Hydrogeology Journal (2014) 22: 1233–1249 DOI 10.1007/s10040-014-1165-6
Multi-scale groundwater flow modeling during temperate climate conditions for the safety assessment of the proposed high-level nuclear waste repository site at Forsmark, Sweden Steven Joyce & Lee Hartley & David Applegate & Jaap Hoek & Peter Jackson Abstract Forsmark in Sweden has been proposed as the site of a geological repository for spent high-level nuclear fuel, to be located at a depth of approximately 470 m in fractured crystalline rock. The safety assessment for the repository has required a multi-disciplinary approach to evaluate the impact of hydrogeological and hydrogeochemical conditions close to the repository and in a wider regional context. Assessing the consequences of potential radionuclide releases requires quantitative site-specific information concerning the details of groundwater flow on the scale of individual waste canister locations (1– 10 m) as well as details of groundwater flow and composition on the scale of groundwater pathways between the facility and the surface (500 m to 5 km). The purpose of this article is to provide an illustration of multi-scale modeling techniques and the results obtained when combining aspects of local-scale flows in fractures around a potential contaminant source with regional-scale groundwater flow and transport subject to natural evolution of the system. The approach set out is novel, as it incorporates both different scales of model and different levels of detail, combining discrete fracture network and equivalent continuous porous medium representations of fractured bedrock. Keywords Fractured rock . Numerical modeling . Temperate climate . Sweden . Forsmark
Received: 4 September 2013 / Accepted: 18 June 2014 Published online: 19 July 2014 * Springer-Verlag Berlin Heidelberg 2014 This article belongs to a series describing hydrogeological safety assessment modeling for the proposed high-level nuclear waste repository site at Forsmark, Sweden
S. Joyce ()) : L. Hartley : D. Applegate : J. Hoek : P. Jackson AMEC, Building 150, Harwell Oxford, Didcot, Oxfordshire OX11 0QB, UK e-mail:
[email protected] Tel.: +44 1635 280300
Introduction Background The Swedish Nuclear Fuel and Waste Management Company (SKB) has applied for a license to construct a geological repository for spent high-level nuclear fuel at Forsmark in Sweden (Thegerström and Olsson 2011). The licensing process is staged; the current application seeks acceptance of the disposal concept and the selection of the site. The proposed disposal concept, known as ‘KBS-3’ (SKB 2011), has a multi-barrier system consisting of copper canisters that contain the spent nuclear fuel, surrounded by a bentonite clay buffer and deposited at approximately 470 m depth in crystalline rock. The rock is relatively impermeable and so groundwater flow is predominantly via a network of connected fractures. A safety assessment, known as ‘SR-Site’, has been conducted (SKB 2011) to demonstrate that the characteristics of the Forsmark site and the engineered barrier systems are such that the risk posed to human populations and the environment is below regulatory criteria. The assessment study is based on site-specific information acquired from phased investigations using surface-based techniques (Andersson et al. 2013). It is understood that not all issues can be resolved at this stage, and detailed characterization of bedrock conditions during construction of the repository together with safety studies of increasing complexity will form part of further investigations. Further details concerning the disposal concept and licensing process are given in the overview essays by Selroos and Follin (2014a, b, see this issue). An important element in a safety assessment is the definition of a set of safety functions that the repository system ideally should fulfill over time (e.g. NEA 2009). For a KBS-3 repository, long-term safety (a one million year period is considered) depends primarily on the ability of the engineered barriers (copper canister and bentonite buffer) to contain the spent fuel. The durability of these barriers requires a hydrogeological environment that exhibits low groundwater flow rates at repository depth and non-detrimental hydrogeochemistry, as well as longterm mechanical stability (SKB 2011). In order to minimize the radiological consequences of hypothetical
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scenarios where some canisters fail, and radionuclides are released into groundwater, the hydrogeological environment has a secondary role to delay the transport of radioactive species to the surface.
