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Hydrogeology (''hydro-'' meaning water, and ''-geology'' meaning the study of the
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
) is the area of
geology Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...
that deals with the distribution and movement of
groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...
in the
soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...
and rocks of the Earth's crust (commonly in
aquifer An aquifer is an underground layer of water-bearing material, consisting of permeability (Earth sciences), permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The s ...
s). 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, 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 Engineering is the practice of using natural science, mathematics, and the engineering design process to Problem solving#Engineering, solve problems within technology, increase efficiency and productivity, and improve Systems engineering, s ...
which is concerned with groundwater movement and design of wells, pumps, and drains. The main concerns in groundwater engineering include groundwater
contamination Contamination is the presence of a constituent, impurity, or some other undesirable element that renders something unsuitable, unfit or harmful for the physical body, natural environment, workplace, etc. Types of contamination Within the scien ...
, conservation of supplies, and
water quality Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through tr ...
.Walton, William C. (November 1990). ''Principles of Groundwater Engineering,'' p. 1. CRC Press. . Wells are constructed for use in developing nations, as well as for use in developed nations in places which are not connected to 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.


Introduction

Hydrogeology is an
interdisciplinary Interdisciplinarity or interdisciplinary studies involves the combination of multiple academic disciplines into one activity (e.g., a research project). It draws knowledge from several fields such as sociology, anthropology, psychology, economi ...
subject; it can be difficult to account fully for the
chemical A chemical substance is a unique form of matter with constant chemical composition and characteristic properties. Chemical substances may take the form of a single element or chemical compounds. If two or more chemical substances can be combin ...
, physical,
biological Biology is the scientific study of life and living organisms. It is a broad natural science that encompasses a wide range of fields and unifying principles that explain the structure, function, growth, origin, evolution, and distribution of ...
, and even
legal Law is a set of rules that are created and are law enforcement, enforceable by social or governmental institutions to regulate behavior, with its precise definition a matter of longstanding debate. It has been variously described as a Socia ...
interactions between
soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...
,
water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
,
nature Nature is an inherent character or constitution, particularly of the Ecosphere (planetary), ecosphere or the universe as a whole. In this general sense nature refers to the Scientific law, laws, elements and phenomenon, phenomena of the physic ...
, and
society A society () is a group of individuals involved in persistent social interaction or a large social group sharing the same spatial or social territory, typically subject to the same political authority and dominant cultural expectations. ...
. The study of the interaction between groundwater movement and geology can be quite complex. Groundwater does not always follow the
surface topography Surface finish, also known as surface texture or surface topography, is the nature of a surface as defined by the three characteristics of lay, surface roughness, and waviness.. It comprises the small, local deviations of a surface from the p ...
; groundwater follows
pressure gradient In hydrodynamics and hydrostatics, the pressure gradient (typically of air but more generally of any fluid) is a physical quantity that describes in which direction and at what rate the pressure increases the most rapidly around a particular locat ...
s (flow from high pressure to low), often through fractures and conduits in circuitous paths. Taking into account the interplay of the different facets of a multi-component system often requires knowledge in several diverse fields at both the
experiment An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into cause-and-effect by demonstrating what outcome occurs whe ...
al and theoretical levels. The following is a more traditional introduction to the methods and nomenclature of saturated subsurface hydrology.


Hydrogeology in relation to other fields

Hydrogeology, as stated above, is a branch of the
earth science Earth science or geoscience includes all fields of natural science related to the planet Earth. This is a branch of science dealing with the physical, chemical, and biological complex constitutions and synergistic linkages of Earth's four spheres ...
s dealing with the flow of water through the subsurface, typically porous or fractured geological material. The very shallow flow of water in the subsurface (the upper 3 m) is pertinent to the fields of
soil science Soil science is the study of soil as a natural resource on the surface of the Earth including soil formation, soil classification, classification and Soil survey, mapping; Soil physics, physical, Soil chemistry, chemical, Soil biology, biologica ...
,
agriculture Agriculture encompasses crop and livestock production, aquaculture, and forestry for food and non-food products. Agriculture was a key factor in the rise of sedentary human civilization, whereby farming of domesticated species created ...
, and
civil engineering Civil engineering is a regulation and licensure in engineering, professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including public works such as roads ...
, as well as to hydrogeology. The general flow of
fluid In physics, a fluid is a liquid, gas, or other material that may continuously motion, move and Deformation (physics), deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are M ...
s (water,
hydrocarbons In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic; their odor is usually faint, and may b ...
, geothermal fluids, etc.) in deeper formations is also a concern of geologists, geophysicists, and petroleum geologists. Groundwater is generally slow-moving; many of the empirically derived laws of groundwater flow can be alternately derived in
fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics of fluids (liquids, gases, and plasma (physics), plasmas) and the forces on them. Originally applied to water (hydromechanics), it found applications in a wide range of discipl ...
from the special case of
Stokes flow Stokes flow (named after George Gabriel Stokes), also named creeping flow or creeping motion,Kim, S. & Karrila, S. J. (2005) ''Microhydrodynamics: Principles and Selected Applications'', Dover. . is a type of fluid flow where advection, advec ...
(viscosity and
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
terms, but no inertial term). The
math Mathematics is a field of study that discovers and organizes methods, Mathematical theory, theories and theorems that are developed and Mathematical proof, proved for the needs of empirical sciences and mathematics itself. There are many ar ...
ematical relationships used to describe the flow of water through porous media are Darcy's law, the
diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
, and
Laplace Pierre-Simon, Marquis de Laplace (; ; 23 March 1749 – 5 March 1827) was a French polymath, a scholar whose work has been instrumental in the fields of physics, astronomy, mathematics, engineering, statistics, and philosophy. He summariz ...
equations, which have applications in many diverse fields. Steady groundwater flow (Laplace equation) has been simulated using
electrical Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...
,
elastic Elastic is a word often used to describe or identify certain types of elastomer, Elastic (notion), elastic used in garments or stretch fabric, stretchable fabrics. Elastic may also refer to: Alternative name * Rubber band, ring-shaped band of rub ...
, and
heat conduction Thermal conduction is the diffusion of thermal energy (heat) within one material or between materials in contact. The higher temperature object has molecules with more kinetic energy; collisions between molecules distributes this kinetic energy u ...
analogies. Transient groundwater flow is analogous to the diffusion of
heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...
in a solid, therefore some solutions to hydrological problems have been adapted from
heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
literature. Traditionally, the movement of groundwater has been studied separately from surface water,
climatology Climatology (from Greek , ''klima'', "slope"; and , '' -logia'') or climate science is the scientific study of Earth's climate, typically defined as weather conditions averaged over a period of at least 30 years. Climate concerns the atmospher ...
, and even the chemical and microbiological aspects of hydrogeology. As the field of hydrogeology has matured, the interactions between groundwater,
surface water Surface water is water located on top of land, forming terrestrial (surrounding by land on all sides) waterbodies, and may also be referred to as ''blue water'', opposed to the seawater and waterbodies like the ocean. The vast majority of surfac ...
, water chemistry, soil moisture, and even
climate Climate is the long-term weather pattern in a region, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteoro ...
have become clearer. California and Washington both require special certification of hydrogeologists to offer professional services to 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 Overdrafting is the process of extracting groundwater beyond the equilibrium yield of an aquifer. Groundwater is one of the largest sources of fresh water and is found underground. The primary cause of groundwater depletion is the excessive pum ...
and the pumping of
fossil water Fossil water, fossil groundwater, or paleowater is an ancient body of water that has been contained in some undisturbed space, typically groundwater in an aquifer, for millennia. Other types of fossil water can include subglacial lakes, such as An ...
may be a contributing factor to sea-level rise.


