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

Vietnam Journal of Earth Sciences, 40(2), 117-125, Doi:10.15625/0866-7187/40/2/11092 117 (VAST) Vietnam Academy of Science and Technology Vietnam Journal of Earth Sciences Human exposure to radon radiation geohazard in Rong Cave, Dong Van Karst Plateau Geopark, Vietnam Nguyen Thi Anh Nguyet1, Nguyen Thuy Duong*1, Arndt Schimmelmann2, Nguyen Van Huong1 1VNU University of Science, Vietnam National University, Hanoi, Vietnam 2Indiana University, Department of Earth and Atmospheric Sciences, Bloomington, Indiana, USA Received 5 October 2017; Received in revised form 28 December 2017; Accepted 13 March 2018 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 neighbor- ing 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 stra- ta along geologic faults provide seasonally elevated concentrations of radon in cave air. Based on repeated measure- ments 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 ex- posure 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. Keywords: annual radioactive dose rate; cave air; geohazard; radon; Rong Cave; thoron. ©2018 Vietnam Academy of Science and Technology 1. Introduction1 Radon is a radioactive noble gas that oc- curs in trace amounts in the atmosphere and consists of radiogenic isotopes 222Rn, 220Rn and 219Rn as intermediate nuclides from radio- active decay chains originating from long- lived nuclides uranium-238 (238U), thorium (232Th), and 235U, respectively (WHO, 2000). Radon’s parental metallic nuclides in the *Corresponding author, Email: duongnt_minerals@vnu.edu.vn earth’s crust decay within minerals in soil, rock, building bricks or concrete to produce radon atoms that can be released from solid phases and enter pore spaces, from where ra- don can be exhaled into the atmosphere. The 3.96 seconds half- life of the relatively rare 219Rn nuclide is too short to allow the exit from a solid phase and significant transfer into air where 219Rn and its progeny can be inhaled by humans. In contrast, the longer-lived radon isotopes 222Rn (half life 3.83 days, decay en- ergy 5.59 MeV) and 220Rn (also called thoron, Nguyen Thi Anh Nguyet, et al./Vietnam Journal of Earth Sciences 40 (2018) 118 half life 55.6 seconds, 6.29 MeV) can more efficiently enter the atmosphere where they and their metallic radioactive progeny can be inhaled (Meisenberg et al., 2017, and refer- ences therein). Both radon and metallic prog- eny are easily dissolved in lymph and blood in lungs or adsorbed to tissue. Radioactive decay results in α, β, and γ-radiation, out of which α- decay is most prominent along the decay chain of radon isotopes. Cumulative radiation- induced damage of tissue can result in carci- noma, most prominently lung cancer (WHO, 2000). Inhalation of radon and its metallic radioac- tive daughter nuclides in air is responsible for about half of the annual average effective dose from natural sources of radiation received by humans (UNSCEAR, 2000). It appears that evolution has equipped humans with biochemi- cal repair mechanisms to avoid negative health effects from low radon concentrations. Howev- er, high levels of radon are known to pose a ra- diation geohazard to human health, for example in poorly ventilated rooms and caves where ra- don and its metallic progeny can accumulate in stagnant air. Monoatomic radon readily diffus- es through porous materials and can be ex- haled from dry soil and limestone in karst en- vironments (Gunn, 2003). Furthermore, radon can be transported along geologic fractures from deeper strata into caves and to earth’s surface with the help of fast-moving water and carrier gases, such as carbon dioxide, CO2 (Etiope and Martinelli, 2002; Walia et al., 2010). Karst caves are frequently aligned with, or intersected by geologic faults that fa- cilitate transport of fluids. The air in many caves is known to contain elevated radon con- centrations that can be problematic for human health (ICRP, 2003; Cigna, 2005; Dumitru et al., 2015). We explored radon concentrations in the air of several karst caves in the Dong Van Karst Plateau Geopark during “warm and wet”, “cold and wet”, and “cold and dry” weather conditions in 2015 and 2016 (Nguyen Thuy Duong et al., 2016). Rong Cave’s radon concentrations in cave air generally fluctuated widely in response to (i) cave air ventilation rates depending on the difference between cave and outside temperatures, and (ii) perco- lating and drip