Radiological Risks Assessment of Ogun State Drinking Water

: The presence of radionuclides in water constitute health risk to man. The consumption of such water increases the likelihood of incurring cancer. Analysis will enhance the detection of significant radionuclides causing harm to the populace and stimulate remediation. This inferred the radioactivity measurements of twenty (20) well water samples from three industrial cities of Ogun State using Hyper Pure Germanium detector (HPGe). The pH and other physiochemical parameters were also determined. The result showed that the mean activity concentrations of 40 K, 210 Pb, 224 Ra, 232 Th and 238 U were found to be 1.86±1.70 Bql − 1 ; 7.93±2.40 Bql − 1 ; 3.60±1.25 Bql − 1 ; 8.48±2.10 Bql − 1 ; and 2.28±0.57 Bql − 1 respectively. The corresponding mean total annual effective dose for ages 0-1 y; 1-2 y; 2-7 y; 7-12 y; 12-17 y and > 17 y are: 9.14 mSv y − 1 ; 9.58 mSv y − 1 ; 6.87 mSv y − 1 ; 6.86 mSv y − 1 ; 11.91 mSv y − 1 ; and 5.68 mSv y − 1 respectively. The mean cancer mortality and morbidity risks respectively are: {(3.16 and 5.00)10 − 5 } for 40 K; {(575.74 and 759.67)10 − 5 } for 210 Pb; {(43.25 and 71.05)10 − 5 } for 224 Ra; {(63.37 and 96.03)10 − 5 } for 232 Th; and {(10.70 and 16.40)10 − 5 } for 238 U. The activity concentrations of 210 Pb, 224 Ra and 232 Th exceeded the World Health Organization (WHO) guidance level in all samples but one. The corresponding total annual effective dose for all the six age groups exceeded the recommended WHO standard of 0.1 mSv y − 1 in all samples. More importantly, there is high radiation risk in drinking the water from these wells and 210 Pb contribution to risk was the highest.


Introduction
Life will not survive on earth without water. It is the most important resource to man after air. Various sources of water exist, but the most accessible is that which is readily available to individual community. Groundwater harnessed as dug or drilled well, is the major source of water for homes and industries in the study area. Several researches have attested to the degree of groundwater usage in the study area (Alausa et al., 2014;Adekunle et al., 2013;Aladejana and Talabi, 2013;Odukoya et al., 2010;Orebiyi et al., 2008). Even in Nigeria, the inadequacy of public water has made well water the resulted alternative in meeting the required need for water. The study of Akujieze et al. (2003) revealed the abundant potential of groundwater resource in Nigeria. Yet, more than half of the population of Nigerian does not have access to safe drinking water (Orebiyi et al., 2010). The same challenge has been noted in the developing countries (Jacobsen et al., 2012;AMCOW, 2012;UNEP, 2010). Ground water is known to be the second smallest of the four main pools of water on earth, yet ground water besides surface water are the components of the hydrologic system that humans use most (Winter et al., 1998). It is therefore necessary to ensure that this indispensable resource is proven healthy for human use. WHO (2011), reported that several radioactive compounds are being released into the environment. These compounds find their way into drinking-water supplies through human activities and human-made sources. Kelleher (2015) reported that industry and agriculture are putting usable groundwater at risk. Literature also confirms varying concentrations of radionuclides in our water body (Rachkova and Shuktomova, 2015;Nwankwo and Balogun, 2013;Ndontchueng et al., 2013;Peterson et al., 2007;Kurttio et al., 2006). Drinking water is one of the pathways to human exposure to radiation. Evidence exist from both human and animal studies that radiation exposure at low to moderate doses may increase the long-term incidence of cancer (WHO, 2011). They noted that, no deleterious radiological health effects are expected from consumption of drinking-water if the concentrations of radionuclides are below the guidance levels (WHO, 2011).
A case-cohort study of drilled wells in Finland witnessed a small evidence of increased risk of bladder or kidney cancers from the ingestion of radionuclides through drinking from these wells (Kurttio et al., 2006). A population based case control study was reported in Thailand. The results showed a strong dose-dependent association between consumption of radium contaminated shallow well water and cancer of the upper digestive tract. Cancer risk also increased with the consumption of fresh water fish in the study area (Phatcha et al., 2006). Similar case control study identified significant correlation between other type of cancer and ingested radionuclides (Auvinen et al., 2002;. Contrariwise, ecological studies in Harris country associated birth defects to ingestion of high radium in water (Cech et al., 2008). Diverse epidemiological studies that estimated cancer risk against ingestion of radionuclides in water has been reported (Canu et al., 2011). Radionuclides embedded in the earth crust are the major contributor to increased radiation in ground water resources (Phatcha et al., 2006;UNSCEAR, 2000).
Ogun State is known for quarrying with rocks blasting and the quest for limestone. These activities among others have left the environment with high radionuclides content. Also, the study area is characterized with varying degrees of geologic formations (Adekunle et al., 2013). Rocks formation known for poor groundwater bearing properties and high concentrations of radionuclides are embedded in this vicinity (Farai and Vincent, 2006). Even, high level of radionuclides concentrations have been anticipated in part of the study area . Groundwater occurrence in the area is limited to the fractured and in-situ weathered portions of the rocks. Movement of water is strongly influenced by topography and springs. Recharge is mainly by percolating rainwater and by seepage from adjacent surface water (Aladejana and Talabi, 2013). According to Winter et al. (1998), surface and ground water are hydraulically connected and commonly interact throughout all landscapes (Winter et al., 1998). They further affirm that groundwater flow paths vary greatly in length, depth and travel time from points of recharge to points of discharge in the groundwater system. When radionuclides are released to the environment, they persist until they are lost through radioactive decay, causing radiation exposures into the future (UNSCEAR, 2000).
Research has shown that access to safe drinking water is a prerequisite to poverty reduction (Orebiyi et al., 2010). Numerous diseases will be averted when ingested water is safe. Naturally occurring radionuclides in drinking-water are often less amenable to control (WHO, 2011). Therefore, it is important to assess the concentration of radionuclides in our drinking water, so that remedial actions can be taken in order to minimize radiation risks.
Hence, this study was embarked upon to assess the magnitude of radiological risks due to radionuclides contamination in drinking water of the study area. So that remediation that will enhance the safety of the populace will be implemented.

