Distribution of Natural Radionuclides in Ama Fatma Oil Shale, Morocco ^†

Abstract

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, and potassium-40 (K-40) in oil shale samples retrieved from the study area. The concentrations of U-238 ranged from 45 ± 36 to 194 ± 50 Bq.kg⁻¹ and those of Th-232 fluctuated between 1.48 ± 0.92 and 9.47 ± 1.06 Bq.kg⁻¹. The measured concentrations for Ra-226, Pb-214, and K-40 oscillated between 33 ± 5 and 138 ± 9 Bq.kg⁻¹, 18 ± 1 and 68 ± 3 Bq.kg⁻¹, and between 17 ± 4 and 279 ± 16 Bq.kg⁻¹, respectively. The obtained values variation appears to be related to the deposit nature and assimilation processes of appropriate radionuclides rather than the retrieved samples’ related depth. The analysis of ²³²Th/²³⁸U and ²²⁶Ra/²³⁸U allowed an understanding of these radionuclides’ behaviors. The ratios ²²⁶Ra/²³⁸U ranged from 0.56 to 0.74 with a mean value of 0.70 which indicates the existence of disequilibrium in the investigated oil shale samples. This disequilibrium can be attributed to significant differences in the mobility of these radionuclides.

1. Introduction

The primary source of natural radionuclides is attributed to the three decay chains of ²³⁵U, ²³⁸U, and ²³²Th, as well as ⁴⁰K. ²³²Th has the longest half-life of 1.405 × 1010 years, followed by ²³⁸U with a half-life of 4.47 × 109 years, and ²³⁵U with a half-life of 7.1 × 108 years. Natural uranium is composed of 99.27% ²³⁸U, 0.72% ²³⁵U, and 0.0055% ²³⁴U [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 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: ⁴⁰K, ²¹⁴Bi, and ²⁰⁸Tl. ⁴⁰K is a radionuclide that has a long lifespan, whereas ²¹4Bi and ²⁰⁸Tl are short-lived daughters in the decay schemes of ²³⁸U and ²³²Th, 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 (²³⁸U, ²³⁵U, ²³²Th, ²²⁶Ra, ²¹⁴Pb, and ⁴⁰K) within Ama Fatma oil shale and assessing the disequilibrium of uranium series radionuclides by studying the ²³²Th/²³⁸U, and ²²⁶Ra/²³⁸U ratios.

2. Materials and Methods

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

Samples	TOC (%) (Rock Eval) *	Depth (m) (from Top of Outcrop) *	Mineralogical Composition **							Lithology *	Age *
			Quartz	Calcite	Dolomite	Clay	Halite	Fluorapatite	Pyrite
S1 (9)	6.19	24.18	6.8	70.2	8.6	5.5	7.3	1.6	-	Marl very rich in organic matter	Upper Cretaceous
S1 (10)	5.84	24.69	7.3	65.3	8.8	4.7	3.1	9.4	1.5
S1 (10–13)	5.84	24.83	17.5	55.8	14.4	7.4	4.4	0.4	0.3
S1 (13–14)	5.84	24.95	3.4	67.8	4.7	3.7	5.7	13.3	1.7
S1 (14)	5.91	25.27	6.7	66.9	4.8	3.9	2.7	5.4	9.7
S1 (15)	-	25.8	17.2	47.9	20.6	8.1	4.1	0.4	1.6
S1 (16)	7.64	26.69	8.4	65.1	14.8	8.3	3.3	-	-

** [18], * [19].

. 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 ²²²Rn and ²²⁰Rn radiogenic gasses to allow radioactive secular equilibrium to be achieved in the decay chains.

2.2. Analytical Methods

The natural radionuclides (²³⁸U, ²³⁵U, ²³²Th, ²²⁶Ra, ²¹⁴Pb, and ⁴⁰K) 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 ^234mPa (1001.03 keV) was used to estimate ²³⁸U specific activities, while the peak area at ²²⁸Ac (911.32 keV) was used to estimate ²³²Th. For the determination of ²³⁵U, ²²⁶Ra, ²¹⁴Pb, and ⁴⁰K, their most intensive lines at 143.76 keV, 186.21 keV, 351.92 keV and 1460.81 keV 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).

3. Results and Discussions

3.1. Radionuclides Distributions

Table 2. Specific activities of ²²⁶Ra, ²³⁸U, ²³²Th, ²¹⁴Pb, ²³⁵U, and ⁴⁰K Bq.kg⁻¹ with the respective standard errors (±σ) and isotopic ratios ²³²Th/²³⁸U and ²²⁶Ra/²³⁸U.

Samples	226Ra	±σ	238U	±σ	232Th	±σ	214Pb	±σ	235U	±σ	40K	±σ	232Th/238U	226Ra/238U
S1 (9)	104.0	7.7	139.9	41.7	2.79	1.15	44.8	2.3	4.02	2.90	145.8	9.0	0.020	0.74
S1 (10)	125.0	8.4	172.8	42.2	2.44	0.90	68.2	3.5	4.40	2.86	110.9	6.9	0.014	0.72
S1 (10–13)	47.7	5.4	67.1	32.6	4.79	0.69	29.8	1.6	4.27	2.78	159.4	9.2	0.071	0.71
S1 (13–14)	33.4	4.9	45.1	36.4	2.25	0.82	25.0	1.4	-	-	63.6	5.0	0.05	0.74
S1 (14)	138.3	9.2	193.7	50.3	1.48	0.92	57.7	3.0	4.75	2.84	86.4	6.7	0.008	0.71
S1 (15)	40.0	4.63	71.2	43.3	-	-	17.8	1.1	-	-	16.9	4.2	-	0.56
S1 (16)	131.0	9.5	185.7	58.5	9.47	1.06	61.5	3.2	4.13	3.83	279.2	16.2	0.05	0.70

