City contractors began work Monday on the remediation of Boulder's 13th Street Plaza, where dangerous chemicals were detected in the groundwater in 2010.
The Colorado Department of Public Health and Environment approved the city's "voluntary cleanup plan" application last October and chose Envirocon for the job.
Beginning this week, the environmental remediators will remove manufactured gas plant infrastructure, soil, fluids and debris associated with the contamination.
"On the 14th Street parking lot, right behind the (Dushanbe) Teahouse, we found a large concrete container, below grade, and it contained an oily liquid substance, which Xcel pumped out and then filled the void back up with a type of foam," Boulder Facilities and Fleet Manager Joe Castro said Monday.
"So part of this follow-up phase in the cleanup is taking out the foamy substances and then testing the tank, and underneath the tank, to see if there's contamination where to the whole tank has to be removed."
Further investigation last March turned up another, older brick tank that contained a similar oily sludge near the pine trees in the 13th Street Plaza.
Castro said workers likely will have to dig down 18 feet to reach that tank and remove the substance. The city also suspects there are two tanks below the middle of the plaza and will excavate that area to investigate.
Six monitoring wells were installed at the site in September 2010 and revealed the presence of benzene and naphthalene, common byproducts of coal gasification. Experts believe the chemicals came from the Federal Gas Company's coal gasification plant that was on the site from 1902 to 1952.
Benzene can damage the immune system or cause cancer, as well stop production of red blood cells, leading to anemia. Naphthalene also can lead to anemia and damage to the liver, and can cause neurological damage in infants.
During the excavation, the site will be covered by a temporary tent to "control any fugitive vapor and odor emissions," according to a news release. A ventilation system will filter the air before it is released, and Castro said the air inside the Dunshanbe Teahouse is still safe.
"We did indoor testing back in September 2011. The levels of impurities in the air were way below EPA standards. We don't have any concerns about indoor air contamination," he said.
The tent is expected to be in place through late February, when the excavation ends and the city will begin to replace and restripe the asphalt. The 13th Street Plaza later will be redesigned to improve circulation and parking for Boulder Farmers' Market patrons, as part of the Boulder Civic Area Plan.
"We're going to go through a design and conceptual phase in 2014," Castro said. "This happened so fast we didn't really have time to design something for that space."
The city hopes to open up the west side of the plaza sooner, depending on how the work goes.
The estimated cost of the cleanup is $4 million, according to the city.
Xcel Energy will help with the costs, but an agreement has not been reached on how much it will contribute. Xcel, which formerly owned the site, contributed $50,000 toward a $210,000 study of the extent of contamination in November 2012.
"We've been working in partnership with the city on the project since it began," Xcel spokeswoman Michelle Aguayo said Monday.
The city will front the cost of the cleanup, Castro said, and reach an agreement with Xcel when the project is completed.
Most of this week will be dedicated to setup at the site, and Castro predicts that crews will begin digging next week.