DETERMINATION OF NATURAL RADIOACTIVITY OF SOME SELECTED ECONOMIC MINERALS FROM QUARRY SITES IN IGARRA AREA, EDO STATE, NIGERIA

ABSTRACT

Activity concentrations of radionuclides in rocks and minerals are of vital importance in geosciences and health physics especially that primordial radionuclides constitute the main source  of  radiation  exposure  risks  externally in  soil,  rocks  and  minerals.  In  this  study, samples of four (4) economic minerals, namely dolomite, feldspar, limestone and calcite, were collected from quarry sites in Igarra area, Edo State, Nigeria and analysed using gamma ray spectrometric technique. From the analytical results, the specific activity concentrations of 226Ra, 232Th and 40K for dolomite ranges from 0.79±0.1 to 4.3±0.24 Bq/kg, 0.27±0.09 to 3.65±0.30 Bq/kg and 27.4±1.91 to 505.19±8.19 Bq/kg with respective mean of 2.88 Bq/kg,1.61Bq/kg  and  227.33  Bq/kg.  In  feldspar,  the  activity  values  range  from  0.69±0.09  to 2.01±0.15  Bq/kg  for  226Ra,  0.52±0.11  to  1.06±0.15  Bq/kg  for  232Th  and  49.29±2.30  to 248.42±4.99 Bq/kg for 40K with respective mean of 1.40, 0.76 and 152.28 Bq/kg. Also in calcite, the activity concentration ranges from 0.54±0.09 to 10.57±0.41 Bq/kg for 226Ra, 1.28±0.19 to 6.12±0.44 Bq/kg for 232Th and 81.37±3.32 to 795.16±11.02 Bq/kg for 40K with respective mean of 5.54, 3.81 and 466.62 Bq/kg. Similarly, in limestone, the activity concentrations range from 0.63±0.09 to 3.91±0.22 Bq/kg for 226Ra, 0.67±0.13 to 2.67±0.26 Bq/kg for 232Th and 57.49±2.60 to 421.39±7.17 Bq/kg for 40K with respective mean of 2.05, 1.59 and 220.02 Bq/kg. Mean specific activities of 226Ra, 232Th and 40K in the four economic minerals were below the world average values of 30, 35 and 400 Bq/kg respectively as stated by the United Nation Scientific Committee on the Effect of Atomic Radiation except calcite that exceeded slightly the value of 40K. Mean absorbed dose computed for dolomite, feldspar, calcite and limestone are 9.99, 7.08, 23.58 and 10.70 nGy/h respectively, which are below the world average of 58 nGy/h. Computed mean annual effective dose equivalent outdoor (occupational) for dolomite, feldspar, calcite and limestone are 0.02, 0.01, 0.04 and 0.02 mSv/y respectively, with corresponding average indoor (residential) values of 0.05, 0.04, 0.12  and  0.05  mSv/y in  sequence.  Mean  external  hazard  index  computed  for  dolomite, feldspar, calcite and limestone are 0.06, 0.03, 0.12 and 0.05 respectively, which are below unity. Similarly calculated excess lifetime cancer risk for both residential and occupational exposures to dolomite, feldspar, calcite and limestone are (0.06, 0.14, 0.43, 0.21)×10-3  and (0.22, 0.04, 0.13, 0.06)×10-3  respectively. These values agree with the worldwide average value of 0.29×10-3. The usage of these economic minerals as aggregate of building materials or for any domestic purposes does not pose any risk to the public from the point of radiation protection.   Continuous   radiological   check   of   the   economic   minerals   is   however recommended to always ensure that the radiation doses are as low as reasonably achievable

