Groundwater

Water that occurs below the ground and is brought to the land surface by wells or springs is referred to as groundwater. Groundwater is a significant part of the hydrologic cycle, containing 21 percent of Earth's freshwater. Groundwater comprises 97 percent of fresh water not tied up as ice and snow in polar ice sheets, glaciers , and snowfields. This greatly exceeds the amount of water in streams, rivers, and lakes.

Groundwater is critically important in supplying water to streams and wetlands, and in providing water for irrigation , manufacturing, and other uses. In the United States, 80 to 90 percent of available fresh water comes from groundwater. Dependence on this resource will continue to grow, particularly in areas where surface water is limited or has already been fully allocated for existing uses, including instream requirements (and particularly ecosystem needs).

Groundwater once was thought to be of unlimited quantity and naturally protected by the soils above it. It is, however, significantly vulnerable to overuse and the improper use and disposal of chemicals at the land surface. The proper use and protection of this resource requires an understanding of how the groundwater system works. In particular, water managers need to know what groundwater is, where it comes from, how it occurs in the subsurface, and how it moves below the ground.

Groundwater Origin and Occurrence

Groundwater is water that occurs below the Earth's surface at depths where all the pore (open) spaces in the soil, sediment , or rock are completely filled with water (see Figure 1). All groundwater, whether from a shallow well or a deep well , originates and is replenished (recharged) by precipitation.

Groundwater is part of the hydrologic cycle, originating when part of the precipitation that falls on the Earth's surface sinks (infiltrates) through the soil and percolates (seeps) downward to become groundwater. Groundwater will eventually come back to the surface, discharging to streams, springs, lakes, or the oceans, to complete the hydrologic cycle.

Groundwater Zones.

A well that is drilled will first pass through a zone called the unsaturated (vadose) zone where the openings in the soil, sediment, or rock are primarily filled with air (Figure 1). Water exists here only in transit downward. The thickness of this zone depends on such factors as climate, elevation, season of the year, and area-wide groundwater withdrawals through pumping. In the rainy season of humid areas, the unsaturated zone may be a fairly thin layer, extending from the land surface to only a few meters (10 feet or so) below the surface. But in drier months of the year, the unsaturated zone may extend deeper as recharge to the aquifer declines and withdrawals increase. In arid regions, the unsaturated zone may be a thick layer, extending from the land surface to 300 meters (1,000 feet) or more below it.

Further drilling will reach a zone called the saturated zone where all of the openings are filled with water, and where the water is known as groundwater. If the saturated zone is permeable enough to supply a well with water under normal hydraulic gradients, this saturated zone is called an aquifer. Importantly, an aquifer is not an underground river, lake, or pool. Rather it consists of geologic materials whose open spaces (pore spaces) are filled with water that moves down a pressure gradient, and which can be tapped productively by wells. The top of the saturated zone is called either the water table (if the aquifer is unconfined) or the potentiometric surface (if the aquifer is confined): see Figure 2.

To visualize the zones, imagine a bucket filled with gravel. Ample pore space exists between the individual pieces of gravel. If water is poured on top of the gravel, the water will percolate down through the pore spaces and begin to fill these spaces from the bottom up. The water in pore spaces at the bottom of the bucket represents groundwater; that is, all the pore spaces are filled with water. If holes were punched in the bottom of the bucket, water would flow out. Using this analogy, the bucket of gravel is like an aquifer: water is stored within in it and will move through it toward a discharge point—in this case, the hole in the bucket.

Permeability is determined by the size of pores and the degree to which they are interconnected, and hence, the ease by which water can flow through the material. Highly permeable aquifers, such as those comprised primarily of coarse sand and gravel, can supply more water than less permeable aquifers, such as those comprised of silts or clays. In this example, the pores in sand and gravel are larger than those in silt and clay, so water moves through sand and gravel more quickly. In some aquifers, especially in sedimentary bedrock, water occurs in fractures (cracks) instead of pore spaces in sediments. The yield from a fractured rock aquifer can vary from less than 1 liter per minute, or about 0.3 gallons per minute (if the well encounters few fractures) to large quantities of groundwater—for example, more than 300 liters per minute, or about 100 gallons per minute (if the fractures are numerous and large).

Aquifer and Well Types.

Aquifers are divided into two types: unconfined and confined. An unconfined aquifer is often shallow, and the vadose zone above it primarily contains permeable material. The top of the aquifer is the water table. The water table moves up and down on a seasonal basis. It is highest during the wet season owing to higher recharge and lower pumping rates (e.g., no irrigation), and lowest during the dry season because of limited recharge and higher use (e.g., a high rate of irrigation).

Confined aquifers may be shallow or deep, and are characterized by being separated from the surface by low-permeability strata (e.g., geologic layers) that confines the groundwater below it. In a confined aquifer, groundwater is generally under pressure. This water pressure may vary seasonally, similar to the water table in an unconfined aquifer.

Because groundwater in a confined aquifer is under pressure, it will rise in a well bore above the level of the aquifer penetrated by that well. One way to visualize this is to squeeze a milk or juice pouch that is punctured at the top by a straw. If the straw fits firmly into the squeezed pouch, the liquid will rise up into the straw, above its level inside the pouch.

Artesian and flowing artesian wells are typical of wells drilled into confined aquifers. An artesian well is one in which the groundwater rises above the level of the penetrated aquifer. The water in an artesian well will rise to an elevation at which the pressure of the water in the aquifer is matched by the pressure reflected by the elevation of the water in the well; this level is known as the hydrostatic level. If groundwater reaches all the way to the surface under its own pressure, the well is called a flowing artesian well (see Figure 2).

