GREEN Engineering Promote LowApplying the principles of green engineering can help create more sustainable development. A LLEN P. DAV IS UNIV ERSIT Y OF M A RY L A ND
Principles Impact Development
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s human populations increase, relocate, and spread from existing urban areas, pressures to build on undeveloped lands have become severe. Unfortunately, traditional development practices put the environment at risk by creating large tracts of impervious area, thereby destroying natural
infiltration and buffer zones. Altering the land in these ways can devastate local water bodies by increasing erosion; mobilizing sediment; and dispersing nutrients, toxic substances, and other pollutants. The large amounts of polluted water that run off urbanized land areas can destroy habitats and disrupt entire ecosystems. However, proper attention to land development methodology and engineering can minimize
The concept of low-impact development (LID) integrates environmental concerns with land development, focusing on water and pollutant balances. Also known by other names, such as environmentally sensitive design, LID represents a fundamental change in the way residential, commercial, and institutional properties are developed and minimizes negative impacts on the environment and local ecology. How land is developed deserves increased attention, especially as nonpoint-source pollution concerns move to the forefront in many water-quality evaluations, and total maximum daily load (TMDL) regulations inch closer to reality. In a fundamental sense, LID represents the application of pollution prevention and waste minimization concepts to land development. LID meshes with the principles © 2005 American Chemical Society
of green engineering as applied to land development for environmental benefit (1).
Consequences of urbanization Converting “natural” land, such as forest, to residential or commercial use drastically changes its hydrological characteristics. Vegetation is uprooted and replaced by impervious surfaces—rooftops, driveways, roadways, parking lots, and sidewalks. Even the remaining vegetated areas in residential developments tend to be smoothed, and small water storage depressions are eliminated. Diverse plant species are replaced with monoculture grasses that are regularly cut short and may be inundated with fertilizers and pesticides. Sacrificing open land for impervious surface alters the water balance by promoting polluted run-
IM AGE S F ROM THE MARYLAND STATE HIGHWAY ADMINISTRATION, RHONDA SAUNDERS, AND PHOTODISC.
these adverse environmental impacts.
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off at the expense of infiltration (2). Development negatively impacts water quality in two ways. First, many of the new materials and components used in land development contribute higher pollutant loads during rainfall and subsequent storm-water runoff. Second, the natural filtering action of wild vegetation (trees, bushes, tall grasses) is replaced by concrete, asphalt, and rooftops, which offer little means for water-quality improvement. Conventional management of urban storm water aims to move runoff away from a developed area as quickly as possible—from impervious surfaces to gutters to storm drains and finally to stream discharge. Water is not permitted to pool. Although this approach minimizes ponding and flooding, it tends to lower water quality. Larger impervious areas lead to higher levels of more diverse pollutants, diminished pollutant removal during overland flow, reduced infiltration, and greater runoff peak flows, which, in turn, can increase stream erosion.
stresses to mobilize deposited pollutants. With the higher volumes and velocities of sheet flow over rooftops, driveways, sidewalks, and roadways, water can dissolve deposited pollutants and mobilize solids and oils that have accumulated on these surfaces between rainfall events. Because of these mechanisms, the fraction of area that is impervious is considered as a primary measure linked to water-quality decline in urban areas. Studies have indicated that modifications in stream ecology occur when the proportion of impervious area is only a few percent. Once the impervious fraction reaches 10–30%, researchers see major declines in habitat and water-quality indicators (3).
