How does porosity and permeability affect aquifers

It has so much air space in the tiny bubbles within the rock that it will float on water [1]. Different types of rock have varying degrees of porosity, meaning that each will have a different permeability.

For example, if particles of gravel are all the same size see Figure 2 , they will stack on top of each other neatly with some gaps. To use a bucket analogy, stacking the gravel pieces shown in Figure 3 would be like filling a bucket with baseballs. Since the baseballs stack neatly, there are spaces in between the balls, allowing water to flow through them fairly easily. If the gravel pieces are all different sizes see Figure 3 then the smaller grains will fill in the spaces between the larger grains, therefore reducing the overall amount of air space in the mix.

Using a similar analogy this would be like filling the same bucket with both baseballs and golf balls. It would be still be possible to add water to the bucket but the addition of the golf balls would mean there is less room to hold water in the bucket.

If grains in rock themselves are porous see Figure 4 giving the rock an appearance of being filled with bubbles, or vesicles, there will be a large amount of air space in the rock. For porosity to affect permeability these bubbles need to be interconnected as shown in Figure 4. Otherwise, water would never be able to reach the air space and the rock would not be very permeable. A wiffle ball, seen in Figure 5, has holes around the outside and is hollow in the middle.

If a bucket was filled with wiffle balls instead of baseballs and then filled with water, the bucket would hold more water than the bucket filled with baseballs because the water would flow through the wiffle balls.

Pumice is a type of rock that has these properties i. Porosity refers to how much air space is in the soil or rock. Permeability refers to the ease with which water can travel through the material. In many cases, rocks or soil with high porosity will also have high permeability. The water will pass quickly through the air spaces in the rock or soil. For the water to travel through the air spaces, those air spaces must be connected.

When rock or soil that has high porosity has air spaces that are not connected, the rock or soil has low permeability even though it has high porosity. A bucket filled with ping pong balls that is then filled with water would not hold very much water. Even though the ping pong balls have air spaces inside of them, the water cannot travel through the ping pong ball. All of the holes in that sponge are water-filled. By squeezing that sponge we force the water out, similarly, by pumping an aquifer we force the water out of pore spaces.

There are lots of terms in hydrogeology, most of which are very simple, but essential. Here are a few of the big ones and their meanings. Porosity is an intrinsic property of every material. It refers to the amount of empty space within a given material. In a soil or rock the porosity empty space exists between the grains of minerals. However, in a material like a gravel, sand and clay mixture the porosity is much less as the smaller grains fill the spaces. The amount of water a material can hold is directly related to the porosity since water will try and fill the empty spaces in a material.

We measure porosity by the percentage of empty space that exists within a particular porous media. Figure 2. Porosity in two different media.

The image on the left is analagous to gravel whereas on the right smaller particles are filling some of the pores and displacing water. Therefore, the water content of the material on the right is less.

Source: Wikipedia. Figure 3. Video showing how connected pores have high permeability and can transport water easily. Note that some pores are isolated and cannot transport water trapped within them. Permeability is another intrinsic property of all materials and is closely related to porosity. Permeability refers to how connected pore spaces are to one another. If the material has high permeability than pore spaces are connected to one another allowing water to flow from one to another, however, if there is low permeability then the pore spaces are isolated and water is trapped within them.

For example, in a gravel all of the pores well connected one another allowing water to flow through it, however, in a clay most of the pore spaces are blocked, meaning water cannot flow through it easily. An aquifer is a term for a type of soil or rock that can hold and transfer water that is completely saturated with water. That means that all it is simply a layer of soil or rock that has a reasonably high porosity and permeability that allows it to contain water and transfer it from pore to pore relatively quickly and all of the pore spaces are filled with water.

Good examples of aquifers are glacial till or sandy soils which have both high porosity and high permeability. Aquifers allows us to recover groundwater by pumping quickly and easily. However, overpumping can easily reduce the amount of water in an aquifer and cause it to dry up. Aquifers are replenished when surface water infiltrates through the ground and refills the pore spaces in the aquifer.

The amount and quality of our groundwater supply is affected by the following:. The porosity of the soil is the percent of the soil that is air space. Porosity ultimately affects the amount of water a particular rock type can hold and depends on a couple of different factors.

The ability of the ground water to pass through the pore spaces in the rock is described as the rock's permeability. Permeable layers of rock that store and transport water are called aquifers. While porosity and permeability usually go hand-in-hand, though some porous rocks are not permeable and some impermeable rocks are porous.

Permeability is affected mostly by the size and arrangement of the grains in the soil. As rain and runoff enter the soil the water begins to fill the pore spaces in the ground.