A national groundwater surveillance programme was started for investigation of natural radioactivity levels in the year 2007. This paper presents (222)Rn radioactivity concentration levels in well waters located in and around the city of Riyadh in Saudi Arabia. Water samples were collected from 171 wells in six different locations. Most of these deep wells have an approximate depth of 1000 m, while shallow wells have a depth of 300 m. The analyses were performed by an ultra-low level liquid scintillation spectrometer equipped with an alpha-beta discrimination device. Efficiency and background calibrations were performed with a (226)Ra aqueous standard homogeneously mixed with a cocktail (high efficiency mineral oil scintillator) which was used after a certain period of time to assure radon equilibrium. The measured (222)Rn activities of deep wells ranged from 0.34±0.05 to 3.52±0.30 Bq l(-1) (average: 1.01±0.10 Bq l(-1)), whereas those of shallow wells ranged from 0.72±0.08 to 7.21±0.58 Bq l(-1) (average: 2.74±0.24 Bq l(-1)). The (222)Rn concentrations levels were found to be in compliance with the proposed national limits of 11.1 Bql(-1) and depend on the water source.
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