Spatial Distribution ofNatural Radionuclides in Soil, Sediment and Waters in Oil Producing Areas inNiger Delta Region of Nigeria
- Avwiri, G.O.1,2Agbalagba, E.O.,1Ononugbo, C.P.
1Department of Physics, University of Port Harcourt, Choba, Rivers State, Nigeria.
2Department of Physics, Federal University of Petroleum Resources, Effurun, Nigeria
Abstract: Activityconcentrations of natural radionuclides (226Ra, 232Th and 40K) in the soil, sediment and water of oil producing communities in Delta and Rivers States were determined using γ-ray spectrometry. The mean soil/sediment activity concentration of 226Ra, 232Th and 40K in onshore west in Delta state is 40.2±5.1Bqkg-1,29.9±4.2Bqkg-1 and 361.5±20.0Bqkg-1respectively, the corresponding values obtainedin onshore east1 of Rivers state is 20.9±2.8Bqkg-1, 19.4±2.5Bqkg-1and 260.0±14.1Bqkg-1 respectively. While the mean activity concentration of 226Ra, 232Th and 40K in onshore east2 of Rivers state is 29.3±3.5Bqkg-1,21.6±2.6Bqkg-1 and 262.1±14.6Bqkg-1 respectively. These values obtained show enhanced NORMs but are well within the world range. All theradiation hazard indices examined in soilhave mean values lower than their maximum permissible limits. In drinking water, the obtained average values of226Ra, 228Ra and 40K is 8.4±0.9, 7.3±0.7 and 29.9±2.2Bql-1 respectively for well water, 4.5±0.6, 5.1±0.4 and 20.9±2.0Bql-1 respectively for borehole water and 11.3±1.2, 8.5±0.7 and 32.4±3.7Bql-1 respectively for river water in onshore west. For onshore east1, average activity concentration of 226Ra, 228Ra and 40K is 8.3±1.0, 8.6±1.1 and 39.6±3.3Bql-1 respectively for well water, 3.8±0.8, 4.9±0.6 and 35.7±4.1Bql-1 respectively for borehole water and 5.5±0.8, 5.4±0.7 and 36.9±3.8Bql-1 respectively for river water. While in onshore east2 average value of 226Ra, 228Ra and 40K is 10.1±1.1, 8.3±1.0 and 50.0±3.9Bql-1 respectively for well water, 4.7±0.9, 4.0±0.4 and 28.8±3.0Bql-1 respectively for borehole water and 7.7±0.9, 6.1±0.8 and 27.1±2.9Bql-1 respectively for river water and the average activity concentrations in the produced water226Ra, 228Ra and 40K is 5.182.14Bql-1, 6.042.48Bql-1 and 48.7813.67Bql-1 respectively. These values obtained are well above world average values of 1.0, 0.1 and 10Bql-1 for 226Ra, 228Ra and 40K respectively, those of the control site values and most reported values around the world. Though the hazard indices (Raeq, Hex, Hin) examined in water is still within tolerable level, the committed effective dose estimated are above ICPR 0.1 mSvy-1 permissible limits. The overall results show that soil and sediment in the area are safe radiologically but the result indicates some level of water pollution in the studied area.
Keywords: Radioactivity, soil, sediment and water, Niger Delta, Gamma detector
Corresponding author: ( +2348033137674)
1.0Introduction
The knowledge of radionuclide distribution and radiation levels in the environment is an important tool for assessing the effects of radiation exposure due to both terrestrial and cosmogenic sources. Natural sources of radiation constitute almost 80% of the collective radiation exposure of the World’s population (UNSCEAR, 2000), while the other 20% comes from industrial, medical, weapon testing, research and other related human activities. Oncethese radionuclides are present in the environment, whether natural or man-made, they are available for uptake by plants and animals and find their way into the human body through the food chain. Terrestrial background radiation represents the main external source of irradiation of the human body. Human beings are exposed also naturally from sources outside their bodies; mainly cosmic rays and gamma ray emitters in soil, building materials, water, food and air (Alaamer, 2008).