Subjects

One of the main tasks a hydrogeologist typically performs 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: *Can the aquifer support another subdivision? *Will the
river A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...
dry up if the farmer doubles his
irrigation Irrigation (also referred to as watering of plants) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has bee ...
? *Did the chemicals from the
dry cleaning Dry cleaning is any cleaning process for clothing and textiles using a solvent other than water. Clothes are instead soaked in a water-free liquid solvent (usually non-polar, as opposed to water which is a Solvent#Solvent classifications, polar ...
facility travel through the aquifer to my well and make me sick? *Will the plume of effluent leaving my neighbor's septic system flow to my drinking
water well A well is an excavation or structure created on the earth by digging, driving, or drilling to access liquid resources, usually water. The oldest and most common kind of well is a water well, to access groundwater in underground aquifers. The ...
? 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, a common task of the hydrogeologist is determining aquifer properties using aquifer tests. In order to further characterize
aquifers An aquifer is an underground layer of water-bearing material, consisting of permeability (Earth sciences), permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The s ...
and
aquitard An aquifer is an underground layer of water-bearing material, consisting of permeability (Earth sciences), permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The s ...
s some primary and derived physical properties are introduced below. Aquifers are broadly classified as being either confined or unconfined (
water table The water table is the upper surface of the phreatic zone or zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with groundwater, which may be fresh, saline, or brackish, depending on the loc ...
aquifers); the type of aquifer affects what properties control 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).


Aquifers

An
aquifer An aquifer is an underground layer of water-bearing material, consisting of permeability (Earth sciences), permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The s ...
A water-bearing layer of rock, or of unconsolidated sediments, that will yield water in a usable quantity to a well or spring. Aquifers can be unconfined, where the top of the aquifer is defined by the
water table The water table is the upper surface of the phreatic zone or zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with groundwater, which may be fresh, saline, or brackish, depending on the loc ...
, or confined, where the aquifer exists underneath a confining bed. There are three aspects that control the nature of aquifers:
stratigraphy Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...
,
lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...
, 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 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.Birzeit University, Groundwater Engineering. ''Groundwater potential and Discharge Areas'' http://www.hwe.org.ps/Education/Birzeit/GroundwaterEngineering/Chapter%204%20-%20Groundwater%20Potential%20and%20Discharge%20Areas.pdf


Hydraulic head

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 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 A transducer is a device that Energy transformation, converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another. Transducers are often employed at the boundaries of automation, M ...
(this value can be negative, e.g., suction, but is positive in saturated aquifers), and ''z'' can be measured relative to a surveyed datum (typically the top of the
well A well is an excavation or structure created on the earth by digging, driving, or drilling to access liquid resources, usually water. The oldest and most common kind of well is a water well, to access groundwater in underground aquifers. The ...
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 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

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 Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...
particles or within a fractured rock. Typically, the majority of groundwater (and anything dissolved in it) moves through 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 Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of extremely vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicula ...
, which, when in its unfractured state, can make 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

Water content (''θ'') 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 The vadose zone (from the Latin word for "shallow"), also termed the unsaturated zone, is the part of Earth between the land surface and the top of the phreatic zone, the position at which the groundwater (the water in the soil's pores) is at ...
hydrology, where the
hydraulic conductivity In science and engineering, hydraulic conductivity (, in SI units of meters per second), is a property of porous materials, soils and Rock (geology), rocks, that describes the ease with which a fluid (usually water) can move through the porosity, ...
is a strongly nonlinear function of water content; this complicates the solution of the unsaturated groundwater flow equation.


Hydraulic conductivity

Hydraulic conductivity (''K'') is the ease with which a
fluid In physics, a fluid is a liquid, gas, or other material that may continuously motion, move and Deformation (physics), deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are M ...
(usually water) can move through the pore space, or fracture network. 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 a well).


