3.1. Radon Concentration
Table 1 shows the radon concentrations for the different measurement periods. The first column contains measurement point id from 1 to 30, the distribution of which is shown in
Figure 2. Columns 2 to 5 contain the measured activity concentrations of radon in the mine.
Table 2 shows descriptive statistics of the results obtained for the measurements of radon concentration in all 30 measurement points of the mine for four seasons and the statistic for the annual average radon concentration.
The arithmetic average of radon concentrations (with standard deviation in brackets) in spring, summer, autumn and winter were found as: 858 (411) Bq m
, 1513 (315) Bq m
, 1315 (416) Bq m
and 397 (337) Bq m
, respectively. The annual average radon concentration was 1021 (568) Bq m
. The results showed higher radon concentrations in summer and autumn compared to spring and winter, as presented in
Figure 3.
Figure 4 shows the radon concentration at 30 measuring points of the mine in four measuring seasons. The highest radon concentration—2280 Bq m
—was measured in the summer time (19 May 2021–26 August 2021) in the “Zawałowa” chamber, with the lowest—80 Bq m
in the winter time (25 November 2021–3 March 2022) in the “Żmija” downcast shaft, the place where fresh air is supplied from the surface. Similarly, to results reported by other authors, the highest concentrations were measured in summer and the lowest in winter [
25,
26,
27,
28]. In winter, the measured concentrations reached the lowest values and ranged from 80–1780 Bq m
, while in summer they were the highest, from 810 to 2280 Bq m
. In spring, radon activity concentrations ranged from 160 to 1990 Bq m
and in autumn from 530 to 2180 Bq m
. Radon concentrations, in addition to varying external atmospheric conditions, can also be influenced by many other factors, such as porosity, permeability, uranium content, hydrological conditions [
29].
In
Figure 4, it can also be seen that the seasonal dynamics or radon change from point to point, but the order the sequence is usually like
, or
.
Concentration changes at a specific measurement site can be assessed as the ratio of standardized deviation over the entire period to total average radon activity concentration as presented in
Figure 5(top). According to the relationship defined as
, in %, where i is measurement point, the concentration of radon at points 5, 6, 7, 8, 11, 13, 18, 21, 24, 25, 26 and 30 does not differ, i.e., less than 10%, from the average radon concentration calculated for all 30 measurement points (1021 Bq m
). However, the relative standard deviations expressed in %, with formula
, as shown in
Figure 5(bottom), are relatively high >30% at these points. It can be concluded that even the average values at these points are close to the annual average, the variation of radon concentration is high. If there is a need to limit costs, these sites could be representative for the assessment of the radiation situation in the facility.
On the other hand, the activity concentration of radon was the most stable inside the “Zawałowa” chamber (point 29, emphasized by light grey) with RSD of 5% (
Figure 5, bottom). The measurement site was there located far from the main stream of the ventilation air, which may indicate low air exchange in this area. Not significant changes of radon concentration can be also observed close to the upcast shaft, measurement points 19 and 20, where all air streams are united before discharging the exhaust air to the surface. In contrast to these places, the radon activity concentration varied most strongly near the downcast shaft, measurement points 1 and 2, where fresh air flows from the surface. The variations in this case reached 90%. In addition, interesting results were obtained in the vicinity of “Srebrna” chamber, measurement points 9, 10, 11 and 12. In these points the values of
d and
parameters are relatively low and varied from 10% to 25% and 25–40%, for
d and
, respectively. Thus, the stream of fresh air entering the mine flows along the galleries but does not reach the depths of the chambers. There are no fresh air inlets in the areas of the chambers, they are not ventilated, in addition these areas are large, so radon exhalation and consequently radon concentrations are consistently high. Either the capacity of mechanical or gravity ventilation does not allow the radon concentration to be maintained at 300 Bq m
.
To check if there is a difference between the seasons, a test of homogeneity of variances was performed in the first step using the Levane test. It has been shown that the variances of radon concentrations between the seasons are homogeneous at p = 0.17. Therefore, the inter-season effect of radon concentration was investigated by ANOVA and other statistical tests. The data analysis of all seasons shows that the radon concentration are statistically significantly different between seasons (t-test, p < 0.05).
Detailed analysis of the relationship between radon concentrations and seasons was conducted by the pairwise comparisons using
t tests with pooled SD and Tukey multiple comparisons of averages with 95% family-wise confidence level and results are presented in
Table 3 and
Table 4.
Both tests indicated that average radon concentration between summer and autumn is not statistically different (p > 0.05) whereas statistically different between other seasons, i.e., Spring–Autumn, Winter–Autumn, Summer–Spring, Winter–Spring and Winter–Summer (p < 0.05).
3.2. Dose Estimation
As the results obtained were quite high in comparison to the reference level, it was decided to determine the effective doses that workers and tourists visiting the mine could encounter. For the calculations, it was assumed that a tourist visits the mine once a year and the exposure time is 60 min. In contrast, the working time used in the calculation is 1800 h/year, the nominal annual working time indicated by the Polish regulations for underground mines when the actual working time cannot be determined [
30]. The effective doses from radon inhalation were determined according to the following formula [
30,
31]:
where
E is the effective dose (mSv),
k is the effective dose per exposure expressed as mSv/(mJhm
),
is the potential alpha energy concentration (
Jm
),
is the uncertainty, (
Jm
), and
t is the working time (h). If the dose was to be estimated on the basis of radon activity concentration measurements, then the equilibrium factor
F should be additionally considered
where
is the radon activity concentration (Bq m
). In this paper, a value of
F = 0.2, as recommended for mines by the ICRP [
32], was adopted.
Table 5 shows the calculated effective doses for workers and tourists. The effective dose determined for a tourist covering the route in 60 min ranges from ca. 1
Sv to 3
Sv, taking into account the uncertainty depending on the season. The annual average dose is equal to ca. 2
Sv. The annual average effective dose for workers is ca. 3 mSv if uncertainty is included (ranging from 1.1 mSv in winter to 4.9 mSv in summer). As the annual value exceeds the legal effective dose limit for the general population (1 mSv in Poland) [
5], counter measures should be taken to reduce the dose received by workers. One possible course of action is to reduce the working time in underground areas.
Table 6 shows the different working time options so that the doses received do not exceed given limits. In order for a worker to receive an effective dose lower than the dose limit for members of the general public, they should not work more than ca. 540 h per year, to be a category B worker not more than 3240 h, and not more than ca. 10,800 h to not exceed the permitted limit of 20 mSv. Therefore, exceeding the limit of 20 mSv or even 6 mSv is impossible because the total number of hours a year is less than the calculated time limit, and the nominal annual working time is 1800 h.