Groundwater Movement

The water table (or the potentiometric surface of a confined aquifer) is not a flat surface: rather, there are high areas and low areas just like the hills and valleys found on land. Just as surface water tends to flow downhill, groundwater tends to move downgradient from water-table areas (or potentiometric regions) of higher elevation to water-table areas (or potentiometric regions) of lower elevation.

Normally, but not exclusively, the higher water-table areas of uncon-fined aquifers coincide with higher elevation at the land surface, and the lower water-table areas coincide with low areas. As a result, groundwater in unconfined aquifers tends to flow towards, and discharge to, streams, lakes, and wetlands, because these waterbodies often occur in low points of the watershed. Even groundwater from confined aquifers tends to discharge to larger area-wide rivers.

Two other common discharge areas for groundwater are springs and wells. A spring is an area where groundwater has access to the land surface. In some cases, precipitation infiltrating downward from the ground surface encounters a relatively impermeable rock or sediment layer as it moves down toward the underlying aquifer. The groundwater, which cannot pass through the low-permeability layer, moves along the top of the layer until the layer is exposed at the ground surface and the water can emerge as a spring. In this typical "gravity spring," the most common form of spring, gravity is the driving force for water movement. Such springs commonly occur at the side of a hill, or at an outcrop such as a bluff or canyon wall (see the small figure below). In other cases, fractures allow groundwater to move from the aquifer to the surface. Groundwater from a spring can issue onto the land surface, or directly into a stream, lake, or ocean.

A well also provides a connection between groundwater and the land surface. In general, a pump is used to draw the groundwater up to the land surface where it can then be used.

Flow Rate.

When referring to how fast surface water moves, hydrologists generally talk in terms of either meters or feet per second. Groundwater moves much more slowly than water in streams, often at rates of only a few centimeters (inches) per day. Groundwater velocity is controlled by the permeability of the aquifer and steepness of the water table (or potentiometric surface). The more permeable the aquifer and the steeper the the slope of the water table or potentiometric surface (i.e., the pressure gradient), the faster groundwater moves. In highly permeable gravels or in fractures, groundwater may move 10 meters (33 feet) per day or more.

Recharge.

Recharge, or replenishment, of an unconfined aquifer occurs at the ground surface directly above the aquifer. In contrast, recharge to a confined aquifer may occur many miles away, typically at a higher elevation where the aquifer is no longer confined; that is, where the overlying materials are permeable and allow percolating rainfall to reach the confined aquifer (See "Recharge to Confined Aquifer" in Figure 2). Once recharged, the groundwater flows downgradient to where the aquifer is confined.

Pumping and Overpumping

Wells supply water by being drilled to a depth below the water table of an unconfined aquifer or into a confined aquifer. When a well is pumping, it lowers the water table (or the potentiometric surface) around it. This causes groundwater to move towards the well, supplying the water that is being pumped out of the well. As long as the pump intake is below the water level in the well, water can be pumped.

Well Interference.

Well interference occurs when the water table (or potentiometric level) in one well is significantly lowered as a result of the pumping of a nearby well. Sometimes the interference can be so great that the water table (or potentiometric level) in the first well is lowered below the pump intakes or even below the bottom of the well itself.

How can wells interfere with one another? When pumping begins, the water table (or potentiometric surface) around the well begins to drop as the aquifer supplies water to the well. This drop in water levels is called drawdown. The area of drawdown is shaped like a three-dimensional, funnel-shaped cone, centered on the well. Pumping rate and aquifer characteristic determine the depth of the drawdown cone and the areal extent (horizontal) of the drawdown.

The following illustration represents the water table as the top of the lower cube; the aquifer as the lower cube itself; and the well as a pipe extending from the land surface (top of the upper cube) down to the aquifer, below the water table. The drawdown cone, also known as a cone of depression, is indicated.

How deep and how wide the drawdown cone is depends on aquifer characteristics and how much water is being pumped. Interference occurs when the drawdown cone from one well reaches another well and lowers the water level in that well. Consider the following two examples of well interference in a water-table aquifer.

First, suppose a high-capacity irrigation well (Well 1 on page 155) is drilled near two shallow private drinking-water wells (Well 2 and Well 3). High pumping volumes from the irrigation well may lower the water table from its original level to a new (lower) level, which is at a depth below the private wells. In this example, Well 2 would be unable to pump water while the irrigation well (Well 1) is pumping. Well 3 barely penetrates the lowered water table, and would likely experience intermittent pumping failures if the irrigation well were also pumping. In other words, the irrigation well pumpage interferes with the water production from both Well 2 and Well 3.

As another example, suppose a city wants to increase its water supply. A new well (Well B below) is drilled very close to an existing well (Well A) that is highly productive. When both wells are pumping, they mutually interfere with one another, and the production of both wells is lowered. Because of this mutual interference, the combined production of the two wells may be only slightly greater than the production of each well by itself.

In the diagram below, the drawdown associated with Well A by itself is shown as a heavy dashed line. Well B pumping by itself would produce a similar drawdown around itself. Note that the amount of water available to Well A (from the bottom of the cone to the bottom of the well) has been substantially reduced by the placement of Well B too close to Well A. Neither Well A or Well B can now produce as much as if they were pumping by themselves.

