Distribution of Natural Radionuclides in Ama Fatma Oil Shale, Morocco

: The present work aims to analyze the distribution of radionuclides naturally occurring in the oil shale of the Ama Fatma coastal zone. For this purpose, using gamma-ray spectrometry, we analyzed the activity concentrations of uranium (U), thorium (Th) series


Introduction
The primary source of natural radionuclides is attributed to the three decay chains of 235 U, 238 U, and 232 Th, as well as 40 K. 232 Th has the longest half-life of 1.405 × 1010 years, followed by 238 U with a half-life of 4.47 × 109 years, and 235 U with a half-life of 7.1 × 108 years.Natural uranium is composed of 99.27% 238 U, 0.72% 235 U, and 0.0055% 234 U [1].The radionuclides' activity concentrations and distribution in natural materials represents high interest as they provide useful information on environmental radioactivity [2].The principal source of external irradiation to the human body is the gamma radiation emitted by naturally occurring radionuclides.The human external exposure to natural radiation doses is related to the geological and geographical conditions as the associated amount of gamma radiation varies from one region to another along the world [3,4].Due to this heterogeneity, radionuclides appear in diverse levels in rocks in each region around the world [5].Higher radiation levels are linked to igneous rocks (e.g., granite) as the content of uranium in a rock increases with silica content [6,7].It is to be noted that some minerals enclose higher levels of natural radionuclides compared to others [8].Less important radiation levels are associated with sedimentary rocks [9], with the exception of certain phosphate and shale rocks that display a relatively high content of radionuclides [10].
The examination of sedimentary rocks for uranium and thorium isotopes typically involves a somewhat laborious process of radiochemical procedures to separate the heavy Eng.Proc.2023, 56, 229 2 of 7 radioactive elements, followed by an analysis of the height of alpha pulses [11][12][13][14][15].It would be desirable to have simpler and faster methods of analysis.One such alternative is gamma ray spectrometric analysis [16,17] which allows for the non-destructive measurement of the natural gamma ray activity of the core, at energies exceeding approximately 1 MeV.In this particular aspect of the natural gamma spectrum, only three nuclides contribute: 40 K, 214 Bi, and 208 Tl. 40 K is a radionuclide that has a long lifespan, whereas 21 4Bi and 208 Tl are short-lived daughters in the decay schemes of 238 U and 232 Th, respectively.
The present study analyzes the radionuclides concentrations naturally occurring in oil shale samples retrieved from the Ama Fatma coastal zone.The study area is situated in the Tarfaya-Boujdour basin, which defines the Atlantic margin area between the towns of Tarfaya and Boujdour.This region is bounded by the Atlantic Ocean on the west and the Republic of Mauritania on the east, as depicted in Figure 1.Located approximately 80 km to the north of the city, this segment commences with a black shale layer exceeding 2 m in thickness [18,19].The geological characteristics of the study area consist of Marl, a sedimentary rock abundant in organic matter while the mineral composition of the samples mainly comprises calcite, dolomite, and quartz [18].
important radiation levels are associated with sedimentary rocks [9], with the exception of certain phosphate and shale rocks that display a relatively high content of radionuclides [10].
The examination of sedimentary rocks for uranium and thorium isotopes typically involves a somewhat laborious process of radiochemical procedures to separate the heavy radioactive elements, followed by an analysis of the height of alpha pulses [11][12][13][14][15].It would be desirable to have simpler and faster methods of analysis.One such alternative is gamma ray spectrometric analysis [16,17] which allows for the non-destructive measurement of the natural gamma ray activity of the core, at energies exceeding approximately 1 MeV.In this particular aspect of the natural gamma spectrum, only three nuclides contribute: 40 K, 214 Bi, and 208 Tl. 40 K is a radionuclide that has a long lifespan, whereas 21 4Bi and 208 Tl are short-lived daughters in the decay schemes of 238 U and 232 Th, respectively.
The present study analyzes the radionuclides concentrations naturally occurring in oil shale samples retrieved from the Ama Fatma coastal zone.The study area is situated in the Tarfaya-Boujdour basin, which defines the Atlantic margin area between the towns of Tarfaya and Boujdour.This region is bounded by the Atlantic Ocean on the west and the Republic of Mauritania on the east, as depicted in Figure 1.Located approximately 80 km to the north of the city, this segment commences with a black shale layer exceeding 2 m in thickness [18,19].The geological characteristics of the study area consist of Marl, a sedimentary rock abundant in organic matter while the mineral composition of the samples mainly comprises calcite, dolomite, and quartz [18].The objective of this study includes investigating the natural concentrations of radionuclide elements ( 238 U, 235 U, 232 Th, 226 Ra, 214 Pb, and 40 K) within Ama Fatma oil shale and assessing the disequilibrium of uranium series radionuclides by studying the 232 Th/ 238 U, and 226 Ra/ 238 U ratios.

