluble TN in the “first flush”. Instead, a first-order decrease in TN runoff concentrations was observed throughout the duration of this study (Fig. 4). When considering the TN mass produced from each tested condition, the largest total nitrogen releases occurred in the winter months, likely due to freeze-thaw, and GAF media had consistently higher loadings of TN than Arkalyte. Both plants and media influenced cumulative TN mass per area (p-values < 0.05). The cumulative TN runoff per area shows a large discrepancy between planted and unplanted Arkalyte (Fig. 5). Planted GAF trays had less TN discharge than unplanted trays. In addition, a 15 increase in TN runoff from planted GAF media, as compared to the control, shows potentially large TN loading from green roofs immediately after installation. TP concentrations varied greatly through the test period, and TP discharged from GAF media was consistently higher than Arkalyte, showing media composition had a large effect on TP (Fig. 6). The planted trays did show lower TP concentrations throughout the testing, likely due to plant roots stabilizing the media and preventing erosion. GAF had much higher TP concentrations in runoff, which was expected given the high P concentrations measured in the media (Table 1). When considering impacts to water bodies downstream, concentrations are much less important than TP load. Despite accounting for the decrease in runoff quantity with a green roof, an elevated load of TP was observed when compared to the control (Fig. 7). Media type significantly influenced TP loading (p-values < 0.001) throughout the year. In addition, plant and media interaction was significant (p-value 0.0011). TOC concentrations, similar to the nutrient concentrations, were elevated for both media when compared to the control. Media type affected TOC concentrations, with unplanted GAF runoff containing on average 208 mg/L TOC as compared to unplanted Arkalyte, which contained 32 mg/L TOC on average. Media type also affected TOC mass loading in cumulative runoff, with Arkalyte loading significantly lower when compared to GAF loading (p < 0.01). GAF TOC concentrations did not drop below 100 mg/L for several months. The presence of plants had minimal impact on TOC, with average concentrations of 36 mg/L and 187 mg/L for Arkalyte and GAF, respectively.
Table 4 2-way ANOVA p-values of plant and media factors on total runoff volume by season. Factor
Fall
Winter
Spring
Combined
Plants Media Planted media
0.0031** 0.0828 0.8387
0.1181 0.0003** 0.8611
0.0445* 0.0779 0.2648
0.016* 0.0013** 0.224
* **
Significant at a = 0.05. Significant at a = 0.01.
Fig. 3. Cumulative runoff from pilot scale tests performed in Green Roof BlocksTM compared to the runoff predicted from a water-balance model with planted GAF.
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Fig. 4. TN concentrations for each storm event, with error bars representing the range of 3 replicate trays.
To assess water quality impacts due to particles in the runoff, TSS were measured for each rain event. TSS remained relatively unchanged over time and at median concentrations <10 mg/L. TSS total mass was also calculated (Fig. 8). TSS data from unplanted and planted media were log transformed and assessed via ANOVA, revealing both media and planted conditions were significant (p < 0.05). The TSS of the Arkalyte media was very similar to the control TSS, whereas GAF TSS was approximately three times larger. 4. Discussion 4.1. Runoff quantity Green roof runoff quantitiy reductions are understandably variable in the literature. A recent review noted average retention values range of 45–78%, with a range of 5–100% (Czemiel Berndtsson 2010). Such variance results from meteorological factors such as antecedant rainfall, rainfall size and intensity, relative humidity and time of year. Green roof properties provide an additional source of variance, with media water holding capacity, vegetation, roof slope and roof age affecting runoff reductions. While the measured data fall comfortably within the literature range, care should be taken in extrapolating the performance of any green roof to a proposed green
roof with one or more differences in meteorological or engineered properties. 4.2. Water balance The water balance model estimated crop coefficients of 0.8 using the pilot green roof data from late summer to spring. Values of crop coefficients from other green roof plants are similar, such as those of Rezaei et al. (2005) who found crop coefficients of CAM plants on extensive green roofs of 0.74 in winter and 1.97 in fall/ spring. Similarly, Starry (2013) found crop coefficients ranging from 0.27 to 0.79 for three sedum species. In the model, several simplifying assumptions were made to generate a parsimonious model. First, the entire soil moisture compartment was considered available for ET; however, some of the moisture will not be accessible by roots (Kasmin et al., 2010). In addition, heat lost by the tray was neglected in the Penman– Monteith energy balance. Depending on the configuration of the roof, heat may be transferred to the