36. Surface mining

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Contents

1. Scope

2. Environmental impacts and protective measures

2.1 Potential environmental consequences of surface mining

2.1.1 Dry extraction
2.1.2 Wet extraction
2.1.3 Nearshore marine mining

2.2 Measures for limiting the environmental consequences of surface mining activities

2.2.1 Measures prior to commencement of mining activities
2.2.2 Measures in the course of mining activities
2.2.3 Measures following termination of mining activities

3. Notes on the analysis and evaluation of environmental impacts

4. Interaction with other sectors

5. Summary assessment of environmental relevance

6. References

1. Scope

Surface mining is the term used to describe diverse forms of raw-material extraction from near-surface deposits. It involves the complete removal of nonbearing surface strata (overburden) in order to gain access to the resource. Depending on the physical characteristics of the raw material and on the site-specific situation, various surface-mining techniques are applied:

Dry extraction of loose or solid raw materials: In hardrock mining, the product must first be "worked" (loosened). Then, it can be loaded, hauled and processed by mechanical means similar to those employed in loose-rock mining. Accordingly, dry surface mines require appropriate dewatering.

In wet-extraction, or dredging, operations, loose raw materials are mechanically or hydraulically extracted and transferred to a processing facility. The entire extraction equipment is normally located on/in the water, often floating on a river or artificial lake.

Offshore, or shelf, mining is the term used to describe the extraction of loose material from nearshore deposits (marine beach placers). Like in wet extraction, the material is excavated and conveyed by mechanical or hydraulic means.

Deep sea mining is a - future - form of mining in which raw materials are extracted from ocean beds; not to be dealt with in the present context.

The various surface mining techniques are applied to different types of raw material reservoirs.

Table 1 - Forms of surface mining and major raw-material products

1) RE-minerals = rare-earth minerals

Surface mines vary in size according to the nature of the deposit and the employed techniques of extraction. Among terrestrial workings, one encounters mines ranging in size from small one-man operations to huge strip mines measuring several kilometers in diameter. Due to the elaborate, expensive technology required, marine workings always strive toward minimum dimensions.

Since mining amounts to a site-bound activity, new and expanding operations often have to compete with other potential users of the premises in question, and the infrastructure required for surface mining operations may still have to be established. As regards the demarcation of surface mining activities, it is inherently difficult to separate them from the required mineral dressing facilities, because such processing normally takes place directly at the place of extraction.

2. Environmental impacts and protective measures

The environmental consequences of surface mining operations are strongly dependent on the project type. Consequently, this section distinguishes between impacts and control measures.

2.1 Potential environmental consequences of surface mining

Common to all surface mining activities is that their environmental impacts are both size-dependent and location-dependent, particularly with regard to climatic, regional and infrastructural contexts. For the sake of simplicity, the potential environmental impacts of surface mining operations are categorized in the following sections according to the employed type of raw-material extraction.

Table 2 - Forms of surface mining and their main environmental impacts

2.1.1 Dry extraction

Differentiation is made between loose-rock and hardrock mines. Wherever necessary, the following sections include reference to specific influences. The environmental consequences are broken down according to physical, biological and social effects.

· Physical environmental impacts of dry surface mining

In essence, the foremost environmental impact of surface mining is the extraction of nonrenewable resources. The processes and activities involved in the extraction of a raw mineral can involve mining losses, free-standing ore pillars, presently uneconomical sections of deposit, overcutting, etc., with resultant destruction of sections to the extent of their becoming inaccessible for future extraction. The strip mining of carbonizable or combustible raw materials such as coal or peat can lead to the destruction of resources by fire (seam fires).

The space requirements of surface mining operations can be quite substantial, comprising the quarry itself and dumps for overburden, which can be very sizable for deep hardrock mines (e.g., open-cast ore mines), tailings heaps, which also can become very large for low-grade ore, and room for infrastructural facilities (miners' lodgings, power supply, transportation, workshops, administration building, processing equipment, etc.). Since surface mining operations are inherently bed-bound, their size and location are determined by the given geological conditions of the bedding and associated strata. And since major disruption of the earth's surface is unavoidable in connection with surface mining operations, the question of tolerability under the prevailing conditions must be given due consideration prior to commencing with any extractive processes.

