From: on behalf of Panel Registry Subject: FW: TNG registration of ...

Stratus Consulting Memorandum (4/16/2010)
�� The EIS shows PAG waste storage above the level of the supernatant pond in year 5 of
operations. As discussed in the previous section, the subaerial exposure of PAG waste
will increase the likelihood of acid generation in the impoundment.
�� The Proponents claim that extra water could be readily obtained from other sources
during low-probability dry events (droughts). In a water balance analysis that they
released last October, they state that the TSF pond volume would dry up completely in a
modeled drought scenario unless flows were supplemented by additional means. They
have not put forth a convincing argument that they will have the “additional means” to
make up for the shortfall.
�� The Proponents have proposed a plan in which water levels in the TSF remain static in
perpetuity, submerging the PAG waste and preventing acid generation without drying out
or overflowing. There is no proposed mechanism to ensure that PAG waste rock remains
submerged in perpetuity, presenting a high likelihood that the water levels will fluctuate
and/or that expensive retrofitting to maintain water levels will be required.
Other aspects of the TSF water balance are addressed below.
2.2 TSF Seepage
Under many of the Proponents’ water balance scenarios, they predict periods during which there
may be insufficient water to ensure waste in the TSF remains submerged. Under these scenarios,
tailings material and PAG waste will be exposed to the atmosphere. However, even these
scenarios are based on a likely underestimate of groundwater seepage out of the TSF. Infiltration
losses from the TSF to groundwater in all versions of the site water balance may be
underestimated for the following reasons:
�� The Proponents assume the hydraulic conductivity of the till underlying a portion of the
TSF is 1 x 10 -6 cm/s in their estimate of TSF seepage, but this value is five times lower
than the geometric mean conductivity from the hydraulic tests that were conducted in this
unit.
�� Hydraulic conductivity estimates from the basalt below the glacial till range across four
orders of magnitude (a factor of 10,000), which likely reflects localized control of
fractures on groundwater flow. However, the Proponents’ sensitivity analyses conducted
to estimate uncertainty in infiltration losses range by only a factor of 25. Seepage through
basalt bedrock will preferentially occur in high permeability zones, such that the actual
seepage rates in bedrock could be substantially higher than their maximum predicted
rates.
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