The performance of point-of-entry granular activated carbon (GAC) and diffused-bubble and bubble-plate aeration systems was evaluated for one year with respect to radon removal efficiency, potential problems, and economics. The radon removal efficiency of the GAC units gradually deteriorated from 99.7 to 79 percent when cationic exchange pretreatment was not used and from 99.7 to 85 percent with pretreatment added. Both aeration techniques were consistently efficient (>99 percent radon removed), despite significant variation in radon loading rates. GAC units are suitable for treating groundwater with radon activities <5,000 pCi/L. At greater radon loading rates, aeration techniques consistently produce an acceptable effluent quality. Includes 34 references, tables, figures.