The vadose Zone and its Structure

 

When rain falls on dry land, it doesn’t become part of the groundwater immediately. First it must penetrate the unsaturated ground (the ground that is not soaked with water and still has space to absorb more water). The water table, or water level, marks the end of the unsaturated zone and the beginning of the saturated (water soaked) zone. Reaching the water table may take considerable time, and not all of the incoming water necessarily gets as far as the water table; usually, after a temporary stay in the unsaturated zone, some of the water returns to the atmosphere.

Water in the unsaturated zone typically is moving, up and down and horizontally; it is seldom at rest. The unsaturated zone has two other names: sometimes it is called the zone of aeration, because its pores usually contain some air, and sometimes the vadose zone (from the Latin vadosus, meaning shallow). From this point on, we will use the term vadose zone; its brevity makes up for its being a jargon term, and it is more accurate, too; the vadose zone is not unsaturated always and everywhere, as we will see. Nor is it a single, homogeneous zone: in most places it has three distinct layers. The topmost layer is the soil; below it is the intermediate zone, consisting of the subsoil together with any solid bedrock that happens to lie between the subsoil and the water table; and at the bottom, in contact with the water table, is the capillary fringe, a layer into which groundwater soaks upward, like water into a sponge.

The soil is the most familiar, and certainly the most complex, of these layers. The distinction between soil and subsoil is that the former contains living organisms larger than bacteria, while the latter does not. The soil is the layer occupied by the roots of plants; it is also the home of myriads of mostly unnoticed soil organisms. This abundance of living material affects the soil’s structure, its color, its chemistry, and the way water flows through it. Indeed, when water descends from soil into subsoil, it encounters entirely different conditions.

The thickness of the soil varies enormously: in some places it is no more than a thin layer of dust on bare rock; in others, it may consist of peat beds, areas made up of partly decomposed plant materials, 4 or 5 meters thick. Soil is continually subject to two opposing forces: dead organic material, accumulating ceaselessly, continually builds it up; at the same time never-ending erosion, as water flows through and over it, continually washes it away. If the soil thickness remains unchanged for long periods, as it does in many places, the reason is that these two processes balance each other.

The intermediate zone is the zone of loose mineral sediments, and sometimes bedrock, sandwiched between the soil and the capillary fringe. In places where the soil is deep or the water table shallow, there may be no intermediate zone; then all that lies between the surface and the capillary fringe is true soil.

The two upper layers of the vadose zone together form a temporary holding ground for water on its way to some other destination: some flows down to join the groundwater, some flows laterally to adjacent streams and lakes, and some is transported back into the atmosphere by the action of plants

The lowermost of the three layers of the vadose zone is the capillary fringe. It is not necessarily different in texture from the material above and below it, but it is permanently wet, saturated with water sucked from the groundwater beneath. The height to which water will rise above the water table because of this spongelike suction depends on the texture of the material holding the water The water moves up to fill the pores and interstices (spaces) of this material and the tiny channels that link them. It is constantly being pulled downward by gravity at the same time as it is being pulled upward by the suction force of surface tension; the level at which these opposing forces balance determines the thickness of the capillary fringe.

1

Water in the unsaturated zone typically is moving, up and down and horizontally; it is seldom at rest. The unsaturated zone has two other names: sometimes it is called the zone of aeration, because its pores usually contain some air, and sometimes the vadose zone (from the Latin vadosus, meaning shallow). From this point on, we will use the term vadose zone; its brevity makes up for its being a jargon term, and it is more accurate, too; the vadose zone is not unsaturated always and everywhere, as we will see. Nor is it a single, homogeneous zone: in most places it has three distinct layers. The topmost layer is the soil; below it is the intermediate zone, consisting of the subsoil together with any solid bedrock that happens to lie between the subsoil and the water table; and at the bottom, in contact with the water table, is the capillary fringe, a layer into which groundwater soaks upward, like water into a sponge.

The word “accurate” in the passage is closest in meaning to

Acorrect

Bpopular

Cunderstandable

Dacademic

 

2

When rain falls on dry land, it doesn’t become part of the groundwater immediately. First it must penetrate the unsaturated ground (the ground that is not soaked with water and still has space to absorb more water). The water table, or water level, marks the end of the unsaturated zone and the beginning of the saturated (water soaked) zone. Reaching the water table may take considerable time, and not all of the incoming water necessarily gets as far as the water table; usually, after a temporary stay in the unsaturated zone, some of the water returns to the atmosphere.

Water in the unsaturated zone typically is moving, up and down and horizontally; it is seldom at rest. The unsaturated zone has two other names: sometimes it is called the zone of aeration, because its pores usually contain some air, and sometimes the vadose zone (from the Latin vadosus, meaning shallow). From this point on, we will use the term vadose zone; its brevity makes up for its being a jargon term, and it is more accurate, too; the vadose zone is not unsaturated always and everywhere, as we will see. Nor is it a single, homogeneous zone: in most places it has three distinct layers. The topmost layer is the soil; below it is the intermediate zone, consisting of the subsoil together with any solid bedrock that happens to lie between the subsoil and the water table; and at the bottom, in contact with the water table, is the capillary fringe, a layer into which groundwater soaks upward, like water into a sponge.

According to paragraphs 1 and 2, which of the following is NOT true about the unsaturated zone?

AIt functions as a temporary water-storage area.

BWater passes through the zone at a rather slow rate.

CWithin the zone, water flows in all directions.

DIts pores are completely filled with water.

