By Bill Albright | Not long ago the Anchorage Daily News published a letter (7 Dec 2017) from Northern Dynasty Minerals VP for External Affairs about the controversial Pebble Mine on the Alaska Peninsula. The author stated that the development team had decided to place a liner underneath the tailings impoundment. This was likely received as good news from those concerned with environmental protection at the mine. A critical analysis of the subject of mining tailings and containment deserves a more careful explanation. Although the subject may appear technical, it needs to be more widely understood.
Photo above: Grading for preparation for a composite liner at a mine site. Photo by Bill Albright.
Tailings can be a source of mobile (dissolved) contaminants that can leak out the bottom of the tailings containment facility to enter the groundwater or surface water systems. Tailings are not the only source of mobile contaminants from mining activities. Heap leach facilities and even unprocessed waste rock (if containing acid generating rock, i.e. sulfide minerals) can be substantial sources of contaminants. The purpose of an underlying liner is to intercept downward moving leachate and thus prevent the release of hazardous contaminants to the environment. Leakage from the bottom of a waste containment facility often is not immediately apparent and can pose an insidious threat. Given the purpose of a liner (i.e. as a barrier to liquid migration), we often add the word “impermeable,” as in “impermeable liner,” sometimes without realizing that these words can have specific definitions in a technical discussion. When we move from a layman’s to a technical perspective these terms should become clearly defined and the associated concepts can have very important ramifications for environmental quality and the cost of conducting mining operations. This brief note explores a few ideas important to mine waste containment and some associated terminology.
Liners differ greatly in performance characteristics depending on technical specifications. Perhaps the most basic design is simply the native soil re-compacted in place with heavy equipment to reduce the permeability. More sophisticated designs consist of clay or other fine-grain soil (often imported), but such soil liners are not “impermeable.” Indeed, soil has a defined (non-zero) permeability, and even carefully designed and constructed soil liners will allow passage of some leachate. Analysis of full-scale soil liners showed that about one in four fail to achieve the design specification for permeability (Benson et al. 1999), and low-permeability soil layers tend to change characteristics (in the direction of increased permeability) with the environmental influences of changes in water content, temperature, and bio-intrusion (Benson et al. 2011). Poorly constructed liners or those that have been damaged by exposure to the site environment can become little more than controlled release features.
The one type of liner that may be considered “impermeable” is called a composite liner. “Composite” refers to a combination of low-permeability soil and an overlying geomembrane. The plastic membrane is typically 1.5-2.5 mm polyethylene. Additional layers are usually required for protection of the geomembrane and to provide lateral drainage to a collection system for intercepted leachate. Extensive field testing shows composite liners are very effective in containing leachate given careful attention to design and construction.
A composite liner is the foundation of contaminant and raises other concerns. First, with effective leachate interception, we have leachate accumulation. To avoid overflowing the containment system leachate collection and treatment is required. There are multiple treatment methods depending on the contaminants of interest, a topic for another day. Second is the question of the period over which leachate interception, collection, and treatment should be conducted. One approach to this question is to conduct remediation activities for as long as the facility poses a threat to human health and/or the environment. For many of today’s “mega-mines” this might point to system maintenance for a very long time. Finally comes the question of funding. The previous three topics – composite liner, leachate collection, and treatment for as long as there is a threat to human health or the environment – are very expensive. In the distant past, mine closure was not a major concern for the mining companies; the west is dotted with 19th century operations that were simply abandoned as they became unprofitable. Costs associated with significant environmental problems have sometimes been borne by government. Today there is a growing awareness that the total cost of operation should be assumed by the mining industry. In addition to operational costs, this includes major expenditures for mine waste containment, long-term maintenance, and operation of contaminant treatment. Appropriate bonding for the financial aspects of closure provides for ongoing maintenance and is a major topic for the industry.
These four bits of terminology – composite liners, leachate collection and treatment, long-term closure with maintenance, and bonding ‑ along with the associated technical details ‑ should be prominent at the beginning of any discussion of new mining activities. As we see from the Pebble Mine example, the word “liner” is not sufficient. All aspects related to containment of mobile mine contaminants begin with proven and reliable containment. Our public lands agencies should have these terms, and associated technical details, front and center at the start of any discussion of new mining activities on public lands. The public should insist on attention to these details.
This brief note addresses some topics related to basal liners which are constructed prior to waste placement. Similar and additional topics apply to final covers, placed atop waste repositories. Covers must provide physical containment from water, wind, and vector erosion, as well as hydraulic control of infiltrating precipitation, aesthetic concerns, and land use characteristics.
Bill Albright grew up in Bishop and spent summers wandering, climbing, and fishing the high Sierra. His formal training includes a BS from UCD (environmental toxicology) and PhD from UNR (hydrology). At the Desert Research Institute in Reno Bill conducted research in containment of hazardous, mine, radioactive, and municipal waste. At retirement Albright returned to Bishop.
References
Benson, C., Daniel, D., and Boutwell, G. (1999), Field Performance of Compacted Clay Liners, J. Geotech. and Geoenvironmental Eng., 125(5), 390-403.
Benson, C., Albright, W., Fratta, D., Tinjum, J., Kucukkirca, E., Lee, S., Scalia, J., Schlicht, P., Wang, X. 2011. Engineered Covers for Waste Containment: Changes in Engineering Properties & Implications for Long-Term Performance Assessment, NUREG/CR-7028, Office of Research, U.S. Nuclear Regulatory Commission, Washington.
