Supervising Scientist Division

Supervising Scientist Annual Report 2002 - 2003: Environmental Radioactivity

Supervising Scientist, Darwin, 2003
ISBN 0 642 24383 2
ISSN 0 158-4030

3 - Environmental research and monitoring (continued)

3.2 Environmental Radioactivity

The Environmental Radioactivity programme provides advice on the protection of people from radiological risk during and after mining activities in the Alligator Rivers Region and uses specialist expertise in remote sensing and isotopes to assist related environmental protection work in the Alligator Rivers Region and elsewhere.

Activities in 2002-03 included:

3.2.1 Radiological assessment of old minesites in the south of Kakadu

National Park

This work is being carried out to assist Parks Australia North with its plans for rehabilitation of old uranium mine and mill locations in the south of Kakadu National Park, and to help in the assessment of any radiological risk to people arising from these sites. Initially, the project concentrated on an area in the upper South Alligator River valley, where the majority of old minesites are located (Figure 3.1). During 2002-03 the work was extended to include an area around the former Sleisbeck minesite, located further south in the Katherine River catchment.

Assessment of minesites in the upper South Alligator River valley

Since 2000, eriss has undertaken a series of sample and dataset collections in the upper South Alligator River valley, including:

One of the aims of the airborne gamma survey and subsequent ground truthing was to estimate the gamma dose rate at ground level, using the counts detected by the sodium iodide detectors aboard the plane. Data for individual image pixels of the airborne survey cannot give an accurate measure of the gamma dose rate on the ground at all points, because of the relatively large effective footprint of the survey (approximately 50 m x 50 m in this case). This footprint is known to be larger than some of the areas of higher gamma radiation on the ground. Correlation with the field data has allowed an overall estimate to be made of average gamma dose rates for each site within the total survey area.

Figure 3.1: Alligator Rivers Region, with a detailed excerpt of the southern area showing the extent of the two airborne gamma surveys and the location of known uranium anomalies

Figure 3.1: Alligator Rivers Region, with a detailed excerpt of the southern area showing the extent of the two airborne gamma surveys and the location of known uranium anomalies

Source: from MODAT database

An advantage of this methodology is that no areas are missed in the dose estimate. In comparison, the more traditional ground survey methods cover only a sample of small areas for each site, and there is a potential for sources to be missed. An additional advantage is that the average dose rate over a large area is generally more representative of the radiation dose which would be received by a person occupying the site over time than spot measurements (which are often carried out at expected points of highest radiation).

In short, the methodology ensures full coverage of the survey area and provides more realistic measurements.

The results of the surveys have been coupled with information from Traditional Owners and Park Rangers on site occupation times and food consumption. This has been used to calculate conservative, but reasonable, estimates of the radiation dose above background received by local Aboriginal People, Park Rangers and tourists visiting the area of approximately 0.3, 0.1 and 0.01 mSv per year, respectively (conservative in this context means the estimates may be higher not lower than actual). These estimates take account of doses received from external gamma radiation, inhalation of dust and radon progeny, and the ingestion of radionuclides in bush foods and water.

The dose estimates are considered to be very small, particularly given that the annual background dose of people living in Australia averages around 2 mSv.

Approximately two thirds, or 0.2 mSv per year, of the dose estimated for local Aboriginal people is due to the consumption of traditional bush foods, including freshwater mussels. This dose has been calculated incorporating an average annual food intake that had been estimated based on five years research conducted as part of the Supervising Scientist's bush food research project and using the latest information on concentration factors (relative to total soil or water activity) for various food items.

Airborne gamma survey of Sleisbeck minesite

In July 2002 an airborne gamma survey (jointly funded with Parks Australia North) was flown over the Sleisbeck area. Due to the small size of the area of interest, the survey used an unusually tight transect spacing of 25 m. This is at the limit of current technology and the resulting spatial resolution (or 'sharpness') of the images was unusually high. A ground truthing survey was carried out during June 2003.

The airborne survey results showed that areas of higher activity are primarily confined to the abandoned Sleisbeck mine, associated with the pit area and low grade ore overburden dumps. Some other anomalies (approximately 200 m from the minesite) were identified in the airborne survey. Ground truthing showed these to be associated with exploration around the old minesite (exploration trenches into naturally occurring anomalies). This outcome showed how refining of our survey technique in this area was particularly useful for highlighting regions of radiation anomalies for subsequent ground truthing and that it may have applications elsewhere. Future analysis of the data will include more detailed integration with other spatial datasets.

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