Table

Coal Production by Type in the United States in 2002

Mining Type

Production (short tons)

Total underground

353,273,486

Total surface

736,546,576

Strip

730,630,458

Auger

5,071,011

Culm bank recovery

398,536

Dredge

446,571

Total coal

1,089,820,062

haul roads, unvegetated spoil surfaces, topsoil stockpiles, and coal stockpiles is a potential problem. Overburden blasting can produce troublesome noise, air shock, and ground vibration. The quality and quantity of surface water and groundwater can be affected if effective reclamation practices are not used. Sedimentation of surface waters may occur. Erosion of reclaimed slopes is often experienced due to the unconsolidated nature of the reclaimed materials. Aesthetically, the disturbed land is very unsightly prior to reclamation.

Surface Disturbance

Surface mining leads to large-scale disturbances on the surface of the Earth. The natural land surface is drastically changed by the mining activities through the removal of soil, rock, and coal. Depending on mining conditions and equipment, widespread changes in the locations of materials will occur [5]. For example, an inadequate amount of material may be available to fill the final pit of a surface coal mine. As a result, the areas will usually be graded to a topography that includes a lake or basins with at least a portion of the area having relatively steep slopes. Reclamation of contour mines and sometimes mountain-top removal mines often results in some very steep slopes [5]. These areas are prone to erosion and mass failure due to the steepness of the slopes and the loose, nonhomogeneous nature of the materials present. Unfortunately, erosion and mass wasting are often found to be the major problems associated with surface mining because these reclaimed land forms are often in states of disequilibrium relative to the natural environment where they were created [5]. The primary role of reclamation, therefore, is to achieve a landscape that approximates pre-mining conditions assumed to be near equilibrium with the local environmental factors.

Generation of Gases

Although a surface mine releases less methane than an underground mine, as discussed earlier, the amount emitted into the atmosphere is still significant due to the large amount of surface-mined coal. Measurements of actual emissions of methane from surface mining are technically difficult, costly, and generally not available for inventory purposes; however, emission factors have been developed from a number of country-specific studies [6]. Irving and Tailakov [6] estimate that surface mining releases 0.3 to 2.0 cubic meters of methane per metric ton of coal mined.

Liquid Effluents/Acid Mine Drainage

Surface mining operations raise some of the same issues that underground mining does. Contour and area mining generates 0.24 and 1.2 tons of liquid effluents per 1000 tons of coal produced, respectively, as compared to 1.0 and 1.6 tons, respectively, for conventional and longwall mining operations [9]. The presence of soluble salts such as sodium in discarded overburden can cause saline and caustic conditions in topsoils if conditions allow the upward migration of these salts. Also, oxidation processes result in significant changes in chemistry. As discussed previously, when sulfides such as pyrite are present, acid is produced, and the solubility of elements tends to increase; hence, acid mine drainage is produced.

Hydrologic Impact

Surface mining affects surface stream runoff. The runoff may increase and subsequent channel erosion may occur as a result of reduced infiltration rates [1]. Conversely, streams may also be affected by decreased surface runoff where more permeable rock strata become exposed by the surface mining. Modifications of the local or regional recharge zones involve changing the infiltration rates by removal of vegetative cover, alteration of soil profiles, and compaction. Reduced infiltration rates decrease groundwater storage and reduce water availability. These disruptions are of particular concern in the semi-arid western region of the United States. Shallow and coal seam aquifers can be drained by mining activity, causing temporary or permanent loss of existing wells near mined areas [1].

The disturbance of the overburden during surface mining also causes significant changes in the chemical nature of the system [5]. Such changes are due to the influence of water on the now-available soluble salts and to the changing redox conditions resulting from the influx of oxygen into the system that was previously oxygen depleted. The movement of high concentrations of salts and/or elements into existing or reestablished ground-water aquifer systems can occur due to the disruption of the consolidated overburden and increased water penetration into reclaimed land.

Solid Waste/Dust

Waste rock is a product of the mining process that influences the post-mining land surface. In the case of mountain-top removal and contour mining methods, waste materials are often used to fill adjacent canyons or hollow areas. When these materials are used for canyon fill, steep slopes are formed that tend to be very erosive. Surface mining produces more solid waste than underground mining techniques, with 10 tons of solid waste produced per 1000 tons of coal removed for both contour and area mining [9]. Conventional and longwall underground mining, on the other hand, produces 3 and 5 tons of solid waste, respectively, per 1000 tons of coal removed. Similarly, dustiness associated with surface mining is significantly greater than that of underground mining. The World Bank Group [9] reports dust generation of 0.1 and 0.06 ton per 1000 tons of coal produced for contour and area mining, respectively, while only 0.0006 and 0.01 ton of dust is generated during conventional and longwall underground mining, respectively.

The Effect of Coal Usage on Human Health and the Environment 89 Health Effects/Miner Safety

Surface miners experience different safety issues from underground miners; in 2002, the annual death rate for surface miners was 0.2 per thousand miners [14]. Historically, underground miners suffered a greater rate of respiratory diseases than did surface miners until regulations improved dust control practices and mandated the use of personal air-purifying systems. Now the fatality rates are similar for surface and underground miners.

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