Review

Hydromechanical effects of continental glaciation ongroundwater systems

C. E. NEUZIL

U.S. Geological Survey, 431 National Center, Reston, VA, USA

ABSTRACT

Hydromechanical effects of continental ice sheets may involve considerably more than the widely recognized

direct compression of overridden terrains by ice load. Lithospheric flexure, which lags ice advance and retreat,

appears capable of causing comparable or greater stress changes. Together, direct and flexural loading may

increase fluid pressures by tens of MPa in geologic units unable to drain. If so, fluid pressures in low-permeability

formations subject to glaciation may have increased and decreased repeatedly during cycles of Pleistocene glacia-

tion and can again in the future. Being asynchronous and normally oriented, direct and flexural loading presum-

ably cause normal and shear stresses to evolve in a complex fashion through much or all of a glacial cycle.

Simulations of fractured rock predict permeability might vary by two to three orders of magnitude under similar

stress changes as fractures at different orientations are subjected to changing normal and shear stresses and some

become critically stressed. Uncertainties surrounding these processes and their interactions, and the confounding

influences of surface hydrologic changes, make it challenging to delineate their effects on groundwater flow and

pressure regimes with any specificity. To date, evidence for hydromechanical changes caused by the last glacia-

tion is sparse and inconclusive, comprising a few pressure anomalies attributed to the removal of direct ice load.

This may change as more data are gathered, and understanding of relevant processes is refined.

Key words: flexural loading, glacial, glacial-hydromechanics, glacial loading, poromechanics, quaternary

paleohydrogeology

Received 11 April 2011; accepted 4 August 2011

Corresponding author: Chris Neuzil, U.S. Geological Survey, 431 National Center, Reston, VA 20192, USA.

Email: ceneuzil@usgs.gov. Tel: 703 648 5880. Fax: 703 648 5274.

Geofluids (2012) 12, 22–37

INTRODUCTION

Expectations of siting nuclear waste repositories in glaci-

ated regions have generated interest in the effects of future

glaciations on groundwater flow (e.g., Talbot 1999; Boul-

ton et al. 2004) and led to the efforts to identify mecha-

nisms by which continental ice sheets might alter

subsurface fluid regimes. Researchers have approached the

problem theoretically, by simulating the effects of climate

and ice sheet forcing processes, and empirically, by

attempting to identify hydrogeological and geochemical

imprints of past glaciation in groundwater systems. Indeed,

the geological recency of Pleistocene glaciations offers an

unusual opportunity to study the response of groundwater

systems to substantial natural forcing.

Hydrologic changes imposed on groundwater systems by

climate excursions and ice cover have been recognized for

some time (e.g., Boulton 1974; Siegel & Mandle 1984;

Grasby et al. 2000; McIntosh & Walter 2005; Person et al.

2007; Lemieux et al. 2008a; also see, Person et al. 2012 and

McIntosh et al. 2011). Elevated subglacial hydraulic heads

appear responsible for the extensive zones of glacial recharge

that can be delineated isotopically, and there is also evidence

that permafrost and ice cover have, at times, slowed or stopp-

ped groundwater recharge and discharge (e.g., McEwen &

de Marsily 1991; Boulton & de Marsily 1997; Lemieux et al.

2008a). From the perspective of groundwater flow, these

effects can be categorized as resulting from changed hydrau-

lic boundary conditions that represent altered pressures or

water fluxes at the ground surface.

Geofluids (2012) 12, 22–37 doi: 10.1111/j.1468-8123.2011.00347.x

� 2011 Blackwell Publishing Ltd

Because of their weight, continental ice sheets also affect

groundwater systems by imposing dramatic and geologi-

cally rapid changes in mechanical stress in the subsurface.

Resulting deformations can alter pore fluid pressure and

fluid transport properties of the rocks themselves. Efforts

to understand hydromechanical perturbations are relatively

new, and a conceptual framework for describing them is

still being established. Attention has thus far focused on

the compression of overridden geologic media by ice

weight (here termed direct loading), a phenomenon readily

studied using widely available groundwater analysis tech-

niques. Here, I argue that stresses resulting from flexure or

bending of the lithosphere under ice weight (flexural load-

ing) and stress-mediated changes in fracture permeability

(Fig. 1) may also be important and that analysts may need

to consider their effects as well. I also discuss the chal-

lenges this presents: flexure and fracture deformation

require specialized descriptive tools and are relatively com-

plex, incompletely understood phenomena. Difficulties are

exacerbated by a lack of observational constraints because

hydromechanical imprints of the last glaciation on today’s

groundwater systems are more equivocal and difficult to

interpret than hydrologic ones.

The first part of the paper presents a brief background

of hydromechanical concepts as a preface to the overviews

of direct glacial loading and flexural loading, their effects

on fracture permeability, and discussion of their potential

effects on groundwater regimes. The second part examines

the evidence for hydromechanical signatures of glaciation

in today’s groundwater systems. Hydromechanical effects

in relatively shallow, deformable sediments caused by the

drag of flowing ice and often called ‘glacial tectonics’ are

not considered here, but are discussed in some detail by

Iverson & Person (2012).

CONCEPTUAL BASES

Stresses induced by glaciation can alter groundwater

regimes by changing both the driving forces for flow and

the fluid transport properties of geologic media. An

increase in all-around compressive stress, for example,

decreases the volume of both solids and pores. If pore vol-

ume is decreased too rapidly for the fluid to drain, fluid

pressure increases. Rates of stress and pore volume change

during glaciation are sufficiently rapid that a significant

fraction of the subsurface probably is largely or partially

unable to drain and presumably experiences fluid pressure

excursions. Pressure changes are governed by changes in

total normal stress, or the first invariant of the stress ten-

sor, which is often denoted rkk to represent the sum of

normal stresses rxx + ryy + rzz. Thus, any process that

changes rkk can change fluid pressure, and an important

goal of hydromechanical analyses is to determine spatial

and temporal changes in rkk through a glacial cycle.

Stress changes accompanying glaciation generally are not

large enough to significantly alter intergranular permeability

in geologic media. However, they may be capable of signifi-

cantly decreasing or increasing fracture permeability.

Changes in aperture and permeability are caused by both

normal and shear stresses on a fracture, and the fact that frac-

tures are themselves mechanical discontinuities must be con-

sidered in analyses. Various approaches to this problem have

been developed, but the impracticality of describing fracture

systems in detail has limited the specificity of analyses.

Effects of mechanical loading on groundwater systems

differ from hydrologically driven changes in three important

respects. First, unlike changes in boundary pressure or fluid

flux, changes in boundary stress and their effects propagate

almost instantaneously throughout the subsurface. Second,

rather than altering flow in the most permeable units, as is

typical of hydrologically driven processes, stress changes

tend to alter fluid pressures primarily in the least permeable

units, which may be of interest in efforts to site radioactive

waste repositories. Third, mechanical loading has the ability

to alter how permeability itself is structured.

Descriptive Tools

Many analyses of hydromechanical effects of glaciation have

been conducted by groundwater specialists using a modified

version of the groundwater flow equation. This approach

implicitly assumes all deformation in the porous medium is

vertical. However, multidimensional deformation has an

especially important role during glaciation, and a more gen-

eral description may be desirable. This involves coupling a

groundwater flow equation with a stress or deformation

equation. The equations combine constitutive relations for

matrix deformation and fluid flow with statements of force

equilibrium and fluid mass conservation, respectively.

