## 5 Discussion
Since the Rb atomic radius is more similar to the K radius, it is a favorable vicariant with respect to Cs, which is often adsorbed on the mineral surface [107]. The analyzed groundwater covers a large area of the Li-Rb-Cs ternary diagram, as shown in Figure 6. However, three different groups of springs (G1, G2, and G3) can be clearly evidenced. The first one (G1), located close to the composition of mean limestone (the \"L\" field in Figure 6) [108], includes springs clearly belonging to the calcium-bicarbonate facies (S3, S4, S6, S7, S9, S10, and S12), where the water chemistry is dominated by simple interactions with the calcites and dolomites of the Mesozoic limestones (Figure 6). In central Italy, the median composition of carbonate aquifers with shallow circulation falls on the Li-Rb side of the diagram, due to a relatively low Cs concentration (0.02 ppb) [109]. Most of the Cs of the mean limestone composition is retained in low solubility impurities of the carbonate rocks. In contrast, limited enrichments in Li in some of the abovementioned springs (S3, S4, and S12) are possibly due to the mixing with groundwater circulating in the bedrock evaporitic deposits, as supported by the increase in sulphates in the same springs or eventually the uprising of  
Figure 5: LogP(CO\({}_{2}\)) at sampling conditions calculated by PHREEQCi [75] versus T, where T = temperature in \({}^{\circ}\)C obtained from an Na-Li geothermometer [96]. A high coefficient of determination (R-squared value = 0.86) for the groundwater samples from the San Vittorino Plain was found. See Figure 1 for spring location. G1, G2 and G3 refer to new spring groups. H\({}_{2}\)S. The second group (G2) is represented by groundwater of the calcium-sulphate domain (S1, S2 and, to a lesser extent, S14; Figure 6), whose chemical compositions are attributable to a more extensive dissolution of the sulphate minerals [33; 38; 88]. Finally, the third group (G3) is characterized by groundwater with the highest concentrations of Li and Cs (S5, S11, S13, and S15) that suggests a different circulation model with respect to the other two groups. It is noteworthy that the samples of the second and third clusters are within or close to the ratios 1.2 \(<\) Rb/Cs \(<\) 3.0, typical of waters interacting with Upper Triassic Burano Formations [110; 111; 112], where the higher ratio is common of sulphate waters such as those from the Antrodoco area (S1, S2). In addition, the Li-Rb-Cs contents of the carbonatitic and silicate fractions of Cupaello rocks display red pentagons [41; 42]. The Rb/Cs ratio seems to be similar to that of the G2 and G3 groups; however, the Li content is very different. Indeed, the Cupaello unit consists of ultrapotassic rocks, and their dissolution should give K/Na \(>\) 1, but this condition is not verified in our sampled groundwater. This fact allows to us to exclude a direct influence of magmatic rocks in the rising fluids of the San Vittorino Plain. The three groups (G1, G2, and G3) recognized through this analysis are the same as those we obtained with the geothermometric analysis (Na-Li geothermometer vs. logPCO\({}_{2}\) in Figure 5). Figure 6: Li-Rb-Cs ternary diagram of the water samples (weight basis), modified after [103].