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
  • Quantified seepage from the mine tailings and analyzed the water budget for the impoundment
  • Tested the attenuation of bedrock materials to evaluate the retardation of selected chemical constituents
  • Calculated downward flux values for the volume of tailings water that is anticipated to seep into the basin-fill aquifer
  • 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 whether the existing mine facilities met conditions for Best Available Demonstrated Control Technology (BADCT); helped apply BADCT standards to the new facilities
  • Installed automated monitoring equipment within the tailings material to characterize active spigot cycles and infiltration in real time