Specific activities of natural rocks and soils at quaternary intraplate volcanism north of Sana'a, Yemen.

The level of natural radioactivity in rocks and soil of 32 samples collected from locations at North Sana'a in Yemen was measured. Concentrations of radionuclides in rocks and soils samples were determined by gamma-ray spectrometer using high purity germanium (HPGe) detector with specially designed shield. The average radioactivity concentrations of (226)Ra, (232)Th, (40)K were determined and expressed in Bq/kg. The results showed that these radionuclides were present in concentrations of 21.79 ± 3.1, 19.5 ± 2.6 and 399.3 ± 16 Bq/kg, respectively, for rocks. For soil, the corresponding values were 48.2 ± 4.4, 41.7 ± 4.5 and 939.1 ± 36 Bq/kg, respectively. Also, the radiological hazard of the natural radionuclide content, radium equivalent activity, total dose rates, external hazard index and gamma activity concentration index of the (rocks/soils) samples in the area under consideration were calculated. The dose rates at 1 m above the ground from terrestrial sources were 38.39 and 86.89 nGy/h for rocks and surface soil, respectively, which present no significant health hazards to humans.

Introduction

Measurements and studies of natural radioactivity in soil and rocks are very important to determine the amount of change of the natural background activity with time as a result of any radioactive release; monitoring of any release of radioactivity to the environment is important for environmental protection.[1]

Quaternary intraplate volcanism north of Sana’a covers an area of ~800 km2; the volcanic samples can be divided into two phases on the basis of relative age: (1) sub-horizontal, plateau-forming basalt flows and (2) a number of lava flows, up to 15 km long.[2] On the basis of morphology, there is a clear distinction in relative age between the older plateau-forming flows and the large stratovolcanoes, and the considerably better preserved strombolian ash and spatter cones with their parasitic resent lava flows. This distinction is used to divide the volcanic samples into two suites, an old and young series.[3]

The aim of the present study is to evaluate health hazards to humans from the radioactivity in rocks and soil in intraplate volcanic fields of western Yemen. The details of natural radioactivity data in rocks and soil (226Ra, 232Th, and 40K) and their radiological hazard as radium equivalent activity, representative level index Ir, external hazard index Hex and dose rate for intraplate volcanic field located in western Yemen, near Sana’a, which is one of such four intraplate volcanic fields in Yemen, are presented in this study.

Experimental

Geological outline

Quaternary intraplate volcanism north of Sana’a, Yemen, covers an area of ~800 km2, located between latitude 15°30’ and 15°54’N and longitude 43°57’ and 44°15’E [Figure 1] and although laterally extensive, is only a thin carapace (<200 m) with an estimated volume of 60 ± 20 km3. This is the volumetric equivalent of about five Oligocene flood basalt flows in this region, and some two orders of magnitude smaller than the total volume of preserved flood volcanism in Yemen.[2] The volcanic field comprises a number of sub-horizontal lava flows, each 5–20 m thick (younger series region A), and in some places, Oligocene flood volcanic flows (older series region B).[3]

Figure 1

Samples’ location

Sampling and sample preparation

A total of 18 rocks and 14 surface soil samples were collected randomly from the study area. Rock sample was crushed into small pieces and ground to a powder. Soil samples were collected with the only constraint that no sampling site should be taken close to a field boundary, a road, a tree or other obstruction. Surface soils were then taken from different places randomly within the marked and cleared area from the ground surface up to 2 cm and mixed together thoroughly in order to obtain a representative sample of that area. Each sample (rock/soil) was dried in an oven at 105°C and sieved through a 100 mesh which is the optimum size enriched in heavy mineral.[4] The samples were packed in plastic containers with dimensions of 75 mm diameter and 90 mm height. The samples were weighed and stored for a minimum period of 1 month to allow daughter products to come into radioactive equilibrium with their parents 226Ra and 232Th and then were counted for 480–720 min depending on the concentration of the radionuclides.

Experimental setup

Each sample was measured with a gamma-ray spectrometer consisting of a high purity germanium (HPGe) setup and multichannel analyzer 8192 channel. The detector used has a coaxial closed end, and a closed facing window geometry with vertical dipstick (500-800 μm). The HPGe detector is of p-type with the following specifications: Resolution [full width at half maximum (FWHM)] at 122 keV 57Co is 1100 eV and at 1.33 MeV 60Co is 2.00 keV – relative efficiency at 1.33 MeV 60Co is 30%. The detector is shielded in a chamber of four layers starting with Plexiglas (10 mm thick), copper (30 mm thick) lead (100 mm thick) and finally cadmium (3 mm thick). This shield serves to reduce different background radioactivity.

