Factors that Influence Groundwater Movement

Lesson Transcript
Instructor: Elizabeth Friedl

Elizabeth, a Licensed Massage Therapist, has a Master's in Zoology from North Carolina State, one in GIS from Florida State University, and a Bachelor's in Biology from Eastern Michigan University. She has taught college level Physical Science and Biology.

The movement of groundwater depends on the circumstances of the environment. Discover the factors that influence groundwater movement such as porosity, permeability, and gravity. Updated: 09/27/2021

Water Flows Underground

When you think of moving water, you likely think of babbling brooks, flowing streams, or raging rivers. Water moves on the surface quite easily, but it also moves underground as well. Groundwater, which is water below Earth's surface, doesn't flow in quite the same way as it does on the surface, but it still gets around! How water moves underground depends on several factors.

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  • 0:07 Water Flows Underground
  • 0:31 Porosity
  • 1:31 Permeability
  • 2:44 Water Moves with Gravity
  • 3:57 Lesson Summary
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Porosity

One factor that influences how groundwater moves is the porosity of a soil. This is the amount of water the soil can hold. You can think of porosity as the spaces between particles, much like the spaces in a jar of jelly beans. All of those nooks and crannies in between the pieces of candy are the pores - the larger the open spaces, the more porous it is.

The size and shape of the soil particles determine porosity in this same way. When particles are about the same size and shape, there tend to be larger open spaces since the particles don't fit together very well. Clay and sand are both very porous materials for this reason. On the other hand, sediment like limestone is less porous because the particles fit together like puzzle pieces, closing up those pores. What does this have to do with groundwater? Well, the more porous the sediment, the more water it can hold. The more water it can hold, the more water can move down into the ground!

Permeability

Porosity tells us how much water the soil can take in, but not how fast it does so. We call the rate of water infiltration into the ground permeability. To better understand this, think of a sponge and a rock. A sponge is very permeable because it absorbs water very quickly. A rock, on the other hand, is not very permeable because it really doesn't absorb water very well at all.

Soil is the same way - some sediments easily absorb water, while others do not. Remember how clay was very porous? Turns out that even though it can hold a lot of water, it's not very permeable, so it takes a long time to absorb it. Sand, on the other hand, is both porous and permeable. It can hold a lot of water and is happy to take it in. Limestone is a tricky one, though, because it is very permeable, but not very porous. So, it will absorb water very well, but its capacity is not as large as something like clay or sand, until it starts absorbing water! Water dissolves limestone easily, so as it is absorbed, it creates new holes inside, making it more porous as time goes on. In fact, most of the world's aquifers are made of limestone for this very reason.

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