Background complementary hydrogeochemical studies. SDM-Site Forsmark

The overall objectives of the hydrogeochemical description for Forsmark are to establish a detailed understanding of the hydrogeochemical conditions at the site and to develop models that fulfil 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 pore water and their evolution with time. The work has involved the development of descriptive and mathematical models for groundwaters in relation to rock domains, fracture domains and deformation zones. Past climate changes are one of the major driving forces for hydrogeochemical changes and therefore of fundamental importance for understanding the palaeohydrogeological, palaeohydrogeochemical and present evolution of groundwater in the crystalline bedrock of the Fennoscandian Shield. Understanding current undisturbed hydrochemical conditions at the proposed repository site is important when predicting future changes in groundwater chemistry. The causes of copper corrosion and/or bentonite degradation are of particular interest as they may jeopardise the long-term integrity of the planned SKB repository system. Thus, the following variables are considered for the hydrogeochemical site descriptive modelling: pH, Eh, sulphur species, iron, manganese, uranium, carbonate, phosphate, nitrogen species, total dissolved solids (TDS), isotopes, colloids, fulvic and humic acids and microorganisms. In addition, dissolved gases (e.g. carbon dioxide, methane and hydrogen) are of interest because of their likely participation in microbial reactions. In this series of reports, the final hydrogeochemical evaluation work of the site investigation at the Forsmark site, is presented. The work was conducted by SKB's hydrogeochemical project group, ChemNet, which consists of independent consultants and university researchers with expertise in geochemistry, hydrochemistry, hydrogeochemistry, microbiology, geomicrobiology, analytical chemistry etc. The resulting site descriptive model version, mainly based on 2.2 data and complementary 2.3 data, was carried out during September 2006 to December 2007. Several groups within ChemNet were involved and the evaluation was conducted independently using different approaches ranging from expert knowledge to geochemical and mathematical modelling including transport modelling. During regular ChemNet meetings the results have been presented and discussed. This report is a compilation of different projects that have been finished independently of each other. Section 1: M3 modelling and 2D visualisation of the hydrochemical parameters by Ioana Gurban. The focus of this part is on updating the hydrochemical model, to make uncertainty tests and to present the final models that can be integrated better with the hydrodynamic models. M3 modelling helps to summarise and understand the measured data, by using the major elements and the isotopes δ18O and δ2H as variables. The visualisation of the mixing proportions along the boreholes helps to understand the distribution of the data in the domain and to check and compare the results of different models; and therefore to choose the model which best describes the measured data. Section 2: Coupled hydrogeological and solute transport, visualisation and supportive detailed reaction modelling by Jorge Molinero, David Arcos, Lara Duro. Reactive mixing and reactive solute transport models are used as quantitative tools in order to evaluate how much disturbance can be allowed for a given groundwater sample at repository depth and still meet the SKB suitability criteria. Spatial analysis and 3D visualisation of available representative samples in Forsmark was performed. The computed M3 mixing fractions show a spatial distribution qualitatively correlated with key hydrochemical signatures, such as strontium (for Deep Saline), magnesium (for Littorina), δ18O and c2H (for Glacial) and tritium (for Modified meteoric). Section 3: Application of the Drilling Impact Study (DIS) to Forsmark groundwaters by Mel Gascoyne, Ioana Gurban. In the Drilling Impact Study (DIS) project a tracer is used as an indicator of contamination to attempt to correct the groundwater composition for dilution or contamination by surface waters. By calculating the drilling water volume lost in the fractures during drilling, it is possible to determine how much water should be pumped out from the section before sampling. Section 4: Analytical uncertainties. Ann-Chatrin Nilsson. There is high confidence in the set of major constituents for each sample. Independent methods were used to check the consistency of the major ions and to confirm the concentrations of chloride, sulphate, bromide and iron.The bromide analyses were found to be more uncertain than most other major ions.