ASSESSMENT OF NATURAL OCCURRING RADIONUCLIDES (40K, 226RA, 232TH) AND HEAVY METALS IN A DISTURBED AND UNDISTURBED SOIL IN THE REGION OF DAKAR, SENEGAL.

Mamadou Lamine Sané 1 , Modou Mbaye 1 , Alassane Traoré 1 , Djicknack Dione 1 , Mamoudou Sall 2 , Ababacar Sadikh Ndao 1 , Papa M. N’diaye 3 and Ahmadou Wagué 1 . 1. Institute of Applied Nuclear Technology, Department of Physics, Faculty of sciences and techniques, University of Cheikh Anta Diop, Dakar, Senegal. 2. Laboratory of X-ray, Department of Physic, Faculty of sciences and techniques, University of Cheikh Anta Diop, Dakar, Senegal. 3. Department of Geology, University of Cheikh Anta Diop, Dakar, Senegal. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 16 July 2019 Final Accepted: 18 August 2019 Published: September 2019


ISSN: 2320-5407
Int. J. Adv. Res. 7 (9), 1341-1351 1342 fate in the environment, is an increasing demand for policymakers; and more importantly, it has been demonstrated that quantification of background levels of radionuclides is necessary to evaluate the potential environmental risk (ICRP, 1993).
The content of heavy metals in the soil is often not constant, especially at the level of industrial and agricultural areas. Heavy metals come from natural processes but also from anthropogenic processes. It is estimated that the contribution of metals from anthropogenic sources to the soil is greater than that of natural metals (Nriagu and Pacyna., 1998). Heavy metals, which are not biodegradable, accumulate in the soil and, depending on biogeochemical conditions, pass through the soil solution (Mench et al., 2000). These metals can be absorbed by plants but also found at the water table; entering the food chain (Loska and Wiechula, 2003). However, their effects on human health or the environment, in general, are dangerous when exposed to higher than normal concentrations. According to existing literature, measurement of concentration of naturally occurring radionuclides and the concentration of chemical elements in the environment have been performed in several countries all around the world (El-kameesy and al., 2015) (Ajayi and al., 2018). However, there are very few data available of occurring radionuclides and elemental concentration in soil from developing countries. This study was conducted with the aim of assessing the specific activities of natural radionuclides, determining the concentration of some chemical elements and assessing the risks of radiological and pollution at ground level of the cities of Dakar and Bargny.

Study area
The disturbed site is in Bargny about 20 km east of Dakar. It lies between 14°43'1.99N//14°41'8.99N and 17°15'1.44W//17°11'35.68W. Its geological cover includes an alternation of marl and limestone. In addition, this eastern part of the Dakar region includes a set of hills and plateaus with altitudes below 50m. Local temperature varies substantially between 18°C to 39°C. This study area soil texture was predominately sandy 63%, with 17% of silt and clay 19% with average organic matter of 76%, with a pH of 6.9 on average. The National cement factory used soil material in Bargny. The second study site is in the central zone of the western end of the Dakar region, which is a local undisturbed area since many decades. The GPS coordinates of this site are between 14°44'30.64N//14°38'57.76N and 17°26'5.16W//17°26'3.49W. This zone has an altitude of less than 10 m and consists soil texture was predominately sandy 66%, with 13% of silt and clay 20% with average coarse fragment of 18%. The soil bulk density was about 1.4 and acidic pH of 6.8.

Gamma spectrometry Analysis
The sample collector equipment is a motorized core drill for the soil column. The samples were dried for 24 h at a temperature of 100° C, crushed, screened (2 mm) and homogenized. A representative part of each sample is taken and placed in a cylindrical plastic container. The whole is sealed for 4 weeks for the realization of the secular equilibrium. The analysis of the samples is carried out by a gamma spectrometer equipped with a high purity germanium detector. The detector is coaxial p-type, which is characterized by an energy resolution of 1.8Kev at 1332Kev with excellent peak symmetry and a relative efficiency of 30%. The detector is calibrated using a certified multi-gamma source and two reference materials (IAEA 327, IAEA 375). The samples are analyzed with duration of 86000 s. The activity calculation, using the spectra, made it possible to determine the content of the radionuclides in each sample. Since secular equilibrium is achieved, radium 226 Ra is obtained from Bismuth 214 Bi (609 Kev); thorium 232 Th from actinium 228 Ac (911Kev); and 40 K potassium from her energy 1461Kev. The activity of a radionuclide is calculated by the following equation:

