Investigators pinpoint, halt likely source of volatile substance found Minneapolis sewers

Monitoring systems now in place after explosion, evacuations on U campus last summer

Workers in the sewers under the streets of Minneapolis heard a tell-tale alarm last August: sniffer devices detected a potentially dangerous explosive vapor.

It was Aug. 8 — just weeks after gases ignited underground near the University of Minnesota on June 30, touching off a blast in a fraternity house, blowing manhole covers out of the street and prompting emergency evacuations and road closures.

And it was less than a week after a second high-profile alarm about petroleum in the sewers, and subsequent evacuation in the same area.

Those involved in the investigation described an exhaustive, multi-agency effort to pinpoint a cause as students flocked back to the U of M campus for the fall semester. They now believe they know where that volatile substance came from, and they’ve taken a wide variety of steps to prevent a recurrence.

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As of last month, crews had installed a new meter in the sewers on campus to monitor for flammable substances, and quickly alert authorities if high levels are detected.

“There was just enormous amount of people coming together to try to make sure that this doesn’t happen again, anytime soon,” said Metropolitan Council spokesperson Bonnie Kollodge.

In documents released at the request of MPR News, the Met Council — which oversees wastewater treatment in the Twin Cities — said that their underground contractors “observed pink flow in a maintenance hole” near Mariucci Arena on Aug. 8.

Tests showed the substance to be potentially volatile.

“We were able to take some samples, and we confirmed that it was a petroleum product,” said Ned Smith, director of pretreatment and finance for Metropolitan Council's Environmental Services Division. “Not conclusive if it was diesel, or gas, or propane, or whatever. But it was a petroleum product. And they did evacuations (of workers) at that point.”

That was a third evacuation related to petroleum in the sewers, after the June 30 explosion and the evacuations on Aug. 2.

The Council also brought in environmental firm Bay West to investigate, and a day later it found similar conditions in northeast Minneapolis and tracked the substance to its source.

That was Zahl-Petroleum Maintenance Company, a petroleum service company at 3101 Spring Street in Minneapolis.

Zahl installs service tanks and equipment for petroleum products, often at gas stations, according to a company description posted online. Its website says the business also collects oily wastewater, collects and recycles used oil, removes sludge from tank bottoms, and removes and recycles contaminated fuels.

The Minnesota Pollution Control Agency and the Metropolitan Council went in for a look last August.

“Inspectors observed pink flow exiting the oil/water separator at the Company: a series of samples were collected from this flow, which also demonstrated an elevated LEL (lower explosive limit) of 20%,” reads a stipulation agreement between the Met Council and the company, signed by Zahl president Jeff Larson. “At the MPCA’s request, the Company voluntarily ceased discharge from the oil/water separator.”

Met Council officials say the equipment was designed to remove petroleum products from contaminated water, then make the water safe to be flushed into the public sanitary sewer system, treated further and eventually discharged into the Mississippi River.

There is no indication in the paperwork that investigators believed the petroleum discharge was intentional.

“That equipment was not working,” said Smith, with the Met Council’s environmental services division. “They are working to fix that. They’re letting us know what their plans are. But in the interim, they are not to discharge into the Metropolitan disposal system.”

Larson, Zahl’s president, did not respond to Thursday email or phone inquiries about the incident and the stipulation agreement.

Smith also noted that staff at the council’s St. Paul wastewater treatment plant, downstream from pipe running under the U, noticed a suspicious “grey-black film” in their primary collection tanks around the time of the June explosion. Smith said the council responded and adjusted its “biologicals,” the organisms used in the treatment process to eat waste in wastewater and to clean up the discharge.

Smith said the treatment plant’s outflow remained “cleaner that the river it was going into.” He said there were no permit violations by the council.

“The operation staff was concerned, but we managed through it,” he said.

Met Council officials were cautious about assigning blame for the June incident, which predated the Zahl discovery in August, although the circumstances were similar.

Kollodge, the Met Council spokesperson, said the suspicious inflows stopped after Zahl stopped discharging. She said the Council knows the company was “a violator. We can't say with 100 percent certainty that it was the violator.”

Smith said the Council was pursuing “cost recovery” for the incident, “in the low five figures.” But he also emphasized the Council was working with the company and that the Council did not want Zahl shut down.

“They're important partners for us, because they take stuff that nobody else will take. And from an environmental perspective, if these companies didn't exist, then who knows where it would end up,” he said. “They perform a valuable service. We want to make sure that we can continue to partner with them to keep our environment clean. Sometimes things happen, and we work together to make sure we come up with a viable solution.”

The Met Council has also installed a monitoring system in its sewers near the University of Minnesota to watch for other spills. A letter to school officials in January said a new metering system was up and running in the Stadium Village area, sniffing for “lower explosive limit” conditions in the sewer.

Photos released to MPR News by the Met Council show the devices hang from an underground utility access hole. Initially they had to be checked daily, but have since been hooked up for continuous electronic monitoring.

“It’s a 24/7 system monitoring application,” Smith said. “And so if LEL (lower explosive limit) were to spike, it would sound an alarm and our dispatch would be notified immediately.”

The January letter to U officials also said the Council was working on updating a formal response plan to such incidents, and would be distributing the plan to the U and other public agencies.