Emerson bought the plant in 1996, three years after the facility stopped using the primary solvent found in the groundwater, said Robert Amberg, a spokesman for Emerson. Manufacturing continued there using different chemicals, he said, but stopped about a year and a half ago.
The state approved the company's plans for cleaning up the site in 2001 under its "brownfields" redevelopment program, which tries to streamline regulatory requirements so contaminated property can be more quickly reused. The company's plans included excavating soil and installing pumps to treat groundwater.
At the time, Carroll said, officials believed the contamination was limited to soil and groundwater relatively near the surface, and that a layer of clay underground was keeping the chemicals from sinking deeper to other aquifers.
But after several years of pumping and treating and seeing no reduction in the chemicals found in the near-surface groundwater, the MDE official said, the company made new inquiries and found that there also had been a "dry well," a pit used to drain liquid out of wastes, on the property.
Further investigation revealed that the clay layer supposedly shielding deeper aquifers from contamination had holes in it, Carroll said.
Deeper test wells were drilled last year, which found more contamination, and a well sunk last fall showed it had spread beyond the plant's boundaries. Three of the first 15 homes tested on Twin Oaks Road in December were found to be "impacted" by the chemicals and were promptly provided with bottled water.
Carroll said Emerson had exercised due diligence when it bought the plant.
Edward J. Bouwer, a professor of environmental engineering at the Johns Hopkins University, said that a layer of clay sometimes does not prevent contamination from spreading. Clay can be breached, he said.
"You have to look at all aquifers," said Bouwer, whose research specialty includes groundwater contamination. "You can't rely on just one location to do the analysis, especially when you have these multiple layers."
The EPA left oversight of the cleanup to the state, said Roy Seneca, a spokesman for the agency's mid-Atlantic regional office in Philadelphia. Federal regulators did inspect the site in 2010 and consulted with the state on the need for more investigation, he said.
Carroll said the company has been directed to drill other monitoring wells to try to pin down the extent of the underground contamination. The company is not being required to hook up or provide bottled water to homes with relatively little or no contamination in their wells, he said, as their water is deemed safe to drink. The two wells with low levels of contamination will be checked every few months.
"We don't know whether they were higher or not in the past," Carroll said, "and what we will do is continue to track the situation going forward." If new contamination is discovered, he said, those homes would get bottled water as well while authorities work to find alternative water for them.
Matt Diehl, a spokesman for the Anne Arundel Department of Public Works, said the county is working with an engineer for the company to extend a water line to serve the affected homes. He could not say when that would occur. As for other residents in the area who might want to hook up for reassurance, he said they would have to pay the usual hookup fee and other charges, which can run into thousands of dollars.
Jim Swingle lives across Twin Oaks Road from homes with badly contaminated wells, but said he's been told his well was clean when tested, apparently because it wasn't drilled into the deeper aquifer. He still has concerns.
"Now we're sitting on a groundwater contamination that's going to impact the sale of our homes," he said.