Bedrock hydrogeochemistry Laxemar – Site descriptive modelling SDM-SiteBedrock hydrogeochemistry Laxemar R-08-93 Bedrock hydrogeochemistry Laxemar Site descriptive modelling SDM-Site Laxemar Marcus Laaksoharju, Geopoint John Smellie, Conterra Eva-Lena Tullborg, Terralogica Bill Wallin, Geokema Henrik Drake, Isochron Geokonsulting HB Mel Gascoyne, Gascoyne Geoprojects Inc Maria Gimeno, University of Zaragoza Ioana Gurban, 3D Terra Lotta Hallbeck, Microbial Analytics Jorge Molinero, Amphos Ann-Chatrin Nilsson, Geosigma Nick Waber, University of Bern June 2009 Svensk Kärnbränslehantering AB Swedish Nuclear Fuel and Waste Management Co Box 250, SE-101 24 Stockholm Phone +46 8 459 84 00 R-08-93 CM Gruppen AB, Bromma, 2009 ISSN 1402-3091 Tänd ett lager: SKB Rapport R-08-93 P, R eller TR. Bedrock hydrogeochemistry Laxemar Site descriptive modelling SDM-Site Laxemar Marcus Laaksoharju, Geopoint John Smellie, Conterra Eva-Lena Tullborg, Terralogica Bill Wallin, Geokema Henrik Drake, Isochron Geokonsulting HB Mel Gascoyne, Gascoyne Geoprojects Inc Maria Gimeno, University of Zaragoza Ioana Gurban, 3D Terra Lotta Hallbeck, Microbial Analytics Jorge Molinero, Amphos Ann-Chatrin Nilsson, Geosigma Nick Waber, University of Bern June 2009 This report concerns a study which was conducted for SKB. The conclusions and viewpoints presented in the report are those of the author(s) and do not necessarily coincide with those of the client. A pdf version of this document can be downloaded from www.skb.se. Preface This Background Report document forms the final component of the hydrogeochemical site descriptive modelling associated with the complete site investigation stage of Laxemar and the larger Laxemar- Simpevarp area leading to a hydrogeochemical site descriptive model version SDM-Site Laxemar, as based on available primary data from the ‘Extended data freeze Laxemar 2.3’ at Laxemar (November 30, 2007). SKB’s ChemNet group, consisting of consultants and university personnel, carried out the data interpretation and modelling during the period November 2007 to November 2008. The Insite and Sierg review comments on the earlier model versions of Laxemar-Simpevarp area were considered in this work. Several groups within ChemNet were involved and the evaluation was conducted using different approaches ranging from expert knowledge to geochemical and transport modelling. During regular ChemNet meetings the results were presented and discussed and directions for subsequent analysis were issued. The underlying original work by the ChemNet participants is presented in six level III reports /Drake and Tullborg 2009a, Hallbeck and Pedersen 2008b, Gimeno et al. 2009, Smellie and Tullborg 2009, Waber et al. 2009, Kalinowski (ed) 2009/ which contain complementary information for the bedrock hydro-geochemistry Laxemar Site Descriptive Model (SDM-Site Laxemar) level II report (this report), as given by the figure below. The former Level III reports include descriptions of fracture mineralogy, explorative analyses of microbes, colloids asnd gases, water-rock interaction modelling and uncertainties of mixing modelling, background complementary studies and porewater in the rock matrix. SDM-Site Laxemar – main report (level I) Main references (level II) Geology Rock mechanics Thermal properties Surface system Hydrogeology Hydrogeochemistry Geological evolution, Transport properties Confidence assessment palaeoclimate and historic development Other references (level III) Geology Hydrogeology Hydrogeochemistry Transport Surface system • Data compilation report • Data interpretation • Fracture mineralogy properties • Limnic ecosystems • Stochastic modelling of and model • Explorative analysis of • Data evaluation and • Terrestrial ecosystems fractures and minor DZ’s parameterisation microbes, colloids and gases retardation model • Marine ecosystems (GeoDFN) • Model testing and • Water-rock interaction synthesis modelling and uncertainties • Hydrochemistry of mixing modelling • Hydrology • Background • Regolith complementary studies • Pore water in the rock matrix 3 The views presented in the level III reports reflect those of the individual authors and do not neccessarily coincide with the views expressed in this integrated Background Report which represents the collective views of the ChemNet Core Group (CCG). The ChemNet members contributing to this report are (in alphabetic order): Patricia Acero, University of Zaragoza, Spain. David Arcos, Amphos, Barcelona, Spain. Luis Auqué, University of Zaragoza, Spain. Henrik Drake, University of Gothenburg, Sweden. Lara Duro, Amphos, Barcelona, Spain. Mel Gascoyne, GGP Inc. Pinawa, Canada. María Gimeno, University of Zaragoza, Spain. Thomas Gimmi, University of Bern, Switzerland. Javier Gómez, University of Zaragoza, Spain. Ioana Gurban, 3D-Terra, Montreal, Canada. Lotta Hallbeck, Micro Analytics, Gothenburg, Sweden. Jorge Molinero, Amphos, Barcelona, Spain. Ann-Chatrin Nilsson, Geosigma AB, Uppsala, Sweden. Karsten Pedersen, Microbial Analytics, Gothenburg, Sweden. John Smellie, Conterra AB, Stockholm, Sweden (CCG). Eva-Lena Tullborg, Terralogica AB, Gråbo, Sweden (CCG). Nick Waber, University of Bern, Switzerland. Bill Wallin, Geokema AB, Sweden (CCG). Marcus Laaksoharju, ChemNet project leader. 