Fig. 1. Types of hydromechanical effects of a continental ice sheet. Vertical

arrows represent direct loading, which causes largely vertical compression

underneath the ice sheet. Horizontal arrows represent flexural loading,

which is predominantly horizontal and causes compression and extension.

The inset represents the effects of changes in normal and shear stresses on

fractures because of both direct and flexural loading. Not to scale.

Hydromechanical effects of continental glaciation 23

� 2011 Blackwell Publishing Ltd, Geofluids, 12, 22–37

Coupling deformation and flow in two and three

dimensions

In an elastic porous medium, the effective stress law can

be used to obtain a constitutive relation for poroelastic

deformation. Neglecting body forces, the latter and force

equilibrium yield the poroelastic deformation equation

(e.g., Ingebritsen et al. 2006),

Gr2u þ G

1� 2�r r � uð Þ ¼ �rP ð1Þ

where u is the displacement vector, P is pore fluid pres-

sure, G is the shear modulus, m is Poisson’s ratio, and a is

the effective stress coefficient. The caret (^) indicates a dif-

ference relative to a reference state, such as preglacial con-

ditions.

Darcy’s law and a statement of fluid mass conservation

yield the poroelastic equation of flow, or (Ingebritsen et al.

2006)

r ��k�f g

�f

rP þ �f grzð Þ� �

¼ s 0S3

oP

ot� �f g�

o

otr � uð Þ ð2Þ

where �k is the permeability tensor, qf is water density, g

is gravitational acceleration, lf is fluid dynamic viscosity,

and s 0s3 is a three-dimensional specific storage. The diver-

gence of displacement, r � u, is the volume change of

the porous medium. Volume strain occurs partly by

changing pore volume, the effects of which are equiva-

lent to adding or removing fluid. Thus, the last term in

the flow equation may be thought of as a fluid source or

sink.

Equation (2) may be written in terms of stress as (Inge-

britsen et al. 2006)

r ��k�f g

�f

rP þ �f grzð Þ� �

¼ sS3oP

ot� sS3B

o

ot

�kk

3

� �: ð3Þ

Here, ss3 is a different form of three-dimensional specific

storage, and B is three-dimensional loading efficiency. As

in Eq. (2), mechanical loads, imposed by glacial ice for

example, affect pressure and flow through the last term,

which in this case includes the rate of change of the mean

of the total normal stress, or rkk ⁄ 3.

Because stress changes and resulting strains occur

throughout media subjected to external load changes, the

last terms in Eqs (2) and (3) act like distributed rather

than point fluid sources. In undrained media, the flow

term is zero and Eq. (3) reduces to

�P ¼ B ��kk

3

� �ð4Þ

showing that loading efficiency describes the fraction of

external load change borne by the pore fluid as a pres-

sure change. In principle, B can assume values between

0 and 1, but in geologic media actually ranges from

about 0.2 in low-porosity rocks at about 10 km depth

to values between 0.5 and slightly less than 1.0 in rocks

and sedimentary media at shallower depths (Kumpel

1991; Ingebritsen et al. 2006). In other words, fluid

pressure changes are about 20 to just under 100% of a

mean total stress change imposed on undrained rocks in

the upper crust.

The preceeding discussion applies when pores are filled

with liquid, in this case water. The presence of gas, for

example methane or carbon dioxide, can significantly

reduce loading efficiency and corresponding pressure

changes. The reduction is relatively minor when little gas is

present and at depths of a few kilometers or more where

pressures are high, but becomes increasingly pronounced

as gas volume increases and pressure decreases (Wang et al.

1998). The presence of a gas phase may itself be a result of

changes in mean stress and the ensuing deformation. In a

water saturated system, pore dilation and resulting fluid

pressure decrease may cause gas to exsolve, while compres-

sion can cause it to redissolve. This was observed, for

example, by Shosa & Cathles (2001) when decreasing and

increasing fluid pressure in sediment containing water and

dissolved carbon dioxide.

A terrain subjected to glaciation is a nonisothermal sys-

tem, and a more rigorous description includes thermal

terms in Eqs. (1–3) and coupling with a heat transport

equation. Thermomechanical effects are probably much

smaller than ice-load effects because subsurface tempera-

ture changes during glaciation are relatively small, and they

are not considered here. Thermomechanical phenomena

may, however, be of interest for specialized problems, and

they are discussed by Boley & Weiner (1960) and Timo-

shenko & Goodier (1987). Other processes not directly

related to hydromechanical behavior may also be included

in the description when circumstances warrant. Examples

include variable fluid density, solute transport, and addi-

tional fluid phases, such as gas.

Solving the coupled equations requires specifying

mechanical and hydraulic properties and their spatial dis-

tributions, as well as mechanical and hydraulic initial and

boundary conditions (Ingebritsen et al. 2006). In prac-

tice, these are typically poorly known and the uncertainty

is exacerbated by the fact that both mechanical and

hydrologic boundary conditions evolve during a glacial

cycle. As a result, fully coupled analyses generally include

a number of explicit and implicit assumptions and approx-

imations.

Partial coupling

Full poroelastic coupling has been used in a few analyses of

direct glacial loading, but partial coupling has seen wider

use. Partial coupling is implemented relatively easily in

groundwater flow simulators and involves solving a version

of the flow equation that includes a deformation or stress

24 C. E. NEUZIL

� 2011 Blackwell Publishing Ltd, Geofluids, 12, 22–37

term but is not coupled with a deformation equation. The

equation can be obtained by assuming strain is solely verti-

cal, in which case Eq. (3) reduces to (Ingebritsen et al.

2006)

r ��k�f g

�f

rP þ �f grzð Þ� �

¼ sSoP

ot� sS �

o�zz

otð5Þ

where ss is uniaxial specific storage, rzz is vertical stress, and

f is one-dimensional loading efficiency. Partial coupling

removes the need to solve for stress or deformation and

essentially preselects all but the upper and lower mechani-

cal boundary conditions.

Inelasticity and fractures

The development outlined posits a linear elastic porous

medium and small strains. This can be restrictive for many

geologic problems. Deformation of geologic media is often

not reversible or linearly related to stress, can be viscous or

plastic, and can be large. Viscoelastic deformation in partic-

ular, which continues after stress changes cease, is hydro-

mechanically interesting because it can cause prolonged

perturbation of fluid pressure. Mechanical behavior of geo-

logic media on timescales relevant to glaciation is not well

understood, so to the extent that inelastic behavior occurs,

there is little on which to base descriptions of it. Generally

speaking, small-strain elastic behavior probably is a useful

approximation for analyzing the effects of glaciation. This

is because affected terrains have experienced prior ice load-

ing, which tends to make subsequent behavior nearly line-

arly elastic and strains relatively small (e.g., Karig & Hou

1992). Fractured rock is an exception in the sense that, at

sufficiently large scales, its behavior is similar to that of an

inelastic continuum. Fractures are more compressible

under normal stress and weaker under shear stress than

intact rock. Mechanical behavior of fractures and interven-

ing blocks can be treated explicitly (e.g., Cundall 1971,

1980), but in practice, this is accomplished using artificial

fracture systems thought to behave like the actual systems

of interest. The alternative continuum approach involves

describing large-scale behavior by summing that of many

fractures and blocks. Rocks with multiple fracture sets of

known orientations can be approximated as elasto-plastic

continua (Chen et al. 2007), with plastic deformation rep-

resenting fractures failing upon reaching a critical shear

stress.

