Rainwater Harvesting for Landscape Irrigation: The Good, the Bad, and the Ugly side of Roof Runoff
by Natasha T. Nicholson, Brett V. Long, Shirley E. Clark, Ph.D., P.E., D. WRE
Penn State Harrisburg, Middletown, PA
One aspect of green engineering and sustainable design is how man interacts with the hydrologic cycle. This includes how stormwater runoff is managed and whether runoff is viewed as a waste product or an opportunity. Rainwater harvesting is a critical component in integrated urban water management. If rainwater harvesting barrels/collection tanks are widely distributed and used in conjunction with other water-sensitive development practices such as low impact development and/or conservation design, a substantial fraction of runoff can be removed from the drainage system. This, in turn, potentially would reduce sewer overflow events and reduce bacterial and other pollutant concentrations in receiving water bodies. In addition, the used of harvested rainwater for outdoor uses such as watering lawns and gardens could reduce domestic potable water consumption by 10% - 30%. The use of roofs (potentially such as those in Figure 1) for rainwater collection is not a new technique, but is becoming more common as people’s awareness of potable water limitations and costs increase. While there are many benefits to using harvested rainwater from roofs, there are also possible negative impacts.
Roofs as a good source for harvested rainwater
Roofs are an ideal location for rainwater harvesting for three main reasons. First, gravity can be used to collect runoff thus eliminating the need for electricity for pumps. Second, when compared to toilet and other household wastewater, water collected from roofs could be relatively clean. Third, a second and separate plumbing system would not be required. Other side benefits of using reclaimed water include cost savings to the consumer via reduction in municipal water consumption and increased efficiency for the drinking water authority in that less water must be treated, especially during hot, dry days when overall water use is likely to be higher.
Potential problems with harvested rainwater from traditional roofing
While roofing seems like an obvious choice for rainwater harvesting, there are some drawbacks in using this resource. Laboratory studies of roofing materials purchased at a local big-box hardware store demonstrated the potential for pollutant leaching into the environment (Clark et al., 2004). A large reservoir of nutrients and metals existed in these materials, and, if the environmental conditions were favorable, some of this reservoir potentially could be released into the runoff. Some common high level contaminants include: pH, nitrate, phosphorus, and heavy metals. Field studies also have confirmed the important role played by roofs, paved surfaces, treated woods, and other construction materials in the pollutant loading of stormwater runoff (Clark et al. 2008a, which contains an extensive reference list and summary table of the literature).
In the past, it has been assumed that, as roofing materials age, their ability to release pollutants decreases to a low-level, steady-state value. However, aging due to, for example, fluctuating temperature and ultra-violet light exposure, and the ongoing interactions between the roof surface and the chemicals in the rainfall can result in newly exposed subsurfaces. These subsurfaces may not have been sealed to prevent degradation, because they were not expected to be exposed to the outdoor environment. Testing at PSH on two old (60+ years) painted, galvanized metal roof panels asphalt shingles (Figure 3), showed that pollutant release could continue for at least 60 years (Clark et al. 2008b).
Although pollutant releases can occur at any point in a roof’s lifespan, generally, it is thought that there are two periods of concern for pollutant release – early and late life. During the early life, excess sealants and potential surface contamination wash off. This release decreases over time with environmental exposure. Because initial pollutant concentrations are likely to be high (exhibiting a first-flush of the lifespan), this time period is of most concern for roof runoff either that enters receiving waters and potentially harms both fauna and flora downstream or that is captured for future use. The second time of concern is the end of life where the material degrades so greatly that the subsurface/subbase materials are exposed and pollutants released from these lower layers of material. Several states and other water authorities have published guidance on the appropriate selection of roofing materials for rainwater harvesting for landscape irrigation. This research investigates the runoff quality from these recommended surfaces.
Potential of Extensive Green Roofs for Rainwater Harvesting
Extensive green roofing, where the media depth is shallow and the plants are typically drought-and heat-tolerant alpine species, has been examined as a means of reducing the volume of and improving the quality of stormwater runoff. The research at Penn State Harrisburg by Long et al. (2007, 2008) has confirmed the ability of the green roof, both through potential plant uptake and growth/drainage media filtration of the rainfall, to neutralize acid rain and to remove certain pollutants from the rainfall. Extensive green roofs, for example, in central Pennsylvania, are documented to return approximately one-half of the annual rainfall to the atmosphere through evaporation and plant transpiration. This is a substantial reduction in runoff volume when compared to a traditional roof. However, this still leaves 50% of the rainfall to become runoff. This runoff, if it is of appropriate quality, may be available for capture and reuse with minimal treatment after passage through a growth and drainage media that also filters out pollutants.
For green roofs, the early stage encompasses plant establishment, washout of fine particles from the media, ... (what we are trying to say is that it takes a few storms for the finer grains of media to wash out. It's like planting a container plant - the first few waterings see some of the soil washing out). The late life definition is still being studied/defined, but it may be when either the plants do not have sufficient nutrients left in the media to be healthy or when the removal capacity of the media for rainfall pollutants is exhausted. Penn State Harrisburg has several pilot-scale green roof tests ongoing to evaluate both runoff generation (compared to rainfall quantities) and pollutant removal of green roofs (Figure 5).
