Elsevier

Ecological Engineering

Volume 73, December 2014, Pages 705-712
Ecological Engineering

Measurements of nutrients and mercury in green roof and gravel roof runoff

https://doi.org/10.1016/j.ecoleng.2014.09.030Get rights and content

Highlights

  • Green roofs leached more N and P than traditional gravel roofs.

  • Although the concentration of Hg was higher in rainwater runoff from green roofs, the load of Hg running off of green roofs was not significantly different from that of gravel roofs due to a reduction in runoff volume.

  • The water-retention and water-drainage underlayments tested did not result in a significant difference in runoff volume, P, or Hg concentrations.

  • Alum and Ultra-Phos Filter were applied in pouches over green roof drains in order to reduce P in runoff: the alum configuration was somewhat successful, but Ultra-Phos Filter was not.

Abstract

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.

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 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, Villarreal and Bengtsson, 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 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, Teemusk and Mander, 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, Czemiel Berndtsson et al., 2008, Czemiel Berndtsson et al., 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 Filter® (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.

Section snippets

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 built-up roof (n = 3) for each treatment (Fig. 1). Plots were constructed as five one-meter high tables that held three plots each,

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

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, Villarreal and Bengtsson, 2005), but it is not always the case (Czemiel Berndtsson, 2010). This study also confirmed that

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,

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