Characterization, Cleanup, and Revitalization of Mining Sites
A range of traditional and innovative technologies may be appropriate for remediation at current and former mining sites. EPA's Office of Research and Development's Engineering Technical Support Center (ETSC) provides assistance to EPA regional offices, states, and communities on the design, function, and application of these technologies. ETSC scientists and engineers work closely with the Superfund program and other EPA programs that address remediation of mining sites, and also collaborate with state governments, universities, and private entities to develop new approaches and remediation technologies for mining wastes.
A resource, Review of Peer Reviewed Documents on Treatment Technologies Used at Mining Waste Sites, by EPA's Office of Superfund Remediation and Technology Innovation, provides a comprehensive evaluation of mine treatment technologies used to remediate waste rock, tailings, pit lakes, water from adits, underground workings, leachate, groundwater and surface water. The guide includes a description of each technology, detailed case study evaluation(s), costs, limitations, lessons learned, and provides references to find additional information on a given technology. Pre-and post-treatment data collected from peer-reviewed case studies for several treatment technologies is provided in the Appendices of the guide to allow users to further assess the effectiveness of various treatment technologies.
EPA's Office of Superfund Remediation and Technology Innovation's 2014 report, Reference Guide to Treatment Technologies for Mining-Influenced Water, highlights select mining-influenced water (MIW) treatment technologies used or piloted as part of remediation efforts at mine sites. The report includes short descriptions of treatment technologies and information on the contaminants treated, pre-treatment requirements, long-term maintenance needs, performance, and costs. Sample sites illustrate considerations associated with selecting a technology. Website links and sources for more information on each topic are also included. An online, searchable library lists technologies provided in Appendix A of the guide, which includes summary information for the technologies discussed in the body of the report, as well as additional technologies or products designed as passive or low-cost treatment options.
The Interstate Technology and Regulatory Council's (ITRC) web-based technical and regulatory guidance site, Mining Waste Treatment Technology Selection, is a tool for selecting an applicable technology or suite of technologies for remediation of mining sites. The guidance uses a series of questions to point users to a set of treatment technologies that may be applicable to a particular site. The website provides an overview of each technology with information about its applicability, advantages, limitations, performance, stakeholder and regulatory considerations, and lessons learned, as well as links to applicable case studies.
The EPA Abandoned Mine Site Characterization and Cleanup Handbook (2000) provides a compendium of information gained during many years of experience on mine site cleanup projects. Chapter 10 summarizes several conventional and innovative (as of 2000) treatment technologies; collection, diversion, and containment technologies; reuse, recycle, and reclamation; and institutional controls.
The technology information below is adapted from the technology overviews on ITRC's Mining Waste Technology Selection site, with a link to the entry for each technology.
Capping, Covers, and Grading — Capping, or covering of solid mining waste, is an effective treatment technology that can be used as a short-term, interim measure or as a long-term or final action. Installation of a cap or cover on solid mining waste can reduce or eliminate erosion, fugitive dust emissions, and infiltration of water to prevent the migration of contaminants. A variety of materials is available and the technology can be modified to adapt to site-specific conditions. Caps and covers can be used alone or with other treatment technologies. The cap or cover must be maintained to ensure its effectiveness. Institutional controls also may be required.
Chemical Stabilization Using Phosphate and Biosolids Treatment — This technology addresses soil, sediment, or mine tailings at remote, rural, and urban locations and can be used for small and large volumes of wastes. Phosphate treatment can be used by itself or with other technologies as an interim or final remedy. Ex situ treatment is more widely used than in situ and frequently is applied in conjunction with off-site disposal. In situ treatment has proven effective at reducing the bioavailability of lead and other heavy metals and providing a relatively nontoxic growth medium for previously barren mine/mill waste. In situ treatment has been used in mines as a coating on exposed ore surfaces but the technology has not been widely used to stabilize lead-contaminated soil in residential settings. Chemical phosphate treatments use a variety of phosphate species, but phosphoric acid has been demonstrated to be the most effective. Organic sources of phosphate such as biosolids or composted animal wastes also are used to stabilize, reclaim, and revegetate barren mine and mill wastes.
Electrokinetics — The electrokinetic remediation (ER) process is an in situ soil processing technology that separates and removes metals and organic contaminants from low-permeability soil, mud, sludge, and marine dredging. ER uses electrochemical and electrokinetic processes to desorb, and then remove, metals and polar organics. Targeted contaminants for electrokinetics are heavy metals, anions, and polar organics. Contaminant concentrations that can be treated range from a few parts per million (ppm) to tens of thousands ppm. There have been few commercial applications of electrokinetic remediation in the United States.
