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 andsafety analysis of the final disposal of radioactive waste into the bedrock. The performance of technical barriers andmigration of possibly released radionuclides depend on chemical conditions. A prerequisite for understanding thesefactors is the ability to specify the water-rock interactions which control chemical conditions in groundwater. Theobjective of this study is to interpret the processes and factors which control the hydrogeochemistry, such as pH andredox conditions. A model of the hydrogeochemical progress in different parts of the bedrock at Kivetty has beencreated and the significance of chemical reactions along different flowpaths calculated. Long term hydrodynamics havealso 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 acomprehensive data set on dissolved chemical species and isotopes was available. Some analyses of dissolved gasesand their isotopic measurements were also utilised. The data covers the bedrock at Kivetty to a depth of 850m. Theresults from groundwater chemistry, isotopes, petrography, hydrogeology of the site, geomicrobial studies, and PCAand speciation calculations were used in the evaluation of evolutionary processes at the site. The geochemicalinterpretation of water-rock interaction, isotope-chemical evolution and C-14 age calculations of groundwater wasgiven a mass-balance approach (NETPATH). Reaction-path calculations (EQ3/6) were used to verify thethermodynamic feasibility of the reaction models obtained.The hydrogeochemistry of Kivetty is characterised by evolution from low-saline-carbonate-rich recharge water towardsNa-Ca-Cl-type water. The salinity remains low. The most important changes in the chemistry of the groundwater aredue to carbonate reactions: oxidising of organic carbon, and dissolution and precipitation of calcite. The carbonatereactions and slight hydrolysis of silicates stabilise the pH value at 8-9. In addition to aerobic oxidation of organicmatter, 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 microbiallymediated sulphate reduction with simultaneous anaerobic respiration of organic carbon. This process buffers the redoxlevel of about -200 ... -300 mV depending on the pH. Even though the salinities of the groundwater samples andmass-transfer along flow paths remain low, the geochemical evolution has fully developed and has reached quite astable 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 deepobserved recently recharged water cannot be demonstrated. Deglacial or subglacial ages (over 9,700 years old atKivetty) are typical below the 150-300m level in the bedrock. Subglacial waters are interpreted to derive from mixingof preglacial water and meltwater, the input of which is estimated to be about 20% at the most. Indications of elevatedoxygen intrusion cannot be observed in groundwater having glacial signals