托福阅读题目练习：Groundwater

　　5. The phrase “glacial outwash” in the passage refers to

　　○Fast rivers

　　○Glaciers

　　○The huge volumes of water created by glacial melting

　　○The particles carried in water from melting glaciers.

　　Paragraph 3: 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.

　　6. All of the following are mentioned in paragraph 3 as places that sediment-laden rivers can deposit their sediments EXCEPT

　　○A mountain valley

　　○Flat land

　　○A lake floor

　　○The seafloor

　　Paragraph 4: 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.

　　7. The word “overlie” in the passage is closest in meaning to

　　○Cover

　　○Change

　　○Separate

　　○Surround

　　Paragraph 5: 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.

　　8. The phrase “so much for” in the passage is closest in meaning to

　　○That is enough about

　　○Now let us turn to

　　○Of greater concern are

　　○This is related to

　　9. The word “plugged” in the passage is closet in meaning to

　　○Washed

　　○Dragged

　　○Filled up

　　○Soaked through

　　Paragraph 6: 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.

　　Paragraph 7: 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.

　　10. According to paragraphs 6 and 7, why is basalt unlike most crystalline forms of rock?

　　○It is unusually solid

　　○It often has high porosity.

　　○It has a low proportion of empty space.

　　○It is highly permeable.

　　11. What is the main purpose of paragraph 7?

　　○To explain why water can flow through rock

　　○To emphasize the large amount of empty space in all rock

　　○To point out that a rock cannot be both porous and permeable

　　○To distinguish between two related properties of rock

　　Paragraph 9: 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.