The emitted X-rays from lead, which contain radioactive impurities due to antimony, can be absorbed by lining the inside of the shield with a graded layer of 0.05 inch cadmium and 0.25 inch perspex.[5] To minimize the effect of the scattered radiation from the shield, the detector was placed in the center of the chamber. Then the sample was placed over the detector for at least 10 h. The spectra were either evaluated with the computer software program Maestro (EG and G ORTEC) or manually with the use of a spread sheet (Microsoft Excel) to calculate the natural radioactivity. 226Ra activity of the samples was determined via its daughters (214Pb and 214Bi) through the intensity of the 295.22 keV, 351.93 keV for 214Pb gamma lines, and 609.31 keV, 1120 keV, 1764.49 keV for 214Bi gamma lines. 232Th activity of the sample was determined from the daughters (228Ac), (212Pb) and (208Ti) through the intensity of 209.25 keV, 338.32 keV, 911.2 keV, 968.97 keV gamma lines for (228Ac), (212Pb) emissions at 238.63 keV and (208Ti) emissions at 583.19 keV, 2614 keV gamma lines. 40K activity was determined from the emissions at 1460.7 keV gamma lines.

Calculation of the radiological effects

The most widely used radiation hazard index Raeq is called the radium equivalent activity. The radium equivalent activity is a weighted sum of activities of the 226Ra, 232Th and 40K radionuclides based on the assumption that 370 Bq/kg of 226Ra, 259 Bq/kg of 232Th and 4810 Bq/kg of 40K produce the same gamma ray dose rate.[6] Radium equivalent activity can be calculated from the following relation suggested by Beretka and Mathew:[7]

Raeq = (ATh × 1.43) + ARa + (Ak × 0.077) ………………… (1)

where ATh is the specific activity of 232Th in Bq/kg, ARais the specific activity of 226Ra in Bq/kg, and AK is the specific activity of 40K in Bq/kg.

Another radiation hazard index called the representative level index, I , is defined from the following formula:[89]

where ARa, ATh and AK have the same meaning as in Equation (1).

External hazard index due to the emitted gamma rays of the samples is calculated and examined according to the following criterion:

where ARa, ATh and AK are the activity concentrations of 226Ra, 232Th and 40K, respectively. The calculated average external hazard index was found to be less than unity.

The total air absorbed dose rate (nGy/h) due to the mean activity concentrations of 226Ra, 232Th and 40K (Bq/kg) can be calculated using the formula of Beck et al.:[110]

D = 0.462ARa + 0.604ATh + 0.042AK ………………………… (4)

where ARa, ATh and AK are the mean activity concentrations of 238U, 232Th and 40K, respectively, in Bq/kg. Beck et al. (1972) derived this equation for calculating the absorbed dose rate in air at a height of 1.0 m above the ground from measured radionuclide concentrations in environmental materials.

Results and Discussion

Quaternary intraplate volcanism north of Sana’a has a sequence ranging from Plio-Quaternary age, which is composed of older plateau-forming basalt flows (region B), to a number of recent lava flows up to 15 km long (region A).[3]

Table 1 illustrates the specific activity of the natural radionuclide (226Ra, 232Th, and 40K) in the sample rocks and soil collected from the area under investigation. The specific activities are given throughout the paper in Bq/kg dry weight. The table also lists the type of rock. The mean activity of 226Ra was found to range from 21.1 ± 4 to 33.3 ± 2.4 Bq/kg with an average value of 26.6 ± 2.9 Bq/kg in rocks (region A), and from 10.3 ± 1.3 to 22.1 ± 2.6 Bq/kg with an average value of 16.98 ± 3.4 Bq/kg (region B). The corresponding values are from 23.9 ± 4.5 to 69.3 ± 4.6 Bq/kg with an average value 48.2 ± 4.4 Bq/kg for soil.

Table 1

Activity concentrations of 226Ra, 232Th and 40K (Bq/kg) of rocks and soils samples

232Th activity concentration in rock samples ranged from 19.7 ± 1 to 30.8 ± 2.5 Bq/kg with an average value of 23.2 ± 2.8 Bq/kg (region A) and from 10.5 ± 1 to 26.9 ± 4 Bq/kg with an average value of 15.1 ± 2.5 Bq/kg (region B).

For soil, the corresponding values are from 17.5 ± 4 to 52.6 ± 5 Bq/kg with an average value of 41.7 ± 4.5 Bq/kg.

40K values ranged from 418.4 ± 15 to 573.7 ± 22 Bq/kg with an average value of 515.6 ± 18 Bq/kg in rock samples (region A) and from 144.7 ± 8.5 to 467.8 ± 19 Bq/kg with an average value of 283.8 ± 13.6 Bq/kg, whereas the corresponding values for soil are from 504.9 ± 21 to 1229.5 ± 45 Bq/kg with an average value of 939.1 ± 36 Bq/kg.

Baker et al. (1997)[2] divided the rocks into two subsets on the basis of relative age into younger and older series. The younger series includes a greater proportion of more silica-undersaturated and alkaline rocks than the older series. It is worth noting that Baker et al. (1997)[2] stated that light rare earth elements, Th, U and K, among other trace elements are generally higher in abundance in the young series compared to the old series at a specific MgO content. This is reflected by the results of the present study [Table 1] showing activity concentration averages for the younger rocks nearly twice as those for the older ones.

