Water-rock interaction modelling and uncertainties of Water-rock R-08-86 Water-rock interaction mixing modelling and uncertainties of mixing modelling modelling SDM-Site Forsmark Maria J Gimeno, Luis F Auqué, Javier B Gómez, Patricia Acero University of Zaragoza, Spain August 2008 Svensk Kärnbränslehantering AB Swedish Nuclear Fuel and Waste Management Co Box 250, SE-101 24 Stockholm Tel +46 8 459 84 00 R-08-86 CM Gruppen AB, Bromma, 2008 ISSN 1402-3091 Tänd ett lager: SKB Rapport R-08-86 P, R eller TR. Water-rock interaction modelling and uncertainties of mixing modelling SDM-Site Forsmark Maria J Gimeno, Luis F Auqué, Javier B Gómez, Patricia Acero University of Zaragoza, Spain August 2008 This report concerns a study which was conducted for SKB. The conclusions and viewpoints presented in the report are those of the authors and do not necessarily coincide with those of the client. A pdf version of this document can be downloaded from www.skb.se. Preface 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 ground­ waters 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 behind of copper corrosion and/or bentonite degradation are of particular interest as they may jeopardise the long­term 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, 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 model­ ling including transport modelling. During regular ChemNet meetings the results have been presented and discussed. The original works by the ChemNet modellers are presented in five level III reports containing complementary information for the bedrock hydrogeochemistry Forsmark Site Descriptive Model (SDM­Site Forsmark, R­08­47) level II report. There is also an additional level III report: Fracture mineralogy of the Forsmark area by Sandström et al. R­08­102. 3 R-08-47 Level II Explorative Explorative Water-rock analysis and expert analysis of interaction judgement of microbes, colloids modelling and major components and gases uncertainties of Level III and isotopes mixing modelling R-08-84 R-08-85 R-08-86 Background Pore Water in the complementary Rock Matrix hydrogeochemical studies studies R-08-87 R-08-105 This report presents the modelling work performed by the University of Zaragoza (UZ) group as part of the work planned for Forsmark during stages 2.2 and 2.3. The chemical characteristics of the groundwaters in the Forsmark and Laxemar areas are the result of a complex mixing process driven by the input of different recharge waters since the last glaciation. The successive penetration at different depths of dilute glacial melt­waters, Littorina Sea waters and dilute meteoric waters has triggered complex density and hydraulically driven flows that have mixed them with long residence time, highly saline waters present in the fractures and in the rock matrix. A general description of the main characteristics and processes controlling the hydrogeo­ chemical evolution with depth in the Forsmark groundwater system is presented in this report: The hydrochemical characteristics and evolution of the Near surface waters (up to 20 m depth) is mainly determined by weathering reactions and especially affected by the presence of limestones. The biogenic CO2 input (derived from decay of organic matter and root respiration) and the associated weathering of carbonates control the pH and the concentrations of Ca and − HCO3 in the near­surface environment. Current seasonal variability of CO2 input produces variable but high calcium and bicarbonate contents in the Forsmark near­surface waters: up to − 240 mg/L Ca and 150 to 900 mg/L HCO3 . These high concentrations are due to the extensive presence of limestones in the overburden, a feature very uncommon in Swedish soils. New samples from the extended Forsmark 2.3 data freeze fit with the general picture already drawn with the previous data. Therefore, their inclusion in the conceptual model does not add or modify any of the previous conclusions. 4 Contents 1 Introduction 7 2 Palaeohydrogeology 9 2.1 Composition of the end­members 9 2.1.1 Deep Saline end­member 10 2.1.2 Glacial end member 18 2.1.3 Littorina end member 22 2.1.4 Altered Meteoric end­member 31 2.1.5 Monte Carlo computation of mixing proportions with uncertain end members 36 2.1.6 General conclusions and summary 46 2.2 Pre­Littorina groundwaters 50 2.2.1 Mixing proportions at present 50 2.2.2 Reconstruction of waters before Littorina stage 52 2.2.3 Feasibility of Littorina penetration: Density calculations 59 2.3 Temporal evolution of mixing 61 3 General overview of the hygrogeochemical system 65 3.1 Fracture domains: implications for the hydrogeochemical modelling 65 3.2 Final set of data for the “shallow” and “deep” groundwater systems 68 3.3 Near­surface and shallow groundwaters 68 3.3.1 Chemical variability and the carbonate system in the shallow and near surface groundwaters from Forsmark 69 3.3.2 Redox characterisation of the near surface and shallow groundwaters at Forsmark 72 3.3.3 Summary of the near surface and shallow groundwaters 73 3.4 Non­redox system in the deep groundwaters 74 3.4.1 Ion­ion plots 76 3.4.2 Distribution with depth 80 3.4.3 Summary of the non­redox system in the deep groundwaters 85 3.5 Redox system in the groundwaters 88 3.5.1 Selection of redox data 88 3.5.2 General trends of redox data 91 3.5.3 Redox pair modelling 95 3.5.4 The iron system 99 3.5.5 The sulphur system 102 3.5.6 New data on the ”uranium problem” in Forsmark groundwaters 106 3.5.7 Manganese in Forsmark groundwaters 120 3.5.8 Summary of the redox modelling 126 3.6 The effect of the extended 2.3 data freeze on groundwater modelling 128 4 Overall conceptual model for groundwater evolution 133 4.1 Temporal evolution of mixing 133 4.2 General conceptual model 134 5 References 137 Appendix A Review of the Chemmac logs in the Forsmark area 147 Appendix B Groundwater composition at repository depth 173 Appendix C Concentration and dilution scenarios in Forsmark evolution and SKB suitability criteria 187 References for Appendices A, B and C 209 5 1 Introduction This report presents the modelling work performed by the UZ group as part of the work plan for Forsmark 2.2 and 2.3. It has been organised in sections corresponding to the different INSITE (Independent Site Investigation Tracking & Evaluation – the authorities advisory group on site investigation issues), issues in which the UZ group is involved. Issues corresponding to predictive modelling or based on the SR­Can (Safety Assessment) results have been included as appendices. The sections included in this report are the following: 1. Introduction. 2. Palaeohydrogeology. This chapter contains three main sections which present (a) a com­ prehensive review of the chemical composition of the end­members, highlighting the main uncertainties encountered during the calculations; (b) the different calculations performed by the UZ group to obtain the chemical composition of the Forsmark groundwaters just before the Littorina stage; and (c) a conceptual model of the temporal evolution of the system. These are basic issues affecting different parts of the work plan: • Issue B-2: Alternative end-member selection and uncertainties in end-members. • Issue F-1: Glacial vs Littorina in the bedrock, establish Littorina (and Deep Saline in Forsmark) compositions. • Issue F-3: Geochemical data used to interpret the groundwater evolution over time. 3. General overview of the system with the updated information from the 2.2 and 2.3 data freezes. This section presents the final picture of the system as envisaged using all available data by the time of the last data freeze. A summary of the main conclusions on the distribu­ tion of chemical elements and on the main factors controlling the system is also presented. The section is divided into three parts concerning the shallow and near surface groundwater system, the non­redox system in the deep groundwater system, and the redox system This last part provides a general overview of the redox processes controlling some of the important parameters in the system.