Assessment of Radioactivity Contents and Associated Risks in Some Soil Used for Agriculture and Building Materials in Cameroon ()
1. Introduction
Radionulides that accumulate in soils constitute a direct route of exposure to human population when the contaminated soils are used. Thus, external exposure of human to ionizing radiation from the use of soil has been noted with great concern [1-7]. Soil is of great important in human life. It constitutes the ground from which man settlement is build and through which foods are produced.
Soils and rocks contain natural radionuclides of a wide range of activity levels. This radioactivity can be transported to the surface by several processes: tectonic movement, volcanic activity, ore/coal mining and groundwater flow. The transportation by groundwater flow and similar mechanism depends on the solubility and chemical reactions of the radioactive substance, and the porosity of the ground. Migration plays an important role in the final storage of radioactive waste.
The most severe civil nuclear reactor accident at Chernobyl power station occurred on 26th April 1986 [8, 9]. The post-impact assessment program for the estimation of migration dose as a component of radiation burden of the populace in Cameroon was put in place because of the worldwide distribution of its radioactive effluents.
Cameroon is surrounded by countries having nuclear reactors and it is not far from the Sahara where some atmospheric weapons tests took place (Figure 1). With particular reference of the lessons and experiences of the Chernobyl-4 accident, there is a growing need to assure the Cameroon public over the concern about the possibility of nuclear accident with transboundary consequences which may nevertheless occur in spite of any adequate nuclear safety criteria by countries operating nuclear (research or power) reactors, particularly in Africa.
During the years after those nuclear accidents, the bioactivity and environmental mobility of radiocesium declines markedly, resulting in large changes in contamination of soils, surface water, foodstuffs, etc. The sorption
Figure 1. Map of Africa showing countries having nuclear reactors.
of radiocesium to soils and sediments has shown that Cs is selectively bound to specific sorption sites on illitic clay minerals where it is available for ion-exchange with ions, which have a similar hydrated radius, specifically potassium and ammonium [10]. Over time, however radiocesium slowly diffuses into the illite lattice know as “fixation”. Radiocesium fixation in soils and sediments are crucial due to its long-term mobility and bioavailability.
In recent times, there is an increased awareness of the effects of environmental radiation from natural radioactivity which has been enhanced by some environmental fields such as rocks, soils, water air, etc. Attention has been given to radioactivity contents and radiation levels in various types of solid due to naturally occurring radioactivity.
Radiation, by its very nature is harmful to life. Depending on the doses, it can induce cancer, genetic and organs damages, cell killing and also rapid death.
Many research works have been carried out on radioactivity measurement throughout the world [1,3,7,11]. Here in Nigeria, Tchokossa and his co-workers reported the results of many investigations on the distribution of radioactivity content in various environmental matrices [12-19], while in Cameroon, Makon et al. carried out similar research in some edible vegetable cultivated and consumed worldwide [20]. Cameroon is located at the center of both operational and non-operational nuclear reactors and industries (Figure 1), and to the best of my knowledge, there is no published or on-going works on the survey of Cameroonian’s soil.
This present work aimed to provide information on the radiation levels in the soil and the resulting contribution of past nuclear operation carried out that may have being accumulated in soil. The data generated will not only help to design a radiation mapping for agricultural lands and building materials, but also will be use as baseline data for future investigations, government, policy makers and the general public.
2. Materials and Methods
A total of 45 soil samples were collected from nine towns across five provinces of the country. At each location, well described elsewhere [20], the samples were collected at the depth between 0 - 30 cm, at the four corners and the center of a square frame of 1 m size and later composed to make a single sample. Samples were collected in their natural form, dried at room temperature to a constant weight, grounded and sieved to a 2 mm size. Each sample was sealed in cylindrical plastic containers. The standard reference sample and the background which was an empty container have the same geometry like the sample containers. The sealed samples were kept for at least 28 days to allow a stage of secular equilibrium to reach before gamma spectrometry analysis commenced.
