The discovery that toxic vapor can rise from contaminated groundwater has undercut decades of pollution cleanup efforts in Minnesota, prompting state regulators to revisit 293 cleanup sites to determine if contaminants that once seemed contained underground are producing health hazards today.

While cleanups in southeast Minneapolis and St. Louis Park have been well publicized, the Minnesota Pollution Control Agency has quietly undertaken a much broader review, and found another 53 pollution sites, from Bagley to Rochester, where vapor intrusion is being addressed. The review is ongoing at 80 additional sites.

State pollution officials also must decide whether to revisit 268 Superfund sites that were closed out years ago, because old cleanup strategies didn't account for vapor risks.

"We will deal with these sites and we will do a professional job of responding to them to protect public health and improve and restore the environment," said John Linc Stine, MPCA commissioner. "Unfortunately, the legacy of past actions is something that all of society is living with."

Among the 53 known problem sites, at least eight were old dry cleaning businesses, where it was common practice to dump cleaning chemicals out the back door.

Others include the Duluth Air Force Base and the Tonka Corp. toy manufacturing site in Mound — where a primary issue had been preventing contamination of nearby Harrison Bay on Lake Minnetonka. The review of these sites has been ongoing since 2008, when the state issued its first guidance for dealing with soil vapor.

Data requested by the Star Tribune regarding the 53 problem sites shows that 68 adjacent residences and 92 businesses received testing. Mitigation systems have been installed in 11 homes.

None of the sites are as large or problematic as Minneapolis' Como neighborhood, which is adjacent to an old General Mills research plant where 1,000 gallons of toxic chemicals such as trichloroethylene, or TCE, were dumped yearly from 1949 to 1962.

Since November, testing of soil vapor has been completed beneath more than 270 Como properties, largely single-family homes or rental apartments for students at the University of Minnesota. General Mills has agreed in 144 cases so far to pay for mitigation systems — mostly negative air-pressure systems that draw vapors out of the soil and prevent them from building up in basements.

Measured in micrograms per cubic meter of air, TCE levels that reached the thousands in a few cases have created anxiety for Como residents who wonder what they have been breathing and whether their houses will be sellable ever again.

"I don't feel safe living there," said Coral Sadowy, 66, who wonders if TCE exposure is related to her uterine cancer. A TCE reading of 3,370 below her basement grossly exceeded General Mills' threshold of 20 for a mitigation system.

Shift in the science

The common thread at these sites is the use of solvents or other compounds known as "volatile organic chemicals" such as TCE and perchloroethylene, or PCE.

Research has linked these compounds to higher rates of cancer, birth defects and other disorders when people are exposed to high levels of them over long periods.

These chemicals tend to sink into the groundwater and sometimes collect in dense globs that contaminate the water unless removed. For years, the primary environmental concern has been whether the chemicals were contaminating drinking water supplies. If not, the strategy often was to contain the chemical plumes below the ground and slowly remove the chemicals by pumping and airing out the groundwater.

While this satisfied scientific knowledge of the time, it didn't address facts discovered in the late 1990s — namely, that these chemicals could vaporize underground and rise through the soil in potentially harmful levels to basements of homes and businesses.

Minnesota is slightly ahead of the curve nationally in reviewing its pollution sites for vapor risks, though New York completed the process several years ago and other states in New England are farther along, said Lenny Siegel of the Center for Public Environmental Oversight in Mountain View, Calif.

"The reason vapor intrusion to me is a high priority is because it is an intrusion," he said. "Something that doesn't belong in people's homes is getting in there and they can't do anything about it."

Minnesota's change in cleanup policy is reflected in sites such as the old Despatch Laundry and Whiteway Cleaners at E. 26th Street and Stevens Avenue S. in Minneapolis. There, in 2000, the public was assured that cleaning chemicals dumped in the ground presented no imminent vapor risk.

Early advice was wrong

"Inhalation exposure to PCE vapors that may have migrated through soil and building foundations is unlikely, particularly any distance from the site," an MPCA advisory letter indicated at the time.

Fast forward a decade, and MPCA data show that vapor tests were conducted at 17 homes — two of which received mitigation systems to prevent vapor risks. Permanganate was injected into the ground because the chemical neutralizes PCE.

State pollution experts have charted an unpopular course in reviewing such sites and disclosing to neighbors that sites once believed safe are presenting the potential for environmental and health hazards.

In Como, many nervous residents have limited their access or their children's access to basements — even if testing has found low TCE levels. In some cases, the properties are basement apartments for U students.

Stine said state officials will work swiftly in each case to notify residents of vapor risks and address them.

"It's a huge undertaking," he said. "There's some good news in the speed that we're moving in getting these sites assessed. We've got more than half of them evaluated for risk."

Among active sites, 162 have either been cleared of vapor risks or cleaned up sufficiently to remove those risks.

Among some of the 80 sites still under investigation, cleanup efforts are underway. Forty mitigation systems were installed in homes in St. Louis Park, where investigators still are tracing the source of TCE contamination that was initially discovered in 2007 in an Edina water well.

A looming issue for the MPCA — one that hasn't been accounted for in its budget — is the cost of reopening investigations into old Superfund sites. The agency is preparing to test a few sites this year to ­determine how severe soil vapor problems might be, Stine said.

Today, Coral Sadowy spends considerably less time in her home. With cancer treatments approaching that will weaken her immune system, she worries about exposure to any contaminants. A negative air pressure system has yet to be installed, because engineers for General Mills have determined that a new basement floor will be needed for the system to work.

A retired Realtor who doubts her house will be sellable for years, Sadowy has another idea: "I wish they'd just tear it down, dig a big hole and suck that stuff out of there."

Jeremy Olson • 612-673-7744