Just like the red stains etched in the pavement, the mystery of the leaking barrel of acid remains an irritant for a north Minneapolis neighborhood.

Sometime before the morning of April 28, someone left a black chemical drum in the alley between the 2900 blocks of James and Knox avenues North.

Tom Specht, who's lived on the block for 25 years, walked into the alley that morning and was surprised to find a clump of neighbors. They were all looking at the barrel, which was up against a fence next to his rhubarb patch.

Some of the reddish-brown liquid had spilled onto the alley and was running down the gutter.

"Why do people think they can just drive down the alley and dump stuff?" an exasperated Specht said last week.

A neighbor called the Minneapolis Fire Department. Soon the alley was filled with emergency vehicles, hazmat specialists and environmental inspectors. They blocked off the alley for the entire day.

They determined the substance was ferric chloride, with a ph of 1, not quite battery acid level, but close.

The Minnesota Pollution Control Agency took charge. Dropping a barrel of acid into a city alley violates a raft of laws designed to keep people safe from hazardous waste. The MPCA had a crime on its hands, and a barrel of dangerous liquid that had to go somewhere.

Typically, these kinds of problems happen at businesses that handle toxic waste on a regular basis, said Stephen Mikkelson, an MPCA spokesman. "To find a stray barrel of hazardous waste is not something you see very often," Mikkelson said.

That wasn't the only strange thing about it. A label on the drum carried the name of Producers Chemical Co., based in Sugar Grove, Ill. When the MPCA contacted the company, it said the ferric chloride was sold in 2004 to the University of Minnesota.

It's used there in the Art Department for etching and printmaking. The department told the MPCA that all of its chemical drums were accounted for, Mikkelson said.

The MPCA asked Hennepin County to take it. The county said no. So the MPCA went back to Producers Chemical, which again pointed the finger at the university as its only Twin Cities customer for the "specialty product," agency records show.

Now convinced that the chemicals belonged to the U, the MPCA once again contacted the University Art Department. This time, the university agreed to take it. A contractor hired by the MPCA picked up the drum, which contained about 30 gallons of the liquid, put it on a truck and mopped up the spill.

"We can't say for certain that it was ours," said Tim Busse, a university spokesman, but the U took it as a "good neighbor gesture."

Busse said ferric chloride is not a "chemical of concern," meaning it isn't monitored as closely as some. "It didn't get into any tracking system that set off bells and whistles when it went missing."

The mystery barrel now lives in the Thompson Center for Environmental Management, the university's hazardous materials storage building, Busse said. The Art Department hopes to reuse the chemicals, so after their sojourn in a north Minneapolis alley, they survive to etch once more.

Without any clue of how the ferric chloride ended up in the alley, the MPCA closed its investigation. Meanwhile, the illegal dumping goes on and on.

Specht said someone left "three pails of gunk" next to the pond around the corner, and on Thursday a stack of rotten boards filled with nails were left in the alley a few feet away from the acid-stained pavement.

Contact James Eli Shiffer at james.shiffer@startribune.com or 612-673-4116. Read his blog at startribune.com/fulldisclosure.