Radioactivity - the "ugly face" of the Mooi river
by Peter Wade
It was a chilly winter's morning in South Africa in the
last year of the
20th century. The sun rose fashionably late, bathing the waters of the
Wonderfonteinspruit in a subtle rosy glow. An icy breeze ruffled the
feathers of the Egyptian geese huddled on the shores of the Blaauwbank dam.
Gamma rays punched skywards from the exposed mud with
many hundreds of thousands of times the energy of the gentle rays falling from
the sky.
In the foreground, four mad scientists wrestled with arcane apparatus
consisting of tubes, wires, nets and sampling dishes. These were
the intrepid researchers from South Africa's Council for Scientific and
Industrial Research, being myself, Petro Vos, Stephan Woodborne and Freda
Morris. Our instruments were to establish the intensity of radiation from
the sediment, and what was generating it.
 Many
months prior, Henk Coetzee of the Council for Geoscience had bravely flown over
the Wonderfonteinspruit at 60 metres' altitude with a gamma
spectrometer coupled to a Global Positioning Satellite system. When Henk
analysed his data he found that the river was a hotspot of radioactivity. In
other words, radioactive material entering the river was coming in at a higher
rate than it was leaving the river.
Even earlier, I had scrutinised monitoring data
from a Department of Water Affairs and Forestry study, and had noticed that
water-borne radionuclides, presumably introduced by
mining activities, were much higher in concentration in the upper catchment of
the Mooi River system than
in the lower end of the catchment. It was obvious that there was a sink for
the radionuclides in the river system.
As an environmental chemist, I thought that the
radionuclides might be
preferentially binding to sediment. Subsequent inspection of Henk's aerial
radiogammagrams (pic above) convinced me that the dams were the best place to
look if you wanted to find the unstable elements that were bathing the river
reaches in radiation.
Did the radionuclides pose a threat to humans in the Mooi River catchment?
This led to the following research questions.
1. Have radionuclides (e.g. uranium) preferentially accumulated in river
mud?
2. Can the radionuclides be remobilised?
3. In what form are the radionuclides in the sediments, so we can work out
under what conditions might they be remobilised?
We toiled for many a month, braving many a danger, including sinking up
to our chests in stinking radioactive mud, spending hours extracting our
4x4 from slime on the banks of a dam, resampling a stretch due to theft of a
cooler box of mud samples which masqueraded as a tasty meal to the eye of a
hungry homeless person, and hasty improvisation due to the engine failure of one
of our transports.
Stephan's gamma specrophotometer had a detector that incorporated sensitive
electronic components, across the junctions of which many thousands of volts
were poised. In order to measure the gamma
emissions in the sediment, Stephan had purchased a plastic PVC pipe and a
two litre bottle of soft drink.
He cut the bottom from the soft-drink bottle, and welded it to one end of
the PVC pipe, thus making a watertight probe, into the end of which may be
gingerly lowered the delicate gamma detector. This is an excellent
example of how cash-strapped scientists innovate their way out of difficult
situations.
At one of our sites, one of us stumbled, and a drop of water made its
way into the open end of the PVC tube, and onto the gamma detector, closing
junctions that were never meant to be closed, and frying the circuits of the
gamma detector. It was a dark day for us, I have to tell, since a replacement
would have cost many thousands of rands. Stephan managed to obtain a
replacement, however, and our work continued apace.
We went from site to site along the river, collecting sediment samples,
water samples and information. We all had our zones of ownership. Stephan
was the custodian of the gamma emission data, I managed the sediment
sampling and water sampling, and Petro collected aquatic insects to measure the
health of the ecosystems we were tramping around in.
Our gruelling sampling trips over, we returned to the labs with our
booty - digital files containing gamma spectra, and ten lovingly labelled
samples of smelly mud, each from a different site along the river in the Mooi
River catchment. The route the sediments took to the lab is summarised below.
Method of handling sediments - three sediment core samples
amalgamated from each of ten sites, stored in the field in anoxic
conditions at low temperature, transport to lab, where stored in a freezer, and
thawed just before sediment chemical extractions.
