Geographic variability in radon exhalation at a rehabilitated uranium mine in the Northern Territory, Australia

In this study, dry season radon flux densities and radon fluxes have been determined at the rehabilitated Nabarlek uranium mine in northern Australia using conventional charcoal canisters. Environmental background levels amounted to 31 ± 15 milli Becquerel per m² per second (mBq m^-2 s^-1). Radon flux densities within the fenced rehabilitated mine area showed large variations with a maximum of 6500 mBq m^-2 s^-1 at an area south of the former pit characterised by a disequilibrium between ²²⁶Ra and ²³⁸U. Radon flux densities were also high above the areas of the former pit (mean 971 mBq m^-2 s^-1) and waste rock dump (mean 335 mBq m^-2 s^-1). The lower limit for the total pre-mining radon flux from the fenced area (140 ha) was estimated to 214 kBq s^-1, post-mining radon flux amounted to 174 kBq s^-1. Our study highlights that the results of radon flux studies are vitally dependant on the selection of individual survey points. We suggest the use of a randomised system for both the selection of survey points and the placement of charcoal canisters at each survey point, to avoid over estimation of radon flux densities. It is also important to emphasize the significance of having reliable pre-mining radiological data available to assess the success of rehabilitation of a uranium mine site.