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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