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Granular Activated Carbon (GAC) filtration removes radon and VOCs (Volatile Organic Compounds) by the process of "adsorption." This is a process whereby a gas or compound adheres to the surface of an adsorbent. In this case, the adsorbent is carbon.
GAC has long been recognized in the water treatment industry as a highly versatile filtering medium, especially when used for taste and odor improvement. It is the primary filtering media found in all the "over the counter" point of use filtering systems available in supermarkets and home improvement centers. In more recent history, its ability to remove radon from water has come to light.
The removal process is simple; as radon contaminated water runs down through a bed of carbon, the carbon "adsorbs" the radon out of the water onto the pore space of the GAC. Caution should be exercised when choosing to use GAC filtration as a waterborne radon mitigation system. Probably the most important aspect to consider is the range of fluctuation of the radon levels in the well under consideration for mitigation. Worst case scenario should always be considered as well as the general overall water quality. GAC can become quickly "loaded" or fouled if installed in a home with a well that has other organic compounds present in the water in excessive amounts. If influent radon levels are too high, a sufficient reduction of the waterborne radon concentration may not be achieved for the duration of the service cycle.
While EPA does not regulate what system should be used at what level, the state of Connecticut currently suggests that GAC be considered to mitigate radon levels up to 10,000 pCi/L. When concentrations exceed 10,000 pCi/L, aeration is the mitigation technology that should be employed.