ASSESSMENT OF AGE-DEPENDENT EFFECTIVE DOSE AND TOXICITY RISKS OF 226 Ra AND 228 Ra IN RIVER WATER SAMPLES IN ONDO STATE. NIGERIA

Activity concentrations of natural radionuclides in some river waters in southwestern Nigeria were measured using high-purity germanium (HPGe) detector. The activity concentrations ranged from 0.12 to 2.31, 0.17 to 2.85, and 7.86 to 65.51 Bq l -1 for 226 Ra, 228 Ra, and 40 K respectively. The calculated mean of the total annual effective dose were 9.86, 2.46, 1.71, 2.43, 5.74 and 0.99 mSv y -1 for age groups ≤1y, 1-2y, 2-7y, 7-12y, 12-17y, and >17y respectively. Estimated cancer mortality and morbidity risks ranged from 0.04x10 -3 to 0.77x10 -3 with a mean of 0.42x10 -3 and 0.04x10 -3 to 0.80x10 -3 with a mean of 0.44x10 -3 respectively for 226 R while they ranged from 0.11x10 -3 to 1.89x10 -3 with a mean of 0.96x10 -3 and 0.16x10 -3 to 2.66x10 -3 with a mean of 1.38x10 -3 respectively for 228 Ra. The lifetime average daily dose (LADD) of 226 Ra and 228 Ra ranged from 9.39x10 -14 µg kg -1 d -1 to 181.01x10 -14 µg kg -1 d -1 with a mean of 100.06x10 -14 µg kg -1 d -1 and 4.82x10 -16 µg kg -1 d -1 to 80.70x10 -16 µg kg -1 d -1 with a mean of 40.90x10 -16 µg kg -1 d -1 respectively.. Radiological hazard indicator of radium is of concern in these drinking river water samples.


