Geochemical modelling of the groundwater at the Olkiluoto site

A preliminary model for probable processes responsible for the evolution of the groundwater at the nuclear waste investigation site Olkiluoto (in Finland) is presented. The hydrological data was collected from boreholes drilled down to 1000-m depth into crystalline bedrock. Based on chemical, isotopic, petrographic and hydrological data as well as ion plots and speciation calculations with PHREEQE the thermodynamic controls on the water composition and trends constraining these processes are evaluated. In order to determine the reactions which can explain the changes along the flow path during the evolution of groundwater system and to determine to which extent these reactions take place, mass-balance calculations with the NETPATH program were used. Mass transfer calculations with the EQ6 program were used to test the feasibility of the model derived, to predict reaction paths and composition of equilibrium solutions for the redox reactions. (57 refs., 43 figs., 10 tabs.)