below living space. Additionally, succulents have the ability to perform CAM photosynthesis, which is unlikely to be well described by the Penman–Monteith equation. Violations of these assumptions likely lead to some of the observed error in fitting the model. Despite these assumptions, the model performed well on the particular roof and meteorological conditions for which it was calibrated, although validation on roofs in different locations needs to be performed. Differing media types, media thicknesses and climatology are needed to robustly validate the model. The benefit of the water balance model is the ability to predict runoff volumes for specific media of a design-specific media depth and in any selected climate, as the Penman–Monteith prediction is based on the local climate data and a crop coefficient. A reasonable range of crop coefficients can be used to simulate runoff using historical or typical meteorological conditions (e.g., Typical Meteorological Year data) for a site considering a green roof application in any climate and utilizing different media and depth. Such a modeling approach to forecast impacts is more powerful and valuable than experiment-specific percent reductions in runoff typically reported. 4.3. Runoff quality
Fig. 5. Cumulative TN per area over the course of the study. Error bars represent the range of 3 replicate trays.
TN values measured in this study did not approach the literature range of 0.2–6.9 mg-N/L for several months, particularly for GAF (Bliss et al., 2009; Czemiel Berndtsson et al., 2009; Hathaway et al., 2008; Moran et al., 2005; Van Seters et al., 2009). This sustained loading is hypothesized to originate from nitrogen moving from non-available forms to a more labile form through slow breakdown of the media and subsequent flushing.
Please cite this article in press as: Harper, G.E., et al., Nine-month evaluation of runoff quality and quantity from an experiential green roof in Missouri, USA. Ecol. Eng. (2014), http://dx.doi.org/10.1016/j.ecoleng.2014.06.004
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Fig. 6. Median TP concentrations over time. Error bars represent the range of replicate trays.
TP concentrations in runoff from extensive green roofs are quite variable in the literature (Czemiel Berndtsson 2010). Teemusk and Mander (2007) reported concentrations ranging from 0.012 mg-P/ L to 0.09 mg-P/L for a 1–2 year old green roof. Others have reported higher concentrations of 0.23–0.45 mg-P/L (Van Seters et al., 2009), 0.6–1.5 mg-P/L (Moran et al., 2005), 0.31 mg-P/L (Czemiel Berndtsson et al., 2009), 0.6–1.4 mg/L (Hathaway et al., 2008) and 2–3 mg-P/L (Bliss et al., 2009). An initial flush of TP from a green roof planted with Sedum was 10.3 mg-P/L (Beck et al., 2011). The concentrations of TP observed from the Arkalyte media fit within the reported range. However, the TP measured in runoff from GAF media were much higher, likely due to excess P reported by the agronomic tests (Table 1). Plants reduced the amount of TP discharged in both media. Discharge of P most often occurs in natural systems from adsorption to soil grains that are then eroded away into a water body (Wendt and Alberts, 1984), suggesting the roots acted as a media stabilizer. Plant uptake of phosphate could also be reducing labile phosphate dissolving into the water. However, the differences remain throughout the winter, when the plants were dormant, supporting hypothesis of roots acting as a media stabilizer. Also of note are the effects of freeze-thaw, particularly
evident for GAF during the winter months, as TP concentrations in runoff notably increased. Initial TOC concentrations bracketed those of Beck et al. (2011); decreasing over time to less than 50 mg/L for all media, however no immediate substantial decrease after the “first flush” was observed. GAF contained higher TOC concentrations, despite containing less organic matter than Arkalyte (Table 1), although GAF contained more fines (Table 2). With TOC concentrations measured as high as 500 mg/L, GAF organic matter was more easily leached than that in the Arkalyte media. Previous literature has reported an initial flush of TSS of 120 mg/L for Arkalyte, decreasing to 10 mg/L after 15 watering events (Morgan et al., 2013). The observation of a first flush is likely dependent on rainfall intensity and the rooting barrier. TSS concentrations are expected to drop over time with TSS values reported less than 2.5 mg/L for a green roof 1–2 years old (Van Seters et al., 2009). TSS concentrations from wastewater treatment plants into water bodies are often set at limits of 20 mg/L (Tchobanoglous et al., 2003). TSS values generally remained below this standard, which shows the effectiveness of the rooting barriers designed to support and retain the growth media. The differences in TSS between the two media is likely due to the age of the media. Arkalyte’s one-year aging likely reduced the number of fines observed in the media (Table 2). The TSS increased for the
Fig. 7. Cumulative TP mass per area of green roof. Error bars denote the range of three replicate trays.