Ice Sheets and Groundwater Hydromechanics

Effects of direct ice sheet loading on groundwater regimes

have been considered by a number of researchers, but this

is not true for flexural loading or stress-mediated fracture

permeability changes. What is known about flexure comes

almost entirely from very large-scale analyses focused on

Earth’s viscoelastic structure, ice sheet history, postglacial

rebound and seismicity, and the mechanics of flexure itself

(e.g., Peltier & Andrews 1976; Morner 1978; Peltier

1996, 2004; Zhong et al. 2003; Hampel & Hetzel 2006;

Steffen et al. 2006; Turpeinen et al. 2008). Studies of the

relation between stress regimes and fracture permeability

are even further removed from a glacial-hydromechanics

and have been motivated mainly by interest in effects of

tectonism and engineered excavations (e.g., Brown & Bru-

hn 1998; Zhang et al. 2007). Nevertheless, it is difficult to

argue that either flexural loading or fracture permeability

changes during glacial cycles can be prudently ignored or,

stated differently, that only direct loading is important.

The basis for this assertion is outlined in this section.

Direct loading: mechanism of choice

As it overrides terrain, a continental ice sheet adds a sub-

stantial vertical load at the ground surface, compressing

underlying geologic media. Thinning and retreat of an ice

sheet removes the load, allowing media to dilate. These

effects are analogous to loading by sedimentary deposition

and unloading by erosion which have been studied in

hydrogeology for some time (e.g., Neuzil 1995). Thus, it

is understandable that this process has been a focus of gla-

ciation-related groundwater research.

Analyzing direct loading is a relatively tractable problem

because it simply tracks ice thickness through time, and

reconstructions of ice thickness history are often available

(e.g., Chan et al. 2005; Lemieux & Sudicky 2010). Direct

effects also lend themselves to partially coupled descrip-

tions via flow equations equivalent to Eq. (5), although

fully coupled analyses of it using Eqs (1) and (2) have also

been done.

With significant interest in the effects of the most recent

glaciation on current groundwater systems, some studies

have considered only the removal of the ice load following

the last glacial maximum. Others have considered a full

interglacial-to-interglacial cycle. None accounted for flex-

ural loads, although studies by Bense & Person (2008),

and Lemieux et al. (2008a,b) incorporated elevation

changes attributed to flexure in their analyses. Most analy-

ses included direct loading as one of a suite of effects

accompanying climate change and glaciation. As a result, it

can be difficult to separate hydromechanical and hydrologic

causes of behavior observed in simulations.

Provost et al. (1998) used a partially coupled approach

to analyze glaciation of fractured crystalline bedrock of the

Fennoscandian shield. The study was part of an effort by

SKi (Statens Karnkraftinspektion, or Swedish Nuclear

Power Inspectorate) to determine how future glaciations

might affect groundwater flow and transport at a proposed

nuclear waste repository. Simulations were conducted using

a modified version of the simulator SUTRA (Voss 1984)

and accounted for fluid density variations attributed to

Hydromechanical effects of continental glaciation 25

� 2011 Blackwell Publishing Ltd, Geofluids, 12, 22–37

brines. An intriguing result was that glacial recharge was

partly controlled by mechanical response to the ice load.

High loading efficiencies (f in Eq. (5)) caused ice loading

to raise fluid pressures at depth at the same time subglacial

water pressure at the ground surface increased. This

reduced hydraulic gradients driving subglacial recharge. To

the extent recharge did occur, it was found that corre-

sponding upward flow and discharge began during glacial

retreat, releasing water added to storage while ice was pres-

ent.

A partially coupled approach was also used by Bense &

Person (2008) to analyze ice advance and retreat over a

generic intracratonic sedimentary basin like the Michigan,

Illinois, and Williston Basins of North America. They

solved equations for variable-density groundwater flow,

and solute and heat transport, using FlexPDE (PDE Solu-

tions Incorporated 2011). The flow equation had a loading

term equivalent to that in Eq. (5). Density variations were

significant because of brine at depth in the basin. Bense &

Person (2008) found that ice load increased pressure in

relatively low-permeability layers, resulting in an outflow of

water from them. Retreat of the ice reversed the process,

leaving pressures more than 5 MPa below local hydrostatic,

or ‘normal’ values. In their simulations, presures more than

3 MPa below normal persist to the present (Fig. 2). Bense

& Person (2008) assumed a loading efficiency of 1.0,

whereas the sedimentary rocks like those in question prob-

ably have loading efficiencies between 0.5 and 0.7, sug-

gesting actual pressure perturbations would be somewhat

smaller. Their results nevertheless clearly demonstrate a

mechanism by which direct loading might leave a detect-

able imprint on modern pressure regimes.

Lemieux et al. (2008a) analyzed the changes in ground-

water systems on an unprecedented scale, simulating the

effects of permafrost and ice sheet cover over North Amer-

ica. Their analysis was guided by detailed reconstructions

of Wisconsinan ice sheet extent and thickness, subglacial

melting, and permafrost extent based on the Memorial

University of Newfoundland and University of Toronto

Glacial Systems Model (see Lemieux et al. 2008a,b).

The reconstructions constrained boundary conditions for

coupled surface- and groundwater flow simulations using

HydroGeoSphere (Therrien et al. 2006), including direct

mechanical loading. An objective was quantifying the frac-

tion of glacial meltwater that recharged to become ground-

water. Their simulations suggested the fraction was almost

half but, like those by Provost et al. (1998), indicated that

direct loading influenced the result. High loading efficien-

cies significantly reduced simulated recharge by raising sub-

surface fluid pressures as the ice advanced (Fig. 3).

A nearly opposite extreme in terms of scale was the focus

of analyses by Chan & Stanchell (2005), Tsang et al.

(2005), Chan et al. (2005), and Vidstrand et al. (2008).

They simulated glaciation of a 103 km2 crystalline rock

mass with properties based on the underground laboratory

in Whiteshell, Canada. The overall aim was to simulate

coupled thermo-hydromechanical (THM) responses

Fig. 2. Present-day groundwater heads simu-

lated by Bense & Person (2008) in a generic in-

tracratonic basin that was half covered by ice at

last glacial maximum. Anomalously subhydrostat-

ic heads and relatively sharp hydraulic gradients

are remnants of fluid expulsion caused by direct

loading. Ice was present long enough for rela-

tively low-permeability sedimentary units and

basement (in gray) to drain to a greater or lesser

degree, but time since deglaciation is insufficient

for flow to accomodate postglacial unloading.

Calculated underpressure exceeds 3 MPa at one

location. After Fig. 9 of Bense & Person (2008).