In most states, laws prohibit significant well interference. A well cannot be operated in such a manner to significantly reduce the production from previously existing wells. The potential of well interference can be calculated if hydrologists know the permeability, thickness and storage characteristics of the aquifer and at what rate the well(s) in question will be pumping.

Groundwater Mining.

Excessive pumping of water from an aquifer may result in an areawide lowering of the water table. This will eventually occur anywhere more water is pumped than is recharged by infiltrating precipitation. Overdrafting an aquifer can result in changes in groundwater quality, a reduction in groundwater availability (and hence the loss of water supply to current and future wells), and perhaps even a permanent loss of the aquifer's capacity to store water. Many states use the water right process to manage groundwater quantity and to ensure that overdrafting does not occur.

Subsidence.

There is a limit to how much groundwater can be pumped out of an aquifer without causing depletion of the resource. If more groundwater is pumped out than is naturally recharged by precipitation, the amount of water stored in the aquifer will decline. In some areas, pumping has resulted in subsidence (sinking) of the land surface. Similar conditions may arise from the pumping of petroleum.

Groundwater occupies volume in an aquifer by filling pore spaces between the mineral grains. Because water is essentially incompressible, that water helps support the weight of the overlying rock and soil. When the water is pumped out, the pore spaces may collapse under the load and the volume of the rock and soil decreases. In many areas, that pore space is forever lost; that is, water cannot reenter the aquifer.

Significant subsidence as the result of excessive pumping has been recorded in areas such as:

• the Houston–Galveston area of Texas (1 to 2 meters, or 3.3–6.6 feet);
• New Orleans (2 meters, or 7 feet);
• Venice, Italy (3 meters, or 10 feet);
• Mexico City (more than 7 meters, or 26 feet); and
• the San Joaquin Valley of California (8 meters, or 26 feet).

Unlike the subsidence caused by cave-ins that result from the collapse of underground mines, sinkholes caused by dissolution of underground carbonate rocks, or human-made sinkholes caused by broken water mains, subsidence as a result of aquifer overdrafting occurs slowly and over a large area. Residents in the region are unlikely to even notice it.

see also Aquifer Characteristics; Groundwater Supplies, Exploration For; Hydrologic Cycle; Land Use and Water Quality; Modeling Groundwater Flow and Transport; Springs; Stream Hydrology; Stream, Hyporheic Zone of a; Wells and Well Drilling.

Dennis O. Nelson

Bibliography

Heath, R. C. Basic Ground-Water Hydrology. U.S. Geological Survey Water-Supply Paper 2220 (1983).

Todd, David Keith. Groundwater Hydrology, 2nd ed. New York: John Wiley & Sons, 1980.

Internet Resources

Drinking Water Program. Groundwater. Portland, OR: Oregon Department of Human Services, 1995. Available online at <http://www.ohd.hr.state.or.us/dwp/gwater.htm>.

GROUNDWATER AND LAND USE

Knowing where groundwater comes from provides a better perspective of how groundwater can become contaminated from land uses. Because groundwater originates as precipitation sinking down from the land surface, downward-infiltrating water must pass through whatever is at or below the surface.

Unconfined aquifers are more vulnerable to contamination than confined aquifers. Hence, it would be unwise to locate a facility that uses hazardous chemicals, for example, above a shallow, un-confined aquifer. Understanding groundwater movement allows a determination of whether a specific land-use activity (e.g., a landfill or septic system) overlies part of the aquifer carrying groundwater that moves towards and supplies a well.

AGATES, GEODES AND PETRIFIED WOOD

Whether in a nature shop, at a rock and mineral show, or in a museum, many people have marveled at the beauty of natural mineral formations such as agates, geodes, and petrified wood. Agates, having exotic names such as blue lace, plume, sagenite, pom pom, crazy lace, fire, snakeskin, sweetwater, and thunder egg, all are variations of a multicolored and banded form of silica (SiO₂), called chalcedony, a microcrystalline form of quartz. Geodes also consist of chalcedony, but commonly are cut in the shape of a bowl to reveal well-formed quartz crystals projecting into the center of the "bowl." In petrified wood, again the dominant material is chalcedony, however in many cases, the original structure of the wood is preserved.

All agates, geodes, and petrified wood share a common origin. They are formed as the result of groundwater. In agates and geodes, the silica is deposited in open spaces in existing rock. Typically, these openings represent "frozen" gas bubbles in lava flows or ash deposits; the rock solidified before the gas bubble was able to escape. The surrounding volcanic deposits, particularly the ash, readily yield silica to circulating groundwater. When the silica-rich groundwater enters larger open spaces, the silica is precipitated as chalcedony, often in layer form. The different colors represent variations in concentrations of elements such as iron, chromium, and titanium. Agates represent cases where the entire open space is filled, whereas geodes are the case where the open space remained filled with silica-rich groundwater and conditions were favorable for slow growth of quartz crystals into the solution.

Naturally occurring agates and geodes are rounded masses or nodules that reflect the geometry of the hole in which they formed. They often are found loose on the ground because the hardness of silica is generally greater than that of the surrounding rock. With time, the agate and geodes are released as the softer surrounding rock is weathered away. The beauty of the agate and geode are revealed only by cutting them open to reveal the internal structure.

Petrified wood is also the result of silica being deposited by groundwater. Though not a completely understood process, the groundwater is able to remove the original organic matter of the wood and deposit silica in a manner that replicates the original structure of the wood. Examples of petrified wood vary from isolated limbs to entire forests such as in the National Petrified Forest Monument in Arizona.

