Determination of thorium, uranium and potassium elemental concentrations in surface soils in Cyprus
Introduction
Gamma radiation emitted from naturally occurring radioisotopes, such as 40K and the radionuclides from the 232Th and 238U series and their decay products (also called terrestrial background radiation), which exist at trace levels in all ground formations, represents the main external source of irradiation to the human body. More specifically, natural environmental radioactivity and the associated external exposure due to gamma radiation depend primarily on the geological and geographical conditions, and appear at different levels in the soils of each region in the world (UNSCEAR, 2000 Report, and further references cited therein). The specific levels of terrestrial environmental radiation are related to the geological composition of each lithologically separated area, and to the content in thorium (Th), uranium (U) and potassium (K) of the rock from which the soils originate in each area. In terms of natural radioactivity, it is well known, for instance, that igneous rocks of granitic composition are strongly enriched in Th and U (on an average 15 μg g−1 of Th and 5 μg g−1 of U), compared to rocks of basaltic or ultramafic composition (<1 μg g−1 of U) (Faure, 1986, Mènager et al., 1993). For that reason, higher radiation levels are associated with igneous rocks and lower levels with sedimentary rocks. There are exceptions, however, as some shales and phosphate rocks have relatively high content of those radionuclides (UNSCEAR, 2000 Report).
From a geological point of view, the island of Cyprus, which is located in the eastern basin of the Mediterranean Sea and extends to an area of about 9300 km2, contains one of the best-preserved and most intensively studied ophiolite complexes in the world, which is known as the Troodos Massif (Moores and Vine, 1971, Robinson and Malpas, 1998). It consists of basic and ultrabasic pillow lavas, fringed by andesitic-sheeted dykes, while the central part of the ophiolite consists of basic and ultrabasic plutonic rocks (gabbros, peridotites, dunites and serpentinised harzburgites). The highly tectonised and fractured conditions of the Troodos mass, as a consequence of its uplift, facilitated deep weathering of the rocks, leading to the development of a smooth, mature topography, mantled with a thick cover of a diversity of soils (Robinson and Malpas, 1998). The soil cover of the central area is highly alkaline, while the soils on the slopes, lower down, are covered by neutral sheeted diabase. The weathering of the sedimentary rocks (chalks, marls, etc.) in the foothills that fringe Troodos, gave rise to alkaline, calcium-rich soils. None of these rock types belongs to the category of silica-oversaturated, which usually is associated with high Th and U elemental concentrations (Faure, 1986).
Since no systematic data on environmental radioactivity in Cyprus were available, a pilot project commenced in 2001 with the objective to systematically measure the terrestrial gamma radiation in the island, and determine its contribution to the annual effective dose equivalent to the population. There are two main features that make this study particularly important and interesting to radiometric studies. Firstly, it provides information on the geomorphological composition and the associated environmental radioactivity of such an area with a large variety of ophiolitic and sedimentary rock types cropping out over a relatively narrow area. Secondly, the analysis of selective elemental abundance (Th/U, K/U and K/Th ratios) may also allow us to study the enrichment/depletion processes as a result of the complex metamorphic history, alteration and/or weathering that affected the investigating rocks (Chiozzi et al., 2002).
Tzortzis et al. (2003a) reported the first results of such measurements and the corresponding average annual effective dose rates equivalent to the population, using 28 soil samples collected from the main geological rock types appearing in the island. A more extensive study of terrestrial gamma radioactivity was performed after the collection of 115 soil samples from all over the island bedrock surface based on the different lithological units of the study area. Such a number of collected samples in combination with the relatively large area portion studied (about 6000 km2) can be considered that they widely cover the various outcropping geological formations. Other parts of the project aimed at measuring the concentration of radon in houses and public buildings (Anastasiou et al., 2003), as well as at detecting α-emitting radioisotopes by utilising radioanalytical techniques and high-resolution α-spectrometry (Pashalidis and Tsertos, 2004). In this paper, the results from this extensive survey regarding Th, U and K elemental concentrations in a wide variety of surface soils by means of high-resolution γ-ray spectrometry are presented. Further results from these investigations concerning activity concentrations and the associated dose rates due to terrestrial gamma radiation are going to be published elsewhere (Tzortzis et al., 2004). The experiments have been carried out in the Nuclear Physics Laboratory of the Department of Physics, University of Cyprus.
Section snippets
Sample collection and preparation
A total of 115 surface soil samples has been collected throughout the whole accessible area1 of the Republic of Cyprus (Fig. 1), which widely covered all the geological surface formations of the island. The official geological map of Cyprus (GSDC, 1995) was used to segregate the accessible island’s area into seven different geological regions indicated in the
Results and discussion
For the purpose of quality assurance, a control sample from a reference soil material (IAEA-326) was prepared in an identical 1000-ml Marinelli beaker (dry soil mass of 1.254 kg), and treated with respect to the measurement and analysis procedure as an unknown sample. The results obtained and the recommended values are summarised in Table 1. One may note that the recommended values (Bojanowski et al., 2001) are given in units of Bq kg−1 for the 232Th, 238U and 40K activity concentrations, which
Conclusions
High-resolution γ-ray spectrometry was used to determine elemental concentrations of the radioactive elements of thorium, uranium and potassium in a number of 115 surface soil samples collected from all over the island of Cyprus. The number and the surface distribution of the samples collected can be considered as widely and sufficiently covering the various geological formation areas. Soils originated from the Troodos ophiolitic complex appear generally to have lower naturally occurring
Acknowledgements
This work is financially supported by the Cyprus Research Promotion Foundation (grant no. 45/2001), and partially by the University of Cyprus.
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2022, Journal of African Earth SciencesCitation Excerpt :For the aerial and spatial distribution of radionuclides, the radiometric survey, especially the gamma-ray spectrometric approach, is used to determine the concentrations of 40K, 238U, and 232Th, and their daughter products, and associated ambient radiations in geological formations and the immediate environment (Mohanty et al., 2004; Ademila et al., 2018; Akingboye and Ademila, 2019; Akingboye et al., 2021; Adebiyi et al., 2021; Faruwa et al., 2021a). The preponderance amount of the radionuclides is detectable with the aid of radiometric instruments (e.g., Gamma-Ray Spectrometer) when naturally occurring radioactive elements (NORMs) in rocks and soils decay spontaneously (Tzortzis and Tsertos, 2004; Chandrasekaran et al., 2014; Cinar et al., 2017). The gamma-ray spectrometric method has been used to analyze radionuclide concentrations in geothermal fields, along geologically active zones, in volcanic areas, and old and current mining sites (Barbuena et al., 2013; Eggeling et al., 2013; Innocent et al., 2013; Youssef and Elkhodary, 2013; Ramola et al., 2014; Erbek and Dolmaz, 2018; Faruwa et al., 2020; Cinelli et al., 2020).