Assessment of Background Ionizing Radiation Dose Levels in Quarry Sites Located in Ebonyi State, Nigeria

The study presents a radiometric survey of Background Ionizing Radiation (BIR) dose levels in ten quarry sites located in Ebonyi State, Nigeria. In-situ BIR dose rate measurements, by means of nuclear radiation survey meter, at 1 m above ground level were carried out at the excavation section (ES) and quarrying section (QS) of the investigated quarry sites. The obtained results indicated dose rates ranging from from 0.14 to 0.18 μSv/h with mean of 0.15±0.01 μSv/h at the ES and 0.16 to 0.19 μSv/h with mean value of 0.18±0.01 μSv/h at the QS. While the values obtained at the QS are respectively higher than those measured at the ES, they are all higher than the worldwide average value of 84 nSv/h signifying BIR elevated environments. The estimated mean annual effective dose (AED) and excess lifetime cancer risk (ELCR) are 0.27±0.03 mSv/y and 0.94×10 respectively at the ES and 0.31±0.02 mSv/y and 1.07×10 at the QS. The obtained AED values for all the sites are well above the outdoor worldwide average value of 0.07 mSv/y but lower than the International Commission on Radiological Protection recommended permissible limits of 1.0 mSv/y for the general public. Generally, the BIR levels of the quarry sites are within acceptable limits and no immediate radiological health threat may be derived from the current levels. However, long-term health effects due to continuous exposure to low-level radiation doses may manifested in future over a lifetime exposure of 70 years as indicated by the ELCR values. DOI: https://dx.doi.org/10.4314/jasem.v24i10.17 Copyright: Copyright © 2020 Echeweozo and Ugbede. This is an open access article distributed under the Creative Commons Attribution License (CCL), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Dates: Received: 15 August 2020; Revised: 22 September 2020; Accepted: 19 October 2020

Increasing demand of quarried products has stimulated investment surge in stone exploration/quarrying and production in both developed and developing nations, including Nigeria (Okeke, 2008). Over 33 solid mineral deposits have been found in Nigeria spreading across almost 400 locations which includes the Southeastern geopolitical zone that include Abia, Anambra, Imo, Enugu and Ebonyi States (Akpokodje, 1992;Lawal, 2010). Ebonyi State is associated with the occurrence of igneous intrusion with volcanic and sedimentary rocks (Chiadikobi et al., 2011). Mineral deposits such as Lead, Zinc, Copper, Gypsum, Granite, Limestone, Marble stone, Aluminum, False gold, Uranium, Igneous rock, among others have been identified in Ebonyi state (Ebure and Ezeribe, 1997). Due to the geological formation of the state, there is a surplus supply of these natural resources (Edet et al., 2011) with series of quarrying activities even as far back as 1950s (Chima et al., 2010). Mineral mining and quarrying activities are parts of anthropogenic activities identified as major contributors to background ionizing radiation (BIR) levels of the human inhibited environment (Mokobia and Balogun, 2004). The various mineral deposits listed above, by nature, contain naturally occurring radioactive materials (NORMs) with constituents of the series radionuclides of 235 U, 238 U and 232 Th and the single radionuclides 40 K (Ugbede and Akpolile, 2019). Usually, when mineral ores with naturally high concentrations of NORMs are mined from their natural depositions to the top surface, they redistribute the radiation levels of the immediate environment in a way that result to an elevation of the natural background level thus resulting in increased exposure of the general public. Consequent upon this, the BIR can therefore be considered as environmental pollution especially when it exceeds safe occupational and public limits (Agbalagba et al., 2016). Although humans are known to be in constant interaction with background ionizing radiation (BIR) (UNSCEAR, 2000), the exposure to increase BIR within quarry site cannot be compared with those of normal environments. Internal exposure to radionuclides associated with the quarry products can also result owing to inadvertent ingestion within the mining/excavation pits, and inhalation of dust particles during quarrying. The continuous exposure by the workers can accumulate radionuclides within the internal body organs (Ugbede, 2018;Ugbede and Benson, 2018) and when in excess doses may result to various health challenges already listed in the literature (UNSCEAR, 1993;Taskin et al., 2009;Jibiri and Okeyode, 2012;Emelue et al., 2014;Enyinna and Onwuka, 2014;Qureshi et al., 2014;Suresh et al., 2014;Rafique et al., 2014;. Notwithstanding the economic benefits of quarrying operations, it is imperative to assess and know the levels of background gamma radiation circulating within quarrying environments. By this, one can examine, quantify and qualify the radiation dose accruing to the workers and the general populace. On this fact, the present study was designed to measure the background ionizing radiation (BIR) dose levels of some quarry sites located in Ebonyi State, Nigeria.

