Simulating the impact of glaciations on continental groundwater flow systems: 1. Relevant processes and model formulation

Abstract

[1] In the recent literature, it has been shown that Pleistocene glaciations had a large impact on North American regional groundwater flow systems. Because of the myriad of complex processes and large spatial scales involved during periods of glaciation, numerical models have become powerful tools to examine how ice sheets control subsurface flow systems. In this paper, the key processes that must be represented in a continental-scale 3-D numerical model of groundwater flow during a glaciation are reviewed, including subglacial infiltration, density-dependent (i.e., high-salinity) groundwater flow, permafrost evolution, isostasy, sea level changes, and ice sheet loading. One-dimensional hydromechanical coupling associated with ice loading and brine generation were included in the numerical model HydroGeoSphere and tested against newly developed exact analytical solutions to verify their implementation. Other processes such as subglacial infiltration, permafrost evolution, and isostasy were explicitly added to HydroGeoSphere. A specified flux constrained by the ice sheet thickness was found to be the most appropriate boundary condition in the subglacial environment. For the permafrost, frozen and unfrozen elements can be selected at every time step with specified hydraulic conductivities. For the isostatic adjustment, the elevations of all the grid nodes in each vertical grid column below the ice sheet are adjusted uniformly to account for the Earth's crust depression and rebound. In a companion paper, the model is applied to the Wisconsinian glaciation over the Canadian landscape in order to illustrate the concepts developed in this paper and to better understand the impact of glaciation on 3-D continental groundwater flow systems.

Glacially generated overpressure on the New England continental shelf: Integration of full‐waveform inversion and overpressure modeling

“…The coefficient ζ in equation (6) is the 1-D loading efficiency, which defines the proportion of surface loading that is transferred to subsurface overpressure [Ingebritsen et al, 2007;Lemieux et al, 2008]. This is similar to the three-dimensional pore pressure buildup coefficient (B) defined by Green and Wang [1986] as the change in pore pressure per unit change in applied stress.…”

“…Person et al, 2007]. Many studies have modeled additional effects of ice sheets on fluid flow such as flexure of the crust and the formation of permafrost Bense and Person, 2008;Lemieux et al, 2008]; because we are only concerned with a 1-D model, and our study region is near the edge of the ice sheet, we do not account for flexure. Permafrost can reduce the permeability beneath and beyond the extent of the ice sheet.…”

Simulation of groundwater age evolution during the Wisconsinian glaciation over the Canadian landscape

“…6). Key processes pertaining to groundwater flow modelling during a glaciation period such as hydromechanical loading, subglacial infiltration, isostasy, sea-level change and permafrost development are also included in the model (see [29,30] for details).…”

“…Groundwater age, which can be defined as the time elapsed since the water infiltrated in a recharge zone, is a measure of the residence time of water in the subsurface and can be used to track the recharge history of an aquifer. The numerical model HydroGeoSphere [27,29,52] is used to simulate three-dimensional groundwater age evolution during the Wisconsinian glaciation over the Canadian landscape and up to a depth of 10 km. The objective is to delineate the recharge history of glacial meltwater into the subsurface during the last glaciation period and to better understand the impact of glaciations on the evolution of deep groundwaters.…”

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.…”