Multi-scale fractured rock hydrogeology Several studies have observed that data obtained for fractured rocks are measured on multiple scales (borehole, lineament, etc.), but there are difficulties in simultaneously representing all scales in a model (e.g. Aarnes et al. 2007; Shapiro et al. 2007). It has been concluded by several authors that a discrete fracture network (DFN) approach is the most appropriate method for modeling fractured rock (e.g. Berkowitz 2002; Samardzioska and Popov 2005; KarimiFard et al. 2006), but due to the computational resources required to model DFNs on a large scale, most studies have used an upscaling approach to calculate equivalent properties for continuous porous medium (ECPM) models (e.g. Niemi et al. 2000; Lee et al. 2001; Öhman et al. 2005; Odén et al. 2008). There are few examples in the literature of models that deal with a local contaminant source within a regional-scale hydrogeological context, which is the topic of this article. Other authors have dealt with some of the challenges faced in the current study. For example, Zhou et al. (2003) describes the effects of small-scale heterogeneity on transport in unsaturated fractured rock at Yucca Mountain (Nevada, USA); Zhang et al. (2010) propose a multidimensional transport model that mimics the effects of a DFN at a regional scale; and Ophori (2004) describes an approach that includes finer cell discretization close to a nuclear waste disposal vault for improved accuracy when modeling large-scale flow and transport. The contribution of this article is to provide an illustration of multi-scale modeling and the results obtained when combining aspects of local-scale flows in fractures around a potential contaminant source with regional-scale flow and transport subject to natural evolution of the system. Assessing the consequences of potential radionuclide releases requires quantitative site-specific information about the details of groundwater flow on the scale of individual waste canister deposition locations (ca. 1– 10 m) as well as details of flow on the scale of groundwater pathways from the facility to the surface (ca. 500 m to 5 km). Within the SR-Site safety assessment, radionuclide migration in the fractured bedrock has been modeled using a particle-tracking approach (e.g. Selroos et al. 2012; Selroos and Painter 2012), and integrated with near surface hydrology (Bosson et al. 2010) and ecological modeling (Berglund et al. 2009; SKB 2010b) by using the same underlying site hydrological descriptions of bedrock and soils and by passing data from the radionuclide migration models to the surface hydrology models. In a study independent of the SR-Site safety assessment, illustrative three-dimensional (3D) radionuclide transport calculations have been made using a homogeneous porous medium representation of the Forsmark site based on preliminary publicly available information (Schwartz 2012). Hydrogeology Journal (2014) 22: 1233–1249
Groundwater flow in the crystalline rock at Forsmark is predominantly in fractures. A quantitative safety assessment requires that the descriptions of groundwater flow and groundwater composition are posed in the context of the site’s geological structural model, heterogeneity needs to be quantified, and each hydrogeological parameter needs to be evaluated at an appropriate scale (e.g. Dershowitz et al. 2003; Neuman 2005). To meet these requirements, the site investigations, data acquisition and numerical modeling methodologies were all developed around the structural framework of a fracture system (Gustafson et al. 2009). In particular, the groundwater flow system at Forsmark is characterized using the discrete fracture network (DFN) concept (Follin et al. 2014). The DFN concept (e.g. Cacas 1989; Dershowitz 1985; Long et al. 1982; Robinson 1984; NRC 1996) aims to explicitly represent the groundwater flow through fractures and considers many of the important observed characteristics of crystalline rock such as the geometry of fracture patterns, variable connectivity, anisotropy, compartmentalization and heterogeneity. It is assumed that flow is predominantly through an inter-connected network of flow-conductive fractures, with groundwater moving from one fracture to another at the intersections between them. It is also assumed that flow through the rock matrix between the fractures can be neglected, but diffusive exchange of solutes between the fractures and the matrix can occur (e.g. Neretnieks 1994). Further information on fractured rock concepts can be found in the comprehensive reviews of Berkowitz (2002) and Neuman (2005). Examples of constructing DFN models based on the interpretation of site fracture data can be found in Niemi et al. (2000), Frampton and Cvetkovic (2010), Andersson et al. (2013) and Follin and Hartley (2014). The DFN approach has been used in other international programs for geological disposal; for example, in the UK (Armitage et al. 1996), France (ANDRA 2005) and Finland (Hartley et al. 2009). Examples of other applications are for oil reservoirs (e.g. Al Qassab et al. 2002; Basquet et al. 2004) and water resources (e.g. Boutt et al. 2010).
