[en] The Mound Radon Program has performed extensive outdoor radon monitoring near six remedial action sites for the Department of Energy since 1980. The results of these environmental measurements indicate that yearly average background concentrations as low as 0.1 to 0.2 picocuries/liter can be determined using Passive Environmental Radon Monitors. This monitoring can also be used to map areas of increased radon concentrations and can be used to provide confirmation of the success of remedial action. Environmental monitoring can provide data that can be used for decision-making by government officials and the general public by replacing uncertainty with knowledge about radon concentrations in the vicinity of remedial action sites. 2 references, 8 figures, 1 table