In order to determine the natural radioactivity in ground water, twenty water samples were collected from most frequently used wells spread in different locations of Al-Baha region. It is located in the south-east of Saudi Arabia (20˚0'0"N, 41˚30'0"E), as shown in Figure 1. The sampling locations were chosen mostly based on population density and accessibility. Al-Baha region is an agricultural area, the main agro products, are wheat, vegetables and fruits. This region was chosen based on the active usage of ground water for drinking and for irrigation purposes made by locals of the area which can also be a source of radionuclides in foods.

The collected water samples were acidified with nitric acid to avoid the collection of organic materials, then each sample was filled into 500 ml capacity polyethylene Marinelli beakers ( IAEA, 1989 ). Before use the containers were washed

with dilute HCl and rinsed with distilled water. The Marinelli beakers were sealed and stored for more than 4 weeks before counting to reach the secular equilibrium between ²²⁶Ra nuclides and ²³²Th nuclides and their daughters for gamma ray measurements ( Abbady, 2004 ). Detection and measurements of the sample concentrations were carried out using a coaxial high-purity germanium (HPGe) detector with relative efficiency of 25% and FWHM 2.0 keV at 1332 keV, of ⁶⁰Co. The detector was housed inside a thick lead shield to reduce the background of the system. Genie 2000 basic spectroscopic software was installed in the computer for data acquisition and analysis. The system was calibrated for energy and efficiency on a regular basis in ( IAEA, 1989 ). Each sample after equilibrium was kept on the top of the HPGe detector and counted for 36,000 s. The background was measured every week under the same conditions of sample measurement.

The activity of ²²⁶Ra was determined based on the gamma ray energies of 351.9 keV from ²¹⁴Pb and 609.3, 1120.3 and 1764.5 keV (from ²¹⁴Bi). The activity concentration of ²³²Th was determined based on gamma ray energies of 338.8, 911.2 and 968.97 keV from ²²⁸Ac and 583.0 keV from ²⁰⁸Tl. ⁴⁰K was determined directly by its emission energy of 1461.8 keV gamma-ray line. The activity concentrations A_w of the natural radionuclides in the measured samples were calculated using the equation ( UNSCEAR, 2000 ):

A w ( Bq ⋅ L − 1 ) = C a / ε P γ m (1)

where: Ca is the net gamma counting rate (counts per second), ε the detector efficiency of the specific γ-ray, Pγ the absolute transition probability of Gamma-decay and m the mass of the sample (kg).

The concentrations in Bq∙L⁻¹ of radionuclides ²²⁶Ra, ²³²Th, and ⁴⁰K measured in the water samples were listed in Table 1. As shown, in this table, ²²⁶Ra activities It ranged from 0.24 Bq∙L⁻¹ (w17) to 1.54 Bq∙L⁻¹ (w11). It was not detected in four samples (w3, w9, w18 and w20) and the measured samples (w5, w6, w7, w10 and w13) were higher than the recommended limits of 1.0 Bq∙L⁻¹ ( WHO, 2006 ). ²³²Th series were detected in eight samples only, the lowest value was 0.15 Bq∙L⁻¹ (sample 15) and the highest value was 0.81 Bq∙L⁻¹ (w16). The results show that the concentration values of ²²⁶Ra were higher than those of ²³²Th. This may because of the radium being more soluble in water ( Kitto & Kim, 2005 ). ⁴⁰K activities ranged from 0.66 Bq∙L⁻¹ (w7) to 5.64 Bq∙L⁻¹ (w4), all samples concentration were below the recommended limit 10.0 Bq∙L⁻¹ of ⁴⁰K reported by WHO 2006 ( WHO, 2006 ). The obtained average concentrations of the three nuclides (²²⁶Ra, ²³²Th, and ⁴⁰K) were 0.85, 0.43 and 2.84 Bq∙L⁻¹, respectively. The average values

ND: not detected.

for ²²⁶Ra and ⁴⁰K do not exceed the WHOO 2006 recommended limit in drinking water, whereas, slightly higher for ²³²Th than the limit. It should be noted that, in the present results, ⁴⁰K is the most abundant concentration, about 76% of the total (²²⁶Ra + ²³²Th + ⁴⁰K). Potassium-40 is an isotope of an essential element which is under homeostatic control ( Jabir et al., 2007 ). The activity concentration of ²²⁶Ra, ²³²Th and ⁴⁰K in the studied ground water samples illustrated in Figure 2.

