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Origin and mobility of major and key trace elements (Cs, YREEs) in fracture groundwater in the upper 1.2 kilometres of coastal granitoids - Implications for future repositories of spent nuclear fuel
Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science. (Geochemistry research group)
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on the chemical and isotopic features of groundwater residing in the upper 1.2 km of sparsely fractured crystalline bedrock of the Baltic Shield. The work is based on previous understanding of the groundwater origin and its evolution in the fractured bedrock of the Äspö Hard Rock Laboratory (underground tunnel and facilities) and in two candidate coastal areas (Laxemar and Forsmark) investigated by the Swedish Nuclear Fuel and Waste Management Company (SKB) for future construction of a nuclear waste repository. In order to assess the origin and the apparent mobility of major elements and key trace elements (Cs and YREEs) in this difficult-to-reach deep environment, new (and previously published) data of SKBs hydrogeochemical monitoring programme was iteratively characterised and integrated in phenomenological models. The overall aim was to improve the integration between groundwater mixing and in situ water-rock interaction processes in deep coastal crystalline bedrock under natural and/or disturbed (i.e., dynamic) flow conditions induced by the presence of a tunnel system.

The multiple origins (glacial, marine, meteoric and allochthonous) of the fracture groundwater resulted in a large range of concentrations for dissolved major and trace elements in the studied bedrock. Dependent on the current flow conditions, the apparent mobility of dissolved elements was generally challenging to identify in the deep fractured bedrock under natural flow conditions. There, the relatively long residence time of most of the various groundwater types prevented to clearly differentiate the (apparent) fast retention processes from slow but active processes on a long-term perspective. Both processes alter the primary hydrochemical composition mainly imposed by the mixing between the dominant sources of groundwater. Nevertheless, in the particular case of YREEs, their generally low natural concentrations and predominant binding to organic colloids in most palaeo- (and modern meteoric) groundwaters (independently of the flow conditions) indicated strong active sorption onto minerals and physical filtration of organic colloids in the fractures. Together, these properties tend to minimise the mobility of dissolved YREEs and to stabilise their concentrations and fractionation patterns during the long residence time of the groundwaters.

At the Äspö HRL, an analogue (in a broad sense) of future repositories for high-level and long-lived radioactive wastes, changes in groundwater origin and salinity took place rapidly in subvertical fracture zones and progressively within the sparsely fractured deep rock domains. The changes resulted either from partial-to-substantial replacement of palaeo-groundwater by modern surface/shallow ground-water or induced dynamic up-flow of deep-lying saline groundwater. The hydrogeochemical instability near the underground facility during excavation to operational phase helped to assess qualitatively – and in some case differentiate quantitatively – the combined role of mixing, short-term and long-term reactions on the chemical composition of groundwater and the mobility of major elements and Cs within fracture zones and the sparsely fractured rock domains.

Collectively, the findings of the individual studies showed that the composition of intruded past or modern marine groundwater was likely to affect the natural retention properties/reactivity of the bedrock towards dissolved species at repository depth. For instance, the intrusion of modern seawater induced a desorption process of some dissolved species originally present on the exchange sites of the clayish fault gouge material in the fractures. This contributed to an apparent increase of the abundance level of dissolved cations naturally occurring in relatively moderate (i.e., K and Mg) and trace (i.e., Cs) concentrations in the fracture groundwaters.

The general understanding of the current hydrogeochemical conditions in deep crystalline bedrock is crucial when predicting future changes in groundwater chemistry (i.e., climatic cycles), which in turn might be of relevance to the long-term integrity of the KBS-3 repository method developed for isolating the nuclear waste from the surficial environment and biosphere.

