– It's usually the summer practice of Mary McConnell to donate some of her vegetable crops to neighbors or sell a portion at the local farmers market to pay for gardening supplies.

This year, after an explosion and fire at the Husky Energy oil refinery north of her house, she's nervous that toxic fallout from the fire contaminated the soil she's carefully tended for years at the homestead she shares with her husband.

The Wisconsin Department of Natural Resources and the city of Superior have said there's no pollution threat, but McConnell said she could smell the fire that day, saw a plume of oily, black smoke billowing in the sky above her home, and has heard warnings from a local chemistry professor not to plant crops this year.

"It's very confusing for the people here who are really, really worried," she said.

A month after the April 26 explosion and fire, locals are getting back to their lives, but with new questions about the accident and its aftermath.

The big fear — a leak of the highly toxic hydrogen fluoride — never materialized, though city officials ordered a mandatory evacuation of thousands of residents in this Twin Ports city of 27,000. The close call left some feeling shaken, not only by the potential for catastrophe, but by the fire that poisoned the air in the hours after the blast.

Hydrogen fluoride use

It's unknown if the company will resume its use of hydrogen fluoride, a chemical used at some oil refineries nationwide under strict safety controls meant to prevent the accidental escape of the potentially lethal gas.

A company spokeswoman said Husky has been talking to experts about alternatives to hydrogen fluoride, but no decision has been made. No other refinery in the country that uses hydrogen fluoride has ever switched, in large part because the process could cost millions of dollars.

The company announced Saturday that it will meet with the public June 5 at Superior Middle School from 4:30 p.m. to 8 p.m. to answer questions about the refinery's status and plans for the future. Along with county, state and federal authorities, the company will also address safety concerns about the air, soil and water near the refinery, according to a statement.

Superior Mayor Jim Paine said last week it could be a year or more before the refinery resumes normal operation, based on what company officials have told him. It will be months before a full report on the cause of the blast comes out, and then the investigation, cleanup and repairs at the refinery will be followed by a "turnaround," a planned cleaning and upgrade of the refinery that was about to begin when the explosion took place.

Paine also said he doesn't know yet how he'll respond if the company wants to resume using hydrogen fluoride. He publicly asked the company to switch to a safer alternative, but Paine said he's going to have to rely on state and federal regulatory agencies to act if the company refuses.

Paine also cautioned against sensationalizing the threat posed by the chemical.

"The worse case scenario is not the likely scenario," he said.

The U.S. Environmental Protection Agency requires every refinery to calculate the number of people who would be at risk if the refinery's largest tank of hydrogen fluoride were to crack open and spill its contents in 10 minutes.

In that scenario, the Husky Energy refinery could jeopardize the lives of as many as 180,000 people living in the region, but it would depend on the amount of hydrogen fluoride gas that escaped and, especially, the wind speed and wind direction at that moment.

"I still maintain that the greatest danger to the public was the evacuation and not the fire," said Paine.

In nearby Duluth, Mayor Emily Larson said the explosion — and the revelation that hydrogen fluoride on site posed a serious threat — was a "wake-up call."

If Husky eventually determines that switching from hydrogen fluoride to another chemical is too costly, Larson said it's too early to say whether she would accept that or continue calling for a change.

"I don't think that's a 'yes' or a 'no.' ... That starts some follow-up questions," she said.

Steve Sternberg, an associate professor in chemical engineering at the University of Minnesota Duluth, has warned residents not to plant vegetable gardens in backyards that were near the refinery or under the plume of smoke it threw off.

Garden or not?

His warning has been far more urgent than what locals heard from Husky Energy, the city of Superior and the Wisconsin Department of Natural Resources, which has said that air monitoring at several locations around Superior and under the path of the smoke plume didn't find a problem.

Sternberg said toxic chemical compounds called polycyclic aromatic hydrocarbons fell out of the plume and contaminated soils for miles southeast of the refinery. He says the chemicals could be dangerous to the parts per trillion level, but that tests of the soil were looking for larger concentrations.

"Once on the ground, they don't just disintegrate and disappear," he said. He recommended that gardeners wait a year or two before planting crops. Fungi and bacteria already in the soil will attack the compounds and eventually break them down, he said.

John Sager, the regional spill coordinator for the Wisconsin DNR, said air monitoring hasn't turned up evidence of worrisome pollution. Trace amounts of chemicals found in firefighting foam are still in Newton Creek, which carries water from the refinery to Lake Superior.

The refinery has an onsite water treatment plant to remove contaminants before water gets flushed out. An unknown number of gallons escaped the refinery area during the initial firefighting attack; water contaminated with petroleum and firefighting foam washed down a ditch along Stinson Avenue, eventually spilling into a nearby creek that leads to Lake Superior.

It's a very small amount, measured in parts per trillion, which is minuscule but still monitored because the toxicity of the firefighting foam chemicals isn't fully understood yet, said Sager.

Nevertheless, the worries about pollution and the revelations about hydrogen fluoride have left people feeling unsettled, said University of Wisconsin-Superior sociology professor Meghan Krausch. A resident of Superior, Krausch said she knows people who want to move out of the city, and others who don't know what to believe about safety.

"We can see a big black cloud in the sky but we're being told that everything is OK. And then later we're told that, 'Wow, this hydrogen fluoride could have blown up and we all could have died.' "

She helped organize a community forum almost two weeks ago that drew about 100 people.

"It seems clear to me, anyway, that people have experienced some trauma around the explosion," Krausch said.