5 System Evolution
5.2 Climatic, surface environmental and geological setting
The natural environment sets the boundary conditions for the SF / HLW / ILW repository constructed deep underground in the Opalinus Clay of the Zürcher Weinland. This section discusses those aspects of the evolution of the natural environment that may exert an influence on the long-term performance of the repository system.
The fundamental controls on the natural (non repository-induced) evolution of the repository site are long-term changes in climatic and geological conditions. These changes are linked – e.g.
overburden and hydrogeology are dependent on uplift/erosion and the periodic formation of ice-sheets in northern Switzerland to be expected in the future. The climate controls the long-term surface environmental conditions, particularly the erosion regime and the dilution of radio-nuclides, and may affect the hydrogeological regime (recharge).
Geological evolution affects the hydrogeological properties of the host rock through its effect on the stress regime, neotectonics, compaction, uplift/erosion, and seismic activity.
Understanding future changes in climatic and geological conditions is, therefore, important in gaining understanding of the evolution of the site. The information collected in the following sections is based on the geological synthesis report for the Opalinus Clay in northern Switzer-land (Nagra 2002a).
5.2.1 Evolution of climate, surface environment and assumptions about future human behaviour
5.2.1.1 Introduction
Future climatic developments are relevant to the evolution of the surface environment. They may also affect to some extent the evolution of the geological setting by driving erosion and groundwater movement. The climate also affects radionuclide transfer in the surface environ-ment and uptake in the food chains.
5.2.1.2 Evolution of climate
The complexity of the earth's climatic system is such that the effects of changes in any part of the system cannot be predicted accurately. However, historic monitoring data, complemented by long-term information recorded in rocks, groundwater, ice-caps and sea-sediments, can all be used to derive climate models which provide both an understanding of past climate and broad-scale predictions of future climate development (IPCC 2001).
The present-day climate of northern Switzerland is described as temperate and is representative of interglacial conditions. Since the late Pliocene and throughout the Quaternary (the last 2.6 million years), the Earth has experienced numerous cycles of cooling and warming that have led to considerable variations in northern hemisphere ice-cover ("icehouse" climate). There is strong evidence that these cycles are related to temporal and spatial changes of the radiation of the sun directly related to orbital parameters (Milankovitch cycles). Until 0.9 million years
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before present, climate changes appeared to be controlled by the 41 000-years-cycle of the orientation of Earth's axis relative to its orbit. During a relatively short period (0.9 – 0.6 million years before present), the evolution of climate seemed to be in a transient regime. Since 0.6 million years, periodic climate changes have been triggered most likely by the 100 000-years-cycling of the eccentricity of Earth's orbit.
At times of greatest cooling, the northern hemisphere has been extensively glaciated and global sea levels have fallen considerably below the present level (e.g. 100 – 120 m during the maximum of the Würm glaciation, according to Burga & Perret (1998)). Arctic sea ice was connected to continental ice caps that covered Scandinavia and much of northern Europe. The Alps have acted as the core of major ice-sheets that extended far into the foreland. The build up to full glacial conditions involved partial freezing of the uppermost part of the sediment sequence (permafrost). During interglacial times, such as the present period, ice sheets retreat until only the upper levels of mountain glaciers remain, permafrost melts and surface drainage patterns become re-established. After each glaciation, there are some changes in the locations of lakes and courses of rivers as a result of glacial and periglacial erosion and sedimentation, but the overall arrangement of the main drainage valleys is believed to have remained more or less constant since the beginning of the Pleistocene. Throughout these cycles, precipitation rates vary in response to major changes in the European and North Atlantic weather system.
Expected evolution of climate
The last glacial period ended about 10 000 years ago. The Earth is presently in the midst of an interglacial period (the Holocene), with sea-levels near their maximum. Permanent snow and ice cover and permafrost in Switzerland are limited to mountainous regions above about 2500 m.
The present climate is temperate, with significant influence from the Atlantic Ocean.