Fig. 3. Subglacial recharge histories for the Wisconsinan glaciation over

North America simulated by Lemieux et al. (2008a) accounting for direct

loading under different loading efficiencies. Fluid pressure increases under

loading, which decrease subglacial recharge, are proportional to loading

efficiency. After Fig. 9 of Lemieux et al. (2008a).

26 C. E. NEUZIL

� 2011 Blackwell Publishing Ltd, Geofluids, 12, 22–37

(although thermomechanical effects were not considered)

and evaluate simulation capabilities as part of Benchmark

Test 3 (BMT3) of the DECOVALEX (DEvelopment of

COupled (THM) models and their VALidation against

Experiments in nuclear waste isolation) project (Tsang

et al. 2005). Of note is the fact that full poroelastic cou-

pling was incorporated using two different numerical simu-

lators, MOTIF and ABAQUS, to solve equivalents of Eqs

(1) and (2). ABAQUS is a commercially available code

(Hibbit, Karlsson and Sorenson, Inc., 1998), while

MOTIF is an AECL (Atomic Energy Of Canada Ltd.) in-

house simulator (Chan & Stanchell 2005; Chan et al.

2005).

Although the DECOVALEX studies included full poro-

elastic coupling, the boundary conditions selected con-

strained deformation to be vertical at side boundaries.

Thus, lateral deformation in the domain was attributed

only to spatial variations in ice load. Both flexural loading

(called ‘large-scale isostasy’ by Chan et al. 2005) and frac-

ture permeability effects (referred to as ‘stress-dependent

permeability’ by Chan et al. 2005) were acknowledged but

not included. Major fracture zones were included, how-

ever. Segregating mechanical and hydrologic effects in their

results is difficult, but Chan et al. (2005) did note a curi-

ous mechanical phenomenon. A brief, unexpected dip in

minimum effective stress occured during deglaciation, per-

haps because direct ice load decreased more rapidly than

fluid pressures. They speculated this mechanism could

cause tensile effective stresses under certain conditions.

Flexural loading: hydromechanical wild card

Earth’s rigid lithosphere – the crust and uppermost mantle

– literally floats on the denser viscoelastic asthenosphere

and is generally in an approximate isostatic state, with sur-

face elevations adjusted to lithosphere thickness and den-

sity. The weight of an ice sheet disturbs the balance,

bending the lithosphere downward into the asthenosphere

and creating a corresponding bulge beyond the ice edge.

Beneath the ice sheet, flexure increases lateral compressive

stresses in the upper and decreases them in the lower litho-

sphere (e.g., Johnston et al. 1998; Klemann & Wolf

1998), with changes in the opposite sense in the forebulge

(Fig. 4, model A). The latter can extend thousands of kilo-

meters beyond the ice sheet (e.g., Peltier & Fairbanks

2006), but causes significant stress changes over much

smaller distances. Removal of the ice load induces stress

changes in the opposite sense as the system returns to an

ice-free isostatic state. There is some debate about whether

ice loads are sufficiently long-lived that flexural stresses

relax viscoelastically while the ice is present. If they do,

flexural stress changes induced by deglaciation are super-

posed on a relaxed stress regime (Grollimund & Zoback

2000) (Fig. 4, model B). Grollimund & Zoback (2000)

note advocates for both models, with Stephansson (1988)

arguing flexural stresses do not relax, and Stein et al.

(1989) appealing to relaxation to explain earthquakes in

Baffin Bay. This uncertainty is more relevant to detecting

flexural stresses than to their effects on groundwater

because the strains in either case are similar.

Like direct loads, flexural loads deform porous media

and can affect fluid pressure. Indeed, both contribute to

changes in mean total stress rkk ⁄ 3, and their combined

effects on fluid pressure are manifested through the term

in Eq. (3) containing this quantity. However, flexural load-

ing differs from direct loading in two important respects.

First, direct loads alter vertical and, to a lesser degree, hori-

zontal stresses, while flexure predominantly changes hori-

zontal stresses. Second, direct loading effects are

concurrent with changes in ice weight, while flexural load-

ing is delayed by viscous flow in the asthenosphere.

Asthenosphere viscosity is such that significant isostatic

disequilibrium persists on the order of 10 000 years (e.g.,

Fig. 4. Models of flexure under loading by an ice sheet. In lithosphere,

converging horizontal arrows indicate increased lateral compressive stress

and diverging arrows indicate decreased lateral compressive stress. Con-

ceptual model A (vertical arrows on left) indicate the sequence unglaciat-

ed–glaciated–unglaciated with no relaxation of flexural stresses; stress

increments added by flexure under ice weight are relieved only during post-

glacial isostatic adjustment. Conceptual model B (vertical arrows on right)

indicate the sequence unglaciated–glaciated–unglaciated with relaxation of

flexural stresses; stress increments added by flexure under ice weight relax

while the ice load is present and stresses in the opposite sense are gener-

ated during postglacial isostatic adjustment. Both models predict compara-

ble fluid pressure perturbations, but suggest different stress signatures as

indicators of flexure during the last glaciation.

Hydromechanical effects of continental glaciation 27

� 2011 Blackwell Publishing Ltd, Geofluids, 12, 22–37

Lemieux & Sudicky 2010; Peltier 2011) while full reequili-

bration takes tens of thousands of years (Turcotte &

Schubert 1982). This is rapid enough to respond to ice

loads during a glacial cycle, but slower than ice sheet

advance and retreat. The lag in flexural response to the last

deglaciation is observable today as ongoing glacial rebound

(e.g., Turcotte & Schubert 1982; Press & Siever 1982).

Orientation and timing differences between direct and

flexural loading mean that magnitudes of principal stresses

presumably evolve continuously during flexural downwar-

ping and rebound, not solely during ice advance and

retreat (Talbot 1999), and the evolution may persist

through an entire glacial cycle. Such persistence is hydro-

mechanically significant because it can help preserve

groundwater pressure perturbations after ice sheet advance

or retreat generated them.

Analyses of flexure by Johnston et al. (1998) seem to

offer a glimpse of its potential hydromechanical signifi-

cance. They invoked a number of ice-loading scenarios

using increasingly rigorous conceptual models, illuminating

many nuances of flexure. Magnitudes of flexural stresses

are controlled in part by intensity of the flexure which, in

turn, depends on both ice thickness and the ratio of its

areal extent to lithosphere thickness. An important contri-

bution of Johnston et al. (1998) was to clarify this rela-

tionship. Using a two-dimensional analysis, they found the

largest increments in horizontal and shear stresses occur

when load wavelength, or twice ice sheet breadth, is about

eight times the lithosphere thickness or, in their model, a

wavelength of about 800 km. Ice sheets smaller or larger

than this critical size exhibited smaller flexural stress

changes with identical ice thicknesses.

Johnston et al. (1998) also analyzed more realistic axi-

symmetrical ice loads. In one example, they considered an

ice sheet slightly less than 700 km across with a parabolic

cross-section and maximum 1 km thickness. Figure 5

depicts their calculated increments in vertical, radial, and

tangential stresses for isostatic equilibrium under ice load,

including direct loading. The plots depict the entire 100-

km-thick lithosphere, so the uppermost few kilometers of

the crust that are of interest here are at the upper edge.

