Most of the world’s potable water----freshwatersuitablefor drinking----is accounted for bygroundwater, which is stored in the pores and fractures in rocks. There is more than 50 times as much freshwater storedundergroundthan in all the freshwater rivers and lakes at the surface. Nearly 50 percent of allgroundwateris stored in the upper 1,000 meters of Earth. At greater depthswithinEarth, thepressureof the overlying rock causes pores and cracks to close, reducing thespacethatporewater canoccupy, and almost complete closure occurs at a depth of about 10 kilometers. The greatest waterstorage,therefore, lies near the surface.

Aquifers,Porosityand Permeability.Groundwateris stored in avarietyof rock types. Agroundwaterreservoirfrom which water can be extracted is called anaquifer. We can effectively think of anaquiferas adepositof water. Extraction of water depends on two properties of the aquifer:porosityand permeability. Betweensedimentgrains are spaces that can be filled with water. Thisporespaceis known asporosityand is expressed as a percentage of the total rockvolume. Porosityis important for water-storagecapacity, but for water to flowthroughrocks, theporespaces must be connected. Theabilityof water, or other fluids, to flowthroughthe interconnectedporespaces in rocks is termed permeability. In the intergranular spaces of rocks, however,fluidmust flow around and between grains in a tortuouspath; this windingpathcauses aresistanceto flow. Therateat which the flowing water overcomes thisresistanceis related to the permeability of rock.

Sedimentsorting and compaction influence permeability andporosity. The more poorly sorted or the more tightly compacted asedimentis ,theloweritsporosityand permeability. Sedimentary rocks----the mostcommonrock type near the surface----are also the mostcommonreservoirs for water because theycontainthe mostspacethat can be filled with water. Sandstones generally make good aquifers, while finer-grained mudstones are typicallyimpermeable. Impermeablerocks are referred to as aquicludes. Igneousandmetamorphicrocks are morecompact, commonlycrystalline, andrarelycontainspaces between grains. However,evenigneousandmetamorphicrocks may act asgroundwaterreservoirs ifextensivefracturing occurs in such rocks and if thefracturesystem is interconnected.

The water table is theundergroundboundarybelow 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 thetopography. Above the water table is the vadosezone,throughwhich rainwater percolates. Water in the vadosezonedrains down to the water table, leaving behind a thincoatingof water onmineralgrains. The vadosezonesupplies plant roots near the surface with water.

Because the surface of the water table is not flat butinsteadrises and falls withtopography,groundwateris affected bygravityin the samefashionas surface water. Groundwaterflows downhill to topographic lows. If the water table intersect the land surface,groundwaterwill flow out onto the surface at springs, whether to be collected there or tosubsequentlyflow fartheralongadrainage. Groundwatercommonly collects instreamdrainages but mayremainentirelybeneaththe surface of dry stream-beds inaridregions. In particularly wet years, short stretches of anotherwisedry stream-bed may have flowing water because the water table rises to intersect the land surface.

Questions:
question_stem: In paragraph 1, why does the author mention “the pressure of the overlying rock”?
choices: tag: A
text: To show how water can be forced deep under Earth’s surface

tag: B
text: To show why groundwater is more plentiful than surface freshwater

tag: C
text: To correct a commonly made error about the location of groundwater

tag: D
text: To explain why most groundwater lies near Earth’s surface
answer_key: D
explanation: D 在高亮部分的描写之后,本段的最后一句出现了,therefore，这里也是在说 the pressure of the overlying rock 导致的结果,也就是文章提到它的目的.其结果是： The greatest water storage, therefore, lies near the surface.从这句话中可以读到，大部分的水储都在比较表面的地方。所以其目的就是要解释为什么水都在表面的地方。因此选 D。

question_stem: According to paragraph 1, groundwater differs from the water in rivers and lakes in terms of its
choices: tag: A
text: portability

tag: B
text: usefulness

tag: C
text: abundance

tag: D
text: cost
answer_key: C
explanation: C 用 rivers and lakes 定位到原文第二句话，也就是将地下水和江河湖泊水进行比较的那句话： There is more than 50 times as much freshwater stored underground than in all the freshwater rivers and lakes at the surface.原句说 more than 50 times，50倍的量的差距。证明是地下水的水量大。这里 A 是饮用性,原文并没有说江水不能；B 是有用，同理；D 是代价，成本，这个就更没提到了。所以答案为 C，丰富。

question_stem: The word “extracted”in the passage is closest in meaning to
choices: tag: A
text: used

tag: B
text: poured

tag: C
text: removed

tag: D
text: kept out
answer_key: C
explanation: C extract: 汲取,取出。对应 C 选项的 remove（移动，去除）。对应原文 A ground water reservoir from which water can be extracted is called an aquifer.从原文后面一句话可以推测,因为后面说储水土层可以看作是对一笔水存款,所以前面应该是说可以取出。另外这个词的词缀 ex-本身就有出的意思。那么 A 是使用,B 是泼出,C 是移动,D 是隔离在外。因此选 C。

question_stem: The word “termed”in the passage is closest in meaning to
choices: tag: A
text: considered

tag: B
text: called

tag: C
text: limited to

tag: D
text: caused by
answer_key: B
explanation: B term: verb.把…叫做。对应 B 选项的 call。对应原文 The ability of water, or other fluids, to flow through the interconnected pore spaces in rocks is termed permeability. term 作名词是“术语”的意思, 这个很常用。而这里用了其动词被动形式, 那么应该是...被术语为,也就是被命名为。那么 A 是认为,B 是叫做,C 是限制于,D 是由于。所以应该选 B。也可以结合该段的语境来推断，整段是介绍两种概念，一个是 porosity，在前文下了一个定义，用了 is known as。这里的这个对应原句很明显也是下定义，is termed 应该和 is known as 是差不多的。这样便可推出正确答案是 B。

