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Encyclopedia > Aquifer test

An Aquifer test is conducted to evaluate an aquifer by "stimulating" the aquifer through constant pumping, 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. An aquifer is an underground layer of water-bearing permeable rock, or permeable mixtures of unconsolidated materials (gravel, sand, silt, or clay) (see also groundwater). ... Drawdown (in hydrogeology) 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. ... For the pioneering virtual community, see The WELL. A well is commonly a pipe or tube installed in an artificial boring in the earth through which water, oil or gas can be obtained. ... Hydrogeology (hydro- meaning water, and -geology meaning the study of rocks) is the part of hydrology that deals with the distribution and movement of groundwater in the soil and rocks of the Earths crust (commonly in aquifers). ...


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. 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. ...


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). Parsimony, in the general sense, means that one has taken extreme care at arriving at a course of action. ...


Aquifer testing (sometimes informally referred to as pump 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 and 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. A pump is a mechanical device used to move liquids or gases. ... This article discusses Water well testing; the testing of other wells, eg. ... An aquifer is an underground layer of water-bearing permeable rock, or permeable mixtures of unconsolidated materials (gravel, sand, silt, or clay) (see also groundwater). ... For the pioneering virtual community, see The WELL. A well is commonly a pipe or tube installed in an artificial boring in the earth through which water, oil or gas can be obtained. ... This is about the hydraulic head of slow-flowing groundwater, for fluid dynamic oriented definitions of hydraulic head, see hydraulic head (disambiguation page) Hydraulic head is a measure of the amount of energy groundwater, flowing through an aquifer, has per unit weight. ... An aquifer is an underground layer of water-bearing permeable rock, or permeable mixtures of unconsolidated materials (gravel, sand, silt, or clay) (see also groundwater). ...

Contents


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. Drawdown (in hydrogeology) 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. ...


The aquifer characteristics which are evaluated by most aquifer tests are:

  • Hydraulic conductivity or transmissivity: the ability of an aquifer to transmit water (how permeable it is);
  • Specific storage or storativity: a measure of the amount of water of a confined aquifer will give up for a certain change in head;

Additional aquifer characteristics which are sometimes evaluated, depending on the type of aquifer, include: Hydraulic conductivity, mathematically represented as , is a property of soil or rock, in the vadose zone or groundwater, that describes the ease with which water can move through pore spaces or fractures. ... Specific storage, storativity and specific yield (Ss, S and Sy) are aquifer properties; they are measures of the ability of an aquifer to release groundwater from storage, due to a unit decline in hydraulic head. ...

  • Specific yield or drainable porosity: a measure of the amount of water an unconfined aquier will give up when completely drained;
  • Leakage coefficient: some aquifers are bounded by aquitards which slowly give up water to the aquitard, providing additional water to reduce drawdown;
  • The presence of aquifer boundaries (recharge or no-flow) and their distance from the pumped well and piezometers.

Specific storage, storativity and specific yield (Ss, S and Sy) are aquifer properties; they are measures of the ability of an aquifer to release groundwater from storage, due to a unit decline in hydraulic head. ...

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 important including: The groundwater flow equation is the mathematical relationship which is used to describe the flow of groundwater through a porous medium (e. ...

  • 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 (typically in fractured rock),
  • anisotropic 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. In geology, the porosity of a rock or sediment is the proportion of the non-solid volume to the total volume of material, and is defined by the ratio: where Vp is the non-solid volume (pores and liquid) and Vm is the total volume of material, including the solid... Look up Anisotropy in Wiktionary, the free dictionary Anisotropy (the opposite of isotropy) is the property of being directionally dependent. ... A heterogeneous compound, mixture, or other such object is one that consists of many different items. ...


Theis solution

The Theis equation was adopted by Charles Vernon Theis (working for the US Geological Survey) in 1935 (see references), from heat transfer 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 The United States Geological Survey (USGS) is a scientific agency of the United States government. ... This article is in the process of being merged into Heat, and may be outdated. ... An aquifer is an underground layer of water-bearing permeable rock, or permeable mixtures of unconsolidated materials (gravel, sand, silt, or clay) (see also groundwater). ...



