## 2 Materials and Methods  
### Experiment  
The experiment conducted in 2023 involved preparing cultures of a selected species of the lichen _C. uncialis_, which inhabits nutrient-poor, sandy, or rocky soils, and which is commonly found in boreal forests, heatlands, sand dunes, coastal areas, rocky outcrops, and inhospitable areas where conventional monitoring methods can be challenging to implement [19; 20; 21]. To achieve this, _C. uncialis_ was collected from the Tuchola Forest region near the Przymuszewo Forest District in Poland, along with the soil on which the lichens were growing. In the laboratory, the lichens and soil were cleaned of dead plant remnants and other vegetation. The soil substrate was classified as medium to fine sand. The largest proportion of soil particles was in the 0.5-0.25 mm size class, comprising 84.2% of the total sample mass. The 1-0.5 mm fraction represented 8.8%, while finer fractions, such as 0.25-0.125 mm, 0.125-0.063 mm, and below 0.063 mm, accounted for 14.5%, 1.1%, and 1.5%, respectively. Coarser fractions were less abundant, with particles larger than 2 mm comprising only 0.4%, and fractions between 2 and 1.6 mm and 1.6 and 1 mm representing 0.2% and 0.5%, respectively. The organic matter content in the soil (LOI) was 1.6%. The experiment was conducted using three aquariums (20 cm \(\times\) 30 cm \(\times\) 20 cm), each with a 5 cm layer of sand at the bottom, onto which the lichens were anchored. Approximately 100 g of lichens were placed in each aquarium. For one week, the lichen cultures were maintained under controlled conditions, with regulated lighting and hydration every three days, using an aerosol generated by a humidifier placed in each aquarium. After a one-week acclimatisation period, a gamma mixed standard solution (Standard solution of gamma-emitting isotopes, code BW/Z-63/19/20) was added to each humidifier on 17 March 2023, to initiate the contamination process. The gamma radionuclides included in the mix were \({}^{241}\)Am (T\({}_{1/2}\) 432.2 years), \({}^{57}\)Co (T\({}_{1/2}\) 0.744 years), \({}^{137}\)Cs (T\({}_{1/2}\) 30.05 years), \({}^{54}\)Mn (T\({}_{1/2}\) 0.855 years), \({}^{65}\)Zn (T\({}_{1/2}\) 0.668 years), and \({}^{60}\)Co (T\({}_{1/2}\) 5.271 years). The experiment was carried out in three parallel replicates, with 1 g of the gamma mix solution added to aquarium A, 2 g to aquarium B, and 3 g to aquarium C. The added gamma mix solution was sprayed along with the water used for hydration, and the deposition of isotopes in each aquarium is presented in Figure 1. Increasing concentrations of isotopes were intended to test experimentally whether there is a dependence of isotope bioaccumulation efficiency on their initial concentration in fallout. Throughout the entire experiment, the lichens were irrigated using a humidifier twice a week, with excess water draining from the aquarium through a specially prepared drain at the bottom. Prior to starting the experiment, the initial activities of the studied isotopes in the lichens and soil were measured to establish reference values, which were used to adjust the observed values of the studied elements during the experiment (Table 1). Samples of contaminated lichens were collected one week after contamination, on 24 March 2023, and then at approximately monthly intervals on the following dates: 20 April 2023; 19 May 2023; 21 June 2023; and 24 August 2023. The activity of the analysed isotopes at all time points was recalculated to the reference date of 17 March 2023, considering the different half-lives (T\({}_{1/2}\)) of each isotope. This approach enabled the direct comparison of long-term trends. Each sample from each aquarium was divided into