Environmental radioactivity measurement in soils of an abandoned potash deposit at Holle, Republic of Congo

The main objective of the present study was to assess natural radioactivity level from soils of an abandoned potash mine and its surroundings at Holle locality in Hinda district, the Republic of Congo. Samples were analyzed using an HPGe gamma spectrometer. Activity concentration values obtained varied from 21.01 ± 6.47 to 117.3 ± 2.69 Bq/kg with a mean value of 46.15 ± 4.69 Bq/kg of K; that of U varied from 17.36 ± 10.74 to 40.04 ± 8.58 Bq/kg with a mean value of 25.14 ± 10.06 Bq/k, while that of Th varied from 8.68 ± 3.7 to 27.33 ± 2.69 Bq/kg with a mean value of 18.16 ± 2.94 Bq/kg. Mean activity concentration values obtained for U, Th and K were relatively low compared to the world average values. Radiological hazard indices associated were also calculated and found to be under recommended limits. Results pointed out that radioactivity level in the region was relatively low. Consequently, soils of the surrounding abandoned potash mine can be used for buildings construction.


Introduction
The Republic of Congo is one of the richest subsoil countries in the world. The mining of these resources by foreign companies is accompanied by a glaring absence of accompanying measurement in the field of public health and radiation protection for the surrounding populations. This disregard of public authorities could lead to the development of diseases in the region such as a cancer. Most of these exploited mines in the Republic of Congo are unfortunately abandoned and left in plain sight in the country, such is the case of Holle potash mine, also called Saint-Paul mine. Works on radioactivity measurement have never been conducted in soils around mine sites in the Republic of Congo. The level of contamination and the daily doses received by the surrounding population, also visitors are unknown and uncontrolled in the region. Then, the Republic of Congo does not have standards or guidelines for the maximum level of radioactivity in soils. However, soils from abandoned mine sites have been the subject of many studies worldwide: [1][2][3][4][5][6] It is also disturbing to note that this abandoned mine is actually turned into economic site by some of the villagers, who engaged activities such as the selling of those potash-salts to those who made salted-fish, to farmers, etc. It is also consumed by the surrounding populations. Herdsmen considered it like a relaxation place, where they spend most of their time to feed their cattle. It is important to note that, this is the first study of measurement of terrestrial gamma radiation doses from soil in the region. So, it is intended that our results would form a baseline scientific data of external terrestrial gamma-radiation doses and an estimate of radiation exposure to public from soils of the study area.
The present study was carried out to determine the activity concentrations of natural radionuclides 40 K, 238 U and 232 Th in soil samples at Holle locality in Hinda district in the Republic of Congo. The results are also compared with those of other regions worldwide and are used to assess the potential radiological hazards associated with these soils by estimating the external hazard index (Hex), internal hazard index (Hin), representative level index (Iꝩr), activity index (I), radium equivalent activity (Raeq), absorbed dose rates in air (DR) and the annual outdoor effective dose equivalent rates (E).

1 Study area
Hinda district is located on latitude 4°36'48" S and longitude 12°02'37" E. It is about 34 m above sea level and has an approximate population of 32995. It is closed to the Kouilou department (Figure 1) and its climate is subject to the influence of the Atlantic Ocean. The great dry season begins in May and ends in mid-September in the coastal zone. The short dry season is hardly felt there in December-January. The great rainy season covers the months of February, March, April and part of May. The short rainy season only lasts two months (October, November). The drop in temperatures in the dry season is more pronounced on the coast than in land beyond Mayombe (20°C). The region is geologically characterized by white quartzite with schistose intercalation. The abandoned potash mine is located at Holle locality, in the district of Hinda. This mine was mined in 1970 and halted when the mine flooded in 1977s. Holle potash mine is an underground mine where the surface is covered by gem salts (Figure 2).

2 Sampling and conditioning
A total of seventeen (17) samples were collected: Four (04) in Holle potash deposit, nine (09) in Moupila village where people lives and three (03) at Nkoungou village (considered as a non-contaminated area: reference) at about 10 km of Holle potash deposit. Samples were collected manually and randomly. Each sample were collected at a maximum depth of 5 cm, with a total fresh mass of about 1 kg. The soil samples were stored in sealed plastic containers, carefully labeled and taken to the Elemental and Radiometric Analysis Laboratory (LAER) in National Center of Energy, Sciences and Nuclear Techniques (CNESTEN), Morocco. In laboratory, after removal of big particles, the samples were oven-dried at 105°C during 24h, then ground and sieved. Each sample was weighed, homogenized and conditioned in 1l Marinelli beakers completely sealed during at least 1 month to avoid radon escape. This ensures secular equilibrium between the heads of the uranium and thorium decay series and their short-lived progenies [7] before gamma counting.

