## 3 Results and Discussion  
### Radioactivity Parameters of Drinking Water in the Studied Area  
All measured values of gross alpha and beta activities and radon-222 were situated within the allowable values. The gross alpha activity increased during the study period from 2017 to 2021 and varied between sources, being higher in groundwater (Ground) than in surface water (Surf). The best fitted model for gross alpha activity and gross beta activity after removing the outlier (sample 61--Tilisca 2017) was the GLM with gamma distribution. By including the locality as a random effect (in mixed models) the quality of the model did not increase, which means that there were no significant differences among localities that were not explained by source. The mean gross alpha activity for the first year of study, 2017, in the localities with groundwater was 0.03 Bq/L. Each year the mean value of this parameter decreased by 25% (\(\chi^{2}\) = 29, df = 1, \(p\) < 0.001), and in localities with surface water the mean gross alpha activity was 71% lower compared to that of groundwater (\(\chi^{2}\) = 49.17, df = 1, \(p\) < 0.001) (Figure 2a). The explained deviation of the gross alpha activity model was 60.8%. The mean gross beta activity of groundwater was 0.122 Bq/L; it was 0.05 lower in surface waters, the difference showing significance (t = \(-\)5.24, df = 62, \(p\) < 0.001) (Figure 2b), and the explained variation being 29.6%. The value of radon-222 from Paltinis source was considered an outlier, with a mean value of 40.4 Bq/L, while the mean value for the other localities was 1.1 Bq/L; therefore, we excluded Paltinis from the data analysis by this statistical modeling. The best fitted model  
Figure 2: Box-and-whisker plots of gross alpha activity (**a**) and gross beta activity (**b**) as a function of water source (Surf—surface water, Ground—groundwater), within the 2017–2021 interval. The circles represent outlier observations. was a GLMM with gamma error distribution. The variance of radon-222 concentration among localities was 1.54 and the mean value for localities with Ground was 1.89. In localities with Surf, the mean radon-222 concentration was 96% lower and the difference was highly significant (t = \(-\)8.83, df = 22, \(p\) \(<\) 0.001). Most of the variation explained by the model was attributed to differences in the water source (0.91), and only a small part was represented by the local variations among the localities (0.05). The three types of water source were the best predictors of the radiation parameters (pseudo-F = 10.8, \(p\) = 0.001), explaining 48.4% (43.9% adjusted) of the variation in the response variables. The comparison between constrained and unconstrained analyses showed a high efficiency of the first constrained (RDA) axis, which summarized 75.7% of the variation explained by the homologous unconstrained (PCA) axis, the correlation between these axes being 0.87. The first constrained axis was the only one significant (pseudo-F = 9.5, \(p\) = 0.001), extracting 93.7% of the explained variation. This axis was defined mainly by the opposition between surface and groundwater sources. Along this axis all radioactivity parameters increased from Surf to Deep and SSurf (Figure 3). The gross alpha and beta activities were higher in Deep compared to Surf (Figure 4a), while all three radioactivity parameters were significantly higher in SSurf compared to Surf (Figure 4b). Figure 3: The biplot of redundancy analysis (RDA) relating the three radioactivity parameters (radon-22, gross alpha and beta activity) to the water sources. Deep—deep water source, SSurf—subsurface water source, Surf—surface water source. The first two constrained axes are illustrated, but the second axis is not significant. However, the high level of variation explained by this model is given mainly by the particularly high radon-222 values of water from Paltinis, which has a SSurf water source.