Performance of Engineered Soil and Trees in a Parking Lot Bioswale Qingfu Xiao a & E
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This article was downloaded by: [University of California Davis] On: 08 August 2011, At: 09:50 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Urban Water Journal Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/nurw20 Performance of engineered soil and trees in a parking lot bioswale Qingfu Xiao a & E. Gregory McPherson b a Department of Land, Air, and Water Resources, University of California, Davis, USA b USDA Forest Service, Pacific Southwest Research Station, Urban Ecosystems & Social Dynamics Program, Davis, CA, 95618, USA Available online: 08 Aug 2011 To cite this article: Qingfu Xiao & E. Gregory McPherson (2011): Performance of engineered soil and trees in a parking lot bioswale, Urban Water Journal, 8:4, 241-253 To link to this article: http://dx.doi.org/10.1080/1573062X.2011.596213 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan, sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. Urban Water Journal Vol. 8, No. 4, August 2011, 241–253 RESEARCH ARTICLE Performance of engineered soil and trees in a parking lot bioswale Qingfu Xiaoa* and E. Gregory McPhersonb aDepartment of Land, Air, and Water Resources, University of California, Davis, USA; bUSDA Forest Service, Pacific Southwest Research Station, Urban Ecosystems & Social Dynamics Program, Davis, CA 95618, USA (Received 23 September 2010; final version received 6 June 2011) A bioswale integrating an engineered soil and trees was installed in a parking lot to evaluate its ability to reduce storm runoff, pollutant loading, and support tree growth. The adjacent control and treatment sites each received runoff from eight parking spaces and were identical except that there was no bioswale for the control site. A tree was planted at both sites. Storm runoff, pollutant loading, and tree growth were measured. There were 50 storm events with a total precipitation of 563.8 mm during February 2007 and October 2008. The bioswale reduced runoff by 88.8% and total pollutant loading by 95.4%. The engineered soil provided a better aeration and drainage for tree growth than did the control’s compacted urban soil. The superior performance of the bioswale demonstrated its potential use for large-scale application in parking lots and roadsides to reduce runoff and support tree growth. Keywords: urban runoff; water quality; engineered soil; bioswale; parking lot Introduction These decentralised BMPs used source-control ap- Increasing urbanisation has resulted in the construc- proaches with potential to significantly reduce urban tion of more roads and parking lots, significantly stormwater runoff quantity (Shuster et al. 2008). increasing the amount of impervious land cover in our Vegetative swales are recommended by the USEPA towns and cities (Arnold and Gibbons 1996). Davis as BMPs to clean pollutants and improve water et al. found parking lots accounted for 5% of urban quality (Barrett et al. 1998, Cheng 2003, Vyslouzilo- land use in the Upper Great Lakes region and 7% of va et al. 2003, USEPA 2004, Matteo et al. 2006, Liu the total urban area in Tippecanoe, Indiana (Davis et al. 2007). Vegetation reduces surface runoff by et al. 2010a, 2010b). In metropolitan Sacramento, canopy interception (Xiao et al. 2000a) and improves parking lots make up 12% of the urban land (Akbari infiltration of compacted subsoil (Bartens et al. et al. 2003). Impervious land cover alters the quantity 2008). Some of the intercepted rainwater will never and quality of surface runoff because of its effects on reach the ground surface to produce surface runoff. surface retention storage, rainfall interception, infiltra- Increasing canopy cover is one approach to reducing tion, runoff temperature, and contaminants. Large surface runoff. Indeed, there are municipal storm- volumes of excess storm water runoff from urbanised water credit programs in several US cities that areas cause flooding, water pollution, groundwater promote tree planting and conservation (Greensboro Downloaded by [University of California Davis] at 09:50 08 August 2011 recharge deficits, destroyed habitat, beach closures, 1996, Haubner et al. 2001, City of San Jose 2006, and toxicity to aquatic organisms (Greenstein et al. City of Austin 2007, City of Portland 2008, City of 2004, USEPA 2005). Runoff from parking lots has Seattle 2008). For example, The City of San Jose, unique flushing effects (Black 1980) and can have California has a program that gives credits for new significant ecological impact. Best