Sample Collection and Physical Preparation
The oil shale samples under investigation were collected from the Ama Fatma region, located within the coastal basin of Tarfaya-Boujdour in southwest Morocco (Figure 1).The geographical coordinates of the sampling area are X = 28.202 and Y = −11.781[19].These samples were collected at different depths, ranging from 24.18 to 26.69 m from the top of the Ama Fatma outcrop, as shown in Table 1.The physical parameters including total carbon, lithological parameters, and mineralogical composition were analyzed and presented in [18,19] as shown in Table 1.The mineralogical composition was measured The objective of this study includes investigating the natural concentrations of radionuclide elements ( 238 U, 235 U, 232 Th, 226 Ra, 214 Pb, and 40 K) within Ama Fatma oil shale and assessing the disequilibrium of uranium series radionuclides by studying the 232 Th/ 238 U, and 226 Ra/ 238 U ratios.

Sample Collection and Physical Preparation
The oil shale samples under investigation were collected from the Ama Fatma region, located within the coastal basin of Tarfaya-Boujdour in southwest Morocco (Figure 1).The geographical coordinates of the sampling area are X = 28.202 and Y = −11.781[19].These samples were collected at different depths, ranging from 24.18 to 26.69 m from the top of the Ama Fatma outcrop, as shown in Table 1.The physical parameters including total carbon, lithological parameters, and mineralogical composition were analyzed and presented in [18,19] as shown in Table 1.The mineralogical composition was measured using X-ray diffractometry while the granulometry was performed using wet Laser Diffraction equipment (Malvern Mastersizer 2000), namely the Hydro 2000G dispersion Unit [18].The total carbon was determined using the Rock-Eval pyrolysis method [19].The obtained oil shale samples were ground to a fine powder using an electric grinder.Then, homogenized samples were weighed and stored in a standard specification plastic container.These containers have been carefully sealed and stored for at least four weeks to prevent contamination of the spectrometer and the escape of 222 Rn and 220 Rn radiogenic gasses to allow radioactive secular equilibrium to be achieved in the decay chains.

Analytical Methods
The natural radionuclides ( 238 U, 235 U, 232 Th, 226 Ra, 214 Pb, and 40 K) activity concentrations were measured by high-resolution gamma-ray spectrometry.The detector used is a low background CANBERRA high-purity coaxial germanium (50% efficiency, 1.8 keV resolution), housed in a 10-cm-thick high-purity lead shield.The gamma spectra delivered by the detector are analyzed by Genie 2000 gamma analysis software.A typical gamma-ray spectrum of an oil shale sample recorded with this system is presented in Figure 2. The 234m Pa (1001.03keV) was used to estimate 238 U specific activities, while the peak area at 228 Ac (911.32 keV) was used to estimate 232 Th.For the determination of 235 U, 226 Ra, 214 Pb, and 40 K, their most intensive lines at 143.76 keV, 186.21 keV, 351.92 keV and 1460.81keV represented their activities, respectively.It worth noting that the spectrometry gamma analytical method requires a significant amount of sample compared to other alternative methods (e.g., Alpha spectrometry).
using X-ray diffractometry while the granulometry was performed using wet Laser Diffraction equipment (Malvern Mastersizer 2000), namely the Hydro 2000G dispersion Unit [18].The total carbon was determined using the Rock-Eval pyrolysis method [19].The obtained oil shale samples were ground to a fine powder using an electric grinder.Then, homogenized samples were weighed and stored in a standard specification plastic container.These containers have been carefully sealed and stored for at least four weeks to prevent contamination of the spectrometer and the escape of 222 Rn and 220 Rn radiogenic gasses to allow radioactive secular equilibrium to be achieved in the decay chains.

