ON A HILLTOP ON MADISON’S far west side, David Liebl stands at the edge of a sea of sparkling new Toyotas. A salesman approaches, but Liebl’s not shopping for cars. A UW-Extension educator and stormwater specialist, he’s more interested in the lot itself, particularly an unpaved corner at the low end of the property. Lined with stones and an assortment of plants, this corner is sculpted into a shallow depression—a high-tech rain garden designed to divert water and the crud it’s carrying from a nearby city storm drain.
“They need to get in here and clean this out,” Liebl says, noting the thick layer of sand, leaves and trash that have collected in the basin. But he’s not complaining too much. Every bit of material that makes its way into the drain becomes his problem. And Liebl has plenty of problems already.
Two and a half miles downhill from the Toyota dealership lies the UW-Madison Arboretum, a 1,200-acre enclave known worldwide for its collection of restored ecological communities. Bordering the shore of Lake Wingra, the arboretum sits at one of the lowest points in Madison, making it the final destination for much of the rain that falls on the city. Decades of rapid development on Madison’s fringes have only worsened the problem. As more fields and woodlands are paved over to create roads, driveways and parking lots, the water arrives at the arboretum faster, dirtier and in greater volume. In one year, 115 million gallons of stormwater pass through an outlet at the western edge of Lake Wingra, reaching speeds of up to 2,244 gallons per second. The torrent has already blown out a pond built to contain it and carved a deep trench leading to the lake. The water brings with it layers of nutrient-rich sediment, which have created a delta that is filling in parts of the lake and are causing algal blooms.
“It’s the same all over the arboretum. We have ponds full of sediment and ponds that are too small to carry these heavy flows,” says Liebl, who chairs the facility’s stormwater committee. The arboretum is already building a larger pond near the west end of Lake Wingra to contain the flow, part of a multimillion dollar effort to deal with stormwater issues. But Liebl says new ponds aren’t the long-term answer.
“The arboretum can’t keep giving up the natural areas it is trying to preserve,” he says. “We need to treat that water and infiltrate that water somewhere closer to where it falls out of the sky and hits the ground.”
Bad things happen WHEN water travels. Most obvious are floods. When something prevents stormwater from soaking into the ground—say, a roof or a couple acres of asphalt—water follows the path of least resistance until it finds a place to settle. Maybe it’s a lake or a stream, but maybe—in cases where there’s too much water or too little penetrable ground to deal with it—it’s a low-lying field or neighborhood.
But flooding is only part of the problem. Running water erodes the soil and picks up all kinds of nasty contaminants, including hydrocarbons from roads and metals such as zinc from galvanized roofing. Just as importantly, water that doesn’t infiltrate the soil doesn’t recharge groundwater, which contributes to the depletion of aquifers.
In 2002, these concerns led the Wisconsin Department of Natural Resources to issue new rules on stormwater management. Now communities and real-estate developers must have plans to infiltrate significant amounts of the rainfall they receive and reduce contamination in whatever does run off. For new housing developments, for instance, the DNR requires infiltration to be 90 percent of pre-development levels.
But the composition of a typical urban neighborhood makes reaching those goals challenging. Asphalt and rooftops are designed to repel water, and they do their job well. According to one estimate, 16 times more water runs off of a one-acre parking lot than a one-acre meadow. Considering that in an average medium-density residential development 20 to 50 percent of the landscape can be paved or shingled, this puts a lot of water on the loose.