The maximum vertical stress increment (�zz) of 9 MPa at

ground surface under the center of the ice sheet corre-

sponds to the direct load imposed by ice weight. Horizon-

tal, radial, and tangential stresses, however, were increased

by more than 20 MPa under the ice sheet, with most of

the change attributable to flexure. Assuming a representa-

tive Poisson’s ratio of 0.25, this suggests mean total stress

changed by less than 5 MPa because of direct loading but

more than 16 MPa when flexure is accounted for. For a

conservative loading efficiency of 0.5, the corresponding

undrained fluid pressure changes would be <2.5 and more

than 8 MPa respectively, or roughly 250 and 800 m of

head. Thus, in this scenario, flexure has a significantly

greater effect on fluid pressure than direct loading. Also,

note in Fig. 5 that changes in radial stress of 2–3 MPa

caused by the forebulge extend hundreds of kilometers

beyond the ice sheet. These stress and pressure changes

may be conservative; the Laurentide ice sheet, for example,

is thought to have reached a thickness of almost 3 km

(Lemieux & Sudicky 2010; Peltier 2011).

The calculations above assumed a flat geometry. How-

ever, Johnston et al. (1998) obtained similar results for a

circular ice sheet loading a lithosphere and asthenosphere

with spherical geometry, multiple lithosphere layers, and a

viscoelastic asthenosphere. Specifically, a critical ice sheet

size of slightly more than 900 km yielded horizontal stress

increments as large as four to five times the direct ice load.

Klemann & Wolf (1998), who analyzed a 1800-km-wide

ice sheet representing Fennoscandian glaciation, also

obtained results of interest here. Their model considered

an asthenosphere with Maxwell viscoelasticity and an elastic

or viscoelastic lithosphere. Figure 6 shows shear stresses

computed for their three-layer model at last glacial maxi-

mum, end of deglaciation, and present. Large shear stres-

ses, indicative of large increases in horizontal stresses, were

Fig. 5. Stress changes attributed to direct and flexural loading calculated by

Johnston et al. (1998). Plots from top to bottom show increments in radial,

tangential, and vertical stress (�rr ; �; �zz ) in a 100-km-thick elastic litho-

sphere in isostaic equlibrium with a load imposed by a circular ice sheet

with a parabolic profile and about 600 km wide. Note that radial stress

increments of a MPa or more extend hundreds of kilometers beyond the

ice sheet. After Fig. 4 of Johnston et al. (1998), who treated compressive

stresses as negative, in this figure and the text they are considered positive.

28 C. E. NEUZIL

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computed in the uppermost crust. Shear stress changes of

several MPa hundreds of kilometers beyond the ice are

shown as still evolving. Klemann & Wolf (1998) consid-

ered multiple glacial cycles and, for their scenario, found

flexural stresses never disappeared.

Almost no attention has been given to flexural loading

in efforts to characterize hydromechanical effects of glacia-

tion on groundwater systems. Although flexural elevation

changes were included in analyses by Bense & Person

(2008) and Lemieux et al. (2008a,b), no groundwater flow

studies have included flexural loading, although as noted

earlier Chan et al. (2005) acknowledged a role for it. Simi-

larly, no analyses of flexure have considered pore fluid,

although Johnston et al. (1998) recognized it affects the

crust’s mechanical response. However, an instance where

flexure may have perturbed pore fluid pressures has been

analyzed. Grollimund & Zoback (2000) described a site in

the North Sea where overpressures coincide with high hor-

izontal stresses and considered whether both were caused

by the retreat of the Fennscandian ice sheet and unbending

of the lithosphere. Using observed stress patterns as a con-

straint, Grollimund & Zoback (2000) considered several

models for the structure and properties of the lithosphere,

finding the best fit using a model with elastic upper and

viscoelastic lower parts. They concluded flexure could have

increased fluid pressures by as much as 3.5 MPa but

invoked other processes, such as sedimentary loading, to

explain observed 15 MPa overpressures. Their study is

notable in the present context for testing different flexure

models, using the observed regional stress field as a con-

straint, and computing expected fluid pressure changes.

However, the analysis did not include flow, and an implicit

assumption was that the system had relaxed hydrodynami-

cally, or drained, while the ice was present but had not yet

drained following deglaciation. This is discussed further in

the second section.

The applicability of most flexure analyses to groundwater

flow is limited by their large scale and the usual assumption

of a homogeneous elastic or viscoelastic lithosphere. Rocks

may fail because they cannot maintain the large shear stres-

ses indicated. Moreover, at the scales and depths of interest

in groundwater studies, mechanical heterogeneity because

of faulting and lithologic differences may cause corre-

sponding heterogeneity in flexural stress regimes. In this

vein, Talbot & Sirat (2001) found stresses at Aspo, Swe-

den, could change dramatically over distances of only

meters, which they attributed to flexural unloading of the

fractured and faulted crystalline rock.

A testament to the crust’s inelastic behavior is the obser-

vation that it does indeed experience shear failure. Postgla-

cial fault scarps have been identified (Arvidsson 1996;

Thorson 2000), with the best known being the Parvie fault

in northern Sweden, which experienced up to 10 m of dis-

placement (Muir-Wood 1989; Lagerback 1992; Backblom

& Munier 2002). Liquefaction features have also been dis-

covered in Sweden that are cited by Morner (2001) as evi-

dence of numerous large earthquakes resulting from

flexural rebound, while analysis of faulting and recorded

seismicity in Fennoscandia led Wu et al. (1999) to attri-

bute postglacial thrusts to the same cause. Hampel & Het-

zel (2006) hypothesized that postglacial seismicity releases

tectonic stresses that accumulate when ice loading slows or

stops slip on faults.

Such complexities can perhaps be represented in models.

Fractures and faults have been incorporated explicitly in

mechanical models (e.g., Chryssanthakis et al. 1991;

Rosengren & Stephansson 1993; Min et al. 2004; Chan

et al. 2005) or implicitly via an effective rheology that

incorporates discontinuities in a continuum at appropriately

large scales (e.g., Oda 1985; Brown & Bruhn 1998). Flex-

ural loading nevertheless presents challenges for analyzing

its effect on groundwater, including uncertainty about how

to appropriately describe it. Such problems aside, it is possi-

ble in principle to simulate flexure as a poromechanical pro-

Fig. 6. Maximum shear stresses attributed to direct and flexural loading

calculated by Klemann & Wolf (1998). From top to bottom, plots contour

values at last glacial maximum, end of deglaciation, and present in a 100-

km-thick lithosphere loaded by a circular ice sheet with an elliptic profile, a

center thickness of 2800 m, and a radius of 900 km. Last glacial maximum

assumes isostatic equilibrium under the ice load, while later plots show tran-

sient states before a new equilibrium without ice load. Shear stresses of

0.5 MPa appear almost 1000 km beyond the maximum ice extent. After

Fig. 2 of Klemann & Wolf (1998).

Hydromechanical effects of continental glaciation 29

� 2011 Blackwell Publishing Ltd, Geofluids, 12, 22–37

cess as Corfdir & Dormieux (1998) did for flexure during

faulting. Alternatively, if the effect of pore fluids on

mechanical response is ignored, computed flexural stresses

can be used as boundary conditions in local-scale porome-

chanical analyses of groundwater flow. Even simpler first-

order analysis is possible using computed stresses to calcu-

late the changes in mean total stress in a partially coupled

description.

