Annual Effective Dose of Radon-222 in Well Water Samples in Male Adults: Idah, Nigeria

A total of 25 well water samples from various locations of Idah and environs including Ofukolo, Ega, Ede Adejo, and Ede Alaba, have been investigated for their 222 Rn concentrations using liquid scintillation counter manufactured by Packard Instrument Company. The concentration of 222 Rn in the well water samples was found to vary in the range 3.0±2.00 – 18.24±1.50 Bq/L and a mean concentration of 9.64 Bq/L with An average mean concentration of 10.23 Bq/L recorded respectively. The results showed that 222 Rn concentration in well water sources were greater than the maximum concentration limit (MCL) of 11.1Bq/L set by USEPA and 10.0 Bq/L set by WHO. The annual effective dose by ingestion for adult male was found to be in the range 0.0198 mSv/y to 0.1198 mSv/yand an average of 0.0721 mSv/y which are lower than the annually received effective dose as set by ICRP.


Introduction
Radon, a natural byproduct of the radioactive decay of uranium, radium and thorium, is analpha-emitting noble gas with a half-life of 3.8 days. Radon gas is soluble in water and consequently the gas may be incorporated into groundwater flows. Radon is extracted from the volcanic deposits in which the aquifer resides, its transport taking place basically through the fissure network in the fractured system or from mantle degassing. The quantity of radon dissolved in groundwater depends on different factors such as the characteristics of the aquifer, water-rock interaction, water residence time within aquifer, material content of radium, etc. ( [1], [2], [3]). Measurements of radon contents in groundwater have been performed in connection with geological, hydro geological and hydrological surveys health hazard studies. On the one hand, the half-life of radon and its solubility have allowed the use of radon gas as a natural groundwater tracer to identify and quantify groundwater discharge to surface waters [4], or to attempt to elucidate the type of rocks through which groundwater's flow [5]. On the other hand, the presence of high levels of radon in drinking water constitutes a major health hazard ( [6], [7], [8]). The Commission of European Communities (CEC) recommends the monitoring of radon levels in domestic drinking water supplies originating from different types of groundwater sources and wells in different geological areas, in order to determine consumer population exposure. The limit is fixed at below 100 Bq/L [9]. Well water in the investigated area plays an important role in guaranteeing water supply for agricultural and domestic purposes. The content of radon in water samples must be determined by reliable methods. Radon is a very mobile gas and it can escape from water with ease during the process of sampling and transportation, hence careful sample preparation is necessary [27]. Several procedures can be found in the literature to perform measurements of radon in groundwater using different techniques such as Lucaas-cell, ionization chambers, solidstate detector, or gamma spectrometry ( [10], [11], [12], [13], [14]). The aim of this study was to measure radon concentration in well water from different locations in Idah and its environs and evaluate the annual effective dose by ingestion from the radon concentrations so measured for the locations.
Idah is a local government in Kogi state Nigeria. In the study area Idah, and its environs there is total dependence and reliance on ground water source for drinking, agricultural purposes and domestic usage. Idah is bounded by latitudes 7° 49N -7° 62 N and longitudes 6° 44 E -6° 75 E. It has a total landmass of 36km 2 (14Sq. mile) and a population of 76, 815 persons at the 2006 census [15]. The study area falls within the forest region of Nigeria [16], the area is underlain by Gneisses, magnetite and metamorphic rocks of Precambrian age which have been intruded by series of granite rocks which are sources of uranium, the parent of radon-222. However, some portion of the study area fall within region underlain by sand stones (sedimentary rock) which could provide a source of water through the tapping of the aquifers while the remaining portion is underlain by igneous rocks [17].

