Radiological Assessment of Radionuclide Contents ... - SAGE Journals

Original Paper

Indoor and Built Environment

Indoor Built Environ 2011;20;2:246–252

Accepted: June 18, 2010

Radiological Assessment of Radionuclide Contents in Soil Waste Streams from an Oil Production Well of a Petroleum Development Company in Warri, Niger Delta, Nigeria N.N. Jibiri

C.M. Amakom

Radiation and Health Physics Research Laboratory, Department of Physics, University of Ibadan, Nigeria

Key Words Activity concentration E Gamma effective dose E Natural radionuclides E Oil production well E Radiological assessment E Soil waste stream

Abstract Activity concentrations of naturally occurring radionuclides in the soil waste stream in the sedimentation tanks of an oil and gas production well site of a major Petroleum Development Company in Nigeria have been measured using gamma-ray spectroscopy. The gamma-ray peaks observed with reliable regularity in all the samples analysed belong to naturally occurring radionuclides. Radioactive elements such as 137Cs were not detected in the samples indicating no contamination by radioactive sources in the operations at the site. The concentrations of the radionuclides in the samples ranged from below detection level to 248.2 14.0, 29.9 8.8 to 122.3 13.6 and 12.4 1.2 to

ß SAGE Publications 2010 Los Angeles, London, New Delhi and Singapore DOI: 10.1177/1420326X10378806 Accessible online at http://ibe.sagepub.com

28.6 1.9 Bq kg1 for 40K, 226Ra and 232Th respectively. The absorbed dose rates ranged from 25.3 to 73.3 nGy h1 with mean 52.6 17.9 nGy h1, while the annual outdoor effective dose varied from 31.1 to 89.9 mSv with mean 64.6 21.9 mSv. The cumulative cancer mortality and morbidity risk due to the radionuclides were 8.64 105 and 1.27 104, respectively. The results of measurements showed low values of the parameters determined, it suffices to say that the soil waste streams do not create any significant health concern for the workers and the environment and hence they can be used for building and development purposes in the area.

Introduction Naturally occurring radioactive material (NORM) deposits have been identified and recognised within the

N.N. Jibiri, P.O. Box 29709 Secretariat Post Office, Tel. þ234-08055216188, Fax þ234-02-8103043, E-Mail [email protected]

Ibadan,

Oyo

State,

Nigeria.

Legislative NORM-waste Control in Nigeria Since the beginning of the twentieth century, research and development in the field of nuclear science and technology have led to wide scale applications in research, medicine, industry and in the generation of electricity by nuclear fission. In common with certain other human activities, these practices generate waste that requires management to ensure the protection of human health and the environment now and in the future, without imposing undue burdens on future generations. Radioactive waste may also result from the processing of raw materials that contain naturally occurring radionuclides. To achieve the objective of safe radioactive waste management requires an effective and systematic approach within a legal

framework within each country in which the roles and responsibilities of all relevant parties are defined [2]. Generally, the purpose of the NORM regulation is to establish requirements for the identification of equipment contaminated with oil and gas NORM, and the disposal of oil and gas NORM waste to protect public health and safety, as well as to protect the environment. The radioactive nature of the material is a hazard commonly arising during the extraction of oil and gas, particularly from mature fields. As such it is governed by legislation requiring operators and contractors to develop awareness training and working procedures to control, protect and minimise exposure to workers. Around the world, strict controls and legislation govern the handling, storage and disposal of NORM waste. In Nigeria, the Nigerian Nuclear Regulatory Authority (NNRA) has drafted a NORM regulation that will soon be operational and whose provisions will be strictly adhered to by operators in oil and gas industries in Nigeria. Over the years, there has been no clear method or monitoring of disposal of radioactive waste (NORM) streams from exploration activities in the country. The NNRA regulations require a specific licence to perform decontamination work. Any person who possesses NORM above the exemption levels is subject to a general licence requirement under the NNRA regulations. NORM have the characteristic that, owing to their ubiquitous nature, they give rise to a very much larger radiological effect on the public than that caused by the nuclear industry and other anthropogenic sources of radiation [3]. Operators and contractors alike have a duty to determine whether or not they possess NORM above the exemption levels. This position hinges on the effectiveness, enforcement and compliance with the provisions of the regulations and the probity of both the operators and contractors. Oil and gas industry in the Niger Delta is a multifaceted industry that includes the construction, exploration, production, downstream and marketing sector. A large percentage of these sectors use radioactive materials. The use of radioactive materials in both the onshore and offshore oil and gas industries include industrial radiography, nuclear gauges, well logging, radio-tracers, mapping and evaluation of geological formations and the extraction of other natural hydrocarbon resources [4]. According to United State Environmental Protection Agency studies in the oil and gas industries have shown that petroleum pipe line scale originating from oil production has very high 226Ra concentration and on disposal exposes the environment to associated radioactive contamination [1,5]. Ughelli and its environs are major oil

