Issue 
Radioprotection
Volume 56, Number 4, October  December 2021



Page(s)  327  336  
DOI  https://doi.org/10.1051/radiopro/2021020  
Published online  14 July 2021 
Article
Measurements of radon exhalation from a South African gold mine tailings using sealed tube method
^{1}
Faculty of Engineering, Built Environment and Information Technology, Central University of Technology, Private Bag X 20539,
Bloemfontein
9300, South Africa
^{2}
Parc Scientific,
PO Box 1045,
Cresta
2118, South Africa
^{3}
Department of Physics, University of Johannesburg,
PO Box 524,
Auckland Park
2006, South Africa
^{*} Corresponding author: fkomati@cut.ac.za; omntwaeaborwa@uj.ac.za
Received:
6
June
2020
Accepted:
28
June
2021
To date, approximately 6000 tons of uranium has been disposed of onto tailings dams per year by gold mining activities in South Africa. These uranium bearing tailings dumps contain significant amounts of radium and are therefore possible sources of substantial amounts of radon gas. Due to the large sizes of these dams, measurements of the radon exhalation for the whole dam using established and conventional methods become time consuming and labour intensive. In this study, radon exhalation rates from a gold mine tailings dam around mining area of Odendalsrus were measured using passive “sealed tube” method and CR39 gas etched track monitors known as Radon Gas Monitors (RGMs). Twenty samples were collected at the depths ranging from 90 cm to 1 m from the tailings for exhalation rate measurements. Radon exhalation rates from these samples were found to vary from 0.0410 ± 0.0042 Bq/m^{2} · s to 0.440 ± 0.045 Bq/m^{2} · s with a mean value of 0.102 ± 0.021 Bq/m^{2} · s and a standard deviation of 0.087 Bq/m^{2} · s. The results of these measurements show very high radon exhalation rate and radium content from one particular sample which requires further investigations.
Key words: radon / exhalation rate / tailings / diffusion / CR39
© SFRP, 2021
1 Introduction
According to UNSCEAR (2008), South Africa’s deep underground gold deposits contain lowgrade uranium (^{238}U). Considering that the gold content in the ore is far much lower compared to uranium (gold: uranium ratio ranges from about 1:10 to 1:100) (Winde and Sandham, 2004), gold mining operations have resulted in substantial amounts of uranium and undesirable radioactive materials brought to the surface and disposed of as waste on tailings dams. Therefore these tailings represents a notable source of radioactive radon (^{222}Rn) gas (Mudd, 2008; Sahoo et al., 2010; IAEA, 2013) with radon concentrations in the tailings up to 1000 times the levels found in natural soils (Ferry et al., 2002). Radon is a colourless and odourless gas that originates from the alpha decay of radium (^{226}Ra) in the decay series of ^{238}U. In spite of its relatively short halflife of 3.8 days, radon is produced in and moves through the tailings until it is released to the atmosphere in large quantities. These continuous radon emissions from gold mine tailings have been and continue to be raised as a cause for concern to those living in the vicinity of tailings dams.
Radon and its alpha emitting progeny, namely ^{218}Po and ^{214}Po are recognised as main causative agent for lung cancer when inhaled in high concentrations (Porstendörfer, 1994; Tomasek et al., 2008; ICRP, 2010). Exposure to radon and its progeny accounts for half of an individual’s total radiation dose from natural and anthropogenic origins, making it the single largest contributor of radiation exposure (Watson et al., 2005; Cooper, 2012). The release and transport of radioactive radon gas from tailings dams to the atmosphere has the potential to result in elevated radiation exposure and health risks due to inhalation by the members of the public at locations up to several kilometres away downwind of the tailings dams. Public radiation