Copper Mine and Mill Tailings Characterization and Seepage Analysis, Central Arizona

Situation

An assessment of the potential groundwater quality impacts of drainage from over 2 square miles of copper mine and mill tailings was required for permitting mine operations. Potential sources of seepage and the acid generating potential of the tailings needed to be defined and appropriate control technologies identified. Large portions of the tailings were inundated by acidic water from on-going remedial activities.

Conceptual Diagram of a Constant Head Seepometer System

Hydro Geo Chem's Approach

We determined early in the project that potential water quality impacts from the tailings would be strongly affected by their depositional history and by natural attenuation processes occurring in the underlying bedrock aquifer. The three-dimensional structure, grain-size distribution, and mineralogy of the tailings were reconstructed from aerial photographs, topographic maps, and operating histories of 90 years of mine activities. Computer-generated solid models and images were then used to guide field investigations.

In-situ cone penetrometer technology was used to conduct a screening-level investigation of the tailings resulting in a focused use of more expensive drilling and well construction methods. The major and trace element chemistry of subsurface solutions were used to define sources of seepage. Measurements of seepage, infiltration, permeability, moisture content, and pressure were made to define the hydraulic conditions in the tailings. Numerical models were constructed to simulate saturated and unsaturated flow in three-dimensions to quantify drainage rates and mass loadings to the underlying aquifer. The natural attenuation capacity of the underlying bedrock aquifer was assessed using appropriate geochemical models.

Results

The investigation concluded that future drainage from the tailings would have a minimal impact on the underlying aquifer because of relatively slow drainage rates and natural attenuation in the bedrock. These findings were incorporated into the aquifer protection permit application. The models are also being used as management tools to develop operating plans and evaluate remedial options.