Groundwater occurs beneath the Earth’s surface in the spaces between soil particles and between the mineral grains that form rocks. According to the U.S. Geological Survey, about 25 percent of the freshwater used in the United States comes from groundwater sources. The other 75 percent comes from surface water. Groundwater is especially important in the arid West where surface water is not as ample as in other parts of the country. Groundwater uses range from providing a freshwater drinking supply to irrigating crops.
The majority of groundwater comes from freshwater aquifers. Usable quantities of water are obtained from these water saturated soils and rocks through wells or springs. Alluvial and glacial sediments composed of silt, sand, and gravel, as well as porous sedimentary bedrock formations of sandstone and limestone, make up most of the world’s aquifers.
Although groundwater can be found in most places on Earth, much of it may be unsuitable for drinking due to poor water quality or low rates of production. The spatial occurrence, production characteristics and groundwater quality of an aquifer is controlled by the presence of a water source, or recharge, and the stratigraphic and structural settings of the local geologic formations. In most cases, groundwater is recharged from precipitation that has infiltrated through the ground surface and into the aquifers.
Aquifers can be confined above and/or below by layers of clay, shale or unfractured granite. These materials typically allow groundwater to flow so slowly that they effectively act as barriers to flow, or aquitards. Although aquitards may store large amounts of groundwater, their low flow does not allow water to be produced from a well at rates that are sufficient for most uses.
Alluvial and shallow bedrock aquifers produce most of Wyoming’s groundwater. Groundwater is recharged naturally by rain and snowmelt. This precipitation plays a key role in replenishing groundwater resources under the Earth’s surface. This hydrologic process involves water moving downward from surface water to groundwater.
Recharge for Wyoming’s aquifers originates largely as rainfall or snowpack in the state’s mountain ranges. During snowmelt in late spring and early summer, the water released infiltrates the ground surface to recharge underlying aquifers, or it turns into runoff that contributes to stream and river flows. Wyoming’s semi-arid basins, characterized by low precipitation, high evaporation, and reduced soil permeability, generally provide much less recharge to its aquifers.
Conceptual cross section of groundwater features in basin margin
to basin center (adapted from WWC Engineering et al., 2007)