More than 1.8 million metro residents get tap water from the ground. Suburbs have a disproportionate share of the metro area's major groundwater chemical plumes, most of them created when the land was an open space.
"They were places that people thought were good places to bury stuff or dump stuff," said Michael Kanner, who heads the state Superfund program for the Minnesota Pollution Control Agency. "Some of the industries needed more land. ... It was easier to move outside the city."
To be sure, many places have groundwater pollution. Cities struggle to clean up so-called brownfields for redevelopment. In rural and some suburban areas, nitrates from fertilizer and other sources often contaminate wells, putting infants at risk.
The Star Tribune found that of the 20 significant pollution plumes in the metro area, 17 are in suburbs. The list is based on interviews with regulators and public health officials and a review of state cleanup and monitoring records and data. The list includes large areas of pollution -- 10 are greater than 1 square mile -- and smaller plumes with notable risks.
Chemicals with abbreviations BTEX, PAHs, TCE and PCP seeped into the ground at rural dumps, industrial plants, refineries, wood-treatment operations and arms plants. Some sites had extensive cleanups, but groundwater still is contaminated.
State Pollution Control Commissioner Brad Moore said cleaning up groundwater is a long-term commitment, "and we have to be vigilant to ensure the resources are there in 10, 20, 30 years from now."
The plumes includes a narrow, mile-long slug of dry-cleaning chemicals beneath Farmington that is drifting toward the Vermillion River, one of the last surviving trout streams in the metro area. Recently drilled monitoring wells show that no pollution has reached the river yet.
In Edina, officials discovered in 2002 that one city well contained low levels of vinyl chloride, a cancer-causing chemical formed by the breakdown of other compounds. The city shut down the well, and plans to drill a new one elsewhere. The estimated replacement cost is $800,000.
Groundwater-dependent cities St. Louis Park, Blaine and Bayport require special treatment of their drinking water to remove chemicals dumped long ago. Cottage Grove and St. Paul Park could be the next cities needing such water purification plants; the 3M coating compound PFBA has been detected in those cities' wells.
At a U.S. Navy munitions plant in Fridley, groundwater containing a degreasing compound called TCE has been pumped out of wells on the bank of the Mississippi River for nearly two decades. Otherwise, TCE likely would enter the river -- just above the Minneapolis water supply intake.
Old dumping, back to haunt
One of the largest groundwater problems is the 7-mile-long blob containing solvents from the former Twin Cities Army Ammunition Plant in Arden Hills that has spread to northeast Minneapolis and the Mississippi River. At its widest point, the plume is 1½ miles across.
The U.S. Army and civilian contractors dumped chemical wastes at the plant from the 1940s on. The pollution seeped into a drinking-water aquifer, and years later reached municipal wells in New Brighton, where it was discovered in 1981.
For 17 years, giant carbon filters have purified the contaminated water for New Brighton's water system, removing enough solvents to fill an estimated 2955-gallon barrels, according to the Army. Ten times that amount has been extracted from groundwater and treated on the site.
"What is being drawn out of the wells now probably was dumped in the 1950s and later," said Mike Fix, director of the Army installation. The plant last produced ammunition in 1975.
The filtered water is considered safe to drink, and all the pumping helps shrink the underground plume. Levels of TCE have dropped. Even so, TCE levels last year in one well were 600 times higher than the federal standard.
So far, the Army has spent $186 million cleaning up pollution from the plant, more than half of that to deal with chemicals in groundwater, Fix said.
The work probably won't be finished for another 20 to 40 years.