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Measurements of nutrients and mercury in green roof and gravel roof runoff
Ecological Engineering 73 (2014) 705–712
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Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Measurements of nutrients and mercury in green roof and gravel roof runoff Elizabeth G. Malcolm a, *, Margaret L. Reese b , Maynard H. Schaus c, Ivy M. Ozmon a , Lan M. Tran a a b c
Department of Earth and Environmental Sciences, Virginia Wesleyan College, 1584 Wesleyan Drive, Norfolk, VA 23502, USA Department of Mathematics and Computer Science, Virginia Wesleyan College, 1584 Wesleyan Drive, Norfolk, VA 23502, USA Department of Biology, Virginia Wesleyan College, 1584 Wesleyan Drive, Norfolk, VA 23502, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 October 2013 Received in revised form 15 August 2014 Accepted 7 September 2014 Available online xxx
Green roofs are a popular stormwater management technique because they are effective in reducing runoff volume from buildings. However, this does not necessarily result in a reduced pollutant load. Runoff from experimental gravel and green roof plots and two sets of real gravel and green roofs, were analyzed for mercury, nitrogen, phosphorus, and metals. The effects of roof type (green versus gravel) and underlayments (drainage layer, water-retention layer) on runoff volume, pollutant concentration, and pollutant load were evaluated. Although mercury concentrations in runoff were often higher from green roofs than gravel roofs, the reduction in runoff volume from green roofs typically resulted in no significant difference in runoff load. The green roofs also leached significantly higher concentrations of the nutrients phosphorus and nitrogen than the gravel roofs, but reduction in runoff volume did not similarly mitigate the nutrient load. Underlayment type had no significant effect on runoff volume, mercury, or phosphorus concentrations. Plots with a water-retention layer embedded with fertilizer initially leached higher nitrogen, but were similar to other green roof treatments two months following installation. Alum and Ultra-Phos Filter (UltraTech International, Inc.) were also tested for their potential to reduce the phosphorus in runoff. The average reduction in phosphorus in runoff directed through alum-filled pouches was 22%. Based on the elevated nitrogen and phosphorus load running off of the green roofs, caution is recommended when considering the application of fertilizer during installation or maintenance of green roofs. When choosing green building technology, materials and methods must be carefully chosen to match the most critical needs of the local environment. ã 2014 Elsevier B.V. All rights reserved.
Keywords: Green roof Vegetated roof Mercury Nitrogen Phosphorus Stormwater Runoff
1. Introduction The last decade has seen an increase in U.S. federal and state policies that promote the installation of “green” vegetated roofs (Carter and Fowler, 2008). The wide-ranging benefits of green roofs can include reduced stormwater runoff (Czemiel Berndtsson, 2010), reduced heat-island effects (Wong et al., 2003a; Kumar and Kaushik, 2005), uptake of atmospheric carbon dioxide (Getter et al., 2009; Rowe, 2011), and improved energy conservation (Del Barrio, 1998; Niachou et al., 2001; Wong et al., 2003a). Despite the high initial costs, green roofs can have a lower lifetime cost over conventional roofs by reducing energy costs associated with heating and cooling and extending roof life (Wong et al., 2003b). Of these benefits, the most apparent has been stormwater runoff
* Corresponding author. Tel.: +1 757 233 8751; fax: +1 757 466 8283. E-mail address: [email protected] (E.G. Malcolm). http://dx.doi.org/10.1016/j.ecoleng.2014.09.030 0925-8574/ ã 2014 Elsevier B.V. All rights reserved.
reduction, which can minimize risks from flooding, and prevent water pollution in areas with combined sewer overflows (VanWoert et al., 2005; Villarreal and Bengtsson, 2004, 2005). As such, vegetated roofs are increasingly used as a best management practice (BMP) in stormwater management. Despite the growing popularity of green roofs in the U.S. and the large number of impaired and sensitive watersheds, few studies on their water quality impact have been conducted in North America. Green roofs have the potential to reduce the load of atmospherically deposited pollutants by filtering the precipitation and reducing overall runoff volume. Additionally, the plants themselves may adsorb nutrients like nitrogen in precipitation. On the other hand, green roof components may leach compounds such as nutrients and metals increasing the concentration and load in runoff. Czemiel Berndtsson’s (2010) review of the water quality of green roof runoff indicates that a variety of pollutants can be leached from green roofs, the quantities of which can be impacted by the composition of green roof materials, fertilizer usage, and the
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age of the roof. For example, Czemiel Berndtsson et al. (2006) observed that green roofs were a moderate source of phosphorus, potassium, copper, and iron. Well-established green roofs (two years old) were sinks for nitrogen, but new green roofs and ones that were recently fertilized were sources of nitrogen in the runoff (Czemiel Berndtsson et al., 2006). Likewise, Teemusk and Mander (2007, 2011),) observed that green roofs can have higher concentrations of Ca, Mg, SO4, P and biochemical oxygen demand (BOD), compared to bituminous roofs and sod roofs. For metals, green roofs have been found to be a moderate source or sink depending on the individual roof and metal, which may result in a higher or lower load versus a conventional roof (Czemiel Berndtsson et al., 2006, 2008, 2009; Van Seters et al., 2009; Alsup et al., 2013). In the only published study of mercury from green roofs, Gregoire and Clausen (2011) found no significant differences in Hg concentrations between precipitation, green roof runoff, and concrete roof runoff. Since they estimated their green roof to retain approximately 50% of the incoming precipitation, their green roofs were a sink for Hg. This study examined the impact of green roofs on runoff water quality with regard to nitrogen, phosphorus, mercury, and other metals. Cultural eutrophication caused by excess nitrogen and phosphorus inputs has been identified as one of the primary environmental problems facing the Chesapeake Bay. Mercury is another pollutant of local and global concern. This toxic metal which is emitted by natural and anthropogenic sources to the atmosphere can be transported to surface waters where mercury accumulates in aquatic organisms. Experimental green roof plots were employed to specifically test the effects of roof type (green versus gravel), underlayments (drainage layer and water-retention layer), and the potential effects of chemicals (alum and Ultra-Phos Filter1 (UltraTech International, Inc.)) to mitigate effects of nutrient pollution. The results from our experimental plots were compared to runoff from full scale green and gravel roofs, two of which included an amendment (Ultra-Phos Filter) marketed for P retention. Underlayment types were investigated because there is no clear consensus regarding the use of underlayments and no previous studies had manipulated underlayment layers to determine whether there is an effect on runoff volume and runoff water quality. Likewise, the potential for chemical adsorbents to mitigate adverse effects on water quality was examined because these products have been utilized in other environmental applications to decrease phosphorus. This approach allowed for comparison of the results from small replicated plots with those from full scale roofs, which are typically unreplicated. 2. Methods 2.1. Experimental plots–Phase I Fifteen one-square-meter test roof plots were constructed on the Virginia Wesleyan College Campus, Norfolk, VA, USA on November 1, 2005. Five different roof configurations were used: green roof, green roof with a water-retention layer, green roof with a drainage layer, green roof with both water-retention and drainage layers, and conventional tar and gravel-covered builtup roof (n = 3) for each treatment (Fig. 1). Plots were constructed as five one-meter high tables that held three plots each, and were set at a 4% slope. Test plots were covered with a polymeric bitumen roofing membrane (Famogreen CU P4), which was attached to the table surface. This roofing membrane has a copper foil inlay that prevents root penetration. A water-retention underlayment, composed of recycled polyurethane foam embedded with clay and slow-release fertilizer (Famoflor Mats) was installed on six of the green roof plots. Six of the plots (including three plots where it was applied over the water-retention layer) received a drainage
Fig. 1. Schematic representation of the green roof configuration showing the drainage and water-retention underlayments.
