New York City’s water supply could be the target of contamination if a water system map made its way into the wrong hands, an environmentalist said.

The threat has arisen since someone broke into a vehicle belonging to a Department of Environmental Protection maintenance supervisor and stole an agency laptop containing a map of the water system. If the map was detailed enough,”there could be the opportunity to pose a threat,” the executive director for the Center for Environmental Information, Cindy Stachowski, said. Even without a map, Ms. Stachowski added, someone pouring biological, chemical, or radiological contaminants into a fresh water source could adulterate the water system.

At about 5 p.m. on Friday, the DEP employee parked his vehicle on his residential block in Staten Island, police said. When he returned four days later he found the front passenger window broken and the computer missing from the floor of the vehicle, police said. The computer contained a New York City “water distribution map,” police said. The chief spokesman for the DEP, Charles Sturcken, said that the employee’s vehicle was one of several broken into on the block. The map was actually a pictorial representation of the citywide water mains, including symbols for such features as fire hydrants and trunk lines. “It’s nothing you wouldn’t observe in the streets,” Mr. Sturcken said, adding, “It’s nothing serious.”

Yet he would not release a copy of the map to a reporter.

Terrorists could strike specific neighborhoods, said a chemical and biological weapons expert who asked not to be named since she was uncertain of the facts of the case. Intruders could pull off manhole covers and tamper with the trunk lines that feed the water into people’s homes.

Mr. Sturcken said someone could not infiltrate the system via the pipes because they are made of cast iron and the water they contain is pressurized.

Future threats of terrorist attacks on the water supply are not far-fetched.

Indeed, President Bush spoke about such plans in his 2002 State of the Union Address. “We have found diagrams of American nuclear power plants and public water facilities, detailed instructions for making chemical weapons, surveillance maps of American cities, and thorough descriptions of landmarks in America and throughout the world,” Mr. Bush said.

Generally, it would not be easy to slip a contaminant through the filtering process, Patrick Garvey, the chairman of Cavalry Security Group/ U. S. Cavalry, a company specializing in military supplies and security assessment, said. “The reality is it’s very difficult because of the purification system to affect the water supply in any serious fashion,” he said. “It’s a biochemical challenge” because of its vastness. He also noted that patrolling police officers monitor the system.

However, a fresh water oceanographer and professor of environmental science and biology at SUNY Brockport, Joseph Makarewicz, said, “Some chemicals could make it through, because the systems are not designed to treat for everything.”

But tainting the water is not the terrorists’ “shtick,” Mr. Garvey said, because they want to execute acts that are “highly visible and “cause pain or suffering.”