water saturating cave sediment and affecting radon exhalation and gas seep- age through geologic faults. Rong Cave has been one of the first caves in the Dong Van Karst Plateau Geopark to be developed for tourism. In contrast to other surveyed caves, Rong Cave’s intensity of direct α-radiation from 222Rn alone, and even more so the cumu- lative radiation from 222Rn, 220Rn, and their progeny exceeded the recommended safety radiation threshold for human health. This raises concern especially for utility workers and tour guides, who spend considerably more time in Rong Cave than visiting tourists. Rong Cave showed the highest radon concentrations of all surveyed caves in the Dong Van Karst Plateau Geopark. While this result spells relief for better ventilated caves in the area, the ex- ample of Rong Cave comes as a warning for caves that have not yet been surveyed during different seasons. This study focuses on estimating the hu- man inhalation dose in the air of Rong Cave from radon isotopes 222Rn and 220Rn during either “warm and wet”, “cold and wet”, or “cold and dry” weather conditions outside of the cave. Specific safety recommendations are based on seasonally different radiation doses that expose utility workers, tour guides, and visitors to the health risks. 2. Geological features and technical infra- structure of Rong Cave Rong Cave is situated close to the Sang Tung Commune in the Dong Van District on the Dong Van Karst Plateau within the first Global Geopark in Vietnam (GGN, 2010). Rong Cave stretches mainly in a Northwest - Southeast direction (Nguyen Van Huong et al., 2016) within the Hong Ngai Formation Vietnam Journal of Earth Sciences, 40(2), 117-125 119 (T1 hn) (Tong-Dzuy Thanh and Vu Khuc, 2011) commonly consisting of dark to grey, thin to medium-bedded marl interbedded with black-grey argillaceous limestone. Argilla- ceous coaly limestone is exposed locally near Rong Cave’s entrance and along some cave walls, with many features being similar to the lower section of the Hong Ngai Formation as described by Tong-Dzuy Thanh and Vu Khuc (2011). Rong Cave has a single narrow entrance with a secured gate at an altitude of 1440 m above sea level (latitude 23°12’43.48” N, lon- gitude 105°14’11.75” E). A relatively straight, ~350 m long and up to 50 m tall passage with a concrete-paved path and short bridges con- nects to a voluminous terminal chamber ex- tending over ~3500 m2 before abruptly sink- ing to a depression holding a ~1500 m2 large subterranean lake (Figure 1A and 1B). The cave features stalagmites and ‘hanging slime threads’ of unknown biological origin (Nguyen-Thuy et al., 2017) in parts of the long passage towards the voluminous terminal room (Figure 1B1 and 1B2). At a distance of ~150 m from the entrance, slickensides indicate a geologic fault inter- secting the passage (Figure 1B4). The floor of some sections of the passage and most of the terminal chamber is covered with red clay- rich sediment (Figure 1B5). The central sec- tion of the large chamber features an extended elevated clay plateau a few meters above the lake level. A laminated sequence of clay dep- osition is visible at an erosional cut along the plateau, which indicates that the water level had occasionally been much higher in the past and even flooded the plateau. The modern lake level fluctuates in response to monsoonal lake recharge and seasonal water withdrawal. A pumping station with a floating water in- take near the center of the lake connects to a steel pipeline that continues through the cave’s passage to the Sang Tung Commune (Figure 1B6). Electric cables run parallel to steel pipes to supply electricity for pumps and lighting along the cave’s path. The commune employs four utility workers who daily access the cave for operation and maintenance of pumps and the water distribution system. 