What caused brown snow in Maine, prompting a warning to 'avoid direct skin contact'?

US state of Maine experienced an unusual activity last week when residents witnessed

brown snow

. The incident occurred due to a mishap at a local paper mill prompting officials to warn residents to avoid direct skin contact to the brown snow.
The incident took place last Tuesday in Rumford city of Maine.
What caused the brown snow?
A local mill in the city released spent black liquor, a byproduct of paper production, into the air. This substance landed on the snow, changing its color to brown or tan.

The discoloration primarily affected areas near the mill, about 80 miles north of Portland. The

Maine Department of Environmental Protection

(DEP) visited the town for investigation, according to officials. After testing the snow, the town officials assured residents the snow wasn't poisonous, but, with a pH of 10, it could irritate skin.

Officials caution residents
The town officials advised residents to avoid touching the snow while the mill pays for more testing.

The school district cautioned students against playing in the discolored snow. Pet owners also received warnings to keep animals away from it.
Town officials released a statement on Facebook, saying, "Based on guidance from (the Department of Environmental Protection), we are confident that the public safety concerns are minimal at this time. In the meantime, out of an abundance of caution please avoid ingesting or direct skin contact with the brown snow."

“We are hopeful the rain tomorrow will wash most of the substance away and flush it off the ground and people’s homes or property,” officials said last Tuesday.
Later tests showed the snow's pH had decreased to 8 or less.
How residents reacted?
Some residents of Maine expressed concern about the brown snow, while others were less surprised about the incident. “It is, you know, odd but at the end of the day, we live in a mill town, we deal with the smell,” one resident told WMTW. “I just think it's part of the mill and what we get here in this area.”

The incident also raised concerns about children playing in the snow. “She's always asking to eat snowballs,” a parent told WMTW reporters. “Thankfully my sister did reach out to me and was like hey don't eat that snow don't play in that snow, but you don't know what's underneath it and you don't know the effects of it either, so that's a little worrisome.”
No adverse health affects reported
A spokesperson of Maine DEP said that there were no reports of adverse health effects because of the brown snow.
“We haven't received any reports of adverse health effects,” the spokesperson told WMTW. “Thankfully, the release was limited in potency and limited in scope," he added.
Rain and winter weather on last Wednesday washed away the remaining brown snow.