Eva-Lena Tullborg

Hydrogeochemical conditions and evolution at the Äspö HRL, Sweden

Abstract The overall hydrogeochemical conditions at and in the near vicinity of the underground experimental Aspo Hard Rock Laboratory (HRL) in SE Sweden have been investigated. Groundwater data from more than 400 samples have been compiled and evaluated. The groundwater samples represent depths down to 1700 m below sea level and sampling has been performed prior to and during the HRL tunnel excavation. Episodic events have to a great extent influenced the hydrochemical evolution since the last glaciation which ended some 13 ka ago. At that time glacial melt water was flushed under hydraulic pressure down into the fracture system to a depth of at least several hundred metres. The next episodic event took place when the Baltic freshwater lake transformed into the brackish Litorina Sea some 7 ka ago. At this time Aspo was covered by the sea and these denser, more saline waters partly replaced the glacial water down to a depth where the density equilibrated with the replacement sea water. At some time around 3–4 ka ago, Aspo started to rise above sea level and meteoric water began to infiltrate the rock. The overall trend of increasing salinity with depth may easily be misinterpreted as a fairly simple groundwater system, evolving from a two component evolution path between non-saline and saline groundwaters. However, when combining the results from environmental isotopes and the chemical parameters using a new modelling tool named M3 (Multivariate Mixing and Mass balance calculations), a higher resolution was obtained and a more complex groundwater pattern, which reflects the present and paleo-hydrogeological events, can be recognised. The measured groundwater composition was modelled to be a mixture of meteoric, past and present Baltic seawater, glacial (or cold climate recharge) and brine type of waters. The modelling result shows that the processes considered to have a dominating impact on the present Aspo groundwater chemistry are mixing, both in disturbed and undisturbed systems, calcite dissolution and precipitation, redox reactions and biological processes. The undisturbed groundwater conditions prior to the HRL tunnel construction at Aspo consisted of: 1. A dominating proportion of meteoric fresh water in the upper 250 m of the aquifer. 2. A brackish–saline water consisting of mixing proportions of present and ancient Baltic Sea water and glacial melt water present to a depth of 250–600 m. 3. Saline water still containing proportions of glacial water which could represent even older glaciations, and brines, a large portion of which have been stagnant for perhaps millions of years, below a depth of 600 m. During the HRL tunnel construction there were changes in the composition of the water flowing into the tunnel at different locations. Although the variation in salinity was relatively small, the variations in the mixing proportions of the different water types were substantial.

Evidence of ancient life at 207 m depth in a granitic aquife

The results of electron-microscopy investigations of calcite precipitated in a water-conducting fracture in a ca. 1800 Ma granitic rock from 207 m below sea level at the island of Aspo on the southeastern (Baltic) coast of Sweden are compared with measurements of carbon, oxygen, and sulfur isotope composition of the calcite and embedded pyrite. Parts of the calcite had extremely low delta 13C values, indicative of biological activity, and contained bacteria-like microfossils occurring in colonies and as typical biofllms. X-ray microanalysis demonstrated these fossils to be enriched in carbon. Our results provide evidence for ancient life in deep granitic rock aquifers and suggest that the modern microbial life found there is intrinsic. Modeling historical and present geochemical processes in deep granitic aquifers should, therefore, preferably include biologically catalyzed reactions. The results also suggest that the search for life on other planets, e.g., Mars, should include subsurface material.

Overview of geological and hydrogeological conditions of the Äspö hard rock laboratory site

Abstract The site of the underground facility, the Aspo Hard Rock Laboratory, was excavated at a depth of 450 m below the island of Aspo and has been extensively investigated by geological, hydrogeological and hydrochemical methods as part of the geoscientific research for disposal of nuclear waste in Sweden. The geological history of the area dates back to 1.85 Ga and is dominated by granitoids belonging to the Trans-Scandinavian Igneous Belt but also includes basic sheets and xenoliths and dikes of fine-grained granite. Seven tectonic episodes, giving rise to fracture mineralization, are recognised. The major discontinuities and fracture zones were characterised from surface investigations before the tunnel construction work started. These structural features were also identified in the tunnel and are, as predicted, the major water conducting features. Sets of open fractures in the NNW–NW and N–S directions and the brittle fine-grained granite are other important water conductors. Groundwater flow modelling shows that the location of Aspo island has a major impact on the current distribution of groundwater salinity due to varying hydraulic/boundary/conditions in the late and post glacial period.

