The sulfidic thermal caves of Acquasanta Terme (Central Italy)

The caves of Acquasanta Terme (central Italy) open at the core of a wide anticline, in the valley of the Tronto River. Cave development is due to the rise of sulfidic thermal water flowing through a thick marine limestone sequence, overlain by thick, low-permeability formations. Some minor caves are developed in the terraced travertines deposited by the thermal water, but the major caves are developed in marine limestone in the small gorge of the Rio Garrafo stream, a tributary of the Tronto River. These caves have a rising pattern, due to the past flow of thermal water toward the surface. The deepening of the Tronto River Valley lowered the regional water table, perching the Rio Garrafo stream 50 m above the thermal groundwater. At present, surface water sinks through the pre-existing karst passages to reach the thermal water flowing in the lower parts of the caves. Where these waters mix, rapid corrosion of the walls through sulfuric acid speleogenesis occurs. Annual temperature and chemistry monitoring of the cave water showed that freshwater contributes up to 45% of the volume at the water table. Dilution events are associated with falling water temperature, which ranges between 44 degrees C and 32 degrees C. At the main spring, 2 km downstream, groundwater dilution was higher resulting in lower temperatures (32 degrees C-21 degrees C) and salinity. The periods of high freshwater dilution correspond with a lowering of pH in the phreatic water and with the release of H(2)S and CO(2) to the cave atmosphere. In the thermal zones, the concentration of H(2)S increased from 40 to over 240 ppm, while CO(2) increased from 0.44% to 2.7%. These data evidence the influence of sinking surface water on the cave environment and speleogenesis.