Human exposure to radon radiation geohazard in Rong Cave, Dong Van Karst Plateau Geopark, Vietnam

Abstract

Rong Cave is one of the more important caves in northern Vietnam’s Dong Van Karst Plateau Geopark (part of the Global Geoparks Network), because its subterranean lake provides agricultural and domestic water for neighboring communities. Maintenance and utilization of Rong Cave’s water reservoir, as well as touristic cave use, require frequent human access to Rong Cave. Depending on the availability of seasonal drip water and the water level of the lake, the abundant clay-rich sediment in the back portion of Rong Cave and possible seepage of gas from deeper strata along geologic faults provide seasonally elevated concentrations of radon in cave air. Based on repeated measurements over 10 months in 2015 and 2016 of the concentrations of radon isotopes (²²²Rn and ²²⁰Rn, also called thoron) with a portable SARAD^® RTM 2200 instrument (SARAD^® GmbH, Germany), the human total annual inhalation dose was estimated according to the UNSCEAR (2000) algorithm. The result indicates that the radon-related radiation exposure is insignificant for short-term visitors but may reach ~1.8 mSv a^-1 for tour guides and ~25 mSv a^-1 for cave utility workers. The latter values exceed the IAEA-recommended safety threshold of 1 mSv a^-1 (IAEA, 1996). We recommend radiation monitoring for cave utility workers and tour guides.  Prolonged human presence in Rong Cave should be avoided during periods of seasonally elevated radon concentrations.

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