 

3

Water in the unsaturated zone typically is moving, up and down and horizontally; it is seldom at rest. The unsaturated zone has two other names: sometimes it is called the zone of aeration, because its pores usually contain some air, and sometimes the vadose zone (from the Latin vadosus, meaning shallow). From this point on, we will use the term vadose zone; its brevity makes up for its being a jargon term, and it is more accurate, too; the vadose zone is not unsaturated always and everywhere, as we will see. Nor is it a single, homogeneous zone: in most places it has three distinct layers. The topmost layer is the soil; below it is the intermediate zone, consisting of the subsoil together with any solid bedrock that happens to lie between the subsoil and the water table; and at the bottom, in contact with the water table, is the capillary fringe, a layer into which groundwater soaks upward, like water into a sponge.

The word “homogeneous” in the passage is closest in meaning to

Asubstantial

Bthick

Cunvarying

Dhard

 

4

The soil is the most familiar, and certainly the most complex, of these layers. The distinction between soil and subsoil is that the former contains living organisms larger than bacteria, while the latter does not. The soil is the layer occupied by the roots of plants; it is also the home of myriads of mostly unnoticed soil organisms. This abundance of living material affects the soil’s structure, its color, its chemistry, and the way water flows through it. Indeed, when water descends from soil into subsoil, it encounters entirely different conditions.

Paragraph 3 supports which of the following statements about subsoil?

AConditions within the subsoil are identical to those elsewhere in the vadose zone.

BThe subsoil is a relatively unstable environment.

CThe organisms living in subsoil are no larger than bacteria.

DScientists know more about the subsoil than about any other part of the vadose zone.

 

5

The thickness of the soil varies enormously: in some places it is no more than a thin layer of dust on bare rock; in others, it may consist of peat beds, areas made up of partly decomposed plant materials, 4 or 5 meters thick. Soil is continually subject to two opposing forces: dead organic material, accumulating ceaselessly, continually builds it up; at the same time never-ending erosion, as water flows through and over it, continually washes it away. If the soil thickness remains unchanged for long periods, as it does in many places, the reason is that these two processes balance each other.

Why does the author mention “peat beds”?

ATo give an example of a product of soil buildup

BTo demonstrate the diverse nature of the soil layer

CTo compare the effects of erosion on peat beds and bare rock

DTo illustrate the process through which plant matter becomes part of the soil

 

6

The soil is the most familiar, and certainly the most complex, of these layers. The distinction between soil and subsoil is that the former contains living organisms larger than bacteria, while the latter does not. The soil is the layer occupied by the roots of plants; it is also the home of myriads of mostly unnoticed soil organisms. This abundance of living material affects the soil’s structure, its color, its chemistry, and the way water flows through it. Indeed, when water descends from soil into subsoil, it encounters entirely different conditions.

The thickness of the soil varies enormously: in some places it is no more than a thin layer of dust on bare rock; in others, it may consist of peat beds, areas made up of partly decomposed plant materials, 4 or 5 meters thick. Soil is continually subject to two opposing forces: dead organic material, accumulating ceaselessly, continually builds it up; at the same time never-ending erosion, as water flows through and over it, continually washes it away. If the soil thickness remains unchanged for long periods, as it does in many places, the reason is that these two processes balance each other.

Paragraphs 3 and 4 support which of the following statements about soil?

ASoil tends to differ relatively little in thickness from one location to the next.

BIn many places, the degree to which soil accumulates is roughly equal to the degree to which it erodes.

CThe rate at which soil erodes depends partly on the amount of dead organic material in it.

DDead organic material does not accumulate in many places due to environmental conditions.

 

7

The two upper layers of the vadose zone together form a temporary holding ground for water on its way to some other destination: some flows down to join the groundwater, some flows laterally to adjacent streams and lakes, and some is transported back into the atmosphere by the action of plants

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.

AThough water is temporarily held in the two upper layers of the vadose zone, most of it is transported down to join the groundwater.

BStreams and lakes, which are located on the same level as the two upper layers of the vadose zone, temporarily hold water until it is transported back into the atmosphere.

CWater in the two upper layers of the vadose zone flows down to the groundwater or into streams and lakes, or is returned to the atmosphere by plants.

DAlthough most water in the two upper layers of the vadose zone is lost to evaporation, some of the water ultimately flows down to join the groundwater.

 

8

The lowermost of the three layers of the vadose zone is the capillary fringe. It is not necessarily different in texture from the material above and below it, but it is permanently wet, saturated with water sucked from the groundwater beneath. The height to which water will rise above the water table because of this spongelike suction depends on the texture of the material holding the water The water moves up to fill the pores and interstices (spaces) of this material and the tiny channels that link them. It is constantly being pulled downward by gravity at the same time as it is being pulled upward by the suction force of surface tension; the level at which these opposing forces balance determines the thickness of the capillary fringe.

According to paragraph 7, what distinguishes the capillary fringe from the other layers of the vadose zones?

AThe texture of its material is uneven.

BIt is always saturated with water that is sucked from below.

CTiny channels connect its pores and spaces.

DBecause of the forces of gravity and surface tension, it is a very thick layer.

 

9

Look at the four squaresthat indicate where the following sentence could be added to the passage

Beneath the soil is a layer of subsoil.

Where would the sentence best fit？Click on a square  sentence to the passage.

10

Before incoming water can become part of groundwater, it must penetrate the complex vadose zone that lies above the water table.

AThe upper two layers of the vadose zone are a temporary holding ground for water while the lowermost layer is permanently saturated with groundwater.

BThe vadose zone is made up of three distinct layers: the soil, the subsoil, and the capillary fringe.

CAn increased rate of evaporation due to environmental changes can have a negative impact on the levels of groundwater and the saturation of the capillary fringe.

DThe vadose zone holds rainwater until this water evaporates and moves into the zone of aeration.

ENot all water reaches the water table: some of the water remains in the unsaturated zone and then is usually recycled back into the atmosphere.

FWater flow typically only moves laterally in rivers and streams, or downward due to the effects of surface tension.

 

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