Bedrock hydrogeochemistry Forsmark. Site descriptive modelling SDM-Site Forsmark

The overall objectives of the hydrogeochemical site description for Forsmark are to establish a detailed understanding of the hydrogeochemical conditions at the site, and to use this understanding to develop models that address the needs identified by the safety assessment groups during the site investigation phase. Issues of concern to safety assessment are radionuclide transport and technical barrier behaviour, both of which are dependent on the chemistry of groundwater and porewater and their evolution with time. The specific aims of the hydrogeochemical work were: To document the hydrogeochemistry at the Forsmark site with focus on the development of conceptual models to describe and visualise the site. To provide relevant parameter values to be used for safety assessment calculations. To provide the hydrogeochemical basis for the modelling work by other teams, in particular hydrogeology. To take account of the feedback from the SR-Can safety assessment work that bears relevance to the hydrogeochemical modelling work. The work has involved the development of descriptive and mathematical models for groundwaters in relation to rock domains, fracture domains and deformation zones. In this report, the groundwaters have been interpreted in relation to their origin, evolution and composition, which require close integration with geological, climatological and hydrogeological information. Past climate changes are one of the major driving forces for long-term hydrogeochemical changes (hundreds to thousands of years) and are, therefore, of fundamental importance for understanding the palaeohydrogeological, palaeohydrogeochemical and present evolution of groundwater in the Fennoscandian crystalline bedrock. In contrast, redox buffer capacity of the bedrock will minimise the effects on changes in alkalinity and redox at repository depths, therefore limiting the variations in pH and Eh significantly, regardless of major changes in groundwater composition. There is generally a high confidence in the description and understanding of the current spatial distribution of groundwater composition, mainly due to the consistency between different analyses and modelling of the chemical data, but also due to the agreement with the hydrogeological understanding of the area. The existence of a near-surface redox reaction zone appears to be plausible, but this is still unclear from uncertainties in data interpretation. Nevertheless, the abundance of calcite suggests there is active buffering capacity on dilute groundwaters low in calcite. Furthermore, even though quantification of the bedrock buffering capacity is not yet achieved, general indications from the Forsmark area as a whole point to a bedrock which has a sufficiently large buffering capacity to maintain pH values in the range 7 to 8.5 and Eh values lower than -140 mV. One important remaining uncertainty concerns the increase in sulphide concentrations measured in the on-going monitoring programme. Initial drilling and pumping may have disturbed the system or may have facilitated sulphate reduction, but this issue remains to be resolved. The monitoring programme will also support the overall understanding of the long term behaviour of other groundwater parameters such as uranium, DOC (dissolved organic carbon) and tritium.