Introduction
Natural radioactivity in water has been attracting widespread attention because of the health problems that radioactive materials cause when they enter the human body through drinking water. Naturally occurring radionuclides such as uranium, thorium, radium and their decay products in drinking water give rise to radiation exposure through the drinking water pathway. It has been reported that the average worldwide radiation exposure to natural sources in foods and drinking water is 0.29 mSv y -1 , made up of about 0.17 mSv y -1 from 40 K and about 0.12 mSv y -1 from uranium and thorium [1]. The World Health Organization (WHO) has recommended safe values for various drinking water quality parameters in its general guidelines [2], which has been used by many countries to formulate their own national water quality guidelines.
Radium is regarded as a highly toxic element in water. It exists in four naturally occurring isotopes 223 Ra , 224 Ra, 226 Ra and 228 Ra. Many studies have been conducted on the occurrence and levels of 224 Ra, 226 Ra and 228 Ra in drinking water [3][4][5][6][7][8][9]. While radium-223 is a decay product of 235 U ( a rare uranium isotope), 224 Ra and 228 Ra originate from the decay of 232 Th.
Radium -226 is a decay product of 238 U. Whereas both 223 Ra and 224 Ra are very short-lived radium isotopes with half live of 11.4 days and 3.6 days respectively , 226 Ra and 228 Ra are more stable with half-lives of 1602 years and 5.75 years respectively and hence the predominant radium isotopes in groundwater. When disintegrating, these radium isotopes emit nuclear radiations that can penetrate and ionize matter to various levels. Although 224 Ra and 226 Ra essentially emit α-radiation, which is believed to be the cause of prevailing deleterious health effects or tissue damage [3,10]. 228 Ra emits β-radiation. Radium also releases some γ-radiation. Radium enters surface and groundwater systems through processes such as aquifer solid weathering, direct recoil over the liquid-solid limit, desorption from the sediment surfaces, etc. The movement of radium in water is dependent on the geochemical properties of solids in the aquifer. Its activity concentration in groundwater depends on its activity concentration in the bedrock, mechanisms like precipitation, dissolution, complexation and adsorption-desorption, which affects its transport in water. All the processes are related to the chemical composition of groundwater [11] Due to its similar metabolism to calcium, 226 Ra is a bone-seeker. Hence it is in the skeleton that the animal ingesting 226 Ra would have the greatest body burden. After ingestion, about 80 to 85% of radium is promptly expelled from the body through faeces while the rest 15 to 20% enters the blood stream and is transported to all parts of the body and is deposited on bone surfaces [8]. The radium deposits on bone surfaces are eventually covered by new deposits and old deposits penetrate deeper into the bone, where it builds up in growing bones and remains in the skeleton for a long time where it can cause bone cancer. Exposure to higher levels of radium than the Maximum Contaminant Level (MCL) over a long period of time may also result in other harmful effects like anaemia, cataracts, fractured teeth, acute leucopenia, necrosis of the jaw cancers (other than bone cancer) and even death. The adverse effect depends on its amount in water (dose), quantity of water consumed, how long an individual is exposed to it (duration) and individual characteristics such as age, state of health, lifestyle etc.
The U.S. Environmental Protection Agency [12] has established MCL of 0.185 Bq l -1 for combined radium ( 226 Ra plus 228 Ra) for drinking water whereas the MCL fixed by the World Health Organization [14] is 1.0 Bq l -1 and 0.1 Bq l -1 for 226 Ra and 228 Ra respectively. Health Canada's [15] MCL for 226 Ra only is 0.5 Bq l -1 .
Since there is no adequate supply of pipe borne or potable water in many rural communities in Nigeria, people of those communities turn to dug wells, rivers and streams (surface waters) as sources of drinking water. No water treatment plant or local water purification system is available for treating their drinking water in these rural areas to reduce or remove the microbial or radioactivity load. River water is used for direct drinking and other purposes without filtering by the local population. The area chosen for this study, with a population of about 510,700, is one of such rural areas in southwestern Nigeria. So, it is necessary to determine the activity concentrations of natural radionuclides in water of these rivers and quantitate radium in the drinking water in order to measure its dose and risk to the target demographic group. Maxwell et al. [9] reported a range of 0.44 to 2.7 mBq l -1 226 Ra activity concentration in groundwater-based drinking water samples in Dawaki,Kuje, Giri and Sabon-Lugbe area of Abuja, North Cental Nigeria. Adekoya [15] reported a range of 1.62 to 3.17 Bq l -1 226Ra activity concentrations in potable drinking water samples from former tinmining areas with elevated activity in Jos, Nigeria. Agaja and Ajisafe [16] reported a range of 0.5 to 5.6 Bq l -1 226 Ra activity concentrations in borehole and surface (river) drinking water samples on coastal communities of Delta state, Nigeria. In a similar investigation carried out in Tanke, Ilorin, Nigeria, Nwakwo [17] reported 226Ra activity concentration ranging from 0.8 to 7.4 Bq l -1 Available literatures on this subject shows that activity concentration of 226 Ra and 228 Ra in drinking water has not been determined in any part of the Bitumen belt of Ondo State, Nigeria. This study was aimed at determining the natural radionuclides content of water of some selected rivers in Okitipupa and Irele areas of Ondo State, southwestern Nigeria as well as the mass concentration of 226 Ra and 228 Ra , and assessing the radiological implications of human exposure to ionizing radiation emitted by these radionuclides.

Study Area
Ondo State lies between longitude 4°30 ' E and 6°0'E and latitude 6°0'N and 7°30'N with mean annual rainfall of 1,150 mm in the northern areas to about 2,000 mm in the southern areas; relative humidity between 70% and 85% in the southern part and less than 78% in the northern part, with mean temperature of 21°C in the south and 32°C in the north [18].

Okitipupa and Irele Local Government Areas fall within the bitumen belt of Ondo
State and are predominantly populated by the Ikales of Yoruba extraction. Rivers in the area include; Ominla, Akeun, Ufara, Otu, Oha and Oni, and the vast River Oluwa. The waters of these rivers serve as a major source of drinking water and livelihood to the people of this region, since fishing is their main occupation. The people also use the waters for drinking, bathing, washing and other domestic purposes without using any water treatment methods to remove radioactivity. Bottom river sediments are also dug and used in building construction in the region.

