ESTIMATION OF NATURAL RADIOACTIVITY AND HEAVY METAL CONCENTRATION IN SOIL SAMPLES FROM RAYFIELD – DU MINING SITE JOS, NIGERIA

ABSTRACT

Mining of tin and other related activities have been active and thus leading to economic growth in the Jos area of Nigeria for more than a century. However, mining of minerals has been confirmed to enhance the concentrations of heavy metals and natural radioisotopes in the soil, air and water bodies in the environment. In an attempt to evaluate the radiological impact of NORM and heavy metals (HM) burdens resulting from tin mining activities at Rayfield-Du area of Jos. Specific activities of natural occurring radioactive nuclei (238U, 232Th and 40K) and HM concentrations were evaluated in soil samples collected from the mining site. The soil samples were classified as normal soil (S), tailings (T) and mineral soils (M) and their corresponding mean activities for 238U, 232Th and 40K were analysed using a well calibrated HPGe detector based gamma spectrometric system. While the HM concentrations in the soils were evaluated using Energy Dispersed X-ray Fluorescence spectrometric system. The mean activity concentration for 238U, 232Th and 40K were 323.44, 877.63 and 864.99 Bqkg-1; 138.84, 469.31 and 578.65 Bqkg-1;  and 168.83, 436.08 and 346.1 Bqkg-1  respectively for M, T and S samples. The calculated radiation dose parameters for the soil samples were all higher than the recommended safety limit. For all the collected soil sample, the external hazard risk Hext were 2.21, 2.81 and 4.44 for S, T and M respectively while the mean calculated radium equivalent was 819, 1057 and 1645 Bqkg-1  for S, T, and M respectively. The excess life cancer risk (ELCR) estimated for the mine was more than the world average value.  The radio ecological dose rate estimate for non-human biota in the mine revealed that all non-human species except lichen and bryophyte had absorbed dose rate less than the 10 µGyh-1 screening dose. Generally, the  potential  of  developing  radiation  induced  health  defects  as  a  result  of  high  radiation absorbed dose rate by the miners and dwellers around the mine is very high. The analysis of eight HM (Cr, Cu, Zn, Pb, Co, As, Cd, and Ni) concentrations showed that they were above the Nigerian reference level except for Co and Ni. For S, T, and M soil classification, the pollution index of the considered HM concentration was 0.67, 1.49, and 0.71 respectively. The corresponding ecological risk indices was 102,172, and 56 for S, T, and M. These can be classified as low pollution except T soil with moderate potential ecological risk index.

CHAPTER ONE

1.0    INTRODUCTION

1.1 Background to the Study

The exposure of man and his environment to natural source of ionising radiation is a continuous process (United Nations Scientific Committee on the Effect of Atomic Radiation (UNSCEAR), 2000). These sources are mainly from external primordial radionuclide which include Uranium (238U), Thorium (232Th), Potassium (40K) and from radionuclide within man himself. Of major concern from the perspective of radiological health is the external (terrestrial) source. Although many radionuclide of natural origin exists, only few have relative abundance and radioactive intensity to contribute largely to the total absorbed dose from natural sources. Long lived radioisotopes of concern, which contribute mostly to radiation dose from terrestrial sources include; radioactive uranium (238U), thorium (232Th), potassium (40K) and their radioactive decay daughters such as radium (226Ra) and Radon (222Rn), (UNSCEAR, 2000). Figure 1.1a and b show the decay chains of the primordial radioisotopes- uranium (238U) and thorium (232Th). These radionuclides are sometimes known as Naturally Occurring Radioactive materials (NORM). The absorbed doses from NORM are directly linked to their distribution in the environment. The distribution of NORM in undisturbed environment is however not uniform  and  their  concentration  in  natural  soils  and  rocks  are  largely  dictated  by geological composition and the geography of the location (Olarinoye et al., 2010). Consequently,  the  distribution  of  natural  radiation  dose  differs  from  one  location  to another.

Some human activities have also been recognised to enhance the distribution of NORM in the  physical  environment.  These  activities  include  nuclear  accidents,  oil  and  gas prospecting, minerals mining and milling processes (Dowdall et al., 2004; Musa and Jiya, 2011; IAEA, 2005). The environmental effect of these anthropogenic activities can be accessed via the measurement of radiation dose within the vicinity where the activities are carried out. In Nigeria, unregulated mining of solid minerals are on-going in different parts of the country. Consequently, mining sites are scattered all over the country where illegal and uncontrolled mining are taking place. For more than a century the Nigerian Mining industry has been very active. One of the oldest mining area in Nigeria is the Jos Plateau in Northern part of Nigeria where mining has been going on for more than a century (Masok et al., 2015).

1.2 Statement of the Research Problem

The negative impact of mining tin or any other minerals on man and his environment cannot be over emphasised. Although, mining brings a lot of economic and sometimes infrastructural  benefits  to  the  immediate  mine  community and  the  nation  at  large,  it however, brings threat to human and animal lives. This threat emanates from the pollution the mining activities bring to the ecological habitats and also the changes in the land scape. Abandoned and active mining pits affect the water table by changing the upstream course brought about by lowered base level. The lowered base level is a major cause of gully erosions. The pits may also serve as snares for unsuspecting animals and humans. These hazards are mostly suffered by the immediate community and the mine workers. Furthermore, the environmental pollution that accompanies tin mining stems from the chemo toxic and radiotoxic chemicals that are released to the environment during mining and milling of minerals. Waste products such as mine tailings and sludge rich in heavy metal and radioactive bearing mineral are the major cause of the environmental pollution resulting from mining. The tin ore itself is a radioactive mineral that contains Zircon, Monazite, Xenotime and thorite (Ibeanu 2003). These are uranium, thorium and heavy metal bearing minerals. Heavy metals associated with tin mine tailings include: Zn, Pb, Cu, Sn, and Ni. These metals are toxic to plants and animals when their concentration in farm  soil  surface  and  underground  water  is  more  than  their  natural  distribution  in geological formations.

