A dense non-aqueous phase liquid or DNAPL is a denser-than-water NAPL , i.e. a liquid that is both denser than water and is immiscible in or does not dissolve in water.
97-450: The term DNAPL is used primarily by environmental engineers and hydrogeologists to describe contaminants in groundwater , surface water and sediments. DNAPLs tends to sink below the water table when spilled in significant quantities and only stop when they reach impermeable bedrock. Their penetration into an aquifer makes them difficult to locate and remediate. Examples of materials that are DNAPLs when spilled include: When spilled into
194-590: A Reynolds number less than unity); many of the empirically derived laws of groundwater flow can be alternately derived in fluid mechanics from the special case of Stokes flow (viscosity and pressure terms, but no inertial term). The mathematical relationships used to describe the flow of water through porous media are Darcy's law , the diffusion , and Laplace equations, which have applications in many diverse fields. Steady groundwater flow (Laplace equation) has been simulated using electrical , elastic , and heat conduction analogies. Transient groundwater flow
291-469: A Taylor series ). For example, the first-order time derivative is often approximated using the following forward finite difference, where the subscripts indicate a discrete time location, Aquifer An aquifer is an underground layer of water -bearing material, consisting of permeable or fractured rock, or of unconsolidated materials ( gravel , sand , or silt ). Aquifers vary greatly in their characteristics. The study of water flow in aquifers and
388-401: A French scientist who made advances in flow of fluids through porous materials. He conducted experiments which studied the movement of fluids through sand columns. These experiments led to the determination of Darcy's law , which describes fluid flow through a medium with high levels of porosity. Darcy's work is considered to be the beginning of quantitative hydrogeology. Oscar Edward Meinzer
485-411: A city water system. Wells are designed and maintained to uphold the integrity of the aquifer, and to prevent contaminants from reaching the groundwater. Controversy arises in the use of groundwater when its usage impacts surface water systems, or when human activity threatens the integrity of the local aquifer system. Hydrogeology is an interdisciplinary subject; it can be difficult to account fully for
582-405: A common task of the hydrogeologist is determining aquifer properties using aquifer tests . In order to further characterize aquifers and aquitards some primary and derived physical properties are introduced below. Aquifers are broadly classified as being either confined or unconfined ( water table aquifers), and either saturated or unsaturated; the type of aquifer affects what properties control
679-630: A confining layer, often made up of clay. The confining layer might offer some protection from surface contamination. If the distinction between confined and unconfined is not clear geologically (i.e., if it is not known if a clear confining layer exists, or if the geology is more complex, e.g., a fractured bedrock aquifer), the value of storativity returned from an aquifer test can be used to determine it (although aquifer tests in unconfined aquifers should be interpreted differently than confined ones). Confined aquifers have very low storativity values (much less than 0.01, and as little as 10 ), which means that
776-541: A considerable effort has been extended to improve our ability to locate and remediate DNAPL present as chlorinated solvents. DNAPLs that are not viscous, such as chlorinated solvents, tend to sink into aquifer materials below the water table and become much more difficult to locate and remediate than non aqueous phase liquids that are lighter than water ( LNAPLs ) which tend to float at the water table when spilled into natural soils. The United States Environmental Protection Agency (USEPA) has focused considerable attention on
873-501: A corresponding steady-state simulation (where the time derivative in the groundwater flow equation is set equal to 0). There are two broad categories of how the (PDE) would be solved; either analytical methods, numerical methods, or something possibly in between. Typically, analytic methods solve the groundwater flow equation under a simplified set of conditions exactly , while numerical methods solve it under more general conditions to an approximation . Analytic methods typically use
970-403: A million cubic kilometers of "low salinity" water that could be economically processed into potable water . The reserves formed when ocean levels were lower and rainwater made its way into the ground in land areas that were not submerged until the ice age ended 20,000 years ago. The volume is estimated to be 100 times the amount of water extracted from other aquifers since 1900. An aquitard
1067-459: A poor aquifer. Porosity does not directly affect the distribution of hydraulic head in an aquifer, but it has a very strong effect on the migration of dissolved contaminants, since it affects groundwater flow velocities through an inversely proportional relationship. Darcy's law is commonly applied to study the movement of water, or other fluids through porous media, and constitutes the basis for many hydrogeological analyses. Water content ( θ )
