Modeling of groundwater flow at depth in crystalline rock beneath a moving ice-sheet margin, exemplified by the Fennoscandian Shield, Sweden

On-going geological disposal programs for spent nuclear fuel have generated strong demands for investigation and characterization of deep-lying groundwater systems. Because of the long time scales for which radiological safety needs to be demonstrated in safety assessment applications, an analysis of the hydrogeological performance of the geosphere system during glacial climate conditions is needed. Groundwater flow at depth in crystalline rock during the passage of an ice-sheet margin is discussed based on performed groundwater-flow modeling of two bedrock sites, Forsmark and Laxemar, in the Fennoscandian Shield, Sweden. The modeled ice sheet mimics the Weichselian ice sheet during its last major advance and retreat over northern Europe. The paper elaborates and analyzes different choices of top boundary conditions at the ice sheet–subsurface interface (e.g. ice-sheet thickness and ice-margin velocity) and in the proglacial area (presence or lack of permafrost) and relates these choices to available groundwater-flow-model hydraulic output and prevailing conceptual hydrogeochemical models of the salinity evolution at the two sites. It is concluded that the choice of boundary conditions has a strong impact on results and that the studied sites behave differently for identical boundary conditions due to differences in their structural-hydraulic properties.     

Effects on groundwater flow of abandoned engineered structures for the safety assessment of the proposed high-level nuclear waste repository site at Forsmark,Sweden

Effects on groundwater flow of abandoned engineered structures in relation to a potential geological repository for spent high-level nuclear fuel in fractured crystalline rock at the Forsmark site, Sweden, are studied by means of numerical modeling. The effects are analyzed by means of particle tracking, and transport-related performance measures are calculated. The impacts of abandoned, partially open repository tunnels are studied for two situations with different climate conditions: a “temperate” climate case with present-day boundary conditions, and a generic future “glacial” climate case with an ice sheet covering the repository. Then, the impact of abandoned open boreholes drilled through the repository is studied for present-day climate conditions. It is found that open repository tunnels and open boreholes can act as easy pathways from repository level to the ground surface; hence, they can attract a considerable proportion of particles released in the model at deposition hole positions within the repository. The changed flow field and flow paths cause some changes in the studied performance measures, i.e., increased flux at the deposition holes and decreased transport lengths and flow-related transport resistances. However, these effects are small and the transport resistance values are still high.     

Exchange and pathways of deep and shallow groundwater in different climate and permafrost conditions using the Forsmark site, Sweden, as an example catchment

This study simulates and quantifies the exchange and the pathways of deep and shallow groundwater flow and solute transport under different climate and permafrost conditions, considering the example field case of the coastal Forsmark catchment in Sweden. A number of simulation scenarios for different climate and permafrost condition combinations have been performed with the three-dimensional groundwater flow and transport model MIKE SHE. Results show generally decreasing vertical groundwater flow with depth, and smaller vertical flow under permafrost conditions than under unfrozen conditions. Also the overall pattern of both the vertical and the horizontal groundwater flow, and the water exchange between the deep and shallow groundwater systems, change dramatically in the presence of permafrost relative to unfrozen conditions. However, although the vertical groundwater flow decreases significantly in the presence of permafrost, there is still an exchange of water between the unfrozen groundwater system below the permafrost and the shallow groundwater in the active layer, via taliks. ‘Through taliks’ tend to prevail in areas that constitute groundwater discharge zones under unfrozen conditions, which then mostly shift to net recharge zones (through taliks with net downward flow) under permafrost conditions.     