Specific storage and specific yield

Specific storage In the field of hydrogeology, ''storage properties'' are physical properties that characterize the capacity of an aquifer to release groundwater. These properties are storativity (S), specific storage (Ss) and specific yield (Sy). According to ' ...
(''Ss'') and its depth-integrated equivalent, storativity (''S=Ssb''), are indirect aquifer properties (they cannot be measured directly); they indicate the amount of groundwater released from storage due to a unit depressurization of a confined aquifer. They are fractions between 0 and 1. Specific yield (''Sy'') is also a ratio between 0 and 1 (''Sy'' ≤ 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 the
porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
or effective porosity is used as an upper bound to the specific yield. Typically ''Sy'' is orders of magnitude larger than ''Ss''.


Fault zone hydrogeology

Fault zone hydrogeology is the study of how brittlely deformed rocks alter fluid flows in different lithological settings, such as
clastic Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks by ...
,
igneous Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...
and
carbonate rock Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3), and Dolomite (rock), dolomite rock (also kn ...
s. Fluid movements, that can be quantified as permeability, can be facilitated or impeded due to 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 Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...
(fresh and marine waters) and
hydrocarbons In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic; their odor is usually faint, and may b ...
(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 Fracture is the appearance of a crack or complete separation of an object or material into two or more pieces under the action of stress (mechanics), stress. The fracture of a solid usually occurs due to the development of certain displacemen ...
and foliations also facilitate the groundwater mainly in hard rock terrains.


Contaminant transport properties

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 Nitrate is a polyatomic ion with the chemical formula . salt (chemistry), Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are solubility, soluble in wa ...
,
chromium Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal. Chromium ...
or
radionuclides A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess numbers of either neutrons or protons, giving it excess nuclear energy, and making it unstable. This excess energy can be used in one of three ...
) or naturally occurring (e.g.,
arsenic Arsenic is a chemical element; it has Symbol (chemistry), symbol As and atomic number 33. It is a metalloid and one of the pnictogens, and therefore shares many properties with its group 15 neighbors phosphorus and antimony. Arsenic is not ...
,
salinity Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...
), can be transported through three main mechanisms,
advection In the fields of physics, engineering, and earth sciences, advection is the transport of a substance or quantity by bulk motion of a fluid. The properties of that substance are carried with it. Generally the majority of the advected substance is a ...
(transport along the main direction of flow at seepage velocity),
diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
(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 dispersion

Hydrodynamic dispersivity (αL, αT) 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 (αL), and some will be "to the sides of" the pure advective groundwater flow, leading to a transverse dispersivity (αT). Dispersion in groundwater arises because each water "particle", passing beyond 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 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 The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying optical instruments. It is the opposite of microscopic. Overview When applied to physical phenome ...
approach (e.g., tiny beds of gravel and clay in sand aquifers); these manifest themselves as an ''apparent'' dispersivity. Because of this, α is often claimed to be dependent on the length scale of the problem — the dispersivity found for transport through 1 m3 of aquifer is different from that for transport through 1 cm3 of the same aquifer material.


Molecular diffusion

Diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
is a fundamental physical phenomenon, which
Albert Einstein Albert Einstein (14 March 187918 April 1955) was a German-born theoretical physicist who is best known for developing the theory of relativity. Einstein also made important contributions to quantum mechanics. His mass–energy equivalence f ...
characterized as
Brownian motion Brownian motion is the random motion of particles suspended in a medium (a liquid or a gas). The traditional mathematical formulation of Brownian motion is that of the Wiener process, which is often called Brownian motion, even in mathematical ...
, that describes the random thermal movement of molecules and small particles in gases and liquids. It 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 Diffusivity, mass diffusivity or diffusion coefficient is usually written as the proportionality constant between the molar flux due to molecular diffusion and the negative value of the gradient in the concentration of the species. More accurate ...
, , is typically quite small, and its effect can often be neglected (unless groundwater flow velocities are extremely low, as they are in
clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...
aquitard An aquifer is an underground layer of water-bearing material, consisting of permeability (Earth sciences), permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The s ...
s). It 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.


Retardation by adsorption

The retardation factor is another very important feature that make the motion of the contaminant to deviate from the average groundwater motion. It is analogous to the retardation factor of
chromatography In chemical analysis, chromatography is a laboratory technique for the Separation process, separation of a mixture into its components. The mixture is dissolved in a fluid solvent (gas or liquid) called the ''mobile phase'', which carries it ...
. 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 Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...
to the soil, which holds the contaminant back and does not allow it to progress until the quantity corresponding to 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.


History and development


Henry Darcy: 19th century

Henry Darcy was 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: 20th century

Oscar Edward Meinzer 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 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.


Governing equations


Darcy's law

Darcy's law is a
constitutive equation In physics and engineering, a constitutive equation or constitutive relation is a relation between two or more physical quantities (especially kinetic quantities as related to kinematic quantities) that is specific to a material or substance o ...
, empirically derived by Henry Darcy in 1856, which states that the amount of
groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...
discharging through a given portion of
aquifer An aquifer is an underground layer of water-bearing material, consisting of permeability (Earth sciences), permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The s ...
is proportional to the cross-sectional area of flow, the hydraulic gradient, and the
hydraulic conductivity In science and engineering, hydraulic conductivity (, in SI units of meters per second), is a property of porous materials, soils and Rock (geology), rocks, that describes the ease with which a fluid (usually water) can move through the porosity, ...
.


Groundwater flow equation

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 The diffusion equation is a parabolic partial differential equation. In physics, it describes the macroscopic behavior of many micro-particles in Brownian motion, resulting from the random movements and collisions of the particles (see Fick's l ...
, and has many analogs in other fields. Many solutions for groundwater flow problems were borrowed or adapted from existing
heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
solutions. It 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 A cylinder () has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infinite ...
. The Theis equation is one of the most commonly used and fundamental solutions to the groundwater flow equation; it can be used to predict 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 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.


