Subglacial Groundwater Flow Research Articles

Groundwater Flow Under a Paleo‐Ice Stream of the Scandinavian Ice Sheet and Its Implications for the Formation of Stargard Drumlin Field, NW Poland

AbstractSubglacial groundwater flow was an integral part of glaciological systems of past ice sheets, but its impact on the origin of active‐ice landforms remains unexplored. Using numerical experiments, we attempt to constrain groundwater flow dynamics under a major paleo‐ice stream of the southern Scandinavian Ice Sheet and its impact on the formation of the Stargard drumlin field. Flow models show a total reorganization of groundwater dynamics under the advancing ice stream to a depth of up to ~200 m. A mosaic of intervening groundwater recharge and discharge areas originates, whereby the same areas may experience multiple shifts in flow directions. A prominent time‐ and space‐transgressive pressure pump recharges groundwater in a subglacial zone up to about 20 km within the ice margin and discharges it in front of the ice sheet. The simulated groundwater flow pattern suggests that drumlins occupying a portion of the ice stream area occur preferentially where groundwater upwells and discharges at the ice/bed interface. Consistent with the geological composition, a possible drumlin‐forming mechanism involves an excess of pressurized water under the ice reducing the strength of the subglacial deposits and facilitating glacial erosion, erosion by turbulent meltwater flows or both, streamlining antecedent deposits into drumlin shapes. This study emphasizes the importance of causal feedback between subglacial groundwater flow and the ice sheet dynamics and suggests an impact of the former on the formation of streamlined subglacial landforms in general.

Representing Glaciations and Subglacial Processes in Hydrogeological Models: A Numerical Investigation

The specific impact of glacial processes on groundwater flow and solute transport under ice-sheets was determined by means of numerical simulations. Groundwater flow and the transport of δ18O, TDS, and groundwater age were simulated in a generic sedimentary basin during a single glacial event followed by a postglacial period. Results show that simulating subglacial recharge with a fixed flux boundary condition is relevant only for small fluxes, which could be the case under partially wet-based ice-sheets. Glacial loading decreases overpressures, which appear only in thick and low hydraulic diffusivity layers. If subglacial recharge is low, glacial loading can lead to underpressures after the retreat of the ice-sheet. Isostasy reduces considerably the infiltration of meltwater and the groundwater flow rates. Below permafrost, groundwater flow is reduced under the ice-sheet but is enhanced beyond the ice-sheet front. Accounting for salinity-dependent density reduces the infiltration of meltwater at depth. This study shows that each glacial process is potentially relevant in models of subglacial groundwater flow and solute transport. It provides a good basis for building and interpreting such models in the future.

THE FACIES ASSOCIATION TILLITE, BOULDER BEDS, BOULDER PAVEMENTS, LIQUEFACTION STRUCTURES AND DEFORMED DRAINAGE CHANNELS IN THE PERMO-CARBONIFEROUS DWYKA GROUP, ELANDSVLEI AREA, SOUTH AFRICA

The Elandsvlei area is situated in the western part of the glaciomarine Karoo basin. A previous investigation established the stratigraphic succession of six cyclic units. Each cycle consists of a basal tillite capped by a retreat facies association. The basal tillite units are characterized by the recurrence of the facies association tillite, boulder beds, boulder pavements and deformed drainage channels. In some basal tillite units liquefaction structures are present. Understanding this facies association in glaciological context is the purpose of this investigation. Systematic follow-up fieldwork focusing on relevant field relationships, was conducted. The observations were compiled and interpreted utilizing the more recent advances made by the theoretical modeling of subglacial groundwater flow and drainage. The facies association is shown to have common cause with respect to the subglacial deforming-bed process. Evidence is presented showing that boulder pavements form at the ice-bed interface by the abrasion of hard rock on hard rock. A simple shear model is utilized to show that deforming-bed shearing can produce displacement without the development of discrete, visible slip surfaces. The deformation of drainage channel-fills is interpreted as the manifestation of subglacial shearing. The liquefaction structures are explained by a water pressure differential between the ice-bed interface and the boulder beds. Sticky spots that may have been streamlined as drumlins or mega-scale glacial lineations, are related to variable drainage development along one particular stratigraphic horizon. The boulders’ composition indicate that the onset area of the Dwyka glaciers, due eastwards, is underlain by a Neoproterozoic igneous province now covered by younger platform sediments.