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#17330927142351164-551: A preferential path in one direction, some other in a different direction, so that the contaminant can be spread in a completely irregular way, like in a (three-dimensional) delta of a river. Dispersivity is actually a factor which represents our lack of information about the system we are simulating. There are many small details about the aquifer which are effectively averaged when using a macroscopic approach (e.g., tiny beds of gravel and clay in sand aquifers); these manifest themselves as an apparent dispersivity. Because of this, α
1261-442: A rock unit of low porosity is highly fractured, it can also make a good aquifer (via fissure flow), provided the rock has a hydraulic conductivity sufficient to facilitate movement of water. Challenges for using groundwater include: overdrafting (extracting groundwater beyond the equilibrium yield of the aquifer), groundwater-related subsidence of land, groundwater becoming saline, groundwater pollution . Aquifer depletion
1358-435: A soil particle, must choose where to go, whether left or right or up or down, so that the water "particles" (and their solute) are gradually spread in all directions around the mean path. This is the "microscopic" mechanism, on the scale of soil particles. More important, over long distances, can be the macroscopic inhomogeneities of the aquifer, which can have regions of larger or smaller permeability, so that some water can find
1455-440: A two-dimensional slice of the aquifer) appear to be layers of alternating coarse and fine materials. Coarse materials, because of the high energy needed to move them, tend to be found nearer the source (mountain fronts or rivers), whereas the fine-grained material will make it farther from the source (to the flatter parts of the basin or overbank areas—sometimes called the pressure area). Since there are less fine-grained deposits near
1552-445: A unit depressurization of a confined aquifer. They are fractions between 0 and 1. Specific yield ( S y ) is also a ratio between 0 and 1 ( S y ≤ porosity) and indicates the amount of water released due to drainage from lowering the water table in an unconfined aquifer. The value for specific yield is less than the value for porosity because some water will remain in the medium even after drainage due to intermolecular forces. Often
1649-416: A well in a fracture trace or intersection of fracture traces increases the likelihood to encounter good water production. Voids in karst aquifers can be large enough to cause destructive collapse or subsidence of the ground surface that can initiate a catastrophic release of contaminants. Groundwater flow rate in karst aquifers is much more rapid than in porous aquifers as shown in the accompanying image to
1746-410: A well is a hydrograph or, the changes in hydraulic head recorded during the pumping of a well in a test are called drawdown . Porosity ( n ) is a directly measurable aquifer property; it is a fraction between 0 and 1 indicating the amount of pore space between unconsolidated soil particles or within a fractured rock. Typically, the majority of groundwater (and anything dissolved in it) moves through
1843-437: A well). Intrinsic permeability ( κ ) is a property of the porous medium alone, and does not change with different fulids (e.g. different density or viscosity; it is used more in the petroleum industry. Specific storage ( S s ) and its depth-integrated equivalent, storativity ( S=S s b ), are indirect aquifer properties (they cannot be measured directly); they indicate the amount of groundwater released from storage due to
1940-418: Is a chlorinated solvent can act as an ongoing pathway for constituents to dissolve into groundwater. Common use of chlorinated solvents in manufacturing operations began during World War II , with the rate of usage for most solvents increasing into the 1970s. By the early 1980s, chemical analyses becoming available that documented widespread contamination of groundwater with chlorinated solvents. Since that time,
2037-463: Is a groundwater flow equation applied to subsurface drainage by pipes, tile drains or ditches. An alternative subsurface drainage method is drainage by wells for which groundwater flow equations are also available. To use the groundwater flow equation to estimate the distribution of hydraulic heads, or the direction and rate of groundwater flow, this partial differential equation (PDE) must be solved. The most common means of analytically solving
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#17330927142352134-566: Is a problem in some areas, especially in northern Africa , where one example is the Great Manmade River project of Libya . However, new methods of groundwater management such as artificial recharge and injection of surface waters during seasonal wet periods has extended the life of many freshwater aquifers, especially in the United States. The Great Artesian Basin situated in Australia
2231-550: Is a zone within the Earth that restricts the flow of groundwater from one aquifer to another. An aquitard can sometimes, if completely impermeable, be called an aquiclude or aquifuge . Aquitards are composed of layers of either clay or non-porous rock with low hydraulic conductivity . Groundwater can be found at nearly every point in the Earth's shallow subsurface to some degree, although aquifers do not necessarily contain fresh water . The Earth's crust can be divided into two regions:
2328-401: Is also a directly measurable property; it is the fraction of the total rock which is filled with liquid water. This is also a fraction between 0 and 1, but it must also be less than or equal to the total porosity. The water content is very important in vadose zone hydrology, where the hydraulic conductivity is a strongly nonlinear function of water content; this complicates the solution of