Then began the arduous task of performing the dreaded "sequential
chemical extractions" from the sediment samples. Freda leapt into the fray,
dutifully soaking the sediments in each of four solutions of noxious
chemicals, which were designed to simulate realistic extreme
environmental conditions in the catchment.
When the chemical reactions were finished, the solutions were filtered
from the solids, and sent off for analysis of the radionuclides 238U, 235U,
234U, 232Th, 226Ra, 224Ra, 223Ra, 210Pb and 210Po.
These radionuclides were carefully selected because their relative
activities would tell us much about how the radionuclides are moving in the
catchment.
Stephan's gamma spectrophotometer had already told us that there was a
high level of radioactivity at the sites we had sampled.
We found that the 238U/234U
ratios in the sediment samples were always
very close to 1.0. This helped us determine that the uranium originated
from recently crushed rock, as in mining, as opposed to a geological uranium
deposit that was slowly leaching into the catchment.
The 238U/235U ratio was always close to 21.7,
meaning that the uranium in the mud did not come from a 235U enrichment plant,
and was therefore of natural origin.
The 234U/226Ra ratio is always very close to 1.0 in a system in which
natural uranium-bearing rock is crushed, and fine crushed rock particles
enter a river. In all of our samples where we found significant radium
concentrations, this ratio was always greater than one, leading us to
believe that the uranium we found in the watercourse had been subjected
to a chemical process that removed the radium, as opposed to environmental
physical processes that would have left the ratio close to 1.0.
The 226Ra/210Pb ratio tells us the short-term age of the disturbances.
We found that the more contaminated the sediments, the greater the excess of
226Ra, implying that the more contaminated sediments were younger. This is not
conclusive, since the great scatter in these numbers we got could mean that
there are chemical processes in the sediments that are removing one or another
of the 226Ra or 210Pb radionuclides. We need to do more studies to find out the
exact age of the radionuclides in the Mooi River system.
We found from our chemical experiments that there was
much more
radioactivity coming from uranium than any of the other radionuclides we
were looking at. Stephan's measurements with his gamma spectrophotometer agreed
that the radiation was mainly coming from uranium daughter
products.
Thus, we decided to focus our investigations on uranium behaviour in the
sediments. The conclusions of the isotope ratio studies above seem to
suggest that uranium in the Mooi River system is of recent origin, and,
while it does not originate in a uranium enrichment process, certainly does
not derive simply from crushed rock. The uranium is probably being rendered
into a water-soluble form from the native rock through a process in the
mining industry.
It seems from the study of radionuclides in the Mooi River
sediments that uranium could be accumulating as a result of gold mining
operations in the catchment.
The first research question "Have radionuclides (e.g. uranium)
preferentially accumulated in river sediments?" had almost completely
been answered. The answer is yes.
This translates into asking if so much uranium has been absorbed into the
mud that the concentration in the sediments may be greater than those in the
water from which the uranium originated. For the sites we chose in the Mooi
River catchment, uranium has accumulated to many orders of magnitude higher
concentration in the sediment than the uranium concentrations in the water.
Is there sufficient uranium in the sediments for us to be concerned
about?
The South African Nuclear Energy Act of 1993 states that if
a site that
has been used for some human activity, then the Council for Nuclear Safety
(CNS) only allows "unconditional deregulation", i.e. for the site to
escape
a detailed CNS risk assessment, if the soils have radioactivity levels below
200 mBq/g (milliBequerels per gram). The Bequerel is a unit of
radioactivity, which is equivalent to the number of radioactive
particles detected per second. Some of the sites that we studied have
radiation levels in excess of this limit. The location of the sites in the
study area may be viewed here.
The chemical separations that Freda did in the lab were supposed to
simulate environmental scenarios in which the river were to be injected with
a pulse of salt water, or acid, or organic matter, like sewage, or if the
sediments were being attacked by oxygen, either through drying out, or by
getting oxygenated river water in by burrowing insect larvae. Freda's
simulations showed that uranium is capable of being emitted from
the sediments under almost all conditions, (acid leakage, reducing conditions
and oxidising conditions in the sediments) except the scenario of a pulse of
salt water entering the river.
The answer to the second research question, i.e. "Can the radionuclides
be remobilised?" is definitely "Yes!".
