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Received 20 July 2012; accepted 25 October 2012
Abstract
The activity concentrations and the gamma-absorbed dose rates of the terrestrial naturally occurring radio nuclides viz. 226Ra, 232Th and 40K were determined in soil samples collected from twelve oil fields and their host communities, using gamma ray spectrometry. The soil activity ranges from 10.10 to 41.23 Bq/kg for 226Ra, 7.42 to 30.31 Bq/kg for 232Th and 92.42 to 482.79 Bq/kg for 40K with mean values of 19.16, 21.26 and 224.29 Bq/kg, respectively for host community soil. In the field soil sample, the activity concentration ranges from 16.27 to 52.19 Bq/kg for 226Ra, 9.72 to 34.13 Bq/kg for 232Th and 134.50 to 395.15 Bq/kg for 40K with mean values of 29.61, 17.41 and 262.63 Bq/kg, respectively. The concentrations of these radionuclides are compared with the values of the control samples and the UNSCEAR, 2000 standards of 35.0, 30.0 and 400 Bq/kg for 226Ra, 232Th and 40K respectively. The measured activity concentration of 226Ra, 232Th and 40K in soil is lower than the world average except in two oil fields that 226Ra and 40K exceeded the world average. Radium equivalent activities are calculated from the results to assess the radiation hazards arising due to the use of these soil samples in the construction of dwellings. All the soil samples have radium equivalent activities lower than the limit set in the UNSCEAR report (370 Bq/kg). The overall mean outdoor Absorbed Dose rate are 32.17 and 35.45nGy?h?1 respectively for host community soil and field soil samples. The corresponding effective dose calculated has mean values less than 1.0 mSvy-1, the limit set by WHO (2008). The hazard indices calculated were all less than unity (1) showing that all the soil/sediment samples sampled are still safe for building purpose since their radiological impact is minimal. The percentage contribution of each of these radionuclide are; 64.77% for radium-226, 3.13% for thorium-232 and 32.10% for potassium-40. The entire radiation hazard indices are within the acceptable limit therefore, no immediate health risk as a result of these radionuclide but continuous exposure may result to a significant health impact.
Key words: Gamma ray spectrometry; Soil samples; Specific activities; Effective dose; NORM; Absorbed dose; Onelga
Avwiri, G. O., & Ononugbo, C. P. (2012). Natural Radioactivity Levels in Surface Soil of Ogba/Egbema/Ndoni Oil and Gas Fields. Energy Science and Technology, 4(2),-0. Available from http://www.cscanada.net/index.php/est/article/view/10.3968/j.est.1923847920120402.427 DOI: http://dx.doi.org/10.3968/j.est.1923847920120402.427
INTRODUCTION
Human beings have always been exposed to natural radiations arising from within and outside the earth. The exposure to ionizing radiations from natural sources occurs because of the naturally occurring radioactive materials (NORM) in the soil and rocks, cosmic rays entering the earth’s atmosphere from outer space and the internal exposure from radioactive elements through food, water and air. Natural radioactivity is wide spread in the earth’s environment and it exists in various geological formations in soil, rocks, plants, water and air (Surinder, Asha, & Rakesh, 2004). The natural radioactivity in soil comes from U and Th series and natural K. Artificial radionuclides can also be present such as 137Cs, resulting from fallout from weapons testing. NORM encountered in hydrocarbon exploration and production operations originate in subsurface formations that may contain radioactive materials such as Uranium and thorium and their daughter products, 226Ra and 228Ra. This can be brought to the surface in the surface in the produced water in conjunction with oil and gas. In addition, radon gas a radium daughter, may be found in produced natural gas. In gas processing activities, NORM generally occurs as radon gas in the natural gas stream (Ajayi et al., 2009; Mokobia et al., 2006).
During exploration and extraction processes, various operational practices contribute to or induce NORM occurrence, namely remote sensing methods of mapping and explosives associated with seismic exploration, drilling equipment and activities and down – the –hole geophysical logging methods. In some instances, radioactive marker bullets are employed as an aid in relative depth determinations. The gamma ray log is used to locate the bullets after casing has been set. Radioactive tracers are also used in evaluating the effective of well cementing and under ground water and crude oil flow direction for the purpose of correlation (Ajayi et al., 2009). In some cases, various amounts of radioisotopes are injected with the secondary recovery flooding fluids to facilitate flow. The wastes originated from these activities are released into the environment, hence an environmental management of the highest quality is needed to reduce the resultant safety problems for both the environment and population. So far under Nigerian legislation, there were no radiological controls on the operation of these industries or restrictions on how waste is discharged (to atmosphere, to landfill, to cellar pits etc) which relate to its radionuclide content. The radiological implication of these radionuclides is due to the gamma ray exposure of the body and irradiation of lung tissue from inhalation of radon and its daughters. The growing worldwide interest in natural radiation exposure has lead to extensive surveys in many countries. External gamma dose estimation due to the terrestrial sources is essential not only because it contributes considerably (0.46 mSvy?1) to the collective dose but also because of the variations of the individual doses related to this pathway. These doses vary depending upon the concentrations of the natural radio nuclides, 238U, 232Th, their daughter products and 40K, present in the soils and rocks, which in turn depend upon the local geology of each region in the world (Ajayi et al., 2009; Mokobia et al., 2006).
To evaluate the terrestrial gamma dose rate for outdoor occupation, it is very important to estimate the natural radioactivity level in soils. The natural radioactivity of soil samples is usually determined from the 226Ra, 232Th and 40K contents (OECD, 79). Since 98.5% of the radiological effects of the uranium series are produced by radium and its daughter products, the contribution from the 238U and the other 226Ra precursors are normally ignored (Zastawny et al., 1979).
