A recent report by the U.S. Natural Resources Defense Council (NRDC), Smart Growth America and American Rivers, notes that "impervious surfaces significantly change natural patterns of water movement, affecting river flows and the recharge of underground water supplies."6 Impervious surfaces are usually developed or compacted surfaces that do not allow water to be absorbed by soils. The U.S. Environmental Protection Agency has found that changes to the hydrology of rivers are second only to the effects of agriculture in the degradation of river systems.
The NRDC report contains the following information on what happens to water supplies when natural areas are replaced with roads, parking lots and buildings:
- A one-acre parking lot produces 16 times more run-off than an undeveloped meadow [of the same size].
- Wide streets and excessive parking around single-family homes in sprawling developments also contribute to run-off.
- Low stream flows are exacerbated by low groundwater levels, which often occur later in the urbanization process. (One study found that groundwater-influenced stream flow fell to 10% of the regional average when the level of imperviousness in the stream watershed reaches 65%.)
- A review of the literature suggests that a watershed becomes badly degraded after a mere 10% is covered by the various impervious surfaces that come with development.
- The rise in vehicle kilometres travelled has been linked to higher polycyclic aromatic hydrocarbon (PAH) levels in some watersheds (due to tire wear, crankcase oil, roadway wear, and car soot and exhaust). In a study of lake soils at ten sites, six exceeded estimates of concentrations that would have adverse impacts on aquatic life.
The NRDC report notes that "low-density, automobile-dependent development is a leading cause of imperviousness. Transportation-related hard surfaces account for 60% of the total imperviousness in suburban areas."7 The creation of large areas planted with turf grass is also a problem associated with urban sprawl, since "soils beneath our developed turf areas are often as impervious as roads and parking lots. Development involves wholesale grading of the site, removal of topsoil, severe erosion during construction, compaction by heavy equipment, and filling of depressions. Indeed, some studies have shown that with these practices, the infiltration rate of urban soils actually approaches those of impervious surfaces."8 While at the local or neighbourhood scale, denser, more compact development may mean a higher percentage of impervious land, at the urban region scale, aggregate levels of imperviousness will be reduced.
In another NRDC study on "Environmental Characteristics of Smart Growth Neighbourhoods," the benefits of smart growth are explored and the conclusion is reached that, "successful new neighbourhoods can be designed and located to perform more efficiently than conventional suburban developments on several environmental indicators, including land consumption, water use and pollution, and energy use and air pollution from residents' travel patterns."9
6. Natural Resources Defense Council, Smart Growth America, and American Rivers. 2002. Paving Our Way to Water Shortages: How Sprawl Aggravates Drought, p. 7.
7. Natural Resources Defense Council, Smart Growth America, and American Rivers. 2002. Paving Our Way to Water Shortages: How Sprawl Aggravates Drought, p. 8.
8. Natural Resources Defense Council, Smart Growth America, and American Rivers. 2002. Paving Our Way to Water Shortages: How Sprawl Aggravates Drought, p. 8.
9. Natural Resources Defense Council. 2002. Environmental Characteristics of Smart Growth Neighbourhoods, executive summary.