In and around the mine and its dumps, some of the soil has to be removed, and some gets covered over. Nearly all industrialized countries have regulations governing the treatment of cultivable soil (topsoil). As a rule, its removal and temporary storage prior to the beginning of direct mining activities is mandatory. In addition, subsequent replacement of the topsoil and recultivation of backfilled ground may also be prescribed.

Surface mining operations also alter the morphological makeup of the mine site as a (temporary) result of shaping the quarry and its dumps and heaps. Once an abandoned mine has been recultivated, some such changes remain behind in the form of permanent, residual (submorphic) hollows, the size of which depends on how much material has been extracted from the mine. Morphological changes can be particularly pronounced in hardrock mines, which tend to have very steep slopes and for which little material is left for refilling (e.g., in stone quarries).

By comparison, the morphological changes occurring in loose-rock mines consist primarily of the overburden dumps established at the time of opening the mine, and ground subsidence caused by dewatering.

Surface mining activities also interfere with the surface water regimen. Relevant intentional intervention aims to keep surface water and groundwater out of the workings by collecting and channelling the water from around the perimeter as well as from the mine proper. Riverbeds are bypassed around the mine, and runoff water from precipitation and drained slopes is collected in ponds and discharged into the natural hydrographic network. Increased sedimentation and altered chemism resulting from such measures can cause qualitative degradation of the recipient water body.

Loose-rock surface mining can also interfere with the groundwater regimen, with resultant loss of groundwater quality due to the infiltration of contaminated wastewater and in washout and leaching of dumps, heaps and the mine itself. If the groundwater level is not lowered in time, groundwater will flow into the pit. Consequently, all around and within the mine, wells are sunk to below the lowest pit bottom in order to enable dry extraction while enhancing the stability of both the slopes and the floor by relieving the effective hydraulic pressure. The well water is generally unpolluted and can be fed directly into the natural river system. Lowering the groundwater level has major consequences for the surrounding area, e.g.:

– drying up of nearby wells,
– settlement/subsidence,
– disturbance of the vegetation due to altered groundwater supply.

When the mine is closed down, hollows resulting from extraction of the resource and removal of overburden during the opening phase remain behind. The hollows eventually form groundwater-fed ponds and lakes reflecting the return of the groundwater level, which may proceed very slowly, depending on the depth of the erstwhile mine and on the given hydrogeological situation. Indeed, it may take more than 50 years for a new state of equilibrium to be achieved. If the zone of contact between the water and the soil contains soluble substances, power-plant ash and/or industrial residues, the water quality may suffer. The most well-known problem in that connection is an excessively low pH in the lakewater. A lack of affluxes and effluxes aggravates the problem, promoting eutrophication, particularly if the surrounding areas are intensively farmed.

The extraction activities impose a noise nuisance on their surroundings, with major noise sources including the machines and devices required for getting, loading, hauling, reloading, etc. In hardrock mines, drilling and blasting constitute two additional sources of noise. In addition to the sound of the explosion, the attendant vibrations and reverberations amount to an additional dynamic burden on the environment that not only annoys the neighbors, but can also cause damage to structures.

Finally, dry surface mining activities also lead to air pollution, the causes and effects of which are multifarious:

– Blasting in hardrock causes dust pollution in that rock dust becomes entrained in the blast damp. The wind can stir up any and all exposed materials, especially during loading, reloading and dumping operations, all of which adds to the dust nuisance;
– Air pollution in the form of gases results from the exhaust of vehicles and engines, which tend to be diesel-driven, as well as from the escape of blast damp. Open-pit coal mines are susceptible to still other, deposit-specific hazards: the extraction of deep-lying coal can give rise to the escape of methane, and spontaneous combustion can release other noxious gases.

