Hydrochemical impact of construction of the western section of the Hallandsås rail tunnel in Sweden
Artikel i vetenskaplig tidskrift, 2017
This paper presents a study of the hydrochemical
changes that took place during construction of a section of
the Hallandsa°s 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 SO4. 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,
FeS2, present as a fracture mineral, caused the formation of
SO4 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.
Groundwater
Tunnels
Hydrochemistry