## 3 Results and Discussion  
### Hydrogeochemical Characteristics  
As shown in Table 1, samples were characterized by high concentrations of Ca\({}^{2+}\), HCO\({}_{3}\)\({}^{-}\) and Cl\({}^{-}\) as well as very low contents of K\({}^{+}\). The fact that for Cl\({}^{-}\), SO\({}_{4}\)\({}^{2-}\) and K\({}^{+}\) the standard deviation represents almost half of the value for the mean, reflects the high variability of these ions. It also shows that the geochemistry of the study area is not homogeneous. This agrees with the chemical characteristics previously described by [14], and could suggest that there have not been significant changes in the forces that control the chemical characteristics of the groundwaters present between Cecina and San Vincenzo villages in the past 10 years. As stated by [14], all of the south-west of Tuscany is characterized by the presence of a pre-neogenic substrate (Ligurian, Subligurian and Tuscany Units) discordantly surmounted by clastic-terrigen strata organized in different sedimentary cycles (Upper Tortorian-Pleistocene). The Tuscany succession was developed during almost 200 MY from the Upper Triassic (Limestones with _Rhatavicula contorta_) to the Upper Oligocene (boulders) and is characterized by the presence above the platform carbonates from the Lower Lias (solid limestones) of a not very thick basin succession that was carbonate at first, and evolved to a marly-clayey (i.e., Marls with _Posidonia_) siliceous rocks from the Dogger to the Eocene. This succession emerges quite well along the coast, in Livorno province and in the higher part of the Cecina valley. This geological setting could explain the dominance of both Ca\({}^{2+}\) and HCO\({}_{3}\)\({}^{-}\) ions in these waters, since the available carbonates might have been dissolved and added to the groundwater system. Further proof of this comes when different ionic relationships are plotted (Figure 5), since according to [29] during rock weathering Ca\({}^{2+}\), Mg\({}^{2+}\), HCO\({}_{3}\)\({}^{-}\) among other ions are added to the groundwater. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & **Unit** & **Ca\({}^{2+}\)** & **Mg\({}^{2+}\)** & **Na\({}^{+}\)** & **K\({}^{+}\)** & **HCO\({}_{3}\)\({}^{-}\)** & **Cl\({}^{-}\)** & **SO\({}_{4}\)\({}^{2-}\)** \\ \hline Mean & meq/L & 7.33 & 3.45 & 3.92 & 0.09 & 6.58 & 5.25 & 1.93 \\ Std. Dev. & meq/L & 1.66 & 1.22 & 1.89 & 0.05 & 1.33 & 3.10 & 1.01 \\ Minimum & meq/L & 0.05 & 0.00 & 0.08 & 0.01 & 0.04 & 0.14 & 0.06 \\ Maximum & meq/L & 12.53 & 6.56 & 9.74 & 0.54 & 16.01 & 20.90 & 5.85 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Descriptive statistics for the dataset. From the Na\({}^{+}\) vs. Cl\({}^{-}\) plot it can be seen there are two different processes taking place. Most of the samples fall into or are very near the 1:1 ratio. This could be a consequence of halite dissolution or sodium chloride entering the system via rainfall infiltration or sea water intrusion. However, considering the geological setting of the study area, the dissolution of halite is unlikely and other sources of both ions need to be considered. According to [30] sea water can be regarded as the main source of Cl\({}^{-}\) whereas rainwater tends to have low concentrations of this ion. Thus, the influence of saline intrusions into the wells near the coast or affected by an extreme lowering of the piezometric level, is considered as a more plausible explanation for this trend. Figure 5: Ionic relationships useful for aquifer matrix characterizations. The points that show a shift from the 1:1 ratio, also have a greater Na\({}^{+}\) concentration. According to [29] this could be a consequence of the occurrence of \"_natural softening_\" of the waters. That is, the removal of Na\({}^{+}\) ions that are saturating the alluvial sediments in exchange for the Ca\({}^{2+}\) ions present in the groundwater due to mineral dissolution.