An additional five million acre-feet enters underground storage areas on the average each year. Groundwater comes from rain or other precipitation which moves by gravity through the layers of earth. The water fills porous layers of rock, sand, sandstone, or limestone. A rock or clay layer below stops the water from moving deeper into the soil profile. If enough water is contained in an area to produce a usable supply, the storage area is called an aquifer.
Aquifers store fresh water beneath over half of all land surface in Texas.
As an aquifer fills, the water level of the aquifer rises towards the earth's surface. In areas with clay or rock above as well as below the water-saturated layer, groundwater is held under pressure. Pumping too much water from an aquifer causes the water level to decline or the pressure to drop. Groundwater offers some tremendous advantages over surface water. Naturally filtered as it makes its way through layers of soil, groundwater is generally free of bacteria and sediments.