The natural terrestrial gamma dose is an important contributor to the average dose received by the world’s population (Tso and Leung, 2000; Senthilkumar et al., 2010). These naturally occurring radionuclides are present at varying concentrations in the Earth’s crust and their concentration on the earth’s surface (soil) can be enhanced by processes associated with the exploitation and recovery of crude oil and gas. On the other hand,several naturally occurring alpha and beta emitting radionuclides such as 238U, 226Ra, 222Ra, 216Pb, 228Ra are frequently dissolved in domestic water supplies and their concentration vary over an extremely wide range. Their concentration depends on the amount of radioelements present in bedrock and soil with which the water comes in contact (Malanca et al., 1998). Butwater is an important parameter of environmental science since it is indispensable to human life. Thus, at the end of the year 2000 United Nations Millennium Summit, member states of which Nigeria is one adopted a set of eight (8) goals and related targets and indicators aimed at helping to end human poverty (Sachs and McArthur, 2005).Among these Goals is a call to halve by the year 2015 the proportion of persons without sustainable access to safe drinking water and basic sanitation in developing nations.Towards the end of March 2005, the UN launched the “International Decade for Action: Water for Life 2005- 2015” (UN, 2005; Bartram et al., 2005). It is hoped that success in reaching these targets will help achieve the other goals. But the occurrence of natural radionuclides in drinking water poses a serious challenge of meeting the UN target of access to safe drinking water in developing nations and the health hazard when these radionuclides are taken to the body by ingestion (Damla et al., 2006). In addition, human activities such as mining, milling and processing of uranium ores and mineral sands, manufacture of fertilizers and oil drilling activities have enhanced the naturally occurring radioactive material concentrations in the environment (Pujol and Sanchez-Cabeza, 2000). During oil exploitation, large amounts of water accompany the production stream. This water is usually referred to as produced water, formation water or oilfield brine (Neff, 2002).Despite treatment before discharge to satisfy regulatory standards, produced water contains a certain amount of Naturally Occurring Radioactive Materials (NORM) such as 226Ra and228Ra and they are difficult to remove from produced water.
The advent of the oil mineral exploitation and exploration activities have resulted in increased pollution of the Niger Delta environment. Some of such pollution problems are increased presence of potentially toxic metals in soil and water bodies of the area and increase in human exposure to ionizing radiation. Studies have shown that almost all the elements in the periodic table including heavy metals are found in the crude oil matrix (Abison, 2000; Avwiri et al., 2007). Thus, the release of gas through flaring, oil spillage on land and water bodies and its derivative may have serious radiological and hazardous effects on man and direct impact on the soil and water (Aroganjo et al., 2004). A study of the ambient air pollutants in the region by Oluwole indicates that the Federal Environmental Standards were grossly exceeded (Otarigho, 2007), while a study of environmental impacts of drilling mud and wastes from onshore oil wells in the region also show a relatively high values (Gbedebo et al., 2010).
The contamination and pollution of soil, sediment and water have been of great concern to many researchers in this region and beyond. This is because if contamination or pollution occurs, damage may be extensive and the effects may be long term. These perceived consequences of drinking water in the region, the consumption of foodstuff and vegetables from these polluted soil and the attendant radiological burden has triggered various environmental studies especially in the Niger Delta region on soil equality, water aquifer quality and aquatic ecosystem (Akpa and Offen, 1993; Udom et al., 1999; Ekpete, 2002; Oguzie et al., 2007, Egila and Terhemen, 2004; Abam et al., 2007; Nwala et al., 2007; Taiwo and Tse, 2009; Tchokossa et al., 2011). Although radioactivity study has been previously carried out in soil, sediment and water samples in a part of the region (Agbalagba and Onoja, 2011), the work covers a very small area (Biseni flood plain ake) of the region, with none to the knowledge of the researchers being dedicated to a wider scope as this paper seek to achieve.
Estimation of the radiation dose distribution is vital in assessing the health risk to a populationand serves as a reference for documenting changes in environmental radioactivity due to natural and anthropogenic activities (Obed et al., 2005). This present study assessed the specific radioactivity and examined some of the radiation hazard indices of these naturally occurring radionuclides (226Ra, 232Th and 40K) in soil and sediment within the oil rich communities in Delta and Rivers States. Moreso, the essence of evaluating specific activity in drinking and produced water is to ensure that the WHO, 2008 permissible level of 1.0, 0.1 and 10 Bql-1for 226Ra, 228Ra and 40K respectively and reference dose level (RDL) of committed effective dose of 0.1mSvy-1 in water is not exceeded in any studied water sample. This study will help to ascertain the safety of produced water, surface and ground water for drinking as well as soil and sedimentin the oil producing communities. The data generated in this study will add to the world data-base of naturally occurring radionuclides in oil and gas producing environments and will be useful for authorities in charge of environmental monitoring like the Niger Delta Environmental Monitor for implementation of radiation protection standards for the oil sector and the general public.