Hot, dry weather poses a considerable fire hazard - by spontaneous combustion - for exposed coal at the bottom of the pit and at the loading and unloading points.

Additionally, self-ignition can cause hard-to-extinguish smoldering fires in overburden dumps and feigh heaps containing small amounts of coal. Such fires can pollute the environment with odors and noxious gases for years or even decades.

– Radiation exposure can occur in special cases, i.e., in connection with the mining of uranium ore or rare-earth pegmatites.

· Interference with the biological environment by dry surface mining

The surface extraction of raw materials necessitates areal exposure of the deposit. Removal of the soil in and around the mine itself, the surrounding dumps and the requisite infrastructure destroys the local flora.

In turn, fauna is driven out of the area by the destruction of its natural habitat.

Aquatic ecosystems can be disrupted by qualitative and quantitative changes in surface water conditions, while wetlands can be emburdened by an altered groundwater level, e.g., its lowering or recovery with subsequent lake/swamp formation. Fragile ecosystems in extreme locations are particularly susceptible to permanent damage or destruction.

Terrestrial ecosystems are also affected by mining-induced situational changes (in connection with the groundwater level, for example). Even after the mine has been abandoned and recultivated, the residual changes in soil physics and chemistry, available water resources, etc. can lead to the appearance of different plant and animal associations constituting an irreversible alteration stemming from the original disruption.

· Effects of dry surface mining on the social environment

The areal nature and deposit dependence of surface mining activities engender the presumably most serious effects on human living conditions. Frequent consequences include:

– the necessity of resettling the inhabitants of the area to be mined. Surface mining operations demand the relocation of settlements as well as traffic routes and communication infrastructure. The consequences range from economic loss to sociological and cultural disruption. The latter will be all the more serious, where the local population feels strongly attached to a limited natural environment, cultural or religious localities, established tribal structures, territorial sovereignties, etc.;
– land-use conflicts when the area to be mined is being used for agricultural or forestry purposes or contains significant cultural monuments, recreation areas/facilities or the like that stand to be destroyed or negatively affected by the mining operations.

If, due either to the large area to be affected by a surface mining operation and/or to attendant damage to the local flora and fauna, farmland and, hence, income potentials are lost, or even the relocation of entire settlements necessitated, those responsible and those affected must investigate in advance which special consequences and impacts the project can be expected to have for existing groups - women in particular. Likewise, the extent to which women will be able to partake of the economic advantages the region stands to gain from the mining operation must be duly investigated.

Moreover, the environmental effects of mining operations can affect the health of both the miners themselves and the people living in the surrounding area.

Finally, the establishment of mining infrastructure can inadvertently induce the uncontrolled generation of settlements in areas which otherwise may have remained undisturbed.

2.1.2 Wet extraction

With regard to the environmental consequences of wet surface mining, the previous subdivision according to physical, biological and social impacts is maintained. In case of identical consequences, the reader is referred to section 2.1.1.

· Physical environmental impacts of wet surface mining

Since the wet extraction of raw materials is a function of site- and mineral-specific factors such as a low degree of consolidation, certain particle-size spectra, well-balanced, shallow topography and adequate quantities of water, the number of potential locations and, hence, the scope of environmental consequences are more limited than for dry extraction.

The differences begin with the space requirement. Wet extraction normally involves a very limited extraction area. Precious-metal and tin dredgers, for example, rarely require more than one hectare, unless overburden has to be removed in advance. On the other hand, the extraction area wanders more or less rapidly over the entire explorated field, which eventually becomes completely modified: when dry land is being worked, the soil is removed, but when a river is being worked, the entire riverbed is altered, and the entire course of the river is likewise affected. Cutting and winning leaves behind rubble containing large amounts of classified material that is extensively lacking in fine and superfine contents. Consequently, pedogenisis, or soil formation, as an essential prerequisite for recolonization by plant associations, is seriously impeded. Meanwhile, the fine and superfine fractions emburden the river with large quantities of muddy wastewater. Such wet-extraction sludge plumes sometimes develop into water pollution loads that remain clearly visible over hundreds of kilometers before the clay fraction finally settles out of suspension. The situation can be additionally aggravated by contaminated wastewater. The escape of mercury from gold-placer processing activities, for example, or the uncontrolled disposal of used oil, constitute serious pollution potentials.