Fig. 8. Cumulative TSS in green roof runoff during the study. Error bars denote the range of three trays.
Please cite this article in press as: Harper, G.E., et al., Nine-month evaluation of runoff quality and quantity from an experiential green roof in Missouri, USA. Ecol. Eng. (2014), http://dx.doi.org/10.1016/j.ecoleng.2014.06.004
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unplanted condition supporting the hypothesis that plants reduce erosion during storm runoff and therefore reduce TP loading. Large variation in the empty control tray resulted from large storm events, especially with high winds, when some media from adjacent trays was observed to have blown into the control trays contributing to control tray TSS. Similar effects would be observed during large events on a full-scale, conventional roof, as nearby debris and leaves would be carried toward the roof drain. 5. Conclusions Green roofs have been championed as an effective resource for urban water management by reducing the stormwater loads reaching grey infrastructure and surface waters. Runoff reduction of over 60% for storms below 5 cm was shown and further support the ability of green roofs to reduce urban stormwater. However, the concentrations of TN and TP leaching out of new green roof media are a concern for water quality downstream, as excess nutrient loads have the potential to increase eutrophication risk for lakes and rivers. A substantial nutrient load was observed in runoff emanating from media tested, likely due to excess nutrients in the manufactured media. In addition, the organics dissolved in the green roof runoff can add to the total biological oxygen demand in the water. Such effects will not be observed on a watershed scale from any single roof, but with city policies encouraging green roofs to be incorporated into urban structures, all effects, both positive and negative, from this implementation must be considered. Altering the amount of organic matter, type of organic matter, and/or fertilizers used could all lead to a “greener” green roof. Alternatively, in areas where nutrient loads are particularly problematic, an alternative media without leachable nutrients but with sufficient water holding capacity may be able to provide much of the same engineering benefits without the risk of lowquality runoff. Acknowledgements The authors thank the USGS Water Center, Columbia Missouri for funding (Project Number: 2011MO122B). We also thank Kelly Luckett of Green Roof Blocks and Helene Hardy-Pierce of GAF for their personal insight and coordinating donation of materials for the study and full scale green roof. We also appreciate input on plant selection and cultivation by Vic Jost of Jost Greenhouses. The authors also greatly appreciate assistance with data collection and analysis by Lea Ahrens, Elise Kittrell, Honglan Shi and Amanda Holmes of Missouri S&T. References Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop Evapotranspiration– Guidelines for Computing Crop Water Requirements. Food and Agriculture Organization of the United Nations, Rome. Alsup, S., Ebbs, S., Retzlaff, W., 2010. The exchangeability and leachability of metals from select green roof growth substrates. Urban Ecosyst. 13, 91–111. American Public Health Association, 2012. Standard Methods for the Examination of Water and Wastewater, twenty-second ed. APHA. Beck, D.A., Johnson, G.R., Spolek, G.A., 2011. Amending greenroof soil with biochar to affect runoff water quantity and quality. Environ. Pollut. 159, 2111–2118. Blanusa, T., Monteiro, Vaz, Fantozzi, M.M., Vysini, F., Li, E., Cameron, Y., R.W.F, 2013. Alternatives to Sedum on green roofs: can broad leaf perennial plants offer better ‘cooling service'? Build. Environ. 59, 99–106.
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Please cite this article in press as: Harper, G.E., et al., Nine-month evaluation of runoff quality and quantity from an experiential green roof in Missouri, USA. Ecol. Eng. (2014), http://dx.doi.org/10.1016/j.ecoleng.2014.06.004