What are Porosity and Permeability?
Groundwater represents about a quarter of the 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 affect 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 a 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 a 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 on 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 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 fill it with each type of sediment. We added a tiny piece of filter paper to prevent the sediment from flowing through the funnel. Then we poured a constant amount of water into each setup 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 down 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 has such low permeability values that they often act as barriers to groundwater flow.