With regard to resources, noise and air, the reader is referred to the hazards discussed in item 2.1.1.

· Effects of wet surface mining on the biological environment

Like dry extraction, wet extraction also destroys flora and drives away fauna. Also and in particular, however, wet extraction disturbs the aquatic ecosystem. The aforementioned mining-induced sludge contamination of affected rivers degrades the water quality, alters the river bed by depositing fine and superfine material, and disrupts the nutrient balance of the rivers, with consequential effects on river fauna and flora. Frequently, such pollution leads to lower fish populations due to dying and migration away from the affected sections of the river.

In tropical areas, wet extraction of mineral resources poses an additional serious environmental hazard in that resultant still waters can serve as breeding places for pathogenic agents such as malaria-carrying mosquitos. Indeed, it can happen that regionally eradicated tropical diseases flare up anew.

· Effects of wet surface mining on the social environment

In otherwise infertile areas, the loss of fertile flood plains or easily irrigated areas to wet surface mines can lead to bitter land-use conflicts. Even if the areas in question are recultivated afterwards, irreversible damage may remain behind on a location- and situation-specific basis. The impairment of fish-farming activities by the aforementioned sludge pollution of rivers counts more as a temporary effect. By contrast, health impairment resulting from the contamination of rivers with mercury, for example, counts as irreversible, permanent damage.

Social conflicts in connection with wet mining activities become particularly serious when boom times (a local gold rush, for example) draw large numbers of small miners (diggers, garimpieros, pirquineros) into a particular area. Many such newcomers lack legal mining titles and either breed or intensify diverse problems (crime, speculation, exploding prices, disease, social tension among the native population, etc.). As the originally rich deposits become harder to work and eventually depleted, such problems tend to intensify.

2.1.3 Nearshore marine mining

In dealing with the environmental consequences of marine mining, deep-sea mining is not gone into separately, because it does not yet actually contribute to the production of raw materials. The environmental effects of deep-sea mining are comparable to those of nearshore marine mining, with the latter limited by definition to the use of bucket chain (scoop) and suction dredgers in waters with a maximum depth of about 50 meters.

· Physical environmental effects of marine mining

The most serious effect of extracting minerals from the ocean is that such activities alter the ocean floor. The ground is removed by mechanical or hydraulic means in order to separate it from its ore in an on-board processing facility. Altering the morphology and composition of the ocean bed amounts to its total restructuring, since natural classifying processes take place when the oversize, tailings and perhaps overburden is re-deposited - assuming, of course, that the raw material in question contains low-grade ore (e.g., heavy mineral sand) and that processing leaves behind large quantities of nonbearing materials. When a large percentage of the material being extracted is commercially valuable (sand, gravel), its removal in large volumes also modifies the seafloor morphology, possibly resulting in intensified coastal erosion and accumulation of sediments, since the "new" ocean floor is less compact and lacking in fine and superfine particles.

The fine and superfine fractions that are left over from ore processing and which swirl up from the ocean floor remain in suspension for a long time, causing turbidity that can be carried off by ocean currents to pollute areas as far as 10 km away from the source.

If the water flows slowly, the fine and superfine particles settle out, covering the ocean floor with a layer of clay.

Moreover, by way of analogy to dry mineral extraction, the mining equipment, machines and apparatus generate noise and pollute the air and water.

