N.M. Admin. Code § 20.6.7.20

Current through Register Vol. 35, No. 23, December 10, 2024

Section 20.6.7.20 - REQUIREMENTS FOR LEACH STOCKPILES AND SX/EW PLANTS

A.Engineering design requirements. At a minimum, the following requirements shall be met in designing leach stockpiles at copper mine facilities unless the applicant or permittee can demonstrate that an alternate design will provide an equal or greater level of containment.

(1)New leach stockpiles. New leach stockpiles shall meet the following requirements.

(a)Liner system. A new leach stockpile shall be placed on an engineered liner system consisting of a subgrade and compacted earthen liner overlain by a synthetic liner which is overlain by a solution collection system designed to transmit process fluids out of the leach stockpile. The liner system shall be approved by the department prior to installation and shall be installed in accordance with a department approved CQA/CQC plan pursuant to Paragraph (1) of Subsection C of 20.6.7.17 NMAC.

(b)Liner system subgrade and earthen liner. A liner system earthen liner shall be prepared and placed upon a stable subgrade. The prepared earthen liner shall consist of a minimum of 12 inches of soil that has a minimum re-compacted in-place coefficient of permeability of 1x10^-6 cm/sec. The top surface of the earthen liner shall be smooth and free of sharp rocks or any other material that could penetrate the overlying synthetic liner.

(c)Liner type. A synthetic liner for a leach stockpile shall provide the same or greater level of containment, including permeability, as a 60 mil HDPE geomembrane liner system. The liner system's tensile strength, tear and puncture resistance and resistance to degradation by ultraviolet light shall be compatible with design loads, exposures and conditions. A licensed New Mexico professional engineer with experience in liner system construction and installation shall identify the basis for the geomembrane composition and specific liner based upon:

(i) the type, slope and stability of the subgrade;

(ii) the overliner protection and provisions for hydraulic relief within the liner system;

(iii) the load and the means of applying the load on the liner system;

(iv) the compatibility of the liner material with process solutions applied to the leach stockpile and temperature extremes of the location at which it will be installed; and

(v) the liner's ability to remain functional for five years after the implementation of closure of the leach stockpile.

(d)Solution collection system. A solution collection system shall be constructed in an overliner protection and drainage system. The solution collection system shall be designed to remain functional for five years after the operational life of the leach stockpile. The overliner protection shall be designed and constructed to protect the synthetic liner from damage during loading and minimize the potential for penetration of the synthetic liner. A sloped collection system shall be designed that will transmit fluids out of the drainage layer of the leach stockpile. The collection system shall be designed to maintain a hydraulic head of less than the thickness of the drainage layer but the drainage layer shall not exceed five feet in thickness. Any penetration of the liner by the collection system through which a pipe or other fixture protrudes shall be constructed in accordance with the liner manufacturer's requirements. Liner penetrations shall be detailed in the construction plans and as-built drawings.

(e)Solution containment systems. PLS flows exiting the leach stockpile shall be collected, contained and conveyed to a process water impoundment(s) or tank(s) using pipelines or lined conveyance systems.

(f)Alternate design. An applicant may propose and the department may approve an alternative design for a leach stockpile located within an open pit surface drainage area provided that the stockpile and solution capture systems are designed to maximize leach solution capture considering the site-specific conditions of the open pit, underlying geology and hydrology, and leach solutions will not migrate outside of the open pit surface drainage area.

(2)Solution extraction/electrowinning (SX/EW) plants. All SX/EW plants shall be designed to contain all associated process fluids within impermeable vessels with secondary containment or process water impoundments meeting the requirements of Subsection D of 20.6.7.17 NMAC. All pipeline and tank systems associated with SX/EW plants shall be designed and operated pursuant to 20.6.7.23 NMAC.

B.Construction.

(1)New leach stockpile and SX/EW plants. Construction of a new leach stockpile or SX/EW plant, including expansion of an existing leach stockpile beyond its ground surface footprint on the effective date of the copper mine rule, shall be performed in accordance with the applicable engineering requirements of Subsection A of 20.6.7.20 and 20.6.7.17 NMAC.

(2)Existing leach stockpiles. A leach stockpile system, including its associated solution collection or containment system, at a copper mine facility in existence on the effective date of the copper mine rule is not required to meet the design and construction requirements of Subsection A of 20.6.7.20 NMAC and may continue to operate as previously permitted under a discharge permit subject to compliance with the contingency requirements of 20.6.30 NMAC. A permit issued for such an existing leach stockpile system after the effective date of the copper mine rule may include the conditions of the existing discharge permit, which shall not be considered to be "additional conditions" under Subsection I of 20.6.7 NMAC.

C.Operational requirements.

(1)Leach stockpile operating requirements. A permittee operating a leach stockpile shall operate the stockpile pursuant to the following requirements.

(a) The stockpile shall remain within the area identified in the discharge permit.

(b) The perimeter of the stockpile and the solution collection system shall be inspected monthly.

(c) Any evidence of instability in the stockpile that could potentially result in a slope failure or an unauthorized discharge shall be reported to the department as soon as possible, but not later than 24 hours after discovery and corrected pursuant to Subsection H of Section 20.6.7.30 NMAC.

(d) Any leaks or spills of PLS or leach solutions outside the leach stockpile or containment system shall be recorded and reported pursuant to 20.6.2.1203 NMAC.

(e) If seeps occur they shall be monitored on a monthly basis and an estimate of the seep flow rate shall be made. Monthly records of the seep inspections and flow rates shall be maintained and included in the site monitoring reports.

(f) Leach solution application rates shall not exceed the maximum rates approved in the discharge permit.

(g) The daily leach solution application and PLS collection rate shall be determined using flow meters installed in accordance with this section and Paragraph (5) of Subsection C of 20.6.7.17 NMAC.

(h) The daily rate and monthly volume of leach solution applied and PLS collected shall be recorded, maintained, and included in the site monitoring reports.

(2)Solution extraction/electrowinning (SX/EW) plants. A permittee operating a SX/EW plant shall operate the SX/EW plant pursuant to the following requirements.

(a) All solution management and extraction operations shall be contained within pipeline and tank systems designed and operated pursuant to 20.6.7.23 NMAC or process water impoundments meeting the requirements of Subsection D of 20.6.7.17 NMAC.

(b) Sludge and spent electrolyte from the SX/EW plant shall be either placed upon the leach stockpile for leaching or disposed of at an approved location.

N.M. Admin. Code § 20.6.7.20

20.6.7.20 NMAC - N, 12/1/13