2. Survey of radon concentrations in cave air Radon-222 and radon-220 concentrations were measured in various locations in Rong Cave on May 5th and from December 2nd to 3rd in 2015, and on March 14th in 2016. A porta- ble SARAD® RTM 2200 instrument (SAR- AD® GmbH, Germany) with an internal dia- phragm pump generated an air flow of 1 L min-1 into the measurement chamber for α- spectroscopic quantification of 222Rn and 220Rn in cave air. 222Rn and 220Rn concentra- tions were calculated based on the signals from the sum of 218Po and 214Po, and from 216Po, respectively. Air was sampled from 1 m above the ground for at least 3 measurement cycles of 10 minutes each. Radon concentrations in the air of Rong Cave were measured during three campaigns in May 2015, December 2015, and March 2016 corresponding to either “warm and wet”, “cold and wet”, or “cold and dry” weather conditions outside of the cave (GSO, 2016). The respective average 222Rn and 220Rn con- centrations were 5956 Bq m-3 and 492 Bq m-3 during “warm and wet” weather conditions, 873 Bq m-3 and 546 Bq m-3 during “cold and wet” conditions, and 206 Bq m-3 and 74 Bq m- 3 during “cold and dry” conditions (Nguyễn Thuy Duong et al., 2016). Radon concentra- tions were also reported by Nguyen Anh et al., (2016) and are shown in Table 1. Vietnam Journal of Earth Sciences, 40(2), 117-126 120 Figure 1. Location (A) and main features (B) of Rong Cave in Dong Van Karst Plateau Geopark. (B1) Stalactites and (B2) ‘slime/silk threads’ of unknown biological origin; (B3) Single narrow entrance with a secured gate; (B4) slick- enside indicating a geologic fault intersecting the passage; the scale is 10 cm long; (B5) the floor of some lower cave sections is covered with red clay-rich sediment; (B6) a depression near the end of Rong Cave with a diameter of ~ 50 m is used as a water reservoir with a central floating water intake A B3 B4 B5 B6 B3 B4 B5 B6 1 m B 2 cm 2 cm B1 B2 B1 B2 Vietnam Journal of Earth Sciences, 40(2), 117-125 121 Table 1. Minimum, maximum, and mean radon concentrations in the air of Rong Cave from different measurement campaigns (including air at the cave entrance, but excluding air in small local depressions and along faults in the cave). The descriptions ‘in’ and ‘out’ refer to air within the cave and external air outside of the cave’s entrance Weather condi- tions outside of the cave Dates of field work Temperature (in; out) (oC) Relative humidi- ty (in, out) (% H) 222Rn (min - max); mean (Bq m-3) 220Rn (min - max); mean (Bq m-3) Warm and wet May 5th, 2015 21; 30 65; 59 (2870 - 8006); 5956 (388 - 1163); 492 Cold and wet December 2 nd- 3rd, 2015 18; 24 69; 62 (178 - 5527); 873 (455 - 910); 546 Cold and dry March 14th, 2016 17; 23 65; 40 (144 - 288); 206 (37 - 111); 74 3. Procedure for assessment of annual radiation dose α-Decay of radon in air generates radioac- tive metal ions that tend to become adsorbed to aerosol and dust particles in the air. Inhala- tion of radon and its radiogenic metallic daughter nuclides causes solution into body fluid and adsorption to lung tissue. Radionu- clides can also enter the human body via eat- ing and drinking, although these pathways are deemed less important in cave environments. All types of radiation from radioactive decay processes can induce harmful random bio- chemical reactions, including damage to DNA (WHO, 2000). Cell damage from exposure to high radon concentrations is known to en- hance the incidence of lung cancer. The World Health Organization recommended an action level of 100 Bq m-3 for dwellings (WHO, 2000), which considers lower levels safe for human habitation (WHO, 2000). This level can be raised to no more than 300 Bq m3 if prevailing country-specific conditions apply (UNSCEAR, 2008). The International Atomic Energy Agency (IAEA, 1996) specified an annual dose limit of 1 mSv a-1 for human ex- posure. Doses from radon and radon progeny can also be calculated using various models. This study uses the following UNSCEAR (2000) algorithm: D = [(kRn + nRn × FRn) × CRn + (kTn + nTn × FTn) × CTn] × H where Rn = 222Rn; Tn = 220Rn - k: solubility coefficient blood (kRn = 0.17; kTn = 0.11) - n: inhalation dose conversion factor (nSv/(Bq h m-3)) (nRn = 9; nTn = 40) - F: equilibrium factor indoor (FRn = 0.4; FTn = 0.3); outdoor (FRn = 0.6; FTn = 0.1) - H: average time exposure in year (h) - C: concentration (Bq m-3) - D: inhalation dose (mSv a-1) The equilibrium factor is the ratio of po- tential α energy concentration (PAEC) of the actual mixture of radon decay products to that which would apply at dynamic equilibrium concentrations of radionuclides (ICRP, 2010). 