Abstracts

First results from stable isotope analyses show groundwater depletion in δ 18 O and δ 2 H
that may result from permafrost thawing which recharges the aquifer with water depleted
in heavy isotopes. In contrast, shallow thermokarst lakes show enriched signals in δ 18 O
and δ 2 H resulting from evaporation. Groundwater hydrogeochemical data revealed mainly
HCO 3
– Ca water types. As expected, the shallow aquifer had lower concentrations in
dissolved ions compared with the deeper aquifer.
These results, combined with further field campaigns including additional site characterization
and water sampling as well as thermal and hydrogeochemical modelling, will
provide better understanding of the flow dynamics in the catchment to evaluate influences
of thawing permafrost on groundwater.
174 - Hydrostratigraphic and Groundwater Flow Models of a
Complex Unconsolidated Aquifer System, Nanaimo Lowlands,
British Columbia, Canada
N. Benoit, D. Paradis, J. Bednarski, & H. Russell
Geological Survey of Canada, Québec City, Québec, Canada
To support sustainable groundwater management, tridimensional (3D) hydrostratigraphic
and groundwater flow models were developed for an unconsolidated aquifer system in
the Nanaimo Lowlands, British Columbia (Canada). The study area is a coastal strip on
eastern Vancouver Island (~580 km 2 ). A 3D hydrostratigraphic model was developed using
existing well logs and published cross sections as well as new data from rotosonic coring,
borehole geophysics, seismic reflection surveys and surficial geology mapping. The detailed
surficial geology consists of 31 different units that are grouped into 8 major hydrostratigraphic
units, of which 5 correspond to aquifers and 3 to aquitards. These are (from the
surface down): Capilano-Salish (sand), Capilano glaciomarine (silty clay), Vashon-Capilano
(sand and gravel), Vashon (till), Quadra (sand), Dashwood-Cowichan (compact silt),
Mapleguard (sand), and siltstone to sandstone bedrock. This succession of Late Pleistocene
to Holocene sediments is up to 140 m thick and is present over most of the study area,
thinning to the southwest with rising topography and bedrock outcrops. Capilano-Salish
and Vashon-Capilano units are shallow aquifers with relatively high vulnerability to surface
contamination and low groundwater potential due to their limited thickness. The
Quadra sand is the most exploited aquifer unit. It underlies the ubiquitous low permeability
Vashon till and overlies Dashwood-Cowichan aquitard or the bedrock. The Quadra
has a thickness of up to 50 m in place and it is predominantly above sea level, which
minimizes issues of seawater intrusion. However, only about one-third of its thickness is
saturated likely due to: (1) the covering by the Vashon aquitard that limits groundwater
replenishment and (2) its deep incision by modern rivers that substantially drain its flanks.
The relatively low permeability sedimentary rock aquifer is also extensively exploited, but
only in areas where the Quadra aquifer is not present. Results of 3D groundwater flow
modelling based on the hydrostratigraphic model show that baseflow to major rivers is
provided mostly by groundwater seepage from heavily incised Quadra sand. Different regional
groundwater flow patterns are also observed reflecting stratigraphic controls: flow
IAH-CNC 2015 WATERLOO CONFERENCE
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