Porosity and Permeability

Summary

This educational video delves into the critical properties of groundwater, focusing on porosity and permeability. It explains how groundwater is formed through the hydrologic cycle and stored in the spaces between soil and rock particles. Demonstrations with gravel and sand illustrate porosity, while a permeability experiment with gravel, sand, and clay shows how water flows through different materials. The video emphasizes the importance of these properties for groundwater extraction and the role of materials like sand and gravel as significant groundwater sources.

Takeaways

• 💧 Groundwater makes up about 25% of the US freshwater supply, primarily used for domestic and agricultural purposes.
• 🔍 Porosity and permeability are two critical properties of groundwater that determine its distribution and accessibility.
• 🌐 The hydrologic cycle connects all Earth's water, starting with evaporation and ending with precipitation and infiltration into the ground.
• 🏞️ Groundwater is commonly stored in the tiny spaces between sand and gravel grains, not typically in caves or underground lakes.
• 📊 Porosity is defined as the proportion of void spaces in an earth material, which can be measured by water displacement in an experiment.
• 🌈 A beaker experiment showed that gravel has about 40% porosity, while sand has approximately 33% porosity.
• 🚰 Wells are used to extract groundwater, similar to how water was extracted from the gravel mixture in the experiment.
• 💧 Permeability is the ability of water to flow through earth materials, which is essential for groundwater extraction.
• 🧪 A funnel experiment demonstrated that gravel has high permeability, allowing water to flow quickly, while clay is almost impermeable.
• 📉 The permeability of materials decreases from gravel to clay, with sand and gravel being excellent groundwater sources due to their combined good porosity and permeability.
• 🚫 Materials like clay, shale, or unfractured granite have low permeability and often act as barriers to groundwater flow.

Q & A

What is groundwater and why is it important in the US?

Groundwater is water that has infiltrated the soil and rocks beneath the Earth's surface, representing about a quarter of the US freshwater supply. It is crucial as it is predominantly used for domestic purposes and agriculture.

What are the two essential properties of groundwater that affect its availability and extraction?

The two essential properties are porosity and permeability. Porosity refers to the proportion of void spaces in an earth material, while permeability is the capacity of water to flow through these materials.

How does the hydrologic cycle relate to groundwater formation?

The hydrologic cycle is linked to groundwater formation through the process where rain falling on land infiltrates the soil and rocks, becoming groundwater.

What is the typical storage location for most usable groundwater?

Most usable groundwater is stored in the tiny spaces between grains of sand and gravel, rather than in caves or underground lakes.

How was porosity demonstrated in the script using a beaker of gravel?

Porosity was demonstrated by pouring dyed water into a beaker filled with gravel. The water filled the empty pore spaces, and the amount of water absorbed indicated that about 40% of the gravel mixture was composed of air spaces.

What is the estimated porosity of the sand in the experiment, and how does it compare to the gravel?

The estimated porosity of the sand was about 33%, which is slightly less than the porosity of the gravel, indicating that the sand had fewer void spaces relative to its volume.

How does permeability differ from porosity, and why is it important for groundwater extraction?

Permeability refers to the ability of water to flow through earth materials, unlike porosity which is about the presence of void spaces. High permeability is important for groundwater extraction because it allows water to flow into wells.

What was the outcome of the permeability experiment with gravel, sand, and clay?

In the permeability experiment, water quickly passed through the gravel, pooled on top of the clay, and passed through the sand more slowly than the gravel, demonstrating that gravel has the highest permeability and clay the lowest among the tested materials.

Why are sand and gravel considered excellent sources of groundwater?

Sand and gravel are considered excellent sources of groundwater due to their combination of good porosity and high permeability, which allows for the storage and extraction of significant amounts of water.

What types of materials can act as barriers to groundwater flow, and why?

Materials like clay, fine-grained sedimentary rocks like shale, and unfractured metamorphic or igneous rocks such as granite can act as barriers to groundwater flow due to their low permeability, which restricts the movement of water through them.