Pollutant sources
Most traditional pollutants, including suspended solids, nutrients, and heavy metals, are found at fairly high levels in urban and roadway runoff (Table 1). Any disturbance that creates bare soil provides the potential for rain or runoff to moTA B L E 1 bilize sediments. In addition, greater overland Typical pollutant concentrations in urban storm-water flows from compacted runoff for different land uses and impervious areas increase erosion rates of Median event mean nearby soils. Loss of vegconcentration for land use etation eliminates the Pollutant (units) Residential Mixed Commercial possibility of attenuaBiological oxygen demand (mg/L) 10 7.8 9.3 tion of suspended solChemical oxygen demand (mg/L) 73 65 57 ids and results in total Total suspended solids (mg/L) 101 67 69 suspended solids levels Total lead (µg/L) 144 114 104 from tens to hundreds Total copper (µg/L) 33 27 29 of milligrams per liter in urban runoff. Fertilizers Total zinc (µg/L) 135 154 226 are applied to managed Total Kjeldahl nitrogen (µg/L) 1900 1288 1179 lawns and other landNitrate and nitrite (µg/L) 736 558 572 scaped areas, possibly Total phosphorus (µg/L) 383 263 201 several times per year. Soluble phosphorus (µg/L) 143 56 80 If not administered apSource: Reference 12. propriately, nitrogen and phosphorus compounds Initially, storm-water management regulations readily wash from these lawns and from spillover tended to focus primarily on controlling quantity, onto paved areas. Nitrate deposition from automowith minimal provisions for improvement of water bile NOx emissions becomes part of the runoff flows. quality. Retention ponds constructed to store runoff In high-density residential areas, wastes from pets from large storm events allowed water to gradually (if not cleaned up), birds, squirrels, and other urenter the stream environment. Little improvement ban animals are washed away by rainfall and add in water quality was expected. Meanwhile, municsignificant organic and pathogen loadings. ipal and industrial point-source water pollution Moreover, a soup of toxic compounds exists in discharges have been increasingly reduced. With urban storm-water runoff. Heavy metals, such as continuing urbanization in sensitive watershed arcopper, lead, and zinc, are released from automoeas, the emphasis must now shift to controlling polbiles, from fluid leaks; brake and tire wear; and lutants originating from nonpoint sources, such as corroded and weathered paints, coatings, decoraurban storm-water runoff. tive flashing, galvanized materials, and structural However, not only are nonpoint sources more components (4–6). Motor oil, fuel, gear fluids, and difficult to manage than point discharges, but the coolants from vehicles are readily washed from imsources, mobilization, and transport of pollutants pervious surfaces, cause oil sheens, and can directly are also much more complex. The complexity arisimpact aquatic life. Numerous toxic hydrocarbons es from the interdependence of pollution, accumuand their oxidized products, which may persist in lation, and mobilization factors with hydrological the environment, are also present in these fluids. behavior. The kinetic energy of rainfall and the moPAHs may be found in leaking fluids, incompletely mentum of flowing water provide sufficient shear combusted fuel in atmospheric deposits, and as-
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phaltic products used in roofing and pavements. Various herbicides and insecticides are frequently used to control weeds and damage from insects in green urban areas. Again, without careful control of application, excess pesticides will leach from the application areas. Collectively, pollutants derived from large developed areas can have major impacts on water bodies and their supported ecosystems during wet-weather flows, resulting in habitat degradation and loss, biological species shifts, and aquatic mortality. Receiving water bodies used for swimming or as a drinking-water supply may be compromised. Some of these pollutant sources are unavoidable with current practices, but development focused on water runoff issues can reduce impacts on the environment.
Elements of LID LID is a philosophy for development that focuses on specific sustainable water conservation goals. Its aim is land use and management that minimize adverse environmental impacts. This process begins with planning at the watershed level. Various factors affect the flow and quality of water as it leaves the watershed; these include development densities; the placement and mixing of developed and undeveloped lands; and how residential, commercial, and other land uses are combined. However, our current understanding of the effects of these “smart-growth” planning features is incomplete and difficult to quantify. The environmental consequences of these planning decisions are very likely to be site-specific and will be influenced by regional development goals. LID designs attempt to replicate pre-development hydrologic conditions as closely as possible. The desired result is not only a reduction in stormwater runoff from the site but improvements in water quality as well. LID is primarily focused