Calculation of groundwater flow

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 In mathematics, a partial differential equation (PDE) is an equation which involves a multivariable function and one or more of its partial derivatives. The function is often thought of as an "unknown" that solves the equation, similar to ho ...
(PDE) must be solved. The most common means of analytically solving the diffusion equation in the hydrogeology literature are: *
Laplace Pierre-Simon, Marquis de Laplace (; ; 23 March 1749 – 5 March 1827) was a French polymath, a scholar whose work has been instrumental in the fields of physics, astronomy, mathematics, engineering, statistics, and philosophy. He summariz ...
, Hankel and Fourier transforms (to reduce the number of
dimension In physics and mathematics, the dimension of a mathematical space (or object) is informally defined as the minimum number of coordinates needed to specify any point within it. Thus, a line has a dimension of one (1D) because only one coo ...
s of the PDE), * similarity transform (also called the Boltzmann transform) is commonly how the Theis solution is derived, *
separation of variables In mathematics, separation of variables (also known as the Fourier method) is any of several methods for solving ordinary differential equation, ordinary and partial differential equations, in which algebra allows one to rewrite an equation so tha ...
, which is more useful for non-Cartesian coordinates, and *
Green's function In mathematics, a Green's function (or Green function) is the impulse response of an inhomogeneous linear differential operator defined on a domain with specified initial conditions or boundary conditions. This means that if L is a linear dif ...
s, which is another common method for deriving the Theis solution — from the
fundamental solution In mathematics, a fundamental solution for a linear partial differential operator is a formulation in the language of distribution theory of the older idea of a Green's function (although unlike Green's functions, fundamental solutions do not ...
to the diffusion equation in free space. 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 In the study of differential equations, a boundary-value problem is a differential equation subjected to constraints called boundary conditions. A solution to a boundary value problem is a solution to the differential equation which also satis ...
(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 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

Analytic methods typically use the structure of
mathematics Mathematics is a field of study that discovers and organizes methods, Mathematical theory, theories and theorems that are developed and Mathematical proof, proved for the needs of empirical sciences and mathematics itself. There are many ar ...
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
coordinate In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine and standardize the position of the points or other geometric elements on a manifold such as Euclidean space. The coordinates are ...
s, 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 simple (yet still very useful) analytic solution to groundwater flow and level around pumping or injecting wells, typically used to analyze the results of an aquifer test or slug test.


Numerical methods

The topic of
numerical methods Numerical analysis is the study of algorithms that use numerical approximation (as opposed to symbolic manipulations) for the problems of mathematical analysis (as distinguished from discrete mathematics). It is the study of numerical methods t ...
is quite large, obviously being of use to most fields of
engineering Engineering is the practice of using natural science, mathematics, and the engineering design process to Problem solving#Engineering, solve problems within technology, increase efficiency and productivity, and improve Systems engineering, s ...
and
science Science is a systematic discipline that builds and organises knowledge in the form of testable hypotheses and predictions about the universe. Modern science is typically divided into twoor threemajor branches: the natural sciences, which stu ...
in general. Numerical methods have been around much longer than
computer A computer is a machine that can be Computer programming, programmed to automatically Execution (computing), carry out sequences of arithmetic or logical operations (''computation''). Modern digital electronic computers can perform generic set ...
s have (In the 1920s Richardson developed some of the
finite difference A finite difference is a mathematical expression of the form . Finite differences (or the associated difference quotients) are often used as approximations of derivatives, such as in numerical differentiation. The difference operator, commonly d ...
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 the availability of fast and cheap
personal computer A personal computer, commonly referred to as PC or computer, is a computer designed for individual use. It is typically used for tasks such as Word processor, word processing, web browser, internet browsing, email, multimedia playback, and PC ...
s. 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 A finite difference is a mathematical expression of the form . Finite differences (or the associated difference quotients) are often used as approximations of derivatives, such as in numerical differentiation. The difference operator, commonly d ...
method and
finite element method Finite element method (FEM) is a popular method for numerically solving differential equations arising in engineering and mathematical modeling. Typical problem areas of interest include the traditional fields of structural analysis, heat tran ...
(FEM) 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.


General properties of gridded methods

Gridded Methods like
finite difference A finite difference is a mathematical expression of the form . Finite differences (or the associated difference quotients) are often used as approximations of derivatives, such as in numerical differentiation. The difference operator, commonly d ...
and finite element methods solve the groundwater flow equation by breaking the problem area (domain) into many small elements (squares, rectangles, triangles, blocks,
tetrahedra In geometry, a tetrahedron (: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular Face (geometry), faces, six straight Edge (geometry), edges, and four vertex (geometry), vertices. The tet ...
, 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 In physics and chemistry, the law of conservation of mass or principle of mass conservation states that for any system closed to all transfers of matter the mass of the system must remain constant over time. The law implies that mass can neith ...
across the boundaries between the elements (similar to the
divergence theorem In vector calculus, the divergence theorem, also known as Gauss's theorem or Ostrogradsky's theorem, reprinted in is a theorem relating the '' flux'' of a vector field through a closed surface to the ''divergence'' of the field in the volume ...
). 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 a
Taylor series In mathematics, the Taylor series or Taylor expansion of a function is an infinite sum of terms that are expressed in terms of the function's derivatives at a single point. For most common functions, the function and the sum of its Taylor ser ...
). For example, the first-order time derivative is often approximated using the following forward finite difference, where the subscripts indicate a discrete time location, : \frac = h'(t_i) \approx \frac. The forward finite difference approximation is unconditionally stable, but leads to an implicit set of equations (that must be solved using matrix methods, e.g. LU or
Cholesky decomposition In linear algebra, the Cholesky decomposition or Cholesky factorization (pronounced ) is a decomposition of a Hermitian, positive-definite matrix into the product of a lower triangular matrix and its conjugate transpose, which is useful for eff ...
). The similar backwards difference is only conditionally stable, but it is explicit and can be used to "march" forward in the time direction, solving one grid node at a time (or possibly in parallel, since one node depends only on its immediate neighbors). Rather than the finite difference method, sometimes the Galerkin FEM approximation is used in space (this is different from the type of FEM often used in
structural engineering Structural engineering is a sub-discipline of civil engineering in which structural engineers are trained to design the 'bones and joints' that create the form and shape of human-made Structure#Load-bearing, structures. Structural engineers also ...
) with finite differences still used in time.


