M&A characterizes the complex flow and geochemical conditions within mine tailings and an underlying aquifer to support ongoing environmental compliance requirements.
Client: International mining company - Time frame: 1986–present
- Location: Southern Arizona
Since 1986, M&A has conducted a range of hydrogeologic investigations at an active copper and molybdenum mine in southern Arizona in support of an APP. The 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. 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 in an effort to:
- Assess areas that may be impacted by the discharge
- Evaluate the efficiency of the interceptor wellfield
- Monitor groundwater to confirm compliance with APP requirements
M&A designed and supervised the construction of 15 of the 23 interceptor wells and prepared an O&M / monitoring manual for the wellfield. We evaluated the wellfield’s performance and recommended options for preventative maintenance, repair, and rehabilitation. In addition, we designed and supervised the construction of about 60 monitoring wells and piezometers, which are completed in the bedrock complex and basin-fill aquifer.
M&A quantified seepage from the mine tailings, analyzing the water budget for both the tailings impoundment and the mill operations. We also 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.
In addition to quantifying seepage from the mine tailings, we developed a flow-and-solute-transport model to identify areas that may be impacted by the seepage, to project conditions at points of compliance, and to simulate capture in the aquifer in the tailings vicinity. The model provided valuable information for siting and designing additional interceptor wells.
M&A initiated a program for monitoring groundwater levels, groundwater quality, and process-solution chemistry throughout the mine area. Data were stored and managed in an extensive electronic database that M&A developed. We also interfaced with ADEQ to identify requirements for permitting and compliance. To support the APP application, we prepared hydrogeologic reports and addendums. We also assessed whether the existing mine facilities met BADCT requirements and helped apply BADCT standards to the new facilities. As part of the permitting process, we tested the attenuation capacity of bedrock materials to evaluate the degree to which the movement of selected chemical constituents would be retarded.
M&A installed automated monitoring equipment within the tailings material to characterize active spigot cycles and infiltration in real time. This information allowed us to calculate downward flux values for the volume of tailings water that is anticipated to seep into the basin-fill aquifer.
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