layer (Enkadrain 3611R), consisting of hard plastic recycled polyethylene rings that provide drainage space below the growing medium. Plants were installed to twelve of the plots as fully established (“instant green”) vegetated pods; a pod consists of a 0.2 m2 flat containing several species of Sedum in a growing medium of 85% expanded slate and 15% compost (EnviroTech GR Vegetation Pod). Species included S. album,S. bithynicum, S. kamtschaticum, S. lineare, and S. spurium. The three gravel roof plots consisted of the same polymeric bitumen sheet with a copper inlay (Famogreen CU P4) covered with Henry #505 Flashmaster Rubber Modified Plastic Roof Cement and Kolor Scape Pea Pebbles. All roofing materials and plants are used commercially and were recommended by green roof company Building Logics, Inc., Virginia Beach, VA, USA. A drain hole with polyvinyl chloride piping was installed at the low end of each test plot to collect runoff from individual plots. Each drainage area was covered with polypropylene filter fabric (EnviroTech GR Filter Fabric) and surrounded by large loose rocks (Kolor Scape Egg Rocks). Runoff samples were collected from each of the test plots using clean 1 L teflon bottles for Hg or metals and using polypropylene bottles when sample collections were made solely for nutrients. On dates where both Hg and nutrient samples were collected, Teflon bottles were used and a subsample was poured into clean polyethylene or polypropylene containers for nutrient analyses. To measure the total amount of runoff, 19 L buckets were placed below the sample bottles to collect overflow, with polyvinyl chloride pipes serving as bottle holders. Holes were drilled in the lids of the buckets so that the runoff would initially fill the sample bottles and then overflow into the buckets. Exceptionally strong rain events caused the excess to overflow the buckets, making load incalculable for those events. Clean glass funnels and Teflon bottles were used to collect precipitation for Hg analysis. Clean polypropylene funnels and bottles were used for precipitation collections made solely for nitrogen and phosphorus. Runoff amount was determined gravimetrically. Rainfall and air temperature were measured by a Vantage Pro2 weather station on the campus of Virginia Wesleyan College. When the weather data were unavailable, data were obtained from the Norfolk Airport, located approximately 2 km from the sampling site. Mean air temperature is reported for the antecedent dry period, which would affect the dryness of the roofs before precipitation. 2.2. Experimental plots–Phase II For the second set of experiments, the roof plots were reconfigured to evaluate the efficacy of alum and the commercial product Ultra-Phos Filter (UltraTech International, Inc.) on
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reducing phosphorus concentrations from green-roof runoff. Alum was selected because it is known to remove phosphorus from wastewater and soil runoff (Dunne et al., 2012; Huang et al., 2012) and because in our pilot laboratory experiment more than 50% of total P was removed from rain water when poured through green roof growing substrate amended with alum. Ultra-Phos Filter was selected because it is used to bind P in other stormwater applications. The experimental roof plots were relocated to a more remote area on campus so that the original location could be restored as a decorative fountain. The plots were placed at least 6 meters from surrounding trees, and all drainage and retention underlayments used in previous experiments were removed. On March 12, 2009, the vegetated pods were rearranged among plots to distribute Sedum coverage evenly among green roof plots and to minimize any plot differences from Phase I. Treatment pouches were made by sewing together two layers of 15 cm 30 cm filter fabric. Two additional lines were sewn through the length of each pouch to divide it into three sections in order to keep any chemical treatments relatively evenly distributed throughout the pouch. Four pouches each received 150 g of Ultra-Phos Filter (UltraTech International Inc.), four received 100 g of alum each, and seven were sewn up empty for the four control green plots and the three gravel plots. The amendment amounts were chosen so that their theoretical binding capacity exceeded the quantity of total P expected to runoff over the experimental period. The treatments (Ultra-Phos Filter, alum, or no amendment) were randomly assigned to the green roof plots. Pouches were positioned directly over the drainage hole, with large loose rocks placed on top of the pouches. 2.3. Full size roofs The Smithdeal dormitory on the VWC campus was retrofit with an extensive green roof (10 cm growing medium depth) in October 2006. This two-story building has a T-shaped roof (415 m2) of flat concrete decking, with no nearby heavy traffic areas or trees taller than the roof. The design of the green roof layers was identical to the “plain green” treatment in our experimental plot study, except that two layers of roofing membrane were heat welded directly to the concrete decking, and that Sedum sp. plants were added to the growing medium as 2 cm diameter plugs planted approximately 21/m2. Thirteen varieties of Sedum were used which included the species S. album, S. kamtschaticum, S. lineare, and S. spurium. The roof has three sections, each of which had its own drain: one was planted with no fertilizer added, one with slow-release fertilizer initially added to the growing medium, and one with slow- release fertilizer plus an Ultra-Phos Filter filled pouch (1.5 m 1.5 m) placed around the drain. The pouch of Ultra-Phos Filter was made with the same materials as those of the plots, but with substantially more Ultra-Phos Filter per square centimeter and a hole in the middle to accommodate the drain. An adjoining gravel roof was also sampled. Runoff water was collected by pipette from inside the drains during rainstorms and therefore, is not a composite sample of the whole runoff event. The Miller Industries building is located at 500 Crawford St., Portsmouth, Virginia, USA, in a moderate- to high-traffic area with many multi-story buildings and few trees, none of which reached the height of the roofs. This building is comprised of one small twostory portion and a larger three-story portion. The lower two-story roof was converted from a gravel to a green roof during the summer of 2008. This smaller green roof (84 m2) has a gravel perimeter on all four sides. Unlike the rest of the Ultra-Phos Filter treated green roofs in this study, Ultra-Phos Filter (1.53 m3) was mixed directly into the growing media of the entire green roof. In all other aspects, the green roof was identical in design to the Smithdeal hall roof and was installed by the same company,
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Building Logics, Inc. The remainder of the building (three-story portion, 904 m2) was retained as a standard gravel roof. Runoff was sampled during rain events from the metal drain pipes that drained the green and gravel roofs. 2.4. Chemical analysis Clean techniques were used throughout sampling and analysis to prevent sample contamination. Sample bottles and glassware used for nutrient analyses were washed in 1.2 M HCl, then rinsed three times in deionized water, inverted and air dried on a drying rack. The teflon sample bottles and glass rain sampling funnels used for Hg and metal analyses were cleaned with 3.5 M HCl and heated to 70 C in a water bath for 4 h, then rinsed five times with deionized water, inverted, and dried in a HEPA filtered cabinet. Total phosphorus was analyzed spectrophotometrically using the molybdenum blue technique following digestion with potassium persulfate (Wetzel and Likens, 2000). A limited set of samples were filtered through a one micrometer pore size glass fiber filter and analyzed for soluble P using the molybdenum blue technique without sample digestion (Wetzel and Likens, 2000). Total nitrogen was analyzed using either the second derivative UV spectroscopy method (Crumpton et al., 1992) or using two point UV spectroscopy following persulfate digestion (APHA, 1995). Olsen (2008) found the results of these two methods to be equivalent in cases without interfering compounds, such as nitrite. The two point method was used to analyze all N samples collected after February 2006, as nearly all total N was found to be in the form of nitrate. Soluble nitrate samples were filtered and analyzed in the same manner but without persulfate digestion. Hg samples were oxidized with BrCl (1% solution) and refrigerated prior to analysis by cold vapor atomic fluorescence spectroscopy (EPA Standard Method 1631). The mean value of the Hg blanks (0.48 0.40 ng/L) was subtracted from all sample values and certified reference materials (ORMS, National Research Council, Canada) indicated a mean percent recovery of 95 15% total Hg. Analytical replicates were conducted on 39% of the samples, with a mean percent difference of 7.7% Hg concentration. Samples collected for metals (from the Phase I plots and Smithdeal hall roof on July 6, 2008) were acidified to pH less than two with nitric acid and were shipped to J.R. Reed Environmental Lab for analysis of Cu, Fe, Mg, Zn and K by inductively coupled plasma–atomic emission spectrometry (EPA Standard Method 2007). Due to low runoff volumes on some sampling dates, the full suite of chemical analyses was not possible for every rain event. 2.5. Statistics The experimental plots were set up as a completely randomized experiment. The standard gravel roof was used as a control treatment to study the effects on volume and water quality from runoff of green roofs as compared to that of gravel roofs. The green roof without additions was used as a control to study the effects of the three treatments that are used by the green roof industry as modifications that address local concerns for vegetated roofs. For the second part of the study, the additional layers were removed and chemicals to mitigate effects of nutrient pollution (alum and Ultra-Phos Filter) were added. Again, the gravel-type roof and the green roof without amendments served as control treatments in a completely randomized experiment. Each storm’s results were analyzed using analysis of variance (F-tests). Storm to storm, no statistically significant differences in runoff volume or pollutants were detected among the different green roof plot configurations. Subsequently, visualization tools and repeated measures analysis (using mixed models) was used to compare green roofs and gravel roofs (Khattree and Naik, 1999).
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by the green roofs. Rain storms of at least 10 mm produced runoff from all the green roof plots while three storms with rainfall ranging from 2.0 to 5.3 mm yielded runoff from all the gravel roofs but only some of the green roof plots. A storm of 1 mm rain only produced runoff from gravel plots. No difference in plant coverage was observed among green roof types, further indicating that the addition of the retention and drainage layer were unnecessary for our location.
Because it differed substantially from storm to storm, the rainfall amount (millimeter) is used as a covariate in all models. Consequently, effects over time were not statistically tested because of sample size restrictions. When comparing runoff quality of green roofs with gravel roofs, the augmented green roof plots in Phase I were set aside in order to simplify the repeatedmeasures analysis. 3. Results and discussion
3.1.1. Nutrients Total nitrogen was analyzed from ten rain events, from which two were also analyzed for soluble nitrogen (Table 1). On dates where both soluble and total N were analyzed, results indicated that a large percentage of N in samples was in the form of soluble nitrate. Concentrations of total nitrogen were high for green roof plots (mean values ranged from 1.0 mg/L to 19.4 mg/L; Table 1), whereas concentrations in the gravel roof runoff were typically less than 1.0 mg/L and in precipitation were usually near or below the detection limit (0.02 mg/L). The concentrations of N measured in the green treatments appeared to decrease over time from a mean of 19.4 mg/L in November 2005 to a mean of 4.5 mg/L in January 2006, with lower concentrations, ranging from 1.0 to 3.5 mg/L after that point (Table 1). This suggests that fertilizer was leaching from the newly installed green roofs. Although no fertilizer was added at the time of the plot construction, the pre-vegetated pods likely contained enough existing fertilizer to account for this leaching. The two green treatments that incorporated the retention layer