In Nigeria and other countries, many studies have been carried out on the natural radioactivity matrices (Tchokossa, 2006; Ajayi et al., 2009; Diad et al., 2008; Fatima et al., 2008; Chukwuocha & Enyinna, 2010; Isinkaye & Shitta, 2010; Senthilkumar et al., 2010). It has been noted that radiation is part of the natural environment and it is estimated that approximately 80% of all human exposure comes from naturally occurring radioactive materials. Hydrocarbon exploration and production activities have the potential to increase the risk of radiation exposure to the environment and humans by concentrating the quantities of naturally occurring radiation beyond normal background levels (Ajayi et al., 2009). The knowledge of the distribution and of the behavior of the radionuclides in soil, in particular of the radium isotopes and its daughter products is important in understanding several aspects of the natural radiation environment, as the exchange of radionuclides between the soil solid matrix and surface and ground waters, the uptake of radioactive nuclides by vegetation, the exchange of radionuclides between the upper soil layers and the atmosphere. Furthermore, the knowledge of the soil radioactivity is important in evaluating the average human exposition to the natural radioactivity. EPA (2005) on environments, health and safety online stated that the more radiation dose a person receives, the greater the chance of developing cancer, leukemia, eye cataracts, Erithemia, hematological depression and incidence of chromosome aberrations. This may not appear until many years after the radiation dose is received (typically, 10-40 years). Ogba/Egbema/Ndoni local government area oil fields produce about 80% of the total crude oil and gas supply in the Niger Delta region of Rivers state. Yet none of the research works done so far has addressed the natural radioactivity in soil samples of oil fields and its health implication on the workers and the general public. This study therefore, seeks to evaluate the natural radioactivity in surface soil samples and also estimate its radiological health implication to the general public and oil/gas workers.
1. GEOLOGY OF THE AREA
Figure 1 shows the geographic location of the Ogba/Egbema/Ndoni (Onelga) oilfields as well as the location of the sampling points. The geographic location of the study area lies within latitude 5°13' N and 5°22' N and longitude 6°33' E and 6°42' North West of the Niger Delta region of Nigeria (UNDP, 2006). It is one of the onshore oil producing area of Rivers State. The area which is one of the highest oil and gas production onshore of Niger Delta has over 900 oil wells with over thirteen active oil fields and playing a host to three multinational companies (Abali, 2009). The area is criss-cross with network of pipelines carrying either oil or gas to the flow stations from the different oil wells (UNDP, 2006). Gas flaring and oil spillage due to rupture of oil/gas pipeline has been the major environmental pollutant in the area.
Ogba/Egbema/Ndoni (Onelga) has topography of flat plains netted in a web of rivers-the Niger, Sombreiro (Nkissa), Orashi and their tributaries as well as dotted creeks. The tertiary lithostratigraphic sequence of the Niger Delta consists in an ascending order of the Akata, Agbada and Benin formations respectively. With the Benin formation making up an overall clastic sequence of about 9000-12,000 m thick deposits (Ajayi et al., 2009). The paralytic Agbada formation is a sequence of alternating sandstone and shales. Major hydrocarbon accumulations are found in the intervals between the Eocene and the Pliocene. The lowest unit (the Akata formation) is a uniform marine shale unit that may contain lenses of abnormally over pressured siltstones or fine grained sandstones. Oil and gas occurrence in the Niger delta are concentrated mainly in the sand stones reservoir at various levels of the Agbada formation (Ajayi et al., 2009). Figure 1
A Sketch of the Onelga Oil Fields Showing Sampling Areas
2. EXPERIMENTAL PROCEDURE
2.1 Estimation of Natural Radioactivity Levels by Gamma Spectrometry Technique
In order to measure natural radioactivity in soil, thirty- six surface soil samples were collected from undisturbed sites at each location. After removing the stones and organic materials, the samples were dried in an oven at about 100 oC for 1-2 h to remove the moisture content and then crushed to pass through a 150 m mesh sieve to homogenize it. Then, a sample of 250 ± 0.05% was weighed and finally, a split of the prepared sample was packed in a standard plastic container (7.5 cm × 6.5 cm diam.) and after properly tightening the threatened lid, the containers were sealed with adhesive tape and left for at least 4 weeks (>7 half-lives of 222Rn and 224Ra) before counting by gamma spectrometry in order to ensure that the daughter products of 226Ra up to 210Pb and of 228Th up to 208Pb achieve equilibrium with their respective parent radionuclides. The standard sources for 226Ra and 232Th (in secular equilibrium with 228Th) have been prepared using known activity contents and mixing with the matrix material of ophthalmic acid powder. Analar grade potassium chloride (KCl) of a known amount in the same geometry has been used as the standard source for 40K. The radionuclides 226Ra, 232Th and 40K contents have been estimated using a low background gamma spectrometry system, which makes use of a gamma ray spectrometer with a 76? 76mm NaI (Ti) detector was used.
The detector is enclosed in a massive 10 cm thick lead shielding lined with 1.5 mm thick cadmium followed by 0.8mm thick copper on the inner surfaces to reduce the contribution due to fluorescence X-rays. The dimensions of the free surface within the shielding enclosure are 44 cm × 44 cm × 65 cm deep. Using different disc-type reference standard sources supplied by M/s ECIL, the gamma ray spectrometer is calibrated up to 3 MeV. The counting time for each sample was 12,000s to get a statistically small error. With appropriate corrections for laboratory background, the activity of 226Ra was evaluated, in all cases, from the 1.76 MeV peak of 214Bi, while the 232Th activity was determined from 2.62 MeV peak of 208Tl, and the 40K peak at 1.46 MeV (Surinder et al., 2004). The characteristics of the gamma ray spectrometer used aregiven in Table 1.
Table 1
Characteristics of the Gamma Ray Spectrometer Parameter For radio nuclides
2.2 Radium Equivalent Activity
The distribution of 226Ra, 232Th and 40K in soil is not uniform. Uniformity with respect to exposure to radiation (radiation hazards associated with them), has been defined in terms of radium equivalent activity (Raeq) in Bq/kg to compare the specific activity of materials containing different amounts of 226Ra, 232Th and 40K. It is calculated through the following relation:
Raeq = ARa + 1.43ATh + 0.077AK , (1)
where ARa, ATh and AK are the activity concentrations of 226Ra, 232Th and 40K in Bqkg?1, respectively (Diab et al., 2008; UNSCEAR, 2000). While defining Raeq activity according to Equation (1), it has been assumed that 370 Bq/kg of 226Ra or 259 Bq/kg of 232Th or 4810 Bq/kg of 40K produce the same gamma dose rate.
2.3 Calculation of Absorbed Dose Rates (D)
The absorbed dose rates (D) due to terrestrial gamma rays at 1m above the ground are calculated from 226Ra, 232Th and 40K concentration values in soil assuming that the other radionuclides, such as 137Cs, 90Sr and the 235U decay series can be neglected as they contribute very little to the total dose from environmental background . The conversion factors used to calculate the absorbed dose a rate is given by (UNSCEAR, 2000) as:
D = 0.462ARa + 0.621ATh + 0.0417AK. (2)
In the above conversions, it is assumed that all the decay products of 226Ra and 232Th are in radioactive equilibrium with their precursors.
