The Army's Fort Detrick is entering a new phase in an effort to clean up environmental hazards it created in Frederick County during years of biological weapons research. The estimated $50 million cleanup and restoration, now three years old and scheduled to finish in 2008, is the biggest and most expensive such project in Fort Detrick's history.

This month, Fort Detrick finished a three-year cleanup of several waste pits. Soon, officials will choose a contractor to assess and contain contamination at 10 other sites, including a former ammunition-storage area and an old painting shed, in a section of Detrick known as Area B.

The cleanup of the 10 sites will prevent potential contaminants from migrating and ensure that groundwater contamination, first detected in 1992, does not exceed federal limits, Detrick officials said. The project follows a cleanup that began in 2001, when Detrick began excavating a series of waste pits whose contents, including vials of nonvirulent anthrax, surprised the Army and unnerved the community.

"This was a very complicated [and] technically challenging cleanup," said Col. John Ball, Fort Detrick garrison commander. Speaking at a ceremony last week to mark the end of the waste pit project, he said the cleanup is also "a cooperation and working-together story."

Area B, a 400-acre area bordered by Montevue Lane, Shookstown Road and Kemp Lane in the city of Frederick, was used by Fort Detrick as a testing, storage and landfill area during the Cold War. For two decades beginning in the late 1940s, Detrick led the United States' secret venture into biological and chemical weapons production, its scientists brewing and testing such substances as anthrax and the Vietnam War-era defoliant Agent Orange.

Until the weapons program was dismantled in 1969 by President Nixon, few knew what Detrick was doing, and fewer still wondered about the program's impact on the environment.

In 1992, two carcinogenic agents -- trichloroethylene, or TCE, and tetrachloroethylene, or PCE -- turned up in residential and Army water wells near Area B. After years of testing and investigation, the Army found a monitoring well near a series of defunct waste pits that was so polluted with PCE, a degreasing compound, that investigators could smell it. The pits, in a section called B-11, had been used heavily in Detrick's bioweapons work.

The U.S. Army Corps of Engineers and a private contractor began emptying the pits in April 2001. They expected to find industrial solvents and refuse. Instead, they uncovered a trove of biological waste.

For three years, workers wearing respirators and digging inside a sealed tent over the site removed 2,005 tons of hazardous waste and contaminated soil. They found more than 100 vials of nonvirulent anthrax and other disease pathogens, drums of chemicals, gas cylinders, even four dead laboratory rats floating in formaldehyde-filled jars. The cleanup budget soared, from $5 million to $26 million.

Scoop by scoop, soil from the site was cleaned and sanitized with bleach, then packed into room-sized containers for travel to a licensed hazardous waste dump in Texas.

Through a community advisory group, media visits and contacts with Frederick Mayor Jennifer P. Dougherty and other local leaders, Detrick officials updated the community on the progress.

Last Monday, a reception to mark the end of the waste pit cleanup drew local and state leaders, including Sen. Paul S. Sarbanes (D-Md.), who, with Sen. Barbara A. Mikulski (D-Md.), helped win federal money for expanding the project. Dougherty, whose initial concern over the perceived Army secrecy surrounding the cleanup helped bring about more openness by Detrick officials in recent years, also attended.

Rep. Roscoe G. Bartlett (R-Md.) said the project is important because it reassures the public that Fort Detrick will no longer be a source of groundwater pollution.

Detrick officials expect the next phase of the Area B cleanup to be less hazardous, though the Army has said that spotty record-keeping and the secrecy of weapons programs make it difficult to predict what lies in the soil. Detrick and state environmental officials believe, based on a preliminary analysis, that sealing off the 10 sites, rather than excavating them, is sufficient, said Lt. Col. Donald Archibald, Detrick's director of safety, environment, and integrated planning.

When that work is finished, environmental workers will test wells near Area B, to ensure that groundwater contamination -- which dropped significantly after the most recent three-year cleanup -- poses no health risk. Although the Army's cleanup will meet federal standards, the water flowing below Area B probably will never be completely free of the toxins, Archibald said.

"There are no absolutes in our business," Archibald said.