Applied Geochemistry (v.23, #7)

The Swedish programme for geological disposal of spent nuclear fuel is approaching major milestones in the form of permit applications for an encapsulation plant and a deep geologic repository. This paper presents an overview of the bedrock and surface modelling work that comprises a major part of the on-going site characterization in Sweden and that results in syntheses of the sites, called site descriptions. The site description incorporates descriptive models of the site and its regional setting, including the current state of the geosphere and the biosphere as well as natural processes affecting long-term evolution. The site description is intended to serve the needs of both repository engineering with respect to layout and construction, and safety assessment, with respect to long-term performance. The development of site-descriptive models involves a multi-disciplinary interpretation of geology, rock mechanics, thermal properties, hydrogeology, hydrogeochemistry, transport properties and ecosystems using input in the form of available data for the surface and from deep boreholes.

Hydrogeochemical evaluation and modelling performed within the Swedish site investigation programme by Marcus Laaksoharju; John Smellie; Eva-Lena Tullborg; María Gimeno; Jorge Molinero; Ioana Gurban; Lotta Hallbeck (1761-1795).
Site studies for SKB’s (Swedish Nuclear Fuel and Waste Management Co.) programme of deep geological disposal of nuclear fuel waste currently involve the investigation of two locations, Forsmark and Laxemar–Simpevarp situated on the eastern coast of Sweden, to determine their geological, hydrogeochemical and hydrogeological characteristics. Present work describes hydrogeochemical methods such as conceptual postglacial modelling, explorative analysis, mathematical modelling and construction of site descriptive models. The complex groundwater evolution and patterns at Forsmark and Laxemar–Simpevarp are a result of many factors such as: (a) the present-day topography and proximity to the Baltic Sea; (b) past changes in hydrogeology related to glaciation/deglaciation, land uplift and repeated marine/lake water regressions/transgressions; and (c) organic or inorganic alteration of the groundwater composition caused by microbial processes or water/rock interactions. The major conclusion is that changes from glacial rebound and hence hydrogeology seems to have a major influence on the groundwater chemistry. These complex data require a multidisciplinary approach to their interpretation which is based on 25 years of experience within the SKB radioactive waste site investigation programme.

Micro-organisms must be included in any hydrogeochemical modelling efforts in the ongoing Swedish programme to characterize potential sites for the geological disposal of spent nuclear fuel. This paper presents the development and testing of several methods for estimating the total numbers of micro-organism groups and amounts of their biomass in groundwater, their diversity, and the rates of microbial processes. The enumeration and cultivation methods were tested and evaluated on groundwater from boreholes at 450 m depth in the Äspö Hard Rock Laboratory (HRL), Sweden, and from two potential sites for a final repository of spent nuclear fuel, Forsmark and Laxemar. The reproducibility of the methods between parallel samples and over time was investigated and found to be excellent. Nitrate-, iron-, manganese- and sulphate-reducing bacteria and acetogens and methanogens were found in numbers up to approximately 87,000 cells L−1 groundwater from the studied sites. A methodology that analysed microbial process rates was developed and tested under open and closed controlled in situ conditions in a circulation system situated 447 m underground in the MICROBE laboratory at the Äspö HRL. The sulphide and acetate production rates were determined to be 0.08 and 0.14 mg L−1  day−1, respectively. The numbers of sulphide- and acetate-producing micro-organisms increased concomitantly in the analysed circulating groundwater. Flushing the sampled circulation aquifer created an artefact, as it lowered the sulphide concentration. Microbial and inorganic processes involved in sulphur transformations are summarized in a conceptual model, based on the observations and results presented here. The model outlines how dissolved sulphide may react with Fe(III) and Fe(II) to form solid phases of iron sulphide and pyrite. Sulphide will, consequently, continuously be removed from the aqeous phase via these reactions, at a rate approximately equalling the rate of production by microbial sulphate reduction.

