Abstracts

and provide the primary contaminant transport pathways, with flow velocities of a few to
several meters per day possible. However, the rock matrix blocks between fractures have high
porosity (~5-20%) and matrix diffusion causes transfer of nitrate from groundwater flowing
in fractures to the matrix. This can be viewed as a positive effect in that rates of transport
in fractures and downgradient nitrate flux is attenuated, which may be reducing short-term
impacts to water supply wells and groundwater discharge areas since the front of the contaminated
zone is expected to move much more slowly. However a negative consequence of the
large nitrate storage in the matrix is potential for slow release via back diffusion, which can
cause long-term nitrate persistence following declines in nitrate inputs due to changes in agricultural
practices. In this study, a portion of an agricultural field was taken out of production
for a five year period to examine downgradient effects. Field activities included instrumentation
of the field from upgradient to downgradient along the flow system using multilevel
monitoring systems for temporal groundwater sampling, continuous coring to assess the nitrate
distribution in detail in overburden and in the bedrock matrix, and an array of core and
borehole measurements to provide matrix and fracture parameters for modeling. Numerical
modeling was conducted to examine matrix diffusion effects using a coupled Equivalent Porous
Media (EPM) – Discrete Fracture Network (DFN) approach. First a 3-D watershed
scale EPM flow model was developed and calibrated using the HydroGeoSphere code. This
flow model provides information on the bulk groundwater flow system (Darcy flux, hydraulic
gradients) needed to inform 2-D DFN transport simulations using the FRACTRAN
code, which incorporates key processes controlling contaminant transport in fractured porous
media. The DFN model was then used to examine matrix diffusion effects for scenarios
with estimated historical nitrate source inputs followed by removal of the source input. Both
the field datasets and numerical modeling show strong matrix diffusion effects, acting as an
impediment to aquifer restoration. Such effects must be considered when assessing efficacy
of implementation of best management practices (BMPs). Sensitivity to fracture network
characteristics is evaluated with the DFN modeling informed from field data.
194 - Decadal scale groundwater nitrate concentrations in
Western Prince Edward Island
Jessica A. Guselle & M.C. Ryan
Department of Geoscience - University of Calgary, Calgary, Alberta, Canada
G. Somers
Department of Environment, Labour, and Justice, Charlottetown, Prince Edward Island,
Canada
Y. Jiang
Crops and Livestock Research Centre, Agriculture and Agri-Food Canada, Prince Edward
Island, Canada
Prince Edward Island (PEI) relies wholly on groundwater for its potable water supply, and
has a large percentage of land under agricultural cultivation (approximately 50%). In addition
to drinking water concerns, elevated groundwater nitrate in PEI is linked with estuarine eutrophication,
as groundwater discharge accounts for 60-70% of the baseflow to rivers. Decadal
scale increases in river nitrate concentrations have been observed in most of PEI’s watersheds.
IAH-CNC 2015 WATERLOO CONFERENCE
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