北京西山潭柘寺地区第四纪冰川与环境问题

第四纪时期,北京西山潭柘寺地区没有发生过任何山地冰川作用。所渭的冰川遗迹、冰碛、冰水沉积及各种冰蚀地貌,实际上是不存在的。因此,所渭的冰期划分也都是不可靠的。在更新世冰期阶段,北京西山属冰缘环境,而非冰川环境。     

Analysis of groundwater flow through low-latitude alpine permafrost by model simulation: a case study in the headwater area of Yellow River on the Qinghai-Tibet Plateau,China

Warming climate and thawing permafrost have profound impacts on groundwater flow regimes in cold regions because of the shrinkage or disappearance of the confining unit formed by the permafrost layers and improving hydraulic connections. Numerical simulations of coupled groundwater flow and heat transfer are often used to characterize the changing permafrost hydrogeology. In this study, a number of scenarios for different hydraulic gradients and lake-water depths have been used to simulate the concordant permafrost evolution and groundwater movement using a two-dimensional cylindrical coordinate model at time scales of decades to centuries in response to a warming climate. The model is applied to a representative headwater catchment in the south-central headwater area of the Yellow River on the northeastern Qinghai-Tibet Plateau, China. The results show that the presence and movement of groundwater and the deeper subpermafrost aquifer can substantially accelerate permafrost degradation, and the disappearance of residual permafrost at depth can result in the sudden establishment of deep groundwater flow paths. All hydrological impacts will become evident after the stabilization of the hydrothermal and flow fields at 100–200 years. The stable discharge rate of groundwater flow varies from 8.0 to 12.4 m³ s⁻¹, and the stable velocity of groundwater flow varies from 1.6 × 10⁻⁷ to 4.4 × 10⁻⁷ m s⁻¹ under different scenarios within the model domain. The modeling results also demonstrate that flow velocity and discharge rate in local groundwater flow systems can be enhanced by an increased hydraulic conductivity, leading to an accelerated degradation of isolated permafrost bodies.

     

Implications of subsurface thermal–hydraulic–mechanical processes associated with glaciation on Shield flow system evolution and performance assessment

A deep geologic repository (DGR) situated on the Canadian Shield will be subject to long-term climate change that will markedly alter surface conditions as a result of glaciation and permafrost penetration. Systematic, two-dimensional and three-dimensional coupled thermal–hydraulic–mechanical finite-element simulations with varying degrees of coupling, including depth-dependent salinity (represented as a change in groundwater density) and temperature-dependent density and viscosity, were undertaken to address the implications of glaciation on groundwater flow system dynamics as it could affect DGR performance. The modelling domain consisted of a 1.6-km deep sub-regional scale (≈100 km²) fractured Shield flow system. Initial and transient thermal, hydraulic and mechanical boundary conditions were developed from two realizations of the University of Toronto Glacial Systems Model of the last Laurentide glaciation. Results indicate that during the glacial loading/unloading cycle, for this particular conceptual model, there is limited penetration of glacial meltwaters to depth and small residual anomalous hydraulic head. During glacial coverage, the mechanical factor of safety increases in the moderately fractured and sparsely fractured rock mass, but principal effective stress reorientation also occurs. Given the assumed nonglacial in situ state of stress and mechanical properties, the fracture zones were predicted to be less stable under glacial conditions.

A 40 ka record of temperature and permafrost conditions in northwestern Europe from noble gases in the Ledo‐Paniselian Aquifer (Belgium)

The Ledo‐Paniselian Aquifer in Belgium offers unique opportunities to study periglacial groundwater recharge during the Last Glacial Maximum (LGM), as it was located close to the southern boundary of the ice sheets at that time. Groundwater residence times determined by ¹⁴C and ⁴He reveal a sequence of Holocene and Pleistocene groundwaters and a gap between about 14 and 21 ka, indicating permafrost conditions which inhibited groundwater recharge. In this paper, a dataset of noble gas measurements is used to study the climatic evolution of the region. The derived recharge temperatures indicate that soil temperatures in the periods just before and after the recharge gap were only slightly above freezing, supporting the hypothesis that permafrost caused the recharge gap. The inferred glacial cooling of 9.5°C is the largest found so far by the noble gas method. Yet, compared to other palaeoclimate reconstructions for the region, recharge temperatures deduced from noble gases for the cold periods tend to be rather high. Most likely, this is due to soil temperatures being several degrees higher than air temperatures during periods with extended snow cover. Thus the noble‐gas‐derived glacial cooling of 9.5°C is only a lower limit of the maximum cooling during the LGM. Some samples younger than the recharge gap are affected by degassing, possibly related to gas production during recharge in part of the recharge area, especially during times of melting permafrost. The findings of this study, such as the occurrence of a recharge gap and degassing related to permafrost and its melting, are significant for groundwater dynamics and geochemistry in periglacial areas. Copyright © 2010 John Wiley & Sons, Ltd.     

