Phytoremediation of Contaminated Groundwater

Ray Douglas: Groundwater is one of our most important natural resources. It provides drinking water to more than half the population in the United States and even a greater percentage of the people in the developing world. So when it becomes contaminated, we have to do our best to find cost-effective ways to restore that contaminated groundwater to conditions not only that we can use now but for future generations. Phytoremediation happens to be one of the options that can be used to meet that goal.

Phytoremediation: what is it and how is it connected to the future of our groundwater?

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Ray Douglas: I'm Ray Douglas, and this is CoreCast, your science podcast for a changing world.

Jim Landmeyer is a USGS research hydrologist for the South Carolina Water Science Center. Jim, thanks for joining us today.

Jim Landmeyer: You're welcome.

Ray Douglas: Before we sat down today, I Googled "define phytoremediation" and the number one link led me to the USGS Toxic Substance Hydrology Program, and at the top of the page was your definition of 'phytoremediation'. For starters, could you tell us just what is phytoremediation?

Jim Landmeyer: Phytoremediation is the use of living plants in the way that they interact with the hydrologic system to either extract contaminated groundwater from the water table or to decrease recharge to the water table through often contaminated, unsaturated zone sediments. Once these contaminants are into a tree, the tree itself can either volatilize the contaminants to the atmosphere or the tree can degrade the contaminants into harmless compounds inside the tree tissue itself.

Ray Douglas: How long have we known about phytoremediation?

Jim Landmeyer: The term 'phytoremediation', and some folks actually like the term 'phytotechnology', in any case, the term itself probably first was used in the early 1990s.

However, the processes for which phytoremediation is the umbrella term for actually had been used by society for generations. In many cases overseas, for instance in Europe, the term 'bog wastes' or 'wetlands' were places obviously containing plants, places where waste were thrown to be remediated, to be cleaned up.

There has been many instances where land farming has been used where wastes are applied to areas that are then planted for the plants to handle the wastes. Those aren't termed phytoremediation but they helped establish the fundamentals for phytoremediation.

Ray Douglas: Jim, it seems to me that there would be some limitations as to which sites could actually use phytoremediation. Aren't contaminants sometimes deep below the earth surface?

Jim Landmeyer: This is true because groundwater can be anywhere from near land surface to as much as 200 to 300 feet below land surface. Most trees, their roots tend to be within the upper few feet of soil because those trees tend to take up rain water as their source of water.

Ray Douglas: So what depth range is best suited for phytoremediation?

Jim Landmeyer: For a phytoremediation of contaminated groundwater to be the most effective, the depth to water table from land surface should be no greater than about 15 to 20 feet below land surface, because any depth greater than that would probably be a less efficient uptake of groundwater.

Ray Douglas: How much uptake of groundwater are we actually talking about?

Jim Landmeyer: Some large trees, tens of years old, say, a poplar tree, can pump tens of gallons of water, some of that groundwater, through their systems on a daily basis. So if you can imagine, if you have a site where 3,000 poplar trees are planted, each pumping each 10 gallons of contaminated groundwater per day, the effect on the water table can be impressive.

Ray Douglas: What are some of the types of groundwater contaminants that can successfully be removed from the water table using phytoremediation?

Jim Landmeyer: Types of contaminants that can be taken up by trees and be phytoremediated include gasoline compounds like benzene, toluene, creosote compounds like naphthalene, chlorinated solvent compounds like perchloroethylene or tetrachloroethylene or TCE. There are various other compounds that can be taken up by plants through phytoremediation such as explosive compounds all the way to including inorganic such as metals.

Ray Douglas: That's quite a list of contaminants. But how is it that the tree can actually survive the uptake of all those compounds?

Jim Landmeyer: Well, as the old saying goes, "It's not the poison that kills you, it's the dose of the poison," and it sort of holds true for phytoremediation of contaminated groundwater. For instance, if you take a plant, be it a young plant or a mature plant, and you put it in a vat of gasoline or a vat of dry-cleaning fluid, it will definitely not survive.

On the other hand, if you put the same type of plant in an area where there are much lower concentrations that are dissolved in water, yes, those plants can not only survive but they will take up those contaminants.