Tab. 5.2-1: Expected climatic evolution in northern Switzerland for the next one million years
There is strong evidence that within the next one million years, the climate in northern Switzerland will continue to oscillate between glacial and interglacial periods ("icehouse", see Tab. 5.2-1), at a rate of one glaciation every 100 000 years, as in the past 600 000 years. This expectation is based on the assumption that past and future human activities may perturb, but not completely alter, the long-term major climatic cycles. Recent modelling indicates that the present interglacial period may continue for about 50 000 years (Loutre & Berger 2000). This may involve periods with higher/lower temperatures, precipitation and evapotranspiration com-pared to present-day conditions. 10 major glaciations are expected to occur in the next
Climate regime Climate states Likelihood of occurrence of climate regime Interglacial
(present-day, possibly with higher/lower temperatures,
precipitation and evapotranspiration)
Periglacial climate (e.g. tundra climate) Glacial/interglacial cycling
("Icehouse")
Orbitally forced climate changes (Milankovitch cycles with a
period of 100 000 years)
Glacial (ice cover)
Very likely (expected evolution)
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one million years, with a possible ice thickness in the Zürcher Weinland of up to 400 m and an average duration of ice cover of 20 000 years. During the transition between interglacial and glacial periods, a periglacial climate (e.g. tundra climate) would develop in northern Switzer-land (Burga & Perret 1998).
It is obvious that glacial cycling will affect the geological setting, the surface environment and, especially, human activities. The effects of repeated glacial loads on the hydromechanical conditions in the Opalinus Clay are discussed in Section 5.2.2.2. The different effects of glacial cycling on the surface environment and human activities are addressed in Section 5.2.1.3 and 5.2.1.4.
Possible deviations from expected evolution of climate
Long-term global changes from the "icehouse" regime to alternative climatic regimes cannot be ruled out. Conceivable alternative climates are:
A Cessation of glaciations and transition to permanent humid temperate climate B Change to permanent humid warm climate induced by anthropogenic activity
("greenhouse")
C Cessation of glaciations and transition to permanent dry climate.
These alternative climatic regimes are unlikely to occur in the next one million years. Moreover, the possible effects of these alternative climates on the surface environment are considered to be covered by the range of climate states involved in the "icehouse" climatic regime: Type A is similar to the present-day climate and type B is similar to interglacial periods involving warmer and more humid conditions. Dry climatic conditions may occur during the transition from glacial to interglacial periods, as can be shown for the late Würm glaciation.
For this reason, the effects of alternative climatic regimes on the surface environment can be evaluated by analysis of the "icehouse" climatic regime, and the alternative climates A – C need not be investigated any further.
5.2.1.3 Evolution of the surface environment
The long-term evolution of the surface environment is far less predictable on a local scale than the evolution of the geological setting. This is because the rates of change at the surface are much larger than those deep underground and are affected by a wider range of phenomena.
Rates of change of the surface environmental characteristics depend on climate (rainfall/ evapo-transpiration), geological processes (tectonic movement, erosion), catastrophic events (e.g.
volcanic eruptions, meteorite impact), and human activities.
In northern Switzerland, deep groundwaters from regional aquifers generally discharge at the lowest levels of terrain (valley bottoms), where deep aquifers are in direct contact with gravel aquifers or surface waters (rivers, lakes). Discharge of deep groundwaters in valleys filled with impermeable lake sediments or in higher lying areas is very unlikely. In most river valleys of present-day northern Switzerland, valley sections with gravel aquifers alternate with sections where the river either flows directly on the bedrock or is embedded in impermeable lake sediments or glacial till (Fig. 5.2-1). As a consequence, river sections where significant ground-water flow occurs alternate with sections where groundground-water flow is insignificant. The distance between water table and terrain surface varies significantly along the river valley. It is greatest in zones with gravel terraces and may decrease to zero in transition zones where the river bed
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passes from gravel aquifers to impermeable ground. In these transition zones, wetlands may exist. Furthermore, valley sections where net erosion of solid materials takes place alternate with sections with net deposition.
