By Aleksandra Robinson
WASHINGTON – The parking lots of Queen Anne’s County’s Bloomfield Farm will soon be modeling the latest in a series of efforts by the county to battle stormwater runoff pollution in the Chesapeake Bay.
Pervious concrete — the technology Queen Anne’s County will use in Bloomfield Farm’s parking lots — allows water to drain straight through into a reservoir below the pavement where it can subsequently soak into the ground. Most concrete and asphalt don’t allow rainwater to penetrate the ground, instead the water runs off and pollutes waterways.
Urban and suburban runoff is one of the top contributors to the Chesapeake Bay’s nitrogen load, and is also the fastest-growing nutrient pollutant source in the bay.
“Urban and suburban runoff account for 16 percent of Maryland’s nitrogen load to the bay,” said Jay Apperson, deputy communications director at the Maryland Department of the Environment.
Pervious concrete, along with other stormwater runoff prevention measures, is becoming more important in Maryland in light of the Stormwater Management Act of 2007, which Stu Schwartz, a senior research scientist at the University of Maryland, Baltimore County, said provides very aggressive criteria for new construction.
The key feature of the law is that it requires Environmental Site Design, also known as ESD, Apperson said.
“What ESD is is the use of planning techniques and design techniques that are designed to replicate runoff conditions, like what you would get from a forest,” Apperson said. “Before that was encouraged, but now it’s required. It’s required to be done, basically as much as possible. The legal term is, ‘to the maximum extent practicable.'”
Excess nitrogen causes algae blooms, which consume the water’s oxygen and lead to fish kills.
In Queen Anne’s County, ESD requirements have translated to the use of more than one innovative technique. The large permeable concrete installation is just the newest, said Lee Edgar, project engineer with Queen Anne’s County public works.
“One of the big things that the county commissioners have been focused on the past few years is leading the push for sustainability in stormwater,” Edgar said. “Some of these ESD techniques that we’ve started in Queen Anne’s County include bioswales, (a design element that allows runoff to flow through rock or other particulate matter before it flows into waterways), an award-winning rain garden and now we’re looking at pervious concrete.”
Concrete is an aggregate material, said Schwartz. It is composed of rocks or gravel, well-mixed cement and water. Where most concrete has fine particles — sand-sized particles — to fill gaps between larger pieces, permeable concrete has none.
“It is sometimes called no-fines concrete,” Schwartz said. “The aggregate used to make pervious concrete doesn’t have sand- or silt-sized particles.”
Or, as Edgar put it: “It kind of looks like a Rice Krispie treat. It’s a little different, but that’s where all those voids are. You have a void where water can run through it.”
Pervious pavement requires more maintenance than regular pavement, usually vacuuming or power washing, but it is quite effective at stopping stormwater runoff.
Schwartz cautioned that pervious paving materials are not always the most-effective way to control runoff.
“It’s probably not a universal substitute for paved surface and like all of the innovative stormwater surfaces, pervious pavement is not a magic bullet,” Schwartz said. “That’s simply because there are no magic bullets.”
By the end of the 2009, Queen Anne’s Bloomfield Farm — once a working farm that now encompasses a 19th-century homestead along with soccer, baseball and lacrosse fields, a fishing pond and a driving range — will sport 30,720-square-feet of pervious cement, along with 16,000-square-feet of permeable pavers, Edgar said.
“We’re trying to make the development as green as possible,” Edgar said. “We’re very enthusiastic about this … the Chesapeake Bay and her tributaries are an invaluable resource on the national scale, while her protection remains on the local level.”