Porous pavement gets another tryout in Maryland

The concrete oozed rather than poured out of the mixer truck, almost as if reluctant to cover the ground — partly because it won't, entirely.

Laborers shoveled pebbly gobs around to form a new sidewalk at a park-and-ride lot in Waysons Corner, one of two where the State Highway Administration is laying "pervious" concrete this summer as a test of its environmental friendliness.


Porous paving surfaces have been around for decades, but they're expensive and often didn't work well. Interest in such surfaces among governments and developers is on the rebound, though, in response to new state regulations aimed at curbing stormwater pollution from pavement smothering the Chesapeake Bay watershed.

"There has been a resurgence in permeable pavement in the state and across the bay watershed in recent years," said Tom Schueler, head of the Chesapeake Stormwater Network, an organization that trains engineers and others how to deal with runoff.


Stormwater runoff is the fastest-growing type of pollution in the region, according to the U.S. Environmental Protection Agency, accounting for one-sixth of all the nitrogen and phosphorus and one-fourth of the sediment fouling the Chesapeake.

Rooftops, roads and other pavement are the conduits, as rainfall washes off their hard surfaces, eroding stream banks and carrying accumulated dirt, oil and other pollutants. The amount of ground covered by pavement and buildings has grown nearly twice as fast as the population, the EPA figures.

Maryland and the other states in the Chesapeake's watershed are under pressure from the EPA to do more to curb polluted runoff.

The pervious concrete being poured in Waysons Corner is a lightly cemented batch of pea gravel with air pockets between the stones intended to let rainfall soak through into the ground rather than run off into storm drains and streams.

Water-absorbing pavement can help mitigate the impact of new development, and it's one of the few ways of curbing runoff in inner-city neighborhoods, where there's little or no open ground.

The use of permeable pavement and pavers has spread gradually in commercial and residential developments, starting in the South, according to Colin Lobo, senior vice president for the National Ready Mixed Concrete Association.

"A lot of universities adopted it, doing permeable parking lots and walkways," said Kelly Lindow, owner of CityScape Engineering in Baltimore. It fit with the institutions' adoption of green building practices, she said, and helped manage runoff when trying to squeeze more buildings and vehicles onto campus.

Municipalities in the Chicago area and nationwide also have embraced permeable pavement.


Its growing popularity represents a turnaround from 15 years ago, when the EPA warned that porous pavement had a high rate of failure, with the pores tending to clog up. Federal officials blamed the problems on improper installation and maintenance. Research since then has shown how to reduce the failure rate.

Cost remains an issue. Standard asphalt or concrete can be installed for $2 to $5 per square foot, on average, while pervious mixes can cost two to four times as much. Paving blocks may run as high as $15 or even $20 per square foot if they have to be laid by hand, experts say.

Much of the extra cost comes from the need to provide a porous base to capture runoff. Often, that means putting down a 6- to 8-inch layer of gravel and maybe tilling or replacing poorly drained, compacted soils. It also requires more and different maintenance than standard pavement, including vacuuming on an annual basis, or more often.

Permeable pavement is more competitive in new development, where the ground has to be prepared for paving anyway, said Greg Hoffman, a water resources engineer with the Center for Watershed Protection, a research and training nonprofit in Ellicott City. But where space is at a premium, he added, permeable pavement soaks up runoff while still allowing parking or traffic on top of it.

Mindful of the learning curve, higher costs and potential snags, the State Highway Administration is taking its first steps to see how pervious concrete will work.

Tim Smith, the agency's materials technology director, said it's being tried out in a limited way. With funds limited, the agency is only looking for now at trying it in new projects, like the park-and-ride lot in Waysons Corner and another one off Interstate 83 in Baltimore County, for a combined cost of $1.7 million. Pervious pavement also is being put down on a hiker-biker path near the C&O; Canal in Allegany County, a $350,000 item in a larger resurfacing and drainage upgrade project there.


Stuart Schwartz, a scientist with the University of Maryland, Baltimore County whose research has focused on how to control runoff, said he's glad to see the state transportation agencies take even those tentative steps.

"I give them real credit for committing to significant projects outside their comfort zone of experience," said Schwartz, who's serving as a consultant to the agencies. Change is hard."

Later this year, the Maryland Transit Administration intends to use pervious concrete on a new commuter bus parking lot in Waldorf. The 555-space lot, more than 1.7 acres, will be covered with the material. The $4.9 million project is designed to soak up a downpour produced by a 100-year storm, nearly 9 inches in a single day, said MTA spokesman Paul Dugan.

In Baltimore, the city is trying porous pavers to see how well they reduce runoff. City officials installed them in the new Upland redevelopment in West Baltimore, and workers are resurfacing an alley grid on East Baltimore's Butchers Hill with concrete paving blocks, aiming to absorb some rainfall in a compact rowhouse neighborhood that has few other options for corralling it.

Working in partnership with Blue Water Baltimore, a local watershed watchdog group; Biohabitats, a local green engineering firm; and the Center for Watershed Protection, the city has contracted to install pervious pavement in a total of three alleys. They're also putting in small rain-absorbing gardens to be extended out from sidewalk into the street between parked cars.

The project, three years in planning, is underwritten in part with a $675,000 grant from the National Fish and Wildlife Foundation.


At the alley off South Collington Street, the existing pavement has been removed, and the ground excavated, with a thick layer of gravel put down as a base. Interlocking gray concrete paving blocks, 4 inches square and 3 inches thick, are being laid on the surface. Different materials are to be tried in two other alleys in Patterson Park.

The "blue alley" project is not designed to get all the rain to soak down into the ground, said Nick Lindow, an engineer with Biohabitats, because that might cause water to seep into residents' basements. The aim, he said, is to slow runoff and filter out at least some of the pollutants — oil, grease, dog waste — before it gets to the storm drain and into the harbor.

Butchers Hill is committed to making the alley project a success, though it's not without challenges, said Sandy Sales, a resident who's coordinated the community's involvement. Parking is in short supply, she noted, and 20 households on the alley haven't been able to use their garages or parking pads since the work began a couple of weeks ago.

"That's a lot of people who are making a lot of sacrifice to do the right thing to try to move forward on our problem of stormwater runoff," she said.