Geochemical and hydrogeological characteristics of the Santa Cesarea terme sulfuric acid cave systems (Apulia, Southern Italy)

Santa Cesarea Terme (Apulia, southern Italy) stands as an important spa area, known since the nineteenth century. Several emergences of slightly thermal sulfidic waters flow out along a coastal sector of the Salento peninsula, and exhibit temperatures ranging between 22–25 °C. The carbonate sequence, consisting of over 5-km-thick Jurassic and Cretaceous limestone and dolostone, rests above Late Triassic evaporites and is unconformably overlain by Cenozoic calcareous successions. Starting from the Early Triassic, the area was part of the Apulian carbonate platform, characterized by shallow-water carbonate sedimentation. Since Cretaceous times, it experienced a number of transgression-regression phases, giving rise to a succession constituted by multiple unconformities. Eventually, during the Middle Pleistocene, the area underwent a severe regional uplift. The Santa Cesarea Terme area shows extensional and trans-tensional structures, with related pull-apart features. The NW–SE trans-tensional faults are the most diffuse, with subordinate presence of other fault systems, and appear to be the youngest set, controlling the development of the main karst landforms in the area. At the surface, these are mostly represented by sinkholes and dry valleys of different sizes and typologies, and by typical low-incised karst valleys. Apart from a number of inland caves (mostly of limited development), the most relevant karst features are distributed along the coast and correspond to three caves within the spa area and an additional cave further south. The four caves are carved at sea level and all have a direct access to the sea. Here, rising sulfidic waters meet and mix with seawater producing solutions with different geochemical contents. The sulfidic springs and the village of Santa Cesarea Terme are located on a structural high, with the caves elongated along the NW–SE fault zones. Inside the caves, that show quite large halls and galleries ending abruptly, it is possible to observe cupolas, rising channels, megacusps, submerged feeders, weathered walls, replacement pockets, and important deposits of gypsum, with abundant native sulphur coatings and jarosite. All these features allow to define the caves as Sulfuric Acid Speleogenesis (SAS) systems. During the last four years, geochemical data have been collected in order to characterize the composition of water flowing inside the caves, and a variety of parameters have been measured (including temperature, dissolved oxygen, conductivity, pH, etc.). They fall into the Na-Cl-SO4 sector of the Ludwig-Langelier diagram, with pH ranging from 7.2 to 7.8 (the latter value being mainly influenced by seawater-deep water mixing), and are colonized by whitish bacterial filaments (streamers). Brownish vermiculation deposits and white gypsum moonmilk diffusely cover walls and ceilings, especially where the amount of H2S-degassing is important. Experiments with limestone tablets are demonstrating the dissolution-corrosion to be an intense on-going process for presentday sulfuric acid speleogenesis.