What is Groundwater? Earth’s Hidden Freshwater Supply


Today, you’re going to learn everything about groundwater.

What is groundwater?

Where can we find groundwater?

And why is groundwater important?

Let’s dive right in.

Table of Contents

Groundwater Definition
Groundwater Numbers
Types of Aquifers
Groundwater Sustainability
Groundwater Wells

Chapter 1. How Groundwater Works

Groundwater fills pore spaces

Groundwater doesn’t exist as a huge lake underground. Instead, it fills in tiny pore spaces within rocks and soil.

So groundwater is more life a sponge with rock and sand. These pore spaces with groundwater are the “saturated zone” or “aquifer”.


Groundwater wells pump water

Because groundwater is below the surface, we use wells to pull it out.

The water we draw from a well was once precipitation that fell as part of the water cycle.

Groundwater well

Aquifers can stretch for miles

Aquifers can stretch for miles serving hundreds of groundwater wells. For example, the High Plains Aquifer extends from South Dakota to Texas.

High Plains Aquifer

All water wells drawing from the same aquifer are connected. So when you pump water from your well, it affects other people miles away.

Rain naturally recharges groundwater

Precipitation seeps into the ground. Over time, it fills in the pore spaces in the ground. Naturally, big particles like leaf chunks that can be found in streams don’t exist in groundwater.

Aquifers can go dry when they are not recharged. Rainwater recharges aquifers by infiltrating the ground surface.

Aquifer Recharge

Sedimentary rocks make great aquifers

In general, sedimentary rocks are ideal for groundwater recharge. But igneous and metamorphic rocks restrict groundwater recharge because pore spaces have been squeezed away. This is due to the nature of how metamorphic and igneous rocks form.

Porosity Pore Spaces

Sedimentary rocks like sandstone and limestone have faster groundwater recharge rates. They make the best aquifers because these types of rocks have connected pore spaces. When it rains, water can travel through easily. The key to recharging groundwater is “porosity”. Porosity measures how much empty space there is between pore spaces of rocks.

Chapter 2. Groundwater in Numbers

The big picture of groundwater
  • About 0.65% of total water on Earth is groundwater.
  • Groundwater is the second largest source of freshwater.
  • We use groundwater mostly for drinking, irrigation and thermoelectric power.
Groundwater is less than 1% of total water on Earth

About 0.65% of total water on Earth is groundwater. In fact, it’s the second largest source of freshwater after glaciers.

Water Allocation

It’s a major source of freshwater

But if you compare only freshwater sources, groundwater has more than 60 times the amount than lakes and streams (combined).

global water distribution usgs

Uses of groundwater

Large cities don’t rely on groundwater because there’s not enough to supply it. Instead, municipal areas use groundwater mostly for drinking and irrigation.

  • Thermoelectric power generation uses groundwater to cool down equipment.
  • Irrigation sprays crops to help grow.
  • Groundwater provides drinking water for millions mostly in rural areas.

Groundwater Uses

Chapter 3. Types of Aquifers

Confined vs unconfined aquifers

The two types of aquifers are:

  • Confined aquifers
  • Unconfined aquifers

The main difference between the two types of aquifers is where it’s located.

Confined aquifers have an impermeable layer

Confined aquifers have an impermeable layer preventing water from seeping into the aquifer. Generally, rainwater recharges this type of aquifer at a location farther away where it’s more porous.

Artesian wells tap into confined aquifers. The confined layer creates pressure squeezing the aquifer. When you tap into an artesian aquifer, the pressure pushes the water upward. If the pressure pushes the water all the way to the surface, it creates a “flowing well”.

Unconfined Aquifer

This is kind of like holding a wet sponge in a plastic bag. If you poke a straw in the bag and squeeze, water would squirt out. The bag is like the confined layer squirting it artesian water.

Unconfined aquifers have no restrictive layers

Unconfined aquifers do not have a layer that restricts water flow. So if you dig beneath your feet, you’d directly hit the water table

After a rainfall, water seeps into the ground directly into the saturated zone. Unlike confined aquifers, these types of aquifers don’t have a layer that restricts flow into the ground.

The top of the saturated zone is called the water table. Generally, unconfined aquifers have a water table closer to the ground. If the water table is high, groundwater can flow to the surface naturally in springs, lakes and wetlands.

Unconfined Aquifer

Chapter 4. Groundwater Sustainability

If we don’t take care of groundwater, we might lose it. For example, here are some ways we can damage groundwater:

  • Pumping too much can make aquifers go dry.
  • We can contaminate if chemicals enter.
  • Climate change will impact groundwater recharge.
Aquifers can run dry

When you extract too much water and too often, then aquifers can run dry. For example, the Ogallala Aquifer stretches from Nebraska to Texas.

From 1980-1995, farmers were pumping too much water to irrigate their fields. During this period, groundwater withdrawal was much faster than it was being recharged.

But as farmers improved their irrigation, they lost less water from evaporation. Eventually, it began to recharge at a more stable rate.

Pollution can contaminate groundwater

Over time, water sinks into the ground through layers of rock that naturally filters groundwater. This makes groundwater some of the purest water available.

But human activities can add contaminants to groundwater. Once contaminated, it’s difficult or nearly impossible to return to its previous state. Here are some examples of groundwater contamination:

  • Fertilizers and pesticides can leach into groundwater from over-fertilization
  • Petroleum can contaminate groundwater from gas stations or storage tanks
  • Septic tanks and waste disposal sites can leak into groundwater
  • Naturally occurring chemicals such as hydrogen sulfide can end up in

Contaminated Aquifer

Climate change will impact groundwater

Overall, the main consensus is that climate is warming. This means that climate change is set to become one of the biggest influences for groundwater recharge.

Although the effects of climate change are still in question, one of the biggest questions is total precipitation.

Wet and dry periods will impact groundwater for human usage and natural recharge rates. And water table will go up and down in response to climate change.

Chapter 5. Importance of Groundwater

Aquifers are like a rainy day fund

Aquifers are important because they can supply water for extended periods of time. Particularly during periods of drought, we can draw in times of need.

Despite the difficulty of pumping water out from the ground, extracting too much can cause it to yield less water. Eventually, it can reach a point where it runs dry

Groundwater is a major source of steam flow

Groundwater and surface water are connected. Discharge from groundwater adds to the total flow of streams and rivers.

But groundwater pumping can reduce the flow of groundwater to streams. This streamflow depletion can impact the availability of aquatic ecosystems in surface water.

Confined vs Unconfined Aquifer

A key indicator of health

Lastly, authorities keep tabs on how much groundwater is pumped. By tracking water use, they monitor risks to groundwater.

For example, groundwater levels indicate the amount available in aquifers. Finally, water quality assesses risk of contamination in groundwater.

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