Place, publisher, year, edition, pages
Växjö: Linnaeus University Press, 2015. , 214 p.
Series
Linnaeus University Dissertations, 235/2015
Keyword [en]
fracture groundwater, groundwater mixing, seawater intrusion, water-rock interactions, cation exchange, Cesium, Rare Earth Elements, colloidal speciation, humic substances, dual porosity, inverse modelling, Äspö HRL, Laxemar, Forsmark
Keyword [fr]
eaux de fractures, mélange d'eaux, intrusion d'eau de mer, interactions eau-roche, échanges cationiques, Césium, Elements Terres Rares, spéciation colloïdale, substances humiques, double porosité, modélisation inverse, Äspö HRL, Laxemar, Forsmark
Keyword [sv]
sprickvatten, grundvatten mixing, havsvattenintrusion, vatten-berg interaction, katjonbyte, cesium, sällsynta jordartsmetaller, kollidal speciation, humusämnen, dual porositet, invers modellering, Äspölaboratoriet, Äspötunneln, Laxemar, Forsmark
National Category
Natural Sciences
Research subject
Natural Science, Environmental Science
Identifiers
URN: urn:nbn:se:lnu:diva-47607ISBN: 978-91-87925-86-3 (print)OAI: oai:DiVA.org:lnu-47607DiVA: diva2:874307
Public defence
2015-12-17, Hörsalen Fullriggaren, Landgången 4, Kalmar, 09:30 (English)
Opponent
Supervisors
Available from: 2015-12-02 Created: 2015-11-26 Last updated: 2017-01-27Bibliographically approved
List of papers
1. Effect of tunnel excavation on source and mixing of groundwater in a coastal granitoidic fracture network.
Open this publication in new window or tab >>Effect of tunnel excavation on source and mixing of groundwater in a coastal granitoidic fracture network.
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2012 (English)In: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 46, no 23, 12779-12786 p.Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Environmental Sciences
Research subject
Environmental Science, Environmental Chemistry
Identifiers
urn:nbn:se:lnu:diva-22805 (URN)10.1021/es301722b (DOI)23088667 (PubMedID)
Available from: 2012-12-12 Created: 2012-12-12 Last updated: 2017-03-08Bibliographically approved
2. High cesium concentrations in groundwater in the upper 1.2 km of fractured crystalline rock - Influence of groundwater origin and secondary minerals
Open this publication in new window or tab >>High cesium concentrations in groundwater in the upper 1.2 km of fractured crystalline rock - Influence of groundwater origin and secondary minerals
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2014 (English)In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 132, 187-213 p.Article in journal (Refereed) Published
Abstract [en]

Dissolved and solid phase cesium (Cs) was studied in the upper 1.2 km of a coastal granitoid fracture network on the Baltic Shield (Aspo Hard Rock Laboratory and Laxemar area, SE Sweden). There unusually high Cs concentrations (up to 5-6 mu g L-1) occur in the low-temperature (<20 degrees C) groundwater. The material includes water collected in earlier hydro-chemical monitoring programs and secondary precipitates (fracture coatings) collected on the fracture walls, as follows: (a) hydraulically pristine fracture groundwater sampled through 23 surface boreholes equipped for the retrieval of representative groundwater at controlled depths (Laxemar area), (b) fracture groundwater affected by artificial drainage collected through 80 boreholes drilled mostly along the Aspo Hard Rock Laboratory (underground research facility), (c) surface water collected in local streams, a lake and sea bay, and shallow groundwater collected in 8 regolith boreholes, and (d) 84 new specimens of fracture coatings sampled in cores from the Aspo HRL and Laxemar areas. The groundwater in each area is different, which affects Cs concentrations. The highest Cs concentrations occurred in deep-seated saline groundwater (median Aspo HRL: 4.1 mu g L-1; median Laxemar: 3.7 mu g L-1) and groundwater with marine origin (Aspo HRL: 4.2 mu g L-1). Overall lower, but variable, Cs concentrations were found in other types of groundwater. The similar concentrations of Cs in the saline groundwater, which had a residence time in the order of millions of years, and in the marine groundwater, which had residence times in the order of years, shows that duration of water-rock interactions is not the single and primary control of dissolved Cs in these systems. The high Cs concentrations in the saline groundwater is ascribed to long-term weathering of minerals, primarily Cs-enriched fracture coatings dominated by illite and mixed-layer clays and possibly wall rock micaceous minerals. The high Cs concentrations in the groundwater of marine origin are, in contrast, explained by relatively fast cation exchange reactions. As indicated by the field data and predicted by 1D solute transport modeling, alkali cations with low-energy hydration carried by intruding marine water are capable of (NH4+ in particular and K+ to some extent) replacing Cs+ on frayed edge (FES) sites on illite in the fracture coatings. The result is a rapid and persistent (at least in the order of decades) buildup of dissolved Cs concentrations in fractures where marine water flows downward. The identification of high Cs concentrations in young groundwater of marine origin and the predicted capacity of NH4+ to displace Cs from fracture solids are of particular relevance in the disposal of radioactive nuclear waste deep underground in crystalline rock. (C) 2014 Elsevier Ltd. All rights reserved.

