When Southdale opened in Edina in 1956, the nation's first enclosed shopping mall bragged that inside it was "perpetual spring." Stores and the giant interior court, with its towering bird cages and tropical plants, would be kept at a balmy 72 degrees by an innovative system that fended off "snow, soot [and] wilting heat."

The heat-pump system that sucked cool water from underground was so innovative that it was featured in Business Week.

Fifty years later, the same system still cools much of Southdale. But not for much longer.

The shopping center is one of the last of 106 Minnesota businesses and other institutions that once used "pump-and-dump" systems, which pulled pristine water from an aquifer and dumped it into storm sewers and ponds after using it for heating and cooling. Once used by General Mills, Honeywell, colleges, government buildings and hospitals, the "once-through" systems fell out of favor after a 1988 drought.

In 1989, state legislators passed a groundwater protection law that required pump-and-dump systems to be phased out.

But the changeover has carried a high price tag, often running into the millions. And there are environmental trade-offs. While the new or reworked systems reduce the drain on the aquifer, they can actually use more energy than the old systems they replaced.

Many of the businesses and institutions that relied on the systems have converted them to "closed-loop" systems that use water over and over again, cutting water consumption by 90 to 95 percent. Since the law passed, use of aquifer water by those systems has dropped from 11 billion gallons a year to less than 2 billion, according to the Department of Natural Resources (DNR).

"This was a painful process, but we've made big changes," said Jim Japs, assistant director for the DNR's division of waters.

Painful indeed. Hutchinson Technology, for example, just made a $3 million transition to a closed-loop cooling system on Christmas Day. Connie Pautz, director of corporate communications, said the firm's engineers described the conversion as "the most complex plant engineering project they've ever done.."

The project took six months to plan and design and six months to install. Two water chillers, two cooling towers and a power substation were bought and put on the roof of the 700,000-square-foot plant, which must be air conditioned most of the year because of the heat generated by manufacturing. The roof as well as supporting columns and floors were reinforced.

Because the old system was driven only by a 50-horsepower motor, the company's new cooling system is nine times more expensive to run, Pautz said.

Protecting the Jordan aquifer is worth the expense, said University of Minnesota geology Prof. Calvin Alexander. The aquifer, a layer of water-rich sandstone, lies beneath southeastern Minnesota, Iowa and parts of Wisconsin and Michigan.

In the Twin Cities area, about two-thirds of residents -- roughly 1.6 million people -- get their drinking water from groundwater, primarily the Jordan aquifer.

"The Jordan is the most important groundwater resource in the southeast third of the state," Alexander said. " It really doesn't make sense to pump clean, pristine groundwater, put heat in it and throw it in the nearest stream or storm sewer. I'm surprised anyone's still doing it."

At Southdale, water from the cooling system flows into a couple of holding ponds and then into the city's storm sewers and lakes. Though the water is still very clean, one of those ponds remains ice-free most winters because of the water's heat.

36 million gallons of water

Southdale is among the last six aquifer users that must convert or shut down their systems by the end of 2010. When Southdale's system was installed, it cost $1.2 million, $300,000 to $600,000 less than a conventional heating and cooling system. It's unclear whether hot water from the summer was ever saved successfully to heat the building in winter, as had been planned, but the cooling system has run for more than 50 years. In 2006, according to the DNR, Southdale's system pulled 36 million gallons of water from the aquifer.

Southdale General Manager Jerry Cohen said the shopping center had just installed new cooling equipment when the law was passed, and it probably could run efficiently for many more years. The current system seems to be very "green" in its operation, he said.

"In my mind [changing the system] is going to add more load to the electric grid," Cohen said.

But consultants have advised that an appeal to the DNR is probably useless. "The feedback I've been given is that it's a fait accompli," Cohen said.

The owners have made no decisions on how to change Southdale's system, and there's no estimate on how much it will cost, though Cohen said "it clearly will be sizable."

A few years ago, Abbott Northwestern Hospital in Minneapolis totally replaced its aging cooling system at a cost of about $7 million, said Daryl Schroeder, vice president for operations. About $500,000 of that cost was linked to retrofitting for a closed-loop system.

Today its cooling system is more efficient than the old one, Schroeder said, and probably saves money. He said that while there was "a lot of angst" at first about the law, the fact that it was a blanket requirement made it easier.

"At first you think, 'Why are they imposing this on us?'" he said. "But if you look at the bigger picture, it does conserve water in the aquifers for consumption rather than just casual use. And everyone has to do it."

Librarian Roberta Hovde contributed to this story. Mary Jane Smetanka • 612-673-7380