Evolution of the Olkiluoto site: Palaeohydrogeochemical considerations

Over the past 20 years a considerable amount of work has been carried out to establish a palaeohydrogeological understanding of the Olkiluoto site and surrounding area, and to integrate this knowledge into the hydrogeochemical and hydrogeological descriptive and modelling programmes. This has involved not only a wide range of well established disciplines such as geology, hydrogeology and hydrochemistry, but also the extraction and determination of rock matrix porewaters by out-diffusion, a relatively new approach in crystalline rock. This required a sophisticated laboratory based input, not only to extract and analyse the porewaters, but also to take into consideration any effects associated to, for example, connected physical porosity and/or geochemical porosity in the rock matrix. In general, there is a good integrated understanding of the Olkiluoto site in terms of the geology, mineralogy, hydrology, hydrochemistry and the overall palaeohydrogeochemical model. The Olkiluoto site has had a complex geological and environmental history from Precambrian to the Quaternary as shown by fluid inclusions in quartz grains and fracture calcites. The Quaternary time period has been dominated by a large climatic variation of cold glacial cycles with temperate interglacials and sea-level changes, all of which have contributed to the hydrogeochemical evolution at the Olkiluoto site. All data indicate that infiltration of aerobic water has systematically been limited to few metres depth in the bedrock at Olkiluoto. Today at about the -300 m elevation level, there exists a distinct change in groundwater chemistry and mean residence time including a redox divide supported by a significant reduction in both the intensity and transmissivity of the water connected fracture networks. These indicate that long term stability (over the time span of glacial cycles) and sufficient buffering capacity of the water-rock system against aerobic infiltration, has dominated continuously until present times at Olkiluoto. Investigation results from matrix porewaters and fracture groundwaters indicate at least five to six different end-member water types that have contributed to current groundwater compositions. Salinity in these end-member waters varies from fresh water to highly saline brine and they seem to represent source waters from different environmental conditions, i.e. meteoric and marine waters from glacial to warm, and humid to arid climates. There are, however, several areas of uncertainty that have been highlighted during the present study that need to be resolved. For example, attempts to integrate the hydrogeology, groundwater chemistry and porewater chemistry, and their relationship to the palaeoevolution of the Olkiluoto site before and since the start of last glaciation, have encountered some difficulties. These include the choice of initial boundary conditions for the hydrogeological modelling, the necessity to consider alternative scenarios to explain present groundwater conditions, the choice of model input parameter values for diffusion and matrix pore diffusivity, and resolving potential problems associated with sampling and analysis of the porewaters. Some uncertainties are also related to the experimentally derived porewater concentrations. For example, one explanation to salinity differences between matrix porewaters and fracture groundwaters may be anion exclusion which has been interpreted in several laboratory experiments based on Olkiluoto and Finnish rock samples. Presently plans and studies are underway to address the many uncertainties that have resulted from this study and additional corroborative (or otherwise) data should be available in the near future.