- Adsorption
- The attraction and adhesion of a layer of ions from an aqueous solution to the solid
mineral surfaces with which it is in contact.
- Alluvium
- Sediments deposited by flowing rivers. Depending upon the location in the flood plain of
the river, different-sized sediments are deposited.
- Aquiclude
- A low-permeability unit that forms either the upper or lower boundary of a ground-water
flow system.
- Aquifer
- Rock or sediment in a formation, group of formations, or part of a formation that is
saturated and sufficiently permeable to transmit economic quantities of water to wells and
springs.
- Aquifer, confined
- An aquifer that is overlain by a confining bed. The confining bed has a significantly
lower hydraulic conductivity than the aquifer.
- Aquifer, perched
- A usually discontinuous region in the unsaturated zone where the sediments are locally saturated because they
overly a low-permeability unit (aquiclude).
- Aquifer, semi-confined
- An aquifer confined by a low-permeability layer that permits water to slowly flow through
it. During pumping of the aquifer, recharge to the aquifer can occur across the confining
layer. Also known as "leaky artesian" or "leaky confined" aquifer.
- Aquifer, unconfined
- An aquifer in which there are no confining beds between the zone of saturation and the
surface. There will be a water table in the unconfined aquifer. Water-table aquifer is a
synonym.
- Aquitard
- A low-permeability geologic unit that, while not an aquifer, can store ground water and
also transmit is slowly form one aquifer to another.
- Artesian well
- A well deriving its water from a confined aquifer in which the static water level in the
well stands above the top elevation of the source aquifer. Where the water level is also above land surface, the well is a flowing artesian well.
- Artificial recharge
- The process by which water can be injected or added to an aquifer. Dug basins, drilled
wells, or simply the spread of water across the land surface are all means of artificial
recharge.
- Backwash (well development)
- The surging effect or reversal of water flow in a well. Backwashing removes fine-grained
material from the formation surrounding the borehole and, thus, can enhance well yield.
- Bailer
- A device used to withdraw a water sample from a small-diameter well or piezometer, A
bailer typically is a piece of pipe attached to a wire and having a check valve in the
bottom.
- Basalt
- A general term for dark-colored iron- and magnesium-rich igneous rocks, commonly
extrusive, but locally intrusive. It is the principal rock type making up the ocean floor.
- Baseflow
- That part of stream flow resulting from ground water seeping into the stream.
- Bentonite
- A colloidal clay, largely made up of the mineral sodium montmorillonite, a hydrated
aluminum silicate.
- Bit
- The cutting tool attached to the bottom of a drill stem.
- Boring
- A hole advanced in the ground by means of a drilling rig.
- Capture zone
- Zone of aquifer contributing flow to a discharging well or wells. Capture zone geometry is
dictated by well construction, aquifer properties, the prevailing hydraulic gradient, and well
discharge.
- Casing
- A solid piece of pipe, typically steel or PVC plastic, used to keep a well open in either
unconsolidated materials or unstable rock.
- Community water system
- Public water supply serving at least 15 service connections used by year-round residents,
or regularly serves at least 25 year-round residents.
- Conceptual model
- Typically the first stage in designing a water balance or ground water model, it comprises
a set of assumptions and concepts based on the known hydraulic conditions in the aquifer. The
conceptual model defines the initial conditions, boundaries, sources, and sinks to be used in
the ground water model.
- Cone of depression
- A depression in the groundwater table or potentiometric surface that has the shape of an
inverted cone and develops around a well from which water is being withdrawn. It defined the
area of influence of a well.
- Confining layer
- A body of material of low hydraulic conductivity that is stratigraphically adjacent to one
or more aquifer. It may lie above or below the aquifer.
- Contamination source inventory
- Inventory of contaminant sources within delineated state water-protection areas that
typically warrant further investigation.
- Darcy’s law
- A derived equation for the flow of fluids on the assumption that the flow is laminar and
that inertia is neglected.
- Development
- The act of repairing damage to the formation caused by drilling procedures and increasing
the porosity and permeability of the materials surrounding the intake portion of the
well.
- Discharge
- The volume of water flowing in a stream or through an aquifer past a specific point in a
given period of time.
- Downgradient
- Hydrogeologic term referring to the direction of decreasing hydraulic head. It is also the
direction in which ground water will flow.
- Drainage basin
- The land area from which surface runoff drains into a stream system.
- Drawdown
- A lowering of the water table of an unconfined aquifer or the potentiometric surface of a
confined aquifer caused by pumping of ground water from well. (The distance between the static
water level and the surface of the cone of depression.)
