Radiation Geophysics - The surface distribution of natural radioelements across the USA and parts of Canada: a contribution to Global Geochemical Baselines

The surface distribution of natural radioelements across the USA and parts of Canada:
a contribution to Global Geochemical Baselines

¹ Hon. Chair, IUGS/IAGC Working Group on Global Geochemical Baselines, Scientist Emeritus, Geological Survey of Canada, Ottawa, ON, Canada
² Geophysicist, United States Geological Survey, Reston, VA, USA
³ Geophysicist, Geological Survey of Canada, Ottawa, ON Canada

A poster version [PDF, 5.1 Mb, viewer] of this information is also available. The poster was first shown at the 6th International Symposium on Environmental Geochemistry, Edinburgh, Scotland, September 2003.

The IUGS/IAGC Working Group on Global Geochemical Baselines was established in 1996 to implement recommendations published in 1995 [1] Most elements can only be determined by direct surface sampling methods, but radioactive elements are amenable to airborne gamma-ray spectrometry (AGRS), which can provide continuous traverses across any terrane. Quantitative methodology developed 30 years ago for measuring the surface distribution of gamma-emitting isotopes, allows data to be expressed in terms of element concentrations and dose rate.

In the 1970s, following the 1973 OPEC oil embargo, the governments of Canada and the USA took steps to stimulate uranium exploration. The Uranium Reconnaissance Program (URP) in Canada, and the National Uranium Resource Evaluation (NURE) program in the USA, included AGRS surveys, with similar technical specifications. The NURE program covered almost all the USA. The Canadian program, with similar aims, ended prematurely after 4 years. Small areas were added during the 1980s and 90s, bringing coverage of Canada to about 35%. The data from each country were published in a variety of formats, at different scales. The work presented here is a unified compilation of the US and Canadian datasets designed to facilitate regional comparisons on a continental scale. Maps show many-fold variations in the surface abundances of potassium, thorium and uranium, and also in their relative abundances. A dose rate map illustrates the resulting variation of background radiation with location. The data have important geological, environmental and public policy implications. For example, the Athabasca Basin, Saskatchewan, Canada, contains the world's largest known uranium deposits, but is an area of very low surface radioactivity. Although incomplete for Canada, this display of radioelement distribution across 60% of North America demonstrates the importance of standardised measurement procedures, and is a significant addition to Global Geochemical Baseline information.

The technique of quantitative airborne gamma ray spectrometry, developed in the period 1967-1970, is described in many publications [2,3]. It measures the mean surface concentrations of potassium, equivalent uranium (eU) and equivalent thorium (eTh). The prefix "equivalent" indicates that determinations are based on gamma rays emitted by daughter products of U and Th; such determinations are accurate provided the radioactive decay series are in equilibrium. Each airborne "measurement" or data point samples an area of several thousand square metres, to a depth of about 30 cm, and equilbrium is normally assumed in samples of this size. At a nominal terrain clearance of 125 metres a continuous strip approximately 250 metres wide is sampled. Typically there are about 18 overlapping data points per km flown. The North American map is based on approximately 100 million data points. Flightline spacing determines the percentage of a survey block that is sampled. The line spacings employed in the North American compilation range from 25 km (equal to 1% sampling of the surface), to 1km, equal to 25% sampling, in limited areas. Most of the area was surveyed with 5 km line spacing. In order to convert the observed count rates to element concentrations a series of corrections have been applied following standard procedures, prior to calibration using manufactured pads [3]. Several hundred blocks of survey data were processed in this way in USA and Canada and national compilations produced [3,4,6]. Minor adjustments were made where necessary to obtain a "best fit" and minimize boundary faults. For the continental compilation, grids of K, eU and eTh values were calculated using a 2x2 km cell size [5]. A grid of the gamma ray absorbed energy dose in air at 1 m above ground, in units of nano-Grays per hour, was calculated from the element grids, using a standard equation [3]

The maps on display are a sample of what can be produced. The digital data from which they are derived can be used as GIS input to complement many types of research, ranging from earth and soil science to environmental and health studies at a wide range of scales. The ternary map shows in a single colour image the relative concentrations of K (magenta), eU (cyan) and eTh (yellow). Variations in total radiation are shown by colour intensity, pale shades indicate low radioactivity [7,8]. The ternary map emphasizes subtle distinctions in the relative concentrations of the radioelements, thereby 'fingerprinting' formations, either individually or collectively. Several of the geological provinces which make up the continent appear to possess distinctive radioelement signatures, shown by the colour contrasts of the ternary map.

