**3. Hydrogeology of the pontine plain**

The carbonate outcrops of the Monte Lepini complex were lowered several 100 m below sea level by a system of vertical faults that accommodated dynamic extension. The depression, thus, formed was filled by clayey sediments dating to the Pliocene and Calabrian (gray-blue clays) during the marine ingress and regression phases. The Quaternary formations of the coastal area consist mostly of sandy continental formations (Upper Pleistocene) deposited by wind in the form of large dunes on preexisting coastal–brackish sandy deposits consisting of organic limestone sands (Middle–Lower Pleistocene). The entire sandy complex, which varies in thickness from a few tens of meters to about a hundred meters, is called the "Duna Antica." Proceeding inland, the recent formations of the continental environment are replaced by deposits formed in fluvial–marsh environments. These consist of alternations of sandy and sandy–clayey strata along with travertine formations. In the marshy environment, formed in the foothill depressions, numerous peaty levels formed and contribute to triggering its marked subsidence [6, 7]. Some additional pertinent information is listed below:


#### **Figure 9.**

*Historic evolution of the piezometric surface within the pontine plain from 1957 to 1977 (Conforto, 1957; Mouton, 1977, Gisotti et al., 1977) [9, 12].*

*Introduction to the Sedimentary Geology, Genesis, and Evolution of the Pontine Plain in Relation… DOI: http://dx.doi.org/10.5772/intechopen.111683*

D. The aquifer of the Duna Antica is considered a hydrogeological basin filled solely by direct rainfall (**Figure 8**) [8, 9]. Broadly, the aquifer of the Pontine Plain evolved considerably from the time of the first hydrogeology studies (Conforto, 1957 [5, 9, 10] to 1977) (Mouton, 1977; Gisotti et al. 1977 [5, 9–11]) (**Figure 9**). **Figure 10** shows a comparison of the hydrogeology between 1957 and 1977. The total error of the comparison process can be assumed to have a maximum value of about 6 m, although the computed error value is usually much lower than 6 m. Given the apparent phenomenon of lowering saturation levels, management decisions are needed.