What are the three learning objectives mentioned in the script, and how do they relate to understanding groundwater?

The three learning objectives are to understand what groundwater is, to examine the properties of porosity and permeability, and to understand how these properties affect the amount and accessibility of groundwater. These objectives are crucial for comprehending groundwater availability and extraction methods.

Outlines

00:00

💧 Understanding Groundwater: Porosity and Permeability

This paragraph introduces the concept of groundwater as a vital component of the US freshwater supply, primarily used for domestic and agricultural purposes. It emphasizes the importance of investigating the properties of porosity and permeability to comprehend the availability and accessibility of groundwater. The hydrologic cycle is briefly explained, highlighting the transition of water from oceans to the atmosphere and back to the land, where it infiltrates to become groundwater. The discussion then shifts to the storage of groundwater in the spaces between soil grains, with porosity defined as the proportion of void spaces in earth materials. An experiment is described to measure porosity by observing how water fills the spaces in a beaker of gravel, indicating approximately 40% porosity. A similar experiment with sand grains reveals a lower porosity of about 33%. The paragraph concludes with the introduction of permeability as a key factor in groundwater extraction, setting the stage for further exploration in the next paragraph.

05:01

🌊 Permeability's Role in Groundwater Flow

This paragraph delves into the concept of permeability, which is crucial for groundwater extraction as it determines the ability of water to flow through earth materials. The paragraph contrasts the permeability of different sediments—gravel, sand, and clay—using an experiment where water is poured into funnels filled with these materials. The results show that water flows quickly through gravel, indicating high permeability, while clay is essentially impermeable due to its tiny particles that prevent water flow. Sand demonstrates an intermediate level of permeability, with only 75% of the water passing through during the experiment. The paragraph highlights that materials like sand and gravel, with good porosity and permeability, are ideal for groundwater storage, whereas materials like clay, shale, or granite act as barriers due to their low permeability. The paragraph concludes by reflecting on the learning objectives of the lesson, inviting the audience to assess their understanding of the concepts presented.

Keywords

💡Groundwater

Groundwater refers to water that is stored underground in the soil or in the pore spaces of rocks. It is a critical component of the Earth's water cycle and an essential source for drinking water, irrigation, and industrial use. In the video, groundwater is highlighted as approximately a quarter of the US freshwater supply, emphasizing its importance in domestic and agricultural sectors. The script discusses how groundwater forms and its properties, which are crucial for understanding its availability and extraction methods.

💡Porosity

Porosity is defined as the proportion of void spaces within a material, such as soil or rock, that can contain fluids like water or air. It is a key property for understanding how much water can be stored in underground materials. The video uses the example of gravel and sand to demonstrate porosity, showing how water fills the empty spaces between the grains. Porosity is crucial for groundwater storage as it determines the volume of water that can be held in the subsurface.

💡Permeability

Permeability is the capacity of a material to allow the passage of fluids, such as water, through its pore spaces. Unlike porosity, which focuses on the presence of voids, permeability is concerned with the connectivity and size of these voids that enable fluid flow. The video explains that permeability is essential for groundwater extraction because it affects how easily water can be drawn from the ground. The experiment with gravel, sand, and clay illustrates how permeability varies among different sediment types.

💡Hydrologic Cycle

The hydrologic cycle is the continuous movement of water on, above, and below the surface of the Earth. It involves processes such as evaporation, condensation, precipitation, and infiltration. The video script mentions the hydrologic cycle to provide context for how water, including groundwater, is part of a larger, interconnected system. Understanding the hydrologic cycle is vital for grasping the origins and behavior of groundwater.

💡Infiltration

Infiltration is the process by which water from precipitation or other sources enters the ground, becoming part of the soil moisture or groundwater. The video script describes how rain falling on land can infiltrate the soil and rocks, contributing to groundwater. Infiltration is a critical process for replenishing groundwater supplies and is influenced by soil type, vegetation, and land use.