4. Estimates of human inhalation dose in Rong Cave Rong Cave is routinely visited by utility workers, tour guides, and tourists. A typical touristic cave visit lasts on average 2 hours and is not repeated in the same year. In contrast, tour guides accompany tourists on multiple oc- casions per year, which is most frequent during the “cold” season and least frequent in the tour- istically unfavorable ‘warm and wet’ monsoon season. Our estimates of inhalation dose in Rong Cave assume (i) a daily average 4-hour presence in the cave by utility workers regard- less of season and outside weather, (ii) occa- sional 2-hour walks through Rong Cave of- fered by tour guides primarily during the tour- ist season from mid-October to March (i.e., 2 weeks in “warm and wet” weather, 3 months in “cold and wet” weather, and 2 months in “cold and dry” weather), and (iii) a one-time 2-hour visit of Rong Cave by a tourist. The various seasonally-adjusted estimates for utility work- ers, tour guides, and one-time visitors entering Nguyen Thi Anh Nguyet, et al./Vietnam Journal of Earth Sciences 40 (2018) 122 Rong Cave, as well as estimated cumulative annual inhalation doses, which are listed in Ta- ble 2, are based on average seasonal concentra- tions of both radon isotopes using the UN- SCEAR (2000) algorithm. Exposure of utility workers, tour guides, and one-time visitors in Rong Cave is less than 20.5, 0.9 and 0.06 mSv a-1, respectively, in the longest “warm and wet” season. The maximum cumulative exposure affects utility workers during the warm and wet season reaching approximately 24.7 mSv a-1 (Table 2). Table 2. Time spent in Rong Cave and estimated total annual inhalation dose from 222Rn and 220Rn in Rong Cave for utility workers, tour guides and visitors by using the UNSCEAR (2000) algorithm. The year is divided into 6 months of ‘warm and wet’ outside weather and 3 months each of two types of ‘cold’ weather Season People enter- ing Rong Cave Average radon concentration (Bq m-3)* Number of hours spent in Rong Cave Seasonal inhalation dose (mSv) Cumulative annual inhala- tion dose(mSv a-1) Time Weather 222Rn 220Rn per day in season 222Rn 220Rn 222Rn+220Rn May to October (i.e. 180 days) Warm and wet Utility workers 5956 492 4 720 16.2 4.3 20.5 24.7 Tour guides 2 30 0.7 0.2 0.9 One-time visi- tors 2 2 0.05 0.01 0.06 November to January (i.e. 90 days) Cold and wet Utility workers 873 546 4 360 1.2 2.4 3.6 1.8 Tour guides 2 90 0.3 0.6 0.9 One-time visi- tors 2 2 0.01 0.01 0.02 February to April (i.e. 90 days) Cold and dry Utility work- ers 206 74 4 360 0.3 0.3 0.6 0.06** Tour guides 2 60 0.05 0.05 0.1 One-time visi- tors 2 2 <0.01 <0.01 <0.01 *(Nguyen Thuy Duong et al., 2016); ** A visitor’s inhalation dose depends on the season of a single 2-hour cave visit once per year 5. Discussion of human exposure to radon radiation in Rong Cave Radon concentrations in Rong Cave varied among measurement campaigns and were highest during ‘warm and wet’ outside weath- er conditions, when the air temperature in Rong Cave was lower than outside air and ventilation was reduced to 0.01 m s-1 near the entrance (Nguyen Thuy Duong et al., 2016), because the cave’s elevated entrance acts like a sill preventing the density-driven outflow of colder air from the cave’s passage. Similar seasonal fluctuations in ventilation rate and radon concentrations have been reported from other caves, for example Postojna Cave in Slovenia (Gregoric et al., 2013) and Luray Caverns in Virginia, USA (Cigna, 2015). Rong Cave’s maximum 222Rn concentration of almost 6 kBq m-3 exceeded Vietnam’s rec- ommended safety threshold of 200 Bq m-3 of natural radon activity in buildings (TCVN 7889: 2008) by a factor of 30 (TCVN, 2008). Even during ‘cold and wet’ and ‘cold and dry’ weather conditions, parts of Rong Cave occa- sionally exceeded the safety threshold several- fold (873 Bq m-3), although at other times the 222Rn concentration essentially matched the TCVN recommendation at values of 206 Bq m-3 (Figure 2). Radon concentrations also of- ten exceeded the UNSCEAR-recommended action safety threshold of 300 Bq m-3 (UNSCEAR, 2008). International organizations and authorities in Vietnam have not yet announced any offi- cial radiation safety standard for 220Rn. How- ever, UNSCEAR (1993) mentions a safety Vietnam Journal of Earth Sciences, 40(2), 117-125 123 level for 220Rn in air of only ~10 Bq m-3, which is far below Rong Cave’s mean 220Rn concentration of 70 to 550 Bq m-3 one meter above the clay-rich floor, close to where peo- ple breathe (Figure 2). 