Radiological impact of an active quarry in the Papuk Nature Park, Croatia

We selected 24 locations along the Brenzberg-Točak and Žervanjska quarries and the road between them and two additional locations by Lake Orahovac, a couple of kilometres away (Figure 2). The locations were selected based on the availability of moss. All the locations except the two by Lake Orahovac were on forest trails.

Figure 2

Sampling and H*(10) measurements took place in April and May of 2018. At each location we registered the coordinates, measured ambient dose rate equivalent H*(10) and sampled moss in order to determine radionuclide content using gamma ray spectrometry. We did not consider investigating the closest quarry to Lake Orahovac, Hercegovac, as it was in the process of remediation. Instead, we selected the next two closest ones connected by a road.

H*(10) measurements were taken with the RDS-31 S/R Multipurpose Survey Meter (San Ramon, CA, USA) (31) at each location at the height of 1 m above the ground in no less than five three-minute gamma dose rate measurements within a radius of five meters from the central location to cover the whole area where moss was sampled. The survey meter was calibrated by the manufacturer ahead of measurements using the reference ¹³⁷Cs source and reference dose rate of 3 mSv/h.

While H*(10) values are a solid indicator of radiological impact on the environment and people, true radiological exposure can only be assessed with a full characterisation of gamma-emitting radionuclides in the area.

Mosses are organisms that efficiently accumulate different elements from the environment and are often used as bioindicators of pollution (or its absence) with metals (32, 33, 34, 35, 36), including radioactive uranium, thorium, and caesium (37, 38, 39, 40) or with potassium (37). Without a developed root system to absorb nutrients form the ground, mosses predominantly absorb nutrients and pollutants from the air (40–42). A recent research (42) suggests that under certain conditions, which include high availability of dust in the area and relatively low precipitation, the main source of nutrients and pollutants is not deposition from the atmosphere, but deposition of local dust.

Moss was collected, cleaned from dead leaves, soil, and other detritus, and packed into 5 L plastic bags for transport. Depending on the quantity available at each location, moss was collected within the radius of up to 5 m from the registered coordinates to ensure at least 1 L of moss for each sample after drying. Moss was dried to a constant mass in the laboratory and then packed and sealed in standard 1 L Marinelli beakers and left undisturbed for 30 days to establish secular equilibrium in the ²³⁸U decay chain. We measured activity concentrations of selected radionuclides (naturally occurring radionuclides and ¹³⁷Cs) using the ORTEC gamma-ray spectrometry system (Advanced Measurement Technology, Inc., Oak Ridge, TN, USA), which uses a high purity Ge (HPGe) coaxial GMX-type detector (relative efficiency of 74.2 % and peak full width at half maximum of 2.24 keV, all at 1.33 MeV ⁶⁰Co). Energy and efficiency were calibrated using certified calibration sources manufactured by the Czech Metrology Institute (Jihlava, Czech Republic), and corrections for true coincidence made using the EFFTRAN program (43). Calibration source was a known mixture of radionuclides dissolved in H₂O packed into the same geometry (Marinelli beaker) that was used for sample measurements.

Each sample was measured for 80,000 seconds. The following peaks were analysed: 46.5 keV (²¹⁰Pb), 63.3 keV and the double peak at 92.4 and 92.8 keV (²³⁸U), 351 keV and 609 keV (²²⁶Ra calculated as ²¹⁴Pb and ²¹⁴Bi average), 662 keV (¹³⁷Cs), 911 keV (²³²Th), and 1461 keV (⁴⁰K).

Ambient dose rate equivalents H*(10) ranged between 0.05 μSv/h and 0.14 μSv/h, which is within the background range measured in Croatia (44, 45).

Moss radionuclide measurements are presented in Table 2. Table 3 shows the averages for the locations grouped as follows: along the Žervanjska quarry (locations 1, 2, 9; group 1), along the Brenzberg-Točak quarry (locations 7, 8, 19–22, 25–26; group 2), along the road between these two quarries (3–6, 10–18; group 3), and by Lake Orahovac (23, 24; group 4) (Figure 2). Moss samples collected by Lake Orahovac showed, on average, lower activity concentrations of naturally occurring radionuclides than samples collected along and between the quarries, save for ¹³⁷Cs. Samples collected along the road showed higher ¹³⁷Cs and lower ⁴⁰K activity concentrations than samples collected along the quarries. Samples collected along the Žervanjska quarry had higher ⁴⁰K, ²³²Th, and ²³⁸U activity concentrations than samples collected at other locations.

To put these numbers in a context, Table 4 shows a comparison with findings from other studies in the same geographic area where possible.

Activity concentrations of ¹³⁷Cs vary greatly, depending on the actual Chernobyl fallout. The values for ¹³⁷Cs activity concentration in Table 4 were measured over the last 15 years. According to Betsou et al. (48) and Cevik & Celik (49), the ecological half-life of ¹³⁷Cs in moss is between 2.1 and 22 years, depending on the species. Our findings indicate that the investigated area was not a hot spot during the Chernobyl fallout.

Compared to other reports (Table 4), our measurements show no indication that ⁴⁰K present in moss is the result of anything else but K present in in all living organisms.

Normally, ²³²Th, ²³⁸U, and ²²⁶Ra are not present in the air in Croatia (45) in measurable quantities, which points to a local source – more specifically dust – in a similar fashion as described for Pb in moss in a park (42) – and explains high values, among the highest reported (53). As activity concentrations of ²³²Th and ²³⁸U are normally independent of each other, the weak correlation (correlation coefficient of 0.78) we found suggests the same origin. While ²²⁶Ra and ²³⁸U are part of the same decay chain, their chemical properties are different enough that their concentrations routinely differ both in soil and in living organisms (58, 59, 60). Therefore, it is no surprise that the correlation between ²²⁶Ra and ²³⁸U values was marginal (correlation coefficient of 0.54).

Measured ²¹⁰Pb activities are high in view of recent literature, even higher than around some coal-fired power plants (56, 57), but not as high as reported for some locations in Bosnia and Herzegovina (55) and Greece (47). ²¹⁰Pb in the atmosphere is the result of the leakage of ²²²Rn from soil and ²¹⁰Pb indicates the presence of ²²²Rn in the air. Without a detailed long-term study of local weather patterns, we cannot be positive if the ²¹⁰Pb detected in moss is the result of local ²²²Rn release or of ²²²Rn transport from afar. While ²²²Rn is considered dangerous to humans, the danger is real in closed spaces, where it can accumulate in the air. Out in the open, ²²²Rn is not considered dangerous, especially not for short exposure times (hours or days) (61).

Our measurements of H*(10) along the Brenzberg-Točak and Žervanjska quarry fields and at two locations by Lake Orahovac show that the Radlovac Quarry complex does not present a radiological threat to Park visitors and environment, as they kept within the typical range of values of background H*(10) in Croatia.

Activity concentrations of naturally occurring radionuclides ²³²Th, ²³⁸U, ²²⁶Ra, ⁷Be, ²¹⁰Pb, and ⁴⁰K and of anthropogenic isotope ¹³⁷Cs in moss support these findings and do not stand out in respect to activity concentrations in moss reported in the rest of Croatia or neighbouring countries.