Hayes, Werner, Worrell Group project

THE EFFECTS OF GREEN ROOFS ON NUTRIENTS IN RUNOFF 1

1. Introduction:

Green roofs (roofs with a vegetated surface and substrate) provide ecosystem services in

urban areas, including improved storm-water management, better regulation of building

temperatures, reduced urban heat-island effects, and increased urban wildlife habitat

(Oberndorfer et al. 2007). Over the years the U.S. Government has been encouraging companies

to use green roofs because, of their benefits to the environment. The starter cost for green roofs

can be pricey, but they result in a lower overall cost compared to a normal roof (Malcolm et al.

2014). In other studies green roofs have been proven to reduce noise pollution from urban traffic

and construction (Czemiel Berndtsson et al. 2006).

Ten years ago, the roof on Smithdeal Hall at Virginia Wesleyan College was transformed

into a green roof (Malcolm et al. 2014). The roof is on a two-story building which is an overall

T-shape with trees taller than the rooftop and a small tree growing in the green roof media. The

green roof consists of seven layers: a base sheet fastened to a plywood base, roof membrane with

a copper root barrier, a drainage layer, a water retention layer, a filter fabric, and a growing

media planted with Sedum (Fig. 1). When the roof was first put in, they planted thirteen species

of Sedum, but now there are an unknown amount species of Sedum and other unknown

vegetation. The original green roof had three sections, but one of the sections has since been

removed due to water damage. A roof adjacent to the green roof on Smithdeal Hall was used as

our gravel roof and the awning of the new Blocker entrance was used as our “normal” or

“control” roof.

The professors at Virginia Wesleyan College wanted to observe if pollutants and

nutrients could be significantly reduced by green roofs. They used fifteen one-square-meter test

roof plots and two green roofs to measure if green roofs reduce pollutants and nutrients in

Ericka Hayes, Andrew Werner, & Christopher Worrell EES 450*01 Spring 2016


stormwater (Malcolm et al. 2014). Five configurations were used on the test plots (all green

plots had Sedum), 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 buildup roof. They found a large percentage of high nitrogen concentrations in the

samples that were in the form of soluble nitrate (1.0 mg/L to 19.4 mg/L), possibly due to the

fertilizer previously used to promote plant growth (Malcolm et al. 2014). They also found that

the nitrogen concentration in the precipitation were typically at or below the detection limit and

the gravel roofs typically lower than 1.0 mg/L (Malcolm et al. 2014).

The goal of our study was to see if green roofs would decrease nutrient runoff. Since our

study took place 10 years after the installation of the green roof, the fertilizer has had a chance to

be absorbed by the Sedum and the natural addition of other plants may have helped to decrease

the volume of runoff. Excess nutrients caused by anthropogenic sources has been a major issue

in the Chesapeake Bay Watershed, because high levels of nitrogen and phosphorus can cause

algal blooms and anoxic zones, which can kill fishes, invertebrates, and subaquatic vegetation.

Fig. 1. Green roof configuration (Malcolm et al. 2014).



Hypothesis:

Our null hypothesis is that the green roof will not alter the concentration of nutrients

runoff. Our alternative hypothesis is green roof will decrease nutrient concentrations in runoff.

2. Methods:

In our experiment, we only collected data from four drains (green 1, green2, gravel,

Blocker) and one rain gauge (precipitation) to investigate the concentration of nutrients in runoff.

To collect the runoff, we used pie pans that we placed inside the four drains before a rain event,

and then we collected the samples in clean 125 mL polyethylene bottles (Malcolm et al. 2014).

We analyzed anions (phosphate, nitrate, nitrite, chloride and sulfate) in each water sample using

ion chromatography (Dionex ACS-2100) to test the effects of nutrient runoff. Single factor

ANOVA was used in Excel to analyze the runoff samples from the three storms, to determine if

the green roof caused any significant variation in nutrient concentrations.

In (Table 2) percent recovery for the SPEX check standards showed that nitrite and

chloride calibration was incorrect because the values we not within the ideal 90-110% range. By

having values outside the ideal range (90-110%), we are unable to be confidence in the

concentrations of the nitrite and chloride samples. In (Table 3) the percent difference in our

replication for nitrite was 23.9%, 14.2% nitrate, 9.6% phosphate, 1.0% chloride, and 2.7%

sulfate, which showed further uncertainty in the nutrient concentrations.