Agates, petrified wood, and geodes may be enhanced by sawing with a diamond-blade, then polishing the cut surfaces.

groundwater Groundwater comprises water that exists beneath the land surface, held within openings, or pores, of soils and geological formations. According to most classical definitions, this term refers exclusively to water occurring at or beneath a surface, known as the water-table (see water-table). Below the water-table, in a region known as the saturated zone, pores are completely filled with water (Fig. 1). Other definitions of groundwater include water held in the unsaturated zone, also known as the soil moisture or vadose zone, which is located above the water-table. Within the unsaturated zone, pores may contain both water and air. This broader definition makes sense when one recognizes that fluids above and below the water-table are intimately connected, being part of a continuous flow field that moves water through the subsurface portion of the hydrological cycle. Although the following discussion will focus primarily on water below the water-table, consideration of water in the unsaturated zone, as well as water in other components of the hydrological cycle, is essential to understanding this important resource.

Groundwater as a resource

Quantitative evaluation of groundwater resources requires information on volumes of water stored in the subsurface, on rates at which groundwater can flow to wells or other extraction points, and on rates at which the resource can be replenished by transfer from other components of the hydrological cycle. The potential importance of groundwater as a resource is indicated by estimates of the world water balance. According to most estimates, saline oceans and seas account for 94 per cent to more than 97 per cent of the world's water, while another 2 per cent is held in glaciers and polar ice caps. Although groundwater constitutes only a few per cent by volume of the total water at or near the Earth's surface, estimated volumes of groundwater are much greater than the combined volumes in lakes and reservoirs, stream channels, and freshwater wetlands. Thus, it becomes apparent that groundwater makes up the majority of the world's utilizable freshwater resources, 95 per cent or more by volume.

Storage and flow properties of geological materials

The amount of water stored in the saturated zone depends on the volume of pore space. This volume is quantified as the porosity, defined as the volume of pores divided by the total volume of the material (see permeability and porosity). The porosity of typical geological materials can vary from less than 0.01 in relatively unfractured igneous or metamorphic rocks to more than 0.5 in silt or clay soils and sediments. High porosities are also found in some limestones and other carbonate rocks in which pores created by depositional processes or fracturing have been enlarged by dissolution of carbonate minerals. The wide range of porosity in geological materials translates into a wide range of water storage potential in the subsurface. Because porosity is determined to a large extent by the type of material, geological maps provide a useful starting point for many evaluations of groundwater resources. The rate at which water can move through the pore space of geological materials is obviously related to porosity, since the greater the volume of pores, the greater the open area through which water can flow. However, the ease with which water can move through a porous material, quantified by the property known as permeability, depends not only on the porosity, but also on the size of pores (see permeability and porosity). Frictional resistance between water and solids where they are in contact at pore walls converts the mechanical energy of flowing water to heat, thereby limiting the rate of flow. For a given porosity, there is more contact area, and hence greater frictional resistance, for materials with smaller pores. In soils and sediments, the sizes of pores vary with sediment size and sorting. The effect of pore size on permeability dominates that of porosity. For example, a clay soil (particle size /< 0.04 mm) with a porosity of 0.5 typically has a permeability many orders of magnitude smaller than a sand (particle size 0.06–2 mm) with a porosity of only 0.3. Permeability also depends on the interconnectedness of the pore network. Relatively isolated pores, such as those created by gas bubbles in cooling lava, can make up a significant portion of the total porosity but will contribute little to flow. In groundwater resource evaluation it is often useful to distinguish between total porosity and the effective porosity, which includes only the well-connected pore space.

Geological formations with sufficient porosity and permeability to supply water to wells are classified as aquifers (see aquifer). Formations through which very little flow can occur under normal conditions have been called aquicludes, a term suggesting the exclusion of water. Because geological materials are rarely (if ever) completely impermeable, many workers prefer to refer to low-permeability geological materials as aquitards, a term that implies retardation of flow rather than exclusion of water. In practice, the distinction between aquifers and aquitards is somewhat subjective, depending on the rates of flow required to meet water supply needs and on the range of permeability and porosities of the local formations.

Rates of replenishment and use

Rates of long-term groundwater use that can be sustained in a given region depend not only on the presence and extent of aquifers, but also on the rates of replenishment, or recharge, through infiltration of rainwater, snow-melt, or other surface water (see recharge and the hydrological cycle). In most lakes and streams, water has a residence time of the order of weeks to tens of years. Groundwater, in contrast, may remain in aquifers for hundreds to thousands of years before discharging into the ocean, lakes, streams, or wetlands, or returning to the atmosphere through evaporation or uptake and transpiration by plants. In many arid parts of the world, existing aquifers contain water that infiltrated during periods of wetter climate thousands of years ago. Current rates of recharge in these areas may be very limited. Extraction of groundwater in arid regions is often referred to as mining because such use is similar to exploitation of a non-renewable mineral resource.

Even in moderately humid areas, where current recharge rates are not negligible, rates of groundwater abstraction for irrigation or other intensive uses can exceed natural rates of recharge. One consequence of excessive rates of groundwater use is a significant decline in the water-table. This occurs as water drains from pores, causing portions of a saturated aquifer to be converted into part of the unsaturated zone. As the water-table drops, wells must be deepened. Pumping costs increase because of the increased energy required to bring water to the surface from greater depths. In coastal areas, a decline in the water-table can also induce flow of saline groundwater from the formation beneath the ocean or sea toward wells on land. Salt-water intrusion into freshwater aquifers has long been recognized as a potentially serious source of groundwater contamination. This problem was first addressed by theoretical analyses in the late 1800s and continues to be a topic of research and practical interest.