Study Area
Three industrial cities of Ogun State namely Abeokuta, Ijebu Ode and Idiroko were used for this study. The population of the study area is about 1.1 million with a growth rate of about 3.9% annually (NBS, 2012;Jacobsen et al., 2012). The geological map of Ogun State showing the study area is as shown in Fig.  1. Abeokuta is basically basement complex in origin, Ijebu Ode belong to the Abeokuta formation while Idiroko is majorly coastal plain sands. The basement complex rocks are of Precambrian age to early Palaeozoic age, consisting of quartzites and biotite schist, hornblende-biotite, granite and gneisses. They are characterized by various folds, structures of various degree of complexity, faults, foliation among others (Adekunle et al., 2013;Okeyode, 2012). The Abeokuta formation consists of grey sand intercalated with brown to dark grey clay (Okeyode, 2012). The coastal sand area of the study environment is in contact with recent alluvium. Groundwater of the study area is subjected to severe degradation due to quarry activities, sand mining and the likes (Adedeji et al., 2014;Aladejana and Talabi, 2013;Jibiri et al., 2010).

Radiation Measurement
A total of 20 well (drilled and dug) water samples were collected randomly at different locations and depths. The locations of the wells are widely spread out to ensure that every region of the study area was covered. This is to ensure a wide coverage for the measurement of the level of radionuclides contaminations of well water in the study area, since the locations are characterized by different geology and topography. Sample preparation was carried out as documented by the International Atomic Energy Agency (IAEA, 1989).
Chemical analysis of all the samples was conducted using standard techniques according to APHA (1999). Gamma spectrometry measurements were carried out with coaxial-type HPGe detectors (Canberra Industries Inc.) of 50% relative efficiency and having a resolution of 2.4 keV at 1.33 MeV peak of 60 Co. The system was set up to cover about 2 MeV photon energy ranges over 4k channels. The detectors were properly shielded in lead castles.  Calibrations of the measuring system were carried out using certified reference standards for various radionuclides. Spectra analyses were performed with the Genie 2k spectrometry software, version 2.1 (Canberra Industries Inc). A library of radionuclides, which contained the energy of the characteristic gamma peaks for each nuclide analyzed and their corresponding emission probabilities, was built from the data supplied in the software. Each sample was counted for 86400 s to achieve minimum counting error. Specific activity of each radionuclide in water were expressed in Bql -1 of the water sample and corrected for the time elapsed time since the samples were collected from the well.

Results
The result of chemical parameters for 20 well water samples from three industrial cities of Ogun State namely Abeokuta, Ijebu Ode and Idiroko is presented in Fig. 2 Fig. 3 showed that the activity concentration of 238 U was below the guidance level in all the water sampled but for Abk 7. However, the activity concentrations of 210 Pb, 224 Ra and 232 Th are above the guidance level in all samples except Abk 4. The Guidance Level (GL) was adopted from the (WHO, 2011) water quality guidelines. Guidance level was not established for 40 K. It is known to be evenly distributed in the body; metabolic balance maintains its concentration in the body irrespective of the amount ingested (UNSCEAR, 2000).
The result depicted in Fig. 4 showed that ages 12-17 y are prone to higher risks from the ingestion of water from these wells than all the other age groups. Next to it is the infants (1-2 y), closely followed by the new born (0-1 y) and the adults (> 17 y) are the least prone.

