Impervious surface areas outpace remaining wetlands – study

Hye Jeong, Greenwire reporter

In a striking example of how significantly human development has altered the U.S. landscape, a new mapping effort by the National Oceanic and Atmospheric Administration shows that impervious surfaces cover nearly 5,500 more square miles in the contiguous United States than all of the nation's remaining herbaceous wetlands, or enough to cover the entire Florida Everglades more than 10 times over.

Moreover, to keep pace with future population growth and associated demand for new housing, roads and other infrastructure, developers will alter hundreds of thousands of additional acres of natural surface every year, according to the research conducted by five scientists and summarized in a brief article in the journal Eos.

The findings are significant, according to Christopher Elvidge, the mapping project's lead researcher and a scientist at NOAA's National Geophysical Data Center, because impervious surfaces -- such as roads, parking lots, sidewalks and rooftops -- prevent stormwater from seeping into the ground where it can be filtered by soil and pollutant-absorbent plants. Rather, rain that falls on such surfaces tends to run directly to the nearest surface water, often carrying a myriad of pollutants from oil and gas residues to sand and silt to common litter.

Such stormwater runoff pollution, including substances that run off nonimpervious surfaces such as farms and fields, is considered by the U.S. EPA to be the greatest source of environmental stress to the nation's waterways and coastal estuaries today. And while many modern developments incorporate stormwater retention basins and other means for trapping runoff, the problem remains one of the hardest to regulate because stormwater collects from virtually any surface where rain falls.

In addition to water pollution problems, scientists believe impervious surfaces contribute to the "heat island effect," in which heat from the sun is absorbed by hardened, and often dark-colored, surfaces where it is released more slowly, thus raising average temperatures in local areas. The heat island effect is most pronounced in large cities where impervious surface covers large areas and increasingly in suburbs where large, asphalt parking lots are common.

"Impervious surfaces are a major part of our land cover right now," noted James Vogelmann, one of the report's authors and a scientist at the U.S. Geological Survey's Earth Resources Observation Systems Data Center. According to estimates offered by Vogelmann and his research team, population growth in the United States will result in more than 1 million new single family homes and 10,000 miles of new roadways annually over the coming decade.

"Given these trends, [impervious surface] is likely to become a more prominent environmental and growth issue in the coming years," the authors conclude.

The mapping effort, based on data from 2003, found that hardened surfaces cover 43,480 square miles, or 1.5 percent, of all land in the lower 48 states. The study notes that if those hardened surfaces were consolidated over a single area, it would cover the state of Ohio. The percentage of impervious surface per square mile is highest along the East and West coasts, Vogelmann said, because that is where population pressure is highest.

More concerning to environmentalists, however, are the findings comparing impervious surface area to wetland area, which the scientists estimate cover 38,020 square miles, or 5,460 fewer square miles than that converted to hard surfaces. Wetlands -- considered vital to environmental health as habitat for wildlife, buffers against storm surges and even as pollution filters -- came under heavy assault during the 19th and 20th centuries, and their destruction was at one time encouraged by the government under the Swampland Acts of 1849 and 1860.

In 1988, responding to the accelerated pace of wetlands loss, then-President George H.W. Bush established a national policy aimed at achieving "no net loss" of wetlands. That policy was amended this year by the current Bush administration, which pledged $349 million to create or safeguard an additional 3 million wetland acres nationwide. The new effort is focused largely on providing financial incentives to private landowners who voluntarily preserve wetlands rather than tightening regulatory controls (Greenwire, April 26).

But Navis Bermudez of the Sierra Club's Environmental Quality Program, said the new policy will likely result in a greater imbalance between wetlands and impervious surface because developers face little in the way of restrictions. "If the study is true, and there is so much more impervious surface area to wetlands, what we really need is for the Bush administration to protect the Clean Water Act ... so we can protect wildlife and communities," Bermudez said.

Vogelmann of USGS said the scientists who contributed to the effort, while not focused on wetlands loss per se, were "simply trying to make people aware of their actions and putting into a map framework showing where impervious surfaces are and where they're not."

Elvidge said a national map of impervious surface could be used to compare the area between regions of the United States. Traditionally, scientists trying to make such comparisons have had to use different data sets, often compiled by state or regional agencies.

The scientists extrapolated the map from satellite images showing artificial light at night as well as data on land cover, land use and roadways. Elvidge said the map should help planners, developers and others "to model runoff and the flow of pollutants and which watersheds are most heavily impacted" by impervious surfaces.