Application of finite difference models

MODFLOW is a well-known example of a general finite difference groundwater flow model. It is developed by the
US Geological Survey The United States Geological Survey (USGS), founded as the Geological Survey, is an agency of the U.S. Department of the Interior whose work spans the disciplines of biology, geography, geology, and hydrology. The agency was founded on March ...
as a modular and extensible simulation tool for modeling groundwater flow. It is
free software Free software, libre software, libreware sometimes known as freedom-respecting software is computer software distributed open-source license, under terms that allow users to run the software for any purpose as well as to study, change, distribut ...
developed, documented and distributed by the USGS. Many commercial products have grown up around it, providing
graphical user interface A graphical user interface, or GUI, is a form of user interface that allows user (computing), users to human–computer interaction, interact with electronic devices through Graphics, graphical icon (computing), icons and visual indicators such ...
s to its input file based interface, and typically incorporating pre- and post-processing of user data. Many other models have been developed to work with MODFLOW input and output, making linked models which simulate several hydrologic processes possible (flow and transport models,
surface water Surface water is water located on top of land, forming terrestrial (surrounding by land on all sides) waterbodies, and may also be referred to as ''blue water'', opposed to the seawater and waterbodies like the ocean. The vast majority of surfac ...
and
groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...
models and chemical reaction models), because of the simple, well documented nature of MODFLOW.


Application of finite element models

Finite Element programs are more flexible in design (triangular elements vs. the block elements most finite difference models use) and there are some programs available (SUTRA, a 2D or 3D density-dependent flow model by the USGS;
Hydrus Hydrus is a small constellation in the deep Southern Celestial Hemisphere, southern sky. It was one of twelve constellations created by Petrus Plancius from the observations of Pieter Dirkszoon Keyser and Frederick de Houtman and it first ap ...
, a commercial unsaturated flow model; FEFLOW, a commercial modelling environment for subsurface flow, solute and heat transport processes; OpenGeoSys, a scientific open-source project for thermo-hydro-mechanical-chemical (THMC) processes in porous and fractured media; COMSOL Multiphysics (a commercial general modelling environment), FEATool Multiphysics an easy to use MATLAB simulation toolbox, and Integrated Water Flow Model (IWFM), but they are still not as popular in with practicing hydrogeologists as MODFLOW is. Finite element models are more popular in
university A university () is an educational institution, institution of tertiary education and research which awards academic degrees in several Discipline (academia), academic disciplines. ''University'' is derived from the Latin phrase , which roughly ...
and
laboratory A laboratory (; ; colloquially lab) is a facility that provides controlled conditions in which scientific or technological research, experiments, and measurement may be performed. Laboratories are found in a variety of settings such as schools ...
environments, where specialized models solve non-standard forms of the flow equation ( unsaturated flow,
density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...
dependent flow, coupled
heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...
and groundwater flow, etc.).


Application of finite volume models

The finite volume method is a method for representing and evaluating partial differential equations as algebraic equations. Similar to the finite difference method, values are calculated at discrete places on a meshed geometry. "Finite volume" refers to the small volume surrounding each node point on a mesh. In the finite volume method, volume integrals in a partial differential equation that contain a divergence term are converted to surface integrals, using the divergence theorem. These terms are then evaluated as fluxes at the surfaces of each finite volume. Because the flux entering a given volume is identical to that leaving the adjacent volume, these methods are conservative. Another advantage of the finite volume method is that it is easily formulated to allow for unstructured meshes. The method is used in many computational fluid dynamics packages. PORFLOW software package is a comprehensive mathematical model for simulation of Ground Water Flow and Nuclear Waste Management developed by Analytic & Computational Research, Inc., ACRi. The FEHM software package is available free from
Los Alamos National Laboratory Los Alamos National Laboratory (often shortened as Los Alamos and LANL) is one of the sixteen research and development Laboratory, laboratories of the United States Department of Energy National Laboratories, United States Department of Energy ...
. This versatile porous flow simulator includes capabilities to model multiphase, thermal, stress, and multicomponent reactive chemistry. Current work using this code includes simulation of methane hydrate formation, CO2 sequestration, oil shale extraction, migration of both nuclear and chemical contaminants, environmental isotope migration in the unsaturated zone, and
karst Karst () is a topography formed from the dissolution of soluble carbonate rocks such as limestone and Dolomite (rock), dolomite. It is characterized by features like poljes above and drainage systems with sinkholes and caves underground. Ther ...
formation.


Other methods

These include mesh-free methods like the Analytic Element Method (AEM) and the Boundary Element Method (BEM), which are closer to analytic solutions, but they do approximate the groundwater flow equation in some way. The BEM and AEM exactly solve the groundwater flow equation (perfect mass balance), while approximating the boundary conditions. These methods are more exact and can be much more elegant solutions (like analytic methods are), but have not seen as widespread use outside academic and research groups yet.


Water wells

A
water well A well is an excavation or structure created on the earth by digging, driving, or drilling to access liquid resources, usually water. The oldest and most common kind of well is a water well, to access groundwater in underground aquifers. The ...
is a mechanism for bringing groundwater to the surface by drilling or digging and bringing it up to the surface with a pump or by hand using buckets or similar devices. The first historical instance of water wells was in the 52nd century BC in modern-day
Austria Austria, formally the Republic of Austria, is a landlocked country in Central Europe, lying in the Eastern Alps. It is a federation of nine Federal states of Austria, states, of which the capital Vienna is the List of largest cities in Aust ...
. Today, wells are used all over the world, from developing nations to suburbs in the United States. There are three main types of wells, shallow, deep, and artesian. Shallow wells tap into unconfined aquifers, and are, generally, shallow, less than 15 meters deep. Shallow wells have a small diameter, usually less than 15 centimeters. Deep wells access confined aquifers, and are always drilled by machine. All deep wells bring water to the surface using mechanical pumps. In artesian wells, water flows naturally without the use of a pump or some other mechanical device. This is due to the top of the well being located below the water table.


