Oxygen and hydrogen isotopic composition of waters in a past-mining area of southern Apuan Alps (Italy): Hydrogeological characterization and implications on the fate of potentially toxic elements

The oxygen and hydrogen isotopic composition of stream water, springs, groundwater tapped from irrigation wells and acid mine drainages were determined during two different surveys in 2015 in an area highly impacted by past-mining activity of Apuan Alps (Italy), as a guidance on good practices for water management. The isotopic local meteoric water line (LMWL) was built by monthly collecting rainwater between 2014 and 2018, given by delta D = 7.02 +/- 0.35 x delta O-18 + 8.54 +/- 2.89. The obtained results indicate that acid mine drainages (AMD) are supplied by freshwater from karst systems which flow throughout the post-mining workings. Such waters contaminate by interactions with sulfides (pyrite) that remained unmined in the ore-bodies. During rainstorms, infiltration rainwater displaces water ponding within mine, sharply increasing the outflow rate of highly-contaminated AMD. Acidic drippings in tunnels show an isotopic shift in both delta O-18 and delta D values attributable to pyrite oxidation and Fe hydrolysis. The data reveal that karst-springs represents the primary supply for the stream. The isotopic data also reveal that waters flowing in the bedrock carbonate aquifer represent the main feeding component for the overlying alluvial aquifer tapped by wells. The prevailing transfer of clean freshwaters from the carbonate aquifer towards the alluvial aquifer system mitigates the possible influence of contaminated water from stream seepage. However, these observations require a monitoring program on water quality to be established.