The annual effective dose equivalents from the consumption of drinking water due to ²²⁶Ra, ²³²Th, and ⁴⁰K were estimated by UNSCEAR (2000) and El-Gamal et al. (2019) as:

D w = A w × C w × F w (2)

where D_w is the annual effective dose equivalent from consumption of drinking water (mSv/year), A_w is the activity concentration of radionuclides in the ingested water (Bq/L), C_w is the consumption rate of water (L/year). According to WHO (2003) , the dose was estimated by considering a consumption rate is 730 L/year for adults. The dose conversion factors F_w (Sv/Bq) for adults were (2.8 × 10⁻⁷, 2.3 × 10⁻⁷ and 6.2 × 10⁻⁹ Sv Bq⁻¹) for ²²⁶Ra, ²³²Th and ⁴⁰K, respectively, ( WHO, 2006 ).

The calculated total annual effective doses for adults ingested radionuclides ²²⁶Ra, ²³²Th and ⁴⁰K from the ground water samples were tabulated in Table 1. The highest value (0.237 mSv/y) of effective dose was calculated in sample w6 due to the radium high concentration. The range of effective doses due to intake of ²²⁶Ra, ²³²Th, and ⁴⁰K were from 0.004 to 0.237 mSv/y, with an average value of 0.058 mSv/y which is below the average limit (0.1 mSv/y) reported by WHO ( WHO, 2006 ). Consequently, we recommended that, the investigated waters are acceptable as drinking water for life-long human without any treatment to reduce the concentrations of radioactive contaminants. Figure 3, shows the effective dose for the ingestion of ²²⁶Ra, ²³²Th and ⁴⁰K by adults.

Lifetime risk assessment was calculated using the Eq ( EPA, 1999 ):

Lifetimerisk ( R ) = D w × D L × R F (3)

where D_w is annual effective dose equivalent (Sv/y), D_L is duration of life (70 years) and R_F is risk factor (Sv⁻¹). For risk assessment, the nominal probability coefficient of 7.3 × 10⁻² Sv⁻¹ was adopted ( ICRP, 1996 ).

The risk levels from the direct ingestion of the natural radionuclides in ground water were estimated as shown in Table 1. The cancer risk R for the adults, i.e. corresponding to the total ingestion dose of 0.058 mSv/y is estimated as 0.299 × 10⁻³. This risk is much lower than the limit 8.4 × 10⁻³, if the total natural radiation dose of 2.4 mSv/y as given by UNSCEAR 2000 UNSCEAR (2000) .

Table 2 shows a comparison between the measured activity concentration values of the groundwater samples in present work and other samples obtained from different countries. Where, the average concentration of ²²⁶Ra (0.56 Bq∙L⁻¹) is acceptable with all countries except the maximum values for Yemen and Nigeria. ²³²Th average value (0.16 Bq∙L⁻¹) is almost matched with Saudi Arabia ( Alseroury et al., 2018 ), higher than Jordan ( Al-Shboul et al., 2017 ) and lower than other countries. The present results show that the average of ⁴⁰K concentration is nearly matching the value of Nigeria ( Ajayi & Owolabi, 2007 ), higher than Yemen ( Abdurabu et al., 2016 ) and Jordan ( Al-Shboul et al., 2017 ), while it is lower than the other observed values. Therefore, we can indicate that the obtained results are in agreement with the data obtained from various locations in the world, taking into account the data of the countries of different geographical locations differ regarding specific mineral.