The third research question, "Under what conditions might they be
remobilised (i.e. in what form are the radionuclides in the sediments)?"
can be answered with chemical modelling.
If you know what chemical reactions are possible in an environmental
system, you can set up a computer model to simulate the reactions, and to
see where chemicals may end up in an environmental system under different
scenarios. Chemical modelling is an extremely useful tool for risk
assessments, because you can essentially perform experiments on a computer that
would be impossible, unethical or impractical to do in a real
environment.
I identified the major reactions of uranium in the
river environment as
depicted here.
 The
results of my computer simulations forced me to conclude that uranium resided in
the sediments of the Wonderfonteinspruit not as uranium solids, which are
depicted here but are adsorbed to particular sediment
components. The stability zones of the sediment components were derived from my
model, and are shown on the Eh/pH diagram .
One can see that the sequential chemical extractions of Freda's removed the
uranium from the carbonates, iron and manganese oxides, and organic matter in
the sediment.
Thus, we had answered the final research question as well, being "Under
what conditions might they be remobilised (i.e. in what form are the
radionuclides in the sediments)?". The answer is "Uranium may be
remobilised from the sediment under any imposed condition that dissolves
the sediment components: carbonate, metal oxides, and organic carbon".
Whether or not the uranium in the sediments actually poses a threat to
humans is a question that could not be properly answered in our study.
A subsequent study is being undertaken by the Council for Geoscience to
shed more light on the human health risks of the radioactive sediments.
More information:
Dr Peter Wade:
At the time that the research in this article took place, Peter was employed at
the CSIR. Peter is now based in PhoKus Technologies, where he works as a
consultant in environmental and industrial chemistry. He is an expert in
computing modelling, conducting environmental risk assessments.
E-mail: pwade@wol.co.za
This research was made possible by financial assistance from the South
African Water Research Commission and the CSIR Parliamentary Grant.
Contributions of the members of the Steering Committee set up by Meiring Du
Plessis of the Water Research Commission are acknowledged gratefully
Research results earlier published as: Tier 1 Risk Assessment Of Selected
Radionuclides In Sediments Of The Mooi River Catchment. Document code -
1095/1/02
Definition of terms:
Gamma spectrometer
Gamma photons have different energies. These energies are characteristic of the
nuclear reactions that produced them. A gamma spectrometer measures the
intensity of the gamma radiation in a situation, and presents the intensity as a
function of the energy of the radiation.
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Radionuclides
Radionuclides are atoms that are particularly susceptible to engaging in nuclear
reactions that produce radioactivity. Water-borne radionuclides are
radionuclides that are transported by the river, as opposed to sticking to
solids in the environment.
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Anoxic
Anoxic conditions are those that exist when we manage to prevent highly reactive
oxygen from the atmosphere (that is all around us all the time) from entering
our system of interest.
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238U and 234U
238U decomposes into 234U reasonably rapidly. 234U decomposes relatively slowly.
Both forms of uranium behave in an almost identical manner in chemical
reactions. In a system where 238U is released e.g. by crushing rock, there would
be quick formation of 234U, and the ratio of 238U to 234U would be close to 1.0.
If the uranium in the system had been there a long time, the 234U would have had
time to decay, would be lower in concentration, and the ratio of 238U to 234U
would be greater than 1.0.
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238U and 235U
In natural uranium the ratio of 238U to 235U is very high, and is reasonably
fixed, due to the fact that the two radionuclides were created by different
processes when the earth formed. 235U is a very valuable form of uranium if you
want to make nuclear reactors or atomic bombs. Thus, there is an industry in
which 235U is concentrated in a uranium sample, (which is now referred to as
"enriched uranium", leaving behind "depleted" uranium that
contains very little 235U). If this industry had been deliberately operating in
the catchment, the ratio of 238U to 235U would be disturbed - if there had been
spills of depleted uranium, the 238U / 235U ratio would be high, if there were
spills of enriched uranium, the 238U / 235U ratio would be low.
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Daughter product
When a radionuclide creates another type of atom by a nuclear reaction, e.g.
238U reacting to form 234U, the originator of the new atom (e.g. 238U) is called
the "parent" radionuclide, and the newly formed atom (e.g. 234U) is
called the "daughter" product.
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