Abstracts

Surface conditions have changed thoroughly over North America during the last glacial-deglacial
cycle and postglacial time (80-5 ka BP). Our study area, Montérégie Est (~9 000 km 2 ),
was covered by the Laurentide Ice Sheet during ~20 ka, until the isostatically depressed St.
Lawrence Valley became ice-free, by about 13 ka BP. These lowlands were briefly occupied by
Glacial Lake Candona, a large freshwater body that preceded the incursion of marine waters
from the Atlantic Ocean. During the early part of this incursion, known as the Champlain
Sea, the deep water body was strongly stratified, with light meltwater-rich surface waters
overlying dense polyhaline seawater. However, the salinity of Champlain Sea water decreased
rapidly, due to the combined effects of sustained meltwater production and isostatic rebound,
which progressively prevented Atlantic seawater from entering the valley upstream of Quebec
City. As the seawater influx had decreased to a minimum, a lacustrine successor basin,
known as Lake Lampsilis, persisted in the central valley. Subsequently, the drainage system
in the lowlands evolved towards its present-day configuration through continued isostatic
adjustment. Along with those spatio-temporal variations in water level and salinity, silts and
clays had deposited during the glaciolacustrine and glaciomarine episodes. These sediments
formed thick low permeability units that retarded the transfer of salt between the marine water
body and the underlying rock aquifer, but also impeded the later flushing of brackish water
from the aquifer, which still contains salty groundwater of marine origin over 2 200 km 2 .
The main objective of this research is to develop a quantitative reconstitution scenario for
the evolution of groundwater salinity within the Montérégie Est regional aquifer system
following deglaciation. To do so, the palaeo-hydrogeological problem was conceptualized
using time-dependent boundary conditions and material properties, while the regional
fractured-rock aquifer system was idealized as a double-porosity equivalent porous medium
with permeability decreasing with depth. A fully coupled 2D density-dependent flow
and mass transport numerical model was set up to simulate the post-marine migration of
salt along a vertical cross-section within the study area. Modeling first assessed the relative
influence of processes and model parameters. Notably, the impacts of transient storage of
salt and fluid in dead-end fractures, on both solute and age mass transport, were investigated.
Overall, results show that salinization of the aquifer below the Champlain Sea was a
density-driven process, with subsequent desalinization being more efficient where silts and
clays had not formed thick confining units.
213 - Nano-scale Colloid Particles Transport in Variable-aperture
Sandstone Rock Fracture
Ertiana Rrokaj, Pulin K. Mondal & Brent E. Sleep
Department of Civil Engineering – University of Toronto, Toronto, Ontario, Canada
In Canada, many communities and over thirty percent of the population rely exclusively on
groundwater for meeting their water demand. These groundwater resources are prone to contamination
by leaking septic tanks, gasoline storage tanks, and municipal landfills. Fractured
rock aquifers are particularly vulnerable to contamination due to the high conductivity pathways
associated with rock fractures. Previous studies have investigated the transport of contaminants
through volcanic rock aquifers, porous shale saprolite, and clay-rich till bedrock, but very
few studies have investigated colloid particles transport through sandstone fractures.
80 IAH-CNC 2015 WATERLOO CONFERENCE