2425-453: Is an empirical factor which quantifies how much contaminants stray away from the path of the groundwater which is carrying it. Some of the contaminants will be "behind" or "ahead" the mean groundwater, giving rise to a longitudinal dispersivity (α L ), and some will be "to the sides of" the pure advective groundwater flow, leading to a transverse dispersivity (α T ). Dispersion in groundwater arises because each water "particle", passing beyond
2522-518: Is an important phenomenon for small distances (it is essential for the achievement of thermodynamic equilibria ), but, as the time necessary to cover a distance by diffusion is proportional to the square of the distance itself, it is less effective for spreading a solute over macroscopic distances on a short time scale. The diffusion coefficient , D , is typically quite small, and its effect can often be neglected (unless groundwater flow velocities are extremely low, as they are in clay aquitards ). It
2619-418: Is analogous to the retardation factor of chromatography . Unlike diffusion and dispersion, which simply spread the contaminant, the retardation factor changes its global average velocity , so that it can be much slower than that of water. This is due to a chemico-physical effect: the adsorption to the soil, which holds the contaminant back and does not allow it to progress until the quantity corresponding to
2716-653: Is analogous to the diffusion of heat in a solid, therefore some solutions to hydrological problems have been adapted from heat transfer literature. Traditionally, the movement of groundwater has been studied separately from surface water, climatology , and even the chemical and microbiological aspects of hydrogeology (the processes are uncoupled). As the field of hydrogeology matures, the strong interactions between groundwater, surface water , water chemistry , soil moisture, and even climate are becoming more clear. California and Washington both require special certification of hydrogeologists to offer professional services to
2813-994: Is arguably the largest groundwater aquifer in the world (over 1.7 million km or 0.66 million sq mi). It plays a large part in water supplies for Queensland, and some remote parts of South Australia. Discontinuous sand bodies at the base of the McMurray Formation in the Athabasca Oil Sands region of northeastern Alberta , Canada, are commonly referred to as the Basal Water Sand (BWS) aquifers . Saturated with water, they are confined beneath impermeable bitumen -saturated sands that are exploited to recover bitumen for synthetic crude oil production. Where they are deep-lying and recharge occurs from underlying Devonian formations they are saline, and where they are shallow and recharged by surface water they are non-saline. The BWS typically pose problems for
2910-484: Is composed of pressure head ( ψ ) and elevation head ( z ). The head gradient is the change in hydraulic head per length of flowpath, and appears in Darcy's law as being proportional to the discharge. Hydraulic head is a directly measurable property that can take on any value (because of the arbitrary datum involved in the z term); ψ can be measured with a pressure transducer (this value can be negative, e.g., suction, but
3007-495: Is considered to be a high rate for porous aquifers, as illustrated by the water slowly seeping from sandstone in the accompanying image to the left. Porosity is important, but, alone , it does not determine a rock's ability to act as an aquifer. Areas of the Deccan Traps (a basaltic lava) in west central India are good examples of rock formations with high porosity but low permeability, which makes them poor aquifers. Similarly,
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3104-421: Is held in place by surface adhesive forces and it rises above the water table (the zero- gauge-pressure isobar ) by capillary action to saturate a small zone above the phreatic surface (the capillary fringe ) at less than atmospheric pressure. This is termed tension saturation and is not the same as saturation on a water-content basis. Water content in a capillary fringe decreases with increasing distance from
3201-422: Is important not to confuse diffusion with dispersion, as the former is a physical phenomenon and the latter is an empirical hydrodynamic factor which is cast into a similar form as diffusion, because its a convenient way to mathematically describe and solve the question. The retardation factor is another very important feature that make the motion of the contaminant to deviate from the average groundwater motion. It
3298-513: Is leached by groundwater, the trichloroethene may preferentially leach out of the oil and the mixture may become less dense than water and become buoyant (e.g. the liquid may become an LNAPL). Similarly changes can be seen at some coal gasification plants or manufactured gas plants where the tar mixtures can be denser than water, be neutrally buoyant or be less dense than water and the densities can change with time. Hydrogeologist Hydrogeology ( hydro- meaning water, and -geology meaning
3395-435: Is often claimed to be dependent on the length scale of the problem — the dispersivity found for transport through 1 m of aquifer is different from that for transport through 1 cm of the same aquifer material. Diffusion is a fundamental physical phenomenon, which Albert Einstein characterized as Brownian motion , that describes the random thermal movement of molecules and small particles in gases and liquids. It