2.4 Calculation of annual effective dose
To estimate the annual effective dose rates, the conversion coefficient from absorbed dose in air to effective dose (0.7SvGy-1) and outdoor occupancy factor (0.2) proposed by UNSCEAR (2000) are used. Therefore, the annual effective dose rate (mSvy-1) was calculated by the following formula (UNSCEAR, 2000):
Effective dose rate (mSvy-1) = D (nGyh-1)? 8760hy-1 ?0.7 ?(103 mSv/109 ) nGy ? 0.2 = D? 1.21 ?10-3 (mSvy-1) (3)
2.5 External and Internal Hazard Indices (Hex and Hin)
A widely used hazard index (reflecting the external exposure) called the external hazard index Hex is defined as follows:
Hex = (A Ra / 370) + (A Th / 259) +( A K / 4810) (UNSCEAR, 2000)
(4)
In addition to external hazard index, radon and its short lived products are also hazardous to the respiratory organs. The internal exposure to radon and its daughter products is quantified by the internal hazard index Hin, which is given by the equation. Hin = ARa/185 + ATh/259 AK/4810 (5)
The values of the indices (Hex, Hin) must be less than unity for the radiation hazard to be negligible.
3. EXPERIMENTAL RESULTS AND DISCUSSION
The results for the activity concentrations of natural radionuclides 226Ra, 232Th and 40K in soil samples of different locations of the oilfields and the host communities are reported in Tables 2 and 3. The ± values shown are because of the 1σ variation due to counting errors. The activity concentrations of 226Ra, 232Th and 40K in host community soil of Table 2 ranges from 10.10 ± 1.32 Bq/kg to 41.23 ± 4.60 Bq/kg , 7.42 ± 1.08 Bq/kg to 30.31 ± 1.94 Bq/kg, 92.42 ± 7.23 Bq/kg to 482.79 ± 18.25 Bq/kg respectively. Except Ebegoro and Erema communities, the obtained results for 226Ra, and 40K have lower values of activity concentrations when compared with worldwide average values of 35.0, 30.0 and 400 Bq/kg (UNSCEAR, 2000). The relative high values of 226Ra, 232Th and 40K obtained at Ebegoro and Erema community soil could be attributed to oil spillage due to pipeline explosion at Ebegoro and Erema oilfield which led to removal and replacement of old oil pipeline, scales from the oil pipeline and so on.
From Table 3, the specific gamma activity concentration of 226Ra, 232Th and 40K in field soil samples ranges from 16.27 ± 2.04 to 52.19 ± 3.62 Bq/kg, 9.72 ± 1.08 to 34.13 ± 3.92 Bq/kg and 134.50 ± 10.24 to 395.15 ± 10.54 Bq/kg respectively. Except samples from Erema, Ogbogene and Agwe oilfields, the activities concentrations of 226Ra, and 40K have lower values of activity concentrations, when compared with worldwide average values of 35.0, 30.0 and 400 Bq/kg (UNSCEAR, 2000) as shown in Figures 2, 3 and 5. However, the higher activity concentration of 226Ra and 40K in field soil at Ogbogene and Agwe could be mainly due to fallout of radionuclide during oil well development which may be rich in radium. The high activity was observed from soil samples collected around the industrial waste pits, near oil wells and around gas stack areas at Erema oil fields. Similarly the mean activity concentration of 226Ra, 232Th and 40K in host community soil is 59.69%, 79.56% and 37.44% respectively higher than the control values while in the field soil, it is 71.95%, 75.59% and 43.93% higher than the control soil samples. This could be attributed to radioactive wastes and effluent from oil and gas exploration and production activities. The overall result indicates that the activity concentration of 40K is higher than that of 226Ra and 232Th in all the host communities soil samples and field soil samples. The high activity of 40K is consistent with Ajayi et al., (2009) who noted that the activity of 40K in sedimentary rocks depends on the relative amounts of feldspar, mica and clay mineral aggregate sediment. The high activity concentration of 40K therefore could be attributed to the presence of feldspar and clay that characterize the formation of the Niger Delta. Table 5 compares the reported values of natural radionuclides in the soil samples, obtained in other countries (UNSCEAR, 2000), with those determined in the present study. On comparison, it is found that the range of 226Ra and 232Th almost match those of the other countries. However, the values of 40K are lower than that of other countries of the world. This could be attributed to the type of rock present in the geological formations of the area which could be rich in potassium and also the type of radionuclide inputs in the industrial activities in the area. With the exception of samples collected from Ebegoro oilfield, the average absorbed dose rates due to the presence of 226Ra, 232Th and 40K in the rest of the soil samples studied were found to be slightly lower than the world average value of 60 nGyh-1 but in the field soil, it is 6.54% higher than the average value of 31.1 nGyh-1 reported in soil around fertilizer factory in Egypt by Diab et al., (2008). The values obtained are comparable with 51.0 nGyh-1 reported in soil around two oilfields in Romania by Elena and Grecea (2004). The average annual effective doses are also within the worldwide average value of 0.07 mSvy-1 (UNSCEAR, 2000) except in Ebegoro oilfield. The high annual effective dose rate along Ebegoro community estimated to be 0.738 mSv which is higher than the World Health Organization maximum (WHO, 2008) safe limit of 0.1 mSvy-1 could be due to local high activity concentration of 226Ra, 232Th and 40K because of oil spillage in the area due to oil pipeline explosion. The experimental results of radium equivalent activity as shown in Tables 2 and 3 indicates that the average Raeq in host community soil and field soil samples are below the internationally acceptable value of 370 Bq/kg. The estimated internal and external hazard indices, alpha and gamma indices were all less than unity (1). This implies that activities involving the use of soil from the host community and oilfields are safe and do not attract any high level of radiation exposure.
