Is Density Good for Water Quality?

Executive Summary

How and where development occurs can affect water quality. Considerable evidence in the literature demonstrates that dispersed, low-density development can exacerbate non-point source pollutant loadings by consuming absorbent open space and increasing impervious surfaces-- relative to compact, mixed use development or redevelopment of brownfields and urban core infill. However, the way this impact is measured is important—population must remain constant because the primary issue state and local governments are addressing is “How will we accommodate future growth?” Some literature and case studies have demonstrated that higher densities can minimize the net water impacts of new growth. However, EPA recognizes that localized hot spots can occur with increased density making best management practices and policy tools necessary in order to prevent degradation or further degradation and to allow water bodies to recover from pervious impairment. The purpose of this paper is to examine the water quality impact from low and high-density development at watershed level.

Background

Streams and their accompanying watersheds provide critical services that sustain larger ecosystems. For example, small watersheds are the ultimate source of our drinking water; watershed soils act as a giant filter to keep drinking water pure; and small streams, and their associated flood plains, serve as conduits for flood waters and act as natural flood control. In addition, the stream corridor, with its rich flood plains, wetlands, and forests, is also home to unique plant and animal species. Streams support diverse aquatic communities and perform vital ecological roles of processing carbon, sediments, and nutrients upon which downstream ecosystems depend.

The services provided by small watersheds are maximized when their land area is maintained in a natural condition. Indeed, the highest and best use of land in a small watershed is forest or native vegetation adapted to the local climate. Watershed services begin to diminish when the natural condition of land is altered within a watershed through development. The loss of free watershed services begins when forests are converted to ranches and farms. However, watershed services begin to decline even more rapidly when these lands are converted to urban uses. The key reason is the transformation of the surface of the watershed. As more land in the watershed is converted to parking lots, roads, buildings, and other impervious surfaces, the area is no longer able to absorb and store rainfall. Consequently, an urban watershed produces a greater volume of stormwater runoff, which in turn degrades the physical, chemical, and biological quality of streams. In part because of urban development, recent water quality data finds that more than 291,000 miles of assessed rivers and streams do not meet water quality standards (USPEA). Across all types of water bodies, states, territories, tribes and other jurisdictions report that poor water quality affects aquatic life, fish consumption, swimming, and drinking water.

Development and its associated activities (e.g., construction activities, increased travel to and from the resulting development, chemical maintenance of the areas in and surrounding the development), also impact water quality. EPA estimates that of the causes of pollution in the states’ impaired waters, only 10 percent is attributable to point source pollution, such as industrial discharges. The rest is attributable to non-point source pollution and combinations of point and non-point source pollution, which includes sedimentation from land development, stormwater runoff, on-site sewage systems, and atmospheric deposition. Clearly, increasing the built environment in a watershed decreases water quality.

Moreover, as metropolitan areas continue to grow, more land will be developed, thereby increasing the region’s built environment. How and where this development occurs will have a profound impact on water quality. EPA believes that water quality impacts from development can be minimized by better planning of where development occurs (e.g., developing urban infill and brownfield sites) and how it occurs (e.g., increasing the density of development, increasing the number of uses (business, residential, commercial, transportation, recreation) within a development, increasing transportation options, preserving some of the development area as open space, building narrower streets, utilizing compact building design). A preliminary scan of the literature suggests that concentrating development, through infill development and brownfield redevelopment in urban areas, should be better for overall watershed water quality than scattering development throughout a watershed (Center for Watershed Protection, EPA’s Chesapeake Bay Program, Trust for Public Land).

The implication of where development occurs is attracting greater attention. Urban sprawl, or low-density development, is increasingly recognized as a primary factor in reducing the quality of streams, lakes, and wetlands in many watersheds. A growing body of research clearly documents that the creation of impervious cover accompanying new growth causes a predicable and profound decline in critical elements of aquatic ecosystems (Center for Watershed Protection). An implication of this research is that low-density development is not only likely to degrade the quality of individual watersheds, but is also likely to degrade a larger number of watersheds than a more compact development pattern. Therefore, concentrating development in urban areas helps to preserve the functions in smaller watersheds at a region’s edge areas because at a regional scale, compact development produces less impervious cover and subjects fewer watersheds to possible degradation (Center for Watershed Protection). In addition, concentrating development in the urban core may help to preserve undeveloped land in edge areas by postponing or eliminating the need to develop the edge areas.

However, developing infill sites or redeveloping brownfield sites may not always be desirable for water quality. For example, while compact urban development may increase watershed-level water quality, this type of development may further degrade site-level or sub-watershed water quality. As EPA has found through its air quality work, there is a potential for creating locational “hot spots,” (e.g., small areas or pockets of degraded environmental quality), with increased brownfield and infill development. Examples of types of infill that may degrade water quality include developing an infill site that is part of an urban greenway; increasing connected impervious surface as a result of infill development; or creating developments, such as golf courses, that may require extensive use of chemicals for maintenance. It is therefore critical that each potential infill and brownfield development project be examined and its projected water quality impacts analyzed. Included in this analysis is the role of site-level and watershed-level water quality. EPA supports a policy of non-degradation of water quality. It is not enough to accept site-level degradation of water quality even though watershed water quality improved. Therefore, mitigation efforts, counter-measures, and strategies must be identified in order to minimize negative impacts to water quality from infill development.

Author and Copyright Information

Ms. Lynn A. Richards
US Environmental Protection Agency– Policy Analyst

Ms. Richards is currently a policy analyst with the United States Environmental Protection Agency. She focuses on all aspects of smart growth, including examining the water quality impacts from different development patterns; developing models for measuring stormwater runoff and associated pollutants from different site designs; and analyzing the regulations and subsidies that support different development patterns. She has recently published, “Alternatives to Subsidizing Edge Development: Strategies for Preserving Rural Landscapes” in Terrain Journal and assisted with the publication of “Getting to Smart Growth: 100 Policies for Implementation” that was recently published by the Smart Growth Network.

Prior to EPA, Richards worked for ICF Consulting and addressed sustainable development, environmental justice, and long-term stewardship. In addition, she also worked with the Government of the Bahamas to establish a Ministry of Environmental Protection. During this time, Richards served on the President’s Council for Sustainable Development Environmental Management Task Force.

Richards holds a MS in Environmental Science and a Masters of Public Affairs from Indiana University.

Lynn Richards
USEPA--Office of Policy, Economics, and Innovation
1200 Pennsylvania Ave., NW [MC 1808]
Washington, DC 20460
T: 202-260-9211
F: 202-260-0174
E: richards.lynn@epa.gov