Radionuclides from an Oil Production Well

Indoor Built Environ 2011;20:246–252

oil and gas exploration and production industry, for a number of decades. NORM is also known as low specific activity scale (LSA). Equipment can be cleaned of scale, sludge or other forms of NORM and the radioactive waste generated should be properly disposed. NORM may also be present in produced water, condensates and radon gas, radium daughters may also be found in produced natural gas. Naturally occurring radioactive material (NORM) encountered in oil and gas exploration, development and production operations originates in subsurface formations, which may contain radioactive materials, such as uranium and thorium and their daughter products, 226Ra and 228Ra. NORM can be brought to the surface in the formation water produced in conjunction with oil and gas [1]. As the NORM levels are typically so low, NORM in produced waters and natural gas is not a problem unless it becomes concentrated in some manner. Through temperature and pressure changes that occur in the course of oil and gas production operations, 226Ra and 228 Ra found in produced waters may co-precipitate with barium sulphate scale in well tubular and surface equipment. Concentrations of 226Ra and 228Ra may also occur in sludge that accumulates in oilfield pits and tanks. These solids become sources of oil and gas NORM waste. In gas processing activities, NORM generally occurs as radon as in the natural gas stream. Workers employed in the area of cutting and reaming oilfield pipe, removing solids from tanks and pits, and refurbishing gas-processing equipment may be exposed to particles containing levels of alpha-emitting radionuclides that could pose health risks if inhaled or ingested. However, these radioactivity levels can only be determined if waste streams from oil and gas activities are suspected to have a possible problem and evaluated for such hazards. This is one of the focal points of this present study.

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producing communities in the Niger Delta region of Nigeria. The areas are crisscrossed with a network of pipelines carrying either oil or gas to the follow stations from oil wells in many locations. Oil and gas operations in the country have generated a lot of health concerns regarding both the population and the environment and as such have attracted a lot of research interest in crude and fuel [6–9]. In our earlier study [10], the soil radionuclide concentrations and radiological assessments in and around a refining and petrochemical company in Warri, NigerDelta area of Nigeria was carried out with a view to determining the effect of the refinery and refining operations on the natural radioactivity of the area which over the years had generated uncertainty on health and fear of radiation exposure on the operations of the petroleum industries in the Niger Delta of the country. Further to which, the elevated levels of 226Ra in petroleum production scales in pipeline and tanks which form soil waste streams are disposed of to the environment. The use of this waste stream for building purposes may be a source of elevated indoor 222Rn, a major source of lung-related cancer from natural sources [2,3]. The chances of the use of these wastes for building purposes by the local population in the area are quite high because of ignorance and lack of available land for development. This present study was therefore considered imperative to complement the effort of our previous research with a view to:

Table 1. The activity concentrations of the radionuclides in the soil waste streams at site.

1. determining the level of radioactivity in soil waste samples from a production oil well; 2. estimate the annual effective dose to the workers; 3. assess the radiological risk associated with the soil wastes; 4. evaluate and ascertain radiometric data indicative of the radiation exposure status of the refining and onshore oil production activities with respect to safety of humans and environment in the area; and 5. determine the suitability of the soil waste steams for building purpose and developmental initiative of the area in the future by the host communities.

on-shore in Ughelli town in Warri, Delta State geopolitically located in the southsouth region of Nigeria. The soil waste stream collected for analysis was from the tank 3 at the UPS oil drilling site which was used for sedimentation of the crude oil from the production well. The tank is cleared of the sediments after the crude oil has been pumped out. The cleared sediments are usually packed in sack containers or sand-bags for disposal. The sediments are usually a mixture of crude oil, sludge, mud, sand and slime. A total of nine samples were collected from the UPS site representative of the soil waste at the site. Three samples were collected from already packed sediments for disposal; another three samples were collected from the mouth 1 sediment clearance of the tank 3 and another three samples from the mouth 2 sediment clearance of the tank. The samples were then given codes to maintain their anonymity as sample A to I (Tables 1 and 2). About 2 kg of the samples was collected at the sampling locations. The collected samples were thereafter transferred to the laboratory for preparation. At the laboratory, the samples were oven dried at 1008C to a constant weight, then

