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

Stratus Consulting Memorandum (4/16/2010)
�� Given the underestimate in lag time to acid generation, we believe there is a substantial
risk that exposed wall rock in pit walls will generate large quantities of acid and
contaminants before the pit is inundated.
�� Similarly, we believe there is a substantial risk that the PAG waste stored within the
tailings impoundment can generate acid before it is submerged, or if the tailings
impoundment becomes unsaturated in the future.
Each of the factors that Dr. Maest will discuss support our conclusion that the Proponents have
greatly underestimated the potential of this mine to generate substantial quantities of acid,
metals, and other contaminants.
2. Mine Site Water Balance and Hydrogeology
As we discussed in our previous report last November, we believe that the limited site-specific
data on which the Proponents rely are not sufficient to support the probabilistic water balance
models that they have generated for the Prosperity Project. Their recent responses to our
November report have not adequately addressed many of our initial concerns. In addition, we
believe that the Proponents’ models of contaminant transport in groundwater fail to account for
the large uncertainty in their underlying data, potentially resulting in unforeseen groundwater
contaminant plumes and discharge of contaminants to Big Onion Lake and, potentially, the
Taseko River.
Dr. Maest will summarize some of the shortcomings in the hydrometeorology modeling and will
review remaining concerns, as well as provide examples of failures in hydrologic predictions at
other mine sites (see Section 3). In summary, we are concerned that if the water balance does not
adequately account for the wide range of potential variations in water input over time, there
remains a significant risk that water levels in the TSF will not be maintained, allowing oxidation
of mine waste materials when water levels drop during dry periods, and subsequent transport of
contaminants via groundwater seepage and from releases during storm events.
2.1 Maintaining Water Levels in TSF
The Proponent has proposed inundating tailings and PAG rock in the TSF, where a static water
level will need to be maintained in perpetuity. If an event such as a drought or unforeseen
seepage causes water levels in the TSF to drop, sulfides in the waste material will oxidize,
resulting in acid generation and metals leaching. Specific criticisms of the Proponents’ plan for
the TSF include the following:
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