Environmental▼News PERSPECTIVE and fish and insect habitats. “NOAA’s estimate of impervious cover is significant because it makes New research from the U.S. Nationleagues used land-cover measures cumulative impacts evident by foal Oceanic and Atmospheric Adminfrom satellites, U.S. Census Bureau cusing on the watershed level,” says istration (NOAA) finds that all the road vectors, and satellite images of Rolf Pendall, an urban planner at impervious surfaces—buildings, nighttime lights (which help correct Cornell University. Across the United roads, parking lots, and roofs—in anomalous results given by the States, many of the decisions aimed the continental United States cover other measures) to model the perat reducing imperviousness on a an area nearly the size of Ohio. Excentage impervious surface cover site-by-site basis have actually made perts call this result a wake-up call, on a 1-km grid. The resulting map the runoff problem worse, he says. arguing that the extent of the counreveals for the first time the distribFor instance, when faced with a try’s most developed lands is roughution and density of the impervious choice of placing either 25 or 15 ly double the level considered houses on a site, local regulators safe for stream health and that have chosen the 15-unit option, policy makers therefore need to reasoning that by building fewer deal with rampant suburban homes they will cut impervioussprawl and development. Yet, in ness at the site level, Pendall exorder to make smarter land-use plains. The dilemma is that choices and implement new inoften the other 10 houses are centives to reduce the area of eventually constructed on large impervious surfaces, decision lots elsewhere, generating a patmakers must know precisely tern of low-density sprawl and how such development affects high levels of impervious surwater quality, and scientists are face area per capita. If decision having a hard time providing makers shifted their focus to the those answers. watershed or regional level, they Measuring the amount of impervious surfaces, Urban runoff—the soup of would see that the better choice such as roads, sidewalks, and buildings, could be nutrients, sediments, metals, would be to reduce imperviousa way for planners to use smart growth to protect local watersheds. and petroleum hydrocarbons ness per capita by focusing that is swept off streets and growth in already-developed roofs by rainfall—is one of the leadsurfaces in the continental United areas with higher density, a pattern ing and most rapidly growing causStates. The total covered area is known as smart growth, he points es of water pollution, according to 112,610 km2, just less than the area out. the U.S. EPA. As more impermeable of the state of Ohio (EOS 2004, 85, Federal and state stormwatermaterials, such as asphalt, concrete, 233–240). control regulations, with their emNOAA’s value for national imperphasis on controlling runoff on a and bricks, cover rainfall-absorbing vious cover compares well to a site-by-site basis, are partly to forests, meadows, and wetlands, Pennsylvania State University estiblame for the lack of regional perthe volume of runoff increases. If Americans continue to gobble up mate made with Landsat satellite spective, says Dana Beach, execuland at twice the rate of population data on a 30-m grid for the state of tive director of the South Carolina growth, impervious surfaces will Pennsylvania, says Toby Carlson, a Coastal Conservation League and cover up another 68 million acres of meteorologist at Pennsylvania State author of the Pew Oceans Comland in the next 25 years for a total University. He selected 40 watermission report. By emphasizing of 174 million acres, an area larger sheds and calculated an average of “end-of-pipe” controls, such as than the state of Texas, concludes 3.2% impervious cover using his 30stormwater detention ponds, the an earlier study by the Pew Oceans m grid, slightly less than the averbest-management practices outCommission. age of 3.7% derived from NOAA’s lined in the regulations create a NOAA’s new research estimates 1-km grid. false sense that they alone will solve the extent of impervious surfaces, Measures of imperviousness are the runoff problem, he says. In fact, or land cover, more explicitly than important because they can be most suburban developments are the Pew study, which used landused to predict the health of water30–35% impervious, which is so far cover classes that included a mix of sheds, Carlson says. In general, above the 10% threshold of damage impervious surfaces as well as perstream quality begins to decline to streams that the best-managemeable areas such as lawns, small when the impervious cover in a wament practices cannot substantially parks, and golf courses, explains tershed reaches 10%. When a waterreduce runoff to safe levels, Beach Chris Elvidge, a physical scientist at shed’s impervious cover approaches says. Cities, of course, are even NOAA and lead author of the new 25%, streams in the watershed are more impervious. study. To get a sharper picture of unable to support basic functions, Calling the NOAA study a “wakeimpermeable areas, he and his colsuch as providing channel stability up call”, Beach notes that if the RHONDA SAUNDERS
Is smart growth better for water quality?
