Groundwater

Groundwater is the water located below the ground's surface. This reservoir of freshwater supports multiple domestic, agricultural, and industrial sectors of society. Groundwater is a renewable resource if properly managed, but it can also be mined faster than it is replenished. Groundwater is susceptible to contamination from pollutants and saltwater intrusion. As the human population continues to grow and intensely develop the Earth, management of groundwater reserves is essential to the survival of the human species.

The cycle of groundwater replenishment includes snowfall and rainfall, and movement of some surface stream and lake water into or out of underground reservoirs. Groundwater removal occurs through the roots of plants, evaporation, and artesian springs. Here, the U.S. Department of Agriculture measures water levels among skunk cabbage.

Water found in soil pore spaces, sedimentary deposits, and fractures in rock formations is called groundwater. The depth at which the soil and sediments become saturated with groundwater is called the water table. The entire volume of groundwater in a geographical region that is held in the soil and porous substrate under the soil is called the groundwater reservoir, or aquifer. The science of groundwater is called hydrogeology, or groundwater hydrology, and experts are known as hydrologists. Groundwater can be liquid and flowing through the aquifer, or it can be virtually immobile such as permafrost or water locked into bedrock formations. Groundwater is 20 percent of the world's freshwater supply, but only 0.61 percent of the Earth's total water resource. The Earth's freshwater is approximately one-third groundwater, less than 1 percent surface water, and two-thirds frozen in glaciers and ice caps. The natural cycle of groundwater replenishment includes snowfall and rainfall infiltration and runoff, and movement of some surface stream and lake water into or out of underground reservoirs. The natural cycle of groundwater removal is through the roots of plants, evaporation, and artesian springs where hydrostatic pressure and the level of the water table force it onto the surface. This water is then evaporated and returns to the Earth's surface as rain or snow to complete the hydrologic cycle. Unnatural ground-water removal occurs through wells for domestic, agricultural, and industrial uses. [Page 189]Occasionally, surface water is pumped back into the ground in an effort to replenish the groundwater.

Groundwater aquifers can be small or extensive and often cross political boundaries. Australia's Great Artesian Basin covers nearly 2 million square kilometers. The water extracted from the western portion of this aquifer may be over 1 million years old. Water travels from its eastern surface recharge sources at about 1 meter per year. Groundwater withdrawal for agricultural and domestic use from the Ogallala aquifer extending from Texas and New Mexico to Wyoming and South Dakota can exceed 230 million liters per square kilometer per year, a rate far greater than the natural infiltration in a short-grass prairie climate. This mining of water has dropped the water table hundreds of feet. As the water table drops, hydraulic pressure is removed, soil and rock particles come closer together, and land subsidence occurs. Land subsidence in San Jose, California, in the early 1900s was almost 4 meters due to overwithdrawal of water. New Orleans, Louisiana, is below sea level in part due to groundwater removal that exceeds its natural replenishment. In some regions of the world, land subsidence has exceeded 6 meters as the aquifer is drawn down. If subsidence occurs near an ocean, the hydraulic pressure of the salt water can infiltrate the freshwater aquifer. This process is called saltwater intrusion, and it permanently contaminates the freshwater aquifer.

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