Sample collection and preparation
In this study, river water samples were taken along the course of five major rivers; Akeun, Oha, Oluwa, Otu and Ufara in Irele and Okitipupa Local Government Areas of Ondo State, Southwestern Nigeria. Ten water samples were taken from the rivers, two samples per river.
The water samples were transferred into 1 litre polyethylene gallons with 1 ml of dilute hydrochloric acid added to it in order to prevent adherence of the radionuclides to the walls of the containers. Each water (with a volume of 1 litre) sample was placed in a Marinelli beaker. The Marinelli beakers were properly sealed with polyvinyl chloride (PVC) tapes to prevent escape of 222 Rn and 220 Rn from the samples. The samples in the Marinelli beakers were stored for four weeks to allow time for 222 Rn to attain a state of secular equilibrium with its short-lived decay products prior to gamma spectroscopy. After these, the samples were taken to the Ghana Atomic Energy Commission (G.A.E.C), Legon, Accra, Ghana for analysis with the hihg-purity germanium (HpGe) detector.

Sample Analysis
The activity concentrations of the samples were measured by using a computerized gamma ray spectrometry system consisting of a high purity germanium (HPGe) detector of 40% relative efficiency coupled to conventional electronics, which was connected to a multichannel cnalyzer card (MCA) set up on a desktop. The resolution of the spectrometer The activity concentrations (Asp) of 226 Ra, 228 Ra and 40 K in Bq kg -1 for the water and sediment samples were determined using the expression [19] ` where, Csam is the final count of the radionuclide in a sample, Pε is the emission probability, E is the efficiency of the detector, CT is the counting time, V is the volume of sample.

Calculation of Mass Concentration of 226 Ra and 228 Ra
The mass concentrations of 226 Ra and 228 Ra in the water samples were calculated from their activity concentrations using where ABq is the activity of 226 Ra or 228 Ra (Bq), MRa is the atomic mass of 226 Ra or 228 Ra (g mol -1 ), NA is Avogadro constant (6.022x10 23 mol -1 ), and t1/2 is the half-life of 226 Ra or 228 Ra (1602 y and 5.75 y respectively

Annual effective dose rates of the river water samples
The annual effective dose from ingestion of radionuclides in the water samples was estimated on the basis of the activity concentrations of the radionuclides, the volume of the water intake, which depends on the age of the person taking the water, and the age-dependent dose conversion factors for the radionuclides. The annual effective dose due to water intake was calculated using the equation [1] Ed=AcAICf (4) The total annual effective dose D (mSv y -1 ) to an individual was established by summing contributions from all radionuclides present in the water samples i.e.
where i are the radionuclides 226 Ra, and 228 Ra, Ac is the activity concentration of the radionuclide in the water (Bq l -1 ), Ai is the annual intake of the drinking water (l y -1 ) and Cf is the ingested dose conversion factor for 226 Ra and 228 Ra (Sv Bq -1 ), which varies with both radionuclides under consideration and the age of individuals ingesting the radionuclides.
The intake rates and conversion factors used in this study were based on the International Commission on Radiological Protection [20] for age groups 0-1 y, 1-2 y, 2-7 y, 7-12 y, 12-17 y, and >17 y old with annual average water intake estimated as 200, 260, 300, 350, 600 and 730 litres respectively.
The dose conversion factors for ingestion of natural radionuclides for members of the public, according to the ICRP are different with respect to different age groups. Table 1 shows the various dose conversion factors for different age groups for the natural radionuclides ( 226 Ra and 228 Ra ) in the water samples analyzed. Toxicity Radiological risk assessment of radium in the water samples.
To assess the health hazards due to the ingestion of radium in the water samples in this study, two types of radium toxicity were evaluatedradiological toxicity due to 226 Ra and 228 Ra as emitters of high ionizing power radiations and chemical toxicity due to their being a heavy elements.

Radiological toxicity assessment
The radiological toxicity of radium was estimated as the life time cancer risk (LTCR) due to ingestion of radium in the water samples using [21,22].
where Ac is activity concentration of radium( 226 Ra, and 228 Ra) in drinking water sample, Cc is Cancer risk coefficient, Vc is Volume of water consumed and Le is Life expectancy.
Cancer risk coefficients of 7.17x10 -9 and 1.04x10 -8 Bq -1 of 226 Ra, and 2.00x10 -8 and 2.81x10 -8 of 228 Ra for mortality and morbidity respectively were taken from EPA [23] and UNSCEAR [1]. The average Nigerian life expectancy at birth is 45.5 y, that is about 16,619 days for both males and females [1] and average adult daily consumption of water is about 2 l [13].

