Aquifer Tests

by Glenn M. Duffield, President, HydroSOLVE, Inc.

What Is An
Aquifer Test?

An aquifer test is a controlled field experiment used to estimate hydraulic properties of aquifer systems such as transmissivity, hydraulic conductivity and storativity (storage coefficient). The three fundamental aquifer testing methods are pumping tests, slug tests and constant-head tests. In each of the testing methods, the control well imposes a different type of controlled hydraulic stress on the aquifer system under investigation.

Methods Of
Aquifer Testing

Figure 1. Aquifer testing methods used to estimate hydraulic properties of aquifers and aquitards.

Three distinct aquifer testing methods are used by groundwater hydrologists for the purpose of estimating hydraulic properties of aquifer systems. Each method differs according to the type of hydraulic stress that the control well imposes on an aquifer system:

  • In a pumping test, a pumping well stresses an aquifer by extracting or injecting water at a controlled rate.
  • In a slug test, the imposed hydraulic stress is due to a sudden change in water level in the control well.
  • In a constant-head test, the head or drawdown in the control well is not allowed to vary.

Since the 1930s, aquifer system properties have been estimated from aquifer tests by fitting mathematical models (type curves) to response data (water-level changes or pumping rates) using a procedure known as curve matching. Modern evaluation of aquifer test data augments traditional curve matching techniques with derivative analysis for more thorough and robust data interpretation.

Pumping Tests

In a pumping test, an aquifer is stressed by pumping a test well at a controlled rate while water levels are measured in one or more surrounding wells. A constant-rate test is the most commonly performed pumping test; however, variable-rate pumping tests such as step-drawdown tests for assessing well performance or designing a constant-rate test are sometimes carried out as well. Pumping tests are conducted to estimate the hydraulic properties of aquifer systems as well as identify aquifer boundaries.

Pumping test
Figure 2. Discharge from a constant-rate pumping test (source: Kansas Geological Survey).

Among groundwater hydrologists, the most familiar curve matching procedure for estimating aquifer properties from pumping tests is due to Theis (1935). The Theis method allows one to estimate the transmissivity and storatvity of a nonleaky confined aquifer having infinite extent by means of matching the Theis type curve to water-level changes (drawdowns) measured in wells during a constant-rate pumping test (Figure 3).

Curve Matching, Theis 1935 method, nonleaky confined aquifer
Figure 3. Estimation of aquifer properties by matching Theis (1935) type-curve solution to time-drawdown data collected in an observation well during a constant-rate pumping test in a nonleaky confined aquifer (data from Walton 1962).

Publication of the work by Theis (1935) marked a major stride forward in groundwater science because of the mathematical rigor applied to the evaluation of transient (nonequilibrium) pumping test data. In the decades since Theis' seminal work, a great number of progressively more sophisticated models have appeared in the literature that further facilitate the interpretation of pumping tests for different well configurations and aquifer geometries. For example, Hantush and Jacob (1955) published the first transient solution for the interpretation of pumping tests in leaky confined aquifers (Figure 4).

Curve Matching, Hatush and Jacob 1955 method, leaky confined aquifer
Figure 4. Estimation of aquifer properties by matching Hantush and Jacob (1955) type-curve solution to drawdown data collected in three fully penetrating observation wells during a constant-rate pumping test in a leaky confined aquifer. Theis (1935) solution for a nonleaky confined aquifer shown by red curve (data from USBR 1995).

Slug Tests

Slug tests, another common aquifer testing technique, are conducted to obtain estimates of aquifer/aquitard properties at a smaller scale than pumping tests. A slug test is initiated by causing the water level in the control well to suddenly rise or fall and subsequently recording the recovery of the water level to its pre-test level. In addition to estimating hydraulic properties, slug tests are sometimes performed to provide an indicator of well performance. These relatively inexpensive tests, which involve little or no water added to or removed from a well, can be advantageous at groundwater contamination sites.

Equipment for conducting a slug test with solid (mechanical) slug
Figure 5. Typical equipment setup for conducting a slug test (source: USGS).

Beginning in the late 1960s, rigorous mathematical models became available for the analysis of slug tests including the KGS Model by Hyder et al. (1994) (Figure 6).

Curve Matching, KGS Method, Overdamped Slug Test
Figure 6. Estimation of aquifer properties from time-displacement data collected during a slug test in an unconfined aquifer using the KGS Model (Hyder et al. 1994) type-curve solution (data from Butler 1998).

Constant-Head Tests

In practice, constant-head tests are performed less frequently than either pumping tests or slug tests. In a constant-head test, the head or drawdown in a control well is maintained at a constant level and the declining discharge rate from the control well is recorded with time; in addition, water level changes may be measured in surrounding wells.

One particular application for a constant-head test is a flowing artesian well in which groundwater under confined or artesian pressure flows at the land surface (Figure 7). In the literature, one finds constant-head tests were performed on flowing artesian wells in the Grand Junction area, Colorado, USA, to estimate the transmissivity and storativity of a confined aquifer (Lohman 1965).

Flowing artesian well
Figure 7. Example of a flowing artesian well near Brunswick, Georgia, USA, prior to increased groundwater use beginning in the 1880s (source: USGS).

The first analytical solution in the groundwater literature for the interpretation of a constant-head test in a nonleaky confined aquifer is due to Jacob and Lohman (1952). Using this solution, one matches type curves to transient discharge data measured at the control well to estimate transmissivity and storativity (Figure 8).

Curve Matching, Jacob and Lohman 1952 Method, Nonleaky Confined Aquifer
Figure 8. Estimation of aquifer properties by matching Jacob and Lohman (1952) type-curve solution to discharge data during a constant-drawdown pumping test in a nonleaky confined aquifer (data from Lohman 1972).

Additional
Resources

Aquifer Testing References

Looking for literature relating to aquifer tests? Check out the annotated aquifer testing reference list to find publications and articles pertaining to pumping tests, slug tests, constant-head tests and more.

Aquifer Testing Glossary

Please visit the glossary for a list of aquifer testing terms and their definitions.