CHAPTER ONE

1.0        INTRODUCTION

1.1       Background to the Study

Natural occurring radioactive materials (NORM) are acceptable and commonly used words  to  express  substances  that  contain  radionuclides  of  a  natural  source.  The primordial radio-nuclides 40K, 238U, 235U, 232Th and their radioactive decay products are typically recognized as NORM. These radio-nuclides are all over in the surroundings and they present, with a broad distinction of concentration levels, in raw rocks, minerals, materials, by-products, products, equipment, residues and waste, as well as in several industrial processes. Individual radionuclides comprise much importance in different aspects of radiological and medical sciences. Dolomite, feldspar, calcite and limestone herein  called  economic  minerals  contain  natural  radioactive  materials  (UNSCEAR, 2000). Exposure to this ionizing radiation emitted by individual NORM can be harmful to both the living and non-living things in the surroundings especially at level above the threshold. The radiation gamma dose rate owing to exposure to NORM is generally small in their undisturbed state in the surroundings ( Fabiano et al., 2011); nevertheless, certain human activities can enhance the concentration of NORM and/or alter exposure circumstances. Examples are mining and quarrying. Subsequently, these can give increase to above background emission dose rate to receivers. Such exposures need to be studied and also controlled through regulations by various bodies to ensure that adequate protections are given to individuals both occupational and residential as well as to the environment.

Though, more than three decades, there has been a rising consciousness resulting from the identification of amplified levels of NORM in non-nuclear industries, such as phosphate mining, coal and petroleum industries (Kolo 2014; Fabiano et al., 2011; Hazou et al., 2018; Kolo et al., 2016; Ali and Ibraheem, 2017). The study and control of exposure rate encompasses both indoor and outdoor surroundings, whether in dwellings (residential) or in workplaces (occupational), as well as in manufacturing companies involving NORM (such as cement, construction and glass companies).

Estimating the activity levels of NORM plays a crucial role in the administration of radiation  exposure  rate.  In  fact,  the  determination  of  activity  concentration  is  a precondition at all levels of a regulation system, beginning from the testing of radio- nuclides to intermittent monitoring checks. It is furthermore essential for the appraisal of radiation dose rate received by non-human biota or certain group of humans. However, the measurements of NORMs are connected with limitations and difficulties since the wider range of radio-nuclides concerned are in huge variations in their physical and chemical properties (UNSCEAR 2000). Generally speaking, quarry sites all the globe are transit point for NORM as the case with Igarra area but occupational radiological and subsequently the residential radiological hazards must be quantified, so as to know the level of exposure of these group mentioned above. These economic minerals are selected because of their uses which range from medicine, animal food, cement aggregates, building materials, glasses, ornament, kitchen utensils, to interior decorations in our homes. Obviously, there is need to quantify these radiological hazards so as to be able to advice the general public of appropriate measures to be taken in limiting exposures rates. Human  beings  exposure to natural  occurring radiations  arises  principally from  two diverse origins (UNSCEAR, 2000). The primary source comes straight from cosmic radiation from the outer space; the interactions of cosmic ray particles in the atmosphere can generate a number of radioactive nuclei such as 3H, 14C and 7Be. The secondary main contributor is the terrestrial radioactive substances that begin from the configuration of the earth and are available universally in the earth’s crust and in the human body itself.

Abdu et al., (2017) has reported that “exposure rate due to external radiation is caused mostly by the activity concentrations of radio-nuclides of natural origin of uranium series, thorium series and natural potassium. These primordial radio-nuclides have long half-lives, they have survived since their formation and decaying to achieve the steady state and produce ionizing radiation in different degrees. The study of the radioactive mechanism in soil, rocks and minerals is a fundamental link to the understanding of the behavior of radioactivity on the ecology since these substances emit radiation by the natural disintegration of natural radio-nuclides and added to the entire absorbed dose rate via inhalation, ingestion and external radiation.

Aside the  natural  exposure from  direct  cosmic rays  and  cosmogenic  radionuclides, natural  exposures  also arises  mainly from  primordial  radionuclides  that  are  spread broadly and are available in nearly all geological substances (dolomite, calcite, feldspar and limestone) in the earth’s surroundings (UNSCEAR, 2000).

The greater part of the natural occurring radionuclides belong to the radio-nuclides in the 238U (226Ra which is the daughter), Thorium 323 series, and the single decay radio- nuclides, 40K. Those radionuclides which emit either beta or alpha particles can be fed into the body by inhalation or ingestion and can give growth to internal exposures. Furthermore, nuclear species of some kinds can emit gamma rays following their radioactive decay; these represent one of the main origins of external (whole-body) exposures to humans systems (UNSCEAR, 2000).

Environmental problems allied with NORM in solid mineral mines such as dolomite, calcite, feldspar and limestone occurs during the processes of blasting, drilling, grinding, handling, storage, leaching, transportation of mineral rocks/ ores and the use of waste media without controls or contaminated equipment. This frequently leads to the stretch of NORM contaminating the surroundings, leading to potential radiation exposure of general public. An instance of negative consequence of contaminated environment on humans, as a result of mining and quarry activities was reported by UNEP (2010).

