Assessment of radon concentrations in selected Tanzanian thermal waters and associated radiological risks to users: implications for environmental radiation protection.

Radon (222Rn) dissolved in geothermal waters contributes to natural radiation exposure through ingestion and inhalation, particularly in thermal spring environments used for bathing, recreation, and tourism. Despite the growing use of geothermal springs in Tanzania, no published data exist on radon concentrations in Tanzanian thermal waters or their radiological implications. This study determined radon activity concentrations in Tanzanian hot springs using an AlphaGUARD radon monitor and evaluated effective doses to users based on accepted radiological protection models. Water samples were collected from Chemka/Kikuletwa, Lake Manyara, Misughaa, Songwe, and Kilambo hot springs in the Kilimanjaro, Manyara, Singida, Songwe, and Mbeya Regions. Radon concentrations varied significantly among sites (p < 0.001), ranging from 0.56 ± 0.39 Bq L-1 at Songwe to 103.34 ± 44.91 Bq L-1 at Misughaa. Intermediate concentrations were recorded at Chemka (4.96 ± 0.76 Bq L-1), Kilambo (9.82 ± 7.58 Bq L-1), and Lake Manyara (13.84 ± 2.34 Bq L-1). All measured concentrations were below the parametric value of 100 Bq L-1 established by Council Directive (International 2013) /51/Euratom for water intended for human consumption, except at Misughaa, where the site mean marginally exceeded the guideline, with individual samples reaching up to 179 Bq L-1. Considerable spatial variability within spring systems, particularly Misughaa and Kilambo, indicated effects of localized hydrogeological controls, including fracture-mediated groundwater flow, groundwater-rock interaction, and radon degassing before emergence. Estimated annual effective doses from incidental ingestion remained below internationally recommended public exposure limits, although higher values occurred at Misughaa under frequent exposure scenarios. These findings provide the first baseline dataset on radon occurrence in Tanzanian thermal waters and support environmental radiation protection and sustainable geothermal resource management.