NORM determination in urban soils from selected cities in Chihuahua, Mexico
Determinación de NORM en suelos urbanos de ciudades seleccionadas en Chihuahua, México
ABSTRACT
Soil radioactivity affects the human body. This study determinates the 238U, 232Th and 40K activity (NORM) in 164 urban soils in 13 cities in the State of Chihuahua, Mexico. Absorbed Dose Rate and the Annual Effective Dose were also calculated. Activities were analyzed using a coaxial hyper-pure germanium detector (HPGe), Ge Model 2020, Thermo and Dewar 7500 SL (CANBERRA brand). A one-way variance analysis (ANOVA) was performed and a Tukey test was applied to identify mean differences. Activity averages were 36.8 Bq kg–1 for 238U, 41.4 Bq kg–1 for 232Th and 805.7 Bq kg–1 for 40K. Calculated absorbed dose rate ranged from 36 nGy h–1 to 83 nGy h–1 while Annual Effective Dose varied from 1.02 mSv y–1 to 0.44 mSv y–1. Results suggest that some levels exceed global averages and are among the highest levels of natural radioactivity in the world.
RESUMEN
La radiactividad del suelo afecta al cuerpo humano. Este estudio determinó la actividad del 238U, 232Th and 40K (NORM) en 164 suelos urbanos de trece ciudades del estado de Chihuahua, México. Se calculó la tasa de dosis absorbida y de dosis efectiva anual. Se usó un detector de germanio hiperpuro (HPGe), modelo Ge 2020, Termo y Dewar 7500 SL marca CANBERRA. Análisis de Varianza (ANOVA) y una prueba de Turkey fueron corridas para detectar las diferencias estadísticas. Los promedios de actividad fueron de 36.8 Bq kg–1 para 238U, 41.4 Bq kg–1 para 232Th y 805.7 Bq kg–1 para 40K. La tasa de dosis absorbida estuvo en un intervalo entre 36 nGy h–1 a 83 nGy h–1, mientras la dosis efectiva anual varió de 1.02 mSv y–1 a 0.44 mSv y–1. Los resultados muestran que algunos valores exceden los promedios mundiales y que están entre los más altos valores mundiales.
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 7th 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).
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).
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).
Figure 1. Main cities of Chihuahua State and its major uraniferous zones.
Source: Villalba et al. (2006).
Figure 1. Main cities of Chihuahua State and its major uraniferous zones.
Source: Villalba et al. (2006). Close
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
Características sociodemográficas
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.
Tabla 1.
Characteristics and samples for the selected cities.
City
North latitude
West longitude
High (mosl)*
Inhabitants**
Type of rocks***
Samples
Aldama
28 50
105 55
1270
19 378
ird
15
Bocoyna
27 51
107 35
2240
27 907
ird
5
Camargo
27 41
105 10
1220
45 852
sll
5
Creel
27 45
107 38
2345
5026
ird
8
Cuauhtémoc
28 24
106 52
2060
124 378
ird
9
Chihuahua
28 38
106 04
1440
671 790
ird
30
Delicias
28 11
105 28
1170
116 426
ird
8
Jiménez
27 08
104 55
1380
38 323
ird
10
Juárez
31 44
106 29
1140
1 218 817
sll
20
Manuel Benavides
29 06
103 54
1060
1746
sll
5
Nuevo Casas Grandes
30 25
107 54
1460
54 390
ird
12
Ojinaga
29 34
104 24
800
24 307
sll
8
Parral
26 56
105 40
1720
100 821
ird
29
Total
2 449 161
164
* 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.
Abrir
* 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. Close
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.
Statistical Analysis
A one-way Analysis of Variance (ANOVA) was performed for each variable and if statistical differences were obtained a Tukey test was used to separate means that using 0.01 as a level of significance (α = 0.01). MINITAB 15 software was used to analyze the data (Rubio & Jiménez, 2012).
Absorbed Dose rate (D) and Annual Effective Dose (AED) calculations
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).
Tabla 2.
Specific activities of 238U, 232Th and 40K (Bq kg-1) in selected cities in Chihuahua State.
City
238U
Min-Max
232Th
Min-Max
40K
Min-Max
Aldama
51.9 ± 0.3a*
44.6 - 58.2
61.1 ± 0.5a*
53.7 - 68.4
1014.5 ± 6.2a*
817.5 - 1106.3
Bocoyna
41.6 ± 0.3ab
34.0 - 47.2
48.1 ± 0.4abc
37.9 - 54.7
818.5 ± 5.8abcd
720.7 - 893.4
Camargo
34.1 ± 0.2b
30.7 - 40.1
38.0 ± 0.3c
33.1 - 44.6
827.0 ± 5.1abcd
766.4 - 909.8
Creel
35.3 ± 0.3b
29.7 - 55.1
38.5 ± 0.4c
33.3 - 55.6
663.7 ± 5.3de
552.9 - 823.4
Cuauhtémoc
37.7 ± 0.3b
32.0 - 43.3
43.9 ± 0.4bc
36.0 - 55.8
797.6 ± 5.5bcd
571.0 - 972.8
Chihuahua
39.8 ± 0.3b
28.9 - 54.6
55.2 ± 0.4ab
23.5 - 102
916.6 ± 6.0ab
653.1 - 1179.2
Delicias
36.5 ± 0.3b
31.3 - 43.6
33.4 ± 0.4c
28.5 - 38.2
772.0 ± 5.8bcde
685.6 - 848.5
Jiménez
40.4 ± 0.3b
38.1 - 45.9
42.3 ± 0.4c
38.2 - 49.5
930.5 ± 6.1ab
842.5 - 1002.8
Juárez
19.8 ± 0.2c
18.5 - 23.0
20.0 ± 0.3d
15.3 - 25.4
570.7 ± 2.9e
512.5 - 631.4
Manuel Benavides
34.8 ± 0.2b
27.3 - 41.1
32.9 ± 0.3cd
20.0 - 39.4
601.7 ± 4.3de
414.4 - 828.9
Nuevo Casas Grandes
41.1 ± 0.3b
37.7 - 44.9
55.9 ± 0.5ab
48.9 - 63.6
1013.7 ± 6.9a
954.0 - 1069.1
Ojinaga
31.8 ± 0.3b
27.6 - 34.1
32.0 ± 0.4cd
27.2 - 36.1
688.3 ± 4.7cde
577.9 - 742.2
Parral
34.8 ± 0.3b
18.3 - 63.3
36.6 ± 0.4c
10.3 - 63.1
859.0 ± 5.9abc
534.2 - 1796.8
Mean
36.8 ± 0.3
41.4 ± 0.4
805.7 ± 5.4
* Means in columns that do not share a letter are statistically different.
Source: Author’s own elaboration.
Abrir
* Means in columns that do not share a letter are statistically different.
Source: Author’s own elaboration. Close
Figure 2. Specific activities of 238U, 232Th and 40K (Bq kg-1) in selected cities in Chihuahua State.
Source: Author’s own elaboration.
Figure 2. Specific activities of 238U, 232Th and 40K (Bq kg-1) in selected cities in Chihuahua State.
Source: Author’s own elaboration. Close
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–1 and 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.
Figure 3. NORM doses in urban soils of selected cities of Chihuahua State (mSv y-1).
Source: Author’s own elaboration.
Figure 3. NORM doses in urban soils of selected cities of Chihuahua State (mSv y-1).
Source: Author’s own elaboration. Close
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).
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.