Natural radioactivity is present in soil and concentrations may be changed by anthropogenic activities (like the use of fertilizers) and transported from one place to another into the ground by the rain or by wind. For this reason, results show an increase of activity concentration of 226Ra, 232Th and 40K for soil rather than rocks.

The mean activity concentrations in soil for 226Ra and 232Th in the present study are 48 and 41 Bq/kg, which agree with the worldwide average concentration of these radionuclides in soils, which is 40 Bq/kg for 226Ra and 232Th as reported by united nation scientific committee on the effect of atomic radiation (UNSCEAR).[1] Although the mean activity concentration of 40K in this study is 939 Bq/kg, this value is higher than the activity concentration (370 Bq/kg) reported by UNSCEAR.[1]

Comparison of activity concentrations with similar studies

The activity concentrations of 226Ra, 232Th and 40K in rocks and soil samples from studied area were compared with those from similar investigations in other countries and summary results are given in Table 2. Bellia et al.[11] studied the natural radioactivity in volcanic island (Ustica), Italy, and the concentration of 226Ra, 232Th and 40K in the basalt rocks ranged from 15 to 164 Bq/kg, from 16 to 174 Bq/kg and from 201 to 1350 Bq/kg, respectively. Batra et al.[12] determined the natural radioactivity in geological matrices around Kaiga environment; the activity concentration in rocks ranged from 1.2 to 14.2 Bq/kg, from 0.5 to 11.5 Bq/kg and from 14.8 to 866.2 Bq/kg for 226Ra, 232Th and 40K, respectively; for soil, the corresponding values ranged from 12.8 to 42.2 Bq/kg, from 19.8 to 45.3 Bq/kg and from 135.8 to 344.6 Bq/kg for 226Ra, 232Th and 40K, respectively. El-Arabi et al.[13] studied the natural radioactivity in some powdered granite rocks; the data showed the concentration values of 226Ra, 232Th and 40K to range from 102 to 640 Bq/kg, from 56 to 161 Bq/kg and from 774 to 1234 Bq/kg, respectively. Also, they investigated variation of radium and thorium with the associated elements, and found the increase in the concentration of 226Ra and 232Th with increase in the Fe2O3, FeO, Al2O3, TiO2, SiO2, Na2O and K2O elements. El-Aydarous[14] studied the radioactivity levels and their corresponding external exposure of some soil samples from Taif, Saudi Arabia. The soil activity ranged from 13 to 33 Bq/kg, from 11 to 27 Bq/kg and from 129 to 230 Bq/kg for 226Ra, 232Th and 40K, respectively. Similar investigations in other countries are also compared with the present study (Ahmed et al.,[15] Nagdia,[16] Al-Jundi et al.,[17] Akhtar et al.,[18] Tzortzis et al.[19] and Narayana et al.[20]).

Table 2

Concentrations of 226Ra, 232Th and 40K in rocks and soils of the present work and other studies

Table 3 shows the comparison of radium equivalent Raeq, representative level index I, external hazard index H and radiation dose rate for rocks and soils from quaternary volcanism of Sana′a, Yemen, with the results in many countries (Bellia et al.,[11] Abbady,[21] Yaboah et al.,[22] Patra et al.,[12] Khaled,[23] Abdul Jabbar et al.,[24] Ziqiang et al.,[25] Malanca et al.[26] and Mustapha et al.[27] ). It can be seen that values of radiological parameter and radiation dose rate for rocks obtained from this study fall within the lowest side of all reported values from other countries except in the case of Egypt (Bir El-Sid). In contrast, values of radium equivalent Raeq, representative level index I, external hazard index Hex and radiation dose rate for soil obtained in this study matched with those of other countries.

Table 3

Radium equivalent activity, representative index I, external hazard index H and the dose rate in air of the present work and other studies

Conclusions

Rock samples and surface soil of the quaternary intraplate volcanism north of Sana’a (Yemen) were measured for their radioactivity content. The results show that the mean concentration values of 226Ra, 232Th and 40K in rocks were 26.6 ± 2.9, 23.2 ± 2.8 and 515.6 ± 18 Bq/kg, respectively, for region A, and 16.98 ± 3.4, 15.1 ± 2.5 and 283 ± 13.6 Bq/kg, respectively, for region B, while that of surface soil were 48.2 ± 4.4, 41.7 ± 4.5 and 939.1 ± 36 Bq/kg, respectively. Also, the results showed that the radioactivity of region A was higher than the radioactivity of region B. On the other hand, the radioactivity of soil samples was higher than the radioactivity of rock samples.

The mean values of radium equivalent activity (Raeq) , representative level index I, and external hazard index Hex for rock samples under investigation were 82.2 Bq/kg, 0.3 and 0.2, respectively, while that of surface soil were 180.1 Bq/kg, 0.7 and 0.5, respectively.

The results indicate that the dose rates at 1 m above the ground from terrestrial sources in all samples under investigation were 38.39 and 86.89 nGy/h for rocks and surface soil, respectively, which present no significant health hazards to humans. These values agree with the worldwide average concentrations of these radionuclides in soils reported by UNSCEAR,[1] which is in the range 24–160 nGy/h. These values present no hazards to humans.