Ground Water Studies Summary

Groundwater

Groundwater Infiltration-Fate Transport Modeling

Groundwater studies looked at the potential for impact on water quality at the site, as well as on surrounding aquifers. The studies also provide predictions for the state of the open pit lake after project operations cease.

22 22

rosemont rosemontcopper copper- -aabridge bridgeto toaasustainable sustainablefuture future

TITLE: Rosemont Area-Wide Fate and Transport and DIA Summary RELEASE DATE: August 2010 PREPARED BY: Tetra Tech Brief Summary: As part of the effort to protect groundwater, Rosemont Copper has studied water movement in an area covering 475 square miles to assess the possible impact of its mine on the area aquifer. This Tetra Tech study looked carefully at pit dewatering (removing water for the mining operation) and aquifer recharge. The analysis concluded that there would be little impact on groundwater quality.

Rosemont carried out a Discharge Impact Area (DIA) analysis by reviewing the “fate and transport” of the materials being managed. (“Fate” refers to possible chemical reactions that will form new substances, and “transport” refers to migration of chemicals.)

The analysis drew conclusions regarding two main issues: 1) Pit dewatering (removal of water from the open pit during mining), and evaporation from the lake formed after closure of the mine, will create a stable groundwater capture zone where water will flow into the lake. The model showed this activity will have no impact on groundwater quality.

2) Aquifer recharge: Outside the capture zone, the operations have the potential to impact down-gradient groundwater quality, but the anticipated quality of recharge from the dry-stack tailings and the mine’s flow-through drain system will be equivalent to or better than the current background groundwater quality for all analyzed parameters. Thus project recharge is not predicted to impact down-gradient groundwater quality.

The DIA report responds specifically to a series of other questions from the Arizona Department of Environmental Quality (ADEQ) concerning hydrology and engineering issues.

This technical memorandum documents the results of Rosemont’s analysis, based on regional groundwater

flow models. Such tests are required by the ADEQ as part of the Aquifer Protection Permit application.

TITLE: Infiltration, Seepage, Fate and Transport Modeling Report RELEASE DATE: February 2010 PREPARED BY: Tetra Tech Brief Summary: In its efforts to protect the environment and groundwater, Rosemont asked Tetra Tech to run modeling analysis on infiltration (entering precipitation) and seepage (outward flow), and on “fate and transport.” The report found there would be little or no seepage under average climate conditions; a passive treatment system will ensure that any drain-down will be non-polluting and a 20-feet-thick cover of waste rock placed over spent ore and ponds will help protect area groundwater.

Here are some key findings: • The Waste Rock Storage Area (WRSA), Heap Leach Facility (HLF), and Dry Stack Tailings Facility (DSTF) are not expected to impact the regional groundwater system in terms of infiltration and seepage.

• After the operations are closed, the former leach solution ponds may be converted into a passive treatment system, and any drain-down would pass through the treatment system before being discharged.

• The high evaporation rate in the area will limit the infiltration and prevent seepage from the WRSA under average climate conditions.

• For the DSTF, results of the modeling were compared to the Arizona Aquifer Water Quality Standard (AWQS); none of the constituents exceeded those standards.

• For the HLF, any seepage from the heap will be captured on the liner and report to the leach solutions ponds for treatment.

• Fate and transport modeling further supports the conclusion that the three facilities are not expected to impact the regional ground water system.

In general, construction and eventual closure plans will result in effective systems.

TITLE: Infiltration, Seepage, Fate and Transport Modeling Report, Revision 1 RELEASE DATE: August 2010 PREPARED BY: Tetra Tech Brief Summary: Rosemont will construct three major facilities in addition to the Open Pit: a Waste Rock

Storage Area (WRSA), a Heap Leach Facility (HLF), and a Dry Stack Tailings Facility (DSTF). Engineering studies show these facilities will have little impact on groundwater quality or quantity.

This Tetra Tech report focuses on potential impact of those three facilities on regional groundwater after closure. Rosemont will re-grade, cover, and revegetate the Rosemont Ridge Landform, which is the consolidated WRSA, HLF and DSTF. Seepage is one environmental concern. Tests showed that no seepage of collected water from precipitation (rainfall or snowmelt) occurred under average conditions from any of the facilities, nor did seepage develop from the WRSA or the HLF for two storm events modeled.

