Engineers battle sprawl to save water
VILLANOVA, Pa. -- A roof that sprouts plants and a parking lot that drains rainwater like a sieve may not be signs that some maintenance work is needed. Instead, you might be looking at the latest in groundwater conservation.
Vegetation to hold water on rooftops and pavement that lets it percolate into the ground instead of racing away through storm drains are some of the latest ways environmental engineers are trying to combat sprawling development and save water tables.
Such strategies are becoming increasingly prevalent -- and needed -- as development covers more land with buildings and pavement, causing the water table to drop, drying up wells and streams.
"The water is not getting back into the soil, so it's not recharging the aquifers," said Maya K. van Rossum of the Delaware Riverkeeper Network in Washington Crossing. "When you're having rainfall, you have water dumped directly into the creek. This is why we've been hearing more and more in recent years about flooding, and drought.
"These problems have the same genesis."
Recent drought years have focused attention on how development spreading into rural areas affects groundwater supplies. When David E. Hess became secretary of the Pennsylvania Department of Environmental Protection a year ago, he declared water resources an urgent priority.
Any threat to groundwater poses problems for about 4.5 million Pennsylvanians who get water from wells and springs, and those who drill about 10,000 new wells every year. More than 100 communities in the Southeastern Pennsylvania Ground Water Protected Area depend on groundwater.
Homes are the biggest users, consuming nearly half the groundwater that is used, with the rest split almost evenly between industry and agriculture/mining.
Even when spreading development covers the land with pavement and concrete, some of the problems can now be avoided, environmental experts say.
Rob Traver, an associate professor of civil and environmental engineering at Villanova University, demonstrated, using a low area of sandy soil and vegetation near a campus parking lot.
Formerly a small hill, it was excavated to a depth of 4 feet, the soil was mixed with sand, and the basin was filled back in to just below parking lot level.
Making little adjustments
Now rain and melting snow from the parking lot and a nearby basketball court drain into the basin, soak into the sandy soil and seep down into the groundwater. The parking lot's storm drains still would carry off any overflow, but Traver said in the several months since it was dug, the infiltration basin has absorbed all the rain, with no overflows.
"It's a little thing," Traver said. "The idea is building a lot of little things and maybe avoiding some of the big detention basins" needed to catch the torrents of water from fast-running storm drain systems.
Tom Cahill of Cahill Associates, an environmental consulting firm in West Chester, has been advocating such practices for years.
"In the current development patterns that we follow, we build structures and they are impervious; we surround them with asphalt, and that is impervious," Cahill said.
On land covered with trees and vegetation, only about 8 inches of a normal year's rainfall of about 45 inches in Pennsylvania runs off, he said.
"When we cover that with a rooftop or pavement, we turn all that rain into immediate runoff. Instead of the 8 to 9 inches of runoff we have 45 inches," Cahill said. "I usually show a community their favorite mall parking lot. I say, picture this area covered with 3 feet of water and you have an idea what it's done to the water balance of the area."
That can change, Cahill said, with projects like a porous-pavement parking lot he helped design as a storage and staging area for new Mustang cars at Ford Rouge Center in Dearborn, Mich.
Adding a garden roof
Porous asphalt or concrete lets stormwater through, flowing into a crushed stone bed where it seeps down into the ground. That can avoid the need for detention basins and channels and preserve wooded or swampy areas that would be scooped out for detention ponds. Morris Arboretum in Philadelphia has a similar parking lot.
On a big mall project, porous pavement can create a huge groundwater recharge area. "All the roof drains go into the underground bed, so there's no roof runoff," Cahill said. "The best measure is to go out in the middle of a thunderstorm and to watch the water drain through."
The Ford plant also includes vegetated roof cover on a million-square-foot assembly plant. An existing storm drain system is retained to carry off any stormwater the roof and porous parking lot can't absorb.
Chicago's City Hall and several Philadelphia buildings have vegetated roof surfaces.