Hydrogeologic Investigations for the Sierrita Mine

Since 1986, M&A has conducted hydrogeologic investigations to support permitting requirements at this copper and molybdenum mine. This work involved designing and installing a wellfield to intercept water seeping from the tailings impoundment, which contains elevated levels of sulfate. We are currently involved in monitoring and evaluating the mine’s tailings water balance.


Located in southern Arizona, the Sierrita Mine 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.


Tailings Water Management

  • Characterized hydrogeologic conditions to assess areas that may be impacted by the tailings seepage
    • Developed water balances for the tailings impoundment using both deterministic approaches and stochastic analyses with GoldSim
    • Utilize GoldSim water balance model to predict the tailings pond size and seepage rates under different impoundment management scenarios
    • Installed subsurface sensors within the operating impoundment for monitoring/analyzing unsaturated/saturated conditions in real time
  • Tested the attenuation of bedrock materials to evaluate the retardation of selected chemical constituents
  • Evaluated the efficiency of the interceptor wellfield

Groundwater Modeling

  • Developed a flow-and-solute transport model using MODFLOW and MT3D to identify areas that may be impacted by tailings seepage, project conditions at points of compliance, and simulate capture in the tailings vicinity
  • Used the model to site and design additional interceptor wells

Environmental Compliance

  • Installed monitoring wells
  • Designed and implemented a program for monitoring groundwater levels / quality and process-solution chemistry throughout mine area
  • Interfaced with state regulatory agencies to identify requirements for permitting and compliance; prepared hydrogeologic reports and addenda to support the application
  • Assessed mine facilities for Best Available Demonstrated Control Technology (BADCT) and helped apply BADCT
  • Estimated tailings seepage volumes through water balance analysis for input into sulfate transport model to meet APP requirements