( )
Where N: Net peak area of the gamma photo peak. ε: efficiency of the detector I: branching ratio m: masse (Kg) of the sample t: time of counting Calculation of the radiological effects Naturally occurring radionuclides are well known to cause significant increase in public exposition. In this study 4 radiological hazard indices associated with the presence the measured naturally occurring radioactive materials were computed using conversion factors. Equation 1 to 6 expressed the following radiological risk index : 1343 1. The total absorbed dose rate in air 2. The radium equivalent index 3. The external and internal hazard index 4. The annual effective dose equivalent for outdoor and indoor The absorbed dose rates in at radionuclides ( 226 Ra, 232 Th, and 40 K) were calculated by using Equation (1) (4) Where A Ra , A Th , A K are the average activity concentrations of 226 Ra, 232 Th and 40 K in Bq.kg -1 respectively. The value of H ex and H in indices must be less than 1mSv.y -1 in order to cause any harmful effect to population. (Quindos, Fernandez and Soto 1987). The outdoor and indoor annual effective dose equivalent are calculated by the equations (5) and (6):  Using the equation (7) of (Muller., 1969) the values of the geo-accumulation index of the metals studied were calculated: Where is the measured concentration of element-n in the soil sample, is the geochemical concentration of the background of a soil element-n (Turekian and Wedepohl; 1961) and 1.5 is a factor in correcting possible variations due to the effects lithogenic. 1981)  The formula established by (Simex and Helz, 1981) on the calculation of the enrichment factor is: Where is the concentration of the element x of the soil sample of the study site, is the concentration of the reference element, is the background concentration of element x and is the Background concentration of the reference element. In this study Iron was used as a reference metal. Iron is distributed independently of other metals and is abundant in the soil. Five categories of contamination are recognized based on the enrichment (Sutherland , 2000) factor:

EF<2
Deficiency to minimal enrichment 2<EF<5 Moderate enrichment 5<EF<20 Significant enrichment 20<EF<40 Very high enrichment EF>40 Extremely high enrichment The measurement of contamination is also carried out by the contamination factor which assesses the anthropogenic contribution of soil pollution by metals. The evaluation is carried out using the formula of (Hakanson, 1980): is the measured concentration of element x in the sample and, is the concentration of element x of the geochemical background. A classification of contamination according to the value of CF is established by (Hakanson, 1980;Loska, 2004)

CF<1
Low contamination factor indicating low contamination 1 CF<3 Moderate contamination factor 3 CF<6 Considerable contamination factor 6 CF Very high contamination factor The pollution load index includes pollution from various heavy metals and directly reflects the level of pollution of all heavy metals at a sampling point (Cao et al., 2016). The PLI was determined by the equation (Tomlinson et al., 1980): Where CF is a factor of contamination and n the number of items studied.

1345
According to the pollution load index, pollution is divided into six levels (Liu, et

Résultats and Discussion:-Activity concentration of radionuclides
The descriptive statistics (mean, minimum, maximum and standard error) of naturally occurring radionuclides are given in Table 2. Correlation Analysis was study the relationship between activity concentrations between sampling sites. In addition, a comparative analysis was conducted across undisturbed and disturbed soil sample by using t.test. Prior to application of statistical inference, the activity concentration data were submitted to a statistical test for homogeneity of variance (F-test) and for normally (Shapiro-Wilk test). Whenever both conditions were satisfied, a parametric t-test was applied otherwise; a non-parametric Wilcoxon-Mann-Whitney test was used. The level of significance of test was set to 5%. ) in the non-disturbed area. Alternatively, the mass concentration of 226 Ra and 40 K in both site areas was similar. No significant differences were found between the undisturbed site and the disturbed agricultural sampling site, with of p-value>0.5, however the null hypothesis of equality of mean activity on radium-226 was stronger compared to 40 K between sites. The reason why 40 K mass concentration was not statistically different may be found in the topsoil organic carbon content at the study sites, while soil texture and lithographical similarity may explain the similarity of the site on 226 Ra concentration. Both sites have an acidic soil pH, similar annual rainfall and relatively similar soil organic carbon. The measured activity of 226 Ra ranged between 22.52 1.16 Bq.kg -1 and 30.95 1.76 Bq.kg -1 in disturbed site and 19.05 1.07 Bq.kg -1 and 28.18 1.54 Bq.kg -1 in the undisturbed area; while the mean activity of 40 K was 153.7 Bq.kg -1 and 168.08 Bq.kg -1 in the disturbed and undisturbed site respectively.
1346 Figure 1: summarizes the statistical correlation, kernel density distribution and comparative boxplot of measured naturally occurring radionuclides and their associated radiological hazard indices. A positively strong correlation was found between 40 K(und) and 232 Th(Und). In addition, the correlation study between sites results shows noncorrelation among radionuclides.

