Claire Chadwick; Lauren Maurizio; Robin Humphreys; and Vijay Vulava

Porosity and Permeability

In this section, we will examine porosity and permeability which determine the productivity of aquifers. Porosity and permeability are the two most important properties in any rock material that allows the material to become an aquifer.

Watch this video to understand these two concepts and how to determine these properties in a lab.

 

Exercise 3 – Properties of an Aquifer

Use the resources below to answer the following questions:

  1. According to the video, most groundwater is stored here.
  2. What are the spaces between grains in a rock that can be filled by air and water called?
  3. What is the proportion of the volume of gravel or sand that is filled by pores called?
  4. In the experiment shown in the video, what was the porosity (in %) of gravel?
  5. In the experiment shown in the video, what was the porosity (in %) of sand?
  6. The porosity of well-sorted sediment is higher than that of poorly-sorted sediment. T/F
  7. Most aquifers in the US are made of the following materials:

Now, let us perform a hypothetical experiment to determine the porosity in a sample of sandstone. In this example, we have two identical-sized samples of sandstone.

  • Sample 1 has water-saturated pore spaces and its mass = 70.5 g
  • Sample 2 has air-filled pore spaces and its mass = 60 g
  • The volume of both sandstone samples = 28 cm3
  1. Determine the mass of water (in grams) that is contained in the pore spaces (subtract the above two masses):
  2. Assuming water density of 1 g/cm3, determine the volume (in cm3) of water in the pore spaces (divide the mass of water by the density of water):
  3. Determine the porosity (in %) of the sandstone block (divide the volume of water by the volume of the sandstone block):

Now, let’s get back to the other property of aquifers:

  1. How is permeability different compared with porosity?
  2. Of the three materials examined in the permeability experiment, order the materials from the largest to smallest pore sizes.
  3. In which material did water flow the fastest?
  4. In which material was permeability the greatest?
  5. Because these materials have high porosity and permeability, they make good aquifers:
  6. Because these materials have low porosity and permeability, they make good aquitards:

Porosity and Permeability in Rocks

Groundwater’s ability to flow in rocks and sediment is dependent on the porosity and permeability of these materials.  Let’s examine these properties in different types of rocks.

Experiment 4 – Porosity and Permeability of Rocks

Let’s examine the porosity and permeability of the following geological materials.  Where needed, use the information from previous labs about different rocks.

  1. Pumice (igneous rock)
  2. Fractured granite (igneous rock)
  3. Shale (sedimentary rock)
  4. Weathered limestone (sedimentary rock)
  5. Unfractured gneiss (metamorphic rock)
  6. Of the above selection of rock formations, the following make good aquifers:
  7. Of the above selection of rock formations, the following make good aquitards:

 

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Environmental Geology Laboratory Copyright © 2021 by Dr. Vijay M. Vulava and colleagues at the Department of Geology and Environmental Geosciences, College of Charleston is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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