Materials and Methods
The following materials were used in this research as listed by [18]  The plastic bottles were first wash cleaned and rinsed with distilled water to avoid radon present in the samples from being contaminated or absorbed. The samples were collected with the aid of a bailer, but the stagnant water in the wells was first purged by drawing it out and allowing the well to refill, this was done severally to ensure fresh samples were obtained. The samples were taken to the laboratory immediately after collection without allowing them to stay long (three days maximum) for analysis. This is done so as to achieve maximum accuracy and not to allow the composition of the sample to change.
10ml each of the water samples were transferred into a 20ml glass scintillation vial to which 10ml of insta-gel scintillation cocktail is added. Having been sealed tightly, the vials were shaken for more than two minutes to extract radon -222 in water phase into the organic scintillate, and the samples collected were then counted for 60minutes in a liquid scintillation counter using energy discrimination for alpha particles.
The prepared samples were analyzed by using Liquid Scintillation Counter (Tri-Carb LSA 1000TR) model manufactured by packard Instrument Company located at the centre for Energy Research and Training (CERT), Ahmadu Bello University, Zaria -Nigeria, after they were allowed to stay for three hours for equilibrium to be attained between radon-222 and its daughter progeny.
The 222 Rn concentration in a sample of water is determined using the formula. (1) Where Rn is Radon level in BqL -1 CS is Sample Count/Second CB is Background Count/Second CF is Conversion factor D is Decay Constant To calculate the annual effective dose of 222 Rn through drinking water, an equation as proposed by the United Nation Scientific Committee on the Effects of Atomic Radiation [19] was used. The highest concentration of radon-222 from well water was recorded from Ofukolo with a range of 12.93Bq/L to 14.12Bq/L and a mean concentration of 13.58Bq/L. The radon concentration of samples from Ede-Adejo area ranges from 8.99Bq/L to 14.03Bq/L and a mean radon concentration of 11.81Bq/L, which are higher than the 11.1Bq/L set by [7] 10.0Bq/L by [20] recorded. This was followed by samples from Ede-Alaba and Ega, both having radon concentration ranging from 7.48Bq/L to 12.97Bq/L, 3.01Bq/L to 6.00Bq/L and a mean concentration of 9.86Bq/L and 5.70Bq/L respectively, which are lower than the 11.1Bq/L set by [7] and 10.0Bq/L by [20]. The frequency distribution of radon concentration in well water is presented on Figure 2. The high radon concentration recorded for these areas may be related to the radon source 234 U and 226 Ra in the water -rock system present in the areas especially in Ofukolo and Ede -Adejo areas which posed a greater health risk when ingested along with water [24], because well sunk in areas with underground rock tend to show high content of granites to which radon is associated, in which radon-222 from fractures and cavities in rocks and in the regions influenced by local and remote anthropogenic radon sources [23]. Another reason that are attributed to this high radon concentration are human activities such as farming and other natural phenomenon such as weathering and volcanic action can also influence water radon content [26]. The results have shown a range of 222Rn concentration between 3.01±2.00 Bq/L to 18.24±1.83 Bq/L with a mean radon concentration of 9.64Bq/L and an average mean radon concentration of 10.23 Bq/L recorded for the study location. 75% of the water samples were found to be above the maximum contamination level of 11.1Bq/L set by [7], 10.0Bq/L set by [20] and [21]. The annual effective dose by ingestion for both adult male and female in the study area range from 0.0198 mSv/y to 0.1198 mSv/y with a mean annual effective dose by ingestion of 0.0.721 mSv/y were recorded. This value is less than the recommended limit by ICRP [22]. The result of annual effective dose by ingestion revealed the likelihood of an associated risk of stomach cancer over many years which call for radon reduction of well water in the study area.

Conclusion
The present study showed that the radon concentration in the well water samples from Idah has been found to have an average mean 222 Rn concentration of 10.23Bq/L for well water. The result show that recorded radon concentrations were above the maximum limit of 11.1Bq/L set by [7] and 10.0Bq/L by [20] and [21] which call for immediate action for radon reduction. Also comparing the result with the value 0.1Bq/L set by the [25], it was observed that all the water samples assayed for radon concentration are not safe for domestic purposes and consumption. It is expected that the people of Idah and its environs may likely suffer from stomach cancer, lung cancer, leukemia, chronic diseases etc. because of the high 222 Rn concentration in well water from these sampled location as they are taking the water directly without no or proper treatment for radon in water. The annual effective dose by ingestion was found to be in the range of 0.0198 mSv/y -0.1198 mSv/y with an average annual effective dose by ingestion found to be 0.0721 mSv/y which is lower than the recommended limit for members of the public of 1.0mSv/y and an intervention level of 3 -10mSv/y set by the International Commission on Radiological Protection [22]. The results showed the samples from well water in this study are not safe for human consumption. Average mean concentration of 222 Rn in well water was found to be higher than the standard limit s set by WHO and USEPA. The highest and lowest concentrations of radon were recorded for Ofukolo and Ega areas. The highest and lowest proportion of effective dose to the standard effective dose by [22] was found to belong to Ofukolo and Ega area. The annually received effective dose through the 222 Rn of well water in Idah was lower than the standard limit of 0.1 mSv/y by [22]. Hence, the data obtained in this study could be used for the study location, since this work pioneer the determination of radon in well water in the study location.