Radiological hazards
Assessment of the radiological hazards includes the calculation of the radium equivalent index (Ra eq ), external and internal hazard index (H ex and H in ), the dose absorbed, the indoor annual effective dose and the outdoor annual effective dose. All the results are indicated in and for the soil samples in Bargny, 28.58nGy.h -1 to 32.74nGy.h -1 and his mean is 30.20nGy.h -1 . Based on the report by UNSCEAR, the dose rate from terrestrial gamma rays under normal conditions is approximately 60 nGy.h -1 for a population-weighted value. The both means values in the studies area are lower than the worldwide average limit value. The minimum and maximum value of the indoor annual effective dose and the outdoor annual effective dose were found to vary from 140.10 μSv.y -1 to 160.62 μSv.y -1 and 35.05 μSv.y -1 to 40.15 μSv.y -1 for Bargny, respectively. And for Dakar City, the indoor annual effective dose and the outdoor annual effective range, respectively, from 105.13 μSv.y -1 to 145.79 μSv.y -1 and from 26.25 μSv.y -1 to 36.45 μSv.y -1 . The mean values are 32.67 μSv.y -1 and 130.69 μSv.y -1 for undisturbed site (Dakar City) respectively for the outdoor annual effective dose and the indoor annual effective dose. Nevertheless, the mean values of the outdoor annual effective and the indoor annual effective dose for Bargny are 37.04 μSv.y -1 and 148.15 μSv.y -1 , respectively. These values in the present work satisfy the safety criterion for the general public.  Table 3:-Descriptive statistics (mean, standard deviation (sd), Minimum (Min), Maximum (Max) and range) of radiological hazard assessment including the Radium equivalent index (Ra eq ), the External and Internal Hazard index (Hex and Hin), the gamma absorbed dose (D) and the outdoor and indoor Annual effective dose equivalent (AEDE) in the disturbed (Dist) and undisturbed reference site (Und).

Elément compositions
The analysis of soil samples from the Bargny site by X-ray fluorescence allowed the determination of the concentrations of some chemical elements. The average concentrations of the major elements showed the following classification Ca>Fe>K>Ti whereas those of the minor elements it is to be noted that their average concentrations followed the following decreasing order P>Zr>Cr>Ni>Mn>As>Zn>Cu. The Calcium Ca is the most present element in the soil of the study area of Bargny. The Calcium concentration ranges from 3.28 to 29.12g/kg, while the least common major element, tritium Ti, ranges from 0.11 to 0.30g/kg. (Table 4). Calcium contributes to the growth of plants and plays an important role in the health of the human body. The concentration of Ca needed for plant growth ranges from 2000 to 4000mg/kg (Steven, 1995). The average Ca concentration in this study, 16.76 g/kg, is above 4,000mg/kg. The geological cover of the study site includes alternating marls and limestones. Copper is not abundant in Bargny's samples. The concentration found was 1.03mg/kg. Perhaps this is since it is very intensely absorbed by plants, essential for their growth. But also, other facts may be responsible for this observation. The highest average concentration at the level of minor elements was for phosphorus P, 479.21mg/kg. It ranges from 277.07 to 589.86 mg/kg. The phosphorus detected in this site has natural but also anthropogenic origins, the use of fertilizers. The average concentration of arsenic As is 12.43mg/kg. In general, the increase in the content of arsenic as in the environment is due to industrial activities, pesticides and fertilizers.  The average concentrations of the elements of the Dakar city soil samples could be ranked in descending order as follows: Fe>Ca>K>Ti>P>Zr>Mn>Zn>Cr>Cu>Ni>As. (Table 5) The average concentrations of the elements determined on the undisturbed site of Dakar city are lower than those of Bargny for Ca, As, Zr and P. The site of Bargny is in an industrial zone, moreover, the market gardening culture which is led there maintain the content of these elements. The concentrations of the elements in the soil are the sum of the concentrations of the elements of natural and anthropic origins. On the other hand, Zn and Cu are more abundant in the study area of Dakar city than in the Bargny study area. This can be attributed to their uptake by plants but also to soil characteristics. Among the minor elements the phosphorus is more abundant then for the major elements the Fe was more abundant.  Table 5:-Descriptive statistics (mean, standard deviation (sd), Minimum (Min), Maximum (Max) and range) of elementals concentrations in the field sampling of undisturbed (Und) soil

