What is the most common or traditional method that is used for groundwater remediation

The most traditional method is referred to as a "pump and treat." This involves pumping the groundwater out of the ground and treatment of the contamination at the surface. Because treatment is done above ground, it's called an "ex situ" method.

In this process, a well is put into the ground where there is contaminated groundwater. A pump is attached to the well and if the pumping is successful it will draw some of the contamination out of the ground to the surface. Then the contaminants are treated in a variety of different ways, usually be adsorption to activated carbon filters. The pump and treat is easy from an engineering standpoint. However, this process is also criticized as being inefficient for two related reasons. The pump and treat method tends not to solve the problem, but just controls it. This means that you have to continue to pump these contaminated areas for a long period. If the well pumps are turned off, the contaminant concentrations tend to rebound.

The constant daily pumping operation is costly. Ex situ process can also be tied up in legal and regulatory issues because once the contaminated water is pumped up to the surface you cannot just dump it back on the ground again because it is hazardous waste, so you need to go through regulatory and legal requirements to remove it.

You have been studying how iron particles break down pollutants since 1992. One technology you and your colleges helped to develop was the use of Permeable Reactive Barriers (PRBs), commonly called "iron walls," to remediate contaminated groundwater. In this process, however, you used macro-sized iron particles. What are iron walls and what are the advantages of using this process for cleanup rather than the traditional remediation technologies?

The "iron wall" or permeable reactive barriers (PRBs) has two main characteristics—it is in situ and it is passive. In situ means that you apply iron particles by placing them into the ground and treat the contamination there. You do not have to pump the contaminated water up to the surface. The other advantage is that this method is passive. Pump and treat is active because you have to constantly keep the pumps working. Ideally passive technologies mean that once the treatment is in place you can cover it up, plant grass, and then you can leave the area—there is nothing else to do.

Passive can be much less expensive in the long haul than pump and treat which is an active process. In situ allows you to put iron particles and materials down into the ground to interact with the groundwa-ter to remove the contaminants. The groundwater will flow through this treatment zone and be treated and what comes out at the end is cleaned. The iron wall also serves the function of an in-ground cutoff wall. This means you can intercept the contaminated groundwater in a very precise way—you cut it off. This cutoff wall is useful in many ways where many properties are near one another. So, if someone spills toxic materials into the ground and contaminates the groundwater, you can use a cutoff wall to keep the contaminants from entering the ground-water of the nearby neighbor's property, or discharging into surface water.

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