3.1. Experimental plots–Phase I The green roof significantly reduced stormwater runoff volume compared to the conventional gravel roof treatment in every analyzed rain event (Table 1). Overall, the amount of rain retained by the green roof plots increased with the amount of rainfall observed in the study period. The gravel roof plots retained about the same amount of rain from storm to storm even after accounting for the amount of rainfall. The retention layer that was specifically designed to increase retention did not significantly reduce runoff volume. The drainage layer which was designed to enhance the drainage of excess rainwater did not significantly increase runoff volume. The percentage of runoff reduced by green roof plots (as compared to gravel roof plots) varied from 45% to 99% (mean = 80%) and was greatest for the storms with the smallest rain depth. The lowest volume rain events in fact were not sampled because there was no runoff from the green roof plots, with all the rain retained
Table 1 Mean, standard error, and sample size of runoff volume, concentration and load of nitrogen and phosphorus from experimental roof plots. DL indicates below detection limit: 0.02 mg/L for N and 0.001 mg/L for P. Each section of this table is arranged in chronological order. Volume
11/17/2005
01/13/2006 03/06/2006
Green plots (mm)
2.7 (0.2) n = 12 8.8 (0.8) n=3 70% 10 5
6.5 (0.3) n = 12 11.9 (0.2) n=3 45% 11 7
14
11
Gravel plots (mm) Runoff reduction Precipitation (mm) Antecedent dry period (days) Mean air temp. ( C)
N concentration 11/17/2005
11/22/2005
04/17/ 2006
05/27/2006
0.03 (0.01) 1.7 (0.2) n = 12 n = 12 2.9 (0.3) n = 3 8.4 (2.7) n=3 99% 79% 3 11 3 1
0.2 (0.1) n=8 4.7 (0.2) n=3 96% 5 1
6
17
01/13/2006
04/04/ 2006
13 03/06/2006
04/17/2006
06/20/2006
07/06/2006
04/12/ 2007
07/11/2007
0.02 (0.01) 0.01 (0.004) n = 12 n = 12 3.9 (0.1) n = 3 3.7 (0.1) n = 3
0.003 (0.003) n=8 1.1 (0.3) n = 3
99.60% 5 8
99.80% 2 5
99.70% 1 5
0.3 (0.1) n = 12 6.4 (0.6) n=3 95% 7 5
3.7 (0.2) n = 12 13.7 (0.5) n=3 73% 14 1
20
23
23
7
28
10/06/2006
10/08/2006
04/12/2007
06/26/2007
07/11/2007
2.6 (0.2) n=9 1.1 (0.0) n=2
1.4 (0.3) n=7 0.5 (1.0) n=3 0.2
1.0 (0.1) n=5 0.3 (0.1) n=3
3.5 (0.5) 3.5 (0.5) n = 12 n = 12 0.3 (0.2) n = 3 0.3 (0.1) n = 3
(mg/L) Green plots Gravel plots Precipitation – N load (mg/m2) Green plots Gravel plots
19.4 (2.0) n = 12 0.1 (0.1) n = 3 DL
8.9 (1.3) 4.5 (1.4) n = 12 2.6 (0.7) n = 12 1.7 (0.4) n = 11 n=7 0.1 (0.1) n = 3 DL (0.1) n = 3 DL n = 3 0.5 (0.6) n=3 DL
53.1 (6.6) n = 12 DL (0.0) n = 3
53.1 (9.0) n = 12 DL (0.0) n = 3
27.9 (3.6) n = 12 DL n = 3
DL
0.5 (0.1) n=5 2.2 (0.7) n=2
P concentration (mg/L)
11/17/2005
11/22/2005
11/29/2005
01/13/2006
06/20/2006
07/06/2006
Green plots Gravel plots Precipitation – P load (mg/m2) Green plots Gravel plots
3.2 (0.2) n = 12 0.1 (0.007) n = 3 0.1
1.1 (0.028) n = 11 0.0 (0.002) n = 3
1.5 (0.03) n = 12 0.1 (0.03) n = 3
1.7 (0.03) n = 12 0.1 (0.02) n = 3 DL
1.0 (0.2) n = 2 0.2 (0.01) n = 3 DL
2.5 (0.08) n = 12 DL (0.01) n = 3 DL
11.3 (0.4) n = 12 0.7 (0.2) n = 2
0.04 (0.01) n = 2 0.6 (0.03) n = 3
51.2 (1.5) n = 12 0.3 (0.3) n = 3
8.7 (0.7) n = 12 0.9 (0.1) n = 3
Ratio N:P
11/17/2005
11/22/2005
01/13/2006
Green plots Gravel plots
6.1 1
8.4 33.3
2.6 Not calculated
DL
13.3 (2.2) n = 12 3.6 (1.3) n = 3
E.G. Malcolm et al. / Ecological Engineering 73 (2014) 705–712
had significantly higher runoff of N on the earliest two dates measured (November 17, 2005 and November 22, 2005). The retention layer had been laced with fertilizer to encourage the plants to put roots into the layer; however, later events had no detectable difference in N for the treatments with the retention layer and upon disassembly of the Phase I plots three years after installation, no root penetration was evident. Much of the fertilizer appears to be washing out and thus, is not being taken up by the plants. Overall, mean N concentration was significantly higher for green roof plots than for gravel roof plots even after accounting for rain amount. On November 17, 2005 and January 13, 2006, the concentration of N from the green roofs was so high that even the substantial reduction of runoff volume from the green roofs did not result in a lower load of N. One year later on April 12, 2007, however, the difference in concentration between green and gravel was not as great (about 90 times higher in 2006 compared with about three times higher in 2007) so that even though the concentration of total N from the green roofs was greater, the mean N load from the green roofs was less than the gravel roofs (Table 1). Thus, green roof plots as constructed in our study can be a source of nitrogen, especially when newly constructed or recently fertilized, but over time this source of N appears to be reduced. Similar findings have been found in previous studies, as green roofs typically are sources of N following installation, but over time the runoff of N is reduced and some studies indicated that green roofs can ultimately function as sinks for N (Czemiel Berndtsson et al., 2006; Teemusk and Mander, 2011). Overall, concentrations of total phosphorus in runoff from the green treatments were consistently much higher (1.0–3.2 mg/L) than that from gravel roofs (less than 0.2 mg/L), even after accounting for rain amount (Table 1). Concentrations did not differ among underlayment types. For half of the storms with a calculated P load, 10 to 180 times more P leached from the green than gravel roof plots. In only one of the analyzed storms (June 20, 2006), the reduction in runoff volume (99.8%) was enough to compensate for the higher concentration of P from the green roofs and to result in a lower load from the green roofs. This suggests that fertilized green roofs are a source of phosphorus, except for light storm events in which runoff volume is greatly reduced by the green roofs. Numerous other studies have observed that extensive green roofs can function as sources of P (e.g., Köhler et al., 2002; Czemiel Berndtsson et al., 2006; Bliss et al., 2009; Teemusk and Mander, 2011). Czemiel Berndtsson et al. (2009) observed that extensive green roofs function as sources of P, whereas intensive green roofs did not. Köhler et al. (2002) observed that over time this load of P decreases, presumably due to improved plant growth and flushing of a portion of the P present during installation. For our roof plots, P concentrations in runoff were still elevated more than three years after construction. In addition, N to P ratios were typically at or below the Redfield ratio, which could adversely impact downstream water quality (Smith, 1983). Thus, although fertilizer can result in faster plant growth (Rowe et al., 2006), it seems prudent to decrease fertilizer use during installation, in order to reduce the negative impacts on water quality during the first few years. 3.1.2. Mercury For each storm, the concentration of Hg in the runoff from the different green roof underlayments did not significantly differ from each other. Overall, the concentration of Hg in the runoff from the green roof plots was significantly higher than the concentration in the runoff from the gravel roof plots, after controlling for rain amount. However, the reduction in volume of runoff from a green roof typically compensated for the higher concentration; that is, the total load of Hg from the green roofs was similar to or less than
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Fig. 2. Mean mercury load (ng/m2) in runoff of the green (n = 3) and gravel (n = 3) roof plots as compared to incoming mercury in precipitation (n = 1).
that of the gravel roofs. Furthermore, the total mass of Hg estimated to have been deposited to the roof by rain was less than that of the runoff for all rain events sampled (Fig. 2). The mean reduction in precipitation load was 61% for gravel and 86% for the green roof plots. This finding suggests that both conventional and green roofs capture some of the Hg being input via wet deposition. This is somewhat analogous to a natural forest ecosystem: precipitation typically increases in Hg concentration after falling through a forest canopy, but then a significant fraction of the Hg is retained in the soil and forest system and not exported to streams (Scherbatskoy et al., 1998). In a forest the increase in Hg falling through the canopy is largely due to the washing off of aerosols that were dry-deposited to the leaves (Rea et al., 1996). This may also occur in a green roof system, which has a greater surface area than a conventional roof. Therefore findings from this study may not be representative of urban or industrial areas with high concentrations of gaseous and particulate Hg which would dry deposit to the roof and could then lead to higher loads running off of the roofs during precipitation events. Many questions remain about what will happen to the retained Hg in a green roof system. Will it re-vaporize, adsorb to the plants and growing medium, or wash out eventually in future storms? Over the three years of sampling in this study, no temporal trend was observed in concentrations of mercury in runoff from the green or gravel roof plots (Tables 2 and 4). 3.2. Phase II and full scale roofs 3.2.1. Nutrients The green roof plots had significantly higher total N and P concentrations compared to gravel roof plots, even after controlling for rainfall amount. The utilization of alum and Ultra-Phos Filter did not significantly reduce the concentration of N in runoff, but the alum amendment consistently reduced the P concentration (22.0% on average; Table 3, Fig. 3). These amendments may work to further reduce runoff P under other conditions, such as with a greater duration of exposure to the amendments or greater volume of amendment. It is possible that the porosity of the amendments prevented adequate runoff flow through the pouches, and therefore another design which forces water to flow through the amendments would improve removal rates. On roofs with drain
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Table 2 Mean, standard error, sample size of runoff concentrations and load of Hg from experimental roof plots. Hg Concentration (ng/L)
11/17/2005
03/06/2006
04/17/2006
07/06/2006
04/12/2007
07/11/2007
Green plots Gravel plots Precipitation – Hg load (ng/m2) Green plots Gravel plots
14.3 (1.3) n = 12 3.7 (0.04) n = 3 13.5
12.0 (3.3) n = 9 8.5 (0.1) n = 3 4
7.0 (2.4) n = 10 5.9 (0.9) n = 3 6.3
20.0 (0.9) n = 12 5.5 (0.9) n = 3
11.9 (2.7) n = 11 3.6 (0.3) n = 3*
19.0 (1.4) n = 12 15.3 (5.2) n = 3 14.7
37.7 (3.2) n = 12 32.5 (3.1) n = 3
0.6 (0.2) n = 9 24.7 (1.8) n = 3
2.0(0.8) n = 10 27.8(4.3) n = 3
5.0(1.5) n = 11 23.2 (3.8) n = 3
70.6 (7.4) n = 12 209.9 (74.2) n = 3
Table 3 Mean and standard error of concentrations in runoff from Phase II, where n is the number of roofs or roof plots sampled. DL indicates a value below detection limit: 0.02 mg/L for N and 0.001 mg/L for P. All green roof plot treatments (i.e., alum, Ultra-Phos Filter, and control) are included in means for greens plots. Phosphorus (mg/L) Green plots Gravel plots Green roofs Gravel roofs Precipitation
10/24/2007
0.59 (n = 1) DL (n = 1) DL (n = 1)
Nitrogen (mg/L) Green plots Gravel plots Green roofs Gravel roofs Precipitation
07/06/2008
0.65 (n = 1) 0.08 (n = 1) DL (n = 1)
10/24/2007
2.7 (n = 1) DL (n = 1) DL (n = 1)
03/01/2009
0.37 (n = 1) 0.05 (n = 1)
03/01/2009
3.33 (n = 1) 0.18 (n = 1)
Mercury (ng/L) Green plots Gravel plots Green roofs Gravel roofs Precipitation
03/16/2009
03/26/2009
1.36 (0.09) n = 12 0.10 (0.02) n = 2 0.33 (0.04) n = 2 0.06 (0.01) n = 2 0.03 (n = 2)
03/16/2009
0.70 (n = 1) 0.61 (n = 1) 0.05 (n = 1)
03/26/2009
2.21 (0.27) n = 12 0.77 (0.33) n = 3 1.37 (n = 2) 0.35 (n = 2) 0.29 (n = 2)
03/01/2009
03/16/2009
6.9 (n = 2) 19.7 (n = 2) 4.0 (n = 1)
21.1 (1.7) n = 12 9.4 (2.8) n = 3 6.5 (n = 2) 6.7 (n = 2) 3.7 (n = 1)
pipes, this may be achieved by modifying commercially available downspout filters, which are currently sold to reduce hydrocarbons from tar roof runoff (McGregor, 2014). Alum has been used successfully to bind to P in many applications, such as in wetland soils, treatment wetlands, and sludge amended soil resulting in measurable reductions in water column P concentrations (i.e., Hoge et al., 2003; Dunne et al., 2012; Huang et al., 2012). Thus adding alum directly to green roof soils may reduce P in runoff, but would require further investigation to determine the optimum application rates to maintain the required levels of phyto-available P. Ultra-Phos Filter additions over Phase II drains, surrounding one of the drains on Smithdeal hall, and as a soil amendment on the Earl Industries roof did not demonstrate any clear reduction of P in runoff. Since our experiment, the manufacturer of Ultra-Phos Filter has found that incorporating zeolite into the Ultra-Phos Filter
04/12/2009
04/15/2009
1.21 (0.11) n = 8 0.10 (0.02) n = 3
1.45 (0.08) n = 9 0.05 (0.003) n = 3
0.0171 (n = 1)
0.006 (n = 1)
04/12/2009
04/15/2009
1.33 (0.55) n = 11 DL (0.02) n = 3
0.46 (0.09) n = 10 DL (0.00) n = 3
DL (n = 1)
DL (n = 1)
03/26/2009
11/11/2009
42.2 (n = 1) 39.9 (n = 1)
5.8 (n = 2) 5.2 (n = 2) 7.6 (n = 1)
improves porosity and removes excess iron which is released when the Ultra-Phos Filter takes up the phosphorus (McGregor, 2014). P runoff from full scale roofs was substantially lower than concentrations in runoff from Phases I and II plots (Tables 1 and 3).
