Tough bacteria lurk under Hanford tank {RADIATION RESISTANT]

Beneath the leak-prone tanks of highly radioactive waste at the Hanford nuclear site lie some of the most inhospitable underground environments in the world.

For seven years in the 1960s, Tank SX-108 there leaked tens of thousands of gallons of a stew of radioactive, toxic and caustic waste left from the past production of plutonium at Hanford.

But when scientists at Pacific Northwest National Laboratory studied the contaminant-soaked soil beneath the buried tank, they found a surprise -- an abundance of life.

In a study to be presented today at the American Society of Microbiology's annual meeting in New Orleans, PNNL chief scientist Fred Brockman will discuss the diverse microorganisms they found living in the highly radioactive and toxic underground environment.

"Despite the extremely harsh conditions, a total of 56 different genera of bacteria were found, representing over 150 different species," Brockman wrote in a paper outlining the work.

Sediments from beneath the tanks are among the most radioactive in the world to have been studied with molecular biological methods, he said.

Some may have survived because they're in microscopic areas that may not be as heavily contaminated as other areas, he said.

As sediments were collected from increasingly contaminated areas along a bore hole run diagonally beneath the tank, fewer and fewer bacteria were found.

But the bacteria that did survive -- what Brockman calls "the real heroes" -- are of a type that scientists know have a much stronger cell wall than other bacteria. They also have the chemical ability to adapt to stressful conditions.

"What they have to do is produce proteins to help stabilize their nucleic acids and cell membrane molecules," he said.

Using methods that have been standard for several decades to analyze a small part of the DNA in the bacteria cells, the microbes did not appear unique, he said.

But using technology only recently available to rapidly sequence all the DNA of the bacteria, it looks very different to other microorganisms in a massive international database.

It shows "how tenacious certain life forms are," Brockman said. "We've never investigated such an extreme environment."

It also indicates that a greater breadth of microbes are able to survive highly toxic environments and that the microorganisms likely contain many novel proteins, he said.

Learning more about how such microorganisms survive could be useful in cleanup projects, as scientists investigate how microorganisms might be one day used to help stabilize or clean up contaminated environments.

No small amount of radiation under there...

Over one million gallons of high-level-waste with more than a million curies of{sup 137}Cs have leaked from Hanford tank farms to the sediments beneath the tanks. Early on, it was assumed that cesium migration would be limited because laboratory experiments had shown that cesium strongly sorbs to phyllosilicate minerals common in soils[1-5]. Additionally, minimal cesium desorption has been observed in contaminated Hanford sediments[6]. However, recent observations beneath the Hanford tank farms show that cesium has migrated to greater depths than expected[7]. Various explanations for enhanced cesium migration include (1) physical processes such as fast flow pathways or bypassing of exchange sites in immobile zones, and (2) chemical processes associated with the very high salt contents and high pH of the tank fluids. Ion exchange processes are clearly indicated in the depth profiles of{sup 137}Cs, and potassium, sodium, calcium, and nitrate (acting as a tracer) from the bore holes beneath tank SX-108 and tank SX-115. Below both tanks, cesium concentration peaks are retarded with respect to potassium and sodium concentration peaks.

http://www.osti.gov/bridge/product.biblio.jsp?posti_id=15001985

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