Roofing materials that pose the least problems in terms of runoff quality
Sustainability initiatives typically focus on rainwater harvesting for landscape irrigation. New studies conducted at Penn State Harrisburg have concentrated on the weathering of roofing materials and the effects of this degradation on roof runoff water quality. Ongoing research highlights the water quality concerns, and potential need for treatment, in the runoff from several common building materials previously advocated for harvesting. If the runoff pollutant loading is high and if the runoff is not treated, potential pollutant accumulation may occur in soils and/or stream sediments – causing problems for the landscaping plants or the aquatic organisms.
The results show that, during the early life of a roofing system, several of the traditional roofing materials (shingles, cedar, uncoated galvanized aluminum) release pollutants into the rainwater (Figures 6 and 7; the box on these box-and-whisker plots bound the 25th and 75th percentile concentrations, while the whiskers show the 10th and 90th percentile concentrations). These pollutants then may be transported in the storm drain system and, eventually, to receiving waters. If the water is harvested for use, several of these materials release pollutants for at least one year after installation at concentrations high enough to be of concern for plant toxicity. Based on these early-life results, it appears that the green roof may not clean the rainwater, but it also does not add pollutants to the water at levels that may be toxic, unlike the uncoated metal roof and metal-impregnated woods. Other findings indicated exposed treated woods and uncoated galvanized metals appear to be inappropriate because of their high metals’ releases for rainwater harvesting, and possibly for direct receiving water discharge as well for a minimum of one year after installation. The roofing material that has shown the least amount of leaching was the coated metal. This research has been performed in the mid-Atlantic region of the United States, and the timing of this early life washoff may vary by region and may be a function of climatic factors, with accelerated aging and pollutant release in areas with high UV exposure and larger temperature fluctuations. It has also measured only total forms of the pollutants and not the potentially more bioavailable dissolved fractions. Further research will be required to determine whether the pollutants seen in this research are bioavailable either to aquatic life or to landscape plants.
Figure 6. Copper (top) and Zinc (bottom) concentrations for several roofing materials © Shirley E. Clark, 2008.Pararagraph 2
Figure 7. Nitrate (top) and Total Phosphorus (bottom) concentrations for several roofing materials © Shirley E. Clark, 2008.
Research also is ongoing to relate the degradation and runoff quality to climatic factors such as interevent period, rainfall intensity, UV exposure, and temperature. Developing relationships between climatic factors and runoff quality will be important, especially if, eventually, an effective series of rapid laboratory tests can be used to predict the environmental friendliness of roofing materials for rainwater harvesting.
Additional information and pdf files of presentations related to this subject can be found at www.personal.psu.edu/sec16/.
The authors would like to thank the multitude of people who made this research possible. From Penn State Harrisburg, this includes the funding from the Graduate Research Council and the School of Science, Engineering and Technology, and the work of the following students: Julia Hafera Spicher, Kelly Franklin Steele, Jim Elligson, Brad Mikula and Christina Y.S. Siu. Dr. Robert Berghage of Penn State University’s Department of Horticulture was invaluable in helping set up this project and educate us about green roofs. This research is being performed as part of Mr. Long and Ms. Nicholson’s thesis projects.
The authors also would like to express their appreciation of the assistance of Dr. Robert Pitt, Ms. Elizabeth Graham and Dr. Pitt’s graduate students for their metals digestion and analysis of a portion of the samples. From the University of Alabama at Birmingham, the authors would like to thank Dr. Melinda Lalor for her support during the entire project, including her work with Dr. Clark on the laboratory testing of the materials in 2004. She also provided support for two students from PSH to participate in the NSF REU program in Environmental Health Engineering, when UAB was attempting to establish a similar setup between hurricanes. Finally, the authors would like to acknowledge the support of the Alabama Water Resources Research Institute for the initial laboratory work performed at UAB.
Clark, S.E., B.V. Long, C.Y.S. Siu, J. Spicher, K. Steele. (2008a). Early-life roof runoff quality: Green vs. traditional roofs. WEFTEC 2008 Conference Proceedings, Chicago, IL, October 2008. Water Environment Federation, Alexandria, VA. CD-ROM.
Clark, S.E., K.A. Steele, J. Spicher, C.Y.S. Siu, M.M. Lalor, R. Pitt. J.T. Kirby. (2008b). Roofing materials’ contributions to stormwater runoff pollution. Journal of Irrigation and Drainage Engineering. Vol. 134. no. 5, pp. 638-645. 2008.
Clark, S.E., M. Lalor, M. Pratap, R. Field, R. Pitt. (2004). Stormwater pollution prevention through appropriate selection of building materials. World Water and Environmental Resources Conference 2004, Environmental and Water Resources Institute of the American Society of Civil Engineers, Salt Lake City, Utah, June 28 – July 1, 2004. CD-ROM.
Long, B., S.E. Clark, K.H. Baker, R. Berghage. (2007). Selecting a green roof medium for water quality benefits. World Environmental and Water Resources Congress 2007. ASCE/EWRI, Tampa, FL, May 15 – 19, 2007. CD-ROM. Long, B., S.E. Clark, K.H. Baker, R. Berghage. (2008). Green roofs – A BMP for urban stormwater quality? 2008 World Environmental and Water Resources Congress Proceedings, Honolulu, HI, May 13 – 16, 2008. American Society of Civil Engineers, Reston, VA. CD-ROM.
McKinstry (2007). Major Natural Resource Issues Facing Pennsylvania. Governor’s Outdoor Conference 2007, State College, PA, March 15-18, 2007. 11 pages. http://www.connectoutdoors.state.pa/us/ McKinstry_GovernorsOutdoorNRIssues_lite.pdf