Fate and transport modeling shows that there may be a potential for other seepage to reach the base of the Waste Rock Storage area. That seepage may have constituents slightly above the Arizona Aquifer Water Quality Standard, but in these cases the concentrations will not be higher than the natural background levels in the area.

In general, detailed laboratory studies and predictive modeling show that the three facilities will have little or no impact on the quality or quantity of water in the regional groundwater system.

TITLE: Rosemont Infiltration, Seepage, Fate and Transport Response to Comments RELEASE DATE: November 2010 PREPARED BY: Tetra Tech

Long Term Pumping Tests TITLE: Analysis of Long-Term, Multi-Well Aquifer Test Volumes 1-4 and Final RELEASE DATE: May 2009

Miscellaneous Technical Memoranda TITLE: Rosemont APP-Regulated Facility Depth to Groundwater RELEASE DATE: August 2010 PREPARED BY: Tetra Tech TITLE: Dewatering for the Planned Rosemont Mine RELEASE DATE: November 2007 PREPARED BY: Call & Nicholas, Inc

Brief Summary: Groundwater in mine slopes can create pressures that can weaken the slopes (resulting in a loss of shear strength), and this in turn can result in flatter mine slopes or slope failures. Rosemont’s approach to pit dewatering (removal of water for mining) thus will depend on slope stability requirements.

Depressurization by dewatering will be required at three locations: the east wall of the Willow Canyon Formation; the south wall alluvial conglomerate; and the northwest wall Bolsa Quartzite.

A memorandum from Call & Nicholas, Inc. (CNI) outlines several alternatives for dealing with mine dewatering: • Use of horizontal drains drilled into bench faces • Pumping from vertical wells • A combination of horizontal drains and pumping wells • An underground drift from which fans of dewatering holes are drilled behind the pit slope • No active depressurization except what results from the mining of the pit CNI recommends that a cost-benefit analysis be done to further study the tradeoff between dewatering costs and reduced stripping costs. CNI also recommends techniques specific to each location.

TITLE: Rosemont Infiltration Analysis RELEASE DATE: April 2010 PREPARED BY: Tetra Tech TITLE: Comparison of Natural Fluctuation in Groundwater Level to Provisional Drawdown Projects, Rosemont Mine RELEASE DATE: March 2010 PREPARED BY: Montgomery & Associates TITLE: Rosemont Well Inventory Update RELEASE DATE: January 2010 PREPARED BY: Tetra Tech TITLE: Responses to SRK’s Technical Review Comments on Tetra Tech’s Groundwater Flow Model Technical Memoranda RELEASE DATE: November 2010 PREPARED BY: Tetra Tech

Monitoring Reports TITLE: Results of Phase 2 Hydrogeologic Investigations and Monitoring Program RELEASE DATE: February 2009 rosemont copper

- a bridge to a sustainable future

23

PREPARED BY: Montgomery & Associates, Inc. SUMMARY: Available (Vol. 1: Text, Tables and Figures) TITLE: Results of Phase 2 Hydrogeologic Investigations and Monitoring Program RELEASE DATE: February 2009 PREPARED BY: Montgomery & Associates, Inc. SUMMARY: Available (Vol. 2: Appendix A and B) TITLE: Results of Phase 2 Hydrogeologic Investigations and Monitoring Program RELEASE DATE: February 2009 PREPARED BY: Montgomery & Associates, Inc. SUMMARY: Available (Vol. 2: Appendix C) TITLE: Analysis of Long-Term, Multi-Well Aquifer Test, November 2008 through January 2009 RELEASE DATE: May 2009 PREPARED BY: Montgomery & Associates, Inc. Brief Summary: Montgomery & Associates conducted a long-term, multi-well aquifer test in and near the planned mining operations site as part of the evaluation of the hydro geologic characteristics of the Rosemont site. To understand the capacity of the aquifer to transmit groundwater, Montgomery & Associates tested a number of wells near the proposed mine site and found the aquifer was moderately transmissive where ground conditions were strongly faulted and fractured, and was less so in other poorly fractured areas.

The study pumped water from five wells and monitored 46 observation wells and piezometers (small-diameter observation wells), four springs and the McCleary Dam underflow. The results showed that with one exception, no apparent change in spring flow occurred as a result of test pumping. Flow at one spring stopped during the test, most likely due to its close proximity to one of the pumped wells.

