238U and 232Th isotopes in groundwater of Jordan: Geological influence, water chemistry, and health impact

https://doi.org/10.1016/j.radphyschem.2019.108660Get rights and content

Highlights

  • The activity concentrations of 238U and 232Th in groundwater of Jordan are below recommended by WHO.

  • The highest mean activity concentration of 238U and 232Th were found for lower Cretaceous geological formation.

  • Geological formations significantly influence 238U, while no influence on 232Th activity concentration was found.

  • The health impact due to intake 238U and 232Th in groundwater of Jordan found lower than the global average.

Abstract

An extensive pioneer study was conducted to determine the activity concentration of 238U and 232Th in 94 groundwater samples to examine the influence of geological formations on the activity concentrations of 238U and 232Th and to investigate a relationship with the major chemical elements in groundwater. The study also aims to estimate the annual effective dose and health impact due to the intake of these radionuclides in the groundwater of Jordan. The mean activity concentrations of 238U and 232Th in groundwater were found to be 33 ± 5 mBq L−1 and 0.017 ± 0.002 mBq L−1, respectively. The highest mean of 238U and 232Th activity concentrations were found for lower Cretaceous geological formation, while Tertiary and Quaternary geological formations have the lowest mean 238U and 232Th activity concentration, respectively. The output of (ANOVA) and post hoc tests revealed the existence of significant influence of geological formations on 238U activity concentrations, while insignificant influence on 232Th activity concentrations was found in the groundwater of Jordan. The groundwater of Jordan was dominated by HCO3−1, Cl−1, Ca+2, and Na+1. The mean annual effective dose due to the ingestion of radionuclides 238U and 232Th in groundwater were found 1.143 μSv lower than recommended by WHO. The ingestion dose deduced is dominated by 238U of 99% where the average contribution from 232Th amounted to 1%. The results of the study will serve as a baseline data of 238U and 232Th activity concentrations in groundwater based on geological formations of Jordan.

Introduction

Natural environmental radioactivity mainly arises from the occurrence of the natural radionuclides of 232Th, 238U, and their decay products, in addition to 40K (UNSCEAR, 2000). As groundwater moves through fractures in the bedrock, natural radionuclides can get into groundwater as a water-rock interaction (Abdurabu et al., 2016b; Kitto et al., 2005; Shabana et al., 2013). Groundwater has acted as a medium of transporting and enhancing uranium in an environment away from its origin (Aliyu et al., 2015). Nearly all the radionuclides occurring in drinking water supplies are naturally occurring (Lopes et al., 2017). The specific elements of concern are 238U and 232Th, which decay mainly by alpha-particle emission to nuclides that themselves are radioactive (Cothern and Reberts, 1990). Natural 232Th has one single isotope that is 100% 232Th and 238U comprises 99.25% of natural uranium (Gilmore, 2008). 238U is widely dispersed in groundwater (Dinh Chau et al., 2011). Human exposure to uranium has long been considered to pose a radiological and chemical hazard (Milvy and Cothern, 1990).

Uranium has many oxidation states (+4, +5 and +6) in nature. In groundwater, uranium is generally found in the +6-oxidation state where the oxidizing environments are prevailed (Kraemer and Genereux, 1998). Uranium (+6) is most soluble in oxidizing water. Naturally occurring uranium components exist in groundwater through the dissolution of the uranium-bearing minerals such as uraninite (UO2) (Langmuir, 1978). Insoluble uraninite is readily oxidized through water-rock interactions to soluble uranyl ions (UO2+2) (Banks et al., 1995). The uranyl ions are the most soluble form of uranium. In the presence of phosphates or carbonates highly soluble complexes are formed mainly in the pH range between 6 and 8. These are typical pH values for a large majority of groundwater systems (Wanty and Schoen, 1992). Under reducing conditions, uranium (+4) is a dominant form. In aquatic environments, under reducing conditions uranium (+4) forms insoluble precipitates strongly adsorbed on suspended particles or mineral surfaces (Hem, 1985). One of the most insoluble radioactive elements in water is 232Th, it has only one oxidation state in nature (+4). 232Th forms insoluble hydroxides under most natural conditions (Dinh Chau et al., 2011).

In the last decades, studies of the natural radioactivity in groundwater and the hosting rock type had received attention worldwide (Abdurabu et al., 2016b; Ammar et al., 2010; Charalambous et al., 2013; Chkir et al., 2009; Cowart, 1987; Da Silva and Bonotto, 2015; Dinh Chau et al., 2011; El-Sharkawy, 2018; Gilkeson et al., 1983; Kanellopoulos et al., 2018; M. Isam Salih et al., 2002; Malov, 2016; Michel, 1990; Porcelli and Swarzenski, 2003; Post et al., 2017; Smith et al., 1996; Szabo and Zapecza, 1991; Wu et al., 2018). Most of these studies were focused on investigating the activity concentration levels of natural radionuclides 238U and 232Th and correlated to rocks type to evaluate the factors that control their presence in groundwater. The concentration of 238U series radionuclides in groundwater may vary over several orders of magnitude depending on hosting rocks and physicochemical parameters that control the release (Dinh Chau et al., 2011; M. Isam Salih et al., 2002). 238U concentration in water wells is directly controlled by the lithology of the aquifer solids adjacent to the wells (Cothern and Reberts, 1990). Consequently, the 238U activity concentration in groundwater should reflect some extent the type of rocks that host the given aquifer system, and this will lead in using aquifer type to predict the occurrence of 238U in groundwater.

