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Scientists and engineers have developed many innovative methods to remove contaminants from soil, surface water, and groundwater. Permeable reactive barriers (PRBs) are a good example of new technology that was created to solve environmental problems. A PRB is a wall built below ground to remove pollutants from contaminated groundwater. The walls are permeable, so water will flow through, but are made of reactive materials that will trap or detoxify pollutants. PRBs made of metallic iron are used to remove chlorinated organic solvents and heavy metals from groundwater. The chemical principle is simple—iron is a good reducing agent. It reduces toxic organic compounds and converts them to less harmful substances. The reaction of iron powder with organic redox indicators (dyes) in this activity demonstrates the “potential” of this method to reduce organic compounds.