Geochemical modelling study on the age and evolution of the groundwater at the Romuvaara site

An understanding of the geochemical evolution of groundwater is an essential part of the performance assessment and
the safety analyses of the final disposal of radioactive waste into the bedrock. The performance of the technical barriers
and the migration of the radionuclides possibly released depend on the chemical conditions. A prerequisite for
understanding these factors is the ability to specify the water-rock interactions which control the chemical conditions
in groundwater. The objective of this study was to interpret the processes and factors which control the
hydrogeochemistry, e.g. the pH and redox conditions. A model of the hydrogeochemical evolution and the chemical
flowpaths in different parts of the bedrock at the Romuvaara site has been created. The significance of chemical
reactions along different flowpaths is calculated. Furthermore, the consistency of the hydrogeochemical model and the
hydrogeological model is examined.
The interpretation and modelling are based on soil groundwater samples and groundwater samples (altogether from 28
packer intervals) obtained from five multi-packered boreholes (KR1-KR5) for which a comprehensive data set on
dissolved chemical species and isotopes was available. Some analyses of dissolved gases and their isotopie
measurements were also utilized. The data covers the bedrock at Romuvaara to a depth of 500 m and is the most
representative and versatile data set used in site scale hydrogeochemical interpretations in Finland. The recently
finished detailed results on the mineralogical investigations were utilized, as well as the conceptualization of
hydrogeology from the preliminary site investigations (Teollisuuden Voima Oy 1992).
The geochemical interpretation of water-rock interaction and flowpaths were evaluated using the results from
groundwater chemistry, isotopes, petrography and thermodynamic speciation calculations (EQ3NR). The geochemical
reaction modelling of the chemical and isotope-chemical evolution of groundwater along the flowpaths was performed
as a mass-balance approach (NETPATH). Mass-transfer modelling (EQ6) was used to verify the thermodynamic
feasibility of the reaction models obtained.
The hydrogeochemistry of Romuvaara is characterized by the evolution from low-saline-carbonate-rich recharge water
towards either Na-Ca-Cl-type or Na-Cl-type water. The salinity remains low in both trends. The former is related to
the interaction of groundwater and gneissic rock, while the latter is related to the interaction with mafic metadiabase
dykes. The most important changes in the chemistry of groundwater are due to carbonate reactions: oxidizing of
organic carbon, and dissolution and precipitation of calcite. Moreover, especially in the Na-Cl-type evolution, cation
exchange is interpreted to be significant. The carbonate reactions and slight hydrolysis of silicates stabilize the pH
value to the level of 8-9. The mafic minerals seem to elevate the pH value slightly over 10 in the Na-Cl trend. The
most important process controlling the redox state was interpreted to be in anaerobic conditions the microbially
mediated sulphate reduction with simultaneous oxidation of organic carbon in anaerobic conditions. This process
buffers the Eh of groundwater below -200 mV during long residence times. Mass-balance reaction models, which were
tested by isotopie calculations of the C-13 of dissolved carbonate (Rayleigh distillations), support the presented
geochemical evolution, as do the thermodynamic mass-transfer calculations. Eventhough the salinities of the
groundwater samples remain low, the geochemical evolution has fully developed and has reached quite a stable
thermodynamic state. Reaction models were also used to adjust the groundwater C-14 ages. The residence time of the
groundwater samples cover the time span from modern (tritium-containing) to about 10 000 years depending on the
flowpath. Young ages are typically connected to the zones of high water conductivity. The geochemical behaviour is
mainly consistent with the hydrogeological model.