- Drawdown curve
- Graphical representation showing drawdown as a function of distance form the well or time
since pumping began.
- Drilling fluid
- A water- or air-based fluid used in the water-well drilling operation to remove cuttings
from the hole, to clean and cool the bit, to reduce friction between the drill string and the
sides of the hole, and to seal the borehole.
- ESA
- The Endangered Species Act, signed into law in 1973
- Effluent
- A waste liquid discharge from a manufacturing or treatment process (treated or
untreated) that discharges into the environment.
- Equipotential line
- A contour line on the water table or potentiometric surface; a line along which the
pressure head of groundwater in an aquifer is the same. Fluid flow is normal to these lines in
the direction of decreasing fluid potential.
- Erosion
- The general process or group of processes whereby the materials of the Earth’s crust are
moved form one place to another by running water (including rainfall), waves and currents,
glacier ice, or wind.
- Evaporation
- The process by which water passes from the liquid to the vapor state.
- Evapotranspiration
- Loss of water from a land area through transpiration of plants and evaporation from the
soil.
- Fault
- A fracture or a zone of fractures along which there has been displacement of the sides
relative to one another parallel to the fracture.
- Filter pack
- Sand or gravel that is smooth, uniform, clean, well-rounded and siliceous. It is placed in
the annulus of the well between the borehole wall and the well screen to prevent formation
material from entering the screen.
- Finite-difference model
- A particular kind of a digital computer model based upon a rectangular grid that sets the
boundaries of the model and the nodes where the model will be solved.
- Finite-element model
- A digital ground-water-flow model where the aquifer is divided into a mesh formed of a
number of polygonal cells.
- Flow lines
- Lines indicating the direction followed by groundwater toward points of discharge. Flow
lines are perpendicular to equipotential lines.
- Flowing well
- Well from which water flows without pumping.
- Fluvial
- 1. Of or pertaining to a river or stream.
- 2. Occuring in a stream or river or produced by the action of a stream or river.
- Fracture trace
- The surface representation of a fracture zone. It may be a characteristic line of
vegetation or linear soil-moisture pattern or a topographic sag.
- Gamma log
- A borehole log that measures the natural gamma radiation emitted by the formation rocks.
It can be used to delineate subsurface rock types.
- Glacial drift
- A general term for unconsolidated sediment transported by glaciers and deposited directly
on land or in the sea.
- Glacial-lacustrine sediments
- Silt and clay deposits formed in the quiet water of lakes that received meltwater from
glaciers.
- Glacial outwash
- Well-sorted sand, or sand and gravel, deposited by the meltwater from a glacier.
- Glacial till
- A glacial deposit composed of mostly unsorted sand, silt, clay and boulders and laid down
directly by the melting ice.
- Glaciofluvial
- Pertaining to the meltwater streams flowing from wasting glacier ice and especially to the deposits and landforms produced by such streams.
- Ground water
- The water contained in interconnected pores located below the water table in an unconfined aquifer or located in a confined aquifer.
- Groundwater basin
- A rather vague designation pertaining to a ground-water reservoir that is more or less separate form neighboring ground-water reservoirs. A ground-water basin can be separated from adjacent basins by geologic boundaries or by hydrologic boundaries.
- Groundwater, confined
- The water contained in a confined aquifer. Pore-water pressure is greater than atmospheric
at the top of the confined aquifer.
- Groundwater divide
- The boundary between two adjacent ground-water basins. The divide is represented by a high
in the water table.
- Groundwater flow
- The movement of water through openings in sediments and rock; occurs in the zone of
saturation.
- Groundwater model
- A physical or mathematical device that represents an approximation of a natural ground
water system. The most commonly used ground water models are numerical models.
- Groundwater, perched
- The water in an isolated, saturated zone located in the zone of aeration. It is the result
of the presence of a layer of material of low hydraulic conductivity, called a perching bed. Perched ground water will have a perched water table.
- Groundwater, unconfined
- The water in an aquifer where there is a water table.
- Grout
- A fluid mixture of cement and water (neat cement) of a consistency that can be forced through a pipe and placed a required. Various additives, such as sand, bentonite, and hydrated lime, may be included in the mixture to meet certain requirements. Bentonite and water are sometimes used for grout.
- Grouting
- The operation by which grout is placed between the casing and sides of the well bore to a
predetermined height above the bottom of the well. This secures the casing in place and
excludes water and other fluids from the well bore.
- Hardness
- A property of water causing formation of an insoluble residue when the water is used with
soap. It is primarily caused by calcium and magnesium ions.
