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BASELINE RADIOMETRIC SURVEY TO ACCESS GAMMA RADIATION DOSE AT SELECTED QUARRY SITES IN IGARRA AND ENVIRONS SOUTHWESTERN NIGERIA
- ABSTRACT
Economic rocks such as limestone, granite, marble etc. have natural radioactivity level. These rocks are being quarried daily in Igarra and environs. The process of quarrying these economic rocks poses health hazard to the environment especially the quarry workers. In the present study, radiations of 238U, 232Th, 40K and dose rate were measured using a new generation self calibrating spectrometer instrument (RS-230 detector) coupled with closer line spacing, which resulted in the acquisition of a higher resolution data. Data were obtained from four quarries, one milling site and two different locations in Igarra town. Data were also obtained 100 meter away from the quarries which served as control. From the result obtained, the concentration of Uranium in part per million (ppm) ranges from 1.1 to 16.8 ppm, Thorium ranges from 0.0 to 19.6 ppm and Potassium concentration in % ranged from 0.0 to 30%. The annual equivalent dose rate in mSv year-1 ranged from 0.3 to 1.4 mSv year-1.The value for the equivalent dose rate for the Petra Quarry Ltd where granite is quarried is 1.4 mSv year-1. This is higher than the Dose limit and Maximum Permissible Dose (MPD) levels for non radiation worker which is 1mSv/y. This Study also compares the data obtained from quarry pit with data obtained 100m away from the quarry pit and along a profile in the town settlement. This was used for a control, so as to ascertain if the quarrying activity affects the radiation dose of the surrounding environment. We can conclude that the quarrying of economic rocks in Igarra and environs poses no immediate threat to the environment noting that longer exposure (for years) of the mine workers in the granite quarry may subject the workers to radioactive hazard.
Keywords: Radiometric survey, Gamma radiation, Quarry sites,radioactive hazard, Igarra southwestern Nigeria
TABLE OF CONTENTS
TITLE PAGE
CERTIFICATION i
DEDICATION ii
ACKNOMLEDGEMENT iii
ABSTRACT iv
TABLE OF CONTENTS v
LIST OF TABLES vi
LIST OF FIGURES vii
LIST OF PLATES viii
LIST OF APPENDICES ix
CHAPTER ONE: INTRODUCTION
1.1 Background Information 1
1.2 Statement of the problem 2
1.3 Aim and objectives 2
1.4 Scope of work 3
1.5 Justification 3
1.6 Location and accessibility 3
1.7 Relief and drainage 4
1.8 Climate and vegetation 7
1.9 Settlement and land use 7
1.10 Geology of the study area 8
CHAPTER TWO: THEORITICAL BACKGROUND AND LITERATURE REVIEW
2.1 Theoretical background 12
2.1.1 Radioactivity 12
2.1.2 Types of radioactive decay 13
2.1.3 Alpha particles 14
| 2.1.4 Beta particles | 16 | |||||
| 2.1.5 Gamma radiation | 16 | |||||
| 2.1.6 Interaction of Gamma radiation with matter | 17 | |||||
| 2.2 Sources of gamma radiation | 19 | |||||
| 2.2.1 Natural sources of gamma radiation | 19 | |||||
| 2.2.2 Artificial sources of gamma radiation | 20 | |||||
| 2.2.3 Background radiation | 20 | |||||
| 2.3 Detectors and instruments | 21 | |||||
| 2.3.1 Geiger Muller Counter | 21 | |||||
| 2.3.2 Scintillation Counter | 22 | |||||
| 2.3.3 Germanium semiconductor | 24 | |||||
| 2.4 Types of radiation | 27 | |||||
| 2.4.1 Effects of radiation to health | 28 | |||||
| 2.5 Principles of gamma ray spectrometry | 29 | |||||
| 2.5.1 Properties of gamma ray spectra | 30 | |||||
| 2.5.2 The detector response | 33 | |||||
| 2.5.3 Source- detector geometry | 33 | |||||
| 2.5.4 Physical models used in gamma ray spectrometry | 36 | |||||
| 2.5.5 Environmental effect on radiation dose | 37 | |||||
| 2.5.6 Instrument calibration | 39 | |||||
| 2.5.7 Background correction | 41 | |||||
| 2.5.8 Stripping ratio | 41 | |||||
| 2.6 Literature review
CHAPTER THREE: METHODOLOGY |
43 | |||||
| 3.1 Desk study | 54 | |||||
3.2 Field work and Instrumentation 57
3.2.1 Field instrumentation 60
3.3 Data processing procedure 62
CHAPTER FOUR: RESULTS AND DISCUSSION
4.1 Results 65
4.2 Discussion 84
CHAPTER FIVE: SUMMARY, CONCLUSION AND RECOMMENDATION
5.1 Summary 89
5.2 Conclusion 89
5.3 Recommendation 90
5.4 Contributions to knowledge 91
REFERENCES 92
APPENDICES 102
CHAPTER ONE
INTRODUCTION
1.1 Background Information
