GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 3.2 653 Electrical resistivity tomography for the hydrogeological characterisation of carbonate lithologies: Gnejna Valley test site (Malta) E. Rizzo 1 , L. Capozzoli 1 , R. Persico 2 , S. D’Amico 3 , A. Micalef 3 1 CNR-IMAA, C.da S. Loja, Tito (PZ), Italy 2 CNR-IBAM, Lecce, Italy 3 University of Malta, Department of Geosciences, Msida, Malta The paper describes the application of the Electrical Resistivity Tomography for hydrogeological studies of carbonate rock close the sea. The objective of this work was to provide more information on role of groundwater seepage in eroding a valley located in Gnejna Valley, along the west coast of Malta. This work forms part of a project (MARCAN) that address the hypothesis that the groundwater is a key geomorphic agent in the formation of terrestrial and submarine theatre-headed valleys in carbonate bedrock in the Maltese Islands. Therefore, the investigation involved the first 40m from the surface, in order to detect the water table inside the carbonate formation. The island of Malta lies in the central Mediterranean Sea, 100 km south from Italy and 290 km east from Tunisia. The Maltese Islands are predominantly composed of marine sedimentary rocks (Fig. 1). The deposition and accumulation of this material, which formed through millions of years, derived from the sea (originated on the sea, i.e. Limestone) or from the land (originated on the surface of the land, and then carried into the sea by action of rain, wind and/ or other agents, i.e.: Blue Clay). The rock formations outcropping on the Maltese Islands are (from top to bottom): Upper Coralline Limestone, Greensand, Blue Clay, Globigerina Limestone, Lower Coralline Limestone (Oil Exploration Directorate, 1993). From hydrogeological point of view, the Upper and Lower Coralline Limestone Formations function as the main aquifer rocks, while the Globigerina Limestone acts locally as an aquifer transmitting water from the surface into the main groundwater bodies along fractures. The Upper Coralline Limestone aquifers are called perched aquifers due to the underlying impermeable Blue Clay formation. (MRA, 2005). The Malta Main Mean Sea Level Groundwater Body is hosted by the Lower Coralline Limestone formation, which makes the latter the most important aquifer rock in the Maltese Islands. When the aquifers were relatively untouched in the central parts of the island, around 70 years ago, hydraulic heads were around 4-5 m; nowadays, however, due to groundwater abstraction from the horizontal galleries excavated at sea level in the Lower Coralline Limestone aquifer, the potentiometric surface in these regions is around 1 m above mean sea level (MRA, 2005). The electrical resistivity method is based on the measurement of a drop of potential (dV) due to an electrical current (A) injected by a pairs of electrodes fixed in the ground. The electrical resistivity is the measured Fig. 1 - Excerpt of the geological map of Maltese Islands (the red square is the investigated area, Gnejna Valley). Mp (green light colour) and Mnt (green dark colour) are members of Upper Coralline Limestone; Mbc (blue colour) is Blue Clay; Mug (orange colour) and Mmg (brown colour) are members of the Globigerina Limestone; Om is a top member of Lower Corraline Limestone and Qs is modern beach sands (from Oil Exploration Directorate, 1993).