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A Road Runs Through It

Image of a Maryland Roadway in the rain

A bird's eye view looking above Rockville Pike in suburban Maryland reveals a seemingly endless stretch of rooftops, roads, and parking lots. It's what U.S. Environmental Protection Agency researchers Linda Exum and Sandra Bird and call impervious cover: when it rains, there's nothing there to absorb the water. Instead it runs directly into adjacent rivers and streams.

Exum and Bird developed a tool to rapidly estimate the amount of impervious cover and the quality of nearby waterways. The tool is a valuable resource for EPA regional offices, state agencies, and local communities to use in order to avoid the high cost of rebuilding damaged waterways.

"There's a very high inverse relationship between the area of impervious cover in a watershed and stream quality," says Exum, a geographer with EPA's Ecosystems Research Division. She explains that as impervious cover increases, troubles rise for nearby streams. "There are some very clear breakpoints. For instance, streams do pretty well until impervious cover gets up to about five or ten percent, and after that you start seeing water quality degradation."

Numerous studies have shown a multitude of negative impacts on watersheds when impervious cover reaches ten percent or more. The troubles include declines in the number of fish and other aquatic life, more intense flooding, and increases in sediments, chemical pollutants, and fecal coliform bacteria.

That breakpoint underscores the need to find a way to quickly and accurately estimate and project impervious cover located near valuable waterways. Researchers Exum and Bird developed such a tool by combining different types of readily available data, including population numbers from the 2000 Census, a map of major highways and interstates, and categorized satellite imagery developed by EPA with partners from U.S. Geological Survey, the National Oceanic and Atmospheric Administration, and others.

By combining information from multiple sources, they produced accurate estimates of the percentage of land area in individual sub-watersheds that are in impervious cover.

In conjunction with EPA Region 4 scientist Jim Harrison, the team applied their research to eight southeastern states (Florida, Georgia, Alabama, Mississippi, Tennessee, Kentucky, and South and North Carolina) with over 10,000 sub-watersheds. Their methods do not rely on site-specific calibration and can be used in other regions of the country to assess the sub-watersheds in those areas.

"Based on population projections in different areas and by looking at the current condition of the landscape, we're able to give a fairly good reading of where watersheds could be in the next 15 to 20 years," says Bird, an environmental engineer with the Ecosystems Research Division. "We're able to say, 'look you're okay now, but in 20 years you could have a problem.' City and county planners can have the peace of mind that their decisions are based on a tool with known accuracy. It's both a planning and an educational tool. One that is flexible, relatively low cost, and well-tested."

Among the things that can be considered in the planning stage are zoning issues such as lot size, frontage, and front yard setbacks. The layout of streets and subdivisions plays a role in the total amount of impervious cover added during development. Use of porous paving materials and rain garden type retention areas, which have yet to gain widespread acceptance, can help mitigate the impacts of development.

Exum and Bird's research provides a highly valuable guide for evaluating current conditions and for identify potential problem areas. By better understanding the detrimental effects of impervious cover, and offering these kinds of new tools to planners and developers, future areas like Rockville Pike can be developed without creating a sea of impervious cover. High rates of development do not have to come at the cost of destroyed local waterways.

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