[en] Earthen cover systems will be used to control radon flux levels at most, if not all, uranium mill tailings disposal sites in the USA. The long-term residual moisture in an earthen cover will determine, to a large degree, the effectiveness of the cover for radon control. The cover moisture content is a function of climate, plant cover, soil type and depth to water table. Estimates of long-term moisture content can be made several ways. Simulations, using deterministic water flow models, provide the most complete description, but require the most detailed information, including long-term climate and plant cover changes, which at present have a high degree of uncertainty. Simpler correlations, related to soil type and expected periodic drought conditions, will likely be more practical for estimating long-term saturation to water retention characteristics of the soil and assumes that residual moisture in the entire soil cover will approach the so called permanent wilting point (15 bar percentage). The expected residual moisture saturation at permanent wilting, ranges from 0.07 to 0.56 as soil texture varies from sands to clays at typical field densities. Radon diffusion coefficients for soil cover materials range from 5.0E-2 cm²/s to 5.0E-3 cm²/s for sands and clays, respectively, at typical field densities. With increased compaction, the residual moisture saturation can increase significantly and radon diffusion coefficients as low as 3.0E-4 cm²/s have been measured. The optimal soil cover design will likely be a two-layer cover system with at least a 1-meter thick plant root zone of uncompacted soil over a moistened, tightly compacted fine textured soil. This design concept has been tested successfully at Grand Junction, Colorado. 11 references, 4 figures, 2 tables