Potentiometrically measured Eh in groundwaters from the Scandinavian Shield by L. Auqué; M.J. Gimeno; J. Gómez; A.-C. Nilsson (1820-1833).
Measurement and interpretation of electrochemical Eh values in natural reducing groundwaters is a complex task. SKB, the company in charge of disposal of nuclear fuel wastes in Sweden, has developed a refined methodology for the determination of this parameter in packered sections in boreholes. The methodology consists of the simultaneous use of three different electrodes (Pt, Au and C) both at depth and at the surface, and maintaining continuous logging over a long period of time. Apart from Eh, the logging also includes other parameters such as pH, dissolved O2, conductivity and temperature. This methodology has been used since the 80s in the framework of the hydrogeochemical characterisation programs supported by SKB at different sites in the Scandinavian Shield. All the existing databases have been revisited in this work using a uniform set of criteria to select Eh values of the Swedish groundwaters as a function of depth.The selected Eh dataset ranges from −140 to −400 mV and corresponds to waters with pH values between 7 and 8.6 at depths between 110 and 1000 m. Eh shows no correlation with depth in any of the fractured aquifers of the studied sites. The Eh values, together with the results obtained from the geochemical modelling and the microbiological studies in the ongoing work at Laxemar and Forsmark have allowed the study of the relationship between the potentiometrically measured Eh and the redox pairs controlling it. The possible control of the redox conditions by the electroactive pair Fe2+/Fe(OH)3(s) in waters with a wide range of measured Eh values suggests either the involvement of different kinds of Fe oxyhydroxides or variations of crystallinity and/or particle size in them. The good fit found between the measured Eh and the results obtained with different S redox pairs, together with the occurrence of SRB and the equilibrium situations with respect to the amorphous Fe(II) monosulphides, suggest the participation of the S system in the measured Eh control.

Characterisation of pore water in crystalline rocks by H.N. Waber; J.A.T. Smellie (1834-1861).
The mass of pore water present in the rock matrix of a crystalline rock mass is significant and its influence on fracture groundwater and future deep repositories needs to be understood. Yet, the rock matrix has a hydraulic transmissivity generally well below 10−10  m2  s−1 inhibiting sampling of pore water by conventional sampling techniques. Various innovative techniques for the chemical and isotopic characterisation of pore water in crystalline rocks are applied and evaluated. Direct sampling of pore water was facilitated by collecting seepage water in a specially designed borehole located at a depth of 420 m in the Äspö underground research laboratory (Äspö HRL), Sweden, over a period of 7.5 a. During the entire time span, seepage waters collected from different sampling sections showed constant, but individual chemical and isotopic compositions. Compositional differences compared to nearby fracture groundwaters indicate that the collected waters originated from the low-permeability rock mass without mutual influence via more conductive microfractures within the metre scale. They are interpreted as representing pore water in a transient state of diffusive interaction with different types of old palaeowaters which have periodically characterised the fracture network at the Äspö HRL over geological times (thousands to hundreds of thousands of years).Pore water compositions derived by indirect methods (out-diffusion and diffusive isotope equilibration experiments) from originally saturated drillcore samples collected at the Laxemar and Forsmark sites, Sweden, can be interpreted within a wider palaeohydrogeological framework. The chemical and isotope compositions of the pore water show distinct trends related to rock type and with depth, becoming more saline at greater depth at both sites. Concentration gradients established between pore water and fracture groundwater commonly coincide with higher fracture frequency and transmissivity in the host rocks. Differences developed in the attainment of near steady-state conditions between pore water and fracture water from Laxemar and Forsmark support a different hydrogeological, and therefore hydrogeochemical evolution, at least during Holocene times.The observed compositional differences between pore water and fracture groundwater support diffusion-dominated solute transport in the low-permeable rock masses, which is consistent with measured and experimentally derived hydraulic properties. From the present investigations it can be concluded that in such rock masses the diffusion-accessible porosity extends over significant distances, of at least metres to tens of metres.

