Detecting preferential seepage along casing installed in fractured rock aquifers:
Examples from the Saguenay-Lac-Saint-Jean region, Canada.
Sandra K. Richard, Romain Chesnaux, Alain Rouleau
Département des sciences appliquées – Université du Québec à Chicoutimi, Chicoutimi, Québec,
Canada
sandra.richard@uqac.ca
KEY WORDS – Preferential flow, casing, constant head permeability test, pumping test, numerical modeling
ABSTRACT
Installing a well in a fractured rock aquifer requires a good sealing of the casing into the bedrock to avoid preferential
seepage along the casing from the granular aquifer to the fractured rock aquifer. An improperly sealed casing can
generate a direct hydraulic connection between both aquifers. Such connections can have important consequences
regarding groundwater quantity and quality. Cross-contamination issues between aquifers can indeed occur if a
hydraulic gradient exists between the two aquifers. This study proposes a methodology to detect a preferential flow
along an improperly sealed casing based on interpreting the observed hydraulic response to a variable-head
permeability test and a pumping test. Three experimental sites in the Saguenay-Lac-Saint-Jean region in Quebec
(Canada) have been considered. At two sites, a well is installed in a crystalline gneissic rock (Precambrian) whereas a
well is installed in a limestone unit (Ordovician) at the third site.
A variable-head permeability test was first conducted in the three wells. The velocity graph allows verifying if the
casing is properly sealed in the bedrock. The theoretical behavior of the descending velocity as a function of the
hydraulic head difference is a straight line. Instead, a curve was obtained for 2 of the investigated wells, suggesting
the possible occurrence of a preferential seepage. Furthermore, the response to the pumping test conducted on each
well was also analysed using the log-derivative of the drawdown as a function of time. The log-derivative drawdown
curve is sensitive to variations induced by the different hydraulic conditions encountered during the test. The different
slopes observed in the log-derivative curve can be attributed to particular sequences of flow dimensions, some of
which being characteristic of hydraulic short-circuits. A numerical model is then used to verify the hydraulic behavior
of the aquifer system constituted of a fractured rock unit and a granular unit, where the two units are hydraulically
connected through the defective seal of the casing.