Large-scale intrusion of shallow water into a vertical fracture zone in crystalline bedrock : Initial hydrochemical perturbation during tunnel construction at the Aspo Hard Rock Laboratory, southeastern Sweden

On March 13, 1991, construction of the entrance tunnel to the Aspo Hard Rock Laboratory opened a vertical fracture zone at a depth of 70 m. This provides an opportunity to study geochemical changes resulting from shallow water inflow into a crystalline bedrock aquifer as anticipated during construction and operation of a deep repository for spent nuclear fuel. Chloride ion is a natural conservative tracer for mixing between the dilute ([Cl−] < 10 mg L−1) shallow groundwater and the saline ([Cl−] = 5000 mg L−1) native groundwater of the fracture zone. A sharp dilution front, corresponding to 80% dilution of the native groundwater, indicated arrival of shallow groundwater in the entrance tunnel after 3 weeks. In spite of this large input of shallow water, the fracture zone remains predominantly anoxic. Major element hydrochemistry and carbon and oxygen stable isotope data indicate large inputs of alkalinity and biogenic CO2(g). Input of organic carbon with shallow groundwater provides a possible energy and carbon source for anaerobic respiration. There is no evidence for sulfate reduction, and Fe(III) oxyhydroxide fracture minerals are the only other dominant electron acceptor observed.

Why Baltic Shield zircons yield late Paleozoic, lower-intercept ages on U-Pb concordia

In the Baltic Shield, the Paleozoic is known as a period of regional Pb mobilization that resulted in new lead deposits and the addition of radiogenic Pb to older deposits. In addition, U-Pb data from zircons in the Baltic Shield commonly yield late Paleozoic lower intercepts on the U-Pb concordia regarded as “geologically insignificant.” Results from fission-track studies reveal that the rocks that form the present bedrock surface in Sweden and Finland were heated at the same time. This fact is explained by upper Paleozoic sedimentary deposits, much thicker than hitherto known, that covered the basement for at least 200 m.y. These erosion products of the Caledonides formed terrestrial or shallow-water (“Old Red”) deposits in a foreland basin that covered Sweden and, to a lesser extent, Finland. Hydrothermal solutions leached metamict zircons in the basement rocks for a long time, resulting in Pb loss and Paleozoic lower-intercept ages on U-Pb concordia. The radiogenic Pb was redistributed and preferentially concentrated into fractures and veins or was added to older sulfide deposits.

Organic carbon oxidation induced by large-scale shallow water intrusion into a vertical fracture zone at the Äspö Hard Rock Laboratory (Sweden)

Entrance tunnel construction at the Aspo Hard Rock Laboratory opened a conductive vertical fracture zone at a depth of 70 m on March 13, 1991. Three weeks later a sharp dilution front corresponding to 80% shallow water inflow to the originally saline fracture zone arrived at the entrance tunnel depth. In spite of this large inflow of shallow water, the fracture zone has remained persistently anoxic over a subsequent 212-year period. Results from gas sampling and 14C dating of dissolved organic and inorganic carbon conclusively show that recent organic carbon is being transported into the fracture zone and oxidized to carbon dioxide. These results are important when considering possible changes of redox status in the deep groundwater environment during construction and operation of a repository for spent nuclear fuel. Opening this fracture zone to large-scale surface water inflow adds reducing capacity in the form of organic carbon. This implies that the soil cover may provide important protection against input of dissolved oxygen to fractures being drained during the open phase of the repository.

Effect of tunnel excavation on source and mixing of groundwater in a coastal granitoidic fracture network.

