Hydrogeologic Investigations for the Sierrita Mine APP
M&A characterizes the complex flow and geochemical conditions within mine tailings and an underlying aquifer to support ongoing environmental compliance requirements.
Client: Freeport-McMoRan Copper & Gold- Time frame: 1986–present
- Location: Southern Arizona
This active copper and molybdenum mine is located in southern Arizona. It features an open pit for ore extraction, a conventional mill and concentrator, a heap-leach system, and a solvent-extraction plant. Tailings from the mill are deposited in an impoundment that partially overlies a regional basin-fill aquifer.
Since 1986, M&A has conducted a range of hydrogeologic investigations in support of an APP. M&A designed and installed a wellfield to intercept water seeping from the tailings impoundment, which contains elevated levels of sulfate. We also compiled and analyzing data to describe regional and site-specific hydrogeologic conditions as part of efforts to:
- Assess areas that may be impacted by the tailings seepage
- Install monitoring and interceptor wells
- Monitor groundwater to confirm compliance with APP requirements
- Evaluate the efficiency of the interceptor wellfield
- Characterize the quantity and quality of tailings seepage
Services
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Designed and supervised the construction of about 60 monitoring wells and piezometers in the basin-fill aquifer and bedrock complex
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Designed and supervised the construction of 15 of the 23 interceptor wells
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Prepared an O&M / monitoring manual for the interceptor wellfield
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Ran pumping tests to determine aquifer parameters, well efficiencies, sustainable pumping rates, and pump requirements and settings for the interceptor wells
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Evaluated the wellfield's performance annually and recommended options for preventative maintenance, repair, and rehabilitation
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Rehabilitated and/or replaced interceptor wells as needed
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Quantified seepage from the mine tailings
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Analyzed the water budget for both the tailings impoundment and the mill operations
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Developed a hydrologic budget and flow analysis for the pit area to demonstrate the potential for passive containment of groundwater impacted by heap-leach seepage
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Tested the attenuation of bedrock materials to evaluate the retardation of selected chemical constituents
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Calculated downward flux values for the volume of tailings water that is anticipated to seep into the basin-fill aquifer
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Developed a flow-and-solute transport model using MODFLOW and MT3D to identify areas in the basin-fill aquifer that may be impacted by tailings seepage, to project conditions at points of compliance, and to simulate capture in the tailings vicinity
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Updated the model periodically using monitoring data
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Used the model to site and design additional interceptor wells
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Designed and implemented a program for monitoring groundwater levels / quality and process-solution chemistry throughout mine area
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Interfaced with ADEQ to identify requirements for APP permitting and compliance; prepared hydrogeologic reports and addenda to support the application
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Assisted with the APP application process and negotiations
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Assessed whether the existing mine facilities met conditions for Best Available Demonstrated Control Technology (BADCT); helped apply BADCT standards to the new facilities
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Installed automated monitoring equipment within the tailings material to characterize active spigot cycles and infiltration in real time
- Stored and managed monitoring data in an extensive electronic database designed by M&A
Outcome
The initial APP application for the mine was submitted to ADEQ in 1994; amended applications were submitted in 1997 and 2005. We are currently assessing hydrogeologic conditions and monitoring tailings seepage to identify areas that may be impacted by the discharge and to evaluate the wellfield's efficiency.
Acronyms & Abbreviations
