*5.3. The Triassic Unit*

– ‰ SW seawards. Transmissivity T ranges from 1 – − −3 – – 000 μS/cm has The Triassic carbonate west and north of the study area is under confined conditions in massive limestone-dolomite (wells # 116, 117, 147, and 162), and it is unconfined in karstified and intensely fractured limestone-dolomitic limestone (wells # 6, 18, 169, and 202), with a hydraulic gradient 0.5–1‰ SW seawards. Transmissivity T ranges from 15–350 m<sup>2</sup> /d (1.74 × 10 <sup>−</sup><sup>4</sup> 4.1 × 10 <sup>−</sup><sup>3</sup> m<sup>2</sup> /s). TDS ranges between 900–3000 mg/L. The confined units appear to maintain their fair quality with TDS between 400–1400 mg/L; instead, in the karstified units, the TDS is up to 2500 mg/L. The seawater front has intruded 8 to 9 km inland. Brackish water with EC = 14,000 µS/cm has been found 10.5 km inland [36].

q' Trying to determine a salt water-fresh water interface, it is assumed the presence of two wells, A and B, located 8000 m and 10 m from the shoreline are tapping the Triassic aquifer, which has a saturated thickness of 50 m. It is also assumed that T=300 m<sup>2</sup> /d and I=0.001; thus, q'=T×i=300×0.001=0.3 m<sup>2</sup> /d and k = 300/50 = 6 m/d. Then, the depth to the interface (i) at well A based on Equation (2), where no vertical flow occurs, is z = (2 × 40 × 0.3 × 8000/6) 0.5 = 179 masl and, (ii) at well B based on Equation (3), where vertical flow occurs, the depth to the interface is z = (2 × 40 × 0.3 × 10/6 + (40 × 0.3/6) 2 ) 0.5 = 6.63 m. In addition, based on Equations (4) and (5), the depth to the interface at the shoreline is zo = 40 × 0.3/6 = 2 masl, and the width of the outflow face xo = 40 × 0.3/(2 × 6) = 1 m, respectively.

The hydrochemical data from such wells as it is reported in the hydrochemical settings section show that groundwater was found brackish during the drilling phase. This means that seawater intruded inland in a past geological time, probably due to Pleistocene seawater fluctuations, or that it is fossil or entrapped (palaeo)seawater. –

In recent years, water samples from wells # 54, 41, 83, 86, 16, 98, and 27 in the Pleistocene deposits show that EC values and chloride concentrations have spectacularly been reduced. On the contrary, in the carbonate aquifers, EC values remain at high elevations, and an increase of chlorides has been observed in almost all the wells of this aquifer, e.g., # 120, 118, 18A, etc. [29–31]. In Figure 8, the TDS versus distance from sea graph is shown. The TDS decreases with the increasing distance from the sea. In the north part of the plain, however, it remains high. − −

**Figure 8.** Total dissolved solids (TDS) decrease with increasing distance from the sea. In the carbonate located north (wells 120 and 147), it remains high.

All the data mentioned above provide the evidence that modern direct seawater intrusion could occur nowadays only in the carbonate aquifers [56] around the plain but not in the Neogene-Quaternary deposits. In addition, the occurrence of high fluoride concentration in the groundwater [50] indicates rock dissolution, which requires hundreds or thousands of years to be accomplished [58]. Besides, the F <sup>−</sup>/Cl <sup>−</sup> molar ratio up to 500 times more than that in modern seawater indicates that there is not an influence of modern seawater. It is very likely the occurrence of palaeo-seawater could explain those values.
