Hydrogeochemical interpretation of baseline groundwater conditions at the Olkiluoto site

Olkiluoto at Eurajoki has been selected as a repository site for final disposal of spent nuclear waste produced on Finland. An understanding of the hydrogeochemical groundwater conditions and evolution is essential in evaluating the long-term safety of the repository. 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 evolution in different parts of the crystalline bedrock at Olkiluoto has been created and the significance of chemical reactions and groundwater mixing along different flow paths calculated. This baseline concept of hydrogeochemistry will perform as a reference in evaluating changes resulting from the ONKALO construction and finally in evaluating hydrogeochemical conditions after the closure of repository. This interpretation and modelling are based on water samples obtained from Baltic sea, precipitation, eight groundwater observation tubes in the overburden, seven shallow boreholes and thirteen deep boreholes in the bedrock for which a comprehensive data set on dissolved chemical species and isotopes was available. Since the previous interpretation report 86 new samples have been collected. Analyses of dissolved gases and fracture calcite and their isotopic measurements were also utilised. The data covers the bedrock at Olkiluoto to a depth of 1000 m. The results from groundwater chemistry, isotopes, petrography, hydrogeology of the site, geomicrobial studies, 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 mixing of palaeo water types were approached by mass-balance calculations (NETPATH). The PHREEQC program was used in the interpretations of the pH and Eh conditions. The hydrochemical data and previous interpretations have already revealed the complex nature of sources of salinity and chemical evolution at the Olkiluoto site. The new data, since 1999, strengthen the previous concept of hydrogeochemical evolution. Actual deviations are not observed compared to the previous interpretations but, clearly, more detailed hydrogeochemical information has been obtained, particularly from shallow depths (the first 10 m), the deep saline groundwater zone (below 500 m), and dissolved gases. Repeated samplings in the bedrock during the years show only minor changes, thus strengthening the hydrogeochemical conceptual model of the site. Changes in past climate and geological environment have caused great variability in the hydrochemical data. Infiltration and mixing of end-members seem to yield a stratified hydrochemical system at least in the waterconducting fracture system. Water-rock interaction, such as carbon and sulphur cycling and silicate reactions, buffer the pH and redox conditions and stabilise groundwater chemistry. Hydrogeochemical interpretations and chemical and isotopic calculations indicate that pH seems to be dominantly controlled by thermodynamic equilibrium with calcite in fractures and there are indications that it may also occur in the overburden layer. Oxic redox conditions, prevailing in recharging groundwater, change abruptly to sulphidic conditions close to the surface, generally in overburden. Methanogenesis may show increasing importance in saline groundwater, although the major part of methane and other hydrocarbons are thermal in origin. The baseline hydrogeochemistry seems to provide the conditions for long-term geochemical stability, which is a requirement for safe long-term disposal of nuclear waste. The concept of hyrogeochemical evolution forms the basis to evaluate changes resulting from the ONKALO construction and finally in evaluating hydrogeochemical conditions after the closure of repository. (orig.)