Groundwater hydraulics

Quantitative estimation of the rates of recharge to and discharge from aquifers requires an understanding of the physics of groundwater flow (see groundwater flow rate). The empirical relationship governing rates of groundwater flow, and more generally the flow of fluids through any porous material, was first demonstrated experimentally by Henry Darcy, an engineer working in Dijon, France in the 1850s. This relationship, which has come to be known as Darcy's law, states that groundwater flow rates are proportional to permeability and to the change along the flow path of a property called hydraulic head. Hydraulic head is a measure of the mechanical energy of the groundwater and can be evaluated by measuring the level to which groundwater rises in an open well. Groundwater flows from areas of high head to areas of lower head, in other words from areas of higher to lower energy.

Measurements in a number of wells are required to map the distribution of hydraulic head within an aquifer. Water in wells open at the water-table will rise to the position of the water-table, while deeper wells may have water levels that are lower or higher than the water-table at the same location (Fig. 2). A contour map of the water-table shows the distribution of shallow hydraulic head and, therefore, provides an indication of the horizontal direction of shallow groundwater flow. Water-table elevations measured relative to a reference elevation, such as sea level, are highest in regional areas of groundwater recharge and are lowest in areas where groundwater discharges to a surface water body or is removed from the subsurface by evapotranspiration.

Darcy's law also provides the basis for predicting the response of water levels in an aquifer to pumping, a problem that has been studied extensively in the field of well hydraulics. Removal of water from a well generates a decline of water levels in a pattern that resembles a downward-pointing cone centred on the well. This water-level pattern is called a cone of depression (Fig. 3). The rate at which the cone of depression spreads away from a well when pumping begins depends on the storage properties and permeability of the aquifer. Pumping tests, which entail monitoring water levels in a pumped well and in nearby observation wells, are commonly used, not only to evaluate the water-supply potential of a particular well, but also to quantify aquifer properties so that groundwater flow rates under other conditions can be estimated.

Groundwater contamination

In recent years, the emphasis of groundwater studies in many parts of the world has shifted from questions of quantity to problems of quality. In addition to contamination associated with natural sources of salinity, such as salt-water intrusion in coastal areas, threats to the quality of groundwater as a source of drinking water result from a variety of chemicals that have been introduced to the subsurface, intentionally or unintentionally, through human activities. Common sources of groundwater contamination include applications of agricultural fertilizers and pesticides, land disposal of municipal solid and sewage wastes, and spills or leaks of fuel and industrial solvents during transport or storage. Because of the long residence times typical of most bodies of groundwater, contaminated aquifers are not readily restored by natural processes.

Attempts to restore water quality of contamination aquifers by pumping are often met with limited success, partly because of the large volumes of water that must be pumped to remove groundwater contaminated even by relatively small, localized sources. Diffusion of dissolved chemicals can spread contaminants originating from a restricted source area, such as a spill, through large volumes of groundwater. Flow of contaminated groundwater through complex pore networks and, on a larger scale, through geological materials of varying porosity and permeability, causes even greater mixing of contaminated and uncontaminated groundwater. In aquifers, flow-induced mixing usually dominates diffusion in the observed spreading, or dispersion, of contaminated groundwater (see diffusion and dispersion in groundwater flow). As a result of dispersion and of the low solubility in water of many contaminants, relatively small sources can generate very long ‘plumes’ of contaminated groundwater. For example, less than 100 litres of organic solvent at a site near Denver, USA generated a plume containing over 4 billion litres of contaminated groundwater that extended over a distance of more than 5 km.

Transport of groundwater contaminants is also affected by the rate and extent of chemical reactions between the dissolved contaminants and the aquifer solids or absorption to the solid surfaces. One possible consequence of chemical reactions is a decreased velocity, or retardation, of contaminants relative to the velocity of the groundwater (see retardation in groundwater). Retardation of contaminants necessitates removing even greater volumes of water in order to clean up an aquifer.

Because of the difficulty of completely removing contaminated groundwater from the subsurface region, limiting the continued migration of contaminants may be a more realistic objective at many sites. Containment techniques applicable to shallow contaminant sources include a variety of passive barriers such as surface or subsurface drains and slurry walls, vertical trenches that have been filled with a low-permeability mixture of clay and other sediment. Containment of deeper plumes generally requires the installation of wells that can be pumped to alter the natural ground-water flow field. The design of these systems is based on the principles of well hydraulics and an understanding of the physical and chemical processes that affect the transport of contaminants. Effective containment also requires adequate characterization of the nature and subsurface distribution of contaminants, which can be highly dependent on the subsurface distribution of porosity and permeability.

While much current research is devoted to the development of more efficient methods for removing and containing contaminated groundwater, there is increasing emphasis on identifying microbial and geochemical mechanisms that can break down or permanently immobilize contaminants within aquifers. Techniques that enhance natural degradation or attenuation of contaminants or provide new mechanism for subsurface treatment might, in the long run, provide the best chance of remedying contaminated aquifers.