National Category
Earth and Related Environmental Sciences
Research subject
Natural Science, Environmental Science
Identifiers
urn:nbn:se:lnu:diva-34476 (URN)10.1016/j.gca.2014.02.001 (DOI)000334832100012 ()
Available from: 2014-05-28 Created: 2014-05-28 Last updated: 2017-03-08Bibliographically approved
3. REE and Y in groundwater in the upper 1.2 km of Proterozoic granitoids (Eastern Sweden) - Assessing the role of composition and origin of groundwaters, geochemistry of fractures, and organic/inorganic aqueous complexation
Open this publication in new window or tab >>REE and Y in groundwater in the upper 1.2 km of Proterozoic granitoids (Eastern Sweden) - Assessing the role of composition and origin of groundwaters, geochemistry of fractures, and organic/inorganic aqueous complexation
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2014 (English)In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 144, 342-378 p.Article in journal (Refereed) Published
Abstract [en]

Yttrium and rare earth elements (YREEs) are studied in groundwater in the shallow regolith aquifer and the fracture networks of the upper 1.2 km of Paleoproterozoic granitoids in boreal Europe (Laxemar and Forsmark areas, Sweden). The study includes groundwater sampled via a total of 34 shallow boreholes reaching the bottom of the regolith aquifer, and 72 deep boreholes with equipment designed for retrieval of representative groundwater at controlled depths in the fractured bedrock. The groundwater composition differs substantially between regolith and fracture groundwater and between areas, which affects the dissolved YREE features, including concentrations and NASC normalized patterns. In the fresh groundwater in the regolith aquifers, highest YREE concentrations occur (10th and 90th percentile; Laxemar: 4.4-82 mu g L-1; Forsmark: 1.9-19 mu g L-1), especially in the slightly acidic groundwater (pH: 6.3-7.2 - Laxemar), where the normalized YREE patterns are slightly enriched in light REEs (La-NASC/Y-NASC: 1.1-2.4). In the recharge areas, where redox potentials of the regolith groundwater is more moderate, negative Ce anomaly (Laxemar: 0.37-0.45; Forsmark: 0.15-0.92) and positive Y anomaly (mainly in Forsmark: 1.0-1.7) are systematically more pronounced than in discharge areas. The significant correlations between the YREE features and dissolved organic carbon, minor elements, and somewhat pH suggest a strong control of humic substances (HSs) together with Al rich colloids and redox sensitive Fe-Mn hydrous precipitates on the dissolved YREE pools. In the bedrock fractures, the groundwater is circumneutral to slightly basic and displays YREE concentrations that are at least one order of magnitude lower than the regolith groundwater, and commonly below detection limit in the deep brackish and saline groundwater, with some exceptions such as La and Y. At intermediate depth (>50 m), where groundwater of meteoric origin percolates, the La-NASC/Y-NASC values moderately to substantially decrease (Laxemar: 0.24-2.65; Forsmark: 0.02-0.06) and Y and Ce anomalies are negligible as compared to the regolith groundwater. Aqueous speciation modeling predicts substantial binding of dissolved Y and La, respectively, to HSs. This, in turn, suggests that the features of the YREE pool in the meteoric fracture groundwater are dominantly controlled by the capacity of fracture minerals to sorb HS ligands inherited from the overlying terrestrial regolith. In the deep bedrock fractures (>100/200 m), the YREE features vary substantially with the groundwater paleo-origin. In Laxemar, where groundwater with pronounced glacial origin percolates, the YREE concentrations decrease with increasing mixing fraction of glacial melt water. There, the dissolved YREEs are mostly bound to HSs, and inherited their fractionation features (La-NASC/Y-NASC: 0.15-2.1) from water-rock interaction in the intermediate bedrock fractures. In Forsmark, the YREE and heavy REE enrichment (La-NASC/Y-NASC: 0.007-0.23) are more systematic in the groundwater with pronounced marine origin, due to water-mineral interactions in the sea sediment and in the fractures while infiltrating and percolating. YREE features significantly change in the deep saline groundwater with a long residence time, which displays La-NASC/Y-NASC similar to those of the local bedrock. The findings of this study are relevant in terms of safety assessment for nuclear waste disposal in crystalline rock carrying groundwater influenced by various paleo-climatic recharges. (C) 2014 Elsevier Ltd. All rights reserved.