The correlation coefficient factor (R2), showed that 226Ra contributed 65%, 232Th contributed 3% while 40K contributed 32% of the total absorbed dose of radiation. The least contribution to the total dose rate of the soil/sediment sampled is made by 232Th. In contrast to 226Ra, 232Th is highly insoluble under all geochemical conditions. This means that it would have been more difficult to mobilize 232Th than to mobilize 226Ra from the source from which they get into the soil and other environmental media even if they had occurred with the same concentration in the source material. This agrees with the NCRP (1987) that radium is more mobile than thorium, and would explain the observation that radium concentration contributed the highest radiation doses more than thorium and potassium soil sampled. The over all result showed an elevated NORM content of the hydrocarbon exploration and production oilfields which was strictly not from the geological formations of the area but might be affected by the radiological degradation and modification due to radionuclide input in daily industrial activities in the oilfields. Table 2
Specific Gamma Activity of 226Ra, 232Th and 40K and Radiation Hazard Parameters Measured in Host Community Soil Samples
Table 3
Specific Gamma Activity of 226Ra, 232Th and 40K and Radiation Hazard Parameters Measured in Oil Field Soil Samples
Oil fields Specific activities Raeq
Table 4
Mean Specific Gamma Activity of 226Ra, 232Th and 40K Together with Mean Radiation Hazard Parameters Measured in Control Samples Collected from Non-Oil Bearing Community
Activity concentration (Bq/kg) Radiation hazard parameters
CSS = Control Soil Sample
Table 5
Comparison of Natural Radioactivity Levels Measured in Soil with Those in Other Countries of the World
Country Concentration in soil (Bq/kg) Absorbed dose in air
Figure 2
Comparison of 226Ra Activity in Host Comm. Soil, and Field Soil with UNSCEAR, 2000
Figure 3
Comparison of 232Th Activity in Host Comm. Soil, and Field Soil with UNSCEAR, 2000
(a) (b)
(c)
Figure 4
(a) Contour Map of 226Ra in Soil; (b) Contour Map of 232Th in Soil; (c) Contour Map of 40K in Soil
Figure 5
Comparison of 40K Activity in Host Comm. Soil, and Field Soil with UNSCEAR, 2000
From the iso-specific map shown in Figures 4a, 4b and 4c, the activity levels were found to follow lognormal distribution showing that natural radioactivity is randomly distributed at varying concentrations in the surface soil under investigation. Some fields have elevated concentration of this radionuclide while some fields have patches of high and low concentration. Therefore, according to the Radiation Protection 112 (European Commission, 1999) report, soil from these regions is safe and can be used as a construction material without posing any significant radiological threat to population but their cumulative effects may have negative consequence on the environment.
CONCLUSION
An analytical method of determining the specific gamma radioactivity of soil samples in Ogba/Egbema/Ndoni oilfields has been carried out using NAI (TI) detector. The following conclusions were made:
● Comparatively, high value of potassium in all the samples may be due to the presence of feldspar and clay that characterizes the formations in the Niger Delta.
● The high values of 226Ra in soil samples may be due to the presence of Uranium mineralization in the Niger Delta region reported earlier by some workers (Ajayi et al., 2009; Makobia et al., 2006) and also due to exploration activities in the area. ● The measured mean activity concentrations of terrestrial gamma ray emitters are compared with the world average values. For 226Ra the concentration in host community soil and field soil are 24.52% and 8.34% respectively higher than the world average; for 232Th the activity concentration are 16.95% and 26.56% respectively lower than the world average. For 40K the activity is 28.15% and 20.73% lower than the world average.
● The results show that all the linear fit of the measured parameters were significantly further away from unity (1) which shows that the concentration of NORM were mainly influenced by the oil exploration and production activities in the area and not from the geological consituent of the area.
REFERENCES
[1] Ajayi, T. R., Torto, N., Tchokossa, P., & Akinlua, A. (2009). Natural Radioactivity and Trace Metals in Crude oils: Implication for Health. Environ Geochem Health, 31, 61-69.
[2] Bright, A. K. (2009). Oil and Gas Exploration: What ONELGA Suffers (pp. 34-43). Port Harcourt: B’ Alive publications co.
[3] Chukwuocha, E. O., & Enyinna, P. I. (2010). Radiation Monitoring of Facilities in Some Oil Wells in Bayelsa State and Rivers State, Nigeria. Scientia Africana, 9(1), 107-111.
[4] Diab, H. M., Nouh, S. A., Hamdy, A., & El-fiki, S. A. (2008). Evaluation of Natural Radioactivity in a Cultivated Area Around a Fertilizer Factory. J Nucl Radiat Phys, 3(1), 53-62.
[5] Environmental Protection Agency. (2008). Retrieved from www.epa.goo/radiation
[6] Fatima, I., Zaidi, J. H. Arif, M., Daud, M., Ahmad, S. A., & Tahir, S. N. (2008). Measurement of Natural Radioactivity and Dose Rate Gamma Radiation of the Soil of Southern Punjab, Pakistan. Radiation protection Dosimetry, 128(2), 206-212.
[7] Isinkaye, M. O., & Shitta, M. B. (2010). Natural Radionuclide Content and Radiological Assessment of Clay Soil Collected from Different Sites in Ekiti State, South Western, Nigeria. Radiation Protection Dosimetry, 139(4), 590-596.
[8] Mokobia, C. E., Edebiyi, F. M., Akpan, I., Olise, F. S., & Tchokossa, P. (2006). Radioessay of Prominent Nigerian Fossil Fuels Using Gamma and TXRF Spectroscopy. Fuel, 85, 1811-1814.
[9] Senthilkumar, B., Dhavamaani, V., Ramkumar, S., & Philominathan, P. (2010). Measurement of Gamma Radiation Levels in Soil Samples from Thanjavur Using Gamma Ray Spectrometry and Estimation of Population Exposure. Journal of Medical Physics, 35(1), 48-53. [10] Tchokossa, P. (2006). Radiological Study of Oil and Gas Producing Areas in Delta State, Nigeria (Unpublished doctoral dissertation). Obafemi Awolowo University, Nigeria.
[11] United Nations Development Programme (UNDP). (2006). Niger Delta Human Development Report: Environmental and Social Challenges in the Niger Delta. Abuja, Nigeria: UN House.
[12] United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). (1993). United Nations Sources and Effects of Atomic Radiation 1993, Report to the General Assembly with Scientific Annexes. New York: United Nations.
[13] United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). (2000). Sources and Effects of Ionizing Radiation (Report to the General Assembly). New York: United Nations.
