Pierre River Mine Project

WATER ERCB SIRS 46 – 79
Question No. 76
Request Appendix 4-1, Section 1.2.2.2, Page 26.
Section 7.1
Shell states, “The Sewetaken Fault property zone (Figures 10 to 12) was
introduced to reflect the possible influence of this fault on the groundwater flow
regime.” Shell further states, “A uniform specific storage value of 3.5 x 10-5 m-1
was applied to all model layers” and “The base of the local and regional
groundwater flow models was also defined as a no-flow boundary condition.”
76a Does the Sewetaken Fault extend to the Fort McMurray formation and further to
the top overburden formations?
Response 76a The Sewetakun Fault does not extend to the McMurray or overburden
formations. Since the Sewetakun Fault occurs on the Precambrian surface and
appears to influence hydraulic heads in the Middle Devonian aquifers, the
Sewetakun Fault property zone was only implemented in the model layers (layers
10 through 12) representing Devonian Formations (see EIA, Appendix 4-1,
Section 1.2.2.3, page 26).
Request 76b What were the geophysical techniques used to delineate the vertical and
horizontal geological structures within the project area?
Response 76b The Sewetakun Fault occurs at a depth below that expected for hydrogeological
interaction with project activities, so no delineation of this structural feature was
deemed necessary.
Request 76c How does a low specific storage value of 3.5 x 10-5 m-1 reflect the higher
Storativity values for the Quaternary formations (unconfined aquifers)?
Response 76c In the numerical model (MODFLOW), the layer type for the upper three of four
layers representing the Quaternary formations was specified to be Type 3. In
MODFLOW, a Type 3 model layer means that the transmissivity of the layer
varies based on the saturated thickness and hydraulic conductivity of the aquifer
and that the storage coefficient can vary between confined (specific storage) and
unconfined (specific yield) values. Therefore, if the model calculated the water
level in a cell to be below the top of the cell, the storage value would be
represented by the specific yield. In the upper three layers of the model, the
specific yield value was 0.1.
Request 76d Why were constant head values not assigned to the local and regional model
boundaries instead of the no-flow boundary?
7-34 Shell Canada Limited April 2010
CR029