This work, which was done within the Swedish nuclear waste management program, was carried out in order to increase the understanding of the mobility and fate of rare earth elements (REEs) in natural boreal waters in granitoidic terrain. Two areas were studied, Forsmark and Simpevarp, one of which will be selected as a site for spent nuclear fuel. The highest REE concentrations were found in the overburden groundwaters, in Simpevarp in particular (median ∑REE 52 μg/L), but also in Forsmark (median ∑REE 6.7 μg/L). The fractionation patterns in these waters were characterised by light REE (LREE) enrichment and negative Ce and Eu anomalies. In contrast, the surface waters had relatively low REE concentrations. They were characterised either by an increase in relative concentrations throughout the lanthanide series (Forsmark which has a carbonate-rich till) or flat patterns (Simpevarp with carbonate-poor till), and had negative Ce and Eu anomalies. In the bedrock groundwaters, the concentrations and fractionation patterns of REEs were entirely different from those in the overburden groundwaters. The median La concentrations were low (just above 0.1 μg/L in both areas), only in a few samples were the concentrations of several REEs (and in a couple of rare cases all REEs) above the detection limit, and there was an increase in the relative concentrations throughout the lanthanide series. In contrast to these large spatial variations, the temporal trends were characterised by small (or non existent) variations in REE-fractionation patterns but rather large variations in concentrations. The Visual MINTEQ speciation calculations predicted that all REEs in all waters were closely associated with dissolved organic matter, and not with carbonate. In the hydrochemical data for the overburden groundwater in particular, there was however a strong indication of association with inorganic colloids, which were not included in the speciation model. Overall the results showed that within a typical boreal granitoidic setting, overburden groundwaters are enriched in REEs, organic complexes are much more important than carbonate complexes, there is little evidence of significant mixing of REEs between different water types (surface, overburden, bedrock) and spatial variations are more extensive than temporal ones.

Palaeohydrogeology: A methodology based on fracture mineral studies by Eva-Lena Tullborg; Henrik Drake; Björn Sandström (1881-1897).
It is concluded that fracture mineral studies can reveal a palaeohydrological record in crystalline rock that is essential to understand the stability or evolution of the groundwater system over a time scale that is relevant to performance assessment for a spent nuclear fuel repository. It is also concluded that the suggested methodology for palaeohydrogeological studies is site-specific.

Red-staining and alteration of wall rock is common around water conducting fractures in the Laxemar–Simpevarp area (SE Sweden), which is currently being investigated by the Swedish Nuclear Fuel and Waste Management Co. (SKB) in common with many other places. Red-staining is often interpreted as a clear sign of oxidation but relevant analyses are seldom performed. The area is dominated by Palaeoproterozoic crystalline rocks ranging in composition from quartz monzodiorite to granite. In this study wall rock samples have been compared with reference samples from within 0.1 to 1 m of the red-stained rock, in order to describe mineralogical and geochemical changes but also changes in redox conditions. A methodology for tracing changes in mineralogy, mineral and whole rock chemistry and Fe3+/Fetot ratio in silicates and oxides in the red-stained wall rock and the reference rock is reported. The results show that the red-stained rock adjacent to the fractures displays major changes in mineralogy; biotite, plagioclase and magnetite have been altered and chlorite, K-feldspar, albite, sericite, prehnite, epidote and hematite have been formed. The changes in chemistry are however moderate; K-enrichment, Ca-depletion and constant Fetot are documented. The Fe3+/Fetot ratio in the oxide phase is higher in the red-stained samples whereas the Fe3+/Fetot ratio in the silicate phase is largely similar in the wall rock and the reference samples. Because most of the Fe is hosted in the silicate phase the decrease in reducing capacity (Fe2+), if any, in the red-stained wall rock is very small and not as high as macroscopic observations might suggest. Instead, the mineralogical changes in combination with the modest oxidation and formation of minute hematite grains in porous secondary minerals in pseudomorphs after plagioclase have produced the red-staining. Increased porosity is also characteristic for the red-stained rock. Moderate alteration in the macroscopically fresh reference rock shows that the hydrothermal alteration reaches further from the fracture than the red-staining. The extent of the red-staining can therefore not be used in the same way as the extent of the alteration adjacent to a fracture. The increase in porosity in the red-stained rock may result in enhanced retention of radio-nuclides due to an increased sorptivity and diffusion close to the fracture.The hydrothermal alteration causing the red-staining is thought to have occurred at temperatures of about 250–400 °C, based on the secondary mineralogy. The major part of this alteration in the area is assumed to be related to fluid circulation associated with the intrusion of the Mesoproterozoic Götemar and Uthammar granites nearby.