Materials Sample Collection and Preparation The site for the present study belonged to the major Oil and Gas Multinational Petroleum Development Company in the country which had been in existence and operated in the Niger Delta region of the country for more than two decades. The site is located 5o300 45.1300 N, 5o560 12.0200 E

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Sample

40

K (Bq kg1)

226

Ra (Bq kg1)

232

Th (Bq kg1)

A B C D E F G H I

BDL 157.8 9.5 38.6 2.7 159.5 10.0 113.1 7.5 170.7 11.0 96.5 6.7 248.2 14.0 44.0 3.2

122.3 13.6 43.7 11.7 114.1 12.8 72.9 11.0 36.0 9.0 98.6 12.2 29.9 8.8 87.6 12.4 75.5 12.6

27.3 1.8 15.0 1.2 22.6 1.7 17.5 1.4 18.3 1.4 24.3 1.8 12.4 1.2 28.6 1.9 25.7 1.9

Mean

128.6 70.1

75.6 33.6

21.3 5.7

Table 2. The total absorbed dose rates and the annual outdoor effective dose rates. Samples

A B C D E F G H I Mean

Absorbed dose rate (nGy h1)

Effective dose rate (mSv y1)

73.3 35.8 66.8 50.9 32.4 67.3 25.3 68.1 52.2 52.6 17.9

89.9 44.0 83.4 62.5 39.8 82.7 31.1 83.6 64.1 64.6 21.9

Jibiri and Amakom

crushed and sieved to pass a 2 mm mesh. Owing to the limited space of the detector shield only 200 g of the soil samples (dry weight) were used for analysis. The samples after weighing were transferred to radon-impermeable cylindrical plastic containers of uniform size (60 mm height 65 mm diameter) and were sealed for a period of about 30 days. This was done in order to allow for Radon and its short-lived progenies to reach secular radioactive equilibrium prior to gamma spectroscopy. The reference soil was also transferred to a container of the same material and dimensions as were used for the soil samples. This is to ensure that the geometry configuration remained the same. The standard reference soil sample used was prepared from Rocketdyne Laboratories California; USA which is traceable to a mixed standard gamma source (Ref. No. 48722-356) by Analytic Inc. Atlanta, Georgia.

Methods The method employed for the measurements of the radioactivity in the samples was gamma-ray spectroscopy and the standard procedures of this method as described in the literature were followed [11–14]. These were also employed in our previous publications [15–17]. The detector used for the radioactivity measurements was a lead-shielded 76 mm 76 mm NaI (TI) detector crystal (Model No. 802 series, Canberra Inc.) coupled to a Canberra Series 10 plus Multichannel Analyzer (MCA) (Model No.1104) through a preamplifier. It has a resolution full width at half maximum (FWHM) of about 8% at an energy of 0.662 MeV (137Cs), which is considered adequate to distinguish the gamma ray energies of interest in the present study. The choice of radionuclides to be detected was predicated on the fact that the NaI (Tl) detector used in this study had a modest energy resolution. Hence the photons emitted by them would only be sufficiently discriminated if their emission probability and their energy were high enough, and the surrounding background continuum low enough. Therefore, the activity concentration of 214Bi (determined from its 1.760 MeV -ray peak) was chosen to provide an estimate of 226 Ra (238U) in the samples, while that of the daughter radionuclide 208Tl (determined from its 2.615 MeV -ray peak) was chosen as an indicator of 232Th. Potassium-40 was determined by measuring the 1.460 MeV -rays emitted during its decay. The samples were placed symmetrically on top of the detector and measured for a period of 10 h. The net area under the corresponding peaks in the energy spectrum was computed by subtracting


counts due to Compton scattering of higher peaks and other background sources from the total area of the peaks. From the net area, the activity concentrations in the samples were obtained using the methods of Mokobia et al. [9] and Jibiri and Emelue [10]. ð1Þ C Bq kg1 ¼ kCn , where k ¼ "P1Ms , C is the activity concentration of the radionuclide in the sample given in Bq kg1, Cn is the count rate under the corresponding peak, " is the detector efficiency at the specific -ray energy, P is the absolute transition probability of the specific -ray, and Ms is the mass of the sample (kg). The detailed calibration procedures of the detector can be found in our publications [15–17]. The detection limit of a measuring system describes its operating capability without the influence of the sample. The detection limit (DL) given in Bq kg1, which is required to estimate the minimum detectable activity in a sample, was obtained: pffiffiffiffiffiffi Cb k, ð2Þ D L Bq kg1 ¼ 4:65 tb where Cb is the net background count in the corresponding peak, tb is the background counting time (s), k is the factor that converts cps (counts per second) to activity concentration (Bq kg1) as given in Equation (1). With the measurement system used in the present work, detection limits obtained were 17.2 Bq kg1, 4.2 Bq kg1 and 5.1 Bq kg1 for 40K, 226Ra and 228Th, respectively. Values below these numbers were taken in this work as being below the detection limit (BDL) of the detector.

