Sasso Pozzo cave is the most important overflow karst emergence of the Umbria-Marche Apennines located in limestone formations sandwiched between poorly permeable marly beds in the limbs of anticlines. The cave was studied together with the nearby Mignano spring to define the conditions of its development and the characteristics of groundwater drainage in this type of aquifer. The cave is a 500 m phreatic tube, developing along the bedding planes, and has a few lateral branches that give it a total length of 600 m. The passage is small (diameter 1-2 m), mainly inclined, with multiple loops often filled with water. The walls are wholly scalloped. The marl beds have influenced the cave development, increasing the recharge of infiltrating water from the karst surface to the limb of the anticline, where the water flows parallel to the bedding strike towards the springs. The cave formed in the deep phreatic zone before the deepening of the surface streams caused the groundwater level to drop and the main spring to be displaced to its present position. At present, the groundwater can be reached at the bottom of the cave, -29 m below the entrance. The high hydraulic gradient measured between this cave pool and the Mignano spring suggests that the karstfication degree in the lower zone of the aquifer is low. The high karstfication of the vadose zone, on the contrary, ensures a fast recharge of meteoric water that, after heavy precipitation, causes a fast rise in the piezometric level (up to 16 m/h, for a total increase that exceeds 45 m), flooding the whole cave and activating the overflow spring. After floods, a large part of the cave empties quickly of water, excluding some perched sumps that can remain long filled with seepage water.
Umbria-Marche Apennines, karst hydrology, speleogenesis, cave.