Publication: La circolazione carsica nel fianco di anticlinali dell’Appennino Marchigiano: l’esempio della grotta Sasso Pozzo (Gagliole), Sandro Galdenzi, Alfredo Campagnoli
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Date
2013
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EUT Edizioni Università di Trieste
Abstract
Sasso Pozzo è la più importante grotta conosciuta negli acquiferi minori
presenti sui fianchi delle anticlinali dell’Appennino Marchigiano, ed è
stata studiata insieme alla vicina sorgente di Mignano (Gagliole, MC) per
acquisire dirette informazioni sulle modalità di drenaggio in questo tipo
di acquiferi.
La grotta si sviluppa in un’unità calcarea compresa entro orizzonti marnosi
al letto e al tetto, la cui giacitura ha condizionato il drenaggio e la
struttura in grande della grotta, che si sviluppa parallelamente al versante,
secondo l’andamento della stratificazione, mantenendosi circa
130 m sotto la superficie topografica per lo più sotto coperture marnose.
La grotta, di origine freatica, si comporta attualmente da emergenza di
troppo pieno ed è formata da un’angusta galleria lunga 500 m, dalla quale
si dipartono poche diramazioni laterali. La grotta ha andamento discendente
e profilo longitudinale irregolare, con tratti sifonanti e frequenti
contropendenze. Raggiunge la superficie dell’acquifero a -29 m rispetto
all’ingresso.
Una livellazione di dettaglio ed il monitoraggio idrologico continuo protrattosi
per un anno, con tre stazioni interne ed una presso la sorgente,
hanno consentito di definire la geometria delle grotta e di seguire le
variazioni assolute dei livelli idrici in risposta agli eventi meteorologici
superficiali.
La temperatura e la conducibilità delle acque nella grotta e alla sorgente
sono molto simili e seguono lo stesso trend stagionale. L’alta permeabilità
dell’ammasso roccioso nella zona vadosa causa risposte molto rapide
agli eventi meteorici di superficie, mentre una riduzione di permeabilità
nelle zone più profonde nell’acquifero giustifica l’elevato gradiente piezometrico
misurato tra la grotta e la sorgente.
In occasione di importanti eventi meteorici si verifica una rapida risalita
dei livelli piezometrici, fino a 16 m/h, per un totale di +45 m rispetto al
livello di magra. In queste situazioni la risalita di acqua dal lago sifone
terminale determina la riattivazione della grotta che viene totalmente invasa
dalle acque fino a defluire dall’ingresso.
In the Umbria-Marche Apennines small aquifers, often perched above the main base level, are present in the limestones interbedded with marls. Many small caves related to this type of aquifers are known in the limbs of anticlines. Sasso Pozzo is the most important cave in a similar setting, and it was studied together with the nearby Mignano Spring to define the conditions of karst caves development and the characteristics of groundwater drainage. The cave is a temporary karst emergence, consists of a main tube 500 m long, with few lateral branches for a total length of ~ 600 m, and reaches the piezometric surface in its termination, -29 m below the entrance. It exhibits typical phreatic features, with prevailing inclined passages and common siphoning zones. The passages have a small size (diameter 1-2 m) and their walls are covered by scallops. The marl Formation underlying limestone has an important role in concentrating the seepage water from the ridge to the limb of the anticline, where the limestone is sandwiched between poorly permeable beds. The cave formed in this geologic setting due to the ground-water flow parallel to bedding in the saturated zone. During the Middle and Upper Pleistocene, the deepening of the surface streams eroded the marl cover and caused the drop of the water table, so that the cave was abandoned by the ground-water. After this event, the invasion of ground-water became occasional and the cave evolved mainly in vadose conditions. The ground-water monitoring and a detailed survey in the cave clarified the hydraulic behaviour of the cave. A high hydraulic gradient exists between the spring and the ground-water in the cave, probably due to the low permeability of the limestone in the phreatic zone of the aquifer. The water characteristics and the hydrodynamic are similar in the cave and in the spring, and are influenced by the fast recharge of infiltration water from the karst surface. The meteoric water reaches the saturated zone in a few hours through a network of fractures and small karst passages existing in the vadose zone. This fast recharge of infiltration water is not drained through the normal pathway due to the low karstfication rate in the phreatic zone, and the flooding events causes important and fast rises (up to 16 m/h) of the ground-water. The piezometric level increases over 45 m and the cave is completely flo-oded by water that rises from the terminal lake and reaches the surface. In the remaining periods, the cave remains dry, excluding some siphons that can be filled by the seepage water descending in the vadose zone.
In the Umbria-Marche Apennines small aquifers, often perched above the main base level, are present in the limestones interbedded with marls. Many small caves related to this type of aquifers are known in the limbs of anticlines. Sasso Pozzo is the most important cave in a similar setting, and it was studied together with the nearby Mignano Spring to define the conditions of karst caves development and the characteristics of groundwater drainage. The cave is a temporary karst emergence, consists of a main tube 500 m long, with few lateral branches for a total length of ~ 600 m, and reaches the piezometric surface in its termination, -29 m below the entrance. It exhibits typical phreatic features, with prevailing inclined passages and common siphoning zones. The passages have a small size (diameter 1-2 m) and their walls are covered by scallops. The marl Formation underlying limestone has an important role in concentrating the seepage water from the ridge to the limb of the anticline, where the limestone is sandwiched between poorly permeable beds. The cave formed in this geologic setting due to the ground-water flow parallel to bedding in the saturated zone. During the Middle and Upper Pleistocene, the deepening of the surface streams eroded the marl cover and caused the drop of the water table, so that the cave was abandoned by the ground-water. After this event, the invasion of ground-water became occasional and the cave evolved mainly in vadose conditions. The ground-water monitoring and a detailed survey in the cave clarified the hydraulic behaviour of the cave. A high hydraulic gradient exists between the spring and the ground-water in the cave, probably due to the low permeability of the limestone in the phreatic zone of the aquifer. The water characteristics and the hydrodynamic are similar in the cave and in the spring, and are influenced by the fast recharge of infiltration water from the karst surface. The meteoric water reaches the saturated zone in a few hours through a network of fractures and small karst passages existing in the vadose zone. This fast recharge of infiltration water is not drained through the normal pathway due to the low karstfication rate in the phreatic zone, and the flooding events causes important and fast rises (up to 16 m/h) of the ground-water. The piezometric level increases over 45 m and the cave is completely flo-oded by water that rises from the terminal lake and reaches the surface. In the remaining periods, the cave remains dry, excluding some siphons that can be filled by the seepage water descending in the vadose zone.
Description
Keywords
Appennino Umbro-Marchigiano, Idrologia carsica, Grotta, Umbro-Marchigiano Appennines, Karst Hydrology, Cave
Citation
Sandro Galdenzi, Alfredo Campagnoli, "La circolazione carsica nel fianco di anticlinali dell’Appennino Marchigiano: l’esempio della grotta Sasso Pozzo (Gagliole)" in: Franco Cucchi e Pino Guidi (a cura di), "Diffusione delle conoscenze: Atti del XXI Congresso Nazionale di Speleologia, Trieste, 2-5 giugno 2011", Trieste, EUT Edizioni Università di Trieste, 2013, pp. 399-410