,

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). Drawdown (in hydrogeology) 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. ... For the pioneering virtual community, see The WELL. A well is commonly a pipe or tube installed in an artificial boring in the earth through which water, oil or gas can be obtained. ... Hydraulic conductivity, mathematically represented as , is a property of soil or rock, in the vadose zone or groundwater, that describes the ease with which water can move through pore spaces or fractures. ... Specific storage, storativity and specific yield (Ss, S and Sy) are aquifer properties; they are measures of the ability of an aquifer to release groundwater from storage, due to a unit decline in hydraulic head. ... In mathematics, the exponential integral Ei(x) is defined as Since 1/t diverges at t=0, the above integral has to be understood in terms of the Cauchy principal value. ...


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. For the pioneering virtual community, see The WELL. A well is commonly a pipe or tube installed in an artificial boring in the earth through which water, oil or gas can be obtained. ... Drawdown (in hydrogeology) 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. ...


The assumptions required by the Theis solution are:

  • homogeneous, isotropic, confined aquifer,
  • well 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,
  • aquifer is infinite in radial extent,
  • horizontal (not sloping), flat, impermeable (non-leaky) top and bottom boundaries of aquifer,

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. An aquifer is an underground layer of water-bearing permeable rock, or permeable mixtures of unconsolidated materials (gravel, sand, silt, or clay) (see also groundwater). ... For the pioneering virtual community, see The WELL. A well is commonly a pipe or tube installed in an artificial boring in the earth through which water, oil or gas can be obtained. ...


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 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: Darcys Law is a phenomologically derived constituative equation that describes the flow of a fluid through a porous medium (typically water through an aquifer). ... A right circular cylinder In mathematics a cylinder is a quadric, i. ...

.

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 transmissibility, 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 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 at a distance R from the pumping well. This is about the hydraulic head of slow-flowing groundwater, for fluid dynamic oriented definitions of hydraulic head, see hydraulic head (disambiguation page) Hydraulic head is a measure of the amount of energy groundwater, flowing through an aquifer, has per unit weight. ... Drawdown (in hydrogeology) 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. ...


Sources of error

Of critical importance in both aquifer and well testing is the accurate recording of data. Not only must water levels be measured carefully, often at millimeter accuracy, and the time of the measurement 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

  • Theis, Charles V., 1935. The relation between the lowering of the piezometric surface and the rate and duration of discharge of a well using ground-water storage, Transactions, American Geophysical Union, 16, 519-524.

Additional Reading

The US Geological Survey has some very useful free references (.pdf) on pumping test interpretation: The United States Geological Survey (USGS) is a scientific agency of the United States government. ...

  • Theory of Aquifer Tests
  • Aquifer-Test Design, Observation, and Data Analysis
  • Type Curves for Selected Problems of Flow to Wells in Confined Aquifers
  • Definition of Boundary and Initial Conditions in the Analysis of Saturated Ground-Water Flow Systems - an Introduction

Some printed references on aquifer test interpretation

  • Batu, Vedat, 1998, Aquifer Hydraulics: a comprehensive guide to hydrogeologic data analysis'. Wiley-Interscience. — Good summary of the most popular aquifer test methods, good for practicing hydrogeologists. ISBN 0471185027
  • Dawson, Karen J. & Istok, Jonathan D., 1991. Aquifer Testing: design and analysis of pumping and slug tests'. Lewis Publishers. — Thorough, a bit more mathematical than Batu. ISBN 0873715012
  • Kruseman, G.P. & de Ridder, N.A., 1990. Analysis and Evaluation of Pumping Test Data second edition. International Institute for Land Reclamation and Improvement, Wagningen, The Netherlands. — Excellent treatment of most aquifer test analysis methods (hard to find book). ISBN 9070754207

More book titles can be found in the Hydrogeology Further Reading section, most of which contain some material on aquifer test analysis or the theory behind these test methods. Hydrogeology (hydro- meaning water, and -geology meaning the study of rocks) is the part of hydrology that deals with the distribution and movement of groundwater in the soil and rocks of the Earths crust (commonly in aquifers). ...


See Also

Aquifer properties used in hydrogeology
hydraulic head | hydraulic conductivity | storativity | porosity | water content

  Results from FactBites:
 
AQTESOLV for Windows v4.5--The World's LEADING Software for the Analysis of Aquifer Tests (Pumping Tests/Slug ... (1476 words)
for analyzing a underdamped slug test with oscillatory response in high-K confined and unconfined aquifers.
Analyze drawdown in the pumped aquifer, the aquitard and the unpumped aquifer!
State-of-the-art diagnostic plots help you to choose appropriate solution methods for your test data and reduce uncertainty by identifying wellbore storage, fracture flow and other important conditions that are essential for thorough analysis of an aquifer test.
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