INTRODUCTION

Soil contains radioactive elements such as uranium (U) and thorium (Th) series elements, and potassium-40 (⁴⁰K), which are known as naturally occurring radioactive materials (NORM) (^{Kathren, 1998}).The most important radioactive isotopes in soils are Uranium-238 (²³⁸U,) Thorium-232 (²³²Th) and potasium-40 (⁴⁰K). ²³⁸U and ²³²Th generate many other radioactive isotopes in soils because they are the beginners of radioactive series (^{Bassioni, Abdulla, Morsy & El-Faramawy, 2012}; ^{Topcuoğlu, Türer, Güngör & Kırbaşoğlu, 2003}). Potassium is the 7^th most abundant element in the earth´s crust, representing approximately 2.4% of its total weight. Most of the K is not radioactive, only the mass 40 isotope is radioactive and it corresponds to 0.0117% of natural potassium (^{Jabbar et al., 2010}).

The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has published the world average activity for ²³⁸U, ²³²Th and ⁴⁰K in soil: 35 Bq kg^-1, 25 Bq kg^-1, and 370 Bq kg^-1, respectively (^{UNSCEAR, 2000}). Moreover, many countries have published their own averages (^{Bassioni et al., 2012}; ^{Dołhańczuk-Śródka, 2012}; ^{García-Talavera, Matarraz, Martínez, Salas & Ramos, 2007}; ^{Kathren, 1998}; ^{Mehra, Singh, Sing & Sonkawade, 2007}; ^{Topcuoğlu et al, 2003}; ^{UNSCEAR, 2000}).

This world average converted into dose values of 55 nGy y^-1 for Absorbed Dose Rate (D) and 0.48 mSv y^-1 for Annual Effective Dose (AED), where the AED (0.48 mSv y^-1) is about 16% of the total dose average that any human body receives from the soil (^{Cancio-Pérez, 2010}; ^{UNSCEAR, 2008}). In addition, the International Commission on Radiological Protection (ICRP) has recommended an annual maximum effective dose of 1 mSv y^-1 for individuals (^{Jabbar et al., 2010}; ^{ICRP, 1993}; ^{Mehra et al., 2007}). This is important because radioactivity can affect human health depending on the level of exposure. It is well known that bone marrow and the blood system are the most radiosensitive parts of the human body and can even be affected by low doses. Genitals are also quite sensitive to radiation. For example, people can suffer temporary sterility if they receive sufficiently high radiation, while prolonged exposure can result in permanent sterility. The lens of the eye is also highly vulnerable and radiation can cause cataracts and opacities after a certain period of exposure (^{Cancio-Pérez, 2010}; ^{Kathren, 1998}).

Mexico uranium deposits are not commercial grade. The State of Chihuahua is considered the most important uraniferous site with 50 natural zones with about 30% of all the uranium of Mexico (^{Bazán-Barrón, 1978}; ^{Burciaga-Valencia et al., 2010}; ^{Colmenero-Sujo et al., 2004}; ^{Domínguez, Hernández, Arango & Medina, 2006}; ^{Rentería-Villalobos et al., 2007}; ^{Villalba et al., 2006}; ^{Villalba, Colmenero-Sujo & Montero, 2012}).

Figure 1 shows the 12 most important uraniferous zones; some of them are near (underled uraniferous zones) large cities. There is little information about radioactivity and absorbed dose rates in Mexico. Therefore, there is no baseline for possible human risk and the potential effects of specific doses on human health have not been calculated (^{Bazán-Barrón, 1978}; ^{Domínguez et al., 2006}).

Source: ^{Villalba et al. (2006)}

Figure 1 Main cities of Chihuahua State and its major uraniferous zones. 

One objective of this study was to determine the specific activity of ²³⁸U, ²³²Th series and ⁴⁰K (NORM) in urban soils in 13 important large cities of Chihuahua State, Mexico. A second objective was to calculate the Dosis Range (D) and the Annual Effective Dosis (AED) that might affect Chihuahua’s inhabitants. This information should be helpful for establishing preventive health measures.

MATERIALS AND METHODS

Study area and Activity

The State of Chihuahua is located in northern Mexico (Figure 1), and has a total population of 3 406 465 (^{Instituto Nacional de Estadística, Geografía e Informática [INEGI], 2010}). Geologically, Northern Mexico is composed mainly of Mesozoic and Cenozoic sedimentary rock, as well as recent continental deposits. Natural uranium is found in Chihuahua in acidic extrusive igneous and rhyolitic and andesitic rock. Some cities are close to known uraniferous zones (^{Domínguez et al., 2006}; ^{INEGI, 2010}).