Research conducted and concluded internationally at different mine area have all come to the conclusion that crops, soil, water and the air of mining areas and their environment are contaminated by NORM and heavy metals (Arogunyo et al., 2009, Ademola, 2008, Jibiri et al., 2007, Jwanbot et al.,  2012, Masok 2015a,b Isikalu et al., 2011). Consequently, it can be concluded that tin mining increases the distribution of these radioactive and chemo- toxic substances in the environment to a large extent. As a result, the background gamma, alpha, and beta, radiation is increased in mining field and vicinity (Ibeanu, 2003; Funtua, 2005; Ademola, 2008; Arogunyo, 2009). Another issue of concern is that these toxic materials are not only concentrated in the mining vicinity. They are dispersed in the soil, air, and water from the source to other areas through weather factors such as wind, surface run off and soil water. They are also re-distributed by man through the transportation of contaminated soil for building and farming purposes.

Mining activities may thus be a major threat to ecological habitat of man and animals. In this regard, continuous monitoring of the extent of contamination of soil in mining area is very important. Mineral extraction negatively impacts the environment, miners and the general public health of the people living around the mine field at Rayfield- Du area of Jos South local government area of Plateau State. Previous research on radiological impact of tin mining in Jos Plateau region has paid little attention to the dose received by non-human biota. However, measured doses in other research has compared with high background radiation regions where noticeable radiation induced defects has been recorded. Dose levels have been set by international organisations towards protecting the non-human biotas (IAEA, 2011; ICRP, 2003; UNSCEAR, 2008). The level of radiation burden to non- human species in an active mine is thus very necessary.

1.3 The Aim and Objectives of the Research

The aim of this research is to access the radiological hazard and heavy concentration due to Natural Occurring Radioactive Material (NORM) and heavy metal concentration in an active tin mining field at Rayfield – Du, Jos South Local Government area of Plateau state, Nigeria.

The objectives of the research are to:

(i)        measure the in situ terrestrial gamma radiation dose in the mining field in Rayfield – Du, Jos South, Nigeria.

(ii)        determine the activity concentrations of 40K, 232Th, and 238U in mine soils, tailings and mineral soils collected from the mine field.

(iii)       calculate    radiological    hazard    indices    associated    with    the    measured radioisotopes concentrations.

(iv)       evaluate the concentrations of selected heavy metals (Chromium (Cr), Copper (Cu), Zinc (Zn), Lead (Pb), Cobalt (Co), Arsenic (As), Cadmium (Cd), and Nickel (Ni)) in the soils, tailings and minerals soils from the mine.

(v)        evaluate the dose to non-human biota based on the measured radionuclide concentrations.

1.4 Justification of the Study

In view of the health hazard associated with uncontrolled increase in radiation exposure to man and his environment, it has become greatly important for continuous environmental impact assessment of mining activities whenever they are carried out. This will reveal the extent of damage done to the environment and the health risk associated with the mining and milling processes. Such impact assessments include the measurement of NORM and heavy metals within the mining company’s vicinity. Also 226Ra another radionuclide of concern is a decay product of 238U which is also widely distributed in many geological formations such as soils, water and rocks. It decays to 222Rn (-a gas) with a half-life of about 4 days). This gas has been reported to be a major cause of lung cancers among non- smokers (IARC 2012). The fact that many active and abandoned mining sites in Jos are located around residential area is a cause for concern. It has thus become important that the radiological risk parameters of the population in the said area be evaluated.

This research will report the concentrations of 238U, 232Th and 40K in the soils around a mining site at Rayfield – Du, area of Jos Plateau. The radiological risk parameters associated with the NORM distribution will be presented. Data from this research is important to government, mining company, and individuals who work or live around the area under study. This will give the level of environmental hazard caused by the mining operations; provide basic radiological data for evaluating environmental impact and for judgment of future impacts.  Data on the future health risk such as cancer associated with working and living in the area will also be provided. The data from this report could be used by government and policy maker to create awareness and legislation in the risk associated with using contaminated soils for building and farming. Also since mining has been established to be a major factor contributing to enhanced radiation exposure in the environment, it is important for baseline studies to be carried out prior to the commencement of mining procedure and routinely from time to time so as to ascertain the impact of mining activities on the environmental radiation level, of the environment. Generally environmental radiation measure are also important for future exploration of minerals,  problem  of  nuclear  waste  management,  location  of  orphan  sources,  and processing of radioactive material. Consequently data from this study will reveal if regulations  and  standard  practice  procedure  with  respect  to  radiation  protection mechanism is adhered to in the selected mine area. The growing international concern with the ecological impact of mining in the environment has made this research relevant and timely.

1.5 Scope of the Study

The research was conducted at an active mine site at the Rayfield – Du area of Jos south in Plateau state in Nigeria. Radiological burden of the mining and tailings of contaminated soil would be restricted to the measurement of the three primordial radionuclide 40K, 232Th and 238U using a High Pure Germanium (HPGe) detector. The heavy metal analysis will be done using Energy Dispersive X-Ray Fluorescence (EDXRF) spectrometer and restricted to (8) eight major poisonous heavy metals. Furthermore, terrestrial dose to non-human elements in the biota would be calculated for birds, amphibians, reptiles, trees mammals and other organisms using the ERICA 1.2 computer simulation.

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