exposure from transported radon is primarily due to the alpha decay of its shortlived radioactive progeny deposited in the lungs, thus increasing the possibility of lung cancer. This correlation between exposure to radon and the induction of lung cancer has led to the International Agency for Research on Cancer (IARC), a subsidiary of the World Health Organisation (WHO), to classify radon as Group 1 carcinogen (IARC, 1988). It is in this light that the nuclear regulatory agencies require mine operators to assess the radiological impact of their operations on members of the public and have imposed monitoring protocols and limits on the radon releases from tailing sites. The main source of environmental radon is its flux from the ground, and it is challenging to distinguish radon released from tailings from this natural source. For this purpose, atmospheric dispersion modelling, which requires a source term, or emission rate of radon from the tailings source is used. Atmospheric dispersion modelling yields the incremental radon concentration levels and public dose that can be apportioned to the tailings dams. Measurements of radon exhalation rates from tailings dams combined with atmospheric modelling then become an important tool for estimation of radiation risk as well as demonstrating compliance with regulatory requirements. This study was aimed at establishing a method for estimation of the exhalation of radon atoms from the surface of the tailings into atmosphere, to establish a source term.
Radon release from the radium bearing tailings dam to the atmosphere is a result of a series of processes, namely: emanation, transport and exhalation (Moed et al., 1988). Only a fraction of the radon atoms (the emanation fraction) created in porous material due to the radium decay will be able to escape from the mineral grains and enter the void space. Upon entering the pore space, emanated radon atoms’ movement from the mineral grains in the tailings to the surface becomes a function of diffusion and advection transport mechanisms. However, advective flow on transport is temporal, rapid and complex to model (Guo et al., 2004) and it fluctuates, resulting in high and low peaks. Radon dose results from longterm exposure, and dose is regulated on an annual basis. The causative exhalation source term must therefore be a longterm one, averaging all temporal variations. In this regard, the dominant average transport mechanism becomes the diffusion mechanism, and a method based on this mechanism, which delivers a timeaveraged source term is preferred. The pore space in the soil has a higher concentration of radon than that on the atmosphere. As a result, radon atoms will migrate from the pore space with high concentration, into the air at low concentration through the diffusion process. The difference in the concentration between the two media is the main cause of the diffusion process.
Following emanation and diffusion to the surface, radon near the soil surface boundary will diffuse into the atmosphere. This atmospheric radon release is called radon exhalation or exhalation flux density (IAEA, 2013). Thus, radon exhalation rate depends on the radium concentration in a material, emanation factor of radon from the material, porosity, convective effects due to pressure differences, permeability, density of the material, diffusion coefficient of the radon in the material, and moisture content and temperature (UNSCEAR, 2000; Guan et al., 2006; Lawrence et al., 2009; IAEA, 2013; Altic, 2011, 2014). Depending on the physical structure of the tailings, the rate of escape of radon from the tailings is usually higher than from the normal soils (Nuclear Energy Agency, 1982). Radon exhalation (E) at the ground surface between the soil and atmosphere is a continuous process, and for a tailings depth of more than 4 m it can be expressed as (IAEA, 1992): (1) where