Results Activity Concentrations Using Equation (1), the activity concentrations of the radionuclides in the samples were calculated. The results of activity concentration of the natural radionuclides in the samples are presented in Table 1. External Gamma Absorbed Dose Rates The external absorbed dose rate, D (nGy h1) in air at 1 m above the ground level for soils containing the concentrations of the radionuclides measured in the samples was calculated using the equation [3]: X AR DCext,R ð3Þ Dext ¼ R


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where DCext,R is the coefficient of dose rate per unit activity concentration of radionuclide R (nGy h1/ Bq kg1) and AR is the concentration of the radionuclide R in the sample (Bq kg1). UNSCEAR (2000) prescribes DCext,R coefficient of 226Ra as 4.27 1010 Gy h1/ Bq kg1, 232Th as 6.62 1010 Gy h1/Bq kg1), 40K 137 as 0.43 1010 Gy h1/Bq kg1 and Cs as 10 1 1 0.30 10 Gy h /Bq kg ). Since Cesium -137 was not detected in any of the samples the last term in Equation (3) was taken as zero. Using Equation (2) and the activity concentrations of the radionuclides in Table 1, the absorbed dose rates were calculated. The results are presented in Table 2.

Table 3. Mortality and morbidity risk coefficients (r) for external exposure from soil at infinite depth (EPA 1999). Nuclide

Mortality (kg Bq s1)

Morbidity (kg Bq s1)

4.66 1016 1.97 1019 1.33 1017

6.83 1016 2.93 1019 1.96 1017

40

K Th 226 Ra 232

Table 4. The average lifetime cancer mortality and morbidity risk due to the radionuclides. Nuclide 40

K Th 226 Ra Cumulative risk 232

Outdoor Gamma Effective Dose Rates The absorbed -dose rates in air are usually related to human absorbed g-dose in order to assess radiological implications. In assessing the outdoor effective dose equivalent to members of the population, two important factors were considered. The first is a factor that converts the absorbed dose rates (Gy h1) in air to human outdoor effective dose rates (Sv y1) while the second factor gives the proportions of the total time for which the typical individual is exposed to outdoor or indoor radiation. The United Nation Scientific Committee on the effect of Atomic Radiation [3] has recommended 0.7 Sv Gy1 as the value of the first factor and 0.2 and 0.8 as the outdoor and indoor occupancy factors, respectively. This second factor implies that the average individual spends only 4.8 h (about 5 h per day) outdoors. In this work, only outdoor exposure from -ray sources due to the concentrations of the primordial radionuclides in the soil samples were considered. The effective dose rate resulting from the absorbed dose rate values was calculated using the following relation: Eair ¼ TfQDair "

ð4Þ

where Eair is the effective dose rate (mSv y1), T is time being 8766 h y1, f is the outdoor occupancy factor that corrects for the average time spent outdoors (0.2), Q is the quotient of the effective dose rate and absorbed dose rate in air (0.7 Sv Gy1), " is a factor converting nano (109) into micro (106) and Dair is the absorbed dose rate in air (nGy h1).The values obtained are also presented in Table 2. Cancer Mortality and Morbidity Risk Assessment The cancer risk associated with radionuclide external exposure was calculated as the product of the appropriate cancer risk coefficient and the corresponding radionuclide

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Mortality risk

Morbidity risk

8.60 105 2.14 108 4.06 107 8.64 105

1.26 104 3.18 108 5.99 107 1.27 104

exposure. In other words, radiation risk follows a linear, no threshold (LNT) model. The risk coefficients were obtained from the EPA Report [18] on cancer risk coefficients for environmental exposures to radionuclides. The risk coefficients are expressed as risk of cancer mortality or morbidity per unit integrated exposure to radionuclide for soil contaminated to an infinite depth (kg Bq s1). The lifetime cancer risk R for exposure to a given radionuclide can be calculated using: R ¼ r X,

ð5Þ

where r is the risk coefficient of a particular radionuclide and X is the time integrated activity concentration of the radionuclide in the exposure scenario. The time integrated activity concentrations were calculated using 45.5 years life expectancy at birth in Nigeria WHO (2008), 3.15 107 s in a year and the activity concentration of each radionuclide. The risk coefficients for the radionuclides are presented in Table 3. The average lifetime cancer mortality and morbidity calculated are presented in Table 4.