Human beings are exposed to ionizing radiation every day from natural radionuclides in the ground, building materials, air, food, the universe and even elements in their own bodies. The assessment of these doses from natural materials is important as external radiation exposures from natural materials contribute about 50 % of the average annual dose to humans from all radiation sources (UNSCEAR, 2010). Gamma radiation emitted from naturally occurring radioisotopes, such as K-40 and the radionuclides from the Th-232 and U-238 series and their decay products (also called terrestrial background radiation), which exist as trace levels in all ground formations, represents the main external source of irradiation to the human body (Harb et al., 2008). Natural radioactivity in soil, sand and rock comes from the Ra- 226 and Th-232 series and K-40, which in turn depends upon the local geology of each region. As natural radionuclides are not uniformly distributed, the knowledge of their concentration and distribution in materials plays an important role in radiation protection. Such investigations can be useful both for the assessment of public dose rates and documentation of reference data (baseline) that is used to ascertain possible changes in the environmental radioactivity due to nuclear, industrial and other human activities (Elissa et al., 2010).
The Nigerian economy has mostly depended on proceeds from the sale of crude oil. This is at the expenses of other sectors such as solid minerals (Olumide et al., 2013). The current trend of dwindling crude oil prices, pipe line vandalisation by the agitating militants of the Niger Delta and the subsequent reduction of daily crude oil production, has necessitated the dire need for diversification of the economy (Sunday, 2013). In this light, the Nigerian government through the office of the Ministry of Solid Minerals has consistently emphasized the urgent need to revive the solid mineral sector to serve as a reliable revenue yielding alternative. This paradigm shift in revenue generation by the Nigerian state has led to a plethora of quarrying activities in the country. Although solid mineral development will add value to the economy of a nation, it has its adverse effect on the environment such as destruction of natural flora and fauna; pollution of air, land, water; degradation of natural landscape; and radiation hazard (Aigbedion & Iyayi, 2007).
Aigbedion and Iyayi (2007) documented such hazardous effect due to radiations associated with mining, which led to the death of some people in contact with the radioactive rocks.
Although, there are other elementary particles such as Alpha particles and Beta particles which are ionizing radiations, Gamma radiation have the most penetrating radiation from natural and man- made sources (IAEA, 2003). Individual radionuclide emits gamma rays of specific energies that are characteristic of an element and isotope. Total count measurement registers gamma ray of all energies. Spectrometers on the other hand, measures both the intensity and the energy of radiation and this enables the source of the radiation to be diagnosed (IAEA, 2003). Gamma ray spectrometry (GRS) is a geophysical sensing technique that provides information about the distribution of naturally occurring radioactive elements (Potassium (K), uranium (U) and Thorium (Th)). It is a surface technique and its interpretation requires an understanding of the nature of the surface materials and their relation to bedrock geology. Although GRS measures a physical phenomenon, it is for geological and exploration purposes (IAEA, 2003). The radiations detected by the spectrometer are in the uppermost region of the electromagnetic spectrum with very short wavelength in air of about 10-13 – 10-10 meters. This ultra high frequency wave in the form of rays is emitted from the atoms and molecules of a radioactive substance as a result of nuclear decay (IAEA, 2003).
1.2 Statement of the problem
In Igarra and environs, there is a plethora of quarrying activities. These quarrying and processing of economic rocks exposes the environment especially the quarry workers to radiation hazard. The effect of radiation poisoning usually takes a long time for physical symptoms to manifest therefore timely monitoring and control is paramount. This thesis proffers solution for environmental risk assessment and monitoring by providing ground radiometric baseline data.
