Rivers, streams, lake, ponds - these are all places where we can find fresh water on the Earth's surface. But, most of the Earth's available fresh, liquid water is actually found underground! We don't see this underground water like we do surface water. However, groundwater is a very important natural resource.

Groundwater is water found under the ground. Groundwater includes all water held within the soil, moving through pore spaces in rocks and soils, and stored underground.

How did it get there? Water falls from the atmosphere as precipitation. Some precipitation falls into rivers and other surface water bodies. Some of the water runs off hard surfaces like roads and into storm drains. Gravity pulls some of the water into the ground. Plant roots can soak up this water. However, some of the water soaks downwards even further. How far the water seeps into the ground depends on the permeability of the soil and rocks that make up the ground.

A material that is permeable allows water to pass through it. Earth materials that are permeable have spaces called pores. If these pores connect, then water can fill in the pores and move into other pores. Think of an ocean wave washing upon a sandy beach. As the wave pulls back into the ocean, the water left on the beach quickly seeps down through the sand. That is because sand is very permeable and porous. Once the water reaches a material through which it cannot easily flow, it collects on top of this layer. For example, clay has very tiny pores and small particles spaced closely together. Clay is an impermeable material. Water cannot easily move through it.

Water can collect on top of impermeable materials like clay and hard bedrock like granite. The water can no longer be pulled down deeper into the Earth. Instead, the water stays within the permeable sediments on top of the impermeable material. Areas where large amount of groundwater move through permeable material are aquifers. Aquifers are importance resources for drinking water. Wells are used to bring water from aquifers to the surface. Many homes in rural places have private wells for water. Water from aquifers is used to irrigate farm crops.

Water in aquifers can be a limited resource. The amount of water entering the aquifer, or recharging it, through precipitation and the water cycle must balance the amount of water removed from it. The amount of available water decreases when water is removed from the aquifer at a faster rate than new water enters. When this happens, the top of the water, or water table, falls deeper into the Earth. Wells may go dry, or no longer bring up water, when the water table drops. The land above the aquifer may even start to collapse because the water is no longer there to support it.

Many of us depend on groundwater without even knowing that we do. Using it without overusing it will help protect this resource for many years to come.