· Effects of marine mining on the biological environment

The altered seafloor interferes with the natural ocean-bottom nutrient balance, both within the mined area and in the emburdened vicinity. The effects are particularly devastating for immobile marine organisms such as corals, which can be partly or completely destroyed by the combination of high turbidity and fine-particle sedimentation.

The clouds of turbidity also impair marine life in the water itself, e.g., by reducing insolation, lowering the available oxygen level due to oxidation of stirred-up particles, obstructing the respiratory passages of marine organisms, and possibly even poisoning them with trace metals.

Mobile marine fauna can evade the polluted environment by moving off, but are nonetheless unable to prevent the destruction of their spawning grounds.

· Effects of marine mining on the social environment

Since marine mining has no direct impact on the human environment, its social effects are limited to usufructuary conflicts, most notably with fish farmers, whose livelihood can be prejudiced by such mining, and with the operators of recreation facilities that can be adversely affected by mining-induced pollution.

2.2 Measures for limiting the environmental consequences of surface mining
activities

A selection of technical options for use in limiting the pertinent environmental impacts prior to, during and subsequent to surface mining activities are pointed out below. Naturally, the limitation of environmental consequences (= pollution control) entails a suitable institutional basis and the existence, adherence to and monitoring of appropriate directives.

2.2.1 Measures prior to commencement of mining activities

The most important precommencement measure is to ascertain the momentary condition of the environment as a basis for evaluating subsequent environmental impacts. The relevant studies should give due consideration to cultural and historical monuments, soil conditions, groundwater and surface water qualities and quantities, as well as flora, fauna, land use, etc.

In the case of marine placers, the marine flora and fauna, prevailing currents, seafloor gradients, etc. also should be determined in advance.

Careful planning of operational sequences enables significant limitation of environmental consequences even before mining activities begin. For example, a suitable time schedule with provisions for the archiving and conservation of archeological finds, the harvesting of standing timber in the area to be worked, and/or keeping the mine open only as long as necessary is extremely useful. Likewise, careful separation and separate storage of humus and the upper soil horizons of the overburden ensure that suitable material will be available for subsequent recultivation. Selective dewatering according to a time scale and the use of modern drainage techniques and/or sealing methods can help minimize the problems arising from groundwater recession.

With a view to precluding potential social tensions, all relevant planning must - in order to protect their interests - involve the groups of persons who will be affected either directly, e.g., by having to resettle, or indirectly, e.g., by impaired fishing conditions. It is particularly important that all parties concerned and affected, as well as the local authorities, be allowed to appropriately participate in the planning and execution of relocating measures, compensation and possible resettlement.

Finally, both the decision makers and the miners must be instructed and sensitized in and toward the environmental and health aspects of surface mining activities prior to their commencement.

2.2.2 Measures in the course of mining activities

In order to avoid excessive land consumption, inside dumps should be established, i.e., the overburden should be stored within the open spaces of the mine.

The noise nuisance must be limited by appropriate soundproofing of individual pieces of equipment. Whole units of equipment can be encapsulated or equipped with special exhaust systems (mufflers). Additionally, the miners must be required to wear personal noise-protection gear, e.g., ear protectors. Finally, time limits can be imposed on noise emissions, e.g., by limiting blasting operations to once a day. Moreover, the propagation of acoustic waves in the near vicinity of noise emitters can be reduced by such measures as noise-control embankments.

In hardrock mines, optimized blasting methods can substantially reduce noise and dust emissions. By optimally matching the explosive quantities to the drilling pattern and by stemming the holes, the overall quantity of explosives and, hence, the magnitude of the explosion (vibrations), the incidence of microfine dust, and the intensity of the blasting noise can be substantially reduced.

Dust control measures in surface workings can encompass such individual measures as sprinkling water on the roads and other conveying routes, washing down transport equipment (trucks, etc.), irrigating and turfing dumps and exposed areas, and applying dust bonding agents as necessary. Also, individual pieces of equipment such as crushers over belt feeders can be encapsulated and surrounded with trees or hedges that filter out dust and reduce the overall drift (deflation). Drills and boring tackle can be fitted with wet or dry dust precipitators.

