Reverse mining—The development of deep geologic isolation of hazardous (chemotoxic) waste in Germany and its international prospects
Published:January 01, 2008
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Hartmut W.J. Schade, 2008. "Reverse mining—The development of deep geologic isolation of hazardous (chemotoxic) waste in Germany and its international prospects", Deep Geologic Repositories, Norbert T. Rempe
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To satisfy the increasing demand by the rapidly growing German industry for environmentally responsible hazardous waste disposal solutions, chemotoxic waste has been, since 1972, geologically isolated in disused portions of underground mines located in geologically stable salt formations beneath impermeable overburden strata. Requirements for permanent safe isolation of hazardous waste, anchored in the concept of multiple barriers, have been incorporated into German and European regulations and applied in three operating underground repositories thus far. On the basis of the same safety concept and a preference for waste avoidance and reuse rather than disposal, the reuse of suitable waste as mine backfill has also increased. Excellent long-term experience with hazardous waste disposal and reuse in German salt and potash mines encourages the practice of deep geologic isolation of chemotoxic waste worldwide.
The Origin of Reverse Mining in Germany
Economic growth in Germany, accompanied by a disproportional amount of growth in the chemical industry since the 1960s, produced rapidly accumulating hazardous, i.e., chemotoxic, wastes that could not be disposed on the surface or at sea, even after treatment by incineration, without environmental contamination. This induced the West German government in 1969 to initiate an environmental protection program including waste legislation. Before the first German law regulating waste disposal was enacted in 1972, a large chemical company tried to solve the problem of disposing of chemotoxic wastes generated by its own factories. Because the company had in 1970 acquired potash and salt mines, it proposed to dispose of its wastes in an inactive mine. That concept derived from the memory of using underground excavations in salt to protect sensitive and precious items during World War II. In a sense, this constitutes a completion of the standard mining cycle by reversing it.
In the inactive potash mine Herfa-Neurode, since 1967 part of the active Wintershall potash mine in Heringen (Werra), suitable excavations have been made available (Fig. 1). At the request of mine safety and groundwater protection authorities of the state of Hessen, the geological and technical conditions for safely disposing of waste were examined by experts. Their assessment confirmed that the 300-m-thick Upper Permian salt formation, deposited more than 250 m.y. ago, is geologically stable. Because it is overlain by 100-m-thick impermeable clay beds (Fig. 2), it can isolate hazardous waste from the hydrological cycle and ensure permanent confinement, provided the waste is deposited at a sufficient distance from the boundaries of the salt formation, and provided all the shafts and boreholes are tightly sealed when the mine is closed.
To gain experience with the underground disposal of hazardous waste, Hessen state authorities in 1972 first permitted the controlled disposal of a few chemotoxic waste categories for only 10 yr in a small room-and-pillar mining section in the northwestern part of the more than 700-m-deep workings in the 3-m-thick Thüringen potash seam (Fig. 2). Permit conditions included only a few principal requirements. The initial restriction to waste generated by the parent company and to packaging in steel drums was soon removed because more companies needed to dispose of their chemotoxic waste, and some waste categories required plastic containers. Facilities for waste transportation by forklift and truck were established on the surface and underground (Fig. 3). The underground repository is operated by a separate management group, but it cooperates closely with the adjacent Winter-shall mine, taking advantage of common infrastructure, such as hoisting and ventilation.
By 1975, waste quantities disposed of in the Herfa-Neurode repository had reached almost 40,000 t/yr (Fig. 4), ∼30% of which came from abroad. To include the repository in the public waste management plan, it was necessary to change the preliminary permit into a permanent one. The successful operating record and the results of a careful geological and technical reexamination that focused on long-term environmental safety led state authorities to grant open-ended approval in 1977 and to permit hazardous waste disposal in the whole west field of the mine.
