STATISTICAL ANALYSIS OF RADON CONCENTRATION IN BOREHOLE WATERS; CORRELATION TO GEOLOGICAL FORMATIONS KATSINA STATE, NORTH WESTERN NIGERIA

Radon in borehole water serves as a source of natural radioactivity exposure through indoor air. Determination of naturally occurring radionuclides concentration in ground and surface waters is useful as a contribution to environmental health studies. In this research, statistical analysis of radon concentration in (borehole) water was conducted in Katsina state. The aim of this study is to analyse statistically the concentration of radon in borehole water in Katsina state and to establish a possible relationship between radon activity concentration in borehole water and underlying rock types. A total of 110 water samples were collected and analysed using Tri-carb 1000 Liquid Scintillation Counter (LSC) at the Centre for Energy Research and Training, Ahmadu Bello University. The overall average concentration of 222 Rn was found to be 69 ± 3 Bql -1 with geological formations, G8 and G7 having the highest and lowest concentrations with values of 75 ± 10 Bql -1 and 57 ± 4 Bql -1 respectively, which are higher than the world average values of 10 Bql -1 set by WHO and 11.1 Bql -1 set by USEPA. The results showed that radon concentrations are clearly correlated to rock types with acidic intrusive rocks associated with values which sedimentary rocks are associated with lower concentrations.


INTRODUCTION
Factors that influence the release, uptake and distribution of natural radionuclides or even the source materials may change the concentration of radionuclides in groundwater over several orders of magnitude (Isam et al., 2002). The radionuclide of concern in drinking water is, ( 222 Rn). Because ( 222 Rn) in groundwater constitutes a source of natural radioactivity to indoor air, therefore, assessment of radon concentration in ground and surface waters is of paramount importance as an input to environmental health research. Although radon is chemically unreactive, but under some rare conditions it can form compounds like clathrates and fluorides (Stein, 1983). It has the highest density and solubility of 9.37 g l -1 and 510 cm 3 l -1 respectively amongst other inert gases (Bunger and Ruhle, 1994;Bello et al., 2020). Lithology of the water bearing layer mostly control the concentration of 222 Rn in groundwater (Banks et al., 1998;Misdaq and Elharti 1997). High concentrations of 222 Rn could lead to detectable levels of 210 Pb and 210 Po, even though, sorption processes may affect the corresponding concentrations (Isam et al., 2002). It has been reported that 222 Rn transfers from bedrock to groundwater through an alpha recoil process followed by diffusion (Bonotto and Andrews, 1999;Sun and Senkow, 1998). In dwellings, 222 Rn is usually transferred according to normal water usage, from water to air by out-gassing, especially if the water is agitated (Isam et al., 2000). The ratio of the escaped 222 Rn from water to the indoor air to the 222 Rn concentration in water is of the order of 10 -1 per Bql -1 (Hess et al., 1985(Hess et al., , 1990. The primary routes of potential human exposure to radon are; inhalation and ingestion from dissolved radon in water. Although, high concentration of radon in groundwater may contribute to radon exposure through ingestion, the exposure risk through inhalation of radon released from water is usually more significant (Pourhabib et al., 2021). When radon gas is inhaled, alpha particles emitted by its short-lived progenies ( 218 Po and 214 Pb) which are highly-ionized, can interact with the biological tissues in the lungs which could ultimately results in carcinogenic effects (Isam et al., 2002). Exposure to radon at any level over a period of time can lead to its related health effect, it is therefore unlikely that there is a threshold concentration below which radon does not have the potential to cause lung cancer (ICRP, 2007). Different studies associated with radon concentration in water have been conducted in different parts of Nigeria (Abba, et al., 2020;Aruwa, et al., 2017;Bello, et al, 2020;Garba et al., 2011;Garba et al., 2012;Joseph et al., 2018). In Katsina State, however, more research associated with radon concentrations and its related health hazards in different water sources need to be conducted in order to fill in the existing data gap. This study been a pioneer, aimed to investigate and establish a possible statistical relationship between radon concentration in borehole water and the underlying geological formations. This result therefore, will serve as a baseline data for regulation and monitoring purposes.