λ is the radon decay constant (2.06 × 10^{−6} s^{−1});

ɛ is the emanation fraction (dimensionless);

R is the radium activity concentration in the residue material (Bq/kg);

ɛR is the effective radium content;

ρ is the bulk density (kg/m^{3});

D_{e} is the effective diffusion coefficient, the characteristic of the material under study.
Equation (1) is the radon source term from the tailings to the atmosphere. There are two basic methods of measuring radon diffusion coefficient from samples (Hassan et al., 2009). The first method is based on the dependence on the time of the increasing of radon concentration called transient method (Sasaki et al., 2006; Jiránek and Fronka, 2008) and the second one is based on the use of stable radon concentration (equilibrium radon concentration) called the steadystate method (Somogyi et al., 1986). Some of the methods that have been used by researchers that fall into the above two categories are: Electrostatic (Aldenkamp et al., 1992), Ionization Chamber (Jiránek and Fronka, 2008) and Lucas Cell (Quindos Poncela et al., 2005) methods.
Radon exhalation studies from gold mine tailings are important to understand the relative contribution of the material to the total radon concentration in the environment. The exhalation rate is the source term or emission rate in the study of radon transport from the source (tailings dam) to the receptor in radon dispersion modelling. In South Africa, a number of studies of radon exhalation rates from the gold mine tailings material are available (Lindsay et al., 2004, 2008; Ongori et al., 2015). Many other studies exist but are in the proprietary domain of regulated mining companies. These published data are few and far between and are mainly based on the Central and the West Rand part of the goldproducing region called Witwatersrand Basin. This elliptical basin shown in Figure 1 is a 400 km long stretch over an arc traversing across the Gauteng (East, South and Central Rand Goldfields), North West (West and Far West Rand Goldfields) and Free State (Free State Goldfields) provinces in South Africa. Each region is characterised by different geological formation in respect of gold to uranium ratio and hence different radium content in the tailings. The current study is a component of a larger investigation on validation of atmospheric radon dispersion modelling. For this purpose, an isolated tailings dam in the Free State goldfields was identified. Currently there is no published data from the Free State goldfields mine tailings and the method proposed in this study was used to measure radon exhalation rate from this dam. The result may be representative of this region.
In this study, a steadystate measurement technique, “diffusion tube” or “sealed tube” technique, which employs the RGM tracketch passive detectors and containers of fixed geometry (diffusion tubes), was used to determine all the characteristic exhalation parameters (effective radium content, effective diffusion length and emanation coefficient) from the representative Free State gold mine tailings’ surface. Apart from being simple and relatively inexpensive, this technique allows for simultaneous radon exhalation measurements at a large number of locations, providing good spatial averaging, while being insensitive to short term temporal variations. This makes the method ideal for sites where repeated access over time is prohibited for safety reasons. The method provides representative and comparatively reliable measurements about the radon exhalation rates (Somogyi et al., 1986). The dimensions of the diffusion tubes, particularly the length of the tubes, were optimised to minimise systematic errors arising from sampling geometry.
Fig. 1
The Witwatersrand Gold Basin. 
2 Theoretical approach
The passive “diffusion tube” or “sealed tube” technique that uses an etched track detector is a simple and efficient method to assess radon exhalation rates from porous materials such as mine tailings (AbuJarad et al., 1980; Samuelsson and Pettersson, 1984; Somogyi et al., 1986; Chen et al., 1993; Ramola and Choubey, 2004; Prasad et al., 2008; Amin, 2015). The geometry of the diffusion tubes is such that the exhalation of radon from a sealed sample surface inside the tube can be modelled by onedimensional diffusion theory. This theory is used to determine both the diffusion coefficient and emanation fraction of the materials. The two parameters are determined from the radium content, bulk density and track density recorded by the etched track detectors sealed in the open space above the sample in the tube. The diffusion coefficient and emanation fraction determined in this way represent the effective quantities for the given physicochemical state of the materials and accounts for the effects of absorption, adsorption and molecular diffusion.
In this study, samples of thickness d were placed at the bottom of closed and sealed cylindrical tubes of crosssectional area A and air height above the specimen h (Fig. 2). An etchedtrack detector called radon gas monitor (RGM) was mounted onto the center of the inner top wall of the container as shown in Figure 2. To avoid gross underestimation and to yield more comparable results of the true exhalation rate from the sealed tubes material, proper attention must be given to dimensions of the samples and the containers (Hassan et al., 2009), more specifically with regard to the diffusion length of the material (Samuelsson and Pettersson, 1984; Samuelsson, 1990). Therefore, the sample and container geometry must be precisely defined to determine the required exhalation parameters for the tailings samples (Solecki and TchorzTrzeciakiewicz, 2011).
In Figure 2, the sample volume is given by V_{s} = A ⋅ d, while the air volume is V_{a} = A ⋅ h. Radon concentration accumulates in the air volume V_{a} from where measurements are taken using the RGM etchedtrack detector, thereby obtaining the exhalation parameters from the shape of the radon growth curve. In the etchedtrack method, the integral under the radon growth curve is obtained by the RGM measurements within the air volume V_{a}.
Assuming that under steady state conditions, the radon concentration gradient exits only in the direction of depth, one dimensional radon diffusion (no convection) will take place in the sample. This steady state radon diffusion is governed by a onedimensional differential equation describing diffusion to the surface of the sample given by (Ishimori et al., 2013): where

C: interstitial radon concentration [Bq/m^{3}];

D_{e}: effective diffusion coefficient [m^{2}/s];

