"We've got belt-and-suspenders all over the place making sure the contractor is doing what they're supposed to be doing," said Jonathan Flesher, senior development director for Beatty Development Group.

The developer plans to open the cap to drive 1,063 hollow steel pilings into the ground below to serve as a foundation for the Exelon building.

The openings will be no more than 400 square feet each, said Flesher, just big enough to drive a pile or two at a time. Once sunk, they will be filled with concrete, and a plastic "boot" will be placed over the top and welded to the restored plastic liner. Then the rest of the cap will be reinstalled.

Some say they are worried about the construction crews who will be closest to any potential release of contaminants.

"Who does that kind of work?" asked Helena Hicks, a civil rights and environmental activist from Grove Park. "Poor whites, blacks and Hispanics. That's who's going to be digging it up."

The developer and regulators say there will be safeguards for the workers and the public.

While areas of the cap are open, Flesher said, any exposed contaminated soil will be sprayed with mist to prevent dust from becoming airborne. Workers will be required to wear long-sleeve shirts and possibly gloves in certain situations. The air around cap openings will be monitored, and an alarm will sound if an increase in dust is detected.

Before pile-driving begins, a porous synthetic fabric will be laid over the exposed soil and a layer of concrete put down to give crews a clean surface on which to work.

Such dust monitoring is not usually required at construction sites. "It's actually safer to build here than virtually anywhere else in the city," said Marco Greenberg, a vice president of Beatty Development.

Edward J. Bouwer, a professor of environmental engineering at Hopkins, believes the pilings can be driven without releasing chromium but recommends that those working near exposed soil wear respirators.

Officials with the EPA and the state say safeguards proved adequate when the seven-story building that houses offices for the investment bank Morgan Stanley was built three years ago. Pilings had to be driven into the ground beneath the cap in that case as well.

"We're comfortable with that," Russell Fish, the EPA's project manager for the site, said of the precautions taken to monitor dust while driving pilings.

Air monitoring detected hexavalent chromium during the Morgan Stanley construction, said Jay Apperson, a spokesman for the Maryland Department of the Environment, and it has been picked up in monitoring conducted in preparation for the Exelon project. But MDE staffers "are not aware" of chromium or dust levels exceeding "action levels" during the first building project, he said.

Apperson said the developer's plan complies with monitoring required by federal regulations for workers at hazardous waste sites, and includes respiratory protection and protective clothing for workers who will be in contact with contaminated soils.

Once the pilings are driven and the cap replaced, a concrete slab will go on top to serve as a floor for the building.

"It'll be more effective, actually," said Flesher. "Not only will we be replacing the cap that's there, we're in essence putting on another cap — putting buildings, roadways and concrete slabs over everything."

Environmental regulators and redevelopers from outside Maryland are closely watching the Baltimore construction project.

Michael McCabe directed the EPA's Mid-Atlantic region during the late 1990s, when the chromium was being entombed beneath the Inner Harbor. Today, McCabe is a court-appointed administrator overseeing the cleanup of 20 chromium sites in New Jersey. There, a 2009 state court settlement forced energy companies to remove the concrete caps put in decades ago — before the Baltimore cleanup — and haul away all of the contaminated soil. At times they are digging 35 feet below the surface to make the sites suitable for redevelopment.

"Capping is an older form of stabilizing the site," McCabe said.  "It does not represent a cleanup; it's containment."

Victoria Streitfeld, a Honeywell spokeswoman, said containment was chosen over removal in Baltimore in part because of "technical difficulties associated with excavation of a site surrounded by water." But she noted that regulators believed capping would protect health and the environment while allowing for redevelopment.