Understanding groundwater chemistry using mixing models by Marcus Laaksoharju; Mel Gascoyne; Ioana Gurban (1921-1940).
For the last 15 a, SKB (the Swedish Nuclear Fuel and Waste Management Company) has been using the Äspö Hard Rock Laboratory (HRL) as the main test site for the development of suitable tools and methods for the final disposal of spent nuclear fuel. Major achievements have been made in the development of a new groundwater modelling technique. The technique described in this paper is used within the ongoing site investigations of Forsmark and Simpevarp in Sweden.The limitations of existing geochemical models used at many sites and the need to decode complex groundwater information in terms of origin, mixing (transport) and reactions at site scale, necessitated the development of a new modelling tool, M3, (multivariate mixing and mass-balance calculations). In M3 modelling, the assumption is made that the groundwater chemistry is a result of mixing as well as water/rock interactions. The M3 model compares groundwater compositions (measured in terms of major ionic components, stable isotopes and 3H) at a site. The model differs from many other standard geochemical models which primarily use water–rock interactions, rather than mixing, to determine groundwater chemical evolution. The tool is not dependent on thermodynamic databases, potentially uncertain redox and pH data, and can deal with the effects of biological reactions. The results of mixing calculations can be compared or integrated with hydrodynamic models.In this paper, the M3 method has been applied to two large hydrogeochemical databases, for the Äspö Hard Rock Laboratory in Sweden, the Underground Research Laboratory in Canada (URL) together with data from the ongoing site investigations of Forsmark and Simpevarp in Sweden. Decreasing amounts of precipitation and increasing proportions of saline and brine waters are seen with increasing depth in both areas. Biogenic waters, caused by uptake of CO2 from organics causing additional formation of HCO3 by water–rock interaction, occur at intermediate depths in both areas but more glacial water is detected in the URL area. The origin and evolution of the groundwater at the Äspö and URL sites have been quantified with the aid of M3. In addition, the conceptual present/post-glacial hydrogeological model has been verified.

One of the most important observations that can be obtained from the study of an aquifer system dominated by mixing is the contribution of each end-member water to the chemical composition of every water parcel in the aquifer. Once the first-order effect of mixing has been taken into account via the mixing proportions, water–rock interaction can be used to explain the remaining variability. There are many sources of uncertainty that can prevent the accurate calculation of the mixing proportions of a mixing-dominated system, but the type and intensity of the chemical reactions that have taken place as a consequence of mixing is one of the most critical. Here the uncertainty in the computed mixing proportions of samples from a “synthetic” aquifer system derived from the actuation of different chemical reactions are assessed (always remembering that the chemical reactions are a second-order effect). These uncertainties are explored using two different geochemical codes in order to infer the limits of both methodological approaches: PHREEQC, as an example of a standard geochemical code; and M3, as an example of a Principal Component-based geochemical code. Several synthetic water samples are created with the direct approach of PHREEQC, both by pure mixing and including different types of chemical reactions. Together with the chemical information of the end-member waters, these samples are then fed into PHREEQC (inverse modelling) and M3 and the mixing proportions and mineral mass transfers are computed. PHREEQC calculations give a reasonable estimate of the real mixing proportions and the chemistry of the groundwaters. However, similar mixing proportions and mass transfers can be obtained using different sets of reactions, indicating a source of uncertainty that should be overcome with additional chemical information. For M3, where synthetic samples have been included in a real data set of groundwater samples from the Scandinavian Shield, mixing proportions are only mildly affected either by the number of compositional variables or the number of samples used for the Principal Component Analysis (PCA). However, the robustness of the output is quite sensitive to whether only conservative compositional variables are used or both conservative and non-conservative compositional variables. Mass balance calculations in M3 are much more sensitive to non-conservative compositional variables and the recommendation here is not to use non-conservative variables with PCA-based codes if any information about reactions is to be obtained.