on design characteristics at the individual lot level, and improvements are expected as a cumulative impact integrated over the entire developed area. LID objectives incorporate as little impervious surface as possible and keep any runoff on site as long as possible with natural approaches. Design. LID design begins with site preparation. Rather than completely clear-cutting and leveling sites, designers leave as much wooded area on each lot as possible and try to avoid disturbing natural topographic depressions. Lots are designed with narrow driveways and minimal sidewalks. Streets are kept as narrow as local zoning and building codes will allow. To minimize soil compaction, the use of heavy equipment is discouraged. Vegetated swales and filter strips are encouraged, instead of elaborate curb-and-gutter systems that rapidly convey runoff. Rooftop downspouts are directed into vegetated areas, and rain barrels capture water for later use. Permeable paving materials are used rather than traditional asphalt and concrete. Collectively, these actions assist in keeping precipitation and runoff on the lot. Fewer impervious surfaces create less runoff. Swales and natural depressions allow for some on-lot storage, thus promot-
ing infiltration and evapotranspiration. Preventing compaction of soils encourages natural infiltration. Overland water flow is slowed by vegetation, depressions, and meandering; this gives water time to seep into the ground, mobilizes fewer pollutants, and allows particulate matter to settle or be filtered. Keeping trees or small, forested areas promotes infiltration, lowers ambient temperatures, and reduces lawn use, which, in turn, decreases applications of fertilizers and pesticides. These ideals follow several of the principles of green engineering (1). Foremost, when land is not paved, roofed, or even converted to lawn, these areas cannot create excess runoff and contribute pollutants from autos and lawn chemicals; this demonstrates Principle 2 (prevention instead of treatment). Care in the land development process, especially with driveways and sidewalks, both with the final design and with the construction phase, falls under Principle 8 (meet need, minimize excess). “One size fits all” designs must be avoided to minimize runoff and maximize on-site water collection and infiltration. Designs to direct downspouts onto lawns and store roof runoff in rain barrels follow Principle 10 (integrate local material and energy flows). Integrating natural areas into land development plans allows ecosystem maintenance and sustainability to be considered and emphasizes Principles 6 (entropy and complexity must figure into design) and 12 (materials should be renewable, not depleting). Management practices. Of course, some impervious area cannot be avoided. Nonetheless, excess runoff can still be managed with LID and green engineering considerations. When some land is sacrificed to impervious cover, the remaining land surface can be engineered for greater infiltration by increasing overland flow time on the lot, creating storage, and modifying soils to promote infiltration. Specific vegetated runoff management practices can be incorporated within individual lots and also used to collect runoff from larger areas, such as several lots, roadways, or parking lots. Vegetated areas are used in LID wherever overland flow is expected. Sheet flow is directed over vegetated filter strips, which include grass or, better yet, wild grass, shrubs, or trees. Vegetated swales are used to convey concentrated flows. Soils should be engineered and managed to encourage infiltration. Studies on grassed swale’s efficiency in removing pollutants emphasize the importance of infiltration, because small flows may be completely infiltrated during transport through the swale (7 ). Bioretention is a vegetated management practice designed to collect, store, infiltrate, and treat runoff. Bioretention areas, also known as rain gardens, may be concave flower gardens or landscaped areas. The rain garden is placed near the boundary of an individual lot to accumulate and manage escaping water. The depression is several centimeters deep, provides storage, and promotes evapotranspiration and infiltration. In a property with multiple lots, runoff from parking areas is conveyed into the bioretention facility. These facilities may be constructed in a parking island or linearly along a roadway AUGUST 15, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 341A
with curb-cut inlets (Figure 1). Overflow drains or swales leading into the adjacent traditional stormdrain infrastructure handle large flows or pooling that exceeds depths of ~15–30 cm. An engineered porous soil mixture encourages infiltration. Larger facilities are generally designed with an underdrain, which is typically a perforated plastic pipe that sits ~75 cm below the soil’s surface. FIGURE 1
Bioretention facilities
ALLEN DAVIS
DEREK WINOGR ADOFF
Also known as rain gardens, bioretention facilities use planted landscapes that are designed to collect, store, filter, and treat runoff. For example, these gardens may be constructed (a) in an island of a parking lot or (b) parallel to a roadway.
