Memo To: Ms. Paula Wilson From: Bryan Ulrich Idaho Department of Environmental Stantec (Denver) Quality File: Nevada TSF Liner Usage Date: January 6, 2020 Reference: Current Liner/Lining Practices for Tailings Facilities in Nevada Bryan Ulrich of Stantec has prepared this brief summary regarding the current liner/lining practices for tailings storage facilities (TSFs) in Nevada. Nevada’s regulations for tailings storage are as follows: NAC 445A.437 Minimum design criteria: Tailings impoundments. 1. A tailings impoundment must utilize a system of containment equivalent to: (a) Twelve inches of recompacted native, imported, or amended soils which have an in place recompacted coefficient of permeability of no more than 1x10-6 cm/sec; or (b) Competent bedrock or other geologic formations underlying the site which has been demonstrated to provide a degree of containment equivalent to paragraph (a). 2. An alternate level of containment may be required by the Department for all of the tailings impoundment or for a portion thereof after considering the following factors: (a) The anticipated characteristics of the material to be deposited; (b) The characteristics of the soil and geology of the site; (c) The degree to which the hydraulic head on the impoundment liner is minimized; (d) The extent and methods used for recycling or detoxifying fluids; (e) Pond area and volume; (f) The depth from the surface to all groundwater; and (g) The methods employed in depositing the impounded material. NAC 445A.438 Minimum design criteria: Liners. 1. When placed on native materials, soil liners must have a minimum thickness of 12 inches and be compacted in lifts which are no more than 6 inches thick. Except when used in tailing impoundments, a soil liner must have a permeability of not more than that exhibited by 12 inches of 1x10-7 cm/sec material. 2. Synthetic liners must be rated as having a resistance to the passage of process fluids equal to a coefficient of permeability of 1x10-11 cm/sec. 3. The Department shall review for completeness the applicant’s evaluation of the following design parameters, where applicable, for a liner: January 6, 2020 Ms. Paula Wilson Page 2 of 5 Reference: Nevada TSF Liner Usage (a) The type of foundation, slope and stability; (b) The over liner protection and provisions for hydraulic relief; (c) The load and means of applying load; (d) The compatibility of a liner with process solutions; (e) The complexity of the leak detection and recovery systems; (f) The depth from the surface to all groundwater; and (g) The liner’s ability to remain functionally competent until permanent closure has been completed. As can be seen, Nevada’s regulations allow either a 12-inch-thick layer of clay or a geosynthetic liner, although the State does have some flexibility to require additional protection in certain cases. The 12-inch clay layer (or equivalent geologic formation, or equivalent geosynthetic layer) constitutes Nevada’s minimum TSF liner design standard; no additional layer beneath or above that is mandated, though in practice a composite system is common – and for gold tailings facilities in particular. The Idaho Department of Environmental Quality (IDEQ1) provides comments from the State of Nevada regarding Nevada’s regulations for TSF liners. IDEQ (2019) states: “…although [Nevada’s] regulations still allow the use of clay (soil) layers for primary liners, we have limited this use to specific applications (e.g., waster [sic] rock facilities, fresh water and sediment ponds), where characterization data has demonstrated to our satisfaction there will be no potential for degradation to waters-of-the-state (WOS). We strongly encourage (and prefer) the placement of the geosynthetic liners over a compacted engineered subbase by our permittees, due to their long-term performance and properties. Two types of geosynthetic we have found to perform acceptably are High-Density Polyethylene (HDPE) and Linear Low-Density Polyethylene (LLDPE): HDPE 60-mil or greater for secondary liners 60-mil or greater for primary liners (80-mil preferred) LLDPE 60-mil or greater for secondary liners 60-mil or greater for primary liners (80-mil preferred)” I personally can speak with some authority on the topic of liner/lining practices for TSFs in the State of Nevada, as I have been involved as an engineer on numerous Nevada TSF designs, at various levels of involvement, including: Big Springs Jerritt Canyon Gold Bar Sleeper Newmont Mill 1 Newmont Mill 2/5 Newmont Mill #3 (Rain) 1 All references are attached. January 6, 2020 Ms. Paula Wilson Page 3 of 5 Reference: Nevada TSF Liner Usage Newmont Mill 4 #1 Getchell North Block Round Mountain Cell 1 Round Mountain Cell 2 Taylor Mine Juniper Hycroft Combined HLF/TSF Cortez Pipeline Newmont Midas Mine Robinson I have also been involved with many other TSF designs elsewhere in the US and abroad. In my personal experience, there has been a very broad variety of liners used in tailings facilities in Nevada. This will include: (i) No liner. (ii) Twelve