Z: sample depth measured perpendicular from surface into the material [m];

P: porosity of the soil.
There are two ways to define the diffusion coefficient of radon in porous media that have been adopted in the literature, namely the effective radon diffusion coefficient (D_{e}) and the bulk radon diffusion coefficient (D). The bulk and the effective radon diffusion coefficients in soil, as stated by Nazaroff and Nero (1988), are related by the total soil porosity, P, according to the expression: (3)
These definitions of D_{e} and D as suggested by Nazaroff and Nero (1988) are the ones adopted in this study. Radon flux density at the material surface inside the tube can then be calculated from the equation (Ishimori et al., 2013): (4)
The solution of equation (4) according to Somogyi et al. (1986) for interstitial radon concentration is given by: (5) where is the diffusion length of radon in the material, and k is a geometrical correction factor.
In the case of an open container (V_{a} → ∞), “free exhalation” will take place from the sample and the geometrical correction factor k would be equal to 1. For a sealed container, k is given by: (6) which is derived from the boundary condition given by (Somogyi et al., 1986): (7)where:

= areal exhalation from the sample surface [Bq/];(m^{2}.s)

A = open surface area of the material [m^{2}].
The areal exhalation rate of radon can be obtained from the solution of the one dimensional diffusion differential equation (5) and from Fick’s first law (Eq. (4)) as: (8) where is the maximum value can attain.
Two important relationships can now be deduced; and , for the bound (sealed container) and the free (open container) areal exhalation rates respectively. The bound areal exhalation rate is related to the free areal exhalation rate by the equation: (9)
From the above relation, the bound exhalation rate is determined by the geometry of the container, k.
The maximum bound exhalation rate will occur when d → ∞, such that tanh (d/L) ≈ 1 and for given h value it is: (10)
The bound mass exhalation rate in terms of (Bq/kg.s) is defined as: (11) where M and γ are sample mass and density, respectively.
From equations (9) and (11), it can be shown that: (12)
Thus, the maximum exhalation rate (d → 0, such that d/L ≈ 0) is given by (13)
From exhalation rate relationships, the two important parameters required for exhalation rate from mine tailings, namely, effective diffusion length (D_{e}) and emanation fraction (ɛ) can then be determined.
Consider the two cases in which one enclosed sample is much shorter than the other, i.e. in one sample, d → 0 and in the other sample d → ∞ (Figs. 2a and 2b respectively). The two containers have the same openair height h. If the open air height is much larger that the diffusion length L in the material, i.e. h ≫ L, then by definition from equations (10) and (13), we can have the relation: (14)
According to equation (14), measurements of , and the density γ from the two enclosed samples will yield the bulk diffusion length L of the material. When radon concentration in the open volume of the sealed can has reached steady state, this concentration is in secular equilibrium with the effective radium content. Knowing , and γ, the effective radium content can be calculated from the following equation: (15)
It therefore follows from equation (1) that: (16)
Both mass and aerial exhalation rates are obtained from the measured integrated concentrations from the RGM in the sample containers.
Fig. 2
Schematic representation of the two sealed containers with different sample lengths and geometry but having same air space (h). 
3 Measurement methods and materials
Samples were collected at a selected tailings dam in Odendaalsrus, in the Free State province (South Africa). Twenty (20) sampling locations were assigned on a gridlike format to the surface area of the tailings dam (see Fig. 3). Sampling points were determined by accessibility, safety and minimal saturation with water.
Taking into account that the diffusion length may vary between 0.002–1.5 m for thick porous body such as tailings, except for dry, coarse material when the value of 2 m may occur (UNSCEAR, 1998; IAEA, 1992). It follows that most of the gas emitted from tailings originates from the top 1 m of material. Twenty plastic 5L buckets with sealable lids were used to collect about 5 kg each of the material from the tailings dam. A TractorLoaderBackhoe (TLB) machinery was used to extract sample materials from a depth of 1 m. Sample materials were collected from different depths over one meter, and each sampling point is described below.
