Variazioni idrogeologiche e riattivazione del carsismo nei gessi di Lesina Marina, Puglia.

The Lesina Lagoon is located in the East-West-trending northern cost of Gargano and is connected with the sea by two channels. The sea-side mouth of these channels was frequently clogged by sand accumulation. In 1927 a long canal was excavated, to ensure a permanent water exchange between the lagoon and the Adriatic Sea for environmental and fish-farming purposes. This N-S trending canal follows the path of the pre-existing canal, except in the northernmost 800 m. In this artificial reach the canal was excavated in evaporite bedrock and in a small outcrop of igneous rocks situated in the coast and protected by a concrete lining. In 1993 in several parts of the canal, the flanks of the canal were smoothed and the concrete lining was replaced by gabions for stability and scenery improvement. The northern reach of the canal is dug in Upper Triassic gypsiferous sediments of the Burano Anhydrite Formation. The evaporite bedrock is mantled by loose sands a few meters thick. The exposures found reveal that the gypsum has a high density of dissolutional conduits and cavities. Locally, it also shows open fractures and brecciated structure caused by dissolution-induced collapse processes and voids are partially filled with detrital sediments. These features seem to correspond to a palaeo-karst, probably developed at several depths controlled by different sea level stands during the Quaternary. The construction of the new canal has caused significant changes in the local hydrology. The groundwater flow, previously controlled by the oscillating sea level and by the lagoon level, has been altered introducing a line pulsating water near the value of the sea level along the canal. As a consequence of the lowering of the water table, slow sub-horizontal phreatic flows have been replaced by more rapid flows. These hydrological changes have induced the reactivation-acceleration of internal erosion and collapse processes leading to the generation of a large number of sinkholes in the vicinity of the canal. The great majority of the sinkholes are located within the canal and on two relatively narrow bands situated on its flanks. The Lesina Marina residential area, whose construction in the western side of the canal started around 1970, is currently suffering from subsidence damage, including the occurrence of collapse sinkholes in streets, destruction of pathways and cracking of walls.