The counting equipment well described elsewhere [12, 16] was a Canberra vertical high-purity coaxial germanium (HPGe) crystal detector, model GC 2018 - 7500, serial number b87063 enclosed in a 100 mm thick lead shield and coupled to a Canberra Multichannel Analysing (MCA) computer system. The energy and efficiency calibrations were done using a well calibrated standard soils sources supplied by the International Atomic Energy Agency (IAEA), Vienna, Austria (Reference Material IAEA-375 for radionuclides and trace elements in soil) and some radioactive point sources from the Isotopes Products Laboratories, Burbank, California, USA. The advantage of gamma ray spectrometry is its ability to measure gamma emitters directly in the original sample without the need for chemical separation. It allows both qualitative identification and quantitative determination of the radionuclides in the sample. The counting time for both the sample and the background was 36,000 s. The photopeaks observed with regularity were identified to belong not only to the naturally occurring series decay radionuclide headed by 238U and 232Th, and non-series natural radionuclide 40K, but also the presence of 137Cs in most of the soils samples investigated. Other radionuclides if present, appeared rather infrequently at low levels or occurred at levels below the maximum detectable limits (MDL) statistically determined a two-standard deviation analytical error. The characteristics of the selected gamma-emitting radionuclides used as monitor lines for this work are as shown in Table 1. The 226Ra (238U) activity was estimated from 214Pb at 351.9 keV, 214Bi at 609.3 and 1120.3 keV; that of 232Th was from 208Tl at 583.2 keV and 228Ac at 911.1 keV. 40K and 137Cs were determined directly from the 1460.3 and 661.6 keV gamma-emission respectively. The activities of the radionuclides in the measured samples were computed from the difference between the net peak and net background areas, accumulation time, absolute peak efficiency, absolute g-ray emission probability and the sample mass. Duplicate analyses were conducted on all soil samples to check for the reproductivity of the results and the stability of the counting system. The total uncertainty in the measured concentrations was estimated from the combined uncertainties in the homogeneity of the samples, their position with respect to the detector, the calibration procedure, the peak area determination and the background correction.
3. Results and Discussion
3.1. Activity Concentration
The average concentrations of the radionuclides observed with regularity in the soil samples analyzed by gammaspectrometry are given in Table 2.
Except for 40K and 137Cs, all the gamma-lines detected came from the 238U and 232Th decay series. The total specific activity from each radionuclide determined at two-standard deviation analytical error was calculated on the basis of a mean value obtained from each sampling location taken from a particular depth. The 238U concentrations ranged from 5.36 ± 0.39 to 51.28 ± 9.67 Bq·kg−1 with an average of 34.52 ± 7.18 Bq·kg−1. 232Th from 4.03 ± 1.03 to 24.74 ± 3.10 Bq·kg−1 with an average of 16.67 ± 4.28 Bq·kg−1. 40K from 16.18 ± 3.11 to 467.40 ± 50.80 Bq·kg−1 with an average of 186.96 ± 16.21 Bq·kg−1 and lastly 137Cs from 0.00 to 4.79 ± 1.75 Bq·kg−1 with an average 2.32 ± 0.99 Bq·kg−1.
For 40K, the highest activity concentration was detected in location 5 (Bamenda), in fact geographically, the town is made up of series of mountains and valleys where most of the agricultural lands lie. The lowest was
Table 1. Characteristics of selected gamma-emitting radio-nuclides [42].
Table 2. Activity concentration (Bq/kg).
detected in location 9 (Limbe), probably because the sampling point is located to the upper slope part of the city, and the soil there are prone to a high level of erosion which may result in leaching of the soil. This pattern has been confirmed as we moved down the slope. This is favourable as a result of erosion which washes out soil to settle at the valley. 40K is a naturally occurring radionuclide present abundantly in the earth crust as one of its major constituents. 40K content in soil shows a more homogeneous distribution in the horizontal micro scale. The variability of 40K among locations could be due to the differences in land management, for example, fertilization, plowing practices, geology and geography.
