Jan 2, 2013 - By Eric Blom, Staff WriterGroundwater cleanup at five locations in downtown Riverton entered a new stage earlier this month. The Wyoming Department of Environmental Quality removed five vapor extraction systems from the blocks bounded by Federal Boulevard, Fremont Avenue, Washington Avenue and Seventh Street in Riverton.
Often referred to as "bubble baths," the vapor extraction systems are housed in small sheds surrounded by wooden fences.
Several more are still in place in Riverton, mostly on the sites of former gasoline stations.
The devices clean groundwater by pushing air into the water table 15 to 25 feet underground. Agitation from the air bubbles vaporizes hydrocarbons such as spilled fuel and strips contaminants such as benzene.
Wells from the bubble baths then pull the polluted air out of the dirt above the water tables and back into the bubble bath shed, where another machine extracts the contaminants before releasing the air.
Mark Thiesse, the DEQ west district groundwater pollution control program supervisor, said that about two years ago, the concentration of pollutants around the five removed devices ceased to decrease.
"We reached the point of diminishing returns," Thiesse said.
So the DEQ turned them off, and monitored the contaminant levels for those years. Concentrations of pollution did not increase, and the DEQ decided it was safe to remove those devices.
Thiesse said each one costs $400 to $600 a month to operate. Shutting down five would yield $24,000 to $36,000 in savings to the state agency every year.
The pipes and wells of the devices are still in place though, in case the agency wants to do more vapor extraction again.
Now the downtown blocks will be in a monitored, natural attenuation phase. Naturally occurring bacteria will turn the pollutants into inert chemicals like carbon dioxide.
The DEQ will continue inspecting the areas to make sure contaminants do not increase or spread.
Thiesse said the DEQ is satisfied with its progress so far, but would always like to do more.
"We've gotten majority of the contamination," he said. "It was a pretty significant mess."
Some of the pollution is under roads and buildings, however, and the agency cannot locate a clean up device.
Thiesse said residents do not need to worry about the pollution.
"In order to have a risk to human health, you have to have a pathway," he said. "(Like) if you've got fumes in a building or kids eating contaminated soil."
So far, he said, those are not happening, and residents do not drink the groundwater.
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