GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.2 145 the cherty limestone, were detected above the bedrock (Siliceous Schists) and they probably contributed to waterproofing the bottom of the lake. The Flinty Limestone and Siliceous Schists are the main reservoir of the Monte Sirino aquifer system. The cherty limestone are characterized by a relatively mid-high permeability, mainly due to the widespread cracking and\or layering and to the major fracture systems oriented N-S and NNE-SSW; instead, the Siliceous Schists can be appreciably permeable along the stratification planes and\or along bands of intense fracturing. All around, the system is wholly or partially buffered by a belt of less permeable soils (Galestrino Flysch) which constitute the main waterproof of the carbonate aquifer formed by Monte Sirino (D’ecclesiis et al. , 1990). Locally, for anisotropy of the structure, also the Siliceous Schists act as impermeable soils (Grassi et al. , 2001a, 2001b). In the last century, the Sirino Lake was affected by many pipings, as a result of sudden openings of sinkholes, which have resulted in the almost total lake depletion. For these reasons, a series of local waterproofing remediation actions were carried out. These, however, did not prevent the phenomenon to recur at different times and in adjacent areas. Moreover, these measures have been often of environmental and especially visual strong impact. The opening of these sinkholes occurs with a not predictable multi-yearly frequency. However, a written record of all the episodes that took place over time lacks. Among the most recent episodes, there are those occurred in 1994, 2009 and 2014. In July 1994, a sinkhole opened in the SW side of the lake, about 3 meters from the shore, with a collapse area of 1 meter in diameter. The water was poured into the hole and in a short time the lake level dropped about 2 m. Simultaneously, there was a sudden increase in the flow rates of underlying sources: in particular the Sotto il Lago II source reached a flow rate value of about 300 l/s. The siphoning stopped naturally, following the collapse of a shore section, with the consequent closure of the hole and the returning, after a few days, at normal flow rates values (Grassi et al. , 2001a, 2001b). In the episode occurred in the summers of 2009 and 2014, a sinkhole opened on the same shore that was affected in the 1994 episode, a few tens of meters away. In this case, the hole was closed artificially with concrete. Geophysical data acquisition and processing. For the study of sinkhole phenomena along the Sirino lake shores an electrical resistivity tomography (ERT) was carried out both on land and water-covered area and a ground penetrating radar and a self-potential surveys (SP) were carried out around the lake shores (Fig. 2). The ER profile was carried out between the northwester and the southeastern lake shores, with a total length of approximately 450 m of which 210 m into the lake with electrodes floating on the water surface. The geoelectrical tomography was performed using the georesistivity meter Syscal Junior (IRIS Instrument) coupled to a multi-electrode system, consisting of two measuring cable at 24 channels with an electrodes spacing of 10 m. For the geoelectrical data acquisition in water, the only measuring cable, floating on the lake surface by means of a series of PVC bottles equally spaced along the cable, was used. For the ground measures, standard stainless steel electrodes were used connected to the cable. The Syscal Junior resistivity meter was placed on board of a small boat, held steady in the lake centre for the whole duration of measures. The ERT was acquired using the Wenner, Wenner-Schlumberger and Dipole-Dipole arrays, with an electrodes spacing of 10 m. Furthermore, water electrical conductivity and temperature were measured along ERT profile. The ETR was inverted by means of the res2Dinv software that uses the smoothness- constrained least-squares inversion method based on the quasi-Newton method (Loke and Barker, 1996). In particular, water electrical resistivity and bathymetry constrains were included in the inversion processes, using a distorted finite element grid, where the upper part of the mesh was used to model the water layer at the assigned electrical resistivity value of 50 Ωm.