Periglacial climate at the 2.5 Ma onset of Northern Hemisphere glaciation inferred from the Whippoorwill Formation, northern Missouri, USA

The Whippoorwill Formation is a gleyed diamicton that is present locally within bedrock depressions beneath the oldest glacial till in northern Missouri, USA. Stratigraphy, paleomagnetism, and cosmogenic-nuclide burial ages show that it was deposited between the Matuyama-Gauss magnetostratigraphic boundary at 2.58 Ma and the first advance of the Laurentide ice sheet into Missouri at 2.47 ± 0.19 Ma. High cosmogenic-nuclide concentrations also show that the constituents of the Whippoorwill Formation experienced long exposure at a stable landscape surface with erosion rates of 1-2 m/Ma. However, cosmogenic-nuclide concentrations are invariant with depth below the Whippoorwill Formation surface, indicating active mixing of the soil profile shortly before burial by till. The Whippoorwill Formation retains numerous features indicative of cryoturbation. Therefore, we interpret it as a buried Gelisol, a soil formed under periglacial conditions in the presence of permafrost. At the onset of Northern Hemisphere glaciation, climate cooling established permafrost conditions and accelerated erosion by inducing landscape instability. Thus, weathered regolith materials were mobilized and redeposited by gelifluction shortly before the ice sheet overrode the landscape.     

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

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.     

Review: Groundwater in Alaska (USA)

Groundwater in the US state of Alaska is critical to both humans and ecosystems. Interactions among physiography, ecology, geology, and current and past climate have largely determined the location and properties of aquifers as well as the timing and magnitude of fluxes to, from, and within the groundwater system. The climate ranges from maritime in the southern portion of the state to continental in the Interior, and arctic on the North Slope. During the Quaternary period, topography and rock type have combined with glacial and periglacial processes to develop the unconsolidated alluvial aquifers of Alaska and have resulted in highly heterogeneous hydrofacies. In addition, the long persistence of frozen ground, whether seasonal or permanent, greatly affects the distribution of aquifer recharge and discharge. Because of high runoff, a high proportion of groundwater use, and highly variable permeability controlled in part by permafrost and seasonally frozen ground, understanding groundwater/surface-water interactions and the effects of climate change is critical for understanding groundwater availability and the movement of natural and anthropogenic contaminants.     

Ice wedge casts and past permafrost distribution in North AmericaFossile eiskeile und dauerfrost in der vergangenheit in NordamerikaPseudomorphoses des coins de glace et distribution du permafrost au passé en Amérique du Nord

Ice wedge casts are the most accurate and widespread indicators of past permafrost. Many ice wedge casts exist in Alaska, some in areas of existing ice wedges. In addition to indicating paleotemperature conditions and a wider distribution of permafrost in Wisconsinan time than now, casts in Alaska also indicate permafrost in Iliinoian and pre-lllinoian time. Hundreds of ice wedge casts are now known in temperate North America and are described from about 22 widespread localities coast to coast in Canada and United States. Permafrost existed in late Wisconsinan time, 20,000 to 10,000 years ago, along the glacial border in temperate United States. Later permafrost formed north of the glacial border as the continental ice sheet withdrew exposing drift to the rigorous periglacial climate. Ice wedge casts indicate that the ? 7 °C mean annual air isotherm was about 2000 km farther south in late Wisconsinan time than now.     

