Strip Mining
Strip mining
This technique is used for near-surface, relatively flat sedimentary mineral deposits. How deeply the mining can occur is essentially determined by the combination of technological capabilities and the economics involved. The latter includes the current value of the mineral, contractual arrangements with the landowner, and mining costs, including reclamation . Strip mining is used for mining phosphate fertilizer in Florida, North Carolina, and Idaho, and for obtaining gypsum (mainly for wallboard) in western states.
However, the most common association of strip mining is with coal . The examples of decimated land in Appalachia have motivated calls for prevention, or at least major efforts at reclamation. Strip mining for coal comprises well over half of the land that is strip-mined, which totaled less that 0.3% of land in the United States between 1930 and 1990. This is far less land than the amount lost to agriculture and urbanization. However, in agriculturally rich areas like Illinois and Indiana there is a growing concern over the onetime disruption of land for mineral extraction, compared to the long term use for food production.
Strip mining has occurred mainly in the Appalachian Mountains and adjacent areas, the Central Plains from Indiana and Illinois through Oklahoma, and new mines for subbituminous coal in North Dakota, Wyoming, and Montana. Important mining is also carried out on Hopi and Navajo lands, notably Black Mesa in northeastern Arizona.
Despite the small amount of land used in strip mining, the process radically alters landforms and ecosystems where it is practiced. Depending on state laws, mining landscapes prior to 1977 were often left as is, dubbed "orphan lands." The 1977 act required the land to be restored as closely as possible to the original condition. This is a nearly impossible task, especially when one considers the reconstruction of the preexisting soil conditions and ecosystem . Even so, reclamation is a vital first step in the healing process. Generally, the steeper the terrain, the greater the impact on the landforms and river systems, and the more difficult the reclamation.
Detailed economic planning precedes any strip mining effort. Numerous cores are drilled to determine the depth, thickness, and quality of the coal, and to assess the difficulty of removing the overburden , which consists of topsoil and rock above the resource. If caprock is encountered, expensive and time-consuming blasting is required, a frequent occurrence in the United States. Economic analysis then determines the area and depth of profitable overburden removal. Finally, contracts must be negotiated with landowners; strip mines commonly end abruptly at property lines.
Two kinds of earth removal equipment are typically used: a front-loading bucket (the classic steam shovel), or a dragline bucket that pulls the material toward the operator. Power shovels and draglines built prior to World War II generally have bucket capacities of 30–50 yd3 (23–38 m3). Post-World-War-II equipment may have a capacity up to 200 yd3 (153 m3). A new development, encouraged by the 1977 reclamation law, is the combination of dozers and scrapers (belly loaders) more commonly seen on road-building or construction sites.
After the overburden is removed, mining begins. The process is conducted in rows, creating long ridges and valleys in the countryside that resemble a washboard. Coal extraction follows behind power shovels, leaving a flat, canyon-like cut. Upon completion of a row, the shovel starts back in the opposite direction, placing the new overburden in the now-empty cut.
In hilly terrain, only a few cuts are all that is usually profitable because the depth of overburden increases rapidly into the hillside. Since the worst complications as a result of strip mining occur on hillsides, the environmental price for a limited amount of coal is very high. Hillside mining such as this is called "contour mining," in contrast to "area mining" on relatively flat terrain. In the latter, the number of rows are limited mostly by contractual arrangements. Consequently, the main difference between area and contour strip mining are the number of rows and the steepness of the terrain.
Both types of strip mining leave behind four basic land configurations: l) spoil bank ridges; 2) a final-cut canyon often partially filled by a lake; 3) a high headwall marking the uphill end of the mining; and 4) coal-haul roads, usually at the base of the outermost spoil bank and through gaps in the spoil-bank rows left for this purpose. In some orphan lands, wilderness-like conditions prevail, where trees populate the spoil banks and aquatic ecosystems thrive in the final-cut lake. Left alone by man, these may afford a surprisingly rich habitat for wildlife , especially birds. Deer thrive in some North Dakota abandoned mines.