Wastewater can be cleansed of suspended solids, neutralized and clarified in wastewater treatment facilities to meet minimum quality standards for release into a recipient body. For each and every solution or suspension, there are appropriate liquid/liquid and solid/liquid separation processes for use in purifying contaminated water. For metal-polluted acid mine drains (a.m.d), electrolysis is indicated, while an ion-exchange technique is more suitable for radioactive wastewater. In general, all means of countering the causes of pollution should be exploited. For example, the use of bypass microfilters in engine lubrication systems can reduce the incidence of used oil by up to 90 % by prolonging its useful life.

The dredges used for working nearshore marine placers should be equipped with long rubbish chutes for use in covering the tailings/trash and oversize with overlay shelf in order to restore a close-to-natural particle-size spectrum to the seafloor.

Wet extraction from an artificial lake is preferable to working directly in a river, because it involves much less of a sludge load for the latter. Wells and other large boreholes that are no longer needed but could disturb groundwater barriers (aquitards) should be sealed.

Particularly for fragile working faces, the angle of slope around the perimeter of the mine must be designed to preclude major flank movements (slides and falling rocks).

In dry coal mines, care must be taken at the planning stage to protect coal-bearing dumps from spontaneous ignition by appropriate surface compaction and air-exclusion measures. The same applies to coal pillars and abandoned working faces, which also require sealing to prevent smoldering fires.

Such special measures as the posting of trespassing notices, installation of fences and blocking of roads can help protect and preserve adjacent ecosystems.

Persons likely to be affected can and must be afforded appropriate protection through, say, the appointment of environmental affairs and/or safety officers and occupational physicians. Since damage to the environment cannot be limited exclusively to the mining area, the right to medical services should also be extended to persons living in the general vicinity.

Continuous monitoring of all important factors must accompany all surface mining activities and attendant pollution-control measures. Such factors include exhaust gases, noise levels, vibrations, water pollution, particulate emissions, slope movement/stability, ground subsidence and groundwater levels.

2.2.3 Measures following termination of mining activities

As soon as any section of a deposit has been fully exploited and refilled with waste from new operations, appropriate rehabilitation measures can and must be taken. Since surface mining operations tend to be quite expansive, ongoing mining operations in one area can be accompanied by rehabilitative measures in another. The same is true of wet mining operations outside of riverbeds. To rehabilitate means to immediately transform the areas concerned into as natural a landscape as possible.

Following wet extraction, particularly in tropical locations, all worked-out areas must be drained and graded to eliminate all open bodies of water that could serve as breeding grounds for pathogenic vectors like malaria-transmitting mosquitos. On the other hand, bodies of water created by surface mining activities can, on a case-by-case basis, also be utilized as dry-season water reservoirs or for such commercial purposes as fish-farming.

Dumps, open-pit perimeters, outside dumps and erstwhile extraction areas require immediate greenbelting or planting with indigenous vegetation in order to limit or prevent erosion, especially in humid, tropical climates, and deflation in arid climates. Special erosion control methods such as drainage and consolidation must be employed in particularly vulnerable areas.

The ultimate aim must be to fully recultivate the worked out areas to enable appropriate and corresponding use, or to renature them for another purpose. To reclaim the land, it must be graded, compacted and covered with soil and humus to allow immediate oversowing and subsequent soil management. It should be borne in mind, however, that recultivation is not the only means of limiting environmental detriment. Recultivation is very time-consuming, and the ultimate success is usually uncertain. Especially recultivation in tropical areas has not yet been adequately researched and developed with regard to planting sequences, site-appropriate species, etc. Moreover, successful recultivation entails extensively natural soil physics (permeability, granularity/type of soil) and soil chemistry (pH, nutrients, absence of pollutants). Otherwise, the soil would not be able to fulfill its diverse functions as a water reservoir, a biotope for plants and animals, and a basis of agricultural production.

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