In the context of global geochemical baselines the data can be used in different ways, either to provide considerable spatial detail for correlation with other parameters, or to provide basic statistics. As examples of the latter, the mean and standard deviations of data for a number of US States are listed in Table 1, together with the same parameters for all Canadian data. Amongst these examples the highest mean exposure rate occurs in Colorado, the lowest in Alaska, by a factor of three. The large standard deviations are a consequence of the diverse geology in each area causing a multi-modal frequency distribution.

An alternative approach is to subdivide the data using the equal area Global Terrestrial Network (GTN) grid, which is superimposed on all of the maps. The GTN grid was devised as part of the recommendations in [1] as a way of planning the collection of global reference samples. Each GTN grid cell has an area of 25600 km². A number of Canadian cells have been selected from different geological provinces, i.e. representative of different geological ages. Two sets of mean values for the principal parameters are shown in Table 2. The first 'mean' column is calculated using all data points within each cell, which over all of northern Canada includes data overlapping many lakes, ponds and bogs. The second column is an 'adjusted mean' which applies a correction designed to delete records with significant amounts of surface water [6]. On average this raises the mean values by about 10%. This is regarded as a more accurate guide to the mean surface concentration of the radioelements in the northern land surface. These mean surface concentrations can be considered as baseline values for the GTN cells, comparable to those obtained for the non-radioactive elements by conventional surface sampling.

As indicated by the maps, there is clearly wide data variance within each cell; as a complement to the mean values it is useful to provide 'box and whisker' plots to indicate the spread. Figure 1 relates to the Canadian data in Table 2. Note that the Athabasca Basin example, with the lowest mean and median, hosts the largest and highest grade uranium deposits known anywhere in the world. (There is a five-fold contrast in dose rate between the Athabasca Basin and cells in the Churchill province of the Shield). The Athabasca deposits are approximately 1400 million years old. The fact that they have no surface expression would seem to be of some relevance to issues concerning the safety of nuclear waste disposal by deep burial.

[1] Darnley, A.G., Björklund, A.J., Bølviken, B., Gustavsson, N., Koval, P.V., Plant, J.A., Steenfelt, A., Tauchid, M. and Xie Xuejing, with contributions by Garrett, R.G. and Hall, G.E.M. 1995: A global geochemical database for environmental and resource management: recommendations for international geochemical mapping. Science Report 19, UNESCO Publishing, Paris. 122pp.

[2] Darnley, A.G., 1991: The development of airborne gamma-ray spectrometry: case study in technological innovation and acceptance. Journal of Nuclear Geophysics, 5, p. 377-402

[3] Grasty, R.L., Carson, J.M., Charbonneau, B.W., and Holman.P.B., 1984: Natural background radiation in Canada. Geological Survey of Canada Bulletin 360 , 39pp.

[4] Duval ,J.S. and Riggle, F.E. 1999: Profiles of gamma-ray and magnetic data from aerial surveys over the conterminous United States: US Geological survey Digital Data Series DDS-31, release 2, 3 CD-ROM.

[5] Duval, J.S., Carson, J.M., Holman, P.B., and Darnley, A.G., 2003: Aerial gamma-ray survey coverage of the United States and Canada. Proceedings of IAEA Technical Committee Meeting, Golden, CO

[6] Darnley, A.G., Carson, J.M., Garrett, R.G., Grant J. A. and Holman P. B. 2003: Commentary on airborne gamma ray spectrometry data from the Canadian portion of the North American compilation. Proceedings of IAEA Technical Committee Meeting, Golden, CO

[7] Duval, J.S., 1983. Composite colour images of aerial gamma ray spectrometric data . Geophysics, 48, p. 722-735.

[8] Broome, J., Carson, J.M., Grant, J.A and Ford, K.L. 1988: A modified ternary radioelement mapping technique and its application to the south coast of Newfoundland. Internal report, Airborne Geophysics Section, Geological Survey of Canada, 13pp.