💡Unconsolidated Sediments

Unconsolidated sediments are sediments that have not been compacted or cemented together into solid rock. They consist of loose particles such as sand, gravel, or silt. The video uses the example of gravel and sand to discuss porosity, which is common in unconsolidated sediments. These materials are significant for groundwater storage because they often have high porosity and permeability, making them suitable for groundwater extraction.

💡Wells

Wells are artificially constructed shafts or holes in the ground that reach aquifers to extract groundwater. The video script mentions wells as a common method for extracting groundwater, similar to how water is extracted from the gravel mixture in the experiment. Wells are essential for providing water to communities and agricultural areas, and understanding their function is key to managing groundwater resources.

💡Aquifers

An aquifer is a subsurface layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be extracted. The video script alludes to aquifers as the source of groundwater when discussing wells. Aquifers are crucial for understanding where and how groundwater can be accessed and are the primary focus of groundwater management and protection efforts.

💡Sandstone

Sandstone is a sedimentary rock composed mainly of sand-sized mineral particles or rock fragments. The video mentions sandstone as an example of a material that can have high porosity and permeability under certain conditions, making it a potential reservoir for groundwater. Sandstone, when properly fractured or porous, can store and transmit significant amounts of groundwater.

💡Clay

Clay is a fine-grained natural rock or soil material that typically has low permeability due to its small particle size and the resulting minimal pore spaces. The video demonstrates clay's impermeability by showing how water does not flow through it in the experiment. Clay often acts as a barrier to groundwater flow, preventing or slowing down the movement of water.

Highlights

Groundwater makes up about 25% of the US freshwater supply, primarily used for domestic and agricultural purposes.

Porosity and permeability are essential properties of groundwater that affect its availability and extraction.

The hydrologic cycle connects all water on Earth, starting with evaporation from oceans and ending with precipitation.

Groundwater is often stored in the spaces between sand and gravel grains, not just in caves or underground lakes.

Porosity is defined as the proportion of void spaces in an earth material's volume.

An experiment with gravel and water demonstrates that approximately 40% of the gravel mixture is composed of air spaces.

Sand has a lower porosity than gravel, with an estimated 33% of its volume being empty spaces.

Porosity values for unconsolidated gravels and sands near Earth's surface are reasonable and significant for groundwater storage.

About 80% of shallow groundwater systems in the US are composed of sand and gravel, highlighting their importance.

Groundwater can be extracted using wells, similar to how water was extracted from the gravel mixture in the experiment.

Permeability is crucial for groundwater flow and its ability to be extracted, not just its presence.

Some igneous rocks have good porosity but low permeability due to non-connected gas bubbles.

An experiment with gravel, sand, and clay shows varying permeability, with gravel having the highest and clay the lowest.

Clay is essentially impermeable at the scale of the experiment, making it a poor groundwater source.

Sand and gravel are excellent groundwater sources due to their good porosity and permeability.

Other materials like sandstone, some limestones, or fractured igneous rocks can also serve as good groundwater reservoirs.

Materials with low permeability like clay, shale, or unfractured rocks act as barriers to groundwater flow.

The lesson aimed to increase confidence in understanding the properties and extraction of groundwater.

Transcripts

Groundwater represents about a quarter of the US freshwater supply.

Most of which is used for domestic purposes or for agriculture.

And there are two essential properties of groundwater that we really need to investigate to understand how it works

and those are porosity and permeability.

Today we are going to describe what we mean when we use the term groundwater

And we are also going to examine those two properties, porosity and permeability,

and how they effect how much groundwater is there and how easy it is to get it out of the ground.

All the water on Earth is linked together by the hydrologic cycle. In brief, this cycle begins when water evaporates from the oceans.

Water vapor rises into the atmosphere and condenses to form clouds. Clouds lose their moisture through precipitation.