Fig. 2. Aerial image of the roof drains, Blocker drain, and the pans in the drains.

3. Results:

There was no significant difference in the nutrient concentrations between sample types

(precipitation, green roof 1, green roof 2, and gravel) (ANOVA nitrate (p=0.3473), nitrite

(p=0.6818), phosphate (p=0.4464), chloride (p=0.1498), sulfide (p=0.6301)). The gravel and

Blocker roof drains had the highest concentrations of nitrite with 0.7295 ppm and 0.6717ppm,

respectively (Table 1). The highest nitrate concentrations were found in gravel and green roof 2

with 3.6887 ppm and 1.6235 ppm, respectively (Table 1). The phosphate concentrations were

mostly 0.000 ppm with gravel having the highest amount 0.5658 ppm (Table 1). Gravel also had

the highest chloride concentration 8.7915 ppm and green roof 1 had the highest sulfate

concentration 11.0210 ppm (Table 1). The average nutrient concentrations in the blanks were

0.000 ppm nitrite, 0.0001 ppm nitrate, 0.000 ppm phosphate, 0.3994 ppm chloride, and 0.0403

ppm sulfate. The R2 values for the calibration curves were 0.9972 nitrite, 1.0 nitrate, 1.0

phosphate.


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Ericka Hayes

Ericka Hayes


Table 1. Volume of samples, weather data, concentrations in ppm for nitrite, nitrate, phosphate, chloride, and sulfate. “n.a” in the 3/13/16 Blocker samples is for unavailable data.

Volume (mL) 3/13/2016 3/15/2016 4/2/2016Rain 119.94 44.50 145.53

Gravel Roof 138.27 32.17 146.23Blocker n.a. 28.36 65.49

Green Roof 1 140.50 164.90 146.63Green Roof 2 137.95 140.29 135.36

Antecedent Dry Period (days) 8 1 4Mean Air Temp ºC 65 58 61Precipitation (mm) 16.51 6.604 17.78

Nitrite Concentration (ppm)Precipitation <DL <DL 0.5879Gravel Roof 0.5793 0.7295 <DL

Blocker n.a. 0.6717 0.5861Green Roof 1 0.5542 <DL 0.5530Green Roof 2 0.5708 0.3506 0.5510

Nitrate Concentration (ppm)Precipitation 0.9410 1.2458 0.6306Gravel Roof 0.4916 3.6887 0.6714

Blocker n.a. 2.9893 0.6450Green Roof 1 0.7933 0.0095 0.4240Green Roof 2 1.6235 0.0170 0.1245

Phosphate Concentration (ppm)Precipitation <DL <DL <DLGravel Roof <DL <DL 0.5658

Blocker n.a. 0.0566 <DLGreen Roof 1 <DL <DL <DLGreen Roof 2 0.0115 <DL <DL

Chloride Concentration (ppm)Precipitation 1.5201 0.8491 5.0309Gravel Roof 3.2103 8.2569 8.7915


Sulfate Concentration (ppm)Precipitation 0.5672 0.5084 6.7662Gravel Roof 1.3068 5.9695 2.7312




Fig. 3. Nitrite concentration (ppm) for all three storms and roof drains. Not significantly different (p= 0.6818).

Fig. 4. Nitrate concentration (ppm) for all three storms and roof drains. Not significantly different (p= 0.3473).



Fig. 5. Phosphate concentration (ppm) for all three storms and roof drains. Not significantly different (p= 0.4464).

Fig. 6. Chloride concentration (ppm) for all three storms and roof drains. Not significantly different (p= 0.1498).



Fig. 7. Sulfate concentration (ppm) for all three storms and roof drains. Not significantly different (p=0.6301).

Table 2. The minimum, maximum, and mean of the SPEX percent recovery.

Nitrite Nitrate

Phosphate

Chloride Sulfate

Min (ppm) 0.0000 0.0000 81.4344 97.9086

91.5975

Max (ppm) 0.0000 1.4440 87.1935

145.6222

95.7126

Mean (ppm) 0.0000 0.1904 84.7477

107.6280

93.8328

Table 3. The minimum, maximum, and mean of the percent difference.