220Rn concentrations in the air closer to clay surfaces are routinely far higher because the parental isotopes of 220Rn reside in minerals and 220Rn’s short half life of ~55.6 seconds limits transport in non- turbulent air (Meisenberg et al., 2017). The ventilation rates of caves have limited influ- ence on near-surface concentrations of 220Rn. Therefore, even during “cold and wet” weath- er conditions, when 222Rn may be limited due to increased ventilation of cave air (0.032 m s- 1 according to Nguyen Thuy Duong et al., 2016), the 220Rn concentration in cave air will remain high near the ground and may signifi- cantly endanger utility workers not only via its own decay, but even more so by the decay of its metallic radioactive daughter nuclides. Figure 2. Average radon concentrations one meter above the ground in Rong Cave compared to TCVN 7889: 2008 for 222Rn and compared to UNSCEAR (1993) for 220Rn safety threshold recommendations The IAEA (1996) recommends a maxi- mum annual inhalation dose of 1 mSv a-1. While exceptional cases may call for an annu- al dose to reach 5 mSv a-1, the average dose over five years should not exceed 1 mSv a-1. Table 2 suggests that touristic short-term vis- its in Rong Cave, whose exposure is less than 0.06 mSv a-1, do not significantly add to a per- son’s annual inhalation dose. The situation for tour guides is less clear, because the radon- related inhalation dose from a 2-hour cave visit is not equally distributed throughout the various weather conditions of a year. The cu- mulative exposure of a tour guide in seasons that reach approximately 1.8 mSv a-1 may ex- ceed the IAEA-recommended annual inhala- tion dose if his activities are centered on months with high radon concentrations in cave air, for instance “warm and wet” and “cold and wet” seasons. The long time spent year-round by utility workers in Rong Cave clearly and unavoidably causes a high annual inhalation dose that stands in violation of IAEA safety guidelines by a factor of up to ~25 (Figure 3). Figure 3. Estimated annual inhalation doses for utility workers, tour guides, and one-time visitors in Rong Cave. The horizontal line represents the recommended safety threshold (IAEA, 1996) The exposure can be marginally reduced if major maintenance and construction activities in the cave can be avoided during times of high radon concentrations during ‘warm and wet’ weather conditions. Ideally most mainte- nance in the cave should be performed during ‘cold and dry’ weather. Moisture enhances the emanation efficiency of radon from sediment (e.g., Markkanen and Arvela, 1992; Moraw- ska and Phillips, 1993). Proper timing of utili- ty work would require feedback from a relia- ble radon monitoring device in the cave to 0.0 5.0 10.0 15.0 20.0 25.0 U lity workers Tour guides Visitors Inh ala on do se (m Sv a‐1 ) Inhalation dose IAEA, 1996 (1 mSv a-1) Nguyen Thi Anh Nguyet, et al./Vietnam Journal of Earth Sciences 40 (2018) 124 workers about dangerous working conditions. Staff should be rotated frequently when work in the cave is unavoidable in the presence of high radon concentrations, or utility workers with known past elevated exposure to radon should work for some years only on infra- structure outside of the cave. Respiratory fil- ters can be employed to reduce the inhaled amount of 220Rn and metallic daughter nu- clides suspended in cave air (Wang et al., 2011). We note that some of the utility work- ers live in mud-built houses that expose their inhabitants to additional significant concentra- tions of 220Rn that is exhaled from dry interior mud walls and the mud floor (Nguyen Thuy, et al., 2017). 6. Conclusions Radon concentrations in the air of Rong Cave exceeded WHO-recommended safety thresholds (UNSCEAR, 1993, 2008) except from February to April during ‘cold and dry’ weather conditions. Rong Cave’s thoron (220Rn) concentrations are far higher than the respective WHO-recommended safety level. 222Rn concentrations in Rong Cave exceed the TCVN-7889: 2008 safety recommendation of 200 Bq m-3 (TCVN, 2008). Radon concentrations were highest during ‘warm and wet’ outside weather conditions and lowest in ‘cold and dry’ weather. Depend- ing on cumulative seasonal and annual expo- sure times in the cave, the inhalation doses for utility workers, tour guides, and touristic visi- tors vary greatly. Short-term visitors are in- significantly affected by radiation in Rong Cave (0.6 mSv a-1) according to IAEA rec- ommendations (1996). However, radon iso- topes and their radioactive decay products may pose a significant health risk to utility workers and tour guides. The estimated total inhalation dose for utility workers and tour guides exceeded IAEA-recommended values (1996), especially for utility workers. 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