A handful of curious Burnsville residents showed up last week at a meeting to find out what had caused the Great Sewer Explosion of 2011.
They came away perhaps a little less confused but still uncertain about what caused the blast in the sanitary sewer system that sent manhole covers flying 8 to 10 feet in the air.
"It certainly was unusual," said Shirley Rasinowich, who along with her husband were among dozens of people evacuated from their homes on July 8. "We still don't know what exactly happened. How often does that happen that you are told to get out of your home?"
According to the city, inspectors have not figured out the cause and have not even been able to determine what exact chemical detonated in the explosion.
"We think more than likely it was gasoline -- perhaps as little as five gallons," Steve Albrecht, city public works director, told a half-dozen people gathered Monday night at River Hills United Methodist Church.
The church was used as a rendezvous spot by residents following the explosion, which happened up the street along River Hills Drive and Ridgewood Court.
The official response that followed the explosion included evacuations, intensive inspections of the sewer system and a block-by-block check for damage and the source of the blast.
In the weeks since, city crews have been monitoring the sewers and air quality to see if there are any lingering effects.
Albrecht said workers have been in the area dozens of times conducting those tests and have found no unusual levels of contaminants.
Fire Chief B.J. Jungmann and Burnsville Police Capt. Eric Werner were at the meeting with Albrecht to update the residents who showed up.
"I was just really curious why the manhole covers jumped," said Gary Wittnebel, one of those attending.
Authorities said a national search revealed that there have been about a half-dozen such unexplained explosions in the past 10 years.
"It isn't unusual that we didn't find the cause of this," Albrecht said. "Shakopee three years ago had the exact same thing happen and they never found a cause."
Albrecht said the two most likely scenarios were that a natural gas leak had trapped gas in the sanitary sewer line, or that someone had poured a flammable liquid such as gasoline directly into the sewer line.
Investigators at the scene found no residue of natural gas, and subsequent inspections found no cracked natural gas lines in the area, Jungmann said.
That leaves gasoline as the most likely culprit, but again, no traces have been found to confirm that suspicion, and investigators have not been able to determine that anyone disposed of gasoline or a similar contaminant in a way that could have caused the explosion.
Investigators did pinpoint the home and basement that they believe was the source of the ignition. They found scorch marks in a home near where the manholes were blown off.
The theory is that the floor drain in the home's basement was dry, so chemical vapors seeped into the basement and the fumes were ignited by the home's hot water heater flame.
One early theory was that the chemicals might have been linked to a meth lab, but investigators have ruled that out.
"It appears to have been a one-time event," Jungmann said. "The police have nothing that would indicate it was something suspicious."
That left the few people at the meeting comforted, though still puzzled as to what happened that July afternoon.
"Hopefully it never happens again," said Lois Ward, another resident who was evacuated.
Heron Marquez • 952-707-9994