Nitrite Nitrate Phosphate

Chloride Sulfate

Min (ppm) 0.0000 0.1276 0.0000 0.1767 0.2023Max (ppm)

100.0000

100.0000 93.9482 2.3722 12.1264

Mean (ppm) 23.9009 14.2036 9.5870 1.0267 2.7193



Table 4. Comparison of phosphate and nitrate concentrations to Malcolm et al.

2007-2009Total P

2016PO4

2007-2009Total N

2016NO3

Green roof 0.33-0.7 <DL-0.015 1.37-3.33 <DL-0.57Gravel <DL-0.61 <DL-0.57 <DL-0.35 <DL-0.7Precipitation <DL-0.05 <DL <DL-0.29 0.63-1.25

4. Discussion:

Over the past decade green roofs have been a topic of interest and debate on whether

they work on improving environmental quality. Green roofs have proven to have effect on the

cycling of nutrients, like phosphorus and nitrogen. In our study, we looked at green roofs ability

to effect anions that are naturally found in runoff. Overall we looked at phosphate, nitrate, nitrite,

sulfate, and chloride, which are important components in biogeochemical cycles. Equilibrium of

these ions are important, because if ions such as nitrogen or phosphorus are limited in the

environment it may result in stunted plant growth and if there is too much it can result in harmful

effects. This is one of the reasons that led a group of VWC professors to conduct a study

measuring the concentrations of nutrients and heavy metals in runoff of the newly installed green

roof. Our hypotheses were to first compare our nutrient levels to the previous study.

4.1. Nitrate/Nitrite

One of the main reasons we chose to study nitrogen in runoff is due to it being an

essential nutrient in organisms and fluxes in biogeochemical cycles. Nitrogen concentrations can

fluctuate due to an increase of natural and anthropogenic sources, and changes in local and

global climate. Higher nitrogen levels in surface runoff can lead to algal blooms and dead zones

in nearby aquatic zones (Hessen et al. 1997). We were limited by the “7-ion standard” we used in



our ion chromatography analysis, so we were only able to analyze the concentrations of nitrate

and nitrite, instead of all forms of nitrogen. Nitrogen ions are highly reactive in the environment,

so to calculate the total N we would need a different ion standard or a different concentration

analysis method. If we were to compare the total N to only nitrate and nitrite there would be

misrepresentation (Cowen et al. 1976). We found that the nitrate and nitrite levels were not

significantly difference between the sample types when we analyzed the samples with a single

factor ANOVA in Excel. In (Fig. 3), we saw that the average nitrite levels were between 0.5 and

0.7 mg/L and the nitrate levels in Blocker and gravel have higher concentrations than the two

green roof samples.

4.2. Phosphate

Similar to nitrogen, phosphorus is a chemical that is essential to life and an abundance of

phosphorus can become a harmful to an ecosystem by causing a rapid growth in algae.

Phosphorus is often found in low levels and can be an important resource to many species

(Wymer et al. 1980). Phosphate might have a lower concentration in the green roof samples, due

to the abundance of vegetation which readily uptake any available phosphorus. Unlike nitrogen

which is highly reactive, the main form of phosphorus is phosphate, which is a reliable

representation of the total P concentration (Howard 2016). Ideally, the roofs on Blocker and

gravel should have the same concentrations as the regional average (0.5ppm, NADP 2014), and a

reduced concentration of phosphate from the green roofs. In (Fig. 5) we found that the

concentrations for phosphate were below the detection limit in a majority of the samples, which

might be a result of the fact that the atmosphere is not a major source of phosphate. Nixon et al



found that the majority of total flux of phosphorus comes from DIP [dissolved inorganic

phosphorus] and 15% of the total P is particulate phosphorus.

4.3. Sulfate

Sulfate can be a harmful ion in the atmosphere and aquatic reservoirs, it is also an

important component in many redox reactions that stimulate many biogeochemical cycles,

important processes are sulfur reduction, pyrite formation, metal cycling, salt-marsh ecosystems,

acid rain, and sulfur emissions. Sulfate is not directly harmful when found in terrestrial

environments, like green roofs (Luther et al. 1986). Atmospheric deposition of sulfate can

become dissolved and enter aquatic ecosystems after a rain storm occurs since it is not used by

terrestrial organisms, the concentrations of the sample type should reflect the regional average

(0.5ppm, NADP 2014). This is seen in (Fig. 7), there is no real trend that can be found. We had a

p-value that was higher than 0.05 which means that we had no difference between sites. It is also

possible for there to be deposited buildup of atmospheric sulfate in between rain events. There

will be increased sulfate deposition when there are more days between rain events.