Excavation and Disposal — Excavation and disposal of contaminated soil, sediment, or tailings is an effective and proven technology that usually involves the removal of contaminated material with heavy equipment. This technology can be modified to adapt to site-specific conditions. Soil, sediment, or tailings can be removed so that the remaining contaminant concentrations meet cleanup goals. Excavated soil, sediment, or tailings can be disposed of either on-site (in an approved repository constructed for this purpose or another location where the exposure pathways allow the material to be beneficially reused) or off-site in a permitted disposal facility. Excavation and disposal can be used by itself as an interim or final remedy or with other technologies.
Soil Amendments — Cleanup treatments at mining sites may involve the addition of amendments to the contaminated soil. Soil amendments are materials added to soils to revitalize and make them suitable for sustaining plant life or development. Mining sites with contaminated or disturbed soils exhibit a variety of problems that often can be addressed effectively and directly through the use of soil amendments. Project managers could evaluate their effects in the subsurface, their potential for eventual transport to surface waters, and their possible subsequent adverse effects on plant and animal communities.↩
Aeration Treatment Systems — Aeration is a relatively simple and effective treatment process in which mechanical introduction of oxygen is used to enhance the oxidation and decrease the solubility of metals in MIW. Aeration can be used with other treatment technologies and often is applied together with acid-neutralizing agents, chemical oxidants, flocculants, and settling basins. The array of aeration technologies can be used at a broad range of sites that vary in site and flow conditions.
Anoxic Limestone Drains (ALD) — ALDs are low-cost, passive treatment systems that can be used to treat the acidity of MIW under specific geochemical conditions. ALDs are easy to construct and maintain, and consist of a buried bed of limestone engineered to intercept anoxic, acidic MIW and add alkalinity through dissolution of the limestone. ALDs can be used alone, but are more commonly used together with other treatment technologies such as constructed wetlands. They can be installed in remote locations and utilities are not required for implementation. The effectiveness of most ALD systems declines over time and they eventually require maintenance or replacement.
Biochemical Reactors — Passive treatment refers to processes that do not require frequent human intervention, operation, or maintenance, and typically employ natural construction materials, natural treatment media, and promote growth of natural vegetation. Biochemical reactors (BCRs) are an engineered passive treatment system that use microorganisms to remove contaminants from MIW. An organic substrate is typically used to promote microbial and chemical reactions that reduce concentrations of metals, acidity and sulfate. BCRs can be lower-maintenance treatment options for mining site cleanups and offer significant opportunities to reduce the environmental footprint associated with treatment of MIW.
Chemical Precipitation — Chemical precipitation is a flexible, permanent technology used to treat MIW, including acid mine drainage, neutral drainage, and pit lake water. Chemical precipitation processes involve adding chemical reagents and then separating the precipitated solids from the cleaned water. Typically, the separation occurs in a clarifier, although separation by filtration or with ceramic or other membranes also is possible. When chemical precipitation is used in pit lakes or other water bodies, the precipitated solids can remain in the bottom of the pool. This technology can be used by itself or in conjunction with other treatments.
Constructed Treatment Wetlands — Constructed treatment wetlands are man-made biologically active systems such as bogs, swamps, or marshes with saturated soils and at least periodic surface or near-surface water designed specifically to treat contaminants in surface water, groundwater, or waste streams. This technology is a valid treatment option for a variety of waste streams, including MIW, remedial wastewaters, agriculture waste streams, and industrial waste streams. Constructed treatment wetlands have also been used for "wet capping" of solid wastes and are often called "capped mine wastes in a wetlands setting." Constructed treatment wetlands can be used with other technologies to extend the operational lifespan of the systems or enhance the removal performance of specific constituents of concern.
Diversionary Structures — Diversionary structures are designed to prevent clean water from coming into contact with mining solid waste (net acid-producing materials) and to divert MIW to treatment or collection systems and away from sensitive environments. These include engineered channels, tunnels, pipelines, or other structures to divert surface water run-on or MIW runoff; engineered slurry walls, sheet pile walls, grouting, or other subsurface structures to divert or contain groundwater; and bulkheads and plugs in mine workings to control influx or discharge of MIW. Diversionary structures can be used to reduce the volume of, or exposure to, MIW. They also can be used to prevent erosion of mining waste and transport of soluble metals into surface water.
