Two-year assessment of radon exposure in the underground tourist route in the Historic Silver Mine in Tarnowskie Góry (Poland)

Geological structure

The ore minerals in the area where the Historic Silver Mine in Tarnowskie Góry is located in carbonate sediments of the Triassic, being part of the Permian-Mesozoic platform cover. It forms the so-called Silesian-Kraków Monocline, overlying older formations of the Palaeozoic (Carboniferous and Devonian). The platform cover is underlain by the Precambrian Upper Silesian block, built of crystalline rocks in the form of granitoids, gneisses, crystalline schists, and bare magmatic rocks. The Triassic in the Upper Silesian region is fully developed. The crumbling of the deposit is associated with a series of so-called crushed-bearing dolomites, characterised by vertical and horizontal variation in formation. The excavations now part of the underground tourist route were made on these rocks. Stratigraphically, they correspond to the Early and Middle Triassic, aged 235–252 million years. It is assumed that they are epigenetic formations formed by secondary dolomitisation occurring after the solidification of the sediments. Structurally, the rocks range from crystalline to compact, generally cavernous and fractured, mostly thick- to medium-bedded, and interlayered with poorly consolidated dolomite or clay layers. In the mine area, they lie relatively shallow, 20–50 m from the surface. As a result of tectonic phenomena, the complex has been heavily fractured (dense network of cracks and fissures). The tectonic phenomena are associated with later karst processes.

The main ore mineral in the area was galena (PbS), called silver-bearing galena because of its high silver content (up to 1.2% max.). The galena deposits were primarily located in the bottom of the ore-bearing dolomites, with the ore occurring in the form of veins and vein-like shoals. In addition, zinc minerals in the form of sphalerite and wurtzite were occasionally present in the deposit. A mineral characteristic of the Tarnowskie Góry area is tarnitite (Ca, Pb)CO₃. Mineralogically, it is aragonite CaCO₃ with an isomorphic admixture of cerusite PbCO₃ (up to 9%). It occurred as radial crystalline clusters on the surface of dolomite and galena. The deposits in question are assumed to be of hydrothermal origin and are associated with alkaline-alkaline Neogene post-magmatic formations. The deposits are overlain by Quaternary post-glacial sediments up to 30 m thick, represented by Pleistocene sands, clays, gravels, and Holocene sands and silts. The Quaternary formations lie on a strongly morphologically differentiated Triassic surface.

Measurement site

Measurements were taken at the Historic Silver Mine in the city of Tarnowskie Góry in Poland. Exploitation of the deposits in this area was completed in 1912, and since 1976, some of the former workings have been open to the public. Since 1784, 150 km of excavations and about 20,000 shafts and fore-shafts were created during the Fryderyk mine operation (The Fryderyk mine was the former name of the Historic Silver Mine). However, most of the old underground workings have been isolated from the tourist part of the mine, and today, the visiting time is only about 1 h. Nevertheless, their influence on radon activity concentrations cannot be excluded. The accessible part of the mine has a specific microclimate characterised by a relatively constant temperature of about 10 °C throughout the year and a relative humidity of 90%. The current tourist route includes galleries of 1,740 m in length (Fig. 1) and 3 mine shafts, “Anioł”, “Żmija”, and “Szczęść Boże”, with the “Szczęść Boże” shaft having been buried and having no technical function. The mine is located at a depth of 40 m^15,16. A diagram of the tourist part of the mine is shown in the figure below (Fig. 2). The 30 selected survey points are also marked on it. A description of the measurement points is given in Table 1. Ventilation of the mine takes place in two ways. For the most part, air exchange in the mine occurs by gravity, with fresh air flowing through the intake shaft (“Żmija” shaft) and used air being removed through the exhaust shaft (“Anioł” shaft). The second way of ventilating the mine is by mechanical ventilation. A fan is installed in the “Anioł” shaft to provide airflow. Mechanical ventilation operates during mine opening hours. The lengths of the galleries, their cross-sections, and the air flows in the mine were also measured during the survey. The highest air flow was measured in the exhaust “Anioł” shaft, where the collective air from the entire mine arrives and is discharged to the surface (78 m³/min), and the lowest in the area of the Low chamber (1.2 m³/min). The chambers, sometimes several tens of meters deep, are located next to galleries along which the air ventilating the mine flows. Therefore, only a tiny part of the air inevitably flows into the chambers. Some of the results obtained are included in the figure below. The airflow direction is indicated in Fig. 2 by arrows, with red representing fresh air and blue representing stale air.

Silver tourist mine in Tarnowskie Góry (source: https://kopalniasrebra.pl/oferta/podziemna-trasa-dla-osob-z-niepelnosprawnosciami).

