The movement of groundwater underneath us is influenced by several factors.
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.
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!
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.
Water Moves with Gravity
There's one more important factor in groundwater movement, and that's gravity. Gravity doesn't just pull objects down to the surface of Earth; it also pulls some things down through the ground.
All water flows downhill because gravity causes it to do so, so both surface and groundwater flow from high to low. However, with groundwater, it moves depending on the elevation of the water table.