3492-419: Is often derived from a physical basis using Darcy's law and a conservation of mass for a small control volume. The equation is often used to predict flow to wells , which have radial symmetry, so the flow equation is commonly solved in polar or cylindrical coordinates . The Theis equation is one of the most commonly used and fundamental solutions to the groundwater flow equation; it can be used to predict
3589-468: Is positive in saturated aquifers), and z can be measured relative to a surveyed datum (typically the top of the well casing). Commonly, in wells tapping unconfined aquifers the water level in a well is used as a proxy for hydraulic head, assuming there is no vertical gradient of pressure. Often only changes in hydraulic head through time are needed, so the constant elevation head term can be left out ( Δh = Δψ ). A record of hydraulic head through time at
3686-419: Is the level to which water will rise in a large-diameter pipe (e.g., a well) that goes down into the aquifer and is open to the atmosphere. Aquifers are typically saturated regions of the subsurface that produce an economically feasible quantity of water to a well or spring (e.g., sand and gravel or fractured bedrock often make good aquifer materials). An aquitard is a zone within the Earth that restricts
3783-432: Is the prediction of future behavior of an aquifer system, based on analysis of past and present observations. Some hypothetical, but characteristic questions asked would be: Most of these questions can be addressed through simulation of the hydrologic system (using numerical models or analytic equations). Accurate simulation of the aquifer system requires knowledge of the aquifer properties and boundary conditions. Therefore,
3880-427: The saturated zone or phreatic zone (e.g., aquifers, aquitards, etc.), where all available spaces are filled with water, and the unsaturated zone (also called the vadose zone ), where there are still pockets of air that contain some water, but can be filled with more water. Saturated means the pressure head of the water is greater than atmospheric pressure (it has a gauge pressure > 0). The definition of
3977-792: The Atlas Mountains in North Africa, the Lebanon and Anti-Lebanon ranges between Syria and Lebanon, the Jebel Akhdar in Oman, parts of the Sierra Nevada and neighboring ranges in the United States' Southwest , have shallow aquifers that are exploited for their water. Overexploitation can lead to the exceeding of the practical sustained yield; i.e., more water is taken out than can be replenished. Along
Dense non-aqueous phase liquid - Misplaced Pages Continue
4074-610: The Guarani people , it covers 1,200,000 km (460,000 sq mi), with a volume of about 40,000 km (9,600 cu mi), a thickness of between 50 and 800 m (160 and 2,620 ft) and a maximum depth of about 1,800 m (5,900 ft). The Ogallala Aquifer of the central United States is one of the world's great aquifers, but in places it is being rapidly depleted by growing municipal use, and continuing agricultural use. This huge aquifer, which underlies portions of eight states, contains primarily fossil water from
4171-467: The Resource Conservation and Recovery Act Dense nonaqueous phase liquids (DNAPLs), have low solubility and are with viscosity markedly lower and density higher than water-asphalt, heavy oils, lubricants and also chlorinated solvents-penetrate the full depth of the aquifer and accumulate on its bottom. "DNAPL movement follows the slope of the impermeable strata underlying the aquifer and can move in
4268-427: The chemical , physical , biological , and even legal interactions between soil , water , nature , and society . The study of the interaction between groundwater movement and geology can be quite complex. Groundwater does not always follow the surface topography ; groundwater follows pressure gradients (flow from high pressure to low), often through fractures and conduits in circuitous paths. Taking into account
4365-661: The depositional sedimentary environment and later natural cementation of the sand grains. The environment where a sand body was deposited controls the orientation of the sand grains, the horizontal and vertical variations, and the distribution of shale layers. Even thin shale layers are important barriers to groundwater flow. All these factors affect the porosity and permeability of sandy aquifers. Sandy deposits formed in shallow marine environments and in windblown sand dune environments have moderate to high permeability while sandy deposits formed in river environments have low to moderate permeability. Rainfall and snowmelt enter
4462-436: The divergence theorem ). This results in a system which overall approximates the groundwater flow equation, but exactly matches the boundary conditions (the head or flux is specified in the elements which intersect the boundaries). Finite differences are a way of representing continuous differential operators using discrete intervals ( Δx and Δt ), and the finite difference methods are based on these (they are derived from
4559-533: The groundwater flow equation , typically used to analyze the results of an aquifer test or slug test . The topic of numerical methods is quite large, obviously being of use to most fields of engineering and science in general. Numerical methods have been around much longer than computers have (In the 1920s Richardson developed some of the finite difference schemes still in use today, but they were calculated by hand, using paper and pencil, by human "calculators"), but they have become very important through