大兴安岭南段赛罕乌拉石海剖面分析及其与蒙山拦马墙石河结构对比

石海（石河/冰石河）作为冰缘地貌的重要标志之一,在国内地学界尚未引起足够的重视,以至于缺少介绍石海结构特征的中文文献,更未建立判别标准,在地貌认知阶段就发生很多误判。为了揭示石海的剖面结构特征,笔者等对位于大兴安岭南段的赛罕乌拉国家级自然保护区乌兰坝石海首次进行了开挖解剖,从而归纳了原生石海的基本结构特征：岩块棱角鲜明,呈镂空状堆积,岩块直径总体上上大下小。根据野外调查确认,赛罕乌拉海拔1800 m以上的区域至少还存在着局部现代石海,也就是说,还残存着不连续冻土带,而且保持年平均气温0℃左右的气候环境。根据赛罕乌拉现今气候记录和石海分布,笔者等估算了全新世初期赛罕乌拉石海开始发育时的地表温度在0~-4℃,而今天年平均气温已经升高到2℃,升温幅度2~6℃。根据赛罕乌拉石海剖面结构特征,笔者等对山东蒙山石河进行了结构对比,认为二者剖面结构基本一致,排除了前人近年来争议明显的冰川侧碛堤和泥石流成因说。结构对比揭示山东蒙山境内全新世早期至少局部存在过冰缘气候环境,而这也为山东境内更新世冰川地貌研究提供了重要的信息。此外,本研究也为中国冰缘地貌解剖和资源调查提供了一个可以参考的研究实例。     

Periglacial structures within fluvio‐aeolian successions of the end of the Last Glaciation – examples from SE Poland and NW Ukraine

The nature of permafrost and related environmental conditions in the Weichselian Late Pleniglacial and Lateglacial are reconstructed based on the assessment of frost structures that are best documented in the Loess Belt and in plateau areas composed of glacial till. Investigations were conducted in the central‐eastern part of the European Sand Belt (SE Poland and NW Ukraine) on a fluvio‐aeolian sedimentary succession and took into account its chronological context given by luminescence dating. Various generations of periglacial structures found in these deposits indicate not only the development of permafrost (ice‐wedge pseudomorphs) and decreased humidity (composite wedge casts) but also the degradation of permafrost (large‐scale involutions) and, finally, the establishment of deep seasonal frost (frost cracks). The diversity of structures in the study region appears to result from local conditions rather than increasing continentality of climate towards the east.     

中国北方冰缘与分期

在描述和分析中国北方残存古冰缘遗迹的基础上,把第四纪晚期不同期冰缘在同一剖面内叠置的结构划分为两种类型:山地剖面冰缘结构型和平原（或高原）剖面冰缘结构型。两种类型的剖面都具有冰缘分期意义,但以平原（或高原）剖面冰缘结构型的分期结果最完整。根据平原剖面冰缘结构型的对比研究和¹⁴C定年数据的综合分析,从45000年以来在中国北方存在七个冰缘期。它们出现的时间顺序为:45000 aB.P.、37000—35000 aB.P.、25000—13000 aB.P.、9500—9000 aB.P.、5500aB.P.前后、2395—1970 aB.P.和1350—1495 A.D.。因此,冰缘现象证明45000年以来在中国北方有七次明显变冷过程,即有七个旋回的冷暖交替。冰缘期与沙地中沙丘的流动期有较好的对应关系,部分地与黄土堆积的加速期相一致,并且与内陆湖泊湖面下降阶段相吻合。     

Sensitivity to long-term climate change of subpermafrost groundwater systems in Svalbard

Deep subpermafrost aquifers are highly climate-dependent, with the permafrost as an aquitard preventing groundwater recharge and discharge. A study from the high-arctic island of Spitsbergen, Svalbard, shows that during a glacial to interglacial phase, both the permafrost and the glacier regime will respond to climatic changes, and a glacier-fed groundwater flow system will vary accordingly. A full glaciation results in the melting of permafrost, and groundwater can flow through pores and fracture systems in the rocks and sediments below the temperate zones of glaciers. These groundwater flow systems will mainly be localized to fjords and valleys and form low-lying terrestrial springs when the relative sea level drops during deglaciation due to glacio-isostatic rise. During an interglaciation, permafrost develops and thickens and the groundwater recharge and discharge areas will thereby be gradually reduced to a minimum reached at the warmest part of an interglaciation. An already frozen spring system cannot reopen before the permafrost melts. Only groundwater springs related to permanently warm-based glacial ice will persist into the next glaciation. During a new glaciation, flow systems that terminated during the previous interglaciation may become revitalized if overridden by warm-based ice causing permafrost thawing.     