The study was carried out in 13 cities (Figure 1). A total of 164 samples were collected (Table 1). The sampling employed was the one used by the Radiation Monitoring Network in Spain according to the ReViRa (for its acronym in spanish) Manual from Spain (^{ReViRa, 1994}), and mentioned in other works (^{Colmenero-Sujo et al., 2004}; ^{Blanco, Vera-Tomé & Lozano, 2005}; ^{Rentería-Villalobos et al., 2007}). This method use a square steel structure, 50 cm length and 5 cm depth, which was placed on the ground to define the soil sample (approximately 12.5l). Large stones and other objects were removed. Two liters of soil were taken to the laboratory, where they were powdered and sieved through a 2 mm mesh. Finally, one liter was packed into a Marinelli beaker and sealed, and then allowed to stand for at least 4 weeks, so that ²³⁸U series was able to reach radioactive secular equilibrium.

Table 1 Characteristics and samples for the selected cities. 

* meters on the sea level; ** INEGI (2010); *** description Uraniferous rocks; ird = acid igneous/rhyolite/dacite; Little uraniferous rocks; sll = sandstone/ limestone/limonite

Source: Author’s own elaboration.

The activity of different isotopes was analyzed by a gamma spectroscopy using a coaxial hyperpure germanium detector (HPGe), Ge Model 2020, Thermo and Dewar 7500 SL (CANBERRA brand), which belongs to the Centro de Investigaciones en Materiales Avanzados (CIMAV-Chihuahua). The detector is cylindrical, 45-mm high and 44 mm in diameter, with an active volume of approximately 80 cm³. The efficiency ratio of the NaI (TI) 3' × 3' detector is 20%. As a measure of quality control, the resolution was checked by determining the full-width at half-maximum (FWHM) for the 1332.47 keV peak of ⁶⁰Co, it was did daily with a source SRM4203D-65 of National Institute of Standards and Technology (NIST) whose value was 2.0 keV.

To calculate the series of ²³⁸U activity, we used the lines of 351 keV of ²¹⁴Pb and 609 keV of ²¹⁴Bi. The series of ²³²Th activity was obtained from lines ²³⁸keV of ²¹²Pb and 912 keV of ²²⁸Ac. For the particular case of ⁴⁰K, we used its unique line of 1460 keV. Every sample was read for 22 h (^{Bassioni et al., 2012}; ^{Burciaga-Valencia et al., 2010}; ^{Mehra et al., 2007}).

A rigorous process of gamma ray testing was carried out using standards to calibrate the HPGe equipment with the objective of assuring consistency. These samples were analyzed, once more by other institutions such as the Centro Regional de Estudios Nucleares of the Universidad Autónoma de Zacatecas, Mexico (CREN-UAZ), and El Colegio de Física Atómica y Nuclear of the Universidad de Sevilla in Spain, corroborating that the values obtained in this study were reliable.

RESULTS

Table 2 shows the mean values, standard deviation and minimum and maximum values for ²³⁸U, ²³²Th and ⁴⁰K found in the soils of the 13 cities evaluated. Concentration of ²³⁸U exceeded the global average (35 Bq kg^-1) in eight of the cities: Aldama, Bocoyna, Creel, Cuauhtémoc, Chihuahua, Delicias, Jiménez y Nuevo Casas Grandes. This agrees to the type of rock around these cities that have acid igneous, rhyolite and dacite rocks. The highest concentration was found in Aldama with 51.9 Bq kg^-1, while the minimum value was in Juárez with 19.8 Bq kg^-1. This last city is according to the type of rock it has around, sedimentary rocks like sandstone, limestone and limonite (Table 2 and Figure 2).

Table 2 Specific activities of ²³⁸U, ²³²Th and ⁴⁰K (Bq kg^-1) in selected cities in Chihuahua State. 

* Means in columns that do not share a letter are statistically different.

Source: Author’s own elaboration.

Source: Author’s own elaboration.

Figure 2 Specific activities of ²³⁸U, ²³²Th and ⁴⁰K (Bq kg^-1) in selected cities in Chihuahua State. 

The ANOVA analysis for this isotope found statistical differences among the cities (P < 0.01). Besides, Table 2 shows the results of the Tukey´s test, which found three groups of cities. With respect to ²³²Th ANOVA analysis found statistical differences among the cities (P < 0.01). It is noted that the concentration of this isotope exceeded the world average (35 Bq kg^-1) in nine of the evaluated cities. The highest value was in Aldama with 61.1 Bq kg^-1, while the lowest value was in Juárez with 20.0 Bq kg^-1 (lowest again). This is the same case of ²³⁸U, the difference founded between cities are the type of rocks around these cities. The mean and all values in the 13 cities were high in ⁴⁰K. The cities of Aldama and Nuevo Casas Grandes had the highest values with 1014.5 Bq kg^-1and 1013.7 Bq kg^-1, respectively, while the lowest value was in Juárez with 570.7 Bq kg^-1. Moreover, the ANOVA analysis detected significant differences among cities concerning the isotope ⁴⁰K.

