What is the water table?

The water table is the upper surface of the saturated zone, where all pore spaces are filled with water. It rises during wet seasons and falls during dry spells, and it generally follows the shape of the land surface.

Can a rock be porous but not permeable?

Yes. Pumice is highly porous but can have low permeability if pores are isolated. Clay has high porosity (up to 50%) but extremely low permeability because its tiny pores are poorly connected.

Groundwater, Aquifers & Karst

Most of Earth's liquid freshwater is hidden underground. Learn how it moves, where it is stored, and what happens when it dissolves limestone.

Intro GeologyUni Year 1

⏱️ About 16 min

Groundwater, Aquifers & Karst — illustration

Illustrative image (AI-generated).

If you gathered every lake, river, and stream on Earth into one sphere, it would be about the size of Lake Baikal. Do the same with groundwater and the sphere would be roughly 250 times larger. Most of the freshwater we rely on — for drinking, irrigation, and industry — is invisible, moving slowly through underground pores and fractures. Understanding groundwater is understanding where our water actually lives.

💡

The big idea: Groundwater fills the pore spaces and fractures in rock and sediment below the water table. An aquifer is a body of rock or sediment that stores and transmits usable amounts of water. Two properties control groundwater flow: porosity (how much open space exists) and permeability (how well those spaces are connected). Where carbonate rocks are present, slightly acidic groundwater dissolves limestone and creates spectacular karst landscapes — caves, sinkholes, and disappearing streams.

🎯 By the end, you'll be able to

Define porosity and permeability and explain why a rock can be porous but not permeable
Contrast confined and unconfined aquifers and sketch their relationship to the water table
Explain how groundwater moves and why it travels much more slowly than surface water
Describe how karst landscapes form and identify their characteristic landforms

📎 Helpful to know first

The hidden reservoir

When rain and snowmelt soak into the ground, they fill the pore spaces between grains of soil and sediment, and the fractures in bedrock. This subsurface water is groundwater, and it makes up about 30% of all freshwater on Earth (and roughly 70% of liquid freshwater excluding ice) — far more than all surface lakes and rivers combined.

The top of the saturated zone — where all pores are filled with water — is called the water table. Above the water table lies the unsaturated zone (or vadose zone), where pores contain both air and water. The water table rises and falls with wet and dry seasons, and it generally follows the shape of the land surface, but more smoothly.

⚠️ Groundwater does not flow in underground rivers

A persistent image from cartoons and fiction is an underground river coursing through a cave. In reality, groundwater moves slowly through pore spaces and fractures in rock and sediment. Open underground rivers are uncommon and limited to large cave passages in karst terrain. Most groundwater trickles through sand grains or fractures at speeds of centimetres to metres per day — not kilometres per hour.

Porosity and permeability: the two keys

Not all rocks make good groundwater reservoirs. Two properties matter:

Porosity (n) is the percentage of a rock's volume that is open space (pores or fractures). Well-sorted sand can have porosity of 30–40%; unfractured granite has porosity under 1%.
Permeability is the ability of a material to transmit fluid. It depends on whether the pores are connected. A rock can be porous but not permeable if its pores are isolated — clay has high porosity but very low permeability because its microscopic pores are poorly connected.

An ideal aquifer has both high porosity (to store water) and high permeability (to let water flow through).

\[ n = \frac{V_{\text{pores}}}{V_{\text{total}}} \times 100\% \]

Porosity n is the volume of pore space divided by total volume, expressed as a percentage. It tells you how much water a rock can hold, but not how easily the water can move.

📝 Worked example: A sandstone sample has a total volume of 200 cm³ and a pore volume of 60 cm³. What is its porosity?

n = (V_pores / V_total) × 100%
n = (60 / 200) × 100%
n = 0.30 × 100% = 30%

✓ The porosity is 30% — a typical value for well-sorted sandstone.

✏️ Practice: A soil sample has a total volume of 500 cm³ and contains 175 cm³ of pore space. Calculate the porosity as a percentage.

Solution

n = (V_pores / V_total) × 100%
n = (175 / 500) × 100%
n = 0.35 × 100% = 35%.

Aquifers: confined and unconfined

An aquifer is a body of saturated rock or sediment that can yield usable water to a well.

Unconfined aquifer: The water table forms its upper boundary. Water enters directly from above through recharge areas. Wells drilled into an unconfined aquifer reach the water table, and the water level in the well equals the water table elevation.
Confined aquifer: The aquifer is sandwiched between impermeable layers called aquicludes or aquitards. Water is trapped under pressure. When a well penetrates a confined aquifer, water may rise above the top of the aquifer — sometimes even to the surface, creating an artesian well.

Unconfined aquifer (water table at top) vs confined aquifer (sandwiched between impermeable layers). Artesian wells tap confined aquifers where pressure drives water upward.

Karst: when groundwater sculpts limestone

Where carbonate rocks (limestone, dolostone) are present, slightly acidic groundwater dissolves the rock and creates karst landscapes. Rainwater absorbs CO₂ from the atmosphere and soil, forming weak carbonic acid:

\[ \text{CaCO}_3 + \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{Ca}^{2+} + 2\,\text{HCO}_3^- \]

Carbonic acid dissolves calcite (CaCO₃), carrying away calcium and bicarbonate ions. Over millennia, this reaction carves caves, sinkholes, and underground drainage networks.

Karst landforms

Karst terrain is distinctive:

Sinkholes: Collapsed or dissolved depressions at the surface, formed where the roof of an underground cavity gives way.
Caves: Large underground passages dissolved by groundwater. When the water table drops, caves drain and speleothems (stalactites, stalagmites) form from dripping, mineral-rich water.
Disappearing streams: Surface streams that vanish into sinkholes and continue underground.

Karst features: sinkholes, caves, and disappearing streams form where acidic groundwater dissolves limestone.

Check your understanding

1. Which property tells you how easily water can flow through a rock?

Permeability measures how well pores are connected, which controls flow. Porosity measures storage capacity, not flow ease.

2. Why does clay make a poor aquifer despite having high porosity?

Clay can hold water (high porosity) but its microscopic, poorly connected pores prevent water from moving through it (low permeability).

3. What is the main agent that creates karst landscapes?

Rainwater absorbs CO₂ and forms weak carbonic acid. This mildly acidic groundwater dissolves limestone and dolostone over long periods, creating caves, sinkholes, and disappearing streams.

✅ Key takeaways

Groundwater makes up most of Earth's liquid freshwater and moves slowly through pores and fractures, not underground rivers.
Porosity is the percentage of open space; permeability is the ability to transmit fluid. A good aquifer needs both.
Unconfined aquifers have a water table at their top; confined aquifers are sandwiched between impermeable layers and may be under pressure.
Karst landscapes form where acidic groundwater dissolves carbonate rocks, creating caves, sinkholes, and disappearing streams.
Carbonic acid (H₂O + CO₂) dissolves calcite: CaCO₃ + H₂O + CO₂ → Ca²⁺ + 2HCO₃⁻.

➡️ Water shapes Earth's surface in all its forms — liquid, underground, and frozen. Next we turn to the most powerful ice sculptor on the planet: glaciers, which carve valleys, deposit ridges, and reshape continents during ice ages.

Want to test yourself on this? Try the Science practice tests →

🎓 Go deeper: university courses & trusted references

Handpicked external material for this module — for when you want the full university treatment of surface processes.

University courses & departments

Trusted references

External sites are listed for reference only. This course is independent and has no affiliation with, or endorsement from, the institutions named.