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ST. PAUL

KEVIN DUCHSCHERE

It’s been nearly a month since a 20-inch water main, buried several feet below Wall Street in downtown St. Paul, burst open at the midnight hour and began spilling 1.75 million gallons of water down the streets of Lowertown.

And it’s still a mystery as to what exactly happened, said Steve Schneider, general manager for St. Paul Regional Water Services. “I don’t know if we’ll ever be able to know,” he said.

The break caused a drop in water pressure that extended beyond downtown, forcing some hospitals to briefly turn to reserve supplies. For a couple of days, residents were told to boil their tap water before using it.

The good news is that this sort of rupture — a double split in a larger-sized, cast-iron pipe — is uncommon and hadn’t happened in at least 20 years. Ninety percent of breaks occur with 6- or 8-inch mains, Schneider said, that are relatively manageable and cause little inconvenience.

But on Feb. 9 and 10, a 9-foot section of the 20-foot pipe was split lengthwise on two sides. Such splits are usually caused by water pressure surges, but the records don’t show such a surge at the time of the break.

It was cold, of course, but the ground temperature was consistent. Although the pipe dates to the 1950s, that’s not terribly old in pipe years, Schneider said.

His best guess is that the break may have been related to a repair made on that section of the pipe in the 1980s. “I do believe this is an isolated incident not predicated on any one event,” he said.

St. Paul water services typically has 140 to 150 main breaks each year (although last year there were just 105, a historic low, he said). The agency oversees more than 1,000 of water mains for St. Paul and for Falcon Heights, Lauderdale, Maplewood and West St. Paul.

St. Paul has an ongoing program to replace its cast-iron mains, which turn brittle and corrode easily, with more flexible ductile iron and plastic. Schneider’s goal is to replace at least 12 miles of mains a year so that the entire system can be turned over every 100 years — the useful life for water mains.

“We’re in the forever business,” he said. “We can’t allow maintenance to be deferred.”