Groundwater Basics

Water Cycle

What is Groundwater? Groundwater is simply water that exists below the earth's surface. Groundwater is often thought of as an underground river or lake. Only in caves or within lava flows does groundwater occur this way. Instead, groundwater is usually held in porous soil or rock materials, much the same way water is held in a sponge.

What happens when Rain falls ? When rain falls to the ground, the water does not stop moving. Some of it flows along the surface in streams or lakes, some of it is used by plants, some evaporates and returns to the atmosphere, and some sinks into the ground.

Groundwater can be found almost everywhere. The water table may be deep or shallow and may rise or fall depending on many factors. Heavy rains or melting snow may cause the water table to rise or an extended period of dry weather may cause the water table to fall.

What is an aquifer ? Groundwater is stored in, and moves slowly through, layers of soil, sand and rocks called aquifers. The speed at which groundwater flows depends on the size of the spaces in the soil or rock and how well the spaces are connected.

Different size particles Different size particles

Ground water flowing from a road cut

The ground beneath our feet is not just rock, or at least, not just one kind of rock. Many different types of rock exist, and they have very different properties. Often, different types of rocks exist in horizontal layers beneath the land surface. Some layers are more porous than others, and at a certain depth below ground the pores and fractures in these rocks can be totally filled with water (an aquifer).porousan aquifer

When precipitation falls and seeps into the ground, it moves downward until it hits a rock layer, which is so dense and unfractured that it won't allow water to easily move through it. When this happens it is easier for the water to start moving horizontally across the more porous rock layer. Sometimes when building a road, the layers are cut into and revealed, and water can be seen dripping out through the exposed layers.