4656-411: The hydraulic gradient , and the hydraulic conductivity . The groundwater flow equation, in its most general form, describes the movement of groundwater in a porous medium (aquifers and aquitards). It is known in mathematics as the diffusion equation , and has many analogs in other fields. Many solutions for groundwater flow problems were borrowed or adapted from existing heat transfer solutions. It
4753-424: The porosity or effective porosity is used as an upper bound to the specific yield. Typically S y is orders of magnitude larger than S s . Fault zone hydrogeology is the study of how brittlely deformed rocks alter fluid flows in different lithological settings , such as clastic , igneous and carbonate rocks . Fluid movements, that can be quantified as permeability , can be facilitated or impeded due to
4850-662: The United States accelerated in the late 1940s and continued at an almost steady linear rate through the end of the century. In addition to widely recognized environmental consequences, groundwater depletion also adversely impacts the long-term sustainability of groundwater supplies to help meet the Nation’s water needs." An example of a significant and sustainable carbonate aquifer is the Edwards Aquifer in central Texas . This carbonate aquifer has historically been providing high quality water for nearly 2 million people, and even today,
4947-413: The aquifer exists underneath a confining bed. There are three aspects that control the nature of aquifers: stratigraphy , lithology , and geological formations and deposits. The stratigraphy relates the age and geometry of the many formations that compose the aquifer. The lithology refers to the physical components of an aquifer, such as the mineral composition and grain size. The structural features are
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#17330927142355044-421: The aquifer is storing water using the mechanisms of aquifer matrix expansion and the compressibility of water, which typically are both quite small quantities. Unconfined aquifers have storativities (typically called specific yield ) greater than 0.01 (1% of bulk volume); they release water from storage by the mechanism of actually draining the pores of the aquifer, releasing relatively large amounts of water (up to
5141-487: The availability of fast and cheap personal computers . A quick survey of the main numerical methods used in hydrogeology, and some of the most basic principles are shown below and further discussed in the Groundwater model article. There are two broad categories of numerical methods: gridded or discretized methods and non-gridded or mesh-free methods. In the common finite difference method and finite element method (FEM)
5238-612: The characterization of aquifers is called hydrogeology . Related terms include aquitard , which is a bed of low permeability along an aquifer, and aquiclude (or aquifuge ), which is a solid, impermeable area underlying or overlying an aquifer, the pressure of which could lead to the formation of a confined aquifer. The classification of aquifers is as follows: Saturated versus unsaturated; aquifers versus aquitards; confined versus unconfined; isotropic versus anisotropic; porous, karst, or fractured; transboundary aquifer. Groundwater from aquifers can be sustainably harvested by humans through
5335-400: The chemical adsorption equilibrium has been adsorbed. This effect is particularly important for less soluble contaminants, which thus can move even hundreds or thousands times slower than water. The effect of this phenomenon is that only more soluble species can cover long distances. The retardation factor depends on the chemical nature of both the contaminant and the aquifer. Henry Darcy was
5432-409: The coastlines of certain countries, such as Libya and Israel, increased water usage associated with population growth has caused a lowering of the water table and the subsequent contamination of the groundwater with saltwater from the sea. In 2013 large freshwater aquifers were discovered under continental shelves off Australia, China, North America and South Africa. They contain an estimated half
5529-487: The complexity of karst aquifers. These conventional investigation methods need to be supplemented with dye traces , measurement of spring discharges, and analysis of water chemistry. U.S. Geological Survey dye tracing has determined that conventional groundwater models that assume a uniform distribution of porosity are not applicable for karst aquifers. Linear alignment of surface features such as straight stream segments and sinkholes develop along fracture traces . Locating
5626-526: The compound Kh and Kv values are different (see hydraulic transmissivity and hydraulic resistance ). When calculating flow to drains or flow to wells in an aquifer, the anisotropy is to be taken into account lest the resulting design of the drainage system may be faulty. To properly manage an aquifer its properties must be understood. Many properties must be known to predict how an aquifer will respond to rainfall, drought, pumping, and contamination . Considerations include where and how much water enters
5723-402: The diffusion equation in the hydrogeology literature are: No matter which method we use to solve the groundwater flow equation , we need both initial conditions (heads at time ( t ) = 0) and boundary conditions (representing either the physical boundaries of the domain, or an approximation of the domain beyond that point). Often the initial conditions are supplied to a transient simulation, by
5820-431: The domain is completely gridded ("cut" into a grid or mesh of small elements). The analytic element method (AEM) and the boundary integral equation method (BIEM — sometimes also called BEM, or Boundary Element Method) are only discretized at boundaries or along flow elements (line sinks, area sources, etc.), the majority of the domain is mesh-free. Gridded Methods like finite difference and finite element methods solve