Discussion As can be seen from Table 1, the concentration of 40K in the samples ranged from BDL to 248.2 Bq kg1, 226Ra from 29.9 to 122.3 Bq kg1 while for 232Th it ranged from 12.4 to 28.6 Bq kg1. The average concentrations of 40 K, 226Ra and 232Th were 128.6 70.1 Bq kg1, 75.6 33.6 Bq kg1 and 21.3 5.7 Bq kg1, respectively. The average concentrations of 40K and 232Th were below

Jibiri and Amakom

the world average values while 226Ra average concentration was found to be slightly higher (420 Bq kg1 for 40K, 33 Bq kg1 for 226Ra and 45 Bq kg1 for 232Th) [3]. The relatively higher concentrations of 226Ra in the samples when compared to other radionuclides is expected as oil and gas activities are known to accumulate this natural radionuclide in higher concentrations [1,4,5]. Generally, the activity concentrations of the radionuclides in the samples indicate only the presence of natural radioactive elements since no artificial radioactive elements such as 137 Cs were detected. This is an indication that suggests that the operations of the Development Company at the site have not brought any artificial sources to the environment. This may also provide a broad base assessment of the company’s safe operations and procedures in the use of radioactive sources at the operational site. Furthermore, the concentrations of the natural radionuclides in the samples only showed marginal difference from the previous work carried out in and around and the environment of a refinery in Warri [10], those reported for the environment of Warri [15,17] and that determined for the oil producing coastal areas of Niger Delta region [4]. From Table 2, the range of the total gamma absorbed dose rates due to the radionuclide concentrations in the samples ranged from 25.3 nGy h1 to 73.3 nGy h1 with a mean 52.6 17.9 nGy h1 while the annual outdoor effective dose rate based on the soil waste stream samples from the site from 31.1 to 89.9 mSv with mean and standard deviation of 64.6 21.9 mSv. The mean value obtained is lower than world average value of outdoor exposure of 70 mSv and also less than 1 mSv annual dose limit for members of the public prescribed by UNSCEAR [3]. Furthermore, the results are relatively higher as would be expected for such a petroleum on-shore drill site when compared with the previous study on the normal and undisturbed soil. The average outdoor effective dose rate based on the soil samples from a refinery premises in Warri was determined as 35.2 mSv y1, whereas for the communities around it ranged between 5.8 mSv y1 and 47.5 mSv y1 [10] and compares well with the values obtained in the area of 37 mSv y1 [15] and 25.8 mSv y1[17]. The internal and external hazard indices for the samples is expected to be much lower than unity and as such the soil waste streams from the oil production well are safe and can be used as a construction material without undue health or any

significant radiological threat to the population and the environment. This is true because the result of the range of absorbed dose rates and the corresponding annual effective dose rates in this study is much lower when compared with those of Mehra [14] (0.11 mSv) for undisturbed soil and 0.24–0.54 mSv obtained in areas of high background radiation in the northern part of Nigeria where the radiation doses were considered safe [16,19,20]. In addition, the results of the effective dose of the soil waste streams in this study are lower than that obtained for the environmental outdoor effective dose (0.34mSv y1) of the site earlier determined by Avwiri et al. [5]. Furthermore, the mortality and morbidity factors as can be seen from Tables 3 and 4 are quite negligible and as a result present no health concerns to the workers and the environment. These results therefore suggest that the soil waste streams can be used for building purposes while the land can be used for redevelopment initiatives in future for the host communities.



Conclusion The gamma radiation peaks identifying the radionuclide contents in the soil waste streams from the oil production well showed regularity of natural radionuclides due to 40K, 226Ra (238U) and 232Th. The natural radioactivity levels of the soil waste streams were found to be normal and the outdoor effective dose rates were less than 1 mSv y1 recommended annual dose limits for the members of the public. In consequence, it may be used safely for building constructions if need be. The cumulative average lifetime cancer risks due to the radionuclides were lower than the 1: 10,000 levels for contribution from a single source activity concentration. It suffices to say, therefore, that these values are low, and do not imply any significant concerns about health effects to the workers and the environment generally. However, if soil waste streams from oil and gas activities exceed the clearance level, they may require proper disposal methods applying best practices with respect to the regulatory requirements of the Nigerian Nuclear Regulatory Authority (NNRA), which should be adopted. Though the present study suggests safe use of the land for redevelopment and the soil waste streams for building construction, there is a need for continuous monitoring to keep that radiation levels under review.

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