Water well design and construction

One of the most important aspects of groundwater engineering and hydrogeology is water well design and construction. Proper well design and construction are important to maintain the health of the groundwater and the people which will use the well. Factors which must be considered in well design are: * A reliable aquifer, providing a continuous water supply * The quality of the accessible groundwater * How to monitor the well * Operating costs of the well * Expected yield of the well * Any prior drilling into the aquifer There are five main areas to be considered when planning and constructing a new water well, along with the factors above. They are: * Aquifer Suitability * "Well Design Considerations * Well Drilling Methods * Well Screen Design and Development * Well Testing"Matlock, Dan. 'Fundamentals of Water Well Design, Construction and Testing.' Pacific Groundwater Group. Aquifer suitability starts with determining possible locations for the well using "
USGS The United States Geological Survey (USGS), founded as the Geological Survey, is an government agency, agency of the United States Department of the Interior, U.S. Department of the Interior whose work spans the disciplines of biology, geograp ...
reports, well logs, and cross sections" of the aquifer. This information should be used to determine aquifer properties such as depth, thickness, transmissivity, and well yield. In this stage, the quality of the water in the aquifer should also be determined, and screening should occur to check for contaminants. After factors such as depth and well yield are determined, the well design and drilling approach must be established. Drilling method is selected based on "soil conditions, well depth, design, and costs." At this stage, cost estimates are prepared, and plans are adjusted to meet budgetary needs. Important parts of a well include the well seals, casings or liners, drive shoes, well screen assemblies, and a sand or gravel pack (optional). Each of these components ensures that the well only draws from one aquifer, and no leakage occurs at any stage of the process. There are several methods of drilling which can be used when constructing a water well. They include: "Cable tool, Air rotary, Mud rotary, and Flooded reverse circulation dual rotary" drilling techniques. Cable tool drilling is inexpensive and can be used for all types of wells, but the alignment must be constantly checked and it has a slow advance rate. It is not an effective drilling technique for consolidated formations, but does provide a small drilling footprint. Air rotary drilling is cost effective and works well for consolidated formations. It has a fast advance rate, but is not adequate for large diameter wells. Mud rotary drilling is especially cost effective for deep wells. It maintains good alignment, but requires a larger footprint. It has a very fast advance rate. Flooded reverse circulation dual rotary drilling is more expensive, but good for large well designs. It is versatile and maintains alignment. It has a fast advance rate. Well screens ensure that only water makes it to the surface, and sediments remain beneath the Earth's surface. Screens are placed along the shaft of the well to filter out sediment as water is pumped towards the surface. Screen design can be impacted by the nature of the soil, and natural pack designs can be used to maximize efficiency. After construction of the well, testing must be done to assess productivity, efficiency and yield of the well, as well as determine the impacts of the well on the aquifer. Several different tests should be completed on the well in order to test all relevant qualities of the well.


Issues in groundwater engineering and hydrogeology


Contamination

Groundwater contamination happens when other fluids seep into the aquifer and mix with existing groundwater. Pesticides, fertilizers, and gasoline are common contaminants of aquifers. Underground storage tanks for chemicals such as gasoline are especially concerning sources of groundwater contamination. As these tanks corrode, they can leak, and their contents can contaminate nearby groundwater. For buildings which are not connected to a
wastewater treatment Wastewater treatment is a process which removes and eliminates contaminants from wastewater. It thus converts it into an effluent that can be returned to the water cycle. Once back in the water cycle, the effluent creates an acceptable impact on ...
system, septic tanks can be used to dispose of waste at a safe rate. If septic tanks are not built or maintained properly, they can leak bacteria, viruses and other chemicals into the surrounding groundwater. Landfills are another potential source of groundwater contamination. As trash is buried, harmful chemicals can migrate from the garbage and into the surrounding groundwater if the protective base layer is cracked or otherwise damaged. Other chemicals, such as road salts and chemicals used on lawns and farms, can runoff into local reservoirs, and eventually into aquifers. As water goes through the water cycle, contaminants in the atmosphere can contaminate the water. This water can also make its way into groundwater.


Controversy


Fracking

Contamination of groundwater due to fracking has long been debated. Since chemicals commonly used in
hydraulic fracturing Fracking (also known as hydraulic fracturing, fracing, hydrofracturing, or hydrofracking) is a well stimulation technique involving the fracturing of Formation (geology), formations in bedrock by a pressurized liquid. The process involves the ...
are not tested by government agencies responsible for determining the effects of fracking on groundwater, laboratories at the
United States Environmental Protection Agency The Environmental Protection Agency (EPA) is an independent agency of the United States government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it began operation on De ...
, or EPA, have a hard time determining if chemicals used in fracking are present in nearby aquifers. In 2016, the EPA released a report which states that drinking water can be contaminated by fracking. This was a reversal of their previous policies after a $29 million study into the effects of fracking on local drinking water.


California

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...
sees some of the largest controversies in groundwater usage due to the dry conditions California faces, high population, and intensive agriculture. Conflicts generally occur over pumping groundwater and shipping it out of the area, unfair use of water by a commercial company, and contamination of groundwater by development projects. In Siskiyou County in northern California, the California Superior Court ruled poor groundwater regulations have allowed pumping to diminish the flows in the Scott River and disturbed the natural habitat of salmon. In
Owens Valley Owens Valley (Mono language (California), Mono: ''Payahǖǖnadǖ'', meaning "place of flowing water") is an arid valley of the Owens River in eastern California in the United States. It is located to the east of the Sierra Nevada (U.S.), Sierra ...
in central California, groundwater was pumped for use in fish farms, which resulted in the death of local meadows and other ecosystems. This resulted in a lawsuit and settlement against the fish companies. Development in southern California is threatening local aquifers, contaminating groundwater through construction and normal human activity. For example, a solar project in
San Bernardino County San Bernardino County ( ), officially the County of San Bernardino and sometimes abbreviated as S.B. County, is a county located in the southern portion of the U.S. state of California, and is located within the Inland Empire area. As of th ...
would allegedly threaten the ecosystem of bird and wildlife species because of its use of up to 1.3 million cubic meters of groundwater, which could impact Harper Lake."Groundwater, Rivers, Ecosystems, and Conflicts" http://waterinthewest.stanford.edu/groundwater/conflicts/index.html In September 2014, California passed the Sustainable Groundwater Management Act, which requires users to manage groundwater appropriately, as it is connected to surface water systems.


