Abstract:
A year-long study of the sulfidic aquifer in the Frasassi caves (central Ita ly) employed chemical analysis of the water and measurements of its level, as well as assessments of the concentration of H2S, CO2, and O2 in the cave air. Bica rbonate water seepage derives from diffuse infiltration of meteoric water into the karst surface, and contributes to sulfidic ground-water dilution, with a percentage that varies between 30% and 60% during the year. Even less diluted sulfidic ground water was found in a localized area of the cave between Lago Verde and nearby springs . This water rises from a deeper phreatic zone , and its chemistry changes only slightly with the seasons with a contribution of seepage water that does not exceed 20'10 . In order to understand how the H2S oxidation, which is considered the main cave forming process, is influenced by the seasonal changes in the cave hydrology, the sulfide/total sulfur ratio was related to ground-water dilution and air composition. The data suggest that in the upper phreatic zone, limestone corrosion due to H2S oxidation is prominent in the wet season because of the high recharge of Oj-rich seepage water, while in the dry season, the H2S content increases, but the extent of oxidation is lower. In the cave atmosphere, the low H2S content in ground water during the wet season inhibits the release of this gas, but the H2S concentration increases in the dry season, favoring its oxidation in the air and the replacement of limestone with gypsum on the cave walls.