222Rn and 220Rn activity flux from the ground in the vicinity of Ranger uranium mine

1998

Internal Report 279
Todd R, Akber RA & Martin P
Supervising Scientist Division
Department of the Environment

About the report

This report is prepared for submission to the Environmental Research Institute of the Supervising Scientist (ERISS) for part fulfillment of the requirement under a QUT-ERISS consultancy agreement.

The report contains up to date details of the work and measurements carried out to study the radon activity flux as a collaborative effort by QUT and ERISS staff. The main body of the text deals with the radon activity flux survey; chapter five describes an initial set up of a radon emanation measurement column and its application to study thoron emanation from a monazite sample; appendix one describes the field sites, appendix two is a site by site description of all measurements, appendix three includes the results of an intercalibration exercise of the equipment.

Extensive research into the production and transport of 222Rn in the environment as been performed, however it is still not possible to determine what set of conditions will lead to elevated levels of 222Rn. Very limited data is available for 220Rn. The majority of studies have been performed in temperate climates with little data obtained in tropical regions. There are large variations in flux between regions. The relative importance of the various parameters on flux also varies between regions. Therefore it is difficult to apply results from previous studies to an unknown region.

A flow through accumulator technique was used to measure 222Rn and 220Rn flux in field and laboratory experiments. An activity flux survey was performed over an area of approximately 10 kilometres square, in the Jabiru region of the Northern Territory. The aim was to determine the association between radon flux and various soil characteristics and meteorological parameters in this tropical region.

The average 222Rn and 220Rn flux in the region were determined to be 64 ± 25 mBq.m-2.s-1 and 2.15 ± 0.21 Bq.m-2.s-1 respectively. The strongest correlation was found between radon activity flux and radium activity concentration in the soil. Weak non-linear relationships were observed between 222Rn and 220Rn flux and the ratio of the >2mm fraction to the <2mm fraction of a core soil sample (O-20cm deep) and with soil moisture. Strong correlation was found between gamma dose rate at 1 m above ground and 220Rn flux but not 222Rn flux and no relationship was observed between 222Rn and 220Rn. Of the meteorological parameters observed weak correlation was observed between 222Rn flux and air temperature. No significant effect was observed due to wind speed, barometric pressure, soil temperature or air soil temperature difference, and no significant correlation was found between 222Rn flux and any of the meteorological parameters.

A study of the diurnal variations in flux was also performed. Significant diurnal variations were observed in both 222Rn and 220Rn flux. 222Rn flux was lower through the middle of the day while 220Rn flux was elevated at this time. No correlation was found between 222Rn and 220Rn flux, suggesting that different factors are dominating variations in these parameters.

A laboratory study was undertaken, into the effect of sample thickness and moisture on 220Rn flux from a monazite sample. Flux increased rapidly with thickness to approximately 5cm after which no increase was observed. Soil moisture had a large effect on flux, causing an increase in flux for water content less than approximately 6% by weight. This was followed by a dramatic decrease with little flux observed for water content over 10%.