The study cautioned that only a small portion of the total Rosemont aquifer, the part nearest the operations can be characterized by this testing program. The report does not address more distant areas.

Montgomery Modeling TITLE: Groundwater Flow Modeling Conducted for Simulation of Proposed Rosemont Pit Dewatering and Post-Closure RELEASE DATE: October 2009 PREPARED BY: Montgomery & Associates, Inc. 24

rosemont copper

- a bridge to a sustainable future

Brief Summary: Montgomery & Associates’ engineering studies simulated the hydrologic impact of the Rosemont operations during operations and 100 years after its closure. The studies found little expected impact on area groundwater during that 122-year period. The company examined groundwater flow during pit development, during pit dewatering and for 100 years of groundwater recovery and potential pit lake development after closure of the mine.

The study was expanded to encompass the Cienega Creek basin using data from previous investigations.

The data also projected that impacts on the basin would be negligible.

Projected pit inflows, pit lake development, and impacts on the ground water system are consistent with observations in similar open pit mines in fractured rock flow systems in arid climates. Projected pit inflow over the 122-year simulation period is approximately 200 gallons per minute (gpm). Based on projections concerning the pit lake 100 years after closure, it is estimated that the pit lake will remain a non-polluting hydraulic sink.

TITLE: Groundwater Flow Modeling Conducted for Simulation of Proposed Rosemont Pit Dewatering and Post-Closure, Vol. 1 RELEASE DATE: August 2010 PREPARED BY: Montgomery & Associates, Inc. Brief Summary: There has been considerable interest in Rosemont’s very long-term effects on the area groundwater. Montgomery & Associates engineering projections indicate that 1,000 years after closure, the open pit lake will remain a complete hydraulic sink, ensuring containment of the pit lake water. More distant from the proposed pit, the extent of the groundwater drawdown contour is less reliable due to the unknown degree of hydraulic connections, variations in aquifer properties and discrete features inherent in the fracture rock system.

Model projections indicate that the groundwater level drawdown 1,000 years after the end of mining will affect local riparian areas and may affect perennial streamflow, but the quantity of any reductions in streamflow will be negligible. Mining is not expected to disrupt the groundwater flow system in the project area and for a majority of the Cienega Creek basin.

Tetra Tech Modeling TITLE: Groundwater Flow Model Construction and Calibration RELEASE DATE: July 2010 PREPARED BY: Tetra Tech Brief Summary: Tetra Tech describes the construction of three regional groundwater flow models representing pre-mining (steady state), active mining, and postclosure mining conditions. Calibration of the steadystate model also is presented. These models were used to assess the potential impact on regional water resources due to the Rosemont project.

TITLE: Hydraulic Property Estimates RELEASE DATE: July 2010 PREPARED BY: Tetra Tech Brief Summary: Tetra Tech reevaluated hydraulic tests conducted by Montgomery & Associates as part of their hydrologic characterization studies. The resulting hydraulic properties (hydraulic conductivity and storage) were used to constrain the flow model calibration.

• The short-term, single-well tests were reanalyzed based on Tetra Tech’s test interpretations. Although there were differences with Montgomery & Associates interpretations, the overall range and average of estimates was essentially unchanged.

• The long-term, multi-well tests were analyzed with two-dimensional, radial-flow models. This analysis resulted in updated horizontal and vertical hydraulic conductivity estimates and specific-storage estimates.

TITLE: Predictive Groundwater Flow Modeling Results RELEASE DATE: July 2010 PREPARED BY: Tetra Tech Brief Summary: In this technical memorandum, Tetra Tech describes how the open pit dewatering and the subsequent permanent lake (pit lake) formation upon mining cessation will affect the regional groundwaterflow system. Predictive simulations at the end of mining, and at 20 years, 50 years, 150 years and 1,000 years post-mining are presented.

At a glance, removing water from the Rosemont open pit (dewatering) will continue throughout the 20 to 25 years of operation, resulting in a cone-shaped depression of the water table. After closure, the pit will naturally refill from groundwater, surface-water and precipitation, and a pit lake will form. It is expected that the pit will remain a perpetual hydraulic sink at a stabilized (equilibrium) condition due to the high

evaporation rate in the area.