- Hard water
- Water with high concentration of calcium and magnesium.
- Head loss
- That part of head energy which is lost because of friction as water flows.
- Head, total hydraulic
- The sum of the elevation head, the pressure head, and the velocity head at a given point
in the aquifer.
- Heterogeneous
- Pertaining to a substance having different characteristics in different locations. A
synonym is nonuniform.
- Homogeneous
- Pertaining to a substance having identical characteristics everywhere. A synonym is
uniform.
- Hydraulic conductivity
- The rate of flow of water in gallons per day through a cross section of one square foot
under a unit hydraulic gradient, at the prevailing temperature (gpd/ft2). In the SI System,
the units are m3/day/m2 or m/day.
- Hydraulic gradient
- The change in total head with a change in distance in a given direction. The direction is
that which yields a maximum rate of decrease in head.
- Hydrogeology
- The study of interrelationships of geologic materials and processes with water, especially
ground water.
- Hydrogeologic
- Those factors that deal with subsurface waters and related geologic aspects of surface
waters.
- Hydrogen sulfide (H2S)
- Gas that is a product of decomposition of sulfur-containing organic compounds and oil
refining and burning.
- Hydrograph
- A graph that shows some property of ground water or surface water as a function of
time.
- Hydrologic cycle
- The circulation of water from the oceans through the atmosphere to the land and ultimately
back to the ocean.
- Hydrology
- The study of the occurrence, distribution, and chemistry of all waters of the earth.
- Infiltration
- The flow of water downward from the land surface into and through the upper soil
layers.
- Interference
- The condition occurring when the area of influence of a water well comes into contact with
or overlaps that a neighboring well, as when two wells are pumping from the same aquifer or
are located near each other.
- In Situ
- Object or substance in its original place.
- Isohyetal line
- A line drawn on a map, all points along which receive equal amounts of precipitation.
- Jacob straight-line method
- A graphical method using semilogarithmic paper and the Theis equation for evaluating the
results of a pumping test.
- Laminar flow
- Water flow in which the stream lines remain distinct and in which the flow direction at
every point remains unchanged with time. It is a characteristic of the movement of
groundwater.
- Leaky confining layer
- A low-permeability layer that can transmit water at sufficient rates to furnish some
recharge to a well pumping from an underlying aquifer. Also called aquitard.
- Lithologic log
- A record of the lithology of the rock and soil encountered in a borehole from the surface
to the bottom. Also known as a well log.
- Losing stream
- A stream or reach of a stream, the flow of which is being increased by inflow of ground
water. Also known as influent stream.
- Maximum contaminant level (MCL)
- The highest concentration of a solute permissible in a public water supply as specified in
the National Interim Primary Drinking Water Standards for the United States.
- Mean sea level
- Mathematical average height of the surface of the sea for all stages of the tide over a
19-year period. Equivalent to sea level midway between mean high water and mean low water. It
is often the most commonly accepted datum plane for the measurement of elevations and
depths.
- Milligrams per liter
- Concentration expressed as a weight of solute per unit volume of water. One mg/L is
loosely equivalent to one part per million.
- Mitigation
- Measures taken to reduce or avoid adverse anthropogenic impacts on the environment.
- Model
- A physical or mathematical device that represents an approximation of a natural
system.
- Model calibration
- The process by which the independent variables of a digital computer modal are varied in
order to calibrate a dependant variable such as a head against a known value such as a
water-table map.
- Model errors
- Difference between modeled results and the field reality due to errors stemming from
numerical approximations, computer accuracy, and mathematical approximation of the natural
system.
- Model verification
- The process by which a digital computer model that has been calibrated against a
steady-state condition is tested to see if it can generate a transient response, such as the
decline in the water table with pumping, that matches the known history of the aquifer.
- Moraine
- A mound, ridge, or other distinct accumulation of unsorted, unstratified glacial drift,
predominantly till, deposited chiefly by direct action of glacier ice.
- Natural gamma radiation log
- A borehole log that measures the natural gamma radiation emitted by the formation rocks.
It can be used to delineate subsurface rock types.
- Naturally developed well
- A well in which the screen is placed in direct contact with the aquifer materials; no
filter pack is used.
- Nominal
- Used to describe standard sizes for pipe from 1/8 in to 12 in (3.2 mm to 305mm). The
nominal size is specified on the basis of the inside diameter. Depending on the wall
thickness, the inside diameter may be less than or greater than the number indicated.
- Numerical model
- A model of ground-water flow in which the aquifer is described by numerical equations,
with specified values for boundary conditions, that are solved on a digital computer.
