The Sisine karst system is one of the coastal underground drainage networks of the Gulf of Orosei that develops within the Mesozoic carbonate succession of coastal Supramonte, along the contact with the underlying impermeable Paleozoic basement. The recharge area of this cave system is located in the Su Canale plateau, which represents a structural high of the monoclinal, dipping towards the sea. The major sinkholes are Lovettecannas and Murgulavò caves that drain the allogenic recharge of small depressions. The typical morphologies of these caves are wide voids that increase their size with depth, with volumes incompatible with the available discharge. Dissolutional features are lacking and parts of these vadose passages are sometimes developed for several metres in granite or metamorphic rocks. Their outflow, hypothesized towards the underground coastal springs, has been recently demonstrated by the direct observation of dye tracer in the Beltorrente Cave, an underwater spring that opens 500 m north of Sisine beach, along the coastal cliff of Gulf of Orosei. This karst system has been recently investigated to determine the geochemical signature of its groundwater. In particular, the water sampling was carried out in the Lovettecannas and Murgulavò streams and at the Beltorrente spring. In addition to the analysis of major elements, a series of periodic measurements of discharge and in situ physico-chemical parameters (pH, conductivity, total dissolved solids, alkalinity and temperature) have been carried out. The preliminary data analysis show that the small stream of Murgulavò has a relatively high water temperature for the altitude at which its entrance opens (900 m asl) and equal to 12.1 °C (internal air temperature of 12.4 °C), with conductivity of 0.29 mS/cm and a pH value of 7.6. In the Lovettecannas stream the pH values remain relatively basic (>8.2), the conductivity around of 0.30 mS/cm and the temperature ranges from about 8 °C in the passages close to the entrance to higher values which increase with depth reaching 11.6 °C at the CinCin Hall at an altitude of 630 m asl. Groundwater geochemistry have been confirmed to have a calcium bicarbonate character, as result of a water-rock interaction that mainly involved rock portions with a magnesium composition (dolomitic lithotypes) even though the monitoring of physico-chemical elements shows clearly heterogeneity in the hydro-chemical feature of the drainage network that suffers the seasonality of the external input, which mitigates its effects gradually penetrating towards the deeper part of the aquifer.
Groundwater geochemistry of the Sisine coastal karst system (Central-East Sardinia, Italy)
Laura Sanna
2018
Abstract
The Sisine karst system is one of the coastal underground drainage networks of the Gulf of Orosei that develops within the Mesozoic carbonate succession of coastal Supramonte, along the contact with the underlying impermeable Paleozoic basement. The recharge area of this cave system is located in the Su Canale plateau, which represents a structural high of the monoclinal, dipping towards the sea. The major sinkholes are Lovettecannas and Murgulavò caves that drain the allogenic recharge of small depressions. The typical morphologies of these caves are wide voids that increase their size with depth, with volumes incompatible with the available discharge. Dissolutional features are lacking and parts of these vadose passages are sometimes developed for several metres in granite or metamorphic rocks. Their outflow, hypothesized towards the underground coastal springs, has been recently demonstrated by the direct observation of dye tracer in the Beltorrente Cave, an underwater spring that opens 500 m north of Sisine beach, along the coastal cliff of Gulf of Orosei. This karst system has been recently investigated to determine the geochemical signature of its groundwater. In particular, the water sampling was carried out in the Lovettecannas and Murgulavò streams and at the Beltorrente spring. In addition to the analysis of major elements, a series of periodic measurements of discharge and in situ physico-chemical parameters (pH, conductivity, total dissolved solids, alkalinity and temperature) have been carried out. The preliminary data analysis show that the small stream of Murgulavò has a relatively high water temperature for the altitude at which its entrance opens (900 m asl) and equal to 12.1 °C (internal air temperature of 12.4 °C), with conductivity of 0.29 mS/cm and a pH value of 7.6. In the Lovettecannas stream the pH values remain relatively basic (>8.2), the conductivity around of 0.30 mS/cm and the temperature ranges from about 8 °C in the passages close to the entrance to higher values which increase with depth reaching 11.6 °C at the CinCin Hall at an altitude of 630 m asl. Groundwater geochemistry have been confirmed to have a calcium bicarbonate character, as result of a water-rock interaction that mainly involved rock portions with a magnesium composition (dolomitic lithotypes) even though the monitoring of physico-chemical elements shows clearly heterogeneity in the hydro-chemical feature of the drainage network that suffers the seasonality of the external input, which mitigates its effects gradually penetrating towards the deeper part of the aquifer.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