Coupled hydrogeological and reactive transport modelling of the Simpevarp area (Sweden) by Jorge Molinero; Juan R. Raposo; Juan M. Galíndez; David Arcos; Jordi Guimerá (1957-1981).
The Simpevarp area is one of the alternative sites being considered for the deep geological disposal of high level radioactive waste in Sweden. In this paper, a coupled regional groundwater flow and reactive solute transport model of the Simpevarp area is presented that integrates current hydrogeological and hydrochemical data of the area. The model simulates the current hydrochemical pattern of the groundwater system in the area. To that aim, a conceptual hydrochemical model was developed in order to represent the dominant chemical processes. Groundwater flow conditions were reproduced by taking into account fluid-density-dependent groundwater flow and regional hydrogeologic boundary conditions. Reactive solute transport calculations were performed on the basis of the velocity field so obtained. The model was calibrated and sensitivity analyses were carried out in order to investigate the effects of heterogeneities of hydraulic conductivity in the subsurface medium. Results provided by the reactive transport model are in good agreement with much of the measured hydrochemical data. This paper emphasizes the appropriateness of the use of reactive solute transport models when water-rock interaction reactions are involved, and demonstrates what powerful tools they are for the interpretation of hydrogeological and hydrochemical data from site geological repository characterization programs, by providing a qualitative framework for data analysis and testing of conceptual assumptions in a process-oriented approach.

Calibration of regional palaeohydrogeology and sensitivity analysis using hydrochemistry data in site investigations by F.M.I. Hunter; L.J. Hartley; A. Hoch; C.P. Jackson; R. McCarthy; N. Marsic; B. Gylling (1982-2003).
A transient coupled regional model of groundwater flow and solute transport has been developed, which allows the use of hydrochemical data to calibrate the model input parameters. The methodology has been illustrated using examples from the Simpevarp area in south-eastern Sweden which is being considered for geological disposal of spent nuclear fuel. The 3-dimensional model includes descriptions of spatial heterogeneity, density driven flow, rock matrix diffusion and transport and mixing of different water types, and has been simulated between 8000 BC and 2000 AD. Present-day analyses of major elemental ions and stable isotopes have been used to calibrate the model, which has then been cross checked against measured hydraulic conductivities, and against the hydrochemical interpretation of reference water mixing fractions. The key hydrogeological model sensitivities have been identified using the calibrated model and are found to include high sensitivity to the top surface flow boundary condition, the influence of variations in fracture transmissivity in different orientations (anisotropy), spatial heterogeneity in the deterministic regional deformation zones and the spacing between water bearing fractures (in terms of its effect on matrix diffusion).

Modelling the evolution of hydrochemical conditions in the Fennoscandian Shield during Holocene time using multidisciplinary information by S. Follin; M.B. Stephens; M. Laaksoharju; A.-C. Nilsson; J.A.T. Smellie; E.-L. Tullborg (2004-2020).
Three-dimensional, large-scale models for groundwater flow and solute transport are used for the low-temperature, fractured crystalline rock sites in Sweden that are being considered for the geological disposal of spent nuclear fuel. It has been suggested that comparisons between measured and simulated present-day hydrochemical data provide a means to constrain the complex influences of past climatic events and to improve the ability to understand the palaeohydrogeological evolution of the physical system studied. Here the authors demonstrate how the integration of multidisciplinary data and models from one of the sites in Sweden (Forsmark) can aid the appraisal of the hydrochemical conditions at 8000 BC, which is the selected starting point for the palaeohydrogeological modelling of the hydrochemical conditions in the Fennoscandian Shield during the Holocene (last 10 ka). Since a firm understanding of the evolution of the hydrochemical conditions is important for the long-term safety assessment, recognition of the initial hydrochemical conditions is essential for the overall build-up of confidence in the modelling process.