Actiuni asupra solului
RECLAMATION OF NATURALLY INACCESSIBLE AREAS
Naturally inaccessible areas are frequently the result of moisture extremes. Wet areas must be either drained or land-filled. Drainage (see drainage systems) is accomplished in several ways: by building a system of channels, by laying drainage pipes, and by pumping. In many cases embankments must be constructed to keep water out of the reclaimed areas. landfill operations require large quantities of suitable fill material such as sand, which is often obtained by dredging adjacent areas. Water is supplied to arid areas either by pumping it from underground sources or by transporting it through an irrigation system.
Reclamation of wetlands has significantly enhanced the well-being of several nations. The Netherlands is an outstanding example. Over a period of centuries the amount of arable land has been significantly increased by the construction of dikes, enclosing portions of the shallow coastal waters, which are then drained off. The Zuider Zee project, the first phase of which was a 29-km (18-mi) dike completed in 1932, has created nearly 200,000 ha (500,000 acres) of polders, which is land reclaimed from the sea. Because much of this land is below sea level, continuous pumping is required to maintain the desired water-table level.
Considerable areas in salt marshes have also been reclaimed in eastern England by constructing embankments that separate the marshes from the sea and draining the marshes both with ditches inside the embankments and through tidal sluices. Because the marshes are above sea level, pumping is not needed. Land reclaimed from the sea has such a high salt level that only salt-tolerant vegetation can be planted during the first few years. However, after several years, rain leaches out most of the sodium ions and soil conditions improve.
An example of large-scale reclamation of freshwater marshes is the drainage in the 1930s of the Pontine Marshes of Italy, a 75,000-ha (195,000-acre) region of dunes and marshes about 70 km (45 mi) southwest of Rome. The project not only opened new land for settlement and agriculture but also helped control malaria, a product of the marshes.
Reclamation of wetlands has been less actively pursued in the United States, because such areas are highly productive habitats for fish, shellfish, birds, and wildlife. Reclamation of arid lands, however, has been a major goal of the U.S. Bureau of Reclamation. The Lower Colorado River Project has transformed desert areas, such as the Imperial Valley of California, into some of the most productive cropland in the world.
The Dutch experience also points a warning, however. Although Dutch farmers have wrested some of the world's highest yields from their reclaimed lands, they have done so at considerable expense. The annual cost of drainage and dike repairs exceeds $400 million; runoff from fertilizers and pesticides adds to the burden of water pollution; the constant pumping has compressed polder soils, causing them to sink while, at the same time, it is feared that sea levels may be rising. In consequence, in 1993 the Netherlands government announced plans to reverse the reclamation process, returning over 240,000 ha (600,000 acres) to marsh, lake, and forest.
RECLAMATION OF ARTIFICIALLY CREATED WASTE AREAS
The major problems associated with waste areas that are the result of human activityŃindustry, mining, and waste disposalŃare slope instability, chemical leachates, and soil nutrient deficiencies. Such waste areas occupy only a small fraction of the land area, but they are frequently concentrated in highly populated regions and, in addition to their ugliness, often cause air and water pollution over widespread areas. In such cases the need for reclamation is particularly pressing; but when it is accomplished, it results in healthier environments as well as useful land. Slope instability can be reduced by reshaping the waste areas into landforms with gentler slopes that will conform with the surrounding landscape features. Erosion is prevented through water-control measures and the establishment of a vegetative cover.
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