Analytical Methods
The natural radionuclides ( 238 U, 235 U, 232 Th, 226 Ra, 214 Pb, and 40 K) activity concentrations were measured by high-resolution gamma-ray spectrometry.The detector used is a low background CANBERRA high-purity coaxial germanium (50% efficiency, 1.8 keV resolution), housed in a 10-cm-thick high-purity lead shield.The gamma spectra delivered by the detector are analyzed by Genie 2000 gamma analysis software.A typical gamma-ray spectrum of an oil shale sample recorded with this system is presented in Figure 2. The 234m Pa (1001.03keV) was used to estimate 238 U specific activities, while the peak area at 228 Ac (911.32 keV) was used to estimate 232 Th.For the determination of 235 U, 226 Ra, 214 Pb, and 40 K, their most intensive lines at 143.76 keV, 186.21 keV, 351.92 keV and 1460.81keV represented their activities, respectively.It worth noting that the spectrometry gamma analytical method requires a significant amount of sample compared to other alternative methods (e.g., Alpha spectrometry).(14).The S1 (16) shows the highest value of 232 Th and 40 K while S1 (10) indicates the highest value of 214 Pb.The obtained activities of 238 U and 226 Ra measured in the Ama Fatma oil shale are greater than the activities recorded in soil samples reported as not contaminated, while the activities of 232 Th and 40 K are less high compared to values recorded in the abovementioned soil [20].The high values of uranium found in the Ama Fatma oil shale samples are related to the oil shale and the sedimentary deposits abundant in the organic materials' abilities to capture redox-sensitive elements, including Re, V, and U, under reductive conditions [10,13].In comparison with the oil shale of Timahdit [13], the Ama Fatma oil shale shows less important activities mainly 238 U, 235 U, and 232 Th.It was expected that 232 Th, 238 U, and 40 K would demonstrate distinct environmental behavior, given their particular solubility and varying chemical reactivity [21].The examination of the exemplary curve (Figure 3) indicates that the specific activities of 226 Ra, 238 U, 232 Th, 214 Pb, 235 U, and 40 K in the oil shale deposits are not related to depth and the irregular recorded variation seems to be related to the lithological factors which probably control the radionuclides' distribution.(14).The S1 (16) shows the highest value of 232 Th and 40 K while S1 (10) indicates the highest value of 214 Pb.The obtained activities of 238 U and 226 Ra measured in the Ama Fatma oil shale are greater than the activities recorded in soil samples reported as not contaminated, while the activities of 232 Th and 40 K are less high compared to values recorded in the abovementioned soil [20].The high values of uranium found in the Ama Fatma oil shale samples are related to the oil shale and the sedimentary deposits abundant in the organic materials' abilities to capture redox-sensitive elements, including Re, V, and U, under reductive conditions [10,13].In comparison with the oil shale of Timahdit [13], the Ama Fatma oil shale shows less important activities mainly 238 U, 235 U, and 232 Th.It was expected that 232 Th, 238 U, and 40 K would demonstrate distinct environmental behavior, given their particular solubility and varying chemical reactivity [21].The examination of the exemplary curve (Figure 3) indicates that the specific activities of 226 Ra, 238 U, 232 Th, 214 Pb, 235 U, and 40 K in the oil shale deposits are not related to depth and the irregular recorded variation seems to be related to the lithological factors which probably control the radionuclides' distribution.

Pearson Correlation Coefficients among Radionuclides, TOC, and Mineralogical Composition
Pearson correlation coefficients between the measured radionuclides, mineralogical composition, and the total carbon (TOC) in the examined oil shale samples are presented in Table 3.All natural radionuclides of the 238U-series ( 226 Ra, 238 U, and 214 Pb) showed a strong positive correlation with each other, which means that the distribution mechanisms of 226 Ra, 238 U, and 214 Pb within the oil shale are more or less the same.On the other hand, no correlation was found with the 40 K, 232 Th, and 238 U series, revealing distinctively different mechanisms of control of 40 K, 238 U, and 232 Th concentrations in Ama Fatma oil shale.There are positive correlations between the organic matter content and 40 K-and 232 Th-specific activities.Organic matter is a component of great importance because it tends to form soluble or insoluble complexes with radionuclides, which can migrate throughout the profile or be retained in the soil [22].Significative positive correlations were found between 232 Th and dolomite and clay, which indicate the association of these radionuclides with dolomite and clay.The 226 Ra, 238 U, and 235 U were found to be positively correlated with pyrite, which is in accordance with earlier results for oil shale from the Timhdit area.These correlations could be attributed to the common affinity of these elements for pyrite minerals.The calculated 232 Th/ 238 U ratios ranged from 0.008 to 0.071 with a mean value of 0.03.The values of the 232 Th/ 238 U are very low as the oil shale samples have a marine origin.The obtained low ratio values of 232 Th/ 238 U suggest that the studied drilling samples were deposited in a reducing environment [23].

226 Ra/ 238 U
If secular equilibrium prevails in the 238 U chain, 226 Ra/ 238 U will be approximately 1. Thus, any values of 226 Ra/ 238 U other than 1 would suggest the existence of disequilibrium.The ratios 226 Ra/ 238 U of Ama Fatma oil shale ranged from 0.56 to 0.74 with a mean value of 0.70.Such ratios being lower than 1 indicate the existence of disequilibrium in the oil shale samples under consideration.This disequilibrium can be attributed to significant differences in the mobility of the radionuclides present.Differential ion mobility has been widely discussed in the literature as a potential cause of such disequilibrium [24][25][26].

Conclusions
The study allowed analyzation of the activity concentrations of the 238 U, 235 U, 232 Th, 226 Ra, 214 Pb, and 40 K in shale oil samples collected from the coastal zone of Ama Fatma.The results show that the distribution of studied radionuclides varied independently in depth where it appears to be associated with the lithological characteristic of the zone.The analysis of the 232 Th/ 238 U and 226 Ra/ 238 U allowed an understanding of these radionu-

Figure 1 .
Figure 1.A sketched map of the study area showing its main geological features.

Figure 1 .
Figure 1.A sketched map of the study area showing its main geological features.

Table 1 .
Chemical and mineralogical characteristics of Ama Fatma oil shale samples *.

Table 1 .
Chemical and mineralogical characteristics of Ama Fatma oil shale samples *.

Table 3 .
Pearson correlation coefficients between mineralogy, total organic content, and radionuclides activity concentrations in oil shale samples collected from Ama Fatma, Morocco.Study of 232 Th/ 238 U and 226 Ra/ 238 U Activity Concentration Ratios The activity concentration ratios 238 U/ 226 Ra and 226 Ra/ 238 U were carried out to analyze their variations as shown in Table 2. 3.3.1. 232Th/ 238 U