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hydrogeology Hydrogeology (''hydro-'' meaning water, and ''-geology'' meaning the study of the Earth) is the area of geology that deals with the distribution and movement of groundwater in the soil and rock (geology), rocks of the Earth's crust (ge ...
, an aquifer test (or a pumping test) is conducted to evaluate 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 ...
by "stimulating" the aquifer through constant
pump A pump is a device that moves fluids (liquids or gases), or sometimes Slurry, slurries, by mechanical action, typically converted from electrical energy into hydraulic or pneumatic energy. Mechanical pumps serve in a wide range of application ...
ing, and observing the aquifer's "response" ( drawdown) in observation wells. Aquifer testing is a common tool that hydrogeologists use to characterize a system of aquifers,
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 and flow system boundaries. A slug test is a variation on the typical aquifer test where an instantaneous change (increase or decrease) is made, and the effects are observed in the same well. This is often used in
geotechnical engineering Geotechnical engineering, also known as geotechnics, is the branch of civil engineering concerned with the engineering behavior of earth materials. It uses the principles of soil mechanics and rock mechanics to solve its engineering problems. I ...
settings to get a quick estimate (minutes instead of days) of the aquifer properties immediately around the well. Aquifer tests are typically interpreted by using an analytical model of aquifer flow (the most fundamental being the Theis solution) to match the data observed in the real world, then assuming that the parameters from the idealized model apply to the real-world aquifer. In more complex cases, a numerical model may be used to analyze the results of an aquifer test. Aquifer testing differs from well testing in that the behaviour of the well is primarily of concern in the latter, while the characteristics of the aquifer are quantified in the former. Aquifer testing also often utilizes one or more monitoring wells, or piezometers ("point" observation wells). A monitoring well is simply a well which is not being pumped (but is used to monitor the
hydraulic head Hydraulic head or piezometric head is a measurement related to liquid pressure (normalized by specific weight) and the liquid elevation above a vertical datum., 410 pages. See pp. 43–44., 650 pages. See p. 22, eq.3.2a. It is usually meas ...
in the aquifer). Typically monitoring and pumping wells are screened across the same aquifers.


General characteristics

Most commonly an aquifer test is conducted by pumping water from one well at a steady rate and for at least one day, while carefully measuring the water levels in the monitoring wells. When water is pumped from the pumping well the pressure in the aquifer that feeds that well declines. This decline in pressure will show up as drawdown (change in hydraulic head) in an observation well. Drawdown decreases with radial distance from the pumping well and drawdown increases with the length of time that the pumping continues. The aquifer characteristics which are evaluated by most aquifer tests are: *
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, ...
The rate of flow of water through a unit cross sectional area of an aquifer, at a unit hydraulic gradient. In US units the rate of flow is in gallons per day per square foot of cross sectional area; in SI units hydraulic conductivity is usually quoted in m3 per day per m2. Units are frequently shortened to metres per day or equivalent. *
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 ' ...
or storativity: a measure of the amount of water a confined aquifer will give up for a certain change in head; * Transmissivity The rate at which water is transmitted through whole thickness and unit width of an aquifer under a unit hydraulic gradient. It is equal to the hydraulic conductivity times the thickness of an aquifer; Additional aquifer characteristics which are sometimes evaluated, depending on the type of aquifer, include: *
Specific yield 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 '' ...
or drainable porosity: a measure of the amount of water an unconfined aquifer will give up when completely drained; * Leakage coefficient: some aquifers are bounded by aquitards which slowly give up water to the aquifer, providing additional water to reduce drawdown; * The presence of aquifer boundaries (recharge or no-flow) and their distance from the pumped well and piezometers.


Analysis methods

An appropriate model or solution to the groundwater flow equation must be chosen to fit to the observed data. There are many different choices of models, depending on what factors are deemed important including: *leaky aquitards, *unconfined flow (delayed yield), *partial penetration of the pumping and monitoring wells, *finite wellbore radius — which can lead to wellbore storage, *dual
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 ...
(typically in fractured rock), *
anisotropic Anisotropy () is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example, many materials exhibit ver ...
aquifers, *
heterogeneous Homogeneity and heterogeneity are concepts relating to the uniformity of a substance, process or image. A homogeneous feature is uniform in composition or character (i.e., color, shape, size, weight, height, distribution, texture, language, i ...
aquifers, *finite aquifers (the effects of physical boundaries are seen in the test), and *combinations of the above situations. Nearly all aquifer test solution methods are based on the Theis solution; it is built upon the most simplifying assumptions. Other methods relax one or more of the assumptions the Theis solution is built on, and therefore they get a more flexible (and more complex) result.