inches of compacted low permeability soil (constructed in two 6-inch layers) with an in-place coefficient of permeability of 1x10-6 cm/sec or less. (iii) Geomembrane liner placed on a prepared subgrade. (iv) Geomembrane liner deployed on (ii) for a portion of the TSF basin. (v) Geomembrane liner deployed on (ii) for the entire TSF basin. With the possible exception of (i) if sufficient geologic containment is absent, each of the above liner combinations satisfies Nevada’s requirements for liners in TSFs. A review of readily accessible information regarding liner usage in Nevada has revealed a few examples. Tortelli and Opperman (2015) provide a good overview of tailings management practices in Nevada. Regarding liners, Tortelli and Opperman report only on the usage of a 12-inch thickness of soil with a compacted hydraulic conductivity of not greater than 1 x 10-6 or an 80-mil HDPE geomembrane. They further state that Geosynthetic Clay Liner (GCL) is sometimes used as a secondary liner or to provide a suitable bedding for geomembrane placement over a rough subgrade. RPA (2017) discussed tailings management at the Goldstrike mine. “The NBTDF [North Block Tailings Disposal Facility] is lined with a composite liner system consisting of low permeability [but greater than 10-6 cm/s] soil overlain by a geosynthetic clay liner (GCL) and a geo-synthetic liner (either high-density polyethylene (HDPE) or linear low-density polyethylene (LLDPE)). TSF3 is lined with a composite liner system consisting of low permeability [without a permeability specification but roughly 5x10-5 cm/s is achieved in practice] soil overlain by a GCL and a geomembrane liner.” CETCO (2017) further discusses the Barrick Goldstrike TSF3, noting additional detail on the bottom liner system - a textured geomembrane overlaying BENTOMAT DN GCL. One standout that exceeds Nevada’s requirement for a low hydraulic permeability layer is presented by ERM- West (2014) regarding the Cortez TSF, in which it is stated that “[t]he tailings impoundment is a fully lined facility with a liner system comprising of smooth, 60-mil HDPE synthetic primary liner placed over a minimum of 24 inches of clayey, second[ary] liner material”. There is no comment regarding the rationale for selection of the 24-inches of clayey, secondary liner material, nor is the permeability specification provided. January 6, 2020 Ms. Paula Wilson Page 4 of 5 Reference: Nevada TSF Liner Usage CONCLUSIONS AND CLOSING As may be observed from the information provided, there is no single preferred practice for lining systems for TSFs in the State of Nevada. Each design appears to be developed based on project-specific requirements and restrictions, material availability and limitations, economic considerations, etc. I understand that the State of Idaho is proposing 36 inches of 10-6 cm/s hydraulic conductivity soil for all cyanidation facilities, placed in 6-inch lifts. This would appear to be a remnant practice from before the advent of modern geosynthetics, which offer far greater consistency of properties – obviating the need for the redundancy sought by multiple lifts of often-inconsistent natural materials such as clay. Not only is mandatory use of clay a considerable economic impact and a step backward in liner technology, but it also provides essentially no improved environmental benefits. For example, a permeability of 10-6 cm/s equates to 0.0028 feet per day, or 352 days per foot. Thus, the breakthrough time for seepage traveling through 1 foot of clay is 352 days (assuming a unit hydraulic gradient, and 1,056 days for 3 feet of clay – either length of time being meaninglessly short relative to the operating life of a typical mine (10+ years), much less post-closure. Further, to my knowledge, such a system has not been utilized in practice for precious metal tailings in Nevada or elsewhere, and would in no way be considered a “minimum design standard,” but rather an (ineffective, per above) attempt at a maximally conservative approach. It has been Stantec’s pleasure to complete this brief summary of the use of liners at Nevada tailings storage facilities. Should you have any questions or comments, please do not hesitate to contact us. Bryan Ulrich, PE Vice President Phone: +1 303 291-2269 [email protected] REFERENCES CETCO, 2017, Project Case Study, Mine tailings storage facility bottom liner system. https://www.mineralstech.com/docs/default-source/performance-materials-documents/cetco/environmental- products/case-studies/cs_barrickgoldstrike2012_am_en_201709_v3.pdf?sfvrsn=47798a91_10 accessed 12/16/2019. ERM-West, 2014, International Cyanide Management Code, Gold Mining Operation Recertification Audit Barrick Cortez, Nevada, Summary Report, https://www.cyanidecode.org/sites/default/files/pdf/BarrickCortezSAR2014_0.pdf Accessed 12/16/2019. Idaho Department of Environmental Quality, 2019, NDEP BMRR Responses to Stakeholders – 0919, http://www.deq.idaho.gov/media/60183391/58-0113-1901-ndep-bmrr-responses-to-stakeholder-group- 0919.pdf, accessed 12/16/2019.