Approximately 1.5 ± 0.5 kg of the material was removed from the first 30 ± 5 cm and deposited next to the sampling hole. Another 1.5 ± 0.5 kg was removed from the next 30 ± 5 cm and the material deposited on top of the previous layer. Lastly, a further 1.5 ± 0.5 kg was removed from the last 30 ± 5 cm. The three samples were thoroughly mixed, transferred to sealed containers to preserve the moisture content.
About 230 g to 320 g of material samples were compacted into 20 polyvinyl chloride (PVC) short tubes of 11 cm in length and 4.2 cm diameter. For practical reasons, a 4.2 cm diameter tube was chosen to minimise the amount of sample material needed for a measurement (typically 5 I of material). The sample containing short tubes were attached to a longer tube of 1.05 m in length to provide a welldefined air space above the sample material. Additional ± 2.3 kg subsamples were compacted into 20 longer tubes of 1.01 m in length and same diameter as the short tubes, referred to as “diffusion tubes”. The long tubes, also made of polyvinyl chloride (PVC) material, were also connected to an air space tube of the same length as the short tubes (1.05 m). This implies that both the long and short tubes in Figures 2a and 2b had the same air space geometry above the compacted material.
According to the Southern African Institute of Mining and Metallurgy (SAIMM), gold tailings in South Africa have an average bulk density of tailings material of between 1250 kg/m^{3} and 1650 kg/m^{3}. Based on this, samples in both short and long tubes were compacted using hand tools to a density in this range of values. The densities of the compacted materials in the short tubes ranged from 1374 kg/m^{3} to 1602 kg/m^{3} while the densities of the compacted materials in the long tubes ranged from 1478 kg/m^{3} to 1590 kg/m^{3}. Similar compaction procedures have been described by Rogers et al. (1980), while complementary measurement systems have been used by Singh and Virk (1996) and Azam et al. (1995). It should be noted that material in the tailings depth is very homogeneous with respect to particle size, compaction and porosity, and there are no micro channels, fissures or air pockets. This validates the approximation that the effective diffusion coefficient is only determined by bulk density and porosity, the parameters which are replicated by the compaction process in the diffusion tubes. After compaction, the diffusion tubes were sealed for twentyone days to attain secular equilibrium. The tubes were then rapidly opened and a CR39 radon gas etched track monitor (RGM) was inserted at the headspace of each tube and then sealed for a period of 6 days. The integrated radon concentration in the headspace was then recorded by the etched track detector in the RGM.
After 6 days of continuous exposure, the RGM etched track detectors were sent to the PARC RGM laboratory in Pretoria (South Africa) for chemical etching, track detection and counting. The etching and analysis technique of the etchedtrack detectors is wellknown phenomenon, and it is therefore not discussed in detail. The calibration of the RGM is traceable to the 2018 Public Health England Intercomparison of passive radon detectors where the PARC RGM laboratory performed very consistently under four different reference exposure values (Miller and Howarth, 2020).
The analysis yielded a trackdensity value S, which is directly proportional to radon exposure through a calibration factor ŋ. The areal exhalation rate from an enclosed sample is given by: (17) where T_{e} = T_{2}−T_{1} is the integration time interval, T_{1} is the starting time of measurement, T_{2} the end time and C = S/ŋ represents the integrated radon exposure (Bq.m^{−3}.h) from the RGMs. The calibration factor ŋ, expressed in terms of (alphatracks.cm^{−2})/(Bq.m^{−3}.h) has a typical value between 0.002 and 0.003, which is inherently included in the concentration results from the PARC RGM laboratory.
Therefore the maximum bound aerial exhalation rate for samples shown in Figure 2b for long tubes was obtained directly from equation (17), whereas the maximum mass bound exhalation rate was calculated from samples depicted in Figure 2a for short tubes using equation (11).
The errors on the exhalation parameters are a combination of random errors emanating from the statistics of track density and the systematic errors, which relate to the sampling geometry, in particular, the length of the tubes. Therefore it is important that the design of the tube dimensions for the measurements should ensure that and are approached as closely as possible with the sampling geometry. For tailings material, the diffusion length may vary between 0.002–1.5 m except for very dry, coarse, material when a value of 2 m or more may occur (UNSCEAR, 1998; IAEA, 1992). The porosity may vary between 0.05–0.5 with a representative value of 0.25.
