| Radon-222 equilibrium activities in karst aquifers are dependent upon the $sp{238}$U content of the rock and fissure aperture, with high $sp{222}$Rn activities in tight fissures and low $sp{222}$Rn activities in conduits. In the vadose zone $sp{222}$Rn activities are low as a result of degassing, especially if flow is turbulent, and activities in the phreatic zone are therefore decoupled. In the phreatic zone, recharge to fissures causes a reduction of residence time below that required for equilibrium ($sim$26 days); $sp{222}$Rn activities fall. In springs and in the vadose zone, initial reductions are followed by increasing activities, as long residence waters with high $sp{222}$Rn activities vary inversely with spring catchment area (discharge) because of the increased unsupported residence time in conduits. Simple mass-balance equations based on $sp{222}$Rn equilibrium activities in ground water have been derived to estimate average micro-fissure apertures, macro-fissure apertures, and conduit apertures in karst aquifers. Basic assumptions of the models include uniform $sp{238}$U and $sp{226}$Ra distribution throughout the aquifer, reductions in $sp{226}$Ra retardation with increasing fissure size, and uniform fissure/conduit aperture dimensions along their length. Application of this model in investigations of hazardous waste sites in karst terranes to estimate subsurface-fissure apertures can be beneficial to environmental managers. |