Colorado

Due to its arid climate, the state of
Colorado Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...
gets most of its water from underground. Because of this, there have been issues regarding groundwater engineering practices. As many as 65,000 people were affected when high levels of PFCs were found in the Widefield Aquifer. Groundwater use in Colorado dates back to before the 20th century. Nineteen of Colorado's 63 counties depend mostly on groundwater for supplies and domestic uses. The Colorado Geological Survey has three significant reports on groundwater in the Denver Basin. The first report Geology of Upper Cretaceous, Paleocene and Eocene Strata in the Southwestern Denver Basin, The second report Bedrock Geology, Structure, and Isopach Maps of the Upper Cretaceous to Paleogene Strata between Greeley and Colorado Springs, The third publication Cross Sections of the Freshwater Bearing Strata of the Denver Basin between Greeley and Colorado Springs.


New trends in groundwater engineering/hydrogeology

Since the first wells were made thousands of years ago, groundwater systems have been changed by human activity. 50 years ago, the sustainability of these systems on a larger scale began to come into consideration, becoming one of the main focuses of groundwater engineering. New ideas and research are advancing groundwater engineering into the 21st century, while still considering groundwater conservation.


Topographical mapping

New advancements have arisen in topographical mapping to improve sustainability. Topographic mapping has been updated to include radar, which can penetrate the ground to help pinpoint areas of concern. In addition, large computations can use gathered data from maps to further the knowledge of groundwater aquifers in recent years. This has made highly complex and individualized water cycle models possible, which has helped to make groundwater sustainability more applicable to specific situations.


The role of technology

Technological improvements have advanced topographical mapping, and have also improved the quality of lithosphere, hydrosphere, biosphere, and atmosphere simulations. These simulations are useful on their own; however, when used together, they help to give an even more accurate prediction of the future sustainability of an area, and what changes can be made to ensure stability in the area. This would not be possible without the advancement of technology. As technology continues to progress, the simulations will increase in accuracy and allow for more complex studies and projects in groundwater engineering.


Growing populations

As populations continue to grow, areas which were using groundwater at a sustainable rate are now beginning to face sustainability issues for the future. Populations of the size currently seen in large cities were not taken into consideration when the long term sustainability of aquifers. These large population sizes are beginning to stress groundwater supply. This has led to the need for new policies in some urban areas. These are known as proactive land-use management, where cities can move proactively to conserve groundwater. In Brazil, overpopulation caused municipally provided water to run low. Due to the shortage of water, people began to drill wells within the range normally served by the municipal water system. This was a solution for people in high socioeconomic standing, but left much of the underprivileged population without access to water. Because of this, a new municipal policy was created which drilled wells to assist those who could not afford to drill wells of their own. Because the city is in charge of drilling the new wells, they can better plan for the future sustainability of the groundwater in the region, by carefully placing the wells and taking growing populations into consideration.


Dependency on groundwater in the United States

In the
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
, 51% of the drinking water comes from groundwater supplies. Around 99% of the rural population depends on groundwater. In addition, 64% of the total groundwater of the country is used for irrigation, and some of it is used for industrial processes and recharge for lakes and rivers. In 2010, 22 percent of freshwater used in US came from groundwater and the other 78 percent came from surface water. Groundwater is important for some states that don't have access to fresh water. most of the fresh groundwater 65 percent is used for irrigation and the 21 percent is used for public purposes drinking mostly.Groundwater Foundation (2018). ''What is groundwater?'' http://www.groundwater.org/get-informed/basics/groundwater.html Perlman, Howard, and USGS. “Groundwater Use in the United States.” Groundwater Use, the USGS Water Science School, water.usgs.gov/edu/wugw.html.


See also

*
Environmental engineering Environmental engineering is a professional engineering Academic discipline, discipline related to environmental science. It encompasses broad Science, scientific topics like chemistry, biology, ecology, geology, hydraulics, hydrology, microbiolo ...
is a broad category hydrogeology fits into; * Flownet is an analysis tool for steady-state flow; * Groundwater energy balance: groundwater flow equations based on the energy balance; * Fault zone hydrogeology: field specifically analyzing hydrogeology in fault zones * Hydrogeophysics: field integrating hydrogeology with geophysics * Hydrology (agriculture) *
Isotope hydrology Isotope hydrology is a field of geochemistry and hydrology that uses naturally occurring stable and radioactive isotopic techniques to evaluate the age and origins of surface and groundwater and the processes within the atmospheric hydrologic cyc ...
is often used to understand sources and travel times in groundwater systems; * Oscar Edward Meinzer is considered the "father of modern groundwater hydrology"; * SahysMod is a spatial agro-hydro-salinity model with groundwater flow in a polygonal network; *
Spring (hydrology) A spring is a natural exit point at which groundwater emerges from an aquifer and flows across the ground surface as surface water. It is a component of the hydrosphere, as well as a part of the water cycle. Springs have long been important f ...
and
water supply network A water supply network or water supply system is a system of engineered hydrologic and hydraulic components that provide water supply. A water supply system typically includes the following: # A drainage basin (see water purification – sour ...
are subjects the hydrogeologist is concerned about; *
Water cycle The water cycle (or hydrologic cycle or hydrological cycle) is a biogeochemical cycle that involves the continuous movement of water on, above and below the surface of the Earth across different reservoirs. The mass of water on Earth remains fai ...
,
hydrosphere The hydrosphere () is the combined mass of water found on, under, and above the Planetary surface, surface of a planet, minor planet, or natural satellite. Although Earth's hydrosphere has been around for about 4 billion years, it continues to ch ...
and
water resources Water resources are natural resources of water that are potentially useful for humans, for example as a source of drinking water supply or irrigation water. These resources can be either Fresh water, freshwater from natural sources, or water produ ...
are larger concepts which hydrogeology is a part of * Coastal hydrogeology


