Pre-mining radiological conditions in the Ranger Project Area

Department of Sustainability, Environment, Water, Population and Communities, 2013

Internal Report 616
Supervising Scientist Division
A Bollhöfer, A Beraldo, K Pfitzner, A Esparon & G Carr

Executive summary

Ground gamma surveys, radon exhalation and soil 226Ra activity concentration measurements were conducted between 2007 and 2009 over an undisturbed analogue site, Anomaly 2, to the south of Ranger mine. This anomaly exhibits radiation levels that are higher than typical background levels in the region, similar to the outcropping orebodies 1 and 3, that have been mined out between 1981 and 2012. The purpose of the measurements was to groundtruth a historic airborne gamma survey (AGS) that was flown over the area in 1976, before mining started, to be able to determine the pre-mining radiation source term and extrapolate to the whole extent of the AGS including the unmined RPA.

Algorithms were developed to upscale the ground gamma data in ArcGIS to make the spatial resolution comparable to the resolution of the 1976 AGS (300 m line spacing). This was done via an intermediate step, correlating the ground data with data from a higher resolution AGS flown in 1997 by Rio Tinto (100 m line spacing) that also covered the area of Anomaly 2.

The minimum footprint area that can be assessed is set by the optimum buffer radius determined when up-scaling the external gamma dose rates measured on ground to the AGS data. For the current case this is approximately 4 ha. Thus, the correlation models developed allow estimates to be made of the pre-mining baseline gamma dose rates, 226Ra soil activity concentrations and 222Rn fluxes for any selected area larger than 4 ha covered by the 1976 AGS over the greater Ranger area.

Comparison with published data on external gamma dose rates, 226Ra soil activity concentrations and 222Rn exhalation flux densities in the Ranger region before mining started shows that our model estimates are in good agreement with radiation and radionuclide activity levels estimated previously via direct measurement on top of orebody 3 and orebody 1, and from previous environmental radiation surveys.

The GIS model will also allow an estimate of pre-mining uptake of uranium series radionuclides into biota over the footprint of the Ranger mine, assuming secular equilibrium of the radionuclides in soils and using uptake factors determined for bushtucker in the region (eg Martin et al 1998, Ryan et al 2005, Doering et al in prep). This will facilitate the calculation of pre-mining ingestion doses to humans from the consumption of traditional foods harvested on site, in addition to an estimate of the internal and external radiation doses to wildlife.

Our approach for determining pre-mining radiological conditions can be used at any mine and industrial site where historical AGS data are available and an undisturbed analogue exists to groundtruth the AGS data. Areal extent and thus resources for the ground survey will ultimately depend on the resolution of the historic AGS.