To be truly green, you have to get down and dirty, it seems. As Baltimore officials begin to tackle the polluted runoff fouling the harbor and the Chesapeake Bay, they are turning to a technique long used by farmers.
It's not enough simply to strip off some of the city's ubiquitous pavement and plant grass. The ground beneath that asphalt and concrete often remains as hard and impervious as the man-made surface it's replacing. And the rainfall will just keep running off — washing fertilizer, pet waste, oil and other contaminants into storm drains and nearby streams.
So to make that urban hardpan act more like a natural sponge and cut down on storm-water pollution, city officials are trying out the agricultural process known as "sub-soiling."
At Yorkwood Elementary School in Northeast Baltimore this week, a tractor plowed deep into a half-acre patch of playground that until recently had been covered in asphalt. The farm vehicle towed a claw-like contraption with long curved blades that slice far into the packed earth.
"That's what Freddy Krueger would have done if he was trained as an agricultural engineer," quips Stuart S. Schwartz, a senior scientist with the Center for Urban Environmental Research and Education at the University of Maryland Baltimore County. Schwartz, who's been studying how to reduce runoff for years, has a contract to monitor the Yorkwood project.
The ground behind the 51-year-old school probably hasn't seen sunlight in decades. It's so tamped down, though, that when Schwartz and his assistant, Brennan Smith, test the dirt's capacity to absorb moisture, it takes almost an hour and a half for an inch of standing water to soak in. Any but the gentlest of rains would just run off, though Schwartz says it's not the most water-resistant soil he's seen.
Standard asphalt or concrete pavement, of course, is practically a runway for water, quickly shunting it into gutters, storm drains and streams. With storm-water pollution a growing threat to the bay, experts say it's necessary to capture that runoff and filter out the pollutants fouling the water. That's what the soil did naturally, before development packed it down and paved it.
In cities — virtual seas of pavement because of their dense development — the challenge is to get rid of what you can.
Schoolyards are fertile ground for Baltimore's quest for pavement it can remove, because many have playgrounds covered with blacktop. Ten have had patches of asphalt and concrete not used for parking vehicles stripped off to make way for planting grass and shrubbery. But none has had the sub-soiling treatment Yorkwood is getting. Runoff from the site eventually drains into Herring Run, a tributary of Back River.
While grass is no doubt more eye-pleasing than pavement, those earlier school greening projects aren't doing as much as they could to curb runoff, said Prakash Mistry, acting engineer supervisor in the surface-water management division of the city's Department of Public Works. The city needs to reduce the runoff fouling streams flowing into the harbor by 20 percent now, and more later.
"You can grow grass on top of asphalt," Mistry says, with enough fertilizer and water. "That will not help us. Our intention is not just to grow vegetation but also to allow runoff to infiltrate into the ground."
One solution in many cases may lie with that old farm practice of sub-soiling.
"We've been sub-soiling for 30-40 years," says Russell B. Brinsfield, an Eastern Shore farmer and director of the University of Maryland's Harry Hughes Center for Agro-Ecology in Queenstown.
Growers sub-soil or deep-till their fields, he explained, when they find the soil has been compacted by repeated passes of heavy farm equipment and other factors. Plowing up not just the top few inches but deep into the ground improves drainage as well as crop yields, Brinsfield said.
Urban and suburban plantings might benefit in the same way, says Schwartz. Loosening up the earth enables grass and shrubs to send their roots deeper into the ground, making them stronger and more resistant to drought.
"Whether you're planting turf grass, making a community garden or putting it back in shrubs or wildflowers, the same holds," Schwartz says. "Compaction of soil determines vitality."
Sub-soiling can pay for itself in reduced fertilizer and maintenance cost on some planted landscapes, Schwartz said. And research in Wisconsin has shown that proper sub-soiling of the ground can reduce runoff 70 percent to 90 percent, he added.
Bill Stack, deputy programs director at the Center for Watershed Protection in Ellicott City, helped arrange for the Yorkwood project while he oversaw the surface-water division at the city's Department of Public Works until earlier this year. He believes this sub-soiling project could teach broader lessons.
"Every urban area has the same issue, and if the results show what a lot of studies have already demonstrated, then this is a tool that can really be used nationwide," said Stack.
The Yorkwood project has not been without glitches. Once the blacktop was removed, contractors found that the soil was not only hard as a rock, but full of rocks, explained Kimberly Burgess, program manager with EA Engineering, a consultant on the project. Fearing that the sub-soil tilling equipment might be damaged, the city directed that the top foot of that rocky earth be excavated, to be replaced with loose dirt.
Staying within the $205,000 budget for the overall project meant limiting the sub-soiling treatment to about half an acre of the schoolyard. When done, there will be a two-foot layer of loosened soil, mixed with compost, on which will be planted a variety of native grasses, shrubs and trees, including redbud, maple, gum and oak. There'll also be a "bioretention" area, or rain garden, specifically to collect runoff piped to it from the roof of the school.
Though not enough to derail the Yorkwood project, such complications indicate there might be cost or practical limits to using sub-soiling for controlling storm-water pollution in all urban and suburban settings, Schwartz says.
"It's not a magic bullet," he said. "There are no magic bullets."
The UMBC researcher says he expects to see "dramatic" reductions in storm-water running off the school yard once the work is finished. "The question is, how long will they last?" he asked.
For Deborah Sharpe, Yorkwood's principal, the project is already yielding educational dividends. She's eager to see what her 485 students can learn and do on the new green landscape they'll have when the work is finished in the next week or so.
Except for a small playground added three or four years ago, her pupils have had to spend their outdoor time on a cracked expanse of asphalt, she noted. Now they'll have a rain garden to tend, a place to take water samples and an outdoor classroom, she said.
"In addition to the blacktop being removed and the whole area being green," Sharpe said, it will be a better place "to explore and learn and play and just be with nature. I think the students will be so much more excited, they will be engaged and looking forward to it."