Calculation of Total Annual Effective Dose
The annual effective dose (E) to an individual due to intake of natural radionuclides of 210 Pb, 224 Ra, 232 Th and 238 U from drinking well waters of Ogun State is estimated using Equation 1: where, E is the annual effective dose to an individual due to the ingestion of radionuclides (Sv y −1 ); A c is the activity concentration of radionuclides in the ingested drinking water (Bql −1 ); I A is the annual intake of drinking water (l y −1 ); and C F is the ingested dose conversion factor for radionuclides (Sv Bq −1 ). For this study, I A (l y −1 ) values for the different age groups are: 200; 260; 300; 350; 600; and 730 for ages 0-1 y; 1-2 y; 2-7 y; 7-12 y; 12-17 y; and > 17 y respectively (IAEA, 1996). C F values are adopted from ICRP (2012).  The total annual effective dose E T (Svy −1 ) to an individual is estimated by summing contributions from 210 Pb, 224 Ra,232 Th and 238 U present in the well water samples using Equation 2: The total annual effective dose E T (Svy −1 ) for six age groups from the ingestion of 210 Pb, 224 Ra, 232 Th and 238 U using Equation 2 is presented in Table 2. Contribution from 40 K was deliberately not included since it is homeostically controlled in the body (UNSCEAR, 2000). The absence of 210 Pb, 224 Ra, 232 Th and 238 U in Abk 4 warranted the blank total effective dose. The total annual effective dose for the six age groups range from 5.21±1.53 to 14.36±4.94 mSvy −1 ; 5.36±1.43 to 14.25±3.84 mSvy −1 ; 3.85±1.04 to 10.22±2.62 mSvy −1 ; 3.82±1.03 to 10.13±2.54 mSvy −1 ; 6.34±1.73 to 16.77±4.10 mSvy −1 ; and 3.31±0.92 to 8.75±1.82 mSvy −1 for ages 0-1 y; 1-2y; 2-7y; 7-12y; 12-17y; and > 17y respectively. All the values for the six age groups exceeded the World Health Organization (WHO) recommended standard of 0.1 mSvy -−1 .

Evaluation of Lifetime Cancer Risks
The lifetime cancer Risks (R) associated with intake of a given radionuclide was evaluated using Equation 3 according to Amakom and Jibiri (2010): where, R means lifetime cancer risks; r is cancer risk coefficient; A C is activity concentration; I A is the annual water consumption; and L E is life expectancy at birth. Pb has the highest value for both mortality and morbidity cancer risks estimate in all water samples of the study area.

Discussion on Activity Concentrations of the Water Samples
The presence of 40 K in all the water samples is indicative of its wide distribution and abundance in nature. Research has shown that 40 K represents 0.012 % of naturally occurring potassium that is found in large amounts throughout nature. Though homeostatically controlled in the body, its decay mechanism is associated with cell damage and renders it potent for cancer induction. Lifetime cancer mortality risk due to its ingestion is estimated as 2.2 ×10 −11 pCu −1 (Peterson et al., 2007). Absence of 210 Pb, 224 Ra,232 Th and 238 U in Abk 4 could be attributed to the depth of the well. It was the only well where the presence of 88 Y, 103 Ru, 131 I, 144 Ce and 192 Ir was detected. Other factors may be due to the wide variations in the concentrations of natural radionuclides even within very small confine (UNSCEAR, 2000). Also, slow movement of groundwater through aquifer resulting in groundwater contamination not been discovered long after it has occurred (USEPA, 2000).
Increased activity concentration in this study is expected. High radionuclides concentrations have been envisaged in water resources of the study area . Also, the geological formation of the area is known to be associated with elevated concentrations of radionuclides (Olurin et al., 2012;Farai and Vincent 2006). Similar studies have confirmed same in other part of the world (Bajwa et al., 2015;Jurgens et al., 2010;Believermis et al., 2009;UNSCEAR, 2000). The type and values of radionuclides recorded in Table 1 conform well to literature. Groundwater exhibits various forms of radionuclides that vary extensively in concentrations due to varying geology and disjointed aquifers. The chemical nature of radionuclide is another factor that influences the disparity in the concentration of radionuclides in ground water. (Kleinschmidt and Akber, 2008;IAEA, 1990). Anthropogenic impacts on the study environment are other factor traceable to the elevated level of their radionuclides. Groundwater pollution of the study area arising from point and non-point sources has been reported in literature (Ojekunle et al., 2016;Adekunle et al., 2013;Olurin et al., 2012).
The presence of 210 Pb and 224 Ra in this study is significant. As far as we know, their concentrations have not been determined or reported in literature in any water resources of the study area. Hence, the value reported in this study will serve as a base line data for future studies. However, several Researchers have investigated these radionuclides in different part of the world (Persson 2014;Jia et al., 2009;Kleinschmidt and Akber 2008;Szabo et al., 2005). Varying concentrations from low to very high levels were reported. High concentration of 210 Pb in well water was considered to be an ingrowth from dissolved 222 Rn (Manu et al., 2014;Kleinschmidt and Akber 2008). Contrarily, the distribution of 224 Ra was related to the presence of other constituents and to the physical characteristics of the aquifer system (Szabo et al., 2005).
Ogun State play host to the highest number of University campuses in Nigeria, it borders with several States and the Republic of Benin. Its proximity to Lagos metropolis subjects it to massive population explosion. The aftermath is rapid urbanization, industrialization and increased waste generation. Invariably, waste management is a major challenge. Therefore, high concentrations of radionuclides are inevitable in the study area because most well in this vicinity are of shallow origin and are liable to surface contamination (Orebiyi et al., 2010;Olurin et al., 2012).