- Observation well
- A nonpumping well used to observe the elevation of the water table or the potentiometric
surface. An observation well is generally of larger diameter than a piezometer and typically
is screened or slotted throughout the thickness of the aquifer.
- Open hole
- Uncased well or borehole.
- Perched water
- Unconfined groundwater separated from an underlying main body of groundwater by an
unsaturated zone.
- Permeability
- The property or capacity of a porous rock, sediment, or soil for transmitting a fluid; it
is a measure of the relative ease of fluid flow under unequal pressure.
- Permeameter
- A laboratory device used to measure the intrinsic permeability and hydraulic conductivity
of a soil or rock sample.
- p.H.
- A measure of the acidity of a solution, numerically equal to 7 for neutral solutions,
increasing with increasing alkalinity and decreasing with increasing acidity. Originally stood
for the words potential of hydrogen.
- Piezometer
- A nonpumping well, generally of small diameter, that is used to measure the elevation of
the water table or potentiometric surface. A piezometer generally has a short well screen
through which water can enter.
- Piezometer nest
- A set of two or more piezometers set close to each other but screened to different
depths.
- Porosity
- The ratio of the volume of void spaces in a rock or sediment to the total volume of the
rock or sediment.
- Potentiometric surface
- A surface that represents the level to which water will rise in tightly cased wells. If
the head varies significantly with depth in the aquifer, then there may be more than one
potentiometric surface. The water table is a particular potentiometric surface for an
unconfined aquifer.
- Potentiometric surface map
- A contour map of the potentiometric surface of a particular hydrogeologic unit.
- Prior-appropriation doctrine
- A doctrine stating that the right to use water is separate from other property rights and
that the first person to withdraw and use the water holds the senior right. The doctrine has
been applied to both ground and surface water.
- Public trust doctrine
- A legal theory holding that certain lands and waters in the public domain are held in
trust for use by the entire populace. It is especially applicable to navigable waters.
- Pumping test
- A test made by pumping a well for a period of time and observing the change in hydraulic
head in the aquifer. A pumping test may be used to determine the capacity of the well and the
hydraulic characteristics of the aquifer. Also called an aquifer test.
- Radius of influence
- The radial distance from the center of a well bore to the point where there is no lowering
of water table or potentiometric surface (the edge of its cone depression).
- Recharge boundary
- An aquifer system boundary that adds water to the aquifer. Streams and lakes are typically
recharge boundaries.
- Recovery
- The rate at which the water level in a well rises after the pump has been shut off. It is
the inverse of drawdown.
- Resistivity log
- A borehole log made by lowering two current electrodes into the borehole and measuring the
resistivity between two additional electrodes. It measures the electrical resistivity of the
formation and contained fluids near the probe.
- Riparian doctrine
- A doctrine that holds that the property owner adjacent to a surface-water body had first
right to withdraw and use water.
- Rock, igneous
- A rock formed by cooling and crystallization of a molten rock mass called magma.
- Rock, metamorphic
- A rock formed by the application of heat and pressure to preexisting rocks.
- Rock, plutonic
- An igneous rock formed when magma cools and crystallizes within the earth.
- Rock, sedimentary
- A rock formed from sediments through a process known as diagenesis or formed by chemical
precipitation in water.
- Rock, volcanic
- An igneous rock formed when molten rock called lava cools on the earth’s surface.
- Safe Drinking Water Act
- Main federal ensuring the quality of Americans’ drinking water. Under SDWA, EPA sets
standards for drinking water quality and oversees the states, localities, and water suppliers
who implement those standards.
- Saline-water encroachment (saltwater intrusion)
- The movement, as a result of human activity, of saline ground water into an aquifer
formerly occupied by fresh water. Passive saline-water encroachment occurs at a slow rate
owing to a general lowering of the fresh-water potentiometric surface. Active saline-water
encroachment proceeds at a more rapid rate owing to the lowering of the fresh-water
potentiometric surface below sea level.
- Sediment
- An assemblage of individual mineral grains that were deposited by some geologic agent such
as, water, wind, ice or gravity.
- Sieve analysis
- Determination of the particle-size distribution of a soil, sediment, or rock by measuring
the percentage of the particles that will pass through standard sieves of various sizes.
- Slurry
- A thin mixture of liquid, especially water, and any of several finely divided substances,
such as cement or clay particles.
- Specific capacity
- An expression of the productivity of a well, obtained by dividing the rate of discharge of
water from the well by the drawdown of the water level in the well. Specific capacity should
be described on the basis of the number of hours of pumping prior to the time the drawdown
measurement is made. It will generally decrease with time as the drawdown increases.