2.0 Materials and Methods
2.1 Description of Study Area
Oil and gas exploration and exploitation activity in the Niger Delta which dates back to 1958 is classified into three zones; the onshore (lands), swamp (riverine area) and offshore (deep sea operation). This delineation is now a recognized easy means of classifying oil field locations and communities within the region. The studied oil producing communities are within the onshore West in Delta state and onshore East 1 and onshore East 2in Rivers State all in the Niger delta region of Nigeria. The oil activities in the areas are operated by Shell Petroleum Development Company (SPDC), Nigerian Agip Oil Company (NAOC) and Total Final Elf respectively (NNPC, 2009). The area also lies within latitude 5013”N and 5068”N and longitude 5033”E and 6042”E in Niger Delta region of Nigeria as shown in figure1.The geology and hydrogeology of the study area have been reported elsewhere (Ajayi et al., 2009; Taiwo and Tse, 2009).
2.2 Sample Collection and Preparation Techniques
One hundred and twenty- six samples were collected and analyzed; three samples of soil and sediment were collected in an oil field in each of the oil producing communities and one soil sample was collected as a control from a non-oil bearing community to have forty- five samples of soil/sediment. Fifty-four domestic water samples were collected in eighteen oil producing communities comprising river/stream water and community public water supply (borehole and well water) and one sample each from the three sources of drinking water ( river water, borehole water and well water) from non-oil bearing community as control samples. Twenty-one produced water samples were also collected from flow stations for analysis.
The bulk soil (stones, vegetations and organic debris removed) samples (1kg) were collected in undisturbed, uncultivated grass covered areas within the oil producing communities (Senthilkumar et al., 2010; Siegesmund et al.,2011). Each soil sample was collected in a black polythene bag within a depth of 0 to 15cm which represents the soil depth variation with the purpose of the soil permeability to particle settlement depth. Sediment samples were collected at the bottom of river. The samples were collected using a steel hand geological auger, which was cleaned with acid, detergent and rinsed with deionized water. Samples were collected in new aluminum foil labeled and placed in black polythene bags. The soil samples were dried by spreading them on polyethylene sheets at room temperature for 1 week under a controlled environment to avoid local dust contamination. Then, the soil samples were dried in oven for 24hours at 1100 C. They were sieved and the selected particle had less than 2 lm diameter. 500 g of each sample was put in marinelli beaker. The marinelli beaker was sealed with aluminum foil and liquid paraffin. This sample preparation procedures have been reported elsewhere (Zarie and Al-Mugren, 2010 and Agbalagba and Onoja, 2011).
For water samples collection, 1.5-liters plastic container was used for the collection of the water sample from source with about 1% air space of the container left for thermal expansion. Sample containers were raised three times with sample water being collected to minimize contamination from the original content of sample container. Well water samples were collected at the early hours of the day from community hand dug wells of varying depths (5-10m). The water samples were collected directly from the wells using manual procedure, which involved the dipping of a clean container firmly tied to a rope long enough to reach the water level in the well. For the tap water, before samples were collected the taps were first turn on at full capacity for few minutes to purge the plumbing system of any water which might contaminates sample (Tchokossaet al., 1999).The taps were turned down to reduce turbulence flow and to reduce radon loss before collection. Sample procedure for river/stream water collection is as reported by Avwiri and Agbalagba, (2007) while for produced water, samples were collected at the recycling treatment plant at recycling sites in the oil field flow stations following standard method (IAEA, 2003), and three samples from non- oil bearing communities as control.At collection point, 10ml of 65% HNO3was added to avoid changes in the state of the ions that are present in the samples. All water samples were evapourated (avoid boiling) in a furnace temperature at 60oC to reduce their volume from approximately 1.5-litres to 0.2litre and poured into 0.2litre cylindrical polyethylene vials that is of the detector geometry. The samples were sealed and stored for about four weeks to reach radioactive equilibrium.
2.3 Radioactivity measurement
The activity concentrations of the natural radionuclides in soil/sediment and water samples were determined by gamma spectrometry using a Canberra HpGe detector, with a relative efficiency of 29% and an energy resolution of 1.8 keV for 60Co γ- ray energy line at 1332 keV.