Groundwater as a geological agent

The flow of groundwater through geological materials also has important effects on a variety of geological processes that shape the land surface and control the distribution of mineral and energy resources. In the absence of water, the entire stress resulting from the weight of overlying material or from tectonic forces must be borne by the solid phases of sediments or rocks. When groundwater fills the pores, however, stress in any direction is reduced to an effective stress equal to the total stress minus the fluid pressure. Changes in effective stress can cause changes in the shear strength of materials, the property that controls the resistance to fracture and deformation. Triggering of landslides and earthquakes has been attributed to changes in fluid pressure of groundwater.

Changes in fluid pressure can be induced by natural events such as heavy rains, or by human activities such as subsurface injection of liquid wastes. Fracturing of rocks and movement along faults can change the permeability of geological formation, generating potential feedbacks between groundwater flow, fluid pressure, and deformation. These feedbacks can become even more complex in geothermal systems, where groundwater circulation is driven in part by a heat source at depth. Hot groundwater and steam generated from groundwater in geothermal areas can provide energy for heating and generation of electricity.

Dissolution and precipitation of minerals and cement by flowing groundwater are important processes leading to low-temperature, or diagenetic, alteration of sediments and sedimentary rocks. This alteration can cause significant changes in porosity and permeability. An extreme case is the dissolution of carbonate rocks to produce caves and the distinctive irregular landscape of sinkholes and steep hills known as karst. Less extreme cases of groundwater-related porosity changes can alter the properties of water supply aquifers and create reservoirs or traps for petroleum resources.

Groundwater flow on a regional scale can transport dissolved metals for long distances and is thought to be responsible for the creation of a wide variety of mineral resources, such as stratabound lead–zinc deposits. Migration of petroleum, either dissolved in groundwater or as a separate immiscible fluid, can also be strongly affected by the groundwater flow field and may accompany the transport of dissolved metals.

Groundwater and engineering problems

The presence of groundwater at shallow depths can create problems for a variety of human activities. Obvious examples are the excavation of building foundation and tunnels that extend below the water-table. The prediction of water infiltration rates and construction of suitable drainage systems or low-permeability barriers requires an understanding of groundwater hydraulics and of the porosity and permeability of the local geological formations. Seepage of water through foundation rocks beneath a dam or through the dam itself can limit reservoir storage and, in some instances, can undermine the stability of the dam. The 1976 failure of the Teton Dam in Idaho, USA, which killed 14 people, resulted from reservoir water that infiltrated through fractures in the dam foundation and then reached the erodible silt core of the earth-fill structure. Even more destructive was the 1963 landslide induced by high fluid pressures in rocks surrounding the reservoir of the Vaiont Dam in Italy. Water was displaced and overtopped the dam as debris from the landslide filled a large area of the reservoir. Some 3000 people died in the resulting flood. Consideration of the effects of reservoir filling on pore pressures and groundwater flow might have helped to prevent both these disasters.

As noted above, excessive abstraction of groundwater can limit the quantity and quality of the water resource. In some geological settings, excessive pumping of groundwater can also create problems for engineering structures by inducing subsidence, or lowering, of the land surface. Compaction of sediments occurs naturally over geological time in areas of active deposition. This compaction can be accelerated when fluid pressures are lowered, increasing the effective stress on the sediments. These effects are particularly pronounced in sedimentary basins containing large thicknesses of compressible clays. Pumping from permeable sands and gravels in the same basins causes a delayed drainage of the clays, which then compact. The resulting subsidence is mostly irreversible. Large areas of Mexico City have subsided up to 7 m over the past century. Subsidence can cause building foundations to crack and buried utility lines to rupture. Venice has subsided by only about 15 cm, but this is sufficient to have greatly increased the frequency of flooding in the city. As the connection between subsidence and groundwater withdrawals became recognized, some areas, such as the Santa Clara Valley in California, began to develop alternative water sources and systems of artificial recharge in attempts to limit the potential for future subsidence.

J. Bahr

Bibliography

Domenico, P. A. and and Schwartz, F. W. (1990) Physical and chemical hydrogeology. John Wiley and Sons, New York.
Freeze, R. A. and and Cherry, J. A. (1979) Groundwater. Prentice Hall, Engelwood Cliffs, NJ.

Groundwater

Groundwater occupies the void space in geological strata. It is one element in the continuous process of moisture circulation on Earth, termed the hydrologic cycle .

Almost all groundwater originates as surface water . Some portion of rain hitting the earth runs off into streams and lakes , and another portion soaks into the soil , where it is available for use by plants and subject to evaporation back into the atmosphere. The third portion soaks below the root zone and continues moving downward until it enters the groundwater. Precipitation is the major source of groundwater. Other sources include the movement of water from lakes or streams and contributions from such activities as excess irrigation and seepage from canals. Water has also been purposely applied to increase the available supply of groundwater. Water-bearing formations called aquifers act as reservoirs for storage and conduits for transmission back to the surface.

The occurrence of groundwater is usually discussed by distinguishing between a zone of saturation and a zone of aeration. In the zone of saturation, the pores are entirely filled with water, while the zone of aeration has pores that are at least partially filled by air. Suspended water does occur in this zone. This water is called vadose, and the zone of aeration is also known as the vadose zone. In the zone of aeration, water moves downward due to gravity , but in the zone of saturation it moves in a direction determined by the relative heights of water at different locations.

Water that occurs in the zone of saturation is termed groundwater. This zone can be thought of as a natural storage area or reservoir whose capacity is the total volume of the pores of openings in rocks.

An important exception to the distinction between these zones is the presence of ancient seawater in some sedimentary formations. The pore spaces of materials that have accumulated on an ocean floor, which has then been raised through later geological processes, can sometimes contain salt water. This is called connate water.