Table 4 Mean, standard error, sample size of runoff concentrations of metals and phosphorus collected on July 6, 2008. DL indicates below detection limit (0.005 mg/L for Cu and 0.005 mg/L for Mg).
Green plots (n = 12) Gravel plots (n = 3) Green roof (3 drains) Gravel roof (n = 1) Precipitation (n = 1)
Copper (mg/L)
Iron (mg/L)
Manganese (mg/L)
0.015 (0.003) 0.002 (0.002) 0.024 (0.002) 0.369
0.061 (0.011) 0.031 (0.003) 0.68 (0.08) 0.097
DL (0.0)
DL
0.072
Zinc (mg/L)
Potassium (mg/L)
Phosphorus (mg/L)
0.040 (0.007) 0.006 0.027 (0.001) (0.003) 0.02 (0.001) 0.037 (0.003) 0.015 0.034
1.301 (0.331) 0.484 (0.076) 11.8 (1.2)
2.539 (0.077) 0.005 (0.019) 0.643 (0.067)
DL
0.125
0.048
1.07 Fig. 3. Concentration of total phosphorus in runoff from the experimental roof plots during Phase II. The roof plots modeled three types of green roof (control, control plus alum filter around drain, and control plus Ultra-Phos Filter around drain) as well as traditional gravel roofs.
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This may be because stormwater on full scale roofs came in contact with more of the green roof surface prior to runoff, and/or because any “first flush” effect is missed by our mid-storm sampling from the real roofs. Previous studies vary dramatically in the concentrations of P in runoff (Czemiel Berndtsson, 2010). P concentrations from our roof plots were similar to those observed by Hathaway et al. (2008) and Bliss et al. (2009), and our full scale roof measures were only slightly higher than the extensive roofs studied by Czemiel Berndtsson et al. (2009). This study adds to the number of studies that have demonstrated substantial runoff of P from green roofs (Köhler et al., 2002; Czemiel Berndtsson et al., 2006; Hathaway et al., 2008; Bliss et al., 2009; Teemusk and Mander, 2011). Thus, a crucial challenge for future green roof applications is to decrease the P in runoff without impacting the positive benefits of green roofs. 3.2.2. Mercury A runoff sample was collected for Hg analysis from all plots and real roofs for the rain event of March 13–16, 2009 (Table 3). No consistent pattern emerged across roofs when comparing green and gravel runoff. Runoff from the green roof experimental plots had the highest concentrations of Hg compared to all other roof types. Unlike the green roof plots, for which Hg concentrations were comparable or higher from green than gravel roofs, Hg concentrations were higher in runoff from the Earl Industries Building gravel roof for two of the four sampling days (8.2 and 3.7 higher). On the other two days, green and gravel concentrations were similar. This building is located closer to industrial areas, including shipyards and dry-docks, than our other sampling sites. Although volume of runoff was not measured from this building, the reduction in runoff volume coupled with comparable or lower concentrations from the green roof would produce lower runoff of Hg load from the green roof over gravel for this building. 3.3. Metals Runoff samples from the Smithdeal dormitory roof and the Phase I roof plots along with precipitation collected on July 6, 2008 were analyzed for metals and phosphorus. Although only one stormwater runoff event was sampled for metals, some interesting results emerged. Copper, iron, zinc, and potassium concentrations were higher from green than gravel roof plots, which is consistent with the measurements for N, P, and Hg. Potassium, iron, and manganese were all higher on average in runoff from the Smithdeal green roof than from any other category of samples (Table 4). This was especially apparent for potassium, which was about ten times more concentrated in the Smithdeal green roof runoff than in the Smithdeal gravel runoff or the green roof plot runoff. This finding is in contrast to that of P, N and Hg for which the green roof plots had higher concentrations than the Smithdeal green roof. Copper was ten times higher in the Smithdeal gravel runoff sample than in any other sample, which may reflect differences in building materials. It is notable that samples from the green and gravel roof plots, constructed with a bitumen layer containing copper film each had 8% or less of the Cu content of the Smithdeal gravel runoff. The real gravel and green roofs on Smithdeal had copper flashing, which may have contributed to runoff concentrations. Zinc, in contrast, showed similar concentrations across all roof types and precipitation. These results are in accordance with the finding of other researchers in that the diversity of roof construction materials can result in incongruous patterns for different metals in runoff. Thus, additional research examining the effect of different building materials and conditions on green roof runoff for toxic metals, such as copper, is warranted.
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4. Conclusion A growing body of literature has examined the potential impact of different types of green roofs on water quality. Previously it was assumed that reduced runoff volume would cause reduced runoff of total pollutants. In some cases, this can be true, such as if stormwater-retention prevents combined sewer overflows (VanWoert et al., 2005; Villarreal and Bengtsson, 2004, 2005), but it is not always the case (Czemiel Berndtsson, 2010). This study also confirmed that nutrient concentrations can be elevated in runoff from green roofs, and the degree to which this can impact downstream systems may depend on the age of the roof, the roof materials, and fertilizer application to the roof. These results are significant for watersheds sensitive to nutrient additions. The special layers for water-retention and drainage offered no significant benefit to runoff volume over the green treatment without these layers. No significant differences were observed between any of these different underlayment treatments, except for elevated N leached from the foam-retention layer during the first two months following installation; thus their use does not seem warranted, at least for the mid-Atlantic area. The chosen method of application of Ultra-Phos Filter did not reduce P in runoff but may work in a different configuration. The chosen method of application of alum reduced P by approximately 20% in runoff. Results of this study indicate that green roofs can moderately reduce Hg runoff by reducing the volume of runoff. Green roofs thus have the potential to reduce the Hg load to watersheds such as the Chesapeake Bay. Reduction of Hg sources to watersheds has become a priority in many states which have consumption advisories for fish due to Hg high concentrations. Results of this study indicate that if green roofs are not deliberately designed and managed, the benefits of green roofs can be diminished by their moderate negative impacts on water quality. An important challenge for sustainability proponents is to optimize green roof design to maximize the positive benefits, without leading to excessive runoff of nutrients and other pollutants in the process. Acknowledgments Funding was provided by the U.S. EPA P3 Program (EPA grant number G5Z70199), Jerry Miller, the VWC Science Undergraduate Research Fund, and a VFIC Mednik Foundation Grant. Funding was provided for the construction of the Smithdeal green roof by the Virginia Department of Conservation and Recreation. Special thanks to VWC students including: John Maravich, Wanda Morris, MariCarmen Korngiebel-Rosique, the VWC Environmental Chemistry, General Ecology, Statistics, and Environmental Biology Classes, as well as VWC Physical plant, VWC Security officers, and Earl Industries. Michael Perry of Building Logics, Inc. was instrumental in the design and installation of the test plots and the Smithdeal hall green roof. Ultra-Phos Filter was provided by UltraTech International. References Alsup, S.E., Ebbs, S.D., Battaglia, L.L., Retzlaff, W.A., 2013. Green roof systems as sources or sinks influencing heavy metal concentrations in runoff. J. Environ. Eng. 139, 502–508. APHA, 1995. Standard Methods for the Examination of Water and Wastewater, 19th Edition American Public Health Association, American Water Works Association, Water Environment Federation, Washington. Bliss, D.J., Neufeld, R.D., Ries, R.J., 2009. Storm water runoff mitigation using a green roof. Environ. Eng. Sci. 26, 407–417. Carter, T., Fowler, L., 2008. Establishing green roof infrastructure through environmental policy instruments. Environ. Manage. 42, 151–164. Crumpton, W.G., Isenhart, T.M., Mitchell, P.D., 1992. Nitrate and organic N analyses with 2nd-derivitave spectroscopy. Limnol. Oceanogr. 37, 907–913.
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Czemiel Berndtsson, J., Emilsson, T., Bengtsson, L., 2006. The influence of extensive vegetated roofs on runoff water quality. Sci. Total Environ. 355, 48–63. Czemiel Berndtsson, J., Bengtsson, L., Jinno, K., 2008. First flush effect from vegetated roofs during simulated rain events. Hydrol. Res. 39, 171–179. Czemiel Berndtsson, J., Bengtsson, L., Jinno, K., 2009. Runoff water quality from intensive and extensive vegetated roofs. Ecol. Eng. 35, 369–380. Czemiel Berndtsson, J., 2010. Green roof performance towards management of runoff water quantity and quality: a review. Ecol. Eng. 36, 351–360. Del Barrio, E.P., 1998. Analysis of the green roofs cooling potential in buildings. Energy Build. 27, 179–193. Dunne, E.J., Coveney, M., Marzolf, E.R., Hoge, V.R., Conrow, R., Naleway, R., Lowe, E.F., Battoe, L.E., 2012. Efficacy of a large-scale constructed wetland to remove phosphorus and suspended solids from Lake Apopka, Florida. Ecol. Eng. 42, 90–100. Getter, K.L., Rowe, D.B., Robertson, G.P., Cregg, B.M., Andresen, J.A., 2009. Carbon sequestration potential of extensive green roofs. Env. Sci. Technol. 43, 7564–7570. 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. Trans. Am. Soc. Agric. Biol. Eng. 51, 37–44. Hoge, V.R., Conrow, R., Coveney, M., Peterson, J., 2003. The application of alum residual as a phosphorus abatement tool within the Lake Apopka restoration area. Proceedings of the Water Environment Federation, WEF/AWWA/CWEA Joint Residuals and Biosolids Management Conference, 2000 1500–1513. Huang, X., Chen, Y., Shenker, M., 2012. Dynamics of phosphorus phytoavailability in soil amended with stabilized sewage sludge materials. Geoderma 170, 144–153. Köhler, M., Schmidt, M., Grimme, F.W., Laar, M., de Assuncão Paiva, V.L., Tavares, S., 2002. Green roofs in temperate climates and in the hot–humid tropics–far beyond the aesthetics. Environ. Manage. Health 13, 382–391. Khattree, R., Naik, D.N., 1999. Applied Multivariate Statistics with SAS Software, second ed. SAS Institute Inc., Cary, NC, USA. Kumar, R., Kaushik, S.C., 2005. Performance evaluation of green roof and shading for thermal protection of buildings. Build. Environ. 40, 1505–1511. McGregor, M., Research and Development Technician, Engineering, UltraTech International, Inc., personal communication, June 10, 2014.