[14] Yousef, M. I., Abu El-Ela, A., & Yousef, H. A. (2007). Natural Radioactivity Levels in Surface Soil of Kitchener Drain in the Nile Delta of Egypt. Journal of Nuclear and Radiation Physics, 2(1), 61-68.
Abstract
The activity concentrations and the gamma-absorbed dose rates of the terrestrial naturally occurring radio nuclides viz. 226Ra, 232Th and 40K were determined in soil samples collected from twelve oil fields and their host communities, using gamma ray spectrometry. The soil activity ranges from 10.10 to 41.23 Bq/kg for 226Ra, 7.42 to 30.31 Bq/kg for 232Th and 92.42 to 482.79 Bq/kg for 40K with mean values of 19.16, 21.26 and 224.29 Bq/kg, respectively for host community soil. In the field soil sample, the activity concentration ranges from 16.27 to 52.19 Bq/kg for 226Ra, 9.72 to 34.13 Bq/kg for 232Th and 134.50 to 395.15 Bq/kg for 40K with mean values of 29.61, 17.41 and 262.63 Bq/kg, respectively. The concentrations of these radionuclides are compared with the values of the control samples and the UNSCEAR, 2000 standards of 35.0, 30.0 and 400 Bq/kg for 226Ra, 232Th and 40K respectively. The measured activity concentration of 226Ra, 232Th and 40K in soil is lower than the world average except in two oil fields that 226Ra and 40K exceeded the world average. Radium equivalent activities are calculated from the results to assess the radiation hazards arising due to the use of these soil samples in the construction of dwellings. All the soil samples have radium equivalent activities lower than the limit set in the UNSCEAR report (370 Bq/kg). The overall mean outdoor Absorbed Dose rate are 32.17 and 35.45nGy?h?1 respectively for host community soil and field soil samples. The corresponding effective dose calculated has mean values less than 1.0 mSvy-1, the limit set by WHO (2008). The hazard indices calculated were all less than unity (1) showing that all the soil/sediment samples sampled are still safe for building purpose since their radiological impact is minimal. The percentage contribution of each of these radionuclide are; 64.77% for radium-226, 3.13% for thorium-232 and 32.10% for potassium-40. The entire radiation hazard indices are within the acceptable limit therefore, no immediate health risk as a result of these radionuclide but continuous exposure may result to a significant health impact.
Key words: Gamma ray spectrometry; Soil samples; Specific activities; Effective dose; NORM; Absorbed dose; Onelga
Avwiri, G. O., & Ononugbo, C. P. (2012). Natural Radioactivity Levels in Surface Soil of Ogba/Egbema/Ndoni Oil and Gas Fields. Energy Science and Technology, 4(2),
Human beings have always been exposed to natural radiations arising from within and outside the earth. The exposure to ionizing radiations from natural sources occurs because of the naturally occurring radioactive materials (NORM) in the soil and rocks, cosmic rays entering the earth’s atmosphere from outer space and the internal exposure from radioactive elements through food, water and air. Natural radioactivity is wide spread in the earth’s environment and it exists in various geological formations in soil, rocks, plants, water and air (Surinder, Asha, & Rakesh, 2004). The natural radioactivity in soil comes from U and Th series and natural K. Artificial radionuclides can also be present such as 137Cs, resulting from fallout from weapons testing. NORM encountered in hydrocarbon exploration and production operations originate in subsurface formations that may contain radioactive materials such as Uranium and thorium and their daughter products, 226Ra and 228Ra. This can be brought to the surface in the surface in the produced water in conjunction with oil and gas. In addition, radon gas a radium daughter, may be found in produced natural gas. In gas processing activities, NORM generally occurs as radon gas in the natural gas stream (Ajayi et al., 2009; Mokobia et al., 2006).
During exploration and extraction processes, various operational practices contribute to or induce NORM occurrence, namely remote sensing methods of mapping and explosives associated with seismic exploration, drilling equipment and activities and down – the –hole geophysical logging methods. In some instances, radioactive marker bullets are employed as an aid in relative depth determinations. The gamma ray log is used to locate the bullets after casing has been set. Radioactive tracers are also used in evaluating the effective of well cementing and under ground water and crude oil flow direction for the purpose of correlation (Ajayi et al., 2009). In some cases, various amounts of radioisotopes are injected with the secondary recovery flooding fluids to facilitate flow. The wastes originated from these activities are released into the environment, hence an environmental management of the highest quality is needed to reduce the resultant safety problems for both the environment and population. So far under Nigerian legislation, there were no radiological controls on the operation of these industries or restrictions on how waste is discharged (to atmosphere, to landfill, to cellar pits etc) which relate to its radionuclide content. The radiological implication of these radionuclides is due to the gamma ray exposure of the body and irradiation of lung tissue from inhalation of radon and its daughters. The growing worldwide interest in natural radiation exposure has lead to extensive surveys in many countries. External gamma dose estimation due to the terrestrial sources is essential not only because it contributes considerably (0.46 mSvy?1) to the collective dose but also because of the variations of the individual doses related to this pathway. These doses vary depending upon the concentrations of the natural radio nuclides, 238U, 232Th, their daughter products and 40K, present in the soils and rocks, which in turn depend upon the local geology of each region in the world (Ajayi et al., 2009; Mokobia et al., 2006).
To evaluate the terrestrial gamma dose rate for outdoor occupation, it is very important to estimate the natural radioactivity level in soils. The natural radioactivity of soil samples is usually determined from the 226Ra, 232Th and 40K contents (OECD, 79). Since 98.5% of the radiological effects of the uranium series are produced by radium and its daughter products, the contribution from the 238U and the other 226Ra precursors are normally ignored (Zastawny et al., 1979).
