Hydrogeological and geochemical overview of the karst aquifers in the Apuan Alps (Northwestern Tuscany, Italy)

Apuan Alps, in north-western Tuscany (Italy), have a very complex geological structure. For this reason karst springs show very different behaviours according to the geological setting of aquifer systems. More than 80 springs are fed by carbonate aquifers; flow rates (Q) range from 10 to 1600 L/s, in average, temperatures range from 8 to 15 °C. Deep and very slow groundwater flow feed some low-thermal springs (20-30 °C). Major karst springs (Q > 100 L/s) are concentrated in two altimetry ranges, one from 200 to 300 m a.s.l. in the seaward side (SW-NW) and a second one from 500 to 600 m a.s.l. on the inner (NE-SE) side of the mountain range. Most of the springs are the final destination of large karst systems developed in meta-dolomite and marbles characterized by a very rapid flow. Some springs have a regular regime and are fed by bathyphreatic systems in metamorphic rocks or by carbonate aquifers with a major contribution of fissured drainage in non-metamorphic rocks. Large physical-chemical variations, both in space and time, are observed as a consequence of lithological heterogeneity, mixing processes and hydrodynamic conditions. Most waters are of the Ca-HCO₃ type, but Ca-SO₄ and Na-Cl facies are also present. A wide range of electrical conductivity is recorded, with values between 0.1 μS/cm and 10 μS/cm. Significant differences in the average isotopic signature (e.g. δ¹⁸O from -5.5 to -8.5‰) of the "base-flow" are registered due to the variability of hydrogeological basins dimension and their distribution in terms of altitude range and side. In some cases, springs with similar chemical features and located close to each other, point out very different isotopes signature, thus highlighting complicated flow path of groundwater. Furthermore, different seasonal evolutions of isotopic signatures are registered.