Abandoned mine reclamation involves a series of steps to address the waste products at a particular mine site. Although there are many reclamation methods and technologies available, they generally focus on cleaning up two different categories of problems:
Mine water refers to the water that accumulates in mines and then leaks out of mine openings. Due to the prevalence of Acid Mine Drainage, reclaiming these discharges is a very complicated and often expensive undertaking. In general, there are three different ways to deal with this problem: closure, passive treatment, and active treatment.
- Closure - Mine closure involves physically sealing the opening with a plug of impervious material to prevent water from escaping. Grouting around the edges usually follows, which refers to the technique of using cement to seal the edges of the plug. At the same time, surface and groundwater must be prevented from getting into the mine, which entails blocking any other openings or water sources. This is done by pumping grout into cracks and fissures inside the mine to cut down on water infiltration. One problem with mine closure is that water often builds up in the mine anyway and eventually finds a way to escape, showing up in seepages on different parts of the hillside or watershed.
- Passive Treatment - Passive treatment involves systems such as wetlands or ponds that improve the water quality of the discharge without using chemicals or machinery. Passive treatment systems are usually less expensive to construct and operate and can be very effective when used in the right situations.
Wetlands are shallow marshy areas that are planted with cattails and aquatic plant species. They are classified as aerobic or anaerobic, depending on their use of oxygen in the system. Anaerobic wetlands use bottom layers of compost such as mushroom, peat moss, or hay to take oxygen out of the water and prevent further oxidation of the metals. They also raise the alkalinity of the water. Aerobic wetlands are used for water discharges that are already alkaline and have lower metal concentrations. Because of the oxygen present, these systems remove metals through oxidation.
Limestone diversion ditches can be as simple as a ditch lined with limestone rock or a more complex system like an anoxic limestone drain, which is a buried trench of limestone that receives the water discharge but prevents oxygen in the system. This type of treatment is often used to deal with small flows and typically requires periodic maintenance due to problems with oxidation, or the coating of the limestone with metal precipitates.
Phyto-remediation involves the use of plants to take metals out of soil and water. An increasing body of research has identified plant species that take dissolved metals in through their roots and pass them on to their leaves and shoots. The plants are then cut and burned to destroy the toxic materials. Although it is a fairly new technique, this process is usually fairly inexpensive, does not involve chemicals, and is more visually satisfying.
- Active Treatment - Active treatment refers to the use of water treatment plants to deal with water discharges. Active treatment plants can be thought of as smaller versions of municipal water treatment plants that clean public drinking water. Most active treatment systems consist of an inflow pipe or ditch, a storage tank or bin holding the treatment chemical, a means of controlling its application rate, a settling pond to capture precipitated metals, and a discharge point. Some of the chemicals commonly used are lime, soda ash, and ammonia. Active treatment often requires a source of electricity and needs constant monitoring. These systems are used for highly toxic discharges and represent the biggest cost in abandoned mine reclamation because they need to be operated “in perpetuity,” which basically means forever.
Solid wastes refer to waste rock, tailings piles, and other materials that have been discarded and scar the landscape. In general, there are two reclamation methods for dealing with solid wastes: removal and capping.
- Removal - Removal involves moving the waste material to another site where there will be little human contact and no exposure to surface or groundwater. Removal is usually accomplished with large excavators and dump trucks which dig out the unwanted material and transport it to a repository. The repository is sort of like a landfill and will often be lined with high density plastic to prevent groundwater contamination. A plastic liner or several feet of topsoil is usually placed on top of the waste material to prevent exposure to rain and wind. To neutralize the acidity of the material, chemicals such as lime may be mixed in with the waste. Water diversion structures or drainages may be placed around the repository to prevent surface water from seeping into the material.
- Capping - When the removal of waste material is cost prohibitive, it is sometimes left in place and capped with a cover of plastic or topsoil. In this situation, the waste is consolidated in one place and may be amended with lime to neutralize the acidity. While capping helps stabilizes the waste materials and prevents erosion, run on controls such as wells, ditches, or culverts minimize the amount of contact with water. A layer of cement may also be added to further stabilize the pile. In some cases, mine tailings will be submerged in water behind an earthen dam to cut off the oxygen supply and prevent oxidation.
Reclamation and Restoration
Reclamation generally refers to the plan for eliminating threats to human health and the environment at an abandoned mine site. In other words, reclamation activities must clean up an area and eliminate any sources of pollution. However, standard reclamation practices do not necessarily take into account the need for habitat restoration. A reclaimed stream consisting of a straight channel, steep gradient, and rock walls may no longer suffer from heavy metal loads and sediment sources, but it will not support fish and other aquatic life.
Whether you are monitoring a Superfund cleanup, serving as a stakeholder in a watershed group, or developing your own reclamation plan, it is important to stress the need for fish and wildlife habitat during the design process, not as an afterthought. The following steps of the reclamation process generally occur after the mine wastes and mine water sources have been dealt with. They all offer opportunities to include habitat improvement.
- Regrading – Regrading consists of changing the slope of the terrain where waste piles, tailings impoundments, and other mine features used to be. This activity helps decrease erosion, increases vegetation, and generally facilitates the restoration process. Large bulldozers and graders change the slope of the land according to its length and the percent grade desired. Recontouring is usually done at the same time, which involves shaping the land to give it more natural features and addressing concerns in geology, hydrology, wildlife habitat, and visual considerations.
- Revegetation - Revegation is a very important part of the mine reclamation process. It is essential to the long term stabilization of capped waste piles, repositories, and other reclaimed mine features. After regrading and recontouring the terrain, revegetation involves adding a layer of topsoil, preparing the surface for seeding, adding a layer of seeds, adding mulch or fertilizer, and then putting in place some kind of erosion control. When possible, revegetation should include native grasses which can also serve as forage for stock or wildlife. After the seed has taken root, other shrubs and trees can be planted at the site.
- Maintenance and Monitoring - Maintenance and monitoring can be a very expensive part of the reclamation process and may be minimized through foresight in the planning process. Maintenance may be required for fences, gates, and other structures at the site that are intended to protect people and wildlife from danger. Water treatment plants or diversion ditches may need to be unclogged or repaired. Monitoring can involve water sampling and reporting procedures, or periodic checks for noxious weed growth at revegetated areas.