Diagram of the ventilation network in the Historic Silver Mine with marked measuring points (L – the branch length, FR – the air flow rate, CS – the cross-section of the branch).

Methods

For long-term measurements of radon activity concentrations in the mine, track detectors CR-39 and a detector etching and reading system from Radosys (Radosys Ltd., Budapest, Hungary) were used. The detectors were deployed at 30 points delineated in the excavations of the Historic Silver Mine. The detectors’ exposure period was about 3 months, after which they were replaced. In total, the measurements lasted 2 years. The first exposure lasted from 9 February 2021 to 19 May 2021 (spring I), the second from 19 May to 26 August 2021 (Summer I), the third from 26 August to 25 November 2021 (Autumn I), and the fourth from 25 November 2021 to 3 March 2022 (Winter I). The second year of measurement is also 4 campaigns: 3 March to 9 June 2022 (Spring II), 9 June to 20 September 2022 (Summer II), 20 September to 15 December 2022 (Autumn II), and 15 December 2022–14 March 2023 (Winter II). After exposure, the detectors were digested in NaOH solution and then read in a dedicated microscope. The calibration factor of the detectors for each batch was verified in the radon chamber of the Silesian Centre for Environmental Radiometry. The lower detection limit for this method was set at 8 Bq/m³ with a 3-month measurement period. The concentrations were calculated according to the formula below.

Where: C_Rn – average radon ²²²Rn concentration in the room [Bq/m³], G_S – trace density read from the CR-39 foil [trace/mm²], G_Sb – background trace density [trace/mm²], T – exposure time [h], C_F – calibration factor, determined in the laboratory [(mm²/trace)(Bqhm^− 3)].

To estimate the uncertainty of the calculated radon concentration, the following factors were taken into account: uncertainty of trace density (10%), uncertainty of the calibration factor (5%), uncertainty of exposure time (6 h), uncertainty of the background trace density (0.1 trace/mm²). The measurement uncertainty is about 20%.

The study also used the AlphaGuard active radon meter (Genitron, Frankfurt, Germany; Bertin Technologies, Montigny-le-Bretonneux, France) to check the diurnal variation in radon activity concentration. The Genitron instrument is based on a 0.56 L ionisation chamber. The radon activity concentration was measured in a mode according to which radon diffused freely into the chamber, where readings were taken at a one-hour frequency. The instrument also allows simultaneous measurement of temperature, humidity, and pressure.

Radioactive analyses were conducted at the Central Mining Institute – National Research Institute. The measurements followed an established internal protocol SCR/ZLGIG/2–004, accredited by the PCA – Polish Centre for Accreditation. Gamma counting was conducted with an HPGe – high purity germanium detector, cooled by LN2 – liquid nitrogen. Planar broad energy detector (BEGe™), model BE5030 Canberra, with a relative efficiency of 50%, was used. The detector performance, expressed as full width at half maximum (FWHM) for the 609 keV line of 214Bi is 1.35 keV. Energy and efficiency calibration utilized a multi-gamma source containing isotopes emitting gamma lines in a wide range (46–2000 keV). This source was prepared using reference materials (RGU-1, RGTh-1, and RGK) containing natural radionuclides provided by IAEA – International Atomic Energy Agency. Calibration relationships were established using Canberra Genie 2000 software. Uncertainties were estimated based on counting statistics and reported with a 2σ counting error. The activity concentrations of ²²⁶Ra, ²²⁸Ra, ²²⁸Th, ²¹⁰Pb, and ⁴⁰K were determined using the following lines: ²²⁶Ra (indirect by: ²¹⁴Pb − 295 keV, 352 keV; ²¹⁴Bi − 609 keV, 1120 keV, 1764 keV), ²²⁸Ra (indirect by: ²²⁸Ac − 338 keV, 912 keV, 969 keV), ²²⁸Th (indirect by: 212Pb: 239 keV), ²¹⁰Pb (46.5 keV), ⁴⁰K (1461 keV).

All uncertainties related to the measured values were evaluated at the confidence level of 95%.

To test whether the differences between the data collected in the different seasons/years are significant, a statistical t-student test was performed. The null hypothesis of the test was that the mean radon concentration in the first year of the study is the same as the mean radon concentration examined in the second year. The alternative hypothesis: the mean radon concentrations measured in subsequent years are different. A two-sided Student’s t-test for equal variances was performed. The significance level α was set as 0.05. The normality of the distribution was checked using the Shapiro-Wilk concordance test (the tested distributions do not deviate from the Gaussian curve). The homogeneity of the distribution was determined using the F-test, and the variances of the compared groups were shown to be homogeneous. All tests and calculations were performed using Microsoft 365 MSO 64-bit software (version 2411 compilation 16.0.18227.20082).