Formations or strata within the saturated zone from which water can be obtained are called aquifers. Aquifers must yield water through wells or springs at a rate that can serve as a practical source of water supply. To be considered an aquifer the geological formation must contain pores or open spaces filled with water, and the openings must be large enough to permit water to move through them at a measurable rate. Both the size of pores and the total pore volume depends on the type of material. Individual pores in fine-grained materials such as clay , for example, can be extremely small, but the total volume is large. Conversely, in coarse material such as sand , individual pores may be quite large but total volume is less. The rate of movement for fine-grained materials, such as clay, will be slow due to the small pore size, and it may not yield sufficient water to wells to be considered an aquifer. However, the sand is considered an aquifer, even though they yield a smaller volume of water, because they will yield water to a well.

The water table is not stationary, but moves up or down depending on surface conditions such as excess precipitation, drought , or heavy use. Formations where the top of the saturated zone or water table define the upper limit of the aquifer are called unconfined aquifers. The hydraulic pressure at any level with an aquifer is equal to the depth from the water table, and there is a type known as a water-table aquifer, where a well drilled produces a static water level which stands at the same level as the water table.

A local zone of saturation occurring in an aerated zone separated from the main water table is called a perched water table. These most often occur when there is an impervious strata or significant particle-size change in the zone of aeration, which causes the water to accumulate. A confined aquifer is found between impermeable layers. Because of the confining upper layer, the water in the aquifer exists within the pores at pressures greater than the atmosphere. This is termed an artesian condition and gives rise to an artesian well.

Groundwater can be pumped from any aquifer that can be reached by modern well-drilling apparatus. Once a well is constructed, hydraulic pumps pull the water up to the surface through pipes. As water from the aquifer is pulled up to the surface, water moves through the aquifer towards the well. Because water is usually pumped out of an aquifer more quickly than new water can flow to replace what has been withdrawn, the level of the aquifer surrounding the well drops, and a cone of depression is formed in the immediate area around the well.

Groundwater can be polluted by the spilling or dumping of contaminants. As surface water percolates downward, contaminants can be carried into the aquifer. The most prevalent sources of contamination are waste disposal , the storage, transportation and handling of commercial materials, mining operations, and nonpoint sources such as agricultural activities. Two other forms of groundwater pollution are the result of pumping too much water too quickly, so that the rate of water withdrawal from the aquifer exceeds the rate of aquifer recharge. In coastal areas, salty water may migrate towards the well, replacing the fresh water that has been withdrawn. This is called salt-water intrusion. Eventually, the well will begin pulling this salt water to the surface; once this happens, the well will have to be abandoned. A similar phenomenon, called connate ascension, occurs when a freshwater aquifer overlies a layer of sedimentary rocks containing connate water. In some cases, over pumping will cause the connate water to migrate out of the sedimentary rocks and into the freshwater aquifer. This results in a brackish, briney contamination similar to the effects of a salt-water intrusion. Unlike salt water intrusion, however, connate ascension is not particularly associated with coastal areas.

Groundwater has always been an important resource, and it will become more so in the future as the need for good quality water increases due to urbanization and agricultural production. It has recently been estimated that 50% of the drinking water in the United States comes from groundwater; 75% of the nation's cities obtain all or part of their supplies from groundwater, and rural areas are 95% dependent upon it. For these reasons every precaution should be taken to protect groundwater purity. Once contaminated, groundwater is difficult, expensive, and sometimes impossible to clean up.

See also Freshwater; Hydrogeology

GROUNDWATER

An estimated 100 million Americans rely on groundwater for their source of drinking water. Approximately one-third of all public supplies and 95 percent of all rural domestic supplies use groundwater sources. In Asia, groundwater provides half of the drinking water, and in Europe the percentage is even much higher, as much as 98 percent in Denmark and 96 percent in Austria.

An aquifer is an underground formation of permeable rock or loose material that can produce useful quantities of water when tapped by a well. Groundwater is held within the tiny pores of the surrounding aquifer material. Aquifers vary in size from a few hectares to thousands of square kilometers of the earth's surface. The rate of groundwater flow is very slow compared to the flow of water on the surface—usually in the range of several inches per year to several feet per year. More than 96 percent of all available fresh water supplies occur in the form of groundwater, which is usually cleaner and more pure than most surface water sources.

Groundwater only partially fills unconfined aquifers. The upper surface of the groundwater, known as the water table, is thus free to rise and fall. The height of the water table will be the same as the water level in a well drilled in an unconfined aquifer. Unconfined aquifers can be vulnerable to contamination, especially if they are close to the surface. In these unconfined aquifers, gravity drives the movement of groundwater. Groundwater can leave the aquifer through the process of discharge, either when it reaches the land surface at a spring or other surface water body, or through the pumping of a well. Discharge can lead to contaminants in groundwater flowing into surface water bodies.

A confined aquifer (also known as an artesian aquifer) occurs between confining beds, which are layers of impermeable materials, such as clay, that impede the movement of water in and out of the aquifer. The groundwater in these artesian aquifers is under high pressure due to the confining beds. A recharge zone occurs where the confined aquifer is exposed to the surface. The confined aquifer is actually unconfined at the recharge zone. Confining beds serve two purposes. The first is to obstruct the movement of water into and out of the aquifer. The second is to bar the entry of contaminants from the overlying unconfined aquifers.