Chemistry of groundwater plays an important role in determining the quality and suitability of water for drinking and other domestic purposes (Sadashivaiah et al., 2008; Subramani et al., 2010). Interaction of groundwater with aquifer minerals through which it flows greatly controls the groundwater chemistry (Subramani et al., 2010). Chemical characters of water may also affect the source and mobility of natural radionuclides in groundwater (Abdurabu et al., 2016b; Sharma et al., 2019; Srilatha et al., 2014). Several studies in recent years have focused on the chemical characters of water to evaluate the chemical factors that may affect the source and mobility of natural radionuclides in groundwater (Abdurabu et al., 2016b; Bajwa et al., 2017; Jobbágy et al., 2009; Mittal et al., 2017; Ramadan et al., 2014; Singh et al., 2003; Srilatha et al., 2014). The studies revealed that there is a strong to the weak link between activity concentrations of natural radionuclides 238U and 232Th and the chemical characters. Understanding 238U and 232Th mobility are vital to minimizing its concentrations in potential drinking water sources (Wu et al., 2018).

Groundwater globally provides 25–40% of the world's drinking water (Selvi et al., 2016). In Jordan, people rely almost 100% on groundwater for drinking, irrigation and other domestic purposes. In recent years, there has developed an increasing national concern for an assessment of the natural radioactivity in drinking water. A few studies have been conducted to evaluate radioactivity levels of drinking water. However, based on existing literature, extensive field measurement of natural radionuclides related to geological formations and to evaluate the effective dose to the public and related health impact of natural radionuclides in groundwater has yet to be conducted.

Therefore, the current study, the first of its kind in Jordan forms part of the pioneer study, represents a comprehensive investigation on 238U and 232Th levels and aims to measure the 238U and 232Th activity concentrations in the groundwater sources to investigate the influence of geological formations as well as the chemical characters of groundwater on 238U and 232Th activity concentrations. Also, this study aims to estimate the annual effective dose and assessment of the lifetime risk related to Jordan groundwater. It can contribute to establishing national recommendations to limit 238U and 232Th activity concentrations as well as chemical characters in drinking groundwater. The results may also be used as baseline data for monitoring possible radioactivity pollution in anticipation of the possibility that Jordan will have a nuclear power plant in the future (Xoubi, 2015).

Section snippets

Study area

Jordan is classified as a Mediterranean country with a total area of about 90,000 km2, and a population of around 10 million people (Jordan Department of statistics, 2017). It lies between latitudes (29° 11' - 33° 22') N and between longitudes (34° 59' - 39° 18') E. Jordan is located at the western edge of the Arabian Plate, and stands out from the remaining part of the Arabian Peninsula by its abundance in radioactive elements, mainly uranium, in a way so far not found elsewhere on the Arabian

Descriptive statistics of 238 U and 232 Th activity concentrations in groundwater

The basic descriptive statistics such as minimum, maximum, mean, standard deviation, Kurtosis, and skewness are presented in Table 1. The activity concentration of 238U ranged from MDA to 312 ± 47 mBq L−1 with a mean value of 33 ± 5 mBq L−1. The obtained values are lower than the recommended level of 360 mBq L−1 (USEPA, 2018; WHO, 2011). The WHO and USEPA guidelines for drinking-water quality recognized 238U as a potential chemical risk.

The frequency distribution of 238U activity concentration

Conclusion

Natural radioactivity of 238U and 232Th in groundwater from the Cambrian-Ordovician, upper Cretaceous, Tertiary, Quaternary and lower Cretaceous geological formations in Jordan was investigated. 238U activity concentrations were found to be in the range of MDA to 312 ± 47 mBq L−1. The activity concentration of 232Th was found to be in the range of MDA to 0.177 ± 0.027 mBq L−1. The highest value of 238U and 232Th activity concentrations were found for upper Cretaceous geological formation

CRediT authorship contribution statement

Ahmad Hussein Alomari: Conceptualization, Methodology, Software, Writing - original draft, Formal analysis, Investigation, Validation, Writing - review & editing. Muneer Aziz Saleh: Conceptualization, Methodology, Software, Formal analysis, Supervision, Writing - review & editing. Suhairul Hashim: Conceptualization, Supervision, Writing - review & editing. Amal Alsayaheen: Methodology, Formal analysis, Investigation, Validation. Ismail Abdeldin: Methodology, Investigation, Validation. Ahmad

Declaration of competing interest

The authors declare that they have no conflict of interest.

Acknowledgement

The authors would like to thank the Water Authority of Jordan. All the measurements were conducted at isotopes and chemical analysis laboratories. The authors would like to thank Mr. Refaat bani Khalaf from the Water Authority of Jordan for his help in preparing the hydrogeological and rainfall level maps of Jordan. The authors would like to thank the ministry of higher education Malaysia (MOHE) and Universiti Teknologi Malaysia (UTM) for support and funding under UTM Research university Grant;

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