Niachou, A., Papakonstantinou, K., Santamouris, M., Tsangrassoulis, A., Mihalakakou, G., 2001. Analysis of the green roof thermal properties and investigation of its energy performance. Energy Build. 33, 719–729. Olsen, K.K., 2008. Multiple wavelength ultraviolet determinations of nitrate concentration method comparisons from the Preakness Brook monitoring project, Oct. 2005–Oct. 2006. Water Air Soil Pollut. 187, 195–202. Rea, A.W., Lindberg, S.E., Keeler, G.J., 1996. Dry deposition and foliar leaching of mercury and selected trace elements in deciduous throughfall. Atmos. Environ. 30, 3453–3462. Rowe, D.B., Monterusso, M.A., Rugh, C.L., 2006. Assessment of heat-expanded slate and fertility requirements in green roof substrates. HortTechnology 16, 471–477. Rowe, D.B., 2011. Green roofs as a means of pollution abatement. Environ. Pollut. 159, 2100–2110. Scherbatskoy, T., Shanley, J.B., Keeler, G.J., 1998. Factors controlling mercury transport in an upland forested catchment. Water Air Soil Pollut. 105, 427–438. Smith, V.H., 1983. Low nitrogen to phosphorus ratios favor dominance by blue–green algae in Lake Phytoplankton. Science 221, 669–671. Teemusk, A., Mander, Ü., 2007. Rainwater runoff quantity and quality performance from a greenroof: the effects of short-term events. Ecol. Eng. 30, 271–277. Teemusk, A., Mander, Ü., 2011. The influence of green roofs on runoff water quality: a case study from Estonia. Water Resour. Manage. 25, 3699–3713. 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. VanWoert, N.D., Rowe, D.B., Andresen, J.A., Rugh, C.L., Fernandez, R.T., Xiao, L., 2005. Green roof stormwater retention: effects of roof surface, slope, and media depth. J. Environ. Qual. 34, 1036–1044. Villarreal, E., Bengtsson, A.S.D.L., 2004. Inner city stormwater control using a combination of best management practices. Ecol. Eng. 22, 279–298. Villarreal, E.L., Bengtsson, L., 2005. Response of a Sedum green-roof to individual rain events. Ecol. Eng. 25, 1–7. Wetzel, R.W., Likens, G.E., 2000. Limnological Analysis, third ed. Springer, New York. Wong, N.H., Chen, Y., Ong, C.L., Sia, A., 2003a. Investigation of thermal benefits of rooftop garden in the tropical environment. Build. Environ. 38, 261–270. Wong, N.H., Tay, S.F., Wong, R., Ong, C.L., Sia, A., 2003b. Life cycle cost analysis of rooftop gardens in Singapore. Build. Environ. 38, 499–509.
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Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Measurements of nutrients and mercury in green roof and gravel roof runoff Elizabeth G. Malcolm a, *, Margaret L. Reese b , Maynard H. Schaus c, Ivy M. Ozmon a , Lan M. Tran a a b c
Department of Earth and Environmental Sciences, Virginia Wesleyan College, 1584 Wesleyan Drive, Norfolk, VA 23502, USA Department of Mathematics and Computer Science, Virginia Wesleyan College, 1584 Wesleyan Drive, Norfolk, VA 23502, USA Department of Biology, Virginia Wesleyan College, 1584 Wesleyan Drive, Norfolk, VA 23502, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 October 2013 Received in revised form 15 August 2014 Accepted 7 September 2014 Available online xxx
Green roofs are a popular stormwater management technique because they are effective in reducing runoff volume from buildings. However, this does not necessarily result in a reduced pollutant load. Runoff from experimental gravel and green roof plots and two sets of real gravel and green roofs, were analyzed for mercury, nitrogen, phosphorus, and metals. The effects of roof type (green versus gravel) and underlayments (drainage layer, water-retention layer) on runoff volume, pollutant concentration, and pollutant load were evaluated. Although mercury concentrations in runoff were often higher from green roofs than gravel roofs, the reduction in runoff volume from green roofs typically resulted in no significant difference in runoff load. The green roofs also leached significantly higher concentrations of the nutrients phosphorus and nitrogen than the gravel roofs, but reduction in runoff volume did not similarly mitigate the nutrient load. Underlayment type had no significant effect on runoff volume, mercury, or phosphorus concentrations. Plots with a water-retention layer embedded with fertilizer initially leached higher nitrogen, but were similar to other green roof treatments two months following installation. Alum and Ultra-Phos Filter (UltraTech International, Inc.) were also tested for their potential to reduce the phosphorus in runoff. The average reduction in phosphorus in runoff directed through alum-filled pouches was 22%. Based on the elevated nitrogen and phosphorus load running off of the green roofs, caution is recommended when considering the application of fertilizer during installation or maintenance of green roofs. When choosing green building technology, materials and methods must be carefully chosen to match the most critical needs of the local environment. ã 2014 Elsevier B.V. All rights reserved.
Keywords: Green roof Vegetated roof Mercury Nitrogen Phosphorus Stormwater Runoff
1. Introduction The last decade has seen an increase in U.S. federal and state policies that promote the installation of “green” vegetated roofs (Carter and Fowler, 2008). The wide-ranging benefits of green roofs can include reduced stormwater runoff (Czemiel Berndtsson, 2010), reduced heat-island effects (Wong et al., 2003a; Kumar and Kaushik, 2005), uptake of atmospheric carbon dioxide (Getter et al., 2009; Rowe, 2011), and improved energy conservation (Del Barrio, 1998; Niachou et al., 2001; Wong et al., 2003a). Despite the high initial costs, green roofs can have a lower lifetime cost over conventional roofs by reducing energy costs associated with heating and cooling and extending roof life (Wong et al., 2003b). Of these benefits, the most apparent has been stormwater runoff
* Corresponding author. Tel.: +1 757 233 8751; fax: +1 757 466 8283. E-mail address: [email protected] (E.G. Malcolm). http://dx.doi.org/10.1016/j.ecoleng.2014.09.030 0925-8574/ ã 2014 Elsevier B.V. All rights reserved.
reduction, which can minimize risks from flooding, and prevent water pollution in areas with combined sewer overflows (VanWoert et al., 2005; Villarreal and Bengtsson, 2004, 2005). As such, vegetated roofs are increasingly used as a best management practice (BMP) in stormwater management. Despite the growing popularity of green roofs in the U.S. and the large number of impaired and sensitive watersheds, few studies on their water quality impact have been conducted in North America. Green roofs have the potential to reduce the load of atmospherically deposited pollutants by filtering the precipitation and reducing overall runoff volume. Additionally, the plants themselves may adsorb nutrients like nitrogen in precipitation. On the other hand, green roof components may leach compounds such as nutrients and metals increasing the concentration and load in runoff. Czemiel Berndtsson’s (2010) review of the water quality of green roof runoff indicates that a variety of pollutants can be leached from green roofs, the quantities of which can be impacted by the composition of green roof materials, fertilizer usage, and the
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age of the roof. For example, Czemiel Berndtsson et al. (2006) observed that green roofs were a moderate source of phosphorus, potassium, copper, and iron. Well-established green roofs (two years old) were sinks for nitrogen, but new green roofs and ones that were recently fertilized were sources of nitrogen in the runoff (Czemiel Berndtsson et al., 2006). Likewise, Teemusk and Mander (2007, 2011),) observed that green roofs can have higher concentrations of Ca, Mg, SO4, P and biochemical oxygen demand (BOD), compared to bituminous roofs and sod roofs. For metals, green roofs have been found to be a moderate source or sink depending on the individual roof and metal, which may result in a higher or lower load versus a conventional roof (Czemiel Berndtsson et al., 2006, 2008, 2009; Van Seters et al., 2009; Alsup et al., 2013). In the only published study of mercury from green roofs, Gregoire and Clausen (2011) found no significant differences in Hg concentrations between precipitation, green roof runoff, and concrete roof runoff. Since they estimated their green roof to retain approximately 50% of the incoming precipitation, their green roofs were a sink for Hg. This study examined the impact of green roofs on runoff water quality with regard to nitrogen, phosphorus, mercury, and other metals. Cultural eutrophication caused by excess nitrogen and phosphorus inputs has been identified as one of the primary environmental problems facing the Chesapeake Bay. Mercury is another pollutant of local and global concern. This toxic metal which is emitted by natural and anthropogenic sources to the atmosphere can be transported to surface waters where mercury accumulates in aquatic organisms. Experimental green roof plots were employed to specifically test the effects of roof type (green versus gravel), underlayments (drainage layer and water-retention layer), and the potential effects of chemicals (alum and Ultra-Phos Filter1 (UltraTech International, Inc.)) to mitigate effects of nutrient pollution. The results from our experimental plots were compared to runoff from full scale green and gravel roofs, two of which included an amendment (Ultra-Phos Filter) marketed for P retention. Underlayment types were investigated because there is no clear consensus regarding the use of underlayments and no previous studies had manipulated underlayment layers to determine whether there is an effect on runoff volume and runoff water quality. Likewise, the potential for chemical adsorbents to mitigate adverse effects on water quality was examined because these products have been utilized in other environmental applications to decrease phosphorus. This approach allowed for comparison of the results from small replicated plots with those from full scale roofs, which are typically unreplicated. 2. Methods 2.1. Experimental plots–Phase I Fifteen one-square-meter test roof plots were constructed on the Virginia Wesleyan College Campus, Norfolk, VA, USA on November 1, 2005. Five different roof configurations were used: green roof, green roof with a water-retention layer, green roof with a drainage layer, green roof with both water-retention and drainage layers, and conventional tar and gravel-covered builtup roof (n = 3) for each treatment (Fig. 1). Plots were constructed as five one-meter high tables that held three plots each, and were set at a 4% slope. Test plots were covered with a polymeric bitumen roofing membrane (Famogreen CU P4), which was attached to the table surface. This roofing membrane has a copper foil inlay that prevents root penetration. A water-retention underlayment, composed of recycled polyurethane foam embedded with clay and slow-release fertilizer (Famoflor Mats) was installed on six of the green roof plots. Six of the plots (including three plots where it was applied over the water-retention layer) received a drainage
Fig. 1. Schematic representation of the green roof configuration showing the drainage and water-retention underlayments.