To approximate optimum tubes’ dimensions and thus minimising systematic errors due to the length of the tubes, a number of simulations were carried out using the diffusion theory, equations (14) and (15) as well as the methods described in theoretical approach above to generate container concentration values and diffusion lengths. The simulations were performed by considering extreme case of diffusion length (L) of 0.25 m and porosity (P) value of 0.5. The maximum error in diffusion length was plotted against the material column length, d, for various heights of openair column (h). The results are shown in Figure 4.
The moisture content of the sample was determined by weighing a sub sample of the material before and after oven drying of the sample. All samples were dried in an oven for 10 h to ensure a complete removal of the moisture by evaporation. The moisture content of the sample was recorded as the mass fraction per dry weight:
where M_{w}, m_{w}, and m_{D} are dry weight mass fraction, wet mass (in kg) of the sample and dry mass (in kg) of the sample respectively. The moisture content S, expressed as the moisture saturation of the material (Rogers and Nielson, 1981) is given by: (19)
The porosity of the material was determined from the following equation: (20) where G is the specific gravity of the tailings material whose value is 2740 kg/m^{3} (Rogers and Nielson, 1981) and ρ is the dry bulk density of the tailings material (kg/m^{3}).
Fig. 3
Twenty sampling points on the tailings dam. 
Fig. 4
Errors in diffusion length for various air column heights for L = 0.25 m and P = 0.5. 
4 Results and discussion
Table 1 lists the values of aerial and mass exhalation rates, porosity and effective radium concentrations in different samples collected from the tailings dam when d → 0 (short tube). In Table 1, the values of the aerial exhalation rates from the surface of the samples vary from 13.6 ± 1.9 to 96.9 ± 10.2 Bq.m^{−2}.h^{−1}. The mass surface exhalation rate values vary from 0.076 ± 0.010 to 0.579 ± 0.061 Bq/kg.s. The values of effective radium content in the samples varied from 9.9 ± 1.4 to 76.6 ± 8.1 Bq/kg in samples 15 and 19 respectively. The percentage error over the whole sample was 12 ± 1.0%.
The calculated values of effective diffusion coefficient and the exhalation rates from the mine tailings in the long tubes are shown in Table 2. The effective diffusion length values ranged from 1.9E07 ± 4.0E08 m^{2}/s in sample 14 to 4.1E05 ± 8.6E06 m^{2}/s in sample 10. Exhalation rate (E) was found to vary from 0.0410 ± 0.0042 Bq/m^{2 }· s to 0.440 ± 0.045 Bq/m^{2 }· s with a mean value of 0.102 ± 0.021 Bq/m^{2 }· s and a standard deviation (St Dev) of 0.087 Bq/m^{2 }· s.
It should however be noted that the values of integrated radon concentration were very high for sample 10 and as such should be treated in a circumspect manner. This has been shown by extremely high values of the aerial exhalation rates, diffusion length, diffusion coefficient and exhalation rates (E) in the sample. This unaccounted for spike in sample 10 is depicted in Figures 5 and 6 for diffusion lengths and exhalation rates (E) respectively. In addition, we observed that the radon exhalation rates from samples 4, 15, 17 and 18 were higher when compared to the all other samples. These samples were characterised by diffusion lengths greater than 1.5 m.
The anomalies in the values of the diffusion lengths and exhalation rates may be partly ascribed to the effect of compaction of the samples in the tubes. The sampling and manual compaction of the material in the tubes using weighted rod with sharpened point may disturb and alter the state of the material in the original tailings. The samples were obtained from the tailings with manual TractorLoaderBackhoe (TLB) machinery and compacted to reach a bulk density values of a typical tailings ranging between 1250 kg/m^{3} and 1650 kg/m^{3}, depending on the moisture content. The sharp pointed tool also ensures that no air voids are trapped in the material (Rogers et al., 1980). This results in the state of the compacted material being close to that in the original tailings. However, we found that when this density was reached, it became impractical to compact further without excessive mechanical force. However, this would have led to over compaction, which would tend to decrease the diffusion length and the exhalation rates. The material of these samples was similar to all others in terms of particle size and porosity. The increased diffusion lengths are therefore considered anomalous and warrant further investigation. Repeated sampling and repeated measurements in such a study would allow detection of possible errors and accounting for them, either in the sampling method or in the method of preparation. Moreover this will make it possible to evaluate the uncertainties associated with the sampling and the measurement methods.
