Granular activated carbon or “GAC” is a material used to filter harmful chemicals from contaminated water or air. It is composed of granules of coal, wood, nutshells or other carbon-rich materials that have been heated to “activate” the surface of the granules. As contaminated water or air flows through GAC, contaminants sorb (stick) to the GAC surface and are removed. GAC can sorb a wide range of contaminants such as fuel oil, solvents, polychlorinated biphenyls (PCBs), dioxins and other industrial chemicals, as well as radioactive materials. It also sorbs low levels of some types of metals. GAC is commonly used as a treatment step in other cleanup methods.