Some groundwater will perpetually flow into the pit lake, although at a much lower rate after the active mining and dewatering process is completed, and that amount will steadily decrease. Groundwater capture is accounted for from several sources: significant recharge increases due to flow-through drains, changes in flows from lateral boundary conditions, and stream flow capture. Additionally, changes to riparian vegetation evapotranspiration accounts for a small percent of groundwater flow to the pit lake.

Any uncertainty in these predictions will be assessed as part of the sensitivity analysis to be documented in companion technical memorandums.

presents the sensitivity analyses performed on three groundwater flow models for the Rosemont Copper project. The models represent pre-mining steady state conditions, active mining conditions, and post-mining/ post-closure conditions. Sensitivity analysis parameters include horizontal and vertical hydraulic conductivity, hydraulic conductivity of the quartz-porphyry dike, specific storage, specific yield, and recharge. In addition, parameters specific to the pit-lake development in the post-closure model were evaluated to determine the sensitivity to changes in precipitation, evaporation, and pit-wall runoff. For completeness, this memorandum also includes the steady-state model sensitivity analysis summarized in a previous memorandum.

TITLE: Hydrogeologic Framework Model RELEASE DATE: July 2010 PREPARED BY: Tetra Tech Brief Summary: This Tetra Tech technical memorandum discussed how the two-dimensional hydrogeologic data developed and reported by Montgomery & Associates was transformed into a three-dimensional (3D) hydrogeologic framework model. Consistency checks confirmed that the three-dimensional model appropriately represented the horizontal hydrogeologic slices and the vertical cross sections developed for the model domain. The 3D framework model was then used as the geologic basis for the Tetra Tech threedimensional regional groundwater flow model.

TITLE: Regional Groundwater Flow Model RELEASE DATE: November 2010 PREPARED BY: Tetra Tech Brief Summary: In addition to the regional groundwater flow model developed by Montgomery & Associates, Tetra Tech prepared a second model for the purpose of predicting possible impacts to groundwater resources due to the planned Rosemont project. This model was developed to predict the potential effects on the surface-water and groundwater hydrology of Davidson Canyon and the Cienega Creek Basin, and included the effects on groundwater levels, spring flows, surfacewater flows, and riparian areas.

TITLE: Steady-State Sensitivity Analyses RELEASE DATE: July 2010 PREPARED BY: Tetra Tech Brief Summary: This Tetra Tech technical memorandum discusses the parameter sensitivity analysis performed on the steady-state groundwater flow model. The model inputs with the most impact on the steady-state model fit and modeling analyses were identified. The sensitivity analysis indicated that nearly optimal parameter values were used in the steady-state model. Only small model fit improvements to the observed data were indicated for select parameters. However, simulations with these parameter values resulted in unrealistic conditions, such as water levels above land surface.

TITLE: Groundwater Flow Model Sensitivity Analysis RELEASE DATE: August 2010 PREPARED BY: Tetra Tech Brief Summary: This Tetra Tech technical memorandum

TITLE: Well Results E-1 RELEASE DATE: April 2007 PREPARED BY: Montgomery & Associates, Inc.

Montgomery Model TITLE: Groundwater Flow Modeling Conducted for Simulation of Rosemont Copper’s Proposed Mine Supply Pumping, Sahuarita, Arizona RELEASE DATE: April 2009 PREPARED BY: Montgomery & Associates, Inc. TITLE: Addendum to Groundwater Flow Modeling Conducted for Simulation of Rosemont Copper’s Proposed Mine Supply Pumping Sahuarita, Arizona RELEASE DATE: October 2010 PREPARED BY: Montgomery & Associates, Inc.

West Side of the Santa Rita Mountains Miscellaneous Technical Memoranda TITLE: ADWR Tech Memo April Update RELEASE DATE: April 2009 PREPARED BY: Montgomery & Associates, Inc. TITLE: Updates to ADWR Model in Sahuarita/Green Valley Area RELEASE DATE: December 2008 PREPARED BY: Montgomery & Associates, Inc.

Monitoring Reports TITLE: Results of Construction, Development and Testing for Production Water Well (D-17-14) 21 add [RC-2] RELEASE DATE: April 2009 PREPARED BY: Errol L. Montgomery & Associates, Inc. rosemont copper

- a bridge to a sustainable future

25