Electrocoagulation — Electrocoagulation refers to a group of technologies that use an electrical current that coagulates organic constituents and suspended solids in water. The coagulated organics have the ability to adsorb certain ionic constituents, making it possible to separate a flocculent with most of the suspended organics and some of the ionic constituents removed. Another variant of this system oxidizes an iron or aluminum anode to form an iron or aluminum hydroxide flocculent that can co-adsorb/co-precipitate some ions. The electrocoagulation process is complex and site- and contaminant-specific. These systems may be effective in certain niche applications. Detailed bench and pilot studies are required before implementing the technique.
In Situ Treatment of Mine Pools and Pit Lakes — This emerging technology for treating MIW involves injecting or placing substances (including carbon sources such as molasses or alcohol with nutrients) or alkaline materials such as lime directly into the mine pool or pit lake to neutralize the MIW and produce anaerobic conditions to precipitate metals in place. The addition of a carbon source leads to the formation of a sulfate-reducing bioreactor. Some metals are less soluble in their reduced form, including selenium, chromium and uranium. These oxidized metals can be removed from the water as solids. In situ treatment of solid mining waste in the form of residual minerals in mine walls, tailings, or waste rock involves the application of amendments such as potassium permanganate, phosphate or biosolids, and carbon substrate to stabilize the metals in place and reduce the formation of leachate or inhibit the migration of metals.
Ion Exchange — Ion exchange is well-established treatment technology that involves the interchange (or exchange) of ions between a solid medium and MIW. The solid medium can be commercially produced or made from naturally occurring substances (e.g., peat or zeolites). Synthetic organic resins are used predominantly because their characteristics can be tailored to specific applications. Ion exchange can be applied to dissolved constituents, cations or anions to treat mine discharges with various flow rates and can be used as a stand-alone technology or with other treatment technologies. The ability to regenerate resin and recover metals provides a potential additional benefit of this approach.
Ion Flotation — Ion flotation is a common separation technology that concentrates metal ions from solution so they can be collected and disposed of or recycled. A surfactant with an opposite charge of the target ions is used to attract metal ions, and air bubbles carry the material to the surface of the solution. Ion flotation can be used to remove heavy metal ions from wastewater, but the process also is being studied to remove uranium from mine water by adding biological or synthetic rhannolipids, a type of biosurfactant. Biological or synthetic rhamnolipids can be made using sustainable production methods, and are biodegradable, recyclable and have low toxicity. As a separation process, ion flotation has low energy requirements, small space requirements, relatively low costs, and occurs quickly. The process is most effective when site-specific conditions, such as water chemistry, are considered.
Microbial Mats — A constructed microbial mat is an aquatic bioremediation system that uses naturally occurring, living organisms (primarily cyanobacteria) to rapidly remove metals from MIW. Cyanobacteria are photosynthetic and can be grown like plants, harvested, and dried until needed. Microbial mats grow rapidly, can survive harsh environmental conditions, and can tolerate high concentrations of compounds that are toxic to plants or algae. They are called "constructed" mats because they are grown using a standard technique that is inexpensive and requires minimal training. Microbial mats can be used as a stand-alone technology or with other technologies to treat dissolved organic and inorganic constituents, including a variety of metals, metalloids, radionuclides, and oxyanions, and can treat mine discharges collected in ponds and slow-flowing leachate. Sunlight intensity is an important requirement, and, like all biological systems, system performance decreases during winter seasons.
Passive Technologies — Passive treatment (passivation) of acid-generating material involves oxidizing or protecting the sulfide surface from water and oxygen. Techniques for reducing metal sulfide oxidation involve removing oxygen, water, bacteria, or the sulfide minerals, all of which contribute to the generation of acid mine drainage. All passivation technologies use a spray-on application, either as a solution (phosphate) or as a slurry (silica). It is one of the few treatment methods that can be used to treat exposed pit walls. Although laboratory and small-scale pilot data are available, this new technology has not been applied on a large scale and there are very limited data on long-term performance. In addition, several studies have indicated that there is an initial release of other constituents into the environment when passivation is applied. Techniques to control and possibly treat this release may be needed and regulatory approval obtained before releasing these constituents. This technology may be used alone or with other technologies.
Permeable Reactive Barrier Systems — A permeable reactive barrier (PRB) is a continuous, in situ permeable treatment zone designed to intercept and remediate a contaminant plume. The treatment zone may be created directly using reactive materials such as iron or indirectly using materials designed to stimulate secondary processes, such as by adding carbon substrate and nutrients to enhance microbial activity. With most PRBs, the reactive material is in direct contact with the surrounding aquifer material. PRBs are designed to be more permeable than the surrounding aquifer materials, enabling contaminants to be treated while groundwater readily flows through. (ITRC 2005).