In Nigeria and other countries, many studies have been carried out on the natural radioactivity matrices (Tchokossa, 2006; Ajayi et al., 2009; Diad et al., 2008; Fatima et al., 2008; Chukwuocha & Enyinna, 2010; Isinkaye & Shitta, 2010; Senthilkumar et al., 2010). It has been noted that radiation is part of the natural environment and it is estimated that approximately 80% of all human exposure comes from naturally occurring radioactive materials. Hydrocarbon exploration and production activities have the potential to increase the risk of radiation exposure to the environment and humans by concentrating the quantities of naturally occurring radiation beyond normal background levels (Ajayi et al., 2009). The knowledge of the distribution and of the behavior of the radionuclides in soil, in particular of the radium isotopes and its daughter products is important in understanding several aspects of the natural radiation environment, as the exchange of radionuclides between the soil solid matrix and surface and ground waters, the uptake of radioactive nuclides by vegetation, the exchange of radionuclides between the upper soil layers and the atmosphere. Furthermore, the knowledge of the soil radioactivity is important in evaluating the average human exposition to the natural radioactivity. EPA (2005) on environments, health and safety online stated that the more radiation dose a person receives, the greater the chance of developing cancer, leukemia, eye cataracts, Erithemia, hematological depression and incidence of chromosome aberrations. This may not appear until many years after the radiation dose is received (typically, 10-40 years). Ogba/Egbema/Ndoni local government area oil fields produce about 80% of the total crude oil and gas supply in the Niger Delta region of Rivers state. Yet none of the research works done so far has addressed the natural radioactivity in soil samples of oil fields and its health implication on the workers and the general public. This study therefore, seeks to evaluate the natural radioactivity in surface soil samples and also estimate its radiological health implication to the general public and oil/gas workers.
1. GEOLOGY OF THE AREA
Figure 1 shows the geographic location of the Ogba/Egbema/Ndoni (Onelga) oilfields as well as the location of the sampling points. The geographic location of the study area lies within latitude 5°13' N and 5°22' N and longitude 6°33' E and 6°42' North West of the Niger Delta region of Nigeria (UNDP, 2006). It is one of the onshore oil producing area of Rivers State. The area which is one of the highest oil and gas production onshore of Niger Delta has over 900 oil wells with over thirteen active oil fields and playing a host to three multinational companies (Abali, 2009). The area is criss-cross with network of pipelines carrying either oil or gas to the flow stations from the different oil wells (UNDP, 2006). Gas flaring and oil spillage due to rupture of oil/gas pipeline has been the major environmental pollutant in the area.
Ogba/Egbema/Ndoni (Onelga) has topography of flat plains netted in a web of rivers-the Niger, Sombreiro (Nkissa), Orashi and their tributaries as well as dotted creeks. The tertiary lithostratigraphic sequence of the Niger Delta consists in an ascending order of the Akata, Agbada and Benin formations respectively. With the Benin formation making up an overall clastic sequence of about 9000-12,000 m thick deposits (Ajayi et al., 2009). The paralytic Agbada formation is a sequence of alternating sandstone and shales. Major hydrocarbon accumulations are found in the intervals between the Eocene and the Pliocene. The lowest unit (the Akata formation) is a uniform marine shale unit that may contain lenses of abnormally over pressured siltstones or fine grained sandstones. Oil and gas occurrence in the Niger delta are concentrated mainly in the sand stones reservoir at various levels of the Agbada formation (Ajayi et al., 2009). Figure 1
A Sketch of the Onelga Oil Fields Showing Sampling Areas
2. EXPERIMENTAL PROCEDURE
2.1 Estimation of Natural Radioactivity Levels by Gamma Spectrometry Technique
In order to measure natural radioactivity in soil, thirty- six surface soil samples were collected from undisturbed sites at each location. After removing the stones and organic materials, the samples were dried in an oven at about 100 oC for 1-2 h to remove the moisture content and then crushed to pass through a 150 m mesh sieve to homogenize it. Then, a sample of 250 ± 0.05% was weighed and finally, a split of the prepared sample was packed in a standard plastic container (7.5 cm × 6.5 cm diam.) and after properly tightening the threatened lid, the containers were sealed with adhesive tape and left for at least 4 weeks (>7 half-lives of 222Rn and 224Ra) before counting by gamma spectrometry in order to ensure that the daughter products of 226Ra up to 210Pb and of 228Th up to 208Pb achieve equilibrium with their respective parent radionuclides. The standard sources for 226Ra and 232Th (in secular equilibrium with 228Th) have been prepared using known activity contents and mixing with the matrix material of ophthalmic acid powder. Analar grade potassium chloride (KCl) of a known amount in the same geometry has been used as the standard source for 40K. The radionuclides 226Ra, 232Th and 40K contents have been estimated using a low background gamma spectrometry system, which makes use of a gamma ray spectrometer with a 76? 76mm NaI (Ti) detector was used.
The detector is enclosed in a massive 10 cm thick lead shielding lined with 1.5 mm thick cadmium followed by 0.8mm thick copper on the inner surfaces to reduce the contribution due to fluorescence X-rays. The dimensions of the free surface within the shielding enclosure are 44 cm × 44 cm × 65 cm deep. Using different disc-type reference standard sources supplied by M/s ECIL, the gamma ray spectrometer is calibrated up to 3 MeV. The counting time for each sample was 12,000s to get a statistically small error. With appropriate corrections for laboratory background, the activity of 226Ra was evaluated, in all cases, from the 1.76 MeV peak of 214Bi, while the 232Th activity was determined from 2.62 MeV peak of 208Tl, and the 40K peak at 1.46 MeV (Surinder et al., 2004). The characteristics of the gamma ray spectrometer used aregiven in Table 1.
Table 1
Characteristics of the Gamma Ray Spectrometer Parameter For radio nuclides
2.2 Radium Equivalent Activity
The distribution of 226Ra, 232Th and 40K in soil is not uniform. Uniformity with respect to exposure to radiation (radiation hazards associated with them), has been defined in terms of radium equivalent activity (Raeq) in Bq/kg to compare the specific activity of materials containing different amounts of 226Ra, 232Th and 40K. It is calculated through the following relation:
Raeq = ARa + 1.43ATh + 0.077AK , (1)
where ARa, ATh and AK are the activity concentrations of 226Ra, 232Th and 40K in Bqkg?1, respectively (Diab et al., 2008; UNSCEAR, 2000). While defining Raeq activity according to Equation (1), it has been assumed that 370 Bq/kg of 226Ra or 259 Bq/kg of 232Th or 4810 Bq/kg of 40K produce the same gamma dose rate.
2.3 Calculation of Absorbed Dose Rates (D)
The absorbed dose rates (D) due to terrestrial gamma rays at 1m above the ground are calculated from 226Ra, 232Th and 40K concentration values in soil assuming that the other radionuclides, such as 137Cs, 90Sr and the 235U decay series can be neglected as they contribute very little to the total dose from environmental background . The conversion factors used to calculate the absorbed dose a rate is given by (UNSCEAR, 2000) as:
D = 0.462ARa + 0.621ATh + 0.0417AK. (2)
In the above conversions, it is assumed that all the decay products of 226Ra and 232Th are in radioactive equilibrium with their precursors.