3 Gamma Spectrometry analysis and measurement
The gamma ray spectrometry analysis was carried out at the Elementary and Radiometric Analysis Laboratory (LAER) in CNESTEN, Morocco. This was done using a Canberra coaxial HPGe detector having a relative efficiency of 30% with resolutions of 1.85 Kev at the 1.33 MeV gamma line of the 60 Co, enclosed in a lead shield. The complete electronic instrumentation was connected to a PC-based multichannel analyzer for gamma spectrum evaluation.
Each sample was counted for a period of 86400 s (24h) to achieve the statistical error of counting of less than 1%. Background spectrum acquisition was measured at 150000 s. The energy calibration of the system was done using a standard mixture source of gamma-emitting isotopes: 241 Am, 109  0.5g of a standard calibration source was deducted in 5 mL flame-sealed ampoule and injected in a 1-liter filled Marinelli beaker containing water. The source was homogeneously distributed in the solution and use for the calibration of the system. The detector calibration and the sample measurement carried out in the same geometry with the same counting time. 238 U was determined through its daughter radionuclides of 234 Th (63,38 kev) and 234m Pa (1001 kev). 226 Ra was obtained from 214 Pb (295.2 kev and 351.9 kev) and 214 Bi (609.3, 1764.5 kev). For 232 Th following radionuclides were used 208 Tl (583.2 Kev), 228 Ac (338.3 kev, 911.2 kev) and 212 Pb (238.6 kev). Finally, the activity concentration of 40 K was determined directly through its line 1460.8 kev [8].
The concentrations of gamma-emitting isotopes were determined using the following relation: Where A (Bq/kg) is activity concentration of the considered radioisotope, N and Nb are net counts of samples and background respectively, Tc and Tb are acquisition times of samples and background respectively, m (kg) is the mass of the sample, Ɛꝩ is the absolute gamma peak detection efficiency and Iꝩ is emission probability of the corresponding gamma-ray energy. However, uncertainty of activity was calculated by the following equation: Where , , Ɛ , , are standard deviation of A, N, Ɛ, Nb and m respectively.

4 Radiation hazard parameters
In order to assess the radiation hazard associated with natural radionuclides ( 40 K, 238 U and 232 Th) in the soils of Holle potash deposit, the following radiological parameters were calculated: the absorbed dose rates in air (DR), representative level index (Iꝩr), activity index (I), radium equivalent activity (Raeq), and the annual outdoor effective dose equivalent rates (E).

1 Measurement of 238 U, 232 Th and 40 K
Results are displayed in Table 1. The activity concentrations of 40 K, 238 U and 232 Th varied from 21.01 ± 6.47 to 117.3 ± 2.69 Bq/kg, 17.36 ± 10.74 to 40.04 ± 8.58 Bq/kg and 8.68 ± 3.7 to 27.33 ± 2.69 Bq/kg respectively. The mean activity concentrations of 40 K, 238 U and 232 Th in the soil samples were observed to be, respectively relatively low compared to the world average values [9].

2. 1 The absorbed dose rates
Since soils of the study area are used as a building material by the local population (bricks making, …), they may lead indoor and outdoor radiation exposure as well as the internal and external exposure of inhabitants. The outdoor absorbed dose rate (Dout), allows to assess the outdoor absorbed gamma dose rates in air, due to terrestrial gamma rays, originating from 226 Ra, 232 Th and 40 K, located in the soil. In this case, we assume that those radionuclides are equally distributed in ground, and the absorbed dose rate is measured at 1.0 m above the ground level. The absorbed dose rate (outdoor) was estimated using the following conversion factors 0.427 nGy/h per Bq per kg for 226 Ra, 0.662 nGy/h per Bq per kg for 232 Th and 0.043 nGy/h per Bq per kg for 40 K. The following equation has been used to calculate Dout according to [9]: Dout (nGy/h) = 0.427ARa + 0.662ATh +0.043AK (2) Where ARa, ATh and AK are respectively the activity concentrations of 226 Ra, 232 Th and 40 K defined previously.
In other hand, the use of contaminated soils as building material, increase the internal radiation. Indeed, according to [9], the indoor absorbed dose rates is 1.4 times higher than outdoors.

2. 2 The annual effective dose (E)
The annual effective dose E (mSv/y) is estimated from previous doses rate calculated. The following equation proposed by [10] taken into an account outdoor and indoor exposure, has been used to calculate the annual effective dose: E (mSv/y) = (DoutOFout +DinOFin).T.CF (3) In this equation, OFout and OFin are the outdoor and indoor occupation factors respectively. In the case of lifestyle in Hinda district, people spend most of their times outside than inside of their houses. This affirmative carried out during our sampling period in the study area. It has been noticed that the population are outside all the day: in their fields, in chase of animals, by feeding their cattle, etc and are usually at home on night for eating, sleeping or taking rapidly some tools for works. Therefore, in the present study, OFout and OFin have been assumed to be 60% (14h outside) and 40% (10h inside). This proposition carried out to adapt the present study to the reality on the spot, contrary to the lifestyle of people in developed countries, where about 80% of people's times is spent in buildings. CF equals to 0.7x10 Sv G/y is the conversion factor from the absorbed dose in the air to the effective dose received by adults at a height 1 m above the ground surface and T = 8760h is the time for one year [9].
Results are displayed in Table 3. The values range from 0,12 to 0,31 mSv/year with a mean value of 0.21 mSv/y, less than the maximum value of 1 mSv/y recommended by [11].