question_stem: According to paragraph 2, what does porosity determine?
choices: tag: A
text: The rate at which the aquifer’s water overcomes resistance to flow

tag: B
text: The amount of water that the aquifer can hold

tag: C
text: The likelihood that fractures and joints will occur in the aquifer

tag: D
text: The depth underground at which the aquifer lies
answer_key: B
explanation: B 用 porosity 定位到该段的第四句话，而 this 也预示着，前一句话也相关： Between sediment grains are spaces that can be filled with water. This pore space is known as porosity and is expressed a percentage of the total rock volume. Porosity is important for water-storage capacity...从这两句话中可以明确读到：porosity 指的是可以储水的空隙。用岩石体积的 percentage 表示，对于储水容量很重要。那么就和 B 选项的内容呼应。A 决定于 permeability of rack, 对应本段最后一句；C 和 porosity 没有直接关系；D 原文就没提到。

question_stem: According to paragraph 2, what is the relationship between permeability and porosity?
choices: tag: A
text: The more pores a rock has, the higher its porosity but the lower its permeability.

tag: B
text: Rocks with many internal spaces that are not connected with each other will have high porosity but low permeability.

tag: C
text: If water flows through a rock easily, it has high permeability but low porosity.

tag: D
text: Rocks that have high permeability have high porosity and vice versa.
answer_key: B
explanation: B  porosity 多孔性；permeability 渗透性。文中提到多孔性意味着岩石中能储蓄的水量；而这些孔联结在一起，则水可以在岩石内流动，这就是渗透性。因此选 B：岩石内有许多空间但这些空间不连接在一起，那么这块岩石是多孔性好但渗透性差的。

question_stem: The word “compacted”in the passage is closest in meaning to
choices: tag: A
text: hard

tag: B
text: compressed

tag: C
text: heavy

tag: D
text: deeply buried
answer_key: B
explanation: compact 是压缩、紧密的意思。通过后面的 and 的并列内容可以推出其词义，因为 and 后面说 rarely contain space，那就是说中间的空间很小。所以选择 B 压缩的。A 是坚硬，C 是重的，D 是深埋的。

question_stem: According to paragraph 3, when can igneous rock serve as an aquifer?
choices: tag: A
text: When it has many connected fractures

tag: B
text: When it lies next to metamorphic rock

tag: C
text: When it lies relatively near the surface

tag: D
text: When it is crystalline
answer_key: A
explanation: 对应本段文章最后一句"even igneous rocks may act as groundwater..."后半句给出的就是答案，那么它说的是要有 fractures 并且要 interconnected。所以选 A。

question_stem: The word “coating”in the passage is closest in meaning to
choices: tag: A
text: stream

tag: B
text: barrier

tag: C
text: amount

tag: D
text: layer
answer_key: D
explanation: D  coating: 涂层，包衣。那么 A 是溪流，B 是障碍物，C 是数量，D 是层。所以 D 符合。对应原文

question_stem: Paragraph 4 implies which of the following about the roots of plants?
choices: tag: A
text: They prevent water from reaching the vadose zone.

tag: B
text: They mark the boundary between the vadose zone and the water table

tag: C
text: They do not typically get their water from the water table.

tag: D
text: They help keep the water table from dropping farther.
answer_key: C
explanation: C  根据 plant roots 定位到本段最后一句： The vadose zone supplies plant roots near the surface with water.说 vadose zone 给接近表面的根部提供了水。这也就是说对于这些植物的根部来说，vadose zone 也可以是一个水源，water table 不是他们唯一的水源选项。这就对应了 C 选项。A,B,D 都没提到。

question_stem: Which of the sentences below best expresses the essential information in the highlighted sentence in the passage paragraph 5? Incorrect choices change the meaning in important ways or leave out essential information.
choices: tag: A
text: Groundwater only flows out of the ground if the water table intersects the land surface.

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

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

tag: D
text: Where the water table meets the land surface, groundwater flows out through surface springs.
answer_key: D
explanation: D 看主要逻辑，either 后面的补充内容为次要内容，可以先忽略不看。主干表达的意思很简单，即：当地下水位和陆地表面交汇时，地下水会流到地表的泉水中。A 选项错在用了 only,这个限制性条件在原句没有体现，所以 A 和原句有出入;B 选项的 flow underground 和原句的 surface 矛盾；C 选项的 may be drained 在原句没有这样的描述，所以错。 D 包含了原文所有的主干部分，正确。

question_stem: Paragraph 5 implies which of the following about the level of the water
choices: tag: A
text: It may rise or fall from year to year, depending on annual rainfall.

tag: B
text: It does not vary in arid regions.

tag: C
text: It rarely intersects the land surface of most regions.

tag: D
text: It is unrelated to the rate at which groundwater flows.
answer_key: A
explanation: A 用 level of the water table 去寻找先关描述，定位到这一段倒数第一二句话。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.根据这段最后两句对于 wet year 的描写可以得出根据降雨量不同, 地下水位是有变化的，在比较多雨的年份，地下水位可能会上涨，和地表交汇。所以 A 正确。B 不正确，原文明显写到在 arid regions 降雨量多时干燥的溪床会有水；C 也不对，整段都有大篇幅在写和 land surface 的 intersects；D 原文没提到。