韋恩食記

開始做這道題目之前，先來看看到底什麼是地下水 
http://nadm.gl.ntu.edu.tw/nadm/cht/class_detail.php?serial=116&serial_type_1=8&serial_type_2=4&serial_type_3=15

Groundwater is the word used to describe water that saturates the ground, filling all the available spaces.
地下水是指滲入到地下並將所有可用空間填滿的水。
By far the most abundant type of groundwater is meteoric water; this is the groundwater that circulates as part of the water cycle.
到現在為止，天水是最豐富的地下水類型，是地下水在水循環中的一個環節。
Ordinary meteoric water is water that has soaked into the ground from the surface, from precipitation (rain and snow) and from lakes and streams.
天水普遍會從地表、降水以及湖泊河流侵入到地下。
There it remains, sometimes for long periods, before emerging at the surface again.
在再次冒出地表之前，這些地下水有時會長時間留在地下。
At first thought it seems incredible that there can be enough space in the "solid" ground underfoot to hold all this water.
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麼些水。

The necessary space is there, however, in many forms.
然而，地下水所需的儲存空間有很多種形式。
The commonest spaces are those among the particles — sand grains and tiny pebbles — of loose, unconsolidated sand and gravel.
最常見的儲存地下水的空間是鬆散的砂子和礫石間的微粒，如沙粒和小石子。
Beds of this material, out of sight beneath the soil, are common.
由這些顆粒組成的水床非常普遍，通常位於看不見的土壤下方。
They are found wherever fast rivers carrying loads of coarse sediment once flowed.
在攜帶粗糙沉澱物的湍急河流曾流過的地方都能找到它們的踪跡。
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.
舉例來說，冰河世紀間覆蓋北美的巨大冰蓋逐漸融化，大量水從那兒流出。
The water was always laden with pebbles , gravel, and sand, known as glacial outwash, that was deposited as the flow slowed down.
水總會攜帶些石子、礫石和沙石，這些顆粒會隨著水流的減速而沉澱，這就是所謂冰河期的冰河沉積。

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.
現代也有發生同樣的事情，儘管規模相對較小，凡是有含沙河流或從山谷湧出的溪流流至相對平坦的土地時，砂石就隨著水流速度的減慢逐漸沉澱；水流通常呈扇形擴散，它們所攜帶的沙石也會沉澱為光滑的扇形斜面。
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.
當河流匯入湖泊和海洋的時候也會有沉澱，這些沉澱物最初在湖底或海底，但將來海平面下降或者陸地崛起時，它們就會分佈於內陸，有時厚達幾千米。

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.
低地區域上的任何位置可能就是曾經的河床，後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於地下水位之下，它們之間的沙子和礫石一定會被大量的地下水填滿。

So much for unconsolidated sediments.
以上說的都是鬆散的沉積物。
Consolidated (or cemented) sediments, too, contain millions of minute water-holding pores.
那些堅固的沉積物，也擁有數以萬計的細微孔洞來容納水。
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.
因為最初顆粒間的縫隙通常並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後被滲入的地下水溶解。
The result is that sandstone, for example, can be as porous as the loose sand from which it was formed.
結果這些砂岩最終變得和形成它的散沙一樣多孔。

Thus a proportion of the total volume of any sediment, loose or cemented, consists of empty space.
因此，不管沉積物是疏鬆還是堅固，它們中一定有空間。
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.
大部分結晶體岩石都非常堅硬，但也有例外，最常見的就是玄武岩，它是一種固化的火山熔岩，經常充滿了微小氣泡，從而變得十分多孔。

The proportion of empty space in a rock is known as its porosity.
孔隙率就是指岩石中空隙的比例。
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.
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，它取決於與單個空隙以及連接孔隙間裂縫的大小。

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.
當充滿水分的沉澱物或者岩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。
☐ But some will remain, clinging to all solid surfaces.
但仍有部分水會繼續附著在堅實的表面上。
☐ It is held there by the force of surface tension without which water would drain instantly from any wet surface, leaving it totally dry.
要不是因為表面張力，這些水分也會立刻從任何潮濕的表面蒸發，僅留下完全乾燥的樣本。
☐ 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. ☐
因此，試驗樣本的含水量既包括可以流乾的水，也包括不能流乾的水。

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.
這兩種水的相對含量因岩石或沉積物種類不同而改變，即便它們有相同比例的孔隙。
What happens depends on pore size.
這取決於孔隙的大小。
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.
如果孔隙很大，其中的水會以水滴的方式存在，但因太重而無法保持表面張力，並且會流走；但如果孔隙夠小，水會像薄膜一樣，太輕而無法克服表面張力，從而穩穩地附著在孔隙表面上。