Earth, Fire & Rainwater
Researchers are using microbes to clean up the highly explosive residue left at the Pantex Nuclear Weapons Plant.
Written by Sally Logue Post
Decades of work on conventional and nuclear weapons at the Pantex Plant, northeast of Amarillo, left behind contaminated soil and water perched above a major drinking water source. Researchers Ken Rainwater, Tony Mollhagen and Caryl Heintz, affiliated with the Texas Tech University Water Resources Center, are using microbes to clean up the high explosive residue at one field site – rather than digging up the place.
The Pantex Plant produced conventional ordnance and weapons from 1942 until the end of World War II. In 1952, the Plant became the Department of Energy’s final assembly and disassembly plant for all nuclear weapons. The work left the soil contaminated with high explosive residue that has seeped into water perched above the Ogallala Aquifer. The Ogallala Aquifer covers 174,000 miles from Texas to the Dakotas. The underground water is a major source of irrigation and drinking water for much of the Texas Panhandle-South Plains area.
“From 1952 until 1987, there was little or no environmental enforcement in these types of facilities,” said Ken Rainwater, Ph.D., Texas Tech professor of civil engineering and Director of the Water Resources Center. “A lot of soil and water contamination has been found at weapon production facilities. Now the question is how best to clean it up.”
Some Department of Energy facilities, such as Rocky Flats in Colorado, have closed. That, Rainwater says, makes cleaning up the soil easier. “At a closed base, you can knock down buildings and dig up the soil to get rid of the contaminants.”
But the Pantex Plant is different in that it will most likely never close. Pantex is the only facility doing nuclear weapon disassembly, Rainwater said. Buried power lines and functioning buildings make it necessary for researchers to find a way to remove some of the contaminants without doing any major excavation.
Rainwater, who is one of the principal researchers on this project, says the good news is that the situation at Pantex has two advantages. When the facility was established in the 1940s, the government bought about 16,000 acres of land a good distance from the city of Amarillo, a situation that still exists today. Secondly, the groundwater contamination has not yet reached the part of the aquifer that is used for drinking water.
“Under the facility, there is a layer that’s called the fine-grained zone within the Ogallala Formation,” said Rainwater. “That layer has low enough permeability that water has ponded on top of it – and that’s called the perched aquifer.”
Rainwater said the high explosives his team has dealt with are RDX, HMX, TATB, TNT and TNB, the chief by-product of TNT. “We know what the facility has used, but because of the high-level security, we don’t know how much of what they’re mixing together or what compounds they put into which weapons,” he said.
Rainwater says HMX has the least environmental risk associated with it, based on regulatory agency assessments. The amounts of HMX found in the soil fall within what have been established as safe levels by the Texas Commission on Environmental Quality.
When the Texas Tech researchers started their work in 1996, TNB levels in soil were above acceptable limits, but that’s changed now. “Toxicologists and regulatory agencies have raised the standard by a factor of 600, which is higher than the levels we typically find in the soil,” he said.
RDX is another matter. “We have found 10 to 20 times the amount of RDX that is considered safe,” he said. “But the levels are not spatially consistent. In some spots we find very high concentrations of RDX. In others, we don’t detect it at all.”
RDX is now the main focus of the clean up, but Rainwater says even that could change. As with TNB, additional work by toxicologists could change the safety standard for RDX and that would mean the clean up effort could change or stop.
“Part of the problem with this effort is that people don’t normally eat explosives and explosives are not used widely,” said Rainwater. “We don’t have a good handle on what contamination levels are safe to leave in the soil or in the water. The risk assessment people tend to be very conservative at first in setting allowable limits, and then they might go up later after new information becomes available.”
The contamination comes from the way the explosives were handled during old assembly procedures. Workers shaped the plasticized explosives to fit into a weapon assembly. “They basically machined these explosives to fit into a certain space,” said Rainwater. “It’s similar to machining metal on a lathe. And if you’ve ever seen a lathe, there are cuttings or excess materials that are carried away by some type of liquid to get them out of the way of the cutting tools.”
Most of those cuttings were collected with the liquids as a wastewater stream and sent through a filtration building for separation of the explosives. The filtered effluent still contained some explosives, and this flow was washed into concrete troughs that emptied into unlined ditches, he said.
A second source of contamination came from workers mixing the explosives with plasticizers in large kettles. When the kettles were cleaned at the end of the shifts, material that spilled from those kettles was later swept or washed out of the buildings onto the ground, said Rainwater.
“Both sources have contaminated the soil in a zone between the surface and the perched aquifer that is more than 250 feet thick,” he said.
Because Pantex continues to operate, buried utility lines make excavation unattractive in some locations, as does the cost to dig up that much soil. “So we had to find a way to remove the contamination in situ, or while leaving the soil in place,” said Rainwater.