Loading fractured rock: does the plumbing change?

The fact that direct and flexural loading probably cause

stress regimes to evolve continuously through much or all

of a glacial cycle was recognized by Talbot (1999), who

also speculated permeability in fractured rock evolved as a

result, changing magnitude, anisotropy, and orientation.

Strong dependence of fracture permeability on stress has

been recognized for some time (e.g., Snow 1968, 1970;

Gale 1977; Neuzil & Tracy 1981; Oda 1986; Barton et al.

1988; Sayers 1990; Chryssanthakis et al. 1991), as mani-

fested, for example, by decreasing fracture apertures with

depth. Indeed, Chryssanthakis et al. (1991) analyzed how

direct loading and drag by an ice sheet decreased fracture

apertures in rock. However, understanding of the stress–

permebility relationship was fundamentally altered in the

1990s when it was discovered that permeable fractures

often tend to be those that are critically stressed, that is,

approaching shear failure.

The striking correlation between stress regime and fluid

conduction first reported by Barton et al. (1995) for frac-

tured rocks at Cajon Pass, California, can be seen in the

three-dimensional Mohr diagrams in Fig. 7. Conductive

fractures cluster in the Coulomb failure envelope for coeffi-

cients of friction between 0.6 and 1.0. Hickman et al.

(1997) and Barton et al. (1998) found a similar relation in

Dixie Valley, Nevada, as did Ito & Zoback (2000) in the

KTB borehole in Germany, Talbot & Sirat (2001) at Aspo,

Sweden, Morin & Savage (2002) in extrusive igneous

rocks in Texas, and Morin and Savage (2003) in sedimen-

tary rocks in New Jersey. The work by Talbot & Sirat

(2001) is notable because they attributed present-day frac-

ture permeability partly to flexural stresses. These field

observations helped inspire later attempts to quantitatively

describe how stress affects permeability in fractured rock.

The underlying conceptual model is one of fracture closure

under increasing normal stress and dilation under increas-

ing shear stress. Dilation is most pronounced approaching

shear failure as aperities begin to ride over one another.

Two approaches were already noted for describing defor-

mation in fractured rock, and these have also been devel-

oped to calculate permeability changes. Fractures and

mechanical interactions of intervening blocks have been

treated explicitly (e.g., Min et al. 2004; Baghbanan & Jing

2008), requiring specification of actual or representative

fracture architecture. Fractured rock has also been analyzed

as equivalent continua in which the contribution of frac-

tures to deformation is accounted for rheologically (e.g.,

Brown & Bruhn 1998; Chen et al. 2007).

Brown & Bruhn (1998) adapted the scheme of Oda

(1985, 1986) for describing mechanical behavior and per-

meability of a continuum that represents specific fracture

architecture based on mapped fractures and added fracture

closure and dilation under increasing normal and shear

stress. They found shear dilatancy increased permeability

by as much as four orders of magnitude and dramatically

changed the orientation of anisotropy under uniaxial stress

changes of about 5 MPa. Treating fractured rock as an

elastic-perfectly plastic continuum, Chen et al. (2007)

arrived at an elasto-plastic rheology, with plastic deforma-

tion accounting for fracture failure after reaching a critical

shear stress, as a description. They separated the response

of fractures and bulk rock, allowing them to develop a

strain–permeability relation. Considering fracture shear up

to 2 cm, they were able to explain dilation and permeabil-

ity changes observed in laboratory tests on artificially frac-

tured granite blocks. A surprisingly small stress ratio

increase, from 1.40 to 1.45, produced a simulated three

order of magnitude permeability increase and four order of

Fig. 7. Three-dimensional Mohr circle plots developed by Barton et al.

(1995) for conductive (top) and nonconductive (bottom) fractures inter-

sected by a drillhole in sedimentary, intrusive, and metamorphic rocks at

Cajon Pass, California. Effective normal stress for each fracture is normal-

ized against the vertical stress and plotted on the horizontal axis. Shear

stress for each fracture is normalized against the vertical stress and plotted

on the vertical axes. The Coulomb failure envelope for coefficients of fric-

tion (tan /) of 1.0 and 0.6 is shown. Stresses on conductive fractures plot

largely within the envelope, while those for nonconductive fractures plot

largely outside of it, indicating that conductive fractures are relatively close

to failure. After Fig. 3 of Barton et al. (1995).

30 C. E. NEUZIL

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magnitude anisotropy increase. Anisotropy then decreased

with further increases in the stress ratio.

A technique for simulating mechanical interaction of

blocks in fractured rock using so-called distinct elements

was pioneered by Cundall (1971, 1980) and later used by

researchers to analyze behavior of rocks with specific frac-

ture patterns (e.g., Chryssanthakis et al. 1991; Rosengren

& Stephansson 1993). Min et al. (2004) adapted the tech-

nique to examine the effect of load changes on fracture

permeability and simulated discrete fractures in a 5 by 5 m

slice meant to represent rock at Sellafield, UK. In one sce-

nario, Min et al. (2004) kept the the ratio of largest to

smallest principal stress at 1.3 as compressive stress

increased. Permeability decreased by two orders of magni-

tude for a 20 MPa stress increase (Fig. 8a), a situation sim-

ilar to direct loading under a 2-km-thick ice sheet before

significant flexure. In the other scenario, one principal

stress was increased to bring the stress ratio from 0.5 to 5.

In this case, an increase in horizontal stress of about

20 MPa led to an increase in horizontal permeability of

nearly an order of magnitude, while the vertical permeabil-

ity increased by a factor of about four (Fig. 8b). This

might compare to conditions after a 2-km-thick ice sheet

retreats and before flexural stresses relax. The results imply

overall permeability and anisotropy changes of orders of

magnitude are possible in fractured rock because of direct

and flexural loading.

The work cited suggests permeability changes are sensi-

tive to choice of model for fracture deformation, character-

istics of the fracture connectivity, and other particulars

and, indeed, this is reinforced by other studies. Baghbanan

& Jing (2008) modified the analysis by Min et al. (2004)

to correlate fracture aperture and length. They found

increasing stress while maintaining stress ratios close to

one had a smaller effect on permeability than Min et al.

(2004) found because large fractures dominated the flow.

Increases in permeability at higher stress ratios were also

less dramatic because of dominant fractures. Sayers (1990)

demonstrated that if connectedness requires two fracture

sets, closure of one set significantly reduces permeability.

This is reminiscent of the abrupt changes in permeability at

flow thresholds in percolation theory (e.g., Berkowitz &

Balberg 1993; Berkowitz & Ewing 1998).

Uncertainties surrounding fracture permeability changes

during a glacial cycle are significant and make it a particu-

larly difficult phenomenon to evaluate. Mapping fracture

networks is notoriously difficult (e.g., Long et al. 1996),

and analysts usually resort to artificial fracture networks.

Yet specific fracture network architecture can control

response to stress changes. Appropriate models for closure

and dilation under normal and shear stress changes are not

fully established (Olsson & Barton 2001; Chen et al.