METHODOLOGY Study Area
The study area is one of the states of Nigeria. It is situated within latitudes 11° 8 1 , 13° 22 1 North and longitudes 6° 52 1 , 9° 20 1 East, with a total land mass of approximately 24,192 km 2 . The study area has nine different geological formations

Samples Collection and Preparation
Borehole water samples were collected based on different geological formations of the state. A total of 110 water samples were collected which widely cover the study area using 2L sampling bottles. The samples were collected after the boreholes were allowed to run for about ten minutes before collection so as to have a more turbulent flow with uniform radon content. The bottles were filled and haematically sealed after concentrated HNO3 acid was added to prevent absorption and precipitation of particulates on the container's wall. A handheld GPS device was also used to record the corresponding coordinates. The collected samples were then transported to laboratory for preparations. 24 About 10 ml from each sample bottle was drawn using syringe and transferred immediately into liquid scintillation vials, in which already there is 10 ml of scintillation cocktail. The vials were then shaken in order to extract 222 Radon from the water phase to the organic scintillate solution. The prepared samples were then kept for about 3hr to allow the 222 Rn and its shortlived decay products to attain radioactive equilibrium.

Determination of 222 Rn Concentrations
After preparations, liquid scintillation counter (LSC, Tri-Carb-LSA1000) was used to analyse the water samples. The counter is equipped with features will aid in achieving a detection limit of 0.407 Bq/l or less in 60 min (Abba, et al., 2020;Bello, et al., 2020). The background and sample count rates (counts min −1 ) were recorded. 222 Rn and its short-lived daughters emit five radioactive particles (3 α and 2 β) per every disintegration of. Since equilibrium was established between 222 Rn and these decay daughters, all the five emissions were used in detection and quantifying 222 Rn in water. The 222 Rn concentration in a sample of water was determined using the Equation 1 (Garba et al., 2012).
where Rn is 222 Rn concentration (Bql -1 ), NS is the sample total count rate (count min. −1 ), NB is the background count rate (count min −1 ), t is the elapsed time between sample collection and counting (4320 min.), λ is 222 Rn decay factor (1.26 × 10 −4 min. −1 ), 100 is a conversion factor from per 10 ml to l − 1 , 60 is conversion factor from min to s , and 0.964 is the fraction of 222 Rn in the cocktail in a vial of 22 ml total capacity, assuming it contains 10 ml cocktail, 10 ml water and 2 ml air.

RESULTS AND DISCUSSION Mean Activity Concentration of 222 Rn
The study area has an overall average groundwater (borehole) 222 Rn concentration of 69 ± 3 Bql -1 with values ranging from 38 Bql -1 to 97 Bql -1 . This showed that the mean value obtained was about six times higher than the recommended limit of 11 Bql -1 set by USEPA (1999) and about ten times higher than the world average value of 10 Bql -1 reported by WHO. Figure 2 below shows the distribution of radon concentration data. It can be noted that, the measured data fits in very well in the bell shape of the normal distribution curve which shows that the distribution of thedata is normal. and as such reliable conclusions can be made. It can be seen that, most of the data are positively skewed with maximum and minimum values obtained on geological formations G8 (Silicified sheared rock) and G7 (Sandstone) as indicated by the upper and lower whiskers respectively. Outliers can also be observed on G8 and G6 as indicated by the dots on their respective lower whiskers.

Figure 3: Box plots for the Measured Radon Activity Concentrations Based on Different Underlying Geological Formations
It can be observed from the Table that, the high F-test values indicates that the 22Rn activity concentrations for the geological formations are normally distributed and are statistically significant with a probability value less than the critical value of (α = 0.05). This means that the null hypothesis is rejected, thus, strong differences exist between the various geological formations in terms 22 Rn activity concentration. Since the null hypothesis is rejected, then it becomes necessary to identify which geological formation differ from another. In line with this, T and multiple comparison tests were also conducted in order to compare the variations in mean 22 Rn activity concentration values for the geological formations.