References


Further reading


General hydrogeology

* Domenico, P.A. & Schwartz, W., 1998. ''Physical and Chemical Hydrogeology'' Second Edition, Wiley. — Good book for consultants, it has many real-world examples and covers additional topics (e.g. heat flow, multi-phase and unsaturated flow). * Driscoll, Fletcher, 1986. ''Groundwater and Wells'', US Filter / Johnson Screens. — Practical book illustrating the actual process of drilling, developing and utilizing water wells, but it is a trade book, so some of the material is slanted towards the products made by Johnson Well Screens. * Freeze, R.A. & Cherry, J.A., 1979. ''Groundwater'', Prentice-Hall. — A classic text; like an older version of Domenico and Schwartz. * de Marsily, G., 1986. ''Quantitative Hydrogeology: Groundwater Hydrology for Engineers'', Academic Press, Inc., Orlando Florida. — Classic book intended for engineers with mathematical background but it can be read by hydrologists and geologists as well. * Good, accessible overview of hydrogeological processes. * Porges, Robert E. & Hammer, Matthew J., 2001. ''The Compendium of Hydrogeology'', National Ground Water Association, . Written by practicing hydrogeologists, this inclusive handbook provides a concise, easy-to-use reference for hydrologic terms, equations, pertinent physical parameters, and acronyms * Todd, David Keith, 1980. ''Groundwater Hydrology'' Second Edition, John Wiley & Sons. — Case studies and real-world problems with examples. * Fetter, C.W. ''Contaminant Hydrogeology'' Second Edition, Prentice Hall. * Fetter, C.W. ''Applied Hydrogeology'' Fourth Edition, Prentice Hall.


Numerical groundwater modeling

* Anderson, Mary P. & Woessner, William W., 1992 ''Applied Groundwater Modeling'', Academic Press. — An introduction to groundwater modeling, a little bit old, but the methods are still very applicable. * Anderson, Mary P., Woessner, William W., & Hunt, Randall J., 2015, ''Applied Groundwater Modeling, 2nd Edition'', Academic Press. — Updates the 1st edition with new examples, new material with respect to model calibration and uncertainty, and online Python scripts (https://github.com/Applied-Groundwater-Modeling-2nd-Ed). * Chiang, W.-H., Kinzelbach, W., Rausch, R. (1998): Aquifer Simulation Model for WINdows – Groundwater flow and transport modeling, an integrated program. - 137 p., 115 fig., 2 tab., 1 CD-ROM; Berlin, Stuttgart (Borntraeger). * Elango, L and Jayakumar, R (Eds.)(2001) Modelling in Hydrogeology, UNESCO-IHP Publication, Allied Publ., Chennai, * Rausch, R., Schäfer W., Therrien, R., Wagner, C., 2005 ''Solute Transport Modelling – An Introduction to Models and Solution Strategies''. - 205 p., 66 fig., 11 tab.; Berlin, Stuttgart (Borntraeger). * Rushton, K.R., 2003, ''Groundwater Hydrology: Conceptual and Computational Models''. John Wiley and Sons Ltd. * Wang H. F., Theory of Linear Poroelasticity with Applications to Geomechanics and Hydrogeology, Princeton Press, (2000). * Waltham T., Foundations of Engineering Geology, 2nd Edition, Taylor & Francis, (2001). * Zheng, C., and Bennett, G.D., 2002, ''Applied Contaminant Transport Modeling'' Second Edition, John Wiley & Sons.


Analytic groundwater modeling

* Haitjema, Henk M., 1995. ''Analytic Element Modeling of Groundwater Flow'', Academic Press. — An introduction to analytic solution methods, especially the Analytic element method (AEM). * Harr, Milton E., 1962. ''Groundwater and seepage'', Dover. — a more
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view on groundwater; includes a great deal on flownets. * Kovacs, Gyorgy, 1981. ''Seepage Hydaulics'', Developments in Water Science; 10. Elsevier. -
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ping well explained. , (series) * Lee, Tien-Chang, 1999. ''Applied Mathematics in Hydrogeology'', CRC Press. — Great explanation of mathematical methods used in deriving solutions to hydrogeology problems (solute transport, finite element and inverse problems too). * Liggett, James A. & Liu, Phillip .L-F., 1983. ''The Boundary Integral Equation Method for Porous Media Flow'', George Allen and Unwin, London. — Book on BIEM (sometimes called BEM) with examples, it makes a good introduction to the method. *


External links


International Association of Hydrogeologists
— worldwide association for groundwater specialists.
UK Groundwater Forum
— Groundwater in the UK
Centre for Groundwater Studies
— Groundwater Education and Research.
EPA drinking water standards
— the maximum contaminant levels (mcl) for dissolved species in US drinking water.
US Geological Survey water resources homepage
— a good place to find free data (for both US surface water and groundwater) and free groundwater modeling software like MODFLOW.
US Geological Survey TWRI index
— a series of instructional manuals covering common procedures in hydrogeology. They are freely available online as PDF files.
International Ground Water Modeling Center (IGWMC)
— an educational repository of groundwater modeling software which offers support for most software, some of which is free.
The Hydrogeologist Time Capsule
— a video collection of interviews of eminent hydrogeologists who have made a material difference to the profession.
IGRAC International Groundwater Resources Assessment Centre US Army Geospatial Center
— For information on OCONUS surface water and groundwater. {{Authority control