2007; Zhang et al. 2007; Baghbanan & Jing 2008) and

depend on shear displacement, fracture history and proba-

bly lithology. It is not clear whether permeability changes

are reversible, an important question in view of glacial

cyclicity. And, while it is possible to test fracture deforma-

tion-permeability models under engineering conditions

(e.g., Barton et al. 1988; Chryssanthakis et al. 1991; Chen

et al. 2007; Zhang et al. 2007), cyclic stress perturbations

that occur over tens of thousands of years with possible

(A)

(B)

Fig. 8. Fracture permeability under changes in stress regime in a synthetic

5 m by 5 m section of rock as calculated by Min et al. (2004). Fractures

properties are based on those at the Nirex site, UK. (A) Changes in horizon-

tal (kx) and vertical (ky) permeability versus mean stress when the ratio of

horizontal stress (rh) to vertical stress (rv) is maintained at 1.3, representing

increasing compressive stress under nearly hydrostatic loading. All fractures

experience mainly increasing normal stress and permeability decrease. (B)

Changes in horizontal (kx) and vertical (ky) permeability versus the ratio of

horizontal stress (rh) to vertical stress (rv) as the former is increased and

the latter is held constant. Solid lines and symbols show behavior when

shear dilation of fractures is allowed as shear stress along fracture plane

increases. Dashed lines and open symbols show behavior under identical

conditions when no shear dilation or failure of fractures is permitted. After

Figs 8 and 11 of Min et al. (2004).

Hydromechanical effects of continental glaciation 31

� 2011 Blackwell Publishing Ltd, Geofluids, 12, 22–37

fracture healing (e.g., Rojstaczer & Wolf 1992) cannot be

emulated experimentally.

IS THERE EVIDENCE FOR HYDROMECHANICALEFFECTS OF GLACIATION?

Other than changes in recharge and flow patterns that can

be delineated geochemically (McIntosh et al. 2011; Person

et al. 2012), tangible evidence of how groundwater sys-

tems have responded to past glacial forcing probably is

limited in practice to remnant perturbations of the hydro-

dynamic state, manifested as pressure anomalies. Fracture

permeability changes, in the unlikely event they are detect-

able at all, would be reflected in the former, and direct and

flexural load changes with the latter. Thus, anomalous

pressure regimes offer the best chance of detecting and

constraining glacier hydromechanical effects. Unfortu-

nately, few pressure anomalies have been attributed to

glaciation and still fewer have been analyzed quantitatively.

This section surveys pressure anomalies that researchers

have linked to glaciation and considers what can be learned

from them.

Generation and persistence of pressure anomalies

One interpretation of anomalous pressure is as a remnant

of a past perturbation – in this context ice loading – that

was both sufficiently long-lived to redistribute fluid mass

and removed recently enough that fluid mass has not been

redistributed to reflect current conditions. This invokes the

notion of the time needed to redistribute fluid mass: the

hydrodynamic response or adjustment time th. It is well

known that this can be approximated by th = l2ss ⁄ K where

K is the hydraulic conductivity, ss is the one- or multi-

dimensional specific storage, and l is half of the shortest

dimension of the subsurface volume in question. Thus, for

an anomaly to be generated by direct ice loading and per-

sist long enough to be observed, th cannot be much

greater than the duration of loading, but must be greater

than the time since removal of the ice load. This was rec-

ognized and discussed by Lerche et al. (1997) for multiple

cycles of glaciation and by Bense & Person (2008) for a

single cycle. Many fine-grained sedimentary units have th

values between 103 and 107 years for l values of tens to

hundreds of meters (e.g., Neuzil 1995) and thus might be

capable of preserving pressure anomalies generated by

retreat of the last ice sheet. The work of Chan et al.

(2005) and Lemieux et al. (2008a) suggests this may also

be true of fractured crystalline rock. This simple conceptual

model illuminates a role evolving fracture permeability

might play by changing th during a glacial cycle. A further

complication is that the time to be compared with th is

somewhat ambiguous for multiple cycles of glaciation.

Specifically, it is possible that systems with very large th

that cannot redistribute fluid mass over a single glacial

cycle may do so over multiple cycles.

Another interpretation of anomalous pressure is as a

response to ongoing perturbation, or forcing, such as flex-

ural unloading following ice sheet retreat. Hydrodynamic

response to ongoing perturbation can be characterized by

dimensionless geologic forcing C* (Neuzil 1995; Ingebrit-

sen et al. 2006), where C* = l C ⁄ K. In the present context,

C is the pore volume strain rate, or the quantity

� o r � uð Þ=ot in Eq. (2). Cast in terms of stress, C is

cb � csð Þ o �kk=3ð Þ=ot and is contained in the last term of

Eq. (3), with cb and cs the porous medium and solid grain

bulk compressibilities. Ongoing stress changes and strain

can generate and maintain pressure anomalies when

C* > 1. This conceptual model might usefully describe the

effects of flexural rebound following deglaciation. Again,

an implicit assumption is that the flexural load was present

long enough that fluid mass redistributed in response to it.

Because both direct and flexural loading occurs during a

glacial cycle, probably neither model alone offers an ade-

quate conceptual framework for present-day pressure

anomalies; actual behavior probably involves a combination

of the two.

Anomalous pressures attributed to glaciation

Pressure anomalies in a number of locations have been

explained, in whole or part, as remnants of mechanical or

hydrologic perturbations of the last glaciation. Few pub-

lished studies have acknowledged a role for flexural stres-

ses, with most focusing on direct loading and hydrologic

changes as perturbing processes. Thus, most analyses posit-

ing a hydromechanical origin have adopted the first con-

ceptual model described above and consider pressure

anomalies a result of direct unloading as the glacier

retreated.

Beginning in about 1990, NAGRA (Nationale Genos-

senschaft fur die Lagerung radioaktiver Abfalle), a Swiss

cooperative for radioactive waste disposal, began investigat-

ing an area in the Swiss Alps for a repository. The study

targeted Cretaceous shaley marls located beneath a topo-

graphic prominence called Wellenberg. Instrumented bore-

holes revealed that fluid pressures in the marls are

anomalously low by as much as 7 MPa, or about 700 m of

fresh-water head (Vinard et al. 1993, 2001). Two hypothe-

ses were advanced to explain the pressure anomalies, both

involving mechanical expansion of the marls. One attrib-

uted the anomalies to long-term erosional unloading and

the other to the more recent and rapid unloading caused

by glacial erosion and ice sheet retreat.

Both partially coupled and fully coupled descriptions

were used to test the hypotheses. Fully coupled simulations

were done in two-dimensional plane-strain domains using

ABAQUS (Hibbit, Karlsson and Sorenson, Inc., 1998).

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Simple mechanical boundary conditions were used (Vinard

et al. 1993, 2001; Vinard 1998) because the rugged

topography and uncertain tectonic history made the actual

boundary conditions uncertain. Simulations were run using

both elastic and and elastoplastic constitutive relations,

with the Cam-clay critical-state model (e.g., Ingebritsen

et al. 2006) used for the latter. Simulations that included

glacial unloading best explained the observed underpres-

sures, leading Vinard et al. (1993, 2001) and Vinard

(1998) to conclude that the pressure regime was at least

partly a response to direct load decreases during deglacia-

tion. They noted, however, it was possible to explain the

underpressures without invoking glacial unloading.