2. 3 The radium equivalent activity (Raeq)
The radium equivalent activity is assessed to the sum of 226 Ra, 232 Th and 40 K activities, and considered like a common radiological index, because of the non-uniform distribution of radionuclides in the material buildings.
The following conversion factors 1 nGy/h per Bq per kg for 226 Ra, 1.43 nGy/h per Bq per kg for 232 Th and 0.077 nGy/h per Bq per kg for 40 K were used to determine this parameter [9,12] (4): Raeq (Bq/kg) = ARa + 1.43ATh + 0.077AK (4) Where Raeq (Bq/kg) is a radium equivalent activity and ARa, ATh and AK are the activity concentrations of 226 Ra, 232 Th and 40 K respectively in Bq/kg. Table 3. The values range from 37.12 to 96.08 Bq/kg with the mean value of 64.7 Bq/kg, relatively low compare to the world mean value 370 Bq/kg [9].

2. 4 Representative level index (Iꝩr)
The representative level index Iꝩr is a helpful parameter for material constructions. It allows to identify material that may be hazardous to health when this last one is used for building constructions. Iꝩr ≤1, corresponds to dose rate ≤ 0.3 mSv/y, whereas Iꝩr ≤3, corresponds to dose rate ≤ 1 mSv/y. The following equation has been used to calculate Iꝩr [13,14]: Where ARa, ATh, AK are the activity concentrations (Bq/kg) of 40 K, 226 Ra and 232 Th, respectively, in the samples.
Results are displayed in the Table 3. The values range from 0.25 to 0.66 with the mean value of 0.44, less than the unit.

2. 5 Gamma activity index
The gamma activity index I was estimated to see the level of dangerousness of material buildings used on the population health in the region. It correlates with Iꝩr The following equation has been used to determined I [15] Eq (6): where AK, ARa and ATh are the activity concentrations (Bq/kg) of 40 K, 226 Ra and 232 Th, respectively, in the samples.
Results are displayed in the Table 3. The values range from 0.13 to 0.33 with a mean value of 0.22, less than the unit.

2. 6 Hazards indexes
The hazard index due to internal and external exposure, respectively denoted Hin and Hex from soils of the study area were assessed using models proposed by [16] Eq. (7) and (8). These parameters are determined to assessed the external and internal radiological risks associated with construction materials. The main objective of these parameters is to limit the dose to an equivalent dose of 1 mSv/y. In general, these parameters must not exceed the limit of unity.
Where AK, ARa and ATh are the activity concentrations (Bq/kg) of 40 K, 226 Ra and 232 Th, respectively, in the samples.
Results are displayed in Table 3
The low activity concentrations of 40 K in the study area, mainly in the mine, is explained in the fact that Holle potash mine is an underground mine. However, potash ore were mined at about 3 km under the surface. After the mine flooded in 1977s, all the mine's surface has been covered by gem salts (Figure 2). And the gem salts are principally composed of Nacl. Considering samples, collected at about 5 cm above the surface, justify the low activity concentrations of 40 K in the mine. So, no radiological health risks are posed in the study area. For comparison, the total activity concentration in soil samples in the study area and some other countries worldwide is displayed in Table 2. The mean concentration of 40 K is within the range and lower than most studies reported in However, a strong positive correlation coefficient is observed between 238 U and 232 Th, and 238 U and 40 K ( Table 5). It means that their origins and behaviors are the same in the study area, while weak positive correlation coefficient is observed between 232 Th and 40 K. This indicates that they may have the same origin but their behaviors in the study area differ. All the three radionuclides have strong positive correlation coefficients with the radiological parameters. This indicates that it exists strong relationship between radionuclides and radiological indices i.e., all the three radionuclides contribute significantly to gamma-ray emission at the sampling points.

Conclusion
Mean concentrations of natural radionuclides and all calculated radiological parameters are below the recommended level. Correlation analysis shows that it exists a positive correlation between all the three radionuclides, also with radiological parameters. Based on these low levels of natural radioactivity concentrations in the study area, it can be concluded that soils of the study area, pose no radiological health risk to the local population and to visitors. Results of the present study may form the baseline scientific data of external terrestrial gamma-radiation doses and an estimate of radiation exposure to public from soils of the study area.
[32] United Nations Scientific Committee on Effects of Atomic Radiation (UNSCEAR) Sources and effects of ionizing radiation. UN, New York (1998).