Some studies have documented significant water-quality improvement through bioretention and infiltration processes. Bench-scale, pilot-scale, and some limited full-scale studies generally show nearly complete removal of suspended solids, oil and grease, and several heavy metals (8, 9). Moderate removal of phosphorus, total Kjeldahl nitrogen, and ammonium has also been noted (8). Vegeta342A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / AUGUST 15, 2005
tion in the bioretention facility is expected to promote evapotranspiration, contribute to the capture and degradation of pollutants, and help maintain the soil infiltration capacity. Impervious rooftops are one of the most difficult runoff problems to address. Green roof technology has significantly advanced in the past decade, and these roofs are now commonplace in Europe. Green roof design generally includes a layer of several centimeters of soil, sand, or another medium to capture the first few centimeters of rainfall. Sedum plants, which have simple growth needs and high evapotranspiration rates, can thrive on rooftops. Through the actions of the plants and soil media, flow volumes and velocities are minimized. Green roofs are an example of applying the principles of green engineering to LID (10). Incorporating simple LID concepts into a site design can significantly reduce runoff flow and pollutant loads (11). For example, ammonia (80–85%), nitrate (66–79%), suspended solids (91–92%), copper (81–94%), iron (92–94%), lead (88–93%), manganese (92–93%), and zinc (75–89%) annual loads were decreased significantly by incorporating porous paving and swales into a parking lot of the Florida Aquarium in Tampa, Fla. The entire water volume was kept on-site during small storm events and eventually infiltrated or evapotranspired; this resulted in no release of water or associated pollutants from the site boundaries. With large storms and saturated soils, however, runoff is not greatly influenced by the LID techniques. Anywhere green space is present, some innovative thought and application of green engineering principles can make it functional for water management. Even “nonfunctioning” space can be made functional. For example, grass swales or bioretention areas can be constructed along the edge of traditional parking spaces (Figure 2). The landscaped area is placed where the front or back of the car would overhang. Generally, this space is not used for wheel contact; thus, no valuable parking space is sacrificed. Obstacles. Unfortunately, several logistic issues hinder complete implementation of the LID philosophy around the world. For example, some LID concepts are not allowed or are highly restricted under many current zoning and regulatory statutes. Easy access for school buses, garbage trucks, and emergency equipment requires a minimum street width. Concrete curbs and gutters may be required to rapidly convey runoff because of safety concerns from ice formation. Runoff collection and ponding may be discouraged because of public-health concerns about mosquito breeding. Continued research, development, and case-study information are needed to address these issues. Economic concerns will always be important in driving LID implementation. LID construction may be more costly than leveling the land, because crews must work around trees and natural vegetated areas. On the other hand, narrower streets, the absence of curbs and gutters, and smaller stormwater management infrastructure may reduce
Benefits and drawbacks Because of the dispersed nature of the pollutant fluxes and the focus on small, dispersed practices, environmental benefits from LID can be difficult to prove. Careful monitoring studies must be implemented so that expected improvements in water quality can be documented within the wide variability in rainfall, runoff flows, background water-quality characteristics, and development options. Both modeling and monitoring programs can assist in determining designs that truly produce the lowest impact. Using LID solves some environmental issues but unfortunately raises others. For example, one particularly prominent aspect of nonpoint-source waterquality improvement practices is the “ownership” of pollutants after they are captured. The traditional development infrastructure encourages the flushing of pollutants from developed lands into the local waterways; thus, these pollutants are transported and diffused throughout the aqueous environment. Capture, however, creates ownership. Toxics such as heavy metals are a particularly tough challenge. For example, data suggest that these metals are efficiently captured via bioretention. Over the lifetime of a facility, the accumulating metals must be periodically removed, or owners must face the unattractive notion of completely excavating contaminated media every few decades. One possible management option is to periodically remove layers of the bioretention mulch and media surface. Another attractive alternative is to carefully manage the vegetation and cut it on a regular basis, thereby creating a controlled pathway for the removal of many pollutants. In addition, novel hyperaccumulating plants could be incorporated into a traditional landscape management program. For sustainable LID systems, natural pollutant degradation mechanisms must be integrated into the design and operation. Nutrient cycling must be emphasized, and opportunities for biological degradation of captured pollutants must be optimized. Biological processes could include “traditional” environmental engineering microbial processes that degrade captured pollutants, such as fuel hydrocarbons and organic pesticides. However, the role of macrophytes (larger plants and trees) must be considered when phytoremediation tools are incorporated into the LID environment. Vegetation plays an important role in the water balance and the fate and cycling of
many pollutants, including nitrogen, phosphorus, and metals. Worms and insects can also affect pollutant fates through tunneling, soil digestion, and other processes. Because suspended solids are accumulated by LID practices, it is important to encourage biological processes that allow the natural integration of these particulates into the soil media structure to prevent clogging of infiltration areas. FIGURE 2
Making use of nonfunctional space In a parking lot, grass swales or bioretention landscapes can be constructed along the edge of traditional parking spaces. No valuable parking area is lost, because the landscaped area is placed where the front or back of the car would overhang. ALLEN DAVIS
costs. In the end, builders will desire the same return on investment through LID as with traditional development. If LID landscaping were to become accepted and commonplace, costs could be similar to those of traditional landscaping. Public education and an understanding of public perception will be critical to the success of LID. Environmental managers must demonstrate to the public the importance of “environmentally friendly” land development and management. Will the public accept and maintain LID lots and subdivisions? Economics and social issues are linked: Landowners will look to the aesthetics, ease of maintenance, and return on investment with owning LID properties.