Modeling North American Freshwater Runoff through the Last Glacial Cycle

The Northern Hemisphere ice sheets decayed rapidly during deglacial phases of the ice-age cycle, producing meltwater fluxes that may have been of sufficient magnitude to perturb oceanic circulation. The continental record of ice-sheet history is more obscured during the growth and advance of the last great ice sheets, ca. 120,000–20,000 yr B.P., but ice cores tell of high-amplitude, millennial-scale climate fluctuations that prevailed throughout this period. These climatic excursions would have provoked significant fluctuation of ice-sheet margins and runoff variability whenever ice sheets extended to mid-latitudes, giving a complex pattern of freshwater delivery to the oceans. A model of continental surface hydrology is coupled with an ice-dynamics model simulating the last glacial cycle in North America. Meltwater discharged from ice sheets is either channeled down continental drainage pathways or stored temporarily in large systems of proglacial lakes that border the retreating ice-sheet margin. The coupled treatment provides quantitative estimates of the spatial and temporal patterns of freshwater flux to the continental margins. Results imply an intensified surface hydrological environment when ice sheets are present, despite a net decrease in precipitation during glacial periods. Diminished continental evaporation and high levels of meltwater production combine to give mid-latitude runoff values that are highly variable through the glacial cycle, but are two to three times in excess of modern river fluxes; drainage to the North Atlantic via the St. Lawrence, Hudson, and Mississippi River catchments averages 0.356 Sv for the period 60,000–10,000 yr B.P., compared to 0.122 Sv for the past 10,000 yr. High-amplitude meltwater pulses to the Gulf of Mexico, North Atlantic, and North Pacific occur throughout the glacial period, with ice-sheet geometry controlling intricate patterns of freshwater routing variability. Runoff from North America is staged in the final deglaciation, with a stepped sequence of pulses through the Mississippi, St. Lawrence, Arctic, and Hudson Strait drainages.     

内蒙古鄂尔多斯高原古冰缘遗迹科学考察研究进展

冰缘遗迹(特别是冷生楔形构造及融冻褶皱)是重建古气候及第四纪晚期多年冻土环境的重要证据.内蒙古鄂尔多斯高原是我国北方地区冰缘现象最为发育的地区之一.为准确了解鄂尔多斯高原冰缘遗迹类型及其分布特征、区域冻土演化历史等,中国科学院西北生态环境资源研究院和荷兰自由大学共同组成科研小组,于2018年5—6月组织了"鄂尔多斯高原...     

Glacial and periglacial buzzsaws: fitting mechanisms to metaphors

The buzzsaw hypothesis refers to the potential for glacial and periglacial processes to rapidly denude mountains at and above glacier Equilibrium Line Altitudes (ELAs), irrespective of uplift rates, rock type or pre-existing topography. Here the appropriateness of the buzzsaw metaphor is examined alongside questions of the links between glacial erosion and ELAs, and whether the glacial system can produce low-relief surfaces or limit summit heights. Plateau fragments in mountains on both active orogens and passive margins that have been cited as products of glacial and periglacial buzzsaw erosion instead generally represent dissected remnants of largely inherited, pre-glacial relief. Summit heights may correlate with ELAs but no causal link need be implied as summit erosion rates are low, cirque headwalls may not directly abut summits and, on passive margins, cirques are cut into pre-existing mountain topography. Any simple links between ELAs and glacial erosion break down on passive margins due to topographic forcing of ice-sheet growth, and to the km-scale vertical swaths through which ELAs have shifted through the Quaternary. Glaciers destroy rather than create low-relief rock surfaces through the innate tendency for ice flow to be faster, thicker and warmer along valleys. The glacial buzzsaw cuts down.