Rain falling on land can run off into streams and lakes or may infiltrate through the soil and into the rocks or sediment below.

This is groundwater. We want to examine the properties of earth materials that allow this water to be present underground.

When people think of groundwater they often imagine water flowing through a cave system or maybe an underground lake

While groundwater does exist in these forms, it is not that common.

Most usable groundwater is actually stored in the tiny spaces between grains of sand and gravel.

Porosity and permeability control the distribution of this water.

We will consider each of these separately, starting with porosity.

We have filled this beaker with about 300 milliliters of relatively unsorted gravel.

Notice that there are grains of different sizes that loosely fill the container,

leaving several visible open spaces.

These spaces represent the porosity of the sediment.

Porosity is the proportion of the volume of an earth material that is composed of void spaces.

We can do a brief experiment to determine the proportion of space in the gravel occupied by porosity.

We have 200 milliliters of water in this smaller beaker.

We dyed the water blue with food coloring to make it easier to see.

When we pour the water into the beaker, it fills up the empty pore spaces from below and the water eventually rises to the top of the gravel.

Now, let’s look to see how much water we used.

Remember that we started with 200 milliliters and now we have about 80 ml left.

So we added 120 milliliters of water to a beaker containing what appeared to be approximately 300 ml of gravel.

That tells us that about 40% of the gravel mixture was composed of air spaces that we subsequently filled with water.

We can try the same experiment with smaller, better sorted sand grains.

In this case, it is more difficult to make out spaces between the smaller individual sand particles.

What proportion of the sand do you think is composed of empty spaces?

This time the sand mixture accommodates 100 ml of water, indicating that the estimated porosity of this sand is about 33%,

a little less than that of the gravel. In both cases, the water lies in the spaces between the grains.

There are a few big, visible spaces in the gravels, and lots of small, less visible spaces in the sand.

Depending how well these materials are sorted, they can have similar porosities.

These porosity values are not unreasonable for unconsolidated gravels and sands near Earth’s surface.

About 80% of shallow groundwater systems in the US are composed of these materials, sand and gravel.

In most cases we can extract this groundwater using wells, in much the same way that we could extract the water from the gravel mixture using a straw.

However, before we make this seem too simple, we have to consider the role that permeability, plays in controlling how groundwater moves through rocks and sediment.

Permeability represents the capacity of water to flow through earth materials.

It is not sufficient that groundwater is present; it must also be able to flow into our well so that we can extract it.

Many rocks have pretty good porosity but their permeability values will differ.

For example, some igneous rocks contain preserved gas bubbles that are not connected.

These rocks would have good porosity but low permeability.

We have designed a little experiment to demonstrate how permeability varies among gravel, sand and clay,

common sediments at or near Earth’s surface

We have taken a funnel and filled it with each type of sediment. We added a tiny piece of filter paper to prevent the sediment flowing through the funnel.

Then we poured a constant amount of water into each set up and watched to see how long it took to collect in the beaker below.

The faster the flow of water, the higher the permeability. Let’s see what happened.

Let's see what happened.

As you can see, water quickly passes through the gravel and almost all of the original water collects in the beaker below.

Water pools on top of the clay, and is unable to flow downward between the tiny clay particles,

making it essentially impermeable at the scale of this experiment.

Finally, water passes through the sand more slowly than the gravel and only about 75%

of the original water makes it to the beaker during the time of the demonstration.

The permeability of these three materials decreases as we move from the gravel on the left to the clay on the right.

Sand and gravel make for excellent groundwater sources because of their combination of good porosity and permeability.

Other materials, such as sandstone, some limestones or fractured igneous rocks

may also have high porosity and permeability values

and serve as good groundwater reservoirs under specific circumstances.

Material like clay or fine grained sedimentary rocks like shale,

or unfractured metamorphic or igneous rocks such as granite

have such low permeability values that they often act as barriers to groundwater flow.

We had three learning objectives for today’s lesson, how confident are you that you could complete these tasks?