Tunnel valleys: Concepts and controversies — A review

Although erosion of valleys by subglacial meltwater was first proposed more than 100years ago, the processes that produced these valleys under past ice sheets have remained a topic of vigorous debate. The most commonly hypothesized mechanisms have included gradual formation by sediment deformation into the incipient valleys under steady state conditions and subsequent transport by meltwater to the ice margin; time-transgressive formation along a retreating ice margin by drainage of surface meltwater to the bed or release of impounded meltwater behind a marginal permafrost wedge, probably catastrophically; and rapid erosion during widespread, catastrophic basal sheetflood events.Recent application of new and enhanced imaging techniques has led to the identification of many new tunnel valleys both on land and offshore throughout the regions glaciated during former ice ages. In addition, new advances in understanding of subglacial hydrology, both from modelling and field instrumentation have helped to constrain the drainage of meltwater from past ice sheet beds. This work elucidates the role of subglacial groundwater flow systems in evacuating meltwater from the bed, but also shows that groundwater flow alone was not enough to maintain ice bed meltwater pore pressures below the level of ice flotation. Both modelling and field observations indicate that subglacial conduits oriented parallel to glacial flow lines reorganize groundwater flow systems into basins containing lateral flow toward conduits cut upward into the ice or downward into the bed. Because of the inability of groundwater flow to account for all the meltwater produced by basal melting, other drainage mechanisms involving both non-channelized and channelized meltwater flow have been proposed. The notion of catastrophic drainage of impounded basal meltwater is still favored by many, and the discovery of hundreds of subglacial lakes beneath the Antarctic Ice Sheet and the likelihood of similar lakes beneath Pleistocene ice sheets provides a source of water for those advocating catastrophic formation for tunnel valleys. Proponents of catastrophic drainage consider the valleys to be channels reflecting the width and perhaps depths of flows that formed them.Tunnel valleys produced by the Scandinavian Ice Sheet have been described and mapped on land as well as on the sea bed. Valleys and networks of valleys, some of which are totally buried, have been linked to 3 or more glaciations. Tunnel valley networks from the last (Weichselian) glaciation in Denmark can be related to specific ice margins developed during ice retreat. In North America, most research has focused on tunnel valleys associated with ice margins formed by the last (Wisconsin) glaciation along the southern margin of the Laurentide Ice Sheet.

Glacier‐bed geomorphic processes and hydrologic conditions relevant to nuclear waste disposal

AbstractCharacterizing glaciotectonic deformation, glacial erosion and sedimentation, and basal hydrologic conditions of ice sheets is vital for selecting sites for nuclear waste repositories at high latitudes. Glaciotectonic deformation is enhanced by excess pore pressures that commonly persist near ice sheet margins. Depths of such deformation can extend locally to a few tens of meters, with depths up to approximately 300 m in exceptional cases. Rates of glacial erosion are highly variable (0.05–15 mm a−1), but rates <1 mm a−1 are expected in tectonically quiescent regions. Total erosion probably not exceeding several tens of meters is expected during a glacial cycle, although locally erosion could be greater. Consolidation of glacial sediments that is less than expected from independent estimates of glacier thickness indicates that heads at the bases of past ice sheets were usually within 30% of the floatation value. This conclusion is reinforced by direct measurements of water pressure beneath portions of the West Antarctic ice sheet, which indicate average heads <7 m below floatation. Landforms of the Laurentide and Scandinavian ice sheets and recent observations in Greenland indicate that high seasonal discharges of surface water are conducted to the bed, despite thick ice at subfreezing temperatures. Therefore, in models of subglacial groundwater flow used to assess sites for nuclear waste repositories, a flux upper boundary condition based on water input from only basal melting will be far more uncertain than applying a hydraulic head at the upper boundary set equal to a large fraction of the floatation value.

Meltwater discharge through the subglacial bed and its land‐forming consequences from numerical experiments in the Polish lowland during the last glaciation

AbstractNumerical experiments suggest that the last glaciation severely affected the upper lithosphere groundwater system in NW Poland: primarily its flow pattern, velocities and fluxes. We have simulated subglacial groundwater flow in two and three spatial dimensions using finite difference codes for steady‐state and transient conditions. The results show how profoundly the ice sheet modifies groundwater pressure heads beneath and some distance beyond the ice margin. All model runs show water discharge at the ice forefield driven by ice‐sheet‐thickness‐modulated, down‐ice‐decreasing hydraulic heads. In relation to non‐glacial times, the transient 3D model shows significant changes in the groundwater flow directions in a regionally extensive aquifer ca. 90 m below the ice–bed interface and up to 40 km in front of the glacier. Comparison with empirical data suggests that, depending on the model run, only between 5 and 24% of the meltwater formed at the ice sole drained through the bed as groundwater. This is consistent with field observations documenting abundant occurrence of tunnel valleys, indicating that the remaining portion of basal meltwater was evacuated through a channelized subglacial drainage system. Groundwater flow simulation suggests that in areas of very low hydraulic conductivity and adverse subglacial slopes water ponding at the ice sole was likely. In these areas the relief shows distinct palaeo‐ice lobes, indicating fast ice flow, possibly triggered by the undrained water at the ice–bed interface. Owing to the abundance of low‐permeability strata in the bed, the simulated groundwater flow depth is less than ca. 200 m. Copyright © 2009 John Wiley & Sons, Ltd.