Chemical Toxicity Assessment
The chemical toxicity (non-carcinogenic) risk due to ingestion of radium in the water samples was assessed in terms of the Lifetime Average Daily Dose (LADD). LADD is expressed as the quantity of the toxic substance taken into the body per kilogram of body mass per day.
It was calculated using [14,21,24,25] = 365 where C is the mass concentration of 226 Ra or 228 Ra (µg l -1 ), IR is water consumption rate (l day -1 ), ED is total exposure duration (y), EF is exposure frequency (days y -1 ), BW is average body mass of consumer (kg) and AT is average time, which is the life expectancy (y). The water consumption rate was set at 2 l day -1 . The total exposure frequency was 365.25 days and total exposure duration was 45.5 y (about 16,619 days ) and average man's mass of 70 kg was used for the calculation of LADD.

Hazard quotient
Hazard quotient of a toxic material is a measure of he extent of damage done as a result of the ingestion of the material. In this context, the materials are 226 Ra and 228 Ra ingested in the investigated drinking water samples. Hazard Quotient (HQ) was calculated using [22,26,27] = (8) where LADD is the Lifetime Average Daily Dose and RfD is reference dose, equal 1.12 µg kg -1 day -1 [13] Results and discussion Activity concentrations of natural radionuclides in river water samples.
Measured activity concentrations of 226 Ra, 228 Ra and 40 K in water samples of the rivers and the total annual effective dose for the six ICRP age groups 0-1 y, 1-2 y, 2-7 y, 7-12 y, 12-17 y and 17 y are displayed in Table 2. The activity concentrations of 226 Ra varied from 0.12±0.02 Bq l -1 in Oluwa river water sample (Opa) to 2.31±0.35 Bq l -1 in Akeun river water    All the river water samples show annual effective doses that exceed the World Health Organization safe limit of 0.1 mSv y -1 from drinking water for all age groups [13,28] except river Oluwa water for age groups 1 -2, 2 -7 y, 7 -12 y and > 17 y . All samples cross the safe limit of annual effective dose of 1.0 mSv y -1 set by the International Commission on Radiological Protection [29]. Water samples from River Ufara present the highest radiation doses to all age groups while River Oluwa water samples impart the least radiation dose from all radionuclides investigated. The variations of total annual effective dose values in the different rivers are shown in Fig. 2.  In like manner, Ajayi and Owolabi [32] reported annual effective doses in the range 0.05 to 481.60 mSv y -1 , 0.02 to 76.84 mSv y -1 and 0.01 to 35.80 mSv y -1 for age groups < 1 y, 2 -7 y and >17 y respectively in drinking water from private dug wells in Akure. Nigeria. Also Ajayi and Achuka [33] reported annual effective doses in the range 0.04 to 6.82 mSv y -1 , 0.01 to 1.36 mSv y -1 and 0.01 to 1.49 mSv y -1 for age groups < 1 y, 2 -7 y and > 17 y respectively in drilled and dug well drinking waters of Ogun State. Nigeria.
Nwankwo [17] reported annual effective doses in the range of 0.81 -1.74 mSv y -1 (with mean value 1.30 mSv y -1 ) in groundwater for adults in Tanke-Ilorin, Nigeria. Ndontchueng et al. [34] reported annual effective doses in the range of 0.009 to 0.159 mSv y -1 ( ̅ = 0.050 mSv y -1 ), 0.008 to 0.147 mSv y -1 ( ̅ = 0.046 mSv y -1 ) and 0.003 to 0.045 mSv y -1 (0.015 mSv y -1 ) for infants, children and adults respectively in some mineral bottled water samples produced in Cameroon. All the results show that infants (from birth to 2 y of age) receive greater radiation doses from drinking water than adults (> 17 years of age). The accumulation of radiotoxic materials like 226 Ra and 228 Ra and their precursors and progenies in growing bones of babies and children can cause bone cancers. Humans of all ages that consume these waters face the risk of some health hazards resulting from significant buildup of radium in their bones and other radiosensitive soft body tissues.
The majority of households in the study area do not have access to pipe borne (treated) water supply, they rely on water from these rivers and streams in the region for drinking. So, water from rivers in the study area is not safe for drinking for those age groups.  Table 4 and Table 5 respectively.   [14,24,35], the 30 µg l -1 limit recommended by WHO [13], the permissible limit set for drinking water in the USA [36] and the 60 µg l -1 limit set for India [37].