The Forsmark site and its hydrogeology The Forsmark site in Sweden is located approximately 120 km north of Stockholm in a flat low-lying area close to the Baltic Sea (Fig. 1a). The area is undergoing postglacial isostatic rebound with a current rate of shoreline vertical displacement of 6 mm/yr (Söderbäck (2008)). Fig. 1 a Map showing the location of the Forsmark site in Sweden relative to neighboring countries and colored by the major tectonic units; b Topographic map of the Forsmark site. The investigation area is shown as a red polygon. The core drill sites are shown as purple dots (labeled DS) and the percussion drill sites are shown as green dots. Thick black lines show the locations of the three regional steeply dipping deformation zones; c The locations of major deformation zones and fracture domains in the north-western part of the investigation area. (modified after Koistinen et al. 2001; Follin et al. 2014) DOI 10.1007/s10040-014-1165-6
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DOI 10.1007/s10040-014-1165-6
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Although various climate scenarios were considered as part of SR-Site (SKB 2010a), the hydrogeological base case only included shore-level displacement due to isostatic rebound and did not include changes in sea level due to climate change. Following deglaciation, the site was covered with glacial melt water, starting at about 8800 BC, followed by Littorina Sea (saline-brackish) water, starting at about 6500 BC. At AD 900 about half the current investigation area was still covered by the Baltic Sea, and by AD 3000, the shoreline will have retreated about 1 km from the current position of the shoreline. The integrated geological and hydrogeological understanding of the bedrock at Forsmark has developed during the site investigations (Follin 2008; Selroos and Follin 2014a) as a result of the comprehensive geological mapping, geophysical surveys, drilling program, hydraulic testing, and hydrochemical sampling (Skagius et al. 2008). The site data provided the basis for an integrated conceptual model of the geological setting, bedrock deformation history, and the evolving hydrogeological and hydrogeochemical processes taking place. The hydraulic testing has been focused on high-density flow logging, specifically the Posiva flow logging (PFL) method (Öhberg and Rouhiainen 2000; Rouhiainen and Sokolnicki 2005; Öhberg 2006), with confirming support from short interval double-packer tests, to provide distributions of hydraulic properties on the scale of individual fractures. The geometrical properties of individual fractures have been obtained from core logging, borehole imaging and outcrop mapping. The dominant rock is metamorphosed, medium-grained granite to granodiorite (metagranite), formed 1.9–1.85 billion years ago during the Proterozoic eon. Large-scale, essentially tabular structures, known as deformation zones, have been identified where there are concentrations of brittle, ductile or combined brittle and ductile deformation (Stephens et al. 2007). Three regional, steeply dipping deformation zones (Forsmark, Singö, and Eckarfjärden) bound the investigation area (Fig. 1b), which is situated in the north-western part of a so-called tectonic lens in which the rock is folded and generally displays lower ductile strain. The hydraulic properties of deformation zones where they intersect boreholes have been interpreted from hydraulic tests (Follin and Stigsson 2014). The interpreted transmissivity values of fractures show a significant decrease with increasing depth and significant lateral heterogeneity. This trend can be described by an exponential relationship for the depth dependency of transmissivity, with the lateral heterogeneity captured by adding a log-normal random deviate to the exponent. Extensive sub-horizontal fracture systems (sheet joints) have been identified in the upper 200 m of the bedrock, which exert a strong influence on groundwater flow and solute transport at these depths. The less fractured rock between the deformation zones was divided into six so-called fracture domains, denoted FFM01 to FFM06 (Fig. 1c), according to fracture frequency (Olofsson et al. 2007). Generally, fracture Hydrogeology Journal (2014) 22: 1233–1249
frequency decreases with increasing depth. The proposed repository is located at an elevation of approximately −470 m above sea level (m.a.s.l) within FFM01 and FFM06. These fracture domains lie below the gently dipping brittle deformation zones ZFMA2 and ZFMF1 and are characterized by a very low frequency of fractures. The strong variations in hydraulic properties with depth and hydraulic anisotropy result in weak hydraulic gradients at depth and shallow circulation of the groundwater flow system. The bedrock is overlain with unconsolidated deposits (regolith) ranging in thickness from 0 to 16 m (Hedenström et al. 2008). The deposits are predominately glacial till of generally less than 5 m in thickness.
Methodology Discrete fracture network parameterization The parameterization of the DFN describing the fracture domains is described in Follin et al. (2014). A power-law distribution relates the fracture size probability density distribution, f(r), to the radius, r (m), of a fracture treated as a disc in terms of a shape parameter, kr (−), and a location parameter, r0 (m): f ðrÞ ¼
k r r0k r rk r þ1
ð1Þ
The value of r0 was taken to be 0.038 m, corresponding to the radius of a core-drilled borehole, i.e. the low end of the scale of observation. The kr parameter for each fracture domain and depth zone was estimated (Follin et al. 2014) by generating realizations of the DFN and optimizing to give the best match between the intensity of connected open fractures and the observed intensity of flow-conductive fractures from hydraulic tests. It seems reasonable to assume that there is some relationship between fracture size and hydraulic aperture/ transmissivity (e.g. Odling 2001; Dershowitz et al. 2003; Baghbanan and Jing 2007; Neuman 2008). The hydrogeological base case for SR-Site assumes a semicorrelated relationship, in which transmissivity increases with fracture size but with some stochastic variation: b T ¼ 10logðar ÞþσN ð0;1Þ
ð2Þ
where T (m2/s) is transmissivity, r (m) is fracture radius, a (−) is the coefficient and b (−) is the exponent of a power-law relationship between r and log10(T), σ is the standard deviation of log10(T), and N(0,1) denotes a normally distributed random deviate with a mean equal to zero and a standard deviation of one. The parameter values were optimized by matching specific capacities (inflow divided by drawdown) calculated from simulated borehole inflows for realizations of the DFN to those obtained from site measurements. DOI 10.1007/s10040-014-1165-6
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Regional-scale ECPM modeling based on upscaling the underlying DFN flow model was used to confirm that it was representative of site conditions based on comparison to independent data obtained from single-hole hydraulic testing, cross-hole hydraulic testing (interference testing), in situ (natural) groundwater levels and hydrogeochemistry (Follin and Hartley 2014). In the hydrogeochemical modeling, groundwater composition was expressed as a mixture of five so-called reference waters (Laaksoharju et al. 2008), each having a defined composition (Table 1) associated with a particular origin. The interpretation of the distribution of reference waters and inferred groundwater residence times confirmed the hydrogeological understanding of slow groundwater circulation and evolution of composition at repository depth (Follin et al. 2008; Follin and Hartley 2014).The resulting set of DFN parameters adopted for SR-Site for each fracture domain, fracture set and depth zone are given in Table 2. For this work, square fractures are used with transmissivities assigned based on the radii of circular fractures with equivalent areas.
