Aquifer Testing 101 : 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 an aquifer system such as transmissivity, hydraulic conductivity and storativity (storage coefficient). Types of aquifer tests include pumping tests, slug tests and constant-head tests which differ according to the form of hydraulic stress applied to an aquifer system at the control well.
In the traditional constant-rate pumping test, perhaps the most commonly used aquifer testing method, an aquifer is pumped at a constant rate of extraction or injection with the aim of estimating the hydraulic properties of aquifers and aquitards as well as identifying aquifer boundaries (no flow and constant head).
Slug tests, another common aquifer testing technique, are conducted to obtain estimates of aquifer/aquitard properties at a smaller scale than pumping tests. Occasionally, slug tests are used to provide an indicator of well performance. These relatively inexpensive tests, which involve little to no water removed from or added to a well, can be advantageous at sites with groundwater contamination.
Aquifer Testing Methods
The three aquifer testing methods used most frequently for the estimation of aquifer properties are as follows:
Estimating Aquifer Properties (Overview)
Typically, aquifer properties are estimated from an aquifer test by fitting mathematical models (type curves) to response data (water-level changes or pumping rates) using a procedure known as curve matching. In addition to curve matching, modern computer software for aquifer test evaluation employs the technique of derivative analysis to obtain a more robust interpretation of response data.
Among hydrogeologists, the most familiar curve matching procedure for pumping tests is due to Theis (1935). The Theis method allows one to estimate the hydraulic properties of nonleaky confined aquifers having infinite extent by matching the Theis type curve to water-level changes (drawdowns) measured in wells during a constant-rate pumping test (Figure 1).
Figure 1. 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 wide range of well configurations and aquifer geometries encountered in the field. For example, Hantush and Jacob (1955) published the first transient solution for the interpretation of pumping tests in leaky confined aquifers (Figure 2).
Figure 2. Estimation of aquifer properties by matching Hantush-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).
Figure 3. 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).
The first analytical solution in the groundwater literature for the interpretation of a constant-head (constant-drawdown) test in a nonleaky confined aquifer is due to Jacob and Lohman (1952). Using the Jacob and Lohman solution, one matches type curves to transient discharge data measured at the control well (Figure 4).
Figure 4. 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).
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.
Please visit the glossary for a list of aquifer testing terms and their definitions.