For 238U, the highest activity concentration was found in location 2 (Ezeka) while the least was obtained in location 4 (Edea). In nature, uranium consists of primary two isotopes 238U and 235U, of which the former is the most abundant 99.73% [21].
While for 232Th, the highest activity concentration was obtained in Edea (location 4) and the least in Limbe (location 9). 238U and 232Th were detectable in all the soils sample with the former almost double that of 232Th except in some few cases.
Geologically, the bedrocks consist of coarse to medium sand, subordinate slit, gneiss and granites which have affinity to 238U and 232Th. Slight variation in the radioactivity content in soil may be observed with different locations mainly due to the type of soil, soil formation and soil transport process.
For 137Cs, the highest activity concentration was found in location 4 (Edea) while it was not detectable in location 3 (Douala). This higher presence of 137Cs in locations 7, 1 and especially 4 may be attributed to the natural reactor in Oklo. In fact, during 1972 a natural nuclear reactor has been detected at Oklo in Gabon (South of Cameroun) [22]. It was critical about 1.8 × 109 years ago, and has generated totally 15,000 MW-years during his life-time of 600,000 years. The exact analysis of the composition of the fission products revealed that it was a light-water-moderated andcooled reactor of the BWR (Boiling Water) type. The steam penetrated the overlaying rocks, dissolved further uranium ore, which was carried into the core by the condensate flow (automatic recharging during operation). The results of this natural long-term waste storage experiment can be compared to shorter man-made experiments of the order of 10 years duration [23]. It was pointed out that this waste was not encapsulated and that the main part of the plutonium is still in the reaction zone. Also Bamenda and Buea at locations 5 and 8 respectively were not too far from another natural disaster namely Lake Nyos.
Lake Nyos which is a crater-lake was formed a few hundred years ago in the highlands of Northwestern Cameroun during one of the volcanic eruptive activity. The Lake, located about 6˚26'N latitude and 10˚16'E longitude is 208 m deep and contains 1.32 × 108 m3 of water [24]. Geologically, the area lies within a volcanic active basement region with volcanic ask material overlying the basement. The lake Nyos Dam was formed at the time of the formation of the Crater Lake. This Dam has been eroding at a geologically alarming rate. Granitic bedrock had therefore been exposed along a 40 m thick dam. This was left at its narrowest point where water overflows and spills from the lake through a surface path down the cliff-face of the dam during the rainy season. A lethal explosion led to the release of gas from Lake Nyos in the night of 21st August 1986, killing 1746 people, several thousand of cattle and other animals by suffocation as the gas cloud swept down the valleys to the North and through the valleys of Cha, Nyos and Subum. The gas explosion generated a huge seiche estimated as high as 75 m [25] which uprooted trees and bamboo forest on the foreshore to the South of the lake. By September 14, 1986, the lake surface was still reddish brown in colour and littered with floating trees and debris. The disaster affected all settlements downstream within 25 km from the Lake. The gas explosion occurred without any previous warming, although two years earlier, on the 15th August 1984, a similar disastrous emission of carbon dioxide occurred in Lake Monoun, killing 37 peoples [26].
The occurrence of this fallout 137Cs in soil in Cameroon may also be due to the Sahara’s atmospheric nuclear weapon tests of the 50’s and 60’s which were the primary causes of high levels of global fallout. Radioactive materials were diffused depending on the extent and form of the emission, and are formed even in areas otherwise uninfluenced by human activity. It is known that small quantities of fission-products residues (137Cs and 90Sr) from atmospheric weapons test (AWTs) are slowly being purged from the food chain [27].
3.2. Estimation of the Radiological Hazard Indices
Table 3 present some radiological effects indices such as the Absorbed dose rate, the Radium Equivalent activity concentration, the Representative Level index, External Hazard Index (Hext), Internal Hazard Index (Hin) and the Caesium-137 Intervention Levels.