Eléments majeurs (g/kg) Eléments mineurs (mg/kg) Ca
The correlation between the concentrations of natural radionuclide activities and the concentrations of chemical elements belonging to the same samples was calculated. Table 7 indicates that the Ca and P have a low and negative correlation with 226 Ra and 232 Th. The correlation between Ca and 40 K is positive low to zero while the correlation between P and 40 K is negative low to zero. The K and Zn are moderately and negatively correlated with the 232 Th.
As and Zn have a strong positive correlation with 226 Ra. Compared to 40 K, the As and Zn have positive but average correlations for As and strong with Zn. The Zr has a strong negative correlation with 226 Ra. So, when 226 Ra decreases in the soil the amount of Zr increases. The correlation of Cu to the three radionuclides is low to zero negative except for 40 K which is positive. Table 8; 232 Th has an average positive correlation with the Ca, Ti, and Zr elements. The correlation of this same radionuclide is positive and strong with Fe. Also, with the 232 Th, the correlation with the K, the As, and Mn is positive and weak while it is with the Zn and cu is negative and weak. Elements K and Zr are correlated with the 40 K radioactive element in a positive and medium way. With the Fe, Zn and P elements the correlation is low and negative except for the Fe which has a positive correlation with the 40 K. Radium 226 has an average negative correlation with Cu, a weak positive correlation with Zn, a low negative correlation with K and Zr.

Pollution Level
With the average concentrations of heavy metals contamination indicators such as the geo-accumulation index, the enrichment factor, the contamination factor and the pollution load index were calculated (see Table 9). The Zn, Mn, Cu, Ni and Cr metal geo-accumulation indices are negative, implying the absence of metal pollution at the two study sites. In contrast, the Arsenic has an Igeo equal to about 2 for the disturbed site and 0.07 for the undisturbed site. This would indicate that moderate pollution by arsenic metal had occurred at the disturbed site and no pollution at the Dakar city study site. This result was confirmed by the enrichment factor. The soil of the Study Area of Bargny was significantly enriched by the metal As, the EF was 15.37, while the soil of the study area of Dakar city had an enrichment factor that ranked it among the sites whose enrichment by Arsenic is Minimum. Arsenic is a toxic and carcinogenic element. Increased risks of lung and bladder cancer, as well as skin changes, have been reported in people exposed to high levels of arsenic (WHO, 2015). The Cr enrichment factor was 9.46 at the disturbed Bargny site, indicating the significant enrichment of chromium in the study area. There is no enrichment by the Cr on the ground of the Dakar city site. Chromium can cause health problems in humans. It attacks the airways and leads to gastric problems and stomach ulcers (Reichl, 2004;Klaassen, 2008). Metal enrichment factors Zn, Mn and Cu indicated deficient enrichment at both sites (see Table 9). The EF being between 0 and 1 show that they were of natural origin i.e. earthly crust. Arsenic, Ni and Cr, whose EF values were higher than one in the Bargny site, indicated anthropogenic origins (Zsefer et al., 1996). The CF contamination factor for all heavy metals measured at the two study areas was less than one, which may suggest no contamination at both sites. The PLI values at both sites were much lower than the unit, indicating a lack of pollution for all heavy metals combined.  Table 8:-Geo-accumulation index (Igeo), Enrichment factor (EF) and Contamination factor (CF) of heavy metals measured in the field sampling of disturbed (dist) and undisturbed (Und) soil 1350

Conclusion:-
The results reported on this paper show that radionuclide activities in both study areas are below the global activity average 35 Bq.kg -1 ; 30Bq.kg -1 and 400Bq.kg -1 226 Ra; 232 Thand 40 K respectively (UNSCEAR, 2000). The average value of doses absorbed in Bargny and Dakar city are below the global average. The same is true for the equivalent radium activity as well as the external and internal hazard index all below their respective global average. The soils of Bargny and Dakar do not present any risk of radiological exposure to population health.
Regarding the assessment of the risks of heavy metal pollution, measurements of geo-accumulation and enrichment factor set to light significant enrichment on the part of elements such as arsenic and chromium at the Bargny's disturbed. The CF and the PLI did not indicate any contamination at the two sites. Concentrations of chemical elements at both sites show similar orders of magnitude for concentrations of major elements except for calcium, which is more abundant in Bargny. The values of global average concentrations of certain elements found in this paper are higher than the concentrations measured in this paper except for the arsenic ace at the Bargny site, which has a concentration almost double that of the global average. The concentration of chromium at the disturbed Site of Bargny is close to the global average concentration of chromium. The global average concentrations of the elements are 7038, 267, 59.5, 29, 488, 6.83, 70 and 38.9mg/kg respectively for Ti, Zr, Cr, Ni, Mn, As, Zn and Cu.(Kabata-Pendias, 2011). This work can serve as a reference on the follow-up to the evolutionary state of natural radioactivity and heavy metal content at both study sites.