1.3 Aim and objectives
The aim of this study is to ascertain if the quarrying of economic rocks in Igarra and environs increases the radiation dose rate of the environment. The objectives include; 1 To determine the radiation dose of the selected quarries.
- To determine the radioelement concentration of Potassium (K), Uranium (U) and Thorium (Th) in the area.
- To determine the most favorable location of radiometric anomalies.
- To identify the source of anomalous radioactivity.
1.4 Scope of work
The scope of this study entails ground radiometric investigation of rock quarries located in Igarra and environs. In this study Gamma radiation is been investigated due to its extensive applicability to the field of geosciences. Measurement of both dose rate and radioelement concentration will be done using a handheld scintillometer.
1.5 Justification
It will provide background information on the levels and distribution of radiation doses in the study area. Data obtained in this study could be useful for future radio-geochemical investigation in the area especially for the search of rare earth element and location of Potassium alterations.
This study will be a reference baseline in situations of nuclear emergency.
1.6 Location and accessibility
The area Igarra and environs lies between Latitudes 7º8’N and 7º 18’N and Longitudes 6º13’E and 6º30’E, fig 1.1. It is situated at the Northern Fringe of the Akoko Edo Local Government area of Edo state, Southwest Nigeria. The landforms comprises of undulating low land separated by hill rocks representing granites. The older granites occur in ridges and rise between 20 meter and 40 meter above the general ground level. Most of the hills occur in the Northeast and Southwest of Igarra. The major highway in the area runs from Auchi through Sobe Ogbe, Ikpeshi, and Igarra to Ibillo. Both the old and new roads were used as access path for the exercise.
Fig. 1.1 Accessibility map of Igarra and Environs
1.7 Relief and drainage
Igarra is made up of undulating high and low relief. Figure 1.2 is a colour enhanced elevation map of the study area. Generally, the highest point is about 526 meter and the lowest is about 106 meter. The highlands vary in sizes and shapes from ridges which can be traced for several kilometers to domes, inselbergs and boulders which may be capping/hanging on the highs or sitting on the low land. Some of the rocks appear as low lying outcrops. Igarra landscape is mostly covered with mosaic croplands/vegetation.
Generally, the area is well drained. The general drainage patterns are trellis and dendrites. Igarra is drained by river Onyami and Orle among others. There is no swamp in this area.
River Ekafe was captured by river Orle through a fault structure diversion at the southern part of Igarra. The topographic map of the area is shown in Figure1.3.
Fig. 1.2 Elevation map of study area (Igarra and Environs)
Fig. 1.3 Topographic map of the study area
1.8 Climate and vegetation
Igarra and environs which is located in the Northern fringe of Edo state has a warm climatic condition, with relatively high temperatures throughout the year. It experiences both wet and dry seasons; the wet season lasting from April to November while, the dry season lasts from December to March. Average rainfall distribution is 350mm per year (SEEFOR, 2015). Igarra lies within the Guinea Savannah vegetation belt characterized by short trees and tall grasses. The vegetation here is prominently made up of sparsely distributed trees, herbs, shrubs, and grasses (SEEFOR, 2015). The trees (mango and orange) help check the activities of erosion. Trees in this area are mostly concentrated along fracture zones within the plutonic bodies and on the Quartzite ridges were adequate soil cover has resulted and there is adequate groundwater retention. The vegetation in this area is mostly secondary i.e. the natural vegetation is being altered and such agricultural crops such as Maize, Yam, Cocoa, Cassava, Pineapple, Cashew, Mango, and Sugar cane are grown.
1.9 Settlement and land use
Igarra area is a medium sized mildly nucleated settlement. According to the 2006 population census, the Igarra area has a population of about 20000 – 50000 people. The Igarra area is classified as a third order urban area (SEEFOR, 2015).There is administrative offices, places of worship, schools, quarry sites and other infrastructural features in the locality. The major occupation of the inhabitants of Igarra and its environs is mainly subsistence farming and the major crops produced are yams, cassava and pineapples, others are maize and cocoa. Most of these farming activities are carried out in the valleys which in most cases have loamy soils and also within a region that has a high water table. Recently, mining activities have become a mainstay of the economy of Igarra and environs. The process of bush burning is followed by hunting of bush animals by the indigenes. Some of the farmers produce palm oil in small quantities from the palm trees. Quarrying is a very common activity within Igarra area. This is purely due to the presence of abundant solid minerals within the area. Some common problems facing the inhabitants of Igarra and its environs are; the problem of erosion and the problem of water scarcity in dry season due to the drying up of rivers that supply water to the fractures which serves as the aquifer.
