This paper presents a study of the hydrochemical changes that took place during construction of a section of the Hallandsås rail tunnel in southwest Sweden based on monitoring from spring 2011 to summer 2012. Leakage into the tunnel during construction caused lowering of the groundwater levels, which in turn resulted in a decrease in or absence of base flow in the streams. The water in the streams became dominated by meteoric water during the drawdown periods. Meanwhile, wetlands were aerated, and oxygen could penetrate to oxidise reduced S, releasing acids and SO 4 . The results for the groundwater in the bedrock differed spatially depending on local geological conditions. In each of the three monitored boreholes, higher redox potentials, higher concentrations of organic matter and lower concentrations of dissolved Mn and Fe were observed. In two of the boreholes, oxidation of pyrite, FeS 2 , present as a fracture mineral, caused the formation of SO 4 and acids with subsequent falls in pH and alkalinity. Leakage into underground constructions generally shortens the residence time of the groundwater significantly. Silicate weathering would thus become less important for the hydrochemistry compared to processes that occur during shorter time frames. As regards the durability of the tunnel, the hydrochemical changes observed in two of the three boreholes indicate a more aggressive environment for several parameters known to increase corrosivity of steel.The recovery of the groundwater levels occurred rapidly following completion of the waterproofing systems in the Tunnel. However, hydrochemical recovery with regard to major ions and pH occurred gradually and with an expected duration of several years.