Nonetheless, the mean value of the exhalation rates (E) were found to be comparable to values obtained by other researchers (Ellis, 1998; Lindsay et al., 2004; Lindsay et al., 2008; Ongori et al., 2015) using other methods in another region.
Values of effective radium content, porosity and the aerial and mass radon exhalation rates for 20 soil samples obtained when d → 0.
Values of diffusion coefficient and the exhalation rates from the mine tailings (E).
Fig. 5
Diffusion lengths for different samples. 
Fig. 6
Radon exhalation rates for different soil samples. 
5 Conclusion
The measurement of average radon flux from large mine tailings dams presents a major challenge to practitioners when estimating their contribution to public radon exposure through atmospheric dispersion modelling. The size of the dams requires time consuming and labour intensive direct radon exhalation measuring techniques and procedures if the usual methods are used (IAEA, 1992, 2013). This is especially true if a sufficient number of samples is needed for an accurate spatial average, and when attempting to average temporal variations. Radium content, emanation fraction, diffusion length and hence radon surface exhalation rates have been successfully measured using RGM plastic track detectors by the sealed tube technique. This passive method is a simple, convenient and inexpensive way of determining the exhalation and diffusion parameters of radon in porous materials like tailings. It is easily standardized with regard to calibration as only a single element, the RGM, was used for concentration measurements.
The range of values obtained in this study agree with the ranges reported in the literature for mine tailings (Ellis, 1998; Lindsay et al., 2004; Speelman et al., 2006; Botha, 2007, Lindsay et al., 2008; Ongori et al., 2015). From the results of the study, it is evident that there is considerable variability in most of the parameters, even for a single tailings dam. It is, however, recognised that given different geological formation in respect of uranium mines as well as those that produce uranium as a byproduct, a wide range of the exhalation values can and will occur throughout South African mine tailings within the Witwatersrand Basin (Wronkiewicz and Condie, 1990; Moshupya et al., 2019). The study further showed a strong positive correlation between the diffusion length and the exhalation rate. Hence the importance of optimising the effective height of the tubes, and in the process, minimising errors associated with the diffusion length.
As a source term for dispersion modelling, radon exhalation is important for understanding the relative contribution of the material to the total radon concentration in the environment. The obtained results can be used as baseline information for future assessments of any variation in the radioactive background level due to mine tailings and geological composition in the Free State province of South Africa.
Acknowledgements
This research was supported by the Department of Higher Education and Training and Central University of Technology, Bloemfontein, South Africa. Special thanks to Harmony Gold Mining Company Limited for permission to access their facility, the late Dirk Venter (MHSRIP) for logistical and technical support, Jaco Cronje from Fraser Alexander for his assist during sampling and PARG RGM for provision and analysis of RMGs.
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Cite this article as: Komati FS, Strydom R, Ntwaeaborwa OM. 2021. Measurements of radon exhalation from a South African gold mine tailings using sealed tube method. Radioprotection 56(4): 327–336
All Tables
Values of effective radium content, porosity and the aerial and mass radon exhalation rates for 20 soil samples obtained when d → 0.
Values of diffusion coefficient and the exhalation rates from the mine tailings (E).
All Figures
Fig. 1
The Witwatersrand Gold Basin. 

In the text 
Fig. 2
Schematic representation of the two sealed containers with different sample lengths and geometry but having same air space (h). 

In the text 
Fig. 3
Twenty sampling points on the tailings dam. 

In the text 
Fig. 4
Errors in diffusion length for various air column heights for L = 0.25 m and P = 0.5. 

In the text 
Fig. 5
Diffusion lengths for different samples. 

In the text 
Fig. 6
Radon exhalation rates for different soil samples. 

In the text 
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