Geochemical processes help reduce chromium levels in aquifer

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New study says chromium pollution tapers off on route to the aquifer

By Roger Snodgrass

Los Alamos National Laboratory and its downstream neighbors may have dodged a chromium bullet.

If the latest analysis of the contaminant found in the regional aquifer is sustained, the chromium problem appears to be taking care of itself by a natural geochemical processes.

“We think things are good on the chromium front,” said Danny Katzman, a water stewardship program manager who has led the chromium response effort at the laboratory. “We’ve learned enough to understand there is nothing imminent as far as human health or ecological endangerment.”

“I complimented them,” said Robert Gilkeson, an independent geologist who has been a leading critic of the laboratory’s groundwater program. “This gets my approval.”

A low-key presentation earlier in the week by Kay Birdsell of the laboratory’s Earth and Environmental Science Division, reviewed the history of the discovery of hexavalent chromium in the regional groundwater in 2005 and the step-by-step investigation of the contaminant sources, pathways and processes, as the effluent settled into the landscape.

As it turned out, the pathways themselves appear to have weakened the toxic effects by converting the chromium from a pollutant into a more stable and benign form.

After the meeting, Gilkeson said, “It is fortunate that those liquid wastes followed the path they did.”

According to Birdsell, a lot of the pollutant became permanently bound up in the sediments in the wetland in Sandia Canyon and then even more was chemically transformed and immobilized in one of the upper geological layers known as the Bandelier tuff.

“And its never going to go back,” said Gilkeson. “It’s just the way the chemistry operates. Nothing is going to remobilize the bound-up chromium.”

Hexavalent chromium, or Cr(VI), is a toxic form of the metallic element found in industrial compounds. It was commonly used in paints and anti-corrosion treatments.

The National Institute of Occupational Safety and Health considers all Cr(VI) compounds to be potential occupational carcinogens, with adverse health effects including increased risk of lung cancer as well as kidney and liver damage.

As the investigators discovered, a great deal of Cr(VI), tens of thousands of kilograms, were released down Sandia Canyon from anticorrosion treatments at a power plant near the administrative center from 1956 to 1972.

The discovery of large concentrations in the regional aquifer came as a shock to the laboratory’s conceptual models at the time. Previously, it was thought that surface releases could not penetrate 1,000 feet of tuff and basalt to get down into the water table, especially not in such a short period of time.

The chromium was found in Mortendad Canyon, not far from a Los Alamos County drinking water well, but went unnoticed for more than another year before it was revealed at the end of 2006.

Concentrations were measured at 400 parts per billion in one well, eight times the state drinking water standards and four times the Environmental Protection Agency’s recommendations. Since then, another well, presumed to have staked out the heart of the plume, brought up samples with twice that amount of the chromium.

The National Academy of Sciences was called in by the Department of Energy and the state hazardous waste regulators of the New Mexico Environment Department demanded accelerated answers about the unknown nature and extent of the chromium problem.

The lab’s groundwater program was reviewed, wells were reevaluated and drilling methods revised.

A new model had to be developed that could allow for rapid pathways through faults and fissures downward and lateral transport which could account for the sideways migration of the pollutant plume from one canyon to another. New wells were installed at strategic locations to test alternative hypotheses about what was going on down there

The chromium study was folded into a comprehensive report on groundwater issues in Sandia Canyon, which was due for delivery to NMED Oct. 15.

“Sometimes, the full realization doesn’t crystallize until we’re writing our reports,” Katzman said, adding that the implications of all the studies that have been going on didn’t come together until then.

“At our best look at it presently, the water supply will be safe,” but he added, “I don’t even want to assume that’s how other people perceive it. It’s important to get NMED’s perspective.”

“The natural environment attenuated much of the chromium,” Katzman said. “That doesn’t mean there won’t be more work on it.”

One recommendation in the new report has to do with a new well that is already underway. Well R-50 is going in to put another peg in the map, intended to mark a boundary for the extent of the contamination toward the south-southeast.