MATERIALS AND METHODS
Study locations: Generally, the study location, Ebonyi State, Nigeria lies between 6°15ꞌN and 8°05ꞌE with low-lying to gentle undulating terrain of about 85-100 m above sea level and punctuated by few isolated lowhills (Ezepue et al., 1984). The state is situated on the plains of Southeastern Savanna belt and southern end of the lower Benue trough of Nigeria (Chiadikobi et al., 2011). The state is associated with the occurrence of igneous intrusion and volcanic with sedimentary rocks.
In achieving the objectives of the study, ten (10) quarry sites within the state were identified as indicated in Table 1. The sites were carefully selected in such a way as to cover all the local government areas of the state where quarrying is actively functional ( Figure 1) and also to cover the different geological formations that exist in the state.  These two sections are referred to as the active sections (Akerblom and Mjones, 1994) of rock quarrying.
The excavation section is the central point of granite excavation which in most of the quarry sites is very deep and the quarrying section is the central point where the excavated granites are crushed to various sizes. Measurements of BIR dose rates in air were carried out using a portable hand survey GQ GMC-320 Geiger Counter nuclear radiation detector (GQ Electronics LLC, USA). The features and functionality of the meter were described in Ugbede and Benson (2018) and Ugbede (2018).
An in-situ approach of BIR measurements was adopted with the standard practice of positioning the meter at height of 1.0 m above ground level (Rafique et al., 2014;Agbalagba et al., 2016;Agbalagba, 2017;Ugbede and Benson, 2018). By this approach, the study areas can still retain their original environmental characteristics (Rafique et al., 2014;Agbalagba et al., 2016;Ugbede and Benson, 2018). The radiation detector window was positioned to facing suspected sources. Four repeated measurements, at interval of three minutes, for a period of twenty days was recorded for each location.
This was done to ensure that the fluctuating nature of background ionizing radiation occasioned by variation in environmental parameters and anthropogenic activities were accounted for (Agbalagba, 2017;Ugbede and Benson, 2018). The average of these repeated measurements was computed and presented in this report.

Estimation of annual effective dose (AED) and excess lifetime cancer risk (ELCR):
The obtained average BIR dose rates in units of µSv/h were used to estimate the annual effective dose (AED) as given in Equ. 1 (Ramli et al., 2014;Jindal et al., 2018;Ibikunle et al., 2019;Okeyode et al., 2019).

AED (mSv/y) = DR × T × OF × 10 1
Where DR is the measured absorbed dose rate in µSv/h, T is the total hours per year (8760 h), and OF is the outdoor occupancy factor, defined as 0.2 (UNSCEAR, 2000).
In order to assess the cancer risk of the quarry workers resulting from the BIR exposure, excess lifetime cancer risk (ELCR) was estimated using wellestablished model given in Equ. 2 (Taskin et al., 2009;Rafique et al., 2014;Agbalagba et al., 2016;Agbalagba, 2017;Ugbede and Benson, 2018;Jindal et al., 2018). The ELCR is a radiological risk assessment parameter that predicts the probability of cancer development by an individual over a lifetime exposure to low-dose radiation (Darwish et al., 2015;Ugbede and Benson, 2018).

