|Updated Jun 23, 2000||[an error occurred while processing this directive]|
[an error occurred while processing this directive]
[an error occurred while processing this directive]
Ma Nature Does Best At Cleaning Toxic Waste
BY MARJORIE DAUGHTRIDGE
When one thinks of cutting-edge technology, bacteria and trees probably arenít the first things that spring to mind.
But maybe they should be.
The cleanup of the Aberdeen Pesticide Dumps Site, a National Priority List Superfund site located off of N.C. 5 in Aberdeen, has involved some of the best available technology for cleanup of soil-borne chemicals.
Now that the cleanup process is almost over, it has taken an unusual twist. Natureís abilities have outstripped the best available human efforts. A little innovation and a willingness to try a new approach have proved to be not only on the cutting edge of a new technology but also a gateway for rethinking similar cleanups elsewhere in the world.
This time it is Mother Nature to the rescue, and thatís what is so high-tech about the process.
"Oh, yeah. This is on the cutting edge," said Bert Cole, senior environmental engineer working on the project. "In many ways, it makes just so much more sense if you can figure out how to do things using the natural systems, because you can basically let Nature do her thing."
In its initial stages, cleanup of the APDS involved the excavation and thermal treatment of more than 127,000 tons of soil. Treatment included heating the soil to 700 degrees Fahrenheit in a closely controlled environment and capturing and removing the toxins from the exhaust. The soil was then returned to its original location.
Contaminants targeted for removal during treatment included pesticides and related chemicals such as DDT, DDE, toxaphene and xylene.
After treatment, the contaminants were gone. The soil, however, which would typically be rife with biotic activity, was dead.
And there were still chemicals to be addressed in the area ó not in the soil, but in the groundwater. Groundwater is the water that flows underground, through the soil, and eventually becomes surface water such as lakes and streams. It tends to move very slowly, measured in feet per year.
Cleaning up groundwater can be a little trickier than cleaning up soil. Typically, the method for removing toxins from groundwater is to pump the water to the surface, treat it for contaminants and return it. This approach is problematic, however.
By design, the pesticides at the Aberdeen site prefer to stick to soil particles, rather than to water. So when one pumps and treats the water, most of what they are hoping to treat remains underground attached to the soil particles.
It has been estimated that it might take more than 100 years to clean the groundwater using the pump-and-treat approach. Not the most effective method, but until recently, really the only method.
But at the APDS, a new approach was given a chance: phytoremediation, in concert with bioremediation.
Phytoremediation is the use of plants to treat and repair. Bioremediation is the use of bacteria to treat and repair. In both cases, the contaminants are converted by the plants or bacteria, to useful and safe compounds.
Healthy populations of bacteria were still living in the lower reaches of the soil at APDS and had actually found ways to digest the pesticides and use them for food.
Bacteria also stick to soil particles, making them perfect candidates to help remove contaminants from the groundwater as it flows through the lower reaches of the soil.
It was determined that the pump-and-treat method actually worsened conditions for the bacteria that were performing this important function, because it lowers the level of the groundwater, drying out areas that need to be moist for the bacteria to stay alive.
Trees on the other hand, do not lower the water level and actually improve conditions for bacteria. Trees also filter water out from between soil particles, removing contaminants more efficiently, and can move water at rates that exceed what wells can do.
Employing this information, scientists at APDS took a leap of faith and planted about 3,700 fast-growing, sterile, hybrid poplars in the areas of the highest pesticide concentration.
Bacteria and nutrients had to be added to the areas with the treated soil in order to revive it and make it livable for the poplars.
The 6- to 18-foot trees were planted between 4 and 12 feet deep, helping them reach the groundwater faster. It is estimated that, once well established, these trees can move water at an average rate of up to 15 gallons per minute per acre.
"We made a leap of faith," said Cole. "We really expected a lot of them to die up-slope. Much to our surprise, they lived."
Less than a year after the last of the full-scale plantings, the results have been stunning.
"Itís actually one of the most amazing things Iíve ever seen in my life," said Cole.
The trees have been growing at a rate of 5 to 10 feet per year and have done exactly what they were supposed to do. Contaminant levels have been dropping, up to a 50 percent reduction for some, while others have already been reduced to "safe" levels and the pesticide "blooms" underground are shrinking.
The experiment has paid off.
"If not the largest, this is one of the largest phytoremediation plantings ever done," said Cole. "Weíre really delighted not only with the trees, but with the results."
Cole wants to assure residents that although the pump-and-treat method has been stopped, the levels of chemicals in the groundwater are still being carefully monitored. Monitoring will continue until all chemicals in the groundwater reach safe levels.
The trees are expected to live for about 25 or 30 years, at which time the groundwater should be restored to health and to be clean.