Dolomite, calcite, feldspar and limestone are among the numerous natural rocks that contains naturally occurring radioactive materials (NORM) today, these NORM or sources deliver a large collective dose to the world population than do all man-made (artificial) sources combined (Fabiano et al., 2011) .

However, rocks contain high activity concentration of radionuclides like uranium, radium, potassium and thorium. Thorium and uranium are integrated into the rocks in the crystallization of the previous magma and residual solutions since their large ionic radii stop them from crystallizing out in the early silicates (Shiva et al., 2008). Quarry activities, involving NORM are potentially sources of radiation exposures to workers and the member of the general public (Mustapha et al., 2007).

Besides, the radio-nuclides contained in these rocks emit ionizing radiation to the environment around the quarry; the workers are also subjected to radiation exposure and also the farming communities. The inhabitants around such sites are also susceptible to radiation exposure.

In order to protect and monitor the health of the public and staff against the radiation hazards originating from NORM, in these cases, dolomite, calcite, feldspar and limestone, therefore, it is important to measure the radiation exposure rate level at these quarry sites.

1.2       Statement of the Research Problem

Quarry and mining activities can enhance the radiation exposure level by bringing out large amount of buried materials containing natural occurring radionuclide materials (NORM) on to the surface of the earth (Saleh et al., 2007; Karangelos et al., 2004). Blasting, crushing and processing of these economic minerals releases radionuclides as that paticles at the quarry sites. these dust particles suspended in the atmosphere forms particulate matter of different sizes and are transported to various locations such as water bodies, communities, farm lands and settlements by air current, thus exposes the general public and the workers to internal and external radiation hazards. The workers in these quarry sites and the general public are exposed through ingestions and inhalations of these radio-nuclides emanating from the NORM at these quarry sites. The ingestion and inhalation of these radioactive dust particles could lead to serious health concern if the radioactive level of the dust is high. Furthermore, the use of calcite for drugs and limestone for animal feed and other consumer goods suggests that these mineral stones can become a source of internal radiological contaminant. Also, the use of some of these economic stones for building material implies that they can act as a source of external radiological risk. The radiological levels of these materials (dolomite, feldspar, calcite and limestone) from  Igarra area, Edo State, Nigeria have not been reported in the literature so far. This study thus intends to establish the presence, activity concentration of radio-nuclides materials and the radiological hazard in these economic minerals form these quarry sites using gamma ray spectrometric technique so that the general public and staff of these quarry sites could be educated on the potential threats pose by these radiological pollutants and to also provide baseline data for future investigation and for regulatory bodies.

1.3       Justification of the Study

This research is basically aim at assessing radiological hazards indices from NORM at selected quarry sites in Igarra area, Edo State, Nigeria. It is imperative that radiation exposure emanating from these minerals are investigated and measured, to ensure that the radiological conditions of these minerals, quarry workers and end users remain acceptable. The data from the study will help to put in place the appropriate control measures with respect to basic radiological protection and will also serve as useful database for regulatory bodies, quarries operators and the general public.

1.4       Aim and Objectives of the Study

The research is aimed at assessing the level of exposure of quarry workers and the general public to ionizing radiation due to quarry activities of economic minerals in Igarra area, Edo State, Nigeria using gamma-ray spectrometric technique.

The objectives of this study are to;

i.      determine the activity concentrations of 226Ra, 232Th and 40K in selected minerals (dolomite, feldspar, calcite and limestone) from the quarry sites in Igarra area,

ii.      evaluate  the  absorbed  dose  rate  and  radiological  hazard  indices  due  to  the radionuclides in these economic minerals, and

iii.      evaluate the occupational and residential excess lifetime cancer risk arising from exposure.

1.5       Scope of the Study

The research was conducted at two major quarry sites in the Study Area. The first quarry site is located at Igarra town called GeoWorks Plc, which houses other quarry companies. The second one is located at Ekpeshi town called Bees Plc, still within the Igarra area, Edo State, Nigeria. Samples are collected at these sites and the radiological analysis was restricted to the measurement of the three primordial radionuclide 40K,

232Th and 226Ra using gamma spectrometer.

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