Simulating the impact of glaciations on continental groundwater flow systems: 2. Model application to the Wisconsinian glaciation over the Canadian landscape

Abstract

[1] A 3-D groundwater flow and brine transport numerical model of the entire Canadian landscape up to a depth of 10 km is constructed in order to capture the impacts of the Wisconsinian glaciation on the continental groundwater flow system. The numerical development of the model is presented in the companion paper of Lemieux et al. (2008b). Although the scale of the model prevents the use of a detailed geological model, commonly occurring geological materials that exhibit relatively consistent hydrogeological properties over the continent justify the simplifications while still allowing the capture of large-scale flow system trends. The model includes key processes pertaining to coupled groundwater flow and glaciation modeling, such a densitydependent (i.e., brine) flow, hydromechanical loading, subglacial infiltration, isostasy, and permafrost development. The surface boundary conditions are specified with the results of a glacial system model. The significant impact of the ice sheet on groundwater flow is evident by increases in the hydraulic head values below the ice sheet by as much as 3000 m down to a depth of 1.5 km into the subsurface. Results also indicate that the groundwater flow system after glaciation did not fully revert to its initial condition and that it is still recovering from the glaciation perturbation. This suggests that the current groundwater flow system cannot be interpreted solely on the basis of present-day boundary conditions and it is likely that several thousands of years of additional equilibration time will be necessary for the system to reach a new quasi-steady state. Finally, we find permafrost to have a large impact on the rate of dissipation of high hydraulic heads that build at depth and capturing its accurate distribution is important to explain the current hydraulic head distribution across the Canadian landscape.

Revised conceptualization of the North China Basin groundwater flow system: Groundwater age, heat and flow simulations

“…In the absence of information allowing transient simulation of the flow system on geological time scales, groundwater residence time indicators in the aquifer can be used to constrain the time scales relevant to the groundwater currently existing in the aquifer, and help to understand the evolution of the system (e.g., Morrissey et al, 2010). Combining such information with groundwater age simulations may prove a powerful new approach to explore the dynamics and evolution of regional groundwater flow systems over geological timescales (e.g., Lemieux et al, 2008;Gupta et al, 2015). Groundwater age (or residence time) is the average time elapsed since recharge of the range of water molecules present in a sample, integrating the effects of advection, hydrodynamic dispersion and mixing (Goode, 1996;Bethke and Johnson, 2002;Massoudieh et al, 2012).…”

“…Using topography solely may lead to misunderstanding of groundwater flow system dynamics and thus problematic conceptualizations. It is therefore necessary to integrate information with respect to all of the controlling factors-including heterogeneous aquifer properties (e.g., Freeze and Witherspoon, 1967), influence of geological structure, and changes in boundary conditions due to geological and geomorphological evolution (e.g., Lemieux et al, 2008)-to more realistically evaluate the groundwater flow regime.…”

Modelling water flow under glaciers and ice sheets

“…Recent work has seen increased use of three-dimensional models [151,152], though compromises are still necessary at the largest scales. In their investigation of groundwater flow beneath the North American ice sheets, Lemieux et al [160,165] use a coarse classification of bedrock geology as one of four types (oceanic crust, orogenic belt, Canadian shield or sedimentary rocks) in order to implement their continentalscale model in HydroGeoSphere. Defining the subsurface hydrostratigraphy (thickness and properties of each hydrogeologic unit) is the only place that data figure prominently in most studies (cf.…”

“…Unless the model domain extends to a hydraulic divide, some upstream boundary condition is required and often takes the form of a prescribed flux (Neumann condition). Most models assume temperate basal conditions, though the potential significance of permafrost, particularly near the ice-sheet margin and beyond, is acknowledged [157] and has been explored through prescribed model boundary conditions [152] as well as coupled modelling of ice-sheet-permafrost dynamics [159,160]). …”

“…The motivation for such studies is varied, ranging from understanding basal effective pressure, till deformation and landform development [148,149] to assessing the influence of glaciation and permafrost on groundwater recharge, circulation and geochemistry [150][151][152][153]. The first theoretical models were hatched with the Pleistocene ice sheets in mind [144] ( figure 3); geographically referenced model applications followed, first to the paleo ice sheets of Europe [154][155][156][157] and then North America [151,[158][159][160][161]. Some of this work has been motivated by the prospective development of deep geologic repositories for nuclear waste [162,163], where the influence of glaciation on subsurface hydrogeology is important in terms of aquifer properties, groundwater geochemistry, saltwaterfreshwater interactions and reorganization of groundwater flow (see Person et al [164] for a review).…”