5917-439: The drainable porosity of the aquifer material, or the minimum volumetric water content ). In isotropic aquifers or aquifer layers the hydraulic conductivity (K) is equal for flow in all directions, while in anisotropic conditions it differs, notably in horizontal (Kh) and vertical (Kv) sense. Semi-confined aquifers with one or more aquitards work as an anisotropic system, even when the separate layers are isotropic, because
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#17330927142356014-403: The elements that arise due to deformations after deposition, such as fractures and folds. Understanding these aspects is paramount to understanding of how an aquifer is formed and how professionals can utilize it for groundwater engineering. Differences in hydraulic head ( h ) cause water to move from one place to another; water flows from locations of high h to locations of low h. Hydraulic head
6111-482: The environment (e.g. spilled trichloroethene does not become lighter than water, it will remain denser than water). However, when the DNAPL is a more complex mixture, the density of the mixture can change over time as the mixture interacts with the natural environment. As an example, a mixture of trichloroethene and cutting oil may be released and originally be denser than water—a DNAPL. As the mixture of trichloroethene and oil
6208-401: The environment, chlorinated solvents are frequently present as DNAPL and the DNAPL can provide a long term secondary source of the chlorinated solvent to dissolved groundwater plumes. Chlorinated solvents are typically immiscible in water, having low solubility in water by definition, yet still have a solubility above the concentrations allowed by drinking water protections. Therefore, DNAPL which
6305-462: The existence of a fault zone . This is because different mechanism and deformed rocks can alter the porosity and hence the permeability within fault zone. Fluids involved generally are groundwater (fresh and marine waters) and hydrocarbons (Oil and Gas). As fault zone is a zone of weakness that helps to increase the weathered zone thickness and hence the help in ground water recharge. Along with faults , fractures and foliations also facilitate
6402-557: The fissures. The enlarged fissures allow a larger quantity of water to enter which leads to a progressive enlargement of openings. Abundant small openings store a large quantity of water. The larger openings form a conduit system that drains the aquifer to springs. Characterization of karst aquifers requires field exploration to locate sinkholes, swallets , sinking streams , and springs in addition to studying geologic maps . Conventional hydrogeologic methods such as aquifer tests and potentiometric mapping are insufficient to characterize
6499-478: The flow of groundwater from one aquifer to another. A completely impermeable aquitard is called an aquiclude or aquifuge . Aquitards contain layers of either clay or non-porous rock with low hydraulic conductivity . In mountainous areas (or near rivers in mountainous areas), the main aquifers are typically unconsolidated alluvium , composed of mostly horizontal layers of materials deposited by water processes (rivers and streams), which in cross-section (looking at
6596-418: The flow of water in that medium (e.g., the release of water from storage for confined aquifers is related to the storativity , while it is related to the specific yield for unconfined aquifers). An aquifer is a collection of water underneath the surface, large enough to be useful in a spring or a well. Aquifers can be unconfined, where the top of the aquifer is defined by the water table , or confined, where
6693-419: The groundwater flow equation by breaking the problem area (domain) into many small elements (squares, rectangles, triangles, blocks, tetrahedra , etc.) and solving the flow equation for each element (all material properties are assumed constant or possibly linearly variable within an element), then linking together all the elements using conservation of mass across the boundaries between the elements (similar to
6790-526: The groundwater from rainfall and snowmelt, how fast and in what direction the groundwater travels, and how much water leaves the ground as springs. Computer models can be used to test how accurately the understanding of the aquifer properties matches the actual aquifer performance. Environmental regulations require sites with potential sources of contamination to demonstrate that the hydrology has been characterized . Porous aquifers typically occur in sand and sandstone . Porous aquifer properties depend on
6887-434: The groundwater mainly in hard rock terrains. Often we are interested in how the moving groundwater will transport dissolved contaminants around (the sub-field of contaminant hydrogeology). The contaminants which are man-made (e.g., petroleum products , nitrate , chromium or radionuclides ) or naturally occurring (e.g., arsenic , salinity ), can be transported through three main mechanisms, advection (transport along
6984-610: The groundwater where the aquifer is near the surface. Groundwater flow directions can be determined from potentiometric surface maps of water levels in wells and springs. Aquifer tests and well tests can be used with Darcy's law flow equations to determine the ability of a porous aquifer to convey water. Analyzing this type of information over an area gives an indication how much water can be pumped without overdrafting and how contamination will travel. In porous aquifers groundwater flows as slow seepage in pores between sand grains. A groundwater flow rate of 1 foot per day (0.3 m/d)