The aim of this study was to assess how the excavation of the Äspö Hard Rock Laboratory tunnel has impacted on sources and mixing of groundwater in fractured crystalline (granitoidic) bedrock. The tunnel is 3600 m long and extends to a depth of 460 m at a coastal site in Boreal Europe. The study builds on a unique data set consisting of 1117 observations on chloride and δ(18)O of groundwater collected from a total of 356 packed-off fractures between 1987 and 2011. On the basis of the values of these two variables in selected source waters, a classification system was developed to relate the groundwater observations to source and postinfiltration mixing phenomena. The results show that the groundwater has multiple sources and a complex history of transport and mixing, and is composed of at least glacial water, marine water, recent meteoric water, and an old saline water. The tunnel excavation has had a large impact on flow, sources, and mixing of the groundwater. Important phenomena include upflow of deep-lying saline water, extensive intrusion of current Baltic Sea water, and substantial temporal variability of chloride and δ(18)O in many fractures.

The occurrence and potential origin of asphaltite in bedrock fractures, Forsmark, central Sweden

Abstract Occurrences of asphaltite in fractures in the Svecofennian basement have been documented in drill cores from the site investigation for a deep repository for nuclear waste in Forsmark, central Sweden. The carbonaceous material has been found at depths down to 124 m and appears as hardened black fracture fillings. The δ13C values of the asphaltite are relatively uniform with values between -29.5 and -30.1‰ (PDB) indicating an organic origin. These results are consistent with previous analyses of asphaltite occurrences in Sweden. One sample collected from a fracture in the Ordovician limestone in Kinnekulle, south-western Sweden was analysed for comparison and shows a similar δ13C signature (-29.5‰). Biomarker analyses of the asphaltite samples confirm an organic origin and the organic-rich early Paleozoic Scandinavian Alum Shale is suggested as the source rock. This is supported by comparison of the asphaltite with published biomarker and carbon isotopic data from the Alum Shale. Downward migration of hydrocarbons into the crystalline basement due to the presence of an overpressured basin is a possible mechanism for the emplacement of the asphaltite in the bedrock fractures. It is suggested that the heat source responsible for the migration of the asphaltite was the overburden of the Caledonian foreland basin.

Low-temperature trace element mobility influenced by microbial activity—indications from fracture calcite and pyrite in crystalline basement

Abstract Stable isotopes and trace elements have been analysed in calcite and pyrite, from a fracture surface exposed at a depth of 200 m in the Precambrian basement of SE Sweden. The fracture filling calcite precipitated from low-temperature solutions of at least four different hydrochemical regimes. In contrast to many fractures in the area, no hydrothermal activity was indicated in this fracture making the effects of low temperature processes easier to evaluate. Local production of HCO 3 − by anaerobic bacteria is indicated from low δ 13 C -values in the calcite. The findings of pyrite with partly low δ 34 S embedded in the calcite suggest that these bacteria may have been sulphate reducers. Inverse correlation of La/Yb ratio with δ 13 C in the calcite suggests release and incorporation of organic complexed light REEs in the calcite produced as a result of biogenic bicarbonate production. High Ba contents in the fracture filling may be related to the presence of organic material. A low but significant mobility of Th is indicated.

Stable isotopes of fissure-filling calcite from Finnsjön, Uppland, Sweden

δ18O and δ13C was analysed for calcite fissure fillings from different depths, down to 533 m, in Svecokarelian rocks of east Sweden. A significant correlation with depth is noticed for δ18O. There also are differences between sealed and open fissure fillings in both δ18O and δ13C. Calcite fillings which originated in high- and low-temperature fluids have been found within single fissures. These calcites have different textures. One population of calcite has δ18O values of −5.7 to −9.0‰ and has been precipitated by water enriched in δ18O relative to that now present in the bedrock. Another population of calcite generally has δ18O values of −17.0 to −19.5‰ and δ13C values of −2.5 to −5.5‰; these calcites have been precipitated under hydrothermal conditions. A third calcite population (δ18O = −9.1 to −16.9‰) precipitated from differentwaters at different temperatures that include present conditions. Calcites which could have been precipitated during present conditions mostly originate from open fissures.