National Category
Earth and Related Environmental Sciences
Research subject
Natural Science
Identifiers
urn:nbn:se:lnu:diva-39152 (URN)10.1016/j.gca.2014.08.004 (DOI)000344945600020 ()
Available from: 2015-01-15 Created: 2015-01-15 Last updated: 2017-03-08Bibliographically approved
4. The role of carbonate complexes and crystal habit on rare earth element uptake in low-temperature calcite in fractured crystalline rock
Open this publication in new window or tab >>The role of carbonate complexes and crystal habit on rare earth element uptake in low-temperature calcite in fractured crystalline rock
2015 (English)In: Chemical Geology, ISSN 0009-2541, E-ISSN 1872-6836, Vol. 391, 100-110 p.Article in journal (Refereed) Published
Abstract [en]

This study focuses on rare earth element (REE) geochemistry of low-temperature calcite coatings occurring on the walls of fractures throughout the upper kilometer of crystalline rocks of the Baltic Shield. Fifty one calcite coatings were sampled from cores drilled with the triple-tube technique which successfully preserved the fragile calcite coatings on the fracture walls. The calcites, which based on geological and isotopic evidence were precipitated over the last 10 million years, had highly variable Sigma REE concentrations (0.61-2276 ppm) that decreased weakly with the depth the calcite was sampled from. When normalized to shale (and host rock), the REE concentrations of habits with c-axis approximate to a-axes and the closely associated c-axis > a-axes, the most abundant crystal morphologies in the system, decreased strongly and smoothly across the series. In contrast, the REEs of habits with c-axis >> a-axes, identified only in fractures in the uppermost 260m of the bedrock, were flatter and occasionally expressed a weak middle REE enrichment. By using calcite-water partition coefficients derived for REEs in previous laboratory experiments, the La/Yb of the paleogroundwater from which the calcites precipitated was back-calculated and found to be overall similar (range 0.15-452) overlap to the corresponding ratio of the present groundwater (range: 2.1-36.4). In terms of REE/Ca, the values for the back-calculated paleogroundwater (La / Ca 9.9 (*) 10(-11)-3.9 (*) 10(-7); Yb / Ca 1.5 (*) 10(-10)-2.2(*)10(-7)) were similar to those of LaCO3+ / Ca (4.5(*)10(-10)-8.5 (*) 10(-7)) and (YbCO3+ + Yb(CO3)(2)(-)) / Ca (5.4 (*) 10(-11)-1.8 (*) 10(-8)), respectively, in the present groundwater. These patterns indicate that the LREE to HREE and REE to Ca ratios in the groundwater at the site are broadly similar to those existing when the calcites precipitated, and that carbonate complexes present in the paleogroundwater played a crucial role in sequestration and fractionation of REEs in calcite. The findings have implications for bedrock storage of high-level radioactivewaste, which contains actinides for which the REEs can be used as natural analogues. (C) 2014 Elsevier B.V. All rights reserved.

Keyword
Low-temperature calcite, Groundwater, Rare earth elements, Stable isotopes, Partition coefficient
National Category
Geochemistry
Research subject
Natural Science
Identifiers
urn:nbn:se:lnu:diva-39118 (URN)10.1016/j.chemgeo.2014.10.030 (DOI)000346565400010 ()
Available from: 2015-01-15 Created: 2015-01-15 Last updated: 2017-03-08Bibliographically approved

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