Drivers cruising through new subdivisions might be surprised to notice what appear to be big pits in the ground not far from new homes. The pits look like craters overgrown with weeds and crawling with insects and other animals. After a rain, those pits fill with water, becoming small ponds for several days until the water trickles away or seeps into the ground.
"There are predominantly many different goals that these facilities obtain," said Scott St. Clair, the director of the Maintenance and Stormwater Management Division at the Department of Public Works and Environmental Services (DPWES).
The ponds prevent flooding downstream, reduce water pollution and help control stream erosion, he said.
AS NEW HOMES gobble up more and more land, retention ponds and other storm-water management efforts are on the front lines of the battle to make sure the water cycle is not overly disrupted.
According to a recent report by three environmental, nonprofit organizations, suburban sprawl is shaking up the way water moves from rain clouds to rivers and streams in ways that have yet to be fully explained.
The three groups — American Rivers, the Natural Resources Defense Council and Smart Growth America — found that sprawl exacerbates drought, erodes streams and hurts water quality.
The results stunned the researchers, according to Betsy Otto, the senior director for watershed programs for American Rivers and a co-author of the report.
"There's something very big going on," she said.
When rain or snow falls in an undeveloped area, a good deal of water percolates into the ground, while some of it runs off into streams and rivers. The water that seeps into the ground, known as the ground water, replenishes underground aquifers over a period that can be as short as less than one day or as long as over a million years, depending on the type of soil. Those underground aquifers will eventually empty into streams and rivers, which constitute surface water.
WHEN LAND is blanketed with roads, parking lots and shopping plazas, rain and snow are immediately drained to streams, cutting off the underground aquifers from their main water source, Otto said. This means that there is less water for plants to suck up through their roots. It also messes up the complex water cycle, according to Otto.
"We're changing the timing of when the water comes into the stream. And how [it enters the stream]," she said.
"If you pave over a site, water is not going to go into the ground, it's going to run off," said Noel Kaplan of the Department of Planning and Zoning's Planning Division. "The water is going to run off into the surface water system."
The water also picks up pollutants as it runs over roads and parking lots. When it is allowed to percolate into the ground, the soil acts like a filter, blocking the pollutant and sending relatively clean water to replenish the underground water tables. When the water is siphoned off straight to the surface water, those pollutants are dumped into the stream.
"It's sort of like a pump of pollutants that goes into our water bodies," said Otto.
Water that runs off a parking lot can also be up to 15 degrees warmer than water that seeps through the ground. Aquatic life forms are affected. Algae bloom in streams, which can choke fish.
Water comes into the stream faster as well, said Kaplan.
"That can have an effect on stream systems," he said.
Otto mentioned smart growth development and better storm-water management as possible solutions. The idea, she said, is to treat storm water as a resource rather than as a waste product.
"The ideal storm-water management approach is designed to infiltrate and to also provide habitat for birds and insects and wildlife," she said.
AND THAT is precisely what Fairfax County is doing with its storm-water retention ponds, said St. Clair.
"We are looking at them to help link ‘greenways’ or greenway corridors," he said.
Rather than having several protected areas strewn about the region, the county is trying to connect its environmentally protected areas to form so-called greenways. Because they are breeding grounds for wildlife, these ponds have proved to be effective links.
The Department of Public Works is also looking into other ways to replenish the ground-water tables through its storm-water planning division, which was created two years ago.
One such way is to include a depression in the corner of a developed lot, where storm water can gather and form a rain garden. The county is also putting these depressions into new parking lots for better drainage or experimenting with new types of pavement that are more porous, according to Carl Bouchard, the director of storm-water planning at DPWES.
"We have to do that because more of the county is being paved," he said.
He also predicted that these new measures will become part of the Virginia codes governing building and development.
"One of these days, those will become standard fare," he said.