10 minutes reading time (2099 words)

    Radioactivity and the Environment

    Alvin Summerton
    South Australia Health Commision

    We often read or hear media items about the risks to human health of exposure to ionising radiation. With the recent attention to items such as the Chernobyl reactor accident, contamination at Maralinga and transport of plutonium by the Japanese, it seems we are being continually provided with new information that suggests we are living in a world which is becoming dangerously radioactive.

    There is no doubt that excessive exposure to ionising radiation harms the health of people. There are many examples of this, such as the "radiation sickness" suffered by those close to Chernobyl during the reactor accident. These people received massive radiation doses - thousands of times greater than the dose received naturally.

    While it is important issues such as the above should be presented to the public and discussed, it is difficult for the reader to place the information in context without some appreciation of the naturally occurring levels of radiation and radioactivity.

    We do indeed live in a radioactive world. In fact all living creatures have been subject to the influence of ionising radiation since life first evolved on this planet, due to the presence of naturally occurring sources of ionising radiation - the "natural background". While it is not possible to state that there is no risk, as scientists we can state that exposure to ionising radiation at or near the natural background level is not likely to prove detrimental to the health of an individual.

    Today some people receive additional exposure as a result of:

    • medical procedures such as X-rays
    • their work or research activities
    • additions to the environment from, for example, mining activities.

    These are monitored by health authorities to ensure such exposures are minimised.

    Where does this "natural background" come from?
    It arises because we are all exposed to cosmic rays and the naturally occurring radioactive isotopes (radionuclides) in our environment.

    While most people are aware of radioactive elements such as uranium and thorium, it is worth noting that:

    • all elements with an atomic number greater than 83 are radioactive ie. all isotopes of these elements are unstable.
    • for those elements with atomic number less than 83, which we normally consider as "stable", some 27 elements have environmental radionuclides.

    Thus overall we find that at least 36 of the naturally occurring elements have radioactive isotopes which are found in the environment. The major sources are:
    1. long lived species - "primordial radionuclides" - such as isotopes of uranium, thorium, rubidium, rhenium and potassium.
    2. shorter lived decay products of (1) such as isotopes of radium, radon, polonium, bismuth and lead.
    3. species produced in the atmosphere by the action of cosmic rays, such as isotopes of hydrogen (tritium), beryllium, carbon, sodium phosphorus, sulphur and chlorine.

    How common are these radionuclides
    While the various radionuclides are widely distributed, the relative amount or concentration of each is usually quite small.

    As an example, the most abundant active species occurring in the crust of the earth are:

    • Potassium - a common element - average 2.6%. The radionuclide of atomic mass 40 (potassium-40) constitutes 0.012% of natural potassium. Thus the abundance of this isotope in the earth's crust is about 0.003% or 3 parts per million (ppm) .
    • Thorium - a relatively common radioactive element - average about 12 ppm.
    • Uranium - again relatively common - average about 4 ppm.

    For comparison, lead averages 16 ppm, copper 70 ppm, and silver 0.1 ppm.

    For my work one practical consequence of this background is the need to shield the sample being analysed from the effects of the radiation environment. About 10 cm of lead shielding will reduce the intensity of the natural gamma radiation by a factor of 10 and allow the analysis of most environmental samples.

    Furthermore, the distribution of these radionuclides in the environment varies considerably. For example, in environmental samples collected within South Australia which I have analysed, the range of uranium concentrations is:

    • between 0.5 and 6 ppm in "normal" soils
    • up to 60 ppm in non-radioactive minerals
    • about 0.003 ppm in sea-water.

    A radioactive ore is defined as one which contains more than 200 ppm of uranium, and commercial grade ores contain more than 1000 ppm or 0.1%.

    This variability means that the analysis of environmental samples never becomes completely routine and predictable. Unexpected results and apparent anomalies occur frequently enough to make sure that I never become bored. It seems that nature is full of surprises which are revealed gradually.

    As an example of one surprising result, a "high grade" ore deposit of particular interest has been found in the African republic of Gabon at Oklo. In 1972, samples of the ore where analysed for radioactive elements. The results were surprising in that they indicated that this deposit "was the fossil remains of a 2-billion-year old natural reactor". The ore grade in a rich lode (40 to 60% uranium) was concentrated enough for the conditions required inside a nuclear reactor to exist naturally. Thus the ore deposit behaved as a natural nuclear reactor for about 500,000 years. The Oklo site has been studied intensively to determine the environmental behaviour of the fission products and trans-uranic elements produced during this period. Because of the "geological" time scale since the reactor was active, the information obtained has been invaluable in assessing the proposed methods of dealing with the long-term disposal of the nuclear reactor waste currently in short-term storage.

    How have these radioactive isotopes been distributed so widely?
    Quite simply this is a consequence of the natural geological processes which have resulted in the physical environment as we observe it. As an example, consider the natural erosive processes which convert rock to soil. Weathering of rock containing a significant amount of uranium over many years would result in:
    • fragmentation of the rock into small particles accompanied by the leaching of some of the soluble elements by water. The residual radionuclide content of the remaining solids (soil) would then be lower than in the original rock.
    • the transport of dissolved elements, such as uranium, in the water to the sea.
    • the transport of very small particles of weathered rock in creeks and rivers and subsequent deposition in sediments.