layer (Enkadrain 3611R), consisting of hard plastic recycled polyethylene rings that provide drainage space below the growing medium. Plants were installed to twelve of the plots as fully established (“instant green”) vegetated pods; a pod consists of a 0.2 m2 flat containing several species of Sedum in a growing medium of 85% expanded slate and 15% compost (EnviroTech GR Vegetation Pod). Species included S. album,S. bithynicum, S. kamtschaticum, S. lineare, and S. spurium. The three gravel roof plots consisted of the same polymeric bitumen sheet with a copper inlay (Famogreen CU P4) covered with Henry #505 Flashmaster Rubber Modified Plastic Roof Cement and Kolor Scape Pea Pebbles. All roofing materials and plants are used commercially and were recommended by green roof company Building Logics, Inc., Virginia Beach, VA, USA. A drain hole with polyvinyl chloride piping was installed at the low end of each test plot to collect runoff from individual plots. Each drainage area was covered with polypropylene filter fabric (EnviroTech GR Filter Fabric) and surrounded by large loose rocks (Kolor Scape Egg Rocks). Runoff samples were collected from each of the test plots using clean 1 L teflon bottles for Hg or metals and using polypropylene bottles when sample collections were made solely for nutrients. On dates where both Hg and nutrient samples were collected, Teflon bottles were used and a subsample was poured into clean polyethylene or polypropylene containers for nutrient analyses. To measure the total amount of runoff, 19 L buckets were placed below the sample bottles to collect overflow, with polyvinyl chloride pipes serving as bottle holders. Holes were drilled in the lids of the buckets so that the runoff would initially fill the sample bottles and then overflow into the buckets. Exceptionally strong rain events caused the excess to overflow the buckets, making load incalculable for those events. Clean glass funnels and Teflon bottles were used to collect precipitation for Hg analysis. Clean polypropylene funnels and bottles were used for precipitation collections made solely for nitrogen and phosphorus. Runoff amount was determined gravimetrically. Rainfall and air temperature were measured by a Vantage Pro2 weather station on the campus of Virginia Wesleyan College. When the weather data were unavailable, data were obtained from the Norfolk Airport, located approximately 2 km from the sampling site. Mean air temperature is reported for the antecedent dry period, which would affect the dryness of the roofs before precipitation. 2.2. Experimental plots–Phase II For the second set of experiments, the roof plots were reconfigured to evaluate the efficacy of alum and the commercial product Ultra-Phos Filter (UltraTech International, Inc.) on
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reducing phosphorus concentrations from green-roof runoff. Alum was selected because it is known to remove phosphorus from wastewater and soil runoff (Dunne et al., 2012; Huang et al., 2012) and because in our pilot laboratory experiment more than 50% of total P was removed from rain water when poured through green roof growing substrate amended with alum. Ultra-Phos Filter was selected because it is used to bind P in other stormwater applications. The experimental roof plots were relocated to a more remote area on campus so that the original location could be restored as a decorative fountain. The plots were placed at least 6 meters from surrounding trees, and all drainage and retention underlayments used in previous experiments were removed. On March 12, 2009, the vegetated pods were rearranged among plots to distribute Sedum coverage evenly among green roof plots and to minimize any plot differences from Phase I. Treatment pouches were made by sewing together two layers of 15 cm 30 cm filter fabric. Two additional lines were sewn through the length of each pouch to divide it into three sections in order to keep any chemical treatments relatively evenly distributed throughout the pouch. Four pouches each received 150 g of Ultra-Phos Filter (UltraTech International Inc.), four received 100 g of alum each, and seven were sewn up empty for the four control green plots and the three gravel plots. The amendment amounts were chosen so that their theoretical binding capacity exceeded the quantity of total P expected to runoff over the experimental period. The treatments (Ultra-Phos Filter, alum, or no amendment) were randomly assigned to the green roof plots. Pouches were positioned directly over the drainage hole, with large loose rocks placed on top of the pouches. 2.3. Full size roofs The Smithdeal dormitory on the VWC campus was retrofit with an extensive green roof (10 cm growing medium depth) in October 2006. This two-story building has a T-shaped roof (415 m2) of flat concrete decking, with no nearby heavy traffic areas or trees taller than the roof. The design of the green roof layers was identical to the “plain green” treatment in our experimental plot study, except that two layers of roofing membrane were heat welded directly to the concrete decking, and that Sedum sp. plants were added to the growing medium as 2 cm diameter plugs planted approximately 21/m2. Thirteen varieties of Sedum were used which included the species S. album, S. kamtschaticum, S. lineare, and S. spurium. The roof has three sections, each of which had its own drain: one was planted with no fertilizer added, one with slow-release fertilizer initially added to the growing medium, and one with slow- release fertilizer plus an Ultra-Phos Filter filled pouch (1.5 m 1.5 m) placed around the drain. The pouch of Ultra-Phos Filter was made with the same materials as those of the plots, but with substantially more Ultra-Phos Filter per square centimeter and a hole in the middle to accommodate the drain. An adjoining gravel roof was also sampled. Runoff water was collected by pipette from inside the drains during rainstorms and therefore, is not a composite sample of the whole runoff event. The Miller Industries building is located at 500 Crawford St., Portsmouth, Virginia, USA, in a moderate- to high-traffic area with many multi-story buildings and few trees, none of which reached the height of the roofs. This building is comprised of one small twostory portion and a larger three-story portion. The lower two-story roof was converted from a gravel to a green roof during the summer of 2008. This smaller green roof (84 m2) has a gravel perimeter on all four sides. Unlike the rest of the Ultra-Phos Filter treated green roofs in this study, Ultra-Phos Filter (1.53 m3) was mixed directly into the growing media of the entire green roof. In all other aspects, the green roof was identical in design to the Smithdeal hall roof and was installed by the same company,
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Building Logics, Inc. The remainder of the building (three-story portion, 904 m2) was retained as a standard gravel roof. Runoff was sampled during rain events from the metal drain pipes that drained the green and gravel roofs. 2.4. Chemical analysis Clean techniques were used throughout sampling and analysis to prevent sample contamination. Sample bottles and glassware used for nutrient analyses were washed in 1.2 M HCl, then rinsed three times in deionized water, inverted and air dried on a drying rack. The teflon sample bottles and glass rain sampling funnels used for Hg and metal analyses were cleaned with 3.5 M HCl and heated to 70 C in a water bath for 4 h, then rinsed five times with deionized water, inverted, and dried in a HEPA filtered cabinet. Total phosphorus was analyzed spectrophotometrically using the molybdenum blue technique following digestion with potassium persulfate (Wetzel and Likens, 2000). A limited set of samples were filtered through a one micrometer pore size glass fiber filter and analyzed for soluble P using the molybdenum blue technique without sample digestion (Wetzel and Likens, 2000). Total nitrogen was analyzed using either the second derivative UV spectroscopy method (Crumpton et al., 1992) or using two point UV spectroscopy following persulfate digestion (APHA, 1995). Olsen (2008) found the results of these two methods to be equivalent in cases without interfering compounds, such as nitrite. The two point method was used to analyze all N samples collected after February 2006, as nearly all total N was found to be in the form of nitrate. Soluble nitrate samples were filtered and analyzed in the same manner but without persulfate digestion. Hg samples were oxidized with BrCl (1% solution) and refrigerated prior to analysis by cold vapor atomic fluorescence spectroscopy (EPA Standard Method 1631). The mean value of the Hg blanks (0.48 0.40 ng/L) was subtracted from all sample values and certified reference materials (ORMS, National Research Council, Canada) indicated a mean percent recovery of 95 15% total Hg. Analytical replicates were conducted on 39% of the samples, with a mean percent difference of 7.7% Hg concentration. Samples collected for metals (from the Phase I plots and Smithdeal hall roof on July 6, 2008) were acidified to pH less than two with nitric acid and were shipped to J.R. Reed Environmental Lab for analysis of Cu, Fe, Mg, Zn and K by inductively coupled plasma–atomic emission spectrometry (EPA Standard Method 2007). Due to low runoff volumes on some sampling dates, the full suite of chemical analyses was not possible for every rain event. 2.5. Statistics The experimental plots were set up as a completely randomized experiment. The standard gravel roof was used as a control treatment to study the effects on volume and water quality from runoff of green roofs as compared to that of gravel roofs. The green roof without additions was used as a control to study the effects of the three treatments that are used by the green roof industry as modifications that address local concerns for vegetated roofs. For the second part of the study, the additional layers were removed and chemicals to mitigate effects of nutrient pollution (alum and Ultra-Phos Filter) were added. Again, the gravel-type roof and the green roof without amendments served as control treatments in a completely randomized experiment. Each storm’s results were analyzed using analysis of variance (F-tests). Storm to storm, no statistically significant differences in runoff volume or pollutants were detected among the different green roof plot configurations. Subsequently, visualization tools and repeated measures analysis (using mixed models) was used to compare green roofs and gravel roofs (Khattree and Naik, 1999).
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by the green roofs. Rain storms of at least 10 mm produced runoff from all the green roof plots while three storms with rainfall ranging from 2.0 to 5.3 mm yielded runoff from all the gravel roofs but only some of the green roof plots. A storm of 1 mm rain only produced runoff from gravel plots. No difference in plant coverage was observed among green roof types, further indicating that the addition of the retention and drainage layer were unnecessary for our location.