Infrastructure materials also demand some discussion within the LID philosophy—even if the land designer has minimal control over what is used to create the developed watershed or subsequently transported within it. Ultimately, we must look to the substitution of building materials and automobile components with those that leach less and are less toxic but still meet their respective performance requirements. Principle 1 of green engineering states that all material inputs should be as inherently nonhazardous as possible. Ultimately, LID plays a role in larger, long-term development philosophies. Pollutant reduction via LID may be less costly than complex structural management practices, and future development in an area may be predicated on lower pollutant loads via new TMDL requirements. Who is responsible for funding pollutant reduction and who benefits from these practices remain to be determined. In the end, local AUGUST 15, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 343A
governments or homeowners’ associations may have to regulate and enforce LID design, construction, and maintenance and subsidize the costs. From a regional development perspective, incorporating LID should not encourage urban sprawl. A forced overimplementation of infiltration practices could propel the development beyond its initial boundaries and result in more land being consumed. The cumulative environmental impact may be greater than that of traditional approaches if more undeveloped land is used and more roadway infrastructure is created to connect the sprawl development. LID practices should be carefully integrated into all development densities without forcing density reduction. High-density LID represents a formidable challenge. Certainly, other important issues exist. Clearly, acceptance of LID will require continued research, performance monitoring, demonstration, and study. Fundamental studies will need to be coupled with practical applications. Expertise from many different disciplines is needed to address important issues, and cooperation among impacted parties is necessary. Conscientious land development decisions based on the best available supporting science are the best ways to protect our aquatic ecosystems. Allen P. Davis is the director of the Maryland Water Resources Research Center and a professor of civil and environmental engineering at the University of Maryland. Address correspondence to him at
[email protected].
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References (1) Anastas, P. T.; Zimmerman, J. B. Design through the 12 Principles of Green Engineering. Environ. Sci. Technol. 2003, 37, 94A–101A. (2) McCuen, R. H. Smart Growth: Hydrologic Perspective. J. Prof. Iss. Eng. Ed. Pr. 2003, 129, 151–154. (3) Wang, L.; et al. Impacts of Urbanization on Stream Habitat and Fish across Multiple Spatial Scales. Environ. Manage. 2001, 28, 255–266. (4) Davis, A. P.; Burns, M. Evaluation of Lead Concentration in Runoff from Painted Structures. Water Res. 1999, 33, 2949–2958. (5) Davis, A. P.; Shokouhian, M.; Ni, S. Loadings of Lead, Copper, Cadmium, and Zinc in Urban Runoff from Specific Sources. Chemosphere 2001, 44, 997–1009. (6) Councell, T. B.; et al. Tire-Wear Particles as a Source of Zinc to the Environment. Environ. Sci. Technol. 2004, 38, 4206–4214. (7) Yu, S. L.; et al. Field Test of Grassed-Swale Performance in Removing Runoff Pollution. J. Water Res. Pl.-ASCE, 2001, 127, 168–171. (8) Davis, A. P.; et al. Laboratory Study of Biological Retention (Bioretention)for Urban Storm Water Management. Water Environ. Res. 2001, 73, 5–14. (9) Davis, A. P.; et al. Water Quality Improvement through Bioretention: Lead, Copper, and Zinc. Water Environ. Res. 2003, 75, 73–82. (10) McDonough, W.; et al. Applying the Principles of Green Engineering to Cradle-to-Cradle Design. Environ. Sci. Technol. 2003, 37, 434A–441A. (11) Rushton, B. T. Low-Impact Parking Lot Design Reduces Runoff and Pollutants Loads. J. Water Res. Pl.-ASCE, 2001, 127, 172–179. (12) U.S. EPA. Preliminary Data Summary of Urban Storm Water Best Management Practices; Report EPA-821-R99-012; Washington, DC, Aug 1999.
