As the Twin Cities suburbs rapidly grew, thousands of homeowners unable to access a water system drilled their own wells to tap the aquifers.

Now the state and counties are sealing the unused wells before they can be used to contaminate the groundwater -- a huge undertaking because there are so many.

Within the last year, Minnesota passed the 250,000 mark for the number of sealed wells in the state, according to Department of Health statistics. But at least that many unused wells are left, and perhaps as many as 500,000 more, officials say.

It's easy for people to forget that the innocuous-looking pipe in their yard or basement is actually a well, says Jill Trescott of Dakota County's Water Resources Office. Even if homeowners know, they may be tempted to use it to toss out things they don't know what to do with.

"Crankcase oil is one of our favorite" things tossed down a well, said Mike Convery, a hydrologist with the well management section of the Minnesota Department of Health.

In Hennepin County, commissioners recently accepted a $42,000 state grant to help homeowners seal 30 wells that extend into or below aquifers used for drinking water. More than 1,100 wells were sealed last year in Hennepin, more than any other metro county.

Anoka County's environmental services department held its 13th annual "Well Water Wise" promotion this week to analyze well water turned in by residents.

Minnesota bears primary responsibility for making sure that groundwater is safe. Eight counties and two cities -- including Dakota County, Bloomington and Minneapolis -- have authority delegated by the state to inspect and monitor wells in their jurisdictions.

But wells in all other cities and counties are handled by the state, which also licenses contractors to perform the complicated and difficult tasks involved in sealing a well.

Under state law, there's nothing wrong with having a well as long as it's being used. "That is certainly their right," said Lynn Moore, a former well inspector who heads Bloomington's environmental health division. "But an unused well is a public health nuisance."

Property owners are not allowed to seal wells themselves. Inspectors sometimes have found wells stuffed with newspapers and capped with cement. However, "We don't see improperly sealed wells as much as used to," Moore said.

The Reilly Tar disaster

The metro area gets water from several aquifers. The one most heavily used is the Prairie du Chien-Jordan, mostly sandy dolostone and Jordan sandstone 100 to 400 feet deep.

Typically, wells were installed in a space off the basement and under the house's front or side steps, surrounded by a glass-block wall. Convery said residential wells usually were built with steel casings or pipes measuring 3 or 4 inches in diameter and drilled into the earth.

In the metro area, well depth ranges from 50 to 300 feet; wells may go as deep as 500 to 600 feet in outstate Minnesota. Some wells needing sealing are monitoring wells, but most in urban areas were formerly water supply wells.

Wells became the predominant source of water in first- and second-tier suburbs, Convery said. Only in the 1960s did inner-ring suburbs begin converting to water supply systems.

The state got serious about sealing wells after the Reilly Tar and Chemical Corp. environmental disaster of the 1970s. The St. Louis Park company had operated a creosote plant for more than 50 years that was found to have polluted groundwater with coal-tar wastes spreading through groundwater aquifers. Wells were closed across the surrounding area.

Home sales impact

When the state required in 1990 that wells be disclosed upon transfer of property, sealing numbers shot up. Disclosure usually prompts property owners to seal a well to avoid liability.

Sealing a well in the metro area can cost up to $1,000. Contractors remove the draw pipe and any debris in the well, clean it out from top to bottom and then fill it in either with cement or bentonite, a clay that expands when wet.

The numbers of Minnesota wells sealed in recent years have fallen, in part because fewer wells are being drilled and because of sluggish housing sales, which means fewer are being identified. Last year, 7,210 wells were sealed in Minnesota, Convery said. In years past the numbers have been as high as 12,000 to 13,000 annually, he said.

Private wells still are being installed, but the state requires that they be drilled outside and connected to a building with piping. Today well casings are made of plastic, with submersible electric pumps.

And the location of new wells is carefully recorded in geographic record systems to eliminate guesswork for the next generation.

Kevin Duchschere • 612-673-4455