Enter Caryl Heintz, Ph.D., professor of biological sciences and associate academic dean in the Texas Tech College of Arts and Sciences. “We had the idea we could use microbes already in the soil to clean up the contamination,” she said.
That idea came from a comment made to Heintz while she was a graduate student. “A professor told me that wherever a compound occurs in nature, there are microbes present that will metabolize it,” she said. “Working from that assumption, we started to look at what microbes were present in the contaminated soil, and to determine experimentally if they could degrade the high explosive residue.”
The science of how the microbes break down the high explosive reside is not complicated, said Tony Mollhagen, Ph.D., director of the Environmental Science Laboratory in the Texas Tech Department of Civil Engineering.
“If there is a chemical bond from which energy can be extracted, the microbes will find a way to do it,” he said. “Microbes exist by finding new resources for food. They find a little niche that no other organism has filled. It’s kind of like in business, if you find a niche no one else is serving, you can get rich.”
So Mollhagen and Heintz set out to find the microbes that could “get rich” from eating high explosive residue. However, the microbes don’t actually eat the high explosive residue. “The microbes secrete enzymes to break down and subsequently absorb the simpler compounds, much like the stomach does to break down food,” said Mollhagen.
Two students who were working in Heintz’s lab confirmed there were microorganisms in the soil that would break down the high explosives, especially if they had added nutrients.
In 1996, Pantex officials allowed Heintz and Mollhagen to collect soil samples from the contamination site. “We determined that the microbes that would do the job best worked more effectively if we could reduce the level of oxygen in the soil,” said Heintz.
The science of bioremediation has been around for a long time. Wastewater treatment plants have used the process for years, and more recently microbes have been used to help clean up oil spills. However, Mollhagen says that taking specific microbes and enhancing them to work in specific, sometimes remote, areas is relatively new.
“There might be a lot more art to this part of science,” he said. “In a laboratory, you can control all the environmental parameters, but you can’t always do that in the field. You can’t always assure the equal distribution of nutrients or water.”
Once Heintz and Mollhagen knew there were organisms that would break down the explosive residue, the next question was how to make the microbes work more rapidly.
“We looked at a lot of possible nutrients,” said Heintz. “About the only form that we could use 100 or more feet below the surface would be a gas of some sort. We settled on nitrogen because it would reduce the oxygen level, and that was what these particular microbes needed.”
At this point Heintz, Mollhagen and Rainwater joined forces to begin planning to test their theories in the field at Pantex. The first steps were laboratory tests to determine whether nitrogen injection would lower the oxygen content enough to stimulate the microbes in the contaminated soil.
In 1998 the Texas Tech group was asked to meet with officials from the Department of Energy Innovative Technology Remediation Demonstration Program.
“There are several facilities that have similar soil contamination problems,” said Rainwater. “The officials at the Innovative Technology Remediation Demonstration Program thought if more people worked together, we could share information.”
One of the scientists Texas Tech researchers met during that collaboration was Corey Radtke with the Idaho National Engineering and Environmental Laboratory. Radtke, who is currently a doctoral student at Texas Tech, offered to perform the nitrogen experiments in his Idaho lab.
“He had more facilities and could do our experiments much more quickly than we could with less experienced graduate students in our labs,” said Rainwater.
With positive results from those experiments, the Texas Tech researchers made the decision to do large-scale field-testing on site at Pantex. Soil samples were taken at the beginning and end of the test.
At the conclusion of that 300-day testing period, there were lower amounts of RDX and TNB in the soil, but Rainwater remained cautious about the results. “The average concentrations were lower,” he said, “but you can’t certifiably say that we made the decrease happen or that we just happened to sample in places where the RDX levels were lower to start with. The problem remains that the contamination levels aren’t uniformly distributed. You can have very high levels in one place and none at all a foot away. Still, the folks at Pantex were encouraged enough to secure Department of Defense funding to build a much larger-scale testing area.”
Besides continuing to examine whether nitrogen and other gasses will stimulate the microbial action, Rainwater says this second site also will help researchers get a real sense of operational costs to clean up a large area. An environmental consulting firm, Caldwell Engineering, led by Tom Caldwell, a 1978 Texas Tech graduate, designed the site and directed its construction and initial operation.
The first samples from that area were taken on Sept. 10, 2001. The terrorist action of the next day shut down the experiment for several weeks. The tests were resumed and results from those are now being analyzed.
As for the Ogallala Aquifer, Rainwater says while there is contamination in the perched aquifer above it, there appears to be no pollution pathway to the drinking water. “So far there have been no consistent reports of contamination in the drinking water,” he said.
Texas Tech’s partnership with Pantex and the government continues, as does the work of disassembling the nation’s nuclear weapons. While much tighter environmental standards have stopped the sources of new contamination, those microscopic “bugs” will hopefully continue to eat their fill of high explosives and help solve a major environmental issue.
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