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Modrič Cave is located at only 29 m above sea level near Starigrad-Paklenica at the Adriatic coast and in close vicinity to the highway. Its total length of about 800 meters in two channels provides a special experience of the underground world in completely intact nature. Although the interior of the cave is passable because both channels are of sufficient height and width for passage, it is not possible to explore it without adequate equipment and expert guidance. Since March 2020, eighteen-month monitoring of natural radioactivity originating from radon and its short-lived progeny has been performed, to estimate the dose received by both cave visitors and the tourist guide(s). Integrated radon measurements were performed by LR115 type II nuclear track etched detectors. Continuous measurements were made using TSR3 detectors and the AlphaGUARD measuring device equipped with the TN-WL-02 module for measurement of equivalent equilibrium concentration of radon progeny. Track etched detectors and TSR3 were exposed for 3 months periods during climatological seasons, while AlphaGUARD and TN-WL-02 measuring devices were exposed for two days at each season which enabled the determination of equilibrium factor between equivalent equilibrium concentration of radon progeny and activity concentration of radon. The average radon concentrations with its standard deviations in the Modrič Cave in the climatological seasons were: cSpring-20 = 3.6 ± 1.5 kBq m-3, cSummer-20 = 5.2 ± 2.7 kBq m-3, cAutumn-20 = 2.9 ± 1.6 kBq m-3, cWinter-20/21 = 0.88 ± 0.96 kBq m-3, cSpring-21 = 2.0 ± 1.3 kBq m- 3 and cSummer-21 = 6.6 ± 3.6 kBq m-3. The measured maximum values of radon concentrations in the summer period rank the Modrič Cave among karst caves with above-average radon concentrations. Radon concentrations in each climatological period had statistically significantly different values in the right and left (tourist) channels, which was confirmed by the corresponding t-tests. No daily variations of radon and its short-lived progeny because of human activity during or after sightseeing were observed. The correlation between radon and partial pressure of carbon dioxide, which is an important factor in the variety of geochemical processes that occur in caves, is analyzed and discussed. The mean equilibrium factors at the beginning (FB) and at the end (FE) of the touristic season of 2020 were also determined and the following values were obtained: FB = 0.432 ± 0.068 and FE = 0.430 ± 0.087. The maximum effective radon dose received by visitors during the two-hour cave tour was 38.7 ± 7.3 μSv, on August 19th, 2001, calculated by using the epidemiological model (ICRP-65) or 161 ± 30 μSv using the biokinetic model (ICRP-137). According to this biokinetic model, the tourist guide received an effective dose of 1045 ± 252 mSv in 2020 and 1410 ± 452 mSv in 2021 for a little bit more than twenty guidance’s in each year due to pandemic restrictions. A comparison of the received dose values with the recommendations of the ICRP and the EU on the maximum annual exposure limit for radon and its short-lived progeny for employees of 50 mSv in one year or 20 mSv averaged over 5 years show that guide in the Modrič Cave in 2020 and 2021 received 19- and 14-times lower doses, respectively than the average reference dose of 20 mSv for the workplaces. Nevertheless, in regular tourist seasons with daily guided cave tours during summertime, the guides would receive a higher dose than the reference level and thus should be monitored. |