4 Summary The overall objectives of the hydrogeochemical description for the Laxemar-Simpevarp area, south- eastern Sweden, are to first 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 group during the site investigation phase /Ström et al. 2008/. Issues of concern to safety assessement are radionuclide transport and technical barrier behaviour, both of which are dependent on the current chemistry of groundwater and matrix porewater and their evolution with time in the future. The specific aims of the hydrogeochemical work were: • To document the hydrogeochemistry in the Laxemar subarea and the larger Laxemar-Simpevarp area, with focus on the development of conceptual understanding to describe and visualise the sites. • 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 /SKB 2006a/ that bears relevance to the hydrogeochemical modelling work. The groundwaters have been interpreted in relation to their composition, origin and evolution, which require close integration with geological, climatological and hydrogeological information. Past climate changes are among the major driving forces for long term hydrogeochemical changes (hundreds to thousands of years) and are, therefore, of fundamental importance for understanding the palaeohydro- geological, palaeohydrogeochemical and present evolution of groundwater in the Fennoscandian crystalline bedrock. The SDM-Site Laxemar hydrochemistry modelling has resulted in improved robustness of model versions Laxemar 1.2 and Laxemar 2.1 concerning site understanding. The many consistent temporal and spatial data support the description concerning the groundwater origin, most of the major end members and major hydrochemical processes. Integration with hydrogeology supports the palaeo- hydrogeological description of the site. Chemical reaction modelling, the use of different isotope ratios (of the elements Sr, S, C, B, Cl, O, H and the U-decay series) and measurements of Eh, pH and microbe population data, support the process understanding. Matrix porewater compositions have now been fairly well established and their compositional variation with depth is determined. Important input from the fracture mineralogical study also supports the palaeohydrogeological and process understanding of the site. Confidence concerning the three-dimensional variability of processes and properties was improved by the addition of new data in previously drilled boreholes and from new boreholes in important key areas. Hydrogeologically the Laxemar subarea is an area of groundwater recharge and shows classic systematic changes in groundwater chemistry with depth which accompany increasingly lower hydraulic conduc- tivity values and lower groundwater flow rates in the bedrock. Although such changes lead to greater water-rock interactions, the major groundwater feature is that the groundwater composition is mainly a result of transport (mixing), of groundwaters from different origins (deep groundwater, glacial water, Littorina Sea water and meteoric water). The groundwater compositions are, or have been, modified by reactions ranging from fast (e.g. redox reactions catalysed by microorganisms, ion exchange, calcite equilibrium) to long term water-rock reactions such as aluminosilicate equilibrium at depth. Despite these changes, the alkalinity and redox buffer capacity provided by the bedrock and the microbial metabolisms is driven by comparatively fast reactions (hundreds of years). Hence, the pH and Eh variability of the contacting groundwaters are restricted to a narrow and stable range provided that appropriate alkalinity and redox buffer capacities are present. Post glacial meteoric water dominates in the depth interval between 100 to 150 metres of the bedrock and may be present as decreasing components also at intermediate depth. Only part of the Laxemar subarea was covered by the Littorina Sea water and even this has had limited impact, resulting in a relatively