N.M. Admin. Code § 20.6.7.33

Current through Register Vol. 35, No. 21, November 5, 2024

Section 20.6.7.33 - CLOSURE REQUIREMENTS FOR COPPER MINE FACILITIES

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.

D.Open pits. The applicant or permittee shall provide detailed information and a closure plan for open pits that demonstrates how the following criteria will be addressed through water management or other activities at open pits to minimize the potential to cause an exceedance of applicable water quality standards:

(1) Open pits in which the evaporation from the surface of an open pit water body is predicted to exceed the water inflow shall be considered to be a hydrologic evaporative sink. If an open pit is determined to be a hydrologic evaporative sink, the standards of 20.6.2.3103 NMAC do not apply within the area of open pit hydrologic containment. This is limited to contaminants associated with standard copper mining practices and found to be present within the open pit, or that can be generated from the natural materials present in the open pit through degradation, oxidation, decay or other expected process.

(2) After closure, if water within an open pit is predicted to flow from the open pit into ground water and the discharge from an open pit may cause an exceedance of applicable standards at monitoring well locations specified by 20.6.7.28 NMAC, then the open pit shall be considered a flow-through pit. In a flow-through pit system the open pit water quality must meet ground water standards of 20.6.2.3103 NMAC or the open pit must be pumped in order to maintain an area of open pit hydrologic containment.

E.Surface water management. The permittee of a copper mine facility shall maintain and implement a plan for the management of all stormwater and sediment generated from the copper mine facility during reclamation and following closure.

F.Cover system. At closure, a permittee shall install a cover system on waste rock piles, leach stockpiles, tailing impoundments and other units that have the potential to generate leachate and cause an exceedance of applicable standards at monitoring well locations specified by 20.6.7.28 NMAC using the following criteria, as appropriate. Any soil cover systems installed before the effective date of the copper mine rule are not subject to the requirements of the copper mine rule unless the department determines that an exceedance of applicable standards has occurred or is likely to occur as a result of the existing installed cover system, and that modification of the cover will prevent further impacts to ground water. Any cover system installed at an existing copper mine facility after the effective date of the copper mine rule shall be a store and release earthen cover system with a thickness of thirty-six inches and shall be constructed in accordance with the applicable requirements of Paragraphs (1) through (3) of this subsection. For leach and waste rock stockpiles inside the open pit surface drainage area, a thirty-six inch cover is only required on the top surfaces.

(1) The cover system shall be constructed of thirty-six inches of earthen materials that are capable of sustaining plant growth without continuous augmentation and have erosion resistant characteristics. Erosion rates shall be equal to or less than stable slopes in the surrounding environment after the vegetation has reached near-equilibrium cover levels. Erosion will be estimated using generally acceptable methods.

(2) Soil cover systems shall be designed to limit net-percolation by having the capacity to store within the fine fraction at least 95 percent of the long-term average winter (December, January and February) precipitation or at least 35% of the long-term average summer (June, July and August) precipitation, whichever is greater. The water holding capacity of the cover system will be determined by multiplying the thickness of the cover times the incremental water holding capacity of the approved cover materials. Appropriate field or laboratory test results or published estimates of available water capacity shall be provided by the permittee to show that the proposed cover material meets this performance standard.

(3) Cover thickness or other design criteria may be reduced or modified if:

(a) the cover system is installed over a lined unit and the design and function of the liner system will complement the cover system, or the permittee proposes a composite, layered or an alternate cover system with an equal or greater level of ground water protection described in Paragraphs (1) and (2) of this section; or

(b) a demonstration is made that an alternate proposed cover system will ensure that an exceedance of applicable standards will not occur in ground water; such a demonstration shall include:

(i) a comprehensive modeling study to estimate the quantity of net-percolation through a cover system that will not result in an exceedance of applicable standards in ground water;

(ii) a plan for performance monitoring of the cover system, including ground water monitoring; and

(iii) an agreement by the permittee to pay for the cost of a third party review of the modeling study and performance monitoring plan.

(4) A CQA/CQC plan shall be submitted for department review as part of the final cover design. The plan shall identify a licensed New Mexico professional engineer as the designated CQA officer and include his or her supervision of the CQA plan and shall identify the methods proposed to ensure that the closure construction will be completed in accordance with the design and specifications. Following the completion of the work, the CQA officer shall prepare a final CQA report. The final CQA report shall provide a detailed description of the installation methods and procedures and document that the work was conducted as designed.

G.Process solution reduction plans. The closure plan shall include a process solution reduction plan for the copper mine facility. The process solution reduction plan shall be a conceptual engineering document that describes the processes and methods that are expected to be used at a copper mine facility to reduce the quantities of process water in storage and circulation inventory at the end of copper production in preparation for long-term water management or treatment. The plan shall describe and list the current or proposed process water management units and inventories of process water. The plan shall describe the modifications to the process water management system required to create an efficient process water reduction system and the operation and maintenance requirements for the system with material take-offs of sufficient detail to prepare an engineering-level cost estimate equivalent to the cost estimate to be provided with the closure plan. The plan shall provide an estimate of the required water reduction period based on the water reduction calculations provided in the plan to be used for planning and operation and maintenance cost calculations.

