Under Arizona's Aquifer Protection Permit (APP), a permit is required for any mining operation that discharges pollutants to the groundwater. The two key requirements of APP are:
A variety of facility types must obtain an APP because they are listed in Arizona statute as categorical discharging facilities. Specific to mining, these facilities include:
- Mine tailing piles and impoundments
- Mine leaching operations
- Process water or non-stormwater
- Surface impoundments
- Leach rock and waste-rock stockpiles
- Injection wells (such as in-situ copper leach operations) and
- Nonpoint source discharges to navigable waters
Meeting Aquifer Water Quality Standards at the Point of Compliance
The point of compliance is a vertical plane downgradient of the facility that extends through the uppermost aquifers underlying that facility. This is the point where the aquifer water quality standards must be met.
Best Available Demonstrated Control Technology (BADCT)
BADCT's purpose is to employ engineering controls, processes, operating methods or other alternatives, including site specific-characteristics (i.e., the local subsurface geology), to reduce discharge of pollutants to the greatest degree achievable before they reach the aquifer.
The Arizona mining industry has two options available when demonstrating BADCT for their facilities:
- Prescriptive BADCT
- "Individual" BADCT
Prescriptive BADCT allows the applicant to meet the second APP requirement in an expedited manner, making review time minimal, thereby expediting the permitting process. The applicant reduces application costs and permitting timeframes by incorporating specific pre-approved design requirements, independent of site specific conditions that automatically demonstrate BADCT. A mining applicant is free to use this process for both new and existing facilities.
By contrast, the "individual" BADCT process is performance-based, with no pre-approved prescribed design. For this process, the reference design approach is used. The reference design concept draws from a menu of demonstrated control technologies and employs a systems approach that considers the project's life cycle and site characteristics, coupled with the designer's own knowledge and creativity to achieve the greatest degree of discharge reduction.
In Situ Leaching
In situ leaching generally includes operations in which mineralized rock is left in place and subjected to infiltration of solutions to dissolve minerals for recovery. The ore body may be artificially fractured or the porosity and permeability altered through precursor injection of acid solutions to enhance infiltration and capture of injected solutions. In situ leaching may target an ore body or portion of it that is located above the water table, below the water table or both.
The objective of BADCT for in-situ leach operations requires the operator to maintain hydrologic control over leaching solutions throughout the process. Unlike other mining operations, BADCT for in-situ leaching cannot be defined in detail due to the site specific factors that relate to design and operation. BADCT for in-situ leach facilities applies to the entire process; i.e., from the point leaching solutions are injected to the point these solutions are recovered.
Some open pit mines may have the potential to form a pit lake either during or after the mining operation ceases. During the operating life of a mine, any groundwater encountered is generally pumped from the mine creating a hydrologic capture zone or groundwater sink. Once mining has ceased, groundwater levels will recover and stabilize at a new elevation below the original land surface. Because Arizona is an arid state with high evaporation rates and relatively deep groundwater, the pit lakes that do form commonly continue to function as a hydrologic capture zone and not discharge to the environment. An evaluation of the mine hydrogeology and the potential to be non-discharging is generally required as part of the aquifer protection permit process.
There is one unusual aspect of BADCT that Arizona uses in some APP permits for open pit mines.
Owing to Arizona's arid climate, mines are allowed to use the concept of "passive containment" as part of their BADCT demonstration to control water pollution from an open pit mine. If a pit lake forms at the bottom after the mine closes, passive containment occurs when a natural hydrologic sink develops in the subsurface around the pit that prevents the pollutants in the lake (such as dissolved metals, low pH water) from migrating away from the pit lake into the aquifer. The hydrologic sink can develop because the high rates of evaporation from the lake match or exceed the inflow of groundwater into the pit. If more water flows into the pit than evaporates, a pit lake forms without a hydrologic sink and the potential exists for groundwater pollution to migrate into the area's aquifer.
Companies that operate mines in Arizona must make a very rigorous demonstration to ADEQ that a hydrologic sink exists, now and / or after the mine closes.
Although there are many abandoned mine properties in Arizona, the great preponderance of them are quite small and at worst, pose only a "local" threat to the underlying aquifer or local surface drainages (many of which flow only a few months out of the year).