Contaminated soil cleanup moving into next phase

  • Published
  • By Patrick Ary
  • AEDC/PA
ARNOLD AIR FORCE BASE, Tenn. --  Environmental workers at Arnold Engineering Development Center (AEDC) are calling ongoing efforts to clean contaminated soil on the base a success.

At the end of January, CH2MHill and subcontractor TerraTherm finished work behind AEDC's Model Shop, where a high concentration of perchlorethylene (PCE) had been found in the groundwater. The contamination goes back to before 1970, when the site was used as a leach pit for a solvent cleaning facility and no environmental regulations were in place.

No one at AEDC was coming into contact with the contaminant, according to Dennis Flatt, Aerospace Testing Alliance (ATA) section manager for environmental restoration and environmental compliance.

"The risk potential was long-term migration to downstream receptors - private wells and springs," Flatt said.

Over the course of 491 days, 165,000 pounds of PCE - more than 90 percent of the estimated amount of contaminants - was removed from the half-acre site and destroyed. The PCE was extracted from 12.7 million gallons of groundwater in 66,700 cubic yards of soil.

"We went from percent levels of contamination down to low part-per-million levels of contamination as a result of the treatment," Flatt said. "And that's huge."

TerraTherm's method of removing the PCE was to insert electric energy heaters into the ground that heated up the soil to an average temperature of 217 degrees Fahrenheit. The PCE and contaminated groundwater were vaporized and brought to the surface, where the water was separated and the contaminants were destroyed through a thermal oxidizer. Before the leftover vapor was discharged into the atmosphere, it was run through an acid gas scrubber. The cleaned water was then discharged into AEDC's retention reservoir.

"We have done some post-treatment sampling in the area, and we're very pleased with the results of what we are seeing," Flatt said.

The thermal remediation technology is so effective it's getting attention around the world. TerraTherm recently was awarded a U.S. Agency for International Development contract to design a similar operation at Da Nang Airport in Vietnam, which has hotspots with high levels of dioxin from the storage and handling of Agent Orange during the Vietnam War.

Because the majority of PCE at the center is gone and it's no longer cost effective to continue with thermal remediation, AEDC workers are looking for more affordable means of dealing with the residual contamination. The Air Force Center for Environmental Excellence, through the Army Corps of Engineers, contracted URS Corporation out of Franklin, Tenn., to reestablish some site characteristics and help determine how much additional work may be needed at the site.

"We know that there's residual contamination present," Flatt said. "But we also have certain factors that we understand about the site with regard to attenuation time from the source to the spring discharges and outcrops. It's likely that this source may never pose a risk to offsite groundwater in its present state. We're trying to study and understand the remnant source regarding how much of a risk may exist."

URS will be working to come up with possible actions for the next phase. Flatt said there are several options on the table. Simply monitoring the site and using land-use controls to prevent digging into contaminated areas may be a viable option.

Pumping out groundwater and treating is also an option. Thermal remediation made more sense before, due to the large amount of contamination that was present.

"In comparison with the effectiveness of groundwater pump-and-treat, this removal was probably as effective as 200 or 300 years of pumping and treating, and ultimately removed more mass than we possibly could have through conventional pump-and-treat methodology," Flatt said.

There's also dual-phase extraction, which would entail extracting both liquid and vapor to clean out contaminants. Flatt said ATA is working with Jacobs Technology on a design for dual-phase extraction as a potential treatment option. That design would be ready for construction when URS is finished in two years.

Until an option is determined, AEDC will continue to monitor the site.

"A lot of work remains to be finished, but we are a great large step toward completion with this effort," Flatt said.