Reclamation of area mining is relatively simple compared to contour strip mining. Prior to mining, the topsoil is removed and stockpiled. The overburden from the initial cut may be used to fill in the final cut, and the top part of the headwall is sometimes cut down to grade into the spoil. The spoil banks are leveled and the topsoil replaced; fertilization and replanting, usually with grasses or trees for erosion control, and subsequent monitoring of revegetation efforts, complete the process. In large operations, the leveling and replanting coincide with mining, which is ideal since this rapidly rebuilds the vegetation cover.
Reclamation of contour mining presents far greater difficulties, primarily because of the slope angles encountered. Research in Great Britain revealed that even well-vegetated slopes were producing 50 to 200 times as much sediment as similar, undisturbed slopes. Furthermore, the greater slope angles allow much more of the sediment to reach the channel below, where it eventually flows into streams and rivers.
Another problem for orphan lands in hilly terrain is the ecological island left when hills are completely enclosed by high headwalls. This is not unlike the ecological islands created in the southwestern United States from climate changes and vertical zonation of vegetation. Though far more recent, ecologists hope these "orphan islands" will allow interesting case studies of genetic isolation.
The long-term effect of strip mining has been the subject of research in Kentucky, Indiana, and Oklahoma. For over a decade the United States Geological Survey studied Beaver Creek Basin, Kentucky, obtaining valuable data before and after contour mining. Their findings were published in a 1970 report authorized by C.R. Collier and others. As expected, mining left a degraded landscape, and resulted in much greater runoff , sediment production, and water quality problems. By contrast, area mining in Indiana trapped vast quantities of groundwater within the loosened soil, reducing peak discharges, extending base flow, and yielding water of acceptable quality.
In Oklahoma, a study conducted by Nathan Meleen dealt with a mix of both flat and hilly terrain. The findings were published as a doctoral dissertation in 1977 by Clark University. Area mining, Meleen found, produced far more benign impacts than did contour stripping. The worst conditions were encountered where contour mining was adjacent to streams below, especially around the end of ridges. The short distances and relatively steep gradients gave ideal conditions for sediment and acid drainage into the channels below. Summer runoff decreased while winter runoff increased. The huge holes left by these unreclaimed operations acted like reservoirs during the drier summer months, but in winter when wetter conditions prevailed, they yielded more outflow than before mining, because of altered infiltration rates.
Strangely enough, a 1971 Oklahoma law produced effects similar to what the Geological Survey found in Kentucky. The incomplete reclamation and lack of sediment retention or topsoil replacement created ideal erosion conditions, with rates approaching 13% sediment by weight. This focuses the crucial role of topsoil replacement and rapid revegetation as the preeminent needs in reclamation. Efforts to revegetate some orphan lands where topsoil replacement is impossible will only result in worse conditions, especially downstream. Given the fact that most impacts from area mining are retained onsite, and that orphan lands possess great potential for recreation , especially fishing and hiking, at least some should be left undisturbed.
See also High-grading; Mine spoil waste; Surface mining[Nathan H. Meleen ]
RESOURCES
BOOKS
Caudill, H. M. Night Comes to the Cumberlands. Boston: Atlantic-Little, Brown, 1963.
Collier, C. R., et al. Influences of Strip Mining on the Hydrologic Environment of Parts of Beaver Creek Basin, Kentucky, 1955-66. USGS Professional Paper 427-C. Washington, DC: U. S. Government Printing Office, 1970.
Doyle, W. S. Strip Mining of Coal: Environmental Solutions. Park Ridge, NJ: Noyes Data Corp., 1976.
Landy, M. K. The Politics of Environmental Reform: Controlling Kentucky Strip Mining. Baltimore: Johns Hopkins University Press, 1976.
OTHER
U.S. Department of the Interior. "Surface Mining and Our Environment: A Special Report to the Nation." Washington, DC: U. S. Government Printing Office, 1967.
U.S. Environmental Protection Agency. "Erosion and Sediment Control: Surface Mining in the Eastern U.S.: Planning." Washington, DC: U. S. Government Printing Office, 1976.