    In addition, the action of rotting vegetation in creek beds and tidal swamps may create a chemically reducing environment. As a result, soluble uranium ions in the +6 oxidation state could be reduced to the less soluble +4 oxidation state, precipitate out of solution and become incorporated into a sedimentary deposit.

    Thus natural processes acting on a rock containing uranium are likely to lead to a number of environmental "products", each of which contains a different amount of uranium. Many of these processes happened a long time ago so we find that the radioactive decay products of uranium, such as radium, are now present in the environmental sample.

    As a consequence the "activity" of uranium (i.e. number of atoms which disintegrate each second) is about the same as that of each of the decay product radionuclides in a solid environmental sample such as soil.

    Unexpected results obtained during a recent environmental survey led me to find out about an interesting natural phenomenon. I'd like to share this with you.

    I collected soil samples from the coastal swamplands near the former uranium treatment plant at Pt. Pirie to see if the environment outside the plant had become contaminated. On analysis I found that the content of uranium in the soils was greater than expected, ranging from 5 to 12 ppm. Further the activity of the uranium was between 2 and 10 times that of its decay product radium. Could this be an indicator of contamination from the treatment plant? The answer is NO - for two reasons:

    • processing of uranium ore removes uranium and leaves behind its decay products such as radium - so contamination from the plant would be indicated by a soil with a radium activity greater than that of uranium.
    • samples obtained from similar environments a long way away from the plant, including sites near Adelaide, gave similar results. The suggestion that measurable levels of contamination could "travel" hundreds of kilometres from Pt. Pirie to Adelaide is just not possible.

    So, having shown where the uranium had NOT come from, I was left with the problem of working out where it had come from and how it had become concentrated into these coastal sediments.

    Further samples were obtained and analysed to try to answer these questions. The data obtained suggest:

    • uranium is concentrated into sediments with a high content of organic residues from decomposing vegetation. Such organic residues seem to be the single most important factor in the incorporation of the uranium.
    • bacterial action is also important - higher uranium concentrations are found in sediments which have an obvious odour of hydrogen sulphide (due to sulphur reducing bacteria).
    • the major source of the uranium appears to be sea-water i.e. uranium at a concentration of 0.003 to 0.004 parts per million is absorbed into the sediment and is concentrated by a factor of about 2000 in the process.

    On discussing my results with other researchers I found that it has been shown recently that a number of other heavy metals also present in sea-water (including iron, copper, chromium, zinc, cadmium and lead) are concentrated into these sediments.

    The significance of mangrove swamps as "a nursery" for fish and other marine creatures is well known. Contamination in the "run-off" from urban development (suburbs and industry) which enters the coastal swamplands can effect the growth of young fish. The absorption of heavy metals into the sediments which collect in the swamplands is of interest for several reasons:

    • environmental monitoring programmes designed to determine the extent of any heavy metal contamination, need to consider the natural processes which lead to the incorporation of these metals into the sediments.
    • the observation that organic residues and bacterial action are significant factors emphasise the need to make sure all samples are collected from a similar environment.

    In addition the fact that organic material seems to contribute to the incorporation of heavy metals may be very significant. Although we don't yet know the precise role of the "organics", preliminary results suggest that the heavy metals can be absorbed into simple organic residues such as mulched lawn clippings. It is just possible that we may be able to obtain an inexpensive natural material which is able to assist in the removal of heavy metal contaminants in waste water.

    From the above discussion, I hope you now have a better understanding of natural radioactivity and can better appreciate the role of one environmental scientist in trying to answer questions asked by the public. I am sure you will have noted that this scientist is still learning something new from his observations of nature after more than 20 years. This, I believe, is one of the most fascinating aspects of science - the more you find out, the more you realise there is yet to learn.

    Suggestions for further reading
    A number of informative pamphlets and booklets are available from the following Australian Government organisations. A selection of material I have found useful follows:

    1. The Information Office, ANSTO, Menai, NSW 2234
      1. "Ionising Radiation" - an ANSTO publication which gives a good summary from the Australian perspective.
      2. "Facts about low-level Radiation" - an International Atomic Energy Agency booklet.
    2. Australian Radiation Laboratory, Lower Plenty Road, Yallambie, Victoria 3085 "Radon in Homes" - Information Bulletin no. 13
    3. I suggest you write to either of the above and ask for details of other publications which may be relevant to your particular interests.

    4. New scientist regularly publishes general articles dealing with practical aspects of radioactivity.In addition, the "Inside Science" series produces articles specifically designed for students.Christine Sutton's "Radioactivity" (New Scientist, 11 Feb 1988) is one of this series and is well worth obtaining

    About the author
    Alvin Summerton was born in South Australia and educated at Woodville High, Adelaide Teachers College and the University of Adelaide gaining degrees in Education and Science, and a PhD in Inorganic Chemistry.

    He is a Fellow of the Royal Australian Chemical Institute and has been a member of the S.A. Branch committee for many years. He has been involved in the teaching of chemistry for over 25 years, initially as school teacher and subsequently at tertiary level lecturing in Analytical Chemistry and the Chemistry of the Environment. He has been the S.A. coordinator of the National Chemical Analysis competition since 1983, a role he enjoys very much.

    For the past 10 years, he has been employed as an environmental scientist involved in the determination of natural radioactive elements in environmental samples. 

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