1.10 Geology of the study area
The geology of Igarra area has been studied at various degrees by many authors who indicate the major rock groups, their distribution and structural relationship. The Igarra area lies within the Pre Cambrian basement complex. The basement complex is one of the three major lithopetrological components that make up the geology of Nigeria (Fig. 1.4).
Fig. 1.4 Geologic Map of Igarra and environs (modified from NGSA, 2006).
The Nigerian basement complex forms a part of the Pan-African mobile belt and lies between the West African and Congo Cratons (Fig. 1.5) and south of the Tuareg Shield (Black, 1980). It is intruded by the Mesozoic calc-alkaline ring complexes (Younger Granites) of the Jos Plateau and is unconformably overlain by Cretaceous and younger sediments (Obaje, 2009).
Fig. 1.5 Generalized geological map of Nigeria within the framework of the geology of West Africa (Adapted from Wright, 1985)
The Nigerian basement (Fig.1.6) was affected by the 600 Ma Pan-African orogeny and it occupies the reactivated region which resulted from plate collision between the passive continental margin of the West African craton and the active Pharusian continental margin (Burke and Dewey, 1972; Dada, 2006). The basement rocks are believed to be the results of at least four major orogenic cycles of deformation, metamorphism and remobilization corresponding to the Liberian (2,700 Ma), the Eburnean (2,000 Ma), the Kibaran (1,100 Ma), and the Pan-African cycles (600 Ma). The first three cycles were characterized by intense deformation and isoclinal folding accompanied by regional metamorphism, which was further followed by extensive migmatization. The Pan-African deformation was accompanied by a regional metamorphism, migmatization and extensive granitization and gneissification which produced syntectonic granites and homogeneous gneisses (Abaa, 1983). Late tectonic emplacement of granites and granodiorites and associated contact metamorphism accompanied the end stages of this last deformation. The end of the orogeny was marked by faulting and fracturing (Gandu et al., 1986; Olayinka, 1992)
Fig. 1.6 Basement Geology of Nigeria (adapted from Obaje (2009); Geology and Mineral resources)
The major structural feature in the Igarra Formation is a series of anticlinal and synclinal over folds trending in the north-south axis (Odeyemi, 1988). The rocks of Igarra Formation were affected by the pan African Orogeny which occurred between 700-750Ma.There are some differences in folding exhibited by the metasediments, migmatite and granite complexes Rahaman (1992).
The rocks that occur in Igarra can be subdivided into three major groups (Oloto & Anyanwu,
2013). These major subdivisions are;
- Migmatite and biotite hornblende gneiss
- Low grade metasediments (Schists, calc-gneiss, marble, metaconglomerate and quartzite)
- Syn-to late tectonic porphyrite biotite hornblende granodiorites adamelites, charnokites gabbros, un-metamorphosed dolerite, pegmatite, aplite and syenite dykes- representing minor rock types.
The contact between the migmatite and the metasediments are fault-bounded in most cases. The meta-sedimentary successions in the Igarra Formation consist of: 1 Quartzite biotite schists
- Mica schists
- Marble and Cal-silicates
- Metaconglomerate
The most easterly schist belts in south-western Nigeria are distributed around the Okene migmatitic nucleus. It trends NNW with a length of only 50 km and in the west is joined to the NW-trending Owo belt and to the Itobe belt in the east which may have extended into the Muro Hills in the north (Obaje, 2009). The presence of both calcareous rocks and conglomerates is peculiar to the Igarra schist belt which set it apart from other schist belt in Nigeria. These rock types together with quartzite occur as bands in the dominant biotite schist. The gneisses at the margins of the metasediments may by highly metamorphosed basal part of the sequence; they are equivalent to the non-migmatitic gneisses described by Furon (1960) from the margins of other schist belts in this easterly region. The main structure is an open synform, but this re-folds earlier E–W folds. Porphyritic granites intrude the centre and margins of the belt. Figure 1.7 is an enlarged geologic map of Igarra showing the different rock types present.
BASELINE RADIOMETRIC SURVEY TO ACCESS GAMMA RADIATION DOSE AT SELECTED QUARRY SITES IN IGARRA AND ENVIRONS SOUTHWESTERN NIGERIA
