Rain and rivers are essential to life on earth. But what about life in earth’s arid regions, where rain rarely falls and rivers are nonexistent?
God provides for life there as well. Beneath our feet flows a priceless supply of water, called groundwater. The Lord designed the chemistry of rocks and soil to provide us with a ready source of fresh water. The wells of Isaac are a good reminder of this blessing (Genesis 26:18), along with the artesian wells in Australia’s Outback and the lush, irrigated fields in North America’s High Plains.
People sometimes assume well water comes from literal caves or underwater streams, but that is not the case. It flows straight out of the soil and rocks.
How Does Water Flow Through Rocks?
Many people often think the earth is made of solid rock impervious to water. In reality, water saturates small spaces between the pebbles, sand, and other tiny particles in the soil and between the grains in rocks. It also fills cracks in rock strata.
Since water is liquid, gravity gradually pulls it along the path of least resistance. Sometimes it seeps out into low-lying streams or lakes. Or if it’s under enough pressure, it can rise up to the surface in seeps and springs.
Where does the water come from? Rain and melting snow—sometimes many miles from its destination—can percolate downward through surface sediments and into the rock layers below. Along the way down, the water is filtered and purified as the soil absorbs chemicals and bacteria break down harmful compounds.
The upper limit of the groundwater is called the water table, with a depth varying from near earth’s surface to hundreds of feet below. You can observe the actual depth by noting the level of water in an open well.
Well diggers look for soil and rocks that allow groundwater to flow very quickly. Sand, gravel, and coarse sedimentary rock are good at this, while fine-grained clay and shale (a rock made out of mud) tend to slow water’s movement. The ease of water flow through underground material is called permeability.
Another important measure of the groundwater environment is its porosity. This term refers to the amount of open space between the particles, where water can accumulate. In places with large particles and little else, more than 50 percent of the space can be empty. Highly compressed rocks, in contrast, have a porosity near zero.
What’s the difference between permeability and porosity? Consider an artificial example: a sponge. It is mainly open space and can hold lots of water, so its porosity is near 100 percent. However, the water doesn’t move out quickly from the sponge unless it is squeezed, so it has low permeability. The ideal place to find groundwater is porous, permeable soil.
Enough Water for the Flood?
In addition to the groundwater, vast amounts of additional water lie within the earth. Off the continental coastlines worldwide, aquifers of semifresh water have been detected beneath the ocean floor.1 Geologists have also found evidence of water in minerals deep within the earth’s mantle, hundreds of miles beneath the surface. Water beneath Asia, for example, is estimated to equal that of the Arctic Ocean, around 4.5 million cubic miles (19 km3) of water.2
Could this explain where water came from at the start of the worldwide Flood, and where some of it went afterward?
Actually, the answer is more complicated than that. The Bible says, “On that day all the fountains of the great deep were broken up” (Genesis 7:11). At the start of the Flood, cracks appeared in the crust. Rising molten rock from the mantle produced new ocean floor that swelled, pushing the ocean water up and over the land. As the new crust later cooled, it caused the ocean floor to sink, and the water flowed back into the ocean.
Some of the water did not end up in the ocean, however. God ensured that it would be accessible in aquifers, which we have not fully tapped, such as the vast Nubian Sandstone Aquifer below the Sahara desert.
See for Yourself . . .
It’s literally possible to get water from a rock—lots of it! Let’s explore an important feature of the ground, called porosity. This measures just how much water can be stored underground.
- A large measuring cup with markings in fluid ounces or milliliters (ml).
- A large, clear drinking glass
- Two different sizes of hard objects that can get wet. One should be marbles or beads. The other can be sand, landscape pebbles, aquarium gravel, or stones.
Fill the measuring cup to a convenient level with marbles (or beads), perhaps 1 cup (236 ml) total. Transfer the marbles to the clear drinking glass. Now fill the measuring cup with a similar amount of water.
Slowly pour the water into the marble-filled glass until the liquid covers the top of the marbles. How much water is gone? Water now fills the openings between the marbles, just as it fills the small spaces between underground particles. The loss of water from the measuring cup shows how much water the marble-filled glass can hold.
You can calculate the exact porosity of the marbles. Simply find the percentage of the water that was dumped out of the measuring cup. For example, if 0.4 cups of water were added to one cup of marbles, then the porosity is 0.4 (100%) = 40%. That means 40% of the volume is open space, now filled with water.
Now measure and calculate the porosity for the other size of objects. It may be surprising that small and large objects often give similar porosity results. You might also try mixing small and large objects. How do your porosity values compare?
For earth materials, porosity ranges from 0% to 50% or more. This shows us that underground rocks hold vast amounts of water. Thank God for supplying us with abundant underground water supplies!