Tides of Adriatic Sea in the caves of Classical Karst: The case of Pozzo dei protei di Monfalcone (Italy)

The effects of the tides of the Adriatic Sea within the Classical Karst are little known. In the years from 1977 to 1980, in the then Yugoslavia, a series of investigations were started in the area of Klariči, today in Slovenia, for the construction of the wells which will then give rise to the Slovenian Karst aqueduct. Thus the effects of the northern Adriatic tides in karst groundwater were documented for the first time. The lack of knowledge on the subject, in addition to the singularity of the phenomenon, was the reason that induced the Karst Research Center “C. Seppenhofer ”of Gorizia to launch a research project on the Pozzo dei Protei of Monfalcone (Classical Karst NW) aimed at investigating the possible effects of tides in a karstic cavity of the area in question, buffered towards the south, ie towards the sea, by a series of alluvial and marine sediments. In the water of the shaft the proteus (Proteus anguinus Laurenti, 1768) has been observed, permanently.

The entrance of the shaft is located in the railway station of Monfalcone (Gorizia, Italy) at 23.18 m a.s.l. The groundwater level is about 2.5 m above the bottom of the cavity at an average altitude of 1.89 m a.m.s.l. 4.4 km southeast there are the Springs of Timavo while 750 m to the south the artificial Valentinis Canal receives water from the Isonzo River and in Monfalcone flows into the lagoon area connected to the Gulf of Trieste (Panzano Bay).

In the water basin of the Pozzo dei Protei di Monfalcone we installed a CTD-Diver probe from Eijleikamp (Netherlands) by the Centre for karst researches “C. Seppenhofer” of Gorizia (Italy) with the purpose to record the variations of level, temperature and electrical conductivity and above all to verify the possible correlations with the tides of the high Adriatic Sea.

The Pozzo dei Protei di Monfalcone is developed in the limestones of Repen formation (Cenomanian-Turonian). In the south, there are the limestone of the Sežana formation (Turonian p.p.-Campanian p.p.) buried by Quaternary and marine sediments. The limestone of Sežana formation crops from the continental and marine Quaternary sediments 2 km SE of the Pozzo dei Protei di Monfalcone, forming the two small hills (Insulae Clarae) of the Roman Baths, where there are thermal water emergencies from a carbonate reservoir deep through the fault system of the Palmanova Line. The shaft has an axis NNE-SSW, corresponding to the large joints that characterize it. This ancient phreatic morphological characteristic is maintained from about 5 meters below its roof to the bottom. In the phreatic part the diving exploration has identified an impracticable conduct with a triangular section of about 30 x 30 cm, developed along a NNE-SSW joint. The shaft is located in the hinge area of a small synform fold with anti-dinaric axis. It could be hypothesized that in the vadose phase the slow water flow occurred along the layers in the depression zone of rocky mass. The karst hydrology to the north of the Pozzo dei Protei di Monfalcone is dominated by the water circulation of the closed or open polja of the lakes of Doberdò, Mucille and Pietrarossa, then from the relationships between superficial infiltration and the contribution of the river Vipacco and above Isonzo.

During the monitoring period in the Pozzo dei Protei di Monfalcone the water level fluctuated between 1.73 m a.m.s.l. and 2.32 m a.m.s.l. (average level 1.89 m a.m.s.l.). Excluding the changes in level caused by tidal oscillations and moderate rainfall in the area that caused an increase in the discharge of the Isonzo River, a decrease in the level of water in the cave was observed. The electrical conductivity (EC K25) fluctuated between 336 and 387 μS/cm (average 348 μS/cm), while the water temperature varied between 12.16 and 12.71 °C (average temperature 12.41 °C).

The level fluctuations observed in the cave are the result of several factors: the allogenic recharge by losses of the Isonzo River inside the Karst and autogenic recharge due to precipitation. These oscillations with periods between 11 and 13 hours are also observable in the Pozzo dei Protei di Monfalcone and have a maximum daily excursion between about 10–12 cm and 5–6 cm respectively during spring tides and neap tides when the variations in sea level in the Panzano Bay are about 130 cm and 50–60 cm respectively. The lag between the height of the tide and the maximum water level reached in the cave is about 4–4.5 h. This phase shift is mainly due to the characteristics of the aquifer and the fact that it comes into contact with the sea at the system’s springs (among which we remember the numerous coastal springs) and indirectly through the gravelly, sandy and silty Quaternary unconsolidated sediments of alluvial and marine origin.

The EC (K25) of the waters of the Pozzo dei Protei di Monfalcone in the period of 348 μS/cm is slightly higher than those measured for Doberdò Lake, and comparable with those of the near caves and karstic springs. The EC (K25) of the phreatic water of three wells in the alluvial aquifer NE of Monfalcone is between 443 and 463 μS/cm  in clear contrast to the karstic aquifer. We also observed variations in the EC (K25), 4–5 μS/cm near the spring tides and 2–3 μS/cm near the neap tides, with a sinusoidal pattern, and a period comparable to that of the tides (between 11 and 14 h), but with a lag of 6.5 h respect to the increase of the water level in the cave due to the tide. It is excluded that the cave is directly affected by a saltwater wedge. The cyclical variations of EC (K25) observed could be due to the entry into the cave of waters with greater mineralization coming from the joints of rock mass or from the karstic conduit discovered at the bottom, mobilized by the increase and release of head loss caused by the high tides.

Authors: Rino Semeraro, Federico Valentinuz, Eduardo Klassen, Mauro Pincin, Stefano Rejc, Luciano Russo, Michele Soranzo, Maurizio Tavagnutti

Centro Ricerche Carsiche “C. Seppenhofer”

Type: poster presentation

Section: Cave Geology, Geomorphology and Geography of Karst




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