displays the specific activities of ²²⁶Ra, ²³⁸U, ²³²Th, ²¹⁴Pb, ²³⁵U, and ⁴⁰K measured in oil shale samples collected at different depths from the Ama Fatma coastal zone. The activities of ²²⁶Ra, ²³⁸U, ²³²Th, ²¹⁴Pb, ²³⁵U, and ⁴⁰K ranged between 33.4, 45.1 and 193.7 Bq.kg⁻¹, 1.48 and 9.47 Bq.kg⁻¹, 17.8 and 68.2 Bq.kg⁻¹, 4.02 and 4.75 and between 16.9 and 279.2 Bq.kg⁻¹, respectively. The highest values of ²²⁶Ra, ²³⁸U, and ²³⁵U were recorded in S1 (14). The S1 (16) shows the highest value of ²³²Th and ⁴⁰K while S1 (10) indicates the highest value of ²¹⁴Pb. The obtained activities of ²³⁸U and ²²⁶Ra measured in the Ama Fatma oil shale are greater than the activities recorded in soil samples reported as not contaminated, while the activities of ²³²Th and ⁴⁰K 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 ²³⁸U, ²³⁵U, and ²³²Th. It was expected that ²³²Th, ²³⁸U, and ⁴⁰K 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 ²²⁶Ra, ²³⁸U, ²³²Th, ²¹⁴Pb, ²³⁵U, and ⁴⁰K 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.

3.2. 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. Pearson correlation coefficients between mineralogy, total organic content, and radionuclides activity concentrations in oil shale samples collected from Ama Fatma, Morocco.

	TOC	226Ra	238U	232Th	214Pb	235U	40K	Quartz	Calcite	Dolomite	Clay	Halite	Fluorapatite	Pyrite
TOC	1.00
226Ra	0.42	1.00
238U	0.44	0.99	1.00
232Th	0.90	0.15	0.18	1.00
214Pb	0.39	0.95	0.94	0.18	1.00
235U	−0.46	0.34	0.36	−0.53	0.30	1.00
40K	0.90	0.50	0.48	0.96	0.54	−0.64	1.00
Quartz	−0.03	−0.44	−0.38	0.34	−0.48	−0.15	−0.10	1.00
calcite	0.10	0.56	0.49	−0.31	0.59	−0.05	0.34	−0.92	1.00
Dolomite	0.58	−0.32	−0.25	0.84	−0.37	−0.63	0.09	0.85	−0.85	1.00
Clay	0.70	−0.17	−0.11	0.91	−0.22	−0.65	0.41	0.75	−0.67	0.94	1.00
Halite	−0.18	−0.42	−0.47	−0.21	−0.46	−0.71	−0.09	−0.17	0.25	−0.15	−0.12	1.00
Fluorapatite	−0.49	0.07	0.03	−0.58	0.26	0.51	−0.22	−0.78	0.56	−0.70	−0.81	−0.09	1.00
Pyrite	0.99	0.68	0.70	−0.70	0.46	0.99	−0.13	−0.40	0.43	−0.51	−0.51	−0.60	0.05	1.00

. All natural radionuclides of the 238U-series (²²⁶Ra, ²³⁸U, and ²¹⁴Pb) showed a strong positive correlation with each other, which means that the distribution mechanisms of ²²⁶Ra, ²³⁸U, and ²¹⁴Pb within the oil shale are more or less the same. On the other hand, no correlation was found with the ⁴⁰K, ²³²Th, and ²³⁸U series, revealing distinctively different mechanisms of control of ⁴⁰K, ²³⁸U, and ²³²Th concentrations in Ama Fatma oil shale. There are positive correlations between the organic matter content and ⁴⁰K- and ²³²Th-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 ²³²Th and dolomite and clay, which indicate the association of these radionuclides with dolomite and clay. The ²²⁶Ra, ²³⁸U, and ²³⁵U 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.

3.3. Study of ²³²Th/²³⁸U and ²²⁶Ra/²³⁸U Activity Concentration Ratios

The activity concentration ratios ²³⁸U/²²⁶Ra and ²²⁶Ra/²³⁸U were carried out to analyze their variations as shown in Table 2.

3.3.1. ²³²Th/²³⁸U

The calculated ²³²Th/²³⁸U ratios ranged from 0.008 to 0.071 with a mean value of 0.03. The values of the ²³²Th/²³⁸U are very low as the oil shale samples have a marine origin. The obtained low ratio values of ²³²Th/²³⁸U suggest that the studied drilling samples were deposited in a reducing environment [23].

3.3.2. ²²⁶Ra/²³⁸U

If secular equilibrium prevails in the ²³⁸U chain, ²²⁶Ra/²³⁸U will be approximately 1. Thus, any values of ²²⁶Ra/²³⁸U other than 1 would suggest the existence of disequilibrium. The ratios ²²⁶Ra/²³⁸U 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].

4. Conclusions

The study allowed analyzation of the activity concentrations of the ²³⁸U, ²³⁵U, ²³²Th, ²²⁶Ra, ²¹⁴Pb, and ⁴⁰K 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 ²³²Th/²³⁸U and ²²⁶Ra/²³⁸U allowed an understanding of these radionuclides’ behavior. The ratios ²³⁸U/²²⁶Ra ranged from 0.56 to 0.74 with a mean value of 0.70 which indicates the existence of disequilibrium in the investigated oil shale samples. This disequilibrium can be attributed to significant differences in the mobility of these radionuclides. This research data can be used as a valuable resource for further related studies and investigations.