Discussion on Radiological Risks
The result of total annual effective dose (E T ) for this study compared well with literature according to UNSCEAR (2013). Their report revealed that infants and children are more sensitive to radiation than adults and oftentimes are prone to higher radiation risks. They also affirmed radiation risks variability in children at different age groups especially in cancer induction. The age dependent factors that contribute to variation in radiation effects and risks includes: Size of individual and organs; growth patterns of the individual and tissues; intake and absorption of radiation; metabolic rates; and physical activities among others (UNSCEAR, 2013).
The high total annual effective dose is due to the high activity concentrations of 210 Pb, 224 Ra and 232 Th, with major contribution from 210 Pb. These finding compared well with literature as the concentration of 210 Pb does not depend on the concentration of its parent isotopes in a medium (Bonczyk, 2013;Health Canada, 2009). Rather, its contribution to dose by ingestion is known to be potentially large (Casacuberta et al., 2010).
The mortality and morbidity cancer risks estimate for 210 Pb was the highest. This is attributable to its high activity concentration in the water samples. Study has shown that 210 Pb concentrate in bones (Health Canada, 2009). Also, being a by-product of 222 Rn, its presence in any tissue where its parent isotope is found cannot be overruled. This calls for proper evaluation of 210 Pb. The mortality and morbidity cancer risks estimate for 238 U in this study compared well with that obtained by Amakom and Jibiri (2010). This is likely due to the close proximity of the two study areas as both areas have similar geological formation. However, the mortality and morbidity cancer risks estimate for 238 U in this study was higher than that obtained by Maxwell et al. (2015). This disparity can linked to the high radionuclides contents in this present study environment. Different geological formation of the two study areas can also be a contributing factor. High levels of 238 U and 232 Th series in drinking water increase the risk of certain cancers in the body (Colorado Health and Environmental Assessment, 2013;Wagner et al., 2011;Kurttio et al. 2006). ICRP (2007) noted that ingested radionuclides are absorbed in the blood stream and accumulate in specific tissues causing damage. Cells in kidney and bladder are irradiated when radionuclides are excreted in urine (Kurttio et al., 2006). Hence, the result of this study should be addressed by the appropriate authorities in the respective level of governance. Adequate measures that will alleviate the high radiation risk associated with drinking water from these wells should be adopted.  (
Consequently, the cumulative annual effective dose resulting from the ingestion of these radionuclides for six different age groups are: 9.14 mSv y −1 ; 9.58 mSv y −1 ; 6.87 mSv y −1 ; 6.86 mSv y −1 ; 11.91 mSv y −1 ; and 5.68 mSv y −1 for ages 0-1 y; 1-2 y; 2-7 y; 7-12 y; 12-17 y and > 17 y respectively. These derived values also exceeded 0.1 m Sv y −1 recommended by the World Health Organization in all the water samples. The mean cancer mortality and morbidity risks respectively were found to have least value for 40 K {(3.16 and 5.00)10 -5 } and highest value for 210 Pb {(575.74 and 759.67)10 -5 }. This is indicative that there is high radiation risk in drinking the water from these wells. Therefore, it is strongly recommended that these wells should be treated especially for 210 Pb whose contribution resulted in the increased risks. Also, further research on the trends of radionuclides in the study area should be investigated.

Funding Information
The processing cost of this manuscript was funded by Covenant University Ota, Ogun State, Nigeria.

Author's Contributions
Justina Ada Achuka: Initiated the idea, collected the data and contributed to the preparation of the manuscript.
Usikalu Mojisola Rachael: Developed the plan and contributed to the preparation of the manuscript.
Oyeyemi Kehinde David: Analyzed the data graphically and contributed to the preparation of the manuscript.

Ethics
There is no known ethical issue or conflict of interest that may arise after the publication of this manuscript.