Hydrochemical impact of construction of the western section of the Hallands\ue5s rail tunnel in Sweden

This paper presents a study of the hydrochemicalchanges that took place during construction of a section ofthe Hallandsa\ub0s rail tunnel in southwest Sweden based onmonitoring from spring 2011 to summer 2012. Leakageinto the tunnel during construction caused lowering of thegroundwater levels, which in turn resulted in a decrease inor absence of base flow in the streams. The water in thestreams became dominated by meteoric water during thedrawdown periods. Meanwhile, wetlands were aerated, andoxygen could penetrate to oxidise reduced S, releasingacids and SO4. The results for the groundwater in thebedrock differed spatially depending on local geologicalconditions. In each of the three monitored boreholes,higher redox potentials, higher concentrations of organicmatter and lower concentrations of dissolved Mn and Fewere observed. In two of the boreholes, oxidation of pyrite,FeS2, present as a fracture mineral, caused the formation ofSO4 and acids with subsequent falls in pH and alkalinity.Leakage into underground constructions generally shortensthe residence time of the groundwater significantly. Silicate weathering would thus become less important for thehydrochemistry compared to processes that occur duringshorter time frames. As regards the durability of the tunnel, the hydrochemical changes observed in two of the three boreholes indicate a more aggressive environment forseveral parameters known to increase corrosivity of steel.The recovery of the groundwater levels occurred rapidlyfollowing completion of the waterproofing systems in theTunnel. However, hydrochemical recovery with regard tomajor ions and pH occurred gradually and with an expectedduration of several years