托福阅读常考话题及练习题推荐四：地下水（3）

1   托福阅读常考话题：地下水练习题

Paragraph 5: Because the surface of the water table is not flat but instead rises and falls with topography, groundwater is affected by gravity in the same fashion as surface water. Groundwater flows downhill to topographic lows. If the water table intersect the land surface, groundwater will flow out onto the surface at springs, weather to be collected there or to subsequently flow farther along a drainage. Groundwater commonly collects in stream drainages but may remain entirely beneath the surface of dry stream-beds in arid regions. In particularly wet years, short stretches of an otherwise dry stream-bed may have flowing water because the water table rises to intersect the land surface.

11. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

A. Groundwater only flows out of the ground if the water table intersects the land surface.

B. If the land surface and the water table intersect, groundwater can flow underground.

C. Groundwater may be drained if springs occur where the water table intersects the land surface.

D. Where the water table meets the land surface, groundwater flows out through surface springs.

12. Paragraph 5 implies which of the following about the level of the water

A. It may rise or fall from year to year, depending on annual rainfall.

B. It does not vary in arid regions.

C. It rarely intersects the land surface of most regions.

D. It is unrelated to the rate at which groundwater flows.

Paragraph 4: The water table is the underground boundary below which all the cracks and pores are filled with water. In some cases, the water table reaches Earth’s surface, where it is expressed as rivers, lakes and marshes. ■Typically, though, the water table may be tens or hundreds of meters below the surface. ■The water table is not flat but usually follows the contours of the topography. ■Above the water table is the vadose zone, through which rainwater percolates. ■Water in the vadose zone drains down to the water table, leaving behind a thin coating of water on mineral grains. The vadose zone supplies plant roots near the surface with water.

13. Look at the four squares [■] that indicate where the following sentence could be added to the passage.

This is a consequence of the slow rate of movement of the groundwater, which often prevents the water table from attaining a flat (horizontal) level.

Where would the sentence best fit?

14. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

Most of the world’s potable water is stored as groundwater in the pores and fractures of underground rock, much of it at depths of less than 1,000 meters.

Answer Choices

A. Sedimentary rock may make poor aquifers because of tightly compacted sediment, which reduces porosity and permeability.

B. Porosity is a measure of the empty space within rock while permeability measures the degree to which water can flow freely through rock.

C. In arid regions, the water tables remain at a constant level far below the surface, preventing stream-beds from filling up even during wet years.

D. Groundwater reservoirs are characterized by the porosity and permeability of the rock in which they lie, and these factors vary according to the type of rock.

E. The vadose zone is typically dry because water does not stay in it, but instead percolates down to aquifers below or drains out through springs and streams.

F. Although the water table usually follows the contours of the land surface, its level may vary from year to year and may intersect to the surface in places.