4.4. Chloride

Chloride is an ion that can be harmful in aquatic systems, because it is highly reactive

and can potentially for dangerous compounds. Urban areas can cause an increase of runoff,

potentially increasing the amount of chloride in aquatic systems. Green roofs are a way to slow

down or prevent the transport of chloride, by allowing the chlorine ions to settle in the

environment without being harmful in mass quantities (Sonzogni et al. 1983). There is an

estimation that 45% of chloride is deposited from rainfall, and a small amount of it comes from

dry deposition alone. The amount of chloride that is not deposited directly into a stream is

transported through as groundwater (Peters and Ratcliffe. 1998). The chloride concentrations in



some of our samples were at or below the regional average (0.6ppm, NADP 2014), however the

higher concentrations may have resulted in poor acid washing methods.

4.5 Comparison to Malcolm et al. 2014

Our experiment was continuation and comparison to a study done by Malcolm et al.

2014, by measuring the nutrient concentrations from the green roof on Smithdeal Hall from their

study. The only nutrients we were able to compare were nitrate, nitrite and phosphate, because

the previous study did not analyze chloride or sulfate (Table 4). Several years ago there were

high concentrations of phosphorus about <DL- 0.7 (Table 4), dependent on the sample type

(Malcolm et al. 2014). In Table 4, the concentrations of phosphate are lower the Malcolm et al.

2014 in our study for all sample types. The main causes for the high concentrations of

phosphorus in the previous study may have been from the fertilizer needed to promote growth of

Sedum when it was first installed (Malcolm et al. 2014). Since the installation fertilizer has not

been reapplied to the green roof, which supports the reduction of phosphorus concentrations in

our samples.

4.6. Sources of Uncertainty

There are many sources of error that could have skewed our results since the data was not

significantly different between the types of samples. We were only able to collect samples from

three rain events, which may have prevented a significant difference due to the small sample

size. Bottle storage and contamination might have also played a role in how the chemistry of the

water may have changed after samples were collected. Our collection design allowed for

precipitation and runoff from the roof media to be caught in the collection pans, therefore our

samples were not purely runoff from the roofs. In a future study, we would design a cover to



prevent direct precipitation so that all water collected would have flowed on or through the roof

media. After the first storm there was an influx of pollen, leaf litter and soil in the runoff that

may have leeched nutrients into the samples. If we could change our experiment it might be

different if we used different methods for analyzing samples. We used ion chromatography in

order to look at individual anions, instead of spectrophotometric analysis using molybdenum

blue (Malcolm et al. 2014), which does not have a precise measurement of total P or total N to

compare to the previous study. We could instead look at only total N and total P to measure how

runoff on green roofs affects nutrients, which would have allowed us to further analyze the

fluxes and reservoirs within the biogeochemical cycling of nutrients.

4.7. Conclusion

Overall we found that even thought our graphs suggested a significant difference, our

ANOVA determined that there was no significant difference between sample types. We were

able to determine how our anions reacted with a green roof by gaining ideas and knowledge from

other studies. Green roofs are helpful because the transport of most nutrients is slowed or the

nutrients are absorbed by sedum on the roof, therefore it is better than other roof types at

mitigating the influx of nutrients. However there is a need for more studies and sampling fully

conclude if the green roof on Smithdeal Hall is effective at reducing the volume of nutrients in

runoff.

Acknowledgments:VWC Physical Plant, Susan (Jake) QuigleyDr. Howard for assistance with the Ion Chromatography Dr. Malcolm for assistance and guidance throughout the study



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Luther, G. W., Church, T. M., Scudlark, J. R., Cosman, M.. 1986. Inorganic and Organic Sulfur Cycling in Salt-Marsh Pore Waters. Science, 232(4751), 746–749.

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Hayes, Werner, Worrell Group project