Lifetime cancer risks
The lifetime cancer risks associated with the ingestion of 226 Ra and 228 Ra radium in the drinking waters of the rivers were evaluated in terms of mortality and morbidity risks. While cancer mortality risk concerns deaths with cancer as the underlying cause in a specified population, morbidity risk is concerned with the amount of cancer within the population. The calculated cancer mortality risk due to the ingestion of 226 Ra are displayed in Table 4, and ranged from 0.04x10 -3 to 0.77x10 -3 with mean value of 0.42x10 -3 whereas cancer morbidity risk ranged from 0.04x10 -3 to 0.80x10 -3 with a mean value of 0.44x10 -3 . Table 5 shows the calculated cancer mortality due to the ingestion of 228 Ra in the water sampled ranged from 0.11x10 -3 to 1.89x10 -3 with a mean value of 0.96x10 -3 while its morbidity risks ranged from 0.16x10 -3 to 2.66x10 -3 with a mean value of 1.38x10 -3 . The mean values of both lifetime cancer risks due to ingestion of both 226 Ra and 228 Ra in the water samples were below the maximum limit of 1.0x10 -3 specified by USNRC [38]. About 40% of the water samples display 228 Ra cancer mortality risk values above the maximum limit of 1.0x10 -3 specified USNRC [38] while up to 70% exhibit cancer morbidity risk above 1.0x10 -3 USNRC limit. The mean carcinogenic risks values of 0.42x10-3, and 0.44x10 -3 (mortality) and1.38x10 -3 (morbidity) in this study area are higher than those reported for Odeda Area (1.46x10 -4 and 2.24x10 -4 respectively) by Amakom and Jibiri [39] and Ago-Iwoye (1.09x10 -4 and 1.68x10 -4 respectively) by Alausa et al. [40] in Nigeria..

Lifetime Average Daily Dose
The calculated lifetime average daily dose (LADD) due to the ingestion of 226 [13]. The data presented in tables 4 and 5 show that all of the river water samples have LADD values that are very much lower than the WHO permissible limit.

Conclusion
The results obtained in this study show that the total annual effective doses of radiation from investigated drinking river water samples to all age groups exceed the 0.1 mSv y -1 safe limit set by WHO [13] and the 1.0 mSv y -1 limit set by ICRP [20] for drinking water. They also indicate that the mass concentrations of 226 Ra and 228 Ra in all the samples are below the 30 µg l -1 permissible limit set by WHO [13]. Whereas about 40% of the water samples display 228 Ra cancer mortality risk values above the maximum limit of 1.0x10 -3 specified USNRC [39], up to 70% exhibit cancer morbidity risk above 1.0x10 -3 USNRC limit. All the river water samples collected from the study area show lifetime average daily dose and hazard quotient values lower than the acceptable limit of 1.0. Therefore, the radiological toxicity of radium should be a matter of more interest to the population in the study area than its chemical toxicity risks.

Conflicts of interest/ competing interest: None
Funding: No funding was received for conducting this study LEGENDS Main Figures   Fig. 1 Average Annual Effective Dose to Different age groups from river water consumption  Tables   Table 1 Dose conversion factors (Sv Bq -1 ) for ingested natural radionuclides for the general public. Table 2 Activity concentrations of radionuclides in the water samples and total annual effective dose to different age groups Table 3 Minimum, maximum and mean activity concentrations of 226 Ra, 228 Ra and 40 K in the river water samples Table 4 Radiological and chemical toxicity due to ingestion of 226 Ra in river waters of the study area. Table 5 Radiological and chemical toxicity due to ingestion of 228 Ra in river waters of the study area.