Because it differed substantially from storm to storm, the rainfall amount (millimeter) is used as a covariate in all models. Consequently, effects over time were not statistically tested because of sample size restrictions. When comparing runoff quality of green roofs with gravel roofs, the augmented green roof plots in Phase I were set aside in order to simplify the repeatedmeasures analysis. 3. Results and discussion
3.1.1. Nutrients Total nitrogen was analyzed from ten rain events, from which two were also analyzed for soluble nitrogen (Table 1). On dates where both soluble and total N were analyzed, results indicated that a large percentage of N in samples was in the form of soluble nitrate. Concentrations of total nitrogen were high for green roof plots (mean values ranged from 1.0 mg/L to 19.4 mg/L; Table 1), whereas concentrations in the gravel roof runoff were typically less than 1.0 mg/L and in precipitation were usually near or below the detection limit (0.02 mg/L). The concentrations of N measured in the green treatments appeared to decrease over time from a mean of 19.4 mg/L in November 2005 to a mean of 4.5 mg/L in January 2006, with lower concentrations, ranging from 1.0 to 3.5 mg/L after that point (Table 1). This suggests that fertilizer was leaching from the newly installed green roofs. Although no fertilizer was added at the time of the plot construction, the pre-vegetated pods likely contained enough existing fertilizer to account for this leaching. The two green treatments that incorporated the retention layer
3.1. Experimental plots–Phase I The green roof significantly reduced stormwater runoff volume compared to the conventional gravel roof treatment in every analyzed rain event (Table 1). Overall, the amount of rain retained by the green roof plots increased with the amount of rainfall observed in the study period. The gravel roof plots retained about the same amount of rain from storm to storm even after accounting for the amount of rainfall. The retention layer that was specifically designed to increase retention did not significantly reduce runoff volume. The drainage layer which was designed to enhance the drainage of excess rainwater did not significantly increase runoff volume. The percentage of runoff reduced by green roof plots (as compared to gravel roof plots) varied from 45% to 99% (mean = 80%) and was greatest for the storms with the smallest rain depth. The lowest volume rain events in fact were not sampled because there was no runoff from the green roof plots, with all the rain retained
Table 1 Mean, standard error, and sample size of runoff volume, concentration and load of nitrogen and phosphorus from experimental roof plots. DL indicates below detection limit: 0.02 mg/L for N and 0.001 mg/L for P. Each section of this table is arranged in chronological order. Volume
11/17/2005
01/13/2006 03/06/2006
Green plots (mm)
2.7 (0.2) n = 12 8.8 (0.8) n=3 70% 10 5
6.5 (0.3) n = 12 11.9 (0.2) n=3 45% 11 7
14
11
Gravel plots (mm) Runoff reduction Precipitation (mm) Antecedent dry period (days) Mean air temp. ( C)
N concentration 11/17/2005
11/22/2005
04/17/ 2006
05/27/2006
0.03 (0.01) 1.7 (0.2) n = 12 n = 12 2.9 (0.3) n = 3 8.4 (2.7) n=3 99% 79% 3 11 3 1
0.2 (0.1) n=8 4.7 (0.2) n=3 96% 5 1
6
17
01/13/2006
04/04/ 2006
13 03/06/2006
04/17/2006
06/20/2006
07/06/2006
04/12/ 2007
07/11/2007
0.02 (0.01) 0.01 (0.004) n = 12 n = 12 3.9 (0.1) n = 3 3.7 (0.1) n = 3
0.003 (0.003) n=8 1.1 (0.3) n = 3
99.60% 5 8
99.80% 2 5
99.70% 1 5
0.3 (0.1) n = 12 6.4 (0.6) n=3 95% 7 5
3.7 (0.2) n = 12 13.7 (0.5) n=3 73% 14 1
20
23
23
7
28
10/06/2006
10/08/2006
04/12/2007
06/26/2007
07/11/2007
2.6 (0.2) n=9 1.1 (0.0) n=2
1.4 (0.3) n=7 0.5 (1.0) n=3 0.2
1.0 (0.1) n=5 0.3 (0.1) n=3
3.5 (0.5) 3.5 (0.5) n = 12 n = 12 0.3 (0.2) n = 3 0.3 (0.1) n = 3
(mg/L) Green plots Gravel plots Precipitation – N load (mg/m2) Green plots Gravel plots
19.4 (2.0) n = 12 0.1 (0.1) n = 3 DL
8.9 (1.3) 4.5 (1.4) n = 12 2.6 (0.7) n = 12 1.7 (0.4) n = 11 n=7 0.1 (0.1) n = 3 DL (0.1) n = 3 DL n = 3 0.5 (0.6) n=3 DL
53.1 (6.6) n = 12 DL (0.0) n = 3
53.1 (9.0) n = 12 DL (0.0) n = 3
27.9 (3.6) n = 12 DL n = 3
DL
0.5 (0.1) n=5 2.2 (0.7) n=2
P concentration (mg/L)
11/17/2005
11/22/2005
11/29/2005
01/13/2006
06/20/2006
07/06/2006
Green plots Gravel plots Precipitation – P load (mg/m2) Green plots Gravel plots
3.2 (0.2) n = 12 0.1 (0.007) n = 3 0.1
1.1 (0.028) n = 11 0.0 (0.002) n = 3
1.5 (0.03) n = 12 0.1 (0.03) n = 3
1.7 (0.03) n = 12 0.1 (0.02) n = 3 DL
1.0 (0.2) n = 2 0.2 (0.01) n = 3 DL
2.5 (0.08) n = 12 DL (0.01) n = 3 DL
11.3 (0.4) n = 12 0.7 (0.2) n = 2
0.04 (0.01) n = 2 0.6 (0.03) n = 3
51.2 (1.5) n = 12 0.3 (0.3) n = 3
8.7 (0.7) n = 12 0.9 (0.1) n = 3
Ratio N:P
11/17/2005
11/22/2005
01/13/2006
Green plots Gravel plots
6.1 1
8.4 33.3
2.6 Not calculated
DL
13.3 (2.2) n = 12 3.6 (1.3) n = 3
E.G. Malcolm et al. / Ecological Engineering 73 (2014) 705–712
had significantly higher runoff of N on the earliest two dates measured (November 17, 2005 and November 22, 2005). The retention layer had been laced with fertilizer to encourage the plants to put roots into the layer; however, later events had no detectable difference in N for the treatments with the retention layer and upon disassembly of the Phase I plots three years after installation, no root penetration was evident. Much of the fertilizer appears to be washing out and thus, is not being taken up by the plants. Overall, mean N concentration was significantly higher for green roof plots than for gravel roof plots even after accounting for rain amount. On November 17, 2005 and January 13, 2006, the concentration of N from the green roofs was so high that even the substantial reduction of runoff volume from the green roofs did not result in a lower load of N. One year later on April 12, 2007, however, the difference in concentration between green and gravel was not as great (about 90 times higher in 2006 compared with about three times higher in 2007) so that even though the concentration of total N from the green roofs was greater, the mean N load from the green roofs was less than the gravel roofs (Table 1). Thus, green roof plots as constructed in our study can be a source of nitrogen, especially when newly constructed or recently fertilized, but over time this source of N appears to be reduced. Similar findings have been found in previous studies, as green roofs typically are sources of N following installation, but over time the runoff of N is reduced and some studies indicated that green roofs can ultimately function as sinks for N (Czemiel Berndtsson et al., 2006; Teemusk and Mander, 2011). Overall, concentrations of total phosphorus in runoff from the green treatments were consistently much higher (1.0–3.2 mg/L) than that from gravel roofs (less than 0.2 mg/L), even after accounting for rain amount (Table 1). Concentrations did not differ among underlayment types. For half of the storms with a calculated P load, 10 to 180 times more P leached from the green than gravel roof plots. In only one of the analyzed storms (June 20, 2006), the reduction in runoff volume (99.8%) was enough to compensate for the higher concentration of P from the green roofs and to result in a lower load from the green roofs. This suggests that fertilized green roofs are a source of phosphorus, except for light storm events in which runoff volume is greatly reduced by the green roofs. Numerous other studies have observed that extensive green roofs can function as sources of P (e.g., Köhler et al., 2002; Czemiel Berndtsson et al., 2006; Bliss et al., 2009; Teemusk and Mander, 2011). Czemiel Berndtsson et al. (2009) observed that extensive green roofs function as sources of P, whereas intensive green roofs did not. Köhler et al. (2002) observed that over time this load of P decreases, presumably due to improved plant growth and flushing of a portion of the P present during installation. For our roof plots, P concentrations in runoff were still elevated more than three years after construction. In addition, N to P ratios were typically at or below the Redfield ratio, which could adversely impact downstream water quality (Smith, 1983). Thus, although fertilizer can result in faster plant growth (Rowe et al., 2006), it seems prudent to decrease fertilizer use during installation, in order to reduce the negative impacts on water quality during the first few years. 3.1.2. Mercury For each storm, the concentration of Hg in the runoff from the different green roof underlayments did not significantly differ from each other. Overall, the concentration of Hg in the runoff from the green roof plots was significantly higher than the concentration in the runoff from the gravel roof plots, after controlling for rain amount. However, the reduction in volume of runoff from a green roof typically compensated for the higher concentration; that is, the total load of Hg from the green roofs was similar to or less than
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Fig. 2. Mean mercury load (ng/m2) in runoff of the green (n = 3) and gravel (n = 3) roof plots as compared to incoming mercury in precipitation (n = 1).
that of the gravel roofs. Furthermore, the total mass of Hg estimated to have been deposited to the roof by rain was less than that of the runoff for all rain events sampled (Fig. 2). The mean reduction in precipitation load was 61% for gravel and 86% for the green roof plots. This finding suggests that both conventional and green roofs capture some of the Hg being input via wet deposition. This is somewhat analogous to a natural forest ecosystem: precipitation typically increases in Hg concentration after falling through a forest canopy, but then a significant fraction of the Hg is retained in the soil and forest system and not exported to streams (Scherbatskoy et al., 1998). In a forest the increase in Hg falling through the canopy is largely due to the washing off of aerosols that were dry-deposited to the leaves (Rea et al., 1996). This may also occur in a green roof system, which has a greater surface area than a conventional roof. Therefore findings from this study may not be representative of urban or industrial areas with high concentrations of gaseous and particulate Hg which would dry deposit to the roof and could then lead to higher loads running off of the roofs during precipitation events. Many questions remain about what will happen to the retained Hg in a green roof system. Will it re-vaporize, adsorb to the plants and growing medium, or wash out eventually in future storms? Over the three years of sampling in this study, no temporal trend was observed in concentrations of mercury in runoff from the green or gravel roof plots (Tables 2 and 4). 3.2. Phase II and full scale roofs 3.2.1. Nutrients The green roof plots had significantly higher total N and P concentrations compared to gravel roof plots, even after controlling for rainfall amount. The utilization of alum and Ultra-Phos Filter did not significantly reduce the concentration of N in runoff, but the alum amendment consistently reduced the P concentration (22.0% on average; Table 3, Fig. 3). These amendments may work to further reduce runoff P under other conditions, such as with a greater duration of exposure to the amendments or greater volume of amendment. It is possible that the porosity of the amendments prevented adequate runoff flow through the pouches, and therefore another design which forces water to flow through the amendments would improve removal rates. On roofs with drain
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Table 2 Mean, standard error, sample size of runoff concentrations and load of Hg from experimental roof plots. Hg Concentration (ng/L)
11/17/2005
03/06/2006
04/17/2006
07/06/2006
04/12/2007
07/11/2007
Green plots Gravel plots Precipitation – Hg load (ng/m2) Green plots Gravel plots
14.3 (1.3) n = 12 3.7 (0.04) n = 3 13.5
12.0 (3.3) n = 9 8.5 (0.1) n = 3 4
7.0 (2.4) n = 10 5.9 (0.9) n = 3 6.3
20.0 (0.9) n = 12 5.5 (0.9) n = 3
11.9 (2.7) n = 11 3.6 (0.3) n = 3*
19.0 (1.4) n = 12 15.3 (5.2) n = 3 14.7
37.7 (3.2) n = 12 32.5 (3.1) n = 3
0.6 (0.2) n = 9 24.7 (1.8) n = 3
2.0(0.8) n = 10 27.8(4.3) n = 3
5.0(1.5) n = 11 23.2 (3.8) n = 3
70.6 (7.4) n = 12 209.9 (74.2) n = 3
Table 3 Mean and standard error of concentrations in runoff from Phase II, where n is the number of roofs or roof plots sampled. DL indicates a value below detection limit: 0.02 mg/L for N and 0.001 mg/L for P. All green roof plot treatments (i.e., alum, Ultra-Phos Filter, and control) are included in means for greens plots. Phosphorus (mg/L) Green plots Gravel plots Green roofs Gravel roofs Precipitation
10/24/2007
0.59 (n = 1) DL (n = 1) DL (n = 1)
Nitrogen (mg/L) Green plots Gravel plots Green roofs Gravel roofs Precipitation
07/06/2008
0.65 (n = 1) 0.08 (n = 1) DL (n = 1)
10/24/2007
2.7 (n = 1) DL (n = 1) DL (n = 1)
03/01/2009
0.37 (n = 1) 0.05 (n = 1)
03/01/2009
3.33 (n = 1) 0.18 (n = 1)
Mercury (ng/L) Green plots Gravel plots Green roofs Gravel roofs Precipitation
03/16/2009
03/26/2009
1.36 (0.09) n = 12 0.10 (0.02) n = 2 0.33 (0.04) n = 2 0.06 (0.01) n = 2 0.03 (n = 2)
03/16/2009
0.70 (n = 1) 0.61 (n = 1) 0.05 (n = 1)
03/26/2009
2.21 (0.27) n = 12 0.77 (0.33) n = 3 1.37 (n = 2) 0.35 (n = 2) 0.29 (n = 2)
03/01/2009
03/16/2009
6.9 (n = 2) 19.7 (n = 2) 4.0 (n = 1)
21.1 (1.7) n = 12 9.4 (2.8) n = 3 6.5 (n = 2) 6.7 (n = 2) 3.7 (n = 1)
pipes, this may be achieved by modifying commercially available downspout filters, which are currently sold to reduce hydrocarbons from tar roof runoff (McGregor, 2014). Alum has been used successfully to bind to P in many applications, such as in wetland soils, treatment wetlands, and sludge amended soil resulting in measurable reductions in water column P concentrations (i.e., Hoge et al., 2003; Dunne et al., 2012; Huang et al., 2012). Thus adding alum directly to green roof soils may reduce P in runoff, but would require further investigation to determine the optimum application rates to maintain the required levels of phyto-available P. Ultra-Phos Filter additions over Phase II drains, surrounding one of the drains on Smithdeal hall, and as a soil amendment on the Earl Industries roof did not demonstrate any clear reduction of P in runoff. Since our experiment, the manufacturer of Ultra-Phos Filter has found that incorporating zeolite into the Ultra-Phos Filter
04/12/2009
04/15/2009
1.21 (0.11) n = 8 0.10 (0.02) n = 3
1.45 (0.08) n = 9 0.05 (0.003) n = 3
0.0171 (n = 1)
0.006 (n = 1)
04/12/2009
04/15/2009
1.33 (0.55) n = 11 DL (0.02) n = 3
0.46 (0.09) n = 10 DL (0.00) n = 3
DL (n = 1)
DL (n = 1)
03/26/2009
11/11/2009
42.2 (n = 1) 39.9 (n = 1)
5.8 (n = 2) 5.2 (n = 2) 7.6 (n = 1)
improves porosity and removes excess iron which is released when the Ultra-Phos Filter takes up the phosphorus (McGregor, 2014). P runoff from full scale roofs was substantially lower than concentrations in runoff from Phases I and II plots (Tables 1 and 3).
