Hydrogeological study of the intensely exploited aquifer of the Santa Croce leather-producing district, Tuscany (central Italy)

A hydrogeological study was undertaken to define the groundwater circulation in the Santa Croce area (Tuscany, central Italy) where the existing multilayered aquifer has long been intensively exploited. Investigations carried out on about 150 wells revealed the existence of a deep piezometric depression (to 20 m below sea level) which drains groundwater from the entire surrounding area. Samples from about 70 water points, collected twice in 2007, were analysed for major elements and stable isotope composition. Three major groups of waters, which mix in the study area, have been distinguished: (1) waters of the Ca–Mg–HCO₃ type mainly flowing along the Arno River Plain; (2) waters of the Na–K–Cl/Ca–Mg–HCO₃–SO_4, type with SO₄ content up to 275 mg/L, inflowing from the Pisane Hills; (3) relatively high-salinity waters mainly of the Na–HCO₃ type which, with Cl concentrations up to 750 mg/L, likely arise from a normal fault located at the foot of the northern hills. The characteristics of the different components are greatly affected by significant modifying processes such as cation exchange and sulfate reduction. The achieved conceptual model suggests the southern hills as the main recharging area of the aquifer system from which water circulation, characterized by pathways of different length and depth, develops.