ELCR = AED × DL × RF 2
Where DL is the average duration of life (70 years) and RF is the cancer risk factor per sievert (Sv −1 ). For low-dose background radiation considered to produce stochastic effects, the International Commission on Radiological Protection (ICRP, 2007) recommended a cancer risk factor of 0.05 Sv -1 for general public exposure. Table 2 shows the obtained results for the average BIR dose rates measured at the excavation section (ES) and quarrying section (QS) of the studied quarry sites. The estimated values of the AED and ELCR are also presented in Table 2. As shown, while the dose rates at the ES ranged from 0.14 to 0.18 μSv/h, that measured at the QS fluctuates between 0.16 and 0.19 μSv/h. Furthermore, the values obtained at the QS are all higher than those at the ES.

RESULTS AND DISCUSSION
The mean BIR dose rates were estimated to be 0.15 and 0.18 μSv/h for ES and QS, respectively with standard deviation of 0.01 μSv/h each. The noticeable difference in dose rates at the ES and QS can be attributed to high content of radioactive dust produced during quarrying/crushing of the excavated granitic rocks.
The radionuclides present in large rock size are dispersed alongside with dust particles into the immediate atmosphere. This observation is thus in consonance with Ugwu et al (2008) who reported high activity concentration of radionuclides 238 U, 232 Th and 40 K in quarry dust samples collected from quarry plants in urban area of Abakaliki, Nigeria. The variation of the BIR dose rates at the ES and QS are presented in Figure 2. This variation can be attributed to the geological location of the sites. As shown in Figure 2, the dose rates are higher than the worldwide average value of 84 nSv/h reported by UNSCEAR (2000). This indicates that the BIR dose levels of the examined quarry sites are elevated, signifying a radiological polluted environment. This is significant when discussing the radiological protection of the worker and the general public. While the current values are higher than those measured in some building types in Ondo State, Nigeria (Oladele et al., 2018), along river Alaknanda and Ganges, India (Sharma et al., 2014) and in Guilan Province, Iran (Basirjafari et al., 2014), they are however within worldwide range of 20 -200 nSv/h from different countries (UNSCEAR, 2000). Though the present dose rates in the quarry sites are elevated, they are still below the levels that can initiate immediate health effects to the workers (Sharma et al., 2014). However, continuous exposure may result to accumulation of low-level radiation doses (Ugbede and Benson, 2018) which may present long-term health effects in future. The estimated AED ranged from 0.25 to 0.32 mSv/y with mean of 0.27±0.03 mSv/y at the ES and 0.28 to 0.33 mSv/y with mean value of 0.31±0.02 mSv/y at the QS. The obtained AED values for all the sites are well above the outdoor worldwide average value of 0.07 mSv/y (UNSCEAR, 2000). This observation is illustrated in Figure 3. This further shows that the BIR levels of the studied quarry sites are quite elevated, an indication of a radiation contaminated environment. For radiological protection practices, ICRP (2007) recommended annual effective dose permissible limits of 20.0 mSv/y for occupational workers and 1.0 mSv/y for the general public. The present values are below these limits, thus indicating that the BIR levels of the studied quarry sites are still within acceptable limits and that no immediate radiological health threat may result from the current levels. However, as earlier noted, long-term health effects resulting from accumulative exposures may be manifested later at advanced age of the exposed individual. The reported AEDs are in similar range with means values of 0.288±0.045 mSv/y and 0.335±0.084 mSv/y in Okposi Okwu and Uburu salt lake areas respectively, both in Ebonyi State . For the ELCR, the obtained values are ranged from 0.86×10 -3 to 1.10×10 -3 with mean of 0.94×10 -3 at the ES and 0.98×10 -3 to 1.17×10 -3 with mean of 1.07×10 -3 . These ELCR values are higher than the world average of 0.29×10 -3 as documented by the United Nations Scientific Committee on the effects of Atomic Radiation (UNSCEAR, 2000). These values indicate probability of cancer incidence over a lifetime exposure of 70 years. Conclusion: Mineral mining and rock quarrying are parts of human activities that can elevate the background ionizing radiation (BIR) levels of the host environment. The present study has shown that the BIR dose levels of the investigated quarry sites in Ebonyi State are elevated, which suggest a radiation contaminated environment. While the current levels of BIR in the studied quarry sites are higher than the worldwide average. Long-term health effects due to accumulation of low-level radiation doses arising from