Elsewhere, overpressure has been attributed to direct ice

loading. Bahr et al. (1994) describes overpressures in sev-

eral formations in the Michigan Basin. Noting the absence

of ongoing compaction or other processes that could

maintain overpressures as well as similarities in the areal

disposition of the anomalies and moraines, Bahr et al.

(1994) suggested the pressures were generated by ice load.

However, they did not explain the mechanism by which

overpressures would persist during unloading.

Subtle overpressure of a few meters in continental shelf

sediments beneath Nantucket island, USA, has been

reported by Marksamer et al. (2007). Although small, the

anomaly is conspicuous because of its setting, and Bense &

Person (2008) suggested it may be a remnant of direct

loading. Again, it is unclear how overpressure would sur-

vive removal of the ice load. Indeed, Mulder & Moran

(1995) proposed high fluid pressures because of direct

loading as a cause of slope failures in now-submerged con-

tinental shelf sediments, but also suggested a return to

normal pressures following ice retreat. Glacial loading was

also proposed as a factor contributing to overpressures at

Cook Inlet, Alaska by Bruhn et al. (2000). However, tec-

tonically active settings host several processes that can

account for the overpressures, making the role of glaciation

especially difficult to evaluate.

Studies by Michael et al. (2000) and Michael & Bachu

(2001) are noteworthy for suggesting flexural unloading

to explain anomalously low pressures in Cretaceous shale

in the Alberta Basin. Other studies (Corbet & Bethke

1992; Neuzil 1993; Bekele et al. 2000) have shown

underpressures in such settings can be explained as

responses to long-term erosional unburdening. Although

Michael et al. (2000) and Michael & Bachu (2001) argued

that flexural rebound is important, they did not quantita-

tively evaluate it.

The work of Grollimund & Zoback (2000) evaluating

overpressure in the North Sea basin, discussed earlier,

deserves inclusion here. Although insufficient to explain

the overpressure, they concluded that flexural unloading

may have contributed up to 3.5 MPa of the 15 MPa total

overpressure.

Interpreting hypothesized linkages

All reported instances of glaciation-generated pressure

anomalies are more or less speculative; even the quantita-

tive analyses (Vinard et al. 1993, 2001; Vinard 1998;

Grollimund & Zoback 2000) are ambiguous. This has

more to do with the nonuniqueness of the problem than

the rigor of the analyses. Perhaps the strongest reason to

link certain anomalous pressures with glaciation is a rather

general and parsimonious one: glaciation is often the most

significant identifiable perturbation in the recent geologic

past. This ‘smoking gun’ argument must be tempered by

noting that no satisfactory explanations have been found

for a number of pressure anomalies in nonglaciated terrains

(e.g., Bredehoeft et al. 1994; Lee & Deming 2002).

CONCLUDING REMARKS

Despite the focus on direct loading in studies of effects of

glaciation on groundwater, other hydromechanical phe-

nomena may be of equal or greater importance. At present,

however, it is difficult to go much beyond generalities

because the phenomena in question are complex and, at

various levels, incompletely understood. These factors hin-

der efforts to describe, analyze, and predict how they may

alter groundwater pressures and flow.

Flexural loading and fracture permeability changes are

subsets of larger unresolved research questions. In the

case of flexure, for example, one finds a surprising lack of

consensus on how to describe it. In fact, Grollimund and

Zoback argue that ‘to investigate the possible stress

changes caused by lithospheric flexure accurately, it is

necessary to have direct stress measurements to calibrate

and test the models.’ But this poses its own difficulties

because it can be difficult to separate flexural and tectonic

stress, a problem Grollimund & Zoback (2000) them-

selves encountered. Lithosphere thickness, structure, and

flexural behavior, moreover, may differ in different global

settings.

Different hydromechanical phenomena can have dis-

tinctly different effects on groundwater flow and solute

mass transport. Substantial fluid pressure changes under

direct and flexural loading appear possible. Indeed, subsur-

face domains that are unable to drain may experience

repeated fluid pressure increases and decreases during suc-

cessive glacial cycles. However, little fluid or solute mass

transport is implied by or likely to result from them.

Rather, such excursions are expected when flow is minimal.

In contrast, stress-mediated changes in fracture permeabil-

ity may significantly increase or decrease groundwater flow

and solute transport. In fractured rock, permeability may

have been larger or smaller with differently oriented anisot-

ropy during glaciation, with corresponding changes in flow

rates and directions. This raises the intriguing possibility

that permeability permits movement of fluid mass during

Hydromechanical effects of continental glaciation 33

� 2011 Blackwell Publishing Ltd, Geofluids, 12, 22–37

part of a glacial cycle, while decreases in a later phase limit

further movement, creating pressure anomalies.

Not addressed in this paper is the two-way nature of

hydromechanical coupling. Just as deformation affects fluid

pressure, fluid pressure affects the mechanical behavior of

porous media. This was acknowledged by Johnston et al.

(1998) and alluded to by Chan et al. (2005) when

they noted abruptly decreased effective stress in their simu-

lations following ice sheet retreat. Such changes can

cause seismicity, fracturing, and faulting. Rock failure, in

turn, can feed back to the fluid pressure by changing per-

meability.

The picture this paper paints may be a daunting one,

and in addition to the complexity of glacial-hydrome-

chanical processes, some (e.g., Morner 2001) have

argued that they make certain geologic terrains too

unstable during glacial cycles to effectively isolate nuclear

waste for the long periods deemed necessary. Certainly,

complexities should not be underestimated nor poten-

tially important processes ignored. However, despite the

difficulties, scoping studies can probably do much to bet-

ter delineate the possible roles played by each hydrome-

chanical process. Also, new data on fluid pressures and

other and other aspects of glaciated terrains may provide

insights into how groundwater systems have responded

to glaciation. Pressure anomalies in particular may be the

hallmark of difficulty draining in response to glacial forc-

ing, a desirable trait for repository siting; a recent exam-

ple is dramatic underpressures in sedimentary rocks in

Ontario (Jensen et al. 2009) that may be glacial-hydro-

mechanical in origin. Such systems may contribute under-

standing if analyzed with all possible perturbing processes

considered.

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Volume 12, Number 1, February 2012ISSN 1468-8115

Geofluids

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Geofluids is abstracted/indexed in Chemical Abstracts

CONTENTS

1 EDITORIAL

REVIEW ARTICLES7 Glacial impacts on hydrologic processes in sedimentary basins: evidence from

natural tracer studiesJ.C. McIntosh, M.E. Schlegel and M. Person

22 Hydromechanical effects of continental glaciation on groundwater systemsC.E. Neuzil

38 Glacier-bed geomorphic processes and hydrologic conditions relevant to nuclearwaste disposalN. Iverson and M. Person

58 Models of ice-sheet hydrogeologic interactions: a reviewM. Person, V. Bense, D. Cohen and A. Banerjee

ORIGINAL ARTICLES79 Glaciation and regional groundwater flow in the Fennoscandian shield

A.M. Provost, C.I. Voss and C.E. Neuzil

97 Paleohydrogeologic simulations of Laurentide ice-sheet history on groundwater at the eastern flank of the Michigan BasinS.D. Normani and J.F. Sykes

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