Aquifer test
Encyclopedia
An aquifer test is conducted to evaluate an aquifer
by "stimulating" the aquifer through constant pump
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, aquitards 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 or engineering 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, but adding complexity does not ensure better results (see parsimony).
Aquifer testing differs from well test
ing 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
in the aquifer
). Typically monitoring and pumping wells are screened across the same aquifers.
(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:
Additional aquifer characteristics which are sometimes evaluated, depending on the type of aquifer, include:
must be chosen to fit to the observed data. There are many different choices of models, depending on what factors are deemed important including:
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.
literature (with the mathematical help of C.I. Lubin), for two-dimensional radial flow to a point source in an infinite, homogeneous aquifer
. It is simply
where s is the drawdown
(change in hydraulic head at a point since the beginning of the test), u is a dimensionless time parameter, Q is the discharge (pumping) rate of the well
(volume divided by time, or m³/s), T and S are the transmissivity
and storativity of the aquifer around the well (m²/s and unitless), r is the distance from the pumping well to the point where the drawdown was observed (m or ft), t is the time since pumping began (minutes or seconds), and W(u) is the "Well function" (called the exponential integral
, E1, in non-hydrogeology literature).
Typically this equation is used to find the average T and S values near a pumping well
, from drawdown (hydrology)
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. As long as none of the additional simplifications which the Theis solution requires (in addition to those required by the groundwater flow equation) are violated, the solution should be very good.
The assumptions required by the Theis solution are:
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.
to cylindrical
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:
In this expression h0 is the background hydraulic head
, h-h0 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
or river
in full contact with the aquifer) surrounding the pumping well at a distance R.
Some commercial printed references on aquifer test interpretation:
More book titles can be found in the further reading section of the hydrogeology article, most of which contain some material on aquifer test analysis or the theory behind these test methods.
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
by "stimulating" the aquifer through constant pump
Pump
A pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...
ing, and observing the aquifer's "response" (drawdown
Drawdown (hydrology)
In water-related science and engineering there are two similar but distinct definitions in use for drawdown.*In subsurface hydrogeology, drawdown is the change in hydraulic head observed at a well in an aquifer, typically due to pumping a well as part of an aquifer test or well test.*In surface...
) in observation wells
Water well
A water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...
. Aquifer testing is a common tool that hydrogeologists
Hydrogeology
Hydrogeology is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust, . The term geohydrology is often used interchangeably...
use to characterize a system of aquifers, aquitards and flow system boundaries.
A slug test
Slug test
A slug test is a particular type of aquifer test where water is quickly added or removed from a groundwater well, and the change in hydraulic head is monitored through time, to determine the near-well aquifer characteristics...
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 or engineering 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, but adding complexity does not ensure better results (see parsimony).
Aquifer testing differs from well test
Well test
This article discusses water well testing; the testing of other wells, eg. petroleum wells, is a separate field.A well test is conducted to evaluate the amount of water that can be pumped from a particular water well...
ing in that the behaviour of the well is primarily of concern in the latter, while the characteristics of the aquifer
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
are quantified in the former. Aquifer testing also often utilizes one or more monitoring wells
Water well
A water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...
, 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
Hydraulic head or piezometric head is a specific measurement of water pressure above a geodetic datum. It is usually measured as a water surface elevation, expressed in units of length, at the entrance of a piezometer...
in the aquifer
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
). 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 drawdownDrawdown (hydrology)
In water-related science and engineering there are two similar but distinct definitions in use for drawdown.*In subsurface hydrogeology, drawdown is the change in hydraulic head observed at a well in an aquifer, typically due to pumping a well as part of an aquifer test or well test.*In surface...
(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 conductivityHydraulic conductivityHydraulic conductivity, symbolically represented as K, is a property of vascular plants, soil or rock, that describes the ease with which water can move through pore spaces or fractures. It depends on the intrinsic permeability of the material and on the degree of saturation...
The rate of flow of water through a unit cross sectional area of an aquifer, at a unit hydraulic gradient. In English units the rate of flow is in gallons per day per square foot of cross sectional area. - Specific storageSpecific storageSpecific storage , storativity , specific yield and specific capacity are material physical properties that characterize the capacity of an aquifer to release groundwater from storage in response to a decline in hydraulic head. For that reason they are sometimes referred to as "storage properties"...
or storativity: a measure of the amount of water of a confined aquifer will give up for a certain change in head; - TransmissivityTransmissivityTransmissivity may refer to:* Transmissivity , the rate which groundwater flows horizontally through an aquifer* Transmittance, in optics...