Table 4 Mean, standard error, sample size of runoff concentrations of metals and phosphorus collected on July 6, 2008. DL indicates below detection limit (0.005 mg/L for Cu and 0.005 mg/L for Mg).
Green plots (n = 12) Gravel plots (n = 3) Green roof (3 drains) Gravel roof (n = 1) Precipitation (n = 1)
Copper (mg/L)
Iron (mg/L)
Manganese (mg/L)
0.015 (0.003) 0.002 (0.002) 0.024 (0.002) 0.369
0.061 (0.011) 0.031 (0.003) 0.68 (0.08) 0.097
DL (0.0)
DL
0.072
Zinc (mg/L)
Potassium (mg/L)
Phosphorus (mg/L)
0.040 (0.007) 0.006 0.027 (0.001) (0.003) 0.02 (0.001) 0.037 (0.003) 0.015 0.034
1.301 (0.331) 0.484 (0.076) 11.8 (1.2)
2.539 (0.077) 0.005 (0.019) 0.643 (0.067)
DL
0.125
0.048
1.07 Fig. 3. Concentration of total phosphorus in runoff from the experimental roof plots during Phase II. The roof plots modeled three types of green roof (control, control plus alum filter around drain, and control plus Ultra-Phos Filter around drain) as well as traditional gravel roofs.
E.G. Malcolm et al. / Ecological Engineering 73 (2014) 705–712
This may be because stormwater on full scale roofs came in contact with more of the green roof surface prior to runoff, and/or because any “first flush” effect is missed by our mid-storm sampling from the real roofs. Previous studies vary dramatically in the concentrations of P in runoff (Czemiel Berndtsson, 2010). P concentrations from our roof plots were similar to those observed by Hathaway et al. (2008) and Bliss et al. (2009), and our full scale roof measures were only slightly higher than the extensive roofs studied by Czemiel Berndtsson et al. (2009). This study adds to the number of studies that have demonstrated substantial runoff of P from green roofs (Köhler et al., 2002; Czemiel Berndtsson et al., 2006; Hathaway et al., 2008; Bliss et al., 2009; Teemusk and Mander, 2011). Thus, a crucial challenge for future green roof applications is to decrease the P in runoff without impacting the positive benefits of green roofs. 3.2.2. Mercury A runoff sample was collected for Hg analysis from all plots and real roofs for the rain event of March 13–16, 2009 (Table 3). No consistent pattern emerged across roofs when comparing green and gravel runoff. Runoff from the green roof experimental plots had the highest concentrations of Hg compared to all other roof types. Unlike the green roof plots, for which Hg concentrations were comparable or higher from green than gravel roofs, Hg concentrations were higher in runoff from the Earl Industries Building gravel roof for two of the four sampling days (8.2 and 3.7 higher). On the other two days, green and gravel concentrations were similar. This building is located closer to industrial areas, including shipyards and dry-docks, than our other sampling sites. Although volume of runoff was not measured from this building, the reduction in runoff volume coupled with comparable or lower concentrations from the green roof would produce lower runoff of Hg load from the green roof over gravel for this building. 3.3. Metals Runoff samples from the Smithdeal dormitory roof and the Phase I roof plots along with precipitation collected on July 6, 2008 were analyzed for metals and phosphorus. Although only one stormwater runoff event was sampled for metals, some interesting results emerged. Copper, iron, zinc, and potassium concentrations were higher from green than gravel roof plots, which is consistent with the measurements for N, P, and Hg. Potassium, iron, and manganese were all higher on average in runoff from the Smithdeal green roof than from any other category of samples (Table 4). This was especially apparent for potassium, which was about ten times more concentrated in the Smithdeal green roof runoff than in the Smithdeal gravel runoff or the green roof plot runoff. This finding is in contrast to that of P, N and Hg for which the green roof plots had higher concentrations than the Smithdeal green roof. Copper was ten times higher in the Smithdeal gravel runoff sample than in any other sample, which may reflect differences in building materials. It is notable that samples from the green and gravel roof plots, constructed with a bitumen layer containing copper film each had 8% or less of the Cu content of the Smithdeal gravel runoff. The real gravel and green roofs on Smithdeal had copper flashing, which may have contributed to runoff concentrations. Zinc, in contrast, showed similar concentrations across all roof types and precipitation. These results are in accordance with the finding of other researchers in that the diversity of roof construction materials can result in incongruous patterns for different metals in runoff. Thus, additional research examining the effect of different building materials and conditions on green roof runoff for toxic metals, such as copper, is warranted.
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4. Conclusion A growing body of literature has examined the potential impact of different types of green roofs on water quality. Previously it was assumed that reduced runoff volume would cause reduced runoff of total pollutants. In some cases, this can be true, such as if stormwater-retention prevents combined sewer overflows (VanWoert et al., 2005; Villarreal and Bengtsson, 2004, 2005), but it is not always the case (Czemiel Berndtsson, 2010). This study also confirmed that nutrient concentrations can be elevated in runoff from green roofs, and the degree to which this can impact downstream systems may depend on the age of the roof, the roof materials, and fertilizer application to the roof. These results are significant for watersheds sensitive to nutrient additions. The special layers for water-retention and drainage offered no significant benefit to runoff volume over the green treatment without these layers. No significant differences were observed between any of these different underlayment treatments, except for elevated N leached from the foam-retention layer during the first two months following installation; thus their use does not seem warranted, at least for the mid-Atlantic area. The chosen method of application of Ultra-Phos Filter did not reduce P in runoff but may work in a different configuration. The chosen method of application of alum reduced P by approximately 20% in runoff. Results of this study indicate that green roofs can moderately reduce Hg runoff by reducing the volume of runoff. Green roofs thus have the potential to reduce the Hg load to watersheds such as the Chesapeake Bay. Reduction of Hg sources to watersheds has become a priority in many states which have consumption advisories for fish due to Hg high concentrations. Results of this study indicate that if green roofs are not deliberately designed and managed, the benefits of green roofs can be diminished by their moderate negative impacts on water quality. An important challenge for sustainability proponents is to optimize green roof design to maximize the positive benefits, without leading to excessive runoff of nutrients and other pollutants in the process. Acknowledgments Funding was provided by the U.S. EPA P3 Program (EPA grant number G5Z70199), Jerry Miller, the VWC Science Undergraduate Research Fund, and a VFIC Mednik Foundation Grant. Funding was provided for the construction of the Smithdeal green roof by the Virginia Department of Conservation and Recreation. Special thanks to VWC students including: John Maravich, Wanda Morris, MariCarmen Korngiebel-Rosique, the VWC Environmental Chemistry, General Ecology, Statistics, and Environmental Biology Classes, as well as VWC Physical plant, VWC Security officers, and Earl Industries. Michael Perry of Building Logics, Inc. was instrumental in the design and installation of the test plots and the Smithdeal hall green roof. Ultra-Phos Filter was provided by UltraTech International. References Alsup, S.E., Ebbs, S.D., Battaglia, L.L., Retzlaff, W.A., 2013. Green roof systems as sources or sinks influencing heavy metal concentrations in runoff. J. Environ. Eng. 139, 502–508. APHA, 1995. Standard Methods for the Examination of Water and Wastewater, 19th Edition American Public Health Association, American Water Works Association, Water Environment Federation, Washington. Bliss, D.J., Neufeld, R.D., Ries, R.J., 2009. Storm water runoff mitigation using a green roof. Environ. Eng. Sci. 26, 407–417. Carter, T., Fowler, L., 2008. Establishing green roof infrastructure through environmental policy instruments. Environ. Manage. 42, 151–164. Crumpton, W.G., Isenhart, T.M., Mitchell, P.D., 1992. Nitrate and organic N analyses with 2nd-derivitave spectroscopy. Limnol. Oceanogr. 37, 907–913.
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