ITRC. 2005. Permeable Reactive Barriers: Lessons Learned/New Directions. PRB-4. Washington, D.C.: Interstate Technology — Regulatory Council, Permeable Reactive Barriers Team. www.itrcweb.org
Pressure-Driven Membrane Separation Technologies — Pressure-driven membrane separation (PDMS) processes are tools used to separate media. Commonly used processes for treatment of MIW include reverse osmosis, nanofiltration, ultrafiltration, and microfiltration. Any of these technologies can be used for surface and groundwater influenced by mining waste, but the particular tool used depends on the cleanup goal for the site. PDMS processes use semi-permeable membranes to reduce the concentration of the selected solutes in a feed solution. They produce a permeate stream containing materials that pass through the membrane, and a concentrate or waste stream that contains the materials filtered out of the feed solution. Passage through the membrane matrix is controlled by the application of a "driving force," which includes mechanical pressure, concentration or chemical potential, and temperature or electrical potential (Mortazavi 2008).↩
Backfilling and Subaqueous Disposal — Backfilling and subaqueous disposal technologies can be effective treatment alternatives for remediation of solid mining wastes and MIW. Subaqueous disposal involves removing surface material and placing it underground and underwater, thus eliminating direct contact exposures. Typically, subaqueous disposal is applied to sulfide-containing solid mining wastes to reduce oxidation, which limits acid generation and release of metals. Subaqueous disposal also is used to dispose of non-acid-generating solid mining wastes through backfilling. Solid mining wastes are disposed of into deep submarine environments, natural lakes, pit lakes, subsidence features, underground mines, and surface mines. Subaqueous disposal also includes injection of MIW and process waters into geologic formations below the depth of fresh groundwater, but this has not been widely practiced.
In Situ Biological Treatment — In situ biological source treatment consists of isolating the source of MIW by establishing an in situ biological layer on exposed metal sulfide surfaces (Jin et al. 2008b). This is typically accomplished by injecting inoculum (e.g., wastewater effluent) and substrate into the subsurface material. The in situ biological source treatment can achieve satisfactory results without the cost of excavation and material handling. The process typically has two components: (a) developing an anaerobic environment through the injection and distribution of inoculums and substrates; and (b) forming and maintaining a biological film that impedes the release of products of iron reduction. A complete analysis of the MIW and the treatment material, including seasonal and formulation variations, must be completed before selecting an in situ biological source treatment system. Bench-scale tests exploring variations in the treatment material and the material to be treated are invaluable when determining whether an in situ biological source treatment system is applicable and the type of treatment material that is suitable for the site.
Jin, S., P. H. Fahlgren, J. M. Morris, and R. B. Gossard. 2008b. "Biological Source Treatment of Acid Mine Drainage Using Microbial and Substrate Amendments: Microcosm Studies," Mine Water and the Environment 27(1): 20-30.
Phytotechnologies — Phytotechnologies use plants to remediate various media impacted with different types of contaminants. There are six basic phytoremediation mechanisms that can be used to clean up mining-contaminated sites: phytosequestration, rhizodegradation, phytohydraulics, phytoextraction, phytodegradation, and phytovolatilization. These technologies can be applied to address certain issues associated with mining solid wastes and MIW, and also can stabilize tailings and act as a hydraulic control for drainage. Phytotechnologies are a common component of mining reclamation and restoration projects that establish a plant cover as a final remedy. Establishing phytotechnologies requires careful selection of plant species and soil amendments. Most phytotechnologies can be applied to both organic and inorganic contaminants and to soil/sediment, surface water, and groundwater. Phytotechnologies also can be applied simultaneously to various combinations of contaminant types and impacted media. Establishment of vegetation can be enhanced by using native soil or other amendments to offset the often poor growing conditions offered by the tailings material.
Reuse and Reprocess Technologies — Reuse and Reprocessing (R2) technologies are applicable where mining wastes can be put to cost-effective, beneficial use directly or following reprocessing or treatment, or where reprocessing of the waste will render it safe for permanent disposal at the mine site. R2 technologies can be used for remediation of many types of mine waste. Examples include direct use of chat-pile material as an asphalt component, reuse of contaminated soil as cover material for site remediation, or use of waste rock and leach-pad material as construction material, either directly or following treatment or reprocessing. R2 technologies can be employed almost anywhere and in any climate as long as a market exists for the beneficial product. The technologies usually are used with mine waste contaminated with metals, but waste containing other contaminants, such as radionuclides, cyanide, and certain organic chemicals, also may be suitable. R2 technologies can be applied alone, but often are applied with treatment technologies that address the contaminants in the material, making it safe for reuse or conversion into a usable form.↩