Read the following passage and mark the letter (A, B, C or D) on your answer sheet to indicate the correct answer to each for the questions.
Ensuring the reliability and purity of the water supply is one of the more significant challenges facing an ever growing world population. Unfortunately, only about 3% of the world’s total water supply is fresh; the rest is sea water and is unusable for most of our purposes. Furthermore, of that 3%, three fourths are largely inaccessible because it exits as frozen ice locked in the polar ice caps or as glacier high in mountains. The remaining percentage of fresh water that is available for human use exits in two forms: surface water and ground water.
Surface water is found in rivers, lakes, and man-made reservoirs fed by either rainfall or snowmelt. Surface water makes up nearly 80% of all the water utilized by human, primarily due to its accessibility. Nearly every river or lake in the world contains one or more pumping stations to divert a portion of its flow to nearby population centers. However, surface water supplies present several disadvantage. First, surface water is easily polluted. Chemical pollution from the air enters surface water in the form of rain, and ground pollution is either dumped directly into lakes and rivers o washed into them by rainwater. In addition, biological pollution, caused by the dumping of raw sewage into a water source, can lead to dangerous levels of bacteria. A vast percentage of rivers and lakes have become so chemically and biologically polluted that the use of their water in untreated form is classifies as hazarduos to human. Another problem in relying on surface water is that its supply is highly variable. Water levels in lakes and rivers can fall drastically in periods of severe drought. In places that are prone to extended dry periods, such as Australia or much of California, some rivers are even known to occasionally run dry due to a combination of drought and overuse.
The other primary source of fresh water is ground water. Although ground water is estimated to be as much as 50 times more abundant than surface water, it constitutes only 20% of all the fresh water used by humans, and much of this usage occurs in rural areas. This is a reflection of the relative difficulty in obtaining ground water. Ground water exits in underground deposits known as aquifers, layers of porous rock in the Earth. As rain water sinks into the ground it eventually reaches the aquifer where it is absorbed, much as a kitchen sponge absorbs water.
To obtain ground water, a well must be drilled down to the level of the aquifer, and then the water must be pumped to the surface. Aquifer occur at different depths in different areas, and the deeper the aquifer, the more difficult and more expensive it is to extract its water. Furthermore, if water is taken from an aquifer at a higher rate than it is recharged naturally, its level will drop, necessitating ever deeper wells. This also creates problems with ground stability. As water is drained out of an aquifer, the ground naturally tends to sink and compress, leading to greater risk of subsidence and landslides. Since aquifers are fed through a slow acting system of drained, they have much slower recharge rates than surface water resources and easily overtaxed. Therefore, ground water is generally only used when surface water is unavailable, even though ground water is far more abundant.
Like surface water, ground water can also become polluted, although not as easily. The soil that water sinks through before reaching the aquifer acts as a natural filter, leaching out some of the pollutants. Furthermore, the lack of oxygen in the aquifer generally restricts the growth of bacteria, so most ground water can be utilized safely without treatment. However, pollution can enter an aquifer when pollution sources are buried underground, such as they are in landfills. In other areas, the presence of heavy metals, nearly all of which are high toxic in the human body, may render ground water undrinkable.
According to paragraph 4, increased danger of subsidence is caused by
A.Excessive drops in the level of an aquifer
B.The holes created by large numbers of wells
C.The compression of water in an aquifer
D.Landslides beneath an aquifer