Management Prac- trees planted within 9.1 m of impervious surfaces tices (BMPs) have been developed to reduce parking and for existing trees kept on a site if the trees’ lot runoff and pollutant loading. These BMPs include canopies are within 6.1 m of impervious surfaces. onsite treatments such as sand filters (USEPA 1999b), The credit for each new deciduous tree is 9.3 m2, and permeable pavement (Booth and Leavitt 1999), low- the credit for each new evergreen tree is 18.6 m2. The impact parking lot designs (Rushton 2001), and other credit for existing trees is the square-footage equal to types of treatment systems (Sonstrom et al. 2002). one-half of the existing tree canopy. Up to 25% of a *Corresponding author. Email: [email protected] ISSN 1573-062X print/ISSN 1744-9006 online Ó 2011 Taylor & Francis DOI: 10.1080/1573062X.2011.596213 http://www.informaworld.com 242 Q. Xiao and E.G. McPherson site’s impervious surface area can be treated through maximise the time water spends in the swale, which the use of trees. aids the trapping of pollutants (USEPA 1999a). Impervious land cover not only causes environ- Biological factors also contribute to the breakdown mental problems downstream but also creates many of certain pollutants. problems that affect our daily quality of life. Urban The engineered soil used in this study offers several heat islands (Swaid and Hoffman 1989, Taha et al. advantages over other engineered soils. It is made of 1991, Simpson et al. 1993) and air quality problems natural materials (75% lava rock and 25% loam soil, (Smith 1978, McPherson and Simpson 1999, Scott porosity: 45.3%) that are readily and inexpensively et al. 1999, Simpson et al. 1999, Nowak et al. 2000) are available in California. Because the main structural directly caused by impervious land cover and the lack element is lava rock, the soil is very porous with high of vegetation cover. Tree planting programs have been water storage capacity, so it stores more stormwater established in many cities across the United States and makes more water available to the trees. The lava (such as Denver, CO, Los Angeles and Sacramento, rock has a very high surface area to volume ratio, with CA, and New York, NY etc.) to improve urban air nooks and crannies that trap pollutants and foster quality and reduce the urban heat island (Hickenloo- growth of bacteria that decompose nutrient pollutants per 2008, Sacramento Tree Foundation 2008, McPher- (Xiao and McPherson 2008). Reducing surface runoff son et al. 2010). Many cities in California have reduces pollutants travelling downstream into the established parking lot ordinances that require 50% receiving water body. The potential of this engineered of paved area shaded within 10 to 15 years (McPherson soil system was tested in laboratory experiments (Xiao 2001) and treatment for 85th percentile 24-hour runoff et al. 2006). The purpose of this study was to evaluate event (California State and Regional Water Quality its effectiveness in the field. In this study a bioswale Control Boards 2008). However, actual shading is was constructed adjacent to an existing parking lot. often less than required. One reason is stunted tree The objective was to evaluate its performance in terms growth due to inadequate soil volume, drainage, and of pollutant removal rate, storm-runoff reduction aeration. Polluted urban stormwater runoff eventually capacity, and tree growth. builds up pollutants in surface soils and sub-surface sediments to cause environmental problems, including Methods groundwater contamination (Mikkelsen et al. 1997) and the growing risk of heavy metal uptake by humans Study site and livestock (Camobreco et al. 1996, Moller et al. The study site was located at the University of 2005). California Davis (UCD) campus (Davis, California, Recently, green infrastructure technology (Bartens 12184603200W, 3883200900N). The campus is located in et al. 2008, Day and Dickinson, 2008, Bartens et al. the heart of the Central Valley, between the Coast 2009) or biofilters (Bratieres et al. 2008, Blecken et al. Range to the west and the Sierra Nevada to the east. 2009, Hatt et al. 2009) has integrated engineered soil The climate is Mediterranean, summers are sunny, hot, and vegetation to treat and store parking lot surface and dry while winters are wet but not cold, it rarely runoff. Engineered soil, a mixture of stones and regular snows. On average, 90% of the average annual soil, is friendly to trees in urban environments because precipitation 446.0 mm occurs between November it is more porous than native backfill urban soil and April.