2.4 Calculation of annual effective dose
To estimate the annual effective dose rates, the conversion coefficient from absorbed dose in air to effective dose (0.7SvGy-1) and outdoor occupancy factor (0.2) proposed by UNSCEAR (2000) are used. Therefore, the annual effective dose rate (mSvy-1) was calculated by the following formula (UNSCEAR, 2000):
Effective dose rate (mSvy-1) = D (nGyh-1)? 8760hy-1 ?0.7 ?(103 mSv/109 ) nGy ? 0.2 = D? 1.21 ?10-3 (mSvy-1) (3)
2.5 External and Internal Hazard Indices (Hex and Hin)
A widely used hazard index (reflecting the external exposure) called the external hazard index Hex is defined as follows:
Hex = (A Ra / 370) + (A Th / 259) +( A K / 4810) (UNSCEAR, 2000)
(4)
In addition to external hazard index, radon and its short lived products are also hazardous to the respiratory organs. The internal exposure to radon and its daughter products is quantified by the internal hazard index Hin, which is given by the equation. Hin = ARa/185 + ATh/259 AK/4810 (5)
The values of the indices (Hex, Hin) must be less than unity for the radiation hazard to be negligible.
3. EXPERIMENTAL RESULTS AND DISCUSSION
The results for the activity concentrations of natural radionuclides 226Ra, 232Th and 40K in soil samples of different locations of the oilfields and the host communities are reported in Tables 2 and 3. The ± values shown are because of the 1σ variation due to counting errors. The activity concentrations of 226Ra, 232Th and 40K in host community soil of Table 2 ranges from 10.10 ± 1.32 Bq/kg to 41.23 ± 4.60 Bq/kg , 7.42 ± 1.08 Bq/kg to 30.31 ± 1.94 Bq/kg, 92.42 ± 7.23 Bq/kg to 482.79 ± 18.25 Bq/kg respectively. Except Ebegoro and Erema communities, the obtained results for 226Ra, and 40K have lower values of activity concentrations when compared with worldwide average values of 35.0, 30.0 and 400 Bq/kg (UNSCEAR, 2000). The relative high values of 226Ra, 232Th and 40K obtained at Ebegoro and Erema community soil could be attributed to oil spillage due to pipeline explosion at Ebegoro and Erema oilfield which led to removal and replacement of old oil pipeline, scales from the oil pipeline and so on.
From Table 3, the specific gamma activity concentration of 226Ra, 232Th and 40K in field soil samples ranges from 16.27 ± 2.04 to 52.19 ± 3.62 Bq/kg, 9.72 ± 1.08 to 34.13 ± 3.92 Bq/kg and 134.50 ± 10.24 to 395.15 ± 10.54 Bq/kg respectively. Except samples from Erema, Ogbogene and Agwe oilfields, the activities concentrations of 226Ra, and 40K have lower values of activity concentrations, when compared with worldwide average values of 35.0, 30.0 and 400 Bq/kg (UNSCEAR, 2000) as shown in Figures 2, 3 and 5. However, the higher activity concentration of 226Ra and 40K in field soil at Ogbogene and Agwe could be mainly due to fallout of radionuclide during oil well development which may be rich in radium. The high activity was observed from soil samples collected around the industrial waste pits, near oil wells and around gas stack areas at Erema oil fields. Similarly the mean activity concentration of 226Ra, 232Th and 40K in host community soil is 59.69%, 79.56% and 37.44% respectively higher than the control values while in the field soil, it is 71.95%, 75.59% and 43.93% higher than the control soil samples. This could be attributed to radioactive wastes and effluent from oil and gas exploration and production activities. The overall result indicates that the activity concentration of 40K is higher than that of 226Ra and 232Th in all the host communities soil samples and field soil samples. The high activity of 40K is consistent with Ajayi et al., (2009) who noted that the activity of 40K in sedimentary rocks depends on the relative amounts of feldspar, mica and clay mineral aggregate sediment. The high activity concentration of 40K therefore could be attributed to the presence of feldspar and clay that characterize the formation of the Niger Delta. Table 5 compares the reported values of natural radionuclides in the soil samples, obtained in other countries (UNSCEAR, 2000), with those determined in the present study. On comparison, it is found that the range of 226Ra and 232Th almost match those of the other countries. However, the values of 40K are lower than that of other countries of the world. This could be attributed to the type of rock present in the geological formations of the area which could be rich in potassium and also the type of radionuclide inputs in the industrial activities in the area. With the exception of samples collected from Ebegoro oilfield, the average absorbed dose rates due to the presence of 226Ra, 232Th and 40K in the rest of the soil samples studied were found to be slightly lower than the world average value of 60 nGyh-1 but in the field soil, it is 6.54% higher than the average value of 31.1 nGyh-1 reported in soil around fertilizer factory in Egypt by Diab et al., (2008). The values obtained are comparable with 51.0 nGyh-1 reported in soil around two oilfields in Romania by Elena and Grecea (2004). The average annual effective doses are also within the worldwide average value of 0.07 mSvy-1 (UNSCEAR, 2000) except in Ebegoro oilfield. The high annual effective dose rate along Ebegoro community estimated to be 0.738 mSv which is higher than the World Health Organization maximum (WHO, 2008) safe limit of 0.1 mSvy-1 could be due to local high activity concentration of 226Ra, 232Th and 40K because of oil spillage in the area due to oil pipeline explosion. The experimental results of radium equivalent activity as shown in Tables 2 and 3 indicates that the average Raeq in host community soil and field soil samples are below the internationally acceptable value of 370 Bq/kg. The estimated internal and external hazard indices, alpha and gamma indices were all less than unity (1). This implies that activities involving the use of soil from the host community and oilfields are safe and do not attract any high level of radiation exposure.