- Split-spoon sample
- A sample of unconsolidated material taken by driving a sampling device ahead of the drill
bit in a boring. The split-spoon sampler is typically advanced by the repetitive dropping of a
weight.
- Spontaneous potential log
- A borehole log made by measuring the natural electrical potential that develops between
the formation and the borehole fluids.
- Static water level
- The level of water in a well that is not being affected by withdrawal of groundwater.
- Stiff diagram
- Graphical representation of major cations and anions of a water sample.
- Storativity
- The volume of water an aquifer releases from or takes into storage per unit surface area
of the aquifer per unit change in head. It is equal to the product of specific storage and
aquifer thickness. In an unconfined aquifer, the storativity is equivalent to the specific
yield. Also called storage coefficient.
- Stream, gaining
- A stream or reach of a stream, the flow of which is being increased by inflow of ground
water. Also known as effluent stream.
- Stream, losing
- A stream or reach of a stream that is losing water by seepage into the ground. Also known
as influent stream.
- Stress period
- Blocks in a transient ground water model that allow the addition, subtraction, or
alteration of parameters or stresses to a model while the simulation is in progress.
- Surface water
- Water found in ponds, lakes, inland seas, streams, and rivers.
- Terrace
- 1. Flat, long, and narrow surface bordered on one side by a steep rise and on the other
side a steep descending slope.
- 2. Horizontal embankment built along a hillside to reduce erosion or conserve moisture.
- 3. Narrow, topographically elevated strip of land adjacent to a stream or other surface
water body that has been produced by differential erosion of the stream channel and adjacent
floodplain.
- Test well
- 1. Well dug or drilled near a surface water body to determine the relation between lake
level and ground water level.
- 2. General term applied to wells installed for a specific investigative purpose.
- Theis equation
- An equation for the flow of ground water in a fully confined aquifer.
- Theis type curve
- A plot on logarithmic paper of well function W(u) as a function of 1/(u).
- Topographic divide
- The boundary between adjacent surface water boundaries. It is represented by a
topographically high area.
- Transmissivity
- The rate at which water is transmitted through a unit width of an aquifer under a unit
hydraulic gradient. Transmissivity values are given in gallons per minute through a vertical
section of an aquifer one foot wide and extending the full saturated height of an aquifer
under a hydraulic gradient of 1 in English Engineering system; in the International System,
transmissivity is given in cubic meters per day through a vertical section of an aquifer one
meter wide and extending the full saturated height of an aquifer under a hydraulic gradient of
1.
- Turbidity
- Quality of opaqueness or reduced clarity of a fluid due to the presence of suspended
matter.
- Turbulent flow
- Water flow in which the flow lines are confused and heterogeneously mixed. It is typical
of flow in surface-water bodies.
- Upgradient
- In direction of increasing hydraulic head. Also, the direction from which water has
flowed.
- Water budget
- An evaluation of all the sources of supply and the corresponding discharges with respect
to an aquifer or a drainage basin.
- Water table
- The surface in an unconfined aquifer or confining bed at which the pore water pressure is
atmospheric. It can be measured by installing shallow wells extending a few feet into the zone
of saturation and then measuring the water level in those wells.
- Water-table map
- A specific type of potentiometric-surface map for an unconfined aquifer; shows lines of
equal elevation of the water table.
- Well development
- The process whereby a well is pumped or surged to remove any fine material that may be
blocking the well screen or the aquifer outside the well screen.
- Wellhead protection area
- Area of the surface and subsurface, surrounding a well or well field supplying a public
water system, protected by a formal plan in order to prevent contaminants from reaching the
well water.
- Well point
- A screening device, equipped with a point on one end, that is meant to be driven into the
ground.
- Well screen
- A tubular device with either slots, holes, gauze, or continuous-wire wrap; used at the end
of a well casing to complete a well. The water enters the well through the well screen. (A
filtering device used to keep sediment from entering a water well.)
- Well yield
- The volume of water discharged from a well in gallons per minute or cubic meters per day.
Definitions were taken from the following sources:
Driscoll, Fletcher, 1987, Groundwater and Wells, Second Edition, Johnson Filtration
Systems, Inc. pgs. 885-892.
Fetter, C.W., 1994, Applied Hydrogeology, Third Edition, Macmillan College Publishing Company, pgs. 635-650.
Porges, Robert, and Hammer, Matthew, 2001, The Compendium of Hydrogeology, National Groundwater Association, pgs. 1-154.
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