Multi-scale groundwater modeling in fractured rock The DFN concept has been applied to provide each input required by the safety assessment at its appropriate scale using a novel approach that combines different scales and representations of the fractured bedrock. It is not practicable to consider the different scales within a single model, so it is necessary to construct a series of related models, each focusing on a different level of detail. Three models are used, each covering either different scales or different levels of detail, but each consistent with one another in terms of hydraulic properties, distribution of fluid density and groundwater flows across the boundaries of the models. For the work described here, this was achieved by basing each model on the same underlying DFN description of the site (i.e. the same structural representation) as represented on the scale of each model, and by interpolating data between the model scales. The three model scales and the relationships between them are shown in Fig. 2. Details of the numerical algorithms used are provided by Hartley and Joyce (2013). Here, the application of these techniques to the
safety assessment of a KBS-3 repository in fractured crystalline rock is presented, illustrating how they were used to quantify distributions of groundwater flow and composition and their evolution during the temperate climate period following closure and sealing of the repository structures. The modeling calculations were carried out using the ConnectFlow groundwater flow and transport software (AMEC 2013a, b).
Regional-scale model A regional-scale porous medium model (Fig. 3) calculates information about the large scale (ca. 150 km2) transient evolution of groundwater flow and composition (i.e. coupled flow and solute transport) over the full temperate time span (8000 BC to AD 12,000) and provides timedependent pressure boundary conditions and the spatial distribution of fluid density for the more detailed scale models. The upstream boundaries of the model correspond to significant surface water divides (Follin et al. 2007). The downstream boundary represents the furthest shoreline position reached during the time period of the simulations. The depth of the model, 1,200 m, is sufficient for the position of the bottom boundary not to significantly influence the results (Follin and Hartley 2014). The model grid consists of approximately one million cells. A local area is defined around the repository, approximately 3 km2, where the grid cells are 20 m square horizontally. Outside the local area, the grid cells are 100 m square horizontally. The vertical cell sizes are similar to the horizontal, except for some thin layers of cells representing the regolith above the bedrock. The upper layers of the model are mapped to follow the surface topography. Continuity of pressure and flow are imposed where there are discontinuities in the grid at the boundaries of the local area (AMEC 2013a). Finite element equations are included to interpolate the pressure from the nodes of the coarser cells on to the nodes of the adjacent finer cells. Finite element equations are also added to ensure that the flux across the faces of the coarse cells is balanced with the sum of the fluxes across the faces of the adjoining fine cells. On this scale, an ECPM is used to represent the fracture domains (FFM01–FFM06) based on an upscaling of the DFN to an ECPM followed by a superposition of
Table 1 Compositions of reference waters Parameter
Deep saline
Littorina Sea
Altered meteoric
Glacial
Old meteoric
Cl (mg/L) Na (mg/L) K (mg/L) Ca (mg/L) Mg (mg/L) HCO3 (mg/L) SO4 (mg/L) Br (mg/L) δ2H (‰ V-SMOW)a δ18O (‰ V-SMOW)a
47,200 8,200 46 19,300 2 14 1 323 −44.9 −8.9
6,500 3,674 134 151 448 93 890 22 −38.0 −4.7
181 274 6 41 8 466 85 1 −80.6 −11.1
1 0 0 0 0 0 1 0 −158.0 −21.0
181 274 6 41 8 14 85 1 −80.6 −11.1
a
Vienna standard mean ocean water
Hydrogeology Journal (2014) 22: 1233–1249
DOI 10.1007/s10040-014-1165-6
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Fracture set name
Orientation set pole: (trend, plunge), conc.
(m, −)
(m.a.s.l)a
FFM01 and FFM06>−200
FFM01 and FFM06 –200 to −400 FFM01 and FFM06−200
FFM03, FFM04 and FFM05>−400
FFM03, FFM04 and FFM05−200 −200 to −400
