There are approximately 500,000 abandoned mines and related or processing facilities across the United States. A total of 130 of these are on the National Priorities List which covers over 1 million acres of contaminated land from past mining activities. The contamination is from waste rock and beneficiation waste including mill-tailing piles which have been scattered in many surface impoundments. Contaminated mining waste has caused problems in surface bodies of water and groundwater through seepage, and potential air pollutants especially from heavy metals and other contaminants from mining and processing. Also there is waste present in the form of slurries and sludge in ponds with questionable retaining walls.
Mining wastes include those wastes which come from extraction, beneficiation, and processing. Beneficiation is a value-added process involving transformation from a primary material to a more finished material of greater value through smelting or an extraction process of the metallic ore from the already extracted material from the ground. The original extraction removes the mass of material including the ore from the earth and then makes it ready for the next step in the process. The mineral processing usually changes the chemical composition or the physical structure of the ore or mineral.
Mining wastes include waste rock which is the material including the overburden (the earth above the deposit) that had to be moved in order to access the ore or mineral; waste rock which is the material that contains the minerals in concentrations that are too low for economic removal; non-metallic byproducts of the metal smelting process; water treatment sludge which comes from the water treatment plants at the mine sites and contains solids which are removed from the water as well as any chemicals that were used to improve the efficiency of the process; gaseous wastes which include particulates from dust, sulfur oxides, and other gases produced during the reactions that occur during processing; tailings which are the residuals usually generated in a slurry form during the beneficiation process; mine water which is groundwater or precipitation that infiltrates during extraction and becomes contaminated; and processing wastes which are the residuals from the processing and beneficiation process. Most processing wastes are water related and are usually placed in surface impoundments.
The mining and mineral processing waste is responsible for a large amount of damage to both human health and the environment. Typically, the wastes are put on the land in unlined units that may leach hazardous materials onto the ground and into surface and groundwater supplies. It can also cause problems of air pollution. The wastes may also be put in heaps which can at some point in time create a landslide with catastrophic results. There are also leaks and spills from pipes that may contribute to the problem. A large variety of potential diseases of either an acute or chronic nature may be a result of these hazards.
Best Practices in the Storage and Disposal of Mine Waste (See endnotes 38, 39)
• Identify the key environmental, health, and risk/safety problems for the given industry and mine in order to be able to establish an appropriate plan for the safe storage and disposal of mine waste.
• Establish a baseline of the quantity and characteristics of the tailings and waste rock which will be or are now being produced and include all information on site selection; assess environmental impacts to air, water, land, and people; develop and carry out a risk assessment study for each potential risk; prepare a plan for routine maintenance and appropriate management of the site; prepare an emergency plan for spills, accidents, and disasters; prepare a disposal plan for treatment, storage, and ultimate disposal of all mine wastes and tailings: determine how best to utilize and dispose of water at the mining site; and develop a decommissioning and closure plan plus lifetime maintenance. Choose drought-resistant plants as part of the cover material.
• Review the best available technologies including emerging technologies for control of the potential hazards from the mine waste and tailings and determine which technologies best suit the specific mine in a reasonably economic manner.
• Develop and implement an environmental management plan that includes: adhering to all environmental laws, rules, and regulations; and establishing necessary structures and responsibilities for implementation of the plan including training, communications, employee involvement, documentation of efforts, establishing maintenance programs, and meeting objectives and goals. Have the program evaluated externally periodically.
• Manage acid rock drainage by placing the material in an impermeable area without earthquake potential, such as solid rock without fissures. If this is not possible, then the material needs to be put in a lined pit that will not corrode, tear, or leak and or be subject to flooding. Choose a liner which is made with low environmental impact. Apply a covering material to this area and supply drainage ditches that will prevent surface water from crossing the area.
• Obtain organic materials from companies that are close to the site to minimize fuel consumption and air emissions from the heavy equipment.
• Determine if there are industrial sources of materials close by that can be used as an appropriate fuel for operations without causing environmental contamination.
• Install remote sensing devices for groundwater supplies, surface water supplies, and air.
• Use solar energy or wind energy where possible for all operations in the mine and processing unit.
• Reuse process water where possible to avoid overconsumption of the potable water supply and release of contaminants to the surface or groundwater.
• Use water treatment systems in which metals or other chemicals may be recovered and reused.
• Mix process water with other substances to neutralize it and make it less hazardous or non-hazardous.
• Use sedimentation ponds to capture the fine materials and then treat the effluent for suspended solids and dissolved metals before releasing into bodies of water.
• Neutralize alkaline waters or contaminants with sulfuric acid or carbon dioxide.
• Remove arsenic from the effluent by adding and mixing with ferric salts.
• Neutralize acid mine water with limestone, hydrated lime, or quicklime.
• Add sodium hydroxide to acid rock drainage and that which has a high manganese content.
• Reduce noise to avoid creating local issues by using continuous working systems, putting the belt drives in an enclosed structure, and creating a barrier around the working face of the mine which will block and absorb the sound.
• Where a dam is needed for retention of the mine waste and tailings, it must be designed to withstand a potential 100-year flood and if there is highly hazardous material present, it must be designed to withstand a 5000- to 10,000-year flood. When building a dam, the natural ground must be stripped of all vegetation and humus soil and be replaced with material that will not weaken under any operational condition or changing climate. All dams for all mine waste and tailings must be inspected on a regular basis by competent engineers, and all recommendations must be implemented as quickly as possible.
• Remove water from tailings where possible and thicken the tailings before disposal.
• In the case of the use of a heap as the disposal method for the mine waste, conduct frequent visual inspections and periodically at least once a year have in-depth studies made by highly competent geotechnical engineers, of the stability and safety of the heap and what needs to be done to stabilize it further. Exercise extreme caution that nothing is permitted below the pathway of the heap in the event of collapse.
• Since mining is an industry with great accident potential, it is essential to develop appropriate plans for accidents and emergencies and test them periodically. Highly competent individuals should evaluate all accidents and continue follow-up for extended periods of time.
• Monitor all pipelines for potential weakness and leaks.
• Prevent the generation of mineral wastes and tailings by conducting in-depth studies by engineers on the best approach to use to remove the material sought with the least amount of waste material being produced.
• Backfill tailings and where the tailings waste rock into mines which are no longer being utilized and may cause damage to the environment.
• Maximize control of soil erosion by wind, rain, or construction activities.