2010年9月11日托福机经英文版原文(康老师)

　　2010年09月11日北美（原文原题）

　　1. 地下水。首先说了地下水总量非常多，深度一般就几百米深（有题，为什么更深的地方没有水了？我选的因为压力大没岩石间空隙了），还有说了下很多种的岩石都能储水（有题，火成岩igneous为什么能储水？）。然后讲了porosity和permeability的联系和区别，最后有说到water table，提到了泉水的产生（有题）和树根吸水（有题）。

　　地下水：讲了地下水和土质分层。
　　讲了两个概念：多孔性（porosity）：容水量，比如有的岩石孔很多，存水就多；和可渗透性（porosity）：水流的能力，孔多不代表水流能力好，这些孔必须相互交叉才成。两者对地下水量有不同影响，前者影响量，后者影响流动性（有题）；又说了sediment（沉积岩吧），对地下水的影响，说一般沉积岩是不储藏水的除非有互相连通的裂纹（有题）；说了植物一般不直接从地下水层吸水，而从上一层（有题）；说地下水位不平整，会因为地表起伏通过泉水的方式流出（有题），可能是在原地，也可能是通过管道流走（有句子meaning题）。

　　groundwater。讲到人们饮用的freshwater，50% 来自于groundwater。从river 和lake 里取的freshwater 比重很少。接着说道是什么东东把groundwater存在的层和其他层分开了。groundwater主要是存在于某层当中。其次说道该层有两个性质。A和B（单词不记得了）。A大概是代表储水量，B代表石头当中的缝隙（channel)。A多的话，该石头储水量就大，B多的话会对水的流动产生阻力就大之类的。（对石头A、B的性质有题）。
还有一段说，有的地方，那个隔开groundwater和其他层的东东不牢固了，导致groundwater从地下冒出来，就成了spring可供取用（有题）。
地下水在很底下，一般不是horizontal的，常常适合地形topography而存在。地下水上面还有一层verbose?~树从这里得到水，因为他们不能直接得到地下水（有题）。

　　字神帝国独家提供新托福阅读真题题源：Groundwater地下水
　　Groundwater, water found below the surface of the land. Such water exists in pores between sedimentary particles and in the fissures of more solid rocks. In arctic regions, groundwater may be frozen. In general such water maintains a fairly even temperature very close to the mean annual temperature of the area. Very deep-lying groundwater can remain undisturbed for thousands or millions of years. Most groundwater lies at shallower depths, however, and plays a slow but steady part in the hydrologic cycle. Worldwide, groundwater accounts for about one-third of one percent of the earth's water, or about 20 times more than the total of surface waters on continents and islands.

　　Groundwater is of major importance to civilization, because it is the largest reserve of drinkable water in regions where humans can live. Groundwater may appear at the surface in the form of springs, or it may be tapped by wells. During dry periods it can also sustain the flow of surface water, and even where the latter is readily available, groundwater is often preferable because it tends to be less contaminated by wastes and organisms.

　　The rate of movement of groundwater depends on the type of subsurface rock materials in a given area. Saturated permeable layers capable of providing a usable supply of water are known as aquifers. Typically, they consist of sands, gravels, limestones, or basalts. Layers that tend to slow down groundwater flow, such as clays, shales, glacial tills, and silts, are instead called aquitards. Impermeable rocks are known as aquicludes, or basement rocks. In permeable zones, the upper surface of the zone of water saturation is called the water table. When heavily populated or highly irrigated arid areas withdraw water from the ground at too rapid a rate, the water table in such areas may drop so drastically that it cannot be reached, even by very deep wells.

　　Although groundwater is less contaminated than surface waters, pollution of this major water supply has become an increasing concern in industrialized nations. In the United States, many thousands of wells have been closed in the late 20th century because of contamination by various toxic substances.

　　The rate of groundwater flow is controlled by two properties of the rock: porosity and permeability. Porosity is the percentage of the volume of the rock that is open space (pore space). This determines the amount of water that a rock can contain. In sediments or sedimentary rocks the porosity depends on grain size, the shapes of the grains, and the degree of sorting, and the degree of cementation.

　　GROUNDWATER (T P O-01)

　　Groundwater is the word used to describe water that saturates the ground, filling all the available spaces. By far the most abundant type of groundwater is meteoric water; this is the groundwater that circulates as part of the water cycle. Ordinary meteoric water is water that has soaked into the ground from the surface, from precipitation (rain and snow) and from lakes and streams. There it remains, sometimes for long periods, before emerging at the surface again. At first thought it seems incredible that there can be enough space in the “solid” ground underfoot to hold all this water.

　　The necessary space is there, however, in many forms. The commonest spaces are those among the particles—sand grains and tiny pebbles—of loose, unconsolidated sand and gravel. Beds of this material, out of sight beneath the soil, are common. They are found wherever fast rivers carrying loads of coarse sediment once flowed. For example, as the great ice sheets that covered North America during the last ice age steadily melted away, huge volumes of water flowed from them. The water was always laden with pebbles, gravel, and sand, known as glacial outwash, that was deposited as the flow slowed down.

　　The same thing happens to this day, though on a smaller scale, wherever a sediment-laden river or stream emerges from a mountain valley onto relatively flat land, dropping its load as the current slows: the water usually spreads out fanwise, depositing the sediment in the form of a smooth, fan-shaped slope. Sediments are also dropped where a river slows on entering a lake or the sea, the deposited sediments are on a lake floor or the seafloor at first, but will be located inland at some future date, when the sea level falls or the land rises; such beds are sometimes thousands of meters thick.

　　In lowland country almost any spot on the ground may overlie what was once the bed of a river that has since become buried by soil; if they are now below the water’s upper surface (the water table), the gravels and sands of the former riverbed, and its sandbars, will be saturated with groundwater.

　　So much for unconsolidated sediments. Consolidated (or cemented) sediments, too, contain millions of minute water-holding pores. This is because the gaps among the original grains are often not totally plugged with cementing chemicals; also, parts of the original grains may become dissolved by percolating groundwater, either while consolidation is taking place or at any time afterwards. The result is that sandstone, for example, can be as porous as the loose sand from which it was formed.

　　Thus a proportion of the total volume of any sediment, loose or cemented, consists of empty space. Most crystalline rocks are much more solid; a common exception is basalt, a form of solidified volcanic lava, which is sometimes full of tiny bubbles that make it very porous.

　　The proportion of empty space in a rock is known as its porosity. But note that porosity is not the same as permeability, which measures the ease with which water can flow through a material; this depends on the sizes of the individual cavities and the crevices linking them.

　　Much of the water in a sample of water-saturated sediment or rock will drain from it if the sample is put in a suitable dry place. ■ But some will remain, clinging to all solid surfaces. ■ It is held there by the force of surface tension without which water would drain instantly from any wet surface, leaving it totally dry. ■ The total volume of water in the saturated sample must therefore be thought of as consisting of water that can, and water that cannot, drain away.

　　The relative amount of these two kinds of water varies greatly from one kind of rock or sediment to another, even though their porosities may be the same. What happens depends on pore size. If the pores are large, the water in them will exist as drops too heavy for surface tension to hold, and it will drain away; but if the pores are small enough, the water in them will exist as thin films, too light to overcome the force of surface tension holding them in place; then the water will be firmly held.

　　以上机经是康老师2011年10月－12月托福预测范围里的。