Is Sand Permeable? (ANSWERED)

Yes, sand is permable.

This article will explain why sand is permeable.

We’ll start by looking at what makes something permeable in general, then discuss how water moves through soil particles or sedimentary grains, before finally stating why sand can be permeable under some set of conditions.

Is Sand Permeable? (EXPLAINED)

What Makes Something Permeable?

The word permeability is generally used to describe how well fluids flow through porous materials.

For instance, you can pour water onto a sandy beach, tromp on it with your boot, and watch the water disappear.

This means that sand is permeable.

But what makes something permeable?

What are the physical characteristics required for permeability?

There are three types of pores (empty spaces) that allow fluid (such as water) movement into or out of the soil; inter-granular, intra-granular, and fissures or fractures.

The fourth type of pore exists but is less important than the first three; shear planes caused by faults in rock due to tectonic activity.

Typically, what we look for is pores larger than 50 micrometers in diameter that continue through the material.

The US Army Corps of Engineers has a great field handbook on this topic.

They break pore spaces into three categories: 

  • Intergranular Pores – Paths created by root and worm channels and animal burrows. 
  • Intragranular Pores – Material between individual sedimentary grains.
  • Fissures and Fractures – Breaks in the rock caused by tectonic activity together with voids left behind when rocks weather away. Hydrogeologists generally use the terms matrix and fracture porosity to describe inter- and intra-granular porosity respectively.

How Does Water Move Through Soil?

Water moves through the soil as a result of what is known as Darcy’s Law.

In an ideal system, think of no friction.

This law states that the velocity of fluid flow is proportional to a pressure gradient (the difference between where you are now versus where you want to go) across a porous material.

Simply put, if the soil is more pressurized on one side compared to another side, water will move towards areas of less pressure.

It also means it would be difficult for water to flow through sand grains.

The key to understanding this is whether or not there are other paths for the water to move along.

If you have something called preferential flow paths, then water moves along these paths instead of through all of the spaces between sedimentary particles (inter-granular pores).

Preferential flow paths can be fractures, animal burrows, root channels, or worm tunnels within the soil.

These tend to form highways that carry the bulk of fluid flow through the soil.

By contrast, rock with no fractures will dominate inter-granular porosity and tend towards becoming an impermeable material because the only way for fluids to move is through all of those small spaces in the soil.

It also means that large volumes of rain or irrigation water can move through a small volume of soil that has been prepared for this to take place.

Why Sand is Permeable

The key here is not having a very high water table which would prevent air from reaching the roots and plant materials, or heavy rainfall which could cause fluid flow along fractures within the rock instead of through all those tiny spaces in the sand.

In addition, if there are preferential pathways such as worm tunnels created by organisms inhabiting those spaces, then water will take the path of least resistance and move through these instead of through all that space between grains.

As long as root channels or other paths don’t line up perfectly with inter-granular pores between individual grains of sedimentary material, you should expect some flow to go through the soil.

To put this into perspective, a gallon of water spread over an acre of land weighs almost 8,000 pounds.

If that water is moving just one inch per day through the soil, it’s going to take 900 days for the water to move from the far edge of your property line to a depth of one foot.

And because a diverse group of plants and animals will be partaking in transpiration and evaporation along with surface filtration and precipitation uptake, you can expect much smaller volumes making it all the way down below 10 feet where most urban soils exist.

This is why it can take anywhere from weeks to months to years for rainwater to make it through the soil before your first irrigation event, depending on rainfall amounts and soil type.

You can increase this timing by adding compost or applying mulch to the soil surface, which reduces evaporation and increases water infiltration rates.

These properties are what define good soils for growing gardens.

Conversely, you can reduce how much time before irrigation takes place by using drip irrigation to deliver water directly to plant roots where it is needed most.

Drip systems also help maintain consistent soil moisture content, which is important for healthy plant growth and root functioning.

What about Unsaturated Flow Paths?

This is where things get a little more complicated.

For our purposes, we’ll define unsaturated flow paths as anything other than preferential flow paths that drive the fluid movement through the soil.

This includes water pathways such as matrix diffusive or interconnected macropores.

While these might not carry much volume of water and will still move at a relatively slow pace (likely measured in days and weeks), these types of flows are steady and do not rely on precipitation rates like preferential flow paths.

Instead, they depend on rainfall amounts, which means you can’t really control how fast this type of movement takes place.

You can only hope to intercept it with your rainwater collection system before it becomes runoff headed for downslope neighbors.

This might be an interesting take-home point for those who think that irrigation is a waste of water because what comes from the tap is supposedly perfectly suitable for plants as it is.

In reality, pre-irrigation or runoff represents water that has been sitting above your soil and likely moving through unsaturated flow paths such as matrix diffusive or interconnected macropores.

It’s this movement that makes all sorts of pollutants along with dirt, grime, and chemicals become mixed together in liquid form, so having access to this type of rainwater before it becomes runoff means you can choose what elements are included in the final product.

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is sand permeable