Hydrogeochemical conditions and evolution at the Äspö HRL, Sweden

Abstract

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. The overall trend of increasing salinity with depth may easily be misinterpreted as a fairly simple groundwater system, evolving from a two component evolution path between non-saline and saline groundwaters. However, when combining the results from environmental isotopes and the chemical parameters using a new modelling tool named M3 ( Multivariate Mixing and Mass balance calculations), a higher resolution was obtained and a more complex groundwater pattern, which reflects the present and paleo-hydrogeological events, can be recognised. The measured groundwater composition was modelled to be a mixture of meteoric, past and present Baltic seawater, glacial (or cold climate recharge) and brine type of waters. The modelling result shows that the processes considered to have a dominating impact on the present Äspö groundwater chemistry are mixing, both in disturbed and undisturbed systems, calcite dissolution and precipitation, redox reactions and biological processes. The undisturbed groundwater conditions prior to the HRL tunnel construction at Äspö consisted of: 1. A dominating proportion of meteoric fresh water in the upper 250 m of the aquifer. 2. A brackish–saline water consisting of mixing proportions of present and ancient Baltic Sea water and glacial melt water present to a depth of 250–600 m. 3. Saline water still containing proportions of glacial water which could represent even older glaciations, and brines, a large portion of which have been stagnant for perhaps millions of years, below a depth of 600 m. During the HRL tunnel construction there were changes in the composition of the water flowing into the tunnel at different locations. Although the variation in salinity was relatively small, the variations in the mixing proportions of the different water types were substantial.