Although deep groundwaters generally discharge into valley bottoms, it cannot be ruled out that, in special circumstances, discharge occurs into springs located at valley sides. This situation is depicted in the leftmost transverse profile in Fig. 5.2-1, where discharge from the regional aquifer does not occur in the impermeable valley infill but higher up at the side of the valley.
Fig. 5.2-1: Schematic cross-sections (rotated 90°) through the valley bottoms along a typical river valley in northern Switzerland that correspond to local geomorphological units (see Tab. 5.2-2)
It is thus important to classify the various valley types in northern Switzerland where discharge of deep groundwaters occurs today and may continue to occur in the future. These discharge areas, termed local geomorphological units for the purpose of biosphere description in the present report, are listed and described in Tab. 5.2-2.
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The mass balance of eroding river sections, such as the Rhine river in the region of interest, is characterised by a small turn-over rate and a net loss of solid materials to downstream locations.
In contrast, braided rivers, meandering rivers, river deltas, lakes and wetlands are local geo-morphological units with medium to large turn-over rates, where net deposition of solid materials takes place72.
Climate evolution controls geomorphological and hydrological conditions in the surface environment. Relevant processes affected by climate are: precipitation, evapotranspiration, glacial and fluvial erosion, river discharge, groundwater flow, capillary rise, and contaminant dilution. The relationship between climatic and geomorphological/hydrological conditions is illustrated in Tab. 5.2-3.
Tab. 5.2-2: Local geomorphological units representing possible discharge areas of deep groundwater in northern Switzerland
72 Net sedimentation of valley-infill in northern Switzerland may be limited to a certain time window in the icehouse climate regime, e.g. during the transition zone from glacial to interglacial conditions.
Local
geomor-phological unit Description Mass balance
of solid material Example Eroding river Relatively narrow, cut-in river section
where solid material balance is dominated by erosion (e.g. V-shaped valleys, gravel terraces). Eroding rivers cause linear erosion and act as regional base level for denudation.
Small turn-over
rate, net erosion Majority of Rhine river sections between Bodensee and Basel
Braided river Relatively flat network of river arms that continuously changes with time due to flooding and deposition. Meandering river Relatively flat river section with a
sequence of bends; local erosion of river banks and episodic short-cuts between
River delta in lake Sedimentation area near inflow of river to
lake. Medium/large
turn-over rate, net sedimentation
Rhine delta in Bodensee Lake (open water) Large surface water body with relatively
low turn-over rate. Lakes act as solid material sinks and significantly reduce the suspended solid load between inlet and outlet rivers. Under oligotrophic (low-nutrient) conditions, lakes are eventually filled with sediments carried by inlet river. Under eutrophic (nutrient-rich) conditions, intensive growth of plants can take place and can turn a lake into land.
Small turn-over rate, net sedimentation
Bodensee
Wetland (including
swamps) Areas with water table at or near the surface, subject to frequent flooding.
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Tab. 5.2-3: Relationship between climate and local geomorphological/hydrological conditions in the surface environment
All geomorphological units listed in Tab. 5.2-2 may exist during future interglacial periods within the region of interest. Significant dilution of radionuclides in the aqueous phase and by mixing with solid material is expected to take place in the surface environment, even for drier/warmer than present-day conditions during interglacial periods. Eroding rivers deserve special attention because of the ongoing uplift/erosion in northern Switzerland (Section 5.2.2.3).
It can be anticipated that the major drainage features, including those of present-day Rhine, Aare, Reuss, Limmat and Thur, will be partly covered by young terraced gravel bodies during
Typical
1 P is the precipitation rate and ETP is the evapotranspiration rate
2 If a lake is present upstream from the investigated biosphere area, then the majority of the suspended solid load is sedimented in the lake, and dilution by mixing with solid material is low.
3 NA = Not applicable
Under glacial conditions, population density and agricultural productivity are either zero (areas covered by ice) or low (periglacial areas). During deglaciation large redistribution of sediments occurs, resulting in significant dilution.