And trees that have been around and have evolved starting about 400 million years ago have developed quite effective ways to deal with threats to their existence. So trees do have the natural ability to take up contaminants that they see both natural and then therefore man-made and have enzyme processes in place that permit those contaminants to be handled by the plant where they turn it into something either innocuous and not harmful or they actually compartmentalize that harmful compound into an area that won't affect their systems.

This process is very similar to how we as humans in our liver detoxify chemicals. In fact, the term 'green liver' has been given to the ability of plants to process and to detoxify these chemicals that they encounter.

Ray Douglas: So I guess we are just pushing the evolution of trees along by introducing phytoremediation.

Jim Landmeyer: Well, in one way, we are. People are actually, and this is something that we haven't specifically done in the USGS, but people are actually taking, for instance, bacterial enzymatic capabilities to grade certain contaminants that plants can't degrade now and through genetic transfer actually transferring from the bacteria to the tree the ability for a tree to now degrade at compound.

Ray Douglas: What key difference is there in this process of phytoremediation of contaminated groundwater and older methods of remediation?

Jim Landmeyer: Well, there are many different ways to remediate contaminated groundwater. The common denominator between many of the more engineered methods is, in order to help remediate, you have to add a lot of energy into that system that you're remediating, the energy in terms of a physical capital investment. Adding wells, injecting slurries, pumping wells, trapping contaminants, all those remediation technologies, the traditional engineering technologies, take a great input of energy, which is an input of funds.

With phytoremediation, the basis for the energy input, other than actually putting the trees in the ground and maybe pruning them from time to time, is actually taken care of by solar radiation.

Ray Douglas: So would you say that phytoremediation is a more cost-effective way to clean up contaminated groundwater?

Jim Landmeyer: Phytoremediation can be more cost-effective than your traditional, classic engineering systems such as a pump-and-treat or vapor extraction recovery system or even oxidant-injected systems.

Ray Douglas: What is it about phytoremediation, the trees, the process, that captured your attention to do this research?

Jim Landmeyer: The interaction with trees, with subsurface sources of water like groundwater, at first blush may not be apparent, because after all roots are hidden below ground out of sight, and groundwater is hidden below ground out of sight. So we tend to think that trees get their water primarily from rain.

In many cases, whether a tree is getting its water from rainfall or from rain that's infiltrated and recharged in aquifer, the tree itself and forests of trees in a given area tend to remove and send back to the atmosphere in some places up to 70% of the water that entered that area. So trees are a huge, important component of processing water through the hydrologic cycle.

Ray Douglas: So I guess now we need to begin thinking of trees as groundwater pumps.

Jim Landmeyer: Yes and no, Ray. Trees do move water from one compartment, the subsurface, to the other, the air, much like a pump would, say, remove water from the ground and put it into your house. But with a pump, you are actually adding energy to turn a lifting mechanism that moves out water with plants.

The only energy that they require is the solar energy from the sun that keeps them alive, but also the difference in wetness between the ground, which tends to be very wet versus the air which can be dry. That's what drives evaporation, and that's what drives the passive movement of water through a tree.

Ray Douglas: I understand that you have a new textbook in the works. Could you tell us a little about that?

Jim Landmeyer: Yes. We've been given the opportunity to compile and prepare and publish a textbook called "The Introduction to Phytoremediation of Contaminated Groundwater" and it will be available sometime in 2011. This is the type of book that I wish was available when I started my first phytoremediation project back in 1998.

Ray Douglas: Jim, thanks for taking time out of your busy schedule to talk with us today. Any closing thoughts on phytoremediation and its future?

Jim Landmeyer: You're welcome. This is an important issue.

Hopefully, someday we'll simply be able to go to a contaminated site, plant trees and grasses, and then walk away knowing that the processes of phytoremediation are occurring and that the groundwater system is being cleaned up. We're not there yet. Right now we still need to collect good reproducible data that shows that these phytoremediation processes are hearing the fundamental principles in physical laws.

Every part of the world has a certain allocation of solar energy, that you can actually measure in kilowatts, that is being input to a system. You can take advantage of that through using phytoremediation to drive the remediation of the system by pumping groundwater, taking advantage of the natural interaction that plants need to have to move water through their system. That is green technology at its best. 

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Ray Douglas: We'd been talking with Jim Landmeyer, a USGS research hydrologist in the South Carolina Water Science Center.

CoreCast is a product of the U.S. Geological Survey, U.S. Department of the Interior. Thank you for listening.