OAR 340-043-0120: Guidelines for the Design, Construction, Operation and Closure of Chemical Mining Operations: Guidelines for Design, Construction, and Operation of Heap-Leach Facilities

(1) This section applies to heap-leach facilities using dedicated, or expanding, pads. Heap-leach facilities using on-off, reusable pads may require variations from these rules; they shall be approved on a case-by-case basis by the Department. (2) The heap-leach facility (pad and associated ponds, pipes and tanks) shall be sized to prevent flooding of any of its components. (3) Table 1 of this Division establishes minimum capacity-sizing criteria for the leach-pad and ponds. The pad and ponds may be designed to act separately or in conjunction with each other to obtain the required storage volumes. Other design criteria may be used, with Department approval, if local conditions warrant. The best available climatic data shall be used to confirm the critical design storm event and estimate the liquid levels in the system over a full seasonal cycle. The liquid mass balance may include provision for evaporation. (4) The heap leach pad liner system shall be designed, constructed, and operated to meet the following criteria: (a) A primary liner consisting, at a minimum, of a continuous flexible-membrane of suitable synthetic material shall be provided. This liner shall function together with the process chemical collection system installed immediately above this liner (see section (8) of this rule) to remove process chemicals from the heap; (b) A leak detection system shall be installed immediately below the primary liner for the purpose of detecting loss of process solutions by leakage through the primary liner. The leak detection system shall be capable of detecting leakage through the primary liner of 400 gallons/day-acre within ten weeks of leak initiation. The leak detection system shall consist of appropriately sized collection piping placed within a minimum thickness of 12 inches of permeable material (minimum permeability of 10-2 cm/sec) that is capable of withstanding the anticipated weight of the heap without loss of function; (c) A secondary liner shall be placed below the leak detection system to provide assurance that any leakage through the primary liner during the operation of the heap and following closure of the heap is not released to the environment. The secondary liner shall be of a composite design with a continuous flexible-membrane of suitable synthetic material in direct contact with an engineered, stable, low permeability soil/clay bottom liner (maximum permeability of 10-7 cm/sec) with a minimum thickness of 36 inches; (d) Each liner system component described in subsections (4)(a)–(c) of this rule addresses a specific need and purpose with respect to environmental protection. For purposes of evaluating alternative facilities and methods of control under OAR 340-043-0050(2), an alternative may be approved if the level of environmental protection intended by each separate liner system component is achieved either within the individual component or on a cross component basis. (5) The processing chemical pond liner system shall be designed, constructed, and operated to meet the following criteria: (a) A primary liner consisting, at a minimum, of a continuous flexible-membrane of suitable synthetic material shall be provided. This liner shall provide for positive containment of processing chemical solutions; (b) A leak detection system shall be installed immediately below the primary liner for the purpose of detecting loss of process chemical solutions by leakage through the primary liner. The leak detection system shall be capable of detecting leakage through the primary liner of 400 gallons/day-acre within ten weeks of leak initiation. The leak detection system shall consist of appropriately sized collection piping placed within a layer of permeable material (minimum permeability of 10-2 cm/sec); (c) A secondary liner shall be placed below the leak detection system to provide assurance that any leakage through the primary liner during the use of the pond is not released to the environment. The secondary liner shall be of a composite design with a continuous flexible-membrane of suitable synthetic material in direct contact with an engineered, stable, low permeability soil/clay bottom liner (maximum permeability of 10-7 cm/sec) with a minimum thickness of 36 inches; (d) Each liner system component described in subsections (5)(a)–(c) of this rule addresses a specific need and purpose with respect to environmental protection. For purposes of evaluating alternative facilities and methods of control under OAR 340-043-0050(2), an alternative may be approved if the level of environmental protection intended by each separate liner system component is achieved either within the individual component or on a cross component basis. (6) Emergency ponds may be constructed as an alternative to larger pregnant and barren ponds. The emergency pond may be constructed to a lesser standard, with the limitation that it is to be used only infrequently and for short periods of time. The Department will specify reporting and use limitations for the ponds in the permit. A between-liner leak detection system is not required for the emergency pond. (7) The emergency pond liner shall be of composite construction consisting of: (a) An engineered, stable, low permeability soil/clay bottom liner (maximum permeability of 10-6 cm/sec) with a minimum thickness of 12 inches; and (b) A single flexible-membrane synthetic top liner of suitable material. (8) The heap-leach pad shall be provided with a process chemical collection system above the upper-most liner that will prevent an accumulation of process chemical within the heap greater than 24 inches in depth. (9) The permittee shall prepare a written operating plan for safe temporary shutdown of the heap-leach facility and train employees in its implementation. (10) The permittee shall respond to leakage collected by the heap-leach and processing-chemical storage pond leak-collection systems according to the process defined in Table 2. (11) The permittee shall determine the acid-generating potential of the spent ore by acid/base accounting and other appropriate static and dynamic laboratory tests. If the spent ore is shown to be potentially acid generating under the conditions expected in the heap at closure, the permittee shall submit a plan for acid correction for Department approval prior to loading the heap. [ED. NOTE: To view attachments referenced in rule text, click here to view rule.]