In Chihuahua´s cities, in general, there are two kinds of cities according of kind of rocks, ten cities have uraniferous rocks around; Aldama, Bocoyna, Camargo, Creel, Cuauhtémoc, Chihuahua, Delicias, Jiménez, Nuevo Casas Grandes and Parral. Three cities have sedimentary rocks (no uraniferous); Juárez, Manuel Benavides and Ojinaga.

Absorbed Dose Rate and Annual Effective Dose

The values for D determined range from 36 nGy h^-1 to 83 nGy h^-1. Nine of the 13 cities had D values equal to or higher than the global average, which is 55 nGy h^-1 (^{UNSCEAR, 2008}). The average D for Chihuahua State was 60.6 nGy h^-1.

Figure 3 shows the AED calculated for NORM for each sampled city. Twelve of the thirteen cities had levels equal to or higher than the world average (0.48 mSv y^-1) (^{UNSCEAR, 2008}). Ciudad Aldama had the highest value with 1.02 mSv C while Ciudad Juárez the lowest value with 0.44 mSv y^-1, lower than the world average. The average AED for Chihuahua State was 0.74 mSv y^-1.

Source: Author’s own elaboration.

Figure 3 NORM doses in urban soils of selected cities of Chihuahua State (mSv y^-1). 

DISCUSSION

The three NORM means were greater than the value by United Nations Scientific Committee of the Effects of Atomic Radiation (UNSCEAR) published (^{UNSCEAR, 2000}). Aldama had the highest values and Ciudad Juarez the lowest values. According to ^{Jabbar et al. (2010)} the abundance of potassium in the soil depends on its characteristics; artificially fertilized soil has high amounts of potassium because of its additive property from potassium in the fertilizers.

It is important to point out that Ciudad Aldama had the highest values of the three natural isotopes: 67% higher than the world average for ²³⁸U, 48% higher for ²³²Th and 174% higher for ⁴⁰K. Comparing the averages in Chihuahua to those in other countries we can see that it has higher NORM levels than Luxemburg, Hungary, Japan, Bangladesh, China, Romania and Spain, and similar values to regions like Punjab, India (^{Dołhańczuk-Śródka, 2012}; ^{Jabbar et al., 2010}; ^{Mehra et al., 2007}). Chihuahua city and Chihuahua State have been mentioned in others papers for its high radioactive levels (^{Colmenero-Sujo et al., 2004}; ^{Colmenero-Sujo & Villalba, 2010}; ^{Villalba et al., 2012}).

In Absorbed dose rate (D), it is important to point out that the value obtained is higher than the world average and higher than levels reported in the United States, Switzerland, Luxemburg, Japan, Romania, Taiwan and others countries (^{Dołhańczuk-Śródka, 2012}; ^{Jabbar et al., 2010}; ^{Mehra et al., 2007}; ^{Quindos, Fernández, Rodenas, Gómez-Arozamena & Arteche, 2004}; ^{Tzortzis, Tsertos, Christofides & Chistodoclides, 2003}). Similar studies carried out in other regions of Mexico have reported lower values than those in Chihuahua. For example, the average D level in the State of Zacatecas is 44.9 nGy h^-1 (^{Mireles et al., 2003}).

The ^{Comisión Nacional de Seguridad Nuclear y Salvaguardias (CNSNS, 2012)} of Mexico reported results in some regions of Mexico, however, the CNSNS did not refer to studies in Chihuahua State.

Some studies like Punjab, India, reported by ^{Mehra et al. (2007)} indicated that AED ranges from 0.28 mSv y^-1 to 0.64 mSv y^-1, In some papers report that Pakistan and Czech Republic average values of 0.43 mSv y^-1 and 0.44 mSv y^-1 respectively have been reported (^{Dołhańczuk-Śródka, 2012}; ^{Jabbar et al., 2010}).

The ^{ICRP (1993)} should declare to Ciudad Aldama a radioactive risk zone according at this work and others.

CONCLUSIONS

Results reported here represent the radioactivity from the geology of selected cities in the Chihuahua State. Eight cities in Chihuahua had higher levels of natural radionuclides than the world average. Nine cities had dose rates equal to or higher than national and international averages, while 12 cities had annual effective doses equal to or higher than national and international averages. These cities have 72% of the inhabitants of Chihuahua State.

We recommend that the Mexican Nuclear Safety Agency consider adopting the averages reported here as they are the results of a rigorous scientific study.