Earth’s land surface is less than monolithically solid. The joints, faults, and weathered rocks described elsewhere in the book allow rainwater to penetrate the crust, sometimes to considerable depths and become groundwater. Stream channels that intersect subsurface layers containing groundwater can add water to or take water from the groundwater supply.
Far from being an isolated source of stored water, the shallower groundwater supplies are intimately related to the rest of the hydrologic cycle in that they are recharged by precipitation. It should be noted that most soil moisture is distinct from groundwater (although there are times and places at which the top of the groundwater supply can be within the soil and so be one in the same). In that groundwater is out of sight and moves much slower than streams, its nature is not appreciated by most people. Yet, its importance looms large in that there is two-and-a-half times more groundwater than in all the streams and lakes on the planet.
Not all groundwater is potable. The contact of underground water with rock leads to the solution of salts into the water. In some cases, the saltiness is much higher than that found in the ocean. In other places with moderately “hard” water containing positively charged ions (cations) of calcium and magnesium, the salts can be dropped out of solution by using water softener, thus making it usable for human purposes.
Groundwater can be found underneath most of Earth’s surfaces but not in consistently developable amounts by either depth or geographical region. Most groundwater is within a kilometer of the surface, but some water has been found as deep as 10 km. With depth, however, the pore spaces in rocks and sediments become considerably smaller because of the pressure of overlying materials, and groundwater is essentially trapped in place. The deeper groundwater is connate or “fossil water” that was trapped as the rock layer was laid down. This deep groundwater is usually brought to the surface as an unavoidable consequence of drilling for oil and natural gas. It is almost invariably salty because of the immense amounts of time the water has had to dissolve surrounding materials. Sometimes these brines can be economically tapped and important materials such as iodine extracted.
Groundwater is sometimes conceived as a gigantic underground lake or stream, but this is not close to the truth. Near Earth’s surface, there are four zones differentiated with respect to groundwater: the zone of aeration, the zone of saturation, the zone of confined water, and the waterless zone. The depths, amount of water, and flow characteristics have immense variations according to the type and structure of the underground materials.
The zone of aeration is the topmost zone and abuts the surface. It is composed of solid materials with pore spaces occupied by air and water. As the sky precipitates, water infiltrates into the zone of aeration, filling the pore spaces for a while. The pore spaces drain of water via gravity and via evaporation directly to the atmosphere and via transpiration from plants whose roots absorbed water from the zone of aeration. The nature of the zone of aeration changes dramatically after each precipitation event. It has vertical depths that can extend to hundreds of meters or much less than a meter. This is not a zone in which one would normally situate the bottom of a groundwater withdrawal well in that the amount of water present is so highly variable.
The zone of saturation is beneath the zone of aeration, and it is this layer into which wells are drilled to tap the groundwater supply. The zone of saturation has gained its water gravitationally from above and all of its spaces are completely filled by water, which is properly known as groundwater. The top of the saturated zone is the water table. The depth to the water table is highly variable around the planet, and humans have gravitated to the areas of large water supply close to the surface. The depth to the water table varies seasonally and topographically. Summer seasons usually result in the drop of the water table with winter recharge. The depth of the water table generally follows the slope of the land above and is closest to the surface in stream valleys. Humans have made impressive changes in the water table. Around wells, there is usually a cone of depression but as many wells tap the zone of saturation the water table is drawn inexorably lower. In Ft. Worth, Texas, the water table has dropped 125 m in the last century. Such drops require increasing energy use to draw the water to the surface and make use of the water less economically efficient.
In some places, layers of impermeable sediments of rocks surround parts of the saturated zone and so impede water from leaving that zone. If the amount of groundwater so confined is large, this confined zone is an aquifer. Aquifers are composed of materials like sandstone that are conducive to the movement of water. Wells tapping an aquifer are kept supplied by groundwater moving toward the wells. Major aquifers around the world include the Ogallala of the U.S. Great Plains, the Great Artesian Basin of Australia, the Guarani aquifer of South America, and the Nubian aquifer of Africa. In some cases, these aquifers do not have substantial recharge, being the relicts of wetter times at the end of the Pleistocene ice age thousands of years ago.
Beneath the saturated zone is the fourth zone, known as the waterless zone. Usually, this zone begins a few kilometers under the surface and exists because pressure from the overlying materials precludes the existence of pores in which water can be stored.
The flow of groundwater is considerably slower than in surface water. Common rates are between 15 and 125 m per day with some places having rates of centimeters per year. Rather than straight flow, groundwater flow is confined to pathways using the tiny openings existing in the surrounding materials. The flow is energy by differential pressures. Unlike surface water, groundwater is able to move up or down depending on the direction of the pressure gradient. Sometimes, there is enough pressure involved that a break in a confining layer (by natural circumstances or by a well) allows the groundwater to escape to the surface where it is known as artesian water.
It has been estimated the world’s groundwater use is 600–700 cubic kilometers per year and represents the greatest tonnage of any extracted material. Use is quite variable by circumstance. Just over a fifth of the United States water consumption is supplied by groundwater withdrawal. In countries like Saudi Arabia, the ratio exceeds four-fifths. In that many aquifers extend beneath national boundaries, the scene is set for potential international tensions as groundwater becomes scarcer.
Oases are interesting examples of situations where the water table is close to or at the desert surface (see Deserts). Oases have played important roles in human geography in diverse countries such as Egypt, Saudi Arabia, Niger, Peru, Libya, and the United States. In the midst of aridity, these contrasting locales of plentiful water have provided people with shade, water and forage for animals, and irrigation for crops since prehistoric times.
Groundwater depletion is a major problem in places dependent on groundwater for water supplies. It is clear that many groundwater withdrawals represent “overdraft” situations in which use is many times that of replenishment. The world groundwater overdraft is estimated at 200 billion cubic meters per year such that the current rate of use cannot be considered sustainable over the long term.
In places like Long Island and Florida, some places proximal to the oceans have had withdrawal of freshwater allowing lenses of salty groundwater from underneath the ocean bottom to supplant the freshwater and make the well water too salty for use. Land subsidence is an issue in areas with substantial withdrawals of groundwater. The groundwater within fine-grained sedimentary rocks and loose sediments forms part of the crustal mass maintaining the level of the surface. Withdrawal of the mass represented by the water allows the compaction of the water-bearing strata by the mass of the materials overhead. Subsidence has been noticeable and damaging in places such as the Central Valley of California, Mexico City, Venice, Tokyo, and Bangkok. Tens of meters of subsidence have been documented. Building foundations can fail, underground pipes can burst, and there can be flooding of low-lying areas. The so-called Leaning Tower of Pisa is perhaps the world’s most famous example of subsidence caused by the great weight of the tower differentially compacting groundwater out from only a meter or two beneath the surface.
Groundwater pollution is a problem that becomes increasingly salient as more human activities take place over shallow groundwater supplies. There are myriad ways in which groundwater can be contaminated to the point where it is nonusable. These include infiltration from septic tank leaching fills, landfills, animal feedlots, and accidental spills of materials. Additionally, injections of toxic wastes down wells, the spread of agricultural chemicals on crops, and the leakage of underground storage tanks can compromise the groundwater supply.
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