Pierre River Mine Project

WATER AENV SIRS 15 – 43
ii. What was the minimum and maximum predicted rate?
Section 12.1
Response 17b i. The predicted seepage rate from the Pierre River Mine external tailings
disposal area (ETDA) during operations was determined using the Pierre
River Mine local groundwater flow model (see EIA, Appendix 4-1, Section
1.2.4, page 93). The hydrogeology modeling information provided in
Appendix 4-1 describes the geological surfaces, model limits, parameters,
boundary conditions and calibration.
ii. During operations, the minimum predicted ETDA seepage rate was 50 L/s,
whereas the maximum predicted ETDA seepage rate was 550 L/s (see EIA,
Volume 4A, Section 6.3.6.2, Table 6.3-18).
Request 17c What is the degree of confidence in the rate of 240 L/s?
Response 17c The degree of confidence in the estimated seepage rate of 240 L/s is considered
moderate. Because of the conservatism built into the prediction of the seepage
rate, as described in the following, the predicted seepage rate of 240 L/s is
expected to be representative of an upper-end estimate.
The seepage rate from the ETDA ultimately depends on:
• the rate at which the ETDA deposits can transmit water vertically
• the hydraulic conductivity of the aquifer underlying the ETDA, reflecting the
capacity of the aquifer to transmit the seepage from the ETDA deposits
The ETDA was represented in the model with a general head boundary (GHB).
Vertical tailings water transmission rates were controlled by the conductance (C)
of the GHB (which depends on the thickness and hydraulic conductivity [K] of
the tailings) and specified fluid levels in the tailings, as discussed in the response
to AENV SIR 18fi.
ETDA Thickness for GHB
The thickness of the ETDA was conservatively represented as 5 m, although the
ETDA design thickness is 45 m. In reality, the thickness of the ETDA will
increase with time through the active lifetime of the ETDA. This assumption of a
reduced thickness leads to more conservative seepage rates.
ETDA Hydraulic Conductivity for GHB
The vertical hydraulic conductivity of the ETDA deposits was assigned one value
(1 x 10 -8 m/s) for the entire ETDA footprint. The hydraulic conductivity of the
tailings deposits will vary based on the composition and deposition method of the
tailings. The vertical hydraulic conductivity would be about 1 x 10 -10 m/s for
thickened tailings (TT), about 1 x 10 -8 m/s for mature fine tailings (MFT), and
about 2.5 x 10 -6 m/s for tailings sand (Volume 3A, Appendix 3-1, Tables 1-6 and
April 2010 Shell Canada Limited 12-7
CR029