The detector was enclosed in a graded lead shield (Model 747, USA) and connected to DSA- 1000 (Canberra, USA) for data acquisition and the spectrum was analyzed by GENIE- 2000 software. The energy calibration was performed using the standard reference radionuclide sources: 60Co, 137Cs and 154Eu, while the efficiency calibration was performed using the reference soil and water (238U, 226Ra, 228Ra, 232Th and 40K) obtained from IAEA laboratory, Vienna. The activity concentration of 40K was directly determined using the 1460.8keV photopeak. For 232Th, the photopeaks of 212Pb (238.6 keV), 208Tl (583.1 kev) and 228Ac (911.1 keV) were used. The 226Ra concentration was derived from 214Bi (609.3 keV) and 214Pb (295.2 and 352.0 keV) in the same pattern. The well known interference between the gamma line of 186.2 keV of 226Ra and 185.7 keV emitted by 235U is inevitable especially in the presence of a high-uranium concentration (Al-Jundi, 2002; Gang et al., 2012). Therefore, the above mentioned lines were not used for the determination of 226Ra High level shielding against the environmental background radiation was achieved by counting in the Canberra 100mm thick lead castle. Since the accuracy of the quantitative measurements is depended on the calibration of the spectrometry system and adequate energy, background measurement and efficiency calibration of the system was made possible using Cs-137 standard source from IAEA, Vienna. Spectrum were accumulated for background for 29000s at 900volts to producestrong peaks at gamma emitting energies of 1461keV for 40K; 609keV of 214Bi and 911keV of 228Ac, which were used to estimate the concentration of 238U(226Ra) and 232Th (228Ra) respectively. The energy resolution of the detector using Cs-137 from International Atomic Energy agency (IAEA) is 18% at 662keV Cs-137 line, while the activity of the standard at the time of calibration is 25.37KBq. The background spectrum measured under the same conditions for both the standard and sample measurements, were used to correct the calculated sample activities concentration in accordance with Arogunjo et al., 2005; Kabir et al., 2009; Zarie and Al-Mugren, 2010. The activity concentrations (C) in Bqkg-1Bql-1 of the radionuclides in the samples were calculated after decay correction using the expression:
(Bqkg-1/ Bql-1) (1)
Where Cs = Sample concentration, NEy= net peak area of a peak at energy, ɛEy= Efficiency of the detector for a γ-energy of interest, Ms =Sample mass, tc= total counting time, Pγ=Emission probability of radionuclide of interest.
2.4 Quality control and assurance
Energy calibration and efficiency calibration of the spectrometer system were performed every two months using the standard reference radionuclide sources and the reference water and soil respectively. The samples of 10% were selected at random for replicate analysis (n = 3) and the relative standard deviations of results were found as <1.4%. It is therefore indicated that the detector system is stable, and the results are reliable (Gang et al., 2012).
2.5 Radiation Hazard Indices Calculation
Different known radiation health hazard indices analysis is been use in radiation studies to arrive at a better and realiable conclusion on the health status of a radiated or irradiated person in previous studies (Diab et al., 2008; Kabir et al., 2009; Zarie and Al Mugren, 2010; Senthilkumar et al., 2010; Agbalagba and Onoja, 2011). To assess the radiation hazards associated with the study soil, sediment and water samples, some quantities (radiation hazard indices) have been used (Zarie and Al Mugren, 2010)
a)Radium equivalent activity index (Raeq):To represent the activity levels of 226Ra, 232Th and 40K by a single quantity, which takes into account the radiation hazards associated with them, a common radiological Index has been introduced (Diab et al., 2008). This Index is called Radium Equivalent (Raeq) activity and is mathematically defined by (UNSCEAR, 2000):
Raeq = CRa + 1.43CTh + 0.077CK (2)
Where CRa, CTh and CK are the activity concentrations of 226Ra, 232Th and 40K respectively. In the above relation, it has been assumed that 10Bqkg-1 of 226Ra, 7Bqkg-1 of 232Th and 130Bqkg-1 of 40K produce equal gamma dose. The maximum value of Raeq in soil must be less than 370Bqkg-1 (Zarie and Al Mugren 2010). The percentage contribution of the three natural radionuclides is estimated using the assumption of the radium equivalent (Gang et al., 2012).
b)Representative level index (Iγ):Another radiation hazard index used for the estimation of gamma radiation associated with the natural radionuclides in soil is called representative level index Iγ, (Alam et al., 1999; Ashraf et al., 2010)