7081-478: The interplay of the different facets of a multi-component system often requires knowledge in several diverse fields at both the experimental and theoretical levels. The following is a more traditional introduction to the methods and nomenclature of saturated subsurface hydrology. Hydrogeology, as stated above, is a branch of the earth sciences dealing with the flow of water through aquifers and other shallow porous media (typically less than 450 meters below
7178-433: The land surface). The very shallow flow of water in the subsurface (the upper 3 m) is pertinent to the fields of soil science , agriculture , and civil engineering , as well as to hydrogeology. The general flow of fluids (water, hydrocarbons , geothermal fluids, etc.) in deeper formations is also a concern of geologists, geophysicists , and petroleum geologists . Groundwater is a slow-moving, viscous fluid (with
7275-481: The left. For example, in the Barton Springs Edwards aquifer, dye traces measured the karst groundwater flow rates from 0.5 to 7 miles per day (0.8 to 11.3 km/d). The rapid groundwater flow rates make karst aquifers much more sensitive to groundwater contamination than porous aquifers. In the extreme case, groundwater may exist in underground rivers (e.g., caves underlying karst topography . If
7372-563: The main direction of flow at seepage velocity), diffusion (migration of the contaminant from high to low concentration areas), and hydrodynamic dispersion (due to microscale heterogeneities present in the porous medium and non-uniform velocity distribution relative to seepage velocity). Besides needing to understand where the groundwater is flowing, based on the other hydrologic properties discussed above, there are additional aquifer properties which affect how dissolved contaminants move with groundwater. Hydrodynamic dispersivity (α L , α T )
7469-641: The mechanical, chemical, and thermal interaction of this water with the porous solid, and the transport of energy, chemical constituents, and particulate matter by flow (Domenico and Schwartz, 1998). Groundwater engineering , another name for hydrogeology, is a branch of engineering which is concerned with groundwater movement and design of wells , pumps , and drains. The main concerns in groundwater engineering include groundwater contamination , conservation of supplies, and water quality . Wells are constructed for use in developing nations, as well as for use in developed nations in places which are not connected to
7566-462: The micro-porous (Upper Cretaceous ) Chalk Group of south east England, although having a reasonably high porosity, has a low grain-to-grain permeability, with its good water-yielding characteristics mostly due to micro-fracturing and fissuring. Karst aquifers typically develop in limestone . Surface water containing natural carbonic acid moves down into small fissures in limestone. This carbonic acid gradually dissolves limestone thereby enlarging
7663-479: The opposite direction to the groundwater gradient." Groundwater remediation technologies have been developed that can address DNAPL in some settings. Excavation is not always practicable due to the depths of the DNAPL, the dispersed nature of the residual DNAPL, mobility caused during excavation, and complexities with near-by structures. Technologies that are emerging for treatment include the following Most DNAPLs remain denser than water after they are released into
7760-416: The phreatic surface. The capillary head depends on soil pore size. In sandy soils with larger pores, the head will be less than in clay soils with very small pores. The normal capillary rise in a clayey soil is less than 1.8 m (6 ft) but can range between 0.3 and 10 m (1 and 33 ft). The capillary rise of water in a small- diameter tube involves the same physical process. The water table
7857-408: The porosity available to flow (sometimes called effective porosity ). Permeability is an expression of the connectedness of the pores. For instance, an unfractured rock unit may have a high porosity (it has many holes between its constituent grains), but a low permeability (none of the pores are connected). An example of this phenomenon is pumice , which, when in its unfractured state, can make
7954-404: The properties of aquifers. Meinzer also highlighted the importance of studying the geochemistry of water, as well as the impact of high salinity levels in aquifers. Darcy's law is a constitutive equation , empirically derived by Henry Darcy in 1856, which states that the amount of groundwater discharging through a given portion of aquifer is proportional to the cross-sectional area of flow,
8051-423: The public. Twenty-nine states require professional licensing for geologists to offer their services to the public, which often includes work within the domains of developing, managing, and/or remediating groundwater resources. For example: aquifer drawdown or overdrafting and the pumping of fossil water may be a contributing factor to sea-level rise. One of the main tasks a hydrogeologist typically performs
8148-402: The recovery of bitumen, whether by open-pit mining or by in situ methods such as steam-assisted gravity drainage (SAGD), and in some areas they are targets for waste-water injection. The Guarani Aquifer , located beneath the surface of Argentina , Brazil , Paraguay , and Uruguay , is one of the world's largest aquifer systems and is an important source of fresh water . Named after
