Geochemical modelling of groundwater evolution and residence time at the Kivetty site

An understanding of the geochemical evolution of groundwater is an essential part of the performance assessment and
safety analysis of the final disposal of radioactive waste into the bedrock. The performance of technical barriers and
migration of possibly released radionuclides depend on chemical conditions. A prerequisite for understanding these
factors is the ability to specify the water-rock interactions which control chemical conditions in groundwater. The
objective of this study is to interpret the processes and factors which control the hydrogeochemistry, such as pH and
redox conditions. A model of the hydrogeochemical progress in different parts of the bedrock at Kivetty has been
created and the significance of chemical reactions along different flowpaths calculated. Long term hydrodynamics have
also been evaluated.
The interpretation and modelling are based on groundwater samples (38 altogether) obtained from the soil layer,
shallow wells in the bedrock, and five deep multi-packered boreholes (KR1-KR5) in the bedrock for which a
comprehensive data set on dissolved chemical species and isotopes was available. Some analyses of dissolved gases
and their isotopic measurements were also utilised. The data covers the bedrock at Kivetty to a depth of 850m. The
results from groundwater chemistry, isotopes, petrography, hydrogeology of the site, geomicrobial studies, and PCA
and speciation calculations were used in the evaluation of evolutionary processes at the site. The geochemical
interpretation of water-rock interaction, isotope-chemical evolution and C-14 age calculations of groundwater was
given a mass-balance approach (NETPATH). Reaction-path calculations (EQ3/6) were used to verify the
thermodynamic feasibility of the reaction models obtained.
The hydrogeochemistry of Kivetty is characterised by evolution from low-saline-carbonate-rich recharge water towards
Na-Ca-Cl-type water. The salinity remains low. The most important changes in the chemistry of the groundwater are
due to carbonate reactions: oxidising of organic carbon, and dissolution and precipitation of calcite. The carbonate
reactions and slight hydrolysis of silicates stabilise the pH value at 8-9. In addition to aerobic oxidation of organic
matter, oxidative dissolution of biotite seems to be an important oxygen consumer at shallow depth during recharge.
The most important process controlling the redox state deeper in the bedrock was interpreted to be the microbially
mediated sulphate reduction with simultaneous anaerobic respiration of organic carbon. This process buffers the redox
level of about -200 ... -300 mV depending on the pH. Even though the salinities of the groundwater samples and
mass-transfer along flow paths remain low, the geochemical evolution has fully developed and has reached quite a
stable thermodynamic state. The residence times of the groundwater samples cover the time span back to glaciation.
Young ages seem to be limited to the upper part of bedrock, and any really dynamic natural flowpath with deep
observed recently recharged water cannot be demonstrated. Deglacial or subglacial ages (over 9,700 years old at
Kivetty) are typical below the 150-300m level in the bedrock. Subglacial waters are interpreted to derive from mixing
of preglacial water and meltwater, the input of which is estimated to be about 20% at the most. Indications of elevated
oxygen intrusion cannot be observed in groundwater having glacial signals.