Requirements For Safe Isolation of Chemotoxic Waste in An Underground Repository
Based on nearly 5 yr of experience, it became possible in 1977 to establish regulations for all stages of waste repository management and operation, on the surface and underground. They became part of the permit. With the concept of multiple barriers serving as the central element of long-term safety, the requirements regulated everything from waste acceptance to disposal (Deisenroth, 2003), starting with affidavits of the waste generators and ending with the watertight sealing of shafts, raises, and drill holes, constantly keeping all elements of the system under tight controls.
Because of rising demand and additional validation in 1985, permission was granted to expand the repository to the east field of Herfa-Neurode and neighboring excavations of the Winter-shall mine, where a vertical basalt dike forms a natural boundary. In 1997, a third large area of potash mining, south of the excavations used before, became part of the repository. Disposal chambers are separated by walls (Fig. 5) and disposal areas by dams as part of the multiple barrier system. In 1985, a supplementary permit granted use of the rail spur to the mine for waste transport (Fig. 6). Fast-growing underground disposal of hazardous waste (Fig. 4), based on the multibarrier concept (K+S GmbH, 2003), gained increasing acceptance in Germany and abroad.
More Underground Waste Repositories in the Context of National and European Regulations
In 1986, increasing demand for chemotoxic waste disposal capacity encouraged a salt mining company in Heilbronn (Fig. 1) to use portions of its salt mine as the second hazardous waste repository. According to an assessment by experts, excavations in the ∼40-m-thick Middle Triassic salt, 170–210 m below the surface (Fig. 7), are suitable because a 40-m-thick anhydrite overlying the salt has protected it for nearly 200 m.y. State authorities first issued a permit until 1994 for the disposal of smoke-cleaning residue from domestic refuse incinerators and of salt residue from chlorine electrolysis. Waste was delivered by truck or rail in steel containers and big (plastic) bags. Waste deliveries kept increasing; by 1990, the successful operating record made it possible to remove the time limit, and since 1993, the repository has been accepting residues from additional domestic waste incineration plants in Germany, Switzerland, and Austria (Fig. 8).
By 1995, the annual quantity of waste disposed in Heilbronn had climbed to almost 85,000 t. Another expert assessment confirmed the repository's capability for accepting additional categories of chemotoxic waste. Since 1999, the Baden-Wuerttemberg state authorities have therefore allowed the disposal of all kinds of hazardous waste that may be disposed underground in conformance with national requirements (Bohnenberger, 1998). In 1991, these requirements, which had been developed for the first (Herfa-Neurode) repository, became (after an almost two-decades–long record of success) part of the general German administrative provisions governing waste TA Abfall (Deutsche Bundesregierung, 1991).
The third repository, in the Zielitz potash mine near Magde-burg (Fig. 1), was permitted after the necessary expert assessment of long-term safety was completed, and work began in 1995. It consists of excavations in the 7-m-thick Ronnenberg potash seam in the lower part of the nearly 300-m-thick sequence of Upper Permian rock salt and anhydrite 450 m below the surface. Thick clay beds in overlying Triassic sandstone isolate the evaporites from aquifers. This repository was designed and operates in compliance with the TA Abfall. All kinds of hazardous waste allowed by that regulation for disposal in salt may be disposed at Zielitz (K+S Entsorgung GmbH, 2004). Waste can be delivered by truck or rail; disposal quantities are shown in Figure 9.
In 1996, Germany passed a new waste law in response to European Union directives (Frenz, 1998), and followed up in 2002 by implementing regulations (Demmich, 2002). They mandate that waste repositories in salt must be constructed and operated according to the TA Abfall to establish proof of long-term safety. Thus, German underground repositories for hazardous waste are in compliance not only with German, but also European laws and regulations.
Reuse of Industrial Waste As Backfill in Mines
Increasing amounts of waste resulting from economic growth has led governments to take measures to reduce waste. In 1986, German waste law and, in 1991, European waste directives established the duty to minimize waste. This mandate is based on the principles of favoring waste avoidance over reuse and of favoring waste reuse over disposal. All industries responded by implementing strategies for waste avoidance and reuse.
