3 Environmental research and monitoring (rehabilitation)
Supervising Scientist, Darwin, 2005
ISBN 0 642 24395 6
ISSN 0 158-4030
Airborne dispersion of radon and the subsequent inhalation of radon progeny is the major pathway of exposure of the public to ionising radiation. UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) reported that radon isotopes contribute to more than 50% of the natural source of radiation exposure and measurements have shown that a dose of approximately 0.7 milliSievert (mSv) per annum is received in Jabiru due to the inhalation of radon progeny. However, it is necessary to distinguish natural and Ranger derived radon. Estimates for the mine-origin radon exposure range from less than 0.01 to more than 0.2 mSv per year at Jabiru.
In order to reliably model radon-222 ( 222Rn) concentrations in air following rehabilitation it is necessary to know the 222Rn exhalation source term in the Ranger area. This work was carried out in collaboration with Queensland University of Technology to determine the current source term and its temporal variation, and to develop algorithms that describe 222Rn exhalation in terms of the key characteristics of soils. This information will be used to predict 222Rn exhalation from the site after rehabilitation. The work program consisted of three main parts:
- Measurements of 222Rn exhalation from surfaces at Ranger during the dry season;
- Measurements of 222Rn exhalation at various sites over a seasonal cycle;
- Measurements of 222 Rn exhalation at various sites over several diurnal cycles.
Conventional charcoal canisters and a radon emanometer (Figure 3.4) were deployed in 2002–03 to measure 222Rn flux densities from a total of 317 sites covering the following areas at Ranger uranium mine: mine pits 1 and 3, wasterock dumps, ore stockpiles and both irrigated and non-irrigated land application areas. 222Rn flux densities were highest for the ore stockpiles with fluxes from the laterite stockpile being three orders of magnitude larger than from environmental areas, which are typically 70 milliBecquerels per m2 per second (mBq�m-2�s-1). 222Rn flux densities from the waste rock dumps and the pits are 1–2 orders of magnitude higher, whereas those from the irrigated land application area were only slightly higher than the environmental areas at 112 mBq�m-2�s-1.
Radon exhalation from soils is influenced by a variety of factors, such as soil moisture, soil porosity and soil radium-226 ( 226Ra) content. As measurements were conducted over the dry season, we were able to investigate the dependency of the 222Rn flux density from soil 226Ra content and soil porosity. Figure 3.5 shows the 222Rn flux density plotted versus the soil 226Ra content for various geomorphic groups: barren (disturbed) areas where compaction has taken place as a result of human influence, vegetated woodland and rehabilitated sites with relatively porous vegetated soils, non-compacted fine grains such as the laterite push zone or overburden zones, and non-compacted boulders. Compaction of the ground and reduction of soil porosity decrease radon exhalation whereas vegetation with established root structures leads to higher exhalation fluxes.
Figure 3.5 222Rn flux density plotted versus the soil 226 Ra content for various sites
Seasonal measurements of radon exhalation
222Rn flux density measurements have been performed over the course of one year at eight sites in the region. Previous studies have indicated that there are large variations in flux between wet and dry season in northern Australia. The influence of soil moisture is the most likely reason for this. Soil moisture profiles (0–1 m) were determined in conjunction with every set of flux measurements at some seasonal sites. The soil moisture data showed large temporal variations throughout the wet season which explains the large variations of 222Rn flux densities during the wet.
Figure 3.6 shows the annual variation of 222 Rn flux densities at four of the eight investigated sites, and the cumulative rainfall during the time. Generally, radon exhalation in the wet season was largely reduced as soil moisture retarded radon exhalation. However, localised variations were also observed. For instance, some of the sampling sites such as the Mudginberri radon station, Jabiru East or Mirray (Figure 3.6) exhibited a peak in radon exhalation during charcoal cup exposure in January, which is likely to be due to evaporation of soil water and the release of trapped radon after a short but intense rain event.
Figure 3.6 Annual variation of 222 Rn flux density and cumulative rainfall plotted versus the date
Diurnal measurements of radon exhalation
The aim of this part of the study was to establish whether there was a correlation between radon exhalation, time of day and meteorological parameters such as atmospheric pressure, site and soil temperature. Diurnal measurements were performed at five of the seasonal sites. To investigate whether diurnal radon flux density variations were dependent upon the soil moisture content, measurements were performed during the wet season and during the dry. The results indicate that there is little or no diurnal variation of 222Rn flux density and that a correlation with soil temperature and atmospheric pressure, if any, is masked by random variations of radon exhalation and measurement uncertainty