Paleo-hydrogeological evolution of a fractured-rock aquifer followingthe Champlain Sea Transgression in the St. Lawrence Valley (Canada)

Marc Laurencelle(1)*, René Lefebvre(1) , John Molson(2) , Michel Parent(3)

(1) Institut national de la recherche scientifique, Centre Eau Terre Environnement, Québec, Canada(2) Université Laval, Department of Geology and Geological Engineering, Québec, Canada(3) Natural Resources Canada, Geological Survey of Canada (GSC), Québec, Canada

① ② ③

1. INTRODUCTION AND OBJECTIVES

The main objectives are to:

3. METHODS 4. RESULTS

2. STUDY AREA & ITS INTRIGUING “BRACKISH AREA”

Qcmax. extent

Conceptualisation of the palaeo-problem:

Numerical modeling is required to represent this coupled flow & transport problem (water density depends on salinity), in a system with complex geometry (e.g. topography) with spatial and temporal variations of model parameters and boundary conditions.

Numerical modeling approach:

5. CONCLUSION

i) Quantitative comparison of mass fluxes for different processes, alone (A, B, C) or coupled (A & C)

ii) A plausible reconstitution of the evolution of groundwater salinity and dynamics following deglaciation (A & C)

2Dxz(t) model flowchart for: C) variable density coupled flow & transport, with fully-transient

conditions at model surface

1Dz(t) model for: A) advective-dispersive salt transport in and below the forming aquitard;

B) expulsion of saltwater from the forming aquitard due to clay accumulation & consolidation

adim

. ele

v. in

aqu

itard

(-)

C. Density-driven free convection in the rock aquifer

dept

h in

rock

(m)

dept

h in

rock

(m)

time (kyr) time (kyr)

time (kyr) early time (kyr) time (kyr)

multi-dimensional axis (-)

q m: m

ass f

lux

(g/a

)

q m(g

/a)

q m(g

/a)

dept

h in

rock

(m)

t: time (kyr)

A. Aquitard salinity evolutionduring and after sedimentation

B. Consolidation and drainage duringsedimentation (S&C) of clays

t ≈ 2.5 ka t ≈ 8.2 ka

t = 13.0 ka ≈ 0 BP

This study aims to better understand changes in regional aquifer dynamics that followed the Champlain Sea marine incursion, ≈13 000 years ago.

1. reconstruct the evolution of groundwater salinity and dynamics following deglaciation, using physically-based numerical models;

2. identify the key palaeo-hydrogeologic processes involved;3. explain the present-day state of the system, especially the persistence

of brackish groundwater even at shallow depths;4. improve our understanding of high-amplitude marine transgression-

regression effects on groundwater systems in general.

Study area in Montérégie Est, Quebec, Canada

• The regional fractured-rock aquifer system underwent a complex late glacial history that left a sedimentary record and ~2 200 km2 of brackish groundwater.

• This brackish groundwater can be used as an indicator of the historical aquifer dynamics.

• The study area also benefits from high-quality data available from a regional hydrogeological assessment (aka “PACES”).

Density-driven free convection dominates salt transport in the rock aquifer, even in clay-confined areas. Consolidation of clay generates advective mass fluxes that bring salt into the rock aquifer. Diffusive fluxes from the base of the clay aquitard to the rock aquifer are negligible compared to other fluxes. Diffusive fluxes from the aquitard to the overlying sea / lake / surface are the dominant transport process

controlling the post-marine evolution of salinity in the aquitard and, hence, underneath. Density-driven saltwater fingers sink until they reach the brine “floor”; afterward, groundwater salinity is

progressively homogenized by spreading and mixing. In higher areas not covered by clay, salty water leaching by fresh groundwater is substantial, whereas leaching

is very limited under the aquitard, thus explaining the presence today of brackish groundwater in the aquifer.

CanadaSW St. Lawrence ValleyMontérégie Est

-120 0-100 -80 -60 -40 -20-500

0500

10001500

ice sheet

global sea level

ice load (thickness)

Ele

vatio

n (m

)

Last glacial period timeline (cal ka BP)

(post-glacial epoch)

(LGM)

Surface conditions during the last glacial cycle(non-local example data for Waterloo, Ontario, Canada; adapted from Lemieux et al., 2008)

t ≈ 1.0 ka

t0

-13130

simul. timecal ka BP

(cla

y aq

uita

rd)

(roc

k)

Brackish delimited groundwater

Fresh GW with signif. salinityThickness of the clay layer (m):

Montérégie Est study areabrackish gw zoneslikely

(similar ctx)well-defined &rel. continuous

possibleresid. sal.

(St. L. River)

30 km

N

(wide cross-section)

(wide cross-section)

Wide cross section A-B-C of St. L. Valley topography

(m)

(km)

Map source: Geol. Surv. Can. O.F. n°6960

red: salinity (TDS); qf: fluid flux (L3/T/L2); qm: mass flux (M/T/L2)

free convection, fingering + sinking,

(hydrodyn. dispersion)qf → qm

forcedconvection

qf → leaching

surface conditions

subsurface processes

RSL(t)

clay acc.(t)consolidation

qf → qmdiffusion

qm

stagnant brines

sea / lake

clay aquitard

REG. ROCK AQUIFER

salinity(t)

Conceptualization of palaeo-conditions (at model surface):

RSL

sal

clay

Relative sea (or lake) level (RSL)flooding lower elevations

≈ 2-stage linear decrease

170

0

Champlain Sea

Lake Lampsilis

20 4

RSL

(m)

Relative time (ka)

Salinity of flooding water(at bottom of the water bodies)

≈ constant at max. then linear decrease late in the C.S. stage

35

0.2520 4

sal (

g/L) max. salinity

seawater at sea bottom (stratified

sea)

incr. vertical mixing of deep & shallow sea waters

lacustrine env. freshwater

Clay & silt accumulation at sea bottom (from settling of fines)

≈ linear thickening, mostly during the C.S. stage

30

020 4

clay

(m) formed

& consolidated

0

30t=2ka

tn

t2t1

. . . movingclay top

rockaquifer z+

BA

time+

GW flow solution → p or hfw → (qx , qz)

ρ ← c ← solute transport solution

changing BC's

qz-dep., changing BC's

≥ csw,max

≤ cfw

relativesalinity

(uniform depth of model base = 1000 m)

AB

C

B

CCBA

excess pressure buildup

Darcy velocities

porosity reduction

t+

c uc

c uc c uc

c uc

dispers.CA →

RA

adv. S&C CA → RA

adv. S&CCA→RA

(bis)

conv. inflow surface → RA (@uc: unconfined)conv. inflow (surface →) CA → RA (@c)

"c" : confined by clay(t) cover "uc" : unconfined (clay-free)

color scale:

IAH – Regional Groundwater Flow CommissionCalgary Symposium 2017"Characterizing regional groundwater flow systems:Insight from practical applications and theoretical development"26 – 28 June, 2017 Calgary, Alberta, Canada