Transient Theis solution

The Theis equation was created by Charles Vernon Theis (working for 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 ...
) in 1935, 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 (with the mathematical help of C.I. Lubin), for two-dimensional radial flow to a point
sink A sink (also known as ''basin'' in the UK) is a bowl-shaped plumbing fixture for washing hands, dishwashing, and other purposes. Sinks have a tap (faucet) that supplies hot and cold water and may include a spray feature to be used for fas ...
in an infinite, homogeneous
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 ...
. It is simply :\begin s &= \fracW(u) \\ .5emu &= \frac \end where ''s'' is the drawdown (change in hydraulic head at a point since the beginning of the test in units of distance), ''u'' is a dimensionless parameter, ''Q'' is the discharge (pumping) rate 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 ...
(volume per unit time), ''T'' and ''S'' are the transmissivity and
storativity 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 '' ...
of the aquifer around the well (distance squared per unit time and dimensionless, respectively), ''r'' is the distance from the pumping well to the point where the drawdown was observed, ''t'' is the time since pumping began, and ''W(u)'' is the "Well function" (called the
incomplete gamma function In mathematics, the upper and lower incomplete gamma functions are types of special functions which arise as solutions to various mathematical problems such as certain integrals. Their respective names stem from their integral definitions, whic ...
, \Gamma(0,u), in non-hydrogeology literature). The well function is given by the infinite series :\begin W(u) = \Gamma(0,u) = -\gamma - \ln(u) + u - \frac + \frac - \frac + \cdots \end where ''γ'' is the Euler constant (=0.577216...). Typically this equation is used to find the average ''T'' and ''S'' values near a pumping
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 ...
, from drawdown data collected during an aquifer test. This is a simple form of inverse modeling, since the result (''s'') is measured in the well, ''r'', ''t'', and ''Q'' are observed, and values of ''T'' and ''S'' which best reproduce the measured data are put into the equation until a best fit between the observed data and the analytic solution is found. The Theis solution is based on the following assumptions: * The flow in the
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 adequately described by Darcy's law (i.e. Re<10). * homogeneous, isotropic, confined
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 ...
, *
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 fully penetrating (open to the entire thickness (''b'') of aquifer), * the well has zero radius (it is approximated as a vertical line) — therefore no water can be stored in the well, * the well has a constant pumping rate Q, * the head loss over the well screen is negligible, * aquifer is infinite in radial extent, * horizontal (not sloping), flat, impermeable (non-leaky) top and bottom boundaries of aquifer, * groundwater flow is horizontal * no other wells or long term changes in regional water levels (all changes in potentiometric surface are the result of the pumping well alone) Even though these assumptions are rarely all met, depending on the degree to which they are violated (e.g., if the boundaries of the aquifer are well beyond the part of the aquifer which will be tested by the pumping test) the solution may still be useful.


Steady-state Thiem solution

Steady-state radial flow to a pumping well is commonly called the Thiem solution, it comes about from application of Darcy's law to
cylindrical A cylinder () has traditionally been a Solid geometry, three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a Prism (geometry), prism with a circle as its base. A cylinder may ...
shell control volumes (i.e., a cylinder with a larger radius which has a smaller radius cylinder cut out of it) about the pumping well; it is commonly written as: : h_0 - h = \frac \ln\left( \frac \right) In this expression ''h0'' is the background
hydraulic head Hydraulic head or piezometric head is a measurement related to liquid pressure (normalized by specific weight) and the liquid elevation above a vertical datum., 410 pages. See pp. 43–44., 650 pages. See p. 22, eq.3.2a. It is usually meas ...
, ''h0''-''h'' is the drawdown at the radial distance ''r'' from the pumping well, ''Q'' is the discharge rate of the pumping well (at the origin), ''T'' is the transmissivity, and ''R'' is the radius of influence, or the distance at which the head is still ''h0''. These conditions (steady-state flow to a pumping well with no nearby boundaries) ''never truly occur'' in nature, but it can often be used as an approximation to actual conditions; the solution is derived by assuming there is a circular constant head boundary (e.g., a
lake A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from ...
or
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 ...
in full contact with the aquifer) surrounding the pumping well at a distance ''R''.


Sources of error

Of critical importance in both aquifer and well testing is the accurate recording of data. Not only must water levels and the time of the measurement be carefully recorded, but the pumping rates must be periodically checked and recorded. An unrecorded change in pumping rate of as little as 2% can be misleading when the data are analysed.


References


Further reading

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 ...
has some very useful free references on pumping test interpretation: * * * * Some commercial printed references on aquifer test interpretation: * **Good summary of the most popular aquifer test methods, good for practicing hydrogeologists * **Thorough, a bit more mathematical than Batu * **Excellent treatment of most aquifer test analysis methods (but it is a hard-to-find book). * **On line

* **On line ANSDIMAT user's guide

More book titles can be found in the '' Hydrogeology#Further reading, further reading'' section of the hydrogeology article, most of which contain some material on aquifer test analysis or the theory behind these test methods.


Analysis software


Water Resources Applications Software
from 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 ...

Schlumberger Water Services
– Pumping test and slug test data analysis software
ANSDIMAT
– advanced commercial software
AQTESOLV
– standard commercial software
MLU for Windows LT
– Free software for pumping test and slug test analysis in one or two aquifer systems
VINMOD Multi-Well
Groundwater pollution Groundwater pollution (also called groundwater contamination) occurs when pollutants are released to the ground and make their way into groundwater. This type of water pollution can also occur naturally due to the presence of a minor and unwant ...
analysis using pumping tests and pollution parameters from pumped groundwater
Hytool
- Open source toolbox for pumping and build up tests interpretation on Matlab
SmartGEO
- advanced commercial software for heterogeneous aquifers characterization, hydraulic tomography and multiple pumping tests


See also

* Dupuit–Forchheimer assumption *
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 ...
*
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 ...
{{Hydrogeology Aquifers Hydrology Hydraulic engineering Water wells