The rate at which water is transmitted through a unit thickness 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 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 equationGroundwater flow equation
Used in hydrogeology, the groundwater flow equation is the mathematical relationship which is used to describe the flow of groundwater through an aquifer. The transient flow of groundwater is described by a form of the diffusion equation, similar to that used in heat transfer to describe the flow...
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 porosityPorosityPorosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
(typically in fractured rock), - anisotropicAnisotropyAnisotropy is the property of being directionally dependent, as opposed to isotropy, which implies identical properties in all directions. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties An example of anisotropy is the light...
aquifers, - heterogeneous 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) in 1935, from heat transferHeat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...
literature (with the mathematical help of C.I. Lubin), for two-dimensional radial flow to a point source in an infinite, homogeneous aquifer
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
. It is simply
where s is the drawdown
Drawdown (hydrology)
In water-related science and engineering there are two similar but distinct definitions in use for drawdown.*In subsurface hydrogeology, drawdown is the change in hydraulic head observed at a well in an aquifer, typically due to pumping a well as part of an aquifer test or well test.*In surface...
(change in hydraulic head at a point since the beginning of the test), u is a dimensionless time parameter, Q is the discharge (pumping) rate of the well
Water well
A water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...
(volume divided by time, or m³/s), T and S are the transmissivity
Transmissivity
Transmissivity may refer to:* Transmissivity , the rate which groundwater flows horizontally through an aquifer* Transmittance, in optics...
and storativity of the aquifer around the well (m²/s and unitless), r is the distance from the pumping well to the point where the drawdown was observed (m or ft), t is the time since pumping began (minutes or seconds), and W(u) is the "Well function" (called the exponential integral
Exponential integral
In mathematics, the exponential integral is a special function defined on the complex plane given the symbol Ei.-Definitions:For real, nonzero values of x, the exponential integral Ei can be defined as...
, E1, in non-hydrogeology literature).
Typically this equation is used to find the average T and S values near a pumping well
Water well
A water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...
, from drawdown (hydrology)
Drawdown (hydrology)
In water-related science and engineering there are two similar but distinct definitions in use for drawdown.*In subsurface hydrogeology, drawdown is the change in hydraulic head observed at a well in an aquifer, typically due to pumping a well as part of an aquifer test or well test.*In surface...
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. As long as none of the additional simplifications which the Theis solution requires (in addition to those required by the groundwater flow equation) are violated, the solution should be very good.
The assumptions required by the Theis solution are:
- homogeneous, isotropic, confined aquiferAquiferAn aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
, - wellWater wellA water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...
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, and the pumping well is 100% efficient, well has a constant pumping rate Q
- 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 lawDarcy's law
Darcy's law is a phenomenologically derived constitutive equation that describes the flow of a fluid through a porous medium. The law was formulated by Henry Darcy based on the results of experiments on the flow of water through beds of sand...
to cylindrical
Cylinder (geometry)
A cylinder is one of the most basic curvilinear geometric shapes, the surface formed by the points at a fixed distance from a given line segment, the axis of the cylinder. The solid enclosed by this surface and by two planes perpendicular to the axis is also called a cylinder...
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:
In this expression h0 is the background hydraulic head
Hydraulic head
Hydraulic head or piezometric head is a specific measurement of water pressure above a geodetic datum. It is usually measured as a water surface elevation, expressed in units of length, at the entrance of a piezometer...
, h-h0 is the drawdown
Drawdown (hydrology)
In water-related science and engineering there are two similar but distinct definitions in use for drawdown.*In subsurface hydrogeology, drawdown is the change in hydraulic head observed at a well in an aquifer, typically due to pumping a well as part of an aquifer test or well test.*In surface...
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
Transmissivity
Transmissivity may refer to:* Transmissivity , the rate which groundwater flows horizontally through an aquifer* Transmittance, in optics...
, 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
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
or river
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...
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.Additional reading
The US Geological Survey 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 : http://www.waterlog.info/sahysmod.htm or http://www.waterlog.info/pdf/satem.pdf.
- On line ANSDIMAT user's guide : http://www.ansdimat.com/help/.
More book titles can be found in the 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 SurveyUnited States Geological SurveyThe United States Geological Survey is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, and the natural hazards that threaten it. The organization has four major science disciplines, concerning biology,...
- 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 analysis using pumping tests and pollution parameters from pumped groundwater
- Online Theis Equation Calculator - Free online calculator solves Theis equation and displays results graphically
See also
- Dupuit assumptionDupuit assumptionThe Dupuit assumption holds that groundwater moves horizontally in an unconfined aquifer, and that the groundwater discharge is proportional to the saturated aquifer thickness...
- GroundwaterGroundwaterGroundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
- Water wellWater wellA water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...
- LinkedIn group on pumping test analysis