托福閱讀作為托福考試中的一個部分，是需要同學們在平時積累托福詞匯和托福素材的，本文小編將為同學們帶來托福閱讀素材之地下水的素材，同學們在備考托福時，也需要要找一些類似的素材進行積累，只有這要才能考出理想的托福成績!

TPO-1-1：Groundwater

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.

譯文：TPO-1-1 地下水

地下水是指滲入到地下并將所有巖石孔隙填滿的水。到現在為止，大氣水是最豐富的地下水資源，是地下水在水循環中的一個環節。普通的大氣水會從地表、降水以及湖泊河流侵入到地下。在再次冒出地表之前，這些地下水有時會長時間留在地下。最初讓人覺得難以置信的是，在我們腳下“堅實的”土地中竟然有足夠的空間能儲存這么些水。

然而，地下水所需的儲存空間多種多樣。松散的砂子和礫石間有許多顆粒，如沙粒和小石子，它們之間的孔隙是最常見的儲存地下水的空間。由這些顆粒組成的水床非常普遍，通常位于看不見的土壤下方，在攜帶粗糙沉淀物的湍急的河流曾流過的地方都能找到它們的蹤跡。比如，冰河時代覆蓋北美的巨大冰層逐漸融化，大量水從那兒流出。水里總會攜帶些石子、礫石和沙石，這些顆粒會隨著水流的減速而沉淀，這就是所謂冰河期的冰水沉積。

現代也有冰水沉積，盡管規模相對較小。凡是有攜帶沙石的河流或者溪流從山谷流至相對平坦的地面時，砂石就隨著水流速度的減慢逐漸沉淀;水流通常呈扇形擴散，它們所攜帶的沙石也會沉淀為光滑的扇形斜面。當河流匯入湖泊和海洋的時候也會有沉淀，這些沉淀最初在湖底或海底，但將來海平面下降或者陸地崛起時，它們就會分布于內陸，通常厚達幾千米。

低地區域上的任何位置可能就是曾經的河床，后續被土壤覆蓋而變成現在的樣子。如果那些過去的河床和沙洲現在位于地下水位之下，一定會有大量的地下水浸在它們的沙子和礫石之間。

以上說的都是松散的沉積物，那些堅固的沉積物，也擁有以數萬計的毛細孔來容納水。因為最初顆粒間的縫隙通常并未完全被黏固的化學物質塞滿，而且部分顆粒很可能在固化時或固化后被滲入的地下水溶解;結果這些砂巖最終變得和形成它的散沙一樣多孔。

因此，不管沉積物是疏松還是堅固，它們中一定有空間。大部分結晶體巖石都非常堅硬，但也有例外，最常見的就是玄武巖，它是一種固化的火山熔巖，經常充滿了微小氣泡，從而變得十分多孔。

巖石的孔隙度就是指其中空間的比例。但需要注意的是，孔隙度與滲透率是不同的。滲透率衡量的是水滲透物質的難易程度，它取決于與單個空隙以及連接孔隙間裂縫的大小。

當充滿水分的沉淀物或者巖石樣本被放置在適宜的干燥環境中時，大部分的水分會流失，但仍有部分水會繼續附著在堅實的表面上。要不是因為表面張力，這些水分也會立刻蒸發，僅留下完全干燥的樣本。因此，試驗樣本的含水量既包括可以流干的水，也包括不能流干的水。

這兩種水的相對含量因巖石或沉積物種類不同而改變，即便它們有相同比例的孔隙，還取決于孔隙的大小。如果孔隙很大，其中的水會形成水滴，太重足以克服吸引它的表面張力，就會流走;但如果孔隙夠小，水會像薄膜一樣，太輕無法克服表面張力，從而穩穩地附著在孔隙表面上。

以上就是小編為同學們整理的托福閱讀素材之地下水的全部內容，希望能夠幫助正在備考托福的同學們!