What is the Difference Between Zone of Aeration and Zone of Saturation

The key difference between zone of aeration and zone of saturation is that the zone of aeration lies between the earth’s surface and water table while the zone of saturation lies beneath the water table saturated with water.

Groundwater that has penetrated the earth’s surface is found in two soil layers. They are the zone of aeration and zone of saturation. The water table acts as the boundary between these two layers. As the amount of groundwater fluctuates, the water table rises and falls accordingly. The amount of water that can be held in soil is called porosity. The rate at which water flows through the soil is permeability. The aeration zone and saturation zone hold different amounts of water and absorb water at different rates.

CONTENTS

1. Overview and Key Difference
2. What is Zone of Aeration  
3. What is Zone of Saturation
4. Similarities – Zone of Aeration and Zone of Saturation
5. Zone of Aeration vs Zone of Saturation in Tabular Form
6. Summary – Zone of Aeration vs Zone of Saturation

What is the Zone of Aeration (Unsaturated Zone)?

The zone of aeration is the region that lies between the earth’s surface and the water table. The main components of zone aeration are soil and rocks. The zone of aeration is also known as the unsaturated zone. The pores in this region are usually filled with air and water. Aeration takes place when air and water come into close contact. The presence of air and water gives rise to the formation of soil moisture. Air indicates the presence of oxygen, which influences the rate of corrosion of metal objects buried under the ground.

The composition and depth of this zone differ from one area to another. That is affected by factors such as altitude, soil type and structure, rock types, climate, human activity, landscape, and vegetation. Groundwater in the zone of aeration comes from several sources, such as infiltration of surface water from rain, river water, and wastewater and the capillary effect of the water from the zone of saturation below the water table. This also affects the moisture content in the zone of aeration since the variation of air and water affects oxygen. Therefore, the rate of corrosion in metallic objects increases with oxygen content. Factors such as other materials present in the soil, the presence of various metals, impurities in water also affect the rate of corrosion in buried objects in the zone of aeration.

What is the Zone of Saturation (Phreatic Zone)?

The zone of saturation is the ground region immediately below the water table. It is also known as the phreatic zone. In this region, the pores are saturated with water but also composed of soil and rocks. The zone of saturation is less corrosive, and the moisture content in this region is at one extreme. Therefore, maximum corrosion occurs in the middle of two extremes. The zone of saturation is usually found between a few feet and thousands of feet below the earth’s surface.

Most drinking water is held in this region in the presence of rivers, springs, and wells. This water is polluted by human activities such as the use of fertilizers, pesticides, landfills, and septic tanks. The depth and size of this zone depend on seasonal change. Therefore, the level of the zone depends on whether it is a dry or wet period. Other factors such as human activities and drawing water from wells, springs, and rivers also affect the depth and size. The low corrosive atmosphere is a result of a low concentration of oxygen in the soil. But factors like dissolved ions such as chloride ions, sulphates, and other aggressive substances influence corrosion in the zone of saturation.

What are the Similarities Between Zone of Aeration and Zone of Saturation?

• Zone of aeration and zone of saturation are on the ground.
• They are composed of soil and rocks.
• They can be affected by human activity and climate.
• Zone of aeration and saturation have water.

What is the Difference Between Zone of Aeration and Zone of Saturation?

Zone of aeration consists of the upper layers of soil where air-filled pores or air pockets are present rather than water. Zone of saturation consists of pores and fractures that are saturated with water. Thus, this is the key difference between zone of aeration and zone of saturation. Moreover, the zone of aeration contains a high amount of oxygen, so they are more prone to corrode objects buried underground. Meanwhile, the zone of saturation is less corrosive than the unsaturated zone since the moisture content and oxygen in soil are less.

The below infographic presents the differences between zone of aeration and zone of saturation in tabular form for side by side comparison.

Summary – Zone of Aeration vs Zone of Saturation

Zone of aeration and zone of saturation are two layers on earth’s surface. The zone of aeration lies between the earth’s surface and the water table. Zone of saturation lies beneath the water table. The pores in the zone of aeration are usually filled with air and water. Aeration takes place when air and water come into close contact. Due to the presence of water and air, there is high moisture content. Therefore, it has the ability to corrode objects easily. In the zone of saturation, pores are saturated with water but also composed of soil and rocks. Most drinking water is held in this region. A low corrosive atmosphere is a result of the low concentration of oxygen in the soil in this zone. So, this summarizes the difference between zone of aeration and zone of saturation.

Reference:

1. “What Is the Zone of Saturation?” Corrosionpedia, 1 Sept. 2018.
2. “What Is the Zone of Aeration?” Corrosionpedia, 13 Feb. 2019.

Image Courtesy:

1. “Water table-season fluctuation” By http://en.wikipedia.org/wiki/User:Fiveless – http://en.wikipedia.org/wiki/File:WaterTable.gif and http://en.wikipedia.org/wiki/File:WTFluctuations.gif (CC BY-SA 3.0) via Commons Wikimedia