The correlation coefficient factor (R2), showed that 226Ra contributed 65%, 232Th contributed 3% while 40K contributed 32% of the total absorbed dose of radiation. The least contribution to the total dose rate of the soil/sediment sampled is made by 232Th. In contrast to 226Ra, 232Th is highly insoluble under all geochemical conditions. This means that it would have been more difficult to mobilize 232Th than to mobilize 226Ra from the source from which they get into the soil and other environmental media even if they had occurred with the same concentration in the source material. This agrees with the NCRP (1987) that radium is more mobile than thorium, and would explain the observation that radium concentration contributed the highest radiation doses more than thorium and potassium soil sampled. The over all result showed an elevated NORM content of the hydrocarbon exploration and production oilfields which was strictly not from the geological formations of the area but might be affected by the radiological degradation and modification due to radionuclide input in daily industrial activities in the oilfields. Table 2
Specific Gamma Activity of 226Ra, 232Th and 40K and Radiation Hazard Parameters Measured in Host Community Soil Samples
Table 3
Specific Gamma Activity of 226Ra, 232Th and 40K and Radiation Hazard Parameters Measured in Oil Field Soil Samples
Oil fields Specific activities Raeq
Table 4
Mean Specific Gamma Activity of 226Ra, 232Th and 40K Together with Mean Radiation Hazard Parameters Measured in Control Samples Collected from Non-Oil Bearing Community
Activity concentration (Bq/kg) Radiation hazard parameters
CSS = Control Soil Sample
Table 5
Comparison of Natural Radioactivity Levels Measured in Soil with Those in Other Countries of the World
Country Concentration in soil (Bq/kg) Absorbed dose in air
Figure 2
Comparison of 226Ra Activity in Host Comm. Soil, and Field Soil with UNSCEAR, 2000
Figure 3
Comparison of 232Th Activity in Host Comm. Soil, and Field Soil with UNSCEAR, 2000
(a) (b)
(c)
Figure 4
(a) Contour Map of 226Ra in Soil; (b) Contour Map of 232Th in Soil; (c) Contour Map of 40K in Soil
Figure 5
Comparison of 40K Activity in Host Comm. Soil, and Field Soil with UNSCEAR, 2000
From the iso-specific map shown in Figures 4a, 4b and 4c, the activity levels were found to follow lognormal distribution showing that natural radioactivity is randomly distributed at varying concentrations in the surface soil under investigation. Some fields have elevated concentration of this radionuclide while some fields have patches of high and low concentration. Therefore, according to the Radiation Protection 112 (European Commission, 1999) report, soil from these regions is safe and can be used as a construction material without posing any significant radiological threat to population but their cumulative effects may have negative consequence on the environment.
CONCLUSION
An analytical method of determining the specific gamma radioactivity of soil samples in Ogba/Egbema/Ndoni oilfields has been carried out using NAI (TI) detector. The following conclusions were made:
● Comparatively, high value of potassium in all the samples may be due to the presence of feldspar and clay that characterizes the formations in the Niger Delta.
● The high values of 226Ra in soil samples may be due to the presence of Uranium mineralization in the Niger Delta region reported earlier by some workers (Ajayi et al., 2009; Makobia et al., 2006) and also due to exploration activities in the area. ● The measured mean activity concentrations of terrestrial gamma ray emitters are compared with the world average values. For 226Ra the concentration in host community soil and field soil are 24.52% and 8.34% respectively higher than the world average; for 232Th the activity concentration are 16.95% and 26.56% respectively lower than the world average. For 40K the activity is 28.15% and 20.73% lower than the world average.
● The results show that all the linear fit of the measured parameters were significantly further away from unity (1) which shows that the concentration of NORM were mainly influenced by the oil exploration and production activities in the area and not from the geological consituent of the area.
REFERENCES
[1] Ajayi, T. R., Torto, N., Tchokossa, P., & Akinlua, A. (2009). Natural Radioactivity and Trace Metals in Crude oils: Implication for Health. Environ Geochem Health, 31, 61-69.
[2] Bright, A. K. (2009). Oil and Gas Exploration: What ONELGA Suffers (pp. 34-43). Port Harcourt: B’ Alive publications co.
[3] Chukwuocha, E. O., & Enyinna, P. I. (2010). Radiation Monitoring of Facilities in Some Oil Wells in Bayelsa State and Rivers State, Nigeria. Scientia Africana, 9(1), 107-111.
[4] Diab, H. M., Nouh, S. A., Hamdy, A., & El-fiki, S. A. (2008). Evaluation of Natural Radioactivity in a Cultivated Area Around a Fertilizer Factory. J Nucl Radiat Phys, 3(1), 53-62.
[5] Environmental Protection Agency. (2008). Retrieved from www.epa.goo/radiation
[6] Fatima, I., Zaidi, J. H. Arif, M., Daud, M., Ahmad, S. A., & Tahir, S. N. (2008). Measurement of Natural Radioactivity and Dose Rate Gamma Radiation of the Soil of Southern Punjab, Pakistan. Radiation protection Dosimetry, 128(2), 206-212.
[7] Isinkaye, M. O., & Shitta, M. B. (2010). Natural Radionuclide Content and Radiological Assessment of Clay Soil Collected from Different Sites in Ekiti State, South Western, Nigeria. Radiation Protection Dosimetry, 139(4), 590-596.
[8] Mokobia, C. E., Edebiyi, F. M., Akpan, I., Olise, F. S., & Tchokossa, P. (2006). Radioessay of Prominent Nigerian Fossil Fuels Using Gamma and TXRF Spectroscopy. Fuel, 85, 1811-1814.
[9] Senthilkumar, B., Dhavamaani, V., Ramkumar, S., & Philominathan, P. (2010). Measurement of Gamma Radiation Levels in Soil Samples from Thanjavur Using Gamma Ray Spectrometry and Estimation of Population Exposure. Journal of Medical Physics, 35(1), 48-53. [10] Tchokossa, P. (2006). Radiological Study of Oil and Gas Producing Areas in Delta State, Nigeria (Unpublished doctoral dissertation). Obafemi Awolowo University, Nigeria.
[11] United Nations Development Programme (UNDP). (2006). Niger Delta Human Development Report: Environmental and Social Challenges in the Niger Delta. Abuja, Nigeria: UN House.
[12] United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). (1993). United Nations Sources and Effects of Atomic Radiation 1993, Report to the General Assembly with Scientific Annexes. New York: United Nations.
[13] United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). (2000). Sources and Effects of Ionizing Radiation (Report to the General Assembly). New York: United Nations.
[14] Yousef, M. I., Abu El-Ela, A., & Yousef, H. A. (2007). Natural Radioactivity Levels in Surface Soil of Kitchener Drain in the Nile Delta of Egypt. Journal of Nuclear and Radiation Physics, 2(1), 61-68.