Isotopes and water chemistry in sulphide-bearing springs of Central Italy

The hydrogeological situation of Central Italy is complicated because of the strong faulting and the local stratigraphy. Three springs were selected on the basis of the occurrence of dissolved sulphides as indicators of deep circulation. The study combines the use of environmental isotope techniques and physico-chemical parameters (temperature, conductivity and silica content) to investigate these groundwater systems. At Lavinio (very low flow of about 1 1/min) the waters exhibit a seasonal amount of exchange with rocks (seasonal mixing or seasonal paths for groundwaters). The location of the recharge area calculated on the basis of the 18O gradient in altitude is reassessed. The conductivity and the silica content show respectively inverse and direct correlation with the thermally induced 18O shift. This suggests that the lowest and hottest parts of the aquifer have already been leached of their soluble salts. At Tivoli (high flow of 3 m3/s) the average area of recharge is at about 640m. However, comparison with conductivity and the silica content shows small amounts of locally derived water in winter and spring and a maximum proportion of deep waters in summer. At Acquasanta (medium flow of about 250 1/s) the comparison of the data leads to a complex seasonal pattern of mixing: spring, summer, autumn: deep, hot, silica and salt-rich supply, evaporated in the karst; winter: dilution with shallow water coming from high altitude and with local water during periods of snow cover. It must be emphasized that: (a) incomplete and sometimes erroneous conclusions can be drawn from an isotope study using only one isotope, e.g. 18O; (b) a large amount of complementary information on groundwater systems can be derived by comparing isotopic data with simple physical and chemical parameters such as conductivity, temperature and silica content. (author)