Pierre River Mine Project

WATER ERCB SIRS 46 – 79
Section 7.1
Response 76d The lateral boundaries of the regional model were chosen to approximate natural
hydrogeologic boundaries (i.e., groundwater flow divides). Given the large
distance from these lateral model boundaries to the stresses (overburden
dewatering and basal aquifer depressurization) investigated in the regional
model, the location of these groundwater divides is not affected by these stresses.
Therefore, the no-flow boundary representation is conceptually more consistent
with the natural functioning of a groundwater divide than a constant head
boundary.
The lateral boundaries of aquifers represented in the local models were assigned
General Head Boundary (GHB) conditions to represent regional hydraulic
gradients. Head values assigned to these GHBs were derived from the regional
model results. Lateral boundaries of aquitards represented in the local models
were assigned no flow boundary conditions (consistent with predominantly
vertical flow in these aquitards).
In the case of the model bases, no significant groundwater flow exchange is
expected between the Precambrian and Devonian formations (i.e. the Devonian
formations are interpreted to form the base of groundwater drainage in the
Athabasca River basin) and, therefore, a no-flow boundary is appropriate.
Request 76e How was lateral horizontal recharge from various sources accounted for (e.g.
Birch Mountain)?
Response 76e As stated in the response to ERCB SIR 76d, the lateral boundaries of the regional
model were chosen to approximate groundwater flow divides, which are natural
hydrogeologic boundaries. This means that lateral groundwater flow does not
occur across these boundaries. Groundwater recharge enters the flow system
vertically through the top model boundary of the regional model.
Question No. 77
Also stated in the response to ERCB SIR 76d, the lateral boundaries of aquifers
represented in the local models were assigned General Head Boundary (GHB)
conditions to represent regional hydraulic gradients. These GHBs account for the
lateral horizontal recharge from other sources, including Birch Mountain.
Request Appendix 4-1, Section 1.2.2.2, Page 28.
Shell states, “Tailings area seepage was not represented in the regional model
because the purpose of the model was to simulate basal aquifer depressurization
and overburden dewatering. Tailings area seepage was represented in the JEMA
and PRMA local models.”
77a As both stresses (source and sink) are occurring simultaneously, how does
simulating basal aquifer depressurization and overburden dewatering provide
April 2010 Shell Canada Limited 7-35
CR029