12.2: What is Groundwater?
What Is Groundwater?
What do you picture when you hear the word “groundwater”? Flowing rivers underground? Hidden lakes in dark caves? The vast majority of groundwater is actually held in porous rock or sediment, and is slowly seeping from one small pore to another, much like how water is absorbed into a sponge (Figure 12.3).
Figure 12.3: Groundwater is simply the subsurface water that fully saturates pores or cracks in soils and rocks. Between the land surface and the aquifer water is a zone that hydrologists call the unsaturated zone. (Public Domain; USGS Links to an external site.)
Have you ever been to the beach and dug a hole, only to have it fill with water from the base? If so, you had reached the water table Links to an external site.: the boundary between the unsaturated and saturated zones. Rocks and soil at and just beneath the land’s surface are part of the unsaturated zone Links to an external site.; pore spaces here are filled with air. Moving down from the surface, we will sooner or later reach the water table, where rocks and soil pore spaces are filled with water in the saturated zone Links to an external site.. The saturated zone beneath the water table is called an aquifer Links to an external site., which are potentially huge storehouses of water.
The water table is said to mimic topography, in that it generally lies near the surface of the ground (often tens of feet below the surface, though this can vary greatly with location) and rises with hills and sinks with valleys, often discharging into streams. The water table receives additional inputs when rainfall infiltrates into the ground; this is called recharge Links to an external site.. The position of the water table is dynamic: during droughts it will lower, and during wet times it will rise.
Two important properties of groundwater that influence its availability and movement are porosity and permeability. Porosity Links to an external site. refers to the open or void space within the rock (Figure 12.4). It is expressed as a percentage of the volume of open space compared to the total rock volume. Porosity will vary with rock type. Many rocks with tight interlocking crystals (such as igneous and metamorphic rocks) will have low porosity since they lack open space. Sedimentary rocks composed of well-sorted sediment tend to have high porosity because of the abundant spaces between the grains that make them up. Envision a room filled from floor to ceiling with basketballs (similar to a rock composed completely of sand grains). Now add water to the room. The room will be able to hold a good deal of water, since the basketballs do not pack tightly due to their shape. That would be an example of high porosity.
Figure 12.4: Porosity and permeability of different geologic materials. (CC-BY 4.0, Emily Haddad, own work)
If the open spaces are connected, water will be able to flow through them; this is why rocks that are permeable Links to an external site. make good aquifers (Figure 12.4). Sedimentary rocks, such as sandstone and limestone, are good aquifers. Rocks that are impermeable make confining layers (aquicludes or aquitards) and prevent the flow of water. Examples of confining layers would be sedimentary rocks like shale (composed of tiny clay and silt grains) or unfractured igneous or metamorphic rock. In a confined aquifer Links to an external site., the aquifer is topped by a confining layer (Figure 12.5). In an unconfined aquifer Links to an external site., the top of the aquifer is the water table.
Figure 12.5: Groundwater flows underground at different rates. The direction and speed of groundwater movement is determined by the various characteristics of aquifers and confining layers of subsurface rocks in the ground. Water moving below ground depends on the permeability and on the porosity of the subsurface rock. If the rock has characteristics that allow water to move relatively freely through it, then groundwater can move significant distances in several days. But groundwater can also sink into deep aquifers where it takes thousands of years to move back into the environment, or even go into deep groundwater storage, where it might stay for much longer periods. (Public Domain, USGS Links to an external site.)
Just like surface runoff, groundwater generally flows from areas of higher elevation to lower elevation in the shallow subsurface. Approximately 20% of the water used in the United States is groundwater, and this water has the potential to become contaminated, mostly from sewage, landfills, industry, and agriculture. The movement of groundwater helps spread the pollutants, making containment and mitigation a challenge.