H.Closure water management and water treatment plan. The applicant or permittee shall submit a closure water management and water treatment plan. The closure water management and water treatment plan shall consist of a conceptual engineering document that describes the processes and methods that are expected to be used at a copper mine facility for long-term management or treatment of process water. The plan shall include an analysis of the expected operational life of each long-term water management or water treatment system, including interceptor systems, until each system is no longer needed to protect ground water quality and applicable standards are met. The plan shall describe the long-term water management and water treatment systems with sufficient detail, including locations of key components, expected operational life, material take-offs, and capital, operational and maintenance costs to prepare an engineering-level cost estimate. The plans shall provide sufficient detail to estimate capital and operating costs to provide the basis for financial assurance for these activities.

I.Impoundments. The permittee shall close all reservoirs and impoundments in a manner that ensures that the requirements of the Water Quality Act, commission rules and the discharge permit are met. Closure activities shall meet the following requirements:

(1) Fluids from reservoirs and impoundments shall be drained and appropriately disposed of.

(2) Sediments in the reservoir or impoundment shall be characterized and abated or appropriately disposed of in a manner that will not cause an exceedance of applicable standards.

(3) Materials underlying the reservoir or impoundment shall be characterized to determine if releases of water contaminants have occurred.

(4) Where characterization results show materials remaining within or beneath any reservoir or other impoundment that are not naturally occurring to be a source or potential source of ground water contamination outside the open pit surface drainage area, the reservoir or impoundment, shall be covered and re-vegetated pursuant to this section.

(5) Based on the characterization conducted pursuant to Paragraph (4) of this subsection, further characterization of ground water beneath and adjacent to the reservoir or impoundment may be required to determine if abatement is necessary.

(6) Reservoirs and impoundments located outside the open pit surface drainage area shall be closed in a manner that creates positive drainage away from the impoundments, unless needed during closure and post closure for storm water retention or seepage interception, post-closure water management and treatment, or unless otherwise approved by the department. Post-closure reservoirs or impoundments to be used for the collection of non-impacted storm water and located over areas where residual wastes, vadose zone contamination or ground water contamination remains shall be synthetically lined pursuant to the design and construction criteria of Paragraph (4) of Subsection D of 20.6.7.17 NMAC.

(7) The department may approve alternative plans for closure of impoundments based on site-specific conditions when the alternative closure method will provide the same level of ground water protection as the methods specified in Paragraphs (1) through (6) of this subsection.

J.Pipelines, tanks and sumps. The permittee shall remove and properly dispose of the tailing, process water, or other materials contained in pipelines, tanks or sumps as soon as they are no longer needed for site operations, water treatment, or other post-closure water management. Any residual tailing, process water, sediments or contaminated water shall be removed from the pipelines, tanks or sumps prior to closure and dispose of the material in a department approved manner. Pipelines may be removed for appropriate disposal or cleaned and buried in place. Sumps may be removed for disposal or cleaned and broken up and buried in place. During pipeline, tank or sump closure, the permittee shall inspect the entire pipeline, tank or sump area for evidence of past spills and characterize the impacts and potential impacts of such spills. The permittee shall document all areas where there is evidence of spills and propose to the department appropriate corrective actions pursuant to 20.6.2.1203 NMAC. Following pipeline, tank or sump removal, the permittee shall remove for disposal or reclaim in place all acid generating pipeline, tank or sump bedding material that has the potential to impact water quality in excess of the applicable standards.

K.Crushing, milling, concentrating and smelting. The permittee shall close all crushing, milling, concentrating or smelting areas in a manner that ensures that the requirements of the Water Quality Act, commission rules and the discharge permit are met. Any remaining materials containing water contaminants that may cause an exceedance of the applicable standards shall be removed or disposed of in a department approved manner or covered pursuant to this section. The permittee shall characterize the crushing, milling, concentrating or smelting area for the presence of any remaining potential water contaminants. If water contaminants are present that may with reasonable probability move directly or indirectly into ground water and cause an exceedance of the applicable standards, the area shall be covered pursuant to this section.

L.Closure monitoring and maintenance. During closure the permittee shall continue monitoring pursuant to 20.6.7.28 and 20.6.7.29 NMAC. The permittee may propose and the department may approve modifications to the required monitoring to reflect changes in conditions during closure, including abandonment of monitoring wells.

M.Exceptions to design criteria. The closure design criteria of this section may be modified if approved by the department. Design criteria required by the office of the state engineer dam safety bureau for regulated units, such as jurisdictional impoundments (including tailing impoundments), shall supersede the criteria in this section.

N.M. Admin. Code § 20.6.7.33

20.6.7.33 NMAC - N, 12/1/13