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AGS / Water / Ground Water / Main

Water that fills the void space between the particles of soil or rock and other more open spaces in bedrock is referred as ground water. The boundary between the saturated zone below (where all the voids are filled with water) and the unsaturated zone above (where not all the voids are filled with water) is called the water table. There are two factors that make ground water available for human use - porosity and permeability. Porosity is the measure of the volume of void space (pore space), their size, and number, and permeability is the measure of how easily the water moves from pore space to pore space through a body such as rock or soil or how well they are interconnected. Porosity will determine how much water is present and permeability how fast that water can pass through material. When both factors are large enough, the medium, like soil or rock, can produce enough water usable by humans to a well placed within it, such material is then called an aquifer.

The highest yielding aquifers are typically in unconsolidated sand and or gravel layers. Aquifers composed of highly porous sandstone or highly fractured rock can also make good aquifers. Contrary to popular belief most ground water is not found in underground lakes or streams. Such large open cavities or caves only occur under very restricted conditions based on the type of bedrock - an example is limestone when it becomes eroded forming what is called karst terrain. Some sedimentary rocks such as clay and shale, metamorphic rocks and massive igneous rocks can hold only small amounts of water and do not allow water to pass through quickly. These types of formations are called aquicludes or aquitards and usually will yield little to no water to a well or spring.

How Groundwater occurs in rocks and sediments
How groundwater occurs in rocks and sediments.

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Howard, J.M., Colton, G.W., and Prior, W.L., eds., 1997, Mineral, fossil-fuel, and water resources of Arkansas: Arkansas Geological Commission Bulletin 24

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