After mine closure in the 1980s and subsequent shutdown of the dewatering system, groundwater rebound led to drainage outflow from the Casargiu gallery (Montevecchio mine, SW Sardinia, Italy) beginning in 1997. Mine drainage had pH 6.0 and dissolved concentrations of sulfate (5000 mg/L) and metals (e.g., 1000 mg/L Zn, 230 mg/L Fe, 150 mg/L Mn) much higher than those previously measured in groundwater under dewatering conditions. As compared with the first stages of rebound at Casargiu, a very high contamination level still persists after more than 15 years of flushing. Mine drainage (20–70 L/s; pH 6.0 ± 0.2; Zn-Mg-Ca-SO₄ composition) flowed into the Rio Irvi. Abundant precipitation of amorphous Fe(III)-(oxy)hydroxides occurred. Moreover, sulfate-bearing green rust was observed to flocculate in the reach of the Rio Irvi where pH was still circumneutral. Water sampling along this stream for about 6 km almost to its mouth in the Mediterranean Sea showed a pH decrease from 6.0 to 4.0 and a significant removal of Fe (46 %) and As (96 %), while sulfate, Zn, Mn, Co, Ni, and Cd showed small variations downstream. Lead was initially adsorbed onto Fe(III)-(oxy)hydroxides, then desorbed as pH dropped below 5. The estimated amount of dissolved metals discharged into the Mediterranean Sea is significant (e.g., 900 kg/day Zn, 1.4 kg/day Cd, 5 kg/day Ni). In particular, a conservative estimation of the amount of Zn discharged to the sea is about 330 ton/year, which would correspond to 1.4 % of the global annual flux of dissolved Zn from uncontaminated rivers to the oceans.