New Mexico Administrative Code › Title 20 - ENVIRONMENTAL PROTECTION › Chapter 6 - WATER QUALITY › Part 7 - GROUND WATER PROTECTION - SUPPLEMENTAL PERMITTING REQUIREMENTS FOR COPPER MINE FACILITIES › Section 20.6.7.33 - CLOSURE REQUIREMENTS FOR COPPER MINE FACILITIES

Current through Register Vol. 35, No. 13, July 16, 2024
An applicant or permittee shall submit a closure plan for all
				  portions of a copper mine facility covered by a discharge permit that addresses
				  the following requirements.
A.
Design storm event.
Permanent storm water conveyances, ditches,
				channels and diversions required for closure of a discharging unit at a copper
				mine facility shall be designed to convey the peak flow generated by the 100
				year return interval storm event. The appropriate design storm duration shall
				be selected based on the maximum peak flow generated using generally accepted
				flood routing methods. Sediment traps or small basins intended as best
				management practices may not be subject to this requirement, based on
				department approval.
B.
Slope
				stability.
At closure, tailing impoundment(s) not regulated by the
				office of the state engineer, leach stockpile(s) or waste rock stockpile(s)
				shall be constructed to promote the long-term stability of the structure.
				Closure of all critical structures at a copper mine facility shall be designed
				for a long-term static factor of safety of 1.5 or greater and non-critical
				structures shall be designed for a long-term static factor of safety of 1.3 or
				greater. The units being closed shall also be designed for a factor of safety
				of 1.1 or greater under pseudostatic analysis. A stability analysis shall be
				conducted for the unit and shall include evaluation for static and seismic
				induced liquefaction.
C.
Surface re-grading.
During closure of any tailing impoundment,
				waste rock pile or leach stockpile at a copper mine facility, the surface shall
				be re-graded to a stable configuration that minimizes ponding and promotes the
				conveyance of surface water off the unit. The operator may propose for
				department approval a grading plan that allows ponding as an appropriate part
				of closure provided additional ground water protection measures, such as
				synthetic liner systems, are included as part of the design.
(1) The top surfaces of all tailing
				impoundments at a copper mine facility shall be constructed to a minimum final
				grade of 0.5% after accounting for the estimated magnitude and location of
				large-scale settlement due to totaling consolidation or differential
				settlement. Prior to final re-grading activities, the permittee shall ensure
				that adequate drainage of the tailing impoundment has occurred to ensure that
				large-scale settlement following grading is minimized. The CQC and CQA plan
				shall provide the methods and procedures to ensure that the design and
				construction activities will be completed according to the approved final
				design and specifications, including design aspects related to potential future
				settlement.
(2) The top surfaces of
				all waste rock and leach stockpiles at a copper mine facility shall be
				constructed to a minimum final grade of 1%.
(3) The outslopes of all tailing
				impoundments, waste rock and leach stockpiles at a copper mine facility shall
				be constructed to an interbench slope no steeper than three horizontal to one
				vertical (3H:1V). Alternative slope gradients may be allowed within an open pit
				surface drainage area, or if the permittee provides information showing that
				the cover performance objectives in Subsection F of this section are met and
				the exception is approved by the department.
(a) At existing copper mine facilities, where
				re-grading of individual outslopes would intersect a highway, cultural
				resource, physical infrastructure or a surface water of the state, outslopes
				may be re-graded no steeper than 2.5:1 or as otherwise approved by the
				department in Paragraph (3) of this subsection.
(b) At existing copper mine facilities, the
				waste rock and leach stockpile outslopes within an open pit surface drainage
				area are not required to be graded and covered.
(4) For design purposes, allowable
				uninterrupted slope lengths shall be calculated using a generally accepted
				erosion estimation method and shall be based on the final slope angle and cover
				material characteristics representative of the cover materials proposed for use
				at the site. The maximum uninterrupted slope lengths shall be no greater than
				300 feet for 4.0:1, 200 feet for 3:1 slopes and 175 feet for 2.5:1 slopes.
				Alternative slope lengths may be allowed if the permittee provides information
				showing that the cover performance objectives specified in Subsection F of this
				section will be achieved and the exception is approved by the
				department.