Radon concentrations were measured in 250 groundwater samples collected from the Goesan area where uraniferous black slate and granites abundantly occur in the formations. The measured radon levels ranged from 0.90 to 7,218.7 Bq/L with the median value of 54.3 Bq/L, similar to the value measured in the nationwide survey in 4,853 wells (52.1 Bq/L). The median value was highest in the Cretaceous granite area (390.0 Bq/L) while it was as low as 20.0~58.8 Bq/L in the Ogcheon meta-sedimentary rock areas. About 23.6% of the total samples exceeded the WHO guideline value of 100 Bq/L established in 2011. The exceeding rate was 69.0, 39.4, and 7.0~13.7% in the Cretaceous granite area, Jurassic granite area, and Ogcheon meta-sedimentary rock areas, respectively.

Keywords: Groundwater;Radon;Concentration;Granite;Ogcheon meta-sedimentary rocks;