The mining industry had nearly given up backfilling as part of the mining cycle because of its high cost. However, the fees companies were willing to pay anyone who could reuse their waste gave mines the incentive to try using suitable industrial waste as backfill, if necessary, after conditioning. Because reuse cannot be subjected to less strict environmental and health regulations than disposal, each mine that proposed backfilling with waste had to undergo an expert assessment of waste categories and conditioning processes that could be used. German mining authorities, in cooperation with waste and water authorities, established requirements and technical rules (Bartke, 1997) for using waste as backfill (Fig. 10). They distinguished three backfill classes. Waste that can contaminate groundwater is permitted only in mines where permanent confinement is as assured as in underground repositories for highly toxic wastes (V2). These include potash and salt mines where long-term safety is assured. Waste with lower toxicity may be used as backfill in other mines, as long as the geologic environment excludes groundwater deterioration (V1). Waste that is nearly free of harmful substances (V0) can be used as backfill in most mines.
The use of waste as backfill can help reduce the danger of fire from gas and dust mixtures, improve ventilation and mine climate, prevent or reduce water inflow by filling and closing cavities, protect the surface by improving mine stability, and reduce subsidence and mining losses. However, it is subject to the same strict long-term environmental assessments and requirements as disposal. Therefore, the European Court of Justice (European Court of Justice, 2002) confirmed waste backfill in mines as an acceptable method of reuse.
As the result of the waste avoidance and reuse mandated by German and European legislators, disposal rates began to decline after reaching a peak of over 170,000 t in 1991 (Fig. 11). By contrast, the re-use of waste as backfill in mines increased considerably (Fig. 12). Hazardous waste was reused (Schade, 2000) in potash mines in Hessen and Thüringen, salt mines in Baden-Württemberg and Sachsen-Anhalt, and in a coal mine in Nordrhein-Westfalen (Fig. 1) more than 800 m underground, where the coal seams are embedded in thick mudstone layers (GDMB, 2001).
In 2002, Germany enacted a waste backfill regulation that allows the backfilling of hazardous waste only in salt mines (including both potash and salt mines), provided permanent isolation from the biosphere is ensured (Wagner, 2002). The detailed instructions for carrying out the long-term safety assessment for the reuse of waste as backfill are the same as for underground waste disposal.
International Prospects For Deep Geologic Isolation of Chemotoxic Waste, Based on the German Experience
This report shows that Germany has had more than 30 yr of experience in the disposal of hazardous, particularly chemotoxic, waste and 10–15 yr of experience in the reuse of such waste as backfill in underground salt mines. Between 1972 and 2003, the three operating hazardous waste underground repositories in German potash and salt mines have disposed of nearly 3.5 million t (Fig. 11). Out of the more than 20 million tons of waste used as backfill in German mines between 1992 and 2003, hazardous waste amounts to ∼9 million t (Fig. 12). Underground disposal of hazardous waste takes place only in potash and salt mines. Backfill of hazardous waste used to take place mostly in potash and salt mines; in the future, the law demands it will take place exclusively in potash and salt mines.
In Germany, laws and regulations conforming to European waste directives mandate and ensure that hazardous waste is reused or—if that is not possible—disposed of in rock salt excavations in an environmentally safe manner (Schade, 1996).
This successful record encourages the use and isolation of hazardous waste in deep rock salt excavations worldwide. The United Nations and its organization for industrial development, UNIDO, gathered information about the proven German hazardous waste reuse and disposal system in 1999 and again in 2004, offering hope that other countries with potash or salt mines will also take advantage of these possibilities for reverse mining.
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Deep Geologic Repositories
Deep Geologic Repositories reviews the success stories of underground waste isolation. It focuses on repositories that did, do, and will permanently and safely isolate dangerous materials from the near-surface biosphere. Complementary topics address the isolation capability of average crustal rock, investigations at one representative underground research laboratory, and the geologic preservation of fission products from Precambrian nuclear reactors. An international cast of contributors presents proven practical solutions to a formerly confounding issue in environmental and engineering geology: What do we do with wastes that retain their dangerous characteristics in human terms forever? The principal answer: Recycling into the lithosphere by “reverse” mining.