Aquifers are replenished with water from the surface through a process called "recharge." This occurs as a part of the hydrologic cycle when water from rainfall percolates into underlying aquifers. The rate of recharge can be influenced by different factors, such as soil, plant cover, water content of surface materials, and rainfall intensity. Groundwater recharge may also occur from surface water bodies in arid areas. Overwithdrawal of groundwater occurs when the discharge of groundwater in an aquifer exceeds the recharge rate over a period of time.

Groundwater can be polluted by landfills, septic tanks, leaky underground gas tanks, and from overuse of fertilizers and pesticides. This pollution poses a great risk to public health since the majority of the fresh water supply occurs as groundwater. Many of the groundwater pollutants are colorless, odorless, and tasteless. Degradation of groundwater supplies also occurs as a result of poor waste-disposal practices or poor land management.

Mark G. Robson

(see also: Ambient Water Quality; Drinking Water; Groundwater Contamination; Water Quality )

Bibliography

Groundwater Foundation. Groundwater and Aquifers, 2000. Available at http://www.groundwater.com/groundwater_aquifer.html.

—— Groundwater Basics, 2000. Available at http://www.groundwater.org/GWBasics/whatisgw.html.

Koren, H., and Bisesi, M. (1996). Handbook of Environmental Health and Safety, 3rd edition, Vol. 2. Boca Raton, FL: Lewis Publishers.

Nadakavukaren, A. (2000). Our Global Environment. Prospect Heights, IL: Waveland Press.

Groundwater

Groundwater is the water that exists below the land surface and fills the spaces between sediment grains and fractures in rocks. A geologic formation saturated with groundwater is considered to be an aquifer if it is sufficiently permeable as to allow the groundwater to be economically extracted. It is replenished naturally through the infiltration of rainfall and artificially through the irrigation of crops. Soluble chemicals in rainwater (like NOx in acid rain) or at the land surface (like pesticides) can be transported downward with percolating water to reach groundwater. Underground petroleum storage tanks (USTs) or buried pipelines also pose threats if they should leak. Over 400,000 leaking USTs have been identified in the United States as of 2001. Dissolved chemicals are transported with the flowing groundwater. Once groundwater is contaminated, remediation can be expensive and time-consuming; billions of dollars are spent annually in the United States on the remediation of contaminated sites and some of the groundwater contamination cannot be reversed. Groundwater discharges naturally into lakes, rivers, oceans, and springs. It is also extracted via pumping wells.

Approximately 80 percent of municipal water systems and close to 99 percent of rural residents in the United States rely on groundwater. In total, approximately 51 percent of the U.S. population depends on it for their water supply. The 1986 amendments to the Safe Drinking Water Act requires that well head protection plans be developed by each state to protect the land around municipal water supply wells from contamination. Individuals can help protect groundwater by disposing of household chemicals properly and fertilizing plants in limited quantities and can help conserve groundwater by limiting water use at home by taking shorter showers, not running water while brushing teeth, running dish and clothes washers with full loads, fixing leaky faucets and pipes, and limiting plant watering in the garden.

If withdrawals exceed recharge over a long period, groundwater levels fall and aquifiers can become depleted. This results in decreased groundwater discharge and may adversely impact on ecosystems dependent on an aquatic habitat. Excess lowering of groundwater levels may result in land subsidence . In central California, for instance, groundwater withdrawals from 1930 to 1955 for crop irrigation caused approximately three meters of subsidence. In some arid regions (like the Middle East), little groundwater recharge occurs because of low amounts of rainfall and high amounts of evaporation. Ancient groundwater that infiltrated thousands of years ago during climates wetter than those of the present is being extracted via pumping. This practice is termed groundwater mining because groundwater at this location is a nonrenewable resource that is being depleted.

see also Drinking Water; Pesticides; Superfund; Underground Storage Tanks; Water Pollution.

Bibliography

U.S. Environmental Protection Agency (1990). Citizen's Guide to Ground-Water Protection. EPA 440/6-90-004. Washington: U.S. Environmental Protection Agency Office of Water.

Alley, William M., Richard W. Healy, James W. Labaugh, and Thomas E. Reilly (2002). "Flow and Storage in Groundwater Systems." In Science Magazine, 296:1985–1990.

Internet Resources

U.S. Environmental Protection Agency Office of Ground Water and Drinking Water Information Page. Available from http://www.epa.gov/safewater.

U.S. Geological Survey Ground Water Information Page. Available from http://water.usgs.gov/ogw.

Karen M. Salvage

groundwater Water that occurs below the Earth's surface, contained in pore spaces within regolith and bedrock. It is either passing through or standing in the soil and underlying strata, and is free to move under the influence of gravity. Most groundwater derives from surface sources (meteoric water); the remainder is either introduced by magmatic processes (juvenile water) or is connate water. See also vadose zone.

groundwater Water that lies beneath the surface of the Earth. It comes chiefly from rain, although some is of volcanic or sedimentary origin. It moves through porous rocks and soil and can be collected in wells. Ground water can dissolve minerals and leave deposits, creating structures such as caves, stalagmites, and stalactites. See also water table

groundwater All the water contained in the void space within rocks. The term is generally taken to exclude vadose water (water travelling between the surface and the water-table). Most groundwater derives from surface sources (meteoric water); the remainder is either introduced by magmatic processes (juvenile water) or is connate water.

ground·wa·ter / ˈgroundˌwôtər; -ˌwätər/ • n. water held underground in the soil or in pores and crevices in rock.