OCTOBER 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 365A
Environmental▼News PERSPECTIVE amount of impervious surface in the United States is roughly 25 million acres, then the 100 million acres of total human-developed land, as calculated by the U.S. Department of Agriculture’s National Resources Inventory, is on average about 25% impervious. “This means that we are not in the margin of likelihood of being successful with best-management practices,” he says. The answer is to plan at the regional level and direct growth into higher-density neighborhoods that free people from dependence on cars, he says. While smart growth is acclaimed as the best method for reducing imperviousness per capita, there is very little hard evidence for its impact on water quality, says Glenn Moglen, hydrologist at the University of Maryland. Part of the challenge of quantifying the relative impacts of sprawl versus smart growth is that no standard method exists for measuring imperviousness, he says. Some researchers rely on satellite images, others on GIS land-use maps; their measures can vary by a factor of two. “Then there are different ‘flavors’ to imperviousness—a parking lot in the middle of a field is not going to have as much impact as one directly connected to a stream,” he points out. In addition, 10% imperviousness as measured by, for instance, satellite, may not be the same as 10% impervious cover found by the study that established the stream damage threshold, Moglen warns. Nevertheless, in theoretical comparisons, smart growth seems to be the best way to protect water quality, Beach says. A stormwater model applied to a 583-acre site in South Carolina revealed that sprawl development there would generate 43% more runoff and 3 times more sediment than a dense town-style settlement with the same number of dwelling and commercial units, according to research by Elizabeth Blood, an ecologist at the Jones Ecological Research Center in Georgia. “However, some smart-growth developments can cause more harm than good to watersheds if they are sited without regard to watershed
conditions,” says Philip Berke, a landuse and environmental planner at the University of North Carolina. New urbanism, a design concept that clusters dwelling units in high densities and promotes a mix of residential and commercial uses, is rapidly gaining ground in the South and has been encouraged in legislation in 10 states, he says. Berke has found that while new urban designs incorporate more bestmanagement practices and have less impervious surface cover per capita than sprawl developments, more than two-thirds of them are built on greenfields rather than urban infill sites. “A low-density sprawl project may generate less impact on a greenfield site in a pristine sub-watershed than a higher-density smart-growth-oriented development,” Berke says. In other words, individual smart-growth projects must be sited based on sound land-use policy as well as sound sitedevelopment practices, he cautions. The problem highlights the need to combine both regional and site planning to reduce runoff, he says. That approach is promoted in a new EPA report, Protecting Water Resources with Smart Growth, released on July 7. The report lists case studies to illustrate 75 policies that local governments can adopt to meet federal stormwater regulations while also achieving other objectives such as downtown revitalization, says Geoffrey Anderson, director of the development, communities, and environment division at EPA. For example, by developing brownfields, communities can reenergize blighted spaces without increasing runoff, Anderson says. The Atlantic Station project on a former brownfield in Atlanta, Ga., accommodates 3100 residences and 7 million square feet of commercial space on 138 acres. It generates 5 times less runoff, 4 times less sediment, and 16 times less phosphorus than it would if located on a suburban greenfield of 1200 acres, the EPA report says. The report, along with NOAA’s data on impervious cover, points to a whole new set of tools to protect water quality at the source before it is impacted, Anderson says. —JANET PELLEY
366A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / OCTOBER 1, 2004