Groundwater Flow and Transport Modelling at Äspö for a Glaciation Period

This study [1] was performed for SKB (Swedish Nuclear Fuel and Waste Management Co) within the framework of the assessment of the long-term safety of a deep repository for spent nuclear fuel.Subsequent to work performed by [5], SKB decided to carry out additional hydrogeological modelling studies related to glaciation effects at Äspö. In particular, subglacial groundwaterflow and the impact assessment on a repository required further studies. The presence of subglacial groundwater flow beneath ice sheets is due to basal melting occurring during glaciation periods.As compared to the previous model [5], the domain geometry and processes involved remained identical but this time numerical calculations are performed with the NAMMU program. Modified assumptions corresponding to specific boundary conditions were implemented and additional variations of the base case were simulated.

Abstract: Subglacial groundwater flow during the last glaciation in the North Sea

Abstract: Subglacial groundwater flow during the last glaciation in the North Sea

Subglacial groundwater flow during the last glaciation in northwestern Germany

Reconstruction of groundwater flow in overpressured soft sediments overridden by the last (Weichselian) ice sheet in northwestern Germany about 18,000 ka B.P, based on numerical modelling, reveals a complete reorganisation of groundwater dynamics as compared to the present system. Groundwater flow, controlled by potentiometric surface high up in the ice sheet and by a steep hydraulic gradient at the glacier periphery, was here up to 30 times faster than at present and a vertical flow component penetrated the entire thickness of Quaternary sediments reaching some 200 m in places.

Groundwater flow beneath ice sheets: Part II — Its impact on glacier tectonic structures and moraine formation

Meltwater flowing as groundwater from beneath the margin of an ice sheet determines the distribution of sub-surface heads and effective pressures. A subglacial groundwater flow model is used, together with an ice sheet loading model, to compute the magnitudes and directions of the principal effective stresses in the subsurface, from which the distribution of different types of sub-surface failure in the subglacial and proglacial zones are deduced. Zones of hydrofracturing, shear fracture and pervasive shear failure are distinguished. Beneath the ice sheet divide area, intact rocks of high tensile strength may fail. Hydrofracturing and liquefaction are two coupled processes which lead to the formation of upward-filled and downward-filled sediment dykes and till wedges. Quicksand conditions are developed where strong vertical seepage pressures occur, producing sediment diapirism. It is suggested that subglacial permeability magnitude may be the product of a self organising process. Certain types of moraine (extrusion moraines) are suggested to be a consequence of upward movement and surface extrusion of sediment driven by rising groundwater. It is suggested that groundwater over pressure associated with narrow proglacial permafrost plates are conductive to the formation of large push moraines, and that many large ancient and modern examples are produced in this setting.

Consolidation of sediments by glaciers: relations between sediment geotechnics, soft-bed glacier dynamics and subglacial ground-water flow

AbstractA theory of subglacial consolidation is developed which shows how the meltwater flux beneath a glacier leaves a consolidation signature from which many important glacier-dynamic properties can be inferred. Conditions under which water is discharged through channels or in a thin film at the ice—bed interface or by ground-water flow are discussed, and it is concluded that glaciers flowing over aquifers of high transmissibility can induce deep circulation patterns quite different from non-glacial circulation. Examples of glacial pre-consolidation profiles in sediments from The Netherlands and England are used to illustrate how basal melting rates, subglacial ground-water flow patterns, ice overburden, basal shear stress, ice-surface profile and the amount of sediment removed by erosion can be inferred.

Consolidation of sediments by glaciers: relations between sediment geotechnics, soft-bed glacier dynamics and subglacial ground-water flow

AbstractA theory of subglacial consolidation is developed which shows how the meltwater flux beneath a glacier leaves a consolidation signature from which many important glacier-dynamic properties can be inferred. Conditions under which water is discharged through channels or in a thin film at the ice—bed interface or by ground-water flow are discussed, and it is concluded that glaciers flowing over aquifers of high transmissibility can induce deep circulation patterns quite different from non-glacial circulation. Examples of glacial pre-consolidation profiles in sediments from The Netherlands and England are used to illustrate how basal melting rates, subglacial ground-water flow patterns, ice overburden, basal shear stress, ice-surface profile and the amount of sediment removed by erosion can be inferred.