8245-460: The remediation of DNAPL which can be costly. Removal or in situ destruction of DNAPLs eliminates the potential exposure to the compounds in the environment and can be an effective method for remediation; however, at some DNAPL sites remediation of DNAPL may not be practicable, and containment may be the only viable remedial action. The USEPA has a program to address sites where DNAPL removal is not practicable for remediation projects under CERCLA under
8342-429: The same geologic unit may be confined in one area and unconfined in another. Unconfined aquifers are sometimes also called water table or phreatic aquifers, because their upper boundary is the water table or phreatic surface (see Biscayne Aquifer ). Typically (but not always) the shallowest aquifer at a given location is unconfined, meaning it does not have a confining layer (an aquitard or aquiclude) between it and
8439-425: The source, this is a place where aquifers are often unconfined (sometimes called the forebay area), or in hydraulic communication with the land surface. An unconfined aquifer has no impermeable barrier immediately above it, such that the water level can rise in response to recharge. A confined aquifer has an overlying impermeable barrier that prevents the water level in the aquifer from rising any higher. An aquifer in
8536-432: The structure of mathematics to arrive at a simple, elegant solution, but the required derivation for all but the simplest domain geometries can be quite complex (involving non-standard coordinates , conformal mapping , etc.). Analytic solutions typically are also simply an equation that can give a quick answer based on a few basic parameters. The Theis equation is a very simple (yet still very useful) analytic solution to
8633-468: The study of the Earth ) is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust (commonly in aquifers ). The terms groundwater hydrology , geohydrology , and hydrogeology are often used interchangeably, though hydrogeology is the most commonly used. Hydrogeology is the study of the laws governing the movement of subterranean water,
8730-419: The surface are not only more likely to be used for water supply and irrigation, but are also more likely to be replenished by local rainfall. Although aquifers are sometimes characterized as "underground rivers or lakes," they are actually porous rock saturated with water. Many desert areas have limestone hills or mountains within them or close to them that can be exploited as groundwater resources. Part of
8827-420: The surface. The term "perched" refers to ground water accumulating above a low-permeability unit or strata, such as a clay layer. This term is generally used to refer to a small local area of ground water that occurs at an elevation higher than a regionally extensive aquifer. The difference between perched and unconfined aquifers is their size (perched is smaller). Confined aquifers are aquifers that are overlain by
8924-605: The time of the last glaciation . Annual recharge, in the more arid parts of the aquifer, is estimated to total only about 10 percent of annual withdrawals. According to a 2013 report by the United States Geological Survey (USGS), the depletion between 2001 and 2008, inclusive, is about 32 percent of the cumulative depletion during the entire 20th century. In the United States, the biggest users of water from aquifers include agricultural irrigation and oil and coal extraction. "Cumulative total groundwater depletion in
9021-425: The transient evolution of head due to the effects of pumping one or a number of pumping wells. The Thiem equation is a solution to the steady state groundwater flow equation (Laplace's Equation) for flow to a well. Unless there are large sources of water nearby (a river or lake), true steady-state is rarely achieved in reality. Both above equations are used in aquifer tests (pump tests). The Hooghoudt equation
9118-440: The unsaturated groundwater flow equation. Hydraulic conductivity ( K ) is a measure of permeability that is a property of both the fluid and the porous medium (i.e. the hydraulic conductivity of water and of oil will not be the same even if in the same geologic formation). Transmissivity is the product of hydraulic conductivity and the aquifer thickness (typically used as an indication of the ability of an aquifer to deliver water to
9215-443: The use of qanats leading to a well. This groundwater is a major source of fresh water for many regions, however can present a number of challenges such as overdrafting (extracting groundwater beyond the equilibrium yield of the aquifer), groundwater-related subsidence of land, and the salinization or pollution of the groundwater. Aquifers occur from near-surface to deeper than 9,000 metres (30,000 ft). Those closer to
9312-410: The water table is the surface where the pressure head is equal to atmospheric pressure (where gauge pressure = 0). Unsaturated conditions occur above the water table where the pressure head is negative (absolute pressure can never be negative, but gauge pressure can) and the water that incompletely fills the pores of the aquifer material is under suction . The water content in the unsaturated zone
9409-407: Was an American scientist who is often called the "father of modern groundwater hydrology". He standardized key terms in the field as well as determined principles regarding occurrence, movement, and discharge. He proved that the flow of water obeys Darcy's law. He also proposed the use of geophysical methods and recorders on wells, as well as suggested pumping tests to gather quantitative information on
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