Modeling of the palaeo-hydrogeological evolution of a fractured-rock aquifer
following the Champlain Sea Transgression in the St. Lawrence Valley (Quebec)
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
(*) (to contact first author: marc.laurencelle@ete.inrs.ca)
Abstract n°2343
① ② ③
1. I
NTRODUCTION
AND
O
BJECTIVES
The main objectives are to:
3. M
ETHODS
4. R
ESULTS
2. S
TUDY
AREA
&
ITS
INTRIGUING
“
BRACKISH
AREA
”
Qc
max. 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. C
ONCLUSION
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
adi m . e le v. in a qui ta rd (-)
C. Density-driven free convection in the rock aquifer
de pt h i n ro ck (m ) de pt h i n ro ck (m )
time (kyr) time (kyr)
time (kyr) early time (kyr) time (kyr)
multi-dimensional axis (-) q m : m ass f lu x ( g/ a) q m (g /a ) q (g m /a ) de pt h i n ro ck (m ) t: time (kyr)
A. Aquitard salinity evolution
during and after sedimentation B. Consolidation and drainage during sedimentation (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. Canada SW St. Lawrence Valley
Montérégie Est
-120 -100 -80 -60 -40 -20 0 -500 0 500 1000 1500 ice sheet global sea level ice load (thickness) E lev at ion ( 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
t
0
-13 simul. time cal ka BP 13 0
(c la y aq uit ar d) (r ock )
Brackish delimited groundwater Fresh GW with signif. salinity Thickness of the clay layer (m):
Montérégie Est study area
brackish gw zones
likely
(similar ctx) rel. continuous well-defined &
possible resid. 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); q
m: mass flux (M/T/L2) free convection, fingering + sinking, (hydrodyn. dispersion) qf → qm forced convection qf → leaching surface conditions subsurface processes RSL(t) clay acc.(t) consolidation qf → qm diffusion 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
2
0 4
RS
L
(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 0.25 2 0 4 sa l ( 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 0 2 0 4 cl ay (m
) & consolidated formed
0 30 tt=2ka n t2 t1 . . . moving clay top rock aquifer z+ B A 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 relative salinity
(uniform depth of model base = 1000 m)
A B C B C C B A excess pressure buildup Darcy velocities porosity reduction t+ c uc c uc c uc c uc di spe rs. CA → RA adv. S&C CA → RA adv. S&C CA→RA (bis)
conv. inflow surface → RA (@uc: unconfined)
conv. inflow (surface →) CA → RA (@c)
"c" : confined by clay(t) cover "uc" : unconfined (clay-free)
