Hydrogeochemical conditions and evolution at the Äspö HRL, Sweden

The overall hydrogeochemical conditions at and in the near vicinity of the underground experimental Äspö Hard Rock Laboratory (HRL) in SE Sweden have been investigated. Groundwater data from more than 400 samples have been compiled and evaluated. The groundwater samples represent depths down to 1700 m below sea level and sampling has been performed prior to and during the HRL tunnel excavation. Episodic events have to a great extent influenced the hydrochemical evolution since the last glaciation which ended some 13 ka ago. At that time glacial melt water was flushed under hydraulic pressure down into the fracture system to a depth of at least several hundred metres. The next episodic event took place when the Baltic freshwater lake transformed into the brackish Litorina Sea some 7 ka ago. At this time Äspö was covered by the sea and these denser, more saline waters partly replaced the glacial water down to a depth where the density equilibrated with the replacement sea water. At some time around 3–4 ka ago, Äspö started to rise above sea level and meteoric water began to infiltrate the rock.