Tests of ground water at the closed Carrs Mill Landfill show levels of a toxic solvent that are 280 times higher than federal drinking water standards.
Residential well tests in the neighborhood near the landfill have not shown any contamination.
But shallow monitoring wells and a creek that runs near the landfill also have turned up unsafe levels of the toxin, called trichloroethylene, or TCE, which is considered a probable carcinogen by the U.S. Environmental Protection Agency.
County public works officials blame the pollution on dozens of drums -- some filled with a TCE-based solvent -- that were discovered last month. The drums were buried or half-buried in a small section of the 30-acre landfill. Cleanup crews have unearthed more than 90 drums, most with their contents already crushed out of them.
The maximum amount of TCE considered safe in drinking water is 5 parts per billion. Tests on a sample taken from the bedrock well Sept. 10 showed 1,400 parts per billion. Another compound, a variety of dichloroethylene considered safe at 70 parts per billion, registered 960 parts per billion in the same sample.
The drums are thought to be about 20 years old because they are close to the surface of the landfill, which closed in 1976.
"This is very typical of an unlined landfill," said Michael Sullivan, spokesman for the state Department of the Environment. Mr. Sullivan said many landfills in the state have similar contamination problems but that the Carrs Mill Landfill situation is made worse by its "hot spot" of concentrated toxins.
Contamination findings from other counties were not available for comparison, he said.
Donald L. Gill, a biochemist and landfill activist who lives in Marriottsville, said he was alarmed by the Carrs Mill findings.
"I would challenge anyone to come up with numbers like that anywhere else in the state," he said. "If anybody had any idea that Carrs Mill was just an old landfill that was going to be a park, it's not. It's a toxic-waste site."
Beverly A. Cahill, who lives on Carrs Mill Road north of the landfill property, said she learned of the contamination a month ago from a neighbor.
"I was hoping that there wasn't anything to it," she said.
Since then, she has called the county health department to ask that her well be tested.
"It's been about a month, and I haven't heard anything yet," she said.
She expressed surprise when she learned that her address was on a list of wells the Public Works Department had asked the county Bureau of Environmental Health to test in June 1991 and periodically since then. She said that in the 17 years she has lived on the property, no one has tested her water.
Frank Skinner, the county environmental health director, said some homes on the list might not have been tested because residents did not respond to requests for permission to test.
The county's other two landfills also have contamination problems with leachate, rainwater that percolates through a landfill and pollutes the ground water below.
GeoTrans Inc., a consultant specializing in ground water contamination, is surveying the landfills, taking samples from new monitoring wells. The $100,000 survey is a $7.8 million capital project aimed at determining how to clean up the pollution.
The nearly 60-acre New Cut Road Landfill in Ellicott City, closed in 1980 after more than 35 years of operation, has shown lower levels of toxic solvents than Carrs Mill has and has contaminated several nearby residential wells. The county has placed treatment systems on those wells and a public water line to the area is now under construction.
The Alpha Ridge Landfill in Marriottsville, the county's only operating landfill, recently became the focus of attention when a bedrock well showed levels of the solvent methylene chloride as high as 430 parts per billion and TCE levels as high as 51 parts per billion.
Marriottsville residents, who learned of shallow-well contamination as early as 1990, had been reassured by county officials that bedrock, from which homes pump their water supplies, would serve as a barrier to the contaminants.
A survey of 70 municipal waste landfills in a 1988 Environmental Protection Agency report showed that trichloroethylene was found in leachate from 17 of the facilities. The amounts ranged from one to 1,300 parts per billion, with a median concentration of 43 parts per billion.
Dr. Gill added that the discovery at Carrs Mill has convinced him that such dumping has also taken place at the Marriottsville landfill.
He said contamination "will get progressively worse, and finally they'll realize that there really are things like Carrs Mill in there, and they'll have to go in and dig them out."
James M. Irvin, the county director of public works, said soil-gas tests done by GeoTrans do not support that belief.
The area where the drums were found "was the only area where we got a significant reading" at Carrs Mill or the other two landfills, he said. the tests are not foolproof, he said, but can be the first indicator of concentrated sources of contamination.
County Councilman Charles C. Feaga, R-5th, who represents the Carrs Mill area, said he was "very much afraid," not only of what had happened already but of what could still be happening in Marriottsville.
"It does prove that landfills need someone to inspect while people are dumping," he said, remembering an incident he witnessed at Alpha Ridge about seven years ago.
"I saw one of those dozers run over a 30-gallon drum. The bung flew out of it, and a black liquid sprayed all over the area. The stench was tremendous," he said. "They just kept rolling right over it. I remember thinking how many of those things were brought in."
"It makes you sick when you think what it could cost the county to clean this up," Mr. Feaga said.